`
`.I
`
`'I
`
`|
`
`|
`
`THIN—FILM
`DEPOSITION
`PRINCIPLES & PRACTICE
`
`.
`
`.
`
`_
`
`GII-'—ETTE 1011
`
`GILLETTE 1011
`
`

`

`
`
`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.T45$65
`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.
`
`111213141516 IBT/IBT 198765432
`
`ISBN-13: 978-0-07-058502-7
`
`ISBN-10: 0-07—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 YBaQCu307 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) KrF excimer laser enter the vacuum chamber from the fight
`and impinge at 45° upon a sintered pellet onBa20u307 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 ofOz 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 Rehnology,
`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
`to 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
`teKnperature. Process gases and vapors are metered into the chamber
`lhI‘OUgh mass flow-controlled supply lines, which are discussed more
`In 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-
`* 35
`
`

`

`
`
`
`36
`Vacuum Technology
`
`
`
`
`process gas supply
`
`load-lock
`chamber
`
`pressure
`
`
`
`control
`throttle
`
`
`loop
`valve toxic
`
` N2 purge vapor
`
`
`
`
`
` ‘
`adsorbent
`
`high
`canister
`roughing
`
`vacuum
`Pump
`PumP
`
`
`
`roughing/backing
`
`pump
`. purge
`N2ballast
`
`
`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
`
`
`
`
`
`
`
`
`type pumps do. Trapping pumps of the cryogenic
`variety are not recommended when pumping flammable vaporS,
`
`

`

`mam—MuseEvEh
`23“553358Bad
`$6»mom$532733%
`
`meaEEau£3,
`
`
`ummnmwcomma—aim323°86
`cmcan«I3m
`331.23503%
`3.55.8.““com$22836
`
`mam8:53
`n.330uni: “3:3th:0
`*3»oz
`ESoz
`3?oz
`
`moawtgomfinoESEE3203.pdWEE.
`T.TII
`385*
`
`
`
`26E 7
`
`mmnewtofisam
`
`1......
`
`
`
`was.6quwaEunhemafia“
`
`.3m2
`
`25me".25.555
`
`Sm._a_m.
`¢5:m:
`____________
`
`:53
`
`
`
`
`
`naggingo:3.5€33.883mam—nonv3.33“:bfiufionmymfi
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`Somecompass—wantmanna?maid“.nouwfififlcoono296Son:955muEmdou095$.
`
`53852»
`
`2%:
`5.522:
`
`_.......T|||'
`
`T.T|llv
`
`
`
`ES:32:2:
`
`
`
`5523..£383.
`
`
`
`
`
`
`
`4
`34
`
`
`
`
`$25..33:8:0
`
`$53qu
`
`
`
`moxmt$3350
`
`.8359mo
`
`
`
`.3633“.>54
`
`«Eii8m
`
`
`.IL
`T
`
`
`IF
`
`
`
`mngomfimaflmsou
`
`
`
`he.“:oEEoO
`
`$50
`
`maflwE—EOO
`
`omuaEwEoum
`
`when?»95mamam
`
`r
`
`._IE
`
`
`
`
`
`
`ESE313.30
`7.1....
`
`ESEha
`
`wEmZ
`
`
`
`woman»0.533.»
`
`
`
` «Ewan
`
`3%
`
`.32
`
`
`
`
`
` I.
`
`$33338%
`
`33835.35
`
`"5?stno
`
`
`
` 1mug315032
`
`magneto
`
`ofiomnlvm
`
`53880an
`75.2...
`
`quameoqugq
`
`
`

`

`
`
`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 Us. 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
`.It they can periodically release
`ln. Unlike the cryogenic trapping
`when saturated but must have
`
`pump operate well at the desired
`2 that the recommended pressure
`ich 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
`2d 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
`3 choices for roughing and backing
`drawbacks. The dry rotary pumps
`ensive. The cryosorption pumps
`itrogen and frequent regeneration
`
`actor. Approximate purchase costs
`Lm-sized pumps are listed in Table
`lng 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-
`
`;, it is integrated with a turbomo-
`is increasing the upper operating
`:till has a wide operating pressure
`v regime. This range encompasses
`position processes as well as some
`
`3.1
`
`PUmp Selection and Exhaust Handling
`
`39
`
`CW3 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 ur. 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 c and bounce off of it having
`the sum of two velocities, namely: c , which is randomly directed; and
`ur, 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,
`
`11:111.
`
`rotor
`
`channel , stator
`(b) fluid flow
`
`(a) molecular flow
`
`Figure 3.2 Molecular-drag pump operation in the two flow regimes.
`
`

`

`
`
`3.2 Problem Gases
`
` 4O
`Vacuum Technology
`
`
`
` 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 TiOg. 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
`
`port.
`_
`.
`.
`
`When tox1c gases are involved, spemal 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 (usually'on
`
`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.
`
`
`
`
`
`
`
`
`
`
`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-
`
`3.3 Gas Throughput
`
`vacuum
`gauge
`
`process
`
`
`chamber
`
`foreline conductance C1
`process
`gas
`
`supp}
`flow (ii
`
`‘
`‘
`p0
`pump
`a
`out assing
`throttle
`conductance C0
`103 Qi
`orifice
`conductance 02
`
`
`
`
`
`
`
`
`
`
`mm
`u!v
`
`SS
`
`30
`re
`
`1e
`
`1e
`iil
`le
`of
`re
`as
`an
`
`id
`
`1e
`
`1d
`of
`ve
`
`)le
`.es
`N0
`re-
`
`Figure 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, Q8, and “outa
`gassing" from the chamber walls, Qi (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, = US), for the
`gas. There is a throttle restriction (C2), a pumping line restriction
`(Cl), and the pump itself (Co). 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 (Falls) or (torr-l/s), which is also
`proportional to mc/s by the ideal-gas law. For the two flow restrictions,
`01 and 02, C is defined by
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`where the conductance Co is known as the pumping “speed” (l/s). Usu‘
`ally, Co varies little with p0 over the pump’s operating range, meaning
`
`that pump throughput is proportional to its inlet pressure, p0. Setting
`
`all of these Q values equal in accordance with Eq. (3.1), and taking the
`
`case where Q, << QS (it had better be!), we have
`
`(3'4)
`
`Qs + Qi = Qs = C2(P2 ~ 131) = C1<P1 - P0) = Copo
`
`Q = CAP
`
`(3.2)
`
`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 = Copo
`
`(3.3)
`
`

`

`
`
`3.3
`
`
`Gas Throughput
`43
`
`t. It
`
`3.1)
`
`mm
`
`the
`ises
`it is
`am-
`
`two
`Jut—
`
`iks,
`:ous
`
`l be
`cm,
`
`ugh
`1th,
`the
`;ion
`
`2 is
`, as
`:ur»
`
`ele-
`llSO
`DDS,
`
`3.2)
`
`l'lV"
`Ien
`
`:.4)
`
`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 02 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 = (Jiz‘Ji1)A = [
`
`N
`
`
`afilmprpo = 02(P2-p1)
`
`(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 Us for air at
`room temperature is
`
`Cm = 12.3o3/L
`
`(3.6)
`
`Where (1) = tube diameter, cm
`L = tube length, cm; L >> 4)
`
`Note that Cm is proportional to (1)3, versus ‘92 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 ‘15-
`FOI‘ gases other than air, Cm scales according to Eq. (2.18). For long
`tUlbes in fluid flow,
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`

`

`
`
`44
`
`Vacuum Technology
`
`
`
`
`0,: 1.41¢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 (1) 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.
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`.,helm!“myML...m.‘,,4.
`
`SEE
`
`The minimum size of foreline tube and pump for a given process can
`be calculated from the anticipated gas load, Q8. 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, Q. In such cases, a
`larger pump than the minimum is desirable. For processes operating
`without a throttle, faster pumping (larger CO) reduces the partial p of
`these contaminant gases, pi, in accordance with Eq. (3.3): that is,
`
`pad/00'
`
`(3.8)
`
`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
`
`
`
`QE‘Q 2%
`
`(3.9)
`
`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 p2,
`whereas in the unthrottled case of Eq. (3.8), p2 does not appear. It does
`appear in the throttled case because it is presumed that Qs 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 02: 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 p0 can be
`selected. The molecular-drag pump is attractive here, because it also
`minimizes oil contamination. Equation (3.9) also applies to atmO'
`
`
`
`

`

` 3.4
`
`45
`Contamination Sources
`
`
`
`(3.7)
`
`,hat C is
`iid flow.
`les from
`
`Ap driv-
`'rom the
`in axial
`l to tube
`
`L air, Cf
`Kn < 1),
`
`:ess can
`
`Lould be
`:e tubes
`LS often
`:ases, a
`
`)erating
`tial p of
`is,
`
`(3.8)
`
`;hrottle,
`(umably
`gassing
`
`(3.9)
`
`her and
`
`Vith p2,
`. It does
`
`already
`nnot be
`ontami-
`ottle to
`
`irity is
`e pump
`
`,’can be
`t it also
`) atmo-
`
`Spheric-pressure CVD, where Qs 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-
`ativeiy high at the high-p end. Thus, using the notation of Fig. 3.3, we
`have p2 = p0, and Eq. (3.1) becomes
`
`dp
`o = Cop2+vf
`
`(3.10)
`
`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
`
` —t
`p2 = P20€xp(V/CO)
`
`(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 US
`roughing pump, the time constant is 10 s. This means that evacuation
`to 10‘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
`fiifl} incorporates or rejects the contaminants arriving at its surface.
`This. in turn, depends on the chemical. reactivity of the surface with
`the Contaminants. For example, gold is not very reactive and can be
`d'alflosited with high purity in a poor vacuum, whereas aluminum
`reacts with and incorporates almost all arriving contaminants except
`the Inert gases. The degree to which contamination needs to be con-
`trolled must be assessed separately for each material system by analy-
`815 Of the resulting film. Whenever film properties are poorer than
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`

`

`
`
`46
`
`
`Vacuum Technology
`
`
`
`nants responsible for this result?
`As discussed in Chap. 1, contaminants can be introduced in all four
`
`
`
`steps in the deposition process: source, transport, deposition, and
`
`analysis. The contaminants associated with the source and analysis
`
`steps will be addressed in Chaps. 4 and 10, respectively. Here, we are
`
`proc:
`char:
`
`((1)1114.
`
`process
`chamb‘
`
`a
`F.
`
`l
`
`(b) fluii
`
`Figure 3.4
`phase at
`
`there. .
`cleanin
`process
`stream
`eventu:
`as we v
`
`diffusio
`backstr
`A mo
`of flow :
`
`in Figs.
`face pa1
`face, b1:
`undocu]
`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 10“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 flow of gas toward the
`pump, oil molecules bounce freely backward toward 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” baffle 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
`
`
`
`
`
`
`
`
`

`

`3.4.1
`
`
`Oil backstreaming
`47
`
`
`ami.
`
`four
`and
`
`lys1s
`a are
`
`nent
`mate
`
`"'-
`process A / / / /
`ber
`/
`, /
`Cham
`//7\/ /
`
`/ /
`
`Oil
`
`'
`
`\\\
`. gas‘O
`
`diffusion
`pump
`
`
`
`
`'ting
`ppli-
`g in
`also
`
`iber,
`:ipal
`will
`the
`
`1 up
`
`)il is
`this
`low
`mol-
`erial
`
`tary
`ump
`
`‘
`
`(a) molecular flow
`
`uncooled creep path W—d '1
`urf
`ace creep
`
`S
`
`A? purge O laminar flow plug flow approximation
`flow Q
`PM #5.“ ,
`/
`I I 5:, ll
`.
` process
`
`chamber
`
`turbopump
`
`
`Ph( Z)
`Z
`
`((3) fluid flow
`
`Figure 3.6% Oil backstreaming behavior in the two flow regimes, showing both gas-
`phase and surface paths
`
`flow
`alec—
`cess
`
`l the
`1am-
`
`. gas
`ther
`)om-
`
`Eemv
`uing
`with-
`3.4a
`ook—
`atter
`
`there. However, the trap will bewarrned 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
`In 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-
`facs, 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
`Stream 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 to be susceptible to surface creep.
`Turbopumps and molecular drag pumps lose their high compression
`ratio when stalled, such as during a power failure or following bearing
`seizure. Then, rapid backstreaming can occur