US RE40195 E1
A chamber housing (2) enclosing a plasma region (20) in a large area plasma source used for performing plasma assisted processes in large area substrates, the chamber housing (2) being composed of: a housing member (2) constituting a substantially vertically extending wall (4) surrounding a space (6) corresponding to the plasma region (10), the housing member (2) having a plurality of openings (32) and electrically conductive elements forming an electrostatic shield around the space; a plurality of dielectric members (36) each having a peripheral edge and each disposed to close a respective opening (23); and sealing members (40, 40′, 42′) forming a hermetic seal between said housing member and said peripheral edge of each of said dielectric members (36).
1. A chamber housing enclosing a plasma region in a large area plasma source used for performing plasma assisted processes on large area substrates, said chamber housing comprising:
a housing member constituting a substantially vertically extending wall surrounding a space corresponding to the plasma region, said housing member having a plurality of openings, and electrically conductive elements forming an electrostatic shield around the space;
a plurality of dielectric members each having a peripheral edge and each disposed to close a respective opening; and
sealing means forming a hermetic seal between said housing member and said peripheral edge of each of said dielectric members; and
a coil configured to generate an RF field that passes through said dielectric members to create a plasma in said plasma region, said coil being disposed outside of said electrostatic shield relative to said plasma region.
2. The chamber housing according to
3. The chamber housing according to
4. The chamber housing according to
5. The chamber housing according to
6. The chamber housing according to
7. The chamber housing according to
8. The chamber housing according to
9. The chamber housing according to
10. The chamber housing according to
11. The chamber housing according to
12. The chamber housing according to
13. A large area plasma source used for performing plasma assisted processes on large area substrates in a plasma region, and source comprising:
the chamber housing according to
a coil surrounding said housing and operative for generating an RF field in the plasma region;
an enclosure member surrounding said housing and the plasma region;
gas injection means extending through said enclosure member for introducing an ionizable processing gas into the plasma region;
substrate support means for supporting a substrate to be processed in the plasma region; and
at least one pump disposed for pumping gas out of the plasma region to maintain a low pressure in the plasma region.
14. The plasma source according to
15. The plasma source according to
16. The plasma source according to
17. The plasma source according to
This application is the National Phase of International Application PCT/US99/27928 filed Dec. 10, 1999 which designated the U.S. and that International Application was published under PCT Article 21(2) in English. This application also claims priority from U.S. provisional application No. 60/114,454 filed on Dec. 30, 1998.
The present invention relates to plasma sources for use in the performance of plasma-assisted processes, including deposition and etching processes performed on substrates in processing chambers. The invention particularly relates to plasma sources which allow processing of large area substrates.
There is a demand for plasma sources that will enable processes of the above-mentioned type to be performed on large size wafers and even more so for flat panel display processing. There are indications in the industry that efforts will be made to manufacture flat panel displays measuring 1 meter on a side and plasma-assisted processing of such substrates will require higher plasma ion density levels than are produced in existing systems. Plasma-assisted processing of such large area substrates requires both high plasma density and high pumping speed to achieve high processing rates.
In plasma sources of the type described above, the plasma deposition or etching rate will depend on the ion flux, or ion density, as long as the process gas throughput, or pumping speed, satisfies the processing chamber requirements. Therefore, the achievement of satisfactory processing rates for large area substrates requires both the gas throughput and the ion flux be sufficiently high.
In addition, a plasma source having the requisite large dimensions must withstand a considerable force from atmospheric pressure and must be capable of providing an optimum geometry for creation of an electric field that will provide a uniform plasma inside the processing chamber of the source.
It is an object of the present invention to provide a large area plasma source which has the above-mentioned capabilities.
Another object of the invention is to provide a large area plasma source housing capable of supporting atmospheric pressure forces while providing a requisite electrostatic shield for the plasma confined within the housing and permitting transmission of RF electromagnetic field energy to the plasma.
The invention achieves these and other objects by providing a plasma source housing having side walls made of: metal electrostatic shield members that provide support against atmospheric pressure; or a ridged dielectric wall that is capable of supporting atmospheric pressure and is combined with electrically conductive elements that provide the electrostatic shield function; or a combination of the two. These walls can be shaped according to any vertical geometry including, but not limited to, straight, tapering in or out, curved in or out, etc. Therefore, a plasma source housing can be constructed to have virtually any dimensions and shape needed, while allowing RF energy to be supplied to the plasma through the housing wall. In addition, this housing will readily accommodate a system for cooling the processing chamber walls.
A further object of the invention is to achieve a high degree of plasma uniformity within the processing chamber. Because the RF electric field which creates and maintains the plasma originates in the region which surrounds the processing chamber, plasma uniformity is attained by diffusion, with gas species, or processing gas, flow and plasma gradient combining to provide process uniformity. Therefore, at any pressure and RF power level, plasma uniformity is a function of the aspect ratio of the processing chamber, i.e., the ratio of the square root of the cross sectional area to the height of the processing chamber. The cress-sectional area is the area of a horizontal plane at a location where the chamber has an average cross-sectional area.
It presently appears that by applying the principles to be disclosed herein together with standard testing procedures, a high degree of plasma uniformity can be achieved.
The above and other objects are achieved, according to the present invention, by a chamber housing enclosing a plasma region in a large area plasma source used for performing plasma assisted processes on large area substrates, the chamber housing comprising:
Space 6 is filled with a liquid coolant and contains an RF coil 8 that is supplied with an RF current which generates an electric field in the region enclosed by housing 2 in order to ignite and sustain a plasma in processing region 10 enclosed by housing 2 and by upper and lower walls of enclosure 4. Upper wall 12 of enclosure 4 carries coolant supply and return lines 16, vacuum pump assemblies 18 for pumping gas molecules and ions out of processing region 10 and maintaining a desired vacuum pressure therein, and passages (not shown) for couplings used to introduce fresh processing gas into region 10.
Wall 12 additionally carries a fast match assembly, or match network, 20, which is a component that is known per se and that is typically made up of an L-network of two variable capacitors and an inductor wherein the variable capacitors are mechanically adjusted by an automatic control network. The purpose of network 20 is to equilibrate the source impedance of the RF generator with the load impedance as seen by the generator looking into the match network and plasma source. Typically, the source impedance of the RF generator is 50Ω and, hence, the variable match network components are varied such that the output impedance of the match network is the complex conjugate of the input impedance to the plasma source. During matched conditions, the forward power at the match network juncture is maximized and the reflected power is minimized. Match network designs, although different in speed, robustness and controllability, are all based upon the same fundamental principles and are often described in the prior art.
As will be described in greater detail below, chamber housing 2 provides the vertical bounding walls for region 10 and is constructed to withstand the forces acting on walls 12 and 14 due to the difference between the atmospheric pressure acting on the outer surfaces of the relatively large area walls 12 and 14 of enclosure 4 and the vacuum pressure established in region 10 and acting on the inner surfaces of those walls. Chamber housing 2 is further constructed to provide an electrostatic shield for region 10 and allow transmission of RF energy from coil 8 to region 10. The vertical wall of enclosure 4 may, but need not, be constructed to assist in withstanding the above-mentioned forces imposed on walls 12 and 14 by the pressure differentials between opposed surfaces of each wall.
Each vacuum pump 18 is part of a vacuum pump assembly that includes a respective gate valve or throttle, 22 mounted on wall 12 with the aid of a coupling flange 24. When gate valve 22 is left wide open, maximum pumping speed can be achieved. However, partial closure can permit a spatial variation of the pumping speed by means of varied flow restriction through the distributed pumping orifice. Gate valve 24 can be of a known design.
Flange 24 is a cylindrical part which provides a flow path between a respective through bore 28 in wall 12 and the inlet end of a respective pump 18. Bores 28 are pumping ports which will each communicate with the inlet of a respective vacuum pump 18. By suitable positioning, and selection of the number, of bores 28, along with suitable selection and control of the operation of pumps 18, the exhaust gas flow can be turned for uniform gas exit from the region 10. Selection and control of pumps and the arrangement of bores 28 can be effected on the basis of principles and practices already well known in the art.
Attainment of the desired process uniformity also requires appropriate control of gas injection. This aspect of the invention will be described, infra.
In addition, the bottom of the source will be provided with a suitable substrate support and means for applying a bias voltage, for example, an RF bias to the support. Here again, such a substrate support can be constructed and installed in the source in accordance with principles and practices already well known in the art.
Being made of a conductive material, housing 2 constitutes an electrostatic shield. Housing 2 is provided with a series of vertically elongated recesses 32 that are spaced at uniform intervals about the housing periphery. At the center of each recess 32, there is a narrow, elongated slot 34 which extends through the remaining thickness of housing 2 to communicate with region 10. Each recess 32 is provided with an inert 36 made of a dielectric material, such as alumina, with a projecting portion that extends into slot 34 of its respective recess 32. Each insert 36 is provided with four elastomeric vacuum seals 40 and 40′ at the inwardly facing surface of insert 36 and 42 and 42′ at the outside of insert 36. Each insert 36 is covered by a respective frame 46 which holds its associated insert 36 and seals 40, 42 in place in recess 32, with the aid of a plurality of screws (not shown) that extend through screw holes 48. Between the two seals 40 and 40′ there is provided at least one passage to atmosphere, shown at 52 in FIG. 4. This passage is fabricated to extend to a location outside of enclosure 4 for access when the system is totally assembled. Passage 52 allows for leak checking both seals 40 and 40′. Thus, both sealing with respect to the coolant fluid 54 between chamber 2 and wall 4 and sealing with respect to the vacuum in region 10 can be checked with one port.
RF energy can flow from coil 8 into region 10 through inserts 36 and slot 34.
Band 68 may be made of Kovar™, which is a trade-name for a metal alloy containing 54% iron, 29% nickel and 17% cobalt. The coefficient of thermal expansion for Kovar is between that of the metal housing and dielectric window. The use of such a material is common to the industry.
In the embodiment of
Housing 72 is further provided with a plurality of elongated load supporting members 86 made of electrically conducting material, such as aluminum. Each member 86 has a T-shaped cross section, is seated between two adjacent ribs 82 and is securely connected at its upper and lower ends to top and bottom edges of housing 72. Members 86 function as the conductive members of the electrostatic shield and it is important these members have good RF electrical contact to housing 72 both at the top and bottom. Satisfactory contact, and a sound mechanical connection, can be provided by using machine screws (not shown) to secure the upper and lower ends of each member 86 to the top and bottom edges of housing 72. Because members 86 are made of metal and are therefore relatively inelastic, a layer of an elastic material is preferably disposed between each member 86 and its associated part of base portion 80. One such member 88 is shown in broken lines in FIG. 6A.
As an alternative to the embodiment illustrated in
Each brace 89 has certain built in features that are shown in FIG. 6C. In particular,
In the case of the embodiment shown in FIG. 6A and the alternative thereto described above, each dielectric panel 78 is hermetically sealed to its associated recessed portion 76 by at least two O-rings held in grooves 41. Preferably, sealing is achieved by the provision of dual elastomeric seals, like rings 40 and 40′, separated by a space which is coupled by a series of passages, like passage 52, internal to the housing to the outside to allow technicians to sense liquid leaks or vacuum leaks.
Outlet portions 94 of tubes 90 can be placed at a height above the substrate to be treated that allows the optimum gas species to arrive at the substrate. As the distance between outlet portions 94 and the substrate is increased, the spacing between tubes 90 can be increased while the density of ionized gas reaching the substrate remains approximately uniform. Of course, an increase in the spacing between tubes 90 results in a reduction in the number of tubes. For certain processes, however, it may be desirable to bring the outlet portions of tubes 90 closer to the substrate. This may be done, for example, if it is desirable to reduce the time between gas ionization and contact of the resulting ions with the substrate.
Each gas injection tube input portion is connected to a flow regulating valve or an individual mass flow controller to control the injection of gas from an inlet manifold (not shown). By controlling the flow of gas into each end of each tube 90, a variety of gas injection profiles are made possible, as will be described below.
At the center of wall 12 there is provided a viewport 99 in the form of a funnel-shaped passage. This funnel-shaped passage is angled such that it provides a field of view encompassing the entire substrate being processed. Viewport 99 may be used simply for visual inspection of the chamber and its process, or it may accommodate a diagnostic system requiring optical access to the inner chamber.
The exterior surfaces of gas introduction tubes 90 will become coated with residue of the processing gasses over the course of time. According to a further feature of the invention, these coatings can be removed from tubes 90 by applying RF bias to tubes 90 during cleaning of the interior of housing 2.
Such cleaning is conventionally performed periodically in present day etch or deposition chambers by a separate cleaning process wherein the chamber is cleaned with a substrate installed in region 10 is filled with a gas which, when ionized in a plasma, is capable of removing residue coating from surfaces within housing 2 and a plasma generating RF field is created in region 10. In wafer processing, this cleaning process is often conducted at very much higher pressure than the normal process pressure to improve the chemical process rate by increasing the number of atoms, or irons, in the plasma. Applying RF bias to parts of housing 2 can also increase the residue removal rate.
It is also known to install a metal electrode outside the housing and behind the dielectric wall of the housing and to apply a voltage to the electrode in order to provide bias to the wall and increase the cleaning rate. An arrangement of this type is disclosed, for example, in pending provisional application No. 60/065,794, by Wayne Johnson, entitled ALL RF BIASABLE AND/OR SURFACE TEMPERATURE CONTROLLED ESRF.
Application of RF bias to tubes 90 can increase the energy of ion bombardment within tubs 90 and thus increase the rate and effectiveness with which residues are etched away from their interior walls. Ion bombardment can be thought of as increasing the surface temperature of surfaces being bombarded and hence can increase the chemical reaction rates.
Preferably, injection tubes 90 are made of anodized aluminum or are composed of a metal tube component sheathed in dielectric tubing made of quartz or alumina.
To allow for application of a cleaning RF bias voltage to tubes 90, it is necessary to isolate tubes 90 electrically from the walls of enclosure 4. Such isolation is needed so that plasma is not created inside the tubes that deliver gas. This bias will not be applied during normal operation, but only during periodic cleaning cycles. The RF bias to the gas injection tubes is applied periodically to clean the exterior surface (or process side). During each process, contaminants may build up on the tube surface. To minimize long-term particulate contamination, the exterior surface of the injection tube (in addition to all surfaces within the chamber) must be cleaned during a cleaning cycle. The RF bias will generate a DC self-bias (and resultant average voltage difference across the sheath) which in-turn affects the average ion energy delivered to the injection tube surface.
If the tubes are made of conductive material, then a capacitor is needed to allow them to charge by self-bias.
In order to prevent the generation of a plasma within the interior volume of the injection tubes during RF bias application, it is possible to use strictures inside the tube that minimize breakdown by use of dielectric surface area. One example is a bundle of capillary tubes of dielectric material (quartz) in the gas flow path as has been used in the semiconductor industry to deliver process gas to an upper electrode-inject plate.
According to the invention, the distribution of processing gas within region 10, using the injection tube arrangement shown in
As shown in
Injection tubes 90 are equally spaced (with spacing d) in one horizontal direction. As illustrated in
The injection system is designed to introduce the processing gas to a large volume chamber region 10 in which a vacuum passage Pc is maintained. The gas is introduced at subsonic speeds, i.e., M=v/a<0.3-0.5 where M is the Mach number, v is the gas velocity at each exit orifice and a is the local speed of sound.
According to the invention, the distribution, or gradient, of gas exit velocities across the length of output portion 94 of a tube 90 can be controlled by proper selection of the flow rate Q and the inlet total pressure Pt at both ends of that tube 90. The particular gradient established will influence the uniformity of the plasma-assisted treatment across the substrate surface.
When the flow rate Q into an injection tube 90 is high, an exit velocity distribution can be obtained such that the velocity is greatest at the mid-point of the length of the outlet portion of the tube and decreases progressively toward the ends thereof. Alternatively, when the flow rate Q is low, the velocity distribution is such that the greatest velocities are achieved at the ends of tube outlet portion 94. Hence, it is possible to control the velocity distribution along the length of the outlet portion of a tube 90, or the span-wise velocity distribution, by adjusting the inlet volume flow rate Q.
For the sake of clarity, the terms “high flow rate” and “low flow rate” will be defined. A “high flow rate” is one in which the gas momentum is large relative to the difference in pressure between the gas in the injection tube and the chamber pressure. Similarly, a “low flow rate” is one in which the relative gas momentum is small. In the case of a high flow rate, the lateral pressure gradient is unable to sufficiently bend the “high” momentum fluid and accelerate it through the adjacent injection nozzle; therefore, the predominant mechanism is deceleration of the gas to a stagnation pressure at the midpoint of the length of the outlet portion 94 of the tube 90 where momentum is cancelled and a sufficient pressure difference can be achieved. The stagnation flow at the outlet portion midpoint is simply a consequence of introducing process gas at both ends of the injection tube. In the case of a low flow rate, the gas momentum is such that gas will tend to exit via openings at the ends of the outlet portion of the injection tube under the effect of the differential between the inlet pressure and the pressure in chamber region 10; as gas exits through successive injection nozzles 94′; the pressure within outlet portion 94 of a tube 90 decreases.
Taking a more rigorous approach, the above explanation can be substantiated in a clearer manner. Consider the transverse equation of momentum for the coordinate system indicated in FIG. 9. Assuming the flow to be steady and two-dimensional and neglecting the viscous terms, the transverse equation of momentum becomes:
The radical pressure gradient is balanced by two terms; the first of which represents the transfer of stream-wise momentum (in the direction z) into radial momentum (in the direction r), and the second of which represents the radial acceleration of the radial flow. The injection tube design depends on an independent set of parameters including ρo, Q, Pt, Pc, A1, A2, Δ1 and L; refer to FIG. 9. Note that this parameter list excludes the number N of injection nozzles 94′ and their respective cross-sectional areas A2 since N=2 L/Δ1 and A2=A2T/N). Neglecting compressibility effects (a good assumption for M<0.3), the radial equation of momentum is non-dimensionalized using the following relationships
When B*>>1, this corresponds to high flow rates at which the pressure gradient is inadequate to substantially turn the stream-wise momentum and, hence, larger velocities exit at the midpoint, or center, of the outlet portion of the injection tube. Conversely, B*<<1 corresponds to low flow rates at which the opposite is true. The resulting velocity distributions are depicted in
Thus, if B*=1, the gas exit velocity will be constant along the length of outlet portion 94 of tube 90. For many processes, this will be the preferred exit velocity distribution. However, there may be situations in which it is preferable that B*≠1. For example, the RF field generated in region 10 may vary in intensity in radial directions perpendicular to the vertical center axis of region 10. In such a case, a gas flow rate variation having a form shown in one of
Close inspection of the definition of B* provides insight into the design of injection tubes 90. For example, the condition B*>>1 can be achieved by performing any one of the following actions while holding all other parameters constant: increase Q (increase the gas momentum ρoV); decrease ΔP (reduce the turning force); and decrease A2T (provide greater flow resistance).
Typically, a fixed relation will exist between a given value for Pe, inlet pressure and flow rate. However, it might be possible to independently control the inlet total pressure and the mass flow rate. This would require adjusting the total pressure losses in the system using throttle valves. For example, the throttle valve upstream of the turbo-molecular pump can adjust the chamber pressure and pressure regulators upstream of the injection tubes can regulate the total pressure.
Considering the list of independent dimensional parameters listed previously, it is sufficient to define a parameter for uniformity u=P(z=0)−P(z=L) such that the non-dimensional uniformity u*=u/ΔP takes the form
We consider the asymptotic limit where the four latter parameters go to zero, i.e., the number of injection nozzles is large (Δ1/L→0), the pressure difference is small relative to the absolute value (ΔP/Pc→0), each injection nozzle area A2 is small relative to the injection tube cross-sectional area (A2/A1→0), and the injection tube is long relative to its diameter (A1/L2→0). Nominal conditions for B*˜1 are: Δ1=1.0 cm, L=50 cm, N=100, A1=1.77 cm2, A2=0.0079 cm2, Pc=500 mTorr; Pt=600 mTorr and Q=160 sccm (or Qtot=320 sccm).
According to various alternatives made possible by the invention, it may be desirable to allow the gas flow to choke at the gas injection nozzles 94′. When the pressure ratio across an injection nozzles 94′ (i.e., the ratio of the total pressure inside the injection tube to the ambient chamber pressure beyond the exit of the injection nozzle) is sufficiently large, the injection nozzle reaches a “choked” condition wherein the volume flow rate is invariant with either further reduction of the back pressure (or chamber pressure) or increase of the inlet total pressure. In fact, the mass flow can only be increased further by increasing the total inlet pressure (hence affecting the gas density). Since the volume flow rate at an injection nozzle exit is invariant and the injection nozzle exit area is constant, this implies that the exit velocity is constant. However, one may redistribute the injection nozzles 94′ in injection tube 90 in order to affect the mass flow distribution entering the chamber. Hence, the mass flow distribution may be designed to behave in either manner as described in
There are several advantages to the use of the gas injection tubes, namely: RF bias of the gas injection tubes allows periodic cleaning of the exterior surface; variable placement of adjacent tubes in separate vertical and/or horizontal planes, allowing each injection tube to be located at a different vertical height above the substrate for improved process control; selectable injection nozzle distribution for modification of inlet mass flow distribution; supersonic or subsonic injection capability; and adjustment of gas momentum in the injection tubes for mass flow redistribution subsonic injection.
As mentioned earlier herein with reference to
As indicated in
While the description above refers to particular embodiments of the present invention, it will be understood that many modifications may be made without departing from the spirit thereof. The accompanying claims are intended to cover such modifications as would fall within the true scope and spirit of the present invention.
A plasma source according to the invention may include a conventional chuck for holding the substrate, or wafer, to be processed. The chuck would be typical of most conventional plasma sources. In addition to holding the substrate, it should be capable of applying a RF bias to and heating the substrate. Therefore, for large area processing, this chuck could consist of multiple segments.
The presently disclosed embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims, rather than the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.