An electrostatic chuck has its electrodes biased with respect to the self-bias potential induced by the plasma on the wafer, thereby providing improved resistance to breakdown in spite of variation of the wafer potential during processing. An alternate embodiment further suppresses the formation of vacuum arcs between the back of the wafer being processed and the body of the chuck by the interposition of a conductive guard ring at the self-bias potential, thereby defining an equipotential area between the closest electrode and the wafer.
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0. 23. An electrostatic chuck system for holding, in a vacuum ambient containing a plasma, by electrostatic attraction of a dc potential a workpiece having a workpiece radius comprising:
at least two circularly symmetric, concentric conductive electrodes having a dielectric coating and together providing a planar clamping surface, at least one of said conductive electrodes having gas feed means therein; and bias means connected to said two electrodes for biasing said two electrodes with respect to a characteristic plasma potential, whereby said two electrodes are maintained at respective bias potentials with respect to said characteristic potential.
0. 18. An electrostatic chuck system for holding, in a vacuum ambient containing a plasma, by electrostatic attraction of a dc potential a workpiece having a workpiece radius comprising:
at least two circularly symmetric, concentric conductive electrodes having a dielectric coating and together providing a planar clamping surface that is a top surface of each of said at least two conductive electrodes, at least one of said conductive electrodes having gas feed means therein; and an outer electrode of said at least two circularly symmetric, concentric conductive electrodes has an electrode outer radius and a gas distribution groove connected to said gas feed means and having an azimuthal gas distribution radius less than said electrode outer radius by a radial impedance distance disposed in said top surface of said outer electrode, whereby, in operation, a cooling gas in said azimuthal gas distribution groove maintains a cooling gas pressure in said azimuthal gas distribution groove by flowing radially outward from said gas distribution radius along said impedance distance and between said clamping surface and said workpiece.
0. 30. An electrostatic chuck system for holding, in a vacuum ambient containing a plasma having a characteristic plasma potential, a workpiece having a workpiece radius by electrostatic attraction of a dc potential between said workpiece and at least one chuck electrode, in which:
said at least one chuck electrode is a circularly symmetric, electrode having gas feed means therein and a dielectric coating on a planar clamping surface, further comprising; voltage bias means for applying a dc clamping voltage to said at least one chuck electrode; and said at least one circularly symmetric, concentric conductive electrode has an electrode outer radius and an azimuthal gas distribution groove connected to said gas feed means and having a gas distribution radius less than said electrode outer radius by a radial impedance distance disposed in said top surface of said at least one electrode, whereby, in operation, a cooling gas in said azimuthal gas distribution groove maintains a cooling gas pressure in said azimuthal gas distribution groove by flowing radially outward from said gas distribution radius to said vacuum ambient along said impedance distance and between said clamping surface and said workpiece. 13. An electrostatic chuck system for holding, in a vacuum ambient containing a plasma, by electrostatic attraction of a dc potential a workpiece having a workpiece radius comprising at least two circularly symmetric, concentric conductive electrodes having a dielectric coating and together providing a planar clamping surface, at least one of said conductive electrodes having gas feed means therein, characterized in that:
an outer electrode of said at least two circularly symmetric, concentric conductive electrodes has an electrode outer radius less than said workpiece radius; said outer electrode is surrounded by a dielectric ring having a dielectric inner radius less than said workpiece radius and having a dielectric ring top surface substantially coplanar with said planar clamping surface, further containing at least one probe member embedded within said dielectric ring and exposed to said plasma, whereby said probe member is maintained by said plasma at a characteristic reference potential; said system further includes bias means connected to said probe member and to said two electrodes for biasing said two electrodes with respect to said characteristic potential, whereby said two electrodes are maintained at respective bias potentials with respect to said characteristic potential.
1. An electrostatic chuck system for holding, in a vacuum ambient containing a plasma, by electrostatic attraction of a dc potential a workpiece having a workpiece radius comprising at least two circularly symmetric, concentric conductive electrodes having a dielectric coating and together providing a planar clamping surface, at least one of said conductive electrodes having gas feed means therein, characterized in that:
an outer electrode of said at least two circularly symmetric, concentric conductive electrodes has an electrode outer radius less than said workpiece radius; said outer electrode is surrounded by a dielectric ring having a dielectric inner radius substantially equal to said workpiece radius, having a dielectric ring top surface substantially coplanar with said planar clamping surface, further containing at least one probe member embedded within said dielectric ring and exposed to said plasma, whereby said probe member is maintained by said plasma at a characteristic reference potential; said system further includes bias means connected to said probe member and to said two electrodes for biasing said two electrodes with respect to said characteristic potential, whereby said two electrodes are maintained at respective bias potentials with respect to said characteristic potential.
6. An electrostatic chuck system for holding, in a vacuum ambient containing a plasma, by electrostatic attraction of a dc potential a workpiece having a workpiece radius comprising:
at least two circularly symmetric, concentric conductive electrodes having a dielectric coating and together providing a planar clamping surface, at least one of said conductive electrodes having gas feed means therein, characterized in that: an outer electrode of said at least two circularly symmetric, concentric conductive electrodes has an electrode outer radius less than said workpiece radius by a guard ring distance; said outer electrode is surrounded by a conductive guard ring having a guard ring top surface substantially coplanar with said planar clamping surface and being dielectrically isolated from said outer electrode; said conductive guard ring has at least one sensing pin extending therefrom and exposed to the plasma, whereby said probe member is maintained by said plasma at a characteristic reference potential; and said system further includes bias means connected to said sensing pin and to said two electrodes for biasing said two electrodes with respect to said characteristic potential, whereby said two electrodes are maintained at respective bias potentials with respect to said characteristic potential.
2. A system according to
gas flows radially outward from a distribution groove in said top surface of said outer electrode and flows between said workpiece and said guard ring.
3. A system according to
a first one of said conductive electrodes has an annular form and is supported in a recess in a second of said conductive electrodes that has a projecting central member within said first electrode and is coplanar with a top surface of said first electrode, said conductive electrodes having at least one interface therebetween; RF power is connected directly to both said conductive electrodes; and said conductive electrodes are decoupled by an annular dielectric member having a greater thickness in a radial region abutting said interface between said conductive electrodes.
4. A system according to
gas flows radially outward from a distribution groove in said top surface of said outer electrode and flows between said workpiece and said guard ring.
5. A system according to
7. A system according to
RF power is fed through said at least two conductive electrodes and coupled therefrom to said workpiece, said RF power being capacitively coupled to said guard ring and capacitively coupled therefrom to that portion of said workpiece having a radius greater than said electrode outer radius.
8. A system according to
a first one of said conductive electrodes has an annular form and is supported in a recess in a second of said conductive electrodes that has a projecting central member within said first electrode and is coplanar with a top surface of said first electrode, said conductive electrodes having at least one interface therebetween; RF power is connected directly to both said conductive electrodes; and said conductive electrodes are decoupled by an annular dielectric member having a greater thickness in a radial region abutting said interface between said conductive electrodes.
9. A system according to
10. A system according to
an annular dielectric field shaping ring is disposed radially outward from said guard ring and having a field shaping inner radius substantially equal to a guard ring outer radius.
11. A system according to
a first one of said conductive electrodes has an annular form and is supported in a recess in a second of said conductive electrodes that has a projecting central member within said first electrode and is coplanar with a top surface of said first electrode, said conductive electrodes having at least one interface therebetween; RF power is connected directly to both said conductive electrodes; and said conductive electrodes are decoupled by an annular dielectric member having a greater thickness in a radial region abutting said interface between said conductive electrodes.
12. A system according to
14. A system according to
a first one of said conductive electrodes has an annular form and is supported in a recess in a second of said conductive electrodes that has a projecting central member within said first electrode and is coplanar with a top surface of said first electrode, said conductive electrodes having at least one interface therebetween; RF power is connected directly to both said conductive electrodes; and said conductive electrodes are decoupled by an annular dielectric member having a greater thickness in a radial region abutting said interface between said conductive electrodes.
15. A system according to
gas flows radially outward from a distribution groove in said top surface of said outer electrode and flows between said workpiece and said guard ring.
16. A system according to
17. A system according to
gas flows radially outward from a distribution groove in said top surface of said outer electrode and flows between said workpiece and said guard ring.
0. 19. A system according to
0. 20. A system according to
0. 21. A system according to
said outer electrode is surrounded by a conductive guard ring having a guard ring outer radius less than said workpiece radius by an overhang amount, having a guard ring top surface substantially coplanar with said planar clamping surface and being dielectrically isolated from to said outer electrode.
0. 22. A system according to
said outer electrode is surrounded by a conductive guard ring having a guard ring outer radius less than said workpiece radius by an overhang amount, having a guard ring top surface substantially coplanar with said planar clamping surface and being dielectrically isolated from to said outer electrode.
0. 24. A system according to
0. 25. A system according to
0. 26. A system according to
said outer electrode is surrounded by a conductive guard ring having a guard ring outer radius less than said workpiece radius by an overhang amount, having a guard ring top surface substantially coplanar with said planar clamping surface and being dielectrically isolated from to said outer electrode.
0. 27. A system according to
said outer electrode is surrounded by a conductive guard ring having a guard ring outer radius less than said workpiece radius by an overhang amount, having a guard ring top surface substantially coplanar with said planar clamping surface and being dielectrically isolated from to said outer electrode.
0. 28. A system according to
0. 29. A system according to
0. 31. A system according to
said voltage bias means applies said dc clamping voltage between said first and second chuck electrodes.
0. 32. A system according to
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This is a continuation of application Ser. No. 08/169,903, filed 12/20/93, now U.S. Pat. No. 5,467,249.
The field of the invention is that of electrostatic chucks for holding a workpiece by electrostatic attraction between the workpiece and one or more electrodes in the chuck.
Extensive work has been done in electrostatic chucks within the last ten years. An example is the chuck illustrated in U.S. Pat. No. 5,055,964, issued to the International Business Machines Corporation.
A chuck adapted to avoid a problem in the prior art of excessive retention of clamping force after power is removed is illustrated in U.S. Pat. No. 5,103,367. That chuck uses an alternating current to avoid polarization of the dielectric.
The invention relates to an electrostatic chuck that has its electrodes biased with respect to the self-bias potential induced by the plasma on the wafer, thereby providing improved resistance to breakdown in spite of variation of the wafer potential during processing.
An embodiment of the invention suppresses the formation of vacuum arcs between the back of the wafer being processed and the body of the chuck by the interposition of a conductive guard ring at the self-bias potential.
Referring now to
It can be seen that ring electrode 100 has an inner vertical surface 155 that will have an inner recess gap between it and corresponding vertical surface 255 of center hub 250 after assembly. There is a corresponding pair of outer surfaces 105 and 205 that define a second outer recess gap. It is important, to provide consistency in clamping force, that these gaps be defined precisely and that they be repeatable. At the bottom of recess 270 there are illustrated two apertures 230 that are used to permit the passage of lifting pins to raise ring electrode 100 up so that top surface 110 is coplanar with surface 210 of base electrode 200. The initial thickness of electrode 100 is made to allow a coupling gap between the bottom of recess 270 (the oxidized recess depth) and the bottom of electrode 100 (i.e. allowing for an oxidized thickness of electrode 100) of nominal thickness 0.001" to 0.003", typically 0.002".
The main requirement of the insulation, whether it be hard-coat anodization, alumina, or any other insulator, is that the coating be as non-porous as possible, so that the electrical breakdown voltage of the insulator is as high as possible. The higher the breakdown voltage the smaller the gaps between the electrodes can be. Preferably, the breakdown strength should be at least 500 volts per mil. Insulators are preferably applied to produce a final thickness of 0.002 inch. Porosity is also important is this application. If the plasma can penetrate through the pores and contact an electrode, then there can be either an arc through the plasma or the electrode can be brought to the plasma potential, thereby declamping that electrode.
Groove 515 extends around the outer portion of electrode 200 in top surface 210 to feed and distribute a gas such as helium into the interstices between the top surface of the electrodes and the back of the wafer for the purpose of providing greater heat transfer than would be provided by mechanical contact between the two surfaces. Those skilled in the art will appreciate that the mechanism used herein to maintain the pressure between the chuck and the wafer at a nominal value (10 Torr, say) that is much greater than the nominal pressure of the chamber (order of magnitude 0.5 mTorr-2 Torr) is that of flowing gas outwardly through the impedance of the short path between rim passage 515 and the ambient vacuum. Pressure within passage 515 is equal to the "impedance" of the constricted passages between the wafer and base electrode 200 times the flow, in analogy to Ohm's law. Thus, it can be seen that, given the impedance set by the roughness of the surfaces and the attractive force between the chuck and wafer 600, it is essential to flow a predetermined amount of gas sufficient to maintain the pressure in the desired range.
Dielectric ring 300, shown as displaced downwardly, has its top surface coplanar with top surface 210 of the gripping electrodes and serves to shape the fringing RF fields passing from the chuck into the plasma radially outside the workpiece.
Referring now to
Wafer 600 is in contact with the plasma and therefore is at a time average "self-bias" potential Vsb that is well below the plasma potential Vp. The exact voltage of the base will depend on the geometric details of the particular apparatus. The time-average plasma potential is always the highest in the system in order to maintain the electron cloud within the plasma. The walls of the vacuum chamber are conventionally at ground, with the chuck at some intermediate voltage depending on the ratio of a wall capacitance between the plasma and the wall and a wafer capacitance between the plasma and the wafer. Since the chuck is much smaller than the wall, and the RF currents through the two capacitors must be equal, the voltage drop over the sheath between the plasma and the chuck must be very much greater than the voltage drop over the wall sheath. (In some chambers having a very symmetric structure such as the Lam Research model 4520, Vsb may be approximately zero.) Thus, the time-average voltage on the wafer Vsb (substantially equal for both the front and back surface) will typically be less than ground.
The two electrodes of the chuck will be biased at some potential above and below Vsb. On the right of
On the left of the figure are found electrical connections for both DC voltage and RF power. The DC voltage is a nominal 600 volts applied between electrodes 200 and 100. The value can range broadly, depending on the application, from nearly zero to about 800 volts. The RF connection is a nominal 1000 watts at 13.56 MHz for a chuck diameter of 200 mm. The RF frequency and power will be determined by the manufacturer of the chamber in which the chuck will be mounted and will vary with the type of etching gas, the material being etched, the size of the wafer, and the size of the chamber. It is fed from generator 630 to two boxes labeled 610 and 620 which represent conventional impedance matching and power distribution subsystems that are connected to electrodes 100 and 200, respectively. A conventional DC power supply 235, isolated by low-pass filters 237 as shown in
In the embodiment illustrated in
In addition, ring 265 of base 260 is close in the vertical dimension to (nominally in contact with) the bottom of wafer 600, reducing the amount of plasma that may make contact with electrode 200 and thus prolonging the life of electrode 200. Since the heat transfer gas is flowing from passage 515 out into the vacuum through the confined space between ring 265 and wafer 600, those skilled in the art would expect that the gas would increase the danger of breakdown in that region, by providing a source of electrons and ions, as gases do in gas discharge apparatus.
There is a drawback to this arrangement in that RF coupling to the portion of wafer 600 above ring 265 is reduced compared to that above electrode 200 by the second capacitor that is introduced between electrode 200 and ring 265. Further, wafer 600 extends over the entire area of ring 265 and there is a deliberate overhang of wafer 600 above dielectric shaping ring 302 that reduces the RF power in that area still further. This overhang reduces the exposure of ring 265 to the plasma, but at the cost of reduced coupling. In a preferred embodiment, the width of guard ring 265 is 1-1.5 mm and the overhang of wafer 600 over shaping ring 302 is 2 mm. In addition, shaping ring 302 serves to shape the fringing RF fields passing into the plasma so that etching uniformity is improved at the rim of the wafer. Suitable materials for shaping ring 302 are alumina or quartz, the horizontal dimension of ring 302 being set to shape the field above the wafer by providing an offset from ground or other low potential, so that the electric field above the wafer remains perpendicular to the surface being etched. The thickness of ring 302 is set to reduce the coupling to the plasma above ring 302 relative to that above wafer 600 so that the plasma is only weakly energized in that area and ring 302 is etched only very slowly. The choice of materials is affected not only by corrosion resistance but also by non-interference of the reaction products that result from etching the ring during processing. There is a frequency-dependent coupling at the edge of the wafer caused by the frequency-dependent conductivity of the wafer. For typical lightly doped substrates having a dopant concentration of 1013/cm3 the wafer has a very high RF conductivity at 400 Khz, a moderate conductivity at 13.5 MHz and a poor conductivity at 40 MHz.
Referring now to
The radial gap between the electrodes should be relatively small, (0.020") in order to have strong fringing fields for a good grip on the workpiece, but a close gap increases the capacitance. The ring is not extended up to the surface because of the above constraint from the fringing fields and also because the thermal conductivity of ceramic is much less than that of aluminum, so that there would be a radial temperature discontinuity if the ceramic did extend up to the surface. Those skilled in the art will appreciate that the final dimensions will depend on the usual engineering tradeoffs, including the sensitivity of the process to radial differences in coupled RF power, differences in temperature and wafer clamping force. In the embodiment illustrated, ring 111 was 0.125 inch thick in the main portion and was 0.340 inch thick in the inner portion. The nominal thickness of electrode 100 at the inner radius was 0.125 inch. Note that this embodiment lacks the outer rim 210 of the embodiment of
The box labelled 615 in
In prior art chucks such as that illustrated in U.S. Pat. No. 4,554,611, the combination of dielectric and wafer was such that there was a significant decay time after the removal of a DC attracting voltage before the wafer could be removed since the very high voltages and dielectric materials used for early chucks caused mobile ions in the dielectric to be trapped polarizing the dielectric. The system described in U.S. Pat. No. 5,103,367 (Horwitz) uses AC drive on the attracting electrodes to alleviate the problem by returning the field to zero repeatedly, thus preventing the formation of a persistent polarization from mobile ions that may be present. If an AC drive were used with the present invention, the pressure of the cooling gas would pop the wafer off the chuck when the voltage dropped below the value that balances the force from the gas pressure. With a 200 mm wafer and a gas pressure of 10 Torr, the voltage required to balance the gas pressure is 200-300 V.
In a system according to the invention, retention of the clamping force is not a problem and there is far more concern with dielectric breakdown between the electrodes and the wafer. With the Very thin dielectric used in the present invention, there is a fine balance between the thickness of the dielectric and the danger of breakdown. As is known, the clamping force is proportional to (V/d)2, where V is the voltage and d is the dielectric thickness. Thus, if the thickness is doubled, so must the voltage, in order to maintain the same clamping force and there is no gain in breakdown resistance. It has been found, contrary to the teachings of the art, that the combination of RF voltage and clamping voltage can cause dielectric breakdown between the wafer and the electrode. This was not a problem in prior art systems that either used very high voltages with correspondingly strong dielectrics and if they used RF, did not feed the RF through the same insulating area as the clamping voltage.
Those skilled in the art will appreciate that the stress on the hard-coat insulation will be frequency-dependent, since there will be an RF voltage superimposed on the DC clamping voltage. In some applications, the RF voltage across the capacitance between the electrode and the wafer (∝1/ωC) together with the DC clamping voltage can exceed the breakdown voltage of the insulator. The danger of this is greatest for low frequency systems, such as the Lam system 4520, in which the plasma is driven at 400 Khz. For example, in a system according to the invention in which the capacitance between the base and the wafer is approximately 6,000 pf, an RF power signal of 400 KHz and a nominal RF current of 2-3 A will produce an RF voltage across the insulator of 200-400 V.
The Horwitz reference also teaches a preferred RF embodiment differing substantially from that of the present invention. In
Those skilled in the art will readily appreciate that different embodiments of the invention may be made in view of the enclosed teaching and the following claims are not meant to be limited to the embodiments disclosed.
Barnes, Michael Scott, Keller, John Howard, Logan, Joseph S., Tompkins, Robert E., Westerfield, Jr., Robert Peter
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Executed on | Assignor | Assignee | Conveyance | Frame | Reel | Doc |
Oct 01 1998 | Dorsey Gage Co., Inc. | (assignment on the face of the patent) | / | |||
Mar 19 2001 | International Business Machines Corporation | DORSEY GAGE CO , INC | ASSIGNMENT OF ASSIGNORS INTEREST SEE DOCUMENT FOR DETAILS | 012698 | /0790 | |
Dec 20 2011 | PRAXAIR SURFACE TECHNOLOGIES, INC | PRAXAIR S T TECHNOLOGY, INC | ASSIGNMENT OF ASSIGNORS INTEREST SEE DOCUMENT FOR DETAILS | 027468 | /0512 | |
Jan 05 2012 | PRAXAIR S T TECHNOLOGY, INC | FM INDUSTRIES, INC | ASSIGNMENT OF ASSIGNORS INTEREST SEE DOCUMENT FOR DETAILS | 028214 | /0615 |
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