Microelectronics
`Processing
`
`Micron et al. Ex?o1o p.1
`
`

`
`ADVANCES IN CHEMISTRY SERIES
`
`Microelectronics Processing
`
`Chemical Engineering Aspects
`
`Dennis; W. Hess, EDITOR
`University of CaEifomio—Berkeley
`
`Klavs IF. Jensen, Enrron
`Univmrsity of Minnesota
`
`
`
`Arnerican Chemical Sooiefy. Washlngfon. DC 1989
`
`Micron et al. Ex.1010 p.2
`
`

`
`Library of Congress Cataloging--in-ruhlicatian Data
`
`Microelectronics Processing: Chemical Engineering: Aspects
`Dennis W. floss. odltor, Klmrs F’. jonson. .t-.i:li'tor
`p.
`cm. -—(Advanocs in chemistry series. ISSN 0055-2393; 221)
`Bibliogmphy: 1:.
`Includes index.
`
`__
`.ISB.h_I o.—s412—1-475-;
`L Miem-slectmnics~Matorials.. 2. Integrated
`Cirt:nits—Des1'gn and Coustmcfion. 3. Surface Chemistry-.
`
`I. Hess, Deumis W.
`HI. Series
`
`TK‘I874.};_t4.B35
`6£1.3S1—dc19
`
`1939
`
`IL Jensen, Klavs E, 1952-
`
`89-6862
`-co»
`
`Copyright o I989
`
`American. Chemical Society
`
`All Rights Reserved. The apps-ananoe ofthe code at the- bottom of the first page of each clung:-{er
`in this volume indicates the "copyright owner's consent that reprogmpliic copies of the chapter
`may be made for personal or internal use or for the personal or 1'nte1-rnfl use of specific clients.
`This consent -is given on the condition, however, that the. copier pay the stated per-copy fee
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`process. or other data be regiirdled as a license [IT as E cmweyauoe of any right or permi.1'isim1
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`HilN'l‘El) IN TILE Imrrno s1'ATn§1 or AMERICA
`
`Micron et al. Ex.1010 p.3
`
`

`
`CONTENTS
`
`¢llI¢IIIOOIItlI§IDIQIllIIII!DIQl'UIlCUIIIIIIODCCOQIIIIHIIIIIIII-I C I O I I l I Ilxi-ii
`
`1. Microelectronics Processing . . . . . . . . . .
`Dennis W. Hess and Iflavs F. Ienson
`
`Iuninaonn-upunchy:napuonadunnunoucnun I
`
`2. Theory of Transport Processes in -Semiconductor
`Crystal Growth from the: Melt
`Robert A. Brown
`
`3'. LiqII_il'1-Phase Epitaxy and Phase Diagrams
`of
`Sfimiflmldllflfifsnun.-unun.--uon-nua--no--o---+-no.
`'I‘im'o'thy J. Anderson
`
`4. Physical Vapor Dopositiorn Reactors
`T. W. Fraser Russell, Biil N. Baron, Scott‘ C. Jackson,
`and Richard E. Rochelaeau
`
`35
`
`171
`
`5'. Chemical Vapor Deposition ..............
`Kla'vs- F. Jonson
`
`....
`
`.....
`
`....... .. 199
`
`6. Dilfusion and Oxidation-ofSilicon........ ................
`Richard B. Fair
`
`....... .....265
`
`7. Resists in Microlithograéflmy.....‘...........,...... .... ............ .... "325
`Michael J. O'Brien on E David 5. Some
`
`8!
`
`Dflpfifiitiflflegnnua Ioulnooo Dial DI IIIIII Incl I
`Dennis W. Hess and David B. Graves
`
`9. Interconnection and Packaging of High—l_1'erformanoe
`Integrated Circuits . . . . .
`. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 441
`Ronald I. Jensen
`
`Ill. Semiconductor Pmcessing Problems Solved by Wet
`(Solution) Chemistry .._ ..... . _. ............. ... .........................
`Marjorie K. Balazs
`
`505
`
`INDEXES
`
`A_ulh_orIndox.....
`
`. . . .
`
`. . . .
`
`. . . . . . . .
`
`. . . . . .
`
`.
`
`.
`
`.
`
`. . . . . . . . . .
`
`. . . . . . . . . . . . . . “H593
`
`MfiliationIndex........-.......
`
`..........
`
`..................... .523
`
`Sn-hfidIndex"""9‘"""‘||'3l-|“I'I|0l¢I|n|IIIotdIltcoc¢¢ o -
`
`I
`
`h
`
`o I r
`
`xi.
`
`Micron et al. Ex.1010 p.4
`
`

`
`362
`
`Ml(JRO.ELEGTl_iUNI(.‘S I’-'!l(l£}ESSlNC: CHEMICAL EP~lGlNEE'fi!N(: Asruous
`
`Resist Stripping. Aitor the additive or .suhtraotive processes of the
`substrate are complete, the resist mask must be. completely removed by
`either wet or dry etching". The selection of resist stripper is determined by
`previous resist history (balms, exposure to plasma, etc.) that rosultsin o'h_oIn—
`ioal alteration and by the underlying substrate stability (175). Wet etohos
`are either solvent-based or inorganic reagents. such as 11.3504, I-INQ3 or
`H303. Solvent-type strippers are typically acetone For positive resists, trio}:-
`loroothylono for negative resists, or commercial products developed to re-
`move hath types of resists. Commeroial. organic strippers were initially
`phenol-based. solvents but have been manufachirod recently with "little or
`uo phenol as a result of health and safety issues associatod with the use of
`this chemical. Plasma stripping or ashing of resist with either 02- or 0-3-"CF,
`gases is clearly the l’nl3ill01El of choice from the standpoint of .con'venio11ce-,
`oost, and safety. However, the method cannot be used with .sul3st-rates that
`are etched by these plasmas.
`
`In addition to the. standard process steps,
`Auxiliary Process Steps.
`auxiliary processes are sonriotitnes necessary. These steps are not used for
`-all situations but as required and will he considered in this section separately.
`‘Certain souiioon_ductor-manufacturing processes are particularly damaging
`to polymeric films and re-qlluire an additional step to harden the resist. For
`example". Al etching with oh]-orine plasma produces A1013. which degrades
`resists. Ion implantation, in which the chalnbor temperature and, hence,
`the wafer ttslnpcrature incnoaso with in'croasin_g implant close, causes tliormal
`deformation of the resist image. One commonly used method to stabilize
`novo_lao-based resists is doop—UV flood exposure after patterning (177). With
`deep-UV exposure, cross-linking of the polymer surface produces a film with
`increased thermal rosistanczo. With this procedure, positive resists can with-
`stand a 180 “C bake for 30 min. Fluorooarbon plasma treatmontalso stabilizes
`resists (I 78), heeluse fluorine insertion impedes subsequent oxidation ofthe
`polymers.
`As discussed previously, an optional postexposura. prodevelopment
`bake can reduce‘ problems-. with the standing-wave elfect in DNQ—novo1ao
`positive resists. However. such a postoxposuro balm stop is indispensable
`in the image reversal of positive resists (37-41) and certain resists based on
`chemical amplification .of.a photogonerated catalyst (64-6?, 77’. 78). For both
`types of resists. the chemistry that dilforontiatos between exposed and unox—
`posed areas does not occur solely during irradiation. Instead, diifereutiation
`occurs -predominantly during a subsequent bake. 'I'horefore_, to obtain a::—
`ooptablo CD control in these systems, the bake oonditions must be carefully
`optimized and monitored.
`
`Rework. Masking shops frequently have the advantage over other IC-
`manufacturing processes of being -able to undergo wafer rework. Rework
`
`Micron et al. Ex.To1o p.5
`
`

`
`Plasma-Enhanced Etching
`
`and Deposition
`
`Dennis W. Hess and David B. Graves
`
`Department ‘of Chemical Engineering, University -of California. Berkeley,
`CA 94720
`
`Cheniical and chemical engineering principles imaalnaal in plasma-
`-enhmmed etching and deposition. are revietned, nmdeling approaches
`to rlescrilm and predict plus-inn: l3!Ellflt)iDf are intimated, and specyfi:
`examples ufplamna-enhiinccd etching and deposition of ll!-I'll-';lll.m ma»-
`terials of intenest to the fabrication of rm'c'raelectrom'c and optical
`devices are discussed.
`
`THE INGREASING COMI‘LE2l€I'I'Y 01-‘ SOLID-STATE -electronic and optical de-
`vices places stringent demands upon the control of thin-film pmcesses-. For
`cxample, as device geometries drop below the 1—p.rn level, previously stan-
`dard processing techniques {hr thimfilm etching and deposition become
`inadequate. For etching, the control of film etcli rate, unifnrrnity, and se-
`lectivity
`no longer sulficiernt; the establi-shment of film cross .sections or
`profiles is crucial to achieving overall reliability and higl1—d-ensity circuits.
`Low-temperature depus-itiun. methods are required to niinimize defect for-
`inatien and solid-smte dilfusiun and to be compatible with low-melting—pO_int
`.sulJst1.'ates 01' films. Th'ere'l'ore, the established techniques of liquid etching
`and, to some extent, chemical -vapor c_lepos'it'1'on (CV13) are being replaced
`by plasma-assisted methutls. Plasmaassistetl etching and pl-flS‘.lI]_i1-BIlllfll1CBCl
`CVD {I’EC\-’D_) take advantage of the high~ené1'g'y electrons‘ present in glow
`discharges to dissociate and i_an:i-ze gaseous molecules to form chemically
`reactive radicals and ions. Because thermal energy is not needed to break
`chemical btmds-, reactions can be promoted at low temperatures (_-<20!) °G).
`Although the chemistry and physics of a glow discharge are extrao'rd1'-
`narily complex. the plasma performs only two basic functianr... First. reactive
`
`006_5—'3393l'39/0221-03778 1 5. 6030
`© 1989 American Chemical Snciety
`
`Micron et al. Ex.1010 p.6
`
`

`
`378
`
`MlC.R0'EL.E.t'."fil0NlCS Pnocussmo: CHEMICAL. Enomesnlso A.=u-1«:m-s
`
`chemical species are generated by electron-impact co'1lisions;- thus they over-
`come ldnetic limitations that may exist in thermally activated processes.
`Second, the discharge supplies energetic radiation (egg. , positive ions, neu-
`tml species, rnetas-table species, electrons, and photons) that bombard sur-
`faces immersed in the plasma and thus alter the surfiioe chemistry during
`etching and deposition. The combination of these physical" processes with
`the -strictly chemical reactions due primarily to atoms, radicals, or molecules-
`yields etch rates, etch profiles, and material properties unattainable with-
`either process indivirlually.
`
`Dry Processing
`
`Liquid etching has been the preferred method for pattern delineation for
`thin films for many years (I). Its pervasive use has been due primarily to
`two considerations. First, -although the exact chemistry is often poorly under-
`stood, the technology of liquid etching is firmly established. Second. the
`selectivity (ratio of film etch rate to -the etch rate of the underlying film or
`substrate) can be es.sentiall)I'- infinite with the proper el1oice.oi"etcha11t so-
`lution.
`Despite-these advantages, several critical problems arise for micrometer
`and subrniorometer pattern sizes. Resist materials‘ often lose. adhesion in the
`acid solutions used ibis most etch processes and thereby alter pattern di-
`mensions and preventlline-. width control. As etching proceeds downward
`into the film, it proceeds laterally at an approximately equal rate. The mask
`is undercut, and an isotropic profile (Figure 1) results. Because film thickness
`and etch rate are often nonuniform across a substrate, overetehing is required
`to ensure complete film removal. Overetohing generates a decrease in pat-
`tern size because of the continued lateral etching and thus afects process
`control. When the film thickness is small relative to the minimum pattern
`dimension, undercutting is insignificant. But when the film thickness is
`comparable with the pattern size, as is the case For current and future devices,
`undercutting is intolerable. Finally, as device geometries. decrease, spacings
`between resist stripes also decrease. With niiororneterand submicrornoter
`patterns, the surface tension of etch solutions can cause the liquid to bridge
`the space between resist stripes. Because the etch solution does not con tact
`the film, etching is precluded.
`The limitations encountered with solution etching can be overcome by
`plasma~enhanced etching. Adhesion is not a major problem with dry—etch
`methods. Ilndercutting can be controlled by varying the -plasma chemistry,
`gas pressure, and electrode potentials (2-6) and thereby generate directional
`or anisotropic profiles.
`Numerous techniques have been developed for the formation of thin-
`Hlm materials (?—9). Because of the versatility and throughput capability of
`CVD, this method has gained wide acceptance for a variety of film materials.
`
`Micron et al. Ex.1010 p.7
`
`

`
`3.
`
`H1353 8:‘ Cmves
`
`Plamza-E1nhc.mcecl Etching and Deposition
`
`379
`
`Musk
`
`Film to be etched
`
`Substrate-
`Etching
`
`,
`
`Isotropic Profile
`' No Ovaretch
`
`Liquid or Plasma
`Etching
`
`Vertical Profile
`
`Plasma or Dry
`
`Figure 1. Gross -.sect'ions- offilms etched with liquid or plasma etchants. The
`isotmpic profile is the result qfzera ooeretah and can be generated with liquid
`or plasma etch techniques. The anisotropic: (vertical) profile requires plasma
`or dry-etch processes. W is the width of the resist pattern. (Heproducedfrom
`reference 2. Copwiglit 1983 American Chemical Society.)
`
`However, deposition rates are often low with CVD, and the presence of
`tompor_ature—se_nsitiv.e- substrates or films (e.g., polymers or low-melting-
`point metals) prior to deposition, along with the possibility of generafing
`defects (e.g.,. vacancies, interstitials, stacking" faults, and dislocations) often
`precludes the use of elevated temperatures (>300 "C) for film growth. In
`such cases. deposition rates can be enhanced by using high-energy electrons
`in a discharge rather than tltermal means to supply the energy for bond
`breaking (10-14).
`
`if Glow Discharges
`
`The rf (radio frequency) glow discharges (2) or plasmas used for plasma
`etching or PECVD are partially ionized gases composed of ions, electrons,
`and st. host of neutral species in both ground and excited states. Typically,
`the plasma is Formed by applying an electric field across a volume of gas.
`Many types "of plasmas exist (15); they differ primarily in electron concen-
`tration rt, and average electmn energy k’1‘,,. A quantity that is useful in
`
`Micron et al. Ex.1010 p.8
`
`

`
`MIUHOELELTRUNLCS Piioensslm:-: Cill£MlC:M. ENC1NEls‘.lllN-G A§_iI'l+‘.CTS
`
`eharnctorizing the average. electron energy is the ratio of the clcc.l:n'¢ field
`to the pressure, E/p (15). As the electric field strength increases, ‘Free elec-
`trons, whose velocities increase because of acceleration by the field, gain
`energy. The electrons lose this energy by inelastic collision-S, so that an
`increase in pressure, which decroases the electron mean free path, decreases
`the electron energy.
`In tl-.1in—film processes for the fabrication of electronic materials and
`de.vices, rfglow discharges. are: primarily used. The application ofan rfvoltage.
`at frequencies between 50 14.1312 and 40 MHz to a low—pressur_e (E-600 Pa]
`gas results in a chemically uniiquc cnvironinent (‘Table I.)
`Eloctmn densities (n,.) and. because the plasma is electrically neutral,
`positive-ion densities (n.-,) range from 10" to 10”/ml“. Ilowevcr, the ratio
`of the neutral-species density ('n.,.,.) to the electron density is usually greater
`than 105-, so that these plasmas -are only weakly ionized. As a result, radicals
`and molecules in the discharge are primarily responsible for etching and
`deposition reactions. That is, radicals and molecules are not inherently more
`reactive than ions, but they are present in significantly higher cont-entrations.
`The glow discharges described by Table I-are termed nonequilihrium plos-
`mas, because the average electron energy (lcT,,) is considerably higher than
`the ion energy (kT,). Tl1(&1'£!lll)l'(3', the discharge cannot be described ade-
`quately by a single temperalture-.
`
`Physical and Electrical Characteristics. The electrical potentials
`established in the reaction chamber determine the energy of ions and elec-
`trons strilwzing the so-rfar-.‘os.im1:nersed in a discharge. Etching and deposition
`of thin films" are usually perfiormecl in a capaeitively coupled parallel-plate
`rl’ reactor (sec Plasma Reactors). Thcreforc, the following -discussion will he
`directed toward this configur:a'lion.
`The important potentials in rf glow discharge systems (16, 17') are the
`plasma potential (potential oil the glow region)-, the floating potential (po-
`tential assumed by a. surface within the plasma that is not extern'a1ly- biased
`or grounded and thus draws no not current). and the potential of the powered
`or externally biased electrode. When‘ the plasma contacts a surface. that
`surface, even if grounded, is usually at a negative potential with respect to
`the plasma (15, I8. 19). 'Ilteref(Jro. positive-ion lJo_Inbard1ne11t.0ecurs. The
`energy of the bombarding ions is estal)lisl)otl by the dillerenco-'i11 potential
`
`Table I. Properties of rf Glow Discharges: (Plasmas)
`Used for Thiin-Film Etching and Depu-iilion
`
`Paronwter
`
`n,, = In
`nu
`kT|'
`JET:
`
`Value
`
`Ill"-ll)"/om"
`** 10 "”'—1U"“/cin"
`]-_
`"'0.U4 EV
`
`Micron et al. Ex.1010 p.9
`
`

`
`8. mass as Gimvss Phisma-Enhanced Etching and Drrpo.sm‘on
`
`381
`
`between the plasma and the surface that the ion striltes, the rf frequency
`(because of mobility considerations), and the gas pressure (because of col~
`lisions). Because ion energies may range from a few volts to more than 500
`V, surface bonds can be brol<:en_. and in certain instances, sputtering oil-llm
`or electrode material may occur (16).
`The reason for the diiferuent potentials within a plasma system becomes
`l3.l)ViO.l1S when electron and ion mobilities are considered (mo). Imagine
`applying an rf field between two plates (electrodes) positioned within a low-
`prcssuro gas. 0-11 the first llalf-cycl'o of the field, one electrode is negative
`and attracts positive ions; the other electrode is positive. and attracts elco
`trons. Because of the frequencies used -and because the mobility of electrons.
`is considerably greater than that ot’pos'it-ive ions, the this (current) ofelecttons
`is much larger than that of positive ions. This situation causes a depletion
`of electrons in the plasma and results in a positive plasma potential.
`On the second half-cycle-, 21 large flux of electrons flows to the electrode
`that previously received the small flux of ions. Because plasma—etching sys-
`tems generally have a dielccl-.'ric coating on the slecnodos or a sories {block-
`ing} capacitor between the power supply and the electrode, no direct current
`(do) can be passed. Therefore, on each subsequent half-cycle, negative charge
`continues to build on the. eluectmtles and on other surfaces in contact with
`
`the plasma, and so electrons are repelled and positive ions are attracted to
`the surface. This transient si_tuatio'n ceases when a sulllcient negative bias is
`achieved on the electrodes such that the fluxes of electrons and positive ions
`striking these surfaces are equal. At this -poin-t,_ ti'1nc—avora'g_'e (positive) plasma
`and (negative) electrode potentials are established.
`A plasma potential that is positive with respect to electrode potentials-
`is; primarily a consequence ‘of the greater mobility of elections cotnparcd
`with positive ions. When there are many more negative ions than electrons
`in the plasma ('e.g., in highly electronegative gases), plasma potentials are
`below electrode potentials, at least during part of the rfcyclc (191)).
`The pl_as'ma_’ potential is nearly uniform lhroilghout the observed -glow
`volume in an if discharge, allthough a small electric field directed from the
`discharge toward the edge at the glow region exists. Between the glow and
`the electrode is a narrow region (typically 0.01—1 cm, depending primarily
`upon. pressure, power, and frequency) wherein a change from the plasma
`potential to the electrode potential occurs. Thisregion is called a sheath or
`dark‘ space -and can be likened to a- depletion layer in a semiconductor device
`in that most of the voltage is dropped across this region.
`Positive ions drift to the sheath edge where they encounter the strong
`field. The ions are then accelerated across. the potential drop -and strike the
`electrode or substrate surface-. Because oftlio series capacitor or the dielectric
`coating of the electrodes, the negative potentials established on the two
`electrodes in a plasma system may not be the same. For instance. the ratio
`of the voltages on the electrodes depends upon the relative electrode areas
`
`Micron et al. Ex.1010 p.10
`
`

`
`383
`
`M1GflUELE'UTflUN£C2S PROCESSING: CHEMICKL ENUINISEHINU A51’-EUFS
`
`(20). The theoretical dependence is given by equation 1, where V is the
`voltage and A is the electrode: area (20).
`
`Vi/V2 =* lAaIlAil4
`
`(ll
`
`If V , is the voltage on the powered electrode and V3 is the voltage on
`the Q-ounded electrode, them the voltage ratio is the inverse ratio of the
`electrode areas raised to the fourth ‘power. However, for typical etch systems,
`the exponent of the. area ratio is generally less than 4-and may be less than
`1.2 (16). This apparent deviation fmm theory is in part due to the reactor
`configuration. Although the physical electrodes in a plasma reactor often
`have the same area, A3 represents the grounded electrode area, that is, the
`area of all grounded sin-faces in contact with the plasma. Because this area
`usually includes the chamber walls, the area ratio can be quite large. Because
`of such oonsiderations, the average potential distribution in a ‘typical com“-
`mercial plasma reactor with two parallel electrodes immersed in the plasma
`is -similar to that shown in Figure 2 (16). In this case, the energy of ions
`striking the powered. electrode or substrates on this electrode will be higher
`than that of ions reaching the grounded electrode. Indeed, equation 1 can
`be used to design electrode areas for reactors su.ch that a particular voltage
`can he established on an electrode surface".
`In addition to the ratio of electrode areas, other plasma parameters can
`
`+V
`
`POWBI”-Ed
`Erectrode
`
`
`
`Gruund
`
`Figure 2. Potential disisibuticiiz in :1 parallel-plate plasma etcher with the
`grounded suifaw area larger than the -powered "electrode area. V is the po-
`tential, and V, is the plasma potential.
`(lieproducecl with pennission. from
`raferenc-:e 16. Copyright 1979 The Elect:-ockemical Society. Inc.)
`
`Micron et al. Ex.1010 p.11
`
`

`
`8. HESS 5: Cimvcs
`
`Pl'asn.1a—Enh:mced Etching and Depositioxn
`
`383
`
`alfect the electrical charactcri.-itics of the discharge. Va_rying_'t_'hc If power
`input will alter plasma and electrode potentials, as well as ion concentrations-.
`and thereby cliangc ion energies and fluxes. Also, radio freq-uency -aflocts
`the kinetic energy of ions that strike surfaces in contact with the plasma.
`This eilect can be readily understood by considering the behavior -of an ion
`experiencing an oscillating plasma potential caused by applied 1-Pvoltages
`(21, 22). Depending upon the ion mobility, some frr-.quon'cy exists above
`which the ion can no longer follow the alternating voltage. Therefore, the
`ion cannot traverse the sheath in one halt‘-cycle. Above this frequency, ions
`experience an accelerating llield (tho diference between the plasma and
`electrode potentials divided by the sheath thickness) that is an average over
`a number of halficyclos. At "lower frequencies, where the ions can respond
`directly to the oscillating fielrfl-, they are accelerated by instantaneous fields.
`Thus, the ions can attain the maximum energy corresponding to the maxi-
`mum instantaneous field across the sheath. As a result, for a constant sheath
`potential,
`ion bombardment energies and fluxes are higher at lower fre-
`quencies.
`
`Chemical Characteristics. Because etching or doposition processes
`are merely ch_ern_ical reactions that yield a volatile or involatile product,
`respectively, the overall procicss can be broken down into the following six
`primary stops:
`
`P’-.°"'."*."3!‘°'."'
`
`Generation of reactive species
`
`Diifusion to the suriiice
`
`Adsorption
`
`Reaction
`
`Desorption of vo1a.til.e products
`
`Diffusion of volatile products away from the surfaoc
`
`First, -reactive atoms, molecules, and ions must be generated by elec-
`tron-m'olc'oule collisions. Because most of the reactant gases or vapors used
`for plasnlacnhanced etching and deposition do not spontaneously undergo
`reaction at the low temperatures involved, radicals or atoms must be formed
`so that heterogeneous chemical reactions can proceed at reasonable rates.
`The reactive species thus gencrotcd dilfuse to surfaces where they can adsorb
`onto a surface site. Sticking coefiicients are believed to be large for free
`radicals, such that chemisorption and surface. reactions -occur readily (23).
`Surface difiixsion of physically adsorbed species or volatile product molecules
`can occur.
`
`The nature of the primary reaction product differentiates pl_as_rna-en-
`hancod etching lrom deposition. In etching. the volatility ofreaction products
`
`Micron et al. Ex.1010 p.12
`
`

`
`334
`
`MIGHOELI-scmonlucs P'noc1:ssmo': Cnemicm. ENGINEERING A-srecrrs
`
`is crucial to film removal. Altrhougll the principal reaction product in dep-
`osition processes is not volatile, secondary products (e-.g.., hydrogen or halide
`molecules) must desorb to avoid incorporation into, and thus contamination
`of, the growing film. Complete elimination ofsuch contamination is. difficult,
`because particle bombardment of adsorbed species can assist incorporation.
`As. indicated previously,
`the chemical reactions taking place in glow
`discharges are exceedingly cornplex. However, two general types ofchem ical
`processes can be categorized; homogeneous gas-phase collisions and hot-
`erogeneous surface _inte_ractions._- To completely understand and characterize
`plasma processes.,
`the fundamental principles -of both processes must he
`understood.
`
`Homogeneous Processes. Homogeneous gas—phase collisions generate
`reactive -free radicals, metastable species, and ions. Therefore, chemical
`dissociation and ionization are independent of the thermodynamic temper-
`ature. Electron impact can result in a nmnber of different reactions de-
`pending upon the electron energy. The following list indicates these reaction
`types in order of increasing energgr requirement (24-26).
`
`I Extzitafion (rotational, vibrational. or electrorlicl
`
`e- + X2--> xi,‘ +- e
`
`0 Dissociative attachment
`
`e+x2-—rK‘+X* +62
`
`.0 D.issoc'iation
`
`e + X2 —-=* 2X + e
`
`.I Ionization
`
`e‘ + X3—+X2“ 4- 2e
`
`o Dissociative ionization
`
`e+X,;-—a~X* +X-I-2e.
`
`Excitation and dissociation processes can occur with mean electron ener-
`gies below a few electronvolts. Thus, the discharge is extremely elfective in
`producing large quantities of free radicals. Many of these species are gen.-
`erated by direct dissociation_._. although if "attachment of an electron to -a
`molecule results in the formation of a repulsive ex_cii:e'cl state, the molecule
`can dissociate by dissociative attachment. These attach1nent -processes are
`prevalent at low electron energies (<11 eV) when eleetronegative gases or
`vapors -ave used. By comparison, the ionization of many molecules or atoms
`requires energies greater than --8 eV, so that relatively few ions exist. The
`generation of reactive species is balanced by losses‘ due to recombination
`processes at surfaces (electrocles and chamber walls) -and in the gas. phase,
`along with dilfusion out of the plasma.
`
`Micron et al. Ex.1010 p.13
`
`

`
`8. Hlsss 5: Gllawns
`
`Plt1.snm-Enlmncezl Etching and Depiositlon
`
`3'35
`
`Electron-_impaot reactions occur at a rate (R) determined by the con-
`centrations of both electrons (21,) and a particular reactant (N) species (24).
`
`R = lm,N
`
`(2)
`
`The proportionality constant k is the rate cooficiellt, which can be expressed
`by
`
`in
`
`k = [
`
`“ Ellllu-.1
`
`(2elm)rr(e)f(£)d£
`
`(3.)
`
`where a and in are the impinging electron onergy and mass, respectively;
`o(c_)
`is the cross section for the specific reaction; and fie) is the. electron
`energy distribution ‘function. The limits of the integral run from the threshold
`energy for the impact reaction to infinity. If an accurate expression for fle)
`and electron collision cross sections for the variousgas-phase species present
`are known, It can be calculated. Unfortunately, such information is generally
`unavailable for many-of the mollccules used in plasma etching and deposition.
`Because of the highly noriequilibrium conditions experienced by else»
`trons in the plasma,
`almost never follows the Ma_xwell.—Boltzmann dise-
`t_1-ihution. In general, the distribution function is determined by the electric
`field that accelerates electrons: and collisions that cause electrons to change.
`energy. Very few direct measurements of have been made under con—
`ditions ofinterest to ‘plasma etching or deposition ;_ consequently, the current
`understanding offle) is limited, -at best. This. fact impedes the ability to make
`quautitat1've' predictions of elerctromimpact rates. As previously described,
`ionization due. to electron impact occurs through the action of the niost
`energetic electrons in the distribution. The number of electrons in the high-
`energy tail of the distribution that are capable of ionizing -neutral species in
`the discharge is considerably less than the number of e1ect_rons- capable of
`molecular dissociation. As a result, the degree of ionization is usually much
`less than the degree of' molecular dissociation.
`A second type of ho-mogemeous impact reaction is that occurring between
`the various heavy species generated by electron collisions, as well as between
`these species and um-eacted gas-phase molecules. (27. 28). Again. di'ssoc'iati'on
`and. ionization processes occur, but in addition, rocombi_na,iion and molecular
`rearrangements are prevalent. Particularly important ‘inelastic collisions are
`those called Penning processes (29). In these collisions, metastable species
`(spades in excite'd states where quantum ‘mechanical selection. rules forbid
`.tr3'.nsitiuu to: the ground -state and thus have long lifetimes") -collide with
`neutral species, transfer their excess energy, and thereby cause dissociation
`or ionization. These processes are particularly important with gases, such
`as argon and helium, that have available a number oflong-lifetime metastable:
`states. Furthermore, Peimingg ionization has a large‘ cross se'ct1'on, which
`enhances the probability of this process.
`
`Micron et al. Ex.1010 p.14
`
`

`
`338
`
`M1cnoE1.Ise'1‘i1oNIt:s Paocasslno: CHEMICAL ENG1N_EEii1N(3 A51»;-zms
`
`Heterogeneous Processes. A variety of heterogeneous processes. can
`occur at solid -su-rfaces exposed to -a glow discharge (28, 30-32).. The primary
`processes of interest in plasma etching and deposition are summarized in
`the following list (23). These. interactions result from the bombardment of
`surfilces by particles.
`
`1» Ion—surl'ace interactions
`
`1. Neutralization and secondary -electron emission
`
`2. Sputtering
`
`3. Ion-induced chemlistry
`
`u Electron-surface interactions
`
`1. Secondary electron emission
`
`2. Electron-induced chemistry
`
`I Radica'l— or atom—su1rface interactions
`
`1. Surface etching
`
`-2. Film deposition
`
`Although vacuum-UV photons and soft X-rays present in -the plasma are
`sufficiently energetic to break chemical bonds. electron and, particularly,
`ion bombardment.-s -are the most efeetive methods of promoting s-m-face
`reactions
`Several theoretical investigations (23, 34, 35) indicate: that nearly all
`incident ions will be neutralized within a few atomic radii of a surface,
`presumably because of electrons arising from Auger emission processes.
`These results suggest that the particles ultimately striking surfaces in contact
`with a glow “discharge are neutral species rather than ions. "To a first ap-
`proximation, elfects due to energetic ions -and neutral species. should be
`similar, provided that the particle energies are the same.
`Auger emission to neutralize incoming ions leaves the solid surface in
`an excited .state;. relaxation of the surface results in secondary electron gen-
`eration (23,
`Secondary electrons are ejected when high-energy ions,
`(electrons-. or neutral species strike the solid surface. ‘These electrons enhance
`the electron density in the plasma and can alter the plasma chemistry near
`a solid surface. Radiation ixnnpingement on a surface can induce a number
`of phenomena that depend upon the bombardment flux and energy.
`As noted previously (33), positive ions (or fast neutral species-) -are ex-
`tremely eficient in enhancing surface processes; thus this chapter will con-
`centrate on ion bombardment" efibcts. The various surface, tltin-film, and
`bulk phenomena alfected by bombarding species are indicated in Figure 3
`(36). The -specific processes taking. place -are designated -above the labeled
`abscissa in Figure 3. along -with the range of particle energies that cause
`such elfeelzs.
`
`Micron et