GB2298084A

GB2298084A - Microwave and rf plasma systems

Info

Publication number: GB2298084A
Application number: GB9524898A
Authority: GB
Inventors: William M Holber; Donald K Smith; Matthew M Besen; Matthew P Fitzner; Eric J Georgelis
Original assignee: Applied Science and Technology Inc
Current assignee: Applied Science and Technology Inc
Priority date: 1995-02-16
Filing date: 1995-12-05
Publication date: 1996-08-21
Anticipated expiration: 2015-12-05
Also published as: GB9524898D0; GB9910653D0; JP2006164991A; GB2298084B; JP2010103121A; JPH08222397A

Abstract

A fluid-cooled plasma chamber for microwave or rf plasma systems includes a discharge tube 50 transparent to microwave or rf energy input through a waveguide 72, and a cooling member 60 surrounding the tube and defining a helical channel 68 for a cooling fluid. The direct contact between the member 60 and the tube 50 between turns of the helix allows energy to enter the tube without being absorbed by the cooling fluid if the fluid is a radiation absorbing fluid such as water. Alternatively the cooling fluid may be air. A dielectric window for a plasma chamber is also described which has a spiral channel for a cooling fluid at its surface.

Description

Microwave Plasma Systems The invention relates generally to the field of microwave plasma systems. In particular, the invention relates to a fluid-cooled microwave plasma applicator for producing reactive gaseous species for processing applications and a fluid-cooled dielectric window for microwave plasma systems.

Reactive gases and gas mixtures are used in many industrial operations including the processing of semiconductor wafers for fabricating electronic and optical devices. Reactive gases can be used, for example, to etch dielectric and semiconductor materials or various masking films such as photoresist and polyimide. In addition, reactive gases can be used to form dielectric films.

Reactive species of gas molecules can be produced by exciting gas molecules in a plasma discharge. The discharge can be created with a plasma source by coupling energy into a discharge tube or a dielectric window on a chamber containing the gas. Microwave energy is often used as the energy source to create and sustain a plasma discharge. A typical microwave frequency used for creating plasma discharges is 2.45 GHz, due to the availability of power sources and system components.

It is desirable to have a plasma source which is capable of producing a large quantity of various reactive gaseous species under very clean conditions.

Examples of desirable species include the various atomic halogens (atomic fluorine chlorine, bromine, etc.), atomic oxygen, and atomic nitrogen. One technical difficulty in using microwave energy for creating a large quantity of reactive gaseous species in a plasma source is cooling the plasma discharge tube or dielectric window. Air cooling can be used for the discharge tube, but it is relatively inefficient compared with liquid cooling. In addition, air cooling requires relatively large and expensive air blowers or compressors to remove a sufficient amount of heat.

Also, air cooling may not be compatible with modern clean room environments used for manufacturing semiconductors.

Liquid cooling is advantageous because it is efficient. Water cooling is particularly desirable because water has good thermal conductivity and it is both safe to handle and environmentally benign. Also, chilled water is readily available in nearly all manufacturing, university and research and development facilities. However, a barrier to using water for cooling microwave plasma discharge tubes is that water also readily absorbs microwave energy. Similarly, many other desirable cooling liquids readily absorb microwave energy.

Certain fluids such as silicone oils, some chlorofluorocarbons, and various hydrocarbon compounds do not absorb microwave energy and thus can be used to cool the outside of a plasma discharge tube.

Unfortunately, these fluids are often environmentally undesirable, hazardous to handle, and expensive. In addition, using these fluids requires closed-loop heat exchangers which further increases the cost and complexity of the system.

It nonetheless remains desirable to utilize water or other desirable microwave absorbing fluids to cool a plasma discharge tube or a dielectric window which passes microwave energy to a chamber.

It has now been found that a microwave electric field oriented in a particular direction can be efficiently coupled to a microwave plasma discharge tube having channels containing a microwave absorbing cooling liquid and surrounding the tube in a certain path. For example, a microwave electric field oriented parallel to a longitudinal axis extending through the centre of the tube will efficiently couple to a plasma discharge tube having cooling channels encircling the tube in a helical path.

It has also been found that a microwave electric field oriented in a particular direction can be efficiently coupled to a dielectric window having one or more channels in contact with the window and containing a microwave absorbing cooling liquid. For example, a microwave electric field oriented parallel to the surface of the window will efficiently couple to a plasma discharge tube having cooling channels encircling the tube in a helical path.

Using such oriented microwave electric fields it is therefore possible to use water or other microwave absorbing cooling liquids to cool plasma discharge tubes and dielectric windows. If the cooling liquid is in parallel channel portions across the window then the microwave electric field is preferably perpendicular to such portions.

Thus viewed from one aspect the present invention provides a plasma chamber apparatus comprising a plasma container member having a radiation transmitting wall with disposed on the outside thereof a cooling member providing a channel for transporting a microwave or RF absorbing cooling fluid over said wall, said cooling member permitting electric field transmission through an electric field transmitting medium adjacent to said channel and thence through said wall into said member.

For such apparatus, the cooled radiation transmitting wall may form a separable component of the apparatus, for example taking the form of a fluid-cooled dielectric window assembly. Such components form a further aspect of the invention and thus viewed from this further aspect, the invention also provides a plasma container window assembly comprising a radiation transmitting dielectric window with disposed on one side thereof a cooling member providing a channel for transporting a microwave or RF absorbing cooling fluid over said window, said cooling member permitting electric field transmission through an electric field transmitting medium adjacent to said channel and thence through said window.

In one preferred embodiment of the invention, the apparatus of the invention features a fluid-cooled plasma applicator comprising a plasma discharge tube formed from a material substantially transparent to microwave energy such as quartz, sapphire, or alumina.

Tubes formed from sapphire are desirable for applications using fluorine based gases. A cooling member surrounds the tube and defines a channel formed along an inner surface of the member and encircling an outer surface of the tube. The channel provides a conduit for transporting a microwave absorbing cooling fluid over the outer surface of the tube. A medium adjacent to the channel allows a microwave electric field to enter the tube and thus create and sustain a plasma therein while the fluid is flowing through the channel.

More particularly, the channel preferably encircles the outer surface of the tube in a helical path. A microwave electric field oriented parallel to a longitudinal axis extending through the centre of the tube enters the tube without being significantly attenuated by the fluid and thus allows a plasma to form and be sustained. The cooling member may be formed from polytetrafluoroethylene which is chemically inert and microwave transparent. The channel within the member is connectable to a pump which forces the fluid over the outer surface of the tube. The fluid may be water which has high thermal conductivity and is convenient to use.

In another embodiment the apparatus of the invention is a liquid-cooled plasma applicator which comprises a plasma discharge tube formed from a material substantially transparent to microwave energy. An elongated cooling member having an outer surface in contact with the tube and an inner surface defining a channel for transporting a microwave absorbing cooling liquid surrounds the tube. The cooling member may be formed from polytetrafluoroethylene, which is chemically inert and microwave transparent, or from high-thermal conductivity material which can be microwave transparent or reflecting. The outer surface of the member can be thermally bonded to the tube. A medium adjacent to the cooling member allows a microwave electric field to enter the tube and sustain a plasma in the tube while the liquid is flowing through the cooling member. The medium may be air.

More particularly, the cooling member may encircle the outer surface of the tube in a helical path. A microwave electric field oriented parallel to a longitudinal axis extending through the centre of the tube enters the tube without being significantly attenuated by the fluid and thus allows a plasma to form and be sustained. The channel within the member is connectable to a pump which forces the fluid through the channel.

In yet another embodiment, the apparatus of the invention includes a source of microwave energy, a discharge tube substantially transparent to microwave energy and coupled to the source, and a cooling jacket circumferentially positioned with respect to the tube and substantially transparent to microwave energy. The jacket defines a channel formed along an inner surface of the jacket in a helical path for transporting water over the outer surface of the tube. A medium adjacent to the channel allows a microwave electric field oriented parallel to a longitudinal axis extending through the centre of the tube to enter the tube and sustain a plasma while the water is flowing through the channel. The system also includes a pump connected to a source of water and the channel which recirculates the water through the channel.

The present invention also features a fluid-cooled dielectric window for use in a microwave plasma system.

A cooling member is in contact with an outer surface of the dielectric window. The window is formed of a material substantially transparent to microwave energy such as quartz, sapphire, or alumina. The cooling member defines a channel for transporting a microwave absorbing cooling fluid over the outer surface of the window and a medium adjacent to the channel. The medium allows a microwave electric field to enter through the window and sustain a plasma in the chamber while the fluid is flowing through the channel.

More specifically, the channel can form a spiral path over the outer surface of the window. An electric field oriented parallel to the surface of the window enters the window without being significantly attenuated by the fluid and thus allows a plasma to form and be sustained. The cooling member can be formed from polytetrafluoroethylene which is chemically inert and microwave transparent. The channel within the member is connectable to a pump which forces the fluid over the outer surface of the window. The fluid may be water.

In another embodiment, an elongated cooling member has an outer surface in contact with the dielectric window and an inner surface defining a channel for transporting a microwave or RF-absorbing cooling fluid.

A medium adjacent to the cooling member allows an electric field to pass through the window to create and sustain a plasma while a microwave absorbing cooling fluid is flowing through the channel. The cooling member may be formed from high-thermal conductivity material and the outer surface of the member may be thermally bonded to the tube.

More specifically, the outer surface of the channel may form a spiral path over the window. A microwave electric field oriented parallel to the surface of the window will enter the tube without being significantly attenuated by the fluid and thus will allow a plasma to form and be sustained.

In yet another embodiment, a plasma applicator includes a chamber having a dielectric window. A cooling member defines a channel having a spiral path for transporting a microwave absorbing cooling liquid over the outer surface of the window. A medium adjacent to the channel allows a microwave electric field oriented parallel to the surface of the window to pass through the window and sustain a plasma while a microwave absorbing cooling liquid is flowing through the channel. A pump connects to a source of liquid and to the channel recirculates the liquid through the channel.

Although the invention specifies microwave energy as the source for creating the plasma discharge, it is noted that the principles of the invention apply to the use of radio frequency (RF) energy sources as well.

Also, although the invention specifies the use of microwave absorbing cooling liquids, it is noted that systems incorporating the invention can utilize nonabsorbing cooling liquids as well.

The invention will now be described further by way of example and with reference to the accompanying drawings in which: FIG. 1 is a cross-sectional view of a prior art liquid-cooled microwave plasma applicator; FIG. 2 is a cross-sectional view of a fluid-cooled microwave plasma applicator using microwave absorbing cooling fluids; FIG. 3 is a cross-sectional view of an alternative embodiment of the cooling jacket of the fluid-cooled microwave plasma applicator using microwave absorbing cooling fluids; and FIG. 4 is a top view of a fluid-cooled dielectric window for a microwave plasma system.

The drawings are not necessarily to scale, emphasis instead being placed on illustrating the principles of the present invention.

Referring to FIG. 1 there is shown a crosssectional view of a prior art liquid-cooled microwave plasma applicator. The applicator includes a dielectric discharge tube 10. The tube is made of material which is substantially transparent to microwave energy and which has suitable mechanical, thermal, and chemical properties for plasma processing. Typical materials include quartz, sapphire, and alumina. A gas inlet 12 positioned at a top of the tube 14 allows process gases to be introduced into the tube. A bottom 16 of the tube is coupled to a vacuum chamber 18. A vacuum pump 19 is used to evacuate the chamber. During processing, reactive gas species generated in the tube flow downstream into the chamber.

A magnetron 20 generates the microwave energy required to create and sustain a plasma in the tube. An output 22 of the magnetron is coupled to a circulator 24 which allows the microwave energy to pass unrestricted to a waveguide 26 which is coupled to the tube. The waveguide transports the energy to the tube. The circulator directs the microwave energy reflected by the tube to a dummy load 28 so as not to damage the magnetron. A tuner 30 minimizes the reflected energy by perturbing the electromagnetic field in the waveguide.

A cooling jacket 32 with an inlet 34 and an outlet 36 surrounds the tube. A pump 38 coupled to the jacket forces cooling liquid into the inlet, through the jacket, and through the outlet back to the pump. The liquid directly contacts the entire outer surface of the tube. Thus, the microwave energy in the waveguide must travel through the liquid to reach the tube. If the liquid significantly absorbs microwave energy, the energy in the waveguide does not sufficiently couple to the tube to form and sustain a plasma.

Thus, only liquids which do not significantly absorb microwave energy are used in a conventional liquid-cooled microwave plasma applicator. Examples of such liquids include silicone oils, certain chlorofluorocarbons, and various hydrocarbon compounds.

Unfortunately, such fluids are both environmentally undesirable and expensive. Many such fluids are also hazardous to workers and require complex handling procedures. In addition, most of these liquids require the use of closed-loop heat exchangers which significantly increase the system cost and complexity.

Furthermore, if the tube were to rupture, these fluids would contaminate the processing equipment.

FIG. 2 is a cross-sectional view of a fluid cooled microwave plasma applicator using microwave absorbing fluids which incorporates the principles of this invention. The applicator is similar to the prior art.

It includes a dielectric discharge tube 50 made of a material which is substantially transparent to microwave energy and which has suitable mechanical, thermal, and chemical properties for plasma processing. Such materials include quartz, sapphire, and alumina. Tubes formed from sapphire are desirable for applications using fluorine based gases. A gas inlet 52 positioned at a top of the tube 54 allows process gases to be introduced into the tube. A bottom 56 of the tube is coupled to a vacuum chamber 58. Reactive gas species generated in the tube flow downstream into the chamber.

A cooling jacket 60 with an inlet 62 and an outlet 64 surrounds an outer surface 66 of the tube. The jacket is formed of a material which is substantially transparent to microwave energy. An example of such a material is polytetrafluoroethylene. The jacket contains a channel 68 formed along an inner surface 70 of the jacket that encircles the outer surface of the tube. The channel provides a conduit for transporting a microwave absorbing cooling fluid directly over the outer surface of the tube. The fluid can be water which is convenient because it is readily available, has high thermal conductivity, and is chemically inert.

The channel forces the cooling fluid to take a particular path around the outer surface of the discharge tube. The path is chosen to maximize the area of the discharge tube exposed to the cooling fluid. The path, however, leaves sufficient space to allow a microwave electric field with a certain orientation to enter the tube and form and sustain the plasma discharge. In one embodiment, the channel encircles the outer surface of the tube in a helical path leaving a small separation between the loops of the path.

A waveguide 72 carries the microwave energy necessary to create and sustain a plasma in the tube from the magnetron (not shown) to the tube 50. In one embodiment, the microwave electric field is oriented parallel to a longitudinal axis 74 extending through a centre of the tube 76. This orientation allows microwave energy to readily penetrate the tube between the loops of the helical channels without being significantly attenuated by the fluid and thus will allow a plasma to form and be sustained.

Although microwave energy is specified as the source for creating the plasma discharge, it is noted that the principles of the invention apply to the use of radio frequency (RF) energy sources. Also, although the use of microwave absorbing cooling liquids is specified, it is noted that systems incorporating the invention can utilize non-absorbing cooling liquids.

FIG. 3 is a cross-sectional view of an alternative embodiment of the cooling jacket. A cooling tube 80 with an inlet 82 and an outlet 84 is wrapped around the discharge tube. The cooling tube preferably encircles the outer surface of the discharge tube 86 in a helical path leaving a small separation between the loops of the path 88. The microwave electric field is oriented parallel to a longitudinal axis 90 extending through a centre of the tube 92. This orientation allows microwave energy to readily penetrate the tube between the loops of the helical channels without being significantly attenuated by the fluid and thus allows a plasma to form and be sustained.

The cooling tube can be either metallic or nonmetallic and is thermally bonded to the outer surface of the discharge tube. This embodiment is useful for situations where direct contact between the fluid and the outer surface of the tube is undesirable.

FIG. 4 is a top view of a fluid-cooled dielectric window for a microwave plasma system which represents another aspect of the present invention. A dielectric window 100 substantially transparent to microwave energy allows microwave energy to enter into a chamber (not shown). The window is typically formed of quartz, sapphire, or alumina.

A cooling member 102 defines a channel 104 for transporting a microwave absorbing cooling fluid over an outer surface of the window 106 and a medium 108 adjacent to the channel. The cooling member may be a cooling jacket surrounding the window. The medium is substantially transparent to microwave energy. The channel is formed in a certain path so as to allow a microwave electric field of a certain orientation to enter the window and create and sustain a plasma in the chamber while the fluid is flowing through the channel.

The channel within the member is coupled to a pump (not shown) which forces the fluid over the outer surface of the window. The fluid can be water which has high thermal conductivity and is convenient to use. In one embodiment, the cooling jacket defines a channel having a spiral path for transporting a microwave absorbing cooling liquid over the outer surface of the window.

The jacket can be formed from polytetrafluoroethylene which is chemically inert. A medium adjacent to the channel between the spiral path is substantially transparent to microwave energy. A spiral pattern is desirable because it minimizes coupling of microwave energy in the radial direction. Thus, an electric field oriented parallel to the surface of the window passes through the window substantially unattenuated and can create and sustain a plasma while a microwave absorbing cooling liquid is flowing through the channel.

Alternatively, the cooling member may be an elongated cooling member having an outer surface in contact with the window and an inner surface defining a channel for transporting a microwave or RF-absorbing cooling fluid. The elongated member is positioned in contact with the window. A medium adjacent to the cooling member allows an electric field to pass through the window to create and sustain a plasma while a microwave absorbing cooling fluid is flowing through the channel. The medium may be air. The cooling member may be formed from high-thermal conductivity material and the outer surface of the member can be thermally bonded to the tube.

Viewed from an alternative aspect the invention provides a microwave or RF plasma generating apparatus having a microwave or RF radiation source, a plasma chamber arranged to receive radiation from said source through a radiation transmitting wall portion thereof, and a cooling means arranged to transport a liquid coolant across an exterior surface of said chamber, characterised in that at said wall portion only a portion of the radiation incident on said cooling means and wall surface is incident on coolant within said cooling means.

Claims

Claims:

1. A plasma chamber apparatus comprising a plasma container member having a radiation transmitting wall with disposed on the outside thereof a cooling member providing a channel for transporting a microwave or RF absorbing cooling fluid over said wall, said cooling member permitting electric field transmission through an electric field transmitting medium adjacent to said channel and thence through said wall into said member.

2. An apparatus as claimed in claim 1 being a fluidcooled plasma applicator comprising: a discharge tube substantially transparent to microwave and RF energy; and a cooling member surrounding said tube defining (i) a channel formed along an inner surface of the member and encircling an outer surface of the tube for transporting a microwave or RF absorbing cooling fluid over the outer surface of the tube, and (ii) a medium adjacent to the channel which allows an electric field to enter the tube and sustain a plasma in the tube while the fluid is flowing through the channel.

3. An apparatus as claimed in claim 2 wherein said cooling member is a cooling tube surrounding said discharge tube and having a channel formed along an inner surface of the cooling tube for transporting said cooling fluid over the outer surface of said discharge tube.

4. An apparatus as claimed in claim 2 wherein said cooling member further comprises a surface covering said channel thereby forming a chamber isolated from said discharge tube to transport said cooling fluid.

5. An apparatus as claimed in claim 1 being a fluidcooled plasma applicator comprising: a discharge tube substantially transparent to microwave energy; an elongated cooling member having an outer surface in contact with and surrounding said tube and an inner surface defining a channel for transporting a microwave absorbing cooling fluid; and a medium adjacent to said elongated cooling member which allows an electric field to enter said tube and sustain a plasma in said tube while said cooling fluid is flowing through said elongated cooling member.

6. An apparatus as claimed in any one of claims 2 to 5 wherein said channel forms a helical path around said discharge tube.

7. An apparatus as claimed in any one of claims 2 to 6 further comprising means for directing radiation into said discharge tube with the electric field direction of said radiation oriented parallel to a longitudinal axis extending through the centre of said discharge tube.

8. An apparatus as claimed in any one of the preceding claims wherein said cooling member is thermally bonded to said discharge tube.

9. An apparatus as claimed in any one of claims 2 to 8 wherein said discharge tube is formed from sapphire.

10. An apparatus as claimed in any one of claims 2 to 8 wherein said discharge tube is formed from quartz or alumina.

11. An apparatus as claimed in any one of the preceding claims further comprising pump means connected to said channel whereby to transport said cooling fluid along said channel.

12. An apparatus as claimed in claim 1 being a plasma applicator comprising: a chamber having a dielectric window; a cooling member in contact with the outer surface of said window defining (i) a channel having a spiral path for transporting a microwave absorbing cooling fluid over the outer surface of said window, and (ii) a medium adjacent to said channel which allows an electric field oriented parallel to the surface of said window to pass through said window and sustain a plasma while a microwave absorbing cooling fluid is flowing through said channel; a pump operatively connected to said channel which recirculates said fluid through said channel; and a source of fluid operatively coupled to said pump.

13. An apparatus as claimed in any one of the preceding claims wherein said cooling fluid is water.

14. An apparatus as claimed in any one of the preceding claims wherein said cooling member is formed from high-thermal conductivity material.

15. An apparatus as claimed in any one of claims 1 to 13 wherein said cooling member is formed from polytetrafluoroethylene.

16. An apparatus as claimed in any one of the preceding claims wherein said medium is air.

17. An apparatus as claimed in claim 1 being a watercooled plasma applicator comprising: a discharge tube substantially transparent to microwave energy; and a cooling jacket circumferentially positioned with respect to said discharge tube and substantially transparent to microwave energy, and which defines (i) a channel formed along an inner surface of the jacket in a helical path for transporting water over the outer surface of said discharge tube, and (ii) a medium adjacent to said channel which allows an electric field oriented parallel to a longitudinal axis extending through the centre of said discharge tube to enter said discharge tube and sustain a plasma therein while water is flowing through said channel.

18. An apparatus as claimed in claim 17 wherein said jacket is formed from polytetrafluoroethylene.

19. An apparatus as claimed in claim 1 being a microwave plasma system comprising: a source of microwave energy; a discharge tube substantially transparent to microwave energy and operatively coupled to said source; a cooling jacket circumferentially positioned with respect to said discharge tube and substantially transparent to microwave energy, and which defines (i) a channel formed along an inner surface of the jacket in a helical path for transporting water over the outer surface of said discharge tube, and (ii) a medium adjacent to said channel which allows an electric field oriented parallel to a longitudinal axis extending through the centre of the tube to enter said discharge tube and sustain a plasma in said discharge tube while water is flowing through said channel; a pump operatively connected to said channel which recirculates water through said channel; and a source of water operatively coupled to said pump.

20. A plasma container window assembly comprising a radiation transmitting dielectric window with disposed on one side thereof a cooling member providing a channel for transporting a microwave or RF absorbing cooling fluid over said window, said cooling member permitting electric field transmission through an electric field transmitting medium adjacent to said channel and thence through said window.

21. A window assembly as claimed in claim 20 comprising: a dielectric window; and a cooling member in contact with an outer surface of said window defining (i) a channel for transporting a microwave or RF absorbing cooling fluid over the outer surface of said window, and (ii) a medium adjacent to said channel which allows an electric field to enter through said window and sustain a plasma in a chamber while said fluid is flowing through said channel.

22. A window assembly as claimed in claim 20 comprising: a dielectric window; an elongated cooling member having an outer surface in contact with said window and an inner surface defining a channel for transporting a microwaveabsorbing cooling fluid; and a medium adjacent to said cooling member which allows an electric field to pass through said window and sustain a plasma while a microwave absorbing cooling fluid is flowing through said channel.

23. A window assembly as claimed in claim 22 further comprising pump means connected to said channel whereby to transport said cooling fluid along said channel.

24. A window assembly as claimed in any one of claims 20 to 23 wherein said channel forms a spiral path.

25. A window assembly as claimed in any one of claims 20 to 24 wherein said cooling fluid is water.

26. A window assembly as claimed in any one of claims 20 to 25 wherein said cooling member is formed from high-thermal conductivity material.

27. A window assembly as claimed in any one of claims 20 to 25 wherein said cooling member is formed from polytetrafluoroethylene.

28. A window assembly as claimed in any one of claims 20 to 27 wherein said window is formed from sapphire.

29. A window assembly as claimed in any one of claims 20 to 27 wherein said window is formed from quartz or alumina.

30. A window assembly as claimed in any of claims 20 to 29 wherein said cooling member is thermally bonded to said window.

31. A microwave or RF plasma generating apparatus having a microwave or RF radiation source, a plasma chamber arranged to receive radiation from said source through a radiation transmitting wall portion thereof, and a cooling means arranged to transport a liquid coolant across an exterior surface of said chamber, characterised in that at said wall portion only a portion of the radiation incident on said cooling means and wall surface is incident on coolant within said cooling means.