`
`(cid:44)(cid:49)(cid:55)(cid:40)(cid:47)(cid:3)EXHIBIT 10(cid:22)(cid:21)
`
`
`
`Copyright © 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—8155-1422-0
`Printed in the United States
`
`Published in the United States of America by
`Noyes Publications
`169 Kinderkarnack Rd., Park Ridge, NJ 07656
`
`1098765432
`
`Library of Congress Cataloging-in-Publication Data
`
`Mattox, D. M.
`Handbook ofphysical vapor deposition (PVD) processing/ by
`Donald M. Mattox.
`p.
`cm
`Includes bibliographical references and index‘
`ISBN 03155-14224)
`I. Vapor-plating--Handbooks, manuals, etc.
`T8695.M38
`I998
`67l.7'35—-dc21
`
`I. Title.
`
`97-44664
`ClP
`
`Page 2 of 20
`Page 2 of 20
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`7
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`228 Handbook of Physical Vapor Deposition (PVD) Processing
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`
`
`CURRENT
`MONITOR
`
`ROTATING
`CAGE
`
`
`
`
`
`CURRENT’
`MONITOR
`
`+ HIGH VOLT-
`AGE SUPFLY
`
`- HIGH VOLTAGE +
`SUPPLY
`
`SF_-UTTERING
`CATHODE
`
`GROUND SHIELD
`
`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—
`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
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`
`
`
`Low-Pressure Plasma Processing Environment 229
`
`The suppression of arcs generated in the DC discharge (are sup-
`pression) are important to obtaining stable performance of the DC power
`supplies particularly when reactively sputter depositing dielectric films.[461
`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-
`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.[48] 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
`Page 4 of 20
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`
`
`
`
`
`
`a. Continuous DC
`b. Unipolar Pulsed DC
`0. 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 electronicsw] 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—
`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).[5°]
`
`4.4.4 Magnetically Confined Plasmas
`
`Balanced Magnetrons
`
`In surface magnetron plasma configurations the electric (E) (vec-
`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
`Page 5 of 20
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`
`
`6
`
`Physical Sputtering and
`Sputter Deposition
`
`(Sputtering)
`
`6.1
`
`INTRODUCTION
`
`The physical sputtering (sputtering) process, or pulvérisation as
`the French call it, involves the physical (not thermal) vaporization of atoms
`from a surface by momentum transfer from bombarding energetic atomic—
`sized particles. The energetic particles are usually ions of a gaseous
`material accelerated in an electric fieldloal 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—
`chemical sputtering” have been associated with the process whereby bom—
`bardment of the target surface with a reactive species produces a volatile
`speciesm This process is now often termed “reactive plasma etching” or
`“reactive ion etching” and is important in the patterning of thin filmsm
`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-
`tion was not widely used in industry until the need developed for reproducible,
`315
`
`
`Page 6 of 20
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`
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`Physical Sputtering and Sputter Deposition 321
`
`
`
`Figure 6-2. Collision of particles.
`
`The maximum energy is transferred when cos 6 = l ( zero degrees)
`andJl/I, =M. 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-
`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
`sputter deposition” of oxides and nitrides.
`It is interesting to note that
`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
`reactive sputter deposition.
`In some cases, energetic ions of the target
`material can bombard the target producing “self-sputtering.” This effect is
`important in ion plating using ionized condensablc ions (“film ions”) formed
`by are 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
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`338 Handbook of Physical Vapor Deposition (PVD) Processing
`
`material is removed from the target layer-by-layer. This allows the deposi-
`tion of some rather complex alloys such as WzTi for semiconductor
`metallization,[86] A1:Si:Cu for semiconductor metallizationlsfl and M(etal)-
`Cr-AlcY alloys for aircraft turbine blade coatings.
`
`6.5.2 Reactive Sputter Deposition
`
`Reactive sputter deposition from an elemental targeti88m9] relies
`on: (a) the reaction of the depositing species with a gaseous species, such 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, 02) 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 surfacelm Poisoning of a target surface greatly reduces the sputter-
`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,[9°]’[93] 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 decreases. 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
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`
`
`
`Physical Sputtering and Sputter Deposition 339
`
`deposition might be 20% nitrogen and 80% argon where the partial pres-
`sure of nitrogen during deposition is 2 x 10'4 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.
`
`.__..>
`
`0 t'm
`Operating
`Conditions
`
`} pl um
`
`
`
`
`
`NitrogenPartialPressure
`
`Continuously Increasing Flow
`
`__....._>
`Nitrogen Flow
`
`Figure 6-8. Nitrogen partial pressure and flow conditions for the reactive sputter deposi-
`tion of TiN with constant target power (adapted from Ref. 51).
`
`In reactive deposition, the reactive gases are being pumped (“get-
`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.[901 The gas
`density (partial pressure) of the reactive gas in the plasma can be monitored by
`optical emission spectroscopy or massspectrometry techniques.[91]'{93]
`
`
`
`Page 9 of 20
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`340 Handbook of Physical Vapor Deposition (PVD) Processing
`
`Since gas pressure is important to the properties of the sputter depos—
`ited film it is important that the vacuum gauge be periodically calibrated and
`located properly and pressure variations in the chamber be minimized.
`
`>Optionai
`
`Microprocessor
` Mass Flow
`Optical Emission
`Meters
`Spectrometer
`
`
`Spectrometer
`
`Vacuum Gauges
`
`1R Pyromeier
`HI-Vac Valve
`
`Alf
`High Vacuum Pump(s)
`
`Variable Conductance By-Pass
`Mass Flow
`Controllers
`
`Roughing Pump(s)
`GI: Manifold
`
`\ Roughing Vatve
`Sputtering
`Targets (4)
`
`Figure 6-9. Typical reactive sputter 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—
`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 + C'—-> 2N° and NH3 + e‘ —>N° + 3H“)
`
`0 Production of new molecular species that are more
`chemically reactive and/or more easily absorbed on surfaces
`
`(e.g., 02 + e‘-—> 20° then 00 + 02 —> 03)
`
`Page 10 of 20
`Page 10 of 20
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`
`
`Physical Sputtering and Sputter Deposition 341
`
`- Production ofions—recombination at surfaces releases energy
`
`0 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—
`tion, application of an rf to 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-
`ated from the plasma (“sputter ion plating” or “bias sputtering”). Concur-
`rent bombardment enhances chemical reactions and can densify the deposit-
`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-
`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 filml94] The lowest resistiv-
`ity is attained with both a high deposition temperature and concurrent
`bombardment although a low-temperature deposition with concurrent bom-
`bardment comes close.
`
`W.”
`
`Page 11 of 20
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`342 Handbook of Physical Vapor Deposition (PVD) Processing
`
`
`
`-H.¥,_I
`
`190
`160
`140
`120
`
`100
`
`
`
`RESISTIVITY(poem)
`
`GOLD COLOR
`,_~...‘
`
`o3888
`
`0
`
`0.01
`
`0.02
`
`003
`
`0.04
`
`0.05
`
`0.06
`
`0.01
`
`0.06
`
`009
`
`0.10
`
`0.11
`
`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.)
`
`1700
`
`0.28
`0.20
`
`reslstlvlly
`Normalized
`
`0.28
`0.20
`
`Blas = 0V
`
`Blas = 400V
`
`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 2102 and
`TiOZ, which are used to form anti-reflection and band—pass coatings on optical
`
`Page 12 of 20
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`Physical Sputtering and Sputter Deposition 343
`
`components, indium—tin-oxide (ITO), is a transparent electrical conductor
`and SiOLs, is a material of interest as a transparent, moisture-permeation-
`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 (Csz) or
`methane (CH4) for carbon, silane (SiH4) for silicon, and diborane (BzHe)
`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 silicideslgs] It should be noted that co-
`
`deposition does not necessarily mean reaction. For example, carbon can be
`deposited with titanium to give a mixture ofTi + 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—
`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—
`tion films are formed by depositing thousands of alternating layers of high-
`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-
`ity of the reactive gas. Thus one can form layers of Ti-TiN-Ti by changing
`the availability of the nitrogen. Since nitrogen has been incorporated in the
`
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`348 Handbook ofPhysical 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-
`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 configurationllou
`
`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 ofien 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 substratell‘m 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 (100G) 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 ofA1203. “031
`
`6.7
`
`TARGETS AND TARGET MATERIALS
`
`For demanding applications, a number of sputtering target proper-
`ties must be controlled in order to have reproducible processing.“041 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
`Page 14 of 20
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`
`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—
`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-
`tions are the planar target, the hollow cylindrical target, the post cathode,
`the conical target, and the rotating cylindrical targetllofluos]
`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 backseattering 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-
`istics in front of each cathodelmn 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-
`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—
`cally rotated in front of the first target and then in front of the substrate.”031
`
`
`
`Page 15 of 20
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`
`
`
`Physical Sputtering and Sputter Deposition 35]
`
`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— '
`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-
`
`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 BaTiO3,[“3] superconducting
`films such as YBazCu3O7, and magnetic materials such as GbTbFeIl 14] In
`the case of alloy deposition, the change in composition may be compen—
`sated for by changing the target composition so as to obtain the desired film
`compOsitionlml
`“
`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—
`deposited material, possibly due to the sputtering of molecular species
`from the targetiué] Second phase precipitates can be detected using
`electrical conductivity measurementsllm
`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
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`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 (Pro-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—
`age-controlled power is first applied to a metal target, the current will be
`high and drop as the discharge comes to equilibrium.“241 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
`plasma. 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-
`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
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`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—
`contaminating tubing such as TeflonTM 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
`
`Whenasputteringsystemisusedforalongtimeorhighvolumes
`
`of materials are sputtered, the film that builds up on the non—removable
`surfaces in the system increases the surface area and porosity. This
`increases the amount of vapor contamination that can be adsorbed and
`retained on the surface. This source of contamination can be reduced by
`periodic cleaning and controlling the availability of water vapor during
`process cycling either by using a load—lock system or by using heated
`system walls when the system is opened to the ambient (Sec.3.12.2).
`The film buildup can also flake-off giving particulate contamina-
`tion in the deposition systemfil‘m Fixturing should be positioned such that
`particulates that are formed do not fall on the substrate surface. The effects
`of contamination from this source can be minimized by having the sub—
`strate facing downward or sideways during deposition. The system should
`be periodically “vacuumed” using a HEPA—filtered vacuum cleaner. The
`use of a “soft—rough” and a “soft-vent” valve minimizes “stirring-up” the
`particulate contamination in the system.
`
`6.10 ADVANTAGES AND DISADVANTAGES OF
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`SPUTTER DEPOSITION
`
`Advantages in some cases:
`
`' Any materia