HANDBOOK OF PHYSICAL
`VAPOR DEPOSITION
`(PVD) PROCESSING
`Film Formation, Adhesion, Surface Preparation
`
`and Contamination Control
`
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`SPUTTER DEPOSITION
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`ION PU.TING
`
`by
`Donald M. Mattox
`
`NOYES PUBLICATIONS
`
`Page 1 of 20
`
`APPLIED MATERIALS EXHIBIT 1032
`
`

`

`Copyright O 1998 by Noyes Publications
`No part of this book may be reproduced or utilized
`in any form or by any means, electronic or
`mechanical, including photocopying, recording or
`by any information storage and retrieval system,
`without permission in writing from the Publisher.
`Library of Congress Catalog Card Number: 97-44664
`ISBN: 0-81SS-1422-0
`Printed in the United Stales
`
`Published in the United States of America by
`Noyes Publications
`169 Kinderkamack Rd., Park Ridge, NJ 07656
`
`10 9 8 7 6 S 4 3 2
`
`Library of Congress Cataloging-in-Publication Data
`
`Mattox, D. M.
`Handbook of physical vapor deposition (PVD) processing/ by
`Donald M. Mattox.
`cm.
`p.
`Includes bibliographical references and index.
`ISBN 0-8155-1422-0
`l. Vapor-plating--Handbooks, manuals, etc.
`TS695.M38
`1998
`671.7'35--dc21
`
`I. Title.
`
`97-44664
`CIP
`
`Page 2 of 20
`
`

`

`228 Handbook of Physical Vapor Deposition (PVD) Processing
`
`CURIIENT
`MONITOII
`
`+
`HIGH VOLT(cid:173)
`AGE IUPPLY
`
`ROTATING
`CAGE
`
`FIELD MAGNET
`SUPPLY
`
`+
`
`Figure 4-6. Barrel ion plating system configuration with a triode DC discharge.
`
`The DC diode discharge cannot be used to sputter dielectric target
`materials, since charge buildup on the cathode surface will prevent bom(cid:173)
`bardment of the surface. If there are reactive gases in the plasma their
`reaction with the target surface can lead to the formation of a surface that
`has a different chemical composition than the original surface. This
`change in composition leads to "poisoning" of the cathode surface and thus
`changes the plasma parameters. In the extreme, poisoning will cause
`bombardment of the cathode to cease due to surface charge buildup. If an
`insulating surface forms on the DC cathode, charge buildup will cause
`arcing over the surface.
`
`Page 3 of 20
`
`

`

`Low-Pressure P'/asma. Processing Environment 229
`
`The suppression of arcs generated in the DC discharge (arc sup(cid:173)
`pression) are important to obtaining stable performance of the DC power
`supplies particularly when reactively sputter depositing dielectric films. l461
`Arcing can occur anytime a hot (thermoelectron emitting) spot is formed or
`when surface charging is different over surfaces in contact with the plasma
`Arc suppression is obtained by momentarily turning off the power supply
`or by applying a positive bias when an arc is detected.
`
`Pulsed DC
`
`When a continuous DC potential is applied to a metal electrode
`completely covered with a dielectric material, the surface of the dielectric
`is polarized to the polarity, and nearly the voltage, of the metal electrode. If
`the surface potential is negative, ions are accelerated out of the plasma to
`bombard the surface giving sputtering, secondary electron emission, "atomic
`peening," and heating. However, since the secondary electron emission
`coefficient is less than one the surface will buildup a positive surface
`charge and the bombardment energy will decrease then bombardment will
`crease. This problem can be overcome by using a pulsed DC rather than a
`continuous DC.
`Pulsed DC uses a potential operating in the range 50-250 kHz
`where the voltage, pulse width, off time (if used), and pulse polarity can be
`varied.£471 The voltage rise and fall is very rapid during the pulse. The
`pulse can be unipolar, where the voltage is typically negative with a no(cid:173)
`voltage (off) time, or bipolar where the voltage polarity alternates between
`negative and positive perhaps with an off time. The bipolar pulse can be
`symmetric, where the positive and negative pulse heights are equal and the
`pulse duration can be varied or asymmetric with the relative voltages being
`variable as well as the duration time.l481 Figure 4-7 shows some DC
`waveforms. Generally in asymmetric pulse DC sputter deposition, the
`negative pulse (e.g., -400 V) is greater than the positive pulse (e.g,. + 100
`V) and the negative pulse time is 80-90% of the voltage cycle and the
`positive pulse is 20-10% of the voltage cycle.
`In pulse DC sputtering, during the positive bias (and off-time),
`electrons can move to the surface from the plasma and neutralize any
`charge build-up generated during the negative portion of the cycle. During
`the negative portion of the cycle, energetic ion bombardment can sputter
`dielectric surfaces.
`
`Page 4 of 20
`
`

`

`230 Handbook of Physical Vapor Deposition (PVD) Processing
`
`C.
`
`a. Continuous DC
`b. Unipolar Pulsed DC
`c. Bipolar Pulsed DC
`d. Asymmetric Bipolar Pulsed DC
`
`Figure 4-7. DC waveforms.
`
`Pulsed DC power can be obtained by switching a continuous DC
`or sinewave power supply with auxiliary electronics149l or can be obtained
`from a specially designed pulsed power supply that generally allows more
`flexibility as to waveform. The pulsed power supply generally incorpo(cid:173)
`rates arc suppression that operates by turning off the voltage or by applying
`a positive voltage when the arc initiates. Pulsed plasmas are also of interest
`in plasma etching and plasma enhanced CVD (PECVD).[SOJ
`
`4.4.4 Magnetically Conrmed Plasmas
`
`Balanced Magnetrons
`
`In surface magnetron plasma configurations the electric (E) (vec(cid:173)
`tor) and magnetic (B) (vector) fields are used to confine the electron path to
`be near the cathode (electron emitting) surface. An electron moving with a
`component of velocity normal to the magnetic field will spiral around the
`magnetic field lines and its direction will be confined by the magnetic field.
`The frequency of the spiraling motion and the radius of the spiral will
`depend on the magnetic field strength. The interaction of an electron
`with the electric and magnetic fields depends on the magnitude and
`vector orientation of the fields (E x B). For example, if the magnetic
`field is parallel to a surface and the electric field is normal to the surface an
`electron leaving the surface will be accelerated away from the surface and
`will spiral around the magnetic field. There will also be a resulting motion
`
`Page 5 of 20
`
`

`

`6
`
`Physical Sputtering and
`Sputter Deposition
`(Sputtering)
`
`6.1
`
`INTRODUCTION
`
`The physical sputtering (sputtering) process, or pulverisation as
`the French call it, involves the physical (not thermal) vaporization of atoms
`from a surface by momentum transfer from bombarding energetic atomic(cid:173)
`sized particles. The energetic particles are usually ions of a gaseous
`material accelerated in an electric field.[OaJ Sputtering was first observed
`by Grove in 1852 and Pulker in 1858 using von Guericke-type oil-sealed
`piston vacuum pumps. The terms "chemical sputtering" and "electro(cid:173)
`chemical sputtering" have been associated with the process whereby bom(cid:173)
`bardment of the target surface with a reactive species produces a volatile
`speciesPl This process is now often termed "reactive plasma etching" or
`"reactive ion etching" and is important in the patterning of thin films.C2l
`Sputter deposition, which is often called just sputtering (a poor use
`of the term), is the deposition of particles whose origin is from a surface
`(target) being sputtered. Sputter deposition of films was first reported by
`Wright in 1877 and was feasible because only a relatively poor vacuum is
`needed for sputter deposition. Edison patented a sputter deposition process
`for depositing silver on wax photograph cylinders in 1904. Sputter deposi(cid:173)
`tion was not widely used in industry until the need developed for reproducible,
`
`315
`
`Page 6 of 20
`
`

`

`Physical Sputtering and Sputter Deposition 321
`
`t
`
`Figure 6-2. Collision of particles.
`
`The maximum energy is transferred when cos e = l ( zero degrees)
`and M; = M 1. Therefore matching the atomic mass of the bombarding ion to
`the target atom is important to the sputtering yield. This makes krypton (84
`amu), xenon (131 amu) and mercury (201 amu) ions attractive for sputter(cid:173)
`ing heavy elements, and light ions such as nitrogen (14 amu) unattractive.
`This advantage is typically outweighed by other considerations such as
`cost of the sputtering gas, health concerns or the desire to perform "reactive
`It is interesting to note that
`sputter deposition" of oxides and nitrides.
`much of the early work on sputtering was done using mercury ions.
`Typically argon ( 40 amu) is used for inert gas sputtering since it is
`a relatively inexpensive inert gas. Mixtures of argon and nitrogen, argon
`and oxygen or argon and methane/acetylene are used for sputtering in
`In some cases, energetic ions of the target
`reactive sputter deposition.
`material can bombard the target producing "self-sputtering." This effect is
`important in ion plating using ionized condensable ions ("film ions") formed
`by arc vaporization or by post-vaporization ionization of sputtered or
`thermally evaporated atoms.
`
`6.2.2 Sputtering Yields
`
`The sputtering yield is the ratio of the number of atoms ejected to
`the number of incident bombarding particles and depends on the chemical
`bonding of the target atoms and the energy transferred by collision. The
`
`Page 7 of 20
`
`

`

`338 Handbook of Physical Vapor Deposition (PVD) Processing
`
`material is removed from the target layer-by-layer. This allows the deposi(cid:173)
`tion of some rather complex alloys such as W:Ti for semiconductor
`metallization,[861 Al:Si:Cu for semiconductor metallization,[871 and M( etal)(cid:173)
`Cr-Al-Y alloys for aircraft turbine blade coatings.
`
`6.5.2 Reactive Sputter Deposition
`
`Reactive sputter deposition from an elemental target[88U891 relies
`on: (a) the reaction of the depositing species with a gaseous species, sucli as
`oxygen or nitrogen, (b) reaction with an adsorbed species, or ( c) reaction
`with a co-depositing species such as carbon to form a compound. The
`reactive gas may be in the molecular state (e.g., N2, 0 2) or may be
`"activated" to form a more chemically reactive or more easily adsorbed
`species. Typically, the reactive gases have a low atomic masses (N=l4,
`0=16) and are thus not effective in sputtering. It is therefore desirable to
`have a heavier inert gas, such as argon, to aid in sputtering. Mixing argon
`with the reactive gas also aids in activating the reactive gas by the Penning
`ionization/excitation processes.
`Typically, a problem in reactive sputter deposition is to prevent the
`"poisoning" of the sputtering target by the formation of a compound layer
`on its surface.l311 Poisoning of a target surface greatly reduces the sputter(cid:173)
`ing rate and sputtering efficiency. This problem is controlled by having a
`high sputtering rate (magnetron sputtering) and controlling the availability
`of the reactive gas, such that there will be enough reactive species to react
`with the film surface to deposit the desired compound, but not so much that
`it will unduly poison the target surface.
`The appropriate gas composition and flow for reactive sputter
`deposition can be established by monitoring the partial pressure of the
`reactive gas as a function of reactive gas flow,(901-[931 or by impedance of
`the plasma discharge. Figure 6-8 shows the effect of reactive gas flow on
`the partial pressure of the reactive gas in the reactive sputter deposition of
`TiN. Under operating conditions of maximum flow and near-minimum
`partial pressure, the deposit is gold-colored TiN and the sputtering rate is
`the same as metallic titanium. At higher partial pressures, the sputtering
`rate decreases and the film is brownish. As the target is poisoned, the
`deposition rate deci;eases. When the nitrogen availability is decreased, the
`target is sputter-cleaned and the deposition rate rises.
`The gas composition should be determined for each deposition
`system and fixture geometry. A typical mixture for reactive sputter
`
`Page 8 of 20
`
`

`

`Physical Sputtering and Sputter Deposition 339
`
`deposition might be 20% nitrogen and 80% argon where the partial pres(cid:173)
`sure of nitrogen during deposition is 2 x 1 o-4 Torr and the total gas flow is 125
`seem. Gases mixtures are typically controlled using individual mass flow
`meters on separate gas sources though specific gas mixtures can be purchased.
`Figure 6-9 depicts a typical reactive sputter deposition system.
`
`Continuously Increasing Flow
`~
`
`}
`
`Optimum
`Operating
`Conditions
`
`~
`Nitrogen Flow
`
`Figure 6-8. Nitrogen partial pressure and flow conditions for the reactive sputter deposi(cid:173)
`tion ofTiN with constant target power (adapted from Ref. 51).
`
`In reactive deposition, the reactive gases are being pumped ("get(cid:173)
`ter pumping") by the depositing film material. Since the depositing film is
`reacting with the reactive gas, changes in the area or rate of the film being
`deposited will change the reactive gas availability and the film properties.
`Thus, it is important to use the same fixture, substrate, and vacuum surface
`areas as well as deposition rate, in order to have a reproducible reactive
`sputter deposition process. Changes in the geometry (loading factor) or
`deposition rate will necessitate changes in gas flow parameters.[90l The gas
`density (partial pressure) of the reactive gas in the plasma can be monitored by
`optical emission spectroscopy or mass_spectrometry techniques.[91l-[93l
`
`Page 9 of 20
`
`

`

`340 Handbook of Physical Vapor Deposition (PVD) Processing
`
`Since gas pressure is important to the properties of the sputter depos(cid:173)
`ited film it is important that the vacuum gauge be periodically calibrated and
`located properly and pressure variations in the chamber be minimized.
`
`Mass Flow
`Meters
`
`Microprocessor
`
`L_-==r::= _ _J---~-'-J Roughing Valve
`
`I---&<~:;,.. Roughing Pump(s)
`
`High
`Voltage
`
`Figure 6-9. Typical reactive spullcr deposition system.
`
`In some reactive deposition configurations, the inert gas is injected
`around the sputtering target and the reactive gas is injected near the
`substrate surface. This inert "gas blanket" over the target surface is helpful
`in reducing target poisoning in some cases.
`In reactive deposition, the depositing material must react rapidly
`or it will be buried by subsequent depositing material. Therefore, the
`reaction rate is an important consideration. The reaction rate is determined
`by the reactivity of the reactive species. their availability, and the tempera(cid:173)
`ture of the surface. The reactive species can be activated by a number of
`processes including:
`• Dissociation of molecular species to more chemically reactive
`radicals (e.g., N2 + e-~ 2N° and NH3 + e- ~N° + 3H0 )
`• Production of new molecular species that are more
`chemically reactive and/or more easily absorbed on surfaces
`(e.g., 0 2 + e-~ 20° then 0° + 0 2 ~ 0 3)
`
`Page 10 of 20
`
`

`

`Physical Sputtering and Sputter Deposition 341
`
`• Production ofions-recombination at surfaces releases energy
`• Adding internal energy to atoms and molecules by creating
`metastable excited states-de-excitation at surfaces releases
`energy
`• Increasing the temperature of the gas
`• Generating short wavelength photons (UV) that can
`stimulate chemical reactions
`• Generating energetic electrons that stimulate chemical reactions
`• Ions accelerated from the plasma to the surface promotes
`chemical reactions on the surface (bombardment enhanced
`chemical reactions)
`The extent to which a plasma can activate the reactive gases and
`provide ions for concurrent bombardment depends on the properties of the
`plasma and its location. In many sputtering systems the plasma conditions
`vary widely throughout the deposition chamber. This is particularly true for
`the magnetron configurations where the sputtering plasma is confined near
`the target.
`In such a case, a plasma needs to be established near the
`substrate surface to activate reactive gases and provide ions for concurrent
`bombardment. This can be done using an unbalanced magnetron configura(cid:173)
`tion, application of an rfto the target, or by establishing a separate auxiliary
`plasma over the substrate surface.
`The reaction probability is also a function of the surface coverage.
`For example, it is easier for an oxygen species to react with a pure titanium
`surface than with a Ti01.9 surface. Figure 6-10 shows the effect of reactive
`nitrogen availability on the electrical resistivity of TiNx films. The films
`have minimum resistivity when the composition is pure titanium and when
`the composition is near TiN.
`Another important variable in reactive deposition is concurrent
`bombardment of the depositing/reacting species by energetic ions acceler(cid:173)
`ated from the plasma ("sputter ion plating" or "bias sputtering''). Concur(cid:173)
`rent bombardment enhances chemical reactions and can densify the deposit(cid:173)
`ing film if unreacted gas is not incorporated into the deposit. Bombardment
`is obtained by having the surface at a negative potential (applied bias or self(cid:173)
`bias) so that ions are accelerated from the plasma to the surface. Figure 6-
`11 shows the relative effects of deposition temperature and applied bias on
`the electrical resistivity (normalized) of a TiNx film. 1941 The lowest resistiv(cid:173)
`ity is attained with both a high deposition temperature and concurrent
`bombardment although a low-temperature deposition with concurrent bom(cid:173)
`bardment comes close.
`
`Page 11 of 20
`
`

`

`342 Handbook of Physical Vapor Deposition (PVD) Processing
`
`e
`~ ......
`~
`== ti
`f3 a:
`
`200
`
`180
`
`180
`
`140
`
`120
`
`100
`
`80
`
`60
`
`40
`
`20
`
`D.111
`
`0,02 O.G3
`
`0.04
`
`0.1111
`
`a.as DJ11
`
`o.ae
`
`IIJl9
`
`a.10
`
`a.11
`
`REACTIVE NITROGEN AVAILABILITY
`(partial pressure • microns)
`
`Figure 6-10. TiN resistivity as a function of nitrogen content (nitrogen availability during
`deposition). (Adapted from Ref. 94.)
`
`35•c
`
`1.00
`
`f ue
`~ uo I ::
`
`0
`
`Blas:OV
`
`Blas= -300V
`
`Figure 6-11. TiN resistivity as a function of deposition temperature and concurrent
`bombardment (adapted from Ref. 94).
`
`Reactive deposition is used to produce colored wear-resistant
`decorative coatings such as TiN (gold), TiCxNy (bronze, rose, violet, or
`black as x and y are varied) and ZrN (brass). Coatings such as TiN and
`(Ti,Al)CxNy are used as wear-resistant coatings on tools, molds, and other
`surfaces. Reactive deposition is used to produce oxide films such as Zr02 and
`Ti02, which are used to form anti-reflection and band-pass coatings on optical
`
`_J
`
`Page 12 of 20
`
`

`

`Physical Sputtering and Sputter Deposition 343
`
`components, indium-tin-oxide (ITO), is a transparent electrical conductor
`and Si0u, is a material of interest as a transparent, moisture-permeation(cid:173)
`barrier materials for packaging applications.
`The co-depositing material for reactive deposition can be from a
`second sputtering target. However it is often in the form of a chemical
`vapor precursor which is decomposed in a plasma and on the surface.
`Chemical vapor precursors are such materials as acetylene (C2H2) or
`methane (CH4) for carbon, silane (SiH4) for silicon, and diborane (B28<;)
`for boron. This technique is thus a combination of sputter deposition and
`plasma enhanced chemical vapor deposition and is used to deposit materials
`such as the carbides, borides, and silicides.l951 It should be noted that co(cid:173)
`deposition does not necessarily mean reaction. For example, carbon can be
`deposited with titanium to give a mixture of Ti + C but the deposit may have
`little TiC.
`In reactive sputtering, the injection of the reactive gas is important
`to insure uniform activation and availability over the substrate surface.
`This can be difficult if, for instance, the film is being deposited over a large
`area such as on 10' x 12' architectural glass panels where the sputtering
`cathode can be twelve feet or more in length. In such an application, it may
`be easier to use quasi-reactive sputtering from a compound target.
`In "quasi-reactive sputter deposition" the sputtering target is made
`from the compound material to be deposited and a partial pressure of
`reactive gas in a plasma is used to make-up for the loss of the portion of the
`gaseous constituent that is lost in the transport and condensation/reaction
`processes. Typically the partial pressure of the reactive gas used in quasi(cid:173)
`reactive deposition is much less than that used for reactive deposition. For
`example, the gas composition might be 10% oxygen and 90% argon.
`
`6.5.3 Deposition of Layered and Graded Composition
`Structures
`
`Layered structures can be deposited by passing the substrate in
`front of several sputtering targets sequentially. For example, X-ray diffrac(cid:173)
`tion films are formed by depositing thousands of alternating layers of high(cid:173)
`Z (W) and low-Z (C) material with each layer being about 30A thick.
`Layered and graded composition structures can be deposited using
`reactive deposition. The composition is changed by changing the availabil(cid:173)
`ity of the reactive gas. Thus one can form layers ofTi-TiN-Ti by changing
`the availability of the nitrogen. Since nitrogen has been incorporated in the
`
`Page 13 of 20
`
`

`

`348 Handbook of Physical Vapor Deposition (PVD) Processing
`
`6.6.4
`
`Ion and Plasma Sources
`
`In some types of reactive sputter deposition, a few monolayers of a
`pure metal are deposited and then the substrate is passed in front of a source
`of the reactive species. By doing this repeatedly, a compound film can be
`built-up. The source for reactive gas is generally a plasma source, such as
`a gridless end-Hall source, where the gas is activated and, in some cases,
`reactive ions are accelerated to the substrate (Sec. 4.5.1). An easy configu(cid:173)
`ration for doing this is to mount the substrates on a drum and repeatedly
`rotate them in front of the sputtering source and the reactive gas source
`such as with the MetaModeTM deposition configuration.C1011
`
`6.6.5 Plasma Activation Using Auxiliary Plasmas
`
`Activation of the reactive species enhances chemical reactions
`during reactive deposition. The plasma used in sputtering will activate the
`reactive gases but often the plasma volume is small or not near the substrate
`surface. Configurations such as the unbalanced magnetron can expand the
`volume. Auxiliary electron sources can be used to enhance the plasma
`density between the target and the substrate.11021 Magnetic fields in the
`vicinity of the substrate can also be used to enhance reactive gas ionization
`and bombardment. For example using a magnetic field (l OOG) in the
`vicinity of the substrate, the ion flux was increased from 0.1 ma/cm2 to 2.5
`ma/cm2 in the unbalanced magnetron reactive sputter deposition of Al20 3. 1io31
`
`6.7
`
`TARGETS AND TARGET MATERIALS
`
`For demanding applications, a number of sputtering target proper(cid:173)
`ties must be controlled in order to have reproducible processin_g.11041 The
`cost of large-area or shaped sputtering targets can be high. Sometimes by
`using a little ingenuity, cheaper configurations can be devised such as
`making large plates from overlapping mosaic tile, rods from stacked
`cylinders, etc. Conformal targets, which conform to the shape of the
`substrate, may be used to obtain uniform coverage over complex shapes
`and in some instances may be worth the increased cost.
`
`Page 14 of 20
`
`

`

`Physical Sputtering and Sputter Deposition 349
`
`6.7.1 Target Configurations
`
`Targets can have many forms. They may have to be of some
`predetermined shape to fit supplied fixtures or be conformal to the sub(cid:173)
`strate shape. For example conformal targets may be a sector of a cone for
`coating a rotating cone, hemispherical to coat a hemisphere, axial rod to
`coat the inside of a tube, etc. The targets may be moveable or be protected
`by shutters to allow "pre-sputtering" and "conditioning" of the target
`before sputter deposition begins. Common sputtering target configura(cid:173)
`tions are the planar target, the hollow cylindrical target, the post cathode,
`the conical target, and the rotating cylindrical target.C105U106J
`A single target may be used to deposit alloys and mixtures by
`having different areas of the target be of different materials. For example,
`the mosaic target may have tiles of several materials, the rod target may
`have cylinders of several materials, etc. The composition of the film can
`then be changed by changing the area ratios. When using this type of
`target, the pressure should be low so that backscattering does not give
`"cross-talk" between the target areas. If cross-talk occurs, the sputtering rates
`may change as one material is covered by the other which has a lower
`sputtering rate.
`Multiple targets allow independent sputtering of materials and can
`be used to allow deposition of layers, alloys, graded compositions, etc. If
`both the targets and the substrates are stationary, the flux distribution from
`each target must be considered. Often when using large area targets, the
`substrates are rotated sequentially in front of the targets to give layered
`structures and mixed compositions
`Targets of different materials can have different plasma character(cid:173)
`istics in front of each cathode.D07J This can be due to differing secondary
`electron emission from the target surfaces. If the substrates are being
`rotated in front of the sputtering target(s), changes in the plasma may be
`observed depending of the position of the fixture, particularly if the fixture
`has a potential on it.
`"Serial co-sputtering" is a term used for a deposition process
`where material from one sputtering target is deposited onto another sputter(cid:173)
`ing target from which it is sputtered to produce a graded or mixed composition.
`Serial co-sputtering can be done continuously if the second target is periodi(cid:173)
`cally rotated in front of the first target and then in front of the substrate.£1081
`
`Page 15 of 20
`
`

`

`r
`
`Physical Sputtering and Sputter Deposition 351
`
`giving a porous material, and act as virtual leaks and contaminant sources.
`Porous targets can adsorb contaminants such as water and introduce a
`processing variable which may be difficult to control. For materials with
`poor thermal conductivity, thin targets are more easily cooled than thick
`targets thus reducing "hot-spots" and the tendency to fracture.
`Targets which have been formed by vacuum melting (metals) or
`chemical vapor deposition (metals, compounds) are generally the most
`dense. Less dense targets are formed by sintering of powders in a gaseous
`or vacuum atmosphere with hot isostatic pressing (HIP) producing the
`most dense sintered product. Sintering sometimes produces a dense
`surface layer ("skin") but the underlying material may be less dense and
`this material becomes exposed with use. In some cases, it may be useful to
`specify the outgassing rate of the target as a function of temperature.
`When using alloy or compound targets care must be taken that the
`target is of uniform composition, that is be homogeneous. This is particu(cid:173)
`larly a problem when sputtering magnetic alloy material such as Co,Cr,Ta;
`Co,Ni,Cr,Ta; CoCr,Pt; Co,Fe,Tb; or Co,Cr,Ni,Pt where material distribu(cid:173)
`tion in the target is extremely important. In some cases, the composition of
`the deposited material may be different from that of the target material in a
`reproducible way due to preferential loss of material. Common examples
`of this problem are: ferroelectric films of BaTi03,C1131 superconducting
`films such as YBa2Cu30 7, and magnetic materials such as GbTbFe.£1141 In
`the case of alloy deposition, the change in composition may be compen(cid:173)
`sated for by changing the target composition so as to obtain the desired film
`composition.£1151
`-
`Second phase particles in the target can lead to the development of
`cones on the target surface during use due to the differing sputtering rates
`of the matrix material and the second phase particles. Also, second phase
`material in the target appears to influence the nucleation of the sputter(cid:173)
`deposited material, possibly due to the sputtering of molecular species
`from the target.C1161 Second phase precipitates can be detected using
`electrical conductivity measurements.£1171
`In some cases, metal plates are rolled to a specific thickness to
`form the sputtering target. This can introduce rolling stresses and texturing
`that should be annealed before the plate is shaped to final dimensions.
`Annealing can cause grain growth which may be undesirable.
`The grain size and orientation of the target material can affect the
`distribution of the sputtered material and the secondary electron emission
`from the target surface. The distribution of sputtered material is important
`
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`Physical Sputtering and Sputter Deposition 355
`
`it may be due to preferential sputtering combined with a high surface
`mobility of the un-sputtered constituent on the surface. The mobile species
`form islands on the target surface and they grow with time. A high target
`temperature contributes to this effect. To restore the target surface the smut
`can be wiped off.
`Surface mobility can also cause the formation of nodules on the
`surface. For example, sputtering targets of indium-tin-oxide develop
`nodules on the surface with use. The origin of these nodules is uncertain
`and they must be machined off periodically.
`
`6. 7. 7 Target Conditioning (Pre-Sputtering)
`
`Generally the surface of the sputtering target is initially covered
`with a layer of oxide or contaminants and may be "pre-sputtered" before
`deposition begins. This pre-sputtering can be done with a shutter between
`the target and the substrate or by moving the substrate out of the deposition
`region while pre-sputtering of the target is being performed. When volt(cid:173)
`age-controlled power is first applied to a metal target, the current will be
`high and drop as the discharge comes to equilibrium.[1241 The initially high
`current is due to the high secondary emission of the metal oxide as
`compared to the clean metal and the high density of the cold gas. As the
`oxide is removed from the surface and the gas heats up, the current density
`will fall. This target conditioning can introduce contaminant gas into the
`plasm.a. One advantage in using a lock-load deposition system is that the
`sputtering target can be maintained in a controlled environment at all times
`and pre-sputtering becomes less of a processing variable from run-to-run.
`
`6. 7 .8 Target Power Supplies
`
`Target power supplies may be DC, AC, pulsed DC, rf, DC+ rf, etc.
`Continuous DC and AC power supplies are generally the most inexpen(cid:173)
`sive. Unipolar pulsed DC can be generated by chopping (interrupting) the
`continuous DC. Bipolar DC requires a special power supply. Continuous
`DC and low-frequency AC power supplies require an arc suppression
`( quenching) circuitry to prevent voltage transients from feeding back into the
`power supply and blowing the diodes. Arc suppression can be done by cutting
`off the voltage or by reversing the voltage polarity for a short period of time.
`
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`Physical Sputtering and Sputter Deposition 363
`
`purity from the supplier. Inert gases can be purified by passing them over
`a hot bed of reactive material such as titanium or uranium. Commercial gas
`purifiers are available that can supply up to 5 x 103 secs. Moisture can be
`removed from the gas stream by using cold zeolite traps. Gas purifiers
`should be routinely used on all sputtering systems in order to ensure a
`reproducible processing gas. Distribution of the gases should be in non(cid:173)
`contaminating tubing such as Teflon TM or stainless steel. For critical
`applications, the stainless steel tubing can be electopolished and a passive
`oxide formed. Particulates in the gas line can be eliminated by filtration
`near the point-of-use.
`
`6.9.7 Contamination from Deposited Film Material
`
`When a sputtering system is used for a long time or high volumes
`of materials are sputtered, the film that builds up on the non-removable
`surf aces in the system increases the surface area and porosity. This
`increases the amount of