United States Patent [19J
`Flamm et al.
`
`[11] Patent Number:
`[45] Date of Patent:
`
`4,918,031
`Apr. 17, 1990
`
`[75]
`
`[54] PROCESSES DEPENDING ON PLASMA
`·GENERATION USING A HELICAL
`RESONATOR
`Inventors: Daniel L. Flamm, Chatham
`Township, Morris County; Dale E.
`Ibbotson, Westfield, both of N.J.;
`Wayne L. Johnson, Phoenix, Ariz.
`[73] Assignee: American Telephone and Telegraph
`Company,AT&T Bell Laboratories,
`Murray Hill, N.J.
`
`[21] Appl. No.: 290,740
`
`[22]
`
`[51]
`
`[52]
`
`[58]
`
`[56]
`
`Dec, 28, 1988
`Filed:
`Int. Cl,4 ..................... H01L 21/00; HOlL 21/02;
`C23F 1/00; C23C 15/00
`U.S. Cl •.................................... 437/225; 437/228;
`437/229; 437/233; 437/235; 156/643;
`156/659.1; 204/192.25; 204/192.32; 330/56;
`315/39.51; 315/39.71
`Field of Search ............... 437/225, 228, 229, 233,
`437/235, 238; 204/192.12, 192.15, 192.25,
`192.32, 192.35; 427/38, 39, 50, 51; 330/56;
`315/39.51, 39.71; 156/643, 646, 657, 659.1, 662
`_ References Cited
`U.S. PATENT DOCUMENTS
`4,123,663 10/1978 Horiike ............................... 204/164
`4,160,690 7/1979 Shibagaki et al. .................. 204/164
`4,175,235 11/1979 Niwa et al. ..................... 204/192.32
`4,233,109 11/1980 Nishizawa ........................... 204/164
`4,298,443 11/1981 Maydan ............................... 204/192
`4,368,092 1/1983 Steinberg et al. .............. 204/192.32
`FOREIGN PATENT DOCUMENTS
`0100079 2/1984 European Pat. Off ..
`
`OTHER PUBLICATIONS
`MacAlpine, W., Coaxial Resonators with Helical Inner
`Conductor, Proceedings of the IRE, Apr. 24, 1959, pp.
`2099-2105.
`C. J. Mogab, VLSI Technology, ed Sze at McGraw-Hill,
`NY 1983, pp. 303-345.
`Suzuki, et al., Journal of the Electrochemical Society, 126,
`1024 (1979).
`W.W. MacAlpine, et al., Proc. of IRE, p. 2099 (1959).
`C. W. Haldeman et al., Air Force Research Lab Techni(cid:173)
`accession No.
`69-0148
`cal Research Report,
`TL501.M41, A25 No. 156.
`D. L. Flamm et al. VLSI Electronics: Microstructure
`Science, vol. 8, N. G. Einspruch and M. D. Brown, eds.,
`Academic Press, NY 1984, Chapter 8.
`L. E. Katz, VLSI Technology, ed. Sze at McGraw Hill,
`NY, 1988, pp. 98-140.
`A. C. Adams, VLSI Technology,
`McGraw-Hill, NY 1988, pp. 238::-248.
`"Electronic and Photonic Applications of Polymers",
`Willson and Bowden eds., pp. 90-108 (American Chem(cid:173)
`ical Society, Washington, D.C.) 1988.
`Primary Examiner-Brian E. Hearn
`Assistant Examiner-Everhart B.
`Attorney, Agent, or Firm-Bruce S. Schneider
`ABSTRACT
`[57]
`Anisotropic plasma etching is accomplished utilizing a
`helical resonator operated at relatively low gas pres(cid:173)
`sure. The use of this combination yields an extremely
`high flux of ionic species with resulting rapid aniso(cid:173)
`tropic etching. A helical resonator in conjunction with
`suitable precursors is also quite useful for plasma in(cid:173)
`duced deposition.
`
`ed. Sze at
`
`38 Claims, 4 Drawing Sheets
`
`Page 1 of 11
`
`Samsung Exhibit 1012
`
`

`

`U.S. Patent Apr. 17, 1990
`
`Sheet 1of4
`FIG. 1
`
`4,918,031
`
`GASES
`5
`
`i
`
`DISCHARGE
`TUBE
`
`POWER
`SOURCE
`
`~~~~tliZzzzzz~~ --1
`I (10
`(~
`' (
`I __ ..J
`
`' '
`
`Page 2 of 11
`
`

`

`U.S. Patent
`
`Apr.17, 1990
`
`Sheet 2 of 4
`FIG. 2
`
`4,918,031
`
`SPLIT
`SHIELD
`
`FIG. 3
`
`,21
`I_.../_,
`I
`I
`I
`I
`L..-,.-...J
`I
`
`~
`
`22~
`
`Page 3 of 11
`
`

`

`U.S. Patent Apr. 17, 1990
`
`Sheet 3 of 4
`
`4,918,031
`
`FIG. 4
`
`POWER
`SOURCE
`
`Page 4 of 11
`
`

`

`4,918,031
`
`US. Patent—Apr. 17, 1990 Sheet 4 of 4 4,918,031
`
`U.S. Patent Apr. 17, 1990
`Sheet 4 of 4
`
`+X
`
`o DISCHARGETUBE 63
`
`FIG.5
`
`Page 5 of 11
`
`I BSra
`
`
`
`
`
`
`cr
`
`
`1a|‘TCTa
`
`
`Page 5 of 11
`
`

`

`1
`
`4,918,031
`
`PROCESSES DEPENDING ON PLASMA
`GENERATION USING A HELICAL RESONATOR
`
`BACKGROUND OF THE INVENTION
`1. Field of the Invention
`This invention, relates to plasma processing and in
`particular to plasma processing of devices.
`2. Description of the Prior Art
`Plasma discharges are extensively utilized in the fab- 10
`rication of devices such as semiconductor devices and,
`in particular, silicon semiconductor devices. For exam(cid:173)
`ple, plasma discharges in appropriate precursor gases
`are utilized to induce formation of a solid on a deposi(cid:173)
`tion substrate (One important embodiment of such a 15
`procedure is called plasma assisted chemical vapor de(cid:173)
`position.) In a second plasma dependent procedure,
`species generated in a plasma are utilized to etch a sub(cid:173)
`strate, e.g. a device substrate being processed which
`generally includes dielectric material, semiconductor 20
`material and/or material with metallic conductivity.
`In plasma-assisted deposition procedures the desired
`solid is commonly formed by the reaction of a gas com(cid:173)
`position in a discharge. In one variation, reactive radi(cid:173)
`cal(s) formed in the plasma region either alone, or as 25
`mixed outside of the discharge region with a second gas,
`are flowed over a deposition substrate remote from the
`discharge to form the desired solid film. In another
`variation, the substrate is surrounded by a plasma which
`supplies charged species for energetic ion bombard- 30
`ment. The plasma tends to aid in rearranging and stabi(cid:173)
`lizing the film provided the bombardment is not suffi(cid:173)
`ciently energetic to damage the underlying substrate or
`the growing film.
`In etching procedures, a pattern is typically etched- 35
`into the substrate by utilizing a mask having openings
`corresponding to this pattern. This mask is usually
`formed by depositing a polymeric photosensitive layer,
`exposing the layer with suitable radiation to change the
`solubility of the exposed regions, and then utilizing the 40
`induced change in solubility to form the desired pattern
`through a salvation process.
`For most present day device applications, it is desir(cid:173)
`able to produce anisotropic etching at an acceptable
`etch rate. (Acceptable etch rates depend upon the mate- 45
`rial to be removed and are generally those that remove
`at least 2% of the layer thickness in a minute. Aniso(cid:173)
`tropic etching for the purpose of this description is an
`etch which undercuts the etch mask a distance less than
`one quarter the layer thickness.) The production of 50
`relatively vertical sidewalls during anisotropic etching
`allows higher packing densities for device structures.
`Additionally, the production of a relatively high etch(cid:173)
`ing rate leads to shorter processing times.
`In one method of anisotropic etching, appropriate 55
`charged species generated in the plasma produce ener(cid:173)
`getic ion bombardment that induces anisotropic etch(cid:173)
`ing. Various sources for producing the desired plasma
`discharge have been employed. For example, parallel
`plate reactors as described in C. J. Mogab, VLSI Tech- 60
`nology, ed Sze at McGraw-Hill, N.Y. 1983, pgs.
`303-345, and reactors having hexagonal electrodes as
`described in U.S. Pat. No. 4,298,443 dated Nov. 3, 1981
`have been employed to induce anisotropic etching.
`Radio frequency resonators such as helical resonators 65
`have been used at pressures above 0.1 Torr as a source
`of etching species solely for isotropic etching. The spe(cid:173)
`cies generated in the resonator are chemically reactive
`
`2
`but have not demonstrated the momentum required for
`anisotropic etching.
`As an alternative, a technique based on electron(cid:173)
`cyclotron resonance (commonly referred to as ECR)
`5 discharges that generate high energy species for aniso(cid:173)
`tropic etching has been described for the generation of
`ions at low pressure. (See Suzuki, et al. Journal of the
`Electrochemical Society 126, I024 (1979).) However, the
`relatively high cost of an ECR is not entirely desirable.
`Additionally the etching of device structures suitable
`for 0.25 µ,m devices has not been reported.
`
`SUMMARY OF THE INVENTION
`It has been found that not only is electron-cyclotron
`resonant etching extremely expensive but also that this
`etching procedure under many circumstances produces
`rapid heating of the substrate being etched and degrades
`extremely fine etching patterns. It has further been
`found that the use of a helical resonator operating at
`pressures below IO mTorr produces sufficiently ener(cid:173)
`getic species to result in downstream anisotropic etch(cid:173)
`ing without any substantial heating of the substrate
`being etched. Additionally the low pressure yields etch
`rates faster than 500 A./min.
`Indeed, a helical resonator operating at low pressure
`is, in general, an excellent source of charged species for
`procedures such as ion implantation, surface modifica(cid:173)
`tion, and downstream reaction to induce deposition. A
`helical resonator is also an excellent source of reactive
`radicals for inducing deposition, etching, surface clean(cid:173)
`ing, and surface modification such as a hydrogen atom
`source, e.g. for molecular or chemical beam epitaxy.
`
`BRIEF DESCRIPTION OF THE DRAWING
`FIGS. 1-5 are illustrative of apparatuses suitable for
`practicing the invention.
`
`DETAILED DESCRIPTION
`As discussed, the invention relies on the use of a
`helical resonator to produce a plasma in a gas at low
`pressure, i.e. a gas at a pressure ofless than IO mTorr for
`processes such as etching procedures or implantation
`procedures. Alternatively, a helical resonator is used to
`maintain a plasma in a precursor gas typically having a
`pressure in the range 10-s to 100 torr for generation of
`species to be employed in procedures such as deposi(cid:173)
`tion. For pedagogic purposes, use of the helical resona(cid:173)
`tor will be described in terms of the etching procedure.
`Conditions that differ for other uses of the generated
`species will subsequently be discussed. -
`Design of helical resonators are generally discussed
`in W. W. MacAlpine et al, Proc. of IRE, page 2099
`(1959) and generation of a plasma with these resonators
`is described in C. W. Haldeman et al, Air Force Re(cid:173)
`search Lab Technical Research Report, 69-0148 acces(cid:173)
`sion No. TL501.M41, A25 No. 156. (Although optimum
`resonance conditions are described by MacAlpine, for
`the procedures of this invention conditions substantially
`deviating from optimal are useful and, in fact, allow use
`of larger resonators. For example, a radius of the spiral
`coil more than 0.6 times the radius of the shield is quite
`useful.) The helical resonator includes an outside enclo(cid:173)
`sure of an electrically conductive material, e.g. a cylin(cid:173)
`der, an internal helical coil of an electrically conductive
`material, if desired, an applied magnetic field in an axial
`direction in the region enclosed by the coil to enhance
`electron confinement, and means for applying an rf field
`
`Page 6 of 11
`
`

`

`4,918,031
`
`3
`to the coil. Typically, the outside enclosure and helical
`coil is of an electrically conductive material such as
`copper.
`It is possible to operate the helical resonator either in
`a half wave mode or a quarter wave mode. It is possible 5
`in the half wave mode to connect both ends of the heli(cid:173)
`cal coil to the outer shield so that the resonator coil is
`grounded at both ends to allow the electrical matching
`tap or coupling to be located toward either end. In the
`quarter wave mode it is possible to connect one end of 10
`the coil to the outer shield and to insulate and separate
`the opposite end from the shield to reduce capacitance
`coupling. Useful processing is performed by positioning
`the floating end of the coil in a quarter wavelength
`configuration at either end.
`The plasma discharge is contained within a low loss
`dielectric, insulating enclosure (e.g., a quartz tube) that
`passes through and is preferably concentric with the
`inner coil of the resonator. It is possible to use gas enclo(cid:173)
`sure materials with higher loss or with both higher loss 20
`and higher dielectric constant. However, the former
`lowers the resonant "Q" of the circuit and the latter
`leads to not only lower "Q", but also lower resonant
`frequency. The enclosure dimensions should be consis(cid:173)
`tent with the diameter of the helical coil and are sized to 25
`provide a relatively uniform plasma flux at the substrate
`that, in turn, provides a concomitantly uniform deposi(cid:173)
`tion or etching. (A control sample is easily employed to
`determine suitable dimensions for a desired flux.) Pre(cid:173)
`cursor gases are flowed into the enclosure, pass through 30
`the discharge and exit.
`The magnetic field utilized in the region of the coil, if
`desired, in conjunction with the helical resonator
`sltould generally be greater than 50 Gauss as measured
`at the axis of the helical coil. Fields weaker than 50 35
`Gauss do not produce substantial plasma enhancement.
`The frequency of the applied rf power is not critical but
`does affect the resulting etching. Generally, frequencies
`above-80 MHz lead to impractically small resonator
`sizes and frequencies below 3 MHz lead to plasma insta- 4o
`bilities and excessive physical dimensions. (It is also
`possible to use a combination of frequencies during
`etching if they are resonant harmonics of each other.
`Resonant harmonics, however, are generally not exact
`multiples and a suitable frequency is obtained by tuning 45
`until a plasma together with a low standing wave ratio
`at the electrical transmission line are obtained.) Typi(cid:173)
`cally a power density generally in the range 0.05
`Watts/cm3 to 1 Watts/cm3 of discharge volume is 'em(cid:173)
`ployed. Power densities below 0.05 Watts/cm3 yield 50
`low specific ion fluxes and power densities above 1
`Watts/cm3 lead to excessive heating of the discharge
`enclosure. (Discharge volume is defined here as the
`volume of dielectric discharge tube enclosed by the
`resonator coil.)
`Generally the larger the outer enclosure, the internal
`coil and the dielectric discharge tube, the greater the
`integral flux of the species produced. Typically, resona(cid:173)
`tor cavities having coil diameters in the range 2.5 cm to
`60 cm are utilized. Cavities smaller than 2.5 cm in diam- 60
`eter are less desirable because of the relatively low
`integral flux of ions and cavities larger than 60 cm,
`although not precluded, are inconvenient because of the
`mechanical size, the lowered resonant frequency, and
`the increased power required. The cavity is brought to 65
`a resonant condition by adding capacitance to the coil,
`adjusting the length of the coil or adjusting the rf fre(cid:173)
`quency to resonance. (It is ·possible to extend the reso-
`
`4
`nance length of a coil by increments of approximately
`the wavelength divided by two, e.g. !, 1, 3/2, 2 of the
`wavelength, etc. for halfwave resonators and !, i, 5/4
`of the wavelength, etc. for quarter wave resonators,
`while maintaining the same discharge mode. This rela(cid:173)
`tionship is not precise because in practice, plasma load-
`ing effects and fringe capacitance influence the resonant
`frequency. Nevertheless, the relationship allows deter(cid:173)
`mination of a suitable range with precise values for a
`desired set of conditions determined with a control
`sample.) Cooling means such as circulating fluid
`through the coil or passing cooling gases through the
`resonator assembly are possible.
`As discussed, it is advantageous to ground one end of
`15 the helical coil, and preferably when used in a half wave
`or multiple mode device both ends are advantageously
`grounded. (Grounding, although not essential to its
`operation, tends to stabilize the plasma operating char-
`acteristics. Additionally, grounding on both ends re(cid:173)
`duces the possibility of coupling stray current to nearby
`metallic objects.) Standard means are employed to cou-
`ple rf power to the resonator. For example, a tap on the
`coil is made at a point where the voltage to current ratio
`is approximately equal to the characteristic impedance
`of the rfsource at operation. Alternatively, it is possible
`to use a coupling loop.
`It is possible to position longitudinally conducting
`elements along the outside of the low loss dielectric
`discharge tube. For example, a heater as shown in FIG.
`1 or a split metallic shield as shown in FIG. 2 are advan(cid:173)
`tageously employed for many applications. The heater,
`in particular embodiments, is useful in deposition proce(cid:173)
`dures to heat the deposition substrate when the sub(cid:173)
`strate is positioned within the discharge tube or to heat
`species generate_d in the plasma for subsequent down(cid:173)
`stream etching or deposition. The shield, in particular
`embodiments, is useful to adjust plasma species concen(cid:173)
`trations by application of a bias or to shield the plasma
`region from radial electric fields. If the longitudinal
`conductor is employed it should not form a low impe(cid:173)
`dance, conducting loop in the circumferential direction.
`Thus the shield is shown split in FIG. 2 and the heater
`although serpentine does not, as shown in FIG. 1, com(cid:173)
`plete a loop within the resonator coil. (It is possible to
`complete the loop outside the conducting coil since the
`impedance of this completed portion is quite high.)
`Gases for etching are introduced in the region of the
`helical electrode at a pressure in the range, 1 X l0-5
`Top- to 10 mTorr. Unexpectedly, relatively low pres(cid:173)
`sures sustain a plasma and yield an intense flux of ions.
`Indeed, pressures above 10 mTorr are not desired for
`etching since the relative flux of ionic species that in(cid:173)
`duce anisotropic etching in proportion to neutral spe(cid:173)
`cies -neutral species tend to cause isotropic etching in
`55 the absence of sufficient ion flux -is substantially
`lower. Pressures below 1X10-s Torr although not
`precluded are also not desirable since the plasma be(cid:173)
`comes difficult to initiate and operate.
`The gas employed depends upon the material to be
`etched. A wide variety of gases have been utilized to
`etch the materials typically employed in devices such as
`semiconductor devices. For a review of suitable etch(cid:173)
`ants, for numerous material utilized in devices see D. L.
`Flamm et al, VLSI Electronics: Microstructure Science,
`Vol. 8, N. G. Einspruch and D. M. Brown, eds, Aca(cid:173)
`demic Press, New York, 1984, Chapter 8. Exemplary of
`such gases are chlorine, utilized to selectively etch sili(cid:173)
`con over Si02, and NF3 for selective etching of Si02
`
`,
`
`Page 7 of 11
`
`

`

`4,918,031
`
`5
`over GaAs. The etching gases are advantageously in(cid:173)
`troduced at one end of the resonator tube such as shown
`at 5 in FIG. 1. It is possible to use the etchant gas itself
`at a suitable pressure or to mix the etchant with other
`gases such as an inert gas, e.g. argon. Irrespective of the 5
`particular gas or combination of gases utilized the pres(cid:173)
`sure should still be maintained elow 10 mTorr.
`A typical configuration for downstream etching is
`shown in FIG. 3. The distance between the discharge
`and substrate depends on (1) coupling between the dis- 10
`charge and the etching chamber (2) the relative areas of
`the discharge tube cross section and the etching cham(cid:173)
`ber, (3) gas pressure and (4) any additional bias em(cid:173)
`ployed. However, typically the substrate is placed a
`distance of at leas_t 0.5 the diameter of the dielectric gas 15
`enclosure from the plasma. (For purpose of this disclo(cid:173)
`sure, bias refers to a d.c. or a.c. electrical potential ap(cid:173)
`plied between a reference surface, e.g. the resonator
`shield or independent electrode, and the substrate.)
`For etching anisotropically in a direction perpendicu- 20
`lar to the surface, it is generally desirable for the major
`surface of the substrate to be positioned perpendicular
`to the direction of the ions emanating from the plasma.
`It is possible to bias the substrate (10 in FIGS. 1 and 3)
`and if desired, to pulse this bias and/or pulse the dis- 25
`charge itself. Pulse rates in the range 0.1 Hz to 150 kHz
`are useful. Pulsing of the bias is of particular use when
`a multilevel resist is employed with a silicon containing
`top level and a planarizing lower level. The use of a
`pulsed bias with oxygen etching species alternates etch- 30
`ing of the underlying resist with formation of an etch
`resistant silicon dioxide layer on the patterned overly(cid:173)
`ing resist. Thus, the pattern is transferred into the un(cid:173)
`derlying resist with substantially no degradation of the
`overlying pattern during this transfer.
`Pulsing of the discharge is advantageous, for exam(cid:173)
`ple, when multiple plasma sources or feed gas flows are
`employed. With suitable pulsing the source of etching
`species (or deposition species in deposition processes)
`are controlled by a time variation in power applied to 40
`different etchant sources, e.g. completely different reso(cid:173)
`nators, one resonator with a time variation in gas flow
`composition or other sources of chemical reactants
`which may optionally be partially dissociated by an
`aqditional plasma device. (Pulsing of the discharge dur- 45
`ing a deposition process also leads to increased deposi(cid:173)
`tion rate under appropriate conditions.)
`The inventive process has been found particularly
`suitable for etching of devices based on extremely strict
`design rules, for example, a device based on 0.25 µ,m 50
`long gate structures of transistors. Dimensions this small
`generally are not adequately etched by available tech(cid:173)
`niques. Nevertheless, by using a helical resonator at low
`pressure, extremely good resolution at an acceptable
`etch rate is obtained. For example, the etching of 55
`polysilicon using a chlorine discharge generated by
`helical resonator at a pressure of lQ-4 Torr yields well
`resolved 0.25 µ,m structures separated by 0.25 µ,m
`spaces. Additionally, this structure is produced at an
`etching rate of approximately 200 A/min. Thus even for 60
`extremely fine structures, anisotropic, well resolved
`etching is produced.
`The parameters employed for species generation for
`other uses such as ion implantation, surface modifica(cid:173)
`tion or multiport processing (such as a source of H 65
`atoms, or H+ for molecular beam epitaxy), and down(cid:173)
`stream deposition based on the use of a helical resonator
`are similar to those utilized for etching. Pressures in the
`
`35
`
`6
`range lQ-5 to 100 Torr are suitable for a variety of
`applications and the precise pressure for a given situa(cid:173)
`tion is determined with a control sample. The gases
`utilized for deposition depends on the species desired. A
`wide variety of gas precursors are well known for pro(cid:173)
`ducing particular deposited material. Exemplary of
`suitable precursors are an 02 plasma for subsequent
`reaction with tetraethoxysilane to deposit Si02.
`Additionally, it is possible to enhance the deposition
`discharge by introducing an axial magnetic field in the
`discharge region, as in the case of etching (e.g. 20 in
`FIG. 3). Moreover, it is possible to further control the
`deposition or etching process by introducing electric
`and magnetic fields near the substrate region (shown in
`phantom at 21 and 22 in FIG. 3). It is possible to employ
`fields that are purely axial, purely radial, or a superposi(cid:173)
`tion of axial and radial fields with respect to the resona(cid:173)
`tor axis.
`These fields are useful as shutters, as a means to direct
`the ions to a particular position on the substrate, as a
`means to alter the radial distribution of the plasma
`stream across the substrate diameter, or as a means to
`regulate impact energy. It is also possible to impose an
`RF electric field onto the substrate to further control
`ion bombardment energy during deposition or etching.
`The conditions of this particular mode of operation are
`fixed so that no discharge, or only a very weak dis(cid:173)
`charge, is sustained by the RF potential unless the reso(cid:173)
`nator plasma is on, i.e., a nonself-sustaining discharge is
`formed. In this instance the helical resonator discharge
`acts as a virtual electrode. Most significantly, deposition
`in the discharge region as shown in FIG. 4 is possible.
`As in etching and other deposition processes, use of a
`heater, 41, around substrates 42 held in a horizontal
`position or as shown in phantom at 43 held in a vertical
`position is suitable if desired. Additionally, as in other
`embodiments a bias, 44, to the substrate support is ac(cid:173)
`ceptable.
`
`EXAMPLE 1
`A 350 A layer of Si02 was grown by the procedure
`described in L E. Katz, VLSI Technology ed. Sze at
`McGraw-Hill, N.Y., 1988, pgs. 98-140, on a 100 mm
`diameter silicon wafer with the major wafer surface
`oriented in the (100) plane. A 3000 A film of undoped
`polycrystalline silicon was deposited by chemical vapor
`deposition (as described in A. C. Adams, VLSI Technol(cid:173)
`ogy ed. Sze at McGraw-Hill, N.Y., 1988, pgs. 238-248),
`onto the silicon dioxide. An etch mask having 0.25 µ,m
`lines and varying spaces was formed by a trilevel pat(cid:173)
`terning scheme, as described in "Electronic and Pho(cid:173)
`tonic Applications of Polymers", M. J. Bowden and S.
`R. Turner, eds., pp. 90-108, (American Chemical Soci(cid:173)
`ety, Washington, D.C.), 1988. The trilevel resist in(cid:173)
`cluded a first layer (4500 A) of a planarizing Novalac
`polymer, an overlying 1200 A thick plasma deposited
`Si02 layer and a top layer of an electron beam sensitive
`resist (chlorinated glycidyl methacrylate). The top
`layer was exposed to an electron beam writing appara(cid:173)
`tus producing 0.25 µ,m features. This pattern was trans(cid:173)
`ferred through the oxide layer by reactive ion etching,
`and the underlying layer of planarizing polymer was
`etched with oxygen reactive ion etching to complete
`the pattern transfer to the polycrystalline silicon.
`The entire wafer was transferred into an etching
`apparatus shown in FIG. 5 via a vacuum loadlock 51,
`using motor drives 52 and 53 as well as wafer cassette,
`54. The substrate 59 was held on a plate insulated from
`
`Page 8 of 11
`
`

`

`4,918,031
`
`8
`source. Temperatures in the range -180° to 20° C. were
`employed. Etching of the substrate was performed as in
`Example 1. In this case the etch selectivities for polysili(cid:173)
`con over gate oxide and the resist were increased com(cid:173)
`pared to those given in Example 1. Lower substrate
`temperature favored etching material with a lower acti(cid:173)
`vation-energy to reaction, e.g. polysilicon.
`
`EXAMPLE 3
`A similar configuration to that described in Example
`1 was used to deposit silicon dioxide films. A portion of
`an undoped (100) silicon wafer was used as the deposi(cid:173)
`tion substrate. The helical resonator employed has a
`primary resonance of 18 MHz. The resonator excited a
`discharge in 02 (100 seem at 0.2 Torr) which passed
`through a quartz tube that was 1.4 in. O.D. The dis(cid:173)
`charge tube was coupled to a quartz reactor having a
`heated substrate holder (430° C.). Tetraethoxysilane
`was introduced downstream of the discharge at a rate of
`5 seem in the region above the substrate. One hundred
`watts of power was applied to the resonator yield a
`deposition rate of 600 A/min.
`The resulting films were analyzed by fourier trans(cid:173)
`form infrared spectroscopy and Rutherford backscat(cid:173)
`tering spectroscopy. The analysis of the films showed
`essentially pure silicon dioxide. Oxide films deposited at
`25° C. had a significant concentration of OH groups and
`a somewhat decreased firm density. However, the film
`composition and density were improved by using a 200
`kHz RF bias (900 V peak-to-peak) on the substrate
`holder, to enhance ion bombardment rearrangement
`and stabilization of the film. The additional RF bias did
`not affect the discharge current flowing from the reso(cid:173)
`nator plasma.
`
`25
`
`7
`ground that could be biased with a separate 13.56 MHz
`rf source. A helium-neon laser, was used to monitor the
`polysilicon etch rate by la8er interferometry. The reac(cid:173)
`tion chamber was evacuated to a pressure of 5 X 10-7
`torr with a diffusion pump, backed by a Roots blower 5
`and mechanical pump. A quarter wave helical resonator
`was employed to sustain a plasma that coupled during
`etching to an underlying aluminum reaction chamber.
`The resonator, 60, was constructed from a 12 in. long, 8
`in. O.D. cylindrical copper shield containing a 27 turn, 10
`6.5 in. long, helical coil, 61, of i in. 0.D. copper tubing,
`4.5 in. O.D .. The fundamental resonance of this struc(cid:173)
`ture (approximately 8.7 MHz) varied slightly with the
`applied RF power and gas pressure. A 64 mm 0.D.
`quartz discharge tube, 63, (498 Gm3 discharge volume 15
`within the coil) passed concentrically through the heli(cid:173)
`cal coil, was mated to the reaction chamber, 64, by
`o-ring seals, and extended 2 in. into the chamber. The
`end of the discharge tube was positioned approximately
`6 in. from the substrate. Gases were passed through the 20
`opposite end of the tube which extended 10 in. beyond
`the resonator shield. The resonator was placed close to
`the top metal flange of the reaction chamber. A flow of
`air was passed through the resonator to cool the quartz
`tube.
`Chlorine was flowed through the quartz discharge
`tube at 15 seem yielding a pressure of approximately
`lQ-4 torr within the reaction chamber. (It is possible to
`use small additions, e.g. 1 to 15% of oxygen to the
`discharge to increase the polysilicon to silicon oxide 30
`etch rate selectivity.) A discharge was initiated by 1)
`coupling an RF amplifier and frequency generator to
`the resonator coil, tuning the sine wave frequency near
`resonance as indicted by a sharp decrease in the voltage
`standing wave ratio at the input to the resonator and the 35
`appearance of a visible glow, and 2) increasing the ap(cid:173)
`plied power to a level of approximately 80 W. Adjust(cid:173)
`ment of frequency and power were normally performed
`in concert. PQwer inputs of the resonator circuit were
`approximately 75 W, (0.15 W /cm3 power density into 40
`the volume of the dielectric tube enclosed by the helical
`~oil) yielding an etch rate in undoped polysilicon of 200
`A/min. (Increasing the power increased the polysilicon
`etch rate. Higher powers were usable but the discharge
`glow in the chamber became somewhat unstable). Etch- 45
`ing was continued 1.6 times the period required to re(cid:173)
`move the exposed 3000 A layer of polysilicon in the
`center of the wafer as measured by the laser interferom(cid:173)
`etry. The discharge and gas flows were then extin(cid:173)
`guished and the wafer was removed for analysis.
`With either Cl2 or Cl2/02 discharge mixtures, the
`etch selectivities for polysilicon over oxide and the
`resist were acceptable, but the selectivities were better
`with oxygen additions. Polysilicon/oxide selectivity
`was approximately 30: 1 with Cl2 and 70: 1 with Clz/02, 55
`while the polysilicon/resist selectivity was 2.5:1 in both
`cases. Scanning electron micrographs of the masked
`regions showed smooth, nearly vertical sidewalls for
`the polysilicon with no undercutting.
`
`EXAMPLE4
`A hotwall, quartz discharge tube 70 cm long and 50
`mm 0.D. was passed through a resonator centered at
`8. 7 MHz. The tube was heated by a cylindrical furnace
`slightly smaller than the resonato"r coil and thermally
`insulated from the resonator volume having the heater
`element in a serpentine array so that continuity was
`avoided around the circumference of the heater. The
`heating element when mounted in this fashion did not
`hinder the operation of the discharge. The tube was
`heated to approximately 500° C. and air cooling kept
`the resonator components from heating excessively.
`Both ends of the helical coil were electrically refer(cid:173)
`enced to the shield, i.e. the resonator was operated in a
`50 half wave mode and the second harmonic (approxi-
`·
`·
`mately 18 MHz) was used.
`Fluorinated silicon nitride was plasma deposited by
`introducing a 200 seem flow of 1 % silane in helium and
`a 4 seem NF3 flow directly on the silicon wafers held in
`the resonator discharge. Pressure in the discharge was
`maintained at 1 torr, the quartz wall was maintained at
`350° C. and the power was 50 W. The resulting deposi(cid:173)
`tion rate was 200 A/min. Analysis of the film showed
`nitrogen, fluorine and silicon.
`
`60
`
`EXAMPLE2
`The same configuration as that described in Example
`1 was used except that the substrate was cooled below
`ambient temperature. This was accomplished by flow(cid:173)
`ing cold fluid through the substrate platen and subse- 65
`quently cooling the wafer to be etched by conduction.
`Temperature was regulated by adjusting the fluid flow