PLASMA PROCESSING APPARATUS

Abstract: There is disclosed a plasma processing apparatus comprising: a) a chamber having a plasma containing region, the chamber having a dielectric portion; b) an antenna (6) for coupling radio frequency (RF) power into the plasma; and c) a shield member (2) which reduces the level of RF power capacitively coupled into the plasma, wherein the shield member (2) comprises a conducting portion and is positioned between the plasma and the dielectric portion. Also disclosed is a shield member, particularly one for use in the described plasma processing apparatus.

Description: Plasma Processing Apparatus This invention relates to a plasma processing apparatus, particularly one in which sputtering from a dielectric material forming part of the apparatus is reduced. Inductively coupled plasma sources require an antenna to couple the RF power into the plasma. This antenna can either be within the chamber in which the plasma is struck (in which case it is provided with an insulated coating or other shielding) , or more usually is located outside of the chamber close to a dielectric window. The insulator is required to prevent direct electrical contact between the antenna and the plasma. For an ideal inductively coupled plasma source, power is purely inductively coupled into the plasma from the antenna, and the potential difference between the undisturbed plasma and the plasma facing surface of the dielectric window is small. Ions from the plasma, accelerated through the small potential difference to impact on the dielectric window gain only a small amount of energy and therefore do not sputter significant amounts of material from the dielectric window. Unfortunately, when RF power is inductively coupled into a real plasma, there will normally be a degree of capacitive coupling of power due to the fact that a poten- tial difference must exist along the antenna in order for the required current to flow, and even if one end of the antenna is grounded there will be a steadily increasing peak potential towards the other end. This capacitive coupling leads to the development of a significant DC potential difference between the plasma and the plasma facing surface of the dielectric window. Ions from the plasma are then accelerated through the potential difference and impact on the dielectric window with sufficient energy to cause significant sputtering of the window material. The sputtered material will be deposited on other surfaces within the plasma processing chamber including the workpiece, which may be a semiconductor wafer or other object. In one known embodiment, the purpose of the plasma processing apparatus is to etch the workpiece or deposit specific material on the workpiece in a controlled fashion. If material is sputtered from the dielectric window, it will interfere with the required process, leading to poor quality product or even completely unacceptable results. It is clearly desirable to reduce the amount of material sputtered from the dielectric window. To do this requires a reduction in the level of RF power capacitively coupled into the plasma, while only having a small effect on the level of RF power inductively coupled into the plasma. A number of methods have been proposed. One of them is to use an arrangement for balancing the feed of RF power to the antenna using a transformer or other means, so that the voltage excursion on a given section of the antenna is reduced, and therefore the capacitive coupling of power from the antenna is reduced. This technique is described in British Patent Application No. 9714142.8. Alternatively, a grounded, slotted, electrostatic shield may be placed between the antenna and the adjacent face of the dielectric window. This arrangement is described in US Patent No. 5,234,529. If the slots were not present in the shield, power would be inductively coupled into the shield instead of the plasma, since it would form a single shorted turn. The presence of the slots greatly reduces the level of power inductively coupled into the shield, but also reduces the effectiveness as a full electrostatic shield, so that some RF power is capacitively coupled from the antenna, through the gaps in the shield and into the plasma. The present invention reduces the effect of dielectric sputtering. According to a first aspect of the present invention, there is provided a plasma processing apparatus comprising: (a) a chamber having a plasma containing region, the chamber having a dielectric portion; (b) an antenna for coupling radio frequency (RF) power into the plasma; and (c) a shield member which reduces the level of RF power capacitively coupled into the plasma, wherein the shield member comprises a conducting portion and is positioned between the plasma and the dielectric portion. Preferably, the chamber is a vacuum chamber of the type known in the art. The dielectric portion may be a dielectric window which is positioned between the antenna and the plasma. The conducting portion acts to screen the plasma from the large electric fields which may be created by the antenna, which would otherwise capacitively couple RF power into the plasma and create large ion accelerating potentials between the plasma and the dielectric portion. The conducting portion may be grounded, allowed to float electrically, or biased to an appropriate DC potential with respect to the chamber wall. In the latter case, the DC potential may be pulsed or continuous. Preferably, the conducting portion is formed such that there is substantially no path through which induced current parallel to the direction of current flow in the antenna may flow. Thus, the conducting portion, whilst reducing the capacitive coupling of RF power into the plasma, does not greatly reduce the inductive coupling of RF power into the plasma. Any conductor which could carry current in the direction of the antenna should be sufficiently well spaced from the antenna, typically greater than 40mm. Conveniently, the dielectric portion and the shield should be in close proximity. However, if the spacing is too small, this may enhance the chances of discharge between conducting elements of the shield. If the spacing is too large, it may result in coupling inefficiency. In a preferred embodiment, the shield member has a plurality of apertures therein. These apertures may take any appropriate form and examples are discussed below. The spacing between the shield member and the dielectric portion may be preset or, alternatively, may be adjustable. The shield member may comprise a first portion formed of a conducting material and a second portion formed of a conducting or non-conducting material. The first and second portions may have apertures therein and may be staggered such that there is no direct line of sight between the dielectric portion and the plasma. There may be an overlap, for example a small overlap, between the conducting or nonconducting material of the second portion and the conducting material of the first portion. When the second portion is formed of a conducting material, the conducting materials of the first and second portions may overlap, but must not complete a circuit into which appreciable amounts of RF power can be inductively coupled. Furthermore, if the second portion is formed of a conducting material, it preferably acts in a similar fashion to the first portion, reducing direct capacitive coupling of power from the antenna to the plasma. The combination of overlapping first and second portions, with both formed from conductive material, will effectively prevent almost all capacitive coupling of RF power from the antenna to the plasma. This is to be compared with the use of a single portion of the shield member or a first portion formed of a conducting material in combination with a second portion formed of a non-conducting material, where capacitive coupling is reduced because of screening by the conducting material, but is still present to some degree because of the presence of apertures . The use of a second portion formed of a non-conducting material is of some advantage, however, particularly where the nonconducting material is capable of absorbing ions. Alternatively, or additionally, the non-conducting material may be less contaminating as a result of sputtering than the material forming the dielectric portion. In any case, the materials chosen for the conducting and/or non-conducting materials may be any suitable materials, particularly those which effectively capture ions. The second portion may be of a similar shape to the first portion, with a suitable change in size, if necessary. However, this is not essential, particularly where the second portion is non-conducting. The shield member may be heated to reduce deposition or residue build up on it during processing. Either or both portions of the shield member may be heated. This may be achieved by a number of techniques. For example, either or both portions of the shield member (the first shield portion only if the second shield portion is formed from insulating material) may be designed so that part of it forms a closed loop into which RF power may be inductively coupled. RF power coupled into the portion of the shield member will cause localised heating, with this heat conducted to the rest of that portion of the shield member. The RF power utilised may be a proportion of that available from the process antenna. As such the power coupled into the plasma will be reduced, and the power coupled into the portion of the shield member will increase proportionate to the process power requested. Alternatively, RF power may be inductively coupled into the portion of the shield member from a second loop antenna outside of the plasma chamber or inside the chamber and suitably insulated from the plasma. This has advantages over utilising a proportion of the power from the process antenna since the power fed to this secondary antenna may be adjusted to set the temperature of the shield member independently of the power coupled into the plasma. By the attachment of suitable pipes to the portions of the shield member, a liquid or gaseous medium may be circulated from outside the plasma processing chamber. The medium would be heated to a selected temperature outside the plasma processing chamber and would utilise an appropriate pump to circulate it. To ensure good heat conduction from a pipe to a portion of the shield member, the pipe would be firmly attached to that portion of the shield member. Vacuum compatible feedthroughs would be required to allow the pipes to pass through the chamber wall . In one embodiment, the shield member comprises protruding fingers, adjacent fingers joined together at one or both ends. When joined at both ends a slotted structure is formed. For example, when the shield member is substantially circular, the fingers may point substantially towards the centre of the circle. Adjacent fingers may be joined at the centre of the circle by a disc. Alternatively the fingers may be joined at alternate ends. When the shield member comprises a first portion and a second portion, the second portion may take a form similar to the first portion, but may be rotated relative thereto such that there is no direct line of sight between the dielectric portion and the plasma, or may be substantially a negative image of the first (with respect to the dielectric shape) achieving the same effects. Such an embodiment has been found to be particularly suitable for use with a flat antenna. Alternatively, or additionally, the apertures in the shield member may be in the form of a plurality of slots. For example, the shield member could be in the form of a cylinder which may be open ended. This embodiment is particularly suitable where the dielectric window is cylindrical and is surrounded by a circular coil antenna where the coil may be of one or more turns . In a further embodiment, a cross section of the shield member is in the form of a trough which at least partially surrounds a corresponding shaped dielectric portion. In this embodiment, or in any other appropriate embodiment, the shield member may be annular. According to a second aspect of the present invention, there is provided a shield member which is capable of reducing the level of radio frequency (RF) power capacitively coupled to a plasma and which comprises a first portion formed of a conducting material and a second portion formed of a conducting or non-conducting material. The shield member is particularly for use in the plasma processing apparatus described above and may have the preferred or alternative features mentioned above. In particular the first and second portions may have apertures therein and may be arranged such that there is no direct line of sight through the shield member, for example when positioned in the plasma processing apparatus between the dielectric portion and the plasma. Whilst the invention has been described above, it extends to any inventive combination of the features set out above or in the following description. The invention will now be described, by way of example, with reference to the accompanying drawings, and in which: Figure 1 is a plan view from the vacuum side in a chamber of a dielectric window in one embodiment of the invention; Figure 2 is a cross section of the structure shown in Figure 1 ; Figure 3 is a perspective view of an alternative embodiment of the invention with no lid on the chamber; Figure 4 is a cross section of the view shown in Figure 3; Figure 5 is a vertical cross section through another embodiment of the invention; Figure 6 is a horizontal cross section of the embodiment shown in Figure 5 ; and Figure 7 is a perspective view, partially cut away, of the embodiment shown in Figures 5 and 6. Referring to Figures 1 and 2 there is shown a shield assembly generally at 1. The shield assembly 1 may be positioned in a plasma generating chamber (not shown) and Figure 1 shows the view from the vacuum side of the plasma generating chamber. The shield assembly 1 comprises a first shield portion 2 formed of a conducting material. The first shield portion 2 is generally circular and comprises fingers 3 extending from an outer region thereof towards the central region thereof. The shield assembly 1 also comprises a second shield portion 4 (not shown in Figure 1) . A circular dielectric window 5 is positioned adjacent the first shield portion 2 on the side remote from the plasma region of the chamber. A flat antenna 6 is positioned adjacent the dielectric window 5 on the atmosphere side thereof, that is remote from the plasma region of the chamber. The first shield portion 2 is formed of any suitable conducting material, and is normally grounded but, as mentioned above, it may be biased to a chosen pulsed or continuous DC potential with respect to the chamber wall. The form of the second shield portion may vary. In one embodiment, the second shield portion 4 (which may be formed of a conducting or a non-conducting material) may be of a similar or identical shape to the first shield portion 2 but is rotated relative thereto such that the gaps between fingers 3 of the first shield portion 2 are covered by corresponding fingers of the second shield portion 4, with a small overlap. The second shield portion 4 therefore shields the sections of the dielectric window 5 which are "visible" to the plasma between fingers 3 of the first shield portion 2. The second shield portion 4 may take any suitable form. For example, the fingers may extend to almost reach the centre of the circle leaving a very small aperture, or the fingers could be joined at both ends, or the fingers could be alternately joined at the outer and inner ends. Alternatively, either the first shield portion 2 or the second shield portion 4 can be formed of fingers 3 extending outward from a central region thereof towards the periphery. When the other of the shield portions is in the form shown in Figures 1 and 2, such an embodiment prevents a "visible" path between the dielectric window and the plasma even in the central region of the shield assembly. If the second shield portion 4 is conducting, then the fingers thereof must not make electrical contact with the fingers of the first shield portion 2, otherwise a path may be formed in to which RF power may be inductively coupled and dissipated. If the second shield portion 4 is nonconducting, it may or may not be of a similar shape to the first shield portion 2, since even if it formed a continuous ring, RF power would not be inductively coupled into it. If the second shield portion 4 is conducting, it will act in a similar fashion to the first shield portion 2, reducing direct capacitive coupling of power from the antenna to the plasma. The combination of overlapping first and second shield portions 2 and 4 when both are formed of a conductive material, will effectively prevent almost all capacitive coupling of RF power to the plasma. The first shield portion 2 and second shield portion 4 have finite thickness which means that the plasma is separated from the antenna by a total distance made up of the thickness of the dielectric window 5, the thickness of both the first and second shield portions 2, 4, and the spacing between the shield portions 2 and 4 and between the first shield portion 2 and the window 5. If this total thickness is great, then the inductive coupling of RF power into the plasma will reduce in efficiency. However, if the spacing between the shield portions 2 and 4 and the antenna 6 is small, the capacitive coupling between the antenna and the shields will increase, leading to RF power dissipation. Turning to Figures 3 and 4, there is shown an alternative arrangement of a shield assembly, dielectric window and antenna. Thus, an annular antenna 7 surrounds a cylindrical dielectric vessel 8 within which is positioned a cylindrical shield assembly shown generally at 9. The shield assembly 9 comprises a first shield portion 10 and a second shield portion 11 which is correspondingly shaped and is positioned within the first shield portion 10. Figure 3 shows a view from outside of the dielectric vessel 8, with no lid on the plasma chamber. The annular antenna 7 is in the form of a circular coil and the coil may be of one or more turns. The first shield portion 10 comprises therein slots 12 which are vertical and are cut in the region of the antenna 7 to allow inductive coupling of RF power into the plasma rather than into the shield. The second shield portion 11 is of the same general shape as the first shield portion 10, but is rotated relative thereto so that the slots 12 in the first and second shield portions are out of alignment, ie . are staggered. As above, the second shield portion 11 may be formed of either a conducting or a non-conducting material. A conducting material has the benefit of i prov- ing the electrostatic screening, reducing the capacitive coupling of power into the plasma, whilst a non-conducting material provides an ion absorbing surface or surface which will not significantly contaminate the workpiece if sputter- ing occurs when in use in a plasma chamber. An alternative embodiment is shown in Figures 5, 6 and 7. A circular coil antenna 13 is positioned within a dielectric window 14 which is in the form of a trough. Around the outer regions of the dielectric window 14 is positioned a shield assembly shown generally at 15. The shield assembly 15 comprises a first shield portion 16 which itself is substantially surrounded on its outer surface by a second shield portion 17 (not shown in Figure 7) . The shield assembly 15 is located outside of the trough-shaped dielectric window 14, which itself is located in a vacuum vessel of a plasma processing apparatus when in use, so that the shield assembly 15 is separated from the antenna 13 by the dielectric window 14. The first shield portion 16 is in the form of a conducting annular construction. The spacing between the first shield portion 16 and the dielectric window 14 may be preset, or adjustable, and this is true of any embodiment of the present invention. The first shield portion 16 has slots 18 cut therein, preferably perpendicular to the local axis of antenna 13. In the embodiment shown, the second shield portion 17 has the same profile as the first shield portion 16, but is of suitably larger dimensions to allow it to be located at a slightly larger distance from antenna 13 than the first shield portion 16. The second shield portion 17 has slots 19 therein which are similar to slots 18 of the first shield portion 16, but are moved relative thereto so that the slots 18 and 19 are staggered. Again, the antenna is shown as a single turn coil, but this is not intended to preclude the use of multiple turns adjacent to either or both of the sides and bottom of the dielectric window 14.

Claims: CLAIMS 1. A plasma processing apparatus comprising: (a) a chamber having a plasma containing region, the chamber having a dielectric portion; (b) an antenna for coupling radio frequency (RF) power into the plasma; and (c) a shield member which reduces the level of RF power capacitively coupled into the plasma, wherein the shield member comprises a conducting portion and is positioned between the plasma and the dielectric portion. 2. A plasma processing apparatus according to Claim 1, wherein the dielectric portion is a dielectric window which is positioned between the antenna and the plasma. 3. A plasma processing apparatus according to Claim 1 or 2, wherein the conducting portion is grounded, allowed to float electrically, or is biased to an appropriate DC potential with respect to the chamber wall. 4. A plasma processing apparatus according to Claim 3, wherein the DC potential is pulsed. 5. A plasma processing apparatus according to Claim 3, wherein the DC potential is continuous. 6. A plasma processing apparatus according to any preceding Claim, wherein the conducting portion is formed such that there is substantially no path through which induced current parallel to the direction of current flow in the antenna may flow. 7. A plasma processing apparatus according to any preceding Claim, wherein the shield member has a plurality of apertures therein. 8. A plasma processing apparatus according to any preced- ing Claim, wherein the spacing between the shield member and the dielectric portion is preset . 9. A plasma processing apparatus according to any one of Claims 1 to 7 , wherein the spacing between the shield member and the dielectric portion is adjustable. 10. A plasma processing apparatus according to any preceding Claim, wherein the shield member comprises a first portion formed of a conducting material and a second portion formed of a conducting or non-conducting material. 11. A plasma processing apparatus according to Claim 10, wherein the first and second portions have apertures therein. 12. A plasma processing apparatus according to Claim 10 or 11, wherein the first and second portions are staggered such that there is no direct line of sight between the dielectric portion and the plasma. 13. A plasma processing apparatus according to any one of Claims 10 to 12, wherein there is an overlap between the conducting or non-conducting material of the second portion and the conducting material of the first portion. 14. A plasma processing apparatus according to any one of Claims 10 to 13, wherein the conducting and/or non-conducting materials effectively capture ions. 15. A plasma processing apparatus according to any one of Claims 10 to 14, wherein the second portion is of a similar shape to the first portion. 16. A plasma processing apparatus according to any preceding claim, wherein the shield member is heated. 17. A plasma processing apparatus according to claim 16, wherein the heating is achieved by inductive coupling of RF power into the shield member. 18. A plasma processing apparatus according to any preceding Claim, wherein the shield member comprises protruding fingers, wherein adjacent fingers are joined together at one or both ends . 19. A plasma processing apparatus according to any preceding Claim, wherein the shield member comprises a plurality of slots formed therein. 20. A plasma processing apparatus according to any preceding claim, wherein the shield member is cylindrical. 21. A plasma processing apparatus according to any one of Claims 1 to 19, wherein a cross-section of the shield is in the form of a trough which at least partially surrounds a correspondingly shaped dielectric portion. 22. A plasma processing apparatus according to any preceding Claim, wherein the shield member is annular. 23. A shield member which is capable of reducing the level of radio frequency power capacitively coupled to a plasma and which comprises a first portion formed of a conducting material and a second portion formed of a conducting or nonconducting material. 24. A shield member according to Claim 23, wherein the first and second portions comprise apertures therein and are arranged such that there is no direct line of sight through the shield member.

Inventors: BHARDWAJ JYOTI KIRON, LEA LESLIE MICHAEL, BHARDWAJ, JYOTI, KIRON, LEA, LESLIE, MICHAEL
Applicants: BHARDWAJ JYOTI KIRON, LEA LESLIE MICHAEL, SURFACE TECH SYS LTD, BHARDWAJ, JYOTI, KIRON, LEA, LESLIE, MICHAEL, SURFACE TECHNOLOGY SYSTEMS LIMITED
Application year: 1999
Main classification: H01J37
IPC codes: B01J19, C23C14, C23F4, H01J37, H01L21, H05H1