United States Patent c191
`Flamm
`
`I lllll llllllll Ill lllll lllll lllll lllll lllll 111111111111111111111111111111111
`US005304282A
`5,304,282
`[11] Patent Number:
`[45] Date of Patent: Apr. 19, 1994
`
`[54] PROCESSES DEPENDING ON PLASMA
`DISCHARGES SUSTAINED IN A HELICAL
`RESONATOR
`
`(76]
`
`Inventor: Daniel L. Flamm, 476 Green View
`Dr., Walnut Creek, Calif. 94596
`
`[21] Appl. No.: 686,763
`
`[22] Filed:
`
`Apr. 17, 1991
`
`Int. Cl,5 ............................................. HOlL 21/00
`[51]
`[52] U.S. Cl ..... ~ ................... ; ........... 156/643; 156/345;
`118/723 E; 427/551
`[58] Field of Search .................. 156/643, 345; 427/38,
`427/551; 118/723 E
`
`[56]
`
`References Cited
`U.S. PATENT DOCUMENTS
`4,033,287 7/1977 Alexander et al ................. 118/49.1
`4,310,380 1/1982 Flamm et al ........................ 156/643
`4,368,092 1/1983 Steinberg et al. .
`4,918,031 4/1990 Flamm et al. ....................... 156/643
`
`OTHER PUBLICATIONS
`"Coaxial Resonators with Helical Inner Conductor";
`
`Proceedings of the IRE; @1958; pp. 2099-2105; Macal(cid:173)
`pine et al.
`"Application of a Ion-Pressure Radio Frequency Dis(cid:173)
`charge Source to Polysilicon Gate Etching"; Cook et
`al.; J. Vac. Science B 8(1); Feb. 1990; pp. 1-4.
`"Silicon Oxide Deposition From Tetraoxysilane in a
`Radio Frequency Downstream Reactor: Mechanisms
`and Step Coverage"; Selamoglu et al.; J. Vac. Science B
`7(6); Dec. 1989; pp. 1345-1351.
`Primary Examiner-William A. Powell
`Assistant Examiner-George Goudreau
`Attorney, Agent, or Firm-Roger S. Borovoy
`[57]
`ABSTRACT
`Plasma etching and deposition is accomplished utilizing
`a helical resonator constructed with an inner diameter
`coil greater than 60 percent of the outer shield diameter.
`The diameter of the conductor used to form the coil is
`not critical and can be less than 40 percent of the wind(cid:173)
`ing pitch in some applications. These parameters permit
`helical resonator plasma sources to be more compact
`and economical, and facilitate improved uniformity for
`processing large substrates.
`
`18 Claims, 1 Drawing Sheet
`
`r--'-10
`
`40
`
`42
`
`~~~-----
`
`
`
`Page 1 of 8
`
`Samsung Exhibit 1013
`
`

`

`U.S. Patent
`U.S. Patent
`
`Apr. 19, 1994
`Apr. 19, 1994
`
`5,304,282
`5,304,282
`
`FIG. 1
`FIG. 1
`y----10
`
`20
`20
`
`2
`
`4
`
`,.
`
`iiiCOC
`
`i,i
`C.
`
`:li
`
`44
`
` ItilIltzlllc535131!
`
`
`32
`
`46
`
`Page 2 of 8
`
`
`
`Page 2 of 8
`
`
`
`
`

`

`1
`
`PROCESSES DEPENDING ON PLASMA
`DISCHARGES SUSTAINED IN A HELICAL
`RESONATOR
`
`2
`Etching An Introduction, ed. D. M. Manos and D. L.
`Flamm, Academic Press, San Diego, 1989, pp. 2-87,
`have been employed to induce anisotropic etching.
`Planar reactors have also been used to produce species
`S for isotropic etching (as described in U.S. Pat. No.
`BACKGROUND AND FIELD OF THE
`4,310,380 dated Jan. 12, 1982) and for the deposition of
`INVENTION
`thin films (as described in U.S. Pat. No. 4,033,287 dated
`1. Field of the Invention
`Jul. 5, 1977). It is well known to the worker in the field
`This invention relates to plasma processing and in
`that when appropriate gaseous chemistries are em-
`particular to plasma processing of devices using a heli- 10 ployed, such as those described by V. M. Donnelly and
`cal resonator.
`D. L. Flamm in Solid State Technology, pp. 161-166
`2. Description of the Prior Art
`(April, 1981), species from practically any plasma dis-
`Plasma discharges are extensively utilized in the fab-
`charge apparatus can be used to induce isotropic etch-
`rication of devices such as semiconductor devices and,
`ing and anisotropic etching can be achieved with appro-
`in particular, silicon semiconductor devices. For exam- lS priate chemistries using suitable pressures and reactor
`pie, plasma discharges in appropriate precursor gases
`geometries. (Representative chemistries and conditions
`are utilized to induce formatfon of a solid on a deposi-
`are described by D. L. Flamm in Plasma Etching An
`tion substrate. (One important embodiment of such a
`Introduction, ed. D. M. Manos and D. L. Flamm, Aca-
`pr~dure is called plasma assisted chemical vapor de-
`demic Press, San Diego, 1989, pp. 91-183.)
`posi~ion.) In a SC?ond plasma del?<'.ndent procedure, 20 Radiofrequency structures such as helicon antenna
`species generated in a plasma are utilized to etch a sub-
`structures and helical resonators have also been used to
`strate, e.g .. a device .substr~te bein~ proces.sed which
`generate plasmas which form appropriate anisotropic
`_gener~ly includes di~lect~c maten~, semicoi;id.uctor
`and isotropic etching species. For example, D. Vender,
`matenal and/or matenal with metallic conductivity.
`"Etching in an Externally Excited RF Plasma" Physics
`I.n plasma-assisted deposition proc~ures the desired 25 Research Laboratory Report No. 87, The Australian
`soh?.is C?mm~nly formed by the rc:ac~on of a ~as co~-
`National University, Oct. 28, 1988 describes isotropic
`position in a d~scharge. In one v.anatic:>n, reactive rad1-
`and anisotropic etching below 10 mTorr in a helicon
`ca!(s) fonD;ed in the Ji>lasma regi~n, cit.her alone or as
`structure while isotropic etching conducted above 10
`·bed· us p t N 4 368 092 d t d J
`mixed outside of the discharge region with a second gas,
`T
`· d
`· · a · 0
`are flowed over a deposition substrate remote from the 30 ~ ~~;s escn
`in
`a e
`an.
`·
`•
`•
`discharge to form the desired solid film. In another
`'
`· .
`.
`.
`.
`The hehcal reson~tor mcludes 8':1 outside ~eld en-
`variation, the substrate is surrounded by a plasma which
`clo~ure of ai;i electncal~y con?uct1ve mate?al, e.g. a
`supplies charged species for energetic ion bombard-
`ment. The plasma tends to aid in rearranging and stabi-
`cyl~der, an ~te~al h~hcal coil of. an electn~ally coi;i-
`lizing the film provided the bombardment is not suffi- 35 duct1ve .matenal, 1f desired, an a~phed magnetic field in
`ciently energetic to damage the underlying substrate or
`the region enclosed by the coil to enhance electron
`the growing film.
`co~finem~nt, and means. for applying an RF ~eld to .t~e
`In some etching procedures, a pattern is etched into
`coil. Typ1~ly, the outs1d~ enclosur~ and hehcal coil 1s
`the substrate by utilizing a mask having openings corre-
`of ~ electnc~ly conductive m~tenal ~uc~ as cop~er.
`sponding to this pattern. This mask is usually formed by 40 Design of. hehcal reso11:ators wi~h cylin.dncal outside
`depositing a polymeric photosensitive layer, exposing
`enclosure 1s generally discussed in W. ~ichak, Proc. of
`the layer with suitable radiation to change the solubility
`IRE, page ~315 (1954~. However helical resonators
`of the exposed regions, and then utilizing the induced
`used to. sustain plas~a ~1sch~ges have been coi;istru~ted
`change in solubility to form the desired pattern through
`~ccording to the ci:itena, design rules and specifications
`a solvation process. In other etching procedures, an 45 in W. W. Macalpine et al., Proc. of IRE, page 2099
`overlayer of material is selectively removed from the
`(1959) and generation of a plasma with these resonators
`sublayers without use of a mask (the polymeric mask
`is described in C. W. Haldeman et al, Air Force Re-
`itself can be removed by this procedure after a pattern
`search Lab Technical Research Report, 69-0148 acces-
`is transferred. This etching procedure is termed strip-
`sion No. TL501.M41, A25 No. 156. The cross section
`so view in FIG . .2 on page 2100 of Macalpine et al. illus-
`ping).
`·
`For most present day device applications, it is desir-
`trates the helical resonator components of a helical
`able to produce etching at an acceptable etch rate. (Ac-
`resonator plasma discharge structure. The symbols used
`ceptable etch rates depend upon the material to be re-
`in the following discussion correspond to those in FIG.
`moved and are generally those that remove at least 2%
`.2 of Macalpine. Macalpine et al. teach that to obtain
`of the layer thickness in a minute.) Additionally, the ss optimum electrical characteristic the ratio ofd/D of the
`production of a relatively high etching rate leads to
`mean diameter of the helical inner coil of the resonator,
`shorter processing times.
`d, to the inside diameter, D, of the outside enclosure is
`In one etching method known as anisotropic etching,
`chosen to have a value between 0.4 and 0.6 and further
`that the ratio, do/T of the diameter of the wire from
`appropriate charged species generated in the plasma
`produce directional energetic ion bombardment that 60 which the coil is wound, d0 to the pitch of the coil, T,
`induces etching on the substrate surface. Another etch-
`(the pitch is the number of turns per lineal inch in a
`ing procedure known as isotropic etching utilizes reac-
`direction parallel to the central axis of the structure) is
`tive neutral species produced by the plasma to induce
`chosen to have a value between 0.4 and 0.7. (For this
`etching of the substrate.
`purpose optimum electrical characteristic is a high un-
`Various structures for producing the desired plasma 65 loaded electrical O. commonly represented by the sym-
`bol 0 11• 0 11 is the O inherent to a helical resonator struc-
`discharges have been employed. For example, planar
`parallel plate reactors and reactors having hexagonal
`ture when there is no plasma present, i.e. a plasma has
`not been ignited. In general, O is defined as the maxi-
`electrodes as described in D. L. Flamm et al., Plasma
`
`
`
`Page 3 of 8
`
`

`

`5,304,282
`
`4
`3
`of Materials in Alicante, Spain, Sep. 4-16, 1988. (An
`mum instantaneous energy stored in the resonator dur-
`abbreviated summary of this process is in G. Cicala et
`ing a cycle of the excitation frequency divided by the
`al., Plasma-Surface Interactions and Processing of Materi-
`power dissipated in the resonator structure during a
`als, edited by 0. Auciello et al., NATO ASI Series E:
`cycle of the RF excitation. For the purpose of measur-
`ing unloaded Q, plasma ignition may be suppressed by 5 Applied Sciences, Vol. 176, Kluwer Academic Publish-
`ers, The Netherlands, 1990, pps. 171-173).
`evacuating the dielectric tube to below lQ-6 Torr, or
`pressurizing the tube to 760 Torr with an inert gas such
`It is well known that pulsing the power to the plasma
`discharge or pulsing the feed gas flow can be advanta-
`as helium).
`lt is well known to workers in the field that the same
`geous for higher deposition rates, improved etching
`design principles utilized for resonators with circular 10 anisotropy or better uniformity under appropriate con-
`outside shields also apply to helical resonators with an
`ditions (for example, G. Cicala et al. describe a pulsing
`outside shield in the form of a simple polygonal cross
`procedure useful to increase deposition rates).
`section. For example, the design of helical resonators
`Helical resonator plasma structures are simple to
`with shield of square cross section is described in Zve-
`manufacture compared to other large diameter plasma
`rev et al., IRE Transactions on Component Parts, pp. 15 sources that are useful for deposition and etching such
`99-110, Sept. 1961. Zverev et al. teach that a square
`as electron cyclotron resonance reactors (see Suzuki et
`al. Journal of the Electrochemical Society, 126, 1024
`shield with side oflength S is equivalent in properties to
`a circular cylindrical shield of diameter 1.2 S.
`(1979) for a description of etching in this type of reac-
`The plasma discharge is contained within a low loss
`tor, commonly referred to as ECR). However helical
`dielectric, insulating enclosure (e.g., a quartz tube) that 20 resonator reactors have not been entirely desirable in
`the past because their design was thought to be limited
`passes through the helical coil and is preferably concen-
`tric with the inner coil of the resonator. The dimensions
`to the range of dimensional ratio and size parameters
`of the dielectric enclosure must be less than the inner
`given by Macalpine et al. Resonators which conform to
`diameter of the helical coil.
`the scaling relationships taught by Macalpine et al. tend
`It is possible to operate the helical resonator dis- 25 to be cumbersome and may be unsuitable for device
`processing. This will be illustrated by example below.
`charge in a quarter wave mode (as described by Halde-
`man) or in a half wave mode as employed in the plasma
`To achieve a highly uniform etching rate (when the
`polymerization coating process described by S. L. Letts
`plasma is used for etching) or a highly uniform rate of
`et al. in "Laser Program Annual Report-1978, Volume
`chemical vapor deposition which is required to grow an
`1, Lawrence Livermore Laboratory Report UCRL- 30 even film thickness over the entire surface of a sub-
`50021-78, edited by M. J. Monsler and B. D. Jarman, pp.
`strate, the diameter of the inner dielectric tube within
`4-7 through 4-11, March 1989. (A detailed blueprint for
`the resonator should be as large or preferably larger
`a production version of this helical resonator plasma
`than the substrate that is to be processed. Plasma
`deposition coating reactor is contained in Lawrence
`sources having a diameter that is smaller than the sub-
`Livermore Laboratory Drawing No. AAA-78-107861- 35 strate diameter tend to produce nonuniform rates.
`00 created by R. Dowrick in 1978. S. Letts of the Law-
`(When the etching rate is nonuniform, it may nonethe-
`rence Livermore Laboratory has informed me that this
`less be possible to etch a film layer for device fabrica-
`design was made freely available to other laboratories
`tion if the inherent chemical selectivities for etching the
`prior to 1985 and units were constructed according to
`film relative to the masking layer and film sublayer are
`this blueprint and operated by KMS Fusion, Inc. of Ann 40 sufficiently high. However such nonuniformity is unde-
`sirable because it reduces process latitude. Precise selec-
`Arbor Mich. and the University of Rochester.) In the
`quarter wave mode it is possible to connect one end of
`tivity requirements corresponding to specified etch rate
`the coil to the outer shield and to insulate and separate
`variability are determined from the mathematical rela-
`tionships published in Flamm et al., Plasma Etching An
`the opposite end from the shield to reduce capacitance
`coupling. In a half wave mode device both ends are 45 Introduction, ed. D. M. Manos and D. L. Flamm, Aca-
`demic Press, San Diego, 1989, pp. 91-183, and incorpo-
`advantageously grounded (Grounding, although not
`essential to its operation, tends to reduce coupling to
`rated herein by reference.) In addition, tube diameters
`metallic objects near the ends and improves confine-
`which are smaller than the substrate diameter tend to
`ment of the plasma).
`produce divergent plasma flows when species from the
`Rather weak magnetic fields may be used to enhance 50 resonator source move radially to reach the surface that
`the plasma density obtained from RF resonant struc-
`is etched. To meet high accuracy pattern transfer re-
`tures. For example, Boswell et al. in Applied Physics
`quirements for submicron device manufacture, trajecto-
`Letters, 50, 1130 (1987) show that the plasma density
`ries of ions impacting the substrate surface should be
`downstream of an inductively coupled source operating
`collinear and perpendicular to the substrate surface. A
`below 1 mTorr is more than doubled when a magnetic 55 divergent plasma flow such as that from a narrow tube
`to a wider diameter substrate tends to induce a system-
`field strength of about 20 gauss is applied.
`It is possible to position longitudinally conducting
`atic variation in the angle between ion trajectories and
`elements along the outside of the low loss dielectric
`a perpendicular to the surface which limits the size of a
`discharge tube. For example, the Lawrence Livermore
`substrate and the minimum feature dimensions which
`Laboratories coating reactor utilizes a split metallic 60 can be processed (the effects on etching characteristics
`are discussed by s, Samukawa et al. in "Proceedings of
`shield between the outside of the quartz tube and the
`resonator coil. A heater formed from longitudinal con-
`the 1989 Dry Process Symposium," pp. 27-32, pub-
`lished by The Institute of Electrical Engineers of Japan,
`ducting elements with relatively high circumferential
`resistance can be advantageously used to heat substrates
`Tokyo, 1989).
`positioned within the discharge tube to permit chemical 65 A high unloaded helical resonator Q has been consid-
`ered essential for the operation of helical resonator
`vapor deposition at elevated temperature as was de-
`scribed by G. Cicala at the NATO Advanced Study
`plasma structures. Consequently, helical resonators
`Institute on Pl3.!1ma-Surface Interactions and Processing
`made to sustain plasma discharges have hitherto been
`
`
`
`Page 4 of 8
`
`

`

`5,304,282
`
`5
`6
`constructed in conformance with the dimensional seal-
`the dielectric tube (26) must be significantly larger than
`ing relationships for optimum Q taught by Macalpine et
`the diameter of the substrate ((42) to allow the wafers to
`al. These scaling relationships require the ratio, d/D, of
`be supported in this volume for processing. Thus a
`the coil diameter, d, to the shield diameter, D, to be
`quartz inner tube with an inside diameter in excess of
`between 0.45 and 0.6. Additionally, Macalpine et al. S 250 millimeters is suitable for processing 200 millimeter
`. teach that the ratio, b/D, of the axial length of the coil,
`diameter wafers. A useful wall thickness for this tube is
`1/16 inch. A space of at least i inch between the inner
`b, to the diameter of the shield, D, be greater than 1
`(b/D > 1 and that the diameter (do) of the conductor
`diameter of the helical coil and the outer wall of the
`used to wind the inner coil is fixed at a value between
`quartz tube is appropriate to accommodate normal vari-
`0.4 and 0.7 times the coil pitch (the coil pitch is defmed 10 ability in tube dimensions and to facilitate tube fusertion
`as the length of the coil divided by the number of turns
`during assembly. (At this point we assume the diameter
`in the coil). Indeed the importance of high Q and the use
`of a wire conductor used to wind the spiral resonator
`of this scaling law for isotropic helical resonator etching
`coil is 1.2 inches. Therefore the diameter, d, of the coil
`reactors are emphasized by Steinberg et al. in U.S. Pat.
`(22) needed to accommodate this 250 mm discharge
`No. 4,368,092 dated Jan. 11, 1983. Cook et al. (in the IS tube will be about 13 inches. The scaling relations given
`Joumal of Vacuum Science and Technology B, pps. 1-4,
`by Macalpine et al. require that the diameter of the
`1990 and also in the Joumal of Vacuum Science and
`shield surrounding a 13-inch coil be at least 1.66 times
`Technology A, pps. 1820-1824, 1991) state that resonator
`this diameter which is in this instance is 21.6 inches.
`structures suitable for helical resonator discharge
`According to the scaling relations, the overall length of
`sources used for anisotropic etching generally have an 20 the resonator structure is chosen to be about two times
`unloaded Q 9f 1000-2000 and a high Z 0 • (Z0 is the char-
`the coil diameter or 21.6 inches. Thus accepted teaching
`acteristic impedance of the helical resonator as given in
`requires that minimum dimensions of a resonator struc-
`Reference Data for Radio Engineers, fourth edition, pp.
`ture for processing 200 millimeter wafers be approxi-
`600-603, ed. H. P. Westman, International Telephone
`mately 21.6 inches in diameter and 21.6 inches in height,
`and Radio Corp., New York, 1956 and incorporated by 25 exclusive of the vacuum chamber dimensions (44) and
`reference herein.) Furthermore helical resonator struc-
`wafer loading mechanisms (loading mechanisms are not
`tures employed for plasma assisted chemical vapor de-
`a feature of this invention and hence are not shown in
`position such as the designs used for polymer deposition
`FIG. 1. FIG. 4 in Macalpine et al. determines that for
`by Letts et al., as well as the resonator employed by
`operation at 13.S6 MHz a helical resonator constructed
`Cicala et al., and the apparatus used for downstream 30 with above dimensions will have approximately 6 turns
`silicon oxide deposition by Selamoglu et al. (as de-
`of wire at a pitch, 'T of about 3 inches per inch for opera-
`scribed in The Journal of Vacuum Science and Technol-
`tion at 13.S6 MHz (this frequency is allocated for indus-
`ogy B 7, 134S, 1989) were constructed with the dimen-
`trial use and is the most common frequency used for
`sional relationships for high Q taught by Macalpine and
`plasma processing). The diameter of the conductor used
`Schildknecht.
`35 to wind this coil then is chosen to be 1.2 inches from the
`However the scaling relationships taught by Macal-
`permitted range of 1.2-2.1 inches satisfying the limits
`pine et al. yield helical resonator structures with dimen-
`0.4<do/T<0.7 given by Macalpine. (Note that this is
`sions that tend to be cumbersome, and are often unsuit-
`consistent with the wire diameter already chosen). The
`able or unduly constraining for device processing. In
`discharge tube thus subtends less than half of the overall
`both of the referenced reports Cook et al. note that the 40 diameter of this structure and the large conductor struc-
`diameters of quartz discharge tubes in the resonators
`tures are massive and bulky.
`they used not only were smaller than the inner coil
`EXAMPLE2
`diameter, but had to be further limited because of the
`space occupied by dielectric material needed to support
`Recently it has been found that RF plasma excitation
`the helical coil. This bulkiness and the tube size con- 45
`frequencies above 13.S6 MHz (for example in the range
`straint limit the usefulness of helical resonator discharge
`of SO MHz) can be advantageous to reduce harmful
`structures conforming to formulae given in Macalpine
`effects of the plasma on electrical characteristics of a
`et al. The seriousness of this limitation is illustrated by
`completed device (described, for example, by Goto et
`the following two examples.
`al. in Solid State Technology, 34(2), pp. S13-Sl6, Febru-
`SO ary 1991). Although helical resonator reactors con(cid:173)
`EXAMPLE 1
`structed for 13.S6 MHz operation according to the
`Films on ISO millimeter and 200 millimeter diameter
`teachings of Macalpine et al. are quite inconvenient,
`silicon wafers are etched using plasma assisted tech-
`corresponding designs for operating at higher fre-
`niques for the production of integrated circuits. In the
`quency are impractical. For example, a resonator struc-
`manufacture of these circuits, films are also deposited SS ture constructed with dimensions suitable for process(cid:173)
`on wafers of this size by plasma assisted chemical vapor
`ing 200 millimeter wafers and operating with 50 MHz
`deposition. When wafers are processed downstream of
`· excitation should contain an inside coil diameter of
`a helical resonator discharge such as that illustrated in
`approximately 13 inches diameter as described above.
`FIG. 1, the inside diameter of the dieleetric discharge
`Thus the required outside shield diameter is about 21.6
`tube (26) in the helical resonator structure is preferably 60 inches by the same reasoning. However for a helical
`larger than the substrate wafer diameter (42) in order to
`resonator structure of these dimensions to be in reso-
`avoid excessive plasma divergence and to achieve a
`nance at SO MHz, the formal design formulae in Macal-.
`high flux of reactive species downstream of the dis-
`pine specify a spiral coil with fewer than 2 turns and
`charge. Furthermore it may be desirable to process · more than 10 inches of axial length along each turn (also
`wafers within the resonator plasma volume (40) as ex- 65 the conductor used to wind the coil should be more
`emplified by the silicon nitride deposition process de-
`than 1.7 inches in diameter). These values are in a pa-
`scribed by Cicala et al. In order to process wafers
`rameter space, according to Macalpine et al., where use
`within the resonator plasma, the inner diameter of the
`of helical resonators is undesirable.
`
`
`
`Page 5 of 8
`
`

`

`5,304,282
`
`7
`It is, therefore, an object of the present invention to
`create a new and improved helical resonator plasma
`discharge structure which is compact and better suited
`for processing substrates, and is in particular better for
`processing substrates for semiconductor device manu(cid:173)
`facture. It is a further object of the present invention to
`provide convenient helical resonator plasma discharge
`structures which operate above 21 MHz and are suit(cid:173)
`able for processing large substrates.
`
`8
`than 500. However, when a plasma discharge suitable
`for substrate processing is present in the resonator struc(cid:173)
`ture, the potentials and currents which determine the
`characteristics of the plasma depend on the Q estab-
`S lished during operation of plasma discharge system.
`(This is known as the loaded Q of the resonator. The
`loaded Q is the ratio of the maximum instantaneous
`energy stored in the resonator during a cycle of the
`excitation frequency to the power dissipated in the reso-
`10 nator structure during a cycle of the RF excitation
`SUMMARY OF THE INVENTION
`when a plasma is ignited.) It has been determined that
`It has been found that not only are the accepted di-
`the loaded Q during plasma processing of substrates in
`mensional scaling relationships for helical resonators
`helical resonator reactors is unexpectedly low, gener-
`given by Macalpine et al. bulky, cumbersome and some-
`ally less than about 50. It will be understood that this
`times impractical for plasma processing reactors, but IS loaded Q is low and is substantially less than the un-
`loaded Q, the magnitude of the unloaded Q is not a
`that this scaling is in fact unnecessary for the operation
`of helical resonator plasma reactors. It has been further
`significant parameter for appropriate operation of the
`plasma reactor. (For this purpose, the unloaded Q is
`found that helical resonator plasma reactors with coil to
`shield diameter ratios greater than those hitherto be-
`considered substantially less than the loaded Q when
`lievcd to be useful can provide a larger plasma diameter 20 the unloaded Q is five times the loaded Q.) Thus this
`and a greater integral flux of species from a specified
`invention involves selecting dimensions for helical reso-
`resonator structure diameter. In fact such helical reso-
`nators used for plasma processing of device substrates
`nator discharges are an excellent source of species for
`that are more convenient and economical than helical
`procedures such as etching, chemical vapor deposition,
`resonators designed according to past art. The inventive
`surface modification and ion implantation.
`2S helical resonators employ dimensional ratios outside of
`Indeed, helical resonator plasma reactors which uti-
`the range taught by Macalpine et al., and in particular
`lize a plasma volume inside a coil diameter that is more
`utilize a coil diameter to shield diameter ratio, d/D,
`which is larger than 0.6. The unloaded Q is generally
`than 60 percent of the outer helical resonator shield
`diameter can readily operate with a loaded Q which is
`chosen to be at least four times the loaded Q. In prac-
`similar to the loaded Q found in reactors that conform 30 tice, this criterion will usually be realized when the
`unloaded Q is designed to be larger than 250. The de-
`to the criteria taught by Macalpine et al. Furthermore,
`coils for helical resonator reactors wound using con-
`sired operating plasma processing conditions in the
`ductors with diameters less than a factor of 0.4 times the
`inventive helical resonator plasma discharge are ad-
`pitch can operate with values ofloaded system Q which
`justed by procedures such as adjusting the position of
`are approximately the same as those found in helical 3S the tap (36) where the power source is connected to the
`resonator plasma reactors conforming to the dimen-
`coil (as shown in FIG. 1) to minimize power reflected to
`sional ratios given by Macalpine et al.
`the RF generator, and adjusting the excitation fre-
`quency to resonance. A high Z 0 value for the resonator
`BRIEF DESCRIPTION OF THE DRAWINGS
`is helpful for anisotropic etching at low pressure. How(cid:173)
`FIG. 1 is illustrative of a helical resonator plasma 40 ever the value of this parameter is not a direct function
`of Q and may be set within wide limits as in prior art.
`discharge apparatus suitable for practicing the inven-
`The main effect of reduced unloaded Q is that the
`tion.
`plasma reactor will exhibit a lower electrical efficiency
`DETAILED DESCRIPTION
`than a similar reactor conforming to the teachings of
`The invention involves the use of a helical resonator 4S Macalpine et al. (For this purpose a similar reactor has
`with an inner coil diameter greater than 0.6 times the
`the same inside dielectric plasma tube diameter.) Of
`outer shield diameter to produce a plasma in a gas at
`course it is clear, according to teachings of this inven-
`low pressure (typically in the range of 10-s Torr to 150
`tion, that Macalpine's criteria are extremely restrictive
`Torr) for processing such as etching procedures or
`and it may not always be possible to build which con-
`deposition procedures. An inner coil dimension substan- SO forms to Macalpine's criteria and is similar in this sense.
`tially larger than 0.6 times the shield has hitherto been
`The electrical efficiency of loaded resonators such as
`considered inadequate for plasma reactors because
`helical resonator plasma reactors is governed by the
`ratio of unloaded to loaded Q values. An electrical
`structures incorporating such coils have lower un-
`loaded Q values than those with coil diameters of 0.45
`· efficiency of 80 percent is generally acceptable for con-
`to 0.6 times the shield diameter. However it has been SS ventional parallel plate or hexode plasma reactor and
`determined that a high unloaded Q is ordinarily super-
`matching network combinations generally used for
`fluous and the practice of optimizing unloaded Q im-
`processing. Helical resonator plasma reactors require
`posed burdensome and unnecessary constraints on heli-
`no matching network and an unloaded Q in excess of
`four times the loaded Q is acceptable.
`cal resonator plasma reactors constructed to the previ-
`ous art.
`60 Operation of an ordinary helical resonator plasma
`Practitioners of the art believed that a high unloaded
`reactor with dimensions conforming to Macalpine et al.
`resonator Q was imperative for efficient and reliable
`illustrate the unexpectedly low Q characterizing past
`operation of helical resonator plasma processing reac-
`practice. A quarter wave resonator was constructed
`tors. For this reason, helical resonator plasma structures
`according to the formulae in Macalpine et al. using a 12
`have hitherto been designed in conformance with the 6S inch long, 8 in. O.D. cylindrical copper shield contain-
`ing a 27 turn 6.5 in. long, helical coil, 22, of i in. O.D.
`optimal Q sizing parameters taught by Macalpine et al.
`In general the unloaded Q of helical resonators used for
`copper tubing, 4.5 in. O.D. centered within the shield.
`plasma reactor structures has been designed to be more
`The bottom of the coil was short circuited to the shield
`
`
`
`Page 6 of 8
`
`

`

`5,304,282
`
`10
`9
`turns comprising the helical inner conductor is reduced
`by a silver solder connection (24) and the top of the coil
`was open circuited. The resonant frequency of this
`and the computation is repeated iteratively. Generally
`an unloaded Q in excess of 200 is desirable to attain a
`structure was approximately 9 MHz when there was no
`resonable power efficiency (approximately 80 percent
`plasma ignited. Power was coupled into the resonator
`by connecting a clip to the coil approximately 1.S turns s or more) when the plasma is operated. The unloaded Q
`above the shorted end. A quartz discharge tube approxi-
`corresponding to a design is also estimated by the rela-
`tionships in Sichak. If the unloaded Q so computed is
`mately 60 mm O.D. passed concentrically through the
`helical coil and was mated to a quartz walled reaction
`less than the desired vlaue, the diameter of the outer
`conducting shield (10) is increased or the pitch of the
`chamber, (44), by o-ring seals. A plasma was ignited in
`a feed gas consisting of 20 percent oxygen and 80 per- 10 inner coil (22) is adjusted to increase the estimated Q
`cent nitrogen and the input power was determined to be
`corresponding to the resonator structure. At high fre-
`46 watts at the operating resonant frequency