`I DEPOSITION
`PRINCIPLES & PRACTICE
`
`
`
`Q DONALD L. SMITH
`
`INTEL 1115
`
`INTEL 1115
`
`
`
`
`
`Thin-Film
`
`Deposition
`
`Principles and Practice
`
`Donald L. Smith
`
`Boston, Massachusetts Burr Ridge, Illinois
`Dubuque, Iowa Madison, Wisconsin New York, New York
`San Francisco, California St. Louis, Missouri
`
`
`
`
`
`
`
`Library of Congress Cataloging-in-Publication Data
`
`Smith, Donald L. (Donald Leonard), date.
`Thin-film deposition: principles and practice / Donald L.
`Smith.
`cm.
`p.
`Includes index.
`ISBN 0-07-058502-4
`1. Thin films.
`2. Vapor-plating.
`I. Includes index.
`II. Title
`TA418.9.T45S65
`1995
`621.3815'2~—dc20
`
`3. Thin film devices.
`
`94-47002
`CIP
`
`Copyright © 1995 by McGraw-Hill, Inc. All rights reserved. Printed in
`the United States of America. Except as permitted under the United
`States Copyright Act of 1976, no part of this publication may be repro-
`duced or distributed in any form or by any means, or stored in a data
`base or retrieval system, without the prior written permission of the
`publisher.
`
`ll 1213141516 IBT/IBT 198765432
`
`ISBN-13: 978-0-O7-058502-7
`
`ISBN-10: O-07-058502-4
`
`The sponsoring editor for this book was Stephen S. Chapman and the
`production supervisor was Suzanne W. B. Rapcavage. It was set in New
`Century Schoolbook by J. K. Eckert & Company, Inc.
`
`INTERNATIONAL EDITION
`
`Copyright © 1995. Exclusive rights by McGraw-Hill, Inc. for manufac-
`ture and export. This book cannot be re-exported from the country to
`which it is consigned by McGraw-Hill. The International Edition is not
`available in North America.‘
`
`When ordering this title, use ISBN 0-07-113913-3.
`
`Cover photo: A thin film ofthe high-temperature superconductor YBa2Cu3O-1 is being deposited from
`a pulsed-laser vaporization source onto a ~750°C MgO substrate shown glowing orange at the top of
`the picture. Pulses from a UV (248 nm) Kr!‘ excimer laser enter the vacuum chamber from the right
`and impinge at 45° upon a sintered pellet ofYBa2Cu3O7 situated near the bottom where the white
`glow originates. Energy from the pulses electronically excites and partially ionizes both the vaporiz-
`ing material and the 4 Pa of02 ambient gas. resulting in a spectacular plume ofglowing plasma. The
`pulsed-laser deposition process is discussed in Sec. 8.4. This photo of Douglas Chrisey‘s apparatus
`was taken by M.A. Savell at the U.S. Naval Research Laboratories, Washington, D.C., and appeared
`on the cover of the MRS Bulletin, February 1992. (Used by permission of MRS and NRL.)
`
`
`
` Information contained in this work has been obtained by McGraw-Hill, Inc., from sources
`believed to be reliable. However, neither McGraw-Hill nor its authors guarantee the accu-
`
`racy or completeness of any information published herein and neither McGraw-Hill nor
`
`its authors shall be responsible for any errors, omissions, or damages arising out of use of
`
`this information. This work is published with the understanding that McGraw-Hill and
`
`its authors are supplying information but are not attempting to render engineering or
`other professional services. If such services are required, the assistance of an appropriate
`professional should be sought.
`
`
`
`
`
`
`
`
`
`Chapter
`
`ration.” In Handbook of Thin Film Technology,
`York: McGraw-Hill.
`
`rmodynamics, the Kinetic Theory of Gases, and
`Lmbridge, Mass.: Addison-Wesley.
`
`Vacuum Technology
`
`Most of the film deposition processes to be discussed in this book oper-
`ate under some degree of vacuum. Only atmospheric-pressure CVD
`does not, but the same vacuum techniques of contamination reduction
`and process control still apply to it. Vacuum technology is a large topic
`which is well treated in textbooks such as those in the recommended
`readings list at the end of this chapter. Our purposes here are more
`specific: first, to become oriented to the general topic and, second, to
`examine certain aspects of vacuum technology that are particularly
`relevant to film deposition and deserve special emphasis. As we know,
`“Nature abhors a vacuum,” so good equipment and techniques are
`needed to create one.
`
`Figure 3.1 is a schematic diagram of a typical vacuum system for
`thin-film deposition. The purpose and functioning of the components
`Shown will be elaborated upon in the subheadings below. Sometimes
`not all of these components will be required for a particular process.
`AS shown, the substrate is introduced through a “load-lock” chamber
`t0 allow the main process chamber to remain under vacuum, because
`this reduces contamination and shortens substrate turnaround time.
`The roughing pump evacuates the load-lock chamber from atmo- .
`Spheric pressure after the substrate has been loaded into it and before
`the Valve is opened into the process chamber. Once the substrate is in
`the process chamber, it is heated and controlled at the film deposition
`temperature. Process gases and vapors are metered into the chamber
`lhrough mass flow-controlled supply lines, which are discussed more
`111 Sec. 7.1.2. Process pressure is measured by a vacuum gauge that
`can be coupled to a motor-driven throttle valve in the pump throat for
`Pressure control. Sometimes, pressure is controlled instead by cou-
`» as
`
`
`
`
`
`process gas supply
`
`load—lock
`chamber
`
`
`36
`Vacuum Technology
`
`
`
`
`l I
`
`roughing
`Pump
`
`pressure
`* control
`l
`cop
`
`N2 purge
`
`
`
`
`
`.
`toxic
`vapor
`adsorbent
`canister
`
`
`roughing/backing
`pump
`
`N2ballast
`
`. purge
`
`
`
`Figure 3.1 Typical vacuum-system components for thin-film deposition.
`
`
`
`pling the vacuum gauge to the gas-supply metering valve, but that
`technique does not allow independent control of gas flow rate and
`pressure. Finally, process and impurity gases are evacuated through a
`high-vacuum pump followed by a “backing” pump which often serves
`to rough out the process chamber from atmosphere as well. For pro-
`ne stage of pumping is needed. The
`
`dled. Pumps fall into two categories by pumping principle: those that
`displace gas from the vacuum chamber and exhaust it to atmosphere,
`and those that trap it within the pump itself. Displacement pumps are
`often oil lubricated, which means that great care must be taken to
`avoid contaminating the process chamber with oil. On the other hand,
`they can pump large gas flows continuously Without becoming satu-
`rated like trapping-type pumps do. Trapping pumps of the cryogenic
`Variety are not recommended when pumping flammable vapors,
`
`
`
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`38
`Vacuum Technology
`
`
`
`
`because usually air is pumped too. When the saturated pump is
`warmed up to regenerate it, the released gas can form an explosive
`mixture. Sputter-ion pumps are trapping pumps that do not release
`chemically active trapped gases, but they can periodically release
`bursts of inert gases during operation. Unlike the cryogenic trapping
`pumps, they cannot be regenerated when saturated but must have
`their internal parts replaced.
`The next consideration is that the pump operate well at the desired
`process pressure. Notice in the table that the recommended pressure
`limits for process operation are much narrower than the ultimate
`pressure limits. The ultimate upper limit is the maximum pressure at
`which the pump can be started, whereas the process upper limit is the
`maximum pressure at which it will operate well on a continuous basis.
`Excessive operating pressure causes overheating or stalling, as well as
`rapid saturation in the case of trapping pumps. If the maximum start-
`ing pressure is less than 1 atm or if a turbomolecular or molecular
`drag pump is being used, the system must first be roughed out with
`another pump. The lower pressure limit of process operation is the
`pressure below which pumping speed drops off, except in the case of
`the oil—sealed rotary pump, which is widely used for both roughing and
`backing. There, the problem at < 10 Pa is contamination from pump
`oil back-diffusing (backstreaming) into the process chamber, as will be
`discussed in Sec. 3.4.1. The other two choices for roughing and backing
`avoid the oil problem but have other drawbacks. The dry rotary pumps
`are larger, noisier, and more expensive. The cryosorption pumps
`require frequent refills with liquid nitrogen and frequent regeneration
`due to saturation with pumped gas.
`Another consideration is the cost factor. Approximate purchase costs
`per unit of pumping speed for medium-sized pumps are listed in Table
`3.1. There is a large difference among pumps, but in many cases the
`higher cost is justified by improved performance. Moreover, there are
`other cost factors involved, such as maintenance, service life, and
`whether a backing or roughing pump is required. The pumps used for
`backing or roughing are the dry and oil—sealed rotary pumps and the
`cryosorption pump. These all have relatively high cost per unit of
`pumping speed in l/s. However, they require a much smaller speed
`than the high-vacuum pump on a given chamber, because of their
`higher operating pressure [see Eq. (3.3) below].
`The molecular-drag pump is the most recent addition to the selec-
`tion of available pumps. Sometimes, it is integrated with a turbomo-
`lecular pump in a single unit, thus increasing the upper operating
`pressure of the latter. By itself, it still has a wide operating pressure
`range that spans the transition-flow regime. This range encompasses
`all of the glow-discharge plasma deposition processes as well as some
`
`
`
`
`
`. When the saturated pump is
`eased gas can form an explosive
`oping pumps that do not release
`lt they can periodically release
`»n. Unlike the cryogenic trapping
`when saturated but must have
`
`pump operate well at the desired
`2 that the recommended pressure
`1Cl'l narrower than the ultimate
`limit is the maximum pressure at
`reas the process upper limit is the
`perate well on a continuous basis.
`overheating or stalling, as well as
`;ng pumps. If the maximum start-
`if a turbomolecular or molecular
`
`11 must first be roughed out with
`limit of process operation is the
`ad drops off, except in the case of
`widely used for both roughing and
`) Pa is contamination from pump
`lto the process chamber, as will be
`) choices for roughing and backing
`drawbacks. The dry rotary pumps
`ensive. The cryosorption pumps
`itrogen and frequent regeneration
`
`actor. Approximate purchase costs
`am-sized pumps are listed in Table
`»ng pumps, but in many cases the
`performance. Moreover, there are
`is maintenance, service life, and
`.p is required. The pumps used for
`d oil-sealed rotary pumps and the
`a relatively high cost per unit of
`ey require a much smaller speed
`given chamber, because of their
`3.3) below].
`most recent addition to the selec-
`
`5, it is integrated with a turbomo-
`LS increasing the upper operating
`Ltill has a wide operating pressure
`v regime. This range encompasses
`position processes as well as some
`
`3.1
`
`Pump Selection and Exhaust Handling
`
`39
`
`CVD processes, so this pump is particularly useful for thin-film work.
`Moreover, its principle of operation is a classic illustration of gas
`kinetic behavior. As illustrated in Fig. 3.2, the pump incorporates a
`channel between the stationary surface of a “stator” and the surface of
`a “rotor,” which is moving to the right at a supersonic velocity 11,. In
`construction, one of the two surfaces is a spiral channel, and the other
`is a cylinder surrounding it and almost touching the channel rim. In
`the molecular-flow regime, molecules bounce from surface to surface
`without encountering collisions along the way. They approach the
`rotor surface with mean thermal velocity (1 and bounce off of it having
`the sum of two velocities, namely: 6 , which is randomly directed; and
`u,, which is directed to the right. Thus, molecules are pumped to the
`right. This behavior is a consequence of the strong interaction
`between gas molecules and surfaces, which causes them to become
`trapped temporarily and thus lose memory of the direction from which
`they approached. (Helium may be an exception, at least on very
`smooth surfaces.)
`The molecular-drag pump can also operate at the lower end of the
`fluid-flow regime. There, a velocity profile from u, to zero develops
`across the channel as shown in Fig. 3.2b. In the absence of pressure
`gradients, this profile would be linear in accordance with a force bal-
`ance using Eq. (2.28) and as discussed in fluid-mechanics texts. How-
`ever, the pumping action causes pressure to decrease from right to
`left, which causes some back flow in that direction and somewhat “col-
`lapses” the velocity profile, as shown. Now, the mass pumping rate is
`proportional to molecular concentration, 11, and thus to pressure, p,
`
`l.1=U.r
`
`rotor
`
`channel , stator
`(b) fluid flow
`
`(a) molecular flow
`
`F59“"8 3.2 Molecular-drag pump operation in the two flow regimes.
`
`
`
`40
`
`Vacuum Technology
`
`
`
`
`
`
`
`
`
`
`
`
`3.2 Problem Gases
`
`
`
`
`
`
`
`
`
`
` whereas it will be seen in Sec. 3.3 that the mass flow rate induced by a
`p drop in the fluid regime is proportional to p2. This means that as p is
`raised, back flow eventually becomes much larger than pumping rate,
`
`
`and this sets an upper limit to pump operating p.
`
`
`
`Some gases present special pumping problems. H2 and He have high
`molecular speeds because of their low mass [Eq. (2.3)]. This limits
`
`
`their compression in molecular-drag, turbomolecular, or oil-diffusion
`
`
`pumps, all of which use supersonic velocity of the pumping medium to
`
`
`push molecules along. Reactive, condensable or toxic gases require
`
`special handling whatever pump is used, and we will discuss these
`three categories in turn below.
`Reactive gases cause several problems. Acidic ones such as the
`chlorinated gases can decompose hydrocarbon pump oil and corrode
`metal pump parts. Oxidizing ones such as 02 can explode the pump—oil
`vapor in the exhaust. Flammable ones such as methane can explode
`when mixed with air in the exhaust. Decomposition and explosion of
`the oil are avoided by using perfluorinated pump oils. These are
`organic molecules that have all of their H replaced by F, which makes
`them chemically inert due to the high strength of the C-F bond. Even
`with these, acidic gases dissolved in the oil can corrode the pump, and
`flammable gases can explode in the exhaust system. These two prob-
`lems are avoided by N2 purging at the pump ballast port and at the
`pump exhaust casing, respectively, as shown in Fig. 3.1. Sufficient and
`reliable N2 purging can also prevent decomposition and explosion of
`hydrocarbon pump oil as an alternative to using the very expensive
`perfluorinated oil.
`N2 purging at the ballast port not only sweeps dissolved corrosives
`out of the pump oil, but also prevents vapor condensation. Conden-
`sates emulsify the oil and thus destroy its sealing and lubricating
`properties. The ballast port addresses this problem by injecting N2
`into the vapor as it is being compressed, which lowers the partial pres-
`sure of the vapor. If the dilution is sufficient, the vapor’s partial pres-
`sure drops below its saturation vapor pressure, at which point no
`condensation can occur. In addition, the equilibrium concentration of
`corrosive gas dissolved in the oil drops roughly in proportion to the
`drop in its partial pressure over the oil. One example of a condensable
`and corrosive vapor situation is the ’I‘iCl4 + H20 mixture sometimes
`used in the CVD of 'I‘iO2. The saturation vapor pressures of these two
`reactants at room T are about 1400 and 3000 Pa, respectively. To pre-
`vent their condensation upon compression of the process stream to 1
`atm (105 Pa), a minimum dilution of 105/1400 = 70/1 is necessary. If
`
`
`
`
`
`
`
`
`
`
`
`
`
`41
`Gas Throughput
`3.3
`
`
`the dilution provided in the process-gas supply and in the pump fore-
`line is less than 70/1, the difference must be made up at the ballast
`POW
`.
`.
`.
`when toxic gases are involved, special procedures are necessary to
`protect personnel and the environment. For example, arsine (AsH3),
`which is used in CVD of the important semiconductor GaAs, is one of
`the most toxic gases known. Oil removed from pumps for disposal or
`recycling should be assumed to contain some amount of any toxic
`gases which have been pumped, and it must be handled and labeled
`accordingly. The dissolved concentration can be reduced, though not
`necessarily to negligible levels, by a long purge with N2 before the oil
`is removed. Regardless of whether any pumps are employed, the pro-
`cess exhaust stream must be tightly sealed and must be treated to
`reduce the concentration of toxics to negligible levels before release to
`the environment. Treatment methods include adsorption (usuallyon
`activated charcoal) as shown in Fig. 3.1, washing in a scrubber, and
`decomposition by burning or by catalysis. Different methods are
`appropriate for each vapor. The chemical manufacturer and local gov-
`ernment authorities need to be consulted prior to selection of process
`equipment. In all cases, continuous environmental-monitoring sensors
`are advised at the gas supply, at the process chamber, and down-
`stream of the exhaust treatment equipment.
`
`3.3 Gas Throughput
`
`The required sizes of the pump and its connecting line to the process
`chamber are determined by the gas load imposed by the film deposi-
`tion process. The essential features of this situation are illustrated in
`Fig. 3.3. The mass flow of gas must satisfy the continuity equation,
`which is an expression of the law of mass (and also energy) conserva-
`tion. This equation is fundamental to all flow and film-deposition pro-
`
`vacuum
`gauge
`
`process
`
`
`chamber
`
`foreline conductance C1
`Process
`gas
`supp},
`flow (5;
`
`P0
`
`
`pump
`conductance Co
`
`..‘”mfl9
`
`F6‘c>'::‘5?/3:3‘
`
`le
`»il
`le
`of
`re
`as
`an
`
`id
`
`1e
`
`Id
`of
`ve
`
`xle
`.es
`N0
`re-
`
`
`
`
`
`
`.
`.
`out assing
`loa Qi
`
`throttle
`orifice
`conductance C2
`
`Flflure 3.3 Geometry for gas throughput calculations.
`
`
`
`
`
`Vacuum Technology
`42
`
`
`
`
`cesses, and it will come up again and again throughout the book. It
`has the general form
`
`
`
`input + generation = output + accumulation
`
`(3.1)
`
`
`
`That is, whatever mass or energy is introduced into a given space
`must either come out again or build up there. Here, the space is the
`process chamber’s gas phase. The generation term typically arises
`with chemical reactions (mass) or heat production (energy), and it is
`zero here. We will first consider operation at a constant process-cham-
`ber pressure, p2, so the accumulation term is also zero. There are two
`input terms: the mass flow rate of process gas supplied, Qs, and “outa
`gassing” from the chamber walls, Q, (neglecting unwanted leaks,
`which should have been plugged). Outgassing is evolution of gaseous
`contaminants from chamber materials, and its minimization will be
`discussed in Sec. 3.4.2. The supply flow, Q5, is ofien expressed in sccm,
`which is proportional to mc/s by the ideal-gas law, Eq. (2.10).
`The output term in Eq. (3.1) is the flow of gas toward and through
`the pump(s). In Fig. 3.3, there are three elements in this flow path,
`each of which has a certain “conductance,” C (liters/s, = l/s), for the
`gas. There is a throttle restriction (C2), a pumping line restriction
`(C1), and the pump itself (C0). The throttle is needed only when p2 is
`larger than the maximum operating p of the pump (see Table 3.1), as
`in the case of low-p CVD or glow-discharge processes run using a tur-
`bomolecular pump. The mass flow or “throughput,” Q, past these ele-
`ments is usually expressed in (Pa-l/s) or (torr-l/s), which is also
`proportional to mc/s by the ideal-gas law. For the two flow restrictions,
`C1 and C2, C is defined by
`
`
`
`where Ap is the pressure difference across the element and is the driv-
`ing force for the gas flow. For the vacuum pump, throughput is given
`by
`
`Q0 = Capo
`
`where the conductance C0 is known as the pumping “speed” (l/s). Usu-
`ally, C0 varies little with po over the pump’s operating range, meaning
`that pump throughput is proportional to its inlet pressure, po. Setting
`all of these Q values equal in accordance with Eq. (3.1), and taking the
`case where Q, << Q3 (it had better be!), we have
`
`(3-4)
`
`Qs + Qi = Qs = C2(P2 ~ 131) = C1(P1 - P0) = Copo
`
`
`
`
`
`3.3
`
`Gas Throughput
`
`43
`
`1. It
`
`3.1)
`
`)ace
`
`the
`ises
`it is
`am-
`
`two
`Jut-
`
`iks,
`:ous
`
`l be
`cm,
`
`ugh
`1th,
`the
`;ion
`
`2 IS
`, as
`;ur-
`
`ele-
`llS0
`ms,
`
`3.2)
`
`l'1V"
`I81’!
`
`
`
`For a given Q5, the three p values will adjust themselves to satisfy
`these equalities. Then, control of the process pressure, p2, is accom-
`plished by throttling down C2 as shown in Fig. 3.1.
`conductances depend on geometry and flow regime. The simplest
`case is an orifice in molecular flow, meaning that the molecular
`mean free path is larger than the orifice diameter (Kn > 1). In this
`case, the flux through the orifice in each direction (downstream and
`upstream) is equal to the impingement flux at the plane of the orifice
`as given by Eq. (2.18). It is an important property of molecular flow
`that these two fluxes are independent of one another. This is because
`of the fact that the molecules cross paths without colliding. (We will
`see in Sec. 3.4.1 how this causes oil backstreaming.) The orifice
`throughput is the difference between the downstream and upstream
`fluxes through the orifice times its area, A; thus
`
`Q2 = (Ji2‘Ji1)A ‘' [
`
`
`NA
`
`fim_fi,l
`
`A
`
`(P2 P1)
`
`—
`
`=0
`
`2(p2 pl)
`
`-
`
`(
`
`3.5)
`
`The term in square brackets is the orifice conductance per unit area,
`CA. For air at room temperature, conversion from SI units to liters
`and cm gives CA = 11.6 l/s~cm2, a very useful quantity to remember,
`since any restriction in the vacuum plumbing can be modeled approxi-
`mately as an orifice. The conductance of an orifice in fluid flow is
`much more complicated to analyze, but it is also much higher. There-
`fore, it usually is not the throughput-limiting element in Fig. 3.3
`unless it is made so deliberately, as in the case of the throttle valve.
`Appendix E gives the conductance of a fluid-flow orifice in the limit of
`sonic velocity in the orifice (“choked” flow).
`For long tubes in molecular flow, the conductance in l/s for air at
`room temperature is
`
`cm = 12.3¢3/L
`
`(3.6)
`
`where q) = tube diameter, cm
`L = tube length, cm; L >> 4)
`
`Note that Cm is proportional to (1)3, versus oz for the orifice in Eq. (3.5).
`TWO powers of (1) come from the area factor as in the case of the orifice,
`and the third power comes from the fact that the axial distance tra-
`Versed by a molecule between bounces off the wall is proportional to 45.
`F0!‘ gases other than air, Cm ‘scales according to Eq. (2.18). For long
`tubes in fluid flow,
`
`
`
`
`
`
`
`This equation assumes that the gases mix well in the chamber and
`that the ideal-gas law holds. Note here that pi also scales with pg,
`whereas in the unthrottled case of Eq. (3.8), pg does not appear. It does
`appear in the throttled case because it is presumed that Q5 is already
`set at the maximum which the pump can handle. Thus, P2 cannot be
`increased by increasing Qs, which would have diluted the contami-
`nant. Instead, p2 can only be increased by closing the throttle to
`reduce C2, which also raises pi. For this reason, film purity is
`improved by reducing process pressure, p2, in cases where the pump
`must be throttled. Alternatively, a pump operable at higher po can be
`selected. The molecular-drag pump is attractive here, because it also
`minimizes oil contamination. Equation (3.9) also applies to atmo‘
`
`44
`
`Vacuum Technology
`
`Cf: 1.41q>4p/L
`
`(3.7)
`
`where p is the average pressure from end to end, in Pa. Note that C is
`independent of p in molecular flow but proportional to p in fluid flow,
`Thus, in fluid flow, Q is proportional to p2. One power of p comes from
`the concentration of the fluid, and the second comes from the Ap driv-
`ing force for flow. The third and fourth powers of 4) here come from the
`viscous drag force, which is proportional to the radial gradient in axial
`flow velocity [Eq. (2.28)]. This velocity is inversely proportional to tube
`area for a given volumetric flow rate. For gases other than air, Cf
`scales inversely with viscosity. In the transition region (0.01 < Kn < 1),
`both of the above equations will estimate C somewhat low.
`
`The minimum size of foreline tube and pump for a given process can
`be calculated from the anticipated gas load, Q5. The tube should be
`sized so that its C is at least as large as that of the pump, since tubes
`are less expensive than pumps. However, depositing films often
`become contaminated by the outgassing load, Qi. In such cases, a
`larger pump than the minimum is desirable. For processes operating
`without a throttle, faster pumping (larger C0) reduces the ‘partial p of
`these contaminant gases, pi, in accordance with Eq. (3.3): that is,
`
`3:-_h-vl..(\.\m.i.u.,...AA..,...
`
`
`5*’3.2.££3..t-Us::m9_..«-2
`
`:5???§‘§-..‘:.:3E~?..?.€.:§
`
`tri
`(/2N.m
`
`pi = Qi/C0.
`
`For processes operating at some fixed pressure p2 with a throttle,
`faster pumping instead allows the supply gas flow, Q3, of (presumably
`pure) process gas to be higher, which dilutes the flow of outgassing
`contaminant, Qi, and thus reduces its pi by
`
`Pi _
`P2
`
`Q,
`Q,
`Q + Q8 z 6;
`
`(3.9)
`
`
`
`
`
` 3.4
`
`45
`
`Contamination Sources
`
`
`Spherjc-pressure CVD, where Q3 can be increased to reduce pi,
`because there is no pump throughput limitation.
`
`The rate of pumpdown from 1 atm can also be calculated from the
`continuity equation, Eq. (3.1). Here we are considering only evacua-
`tion of the air in the process-chamber volume, V; we are not supplying
`any additional gas. Therefore, the input term is zero. The accumula-
`tion term is the evacuation rate. We also assume that the output term
`is limited by the pump throughput, since (1) the throttle valve will be
`open, and (2) the tube throughput is proportional to p2 and will be rel-
`atively high at the high-p end. Thus, using the notation of Fig. 3.3, we
`have p2 = po, and Eq. (3.1) becomes
`
`
`
`(3.7)
`
`hat C is
`iid flow.
`ies from
`
`Ap driv-
`'rom the
`in axial
`l to tube
`
`L air, Cf
`Kn < 1),
`
`:ess can
`
`L011ld be
`:e tubes
`LS often
`:ases, a
`
`)erating
`tial p of
`is,
`
`(3.8)
`
`;hrottle,
`iumably
`gassing
`
`(3.9)
`
`ber and
`
`Vlth p2,
`. It does
`
`already
`nnot be
`ontami-
`ottle to
`
`1rity is
`e pump
`
`{can be
`: it also
`) atmo-
`
`0=Cp+V-C-1-B2
`02
`dt
`
`(310)
`'
`
`where both terms have the units of throughput (Pa-l/s). Rearranging,
`integrating, and applying the initial condition that p2 = p20 at time t =
`0, we have
`
`-1:
`
`p2 = p20eXp(V/C0)
`
`(3.11)
`
`This is a classic exponential-decay situation and is analogous to dis-
`charging a capacitor through a resistor. V/Co is the time constant for
`the process, and for a typical case of a 100 l chamber and a 10 l/s
`roughing pump, the time constant is 10 s. This means that evacuation
`to lO‘5 Pa would take place in a mere 4 min. In practice, however,
`Eq.. (3.11) is obeyed only down to 10 Pa or so. At lower p, evacuation
`slows down progressively as the added load from outgassing of the
`chamber walls now dominates the situation. Outgassing is one of the
`major contamination sources that we will discuss in the next section.
`
`3.4 Contamination Sources
`
`The sensitivity of a deposition process to contamination varies greatly
`with the materials involved and depends on whether the depositing
`film} incorporates or rejects the contaminants arriving at its surface.
`T1115, in turn, depends on the chemical. reactivity of the surface with
`thfi contaminants. For example, gold is not very reactive and can be
`deposited with high purity in a poor vacuum, whereas aluminum
`reacts with and incorporates almost all arriving contaminants except
`the lnert gases. The degree to which contamination needs to be con-
`trolled must be assessed separately for each material system by analy-
`515 Of the resulting film. Whenever film properties are poorer than
`
`
`
`
`
`46
`
`Vacuum Technology
`
`proc-
`chan
`
`(a) mi
`
`process
`charnb<
`
`a
`".
`
`l
`
`(6) flui«
`
`Figure 3.4
`phase at
`
`there. .
`cleanin
`process
`stream
`eventu:
`as we v
`
`diffusio
`backstr
`A mo
`of flow :
`
`in Figs.
`face pai
`face, bl.‘
`undocui
`from th
`stream.
`
`
`
`pumps, gas evolution from chamber materials, and dust stirred up
`from surfaces.
`
`3.4.1 Oil backstreaming
`Oil backstreaming into the process chamber can occur whenever oil is
`used as the pump operating fluid or lubricant. The rate at which this
`occurs is much higher than one would predict from the relatively low
`room-temperature vapor pressures (pv) of oils used in vacuum technol-
`ogy. This is because pumps usually run hot, and the pv of any material
`rises very steeply (exponentially) with T. For example, a typical rotary
`pump oil has a pv of 1O’4 Pa at 25° C but a pv of 10"1 Pa at a pump
`operating temperature of 85° C.
`
`between the process
`chamber and an oil diffusion pump. Despite the fl
`ow of gas toward the
`pump, oil molecules bounce freely backward towa
`rd the process cham—
`ber without encountering any resistance from the countercurrent gas
`flow, because the gas and oil molecules do not collide with each other
`when Kn > 1. Some but not all of the oil will condense on the room-
`temperature walls of the tube before reaching the chamber. The rem‘
`edy is to place in the tube a baffle which is optically opaque, meaning
`that there is no line-of-sight path for an oil molecule through it with-
`out encountering at least one surface (two is better). Figure 3.441
`shows the traditional “chevron” bafiie or trap, named after the look—
`alike sergeant’s stripes. The trap is cooled with chilled water or, better
`yet, liquid nitrogen (LN2), so that the oil condenses on it and stays
`
`
`
`
`
`
`
`can be introduced in all four
`transport, deposition, and
`th the source and analysis
`0, respectively. Here, we are
`
`nants responsible for this result?
`As discussed in Chap. 1, contaminants
`steps in the deposition process: source,
`analysis. The contaminants associated wi
`steps will be addressed in Chaps. 4 and 1
`
`
`
`
`
`
`
`3.4.1
`
`
`Oil backstreaming
`47
`
`
`
`/ / /
`
`Oil
`
`'
`
`2"
`process A I ’ / ’
`chamber//7\/ / / /
`
`l
`olecular flow
`
`a m
`)
`
`(
`
`uncooled creep path
`
`\\\
`' gaS\O
`
`diffusion
`Pump
`
`~—~—-—d '1
`surface creep
`
`ami.
`
`four
`and
`
`lyS1S
`2 are
`
`nent
`;rate
`
`‘ting
`pp1i-
`g in
`also
`
`iber,
`:ipal
`will
`the
`
`1 up
`
`)il is
`this
`low
`mol-
`erial
`
`tary
`ump
`
`flow
`elec-
`cess
`
`l the
`lam-
`
`. gas
`ther
`wom-
`
`rem-
`riing
`with-
`3.4a
`ook—
`atter
`
`A1’ purge O laminar flow plug flow approximation
`fl0W Q
`,..--vh.--5
`,._.as__.‘
`>
`/
`I I L/_, /I
`.
` process
`
`chamber
`
`turbopump
`
`
`P1-1(2)
`Z
`
`(b) fluid flow
`
`Figure as Oil backstreaming behavior in the two flow regimes, showing both gas-
`phase and surface paths
`
`there. However, the trap will bevwarmed up on occasion, either for
`cleaning or due to coolant failure. A valve can be used to block off the
`process chamber on such occasions, but then oil still reaches the down-
`stream face of the valve and is bound to work its way into the chamber
`eventually. Gas purging of the warm trap can stop the backstreaming,
`as we will see below, but this is getting complicated. It is clear why oil
`diffusion pumps are listed in Table 3.1 as having the highest risk of oil
`backstreaming.
`A more insidious backstreaming mechanism which is independent
`9f flow regime is surface diffusion (or “creep,” or migration), as shown
`1“ Figs. 3.4a and b. It is possible to stop this with a trap having no sur-
`face Pathway through it that does not encounter an LN2-cooled sur-
`face» but most traps do not have this feature. Another possible but
`undocumented remedy is to heat the surface so that the oil evaporates
`mm the surface and into the cold trap or, in fluid flow, into the gas
`St’°eam- Even turbomolecular pumps, which are generally accepted as
`
`
`
`
`
`
`
`
` 48
`
`Vacuum Technology
`
`
`
`
`
`being clean due to the very high compression ratio for heavy molecules
`such as oil, would seem t