PLASMA DISCHARGE DEVICE AND METHOD WITH DYNAMIC TUNING

Abstract: A plasma discharge device and method for removing material from a substrate having dynamic tuning, which permits operation with a variety of process gasses over a range of operating conditions. A longitudinally extending microwave cavity (2) is defined at the ends by microwave traps (20, 22), the positions of which are automatically adjustable to provide dynamic tuning. An adjustable antenna is provided, and operation utilizes the TM012 mode.

Description: Title of Invention: PLASMA DISCHARGE DEVICE AND METHOD WITH DYNAMIC TUNING BACKGROUND OF THE INVENTION The present invention is directed to an improved plasma discharge device and method for removing material from a substrate. In the manufacture of semiconductor devices, it is frequently necessary to remove a substance from a substrate. The present invention is broadly applicable to such processes, and for example, would include specific processes relating to residue removal, chemical downstream etching (CDE), and etching. It is known to use plasma discharge devices to remove a substance from a substrate, and these may be of the "afterglow" type, where it is the afterglow of the plasma rather than the plasma itself which accomplishes removal. While the gas used in the plasma is frequently oxygen, as for ashing applications, it may be a different gas such as a fluorine containing gas for other applications, as when materials such as heavily metallized polymeric residues are to be removed. ln a plasma discharge device, a gas is flowed through a plasma tube which is located in a microwave cavity, and a plasma is excited in the gas by microwave energy. Then, the plasma, or the afterglow therefrom is used to remove the material from the substrate. One drawback of many plasma discharge devices for material removal is that they are designed for use with only a single type of gas, e.g., oxygen, or fluorine containing gas, as the case may be. This results in added expense since, when a process using a different gas is to be performed, a new piece of equipment must be used. SUMMARY OF THE INVENTION In accordance with the present invention, a plasma discharge device is provided which may be used with different fill gases over a wide range of process conditions. This is accomplished by providing a device which is broadly tunable, so that an appropriate resonant microwave mode may be achieved even when different gases and different operating conditions are present. The dynamic tuning of the present invention is achieved by defining at least one end of a longitudinally extending microwave cavity with a microwave trap, and arranging for the longitudinal position of the microwave trap to be adjustable. In a preferred embodiment of the invention, each end of the cavity is defined by a microwave trap, and the longitudinal positions of both traps are adjustable. In accordance with a further aspect of the invention, the microwave power is coupled to the cavity with an antenna which extends into the cavity, the degree of insertion of which into the cavity is adjustable. This provides a further tuning adjustment, so that coupling of the desired microwave mode may be enhanced while the operating window is enlarged. In accordance with a further aspect of the invention, a moveable microwave trap is provided which does not require finger stock between the trap and the microwave cavity. In accordance with a still further aspect of the invention, a system for automatically adjusting the respective positions of the trap and antenna to the optimum positions for any given process is provided. In accordance with a still further aspect of the invention, the cavity is excited with microwave power in the resonant cylindrical TM 012 mode. In a prior plasma discharge device, which provided microwave modes having azimuthal and longitudinal uniformity to prevent cracking of the plasma tube, it was necessary to separate the elongated microwave cavity into sections with metallic partitions to create sub-cavities which were small enough to support such modes. By providing an adjustable cavity length to excite the cylindrical TM 012 mode, the need for such cavity partitioning is avoided, while azimuthal uniformity and an adequate degree of longitudinal uniformity is provided. BRIEF DESCRIPTION OF DRAWINGS The invention will be better appreciated by referring to the drawings, wherein: Figure 1 is a cross-sectional view of a plasma discharge device. Figure 2 is a perspective view with partial cutaway, of the plasma discharge device of Figure 1. Figure 3 shows the structure relating to the antenna configuration in greater detail. Figure 4 shows alternative structure relating to the antenna configuration. Figure 5 shows a slidable microwave trap. Figures 6, 7 and 8 show components of the slidable trap. Figure 9 is a block diagram of a system for automatically adjusting the positions of the movable trap and the antenna. Figure 10 shows an embodiment of a microwave feed for a plasma discharge device. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Referring to the Figures, longitudinally extending cylindrical microwave cavity 2 is depicted, which has cylindrical plasma tube 4 running therethrough. The plasma tube may be made of quartz or sapphire, or other substance which is appropriate for the gas which is used. In order to excite the gas flowing in plasma tube 4 to a plasma state, microwave excitation must be provided. To this end, microwave source 6, e.g., a magnetron, generates microwave power which is fed through directional coupler 7 to a waveguide having mutually perpendicular sections 8 and 9. The length of section 9 is controlled with moveable short 10, which may be a fixed rather than a moveable member if desired. An antenna 12 in the form of a metallic rod is provided for coupling microwave power from the waveguide to the cavity via a waveguide to coaxial transition. The antenna extends through a circular opening in the cavity wall into the cavity, and its degree of insertion is adjustable. The adjustment is accomplished by pushing or pulling on member 13 which is attached to antenna 12 with pin 15, thus causing antenna 12 to move in and out of the cavity. Member 13 is made of a low loss electrical insulator of low dielectric constant, e.g., polytetrafluorethylene, to prevent microwave leakage through the insertion hole. A screw mechanism including a stepping motor may be provided for automating the insertion and retraction of the antenna. As described above, the antenna adjustment allows the coupling of the field to the cavity to be optimized for specific operating conditions and increases the operating window of the system. Figure 3 is a detail of coaxial structure 70 shown in Figure 1. The coaxial structure is comprised of outer conductor 72 and an inner conductor comprised of sleeve 74 and antenna 12 which slides in sleeve 74 with the aid of finger stock contact ring 76. An alternative embodiment is shown in Figure 4, wherein like parts are identified with the same numerals. In Figure 4, coaxial structure 70' is comprised of outer conductor 72' and an inner conductor comprised of sleeve 74' and antenna 12' which slides therein. In the embodiment of Figure 4, the inner surface of outer conductor 72' and the outer surface of sleeve 74' are tapered so as to provide a gradual change in impedance, thereby providing better impedance matching between the waveguide 9 and the cavity 2. The ends of the microwave cavity are defined by microwave traps 20 and 22. Such traps are a very effective way of preventing microwave leakage since they electrically reduce the microwave current to zero. The traps may be of the type disclosed in U.S. Patent 5,498,308, the entire disclosure of which is incorporated herein by reference. In accordance with an aspect of the present invention, the longitudinal position of microwave traps 20 and 22 is adjustable. For example, the adjustment may be facilitated by disposing finger stock 26 and 27 between the traps and the cavity wall, which are compressible protrusions, thus rendering the microwave traps longitudinally slidable. Additionally, a screw mechanism may be provided for conveniently effecting the adjustment of the traps, and such a mechanism is shown in connection with the top trap 20 in the Figures. It is comprised of horizontal (in the Figures) members 28 and 29, and vertical (in the Figures) cylindrical members 23, 65, and 60. There are three members 23, (not all shown) which connect the top of top trap 20 to member 29. There are three members 65, (not all shown) which connect the top of the microwave cavity to member 28, where they are secured with nuts 30 (only one shown). Member 60 is threaded, as is an opening in member 29 through which in turn raises or lowers the microwave trap 20. If desired a stepper motor may be employed in a configuration for automating the raising and lowering of the trap. A similar mechanism (not shown) can be employed for moving the bottom trap 22. The adjustment of the cavity length provides a broad range of dynamic cavity tuning, and is what permits the device to be operated with different gasses over a wide range of process conditions. As is known to those skilled in the art, utilizing a different gas will change the load impedance and resonant frequency of the cavity, and effectively of the cavity, thereby de¬ tuning it. Re-tuning may be accomplished by experimenting with the positions of the slidable traps to establish a resonant TM 012 mode in the cavity. That is, the cavity is nominally dimensioned to result in the TM 012 mode, and the dynamic tuning permits resonance to be maintained over a variety of process gasses. In addition to the use of different gasses, a broad range of process conditions, e.g., gas flow, pressure, input power, etc. can be accommodated. While the invention is operable by making the position of only one trap adjustable, a big advantage is afforded when both traps are moveable. This is because the position of each trap may be independently changed in relation to the position of the antenna, thus affording a greater range of effective tuning. The device may be air cooled by providing a quartz tube close to and concentric with plasma tube 4, and feeding pressurized air in the space between the tubes, or if desired, may be liquid cooled with a high specific heat liquid that is microwave transparent and clean room friendly by feeding such substance between the concentric tubes. Referring to Figure 1 , it is seen that end cap 40 is provided which abuts the top of the cavity, while fitting 42 having a central orifice for admitting gas to the plasma tube extends into the end cap. The plasma tube is supported at this end by O ring 44 in the end cap. End member 46 is provided at the other end to provide the proper spacing in relation to bottom plate 47 and the process chamber 48, while this end of the plasma tube has an opening in it for emission into the process chamber. In the operation of the device, oxygen, fluorine containing gas, or other gas is fed into gas inlet 42, which feeds plasma tube 4. The microwave cavity is tuned to achieve a resonant TM 012 mode by adjusting the position of slidable microwave traps 20 and 22, and matching of the cavity impedance with plasma to the characteristic impedance of the coaxial section is accomplished by adjusting the insertion of antenna 12 into the cavity. A plasma is excited in the gas, the afterglow of which is emitted from an opening at the end of the plasma tube into process chamber 48. While the invention is applicable to devices where either the plasma or the afterglow from the plasma is used to remove material, the preferred embodiment is an afterglow device. The TM 012 field configuration is depicted in Figure 1 at reference numeral 25. To prevent cracking of a sapphire plasma tube, it is necessary to prevent unequal heating of the tube, and a field configuration having azimuthal and longitudinal uniformity will accomplish this. The TM 012 field has azimuthal uniformity (not shown), and as seen in Figure 1 also has adequate longitudinal uniformity- Thus, it has been found that use of the TM 012 field configuration obviates cracking of the plasma tube, and as mentioned above, does not require a cavity which uses partitioning. The process chamber 48 includes retractable wafer support pins which support wafer 54, to be processed. A chuck is arranged to provide is for providing the correct heating to the wafer during processing. One or more baffle plates may be present above the wafer to promote even distribution of the gas. A disadvantage of the slidable microwave trap shown in Figure 1 is that it utilizes mechanical finger stock between it and the microwave cavity for enabling sliding motion. The finger stock may grind against the microwave cavity and create particles which could contaminate the processing, it may wear out and have to be replaced, and it may otherwise fail, so it is desirable to be able to eliminate it. This is what is accomplished in the embodiment depicted in Figure 5, wherein a particular sliding trap 80 is utilized in microwave cavity 82 having plasma tube 84. The trap is comprised of member 86, shown in perspective in Figure 6, and member 88 shown in perspective in Figure 7, and in cross- section in Figure 8. Member 86 is seen to have slits 90 and circular groove 92. Member 88 is inserted in member 86 (see Figure 5), so that cylindrical flange 94 of member 88 fits in groove 92. The structure is electrically two concentric microwave sub-traps, each of the type shown in Figure 1 and described in detail in U.S. Patent No. 5,498,308. Thus, the inner sub-trap, referring to Figure 5, which includes cylinders 96 and 94 brings the microwave field in gap 98 to zero, while the outer trap, comprised of cylinders 94 and 99, brings the field in gap 100 to zero. Plasma tube 84 passes through central openings 93 and 95 in members 86 and 88 respectively. The length of the trap is substantially one quarter wavelength, and the spacing between cylinders 96 and 94, and between cylinders 94 and 99 is substantially equal. Since the structure does not contact the microwave cavity, it is readily slidable therein, while effectively preventing microwave leakage. Turning to a further aspect of the invention, it is desirable for the respective positions of the moveable trap and antenna to be automatically adjusted to the best positions for any given process. The system which is shown in Figure 9 accomplish this. As previously mentioned, an advantage of the invention is that the dynamic tuning feature permits the device to be used over a wide range of specific processes. For each different process, e.g., different fill gas or gasses and/or power level, there is a unique adjustment of trap and antenna positions which provides optimum operation, i.e., minimum reflected power. Referring to Figure 9, look-up table 102 is provided, which may be in the form of a read only memory or software. The look-up table has programmed in it those values for trap and antenna positions which result in optimum operation. The data in look-up table 102 is established for each specific process by performing the following steps: a) selecting an antenna position and while the process is performed, moving the trap through a range of positions, and recording the reflected power for each combination of positions, b) moving the antenna position incrementally and again moving the trap through a range of positions and recording the reflected power for each combination of positions, and c) repeating step b) until the reflected power value for each incremental combination of trap and antenna position is determined. These values comprise the look-up table. In use of the system of Figure 9, an operator identifies the desired process to be performed to the processor 104, which then accesses look-up table 102. The processor is programmed to select the combination of trap and antenna positions which has the minimum value of reflected power recorded against them for the particular process involved. Such information is fed back to the processor, which then controls trap stepping motor 106 and antenna stepping motor 108 to move the trap and antenna to the indicated positions. If there are a few combinations of trap - antenna positions which result in very low reflected power, i.e., less than 5%, the processor may be programmed to provide an indication of all such positions, so that each may be empirically tried. Figure 10 shows an embodiment of a microwave feed for the plasma discharge device of the invention. Referring to the Figure, magnetron 110 is coupled to circulator 112 and to waveguide 114 which drives antenna 116. Directional coupler 118 is also provided, as is reflected power sensor 118. There is a twist 120 in waveguide 114 to enable more compact packaging of the apparatus. In an actual embodiment according to Figures 1 and 2, which was built, the magnetron frequency was 2443 Mhz, the microwave cavity had an internal diameter of 5.43 inches and was approximately 8.25 inches long with the microwave traps positioned to make the cavity as long as possible. The top microwave trap could be moved to shorten the cavity by about 3.5 inches, while the bottom trap could be so moved by about .5 inch. In use for removing polymeric residues, a plasma tube made of sapphire was used having an ID of 1.37 inches and a fluorine containing gas was flowed through the tube. The microwave power was about 2000 watts and the power density was about 11.8 watts/cc. It should be understood that while the invention has been described in connection with illustrative embodiments, variations will occur to those skilled in the art, and the scope of the invention is to be limited only by the claims which are appended hereto and equivalents.

Claims: CLAIMS 1) A dynamically tunable plasma discharge device for removing material from a substrate, comprising a longitudinally extending microwave cavity, a plasma tube passing through said microwave cavity for containing gas when in operation, means for providing a microwave field in said cavity for exciting gas when in said plasma tube to a plasma, and an opening in said plasma tube for permitting said gas to exit, wherein at least one end of said microwave cavity is defined by a microwave trap and the position of said microwave trap is longitudinally adjustable to provide dynamic tuning of said cavity. 2) The device of claim 1 wherein both ends of said microwave cavity are defined by microwave traps and the respective positions of both microwave traps are longitudinally adjustable to provide dynamic tuning of said cavity. 3) The device of claim 2 wherein said means for providing a microwave field in said cavity comprises microwave source means for providing microwave power and means for coupling said microwave power from said source means to said cavity, wherein said means for coupling includes an antenna which extends into said cavity, the degree of insertion of which into said cavity is adjustable. 4) The device of claim 2 wherein said means for providing a microwave field in said cavity provides a field having a resonant cylindrical TM 012 mode. 5) The device of claim 3 wherein said means for providing a microwave field in said cavity provides a field having a resonant cylindrical TM 012 mode. 6) The device of claim 2 wherein said gas may be selected from a plurality of gases including oxygen containing gas and fluorine containing gas. 7) The device of claim 2 further including means for utilizing said plasma or the afterglow therefrom to remove said material from said substrate. 8) The device of claim 5 further including means for utilizing said plasma or the afterglow therefrom to remove said material from said substrate. 9) The device of claim 1 wherein the microwave trap is moveable within the cavity along the length thereof, and causes the field between the trap and the cavity to be zero without the use of any physical member between the trap and the cavity. 10) The device of claim 9 wherein the microwave trap is comprised of three concentric metallic cylindrical members. 11 ) The device of claim 10 wherein the three concentric metallic cylindrical members an inner cylindrical member, an outer cylindrical member, and an intermediate cylindrical member between the inner and outer members, and wherein the plasma tube passes through the inner cylindrical member. 12) The device of claim 11 wherein the microwave trap causes the field between the plasma tube and the inner cylindrical member to be zero. 13) The device of claim 12 wherein the inner and outer cylindrical members are parts of a first piece, while the intermediate cylindrical member is part of a second piece, wherein the second piece is inserted in the first piece. 14) The device of claim 13 wherein the means for providing a microwave field in the cavity comprises microwave source means for providing microwave power and means for coupling the microwave power from the source means to the cavity, wherein the means for coupling includes an antenna which extends into the cavity, the degree of insertion of which into the cavity is adjustable. 15) The device of claim 14 wherein the means for providing a microwave field in the cavity provides a field having a resonant cylindrical TM 012 mode. 16) The device of claim 1 , wherein the means for providing a microwave field in the cavity comprises microwave source means for providing microwave power and means for coupling the microwave power from the source means to the cavity, wherein the means for coupling includes an antenna which extends into the cavity, the degree of insertion of which into the cavity is adjustable, further including means for any selected material removal process, for automatically adjusting the respective positions of the microwave trap and the antenna such that the reflected microwave power is at a minimum. 17) The device of claim 16 further including data storage means for storing data corresponding to microwave trap and antenna positions for various material removal processes. 18) The device of claim 17 wherein the microwave trap is comprised of an inner cylindrical member, an outer cylindrical member, and an intermediate cylindrical member between the inner and outer cylindrical members, and wherein the plasma tube passes through the inner cylindrical member. 19) A plasma discharge device for removing material from a substrate, which is dynamically tunable for use with different gases over a range of process conditions, comprising a longitudinally extending microwave cavity, a plasma tube passing through said cavity, means for providing gas to said plasma tube, means for providing a microwave field having a resonant TM 012 mode in said cavity which includes an antenna extending into said cavity the degree of insertion of which into said cavity is adjustable, for exciting said gas to a plasma, an end of said longitudinally extending cavity being defined by a microwave trap, and the longitudinal position of the trap being adjustable to provide dynamic tuning of the cavity. 20) The device of claim 19 wherein the microwave trap is comprised of a device for bringing the microwave current to zero. 21) The device of claim 20 further including means for utilizing said plasma or the afterglow therefrom to remove said material from said substrate. 22) A plasma discharge device for removing material from a substrate, comprising, a longitudinally extending microwave cavity, a longitudinally extending plasma tube passing through said cavity for containing gas during operation, means for coupling microwave energy in the TM 012 mode to said cavity for exciting a plasma in said gas during operation, and means for utilizing said plasma or the afterglow therefrom to remove said material from said substrate. 23) The plasma discharge device of claim 22 wherein said plasma tube is made of sapphire.

Inventors: PELLICIER VICTOR, PINGREE RICHARD E, SRIVASTAVA ASEEM K, PELLICIER, VICTOR, PINGREE, RICHARD, E., SRIVASTAVA, ASEEM, K.
Applicants: EATON CORP, PELLICIER VICTOR, PINGREE RICHARD E, SRIVASTAVA ASEEM K, EATON CORPORATION, PELLICIER, VICTOR, PINGREE, RICHARD, E., SRIVASTAVA, ASEEM, K.
Application year: 1998
Main classification: B44C1
IPC codes: B23K10, H01J37, H01L21, H05H1