United States Patent [191
`Miller
`
`[75]
`
`[54] LOW PASS FILTER FOR PLASMA
`DISCHARGE
`Inventor: Paul A. Miller, Albuquerque, N.
`Mex.
`[73] Assignee: Sematech, Inc., Austin, Tex.
`[21] Appl. No.: 965,629
`[22] Filed:
`Oct. 22, 1992
`
`[56]
`
`Related U.S. Application Data
`[63] Continuation of Ser. No. 756,649, Sep. 9, 1991, aban-
`doned.
`·
`Int. a.5 •••••••••••••••••••••••••••••••••••••••••••••••• ROlJ 7/24
`[51]
`[52] U.S. a ........................... 315/111.21; 315/111.71;
`315/111.81; 313/231.31; 333/99 PL
`References Cited
`U.S. PATENT DOCUMENTS
`4,207,137 6/1980 Tretola ................................ 156/627
`4,579,618 4/1986 Celestino et al. ................... 156/345
`4,589,123 5/1986 Pearlman et al. .......... 315/111.71 X
`4,602,981 7/1986 Chen et al. .......................... 156/627
`4,617,079 10/1986 Tracy et al. ......................... 156/345
`4,622,094 11/1986 Otsubo ................................ 156/627
`4,795,529 1/1989 Kawasaki et al. .................. 156/643
`4,812,712 3/1989 Ohnishi et al. ...................... 315/176
`4,824,456 4/1989 Ohmi ................................... 204/298
`4,824,546 4/1989 Ohmi ................................... 204/298
`4,846,920 7/1989 Keller et al. ........................ 156/345
`4,874,494 10/1989 Ohmi .............................. 204/192.12
`4,877,999 10/1989 Knapp et al. .............. 315/111.21 X
`4,935,661 6/1990 Heinecke et al. .......... 315/111.91 X
`4,950,376 8/1990 Hayashi et al. ................ 204/192.32
`4,956,043 9/1990 Kanetomo et al. ................. 156/345
`4,968,374 11/1990 Tsukada etal. ..................... 156/345
`
`I IIIII IIIIIIII Ill lllll lllll lllll lllll lllll lllll lllll lllll llllll Ill lllll llll
`US005302882A
`5,302,882
`[11] Patent Number:
`[45] Date of Patent: Apr. 12, 1994
`
`4,973,883 11/1990 Hirose et al. ................... 315/111.41
`5,028,847 7/1991 Greb et al ...................... 315/111.21
`5,111,111 5/1992 Stevens et al. ................... 315/111.4
`
`FOREIGN PATENT DOCUMENTS
`0167703 1/1986 European Pat. Off ..
`2663806 12/1991 France .
`58-209117 6/1983 Japan .
`63-14863 1/1988 Japan.
`
`OTHER PUBLICATIONS
`"Electrical Isolation of Radio-Frequency Plasma Dis(cid:173)
`charges", J. Appl. Phys., vol. 71, No. 3, Feb. 1992, Paul
`A. Miller, pp. 1171-1176.
`"MOVPE Growth of II-VI Compounds in a Vertical
`Reactor with High-Speed Horizontal Rotating Disk",
`2300 Journal of Crystal Growth, 107 (1991) Jan. 1, Nos.
`1/4, Amsterdam, Netherlands.
`Primary Examiner-Robert J. Pascal
`Assistant Examiner-Michael B. Shingleton
`Attorney, Agent, or Firm-William W. Kidd
`[57]
`ABSTRACT
`An isolator is disposed between a plasma reactor and its
`electrical energy source in order to isolate the reactor
`from the electrical energy source. The isolator operates
`as a filter to attenuate the transmission of harmonics of
`a fundamental frequency of the electrical energy source
`generated by the reactor from interacting with the en(cid:173)
`ergy source. By preventing harmonic interaction with
`the energy source, plasma conditions can be readily
`reproduced independent of the electrical characteristics
`of the electrical energy source and/or its associated
`coupling network.
`
`13 Qaims, 4 Drawing Sheets
`
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`Page 1 of 11
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`APPLIED MATERIALS EXHIBIT 1023
`
`

`

`U.S. Patent
`
`Apr. 12, 1994
`
`Sheet 1 of 4
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`5,302,882
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`U.S. Patent
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`Apr. 12, 1994
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`Page 3 of 11
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`

`

`U.S. Patent
`
`Apr. 12, 1994
`
`Sheet 3 of 4
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`5,302,882
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`Page 4 of 11
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`

`

`U.S. Patent
`
`Apr. 12, 1994
`
`Sheet 4 of 4
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`5,302,882
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`Page 5 of 11
`
`

`

`1
`
`5,302,882
`
`LOW PASS FILTER FOR PLASMA DISCHARGE
`
`This application is a continuation of U.S. patent appli(cid:173)
`cation Ser. No. 756,649, filed Sep. 9, 1991, now aban- S
`doned.
`
`2
`ply must be largely prevented from reaching the other
`power supply, the mixing products caused by the cou(cid:173)
`pling of two different frequencies to a non-linear load
`must be attenuated and the radiation emitted by the
`reactor and the various interconnections must be mini(cid:173)
`mized.
`A key feature of most plasmas, is that the plasmas
`have "non-linear" impedance characteristics. Non(cid:173)
`linearity is a mathematical definition signifying that the
`10 magnitude of the voltage (electric field) in the plasma is
`not directly proportional to the magnitude of the cur(cid:173)
`rent (magnetic field). Typically, the generators em(cid:173)
`ployed in various plasma systems are designed to gener(cid:173)
`ate an output of predominantly single-frequency. How-
`15 ever, because of the non-linearity of the plasma, signals
`at multiples of the fundamental generator frequency are
`generated by the plasma. These multiple frequencies of
`the fundamental frequency are called harmonic fre-
`quencies (or harmonics). The amplitude of the harmon(cid:173)
`ics affect certain properties of the plasma, such as direct
`current (DC) bias, which impact the particular plasma
`process. The amplitude of the harmonics is determined
`by the interaction of the plasma with the generator and
`the coupling system and is difficult to control simply by
`adjusting the amplitude of the fundamental frequency
`component.
`Plasma non-linearity is a phenomenon which plays an
`important role in causing the plasma conditions to be
`dependent upon the electrical characteristics of the
`generator, as well as the coupling system, at both the
`operating (fundamental) frequency of the generator and
`at the various harmonic frequencies. That is, if satisfac(cid:173)
`tory operation of a plasma reactor is achieved for a
`given generator and coupling system, the parameters of
`the generator and the coupling system cannot be readily
`changed without affecting the plasma itself. Thus, gen-
`erally it is impractical, if not possible, to make changes
`to the electrical parameters of the generator and/or the
`coupling system and still be able to reproduce the de(cid:173)
`sired plasma conditions, simply by readjusting the am(cid:173)
`plitucle of the generator output. Typically, what is re-
`quired is a considerable retuning of the system in order
`to satisfactorily couple the reactor to the generator
`and/or the coupling system to obtain the desired plasma
`conditions.
`Therefore, it is difficult to replace a generator from a
`first manufacturer with a generator from a second man(cid:173)
`ufacturer and obtain the same plasma conditions, unless
`the electrical parameters of the two generators are iden(cid:173)
`tical. Similarly, if a change is made to an impedance
`matching network in the coupling system, due to a
`component change for example, the same plasma condi(cid:173)
`tions cannot be reproduced unless the networks are
`substantially identical. Merely changing the amplitude
`of the generator output will not compensate for the
`impedance differences in the generator and/or the cou-
`pling system.
`An added problem also exists when certain parame(cid:173)
`ters associated with two systems are not substantially
`identical. If two installations of plasma systems are
`made using identical generators and reactors but differ-
`ent lengths of coaxial cable (wave guides and/or other
`transmission mediums) are utilized in the systems then
`generally identical plasma conditions cannot be repro(cid:173)
`duced. In actual practice, this lack of reproducibility of
`desired operating plasma conditions under realistic con-
`ditions presents significant difficulties to the user. For
`example, if an RF generator requires service and/or
`
`60
`
`20
`
`BACKGROUND OF THE INVENTION
`1. Field of the Invention
`The present invention relates to the field of plasma
`processing and, more particularly, to the use of plasma
`equipment for modification of materials.
`2. Prior Art
`Pb1sma processing equipment is used extensively in
`the industry for the modification of materials. These
`modifications include etching and deposition of films
`for fabrication of microelectronic circuits and semicon(cid:173)
`ductor devices. The modifications also may include
`implantation of chemical species that change the fric-
`tion and wear properties of surfaces.
`A plasma is a gas (or a gas mixture) which is ener(cid:173)
`gized so that it is partially decomposed into species that
`are electrically charged. A variety of techniques are
`known for energizing the gas. One commonly used
`technique is the energizing of the gas by imposing an 25
`electric field on the gas from an external source. A
`common practice is to use high frequency alternating(cid:173)
`current (AC) fields to energize or excite the gas. For
`example, radio-frequency (RF) fields are generated at
`frequency ranges near 10 MHz. At still higher frequen- 30
`cies, in the order of 1000 MHz, microwave fields are
`generated. In some instances, these electric fields are
`utilized in combination with magnetic fields which are
`used for the purpose of confining the plasma. Electron
`cyclotron resonance (ECR) plasma processing is one 35
`technique for controlling the plasma with the use of
`electric and magnetic fields.
`The plasma is typically retained in a chamber of a
`processing equipment and isolated from the surround(cid:173)
`ing ambient and this plasma usually contains species that 40
`undergo chemical reactions. The plasma chamber and
`its gas-handling equipment are typically referred to as a
`reactor. The source of the electrical power that ener(cid:173)
`gizes the plasma is commonly referred to as a generator.
`Usually, there are a number of components, including 45
`cables, wave guides, inductors, capacitors, matching
`network, tuner and/or an impedance transforming net(cid:173)
`work coupling the generator to the reactor. These com(cid:173)
`ponents are included in a system sometimes referred to
`as a coupler or a coupling system. The generator and so
`the coupling system together comprise the AC source
`that energizes the plasma.
`Various schemes have been devised in the prior art
`for coupling the generator, coupler, and the reactor to
`operate as a plasma processing system for example, in S5
`U.S. Pat. No. 4,824,546 (Ohmi) an RF power source is
`coupled to a vacuum vessel through a matching circuit
`in order to provide a sputtering apparatus for forming
`an insulating thin film. Band reject filters are provided
`to permit only high frequencies to be supplied.
`·
`Another example is disclosed in U.S. Pat. No.
`4,579,618 (Celestino et al.) in which two power sources
`are each coupled through a matching network to a
`plasma reactor. A filter/combiner is coupled between
`the low frequency power supply and the high frequency 65
`power supply. The filter/combiner serves three pur(cid:173)
`poses which are unique to a single electrode, dual fre(cid:173)
`quency plasma reactor. The power of each power sup-
`
`Page 6 of 11
`
`

`

`3
`corrective repair, it can only be replaced with another
`identical RF generator without undue tuning and ad(cid:173)
`justment.
`Accordingly, it is appreciated that a plasma system
`which is flexible in design to accommodate a multitude 5
`of generator sources, as well as coupling systems, such
`that the reactor could repeatedly reproduce desirable
`plasma operating conditions, will provide for an im(cid:173)
`provement over the prior art.
`
`5,302,882
`
`4
`that the present invention may be practiced without
`these specific details. In other instances, well-known
`processes and structures have not been described in
`detail in order not to unnecessarily obscure the present
`invention.
`
`SUMMARY OF THE INVENTION
`The present invention describes an isolator for isolat(cid:173)
`ing a plasma reactor from its electrical energy source.
`The isolator is a low-pass electrical filter which permits
`the passage of the fundamental frequency of an electri- 15
`cal energy source supplying electrical energy to the
`reactor, but blocks transmission of harmonic frequen(cid:173)
`cies. Because the plasma operates with non-linear impe(cid:173)
`dance characteristics and the amplitudes of these har(cid:173)
`monics affect properties of the plasma, the plasma con- 20
`ditions are usually dependent upon the electrical char(cid:173)
`:acteristics of the generator, as well as the entire cou(cid:173)
`pling system. However, by attenuating and substantially
`preventing the harmonics from interacting with the
`generator and with the coupling circuitry that couples 25
`the generator to the isolator, this dependence is elimi(cid:173)
`nated. The reactor is made to operate substantially inde(cid:173)
`pendent of the effects in the change of the generator
`and/or the coupling system due to the harmonic isola(cid:173)
`tion and permits substitution of the generator and/or 30
`the coupling system without undue hardship in tuning
`the system to reproduce the desired plasma conditions.
`
`Prior Art
`Referring to FIG. 1, a prior art plasma reactor system
`is shown. A generator 10 for providing an alternating
`10 current electric field to energize or excite the gas (or gas
`mixture) to form the plasma is coupled to reactor 12
`through a coupler 11. The generator 10 is typically of
`RF or microwave frequency in which the desired oper-
`ating (fundamental) frequency is selected. The ampli(cid:173)
`tude of the output of generator 10 is adjustable.
`Reactor 12 includes the equipment containing the
`plasma chamber, as well as its gas handling apparatus.
`The plasma gas (or gas mixture) is introduced into the
`chamber for it to operate on a target device. The target
`device for whose properties are to be modified is also
`present in the chamber. The coupler 11 can be of a
`variety of couplers utilized in coupling generator 10 to
`reactor 12. For example, coupler 11 can be a blocking
`capacitor or an impedance matching network. Al(cid:173)
`though shown as coupler 11 it also includes the com(cid:173)
`plete coupling system, including the various transmis-
`sion cables, wave guides, connectors, etc., which com(cid:173)
`prise the transmission medium between generator 10
`and reactor 12. The purpose of the coupler 11 is to
`match the impedance, as well as other circuit parame(cid:173)
`ters, between the generator 10 and reactor 12, in order
`to provide for an efficient transfer of electrical energy
`from generator 10 to reactor 12.
`As was earlier described in the background of the
`invention, a particular reactor 12 is coupled to operate
`with a particular generator 10 and coupler 11. In order
`to obtain the desired plasma conditions, considerable
`amount of tuning is required to obtain those desired
`plasma conditions in reactor 12. During operation of the
`system in FIG. 1, the amplitude of generator 10 can be
`adjusted to vary the plasma conditions in reactor 12.
`A significant disadvantage of the prior art plasma
`system of FIG. 1 is that the desired plasma conditions
`typically cannot be reproduced readily, if any signifi(cid:173)
`cant characteristic of the generator 10 and/or the cou(cid:173)
`pler 11 is changed. If, for example, another generator is
`substituted in place of generator 10 and/or another
`coupler is substituted for coupler 11, then in most in(cid:173)
`stances, unless the new generator and/or coupler is
`identical in electrical characteristics to the one substi(cid:173)
`tuted, the desired plasma conditions typically cannot be
`reproduced in reactor 12 without further adjustment.
`In order to obtain the desireq plasma conditions
`again, the system of FIG. 1 must be retuned to accom(cid:173)
`modate the new generator and/or coupler. Thus, the
`system of FIG. 1 must necessarily depend on the partic-
`ular generator 10 and coupler 11 to be tuned to operate
`with reactor 12. In the event a component having differ(cid:173)
`ent electrical characteristics is to be substituted, consid-
`60 erable amount of time and effort are required to retune
`the system. Thus, anytime generator 10 and/or coupler
`11 require repair and/or service, the plasma system will
`necessarily require a complete "shut-down" while the
`reactor is reconfigured and retuned to the new system.
`In practice, the lack of reproducibility of desired plasma
`conditions in reactor 12 provides for an inflexible sys-
`tem which may pose economic hardship to the user of
`the.plasma equipment.
`
`50
`
`40
`
`BRIEF DESCRIPTION OF THE ORA WINGS
`FIG. 1 is a block diagram of a prior art plasma reactor 35
`showing a generator and a reactor coupled by a cou(cid:173)
`pler.
`FIG. 2 is a block diagram of a plasma reactor system
`of the present invention utilizing an isolator to isolate
`the reactor from the generator and the coupler.
`FIG. 3 is a circuit schematic diagram of a low-pass
`filter which is utilized as one embodiment for the isola(cid:173)
`tor of FIG. 2.
`FIG. 4 is a graphic representation of a frequency
`response curve V our!V IN of an ideal filter and mea- 45
`sured values for the circuit of FIG. 3.
`FIG. 5 is a block diagram showing four different
`plasma system arrangements with and without the isola(cid:173)
`tor of the present invention which were used in provid-
`ing experimental results.
`FIG. 6 is a graphical representation of DC Bias volt(cid:173)
`age measured for the eight systems shown in FIG. 5.
`FIG. 7 is a graphical representation of plasma volt(cid:173)
`ages measured for the eight systems shown in FIG. 5.
`FIG. 8 is a graphical representation of plasma cur- 55
`rents measured for the eight systems shown in FIG. 5.
`FIG. 9 is a graphical representation of phase differ(cid:173)
`ences for plasma voltages and currents measured for the
`eight systems shown in FIG. 5.
`
`DETAILED DESCRIPTION OF THE
`PREFERRED EMBODIMENTS
`An apparatus and method for providing an isolator
`for a plasma reactor is described. In the following de(cid:173)
`scription, numerous specific details are set forth, such as 65
`specific circuits, reactors, processes, etc., in order to
`provide a thorough understanding of the present inven(cid:173)
`tion. However, it will be obvious to on skilled in the art
`
`Page 7 of 11
`
`

`

`5,302,882
`
`6
`of FIG. 3 as curve 29. As is noted, the fundamental
`frequency is set at 13.56 MHz. As is also noted in the
`graph of FIG. 4 the second harmonic frequency of
`27.12 MHz is well below the -3 db point. Thus, by
`utilizing a low-pass filter for isolator 19, the harmonic
`signals generated by the reactor 12 are largely pre-
`vented from interacting with the generator 10 and/or
`coupler 11. The fundamental frequency component
`from generator 10 is passed through coupler 11 and
`10 through the isolator 19 to energize the plasma in reactor
`12. The desired operating conditions can be readily
`achieved by adjusting the amplitude of generator 10.
`- Accordingly, substitution of generator 10, coupler 11
`and/or other components in the transmission medium
`15 can be easily compensated by adjusting the amplitude of
`generator 10 to obtain the desired plasma conditions in
`reactor 12. The harmonics generated by the reactor 12
`are essentially "trapped" by isolator 19 and are substan-
`tially prevented from interacting with generator 10 and
`coupler 11.
`It is appreciated then that reactor 12 can be readily
`coupled to a variety of generators, couplers, and/or
`transmission medium, wherein the desired plasma con(cid:173)
`ditions can be readily reproduced by simply adjusting
`the frequency of the generator 10 to the desired funda(cid:173)
`mental frequency and adjusting the amplitude of the
`electrical signal from generator 10.
`
`5
`Present Invention
`Referring to FIG. 2, a plasma reactor system of the
`present invention is shown. The apparatus of the pres(cid:173)
`ent invention is comprised of the same prior art genera- 5
`tor 10, coupler 11 and reactor 12. However, isolator 19
`of the present invention is inserted between coupler 11
`and reactor 12. The purpose of isolator 19 is to isolate
`the reactor 12 from the electrical energy generating
`source and transmission medium provided by generator
`10 and coupler 11.
`Isolator 19 is designed to permit the transmission of
`the electrical energy at the fundamental operating fre(cid:173)
`quency of the generator 10, but to inhibit the transmis(cid:173)
`sion of higher frequencies, predominantly the harmon(cid:173)
`ics. Therefore, the harmonic content of the electrical
`signal from reactor 12 is significantly prevented from
`reaching coupler 11 and generator 10. Because of the
`harmonic isolation, the plasma in reactor 12 cannot
`interact with, nor respond to changes in, the imped- 20
`ances of generator 10 and coupler 11 at the harmonic
`frequencies. Changes made to generator 10 and/or cou(cid:173)
`pler 11 can be readily compensated by the adjustment of
`the amplitude of the output signal from generator 10,
`which is for the purpose of adjusting the amplitude of 25
`the fundamental frequency component.
`Thus, substitutions for generator 10 and coupler 11
`can be readily made by non-identical generators and
`couplers, wherein the desired plasma conditions in the
`reactor 12 can be reproduced by adjusting the ampli- 30
`tude of the output signal from generator 10. The har(cid:173)
`monics generated due to the nonlinearity of the plasma
`ar!! prevented from substantially interacting with the
`generator 10 and/or the coupler 11. A variety of inter(cid:173)
`actions can occur, one such being the change of the 35
`impedance of the generator 10 and/or coupler 11
`caused by the harmonics. Another interaction being the
`feedback of harmonics generated by reactor 12, trans(cid:173)
`mitted to generator 10 and coupler 11, and reflected
`from generator 10 and/or coupler 11, so as to either 40
`strengthen or cancel the harmonics at the reactor 12.
`Although a variety of electrical devices can be uti(cid:173)
`lized for isolator 19, the preferred embodiment utilizes a
`tuned electrical filter. The tuned electrical filter of the
`preferred embodiment is a low-pass filter and is shown 45
`in FIG. 3. Referring to FIG. 3, the particular low-pass
`filter utilized in the preferred embodiment is a Cheby(cid:173)
`shev filter, which is comprised of five circuit compo(cid:173)
`nents 22-26. Two ?T-sections are utilized between input
`terminals 20 and output terminals 21. The input termi- so
`nals 20 are coupled to the coupler 11 (actually the trans(cid:173)
`mission medium), while the output terminals 21 are
`coupled to reactor 12. One of the input terminals 20 and
`one of the output terminals 21 are coupled together to
`operate as an electrical return (typically ground poten- 55
`tial of the electrical system). Capacitor 22 is coupled
`across the input terminals 20, while capacitor 24 is cou(cid:173)
`pled across the output terminals 21. Two inductors 25
`and 26 are coupled in series between the non-returning
`input and output terminals. A third capacitor 23 is cou- 60
`pied between the junction of the two inductors and the
`return line. In the preferred embodiment, capacitors 22
`and 24 have the values of 220.9 pF, while capacitor 23
`has the value of 310.6 pF. The inductors 25 and 26 each
`have a value of 935.1 nH.
`FIG. 4 shows a graphical representation of the theo(cid:173)
`retically designed response of the filter of FIG. 3 as
`curve 18 and the actual measured response of the filter
`
`65
`
`Experimental Results
`Referring to FIG. 5, block diagrams for four different
`plasma systems with and without the isolator 19 are
`shown. These eight different' arrangements provided
`the experimental results illustrated in FIGS. 6-9. In
`Configuration I, generator 31 is coupled to reactor 33
`(designated also as "Reactor A"), wherein blocking
`capacitor 35 is utilized as part of coupler 11. In Configu(cid:173)
`ration II, a second generator 32 is coupled to reactor 33
`through the blocking capacitor 35. In Configuration III,
`generator 32 is coupled to a second reactor 34 (desig(cid:173)
`nated also as "Reactor B") through the blocking capaci(cid:173)
`tor 35. In Configuration IV, generator 32 is coupled to
`the same reactor 34, but a matching network 36 is uti(cid:173)
`lized instead of blocking capacitor 35. These four con(cid:173)
`figurations which do not include isolator 19 are desig(cid:173)
`nated as Io, Ilo, Illo, and IVo and represent four different
`prior art arrangements. With the same four configura(cid:173)
`tions, isolator 19 (shown as dotted in FIG. 5) is now
`included and represent four arrangements IF, IIF, IIIF,
`and IVF.
`The results of the four configurations with and with(cid:173)
`out the filter of the present invention are shown in the
`resultant graphs of FIGS. 6-9. All data represent dis(cid:173)
`charges in argon gas at 100 mTorr pressure and 200
`volts peak-to-peak excitation at fundamental frequency
`of 13.56 MHz. In the particular example, the first gener(cid:173)
`ator 31 is model SG-1250 manufactured by R. D. Mathis
`Co., while the second generator 32 is model ACG-5
`manufactured by ENI Power Systems. The matching
`network 36 is "Matchwork MW-5", also from ENI
`Power Systems.
`Referring to FIGS. 6-9, in all four of these graphs,
`the results obtained from the first two configurations (I
`and· II) pertaining to reactor A are shown on the left
`half portion of the diagram, while configurations III
`and IV, pertaining to reactor B are shown on the right
`half portion of the diagram. FIG. 6 shows the measure(cid:173)
`ment of the DC Bias voltage in each of the configura(cid:173)
`tions. FIG. 7 shows the magnitude of the Fourier coeffi-
`
`Page 8 of 11
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`

`5,302,882
`
`8
`7
`an electrical filter coupled between said reactor and
`cients of the fundamental (VI) and the second harmonic
`said first electrical source for passing said fre-
`(V2) of the plasma voltage in each of the four configura-
`tions with and without the filter. FIG. 8 shows the
`quency f, but inhibiting harmonics of said funda-
`mental frequency f generated due to a non-linear
`magnitude of the Fourier coefficient of the plasma cur-
`rent at the fundamental (11) and at the second harmonic 5
`response characteristic of said plasma in said reac-
`frequency (12) in each of the four configurations with
`tor from interacting with electrical circuit parame-
`and without the filter. FIG. 9 shows the phase cf> of the
`ters of said first electrical source, such that a sec-
`Fourier coefficients of the voltages Vl, V2 and current
`ond electrical source can be readily substituted in
`12. The phase of the current 11 is not indicated on the
`place of the first electrical source and wherein
`graph simply because the selected value for the phase of 10
`original plasma characteristics can be substantially
`11 is chosen as zero degrees.
`restored by adjusting the amplitude of an electrical
`signal from said second electrical source at fre-
`Notice that in FIG. 6, for Reactor A, the DC Bias
`voltage of the reactor is measured at approximately 155
`quency f, but without requiring retuning of said
`second electrical source.
`volts for configuration Io (without the filter). When
`2. The improvement of claim 1 wherein said electrical
`generator 31 is substituted by a different generator 32, 15
`which is the condition shown in Configuration Ilo, the
`filter is a low-pass electrical filter.
`3. In a pl_asma proc~ssing apparatus, ~avi~g a react~r
`DC Bias voltage in the reactor drops to approximately
`129 volts. However, when the isolator l8, in the form of
`for processmg a reactive.gas and wherem said r7actor 1s
`c~>Upled to a ~rst ~lectrical energy source whtch pro-
`the low-pass Chebyshev filter is used, the same DC bias,
`approximately 138 volts, is measured regardless of 20 v1des an electrical s!gnal at a fundamental frequen~y fto
`which generator 31 or 32 is used to energize the reactor
`generat~ an_ electrical energy fie!~ for gen:rat1on of
`(Configurations Ipand lip). This illustrates the fact that
`plasma m s~1d reactor, and an ~lectrical filte~ 1s coupled
`.
`.
`.
`between said reactor and said first electrical energy
`fi
`·
`"d f
`f b t · h"b"t"
`the presence of isolator 19 of the present mvent1on
`h r
`fi 3
`·
`fi
`source or passmg sa1
`b
`requency , u m 1 1 mg a -
`·
`·
`h
`permits or. the su st1tut1?n ?f t e generator 32 o!
`l, 25 monies of said fundamental frequency f generated due
`to a non-linear response characteristic of said plasma in
`but wherem sue~ substitution does not appr7ctably
`change the DC Bias voltage o'. Reactor ~- S1mll~~ re-
`said reactor from interacting with electrical circuit pa-
`suits ar7 shown for Reactor B m comparmg cond1t10ns
`rameters of said first electrical energy source, such that
`shown m lllf~nd IVpof FIG. 6·
`when a second electrical energy source is substituted in
`In FIG. 7, it JS noted t?at t_he pla~ma volt~ge Vl at _the 30 place of the first electrical energy source, original
`fundame~tal frequency is fairly umform with and Wit~-
`plasma characteristics are substantially restored by ad-
`out the 1so~ato~ 1~. However, the second harmomc
`justing he amplitude of an electrical signal from said
`cont_ent v~ries _s1gmfica_n~ly when the filter of the pres-
`second electrical source at said frequency f without
`ent mvent1on 1s not utthzed (Compare Io and Ilo; and
`requiring retuning of said second electrical energy
`compare IIIoand IVo). This fact is significantly noted in 35 source.
`Io and ~lo, wherei~ the plasma voltage of the second
`4. The apparatus of claim 3 wherein each of said
`harmomc _(V2~ ~aries fro~ 50 :101ts to 15 volts. When
`electrical energy sources is comprises of an electrical
`the filter 1s utJl~zed, the ~1spanty of ~he. values of the
`generator and a coupling means for electrically cou-
`sec~n~ harmomc ~oltage 1s reduced ~1gmficantly.
`piing said electrical generator to said reactor.
`s. The apparatus of claim 4 wherein said electrical
`Similar comparisons can be readily made for the 40
`plasma curr~nt graphs of FIG. 8 and the overall resul-
`filter is a !ow-pass electrical filter.
`tant phase ~hfferences of curre!1t and ~oltage phas~s (cf>)
`6. The apparatus of claim 5 wherein said low-pass
`as shown m FIG. 9. These 11lustrat1ons conclusively
`filter is a Chebyshev filter.
`7. A plasma processing apparatus for processing a
`exemplif~ the insensitivity of the react?r conditions to
`changes m generator and coupler that 1s caused by the 45 reactive gas and in which said apparatus is coupled to a
`presence of the isolator 19 of the present invention. The
`first electrical energy source which provides an electri-
`plasma system operates to provide substantially uniform
`cal signal at a fundamental frequency f to generate an
`plasma conditions for a given reactor even when the
`electrical energy field for generation of plasma in said
`electrical energy source or the energy transfer medium
`apparatus comprising:
`is varied or substituted.
`a reactor for processing said reactive gas therein;
`It is appreciated that although one particular Cheby-
`an electrical filter coupled between said reactor and
`shev low-pass filter is shown in four experimental con-
`said first electrical energy so