The
`
`Materials Science
`
`
`
`Milton Ohring
`Stevens Institute of Technology
`Department of Materials Science and Engineering
`Hoboken, New Jersey
`
`of
`
`Thin Films
`
`ACADEMIC PRESS, INC.
`Harcourt Brace Jovanovich, Publishers
`
`Boston San Diego New York
`London Sydney Tokyo Toronto
`
`INTEL 1214
`
`INTEL 1214
`
`

`
`
`This book is printed on acid—free paper.
`Copyright © 1992 by Academic Press, Inc.
`All rights reserved.
`No part of this publication may be reproduced or
`transmitted in any form or by any means, electronic
`or mechanical, including photocopy, recording, or
`any information storage and retrieval system, without
`permission in writing from the publisher.
`
`
`
`Designed by Elizabeth E. Tustian
`
`ACADEMIC PRESS, INC.
`1250 Sixth Avenue, San Diego, CA 92101
`
`United Kingdom Edition published by
`ACADEMIC PRESS LIMITED
`24-28 Oval Road, London NW1 7DX
`
`Library of Congress Cataloging—in—Publication Data
`
`Ohring, Milton, date.
`The materials science of thin films / Milton Ohring.
`p.
`cm.
`Includes bibliographical references and index.
`ISBN O—12—524990—X (Alk. paper)
`1. Thin films.
`I. Title.
`TA4l8.9.T45047
`1991
`620’.44—dc2O
`
`91-9664
`CIP
`
`Printed in the United States of America
`91929394
`987654321
`
`Fc
`
`Pr:
`
`Ac
`
`Th
`
`0\~
`
`>—*I--|b—|t—4y-—A;...t)—t)._
`
`oo\lnxn.'r\i.\Lxl_.
`
`Ch
`
`Va
`2.1
`2.2
`
`

`
`physical Vapor Deposition
`
`3.1.
`
`INTRODUCTION
`
`1
`
`.
`
`two of the most
`3 chapter we focus on evaporation and sputtering,
`rtant methods for depositing thin films. The objective of these deposition
`teases is to controllably transfer atoms from a source to a substrate where
`formation and growth proceed atomistically. In evaporation, atoms are
`£:§g‘:1df;O1ll thepsource by thermal means, whereas in sputtering they are
`eamestF<>er)I(1 solid target (source) surfaces through impact of gaseous ions.
`be tmcedieflrfilientation in both of these deposition techniques can appar-
`1) Ob§erV0dt 6 same decade of the nineteenth century. In 1852, Grove
`rgfi F~iVe Ge rneltal deposits sputtered from the cathode of a glow
`Wpike metal Wi)1:e:rs ater Faraday (Ref. 2), experimenting with exploding
`es in the d In elm inert atmosphere, produced evaporated thin films.‘
`filidlianc
`> on of Suitable JOE;/6 Cilpment of vacuum—pumping equipment and the fabri-
`giggle“ Wire Spurreg
`eating sources, first made from platinum and then
`9
`the progress of evaporation technology. Scientific
`,i £35‘
`in the
`fhenomenon of evaporation and the properties of thin metal
`firm
`r was SO0
`ollowed by industrial production of optical components such as
`n
`fr)
`, rs, beam
`Sphttarsy and,
`later, antireflection coatings. Simultaneously,
`
`L
`
`.
`
`79
`
`

`
`80
`
`Physical Vapor Deposition
`
`sputtering was used as early as 1877 to coat mirrors. Later applications
`included the coating of flimsy fabrics with Au and the deposition of metal films
`on wax masters of phonograph records prior to thickening. Up until the late
`1960s, evaporation clearly surpassed sputtering as the preferred film deposition
`technique. Higher deposition rates, better vacuum, and, thus, cleaner environ.
`ments for film formation and growth, and general applicability to all classes of
`materials were some of the reasons for the ascendancy of evaporation methods.
`However, films used for magnetic and microelectronic applications necessi-
`tated the use of alloys, with stringent stoichiometry limits, which had to
`conformally cover and adhere well to substrate surfaces. These demands plus
`the introduction of radio frequency (RF), bias, and magnetron variants, which
`extended the capabilities of sputtering, and the availability of high-purity
`targets and working gases, helped to promote the popularity of sputter deposi—
`tion. Today the decision of whether to evaporate or sputter films in particular
`applications is not always obvious and has fostered a lively competition
`between these methods. In other cases, features of both have been forged into
`hybrid processes.
`
`through which the gaseous species are
`
`the term that includes both evaporation
`),
`and sputtering, and chemical vapor deposition (CVD), together with all of their
`variant and hybrid processes, are the basic film deposition methods treated in
`this book. Some factors that distinguish PVD from CVD are:
`1. Reliance on solid or molten sources
`2. Physical mechanisms (evaporation or collisional impact) by which‘ source
`atoms enter the gas phase
`3. Reduced pressure environment
`transported
`4. General absence of chemical reactions in the gas phase and at the substrate
`surface (reactive PVD processes are exceptions)
`The remainder of the chapter is divided into the following sections:
`3.2. The Physics and Chemistry of Evaporation
`3.3. Film Thickness Uniformity and Purity
`3.4. Evaporation Hardware and Techniques
`3.5. Glow Discharges and Plasmas
`3.6. Sputtering
`3.7. Sputtering Processes
`3.8. Hybrid and Modified PVD Processes
`
`Additional excellent reading material on the subject can be found in Refs»
`3-6. The book by Chapman is particularly recommended for its entertaining j
`
`

`
`
`he Physics and Chemistry of Evaporation
`81
`3.2 T
`
`
`
`and Very readable presentation of the many aspects relating to phenomena in
`rarefied gases, glow discharges, and sputtering.
`
`3.2. THE PHYSICS AND CHEMISTRY OF EVAPORATION
`
`3.2.1. Evaporation Rate
`
`Early attempts to quantitatively interpret evaporation phenomena are connected
`with the names of Hertz, Knudsen, and, later, Langmuir (Ref. 3). Based on
`experimentation on the evaporation of mercury, Hertz, in 1882, observed that
`evaporation rates were:
`
`limited by insufficient heat supplied to the surface of the molten
`1. Not
`evaporant
`2. Proportional to the difference between the equilibrium pressure P9 of Hg at
`the given temperature and the hydrostatic pressure Ph acting on the
`evaporant.
`
`Hertz concluded that a liquid has a specific ability to evaporate at a given
`temperature. Furthermore, the maximum evaporation rate is attained when the
`number of vapor molecules emitted corresponds to that required to exert the
`equilibrium vapor pressure while none return. These ideas led to the basic
`equation for the rate of evaporation from both liquid and solid surfaces,
`namely,
`
`<1>e
`
`aeNA(Pe '‘ Ph)
`.—. ————————-,
`21rMRT
`
`(3-1)
`
`Where <I>e is the evaporation flux in number of atoms (or molecules) per unit
`area per unit time, and are is the coefficient of evaporation, which has a value
`between 0 and 1. When ore = 1 and Ph is zero, the maximum evaporation rate
`is realized. By analogy with Eq. 2-9, an expression for the maximum value of
`‘IQ is
`
`P
`=13, = 3.513 x 1022~—-5- molecules/cmz-sec.
`\/MT
`
`(3-2)
`
`when P9 is expressed in torr, a useful variant of this formula is
`
`re = 5.834 X 10-2 s/M/TPe g/cm2—sec,
`
`(3-3)
`
`
`
`

`
`
`
`
`
`!LOC-3:10VAPORPRESSURE(ATM)
`
`3.2.2. Vapor Pressure of the Elements
`
`A convenient starting point for expressing the connection between temperature
`and vapor pressure is the Clausius—Clapeyron equation, which for both
`solid—vapor and liquid—vapor equilibria can be written as
`
`AH( T)
`dP
`217 = 71;?
`
`<34)
`
`
`3;?
`
`I
`
`I
`
`
`
`82
`
`Physical Vapor Deposition g
`
`3.2
`
`
`
`
`
`where Fe is thelmass evaporation rate. At a pressure of 10-2 torr, a typical
`it
`value of <I>e for many elements is approximately 10-4 g/cm2—sec of evaporant
`
`area. The key Variable influencing evaporation rates is the temperature, Which if f
`has a profound effect on the equilibrium vapor pressure.
`
`.AH(T), and volume, AV, refer to differences it
`The changes in enthalphy,
`between the vapor (v) and the particular condensed phase (C) from which it
`‘V,’
`originates, and T is the transformation temperature in question. Since AV =
`VU — Vc, and the volume of vapor normally considerably exceeds that of the
`condensed solid or liquid phase, AV = V”. If the gas is assumed to be perfect, ;}f
`V; z RT/ P, and Eq. 3-4 may be rewritten as
`
`PAH(T)
`dP
`E = 7?‘
`
`e
`
`(35)
`
`As a first approximation, AH( T) = AHe, the molar heat of evaporation (a 7
`constant), in which case simple integration yields
`
`AHe
`lnPz -——————+I,
`
`RT
`
`(36) it
`
`where I is a constant of integration. Through substitution of the latent heat of Le
`vaporization for AHe, the boiling point for T, and 1 atm for P, I can be
`evaluated for the liquid—vapor transformation. For practical purposes, Eq. 3—6 L
`adequately describes the temperature dependence of the vapor pressure in
`many materials.
`It
`is rigorously applicable only over a small
`temperature
`range, however. To extend the range of validity, we must account
`for
`the temperature dependence of AH( T). For example, careful evaluation Of
`thermodynamic data reveals that the vapor pressure of liquid Al is given by
`(Ref. 3)
`"
`
`
`
`log PM) = 15,993/T+ 12.409 — 0.999 log T — 3.52 x 10-67".
`
`(3—7) ;
`
`

`
`
`
` 3 2 The Physics and Chemistry of Evaporation
`‘ Deposition
`
`)
`
`O F
`
`4000
`
`2500 2000
`
`1500
`
`I200
`
`
`
`I4
`
`b
`
`I
`
`I
`
`ox
`
`I
`
`
`TEMPERATURE (OK)
`
` 700
`I000
`900
`800
`600
`
`
`
`
`
`
`4
`1o /r(°K-1)
`Vapor pressures of selected elements. Dots correspond to melting points.
`
`20
`
`15
`
`Figure 3-1.
`(From Ref. 7).
`
`The Arrhenius character of log P vs. 1/ T is essentially preserved, since the
`last two terms on the right—hand side are small corrections.
`Vapor—pressure data for many other metals have been similarly obtained and
`conveniently represented as a function of temperature in Fig. 3-1. Similarly,
`Vapor—pressure data for elements important in the deposition of semiconductor
`films are presented in Fig. 3-2. Much of the data represent direct measure-
`ments of the vapor pressures. Other values are inferred indirectly from
`thermodynamic relationships and identities using a limited amount of experi-
`mental data. Thus the vapor pressures of refractory metals such as W and Mo
`can be unerringly extrapolated to lower temperatures, even though it may be
`Impossible to measure them directly.
`Two modes of evaporation can be distinguished in practice, depending on
`Whether the Vapor effectively emanates from a liquid or solid source. As a rule
`Of thumb, a melt will be required if the element in question does not achieve a
`VaP0r pressure greater than 1O‘3 torr at its melting point. Most metals fall into
`this category, and effective film deposition is attained only when the source is
`heated into the liquid phase. On the other hand, elements such as Cr, Ti, Mo,
`Fe: and Si reach sufficiently high vapor pressures below the melting point and,
`erefore, sublime. For example, Cr can be effectively deposited at high rates
`fmm a solid metal source because it attains vapor pressures of 10‘2 torr some
`500 °C below the melting point. The operation of the Ti sublimation pump
`mentioned in Chapter 2 is, in fact, based on the sublimation from heated Ti
`
`1
`
`I
`
`I
`
`
`
`
`
`‘LOG10VAPORPRESSURE(ATM_.I
`
`
`
`a typical
`evaporant
`
`re, which
`
`iperature
`
`for both
`
`tion (a
`
`

`
`
`
`
`
`200
`
`400
`
`600
`
`
`8001000
`1500 2000
`
`TEMPERATURE (OK)
`Figure 3-2. Vapor pressures of ‘elements employed in semiconductor materials. Dots
`correspond to melting points. (Adapted from Ref. 8).
`
`filaments. A third example is carbon, which is used to prepare replicas of the
`surface topography of materials for subsequent examination in the electron
`microscope. The carbon is sublimed from an arc struck between graphite
`electrodes.
`
`3.2.3. Evaporation of Compounds
`
`While metals essentially evaporate as atoms and occasionally as clusters of
`atoms,
`the same is not true of compounds. Very few inorganic compounds
`evaporate without molecular change, and, therefore, the vapor composition i8
`usually different from that of the original solid or liquid source. A consequence
`of this is that the stoichiometry of the film deposit will generally differ from
`that of the source. Mass spectroscopic studies of the Vapor phase have shown
`that the processes of molecular association as well as dissociation frequentl)’
`occur. A broad range of evaporation phenomena in compounds occurs, and
`these are categorized briefly in Table 3-1.
`
`.3:7?(D('>n'».n--v-x—
`
`
`Physical Vapor Deposition
`
`~$>-
`
`
`
`
`
`
`
`LOG10VAPORPRESSURE(TORR)
`
`

`
` 3_2 The Physics and Chemistry of Evaporation
`
`Table 3-1. Evaporation of Compounds
` —1—“
`Reaction
`
`
`
`
`
`Type
`
`Evaporation
`without
`dissociation
`
`Chemical Reaction
`
`Examples
`
`Comments
`
`MX(s or I) —> MX(g)
`
`Compound
`Si0, B203
`GeO, SnO, AlN stoichiometry
`CaF2, MgF2
`maintained in
`deposit
`Separate
`sources are
`
`Ag2S, Ag2Se
`
`Decomposition
`
`MX(s) —> M(s) + (1 /2)X2( g)
`
`MX(s) -> M(l) + (1 /r1)X ,,(g)
`
`Ill-V
`semiconductors
`
`Evaporation
`with dissociation
`a. Chalcogenides MX(s) —> M( g) + (1 /2)X2(g)
`X = S, Se, Te
`
`b. Oxides
`
`MO2(s) —> MO(g) + (l/2)02(g)
`
`CdS, CdSe
`CdTe
`
`SiO2, GeO2
`TiO2, SnO2
`ZrO2
`
`required to
`deposit these
`compounds
`Deposits are
`metal-rich;
`separate
`sources are
`required to
`deposit these
`compounds
`Metal-rich
`discolored
`deposits;
`dioxides are
`
`best deposited
`in 02 partial
`pressure
`(reactive
`evaporation)
`_______._____________________._____________
`Note M = metal, X = nonmetal.
`Adapted from Ref, 3.
`
`3.2.4. Evaporation of Alloys
`
`EVap0rated metal alloy films are widely utilized for a variety of electronic,
`magnetic, and optical applications as well as for decorative coating purposes.
`Important examples of such alloys that have been directly evaporated include
`A1-Cu, Permalloy (Fe—Ni), nichrome (Ni—Cr), and Co—Cr. Atoms in metals
`0f such alloys are generally less tightly bound than atoms in the inorganic
`Compounds discussed previously. The constituents of the alloys,
`therefore,
`eVapOrate nearly independently of each other and enter the vapor phase as
`Single atoms in a manner paralleling the behavior of pure metals. Metallic
`melts are solutions and as such are governed by well-known thermodynamic
`
`
`
`

`
`
`
`
`86
`
`Physical Vapor Deposition
`
`
`laws. When the interaction energy between A and B atoms of a binary AB
`then no
`alloy melt are the same as between A-A and B—B atom pairs,
`
`preference is shown for atomic partners. Such is the environment in an ideal
`solution. Raoult’s law, which holds under these conditions, states that the
`
`
`
`vapor pressure of component B in solution is reduced relative to the vapor
`
`pressure of pure B (PB(O)) in proportion to its mole fraction XB. Therefore,
`
`PB = XBPB(0).
`
`(3-8)
`
`
`
`
`
`
` Metallic solutions usually are not ideal, however. This means that either
`more or less B will evaporate relative to the ideal solution case, depending on
`whether the deviation from ideality is positive or negative, respectively. A
`positive deviation occurs because B atoms are physically bound less tightly to
`the solution, facilitating their tendency to escape or evaporate. In real solutions
`
`
`
`
` where a3 is the effective thermodynamic concentration of B known as the
`activity. The activity is, in turn, related to XB through an activity coefficient
`
`73; i.e.,
` a3 = yBXB.
`
`By combination of Eqs. 3-2, 3-9, and 3-10, the ratio of the fluxes of A and
`
`PB = aBPB(O)=
`
`(3‘9)
`
`(3-10)
`
`B atoms in the vapor stream above the melt is given by
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`MB
`(EA __ 7AXAPA(0)
`———-——— —.
`<r>B
`7BXBPB(O)
`MA
`
`3-11
`
`(
`
`)
`
`Practical application of this equation is difficult because the melt composition
`changes as evaporation proceeds. Therefore,
`the activity coefficients, which
`can sometimes be located in the metallurgical literature, but just as frequently
`not, also change with time. As an example of the use of Eq. 3~ll, consider the
`
`problem of estimating the approximate Al—Cu melt composition required to
`evaporate films containing 2 wt% Cu from a single crucible heated to 1350 K.
`Substituting gives
`
`98/MAI
`‘DA1
`<pCu W 2/MCu ’
`
`10-3
`PA1(0)
`Fcum) — 2 X 10-4
`
`and assuming 79, = 7A1’
`
`982><10”“ /63.7
`XA1
`LI
`XC _ 2
`10*
`27.0 "
`
`15
`
`'
`
`This suggests that the original melt composition should be enriched to 13.6
`wt% Cu in order to compensate for the preferential vaporization of A1. It is,
`therefore, feasible to evaporate such alloys from one heated source. If the alloy
`
`
`
`
`
`

`
` 3_3 Film Thickness Uniformity and Purity
`
`87
`
`melt is of large volume, fractionation—induced melt composition changes are
`minimal. A practical way to cope with severe fractionation is to evaporate
`from dual sources maintained at different temperatures.
`
`
`
`
`
`3.3. FILM THICKNESS UNIFORMITY AND Pumrv
`
`3.3.1. Deposition Geometry
`
`In this section aspects of the deposition geometry, including the characteristics
`of evaporation sources, and the orientation and placement of substrates are
`discussed. The source-substrate geometry, in turn, influences the ultimate film
`uniformity, a concern of paramount importance, which will be treated subse-
`quently. Evaporation from a point source is the simplest of situations to model.
`Evaporant particles are imagined to emerge from an infinitesimally small
`region (dA 6) of a sphere of surface area A8 with a uniform mass evaporation
`rate as shown in Fig. 3—3a. The total evaporated mass 17,, is then given by the
`double integral
`
`17,=/t/ I‘edAedt.
`
`0 A,
`
`(3-12)
`
`Of this amount, mass d]i7IS falls on the substrate of area dAS. Since the
`projected area dAS on_t_he surface of the sphere is dA C, with dAc = (1/1 scos 6,
`the proportionality dMS : Me = dAC : 47rr2 holds. Finally,
`
`d175
`S
`dA "
`
`Ii-4_ecos 0
`47rr2
`
`(3-13)
`
`time basis we speak of deposition rate R
`is obtained. On a per unit
`(atoms /cm2—sec), a related quantity referred to later in the book. The deposi-
`
`
`
`
`
`
`Figure 3-3. Evaporation from (a) point source, (b) surface source.
`
`
`SURFACE SOURCE
`
`POINT SOURCE
`
`

`
`
`
`Physical Vapor Deposition
`
`88
`
`
`
`
`tion varies with the geometric orientation of the substrate and with the inverse
`square of the source—substrate distance. Substrates placed tangent
`to the
`surface of the receiving sphere would be coated uniformly, since cos 0 = 1.
`An evaporation source employed in the pioneering research by Knudsen
`made use of an isothermal enclosure with a very small opening through which
`
`the evaporant atoms or molecules effused. These effusion or Knudsen cells are
`frequently employed in molecular—beam epitaxy deposition systems, where
`precise control of evaporation variables is required. Kinetic theory predicts that
`the molecular flow of the vapor through the opening is directed according to a
`cosine distribution law, and this has been verified experimentally. The mass
`deposited per unit area is given by
`
`
`
`
`
`d]7S
`
`17ecos <1) cos 0
`
`dAs _
`
`7:-r2
`
`(3-14)
`
`and now depends on two angles (emission and incidence) that are defined in
`Fig. 3-3b. Evaporation from a small area or surface source is also modeled by
`Eq. 3-14. Boat filaments and wide crucibles containing a pool of molten
`material to be evaporated approximate surface sources in practice.
`From careful measurements of the angular distribution of film thickness, it
`has been found that, rather than a cos <25 dependence, a cos"<;5 evaporation law
`is more realistic. As shown in Fig. 3-4,
`71
`is a number that determines the
`
`geometry of the lobe—shaped vapor cloud and the angular distribution of
`evaporant flux from a source. When n is large,
`the vapor flux is highly
`directed. Physically n is related to the evaporation crucible geometry and
`scales directly with the ratio of the melt depth (below top of crucible) to the
`
`1.0. .8 .7 .6 .5 .4 .3 .2 .1 O .1 .2 .3 .4 .5 .6 .7 .8 .9100’
`Figure 3-4. Calculated lobe-shaped vapor clouds with various cosine exponents.
`(From Ref. 9).
`
`

`
` 3_3 Film Thickness Uniformity and Purity
`
`
`
`89
`
`
`
`
`
`
`
`melt surface area. Deep narrow crucibles with large n have been employed to
`confine evaporated radioactive materials to a narrow angular spread in order to
`minimize chamber contamination. The corresponding deposition equation is
`(Ref. 9)
`_
`_
`dMS Me(n + l)cos"q5 cos 0
`=
`2
`dA S
`2 7rr
`
`(n 2 0).
`
`(3-15)
`
`the surface area effectively
`As the source becomes increasingly directional,
`exposed to evaporant shrinks (i.e., 27rr2, 7rr2, and 271-r2/(n + 1) for point,
`cos a5, and cosӢ sources, respectively).
`
`3.3.2. Film Thickness Uniformity
`
`While maintaining thin—f11m thickness uniformity is always desirable, but not
`necessarily required,
`it is absolutely essential for microelectronic and many
`optical coating applications. For example, thin—film, narrow—band optical inter-
`ference filters require a thickness uniformity of : 1%. This poses a problem,
`particularly if there are many components to be coated or the surfaces involved
`are large or curved. Utilizing formulas developed in the previous section, we
`can calculate the thickness distribution for a variety of important source—sub—
`strate geometries. Consider evaporation from the point and small surface
`source onto a parallel plane—receiving substrate surface as indicated in the
`insert of Fig. 3-5. The film thickness d is given by dM—s/p dA5, where p is
`the density of the deposit. For the point source
`l\7eh
`l7ec0s 0
`ll/.7eh
`)
`(
`47rp(h2 +12)3/2
`47rpr2
`47rpr3
`The thickest deposit (do) occurs at I = 0, in which case d = 178 /41rph2,
`and, thus,
`
`.
`
`3-16
`
`=
`
`=
`
`d
`do
`Similarly, for the surface source
`
`
`
`1
`{1+(1/h)2}3/2'
`
`(3 17)
`
`./Wcosficosqb
`
`d= ————————~—"’2
`7rpr
`
`=
`
`17,, h h
`l\—l—h2
`
`,—— =
`”’
`7:-pr
`r r
`1rp(h2 + 12)2 ’
`since cos 0 = cos q5 = h /r. When normalized to the thickest dimensions, or
`do = Me / 7rp/12,
`
`(3—18)
`
`d
`
`1
`
`
`6
`
`*1
`h
`6
`
`e i
`
`t
`a
`
`1
`,»
`1
`
`t
`v
`3
`f
`1
`1
`ua
`
`do
`
`
`{1+(l/h)2}2.
`
`(3-19)
`
`
`
`

`
`Physical Vapor Deposition
`
`SOURCE
`
`POINT
`SOURCE
`
`SURFACE
`SOURCE
`
`l
`0.5
`
`O
`
`'1.0
`
`6/h
`
`Figure 3-5. Film thickness uniformity for point and surface sources. (Insert) Geome-
`try of evaporation onto parallel plane substrate.
`
`A comparison of Eqs. 3-17 and 3-19 is made in Fig. 3-5, where it is apparent
`that less thickness uniformity can be expected with the surface source.
`A couple of practical examples (Ref. 10) will demonstrate how these film
`thickness distributions are used in designing source—substrate geometries for
`coating applications.
`In the first example suppose it
`is desired to coat a
`150-cm—wide strip utilizing two evaporation sources oriented as shown in the
`insert of Fig. 3-6. If a thickness tolerance of i 10% is required, what should
`the distance between sources be and how far should they be located from the
`substrate? A superposition of solutions for two individual surface sources (Eq.
`3-19) gives the thickness variation shown graphically in Fig. 3-6 as a function
`of the relative distance r from the center line for various values of the source
`
`spacing D. All pertinent variables are in terms of dimensionless ratios r / h
`and D/ hp. The desired tolerance requires that d/ do stay between 0.9 and
`1.1, and this can be achieved with D /,1” = 0.6 yielding a maximum value of
`r/hv = 0.87. Since r = 150/2 = 75 cm, hv = 75/0.87 = 86.2 cm. The re-
`quired distance between sources is therefore D = 2 X 0.6 X 86.2 = 103.4
`cm. There are other solutions, of course, but we are seeking the minimum
`value of hv. It is obvious that the uniformity tolerance can always be realized
`by extending the source—substrate distance, but this wastes evaporant.
`As a second example, consider a composite optical coating where a i 1%
`film thickness variation is required in each layer. The substrate is rotated to
`even out source distribution anomalies and minimize preferential film growth
`
`

`
` 3,3
`
`
`Film Thickness Uniformity and Purity
`91
`
`
`
`
`
`
`
`
`
`that can adversely affect coating durability and optical properties. Since
`multiple films of different composition will be sequentially deposited,
`the
`necessary fixturing requires that the sources be offset from the axis of rotation
`by a distance R = 20 cm. How high above the source should a 25-cm-diame-
`ter substrate be rotated to maintain the desired film tolerance? The film
`thickness distribution in this case is a complex function of the three-dimen-
`sional geometry, which, fortunately, has been graphed in Fig. 3-7. Reference
`to this figure indicates that
`the curve ht,/R = 1.33 in conjunction with
`r/R = 0.6 will generate a thickness deviation ranging from about -0.6 to
`+0.5%. On this basis, the required distance is h, = 1.33 X 20 = 26.6 cm.
`A clever way to achieve thickness uniformity, however, is to locate both the
`surface evaporant source and the substrates on the surface of a sphere as shown
`in Fig. 3-8. In this case, cos 6 = cos qfl = r/2r0, and Eq. 3-14 becomes
`dill:
`178
`r
`r
`[We
`3 20
`dA5 _- 7rr2 2r0 2r0 — 41rr§'
`_
`
`(
`
`)
`
`The resultant deposit thickness is a constant clearly independent of angle. Use
`is made of this principle in the planetary substrate fixtures that hold silicon
`wafers to be coated with metal (metalized) by evaporation. To further promote
`uniform coverage,
`the planetary fixture is rotated during deposition. Physi-
`
`THICKNESS(d/d0) 2
`RELATIVE
`
`RELATIVE DISTANCE FROM CENTER LINE
`Figure 3-6. Film thickness uniformity across a strip employing two evaporation
`S°Urces for various values of D / hu. (From Ref. 10).
`
`
`
`
`
`

`
`
`
`
` Physical Vapor Deposition
`
`
`
`
`PERCENTTHICKNESSDEVIATIONFROMCENTERTHICKNESS
`
`ANGLE OF INCIDENCE
`
`Figure 3-7.
`(From Ref. 10).
`
`'
`
`'
`
`’
`
`'
`
`a rotating disk.
`
`WAFER
`SUBSTRATES
`
`9=¢
`CosG=Cosq>= -—2rT0
`
`
`
`
`
`
`
`SURFACE SOURCE
`Figure 3-8. Evaporation scheme to achieve uniform deposition. Source and sub’
`strates lie on sphere of radius r0.
`
`

`
` 3_3 Film Thickness Uniformity and Purity
`
`93
`
`parabolic, and hyperbolic substrate surfaces (Ref. 9). Similarly, cylindrical,
`wire, and ring evaporation source geometries have been treated (Ref. 11). For
`the results, interested readers should consult the appropriate references.
`
`sk.
`
`3.3.3. Conformal Coverage
`
`An issue related to film uniformity is step or, more generally, conformal
`
`coverage, and it arises primarily in the fabrication of integrated circuits. The
`required semiconductor contact and device interconnection metalization deposi-
`tions frequently occur over a terrain of intricate topography where microsteps,
`grooves, and raised stripes abound. When the horizontal as well as vertical
`
`
`
`cally, deposition uniformity is achieved because short source—substrate dis-
`tances are offset by unfavorably large vapor emission and deposition angles.
`Alternately, long source—substrate distances are compensated by correspond-
`ingly small emission and reception angles. For sources with a higher degree of
`directionality (i.e., where cosӢ rather than cos <1) is involved), the reader can
`easily show that thickness uniformity is no longer maintained.
`Two principal methods for optimizing film uniformity over large areas
`involve varying the geometric location of the source and interposing static as
`well as rotating shutters between evaporation sources and substrates. Computer
`calculations have proven useful
`in locating sources and designing shutter
`contours to meet the stringent demands of optical coatings. Film thickness
`uniformity cannot, however, be maintained beyond : 1% because of insuffi-
`cient mechanical stability of both the stationary and rotating hardware.
`In addition to the parallel source—substrate configuration, calculations of
`thickness distributions have also been made for spherical as well as conical,
`
`
`
`/SOURCE
`
`Lllb‘
`
`Figure 3-9. Schematic illustration of film coverage of stepped substrate: (A) uniform
`coverage; (B) poor sidewall coverage; (C) lack of coverage~discontinuous film.
`
`

`
` 94
`
`Physical Vapor Deposition
`
`
`
`
`
`surfaces of substrates are coated to the same thickness, we speak of conformal
`coverage. On the other hand, coverage will not be uniform when physical
`shadowing effects cause unequal deposition on the top and sidewalls of steps.
`Inadequate step coverage can lead to minute cracks in the metalization, which
`have been shown to be a major source of failure in device reliability testing.
`Thinned regions on conducting stripes exhibit greater Joule heating, which
`sometimes fosters early burnout. Step coverage problems have been shown to
`be related to the profile of the substrate step as well as to the evaporation
`source—substrate geometry. The simplest model of evaporation from a point
`source onto a stepped substrate results in either conformal coverage or a lack
`of deposition in the step shadow, as shown schematically in Fig. 3-9. Line-of-
`sight motion of evaporant atoms and sticking coefficients of unity can be
`assumed in estimating the extent of coverage.
`More realistic computer modeling of step coverage has been performed for
`the case in which the substrate is located on a rotating planetary holder (Ref).
`12). In Fig. 3-10 coverage of a 1—um—wide, l—um-high test pattern with 5000 A
`
`
`
`0
`
`1-0,“
`
`2.0#
`
`O
`
`l.O;1
`
`2.0;:
`
`IJJI
`
`1.0/1
`
`E
`
`In
`
`1.0;:
`
`2.0,u
`
`)7
`
`
`
`
`
`Figure 3-10. Comparison of simulated and experimental Al film coverage of ll-um
`line step and trench features. (Left) Orientation of most symmetric deposition. (Right)
`Orientation of most asymmetric deposition. (Reprinted with permission from Cowan
`Publishing C0,, from C. H. Ting and A. R. Neureuther, Solid State Technology 25,
`115, 1982).
`
`
`
`

`
` 3.3 Film Thickness Uniformity and Purity
`
`95
`
`of evaporated Al is simulated and compared with experiment. In the symmetric
`orientation the region between the pattern stripes always manages to “see” the
`source, and this results in a small plateau of full film thickness.
`In the
`asymmetric orientation, however,
`the substrate stripes cast a shadow with
`respect
`to the source biasing the deposition in favor of unequal sidewall
`coverage. In generating the simulated film profiles, the surface migration of
`atoms was neglected, a valid assumption at low substrate temperatures. Heat-
`mg the substrate increases surface diffusion of depositing atoms, thus promot-
`ing the filling of potential voids as they form.
`Interestingly, similar step
`coverage problems exist in chemical—vapor—deposited SiO2 and silicon nitride
`films.
`
`
`
`
`
`
`Terms Ma and Mg refer to evaporant and gas molecular weights, respec-
`tively, and P is the residual gas vapor pressure in torr.
`Table 3-2 illustrates the combined role that deposition rate and residual
`pressure play in determining the oxygen level that can be incorporated into thin
`tin films (Ref. 13). Although the concentrations are probably overestimated
`because the sticking probabiltiy of O2 is about 0.1 or less, the results have
`Several important implications. To produce very pure films, it is important to
`deposit at Very high rates while maintaining very low background pressures of
`residual gases such as H20, CO2, CO, 02 , and N2. Neither of these
`requirements is too formidable for vacuum evaporation, where deposition rates
`from electron—beam sources can reach 1000 A/sec at chamber pressures of
`~ 1O‘8 torr.
`On the other hand, in sputtering processes, discussed later in the chapter,
`
`
`3.3.4. Film Purity
`
`The chemical purity of evaporated films depends on the nature and level of
`impurities that (1) are initially present in the source, (2) contaminate the source
`from the heater, crucible, or support materials, and (3) originate from the
`residual gases present in the Vacuum system. In this section only the effect of
`residual gases on film purity will be addressed. During deposition the atoms
`and molecules of both the evaporant and residual gases impinge on the
`substrate in parallel,
`independent events. The evaporant vapor impingement
`rate is pNAa"/Ma atoms/cm2—sec, where p is the density and ci’
`is the
`deposition rate (cm/sec). Simultaneously, gas molecules impinge at a rate
`given by Eq. 2-9. The ratio of the latter to the former is the impurity
`concentration C,-:
`
`P M
`C, = 5.82 x 10-2 ————— ”
`,/Mgr" pd’
`
`(3-21)
`
`

`
`
`
`Physical Vapor Deposition
`
`Table 3-2. Maximum Oxygen Concentration in Tin Films Deposited
`at Room Temperature
`
`
`
`deposition rates are typically more than an order of magnitude less, and
`chamber pressures five orders of magnitude higher than for evaporation.
`Therefore, the potential exists for producing films containing high gas concen-
`trations. For this reason sputtering was traditionally not considered to be as
`“clean” a process as evaporation. Considerable progress has been made in the
`last two decades, however, with the commercial development of high—deposi—
`tion—rate magnetron sputtering systems, operating at somewhat
`lower gas
`pressures in cleaner vacuum systems. In the case of aluminum films, compara-
`ble purities appear to be attained in both processes. Lastly, Table 3—2 suggests
`that very high oxygen incorporation occurs at residual gas pressures of l()‘3
`torr. Advantage of this fact is taken in reactive evaporation processes where
`intentionally introduced gases serve to promote reactions with the evaporant
`metal and control the deposit stoichiometry.
`The presence of gaseous impurities within metal films sometimes has a
`pronounce