WO2002031932A1 - Very narrow band, two chamber, high rep rate gas discharge laser system - Google Patents
Very narrow band, two chamber, high rep rate gas discharge laser system Download PDFInfo
- Publication number
- WO2002031932A1 WO2002031932A1 PCT/US2001/030164 US0130164W WO0231932A1 WO 2002031932 A1 WO2002031932 A1 WO 2002031932A1 US 0130164 W US0130164 W US 0130164W WO 0231932 A1 WO0231932 A1 WO 0231932A1
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- Prior art keywords
- laser
- pulse
- gas
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- discharge
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Definitions
- the present invention is a continuation-in-part of Serial No. 09/854,097, filed May 11, 2001, Serial No. 09/848,043, filed May 3, 2001, Serial No. 09/459,165, filed December 10, 1999, Serial No. 09/834,840, filed April 13, 2001, Serial No. 09/794,782, filed February 27, 2001, Serial No. 09/771,789, filed January 29, 2001, Serial No. 09/768,753, filed January 23, 2001, Serial No. 09/684,629, filed October 6, 2000, Serial No. 09/597,812, filed June 19, 2000 and Serial No. 09/473,852, filed December 27, 1999.
- This invention relates to electric discharge gas lasers and in particular to very narrow band high repetition rate injection seeded gas discharge lasers.
- Electric discharge gas lasers are well known and have been available since soon after lasers were invented in the 1960s.
- a high voltage discharge between two electrodes excites a laser gas to produce a gaseous gain medium.
- a resonance cavity containing the gain medium permits stimulated amplification of light which is then extracted from the cavity in the form of a laser beam.
- Many of these electric discharge gas lasers are operated in a pulse mode.
- Excimer lasers are a particular type of electric discharge gas laser and they have been known since the mid 1970s.
- a description of an excimer laser, useful for integrated circuit lithography, is described in U.S. Patent No. 5,023,884 issued June 11, 1991 entitled “Compact Excimer Laser.” This patent has been assigned to Applicants' employer, and the patent is hereby incorporated herein by reference.
- the excimer laser described in Patent '884 is a high repetition rate pulse laser.
- These excimer lasers, when used for integrated circuit lithography are typically operated in an integrated circuit fabrication line "around-the-clock" producing many thousands of valuable integrated circuits per hour; therefore, down-time can be very expensive. For this reason most of the components are organized into modules which can be replaced within a few minutes.
- Excimer lasers used for lithography typically must have its output beam reduced in bandwidth to a fraction of a picometer. This "line-narrowing" is typically accomplished in a line narrowing module (called a “line narrowing package” or “LNP”) which forms the back of the laser's resonant cavity.
- LNP line narrowing package
- This LNP is comprised of delicate optical elements including prisms, mirrors and a grating.
- Electric discharge gas lasers of the type described in Patent '884 utilize an electric pulse power system to produce the electrical discharges, between the two electrodes. In such prior art systems, a direct current power supply charges a capacitor bank called “the charging capacitor” or “Go” to a predetermined and controlled voltage called the “charging voltage” for each pulse.
- This charging voltage may be in the range of about 500 to 1000 volts in these prior art units.
- a solid state switch is closed allowing the electrical energy stored on Co to ring very quickly through a series of magnetic compression circuits and a voltage transformer to produce high voltage electrical potential in the range of about 16,000 volts (or greater) across the electrodes which produce the discharges which lasts about 20 to 50 ns.
- Excimer lasers such as described in the '884 patent have during the period 1989 to 2001 become the primary light source for integrated circuit lithography. More than 1000 of these lasers are currently in use in the most modern integrated circuit fabrication plants. Almost all of these lasers have the basic design features described in the '884 patent.
- a single, pulse power system for providing electrical pulses across the electrodes at pulse rates of about 100 to 2500 pulses per second;
- a single resonant cavity comprised of a partially reflecting mirror-type output coupler and a line narrowing unit consisting of a prism beam expander, a tuning mirror and a grating;
- a single discharge chamber containing a laser gas (either KrF or ArF), two elongated electrodes and a tangential fan for circulating the laser gas between the two electrodes fast enough to clear the discharge region between pulses, and (4) a beam monitor for monitoring pulse energy, wavelength and bandwidth of output pulses with a feedback control system for controlling pulse energy, energy dose and wavelength on a pulse-to-pulse basis.
- a laser gas either KrF or ArF
- a well-known technique for reducing the band-width of gas discharge laser systems involves the injection of a narrow band "seed" beam into a gain medium.
- a laser producing the seed beam called a "master oscillator” is designed to provide a very narrow bandwidth beam in a first gain medium, and that beam is used as a seed beam in a second gain medium. If the second gain medium functions as a power amplifier, the system is referred to as a master oscillator, power amplifier (MOPA) system.
- MOPA master oscillator, power amplifier
- the system is referred to as an injection seeded oscillator (ISO) system or a master oscillator, power oscillator (MOPO) system in which case the seed laser is called the master oscillator and the downstream system is called the power oscillator.
- ISO injection seeded oscillator
- MOPO power oscillator
- Laser systems comprised of two separate systems tend to be substantially more expensive, larger and more complicated than comparable single chamber laser systems. Therefore, commercial application of two chamber laser has been limited.
- the present invention provides an injection seeded modular gas discharge laser system capable of producing high quality pulsed laser beams at pulse rates of about 4,000 Hz or greater and at pulse energies of about 5 to 10 mJ or greater.
- Two separate discharge chambers are provided, one of which is a part of a master oscillator producing a very narrow band seed beam which is amplified in the second discharge chamber.
- the chambers can be controlled separately permitting optimization of wavelength parameters in the master oscillator and optimization of pulse energy parameters in the amplifying chamber.
- a preferred embodiment is an ArF excimer laser system configured as a MOPA and specifically designed for use as a light source for integrated circuit lithography.
- each chamber comprises a single tangential fan providing sufficient gas flow to permit operation at pulse rates of 4000 Hz or greater by clearing debris from the discharge region in less time than the approximately 0.25 milliseconds between pulses.
- the master oscillator is equipped with a line narrowing package having a very fast tuning mirror capable of controlling centerline wavelength on a pulse-to-pulse basis at repetition rates of 4000 Hz or greater and providing a bandwidth of less than 0.2 pm (FWHM).
- FIG. 1 is a perspective drawing of a preferred embodiment of the present invention.
- FIGS. 1 A and IB show a U-shaped optical table.
- FIGS. 2 and 2 A show chamber features.
- FIGS. 3 A and 3B show a two-pass MOPA.
- FIGS. 4, 4A, 4B and 4C show features of a preferred pulse power system.
- FIGS. 5, 5A and 5B show additional features of the pulse power system.
- FIGS. 6A1 and 6A2 show various MOPA configurations and test results.
- FIGS. 6B, 6C, 6D and 6E show test results of prototype MOPA systems.
- FIGS. 7, 7A, 8, 9A, 9B, 10, 10A, 11, 12, 12A, 12B show features of pulse power components.
- FIG. 13 shows a technique for minimizing jitter problems.
- FIG. 14 shows elements of a wavemeter.
- FIGS. 14 A, 14B and 14C through 14H demonstrate a technique and components for measuring bandwidth.
- FIG. 15 shows a technique for fast reading of a photodiode array.
- FIG. 16 shows a technique for fine line narrowing of a master oscillator.
- FIGS. 16A, 16B1 and 16B2 show a PZT controlled LNP.
- FIG. 16C shows the result of the use of the PZT controlled LNP.
- FIGS. 16D and 16E show techniques for controlling the LNP.
- FIGS. 17, 17 A, 17B and 17C show techniques for purging a grating face.
- FIG. 18 shows a fan motor drive arrangement
- FIG. 18A show a preferred fan blade.
- FIGS. 19, 19A and 19B show features of a purge system.
- FIGS. 20, 20 A and 20B show features of a preferred shutter.
- FIGS. 21 and 21 A show heat exhanger features.
- FIGS. 22 A, 22B, 22C and 22D show a technique for support of discharge chambers on rails.
- FIGS. 22E, 22F, 22G and 22H show a technique for sealing a beam interface.
- FIGS. 221, 22 J, 22K and 22L show another technique for sealing a beam interface.
- FIGS. 22M show a purge technique for sealing and merging a portion of a beam path.
- FIG. 23 shows a technique for spatially filtering a seed beam.
- FIG. 1 is a perspective view of a first preferred embodiment of the present invention.
- This embodiment is an injection seeded narrow band excimer laser system configured as a MOPA laser system. It is specially designed for use as a light source for integrated circuit lithography.
- the major improvement in the present invention as exemplified in this embodiment over the prior art lithography lasers is the utilization of injection seeding and in particular a master oscillator-power amplifier (MOPA) configuration with two separate discharge chambers.
- MOPA master oscillator-power amplifier
- This first preferred embodiment is an argon-fluoride (ArF) excimer laser system; however, the system utilizes a modular platform configuration which is designed to accommodate either krypton-fluoride (KrF), ArF or fluorine (F 2 ) laser components.
- This platform design permits use of the same basic cabinet and many of the laser system modules and components for either of these three types of lasers. Applicants refer to this platform as tlieir "three wavelength platform" since the three laser designs produce laser beams with wavelengths of about 248 nm for KrF, about 193 run for ArF and about 157.63 for F 2 .
- This platform is also designed with interface components to make the laser systems at each of the three wavelengths compatible with modern lithography tools of all the major makers of such tools.
- FIG. 1 The major components of this preferred laser system 2 are identified in FIG. 1. These include:
- (1) laser system frame 4 which is designed to house all modules of the laser except the AC/DC power supply module,
- a resonant charger module 7 for charging two charging capacitor banks to about 1000 volts at rates of 4000 charges per second
- two commutator modules 8 A and 8B each comprising one of the charging capacitor banks referred to above and each comprising a commutator circuit for forming very short high voltage electrical pulses, of about 16,000 volts and about 1 ⁇ s duration from the energy stored on the charging capacitor banks,
- each module includes a discharge chamber 10A and 12A and a compression head 10B and 12B mounted on top of the chamber.
- the compression head compresses (time-wise) the electrical pulses from the commutator module from about 1 ⁇ s to about 50 ns with a corresponding increase in current,
- a wavefront engineering box 14 including optics and instruments for shaping and directing the seed beam into the power amplifier, and monitoring the MO output power
- beam stabilizer module 16 including wavelength, bandwidth and energy monitors
- a line selector unit is used instead of the LNP described herein since the natural F 2 spectrum comprises two primary lines one of which is selected and the other of which is deselected.
- the optics of both the MO and the PA are mounted on a U-shaped optical table as shown in FIGS. 1A and IB.
- the U-shaped optical table is kinematically mounted to the base of the laser in the manner described in U.S. Patent No. 5,863,017 incorporated herein by reference.
- Both chambers of the MO and the PA are not mounted on the table but each is supported by three wheels (two on one side and one on the other) on rails supported from the bottom frame of chamber 2. (The wheel and rails are preferably arranged as described in U.S. Patent No. 6,109,574 incorporated herein by reference.) This arrangement provides isolation of the optics from chamber caused vibrations.
- the master oscillator 10 is in many ways similar to prior art ArF lasers such as described in the '884 patent and in U.S. Patent No. 6,128,323, which is incorporated by reference herein. However, major improvements are provided to permit operation at 4000 Hz and greater, and it is optimized for spectral performance including bandwidth control. This result is a much more narrow bandwidth and improved bandwidth stability.
- the master oscillator comprises, discharge chamber 10A as shown in FIG. 1, FIG. 2 and FIG. 2 A in which are located a pair of elongated electrodes 10 A-2 and 10A-4, each about 50 cm long and spaced apart by about 0.5 inch. Anode 10A-4 is mounted on flow shaping anode support bar 10A-6.
- a tangential fan lOA-10 is driven by two motors (not shown) for providing a laser gas flow at a velocity of about 80 m/s between the electrodes.
- the chamber includes window units (not shown) with CaF 2 windows positioned at about 45° with the laser beam.
- An electrostatic filter unit having an intake at the center of the chamber, filters a small portion of the gas flow as indicated at 11 in FIG. 2 and the cleaned gas is directed into window units in the manner described in U.S. Patent No. 5,359,620 (incorporated herein by reference) to keep discharge debris away from the windows.
- the gain region of the master oscillator is created by discharges between the electrodes through the laser gas which in this embodiment is comprised of about 3% argon, 0.1% F and the rest neon.
- the gas flow clears the debris of each discharge from the discharge region prior to the next pulse.
- the resonant cavity is created at the output side by an output coupler 100 which is comprised of a CaF 2 mirror mounted perpendicular to the beam direction and coated to reflect about 30% of light at 193 nm and to pass about 70% of the 193 nm light.
- the opposite boundary of the resonant cavity is a line narrowing unit 10C as shown in FIG. 1 similar to prior art line narrowing units described in U.S. Patent No. 6,128,323.
- the LNP is described in more detail below as in FIGS.
- Important improvements in this line narrowing package include four CaF beam expanding prisms 10C1 for expanding the beam in the horizontal direction by 35 times and a tuning mirror 10C2 controlled by a stepper motor for relatively large pivots and a piezoelectric driver for providing extremely fine tuning of the mirror echelle grating 10C3 having about 80 facets per mm is mounted in the Litrow configuration reflects a very narrow band of UV light selected from the approximately 300 pm wide ArF natural spectrum.
- Another important improvement is a narrow rear aperture which limits the cross section of the oscillator beam to 1.1 mm in the horizontal direction and 7 mm in the vertical direction. Control of the oscillator beam is discussed below.
- Power amplifier 12 is comprised of laser chamber 12A which is in this embodiment is very similar to discharge chamber 10 A. Having the two separate chambers allows the pulse energy and integrated energy in a series of pulses (called dose) to be controlled, to a large extent, separately from wavelength and bandwidth. This permits better close stability. All of the components of the chamber are the same and are interchangeable during the manufacturing process. However, in operation, the gas pressure is substantially lower in the MO as compared to the PA.
- the compression head 12B of the power amplifier is also substantially identical in this embodiment to the 10B compression head and the components of the compression head are also interchangeable during manufacture. One difference is that the capacitors of the compression head capacitor bank are more widely positioned for the MO to produce a substantially higher inductance as compared to the PA. This close identity of the chambers and the electrical components of the pulse power systems helps assure that the timing characteristics of the pulse forming circuits are the same or substantially the same so that jitter problems are minimized.
- the power amplifier is configured for two beam passages through the discharge region of the power amplifier discharge chamber.
- FIGS. 3 A and 3B show the beam path through the master oscillator and the power amplifier.
- the beam oscillates several times through the chamber 10A and LNP IOC of the MO 10 as shown in FIG. 3 A and is severely line narrowed on its passages through LNP IOC.
- the line narrowed seed beam is reflected upward by mirror 14A and reflected horizontally at an angle slightly skewed (with respect to the electrode orientations) through chamber 12A by mirror 14B.
- two mirrors 12C and 12D reflect the beam back for a second pass through PA chamber 12A horizontally in line with the electrode orientation as shown in FIG. 3B.
- FIGS.6A through 6E display some of the results of this proof of principal testing.
- FIG. 6A shows how well the skewed double pass amplifier design performs as compared with other amplifier designs. Other designs that have been tested are single pass, straight double pass, single pass with divided amplifier electrodes, tilted double pass.
- FIG. 6B shows system output pulse energy as a function of PA input energy for the skewed double pass configuration at charging voltage ranging from 650 V to 1100 V.
- FIG. 6C shows the shape of the output pulse as a function of time delay between beginning of the oscillator and the amplifier pulses for four input energies.
- FIG. 6D shows the effect of time delay between pulses on output beam bandwidth. This graph also shows the effect of delay on output pulse energy. This graph shows that bandwidth can be reduced at the expense of pulse energy.
- FIG. 6E shows that the laser system pulse duration can also be extended somewhat at the expense of pulse energy.
- the basic pulse power circuits are- similar to pulse power circuits of prior art excimer laser light sources for lithography. Important differences and improvements include the provision of separate pulse power circuits ⁇ downstream of the charging capacitors for each discharge chamber. A single resonant charger charges two charging capacitor banks connected in parallel to assure that both charging capacitor banks are charged to precisely the same voltage. Important improvements are also provided to regulate the temperature of components of the pulse power circuits.
- FIGS. 5, 5A and 5B show important elements of a preferred basis pulse power circuit which is used for the MO. The same basic circuit is also used for the PA. The only significant difference being the values chosen for some of the capacitors and inductors in the circuit. The circuits shown in FIGS. 5, 5A and 5B are described below.
- Applicants have utilized two types of resonant charging systems for very fast charging of C 0 . These systems can be described by reference to FIGS. 5A and 5B.
- FIG 5A An electrical circuit showing this preferred resonant charges is shown in FIG 5A.
- a standard dc power supply 200 having a 208 VAC/90 amp input and an 1500 VDC 50 amp output is used.
- the power supply is a dc power supply adjustable from approximately 600 volts to 1500 volts.
- the power supply is attached directly to C-l eliminating the need for voltage feedback to the supply.
- the performance of the system is somewhat independent of the voltage regulation on C-l therefore only the most basic control loop is necessary in the power supply.
- the supply will be adding energy into the system whenever the voltage on C-l falls below the voltage setting.
- Control Board This power supply continuously charges a 1033 iF capacitor 202 to the voltage level commanded by the control board 204.
- the control board 204 also commands IGBT switch 206 closed and open to transfer energy from capacitor 202 to capacitor 42.
- Inductor 208 sets up the transfer time constant in conjunction with capacitor 202 and 42 and limits the peak charging current.
- Control board 204 receives a voltage feedback 212 that is proportional to the voltage on capacitor 42 and a current feedback 214 that is proportional to the current flowing through inductor 208. From these two feedback signals control board 204 can calculate in real time the final voltage on capacitor 42 should IGBT switch 206 open at that instant of time.
- control board 204 Therefore with a command voltage 210 fed into control board 204 a precise calculation can be made of the stored energy within capacitor 42 and inductor 208 to compare to the required charge voltage commanded 210. From this calculation, the control board 204 will determine the exact time in the charge cycle to open IGBT switch 206.
- IGBT switch 206 opens the energy stored in the magnetic field of inductor 208 will transfer to the two capacitor banks 42 (Co's for the MO and the PA) through the freewheeling diode path 215.
- the accuracy of the real time energy calculation will determine the amount of fluctuation dither that will exist on the final voltage on capacitor banks 42. Due to the extreme charge rate of this system, too much dither may exist to meet a desired systems regulation need of ⁇ 0.05%). If so, additional circuitry may be utilized, such as for example, a de-qing circuit or a bleed-down circuit as discussed below.
- FIG. 5B Second Resonant Charger A second resonant charger system is shown in FIG. 5B. This circuit is similar to the one shown in FIG. 5 A. The principal circuit elements are:
- C-l - A source capacitor 302 that is an order of magnitude or more larger than the existing Co capacitor 42.
- Ql, Q2, and Q3 - Switches to control current flow for charging and maintaining a regulated voltage on Co- Dl, D2, and D3 - Provides current single direction flow.
- Rl, and R2 - Provides voltage feedback to the control circuitry.
- R3 - Allows for rapid discharge of the voltage on Co in the event of a small over charge.
- Control Board 304 - Commands Ql, Q2, and Q3 open and closed based upon circuit feedback parameters.
- V f The voltage on Co after Ql opens and the current in LI goes to zero.
- Nco s The voltage on Co when Ql opens.
- I L i s The current flowing through L! when Ql opens.
- Switch Q3 of bleed down circuit 216 will be commanded closed by the control board when current flowing through inductor LI stops and the voltage on Co will be bled down to the desired control voltage; then switch Q3 is opened.
- the time constant of capacitor C 0 and resistor R3 should be sufficiently fast to bleed down capacitor C 0 to the command voltage without being an appreciable amount of the total charge cycle.
- the resonant charger can be configured with tliree levels of regulation control. Somewhat crude regulation is provided by the energy calculator and the opening of switch Ql during the charging cycle. As the voltage on the CO capacitor banks nears the target value, the de-qing switch is closed, stopping the resonant charging when the voltage on C 0 is at or slightly above the target value.
- the switch Ql and the de-qing switch is used to provide regulation with accuracy better than +/- 0.1%. If additional regulation is required, the third control over the voltage regulation could be utilized. This is the bleed-down circuit of switch Q3 and R3 (shown at 216 in FIG. 5B) to discharge the CO's down to the precise target value. Improvements Downstream of the CO's
- the pulse power system of the MO and the PA of the present invention each utilizes the same basic design as was used in the prior art systems.
- some significant improvements in that basic design were required for the approximate factor of 3 increase in heat load resulting from the greatly increased repetition rate. These improvements are discussed below.
- Solid State Switch Solid state switch 46 is an P/N CM 800 HA-34H IGBT switch provided by Powerex,
- Inductors Inductors 48, 54 and 64 are saturable inductors similiar to those used in prior systems as described in U.S. Patents 5,448,580 and 5,315,611.
- FIG. 7 shows a preferred design of the L 0 inductor 48. In this inductor four conductors from the two IGBT switches 46B pass through sixteen ferrite toroids 49 to form part 48A an 8 inch long hollow cylinder of very high permability material with an ID of about 1 inch and an Od of about 1.5 inch.
- Each of the four conductors are then wrapped twice around an insulating doughnut shaped core to form part 48B.
- the four conductors then connect to a plate which is in turn connected to the high voltage side of the Ci capacitor bank 52.
- FIG. 8 A preferred sketch of saturable inductor 54 is shown in FIG. 8.
- the inductor is a single turn geometry where the assembly top and bottom lids 541 and 542 and center mandrel 543, all at high voltage, form the single turn through the inductor magnetic cores.
- the outer housing 545 is at ground potential.
- the magnetic cores are 0.0005" thick tape wound 50-50%) Ni-Fe alloy provided by Magnetics of Butler, Pennsylvania or National Arnold of Adelanto, California. Fins 546 on the inductor housing facilitate transfer of internally dissipated heat to forced air cooling.
- a ceramic disk (not shown) is mounted underneath the reactor bottom lid to help transfer heat from the center section of the assembly to the module chassis base plate.
- FIG. 8 also shows the high voltage connections to one of the capacitors of the C ⁇ capacitor bank 52 and to a high voltage lead on one of the induction units of the 1:25 step up pulse transformer 56.
- the housing 545 is connected to the ground lead of unit 56.
- FIGS. 9 A and 9B A top and section view of the saturable inductor 64 is shown respectively in FIGS. 9 A and 9B.
- flux excluding metal pieces 301, 302, 303 and 304 are added as shown in FIG. 9B in order to reduce the leakage flux in the inductors.
- These flux excluding pieces substantially reduce the area which the magnetic flux can penetrate and therefore help to minimize the saturated inductance of the inductor.
- the current makes five loops through vertical conductor rods in the inductor assembly around magnetic core 307.
- the current enters at 305 travels down a large diameter conductor in the center labeled "1" and up six smaller conductors on the circumference also labeled "1" as shown in FIG. 9 A.
- the current then flows down two conductors labeled 2 on the inside, then up the six conductors labeled 2 on the outside then down flux exclusion metal on the inside then up the six conductors labeled 3 on the outside, then down the two conductors labeled 3 on the inside, then up the six conductors labeled 4 on the outside, then down the conductor labeled 4 on the inside.
- the flux exclusion metal components are held at half the full pulsed voltage across the conductor allowing a reduction in the safe hold-off spacing between the flux exclusion metal parts and the metal rods of the other turns.
- the magnetic core 307 is made up of three coils 307A, B and C formed by windings of 0.0005" thick tape 80-20% Ni-Fe alloy provided by Magnetics, Inc.
- nano-crystoline materials such as VITROPERMTM available from VACUUM SCHITELZE GmbH, Germany and FINEMETTM from Hitachi Metals, Japan could be used for inductors 54 and 64.
- oil leakage from electrical components has been a potential problem.
- oil insulated components are limited to the saturable inductors.
- the saturable inductor 64 as shown in FIG. 9B is housed in a pot type oil containing housing in which all seal connections are located above the oil level to substantially eliminate the possibility of oil leakage.
- the lowest seal in inductor 64 is shown at 308 in FIG. 8B. Since the normal oil level is below the top lip of the housing 306, it is almost impossible for oil to leak outside the assembly as long as the housing is maintained in an upright condition.
- Capacitors Capacitor banks 42, 52, 62 and 82 (i.e., C 0 , C ⁇ , C p . ⁇ and C p ) as shown in FIG. 5 are all comprised of banks of off-the-shelf capacitors connected in parallel.
- Capacitors 42 and 52 are film type capacitors available from suppliers such as Vishay Roederstein with offices in Statesville, North Carolina or Wima of Germany. Applicants preferred method of connecting the capacitors and inductors is to solder them to positive and negative terminals on special printed circuit board having heavy nickel coated copper leads in a manner similar to that described in U.S. Patent No. 5,448,580.
- Capacitor bank 62 and 64 is typically composed of a parallel array of high voltage ceramic capacitors from vendors such as Murata or TDK, both of Japan.
- capacitor bank 82 i.e., C p
- C p capacitor bank 82
- Cp-j capacitor bank 82
- C ⁇ is a 5.7 ⁇ F capacitor bank
- C 0 is a 5.3 ⁇ F capacitor bank.
- Pulse Transformer Pulse transformer 56 is also similar to the pulse transformer described in U.S. Patent Nos. 5,448,580 and 5,313,481; however, the pulse transformers of the present embodiment has only a single turn in the secondary winding and 24 induction units equivalent to 1/24 of a single primary turn for an equivalent step-up ratio of 1:24.
- a drawing of pulse transformer 56 is shown in FIG. 10.
- Each of the 24 induction units comprise an aluminum spool 56A having two flanges (each with a flat edge with threaded bolt holes) which are bolted to positive and negative terminals on printed circuit board 56B as shown along the bottom edge of FIG. 10.
- Insulators 56C separates the positive terminal of each spool from the negative terminal of the adjacent spool. Between the flanges of the spool is a hollow cylinder 1 1/16 inches long with a 0.875 OD with a wall thickness of about 1/32 inch. The spool is wrapped with one inch wide, 0.7 mil thick MetglasTM 2605 S3A and a 0.1 mil thick mylar film until the OD of the insulated MetglasTM wrapping is 2.24 inches. A prospective view of a single wrapped spool forming one primary winding is shown in FIG. 10 A.
- the secondary of the transformer is a single OD stainless steel rod mounted within a tight fitting insulating tube of PTFE (Teflon®).
- the winding is in four sections as shown in FIG. 10.
- the low voltage end of stainless steel secondary shown as 56D in FIG. 10 is tied to the primary HV lead on printed circuit board 56B at 56E, the high voltage terminal is shown at 56F.
- the transformer assumes an auto-transformer configuration and the step-up ratio becomes 1 :25 instead of 1 :24.
- an approximately -1400 volt pulse between the + and - terminals of the induction units will produce an approximately -35,000 volt pulse at terminal 56F on the secondary side.
- This single turn secondary winding design provides very low leakage inductance permitting extremely fast output rise time.
- the Cp capacitor 82 is comprised of a bank of thirty-three 0.3 nf capacitors mounted on top of the chamber pressure vessel. (Typically an ArF laser is operated with a lasing gas made up of 3.5% argon, 0.1% fluorine, and the remainder neon.)
- the electrodes are about 28 inches long which are separated by about 0.5 to 1.0 inch preferably about 5/8 inch. Preferred electrodes are described below.
- the top electrode is referred to as the cathode and the bottom electrode is connected to ground as indicated in FIG. 5 and is referred to as the anode.
- ArF, KrF and F 2 electric discharge lasers the electric discharge lasts only about 50 ns (i.e., 50 billionths of a second). This discharge creates a population inversion necessary t for lasing action but the inversion only exists during the time of the discharge. Therefore, an important requirement for an injection seeded ArF, KrF or F 2 laser is to assure that the seed beam from the master oscillator passes through discharge region of the power amplifier during the approximately 50 billionth of a second when the population is inverted in the laser gas so that amplification of the seed beam can occur.
- An important obstacle to precise timing of the discharge is the fact that there is a delay of about 5 microseconds between the time switch 42 (as shown in FIG.
- Applicants have developed electrical pulse power circuits that provide timing control of the discharges of the two discharge chambers within a relative accuracy of less than about 2 ns (i.e., 2 billionths of a second).
- a block diagram of the two circuits are shown in FIG. 4.
- Applicants' preferred solution to this problem is to charge the charging capacitor of both the MO and the PA in parallel from the single resonant charger 7 as indicated in FIG. 1 and FIG. 4 and as described above. It is also important to design the two pulse compression/amplification circuits for the two systems so that time delay versus charging voltage curves match as shown in FIG. 4A. This is done most easily by using to the extent possible the same components in each circuit. Thus, in order to minimize timing variations (the variations are referred to as jitter) in this preferred embodiment, Applicants have designed pulse power components for both discharge chambers with similar components and have confirmed that the time delay versus voltage curves do in fact track each other as indicated in FIG. 4A.
- Trigger Control The triggering of the discharge for each of the two chambers is accomplished separately utilizing for each circuit a trigger circuit such as one of those described in U.S. Patent No. 6,016,325. These circuits add timing delays to correct for variations in charging voltage and temperature changes in the electrical components of the pulse power so that the time between trigger and discharge is held as constant as feasible. As indicated above, since the two circuits are basically the same, the variations after correction are almost equal (i.e., within about 2 ns of each other).
- performance of this preferred embodiment is greatly enhanced if the discharge in the power amplifier occurs about 40 to 50 ns after the discharge in the master oscillator. This is because it takes several nanoseconds for the laser pulse to develop in the master oscillator and another several nanoseconds for the front part of the laser beam from the oscillator to reach the amplifier and because the rear end of the laser pulse from the master oscillator is at a much narrower bandwidth than the front part. For this reason, separate trigger signals are provided to trigger switch 46 for each chamber. The actual delay is chosen to achieve desired beam quality based on actual performance curves such as those shown in FIGS. 6C, D and E.
- the timing of the discharges can be monitored on a pulse-to-pulse basis and the time difference can be used in a feedback control system to adjust timing of the trigger signals closing switch 42.
- the PA discharge would be monitored using a photocell to observe discharge fluorescence (called ASE) rather than the laser pulse since very poor timing could result if no laser beam being produced in the PA.
- ASE discharge fluorescence
- the MO either the ASE or the seed laser pulse could be used.
- the pulse timing can be increased or decreased by adjusting the bias currents through inductors L B I LB 2 and L ⁇ 3 which provide bias for inductors 48, 54 and 64 as shown in FIG. 5.
- Other techniques could be used to increase the time needed to saturate these inductors.
- the core material can be mechanically separated with a very fast responding PZT element which can be feedback controlled based on a feedback signal from a pulse timing monitor.
- An adjustable parasitic load could be added to either or both of the pulse power circuits downstream of the CO's.
- Charging voltage and inductor temperature signals in addition to the pulse timing monitor signals can be used in feedback controls to adjust the bias voltage or core mechanical separation as indicated above in addition to the adjustment of the trigger timing as described above.
- water cooling of pulse power components is provided in addition to the normal forced air cooling provided by cooling fans inside the laser cabinet in order to support operation at this higher average power mode.
- a water cooled saturable inductor 54A is provided as shown in FIG. 11 which is similar to the inductor 54 shown in FIG. 8 except the fins of 54 are replaced with a water cooled jacket 54 A 1 as shown in FIG. 11.
- the cooling line 54A2 is routed within the module to wrap around jacket 54A1 and through aluminum base plate where the IGBT switches and Series diodes are mounted. These three components make up the majority of the power dissipation within the module.
- Other items that also dissipate heat are cooled by forced air provided by the two fans in the rear of the module.
- the jacket 54A1 Since the jacket 54A1 is held at ground potential, there are no voltage isolation issues in directly attaching the cooling tubing to the reactor housing. This is done by press-fitting the tubing into a dovetail groove cut in the outside of the housing as shown at 54A3 and using a thermally conductive compound to aid in making good thermal contact between the cooling tubing and the housing.
- Cooling High Voltage Components Although the IGBT switches "float" at high voltage, they are mounted on an aluminum base electrically isolated from the switches by a 1/16 inch thick alumina plate. The aluminum base plate which functions as a heat sink and operates at ground potential and is much easier to cool since high voltage isolation is not required in the cooling circuit. A drawing of a water cooled aluminum base plate is shown in FIG. 7A. In this case, the cooling tubing is pressed into a groove in an aluminum base on which the IGBT's are mounted. As with the inductor 54a, thermally conductive compound is used to improve the overall joint between the tubing and the base plate.
- the series diodes also "float" at high potential during normal operation.
- the diode housing typically used in the design provides no high voltage isolation.
- the diode "hockey puck” package is clamped within a heat sink assembly which is then mounted on top of a ceramic base that is then mounted on top of the water-cooled aluminum base plate.
- the ceramic base is just thick enough to provide the necessary electrical isolation but not too thick to incur more than necessary thermal impedance.
- the ceramic is 1/16" thick alumina although other more exotic materials, such as beryllia, can also be used to further reduce the thermal impedance between the diode junction and the cooling water.
- a second embodiment of a water cooled commutator utilizes a single cold plate assembly which is attached to the chassis baseplate for the IGBT's and the diodes.
- the cold plate may be fabricated by brazing single piece nickel tubing to two aluminum "top” and “bottom” plates.
- the IGBT's and diodes are designed to transfer their heat into the cold plate by use of the previously mentioned ceramic disks underneath the assembly.
- the cold plate cooling method is also used to cool the IGBT and the diodes in the resonant charger.
- Thermally conductive rods or a heat pipe can also be used to transfer heat from the outside housing to the chassis plate.
- the water-cooled compression head is similar in the electrical design to a prior art air- cooled version (the same type ceramic capacitors are used and similar material is used in the reactor designs). The primary differences in this case are that the module must run at higher rep-rates and therefore, higher average power. In the case of the compression head module, the majority of the heat is dissipated within the modified saturable inductor 64 A. Cooling the subassembly is not a simple matter since the entire housing operates with short pulses of very high voltages. The solution to this issue as shown in FIGS. 12, 12A and 12B is to inductively isolate the housing from ground potential. This inductance is provided by wrapping the cooling tubing around two cylindrical forms that contain a ferrite magnetic core. Both the input and output cooling lines are coiled around cylindrical portions of a ferrite core formed of the two cylindrical portions and the two ferrite blocks as shown in FIGS. 12, 12A and 12B.
- the ferrite pieces are made from CN-20 material manufactured by Ceramic Magnetics, Inc. of Fairfield, New Jersey.
- a single piece of copper tubing (0.187" diameter) is press fit and wound onto one winding form, around the housing 64 A 1 of inductor 64 A and around the second winding form. Sufficient length is left at the ends to extend through fittings in the compression head sheet metal cover such that no cooling tubing joints exist within the chassis.
- the inductor 64A comprises a dovetail groove as shown at 64A2 similar to that used in the water-cooled commutator first stage reactor housing. This housing is much the same as previous air-cooled versions with the exception of the dovetail groove.
- the copper cooling-water tubing is press fit into this groove in order to make a good thermal connection between the housing and the cooling-water tubing. Thermally conductive compound is also added to minimize the thermal impedance.
- inductor 64A The electrical design of inductor 64A is changed slightly from that of 64 shown in FIGS 9 A and 9B.
- Inductor 64A provides only two loops (instead of five loops) around magnetic core 64A3 which is comprised of four coils of tape (instead of tliree).
- bias current is supplied by a dc-dc converter in the commutator through a cable into the compression head. The current passes through the "positive" bias inductor L ⁇ 2 and is connected to the Cp-1 voltage node.
- the current then splits with a portion returning to the commutator through the HV cable (passing through the transformer secondary to ground and back to the dc-dc converter).
- the other portion passes through the compression head reactor Lp- 1 (to bias the magnetic switch) and then through the cooling-water tubing "negative" bias inductor L B3 and back to ground and the dc-dc converter.
- the "positive" bias inductor B2 is made very similarly to the "negative” bias inductor L B3 .
- the same ferrite bars and blocks are used as a magnetic core.
- two 0.125" thick plastic spacers are used to create an air gap in the magnetic circuit so that the cores do not saturate with the dc current.
- 18 AWG teflon wire is wound around the forms.
- three of the pulse power electrical modules utilize blind mate electrical connections so that all electrical connections to the portions of the laser system are made merely by sliding the module into its place in the laser cabinet.
- These are the AC distribution module, the power supply module and the resonant charges module.
- a male or female plug on the module mates with the opposite sex plug mounted at the back of the cabinet.
- two approximately 3 -inch end tapered pins on the module guide the module into its precise position so that the electrical plugs properly mate.
- the blind mate connectors such as AMP Model No. 194242-1 are commercially available from AMP, Inc. with offices in Harrisburg, Pennsylvania.
- connectors are for the various power circuits such as 208 volt AC, 400 volt AC, 1000 Volt DC (power supply out and resonant charges in) and several signal voltages. These blind mate connections permit these modules to be removed for servicing and replacing in a few seconds or minutes. In this embodiment blind mate connections are not used for the commutator module the output voltage of the module is in the range of 20 to 30,000 volts. Instead, a typical high voltage connector is used.
- FIGS. 2 and 2 A show details of an improved discharge configuration utilized in preferred embodiments of the present invention.
- This configuration includes an electrode configuration that Applicants call a blade-dielectric electrode.
- the anode 10A4 comprises a blunt blade shaped electrode with dielectric spaces mounted on both sides of the anode as shown to improve the gas flow in the discharge region.
- the anode is 26.4 inches long and 0.439 inches high. It is 0.284 inches wide at the bottom and 0.141 inches wide at the top. It is attached to flow shaping anode support bar 10A6 with screws through sockets that allow differential thermal expansion of the electrode from its center position.
- the anode is comprised of a copper based alloy preferably C36000, C95400, or C19400.
- Cathode 10A2 has a cross section shape as shown in FIG. 2A which is slightly pointed at the anode facing position.
- a preferred cathode material is C36000. Additional details of this blade dielectric configuration are provided in U.S. Patent Application Serial No. 09/768,753 incorporated herein by reference.
- the current return 10A8 in this configuration is comprised of a single long section of thin (about 1/16" diameter) copper or brass wire formed into a whale bone shaped with 27 ribs equally spaced along the length of electrode, the cross section of which is shown in FIGS. 2 and 2 A. The wire is clamped into line grooves at the bottom of anode and semi-circular grooves at the chamber top inside surface.
- Applicants is in the range of about 20 ns and is to some extent a function of the relative timing of the two discharges.
- a longer pulse length (other things being equal) can increase the lifetime of optical components of lithography equipment.
- the relative time between discharges can be optimized for pulse length.
- the pulse power circuits of both the MO and the PA could be optimized for longer pulses using techniques such as those described in U.S. Patent Application Serial No. 09/451,995 incorporated herein by reference.
- An optical pulse multiplier system such as one of those described in U.S. Patent No. 6,067,311, incorporated by reference herein, could be added downstream of the PA to reduce the intensity of individual pulses.
- This pulse multiplier could be made a part of the beam path to lens components of a lithography tool.
- the chamber could be made longer and the electrodes could be configured to produce traveling wave discharges designed for longer pulse lengths.
- Pulse energy and dose energy are preferably controlled with a feedback control system and algorithm such as that described above.
- the pulse energy monitor can be at the laser as closer to the wafer in the lithography tool.
- charging voltages are chosen to produce the pulse energy desired.
- both the MO and the PA are provided with the same charging voltage since the CO's are charged in parallel.
- the present invention provides a laser system capable of much greater pulse energy and output power than prior art single chamber high repetition rate gas discharge lasers. This additional pulse energy provides opportunities for using certain techniques for improving beam quality which are not particularly energy efficient. These techniques include:
- Pulse trimming as described in U.S. Patent No. 5,852,621, incorporated herein by reference.
- the pulse energy is monitored, the pulse is delayed and a portion of the delayed pulse is trimmed using a very fast optical switch such as a Pockels cell.
- Intercavity wavefront correction in addition to the single bend of the grating as shown in U.S. Patent No. 6,094,448 can be added to the master oscillator. This could include multiple bends of the grating as described in U.S. Patent Application Serial No. 09/703,317 incorporated herein by reference, a deformable tuning mirror 14, (as described in U.S. Patent No. 6,192,064 incorporated herein by reference), wavefront correction can also be a static correction such as a non-flat prism face configured to correct a known wavefront distortion.
- Beam filters such as a spacial filter as described in U.S. Patent Application Serial No. 09/309,478, incorporated by reference herein, and shown at 11 in FIG. 23 could be included between the MO and the PO. Beam filters can also be added downstream of the PA.
- Coherence of the laser beam can be a problem for integrated circuit fabricators.
- Gas discharge lasers typically produce a laser beam which has low coherence.
- a consequence is greater coherence of the output beam.
- some induced spacial in-coherence may be desired.
- optical components for reducing the coherence would be added either in the MO resonance cavity or between the MO and the PA.
- Several optical components are know for reducing coherence such as moving phase plates or acoustic-optic devices. • Aperturing
- Beam quality of the seed beam can also be improved by tighter aperturing of the beam.
- the preferred embodiment of this invention has a gas control module as indicated in FIG. 1 and it is configured to fill each chamber with appropriate quantities of laser gas.
- appropriate controls and processor equipment is provided to maintain continuous flow of gas into each chamber so as to maintain laser gas concentrations constant or approximately constant at desired levels. This may be accomplished using techniques such as those described in U.S. Patent No. 6,028,880 or U.S. Patent No. 6,151,349 or U.S. Patent No. 6,240,117 (both of which are incorporated hereby reference).
- Another technique for providing continuous flow of laser gas into the chambers which Applicants call its binary fill technique is to provide a number (such as 5) fill lines each successive line orificed to permit double the flow of the previous line with each line having a shut off valve.
- the lowest flow line is orificed to permit minimum equilibrium gas flow.
- Almost any desired flow rate can be achieved by selecting appropriate combinations of valves to be opened.
- a buffer tank is provided between the orificed lines and the laser gas source which is maintained at a pressure at about twice the pressure of the laser chambers.
- this preferred embodiment of the present invention produces laser pulses much more narrow than prior art excimer laser bandwidths.
- the bandwidth is more narrow than desired giving a focus with a very short depth of focus.
- better lithography results are obtained with a larger bandwidth. Therefore, in some cases a technique for tailoring the bandwidth will be preferred.
- Such a technique is described in detail in U.S. Patent Application, Serial No. 09/918,773 and 09/608,543, which are incorporated herein by reference.
- This technique involves use of computer modeling to determine a preferred bandwidth for a particular lithography results and then to use the very fast wavelength control available with the PZT tuning mirror control shown in FIGS. 16B1 and 16B2 to quickly change the laser wavelength during a burst of pulses to simulate a desired spectral shape. This technique is especially useful in producing relatively deep holes in integrated circuits.
- Prior art excimer lasers used for integrated circuit lithography are subject to tight specifications on laser beam parameters. This has typically required the measurement of pulse energy, bandwidth and center wavelength for every pulse and feedback control of pulse energy and bandwidth.
- the feedback control of pulse energy has been on a pulse-to-pulse basis, i.e., the pulse energy of each pulse is measured quickly enough so that the resulting data can be used in the control algorithm to control the energy of the immediately following pulse. For a 1,000 Hz system this means the measurement and the control for the next pulse must take less than 1/1000 second. For a 4000 Hz system speeds need to be four times as fast.
- a technique for controlling center wavelength and measuring wavelength and bandwidth is described in U.S. Patent No. 5,025,455 and in U.S. Patent No. 5,978,394. These patents are incorporated herein by reference.
- Control of beam parameters for this preferred embodiment is also different from prior art excimer light source designs in that the wavelength and bandwidth of the output beam is set by conditions in the master oscillator 10 whereas the pulse energy is mostly determined by conditions in the power amplifier 12.
- wavelength bandwidths and pulse energy are measured on a pulse to pulse basis at the output of the power amplifier and the measurements are used in a feedback control system to control wavelength and pulse energy.
- the beam parameter measurement and control for this laser is described below.
- the wavemeter used in the present embodiment is similar to the one described in U.S. Patent No. 5,978,394 and some of the description below is extracted from that patent.
- FIG. 14 shows the layouts of a preferred wavemeter unit 120, an absolute wavelength reference calibration unit 190, and a wavemeter processor 197.
- the optical equipment in these units measure pulse energy, wavelength and bandwidth. These measurements are used with feedback circuits to maintain pulse energy and wavelength within desired limits.
- the equipment calibrates itself by reference to an atomic reference source on the command from the laser system control processor.
- the laser output beam intersects partially reflecting mirror 170, which passes about 95.5% of the beam energy as output beam 33 and reflects about 4.5% for pulse energy, wavelength and bandwidth measurement.
- Pulse Energy About 4% of the reflected beam is reflected by mirror 171 to energy detector 172 which comprises a very fast photo diode 69 which is able to measure the energy of individual pulses occurring at the rate of 4,000 pulses per second.
- the pulse energy is about 10 mJ
- the output of detector 69 is fed to a computer controller which uses a special algorithm to adjust the laser charging voltage to precisely control the pulse energy of future pulses based on stored pulse energy data in order to limit the variation of the energy of individual pulses and the integrated energy of bursts of pulses.
- Linear Photo Diode array 180 The photo sensitive surface of linear photo diode array 180 is depicted in detail in FIG. 14 A.
- the array is an integrated circuit chip comprising 1024 separate photo diode integrated circuits and an associated sample and hold readout circuit (not shown).
- the photo diodes are on a 25 micrometer pitch for a total length of 25.6 mm (about one inch). Each photo diode is 500 micrometer long.
- Photo diode arrays such as this are available from several sources.
- a preferred supplier is Hamamatsu.
- we use a Model S3903-1024Q which can be read at the rate of up to 4 x 10 6 pixels/sec on a FIFO basis in which complete 1024 pixel scans can be read at rates of 4,000 Hz or greater.
- the PDA is designed for 2 x 10 6 pixel/sec operation but Applicants have found that it can be over-clocked to run much faster, i.e., up to 4 x 10 6 pixel/sec. For pulse rates greater than 4,000 Hz, Applicants can use the same PDA but only a fraction (such as 60%) of the pixels are normally read on each scan.
- Coarse Wavelength Measurement About 4%> of the beam which passes through mirror 171 is reflected by mirror 173 through slit 177 to mirror 174, to mirror 175, back to mirror 174 and onto echelle grating 176.
- the beam is collimated by lens 178 having a focal length of 458.4 mm.
- Light reflected from grating 176 passes back through lens 178, is reflected again from mirrors 174, 175 and 174 again, and then is reflected from mirror 179 and focused onto the left side of 1024-pixel linear photo diode array 180 in the region of pixel 600 to pixel 950 as shown in the upper part of FIG.
- the spatial position of the beam on the photo diode array is a coarse measure of the relative nominal wavelength of the output beam. For example, as shown in FIG. 14B, light in the wavelength range of about 193.350 pm would be focused on pixel 750 and its neighbors.
- the coarse wavelength optics in wavemeter module 120 produces a rectangular image of about 0.25 mm x 3 mm on the left side of photo diode array 180.
- the ten or eleven illuminated photo diodes will generate signals in proportion to the intensity of the illumination received (as indicated in FIG. 14C) and the signals are read and digitized by a processor in wavemeter controller 197.
- controller 197 uses this information and an interpolation algorithm controller 197 calculates the center position of the image. This position (measured in pixels) is converted into a coarse wavelength value using two calibration coefficients and assuming a linear relationship between position and wavelength.
- These calibration coefficients are determined by reference to an atomic wavelength reference source as described below. For example, the relationship between image position and wavelength might be the following algorithm:
- Fine Wavelength Measurement About 95% of the beam which passes through mirror 173 as shown in FIG. 14 is reflected off mirror 182 through lens 183 onto a diffuser (preferably a diffraction diffuser as explained in a following section entitled "Improved Etalon") at the input to etalon assembly 184.
- the beam exiting etalon 184 is focused by a 458.4 mm focal length lens in the etalon assembly and produces interference fringes on the middle and right side of linear photo diode array 180 after being reflected off two mirrors as shown in FIG. 14.
- the spectrometer must measure wavelength and bandwidth substantially in real time. Because the laser repetition rate may be 4,000 Hz to 6,000 Hz or higher, it is necessary to use algorithms which are accurate but not computationally intensive in order to achieve the desired performance with economical and compact processing electronics. Calculational algorithm therefore preferably should use integer as opposed to floating point math, and mathematical operations should preferably be computation efficient (no use of square root, sine, log, etc.).
- FIG. 14D is a curve with 5 peaks as shown which represents a typical etalon fringe signal as measured by linear photo diode array 180.
- the central peak is drawn lower in height than the others. As different wavelengths of light enter the etalon, the central peak will rise and fall, sometimes going to zero. This aspect renders the central peak unsuitable for the wavelength measurements.
- the other peaks will move toward or away from the central peak in response to changes in wavelength, so the position of these peaks can be used to determine the wavelength, while their width measures the bandwidth of the laser.
- Two regions, each labeled data window, are shown in FIG. 14D. The data windows are located so that the fringe nearest the central peak is normally used for the analysis.
- the hardware includes a microprocessor 400, Model MPC 823 supplied by Motorola with offices in Phoenix, Arizona; a programmable logic device 402, Model EP 6016QC240 supplied by Altera with offices in San Jose, California; an executive and data memory bank 404; a special very fast RAM 406 for temporary storage of photodiode array data in table form; a third 4X1024 pixel RAM memory bank 408 operating as a memory buffer; and an analog to digital converter 410.
- the basic technique is to use programmable logic device 402 to continuously produce a fringe data table from the PDA pixel data as the pixel data are produced.
- Logic device 402 also identifies which of the sets of fringe data represent fringe data of interest. Then when a calculation of center wavelength and bandwidth are needed, microprocessor merely picks up the data from the identified pixels of interest and calculates the needed values of center wavelength and bandwidth. This process reduces the calculation time for microprocessor by about a factor of 10.
- PDA 180 With PDA 180 clocked to operate at 2.5 MHz, PDA 180 is directed by processor 400 to collect data from pixels 1 to 600 at a scan rate of 4,000 Hz and to read pixels 1 to 1028 at a rate of 100 Hz.
- the analog pixel intensity data produced by PDA 180 is converted from analog intensity values into digital 8 bit values (0 to 255) by analog to digital converter 410 and the digital data are stored temporily in RAM buffer 408 as 8 bit values representing intensity at each pixel of photodiode array 180.
- Programmable logic device 402 analyzes the data passing out of RAM buffer 408 continuously on an almost real time basis looking for fringes, stores all the data in RAM memory 406, identifies all fringes for each pulse, produces a table of fringes for each pulse and stores the tables in RAM 406, and identifies for further analysis one best set of two fringes for each pulse.
- the technique used by logic device 402 is as follows:
- A) PLD 402 analyzes each pixel value coming through buffer 408 to determine if it exceeds an intensity threshold while keeping track of the minimum pixel intensity value. If the threshold is exceeded this is an indication that a fringe peak is coming.
- the PLD identifies the first pixel above threshold as the "rising edge” pixel number and saves the minimum pixel value of the pixels preceding the "rising edge” pixel. The intensity value of this pixel is identified as the "minimum" of the fringe.
- PLD 402 then monitors subsequent pixel intensity values to search for the peak of the fringe. It does this by keeping track of the highest intensity value until the intensity drops below the threshold intensity.
- the PLD When a pixel having a value below threshold is found, the PLD identifies it- as the falling edge pixel number and saves the maximum value. The PLD then calculates the "width" of the fringe by substracting the rising edge pixel number from the falling edge pixel number.
- PLD 402 also is programmed to identify with respect to each pulse the "best" two fringes for each pulse. It does this by identifying the last fringe completely within the 0 to 199 window and the first fringe completely within the 400 to 599 window.
- microprocessor 400 secures the raw pixel data in the region of the two fringes from RAM memory bank 406 and calculates from that data the bandwidth and center wavelength. The calculation is as follows:
- Typical shape of the etalon fringes are shown in FIG. 14D. Based on the prior work of PLD 402 the fringe having a maximum at about pixel 180 and the fringe having a maximum at about pixel 450 will be identified to microprocessor 400. The pixel data surrounding these two maxima are analyzed by microprocessor 400 to define the shape and location of the fringe. This is done as follows:
- a half maximum value is determined by subtracting the fringe minimum from the fringe maximum dividing the difference by 2 and adding the result to the fringe minimum. For each rising edge and each falling edge of the two fringes the two pixels having values of closest above and closest below the half maximum value are calculated. Microprocessor then extrapolates between the two pixel values in each case to define the end points of Dl and D2 as shown in FIG. 18B with a precision of 1/32 pixel. From these values the inner diameter Dl and the outer diameter D2 of the circular fringe are determined.
- Fine Wavelength Calculation The fine wavelength calculation is made using the course wavelength measured value and the measured values of Dl and D2.
- ⁇ is the wavelength, in picometers
- n is the internal index of refraction of the etalon, about 1.0003
- d is the etalon spacing
- m is the order, the integral number of wavelengths at the fringe peak, about 12440 for KrF and 9,664 for ArF,
- R is the fringe radius, 130 to 280 PDA pixels, a pixel being 25 microns, f is the focal distance from the lens to the PDA plane.
- the wavemeter' s principal task is to calculate ⁇ from D. This requires knowing f, n, d and m. Since n and d are both intrinsic to the etalon we combine them into a single calibration constant named ND. We consider f to be another calibration constant named FD with units of pixels to match the units of D for a pure ratio. The integer order m varies depending on the wavelength and which fringe pair we choose, m is determined using the coarse fringe wavelength, which is sufficiently accurate for the purpose.
- FRAC Internally FRAC is represented as an unsigned 32 bit value with its radix point to the left of the most significant bit. FRAC is always just slightly less than one, so we get maximal precision there. FRAC ranges from [1 - 120E-6] to [1 - 25E-6] for D range of ⁇ 560 ⁇ 260 ⁇ pixels.
- m nearest integer ( FRAC * 2ND/CWL ) ( 6 ) Taking the nearest integer is equivalent to adding or subtracting FSRs in the old scheme until the nearest fine wavelength to the coarse wavelength was reached. Calculate wavelength by solving equation (4) then equation (6) then equation (5). We calculate WL separately for the inner and outer diameters. The average is the line center wavelength, the difference is the linewidth.
- the bandwidth of the laser is computed as ( ⁇ 2 - ⁇ )/2.
- a fixed correction factor is applied to account for the intrinsic width of the etalon peak adding to the true laser bandwidth.
- a deconvolution algorithm is the formalism for removing the etalon intrinsic width from the measured width, but this would be far too computation- intensive, so a fixed correction ⁇ e is subtracted, which provides sufficient accuracy. Therefore, the bandwidth is:
- ⁇ e depends on both the etalon specifications and the true laser bandwidth. It typically lies in the range of 0.1-1 pm for the application described here.
- This embodiment utilizes an improved etalon.
- Conventional etalon mounting schemes typically employ an elastomer to mount the optical elements to the surrounding structure, to constrain the position of the elements but minimize forces applied to the elements.
- a compound commonly used for this is room-temperature vulcanizing silicone (RTV).
- RTV room-temperature vulcanizing silicone
- various organic vapors emitted from these elastomers can deposit onto the optical surfaces, degrading their performance.
- a preferred embodiment includes an improved etalon assembly shown at 184 in FIGS. 14 and 14E.
- the fused silica etalon 79 shown in FIG. 14G itself is comprised of a top plate 80 having a flange 81 and a lower plate 82, both plates being comprised of premium grade fused silica.
- the etalon is designed to produce fringes having free spectral range of 20.00 pm at 193.35 nm when surrounded by gas with an index of refraction of 1.0003 and a finesse equal to or greater than 25.
- Three fused silica spacers 83 with ultra low thermal expansion separate the plates and are 934 micrometer ⁇ 1 micrometer thick.
- the reflectance of the inside surfaces of the etalon are each about 92 percent and the outside surfaces are anti-reflection coated.
- the transmission of the etalon is about 50 percent.
- the etalon 79 is held in place in aluminum housing 84 only by gravity and three low force springs 86 pressing the flange against three pads not shown but positioned on 120 degree centers under the bottom edge of flange 81 at the radial location indicated by leader 85.
- a clearance of only 0.004 inch along the top edge of flange 81 at 87 assures that the etalon will remain approximately in its proper position. This close tolerance fit also ensures that if any shock or impulse is transferred to the etalon system through the mounting, the relative velocities between the optical components and the housing contact points will be kept to a minimum.
- Other optical components of etalon assembly 184 include diffuser 88, window 89 and focusing lens 90 having a focal length of 458.4 mm.
- the diffuser 88 may be a standard prior art diffuser commonly used up-stream of an etalon to produce a great variety of incident angles needed for the proper operation of the etalon.
- a problem with prior art diffusers is that about 90 percent of the light passing through the diffuser is not at a useful angle and consequently is not focused on the photo diode array. This wasted light, however, adds to the heating of the optical system and can contribute to degradation of optical surfaces.
- a diffractive lens array is used as the diffuser 88. With this type of diffuser, a pattern is produced in the diffractive lens array which scatters the light thoroughly but only within an angle of about 5 degrees.
- the incident light can be greatly reduced which greatly increases optical component life. Applicants estimate that the incident light can be reduced to less than 5% or 10% of prior art values with equivalent light on the photo diode array.
- FIG. 14H shows features of a preferred embodiment providing even further reduction of light intensity passing through the etalon.
- This embodiment is similar to the embodiment discussed above.
- the sample beam from mirror 182 (approximately 15mm x 3mm) passes upward through condensing lens 400 and is then re-collimated by lens 402.
- the . beam now colliminated and reduced in dimension to about 5mm x 1mm passes through etalon housing window 404 and then passes through a diffractive diffusing element 406 which in this case (for an ArF laser) is a diffractive diffusing element provided by Mems Optical, Inc. with offices in Huntsville, Alabama.
- the element is part number D023-193 which converts substantially all 193nm light in any incoming collimated beam of any cross sectional configuration into a beam expanding in a first direction at 2° and in a second direction perpendicular to the first direction at 4°.
- Lens 410 then "focuses" the expanding beam onto a rectangular pattern covering photodiode array 180 shown in FIG. 14.
- the active area of the photo diode array is about 0.5mm wide and 25.6 mm long and the spot pattern formed by lens 410 is about 15mm x 30mm.
- Diffractive diffusing element thoroughly mixes the spacial components of the beam but maintains substantially all of the beam energy within the 2° and 4° limits so that the light passing through the etalon can be substantially reduced and efficiently utilized.
- lens 400 has a focal length of about 50mm.
- the lens is Melles Griot Corporation part number OILQP001.
- Collimating lens 402 has a focal length of -20mm (EVI Laser Corporation part number PLCC-10.0-10.3-UV).
- the diffractive diffusing element 406 is Mems Optical Corporation part number DO23-248.
- the spacing between the two lenses can be properly positioned with spacer 416. Applicants estimate that the energy of the beam passing through the etalon with the laser operating in this design range is not sufficient to cause significant thermal problems in the etalon.
- the beam could be allowed to come to a focus between lenses 400 and 402.
- Appropriate lenses would in this case be chosen using well known optical techniques.
- the pulse energy of subsequent pulses are controlled to maintain desired pulse energies and also desired total integrated dose of a specified number of pulses all as described in U.S. Patent No. 6,005,879, Pulse Energy Control for Excimer Laser which is incorporated by reference herein.
- Wavelength of the laser may be controlled in a feedback arrangement using measured values of wavelengths and techniques known in the prior art such as those techniques described in U.S. Patent No. 5,978,394, Wavelength System for an Excimer Laser also incorporated herein by reference.
- Applicants have recently developed techniques for wavelength tuning which utilize a piezoelectric driver to provide extremely fast movement of tuning mirror. Some of these techniques are described in United States Patent Application Serial No. 608,543, Bandwidth Control Technique for a Laser, filed June 30, 2000 which is incorporated herein by reference. The following section provides a brief description of these techniques.
- the piezoelectric stack adjusts the position of the fulcrum of the lever arm.
- FIG. 16 is a block diagram showing features of the laser system which are important for controlling the wavelength and pulse energy of the output laser beam.
- Line narrowing is done by a line narrowing module 110, which contains a four prism beam expander (112a - 112d), a tuning mirror 114, and a grating 10C3.
- a line narrowing module 110 which contains a four prism beam expander (112a - 112d), a tuning mirror 114, and a grating 10C3.
- This beam expansion is 45X as compared to 20X - 25X typically used in prior art microlithography excimer lasers.
- the horizontal size of front (116a) and back (116B) apertures are made also smaller, i.e., 1.6 and 1.1 mm as compared to about 3 mm and 2 mm in the prior art.
- the height of the beam is limited to 7 mm. All these measures allow to reduce the bandwidth from about 0.5 pm (FWHM) to about 02 pm (FWHM).
- the laser output pulse energy is also reduced, from 5 mJ to about 1 mJ. This, however, does not present a problem, because this light will be amplified in the amplifier to get the 10 mJ desired output.
- the reflectivity of the output coupler 118 is 30%, which is close to that of prior art lasers.
- FIG. 16B1 is a drawing showing detail features of a preferred embodiment of the present invention.
- Large changes in the position of mirror 14 are produced by stepper motor through a 26.5 to 1 lever arm 84.
- a diamond pad 81 at the end of piezoelectric drive 80 is provided to contact spherical tooling ball at the fulcrum of lever arm 84.
- the contact between the top of lever arm 84 and mirror mount 86 is provided with a cylindrical dowel pin on the lever arm and four spherical ball bearings mounted (only two of which are shown) on the mirror mount as shown at 85.
- Piezoelectric drive 80 is mounted on the LNP frame with piezoelectric mount 80A and the stepper motor is mounted to the frame with stepper motor mount 82 A.
- Mirror 14 is mounted in mirror mount 86 with a three point mount using three aluminum spheres, only one of which are shown in FIG. 16B1. Three springs 14A apply the compressive force to hold the mirror against the spheres.
- FIG. 16B2 is a second preferred embodiment slightly different from the one shown in FIG. 16B1.
- This embodiment includes a bellows 87 (which functions as a can) to isolate the piezoelectric drive from the environment inside the LNP. This isolation prevents UV damage to the piezoelectric element and avoid possible contamination caused by out- gassing from the piezoelectric materials. Pretuning and Active Tuning
- the embodiments described above can be used for purposes other than chirp corrections.
- the operator of a integrated circuit lithography machine may desire to change wavelength on a predetermined basis.
- the target center wavelength ⁇ j may not be a fixed wavelength but could be changed as often as desired either following a predetermined pattern or as the result of a continuously or periodically updating learning algorithm using early historical wavelength data or other parameters.
- Adaptive Feedforward Preferred embodiments of the present invention includes feedforward algorithms. These algorithms can be coded by the laser operator based on known burst operation patterns. Alternatively, this algorithm can be adaptive so that the laser control detects burst patterns such as those shown in the above charts and then revises the control parameters to provide adjustment of mirror 14 in anticipation of wavelength shifts in order to prevent or minimize the shifts.
- the adaptive feedforward technique involves building a model of the chirp at a given rep rate in software, from data from one or more previous bursts and using the PZT stack to invert the effect of the chirp.
- the pulse response of the PZT stack For each repetition rate, deconvolution of the chirp waveform by the pulse response of the PZT stack will yield a sequence of pulses, which, when applied to the PZT stack (with appropriate sign), will cancel the chirp.
- This computation can be done offline through a survey of behavior at a set of repetition rates.
- the data sequences could be saved to tables indexed by pulse number and repetition rate. This table could be referred to during operation to pick the appropriate waveform data to be used in adaptive feedforward inversion.
- the chirp shape model could then be updated (e.g. adapted) every N-bursts based on accumulated measured error between model and data.
- a preferred algorithm is described in FIG. 16E.
- the chirp at the beginning of bursts of pulses can be controlled using the algorithm described in FIG. 16E.
- the letter k refers to the pulse number in a burst. The burst is separated into two regions, a k region and an 1 region.
- the k region is for pulse numbers less than k th (defining a time period long enough to encompass the chirp).
- proportional constant P k integral constant I k and integral sum of the line center error ⁇ LCE k are used for each pulse number.
- the PZT voltage for the corresponding pulse number in the k region in the next burst is determined by these constants and sums.
- a traditional proportional integral routine controls the PZT voltage.
- the voltage for next pulse in the burst will be the current voltage plus P*LCE + I* ⁇ LCE.
- This preferred embodiment is designed to operate at pulse repetition rates of 4,000 pulses per second. Clearing the discharge region of discharge affected gas between pulses requires a gas flow between the electrodes 18 A and 20A of up to about 67 m/s. To achieve these speeds, the diameter of tangential fan unit has been set at 5 inches (the length of the blade structure is 26 inches) and the rotational speed has been increased to about 3500 rpm. To achieve this performance the embodiment utilizes two motors which together deliver up to about 4 kw of drive power to the fan blade structure. At a pulse rate of 4000 Hz, the discharge will add about 12 kw of heat energy to the laser gas. To remove the heat produced by the discharge along with the heat added by the fan four separate water cooled finned heat exchanger units 58A are provided. The motors and the heat exchangers are described in detail below.
- a preferred embodiment of the present invention utilizes four finned water cooled heat exchangers 58 A shown generally in FIG. 4. Each of these heat exchangers is somewhat similar to the single heat exchangers shown at 58 in FIG. 1 having however substantial improvements.
- Heat Exchanger Components A cross sectional drawing of one of the heat exchangers is shown in FIG. 21. The middle section of the heat exchanger is cut out but both ends are shown. FIG. 21 A shows an enlarged view of the end of the heat exchanger which accommodates thermal expansion and contraction.
- the components of the heat exchanger includes a finned structure 302 which is machined from solid copper (CU 11000) and contains twelve fins 303 per inch. Water flow is through an axial passage having a bore diameter of 0.33 inch.
- a plastic turbulator 306 located in the axial passage prevents stratification of water in the passage and prevents the formation of a hot boundary layer on the inside surface of the passage.
- a flexible flange unit 304 is a welded unit comprised of inner flange 304A, bellows 304B and outer flange 304C.
- the heat exchanger unit includes three c-seals 308 to seal the water flowing in the heat exchanger from the laser gas. Bellows 304B permits expansion and contraction of the heat exchanger relative to the chamber.
- a double port nut 400 connects the heat exchanger passage to a standard 5/16 inch positional elbow pipe fitting which in turn is connected to a water source.
- O-ring 402 provides a seal between nut 400 and finned structure 302.
- cooling flow direction in two of the units is opposite the other two minimizing axial temperature gradients.
- the turbulator is comprised of four off-the-shelf, long in-line mixing elements which are typically used to mix epoxy components and are available from 3M Corporation (Static Mixer, Part No. 06-D 1229-00).
- the in-line mixers are shown at 306 in FIG. 21 and 21 A.
- the in-line mixers force the water to flow along a generally helical path which reverses its clockwise direction about every pitch distance (which is 0.3 inch).
- the turbulator substantially improves heat exchanger performance. Tests by Applicants have shown that the addition of the turbulator reduces the required water flow by a factor of roughly 5 to maintain comparable gas temperature conditions.
- gas flow into and out of the discharge region has been greatly improved over prior art laser chambers.
- the region upstream of the discharge and adjacent to the exit of the cross flow fan is shaped to form a smooth transition from a large cross section to the small cross section of the discharge.
- the cross section of the region directly downstream of the discharge increases smoothly for the small value of the discharge to a much greater value before the gas is forced to turn 90° into the heat exchangers. This arrangement minimizes the pressure drop and associated turbulence caused by high velocity flow over sharp steps.
- This first preferred embodiment of the present invention provides a large tangential fan driven by dual motors for circulating the laser gas.
- This preferred arrangement as shown in FIG. 24 provides a gas flow between the electrode of 67 m/sec which is enough to clear a space of about 1.7 cm in the discharge region between 4,000 Hz pulses.
- FIG. 18 A A cross section blade structure of the fan is shown as 64A in FIG. 4.
- FIG. 18 A A prospective view is shown in FIG. 18 A.
- the blade structure has a 5 inch diameter and is machined out of a solid aluminum alloy 6061-T6 bar stock.
- the individual blade in each section is slightly offset from the adjacent section as shown in FIG. 18 A. The offset is previously made non-uniform so as to avoid any pressure wave front creation.
- the individual blades can be slightly angled with respect to the blade axis (again to avoid creation of pressure wave fronts).
- This embodiment as shown in FIG. 18 utilizes two 3 phase brushless DC motors each with a magnetic rotor contained within a metallic pressure cup which separates the stator portion of the motors from the laser gas environment as described in U.S. Patent No. 4,950,840.
- the pressure cup is thin- walled nickel alloy 400, 0.016 inch thick which functions as the laser gas barrier.
- the two motors 530 and 532 drive the same shaft and are programmed to rotate in opposite directions. Both motors are sensorless motors (i.e., they operate without position sensors).
- Right motor controller 534 which controls right motor 530 functions as a master controller controlling slave motor controller 536 via analog and digital signals to institute start/stop, current command, current feedback, etc. Communication with the laser controller 24A is via a RS-232 serial port into master controller 534.
- This first embodiment of the present invention includes an ultra-pure N 2 purge system which provides greatly improved performance and substantially increases component lifetime.
- FIG. 19 is a block diagram showing important features of a first preferred embodiment the present invention.
- Five excimer laser components which are purged by nitrogen gas in this embodiment of the present system are LNP 2P, high voltage components 4P mounted on laser chamber 6P, high voltage cable 8P connecting the high voltage components 4P with upstream pulse power components 10P, output coupler 12P and wavemeter 14P.
- Each of the components 2P, 4P, 8P, 12P, and 14P are contained in sealed containers or chambers each having only two ports an N 2 inlet port and an N 2 outlet port.
- An N 2 source 16P which typically is a large N 2 tank (typically maintained at liquid nitrogen temperatures) at a integrated circuit fabrication plant but may be a relatively small bottle of N 2 .
- FIG. 19A is a line diagram showing specific components of this preferred embodiment including some additional N 2 features not specifically related to the purge features of the present invention.
- N 2 Filter An important feature of the present invention is the inclusion of N 2 filter 18.
- a filter for N 2 purge gas was not necessary since N 2 gas specification for commercially available N is almost always good enough so that gas meeting specifications is clean enough.
- the source gas may be out of specification or the N 2 lines leading to the purge system may contain contamination. Also lines can become contaminated during maintenance or operation procedures.
- the cost of the filter is very good insurance against an even low probability of contamination caused damage.
- a preferred N 2 filter is Model 500K Inert Gas Purifier available from Aeronex, Inc. with offices in San Diego, California. This filter removes H O, O , CO, CO 2; H 2 and non- methane hydrocarbons to sub-parts-per-billion levels. It removes 99.9999999 percent of all particulate 0.003 microns or larger.
- a flow monitor in unit 22 is provided for each of the five purged components. These are commercially available units having an alarm feature for low flow.
- Piping Preferably all piping is comprised of stainless steel (316SST) with electro polished interior. Certain types of plastic tubing, comprised of PFA 400 or ultra-high purity Teflon, may be also used.
- a portion or all of the purge gas could be recirculated as shown in FIG. 19B.
- a blower and a water cooled heat exchanger is added to the purge module.
- purge flow from the optical components could be recirculated and purge flow from the electrical components could be exhausted or a portion of the combined flow could be exhausted.
- This first preferred embodiment includes a sealed shutter unit 500 with a built in power meter as shown in FIGS. 20, 20 A and 20B.
- the shutter has two functions, first, as a shutter to block the laser beam and, second, as a full beam power meter for monitoring beam power whenever a measurement is needed.
- FIG. 20 is a top view showing the main components of the shutter unit. These are shutter 502, beam dump 504 and power meter 506.
- the path of the laser output beam with the shutter in the closed position is shown at 510 in FIG. 20.
- the path with the beam open is shown at 512.
- the shutter active surface of beam stop element 516 is at 45° with the direction of the beam exiting the chamber and when the shutter is closed the beam is both absorbed in the shutter surface and reflected to beam dump 504. Both the beam dump active surface and the shutter active surface is chrome plated for high absorption of the laser beam.
- beam stop element 516 is mounted on flexible spring steel arm 518. The shutter is opened by applying a current to coil 514 as shown in FIG.
- the shutter is closed by stopping the current flow through coil 514 which permits permanent magnets 520 to pull beam stop element 516 and flexible arm 518 back into the close position.
- the current flow is carefully tailored to produce an easy transmit of the element and arm between the open and close positions.
- Power meter 506 is operated in a similar fashion to place pyroelectric photo detector in the path of the output laser beam as shown in FIGS. 20 and 20A.
- coil 520 and magnets 522 pull detector unit 524 and its flexible arm 526 into and out of the beam path for output power measurements.
- This power meter can operate with the shutter open and with the shutter closed. Current to the coil is as with the shutter controlled to provide easy transit of unit 524 into and out of the beam path.
- Ultraviolet light in the spectral range of the ArF laser can damage sensitive optical components in the presence of oxygen or a wide variety of other chemical components. Also, oxygen is a significant absorber of the ArF laser beam.
- special provisions are made to isolate the laser beam line between laser modules from atmospheric air while permitting quick and easy replacement of the modules. Since a substantial amount of vibrational forces are generated in the laser chamber the beam seal components between the chamber and the LNP and between chamber, and the output coupler are preferably designed to minimize the transfer of vibrational forces to the optical components in the LNP and output coupler unit.
- Two special beam sealing bellows unit are described below which can be used on both the LNP side of the chamber and the output coupler side of the chamber. These seal units: 1) contain no elastomers 2) provide vibration isolation for the LNP and the OC from chamber vibration
- each laser chamber weighs more than 200 pounds and is typically rolled into position in the laser cabinet on little wheels as shown in FIGS. 22, A, 22B, 22C and 22D.
- the seal unit (between the chamber window block 156A and the LNP front plate 178) comprises a metal bellows 158, bellows flange 159 and protector bracket 160.
- FIGS. 22E, 22F and 22G show the bellows unit being compressed as the chamber is rolled into position in the direction indicated by arrow 22F.
- An exploded view showing this bellows seal unit is shown in FIG. 22H a back side flange of bellows 158 is bolted to LNP front plate 157 A.
- protection bracket 160 is attached to plate 178 at threaded location 15A with bolts that allow side- ays slipping of bracket 160 as it is compressed.
- the purge gas enters the LNP at a port at 16A and flows up across the grating face as described above by reference to FIG. 17.
- purge gas flows into the bellows region through LNP aperture 18A and diffusion slots 19A then into window block 156 A and from there to purge module 17 as shown in FIG. 19 where the flow can be monitored and then exhausted.
- This bellows provides a very good seal reducing the oxygen content in the LNP to less than 100 parts per million while at the same time permitting easy chamber replacement and minimizing vibration transfer from the chamber to the LNP.
- the seal is not 100% effective since it relies on a surface to surface contact between the bellows flange 159 and the facing surface of window block 156A with only moderate contact force of about 2 pounds applied by the bellows spring force.
- FIGS. 221, 22 J, 22K and 22L An alternative seal unit is described in FIGS. 221, 22 J, 22K and 22L.
- This bellows unit as shown in FIG. 221 is sealed with metal seals at both interfaces with the chamber window block and the LNP front plate.
- This unit includes two bellows 26A and 26B separated by metal cylinder 24A.
- Flange 20 is bolted to the chamber window block 56A and sealed with a metal "C" seal positioned at 28A as shown in FIG. 221 prior to insertion of the chamber unit into the laser cabinet.
- the chamber is then rolled into the laser cabinet as indicated in FIGS. 22A-D.
- flange 22 A is clamped to LNP front plate 157 A with V-clamp unit 31 A and sealed with a metal "C" seal located at 30A as shown in FIG. 221.
- the V-clamp unit which is a part of LNP module is shown in FIG. 221 and 22K.
- the V- clamp works as follows.
- the V-clamp shown in FIG. 221 is mounted on LNP frame 178 with bolts at 50A.
- Torsion spring 52A holds the front edge 47A of yoke-like lever 46A about 1 cm off the surface of LNP frame 178 (not shown here).
- clampable flange 22A passes very close to the surface of LNP frame 178 until the outer edge of clampable flange 22A is positioned between yoke-like lever 46A and the surface of LNP frame 178.
- FIG. 22K shows the operation of the V-clamp unit.
- FIG. 22L shows the bellows unit in place sealing the chamber-LNP interface. This is a cross-section top view. Shown on the drawing are metal "C" seals at 54A and 56A, chamber window block 156 A, purge vent hole 58 A, chamber window 61 A with seal 60A. Arrow 62 shows where the outer edge 22A of clampable flange 22 is clamped against LNP frame 178 by yoke-like lever 46 A (not shown here).
- FIGS. 22I-L provides a much tighter seal between the chamber and the LNP and between the chamber and the output coupler than the one shown in FIGS. 22E- H; however, it is somewhat more expensive and the transfer of vibrational components through it may be somewhat greater.
- the same unit and technique is preferably used to seal the chamber-output coupler interface and other optical interfaces in the beam path.
- a special N 2 purge technique is used to provide extra purging of the high ultraviolet flux portions of the wavemeter as well as the output coupler and the chamber output window block. This technique is shown in FIG. 22M.
- the laser output beam intersects partially reflecting mirror 170 (see also FIG. 14) which passes 95% of the energy in the beam as an output beam. About 4% of the reflected beam is reflected from mirror 171 to energy detector 172 where the pulse energy is measured.
- the purge flow 62 A is confined in the wavemeter by seals at mirrors 170, 171 and the 172 detector window.
- the purge flow exits this region along the laser output beam path through a bellows region 6A back to the output coupler module 68A to purge it.
- the flow then flows through bellows unit 70A and into window block 72A, out through an exit port in the window block and an exit port in bellows unit 70A then back through a tube to N 2 purge module 17 as shown at 74 A and in FIG. 19.
- the downstream side of window 170 is purged with purge flow from shutter module 5K.
- the purge flow may be from module 17 as shown in FIG. 19 or in some cases window 76A is removed and the output of shutter module is openly connected with a purged customer beam line in which case the exit purge line at 78A may be directed to a customer purge return system or exhausted to atmosphere.
- the pulse power circuit could be a common circuit up to the output of pulse transformer 56 as shown in FIG. 5.
- This approach provides for a further reduction in jitter as explained in U.S. Patent Application Serial No. 09/848,043 which is incorporated herein by reference.
- FIG. 3B of that patent application showing the input and output to the pulse transformer is included herein as FIG. 13 for the convenience of the reader.
- Active feedback control of bandwidth can be provided by adjusting the curvature of the line narrowing grating using a motor driver to adjust the bending mechanism shown in FIG. 22A.
- control of bandwidth could be provided by using piezoelectric devices to control the curvature of the grating.
- Other heat exchanger designs should be obvious modifications to the one configuration shown herein. For example, all four units could be combined into a single unit. There could be significant advantages to using much larger fins on the heat exchanger to moderate the effects of rapid changes in gas temperature which occurs as a result of burst mode operation of the laser.
- the reader should understand that at extremely high pulse rates the feedback control on pulse energy does not necessarily have to be fast enough to control the pulse energy of a particular pulse using the immediately preceding pulse.
- control techniques could be provided where measured pulse energy for a particular pulse is used in the control of the second or third following pulse.
- the second laser unit could be configured as a slave oscillator by including an output coupler such as a partially reflecting mirror.
- Fans other than the tangential fans could be used. This may be required at repetition rates much greater than 4 kHz.
- the fans and the heat exchanger could be located outside the discharge chambers.
- Pulse timing techniques described in U.S. Patent Application Serial No. 09/837,035 could also be utilized. Since the bandwidth of the preferred embodiment can be less than 0.2 pm, measurement of the bandwidth with additional precision may be desired. This could be done with the use of an etalon having a smaller free spectral range than the etalons described above. Other techniques well known could be adapted for use to precisely measure the bandwidth. Accordingly, the above disclosure is not intended to be limiting and the scope of the invention should be determined by the appended claims and their legal equivalents.
Abstract
Description
Claims
Priority Applications (3)
Application Number | Priority Date | Filing Date | Title |
---|---|---|---|
KR1020037004934A KR100850450B1 (en) | 2000-10-06 | 2001-09-25 | Very narrow band, two chamber, high rep rate gas discharge laser system |
EP01977198A EP1323216A4 (en) | 2000-10-06 | 2001-09-25 | Very narrow band, two chamber, high rep rate gas discharge laser system |
AU2001296334A AU2001296334A1 (en) | 2000-10-06 | 2001-09-25 | Very narrow band, two chamber, high rep rate gas discharge laser system |
Applications Claiming Priority (16)
Application Number | Priority Date | Filing Date | Title |
---|---|---|---|
US09/684,629 | 2000-10-06 | ||
US09/684,629 US6442181B1 (en) | 1998-07-18 | 2000-10-06 | Extreme repetition rate gas discharge laser |
US09/768,753 | 2001-01-23 | ||
US09/768,753 US6414979B2 (en) | 2000-06-09 | 2001-01-23 | Gas discharge laser with blade-dielectric electrode |
US09/771,789 US6539042B2 (en) | 1999-11-30 | 2001-01-29 | Ultra pure component purge system for gas discharge laser |
US09/771,789 | 2001-01-29 | ||
US09/794,782 US6532247B2 (en) | 2000-02-09 | 2001-02-27 | Laser wavelength control unit with piezoelectric driver |
US09/794,782 | 2001-02-27 | ||
US09/834,840 | 2001-04-13 | ||
US09/834,840 US6466601B1 (en) | 2001-04-13 | 2001-04-13 | Beam seal for line narrowed production laser |
US09/848,043 US6549551B2 (en) | 1999-09-27 | 2001-05-03 | Injection seeded laser with precise timing control |
US09/848,043 | 2001-05-03 | ||
US09/854,097 US6757316B2 (en) | 1999-12-27 | 2001-05-11 | Four KHz gas discharge laser |
US09/854,097 | 2001-05-11 | ||
US09/943,343 US6567450B2 (en) | 1999-12-10 | 2001-08-29 | Very narrow band, two chamber, high rep rate gas discharge laser system |
US09/943,343 | 2001-08-29 |
Publications (1)
Publication Number | Publication Date |
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WO2002031932A1 true WO2002031932A1 (en) | 2002-04-18 |
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Application Number | Title | Priority Date | Filing Date |
---|---|---|---|
PCT/US2001/030164 WO2002031932A1 (en) | 2000-10-06 | 2001-09-25 | Very narrow band, two chamber, high rep rate gas discharge laser system |
Country Status (6)
Country | Link |
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US (1) | US6567450B2 (en) |
EP (1) | EP1323216A4 (en) |
JP (2) | JP2002198604A (en) |
KR (1) | KR100850450B1 (en) |
AU (1) | AU2001296334A1 (en) |
WO (1) | WO2002031932A1 (en) |
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Also Published As
Publication number | Publication date |
---|---|
US6567450B2 (en) | 2003-05-20 |
KR20030041151A (en) | 2003-05-23 |
AU2001296334A1 (en) | 2002-04-22 |
JP2002198604A (en) | 2002-07-12 |
EP1323216A1 (en) | 2003-07-02 |
US20020044586A1 (en) | 2002-04-18 |
JP2006261689A (en) | 2006-09-28 |
EP1323216A4 (en) | 2006-03-22 |
KR100850450B1 (en) | 2008-08-07 |
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