Plasma processor for large workpieces

What is claimed is:

1. A device for treating a workpiece with a plasma comprising a vacuum chamber in which the workpiece is adapted to be mounted, means for introducing into the chamber a gas which can be converted into the plasma for treating the workpiece, means for converting the gas into the plasma including an electric source for producing an r.f. field originating outside of the chamber, plural individually supported dielectric windows on an exterior surface of the chamber positioned to couple the r.f. field to the gas so the field coupled through the windows excites the plasma, the windows having a thickness substantially less than the thickness of a single window having the same combined area as the plural windows to withstand the differential pressure between the interior and exterior of the chamber.

2. The device of claim 1 wherein the electric source includes a single excitation device for producing the r.f. field that is coupled through the plural windows.

3. The device of claim 2 wherein the excitation device includes a single coil array that extends over the plural windows, the r.f. field being a magnetic field derived from the array.

4. The device of claim 3 wherein the coil array includes a planar coil that extends over the plural windows.

5. The device of claim 3 wherein the coil array has a pair of terminals connected to several electrically parallel segments via a pair of leads, the electrical length for current flow from the terminals through each of the segments being about the same.

6. The device of claim 3 wherein the coil has a pair of terminals connected to several electrically parallel segments via a pair of leads, the electrical and physical lengths for current flow from the terminals through each of the segments being about the same.

7. The device of claim 1 wherein the electric source includes plural excitation devices, one for and associated with each window, each excitation device being positioned to produce the r.f. field that is coupled through the associated window.

8. The device of claim 7 wherein each of the excitation devices includes a coil array positioned adjacent the window associated with the excitation device, the r.f. field including magnetic lines of flux derived from the coil arrays associated with the plural windows.

9. The device of claim 8 wherein each coil array includes a substantially planar coil that is positioned adjacent a particular window.

10. The device of claim 8 wherein the coil arrays are electrically connected in parallel.

11. The device of claim 10 wherein each of the coil arrays has about the same electrical length.

12. The device of claim 11 wherein each coil array has a pair of terminals connected to several electrically parallel segments via a pair of leads, the electrical length for current flow from the terminals through each of the segments being about the same.

13. The device of claim 11 wherein each coil array has a pair of terminals connected to several electrically parallel segments via a pair of leads, the electrical and physical lengths for current flow from the terminals through each of the segments being about the same.

14. The device of claim 1 wherein the surface includes a frame having plural openings, each with a separate window support structure, one of the windows being located in each of the openings and being carried by the support structure of the associated opening.

15. The device of claim 1 wherein the surface includes a frame having four openings arranged in quadrants, each opening including a separate window support structure, one of the windows being located in each of the openings and being carried by the support structure of the associated opening.

16. A device for treating a workpiece with a plasma comprising a vacuum chamber in which the workpiece is adapted to be mounted, means for introducing into the chamber a gas which can be converted into the plasma for treating the workpiece, means for converting the gas into the plasma including a dielectric window on an exterior surface of the chamber, a coil positioned to couple an r.f. magnetic field to the gas via the window for exciting the gas to a plasma state, the coil including first and second terminals adapted to be connected to an r.f. source that causes the r.f. magnetic field to be derived and plural winding segments connected in parallel between the first and second terminals, at least two of the winding segments being in paths having about the same electric length between the first and second terminals.

17. The device of claim 16 wherein a plurality of dielectric windows are included, the coil extending over said plural dielectric windows.

18. The device of claim 16 wherein a plurality of dielectric windows are included, a separate one of said coils being adjacent each of said windows.

19. The device of claim 18 wherein said separate coils are connected in parallel with each other to said r.f. source.

20. The device of claim 16 wherein the paths have about the same physical lengths between the first and second terminals.

21. The device of claim 20 wherein there are several of said winding segments and an equal number of said paths having about the same electric length between the first and second terminals.

22. The device of claim 21 wherein said several paths and winding segments are arranged so current from the r.f. source generally flows in the same direction through all of the segments at a particular time.

23. The device of claim 22 wherein the coil includes first and second elongated spatially parallel leads having the same cross section geometry, the first and second terminals being at opposite ends of the first and second leads, respectively, each of the several segments including an elongated element extending between the leads and having opposite ends connected to the leads, each of the elements having the same length and cross section geometry.

24. The device of claim 23 wherein each element has a length of no greater than about a 1/16 of a wavelength of a wave applied by the r.f. source to the coil.

25. The device of claim 21 wherein each segment includes at least one element, the paths, segments and elements being arranged so the elements extend generally parallel to each other and being arranged so the elements extend generally parallel to each other and current from the r.f. source generally flows in opposite directions in the elements that are next to each other.

26. The device of claim 25 wherein the coil includes first and second elongated spatially parallel leads having the same cross section geometry, the first and second terminals being at opposite ends of the first and second leads, respectively, each of the several segments including a pair of series connected elongated elements, the leads, elements and segments being arranged so the leads are adjacent each other generally to one side of the elements.

27. The device of claim 20 wherein the two paths having about the same physical and electrical lengths include: (a) first and second generally parallel elongated leads respectively connected to the first and second terminals, and (b) first and second coil elements that extend between the first and second leads, the terminals being connected to the leads at locations between the two coil segments.

28. The device of claim 16 wherein at least some of the paths having about the same electrical lengths have substantially different physical lengths across the terminals, the paths having substantially different physical lengths and about the same electrical lengths having reactances with different values causing the electrical lengths to be about the same.

29. The device of claim 25 wherein each of the paths has the same type of dominant reactive impedance value at the frequency of the current applied by the r.f. source to the coil.

30. The device of claim 29 wherein each of the paths includes an element connected between a pair of leads connected to the first and second terminals, each element having about the same physical and electrical length.

31. The device of claim 30 wherein each element has a length of no greater than about a 1/16 of a wavelength of a wave applied by the r.f. source to the coil.

32. The device of claim 29 wherein at least one of said leads has differing values of inductance between connections with adjacent pairs of said elements.

33. The device of claim 32 wherein the differing values of inductance are attained by providing the leads with different cross sectional areas between connections with adjacent pairs of said elements.

34. The device of claim 29 wherein at least some of the paths include a series capacitor having a reactive impedance value at the frequency of the current applied by the r.f. source to the coil, the series capacitors causing the paths to have about the same lengths.

35. The device of claim 34 wherein the series capacitors have values causing each path to have a dominant capacitive impedance value at the frequency of the current applied by the r.f. source to the coil.

36. The device of claim 34 wherein the series capacitors have values causing each path to have a dominant inductive impedance value at the frequency of the current applied by the r.f. source to the coil.

Description Submit all comments and votes

FIELD OF INVENTION

The present invention relates generally to processors for treating workpieces in a vacuum chamber with a plasma and more particularly to such a processor having plural individually supported dielectric windows for coupling an r.f. field originating outside of the chamber into the chamber to excite the plasma and/or a coil for inductively deriving the field, wherein the coil has plural segments with the same electrical length, each including an element connected in parallel with an element of another segment.

BACKGROUND ART

Various structures have been developed to supply r.f. fields from devices outside of a vacuum chamber to excite a gas in the chamber to a plasma state. The r.f. fields have been derived from electric field sources including capacitive electrodes, electromagnetic field sources including electron cyclotron resonators and induction, i.e. magnetic, field sources including coils. The excited plasma interacts with the workpiece to etch the workpiece or deposit materials on it. Typically, the workpiece is a semiconductor wafer having a planar circular surface.

A processor for treating workpieces with an inductively coupled planar plasma (ICP) is disclosed, inter alia, by Ogle, U.S. Pat. No. 4,948,458, commonly assigned with the present invention. The magnetic field is derived from a planar coil positioned on or adjacent a single planar dielectric window that extends in a direction generally parallel to the workpiece planar surface. In commercial devices the window is usually quartz because this material has low impurity content and provides optimum results for r.f. field coupling. The coil is connected to be responsive to an r.f. source having a frequency in the range of 1 to 100 MHz and coupled to the coil by an impedance matching network including a circuit resonant to the frequency of the source. The coil is disclosed as a planar spiral having external and internal terminals connected to be responsive to the r.f. source. The circular spiral coil disclosed by Ogle has been modified to include linear, elongated elements generally in a spiral configuration, to process workpieces having square and rectangular shapes. Coultas et al., U.S. Pat. No. 5,304,279 discloses a similar device employing permanent magnets in combination with the planar spiral coil.

Cuomo et al., U.S. Pat. No. 5,280,154 and Ogle, U.S. Pat. No. 5,277,751 disclose a variation of the aforementioned processor wherein the linear spiral coil is replaced by a solenoidal coil. The solenoidal coil is wound on a dielectric mandrel or the like and includes plural helical-like turns, a portion of which extend along the dielectric window surface. The remainder of the coil extends above the dielectric window. Opposite ends of the solenoidal coil are connected to an r.f. excitation source.

None of the prior art plasma processors with which we are familiar is well adapted to excite plasmas for processing very large substrates, for example, substrates used in forming rectangular flat panel displays having sides in the range of 30-100 cm. Excitation of plasmas for treating, i.e., processing, such large substrates requires coils having correspondingly large surface areas in contact with or adjacent a dielectric window structure having a large surface area, commensurate with the areas of the workpieces to be treated. If these prior art structures are used for exciting plasmas for treating large workpieces, numerous problems which apparently have not been previously considered or resolved arise.

A problem common to all of the prior art processor designs is that the windows must be increased to a substantial thickness as the area thereof increases. Otherwise, the windows would not withstand the differential pressure between the atmospheric pressure outside of the chamber and the vacuum in the chamber; e.g. to process workpieces having rectangular treatment surfaces of about 75 cm.times.80 cm, a single quartz window having a surface of approximately 80 cm.times.85 cm must have a thickness in excess of 5 cm. Quartz windows of the stated area and thickness are also very expensive and fragile so use thereof considerably increases the cost of the processor. In addition, we have found that the r.f. fields derived from excitation sources using prior art processor designs are not usually capable of effectively exciting the plasma in a vacuum chamber with a large area, thick window. This is because the r.f. fields do not have sufficient flux density, after penetrating the thick window, to provide the required excitation. For example, the magnetic flux density penetrating a 5 cm thick dielectric window from a coil has a much smaller number of effective magnetic lines of flux than the magnetic field penetrating a 2.5 cm thick window of a prior art device for treating circular wafers having a 20 cm diameter. It is not feasible to simply increase magnetic flux density by increasing current from an r.f. source driving the coil because the increased current can cause excessive heating of the coil as well as other components and because of the difficulty in obtaining suitable high power r.f. sources.

A problem peculiar to the use of prior art induction coils for exciting a plasma having a large surface area is non-uniform excitation of the plasma, resulting in non-uniform plasma density and uneven workpiece processing. We have realized this non-uniform distribution occurs in part because the prior art coils function as transmission lines likely to have lengths, when laid over a large surface window, approaching or exceeding one-eighth wavelength of the r.f. driving sources. Because of the coil length there are significant voltage and current variations along the coil, resulting in appreciable magnetic flux density variations in the plasma. If the coil has a length in excess of one-eighth wavelength of the r.f. source there is an RMS voltage null in a coil driven by a current having an RMS peak value because of the substantial mismatch between the source and the load driven thereby. The mismatch causes the coil voltage and current to be phase displaced by close to 90.degree., resulting in the voltage null. These magnetic flux density variations cause the non-uniform gas excitation and uneven workpiece processing.

We have realized that the length of the coil between terminals thereof connected to the r.f. source must be considerably less than one-eighth of a wavelength of the r.f. source output and that such a result can be achieved by providing a coil with plural parallel branch elements or segments. While Hamamoto et al., U.S. Pat. No. 5,261,962 discloses a planar plasma excitation coil having plural parallel branch segments connected in a ladder configuration to a pair of physically opposed terminals connected to the same ends of leads connected to the branch segments, the structure in Hamamoto et al. is not suitable for use over a large surface area window. If Hamamoto et al. were used on large area windows there would be a tendency for uneven flux distribution and non-uniform plasma density because the different branches are included in r.f. transmission lines with different lengths across the opposed terminals. Hence, the branch segment physically closest to the terminals is in the shortest length line, while the branch segment physically farthest from the terminals is in the longest length line. The different length lines draw different currents from the source so the portion of the plasma adjacent the shortest length line is excited to a considerably greater degree than the plasma portion adjacent the longest length line. This causes non-uniform plasma excitation in processors for treating large surface area workpieces.

It is, accordingly, an object of the present invention to provide a new and improved r.f. field excited plasma processor particularly adapted for treating large workpieces.

A further object of the invention is to provide a new and improved r.f. field excited plasma processor for large workpieces wherein the plasma is uniformly distributed over the workpiece.

Another object of the invention is to provide a new and improved r.f. field excited plasma processor vacuum chamber arrangement particularly adapted for relatively large workpieces wherein dielectric coupling windows are arranged to withstand the differential pressure between the chamber interior and exterior while being thin enough to couple r.f. fields with sufficient density to effectively excite the plasma.

An additional object of the invention is to provide a new and improved r.f. field excited plasma workpiece processor wherein a plasma is inductively excited in an efficient manner to provide relatively uniform plasma distribution for large workpieces.

An added object is to provide a new and improved r.f. field excited plasma processor having plural electrically parallel coil segment branches arranged to supply about the same excitation flux to the plasma.

Yet a further object is to provide a new and improved r.f. field excited plasma processor having plural electrically parallel coil segment branches having about the same electrical and physical lengths to provide uniform flux distribution to the plasma and simplify design of the coil.

THE INVENTION

In accordance with one aspect of the present invention, some of the foregoing objects are attained by providing a processor for treating a large workpiece with a plasma comprising a vacuum chamber in which the workpiece is adapted to be mounted. A gas which can be converted into the plasma for treating the workpiece is supplied to the chamber. The gas is excited into the plasma state by an r.f. electric source outside of the vacuum. The r.f. source derives a field that is coupled to the plasma via plural individually supported dielectric windows on a wall of the chamber. Because there are plural individually supported windows, rather than a single large window, each window can be thin enough, e.g. 2.5 cm, to provide effective coupling of the r.f. field to the plasma.

In accordance with another aspect of the invention, other objects of the invention are attained by providing a processor for treating a workpiece with a plasma comprising a vacuum chamber in which the workpiece is adapted to be mounted. The chamber has introduced into it a gas which can be converted into the plasma for treating the workpiece. A means for converting the gas into the plasma includes a coil positioned to couple an r.f. magnetic field to the gas via a dielectric window structure on a wall of the chamber to excite the gas to produce and maintain the plasma. The coil includes first and second terminals adapted to be connected to an r.f. source that causes the r.f. magnetic field to be derived, as well as plural winding segments electrically connected between the first and second terminals so they have about the same electric length. Each segment includes an element that is electrically in parallel with elements of the other segments. Thereby, the RMS amplitude of the AC current flowing in the different coil elements is about the same to provide a relatively uniform magnetic flux distribution in the plasma.

In certain preferred embodiments, first and second terminals of the coil and the coil segments are positioned and arranged so the electrical and physical. lengths of current paths are approximately the same between the first and second terminals via at least two, and in some embodiments all, of the coil segments. A particularly advantageous arrangement including this feature comprises plural physically and electrically parallel branch conductor elements connected to leads extending at right angles to the elements, wherein the first and second terminals are at diagonally opposite ends of the leads. The like electric length lines can also be attained by proper design of the cross section geometry of conductors in the lines to provide lines with different inductive values and/or by inserting capacitors having appropriate values in series with the parallel coil elements.

The above and still further objects, features and advantages of the present invention will become apparent upon consideration of the following detailed descriptions of specific embodiments thereof, especially when taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a side sectional view of a plasma processor in accordance with one embodiment of the present invention;

FIG. 1a is a side sectional view, at right angles to the view of FIG. 1 of a portion of the plasma processor illustrated in FIG. 1;

FIG. 2 is top view of a coil employing plural parallel linear conductor segments or elements, wherein all of the currents flow in the same direction through the segments;

FIG. 2a is a top view of a portion of a modified version of FIG. 2;

FIG. 3 is a top view of a coil including parallel segments having currents flowing through them in the same direction, wherein the segments are in paths having equal physical and electrical lengths between diagonally opposite first and second terminals connected to be responsive to an r.f. excitation source;

FIG. 4 is a top view of a further coil configuration wherein all of the currents flow in parallel branches in the same direction between first and second adjacent terminals connected to an AC excitation source;

FIG. 5 is a top view of a coil arrangement including multiple parallel coil segments including adjacent elements having current flowing through them in opposite directions, wherein the segments are in paths having equal physical and electrical lengths between first and second terminals at opposite ends of adjacent lead lines;

FIG. 6 is a top view of a coil including parallel elements arranged in a woven pattern so current flows in opposite directions in adjacent elements;

FIG. 7 is a modification of the woven pattern structure illustrated in FIG. 6;

FIG. 8 is a top view of a coil configuration having plural coil portions, each occupying a mutually exclusive area on a different individually supported window and connected in parallel to an excitation source;

FIG. 9 is a top view of a coil including plural parallel linear segments having differing lengths;

FIG. 10 is a top view of a coil including plural linear elements connected in series between external terminals connected to be responsive to an r.f. source;

FIG. 11 is a side view of magnetic flux lines produced as a result of excitation of the coil configurations of FIGS. 2-4 and 9;

FIG. 12 is a side sectional view of magnetic flux lines resulting from excitation of the coil configurations of FIGS. 5-8 and 10; and

FIGS. 13a-13c are top views of alternate window configurations.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference is now made to FIGS. 1 and 1(a) of the drawing, wherein a workpiece processor is illustrated as including vacuum chamber 10, shaped as a right parallelepiped having electrically grounded, sealed exterior surfaces formed by rectangular metal, preferably anodized aluminum, sidewalls 12 and 14 that extend parallel to each other and at right angles to rectangular metal sidewalls 13 and 15. Vacuum chamber 10 also includes rectangular metal, preferably anodized aluminum, bottom end plate 16 and rectangular top end plate structure 18, including four individually supported dielectric, rectangular windows 19 having substantially the same size. Sealing of these exterior surfaces of chamber 10 is provided by conventional gaskets (not shown).

Windows 19, preferably made of quartz, are individually supported by one-piece, rigid frame 23, made of a non-magnetic metal, such as anodized aluminum. Frame 23 includes peripheral, mutually perpendicular legs 25 and interior mutually perpendicular rails 21, connected to the centers of the legs. Rails 21 and legs 25 include notches 27, which individually support each of windows 19 since the side walls of the windows and the bottom portions of the windows adjacent the side walls fit in and rest on gaskets (not shown) on the bottoms and side walls of the notches. Legs 25 of frame 21 are bonded to side walls 12-15 of chamber 10. Because windows 19 are individually supported by rails 21 and legs 25, the thickness of windows 19 can be less than about 2.5 cm and withstand the pressure differential between the atmospheric air on the exterior of chamber 10 and the vacuum inside the chamber, which is typically in the 0.5-5 milliTorr range. If windows 19 were not individually supported and a single window were employed, such a single window would have to have a thickness of at least 5 cm to be able to withstand the differential pressure. Such a thick window would significantly reduce the amount of r.f. field energy that could be coupled through the windows and would be very expensive. In one configuration of chamber 10 for processing large workpieces, e.g. television receiver active matrix liquid crystal displays having a planar rectangular configuration with sides as large as 75 cm.times.85 cm, each of windows 19 has an area of about 40 cm..times.43 cm.

Sidewall 12 includes port 20, connected to a conduit (not shown) leading to a vacuum pump (not shown) which maintains the interior of chamber 10 at a pressure on the order of 0.5-5 milliTorr. A gas which can be excited to a plasma, of a type well known in the prior art, is introduced from a suitable source (not shown) into chamber 10 via port 22 on sidewall 14.

Workpiece 24, e.g. a large semiconductor substrate wafer having a rectangular shape as specified supra, is mounted on metal chuck 26 in a plane parallel to the planes of bottom end plate 16 and windows 19, and close to plate 16. An electric field, typically having a frequency of about 30 MHz, is applied to workpiece 24 by r.f. source 28 via impedance matching network 30 and chuck 26. Chuck 26 is electrically insulated from the remaining metal parts of chamber 10 because it rests on electric insulator pad 29. Dielectric end plate structure 18 carries planar coil 34, connected to r.f. excitation device 33 including impedance matching network 36 and r.f. source 38, having a frequency different from r.f. source 28, and preferably equal to approximately 13.3 MHz. Both terminals of source 38 can float or one of them can be grounded to the metal walls of chamber 10. Matching network 36 includes circuitry tuned to the frequency of source 38 to form a resonant coupling circuit. Coil 34 is positioned and responds to source 38 to supply r.f. magnetic lines of flux to the gas coupled through port 22, to excite the gas to a plasma state. The plasma treats workpiece 24 to etch the substrate or to deposit molecules thereon.

Planar coil 34 can have many different configurations, as illustrated, for example, in FIGS. 2-10. Each of these coil configurations includes multiple linear electrically conducting, metal (preferably silver coated copper) stripe elements or segments for inductively supplying magnetic lines of flux to the gas in chamber 10 to sustain and generate a planar plasma that processes workpieces 24 in chamber 10. The linear elements of coil 34 preferably have a rectangular cross section with a broad side fixedly positioned on dielectric end face structure 18, although the narrow sides of the elements could be fixedly mounted on window 19. Coil 34 is basically an r.f. transmission line including distributed series inductances resulting from the self inductance of the metal elements and shunt capacitances between the metal elements and the grounded chamber exterior walls. To excite and maintain the plasma for these purposes, source 30 supplies up to 30 amperes to coil 34.

To confine and concentrate magnetic field lines resulting from current flowing through the linear conductors of coil 34, magnetic shield cover 40, preferably made of aluminum in which r.f. eddy currents are induced by the r.f. magnetic flux lines, surrounds the sides and top of the coil. Cover 40 has a roof 42 and four sidewalls 44, that are fixedly attached to vacuum chamber 10.

According to one embodiment, illustrated in FIG. 2, coil 34, that extends over all four of windows 19, has a configuration including eight elongated, straight, linear, metal conducting elements 51-58 having opposite ends connected to elongated straight, metal (preferably silver coated copper) leads 59 and 60 which extend parallel to each other and at right angles to elements 51-58. The bottom faces of elements 51-58 and leads 59, 60 are bonded to windows 19, except the portions of elements 51-58 which span gaps 31 across rail 21, between interior edges of the windows, as illustrated in FIG. 1a. Conducting elements 51-58 are approximately equidistant from each other (except for the spacing between central elements 54 and 55 which is somewhat different because of center rail 21), have about the same length and extend parallel to each other. Leads 59 and 60 include central terminals 62 and 64, located midway between central conductors 54 and 55. Terminals 62 and 64 are respectively connected to terminal 66 of r.f. source 38 by cable 68 and to output terminal 70 of matching network 36 by cable 72. Matching network 36 is connected to output terminal 74 of r.f. source 38.

In response to the output of r.f. source 38, current flows through each of conducting elements 51-54 generally in the same direction at any instant to produce r.f. magnetic flux lines 124, 128, 130 and 132, FIG. 11. Because the lengths of each of conducting elements 51-58 is a relatively small fraction, e.g. about 1/16th, of a wavelength (.lambda.) of the frequency derived from r.f. source 38, the instantaneous current and voltage variations across each of the conducting elements is not substantial. Because central conducting elements 54 and 55 have the same length, same cross sectional geometry and are equispaced from terminals 62 and 64, the lengths of the current paths formed by the transmission lines from terminal 62 to terminal 64 through conducting elements 54 and 55 are the same, whereby the magnetic flux densities resulting from the substantially equal RMS amplitude r.f. currents flowing through conducting elements 54 and 55 are approximately the same. Similarly, slightly off-center conducting elements 53 and 56 have equal length transmission lines and current paths between terminals 62 and 64 so the magnetic flux densities resulting from the substantially equal RMS amplitude currents flowing through them are about equal.

Because the lengths of the transmission lines and current paths through conducting elements 53 and 55 are somewhat greater than those through elements 54 and 55, there is a tendency for the RMS values of the r.f. currents flowing through elements 53 and 56 to be somewhat less than those through elements 54 and 55, whereby the magnetic flux densities derived from elements 53 and 56 tend to be less than those from elements 54 and 55. By the same reasoning, magnetic flux densities resulting from r.f. excitation of conducting elements 52 and 57 are approximately the same and tend to be less than those resulting from current flowing through conducting elements 53 and 56; the same is true for conducting elements 51 and 58.

As a result of the differential lengths of the transmission lines and the resulting differences in current path lengths from terminals 62 and 64 through different ones of elements 51-58 there are differences in the excitation and distribution of the plasma in chamber 10. This is likely to lead to uneven plasma processing of the large surface area workpiece because there is greater plasma density in the workpiece central region (beneath elements 54 and 55) than the workpiece periphery (beneath elements 51 and 58).

According to one aspect of the invention, the lengths of the transmission lines including elements 51-58 are approximately electrically equalized by providing the different lines with reactances having different values. Since the self inductance of a single electric line is inversely proportional to the line cross sectional area and the inductance of a line increases as the line length increases, the lines closest to terminals 62 and 64 can be made electrically longer by decreasing the cross sectional areas thereof relative to the cross sectional areas of the lines farther from the terminals. It is also desirable to maintain the electrical length of each of elements 51-58 the same so the RMS voltage and current variations across them are equalized to provide the same plasma distribution below these elements.

To these ends, the cross sectional areas of leads 59 and 60 progressively increase between adjacent pairs of segments 55-58 and 51-54 while the cross sectional areas of segments 51-58 are the same. Hence, leads 59 and 60 have relatively small cross sectional areas between segments 55 and 56 as well as between segments 53 and 54 and relatively large cross sectional areas between segments 57 and 58 as well as between segments 51 and 52.

Alternatively, capacitors 81-88 are connected in series with elements 51-58 to equalize the lengths of the transmission lines. As illustrated in FIG. 2a, capacitors 81-88 are connected in series with elements 51-58 and lead 59, at the end of each element adjacent the lead. These locations for capacitors 81-88 do not affect the effective physical lengths of elements 51-58 because of the relatively small physical size of the capacitors.

To enable the phase of the currents in each of elements 51-58 to be generally the same (either leading or lagging the voltage across the element) the geometry of elements 51-58 and the values of capacitors 81-88 are selected so the net impedance at the frequency of source 38 of each of the branches including elements 51-58 is of the same reactance type, i.e.