United States Patent c191
`Patrick et al.
`
`[54] COIL CONFIGURATIONS FOR IMPROVED
`UNIFORMITY IN INDUCTIVELY COUPLED
`PLASMA SYSTEMS
`
`[75]
`
`Inventors: Roger Patrick, Santa Clara, Calif.;
`Frank Bose, W ettingen, Switzerland;
`Philippe Schoenborn, San Jose; Harry
`Toda, Santa Clara, both of Calif.
`
`[73] Assignee: LSI Logic Corporation, Milpitas,
`Calif.
`
`[21] Appl. No.: 27,995
`
`[22] Filed:
`
`Mar.8, 1993
`
`Int. CI.6 ............................................... H05H 1/00
`[51]
`[52] U.S. Cl •.................................... 156/345; 156/643;
`118/723 I; 204/298.06; 204/298.34
`[58] Field of Search ............................... 156/345, 643;
`118/723 I, 723 IR; 204/298.34, 298.06;
`315/111.41, 111.21
`
`I lllll llllllll Ill lllll lllll lllll lllll lllll 111111111111111111111111111111111
`US005401350A
`5,401,350
`[11] Patent Number:
`[45] Date of Patent: Mar. 28, 1995
`
`[56]
`
`References Cited
`U.S. PATENT DOCUMENTS
`5,226,967 7/1993 Chen et al ....................... 156/345 X
`5,234,529 8/1993 Johnson .............................. 156/345
`5,241,245 8/1993 Barnes et al .................... 156/643 X
`Primary Examiner-Thi Dang
`[57]
`ABSTRACT
`The present invention relates to an apparatus for gener(cid:173)
`ating a low pressure plasma circulating in a planar di(cid:173)
`rection within a process enclosure. The invention gen(cid:173)
`erates plasma having substantially uniform density char(cid:173)
`acteristics across a planar axis. The invention achieves
`improved uniformity of the plasma density by deliver(cid:173)
`ing more radio frequency power toward the periphery
`of the circulating plasma than toward the center of the
`plasma. Increasing the periphery power to the circulat(cid:173)
`ing plasma compensates for increased plasma losses due
`to interaction with the side walls of the process contain(cid:173)
`ment enclosure.
`
`8 Claims, 14 Drawing Sheets
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`MATCH
`NETWORK
`
`Ex.1014 p.1
`
`

`
`U.S. Patent
`
`Mar. 28, 1995
`
`Sheet 1of14
`
`5,401,350
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`Ex.1014 p.2
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`U.S. Patent
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`Mar. 28, 1995
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`U.S. Patent
`
`Mar. 28, 1995
`
`Sheet 3 of 14
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`32
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`F I G . 3 (PRIOR ART)
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`Ex.1014 p.4
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`

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`U.S. Patent
`
`Mar. 28, 1995
`
`Sheet 4of14
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`5,401,350
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`58_(
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`FIG. 4
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`(PRIOR ART)
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`(PRIOR ART)
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`Ex.1014 p.5
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`

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`U.S. Patent
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`Mar. 28, 1995
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`Sheet 5of14
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`Ex.1014 p.6
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`Mar. 28, 1995
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`Mar. 28, 1995
`Mar. 28, 1995
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`Sheet 9‘of 14
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`Mar. 28, 1995
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`Mar. 28, 1995
`Mar. 28, 1995
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`U.S. Patent
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`Mar. 23, 1995
`Mar. 28, 1995
`
`Sheet 12 of 14
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`Mar. 28, 1995
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`

`
`COIL CONFIGURATIONS FOR IMPROVED
`UNIFORMITY IN INDUCTIVELY COUPLED
`PLASMA SYSTEMS
`
`20
`
`BACKGROUND OF THE INVENTION
`1. Field of the Invention
`The present invention relates to low pressure plasma
`generation systems and, more particularly, to coil con(cid:173)
`figurations for improving plasma uniformity in a plasma 10
`generation system.
`2. Description of the Related Technology
`Ionized gas or "plasma" may be used during process(cid:173)
`ing and fabrication of semiconductor devices. Plasma is
`used to etch or remove material from semiconductor 15
`integrated circuit wafers. Plasma may also be used to
`deposit or sputter material onto integrated circuit wa(cid:173)
`fers. Use of plasma gases in the fabrication of integrated
`circuits is widespread in the semiconductor manufactur-
`ing industry.
`During fabrication, a semiconductor integrated cir(cid:173)
`cuit wafer may require materials to be added or re(cid:173)
`moved, or selectively etched through a mask. To use
`plasma in the integrated circuit fabrication process,
`typically, a low pressure process gas is introduced into 25
`a process vessel chamber surrounding an integrated
`circuit wafer. The process vessel is used to maintain the
`low pressures required for the. plasma and to serve as a
`structure for attachment of the energy source. The
`process gas is ionized into a plasma by the energy 30
`source, either before or after entering the chamber, and
`the plasma flows over the semiconductor integrated
`circuit wafer.
`Ideally, uniformly ionized plasma would flow over
`the entire surface of the wafer. Any difference in the 35
`plasma ionization strength will cause uneven reaction.
`characteristics along the surface of the wafer. Uneven
`reaction characteristics may cause problems when etch(cid:173)
`ing thin films associated with semiconductor manufac(cid:173)
`turing. Some of the problems created are etch rate non- 40
`uniformity across a substrate from edge to center, pro(cid:173)
`file or line width variation across the substrate, and
`.semiconductor device damage.
`Plasma may be created from a low pressure process
`gas by inducing an electron flow which ionizes individ- 45
`ual gas molecules through the transfer of kinetic energy
`through individual electron-gas molecule collisions.
`Various methods of inducing an electron flow in the
`process gas are well known to those skilled in the art.
`Typically, electrons are accelerated in an electric field 50
`such as one produced by radio frequency energy. Low
`frequencies (below 550 KHz), high frequencies (13.56
`MHz), or microwaves (2.45 GHz).
`Using microwave radio frequency energy to generate
`plasma has the advantage of niore readily transferring 55
`energy to the process gas rather than to surrounding
`objects such as the walls of a process chamber or the
`semiconductor wafer. Another way of generating a
`plasma is with an electron cyclotron resonance (ECR)
`system. The ECR system requires a controlled mag- 60
`netic field to induce circular electron energy into the
`process gas and not into the process chamber walls.
`Other methods for improving the efficiency of
`plasma generation are magnetically enhanced plasma
`generation systems and inductively coupled electron 65
`acceleration, more commonly called inductively cou(cid:173)
`pled plasma. Magnetically enhanced plasma systems use
`a constant magnetic field parallel to the integrated cir-
`
`1
`
`5,401,350
`
`2
`cuit wafer surface and a high frequency electric field
`perpendicular to the wafer surface. The combined mag(cid:173)
`netic and electric forces cause the electrons to follow a
`cycloidal path, thus, increasing the distance the elec-
`5 trons travel relative to the more direct straight path
`induced by the electric field alone. A major drawback
`in using a magnetic field to increase the electron travel
`distances is the requirement of a strong magnetic field
`which is both costly and difficult to maintain.
`In the inductively coupled plasma process, the elec(cid:173)
`trons also follow an extended circular path. Two tech(cid:173)
`niques may be used to generate plasma by inductive
`coupling, both of which use alternating current to trans(cid:173)
`fer energy to the gas by transformer coupling. The first
`technique utilizes a ferrite magnetic core to enhance
`transformer coupling between primary and secondary
`windings, and uses low frequencies, for example, below
`550 KHz. The second technique uses a solenoid coil
`surrounding the gas to be ionized. This technique may
`use either low frequencies or high frequencies in the
`range of 13.56 MHz. Neither of these techniques pro-
`vides a uniform plasma proximate and substantially
`parallel with the surface of an integrated circuit wafer.
`U.S. Pat. No. 4,948,458 describes a method and appa-
`ratus for obtaining a more uniform and substantially
`parallel (planar) plasma for use during processing of
`integrated circuit wafers. The invention disclosed in
`this patent comprises an enclosure having an interior
`bounded at least in part by a radio frequency transpar(cid:173)
`ent window. A planar coil is disposed proximate to the
`window, and a radio frequency energy source is cou(cid:173)
`pled through an impedance matching circuit to the coil.
`The planar coil radiates the radio frequency energy
`such that a planar magnetic field is induced in the inte(cid:173)
`rior of the enclosure. This planar magnetic field causes
`a circulating flow of electrons to be induced into the
`process gas.
`The circulating flow allows the electrons to travel a
`path a much greater distance before striking the enclo(cid:173)
`sure. The circulating electrons flow is substantially
`planar and has minimal kinetic energy in the non-planar
`direction. The planar coil is substantially parallel with a
`support surface. The support surface, therefore, is ori(cid:173)
`ented substantially parallel to the circulating electron
`flow and is adapted to hold a semiconductor integrated
`circuit wafer during process fabrication. Thus, the sup(cid:173)
`port surface holds the semiconductor wafer substan(cid:173)
`tially parallel to the electron flow.
`The purpose of the invention disclosed in the above
`mentioned patent is to generally limit the wafer treat(cid:173)
`ment to only the chemical interaction of the plasma
`species with the integrated circuit wafer. This is accom(cid:173)
`plished by minimizing the kinetic velocity of the plasma
`flux in the non-planar direction, thus reducing the ki(cid:173)
`netic impact on the wafer.
`Referring to FIGS. 1 and 2, isometric and cross-sec-
`tional views of the prior art, respectively, are illustrated
`schematically. A plasma treatment system 10, for etch(cid:173)
`ing individual semiconductor wafers W, includes an
`enclosure 12 having an access port 14 formed in an
`upper wall 16. A radio frequency transparent window
`18 is disposed below the upper wall 16 and extends
`across the access port 14. The window 18 is sealed to
`the wall 16 to define a vacuum-tight interior 19 of the
`enclosure 12.
`A planar coil 20 is disposed within the access port 14
`adjacent to the window 18. Coil 20 is formed as a spiral
`
`Ex.1014 p.16
`
`

`
`5,401,350
`
`3
`having a center tap 22 and an outer tap 24. The plane of
`the coil 20 is oriented parallel to both the window 18
`and a support surface 13 upon which the wafer W is
`mounted. In this way, the coil 20 is able to produce a
`planar plasma within the interior 19 of the enclosure 12 5
`which is parallel to the wafer W.
`Referring now to FIGS. 1-3, the planar coil 20 is
`driven by a radio frequency (RF) generator 30. The
`output of the generator 30 is fed by a coaxial cable 32 to
`a matching circuit 34. The matching cirquit 34 includes 10
`a primary coil 36 and a secondary loop 38 which may be
`mutually positioned to adjust the effective coupling of
`the circuit and allow for loading of the circuit at the
`frequency of operation. Conveniently, the primary coil
`36 is mounted on a disk 40 which may be rotated about 15
`a vertical axis 42 in order to adjust the coupling therebe(cid:173)
`tween.
`A variable capacitor 44 is also provided in series with
`the secondary loop 38 in order to adjust the circuit
`resonant frequency with the frequency output of the RF 20
`generator 30. Impedance matching maximizes the effi(cid:173)
`ciency of power transfer to the planar coil 20. An addi(cid:173)
`tional capacitor 46 is provided in the primary circuit in
`order to cancel part of the inductive reactance of coil 36
`in the circuit. .
`Referring now to FIGS. 2 and 4, process gas is intro(cid:173)
`duced into the interior 19 of the enclosure 12 through a
`port 50 formed through the side of the enclosure 12.
`The gas is introduced at a point which provides for
`distribution throughout the interior 19.
`The flat spiral coil 20 may consist of equally spaced
`turns. Referring to FIG. 8, a graph representing test
`measurements of the current density versus position
`relative to the center of an equally spaced planar coil is
`illustrated. The graph of FIG. 8 illustrates maximum 35
`plasma density at or near the center of the equally
`spaced planar coil. This is also described in U.S. Pat.
`No. 4,948,458, column 6, lines 35 to 41.
`Further tests, however, indicate that the equally
`spaced turns of the coil create a non-uniformity in the 40
`plasma generated. This is so because the side walls of
`the enclosure 12 cause more losses to the periphery of
`the plasma than toward the center of the plasma. Refer(cid:173)
`ring to FIG. 9, the current density versus position of an
`unmodified equally spaced spiral planar coil and a mod- 45
`ified planar coil having unequal spacing of the turns, is
`illustrated. The current density 90 of the unmodified
`coil has a lower current density at the outer periphery
`92 and 94 than does the modified coil current density 96.
`Thus, in contrast to the invention claimed in U.S. Pat. 50
`No. 4,948,458, a more uniform plasma over the entire
`surface of the semiconductor wafer requires not more
`but less RF power near the center of a planar spiral
`wound. coil.
`
`25
`
`30
`
`4
`wafer from edge to center, profile or line width varia(cid:173)
`tion across the wafer from edge to center, and possible
`wafer surface damage.
`The present
`invention utilizes a non-uniformly
`wound planar coil or coils to create a current density in
`the plasma such that the plasma has consistently even
`characteristics from the periphery to the center of the
`surface of a semiconductor wafer. A number of embodi(cid:173)
`ments may be utilized to produce the required current
`density throughout the plasma.
`
`Unevenly Spaced Spiral Coil
`A non-uniformly spaced spiral coil having wider
`spacing between the turns toward the center and closer
`spacing of the turns toward the outside radius of the coil
`is illustrated in FIG. 10. The more closely spaced turns
`toward the outside of the coil create a higher density
`radio frequency field than the field produced by the
`wider spaced turns toward the center of the coil. By
`careful selection of the number of turns of the coil and
`the various spacings between the different turns, a cur(cid:173)
`rent density may be configured that results in a uniform
`plasma density across the entire working surface of the
`enclosure. This is especially important when processing
`the newer eight inch diameter semiconductor wafers,
`flat panel displays and future larger diameter semicon(cid:173)
`ductor wafers.
`Radio frequency power from an RF source is cou(cid:173)
`pled to a matching network by means of coaxial cable.
`The matching network is used to insure maximum trans(cid:173)
`fer of RF power from the source into the coil. The
`transferred RF power is radiated from the matched coil
`into the process gas flowing into the work chamber
`where the gas becomes plasma.
`An object of the present invention is to compensate
`for plasma energy loss near the side walls of the enclo(cid:173)
`sure by having more RF energy available toward the
`circumference of the plasma than toward the center.
`This results in the plasma having a uniform energy
`density throughout.
`
`Doughnut Coil Configuration
`A partially spiral coil in the form of a doughnut hav(cid:173)
`ing coil turns predominately toward the outside radius
`of the coil is illustrated in FIG. 11. The turns in dough(cid:173)
`nut coil configuration create a higher density radio
`frequency field around the periphery of the coil. By
`careful selection of the number of turns of the coil and
`the tum spacing, a current density may be configured
`that results in a uniform plasma density across the entire
`working surface of the enclosure.
`In similar fashion to the non-uniform spiral coil de(cid:173)
`scribed above, radio frequency power from an RF
`source is coupled to a matching network by means of
`55 coaxial cable. The matching network is used to insure
`maximum transfer of RF power from the source into the
`coil. The transferred RF power is radiated from the
`matched coil into the process gas flowing into the work
`chamber where the gas becomes plasma.
`
`SUMMARY OF THE INVENTION
`The present invention utilizes new, novel and non(cid:173)
`obvious coil configurations for the purpose of enhanc(cid:173)
`ing the RF power delivered toward the periphery of the
`circulating plasma and reducing the RF power deliv- 60
`ered toward the center of the plasma. By delivering
`inversely gradiated RF power to the plasma stream, a
`more uniform plasma density results in practice. The
`present invention more effectively compensates for loss
`of plasma at the walls of the enclosure without creating 65
`plasma "hot spots" toward the center of the plasma that
`lead to problems in etching of thin films. These prob(cid:173)
`lems may include et.ch rate non-uniformity across the
`
`Doughnut Coil Plus Separate Independently Powered
`Center Coil
`A partially spiral doughnut shaped exterior coil and
`an interior coil concentric to the exterior coil, both
`having separate RF sources, are illustrated in FIG. 12.
`The exterior and interior coils allow a current density
`pattern to be created that generates a uniform plasma
`field by adjusting each of the respective power sources.
`
`Ex.1014 p.17
`
`

`
`5,401,350
`
`6
`50 ohm impedance of the coaxial cable used to connect
`the RF power source to the matching network.
`A third RF power source may be connected to the
`wafer support surface to impart plasma energy in the
`tangential direction to the wafer surface. RF frequen(cid:173)
`cies in the high frequency region of 13.56 MHz, the
`microwave region of 2.45 GHz, or the low frequency
`region below 540 KHz may be utilized separately, or in
`combination, to produce a desired result during pro(cid:173)
`cessing of the integrated circuit wafer or other objects
`such as, for example, flat screen display panels.
`
`5
`The power of each power source may be independently
`adjusted for best current density pattern. Each RF
`source may be phase locked together so as to maintain
`the same frequency. Phasing of the two RF sources may
`be adjusted over a 0-180 degree range for fine tuning of 5
`the resulting plasma density. Individual matching net(cid:173)
`works are used to insure maximum transfer of power
`from the respective RF power sources to respective
`coils.
`A single RF power source may be utilized with the 10
`two coil embodiment of the present invention. FIG.12a
`illustrates a simplified schematic block diagram of the
`S-Shaped Coil
`single RF power source embodiment. The power
`source connects to a RF power divider which supplies
`An S shaped coil is illustrated in FIG. 15. This S
`a portion of the RF power source to each matching 15 shaped coil more evenly distributes the RF energy into
`the process gas than does a circular spiral coil. The S
`network. The power divider may be utilized to balance
`the power distribution between the interior and exterior
`shaped coil may be utilized at microwave (2.45 Ghz) or
`coils. Phasing between coils may be varied over a 0-180
`high frequency (13.56 MHz). Adjustment of the spacing
`degree range by varying the length of one of the coaxial
`between the coils may be used to adjust the RF radia-
`cables between the RF power divider and the respec- 20 tion pattern into the process gas to create the uniformly
`energized plasma. A matching network and RF source
`· rive matching networks.
`which function as mentioned above are also illustrated.
`Other and further objects, features and advantages
`will be apparent from the following description of pres(cid:173)
`ently preferred embodiments of the invention, given for
`the purpose of disclosure and taken in conjunction with
`the accompanying drawings.
`
`25
`
`Spiral Coil With Moveable Tap
`A spiral coil having an adjustable tap connected to an
`RF power source is illustrated in FIG. 13. The adjust(cid:173)
`ment of the coil tap results in a radio frequency field
`that results in a uniform plasma density across the entire
`working surface of the enclosure. The RF power flows
`mainly between the tap connection and ground. The 30
`ungrounded portion of the coil does not radiate a signif(cid:173)
`icant amount of RF power but may produce a phase
`inversion feedback that beneficially modifies the RF
`current density toward the center of the tapped coil.
`Radio frequency power from an RF source is cou- 35
`pied to a matching network by means of coaxial cable.
`The matching network is used to insure maximum trans(cid:173)
`fer of RF power from the source into the coil. The
`transferred RF power is radiated from the matched coil
`into the process gas flowing into the work chamber 40
`where the gas becomes plasma.
`An object of the present invention is to easily adjust
`a planar coil so as to compensate for plasma energy loss
`near the side walls of the enclosure by having more RF
`energy available toward the circumference of the 45
`plasma than toward the center.
`Additional Independently Powered coil Around Exte(cid:173)
`rior Wall of Plasma Chamber
`As illustrated in FIG. 14, a spiral coil may be placed
`on top of the chamber housing and a side coil may be 50
`placed around the side wall of the chamber biased
`toward the top coil. Independent RF power sources
`may be utilized for adjusting the amount of RF intro(cid:173)
`duced into the chamber interior for creation of the
`plasma. The two power sources may be phase locked 55
`together to maintain the same frequency. Phase adjust(cid:173)
`ment of 0-180 degrees may be made between the two
`RF sources by phase adjustment means well known in
`the art of signal generators and transmission lines. One
`RF power source, a power divider and coaxial phasing 60
`lines may also be utilized as illustrated in FIG. 12a.
`The side coil adds RF energy to the outer circumfer(cid:173)
`ence of the plasma field where there is the most plasma
`energy loss due to side wall absorption. Matching net(cid:173)
`works are utilized for maximum power transfer from 65
`the RF power sources which may be, for example, 50
`ohms impedance. The matching networks adjust the
`impedance of the respective coils to match the typical
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`FIG. 1 is an isometric view of a prior art apparatus
`for producing a planar plasma;
`FIG. 2 is a cross-sectional view of the apparatus of
`FIG.1;
`FIG. 3 is a schematic view of the circuitry of the
`apparatus of FIG. 1;
`FIG. 4 is a detailed view of a process gas introducing
`ring employed in the apparatus of FIG. 1;
`FIG. 5 is a partial elevational view illustrating the
`magnetic field strength produced by the apparatus of
`FIGS. 1 and 2;
`FIGS. 6 and 7 are schematic views of prior art appa(cid:173)
`ratus for producing a planar plasma;
`FIG. 8 is a graphical representation oftest data taken
`with the apparatus of FIG. 1;
`FIG. 9 is a graphical representation oftest data taken
`with the apparatus of FIG. 1 and a modified planar coil
`in accordance with the present invention;
`FIG. 10 is a schematic block diagram of a non(cid:173)
`uniformly spaced spiral coil embodiment of the present
`invention;
`FIG. 11 is a schematic block diagram of a doughnut
`spiral coil embodiment of the present invention;
`FIG. 12 is a schematic block diagram of a doughnut
`spiral coil and concentric spiral coil embodiment of the
`present invention having independent RF power
`sources;
`FIG. 12a is a schematic block diagram of a doughnut
`spiral coil and concentric spiral coil embodiment of the
`present invention having a common RF power source
`connected through an adjustable power divider and
`phasing coaxial cables;
`FIG. 13 is a schematic block diagram of a spiral coil
`embodiment of the present invention having an adjust(cid:173)
`able tap and connected to an RF power source;
`FIG. 14 is a schematic elevational view of an embodi(cid:173)
`ment of the present invention illustrating top and side
`coils both having independent RF power sources; and
`
`Ex.1014 p.18
`
`

`
`5,401,350
`
`8
`The current density of the prior art coil 20 is greatest
`toward the center region 802. Current densities 804 are
`illustrated on the vertical axis of the graph for different
`RF power levels 806 to the coil 20. The variation of the
`5 current density is plotted on the horizontal axis of the
`graph for various distances 808 from one edge of the
`chamber to the other. Distance 802 is representative of
`the current density at the center of the coil 20.
`The graph of FIG. 9 illustrates the difference in cur(cid:173)
`rent densities between the unmodified prior art coil 20
`and the coil 1002 of the present invention. The current
`density curve 906 of coil 1002 is greater and more
`evenly distributed than the current density curve 900 of
`the prior art coil 20. At end points 902 and 904, the
`curve 906 has a greater current density than does curve
`900. This increase in current density at the outer periph-
`ery greatly helps in overcoming the plasma energy loses
`from the proximate enclosure 12 walls.
`
`7
`FIG. 15 is a schematic block diagram illustrating an S
`shaped coil embodiment of the present invention.
`
`DESCRIPTION OF THE PREFERRED
`EMBODIMENTS
`, The development and characterization of uniform,
`large area, high density plasma sources capable of clean
`and rapid processing of integrated circuit substrates is
`crucial to the semiconductor industry. The present in(cid:173)
`vention is used in a radio frequency induction (RFI) 10
`plasma processing system. The RFI system is used to
`economically produce a uniform planar plasma during
`the process fabrication of modern integrated circuit
`semiconductor wafers.
`Referring now to the drawings, the details of the 15
`preferred embodiments are schematically illustrated. In
`the drawings, the letter C designates generally an appa(cid:173)
`ratus for enhancing the RF power delivered toward the
`periphery of the circulating plasma and reducing the
`RF power delivered toward the center of the plasma. 20
`Like elements are numbered the same, and similar ele(cid:173)
`ments are represented by the same number and a differ(cid:173)
`ent lower case letter thereafter.
`
`Non-Uniformly Spaced Spiral Coil
`Referring now to FIG. 10, a non-uniformly spaced
`spiral coil embodiment of the present invention is illus(cid:173)
`trated schematically. An RF generation system having
`even distribution of RF energy for creating a planar gas 30
`plasma is generally referenced by the letter C. The RF
`generation system C comprises a non-uniformly spaced
`spiral coil 1002, an impedance matching network 1004,
`coaxial cable feed line 1006 and an RF power source
`1008. Power source 1008 typically has an output power 35
`of from 100 to 2000 watts into a 50 ohm load. Coaxial
`cable 1006 may have 50 ohm impedance and have a
`power handling capacity at the frequency of use suffi(cid:173)
`cient for the RF source 1008. The cable 1006 feeds RF
`power from the source 1008 to the matching network 40
`1004. Network 1004 is used to obtain a proper match to
`the coaxial cable 1006 and source 1008.
`In addition, network 1004 may also tune coil 1002 to
`resonance at the frequency of source 1008. A matched
`and tuned condition results in maximum transfer of RF 45
`power from the source 1008 to the coil 1002. RF power
`is radiated by the coil 1002 into the chamber 19 (FIG. 2)
`where the RF energy causes the process gas to become
`a plasma.
`The coil 1002 has a winding pitch which varies ac- 50
`cording to the distance from the center such that the
`windings become more tightly wound further away
`from the center. A center winding spacing distance
`1010 is greater than an outer circumference winding
`spacing distance 1012. Thus, the winding spacing dis- 55
`tances between the spiral coil 1002 turns starts widest at
`the center 1014 and decreases toward the outer circum(cid:173)
`ference 1016. Ground return connections are illustrated
`by grounds 1018 and 1020.
`The coil 1002 produces an RF energy field that is 60
`greater toward the outer circumference 1016 than
`toward the center 1014. Having the RF energy field
`biased toward the outer circumference of the coil 1002
`compensates for the greater plasma energy loses nearer
`the walls of the housing 12 (FIG. 1). Referring now to 65
`FIGS. 8 and 9, current densities of the prior art coil 20
`and the coil 1002 of the present invention are graphi(cid:173)
`cally illustrated.
`
`Doughnut Coil Configuration
`Referring now to FIG. 11, a partially spiral coil in the
`form of a doughnut having coil turns predominately
`toward the outside radius of the coil is schematically
`illustrated. The turns in doughnut coil 1102 create a
`25 higher density radio frequency field around the periph(cid:173)
`ery of the coil 1102. By careful selection of the number
`of turns of the coil and the turn spacing 1104, a current
`density may be configured that results in a uniform
`plasma density across the entire working surface of the
`enclosure 12 (FIG. 1).
`In similar fashion to the non-uniform spiral coil 1002
`described above, radio frequency power from the RF
`source 1108 is coupled to a matching network 1104 by
`means of coaxial cable 1106. The matching network
`1104 is used to insure maximum transfer of RF power
`from the source 1108 into the coil 1102. The transferred
`RF power is radiated from the matched coil 1102 into
`the process gas flowing into the work chamber 19 (FIG.
`2) where the gas becomes plasma.
`Doughnut Coil Plus Separate Independently Powered
`Center Coil
`Referring to FIG. 12, a partially spiral doughnut
`shaped exterior coil and an interior coil concentric to
`the exterior coil, both having separate RF sources, are
`schematically illustrated. The· exterior coil 1202 and
`interior coil 1204 allow a current density pattern to be
`created that generates a uniform plas