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Publication numberUS20050205212 A1
Publication typeApplication
Application numberUS 10/905,172
Publication dateSep 22, 2005
Filing dateDec 20, 2004
Priority dateMar 22, 2004
Also published asUS20050205211, WO2005093780A2, WO2005093780A3
Publication number10905172, 905172, US 2005/0205212 A1, US 2005/205212 A1, US 20050205212 A1, US 20050205212A1, US 2005205212 A1, US 2005205212A1, US-A1-20050205212, US-A1-2005205212, US2005/0205212A1, US2005/205212A1, US20050205212 A1, US20050205212A1, US2005205212 A1, US2005205212A1
InventorsVikram Singh, Harold Persing, Timothy Miller
Original AssigneeVarian Semiconductor Equipment
Export CitationBiBTeX, EndNote, RefMan
External Links: USPTO, USPTO Assignment, Espacenet
RF Plasma Source With Conductive Top Section
US 20050205212 A1
Abstract
A plasma source includes a chamber that contains a process gas. The chamber has a chamber top comprising a first section formed of a dielectric material that extends in a horizontal direction. A second section of the chamber top is formed of a dielectric material that extends a height from the first section in a vertical direction. A top section of the chamber top is formed of a conductive material that extends a length across the second section in the horizontal direction. A radio frequency antenna is positioned proximate to at least one of the first section and the second section. The radio frequency antenna induces radio frequency currents into the chamber that excite and ionize the process gas so as to generate a plasma in the chamber.
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Claims(33)
1. A plasma source comprising:
a chamber that contains a process gas, the chamber having a chamber top comprising: a first section formed of a dielectric material that extends in a horizontal direction; a second section formed of a dielectric material that extends a height from the first section in a vertical direction; and a top section formed of a conductive material that extends a length across the second section in the horizontal direction; and
a radio frequency antenna that is positioned proximate to at least one of the first section and the second section, the radio frequency antenna inducing radio frequency currents into the chamber that excite and ionize the process gas so as to generate a plasma in the chamber.
2. The plasma source of claim 1 wherein a ratio of the height from the first section in the vertical direction to the length across the second section in the horizontal direction is approximately between 1.5 and 5.5.
3. The plasma source of claim 1 wherein the radio frequency antenna comprises a planar coil that is positioned proximate to the first section.
4. The plasma source of claim 1 wherein the radio frequency antenna comprises a coil that surrounds the second section.
5. The plasma source of claim 1 wherein the radio frequency antenna comprises a planar coil that is positioned adjacent to the first section and a coil that surrounds the second section.
6. The plasma source of claim 5 wherein the planar coil that is positioned adjacent to the first section and the coil that surrounds the second section are electrically connected.
7. The plasma source of claim 5 wherein the planar coil that is positioned adjacent to the first section and the coil that surrounds the second section are positioned in electromagnetic communication.
8. The plasma source of claim 1 wherein the radio frequency antenna comprises an active antenna that is electrically coupled to a radio frequency power supply and a parasitic antenna that is electromagnetically coupled to the active antenna.
9. The plasma source of claim 8 wherein one end of the parasitic antenna is electrically coupled to ground potential.
10. The plasma source of claim 1 wherein the top section comprises a fluid cooling system that regulates a temperature of the top section.
11. The plasma source of claim 1 wherein the chamber top is electrically connected to ground potential.
12. The plasma source of claim 1 further comprising a platen for holding a target that is positioned adjacent to the top section.
13. The plasma source of claim 12 further comprising a bias voltage power supply having an output that is electrically connected to the platen, the bias voltage power supply generating a voltage on the platen that attracts ions in the plasma to the target.
14. A plasma source comprising:
a chamber that contains a process gas, the chamber having a chamber top comprising: a first section formed of a dielectric material that extends in a first direction; a second section formed of a dielectric material that extends a height from the first section in a second direction; and a top section formed of a conductive material that extends a length across the second section; and
a radio frequency antenna that is positioned proximate to at least one of the first section and the second section, the radio frequency antenna inducing radio frequency currents into the chamber that excite and ionize the process gas so as to generate a plasma in the chamber.
15. The plasma source of claim 14 wherein the first section is curved.
16. The plasma source of claim 14 wherein the first and second directions are not orthogonal.
17. A method of generating a uniform plasma, the method comprising:
introducing a process gas into a chamber;
inducing a radio frequency current through at least one of a horizontal dielectric window and a vertical dielectric window of the chamber, the radio frequency current exciting and ionizing the process gas so as to generate a plasma in the chamber; and
biasing a target so that ions in the plasma are attracted to the target, wherein secondary electrons generated when the ions hit the target are dissipated by a top section of the chamber that is formed of a conductive material, thereby reducing charging effects and improving a uniformity of the plasma.
18. The method of claim 17 further comprising cooling the top section of the chamber to dissipate heat generated when the secondary electrons hit the target.
19. A method of generating a uniform plasma, the method comprising:
introducing a process gas into a chamber;
inducing a radio frequency current through one of a horizontal dielectric window and a vertical dielectric window of the chamber, the radio frequency current exciting and ionizing the process gas so as to generate a plasma in the chamber;
electromagnetically coupling the induced radio frequency current from the one of the horizontal dielectric window and the vertical dielectric window to the other of the horizontal dielectric window and the vertical dielectric window; and
biasing a target so that ions in the plasma are attracted to the target, wherein secondary electrons generated when the ions hit the target are dissipated by a top section of the chamber that is formed of a conductive material, thereby reducing charging effects and improving a uniformity of the plasma.
20. The method of claim 19 further comprising adjusting the electromagnetic coupling to improve the uniformity of the plasma.
21. A plasma source comprising:
a chamber that contains a process gas, the chamber having a chamber top comprising: a first section formed of a dielectric material that extends in a horizontal direction; a second section formed of a dielectric material that extends a height from the first section in a vertical direction; and a top section formed of a conductive material that extends a length across the second section in the horizontal direction;
an anode that is positioned in the chamber adjacent to the top section; and
a radio frequency antenna that is positioned proximate to at least one of the first section and the second section, the radio frequency antenna inducing radio frequency currents into the chamber that excite and ionize the process gas so as to generate a plasma in the chamber.
22. The plasma source of claim 21 wherein the position of the anode in the chamber relative to the top section is adjustable.
23. The plasma source of claim 21 wherein the position of the anode in the chamber relative to the top section is chosen to achieve a predetermined plasma uniformity.
24. The plasma source of claim 21 wherein a ratio of an area of the anode to an area of the top section is less than one.
25. The plasma source of claim 21 wherein the anode comprises a baffle that disperses the process gas.
26. The plasma source of claim 21 wherein the anode comprises a shower head that dispenses the process gas.
27. The plasma source of claim 21 wherein the anode is electrically connected to ground potential.
28. A method of generating a plasma, the method comprising:
introducing a process gas into a chamber;
biasing an anode that is positioned in the chamber to emit electrons from the anode;
inducing a radio frequency current through a horizontal dielectric window of the chamber; and
inducing a radio frequency current through a vertical dielectric window of the chamber, wherein at least one of the electrons emitted by the anode and the induced radio frequency currents exciting and ionizing the process gas to ignite a plasma in the chamber.
29. The method of claim 28 wherein the biasing the anode comprises applying a pulsed direct current to the anode.
30. The method of claim 28 wherein the biasing the anode comprises applying a RF field to the anode.
31. The method of claim 28 wherein the biasing the anode comprises applying a combination of pulsed DC and RF signals.
32. The method of claim 28 further comprising sustaining the plasma with the induced radio frequency currents.
33. The method of claim 28 further comprising sustaining the plasma with the electrons generated from the anode.
Description
    RELATED APPLICATION SECTION
  • [0001]
    This application is a continuation-in-part of U.S. patent application Ser. No. 10/805,966, filed Mar. 22, 2004 entitled “Plasma Immersion Ion Implantation Apparatus and Method,” the entire application of which is incorporated herein by reference.
  • INTRODUCTION
  • [0002]
    The section headings used herein are for organizational purposes only and are not to be construed as limiting the subject matter described in any way.
  • [0003]
    The present invention relates to plasma sources. Plasma sources are commonly used in the semiconductor industry and other industries for performing etching and deposition. Plasma immersion systems immerse a substrate or target in a plasma for processing. The substrate or target is biased with respect to the plasma potential in order to attract ions for processing.
  • [0004]
    Recently plasma immersion systems have been used for performing ion implantation of semiconductor wafers. Ions are not accelerated toward the wafer like in conventional ion implantation systems. Instead the wafer is immersed in a plasma containing dopant ions. The ion penetration depths can be very shallow. Therefore, plasma immersion systems can perform very shallow ion implantations that can be used for fabricating modern electronic and optical components.
  • [0005]
    One type of plasma immersion source uses a pulsed DC power supply to generate the plasma. The DC power supply generates a voltage that creates a plasma discharge from a process gas in a chamber. The DC voltage and secondary electrons generated from collisions with chamber surfaces and with the target sustain the plasma. Other types of plasma immersion sources use a radio frequency (RF) source to generate the plasma. The RF source generates a RF voltage. The RF voltage generates and maintains the plasma by capacitively coupling RF energy from an electrode across the plasma sheath to electrons in the plasma. Other types of plasma immersion sources use microwave power applicators to generate and maintain the plasma.
  • [0006]
    Plasma sources for ion implantation have more stringent requirements than plasma sources for other plasma processing applications, such as plasma etching and plasma deposition. For example, plasma immersion sources used for ion implantation must generate plasma with highly uniform plasmas ion flux in both the radial and the azimuthal direction so that uniform ion flux impinges on the wafer surface.
  • [0007]
    In addition, plasma immersion systems must dissipate the heat load and minimize charging effects that results from secondary electron emission from the wafer. Typically secondary electrons are accelerated away from the surface of the substrate at the implant voltage and the power carried by these electrons is deposited in the chamber top. Conventional plasma immersion sources are used with chamber tops that are formed of insulating materials. The secondary electrons tend to heat and to charge the chamber tops, which can adversely affect ion energy uniformity and process repeatability.
  • BRIEF DESCRIPTION OF THE DRAWINGS
  • [0008]
    The aspects of this invention may be better understood by referring to the following description in conjunction with the accompanying drawings, in which like numerals indicate like structural elements and features in various figures. The drawings are not necessarily to scale. The skilled artisan will understand that the drawings, described below, are for illustration purposes only. The drawings are not intended to limit the scope of the present teachings in anyway.
  • [0009]
    FIG. 1 illustrates a RF plasma source having vertical and horizontal RF coils and a conductive top section according to the present invention.
  • [0010]
    FIG. 2 illustrates a RF plasma source having a first RF coil in a first direction, a second RF coil in a second direction, and a conductive top section according to the present invention.
  • [0011]
    FIG. 3 illustrates a RF plasma source having vertical and horizontal RF coils, a conductive top section, and an anode according to the present invention.
  • [0012]
    FIGS. 4A-C illustrate graphs of radial plasma density profiles for two different coil adjuster positions.
  • [0013]
    FIGS. 5A-B illustrate graphs of plasma uniformity and mean ion current as a function of chamber pressure for a constant RF power level.
  • DETAILED DESCRIPTION
  • [0014]
    A plasma source of the present invention provides a uniform ion flux and also dissipates the effects of secondary electrons. Some aspects of the plasma source of the present invention are described in connection with plasma doping for the purpose of illustrating the invention. However, it is understood that the plasma source of the present invention has many applications and is not limited to plasma immersion sources for plasma doping.
  • [0015]
    FIG. 1 illustrates a RF plasma source 100 having vertical and horizontal RF coils and a conductive top section according to the present invention. The plasma source 100 includes a chamber 102 that contains a process gas. A gas source 104 that is coupled to the chamber 102 through a proportional valve 106 supplies the process gas to the chamber 102. A pressure gauge 108 measures the pressure inside the chamber 102. An exhaust port 110 in the chamber 102 is coupled to a vacuum pump 112 that evacuates the chamber 102. An exhaust valve 114 controls the exhaust conductance through the exhaust port 110.
  • [0016]
    A gas pressure controller 116 is electrically connected to the proportional valve 106, the pressure gauge 108, and the exhaust valve 114. The gas pressure controller 116 maintains the desired pressure in the chamber 102 by controlling the exhaust conductance with the exhaust valve 114 and controlling the process gas flow rate with the proportional valve 106 in a feedback loop that is responsive to the pressure gauge 108.
  • [0017]
    In some embodiments, a ratio control of trace gas species is provided by a mass flow meter (now shown) that is coupled in-line with the process gas that provides the primary gas species. Also, in some embodiments, a separate gas injection means (not shown) is used for in-situ conditioning species. For example, silicon doped with an appropriate dopant can be used to provide a uniform coating in the chamber 102 that reduces contaminants. Furthermore, in some embodiments, a multi-port gas injection means (not shown) is used to provide gases that cause neutral chemistry effects that result in across wafer variations.
  • [0018]
    The chamber 102 has a chamber top 118 including a first section 120 formed of a dielectric material that extends in a generally horizontal direction. A second section 122 of the chamber top 118 is formed of a dielectric material that extends a height from the first section 120 in a generally vertical direction. The dimensions of the first and the second sections 120, 122 of the chamber top 118 can be selected to improve the uniformity of plasmas generated in the chamber 102.
  • [0019]
    The dielectric materials in the first and second sections 120, 122 provide a medium for transferring the RF power from the RF antenna to a plasma inside the chamber 102. In one embodiment, the dielectric material used to form the first and second sections 120, 122 is a high purity ceramic material that is chemically resistant to the process gases and that has good thermal properties. For example, in some embodiments, the dielectric material is 99.6% Al2O3 or AlN. In other embodiments, the dielectric material is Yittria and YAG.
  • [0020]
    A top section 124 of the chamber top 118 is formed of a conductive material that extends a length across the second section 122 in the horizontal direction. In many embodiments, the conductivity of the material used to form the top section 124 is high enough to dissipate the heat load and to minimize charging effects that results from secondary electron emission. Typically, the conductive material used to form the top section 124 is chemically resistant to the process gases. In some embodiments, the conductive material is aluminum.
  • [0021]
    The top section 124 can be coupled to the second section 122 with a halogen resistant O-ring made of fluoro-carbon polymer, such as an O-ring formed of Chemrz and/or Kalrex materials. The top section 124 is typically mounted to the second section 122 in a manner that minimizes compression on the second section 122, but that provides enough compression to seal the top section 124 to the second section. In some operating modes, the top section 124 is RF and DC grounded as shown in FIG. 1.
  • [0022]
    In some embodiments, the top section 124 comprises a cooling system that regulates the temperature of the top section 124 in order to dissipate the heat load generated during processing. The cooling system can be a fluid cooling system that includes cooling passages 128 in the top section 124 that circulate a liquid coolant from a coolant source. Some processes, such as plasma doping processes, generate a considerable amount of non-uniformly distributed heat on the inner surfaces of the plasma chamber because of secondary electron emissions. The non-uniformly distributed heat creates temperature gradients that are high enough to cause thermal stress points within the chamber 102 that can result in a chamber 102 failure.
  • [0023]
    In one embodiment, a ratio of the height 130 of the first section 122 of the chamber top 118 in the vertical direction to the length 132 across the second section 122 of the chamber top 118 in the horizontal direction is approximately between 1.5 and 5.5. In the embodiment shown in FIG. 1, the second section 122 is formed in a cylindrical shape. However, in other embodiments of the invention, the first section 120 of the chamber top 118 does not extend in exactly a horizontal direction. Also, in other embodiments, the second section 122 of the chamber top 118 does not extend in exactly a vertical direction.
  • [0024]
    A platen 134 is positioned in the chamber 102 a height 136 below the top section 124 of the chamber top 118 and a height 138 below the first section 120 of the chamber top 118. The platen 134 can be a substrate holder that holds a wafer 140 for processing. For example, if the plasma source 100 is configured as a plasma immersion ion implantation source, the platen 134 holds a target, such as a semiconductor wafer to be implanted. In one embodiment, the platen 134 is dimensioned so that it is positioned within the inner diameter 142 of the chamber top 118.
  • [0025]
    In some embodiments, a bias voltage power supply 144 is electrically connected to the platen 134. The bias voltage power supply 144 biases the platen 134 at a voltage that attracts ions in the plasma to the wafer 140. The bias voltage power supply 144 can be a DC power supply or a RF power supply.
  • [0026]
    A RF antenna is positioned proximate to at least one of the first section 120 and the second section 122 of the chamber top 118. The plasma source 100 illustrated in FIG. 1 shows two separate RF antennas that are electrically isolated. A planar coil antenna 146 having a plurality of turns is positioned adjacent to the first section 120 of the chamber top 118 and a helical coil antenna 148 having a plurality of turns surrounds the second section 122 of the chamber top 118.
  • [0027]
    A RF source 150, such as a RF power supply, is electrically connected to at least one of the planar coil antenna 146 and the helical coil antenna 148. The RF source 150 is coupled to the RF antennas 146, 148 by an impedance matching network 152 that maximizes the power transferred from the RF source 150 to the RF antennas 146, 148. Dashed lines from the output of the impedance matching network 152 to the planar coil antenna 146 and the helical coil antenna 148 are used to indicate that electrical connections can be made from the output of the impedance matching network 152 to either or both of the planar coil antenna 146 and the helical coil antenna 148.
  • [0028]
    The RF source 150 resonates RF currents in the RF antennas 146, 148. The RF current in the RF antennas 146, 148 induces RF currents into the chamber 102. The RF currents in the chamber 102 excite and ionize the process gas so as to generate a plasma in the chamber 102.
  • [0029]
    The plasma source of the present invention can have many different antenna configurations. At least one of the planar coil antenna 146 and the helical coil antenna 148 is an active antenna. The term “active antenna” is herein defined as an antenna that is driven directly by a power supply. In other words, a voltage generated by the power supply is directly applied to an active antenna.
  • [0030]
    In some embodiments, at least one of the planar coil antenna 146 and the helical coil antenna 148 is formed such that it can be liquid cooled. For example, the planar coil antenna 146 and the helical coil antenna 148 can be tubular members that are connected to a pressurized fluid source. Cooling at least one of the planar coil antenna 146 and the helical coil antenna 148 will reduce temperature gradients caused by the RF power propagating in the RF antennas 146, 148.
  • [0031]
    In some embodiments, one of the planar coil antenna 146 and the helical coil antenna 148 is a parasitic antenna. The term “parasitic antenna” is defined herein to mean an antenna that is in electromagnetic communication with an active antenna, but that is not directly connected to a power supply. In other words, a parasitic antenna is not directly excited by a power supply, but rather is excited by an active antenna.
  • [0032]
    For example, in one embodiment, the planar coil antenna 146 is an active antenna that is electrically connected to the output of the power supply 150 and the helical coil antenna 148 is a parasitic antenna that is positioned in electromagnetic communication with the planar coil antenna 146. In another embodiment, the helical coil antenna 148 is an active antenna that is electrically connected to the output of the power supply 150 and the planar coil antenna 146 is positioned in electromagnetic communication with the helical coil antenna 148.
  • [0033]
    In some embodiments of the invention, one end of the parasitic antenna is electrically connected to ground potential in order to provide antenna tuning capabilities. In this embodiment, the parasitic antenna includes a coil adjuster that is used to change the effective number of turns in the parasitic antenna coil. Numerous different types of coil adjusters can be used. For example, the coil adjuster 154 shown in FIG. 1 is a metal short that is positioned between a floating end of the parasitic coil and a desired number of turns in the helical coil antenna 148. In other embodiments, the parasitic antenna is electrically floating at both ends. In these other embodiments, a switch (not shown) is used to select the desired number of turns in the parasitic antenna coil.
  • [0034]
    In some embodiments, the plasma source 100 includes a plasma igniter 156. Numerous types of plasma igniters can be used with the plasma source of the present invention. In one embodiment, the plasma igniter 156 includes a reservoir 158 of strike gas, which is a highly-ionizable gas, such as argon (Ar), that assists in igniting the plasma. The reservoir 158 can be a relatively small reservoir of known volume and known pressure. The reservoir 158 is coupled to the plasma chamber 102 with a high conductance gas connection 160. A burst valve 162 isolates the reservoir 158 from the chamber 102. In another embodiment, a strike gas source is plumbed directly to the burst valve 162 using a low conductance gas connection.
  • [0035]
    In operation, the chamber 102 is evacuated to high vacuum. The process gas is then introduced into the chamber 102 by the proportional valve 106 and exhausted from the chamber 102 by the vacuum pump 112. The gas pressure controller 116 is used to maintain the desired gas pressure for a desired process gas flow rate and exhaust conductance.
  • [0036]
    The RF source 150 generates a RF signal that is applied to the RF antennas 146, 148. In some embodiments, the RF source 150 generates a relatively low frequency RF signal. Using a relatively low frequency RF signal will minimize capacitive coupling and, therefore will reduce sputtering of the chamber walls and the resulting contamination. For example, in these embodiments, the RF source 150 generates RF signals below 27 MHz, such as 400 kHz, 2 MHz, 4 MHz or 13.56 MHz.
  • [0037]
    The RF signal applied to the RF antennas 146, 148 generates a RF current in the RF antennas 146, 148. Electromagnetic fields induced by the RF currents in the RF antennas 146, 148 couple through at least one of the dielectric material forming the first section 120 and the dielectric material forming the second section 122 and into the chamber 102. In some operating modes, RF current is induced through the first section 120 of the chamber top 118 with an active antenna that is electrically coupled to the RF source 150 and through the second section 122 of the chamber top 118 with a parasitic antenna. In other operating modes, RF current is induced through the second section 122 of the chamber top 118 with an active antenna that is electrically coupled to the RF source 150 and through the first section 120 of the chamber top 118 with a parasitic antenna.
  • [0038]
    The electromagnetic fields induced in the chamber 102 excite and ionize the process gas molecules. Plasma ignition occurs when a small number of free electrons move in such a way that they ionize some process gas molecules. The ionized process gas molecules release more free electrons that ionize more gas molecules. The ionization process continues until a steady state of ionized gas and free electrons are present in the plasma. In some embodiments, the characteristics of the plasma are tuned by changing the effective number of turns in the parasitic antenna coil with the coil adjuster 154.
  • [0039]
    Plasma ignition is difficult for some process gases, such as diborane in helium (15% B2H6 in 85% He). For these gases, it is desirable to use a strike gas to initiate the plasma. In one embodiment, a strike gas is controllably introduced into the plasma chamber 102 at a predetermined time by opening and then closing the burst valve 162. The burst valve 162 passes a short high-flow-rate burst of strike gas into the plasma chamber 102 in order to assist in igniting the plasma.
  • [0040]
    The burst gas profile is characterized by the amplitude, shape, and duration of the burst. The burst gas profile is defined by several factors, such as the length of time the burst valve 162 is open, the pressure and the volume of the strike gas in the reservoir 158, the conductance of the gas connection 160, the pumping speed of the vacuum pump 112 and the position of the exhaust value 114. In some embodiments, a portion of the reservoir 158 is separated by a limited conductance orifice 164 or metering valve that provides a steady flow rate of strike gas after the initial high-flow-rate burst.
  • [0041]
    The gas pressure controller 116 senses an increase in chamber pressure and a corresponding decrease in the process gas flow that results from the burst of strike gas. The pressure controller 116 then adjusts the exhaust conductance and varies the process gas flow rate in a feedback loop that is responsive to the pressure gauge 108 so that the chamber pressure recovers to the desired processing conditions within the desired response time.
  • [0042]
    For example, a strike gas comprising argon (Ar) can be used to ignite diborane in helium (15% B2H6 in 85% He). In this example, a plasma can be struck with a burst of argon that is introduced during a 0.5-5.0 second time interval from a limited conductance gas supply having a pressure that is approximately 500 Torr. The burst of argon increases a pressure in the chamber by about 20 mTorr, which provides reliable ignition of the plasma.
  • [0043]
    FIG. 2 illustrates a RF plasma source 200 having a first RF coil in a first direction, a second RF coil in a second direction, and a conductive top section according to the present invention. The RF plasma source 200 is similar to the RF plasma source 100 that was described in connection with FIG. 1. The plasma source 200 includes a chamber 102 that contains a process gas. A gas source 104, which is coupled to the chamber through a proportional valve 106, supplies the process gas to the chamber 102.
  • [0044]
    A pressure gauge 108 measures the pressure inside the chamber 102. A gas pressure controller 116 is used to maintain the desired pressure in the chamber 102 by establishing an exhaust conductance and varying the process gas flow rate in a feedback loop that is responsive to the pressure gauge 108. The chamber 102 includes an exhaust port 114 that is coupled to a vacuum pump 112 that evacuates the chamber 102.
  • [0045]
    The chamber 102 has a chamber top 202 that including a first section 204 formed of a dielectric material that extends in a generally curved direction. A second section 206 of the chamber top 202 is formed of a dielectric material that extends in a generally vertical direction. The first and second sections 204, 206 are not orthogonal. The shape and dimensions of the first and the second sections 204, 206 can be selected to improve the uniformity of plasmas generated in the chamber 102. A top section 124 of the chamber top 118 is formed of a conductive material and extends a length 132 across the second section 206. In some embodiments, the top section 124 of the chamber top 202 includes cooling passages 128 for passing cooling fluid to control the temperature of the chamber top 124.
  • [0046]
    A platen 134 is positioned in the chamber 102 a height 136 below the top section 124 of the chamber 102. The platen 134 can be a substrate holder that holds a wafer 140 for processing as described herein. In some embodiments, a bias voltage power supply 144 is electrically connected to the platen 134.
  • [0047]
    A RF antenna is positioned proximate to at least one of the first section 204 and the second section 206. The RF antenna can have many different antenna configurations as described herein. The plasma source 200 illustrated in FIG. 2 shows two separate RF antennas that are electrically isolated. A coil antenna 208 having a plurality of turns surrounds the curved portion of the first section 204 of the chamber top 202. A helical coil antenna 210 having a plurality of turns surrounds the second section 204 of the chamber top 202. At least one of the coil antenna 208 and the helical coil antenna 210 is an active antenna as described herein. In some embodiments, at least one of the coil antenna 208 and the helical coil antenna 210 is formed such that it can be liquid cooled.
  • [0048]
    A RF source 150, such as a RF power supply, is electrically connected to at least one of the coil antenna 208 and the helical coil antenna 210. The RF source 150 is coupled to the RF antennas 208, 210 by an impedance matching network 152 that maximizes the power transferred from the RF source 150 to the RF antennas 208, 210. In some embodiments, the plasma source 200 includes a plasma igniter 156 that assists in igniting the plasma. The operation of the plasma source 200 is similar to the operation of the plasma source 100 that was described in connection with FIG. 1.
  • [0049]
    FIG. 3 illustrates a RF plasma source 300 having vertical and horizontal RF coils, a conductive top section, and an anode according to the present invention. The RF plasma source 300 is similar to the RF plasma source 100 that was described in connection with FIG. 1. The plasma source 300 includes a chamber 102 that contains a process gas. A gas source 104, which is coupled to the chamber through a proportional valve 106, supplies the process gas to the chamber 102.
  • [0050]
    A pressure gauge 108 measures the pressure inside the chamber 102. A gas pressure controller 116 is used to maintain the desired pressure in the chamber 102 by establishing an exhaust conductance and varying the process gas flow rate in a feedback loop that is responsive to the pressure gauge 108. The chamber 102 includes an exhaust port 114 that is coupled to a vacuum pump 112 that evacuates the chamber 102.
  • [0051]
    The chamber 102 has a chamber top 118 including a first section 120 formed of a dielectric material that extends in a horizontal direction. A second section 122 of the chamber top 118 is formed of a dielectric material that extends a height from the first section in a vertical direction. A top section 124 of the chamber top 118 is formed of a conductive material that extends a length across the second section 122 in the horizontal direction. In some embodiments, the top section 124 of the chamber top 118 comprises a cooling system as described herein. The dimensions of the first and the second sections 120, 122 can be selected to improve the uniformity of plasmas generated in the chamber 102 as described herein.
  • [0052]
    An anode 302 is positioned in the chamber 102 adjacent to the top section 124 of the chamber top 118. In some embodiments, the ratio of an area of the anode 302 to an area of the top section 124 of the chamber top 118 is less than one. In some embodiments, the anode 302 has a planar geometry as shown in FIG. 3. However, there are many other anode geometries that are within the scope of the present invention. For example, in some embodiments, the anode 302 forms a baffle that disperses the process gas in the chamber 102. Also, in some embodiments, the anode 302 forms a shower head that dispenses the process gas into the chamber 102. Furthermore, in some embodiments, the position of the anode 302 in the chamber 102 relative to the top section 124 of the chamber top 118 is adjustable. For example, the position of the anode 302 in the chamber 102 relative to the top section 124 can be chosen to achieve a particular plasma uniformity.
  • [0053]
    In one embodiment, a power supply 304 is electrically connected to the anode 302 as shown in FIG. 3. The power supply 304 can be a pulsed DC power supply, a RF power supply, or a combination of a pulsed DC power supply and a RF power supply. The power supply 304 biases the anode 302 to emit electrons. In other embodiments, the anode 302 is electrically connected to ground potential or is electrically floating.
  • [0054]
    A platen 134 is positioned in the chamber 102 a height 306 below the anode 302 and a height 136 below the first section 120 of the chamber 102 top. The platen 134 can be a substrate holder that holds a wafer 140 for processing as described herein. In some embodiments, a bias voltage power supply 144 is electrically connected to the platen 134.
  • [0055]
    A RF antenna is positioned proximate to at least one of the first section 120 and the second section 122. The plasma source 300 illustrated in FIG. 3 shows two separate RF antennas that are electrically isolated. A planar coil antenna 146 having a plurality of turns is positioned adjacent to the first section 120 of the chamber top 118 and a helical coil antenna 148 having a plurality of turns surrounds the second section 122 of the chamber top 118 as described in connection with FIG. 1. At least one of the planar coil antenna 146 and the helical coil antenna 148 is an active antenna. In some embodiments, at least one of the planar coil antenna 146 and the helical coil antenna 148 is formed such that it can be liquid cooled.
  • [0056]
    A RF source 150, such as a RF power supply, is electrically connected to at least one of the planar coil antenna 146 and the helical coil antenna 148. The RF source 150 is coupled to the RF antennas 146, 148 by an impedance matching network 152 that maximizes the power transferred from the RF source 150 to the RF antennas 146, 148. In some embodiments, the plasma source 300 includes a plasma igniter 156 that assists in igniting the plasma.
  • [0057]
    The operation of the RF source 300 is similar to the operation of the RF source 100. However, the anode 302 is biased to emit electrons. The RF power 150 resonates RF currents in the RF antenna 146, 148. The RF current in the RF antenna 146, 148 induces radio frequency currents into the chamber 102. The power supply 304 applies a pulsed direct current and/or a RF field the anode 302 at a voltage that causes the anode 302 to emit electrons. Both the electrons emitted by the anode and the electrons induced radio frequency currents excite and ionize the process gas, which ignites a plasma in the chamber 102. A plasma igniter 156 can be used to assist in igniting the plasma. The plasma is sustained by one or both of the electrons emitted from the anode 302 and the induced radio frequency currents.
  • [0058]
    Plasma sources according to the present invention can be used to perform numerous types of plasma processes. Some plasma processes are performed under isobaric and isothermal conditions to minimize shock to the processing system. Minimizing shock to the system will reduce particulate contamination in the chamber and on the wafer. For example, plasma sources according to the present invention can be used for plasma immersion ion implantation. Plasma immersion ion implantation requires the generation of a highly uniform plasma. Also, plasma immersion ion implantation requires that the power supply 144 biases the platen 134 with a negative voltage so that ions are attracted to the wafer or target 140.
  • [0059]
    A method of generating a uniform plasma for ion implantation according to the present invention includes introducing a process gas into a chamber 102. A radio frequency current is induced through the dielectric material of at least one of the first and second sections 120, 124 of the chamber top 118. The radio frequency current excites and ionizes the process gas so as to generate a plasma in the chamber 102. The geometry of the first and the second sections 120, 124 of the chamber top 118 and the configuration of the RF antenna is chosen so that a uniform plasma is generated. In addition, the electromagnetic coupling can be adjusted with the coil adjuster 154 to improve the uniformity of the plasma. A wafer or target 140 positioned on the platen 134 is biased so that ions in the plasma are attracted to the wafer or target 140.
  • [0060]
    Secondary electrons are generated when the ions in the plasma impact the wafer or target 140. These secondary electrons are dissipated by the conducting material forming the top section 124 of the chamber top 118. Dissipating the secondary electrons reduces or eliminates charging effects caused by the secondary electrons and, therefore, improves the uniformity of the plasma. The top section 124 of the chamber top 118 may require fluid cooling in order to dissipate the heat generated when the secondary electrons impact the conducting material.
  • [0061]
    The dimensions of the chamber top 118 of the plasma sources 100, 200, 300 described in connection with FIGS. 1-3 can be chosen so that the plasma sources 100, 200, 300 achieve exceptionally high radial and azimuthal plasma uniformity. The radial and azimuthal plasma uniformity can be adjusted by varying the ratio of the height 130 of the first section 120 of the chamber top 118 to the length 132 of the second section 122 of the chamber top 118. Varying the ratio of the height 130 of the first section 120 to the length 132 of the second section 122 of the chamber top 118 will affect the RF coupling into the plasma and, therefore, the uniformity of the ion flux at the platen 134.
  • [0062]
    The dimensions of the chamber top 118 of the plasma sources 100, 200, 300 described in connection with FIGS. 1-3 can also be chosen so that the plasma sources 100, 200, 300 minimize the effects of secondary electrons on the plasma density, plasma uniformity, and the plasma chemistry. Furthermore, the dimensions of the chamber top 118 of the plasma sources 100, 200, 300 described in connection with FIGS. 1-3 can be chosen so that the chamber volume and, therefore, the gas residence time improves or maximizes the plasma uniformity and repeatability. It is understood, however, that the optimal ratio of the dimensions of the chamber top 118 of the plasma sources 100, 200, 300 is also a function of several non-geometrical factors, such as the chamber material, the process gas, and the RF power level.
  • [0063]
    In some embodiments of the invention, the coil adjuster 154 illustrated in FIGS. 1-3 is used to adjust the number of parasitic coil turns in order to change the properties of the plasma generated in the chamber 102. FIGS. 4A-C illustrate graphs of radial plasma density profiles for two different coil adjuster 154 positions.
  • [0064]
    FIG. 4A illustrates a graph 400 of ion saturation current as a function of radius for a one turn vertical parasitic coil with a chamber pressure of 2 mTorr and a RF power level of 750 W. Also, FIG. 4A illustrates a graph 402 of ion saturation current as a function of radius for a four turn vertical parasitic coil with a chamber pressure of 2 mTorr and a RF power level of 750 W. The graphs 400, 402 in FIG. 4A illustrate that adjusting the coil adjuster 154 to a position that results in four vertical turns will result in a relatively uniform plasma over about a 15 cm radius.
  • [0065]
    FIG. 4B illustrates a graph 406 of ion saturation current as a function of radius for a one turn vertical parasitic coil with a chamber pressure of 4 mTorr and a RF power level of 750 W. Also, FIG. 4B illustrates a graph 408 of ion saturation current as a function of radius for a four turn vertical parasitic coil with a chamber pressure of 4 mTorr and a RF power level of 750 W. The graphs 406, 408 in FIG. 4B illustrate that adjusting the coil adjuster 154 to a position that results in four vertical turns will result in a relatively uniform plasma over about a 12 cm radius.
  • [0066]
    FIG. 4C illustrates a graph 410 of ion saturation current as a function of radius for a one turn vertical parasitic coil with a chamber pressure of 8 mTorr and a RF power level of 750 W. Also, FIG. 4C illustrates a graph 412 of ion saturation current as a function of radius for a four turn vertical parasitic coil with a chamber pressure of 8 mTorr and a RF power level of about 750 W. The graphs 410, 412 in FIG. 4C illustrate that adjusting the coil adjuster 154 to a position that results in four vertical turns will result in a relatively uniform plasma over about a 8 cm radius. Comparing FIGS. 4A-C indicates that both the number of turns of the vertical parasitic coil and the chamber pressure affect the uniformity of the plasma and that the chamber pressure affects the ion saturation current.
  • [0067]
    FIGS. 5A-B illustrate graphs of plasma uniformity and mean ion current as a function of chamber pressure for a constant RF power level. FIG. 5A illustrates a graph 500 of percent ion saturation current change for a one turn vertical parasitic coil as a function of chamber pressure over a 15 cm distance. The RF power is 750 W. The graph 500 indicates that the percent ion saturation current change is at a minimum in the pressure range of about 4-8 mTorr.
  • [0068]
    FIG. 5A also illustrates a graph 502 of percent ion saturation current change for a four turn vertical parasitic coil as a function of chamber pressure over a 15 cm distance. The RF power is 750 W. The graph 502 indicates that the percent ion saturation current change is at a minimum when the chamber pressure is about 4 mTorr. The graphs 500, 502 indicate that increasing the number of parasitic coil turns will improve the uniformity of the plasma.
  • [0069]
    FIG. 5B illustrates a graph 504 of mean ion saturation current as a function of chamber pressure for a one turn vertical parasitic coil with a RF power level of 750 W. FIG. 5B also illustrates a graph 506 of mean ion saturation current as a function of chamber pressure for a four turn vertical parasitic coil with a RF power level of 750 W. The graphs 504, 506 indicate that increasing the number of turns in the parasitic coil will decrease the plasma density.
  • [0000]
    Equivalents
  • [0070]
    While the present teachings are described in conjunction with various embodiments and examples, it is not intended that the present teachings be limited to such embodiments. On the contrary, the present teachings encompass various alternatives, modifications and equivalents, as will be appreciated by those of skill in the art.
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Classifications
U.S. Classification156/345.48, 118/723.00I, 216/68
International ClassificationH01J37/32
Cooperative ClassificationH01J37/32412
Legal Events
DateCodeEventDescription
Apr 6, 2005ASAssignment
Owner name: VARIAN SEMICONDUCTOR EQUIPMENT ASSOCIATES, INC., M
Free format text: ASSIGNMENT OF ASSIGNORS INTEREST;ASSIGNORS:SINGH, VIKRAM;PERSING, HAROLD;MILLER, TIMOTHY;REEL/FRAME:015866/0607
Effective date: 20050404