Microelectronics
`Processing
`
`Micron et al. Ex.1010 p.1
`
`

`
`ADVANCES IN CHEMISTRY SERIES
`
`Microelectronics Processing
`
`Chemical Engineering Aspects
`
`Dennis: W. Hess, EDITOR
`Unive-rsity of Caliform'.a—Berkeley
`
`Klavs JF. Iensen, EDITOR
`Univevsity of Minnesota
`
`
`
`American Chemical .SocieTy. Wcshlngion. DC 1989
`
`Micron et al. Ex.1010 p.2
`
`

`
`library of Congress Cataloging--in»Puhlication Data
`
`Microelectronics Processing: Chemical Engineering. Aspects
`Dennis W. Hess, editor, Klaus F‘. ]en:ien, editor
`1:.
`cm.—-(akdvanoos in chemistry series. ISSN 0065-2393.; 221)
`Bibliography: 1).
`
`Includes -index.
`
`_
`ISBN IJ-—t_i412—l_475—1_
`1- Microele-otronicn~Mntcrials:. 2. Integrated
`Circnils—Design and Construction. 3. Surface Chemistry-.
`
`I. Hess, Dennis W.
`111. Series
`
`'1‘K'3T874.M453s
`e21.381—dc19
`
`1989
`
`11. Iennen, Klavs 17., 1952-
`
`89-6862
`CIP
`
`Copyright @ 1989
`
`American Cheinieai Society
`
`All Rights Reserved. The appearance of the code at the bottom of the first page of each chapter
`in this volume iilriieates the oopyright owner's consent that reprographic copies of the chapter
`may he made for personai or internal use or for the personal or internal use ofspocilic clients.
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`does not extend to copying or transmission by any n1enns—grnphic or oIe'ctronic—i'or my other
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`a new collective work, for resale.. or for information storage-and retrieval systems. The copying‘
`Fee for ‘each chapter is indicated in the code at the bottom of the first-page of the chapter-
`
`The citation of trade names andlor narn-as of znonutiicturers in this publication is not to be
`construed 115- an endorsement or as approval by ACS of the commercial products or services
`referenced herein; nor should the more reference herein to any drawing, specification. chemical
`process, or other data he regardled as a license or as a conveyance of any right or permission
`to. the holder, render-, or any other person or corporation, to manufacture, reproduce, use... or
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`Registered names. trademarks. etc.. used in this publication. even without specific inclination
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`PRINTED IN THE UNITED" STATESI OF I\MEiiIC.*a
`
`Micron et al. Ex.1010 p.3
`
`

`
`CONTENTS
`
`ciucoohcttcooncuIOIIIuseunutvcvlullcuui-IUIIUIIIvutlououiullcII I I I I I l I Ilxiii
`
`l. Microelectronics Process:ing......................................... . . . . . .. 1
`Dennis W. Hess and Klavs F. Jcnsen
`
`2. Theory of Transport Processes in Semiconductor
`Crystal Growth from the Melt . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
`Robert A. Brown
`
`3. Liquid+-Phase Epitaxy and Phase Diagrams
`offlompound Semiconductors...........................................105
`Timothy J. Anderson
`
`.......
`4. Physical Vapor Deposition Reactors
`T. W. Fraser Russell, Bill N. Baron, Scott C. Jackson.
`and Richard E. Roche!-eau
`
`171
`
`5. Chemical Vapor Deposition .................................... ....... .. 199
`Klavs F. Jensen
`
`6. Diffusion and Oxidation of Silicon........ ....
`Richard B. Fair
`
`............. .....265
`
`7. ResistsinMicrolithography....................... .... ............ .... "325
`Michael]. O'Brien and David 5. some
`
`8.
`
`and DBp0Siti0fl.....u..au.o..nun-nu-uu.n377
`Dennis W. Hess and David B. Graves
`
`9. Interconnection and Packaging of High-Perform_anee
`Integrated Circuits . . . . .
`. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 441
`Ronald I. Jensen
`
`lfl. Semiconductor Processing Problems Solved by Wet
`(Solution) Chemistry ................. ... .............................. .. 505
`Marjorie K. Balazs
`
`INDEXES
`
`Author Index . . .
`
`.
`
`. . . .
`
`. . . .
`
`. . . . . .
`
`. .
`
`. . . .
`
`.
`
`.
`
`.
`
`.
`
`.
`
`. . . . . .
`
`. . . _
`
`. .
`
`. , . , . .
`
`. . . _ , , , _ , .523
`
`Affiliation Index . . . . .
`
`. . .. .
`
`. . . .
`
`.
`
`.
`
`.
`
`.
`
`.
`
`. . . . . . . .
`
`. . . . . . . . . .
`
`. .
`
`. . .
`
`.
`
`.
`
`. . . . .
`
`.
`
`. .
`
`.
`
`. .523
`
`S‘-1h!wctIndexIIIIIUIIIIIIIIIIIIIIIIIOHIIIII|IIIIIIOiI I I I I C1! O I
`
`I I k I I llsm
`
`xi.
`
`Micron et al. Ex.1010 p.4
`
`

`
`362
`
`M1caos.i.ec1'1_msIt:s Pnocsssmc: CHEMICAL E-wolunamno Asrecrs
`
`Resist Stripping. After the additive or subtractive 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 (l3fll{IBS, exposure to plasma, etc.) that results in cher:n—
`ical alteration and by the underlying substrate stability (176). Wet etches
`are either solvent-based or inorganic reagents such as H,SD,,, HN03 or
`H 20,. Solven't~type strippers are typically acetone For positive resists, triall-
`loroethylene for negative resists, or commercial products developed to re-
`move both types of resists. Commercial organic strippers were initially
`phenol—b_ased solvents but have been manufactured recently with little or
`no phenol as a result of health and safety issues associated with the use of
`this chemical. Plasma stripping or ashing of resist with either 0% or 03-CF;
`gases is clearly the method of choice from the standpoint of convenience,
`cost, and safety. However, the method cannot be used with substrates that
`are etched by these plasmas.
`
`In addition to the standard process steps,
`Auxiliary Process Steps.
`auxiliary processes are sometimes necessary. These steps are not used for
`all situations but as required and will be considered in this section separately.
`Certain semiconductor—ma.nufactu'ri'ng processes are particularly damaging
`to polymeric films and require an additional step to harden the resist. For
`example, Al etching with chlorine -plasma produces AICI3. which degrades
`resists. Ion implantation, in which the chamber temperature and, hence,
`the water temperature increase with increasing implant dose, causes thermal
`deformation of the resist image. One commonly used method to stabilize
`novolac-based resists is deep-UV flood exposure after patterning (177). With
`deep~UV exposure, cross-linking of the polymer surface produces afilm with
`increased thermal resistance. With this procedure, positive resists can with-
`stand a 180 “C bake for 30 min. Fluorocarbon plasma treatment also stabilizes
`resists (I 78), because fluorine insertion impedes subsequent oxidation of the
`polymers.
`As discussed previously, an optional postexposure, preclevelopment
`bake can reduce problems with the standing-wave elfect in DNQ—novolac
`positive resists. However, such a postexposure bake step is indispensable
`in the image reversal of positive resists (37-41) and certain resists based on
`chemical amplification of a photogenerated catalyst (64-6?, 77. 78). For both
`types of resists, the chemistry that diferentiates between exposed and unex-
`posed areas does not occur" solely during irradiation. Instead, diiferentiatiou
`occurs predominantly during a subsequent bake. Therefore, to obtain ac-
`ceptable CD control in these systems, the bake conditions must be carefully
`optimized and monitored.
`
`Rework. Masking steps frequently have the advantage over other [C-
`manufacturing processes of being able to undergo wafer rework. Rework
`
`Micron et al. Ex.l010 p.5
`
`

`
`Plasma-Enhanced Etching
`
`and Deposition
`
`Dennis W. Hess and David B. Graves
`
`Department of Chemical Engineering, University of California, Berkeley,
`CA 94720
`
`Chemical and chemical engineering p-rinciples involved in plasma-
`enhanced etching -and disposition are moiewed, modeling approaches
`to describe and predict plasma behavior are indicated, and specific
`snzainples ufgnlannai-cnhtmccd etching and deposition of thin-film ma-
`terials of interest to H113 fabrication of nzicraelecironic and optical
`denices are discussed.
`
`THE INCREASING COMPLEXITY on S,0l..1D—STATE -electronic and optical de-
`vices places stringent demands upon the control of thin-film processes. For
`cxample, as device geometries drop below the lvpm level, previously stan-
`dard processing techniques for thin~film etching. and deposition become
`inadequate. For etching, the control of film etch rate, uniformity, and se-
`lectivity is no longer sulfi-ciernt; the establisliment of film cross sections or
`profiles is crucial to achieving overall reliability and higlrclonsity circuits.
`Low-temperature deposition. methods are required to ininimize defect for-
`mation -and solid-state dilfiision and to be compatible with low-n'1elting—point
`substrates or films. Therefore, the established techniques of liquid -etching
`and, to some extent, c-hcmioal -vapor deposition (CVD) are being replaced
`by plasma—assisteI:l methods. Plasmasassisted etching and plasnia-enhanced
`CVD (PECVD) take advantage of the high—ene‘1'gy electrons present in glow
`discharges to dissociate and ionize -gaseous molecules to form chemically
`reactive radicals and ions. Because thermal energy is not needed to break
`chemical l)0l!(lS-, reactions can be promoted at low temperatures ((200 °C).
`Although the chemistry and physics of a glow discharge are extraordi-
`narily complex. the plasma performs only two basic Functions. First. reactive
`
`0ll‘55—2393l39al022l.—0377$15.60:"0
`© 1989 American Chemical Society
`
`Micron et al. Ex.1010 p.6
`
`

`
`378
`
`MlCR()EL.EC'I'RONICS Pnocnssmc: CHEMICAL ENGINEERING AsPt<:c'rs
`
`chemical species are generated by electron-impact collisions; thus they over-
`-come kinetic limitations. that may ‘exist in thermally activated processes.
`Second, the discharge supplies energetic radiation (e.g. , positive ions, neu-
`tral species, metastable species, electrons, and photons) that bombard sur-
`faces immersed in the plasma and thus alter the surface 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 profilles, and material properties unattainable with
`either process individually.
`
`Dry Processing
`
`Liquid etching has been the. preferred method for pattern delineation for
`thin Films for many years (if). 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 essentially infinite with the proper choice of ctchant so-
`lution.
`Despite these advantages, several critical problems arise for micrometer
`and. submicrorneter pattern sizes. Resist materials often lose adhesion in the
`acid solutions used tin‘ most etch processes and thereby alter pattern di-
`mensions and preventbline 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, over-etching is required
`to ensure complete film removal. Overetching 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 micrometer and submicrometer
`patterns, the surface tension of etch solutions can cause the liquid to bridge
`the space between resist stripes. Because the etch solution does not contact
`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-
`lilm materials (7-9). 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
`
`

`
`8. HESS 6: Gmwr-:3 Plasma-Enhanced Etching and Deposition
`
`379
`
`Musk
`
`Film 10 be etched
`
`Substrate
`Etching
`
`Isotropic Profile
`No Overeich
`
`Liquid or Plasma
`Etching
`
`Vertical Profile
`
`Plasma or Dry
`
`Figure 1. Cross sections offilms etched with liquid or plasma eta-hunts. The
`isotropic profile is the result qfzero ooeretoh and can be generated with liquid
`or plasma etch techniques. The anisotmpic: (vertical) profile requires plasma
`or dry-etch processes. W is the width of the resist pattern. (Reproducedfrom
`reference 2. Copgnrlglll 1983 American Chemical Society.)
`
`However, deposition rates are often low with CVD, and the presence of
`temporature—sensitive substrates or films (e.g., polymers or low—melting'-
`point metals) prior to depos:ition, along with the possibility of generating
`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 thermal means to supply the energy for bond
`breaking (10-14).
`
`if Glow Discharges
`
`or plasmas used for plasma
`The rf (rad-‘io {'1-oquency) glow discharges
`etching or I’-ECVD are partially ionized gases composed of ions, electrons,
`and a 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 n, and average electron energy kT,,. A quantity that is useful in
`
`Micron et al. Ex.1010 p.8
`
`

`
`380
`
`M1£.:HoELEL.‘raL-mics Piiocassznc-; Cmsmciu. ENC1N.I-!'.E1llNC As:-ems
`
`characterizing the average electron energy is the ratio of the electric field
`to the pressure, E/p
`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 collisions, so that an
`increase. in pressure, which decreases the electron mean free path, decreases
`the .elect'ron energy.
`In thin-film processes for the fab-rication of electronic materials and
`devices, rf glow discharges are primarily used. The application of an rf voltage
`at frequencies between 50 kHz and 40 MHz to a low—pre_ssure (6—600 Pa)
`gas results in a_ c'hemical'ly unique environment (Table 1.)
`Electron densities (rte) and, because the plasma is electrically neutral,
`positive-ion densities (11,) range from 10“ to 10"/crn“. However, the ratio
`of the neutral-species density" (n N) to the electron density is usually greater
`than 103, 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 concentrations.
`The glow discharges described by Table I are termed nonequilibriurn plas-
`mas, because the average electron energy (lcT,.) is considerably higher than
`the ion energy (k'I‘,). Thercfbrc, the discharge cannot be described “ade-
`quately by a single temperature.
`
`Physical and Electrical Characteristics. The electrical potentials
`established in the reaction chamber -determine the energy‘ of ions and elec-
`trons strilcing the surfaces. immersed in a discharge. Etching and deposition
`of thin films are usually perfiarmcd in a capacitivcly coupled parallel—plate
`rf reactor (see Plasma Reactors). Therefore, the following -discussion will be
`directed toward this configuration.
`The important potentials in rt 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 externally biased
`or grounded and thus draws no net cnrren t), and the potential of the powered
`or externally biased electrode. When the plasma contacts a surihce, that
`surface, even if grounded, is usually at a negative potential with respect to
`the plasma (16, I8, 19). Therefore, positive-ion bombardment occurs. The
`energy of the bombarding ions is established by the dilferenoe in potential
`
`Table I. Properties of rf Glow Discharges (Plasmas)
`Used for Thiln-Film Etching and Deposition
`
`Parrnneter
`11,. = n.
`nu
`kT.,
`HI":
`
`_
`
`Value
`10“-—1lJ”‘lc1n"
`~10 ”‘-10"‘/cm”
`I-10 CV
`"'0.ll4 (EV
`
`Micron et al. Ex.1010 p.9
`
`

`
`8. H355 or Cnavss Plasma-Enhanced Etching and Deposition
`
`381
`
`between the plasma and the surface that the ion strikes, 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 broken, and in certain instances, sputtering of film
`or electrode material may’ occur (.16).
`The reason for the diifenent potentials within a plasma system becomes
`obvious when electron and ion mobilities are considered (I-9:1). Imagine
`applying an rt licld between two plates (electrodes) positioned within a low-
`pressure gas. On the first liall’-cycle of the field, one electrode is negative
`and attracts positive ions; the other electrode is positive and attracts elec-
`trons. Because ofthe frequencies used and because the mobility of electrons
`is considerably greater than that ofpositive ions, the flux (current) ofelectrons.
`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-, a large flux of electrons flows to the electrode
`that previously received the small flux of ions. Because plasma-etching sys-
`tems generally have a dielectric coating on the electrodes or a series (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. electrodes and on other surfaces in contact with
`
`the plasrna, and so electrons are repelled and positive ions are attracted to
`the surface. This transient situation ceases when a sufllcient negative bias is
`achieved on the electrodes such that the fluxes of electrons and positive ions
`striking these surfaces are equal. At this point, tiine-average (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 electrons compared
`with posi'ti've ions. When there are many more negative ions than electrons
`in the plasma (c.g., in highly electronegative gases), plasma potentials are
`below electrode potentials, at least during part of the if cycle (1912).
`The plasma potential is nearly uniform throughout the observed glow
`volume in an rf discliarge, although a small electric field directed from the
`discharge toward the edge of 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. This region is called a sheath or
`dark space and can he liltencd 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
`Hold. The ions are then accelerated across the potential drop and strike the
`elcctrode or substrate surface-. Because ofthe 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
`
`

`
`382
`
`NIIUROE-LEUTHONIUS Psoosssmo: CHEMICAL ENUINIH-JBlNl’.' ASPli2C'l'S
`
`(20). The theoretical dependence is given by equation 1, wliere V is the
`voltage and A is the electrode: area (20).
`
`V1 [V2 = (A21A1)‘
`
`(1)
`
`If V, is the voltage on the powered electrode and V2 is the voltage on
`the grounded 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 from theory is in part due to the reactor
`configuration. Although the physical electrodes in a plasma reactor often
`have the same area, As represents the grounded electrode -area, that is, the
`area ofall grounded surfaces in Contact with the plasma. Because this area
`usually includes the chamber walls, the area ratio can be quite large. Because
`of such considerations, 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
`he used to design electrode areas for reactors such that a particular voltage
`can he established on an electrode surface.
`In addition to the_ ratio of electrode areas, other plasma parameters can
`
`-I-V
`
`
`
`Powered
`Erecirode
`
`G-round
`
`Figure 2. Potential distributicm in a parallel-plate plasnm -ctcher -with the
`grounded suirfaoe area larger than the powered electrode area. V is the po-
`tential, and V, is the plasma potential. (Reproduced with permission. from
`reference 16. Copyright 1979 The Electrochemical Society. Inc.)
`
`Micron et al. Ex.1010 p.11
`
`

`
`8. Bass 5: GRAVES Plasnm-Enhanced Etching and Deposition
`
`383
`
`alfect the. electrical characteristics of the discharge. Varying the rf power
`input will alter plasma and electrode potentials, as well as ion concentrations,
`and thereby change ion energies and fluxes. Also, radio frequency altects
`the kinetic energy of ions that strike surfaces in contact with the plasma.
`This elect can be readily undlerstood by considering the behavior of an ion
`experiencing an oscillating plasma potential caused by applied rf voltages
`(21, 22). Depending upon the ion mobility, some frequency exists -above
`which the ion can no longer follow the alternating voltage. Therefore, the
`ion cannot traverse the sheath in one half~cycle. Above this frequency. ions
`experience an accelerating fiield (the dilierence between the plasma and
`electrode potentials divided by the sheath thickness) that is an average over
`a number of half-cycles. At lower frequencies, where the ions can respond
`directly to the oscillating ficlél, 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 deposition processes
`are merely chemical reactions that yield a volatile or in-volatile product,
`respectively, the overall process can be broken down. into the following six
`primary steps:
`
`1. Generation of reactive species
`
`2. Dilfusion to the surface-
`
`3. Adsorption
`
`4. Reaction
`
`5. Desorption of volatile products
`
`6. Diffusion of volatile products away from the surface
`
`First, reactive atoms, molecules, and ions must be generated by elec-
`tron—molecule collisions. Because most of the reactant gases or vapors used
`for plasma-enhanced etching and deposition do not spontaneously undergo
`reaction at the low temperatures involved, radicals or atoms must he formed
`so that heterogeneous chcinical reactions can proceed at reasonable rates.
`The reactive species thus generated dilfuse to surlhces-where they can adsorb
`onto a surface site. Sticking coelhcients are believed to "be large for free
`radicals, such that chemisorption and surface reactions occur readily (23).
`Surface dillusion ofphysioally adsorbed species or volatile product molecules
`can occur.
`
`The nature of the prirnary reaction product differentiates plasma-em
`hanced etching from deposition. In etching, the volatility of reaction products
`
`Micron et al. Ex.1010 p.12
`
`

`
`334
`
`MICRDELEL‘I'fl0N'IiGS Pnoccssme: CHEMICAL ENGINEERING ASPECTS
`
`is crucial to film removal. Althouggh the principal reaction product in dep-
`osition processes is not volatile, secondary products (e._g. , hydrogen or halide
`molecules) must desorh to avoid incorporation into, and thus contamination
`of, the growing film. Cornplebe elimination of such contamination is-diiiicult,
`because particle bombardment of adsorbed species can assist incorporation.
`As indicated previously, the chemical reactions taking place in glow
`discharges are exceedingly complex. However, two general types ofchemical
`processes can be categorized: homogeneous gas—p'ha.se collisions and het-
`erogeneous surface interacticns. To completely understand and characterize
`plasma processes,
`the fundamental principles of both processes must be
`understood.
`
`Homogeneous Processes. Homogeneous gas—ph_ase collisions generate
`reactive -free radicals, metastable species, and ions. Therefore, chemical
`dissociation and ionization are independent of the thermodynélmic temper-
`ature. Electron impact can result in a number of" diilerent reactions de-
`pending upon the electron energy. The following list indicates these reaction
`types in order of increasing energy requirement (24-25).
`
`- Excitation (rotational. vibrational. or electronic)
`
`e + X34 X2* ~l-- e
`
`0 Dissociative attachment
`
`e+J.{2->X‘+X* +e
`
`an Dissociation
`
`c + X3-+ 2X + e
`
`in Ionization
`
`c‘ + X3—+X,‘" + 2c
`
`I Dissociative ionization
`
`e+x2—->X*+X-I-2e
`
`Excitation and dissociation processes can occur with mean electron ener-
`gies below a few electronvolts. Thus, the discharge is extremely eilective 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 oi" a repulsive excited state, the molecule
`can dissociate by dissociative attachment. These attachment processes are
`prevalent at low electron energies (<1 -eV) when electronegative gases or
`vapors are 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 (electrodes and chamber walls) and in the gas. phase,
`along with diifusion out of the plasma.
`
`Micron et al. Ex.1010 p.13
`
`

`
`8. HESS {St GRAVES
`
`i’lc.snm-Enzhcncecl Etching and Dciiositlon
`
`335
`
`Electron-impact reactions occur at a rate (R) determined by the con-
`centrations of both electrons (n,.) and a particular reactant (N) species (24).
`
`R = mm
`
`(2)
`
`The proportionality constant k is the rate cocliicient, which can be expressed
`by
`
`5)
`
`k = [
`
`" Elllws
`
`(2efm)rr[e)f(£)d£
`
`(3)
`
`where e and m. are the impinging electron energy and mass, respectively;
`o'(e) is the cross section for the specific reaction; and fle) 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 fbr fle)
`and electroiicollision cross sections for the various gas-phase species present
`are lcnown, k can be calculated. Unfortunately, such information is generally
`unavailable for many of the molecules used in plasma etching and deposition.
`Because of the highly nonequilil:vr.iur_n conditions experienced by elec~
`trons in the plasma, flc) almost never follows the Maxwell-.—Boltzmann dis.-
`tributicn. 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 offie) is limited, at best. This fact impedes the ability to make
`quantitative" predictions of elE:otron~impact rates. As previously described,
`ionization due. to electron impact occurs through the action of the most
`energetic electrons in the distribution. The number of electrons in the high-
`encrgy tail of the distribution that are capable of ionizing neutral species in
`the discharge is considerably less than the number of electrons 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 homogeneous impact reaction is that occurring between
`the various heavy speciesgenesrated by "electron collisions, as well as between
`these species and um-eacted gas-phase molecules (2.7. 28). Again, dissociation
`and ionization processes occur, but in addition, recombination and molecular
`rearrangements are prevalent. Particularly important inelastic collisions are
`those called Penning processes (29). In these collisions, metastable species
`(species in excited states where quantum mechanical selection rules forbid
`transition 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 oi'long-.li‘fetime metastable
`states. Furthermore. Penning; ionization has a large cross section, which
`enhances the probability of this process.
`
`Micron et al. Ex.1010 p.14
`
`

`
`385
`
`Mlcnoemmntauics Pnooessmo: CHEMICAL ENGINEERING ASPEC'1S
`
`Heterogeneous Processes. A variety of heterogeneous processes can
`occur at solid surfaces 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 bomharclxnent of
`surfaces by particles.
`
`u lon—surface- interactions
`
`1. Neutralization and secondary electron _emission
`
`2. Sputtering
`
`3. Ion-induced chemistry
`
`u Electron—surface interactions
`
`1. Secondary electron emission
`
`2. Electron-induced chemistry
`
`I I{adica'l— or atom—surface interactions
`
`1. Surface etching
`
`-2. Film deposition
`
`Although vacuum-UV photons and soft X-rays present in the plasma are
`sulliciently energetic to break chemical bonds, electron and, particularly,
`ion bombardments are the most eifiactive methods of promoting surface
`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 1'esults'sI.lggest that the particles ultimately striking surfaces in contact
`with a glow discharge are neutral species rather than ions. To a first ap-
`proximation, effects 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, 24). 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 impingement 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 efioient in enhancing surface processes; thus this chapter will con-
`centrate on ion bombardment efiects. The various surface,
`thin—film, and
`bull: phenomena afected 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 eifects.
`
`Micron et al. Ex.1010 p.15
`
`

`
`8. Hess or GRAVES Plasma-Enlmnced Etching and Deposition
`
`387
`
`I022
`
`
`
`
`
`%O-
`
`
`
`-..----—--n
`
`Ion Beam
`Modification
`
`(Implofntotion)
`.ssss..
`
`Chemistry
`
`20
`IO
`
`. {B
`I0
`
`a