`

`

`

`• Library of Congress Cataloging-in-Publication Data
`
`Microelectronics Processing: Chemical Engineering Aspects
`Dennis W. Hess, editor, Klavs F. Jensen, editor
`cm.-(Advances in chemistry series, ISSN 0065-2393; 221)
`p.
`Bibliography: p.
`Includes index.
`ISBN 0- 8412-1475- 1
`l. Microelectronics-Materials. 2. Integrated
`Circuits- Design and Construction. 3. Surface Chemistry.
`I. Hess, Dennis W.
`II. Jensen, Klavs F., 1952-
`III. Series
`TK7874. M4835 1989
`621.381- dcl9
`
`89-6862
`CIP
`
`Copyright © 1989
`American Chemical Society
`All Rights Reserved. The appearance of the code at the bottom of the first page of each chapter
`in this volume indicates the copyright owner's consent that reprographic copies of the chapter
`may be made for personal or internal use or for the personal or internal use of specific clients.
`This consent is given on the condition, however, that the copier pay the stated per-copy fee
`through the Copyright Clearance Center, Inc., 27 Congress Street, Salem, MA 01970, for
`copying beyond that permitted by Sections 107 or 108 of the U.S. Copyright Law. This consent
`does not extend to copying or transmission by any means-graphic or electronic-
`for any other
`purpose, such as for general distribution, for advertising or promotional purposes, for creating
`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 and/ or names of manufacturers in this publication is not to be
`construed as an endorsement or as approval by ACS of the commercial products or services
`referenced herein; nor should the mere reference herein to any drawing, specification, chemical
`process, or other data be regarded as a license or as a conveyance of any right or permission
`to the holder, reader, or any other person or corporation, to manufacture, reproduce, use, or
`sell any patented invention or copyrighted work that may in any way be related thereto.
`Registered names, trademarks, etc., used in this publication, even without specific indication
`thereof, are not to be considered unprotected by law.
`
`PRINTED IN THE UNITED STATES OF AM ERICA
`
`Page 3 of 69
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`CONTENTS
`
`Pref ace ....................................................................... xiii
`
`1. Microelectronics Processing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I
`Dennis W. Hess and Klavs F. Jensen
`
`2. Theory of Transport Processes in Semiconductor
`Crystal Growth from the Melt ........................................... 35
`Robert A. Brown
`
`3. Liquid-Phase Epitaxy and Phase Diagrams
`of Compound Semiconductors ................ . .......................... 105
`Timothy J. Anderson
`
`4. Physical Vapor Deposition Reactors ...................... . . .. ........... 171
`T. W. Fraser Russell, Bill N. Baron, Scott C. Jackson,
`and Richard E. Rocheleau
`
`5. Chemical Vapor Deposition .................. . ..................... . .... 199
`Klavs F. Jensen
`
`6. Diffusion and Oxidation of Silicon ....................................... 265
`Richard B. Fair
`
`7. Resists in Microlithography ... .. ............. . .......................... 325
`Michael J. O'Brien and David S. Soane
`
`8. Plasma-Enhanced Etching and Deposition ............................... 377
`Dennis W. Hess and David B. Graves
`
`9. Interconnection and Packaging of High-Performance
`Integrated Circuits ...................................................... 441
`Ronald J. Je nsen
`
`10. Semiconductor Processing Problems Solved by Wet
`(Solution) Chemistry ..... .. ..................... ... .. . .................. 505
`Marjorie K. Balazs
`
`INDEXES
`
`Author Index . .............. .... .... . ............... ....... ....... . . . . 523
`Affiliation Index .......... . ........................................... 523
`
`Subject Index ................................................ .. ...... 523
`
`xi
`
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`8
`
`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 principles involved in plasma(cid:173)
`enhanced etching and deposition are reviewed, modeling approaches
`to describe and predict plasma behavior are indicated, and specific
`examples of plasma-enhanced etching and deposition of thin-film ma(cid:173)
`terials of interest to the fabrication of microelectronic and optical
`devices are discussed.
`
`THE INCREASING COMPLEXITY OF SOLID-STATE electronic and optical de(cid:173)
`
`vices places stringent demands upon the control of thin-film processes. For
`example, as device geometries drop below the 1-µ.m level, previously stan(cid:173)
`dard processing techniques for thin-film etching and deposition become
`inadequate. For etching, the control of film etch rate, uniformity, and se(cid:173)
`lectivity is no longer sufficient; the establishment of film cross sections or
`profiles is crucial to achieving overall reliability and high-density circuits.
`Low-temperature deposition methods are required to minimize defect for(cid:173)
`mation and solid-state diffusion and to be compatible with low-melting-point
`substrates or films. Therefore, the established techniques of liquid etching
`and, to some extent, chemical vapor deposition (CVD) are being replaced
`by plasma-assisted methods. Plasma-assisted etching and plasma-enhanced
`CVD (PECVD) take advantage of the high-energy 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 bonds, reactions can be promoted at low temperatures (<200 °C).
`Although the chemistry and physics of a glow discharge are extraordi(cid:173)
`narily complex, the plasma performs only two basic functions. First, reactive
`
`0065-2393/89/0221- 0377$15.60/0
`© 1989 American Chemical Society
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`378
`
`MICROELECTRONICS PROCESSING: CHEMICAL ENGINEERING ASPECTS
`
`chemical species are generated by electron-impact collisions; thus they over(cid:173)
`come kinetic limitations that may exist in thermally activated processes.
`Second, the discharge supplies energetic radiation (e.g., positive ions, neu(cid:173)
`tral species, metastable species, electrons, and photons) that bombard sur(cid:173)
`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 profiles, 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 y~ars (1). Its pervasive use has been due primarily to
`two considerations. First, although the exact chemistry is often poorly under(cid:173)
`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 etchant so-
`lution.
`.
`Despite these advantages, several critical problems arise for micrometer
`and submicrometer pattern sizes. Resist materials often lose adhesion in the
`acid solutions used fo,r mos.t etch processes and thereby alter pattern di(cid:173)
`mensions and prevent'Bne 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, overetching is required
`to ensure complete film removal. Overetching generates a decrease in pat(cid:173)
`tern size because of the continued lateral etching and thus affects 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. Undercutting can be controlled by varying the plasma chemistry,
`gas pressure, and electrode potentials (2-6) and thereby generate directional
`or anisotropic proµIes.
`Numerous techniques have been developed for the formation of thin(cid:173)
`film materials (7-9). Because of the versatility and throughput capability of
`CVD, this method has gained wide acceptance for a variety of film materials.
`
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`380
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`MlCHOELECTRONICS PROCESSING: CHEMICAL ENGINEERING ASPECTS
`
`characterizing the average electron energy is the ratio of the electric field
`to the pressure, Elp (15). As the electric field strength increases, free elec(cid:173)
`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 electron energy.
`In thin-film processes for the fabrication 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-pressure (6- 600 Pa)
`gas results in a chemically unique environment (Table I. )
`Electron densities (ne) and, because the plasma is electrically neutral,
`positive-ion densities (ni) range from 108 to 10 12/cm 3
`. However, the ratio
`of the neutral-species density (nN) 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 nonequilibrium plas(cid:173)
`mas, because the average electron energy (kT e) is considerably higher than
`the ion energy (kTJ Therefore, the discharge cannot be described ade(cid:173)
`quately by a single temperature.
`
`Physical and E1e.ctrical Characteristics. The electrical potentials
`established in the reaction chamber determine the energy of ions and elec(cid:173)
`trons striking the surfaces immersed in a discharge. Etching and deposition
`of thin films are usually performed in a capacitively coupled parallel-plate
`if reactor (see Plasma Reactors). Therefore, the following discussion will be
`directed toward this configuration.
`The important potentials in rf glow discharge systems (16, 17) are the
`plasma potential (potential of the glow region), the floating potential (po(cid:173)
`tential assumed by a surface within the plasma that is not externally biased
`or grounded and thus draws no net 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 (16, 18, 19). Therefore, positive-ion bombardment occurs. The
`energy of the bombarding ions is established by the difference in potential
`
`Table I. Properties of rf Glow Discharges (Plasmas)
`Used for Thin-Film Etching and Deposition
`Parameter
`Value
`ne = nj
`108- 10 12/cm 3
`- 10 15- 10 16/cm 3
`nN
`1- 10 eV
`kT.
`- 0.04 eV
`kTj
`
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`8. HESS & GRAVES 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(cid:173)
`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 different potentials within a plasma system becomes
`obvious when electron and ion mobilities are considered (19a). Imagine
`applying an rf field between two plates (electrodes) positioned within a low(cid:173)
`pressure gas. On the first half-cycle .of the field, one electrode is negative
`and attracts positive ions; the other electrode is positive and attracts elec(cid:173)
`trons. Because of the frequencies used and because the mobility of electrons
`is considerably greater than that of positive ions, the flux (current) of electrons
`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(cid:173)
`tems generally have a dielectric coating on the electrodes or a series (block(cid:173)
`ing) capacitor between the power supply and the electrode, no direct current
`(de) 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 plasma, and so electrons are repelled and positive ions are attracted to
`the surface. This transient situation ceases when a sufficient negative bias is
`achieved on the electrodes such that the fluxes of electrons and positive ions
`striking these surfaces are equal. At this point, time-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 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 rf cycle (19b).
`The plasma potential is nearly uniform throughout the observed glow
`volume in an rf discharge, 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 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 of the 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
`
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`382
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`MICROELECTRONICS PROCESSING: CHEMICAL ENGINEERING ASPECTS
`
`(20). The theoretical dependence is given by equation 1, where V is the
`voltage and A is the electrode area (20).
`
`(1)
`
`If V 1 is the voltage on the powered electrode and V 2 is the voltage on
`the grounded electrode, then 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, A 2 represents the grounded electrode area, that is, the
`area of all 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(cid:173)
`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 such that a particular voltage
`can be established on an electrode surface.
`In addition to the1 r~tio of electrode areas, other plasma parameters can
`
`+v
`
`0
`
`- v
`
`Powered
`Efectrode
`
`Ground
`
`Figure 2. Potential distribution in a parallel-plate plasma etcher with the
`grounded surf ace area larger than the powered electrode area. V is the po(cid:173)
`tential, and VP is the plasma potential. (Reproduced with permission from
`reference 16. Copyright 1979 The Electrochemical Society, Inc.)
`
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`8. HESS & GRAVES Plasma-Enhanced Etching and Deposition
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`383
`
`a[ect 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 aH'ects
`the kinetic energy of ions that strike smfaces in contact with the plasma.
`This e[ect can be readily understood 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 field (the diH'erence 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 field, they are accelerated by instantaneous fields.
`Thus, the ions can attain the maximum energy corresponding to the maxi(cid:173)
`mum instantaneous field across the sheath. As a result, for a constant sheath
`potential, ion bombardment energies and fluxes are higher at lower fre(cid:173)
`quencies.
`
`Chemical Characteristics. Because etching or deposition processes
`are merely chemical reactions that yield a volatile or involatile product,
`respectively, the overall process can be broken down into the following six
`primary steps:
`
`1. Generation of reactive species
`2. Di[usion 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(cid:173)
`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 be formed
`so that heterogeneous chemical reactions can proceed at reasonable rates.
`The reactive species thus generated diffuse to surfaces where they can adsorb
`onto a surface site. Sticking coefficients are believed to be large for free
`radicals, such that chemisorption and surface reactions occur readily (23).
`Surface diffusion of physically adsorbed species or volatile product molecules
`can occur.
`The nature of the primary reaction product differentiates plasma-en(cid:173)
`hanced etching from deposition. In etching, the volatility of reaction products
`
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`384
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`MICROELECTRONICS PROCESSING: CHEMICAL E NGINEERING ASPECTS
`
`is crucial to film removal. Although the principal reaction product in dep(cid:173)
`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 of such 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 complex. However, two general types of chemical
`processes can be categorized: homogeneous gas-phase collisions and het(cid:173)
`erogeneous surface interactions. To completely understand and characterize
`plasma processes, the fundamental principles of both processes must be
`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(cid:173)
`ature. Electron impact can result in a number of different reactions de(cid:173)
`pending upon the electron energy. The following list indicates these reaction
`types in order of increasing energy requirement (24- 26).
`
`• Excitation (rotational, vibrational, or electronic)
`e + X 2 ~ X 2* + e
`• Dissociative attachment
`e + X2 ~ x- + X + + e
`• Dissociation
`e + X 2 ~ 2X + e
`• Ionization
`e + X 2 ~ X 2 + + 2e
`• Dissociative ionization
`e + X 2 ~ X + + X + 2e
`
`Excitation and dissociation processes can occur with mean electron ener(cid:173)
`gies below a few electronvolts. Thus, the discharge is extremely effective in
`producing large quantities of free radicals. Many of these species are gen(cid:173)
`erated by direct dissociation, although if attachment of an electron to a
`molecule results in the formation of a repulsive excited state, the molecule
`can dissociate by dissociative attachment. These attachment processes are
`prevalent at low electron energies (<l 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 smfaces (electrodes and chamber walls) and in the gas phase,
`along with diffusion out of the plasma.
`
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`8. HESS & GRAVES Plasma-Enhanced Etching and Deposition
`
`385
`
`Electron-impact reactions occur at a rate (R) determined by the con(cid:173)
`centrations of both electrons (nJ and a particular reactant (N) species (24).
`
`(2)
`
`The proportionality constant k is the rate coefficient, which can be expressed
`by
`
`k = f"'
`
`Ethr«.'s
`
`(2E/m)cr(E)f(E)de
`
`(3)
`
`where e and m are the impinging electron energy and mass, respectively;
`cr(e) is the cross section for the specific reaction; and fi.e) 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 fi.e)
`and electron collision cross sections for the various gas-phase species present
`are known, 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 nonequilibrium conditions experienced by elec(cid:173)
`trons in the plasma, fie) almost never follows the Maxwell-Boltzmann dis(cid:173)
`tribution. 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 f(e) have been made under con(cid:173)
`ditions of interest to plasma etching or deposition; consequently, the current
`understanding offiE) is limited, at best. This fact impedes the ability to make
`quantitative predictions of electron-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(cid:173)
`energy 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 species generated by electron collisions, as well as between
`these species and unreacted gas-phase molecules (27, 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 oflong-lifetime metastable
`states. Furthermore, Penning ionization has a large cross section, which
`enhances the probability of this process.
`
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`386
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`MICROELECTRONICS PROCESSING: CHEMICAL ENGIN EERING ASPECTS
`
`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 bombardment of
`surfaces by particles.
`
`• Ion-surface interactions
`1. Neutralization and secondary electron emission
`2. Sputtering
`3. Ion-induced chemistry
`• Electron-surface interactions
`1. Secondary electron emission
`2. Electron-induced chemistry
`• Radical- or atom-surface 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 bombardments 'are the most effective methods of promoting surface
`reactions (33).
`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(cid:173)
`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(cid:173)
`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(cid:173)
`tremely efficient in enhancing surface processes; thus this chapter will con(cid:173)
`centrate on ion bombardment effects. The various surface, thin-film, and
`bulk phenomena affected by born barding 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 effects.
`
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`8. HESS & GRAVES Plasma-Enhanced Etching and Deposition
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`389
`
`should be prevalent with particle energies above 20 eV. However, the various
`effects of ion bombardment (Figure 3) on the primary processes occurring
`during growth are extremely difficult to separate (32). Furthermore, although
`the basic plasma chemistry, physics, and synergistic effects for both etching
`and deposition are analogous, PECVD introduces one additional compli(cid:173)
`cation: Film bonding configurations must be controlled if films with specified
`and reproducible properties are to be formed.
`The previous discussions indicate that a fundamental understanding of
`gas-phase plasma chemistry and physics, along with surface chemistry mod(cid:173)
`ified by radiation effects, is needed in order to define film etch and growth
`mechanisms. These phenomena ultimately establish etch rates and profiles"
`as well as film deposition rates and properties. The complex interactions
`involved in PECVD are outlined in Figure 5 (14, 40). If the basic or micro(cid:173)
`scopic plasma parameters (neutral-species, ion, and electron densities; elec(cid:173)
`tron energy distribution; and residence time) can be controlled, the gas(cid:173)
`phase chemistry can be defined. Many macroscopic plasma variables (gas
`flow, discharge gas, pumping speed, rf power, frequency, etc.) can be
`changed to alter the basic plasma conditions. However, the precise manner
`in which a change in any of these variables affects basic plasma parameters
`is currently unknown.
`The variation of a macroscopic variable usually results in a change in
`two or more basic gas-phase parameters, as well as surface potential, particle
`flux, and surface temperature. For instance, rf power determines the current
`and voltage between the electrodes in a parallel-plate plasma reactor. Varying
`the rf frequency changes the number and energy of ions (because of mobility
`considerations) that can follow the alternating field; thus, bombardment flux
`and energy are affected. The gas flow rate, the pump speed, and the pressure
`are interrelated, and two ways of changing the gas pressure can be envi(cid:173)
`sioned. The gas flow rate can be varied at constant pump speed, or the
`pump speed can be varied (by throttling the pump) at constant gas flow rate.
`These two methods of pressure variation yield different residence times for
`the chemical species in the reactor, so that the precise chemistry is altered.
`The particular reactant gas and the surface temperature (not necessarily
`equal to the electrode temperature) are critical parameters because of the
`dependence of the process on the type and concentration of reactive species
`and because of the observation that most deposition and etching processes
`follow an Arrhenius rate expression. Electrode and chamber materials can
`alter the chemistry occurring in glow discharges because of chemical reac(cid:173)
`tions (adsorption, recombination, etc.) on or with the surfaces. Electrode
`potential and reactor configuration (equation 1) determine the energy of ions
`and electrons that strike the surfaces in contact with the discharge. Synergism
`between these numerous processes results in specific film growth (and etch)
`mechanisms. Ultimately, these factors establish film composition, bonding
`structure, and thus film properties.
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`MICROELECTRONICS PROCESSING: CI-U.MICAL ENGINEERING ASPECTS
`
`back-scattering spectrometry (RBS) were used to characterize the carbona(cid:173)
`ceous residue and silicon surface region (43). Such damage degrades material
`properties and alters the characteristics of fabricated devices. In addition,
`charge formation and accumulation in insulators subjected to radiation during
`plasma etching can lower the dielectric breakdown strength of device struc(cid:173)
`tures (44).
`Few studies have been reported that address radiation damage in
`PECVD processes. Recent work comparing sputter deposition and PECVD
`for dielectric film deposition indicates that structural damage is minimal in
`PECVD, although substrate damage is noted for sputtered coatings (45).
`These differences are probably due to lower ion energies resulting from the
`higher pressures and lower power densities used in PECVD compared with
`sputter deposition. Also, substrate temperatures above 200 °C are generally
`used in PECVD, so that any damage incurred may be annealed during
`deposition. Finally, in plasma etching, the underlying film or substrate is
`exposed to the discharge at the end of the etch cycle. With PECVD, un(cid:173)
`derlying surfaces are only briefly exposed at the start of the deposition cycle.
`
`Plasma Reactors
`
`Like CVD units, plasma etching and deposition systems are simply chemical
`reactors. Therefore, flow rates and flow patterns of reactant vapors, along
`with substrate or film temperature, must be precisely controlled to achieve
`uniform etching and deposition. The prediction o