-
UV/ozone cleaning of surfacesJohn R. Vig
Citation: Journal of Vacuum Science & Technology A 3, 1027
(1985); doi: 10.1116/1.573115 View online:
http://dx.doi.org/10.1116/1.573115 View Table of Contents:
http://scitation.aip.org/content/avs/journal/jvsta/3/3?ver=pdfcov
Published by the AVS: Science & Technology of Materials,
Interfaces, and Processing
Articles you may be interested in Isolation of organic
field-effect transistors by surface patterning with an UV/ozone
process J. Vac. Sci. Technol. B 27, 1057 (2009);
10.1116/1.3117350
Effects of UV/ozone treatment of a polymer dielectric surface on
the properties of pentacene thin films fororganic transistors J.
Appl. Phys. 104, 013715 (2008); 10.1063/1.2951905
The role of proximity caps during the annealing of UV-ozone
oxidized GaAs J. Appl. Phys. 101, 114321 (2007);
10.1063/1.2740359
Growth and characterization of hafnium silicate films prepared
by UV/ozone oxidation J. Vac. Sci. Technol. A 22, 395 (2004);
10.1116/1.1649346
UVOzone Cleaning of GaAs for MBE J. Vac. Sci. Technol. 20, 241
(1982); 10.1116/1.571365
Redistribution subject to AVS license or copyright; see
http://scitation.aip.org/termsconditions. Download to IP:
128.114.34.22 On: Mon, 01 Dec 2014 15:02:16
-
UV /ozone cleaning of surfaces John R. Vig u. S. Army
Electronics Technology and Devices Laboratory, ERADCOM, Fort
Monmouth, New Jersey 07703-5302
(Received 17 September 1984; accepted 29 October 1984) The
ultraviolet (UV)lozone surface cleaning method is reviewed. The UV
/ozone cleaning procedure is an effective method of removing a
variety of contaminants from surfaces. It is a simple-to-use dry
process which is inexpensive to set up and operate. It can rapidly
produce clean surfaces, in air or in a vacuum system, at ambient
temperatures. By placing properly precleaned surfaces within a few
millimeters of an ozone producing UV source, the process can
produce clean surfaces in less than 1 min. The technique is capable
of producing near-atomically clean surfaces, as evidenced by Auger
electron spectroscopy, ESCA, and ISS/SIMS studies. Topics discussed
include: the variables of the process,the types of surfaces which
have been successfully cleaned, the contaminants which can be
removed, the construction of a UV /ozone cleaning facility, the
mechanism of the process, UV /ozone cleaning in vacuum systems,
rate enhancement techniques, safety considerations, effects ofUV
/ozone other than cleaning, and applications.
I. INTRODUCTION The ability of ultraviolet (UV) light to
decompose organic molecules has been known for a long time, but it
is only during the past decade that UV cleaning of surfaces has
been explored.
In 1972, Bolon and Kunz! reported the ability ofUV light to
depolymerize a variety of photoresist polymers. The poly-mer films
were enclosed in a quartz tube, the tube was evacu-ated, then
backfilled with oxygen. The samples were irra-diated with UV light
from a medium pressure mercury lamp which generated ozone. The
polymer films, which had been several thousand angstroms thick,
were successfully depoly-merized in less than 1 h. The major
products of depolymeri-zation were found to be water and carbon
dioxide. Subse-quent to depolymerization, the substrates were
examined by Auger electron spectroscopy (AES) and were found to be
free of carbonaceous residues. Only inorganic residues such as tin
and chlorine were found. When a Pyrex filter was placed between the
UV light and the films, or when a nitrogen at-mosphere was used
instead of oxygen, the depolymerization was hindered. Thus, Bolon
and Kunz recognized that oxy-gen and wavelengths shorter than 300
nm played a role in the depolymerization.
In 1974, Sowell et al. 2 described UV cleaning of adsorbed
hydrocarbons from glass and gold surfaces, in air and in a
vacuum system. A clean glass surface was obtained after 15 h of
exposure to the UV radiation in air. In a vacuum system at 10-4
Torr of oxygen, clean gold surfaces were produced after about 2 h
ofUV exposure. During cleaning, the partial pressure of O2
decreased, while that of CO2 and H 20 in-creased. The UV also
desorbed gases from the vacuum chamber walls. In air, gold surfaces
which had been con-taminated by adsorbed hydrocarbons could be
cleaned by "several hours of exposure to the UV radiation." Sowell
et al. also noted that by storing clean surfaces under UV
radi-ation it was possible to maintain the surface cleanliness
in-definitely.
Starting in 1974, Vig et al. 3-5 described a series of
experi-ments aimed at determining the optimum conditions for
pro-ducing clean surfaces by UV irradiation. The variables of
cleaning by UV irradiation were defined, and it was shown that,
under the proper conditions, UV /ozone cleaning was capable of
producing clean surfaces in less than 1 min.
II. THE VARIABLES OF UV/OZONE CLEANING
A. The wavelengths emitted by the UV sources To study the
variables of the UV cleaning procedure, Vig
and LeBus5 constructed the two UV cleaning boxes shown in Fig.
1. Both were made of aluminum and contained low-
AlZAK REFlEC;T IIO:R~~~~~~~5F~9 l----l_!ll:::;i~--I-SAMPlE -
U V BOX I FIG. I. Apparatus for UV lozone cleaning
experiments.
1027 J. Vac. Sci. Technol. A 3 (3), May/Jun 1985
Al BOX Al STAND
TRANSFORMER U V BOX 2
1027
Redistribution subject to AVS license or copyright; see
http://scitation.aip.org/termsconditions. Download to IP:
128.114.34.22 On: Mon, 01 Dec 2014 15:02:16
-
1028 John R. Vlg: UV I ozone cleaning of surfaces
pressure mercury discharge lamps and an aluminum stand with
Alzak6 reflectors. The two lamps produced nearly equal intensities
of short wavelength UV light, about 1.6 mW /cm2 for a sample 1 cm
from the tube. Both boxes con-tained room air (in a clean room)
throughout these experi-ments. The boxes were completely enclosed
to reduce recon-tamination by air circulation.
Since only the light which is absorbed can be effective in
producing photochemical changes, the wavelengths emitted by the UV
sources are important variables. The low-pressure mercury discharge
tubes generate two wavelengths of inter-est, 184.9 and 253.7 nm.
The 184.9 nm wavelength is impor-tant because it is absorbed by
oxygen, and it thus leads to the generation of ozone. 7 The 253.7
nm radiation is not absorbed by oxygen; it therefore does not
contribute to ozone genera-tion. However, it is absorbed by most
hydrocarbons89 and also by ozone.7 The absorption by ozone is
principally re-sponsible for the destruction of ozone in the UV
box. There-fore, when both wavelengths are present, ozone is
continual-ly being formed and destroyed. An intermediate product of
both the formation and destruction processes is atomic oxy-gen,
which is a very strong oxidizing agent.
The tube of the UV lamplO in box 1 consisted of91 cm of
"hairpin-bent" fused quartz, which transmits both the 253.7 and
184.9 nm lines. The lamp emitted about 0.1 mW /cm2 of 184.9 nm
radiation, measured at 1 cm from the tube.
The lamp in box 2 had two straight and parallel, 46 cm long,
high-silica glass tubes. The glass was Corning UV Glass No. 9823
which transmits at 253.7 nm but not at 184.9 nm. Since this lamp
generated no measurable ozone, a sepa-rate Siemens-type ozone
generator ll was built into box 2. This ozone generator did not
emit UV light. Ozone was pro-duced by a "silent" discharge when
high-voltage ac was ap-plied across a discharge gap formed by two
concentric glass tubes, each of which was wrapped in aluminum foil
elec-trodes. The ozone-generating tubes were parallel to the UV
tubes, approximately 6 cm away.
UV box 1 was used to expose samples, simultaneously, to 253.7
nm, 184.9 nm, and the ozone generated by the 184.9 nm radiation. UV
box 2 permitted the options of exposing samples to 253.7 nm plus
ozone, 253.7 nm only, or ozone only.
Vig et al. used contact angle measurements, wettability tests,
and Auger electron spectroscopy (AES) to evaluate the results of
cleaning experiments. Most of the experiments were conducted on
polished quartz wafers, the cleanliness of which could be evaluated
by the "steam test," a highly sensi-tive wettability
test.5.12,15
A "black-light" long wavelength UV source, which emit-ted
wavelengths above 300 nm only, was also tried. It pro-duced no
noticeable cleaning even after 24 h of irradiation.
It was found early in the studies ofVig et al. that samples
could be cleaned consistently by UV irradiation only if gross
contamination was first removed from the surfaces. Their
precleaning procedure consisted of the following steps:
(1) Scrub the samples with a swab while it is immersed in ethyl
alcohol.
(2) Degrease ultrasonically in a good solvent. (3) Boil in fresh
ethyl alcohol, then agitate ultrasonically.
J. Vac. Sci. Technol. A, Vol. 3, No.3, May/Jun 1985
1028
(4) Rinse in running ultrapure (18 M cm) water. (5) Spin-dry
immediately after the running water rinse. Subsequent to this
precleaning procedure, the steam test
and contact angle measurements invariably indicated that the
surfaces were contaminated. However, after exposure to UV /ozone in
box 1, the same tests always indicated clean surfaces. On numerous
occasions the cleanliness of such UV /ozone cleaned surfaces has
been verified in the author's laboratory, and elsewhere by AES and
electron spectroscopy for chemical analysis (ESCA).I,3,4,13-15 The
effectiveness of UV /ozone cleaning has also been confirmed by ion
scatter-ing spectroscopy/secondary ion mass spectroscopy (ISS/
SIMS). 16
A number of quartz wafers were precleaned and exposed to the UV
light in box 1 until clean surfaces were obtained. Each of the
wafers was then thoroughly contaminated with human skin oil which
has been a difficult contaminant to remove. The wafers were
precleaned again, groups were ex-posed to each of the four UV
/ozone combinations men-tioned earlier, and the time to attain a
clean surface, as indi-cated by the steam test, was measured. In
each UV box, the samples were placed within 5 mm of the UV source
(where the temperature was about 70C).
The wafers exposed to 253.7 nm + 184.9 nm + ozone in UV box 1
became clean in 20 s. The samples exposed to 253.7 nm + ozone in UV
box 2 reached the clean condition in 90 s. Samples exposed to 253.7
nm without ozone and to ozone without UV light took about 1 hand 10
h, respectively, be-fore clean surfaces were obtained.
The conclusion one can draw is that, while both UV light without
ozone and ozone without UV light can produce a slow cleaning effect
in air, the combination of both short wavelength UV light and
ozone, such as is obtained from a quartz UV lamp, produces a clean
surface orders of magni-tude faster. Although the 184.9 nm
radiation is also ab-sorbed by many hydrocarbons, it was not
possible from these experiments to isolate the cleaning effect of
the 184.9 nm radiation. The ozone concentrations had not been
measured. As is discussed below, the concentrations vary within
each box with distance from the UV source.
B. Distance between sample and UV source Another variable which
can greatly affect the cleaning
rate is the distance between the sample and the UV source.
Because of the shapes of the UV tubes and of the Alzak reflectors
above the tubes and below the samples, the lamps in both boxes were
essentially plane sources. Therefore, one might have expected that
the intensity ofUV light reaching a sample would be nearly
independent of distance. This was not so, however, when ozone was
present, because ozone has a broad absorption band7,I7,I8 centered
at about 260 nm. At 253.7 nm, the absorption coefficient of ozone
is 130 cm -I atm -I. The intensity I of the 253.7 nm radiation
reaching a sample therefore decreases as
I = IDe - 130pd,
where p is the average ozone pressure between the sample and the
UV source in atmospheres at 0 C, and d is the dis-
Redistribution subject to AVS license or copyright; see
http://scitation.aip.org/termsconditions. Download to IP:
128.114.34.22 On: Mon, 01 Dec 2014 15:02:16
-
1029 John R. Vig: UV/ozone cleaning of surfaces
tance to the sample in centimeters. When a quartz UV tube is
used, both the ozone concentration and the UV radiation intensity
decrease with distance from the UV source.
Two sets of identically precleaned samples were placed in UV box
2. One set was placed within 5 mm of the UV tube, the other, at the
bottom ofthe box, about 8 cm from the tube. With the ozone
generator off, there was less than a 30% difference in the time it
took for the two sets of samples to attain a minimal contact angle
(about 60 min vs 75 min). When the experiment was repeated with the
ozone generator on, the samples near the tube became clean nearly
ten times faster (about 90 s vs 13 min). Similarly, in UV box 1,
samples placed within 5 mm of the tube were cleaned in 20 s vs
20-30 min for samples placed near the bottom of the box, 13 cm
away. Therefore, to maximize the cleaning rate, the samples should
be placed as close to the UV source as possible.
C. Contaminants Vig et al. tested the effectiveness of the UV
lozone clean-
ing procedure on a variety of contaminants. Among the
con-taminants were: human skin oils, contamination adsorbed during
prolonged exposure to air, a cutting oil, 19 a beeswax and rosin
mixture, a lapping vehicle,20 a mechanical vacuum pump oil,21 DC
704 silicone diffusion pump oil,22 DC 705 silicone diffusion pump
oil,22 a silicone vacuum grease,22 an acid (solder) flux,23 and a
rosin flux from a rosin core lead-tin solder. The contaminants were
applied to clean, polished quartz wafers. After contamination, the
wafers were pre-cleaned, then exposed to UV lozone by being placed
within a few millimeters of the tube in UV box 1. After a 60 s
expo-sure, the steam test and AES indicated that all traces of the
contaminants had been removed.
Since AES could not differentiate between the silicon peaks due
to quartz and those due to the silicon containing contaminants, the
removal of silicone diffusion pump fluids was also tested on Alzak,
which normally has a silicon-free oxide surface, and on gold. AES
examination of the Alzak and gold surfaces following UV lozone
cleaning showed no traces of silicon present.
During the course of their studies, Vig et al. learned from
colleagues working on ion implantation for integrated cir-cuits
that the usual wet-cleaning procedures (with hot acids) failed to
remove the photoresist from silicon wafers which had been exposed
to radiation in an ion-implantation accel-erator, presumably
because of crosslinking of the photore-sist. Ion-implanted silicon
wafers, each with approximately a 1 }lm coating of exposed Kodak
Micro Resist 747,24 were placed within a few millimeters of the
source in UV box 1. After an overnight (~ 10 h) exposure to UV
lozone, all traces of the photoresist were removed from the wafers,
as con-firmed by AES.
Films of carbon, vacuum deposited onto quartz to make the quartz
surfaces conductive for study in an electron mi-croscope, were also
successfully removed by exposure to UV lozone. Inorganic
contaminants such as dust and salts cannot be removed by UV I
ozone. Such contaminants should be removed in the precleaning
procedure.
UV lozone has also been used for waste-water treatment and for
destruction of highly toxic compounds.25-28 Experi-
J. Vac. Sci. Technol. A, Vol. 3, No.3, May/Jun 1985
1029
mental work in connection with these applications has shown that
UV lozone can convert a wide variety of organic and some inorganic
species to relatively harmless, mostly volatile products such as
CO2, CO, H20, N2, etc. Com-pounds which have been successfully
destroyed in water by UV lozone include: ethanol, acetic acid,
glycine, glycerol, palmitic acid; organic nitrogen, phosphorous and
sulfur compounds; potassium cyanide; complexed Cd, Cu, Fe, and Ni
cyanides; photographic wastes, medical wastes, secon-dary
effluents; plus chlorinated organics and pesticides such as
pentachlorophenol, dichlorobenzene, dichlorbutane, chloroform,
malathion, Baygon, Vapam, and DDT. It has also been shown29 that
the combination ofUV and ozone is more effective in destroying
microbial contaminants in wa-ter, such as E. coli and streptococcus
faecalis, than UV or ozone alone.
D. The precleaning Although UV lozone is able to remove
contaminants such
as thick photoresist coatings and carbon films, without any
precleaning, the procedure cannot, in general, remove gross
contamination. For example, when a clean quartz wafer was coated
thoroughly with human skin oils and placed in UV box 1 (Fig. 1)
without any precleaning, even prolonged expo-sure to UV lozone
failed to produce a low-contact-angle sur-face. This is possibly
due to the fact that human skin oils contain materials, such as
inorganic salts, which cannot be removed by photosensitized
oxidation.
The UV lozone removed silicones from surfaces which had been
precleaned, as described earlier, and also from sur-faces which had
been just wiped with a cloth to leave a thin film. However, when
the removal of a thick film was at-tempted, the UV lozone removed
most of the film upon pro-longed exposure; but it also left a hard
cracked residue on the surface. This may be due to the fact that
many chemicals respond to radiation differently, depending on
whether or not oxygen is present. In the presence of oxygen many
poly-mers, for instance, degrade when irradiated; whereas, in the
absence of oxygen (as would be the case for the bulk of a thick
film) these same polymers crosslink. In the study of the radi-ation
degradation of polymers in air, the "results obtained with thin
films are often markedly different from those ob-tained using thick
specimen."30
For the UV lozone cleaning procedure to work reliably, the
surfaces must be precleaned: first, to remove contamin-ants such as
dust and salts which cannot be changed to vola-tile products by the
oxidizing action ofUV lozone, and, sec-ond, to remove thick films
the bulk of which could be transformed into a UV resistant film by
the crosslinking ac-tion of the UV light that penetrates the
surface.
E. The substrate The UV lozone cleaning process has been
successfully
used on a variety of surfaces including: glass, quartz, mica,
sapphire, ceramics, metals, semiconductors, and a conduc-tive
polyimide cement.
Quartz and sapphire are especially easy to clean with UV I
ozone, since these materials are transparent to short wave-length
UV. For example, when a pile of thin quartz plates, a
Redistribution subject to AVS license or copyright; see
http://scitation.aip.org/termsconditions. Download to IP:
128.114.34.22 On: Mon, 01 Dec 2014 15:02:16
-
1030 John R. Vig: UV I ozone cleaning of surfaces
couple of centimeters deep, was cleaned by UV /ozone, both sides
of all the blanks, even the ones at the bottom of the pile, were
cleaned by the process. Since sapphire is even more transparent, it
too could probably be cleaned the same way. When flat quartz plates
were placed one on top of the other so that there could have been
little or no ozone circulation between the plates, the UV /ozone
cleaning was able to clean both sides of these plates also. (There
is evidence in the litera-ture31 that photocatalytic oxidation of
hydrocarbons, with-out the presence of gaseous oxygen, can occur on
some oxide surfaces.)
When white alumina ceramic substrates were cleaned by UV /ozone,
the surfaces could be cleaned properly. How-ever, the sides facing
the UV turned yellow, probably due to the production of
(UV-induced) color centers. After several days at room temperature,
or a few minutes at high tempera-tures, the white color
returned.
Metal surfaces could be cleaned by UV /ozone without any
problems as long as the UV exposure was limited to the time
required to produce a clean surface. (This time should generally be
about 1 min or less for surfaces which have been properly
precleaned.) However, prolonged exposure of ox-ide-forming metals
to UV light can produce rapid corrosion. Silver samples, for
example, turned black in UV box 1 within 1 h. Experiments with
sheets of Kovar, stainless steel (type 302), gold, silver, and
copper showed that, upon extended UV irradiation, the Kovar, the
stainless steel, and the gold appeared unchanged; the silver and
copper oxidized on both sides, but the oxide layers were darker on
the sides facing away from the UV source than on the sides facing
the UV. When electroless gold-plated nickel parts were stored under
UV /ozone for several days, a black powdery coating gradu-ally
appeared on the parts. Apparently, nickel diffused to the surface
through pinholes in the gold plating, and the oxi-dized nickel
eventually nearly completely covered the gold. The corrosion was
observed even in UV box 2, when no ozone was being generated. The
rate of corrosion increased substantially when a beaker of water
was placed in the UV boxes to increase the humidity. Even Kovar
showed signs of corrosion under such conditions.
The corrosion may possibly be explained by the fact that, as is
known in the science of air-pollution control, in the presence of
short wavelength UV light, impurities in air, such as oxides of
nitrogen and sulfur, combine with water vapor to form a corrosive
mist of nitric and sulfuric acids. The use of controlled
atmospheres in the UV box should, therefore, minimize the corrosion
problem.
Since UV /ozone dissociates organic molecules, it may be useful
for cleaning some organic materials just as etching and
electropolishing are sometimes useful for cleaning met-als. The
process has been used successfully to clean quartz crystal
resonators which have been bonded with silver-filled polyimide
cement.32 Teflon (TFE) tape exposed to UV / ozone in UV box 1 for
10 days experienced a weight loss of 2.5%.33 Also, the contact
angles measured on clean quartz plates increased after a piece of
Teflon was placed next to the plates in a UV box.34 Similarly,
Viton shavings taken from an 0 ring experienced a weight loss of
3.7% after 24 h in UV box 1. At the end of the 24 h, the Viton
surfaces had become
J. Vac. Sci. Technol. A, Vol. 3, No.3, May/Jun 1985
1030
sticky.33 Semiconductor surfaces have been successfully UV /
ozone-cleaned without adversely affecting the functioning of the
devices. For example, after a 4K static RAM integrated circuit was
exposed to UV /ozone for 120 min in a commer-cial UV /ozone
cleaner, the device continued to function without any change in
performance. (This IC has been made using n-channel silicon gate
technology, with 1 to 1.5 f-lm junction depths.35)
F. Rate enhancement techniques UV /ozone cleaning "rate
enhancement" techniques have
been investigated by Zafonte and Chiu.36 Experiments on gas
phase enhancement techniques included a comparison of the cleaning
rates in dry air, dry oxygen, moist air, and moist oxygen. The
moist air and moist oxygen consisted of gases that had been bubbled
through water. Oxygen that had been bubbled through hydrogen
peroxide was also tried. Experi-ments on liquid-enhancement
techniques consisted of drop-wise addition of either distilled
water or of hydrogen perox-ide solutions of various concentrations
to the sample surfaces. Most of the sample surfaces consisted of
various types of photoresist on silicon wafers.
The gas phase "enhancement" techniques resulted in neg-ligible
to slight increases in the rates of photoresist removal (3-20 A/min
without enhancement vs 3-30 A/min with en-hancement). The water and
hydrogen peroxide liquid phase enhancement techniques both resulted
in significant rate en-hancements (100-200 A/min) for non-ion
implanted resists. The heavily ion implanted resists (1015_1016
atoms/cm2) were not significantly affected by UV /ozone, whether
"en-hanced" or not.
III. THE MECHANISM OF UV/OZONE CLEANING The available evidence
indicates that UV /ozone cleaning
is primarily the result of photosensitized oxidation pro-cesses,
as is represented schematically in Fig. 2. The con-taminant
molecules are excited and/or dissociated by the absorption of short
wavelength UV light. Simultaneously, atomic oxygen and ozone l718
are produced when O2 is disso-ciated by the absorption of UV with a
wavelength less than 245.4 nm. Atomic oxygen is also produced l718
when ozone is dissociated by the absorption of the UV and longer
wave-lengths of radiation. The excited contaminant molecules, and
the free radicals produced by the dissociation of con-
1 IONS CONTAMINANT FREE RADICALS MOLECULES + hv 1 EXCITED
MOLECULES NEUTRAL MOLECULES VOLATILE MOLECULES
(C02,H20,N2,etc.1
FIG. 2. Simplified schematic representation ofUV /ozone cleaning
process.
Redistribution subject to AVS license or copyright; see
http://scitation.aip.org/termsconditions. Download to IP:
128.114.34.22 On: Mon, 01 Dec 2014 15:02:16
-
1031 John R. Vig: UV I ozone cleaning of surfaces
taminant molecules, react with atomic oxygen to form simpler
volatile molecules, such as CO2, H 20, N 2, etc.
The energy required to dissociate an O2 molecule into two ground
state 0 atoms corresponds to 245.4 nm. However, at and just below
245.4 nm the absorption of O2 is very weak.7,17,18 The absorption
coefficient increases rapidly with decreasing wavelengths. A
convenient wavelength for pro-ducing 0 3 is the 184.9 nm emitted by
low-pressure Hg dis-charge lamps in fused quartz envelopes.
Similarly, since most hydrocarbons have a strong absorption band
between 200 and 300 nm, the 253.7 nm wavelength emitted by the same
lamps is useful for exciting or dissociating contaminant molecules.
The energy required to dissociate ozone corre-sponds to 1140 nm.
The absorption by ozone reaches a maxi-mum near the 253.7 nm
wavelength. The actual photo-chemical processes occurring during UV
lozone cleaning are more complex than shown in Fig. 2. For example,
the rate of production of ozone by 184.9 nm photons is promoted by
the presence of other molecules, such as N2 and CO2,
As described earlier, the combination of short wavelength UV
light and ozone produced clean surfaces about 200 to 2000 times
faster than UV light alone or ozone alone. Simi-larly, Prengle et
al. 25 ,28 had found in their studies of waste-water treatment that
UV enhances the reaction with ozone 102-fold to 104-fold, and the
products of the reactions are materials such as CO2, H20, and N 2.
Increasing the tem-perature was found to increase the reaction
rates. Mattox37 has also found that mild heating increases the UV
lozone cleaning rates. Bolon and Kunz,l on the other hand, had
found that the rate ofUV lozone depolymerization of photo-resists
did not change significantly between 100 and 300 DC. The rate of
destruction of microorganisms was similarly in-sensitive to a
temperature increase from room temperature to 40 Dc. 29
IV. UV/OZONE CLEANING IN VACUUM SYSTEMS Sowell et ae reported
that, when 10-4 Torr pressure of
oxygen was present in a vacuum system, short wavelength UV
desorbed gases from the walls of the system. During UV irradiation,
the partial pressure of oxygen decreased, while that of CO2 and H
20 increased.
One must exercise caution in using a mercury UV source in a
vacuum system because, should the lamp envelope break or leak,
mercury would enter and ruin the usefulness of the system. Mercury
has a high vapor pressure; its complete re-moval from a vacuum
chamber is a difficult task. Other types ofUV sources, such as
xenon or deuterium lamps, may be safer to use in vacuum systems.
The UV light can also be radiated into systems through sapphire or
quartz windows, or through deep-UV fiber-optic bundles. A small
partial pressure of oxygen should be present during UV
cleaning.
Caution must also be exercised when using UV lozone in a
cryopumped vacuum system, since cryopumped ozone is po-tentially
explosive,39 particularly during regeneration of the cryopump. A
convenient method of dealing with this poten-tial hazard is to use
two kinds of UV sources, one an ozone-generating source, the other
an "ozone killer" source40 (see Sec. V.).
J. Vac. Sci. Technol. A, Vol. 3, No.3, May/Jun 1985
1031
V. SAFETY CONSIDERATIONS In the construction of a UV cleaning
facility, one should
be aware of the safety hazards associated with short wave-length
UV light. Exposure to intense short wavelength UV light can cause
serious skin and eye injury within a short time. For the UV boxes
used in the Vig et al. experiments, switches are attached to the
doors in such a manner that when the doors are opened, the UV lamps
are shut off auto-matically. If the application demands that the UV
lamps be used without being completely enclosed (for example, as
might be the case if an UV cleaning facility is incorporated into a
thermocompression bonder), then proper clothing and eye protection
should be worn to prevent skin burns and eye injury.
Another safety hazard is ozone, which is highly toxic. In
setting up a UV cleaning facility, one must ensure that the ozone
levels to which people are exposed do not exceed 0.1 ppm, the OSHA
standard.38 Ozone is also a potential hazard in a cryopumped vacuum
system because cryopumped ozone can become explosive under certain
conditions. 39
One method of minimizing the hazards associated with ozone is to
use two types of short wavelength ultraviolet sources for UV lozone
cleaning.40 One, an ozone generating UV lamp, e.g., a low-pressure
mercury light in a fused quartz envelope. The other, a UV lamp that
does not generate ozone but one which emits one or more wavelengths
that are strongly absorbed by ozone, e.g., a low-pressure mercury
light in a high silica glass tube, which emits primarily at 253.7
nm. Such a nonozone generating UV source can be used as an "ozone
killer." For example, in one cryopumped vacuum system it was found
that after the UV lozone clean-ing, in up to 20 Torr of oxygen, was
completed and the ozone generating UV lamp was turned off, 10 min
of "ozone killer" UV light reduced the concentration of ozone to
less than 0.01 ppm, a level that is safe for immediate
cryopumping.41 With the ozone killer lamp, ozone concentrations
were re-duced by at least a factor of 100 within 10 min. Without
the ozone killer lamp, the half life of ozone is 3 days at 20
DC.42
VI. UV/OZONE CLEANING FACILITY CONSTRUCTION
The material chosen for the construction of a UV lozone cleaning
facility should be one which is not corroded by ex-tended exposure
to UV lozone. One material that can be used is polished aluminum
with a relatively thick anodized oxide layer, such as Alzak. 6 Such
materials are resistant to corrosion, have a high thermal
conductivity which helps to prevent heat buildup, and are also good
reflectors of short wavelength UV. Most other metals, including
silver, are poor reflectors in this range.
Vig et al. initially used ordinary, shop variety aluminum sheet
for UV box construction. After a while, however, it was noticed
that a thin coating of a white powder (probably aluminum oxide
particles) appeared at the bottom of the boxes. Even in a UV box
made of standard Alzak, after a couple of years' usage white spots
started appearing on the Alzak. To avoid the possibility of
particles being generated inside the UV lozone cleaning facility,
the facility should be
Redistribution subject to AVS license or copyright; see
http://scitation.aip.org/termsconditions. Download to IP:
128.114.34.22 On: Mon, 01 Dec 2014 15:02:16
-
1032 John R. Vig: UV/ozone cleaning of surfaces
inspected periodically for signs of corrosion. The use of "class
M" Alzak may also help, since this material has a much thicker
oxide coating. It is made for "exterior marine service," instead of
the "mild interior service" specified for standard Alzak.
Commercially available UV lozone clean-ers are constructed of
stainless steel. To date, no corrosion problems have been reported
with such cleaners.
Organic materials should not be present in the UV clean-ing box.
For example, the plastic insulation usually found on the leads of
UV lamps should be replaced with inorganic insulation, such as
glass or ceramic. The box should be en-closed so as to minimize
recontamination by circulating air, and to prevent accidental UV
exposure.
The most widely available sources of short wavelength UV light
are mercury arc lamps. Low-pressure mercury lamps in pure, fused
quartz envelopes operate near room temperature, emit approximately
90% at 253.7 nm, and gen-erate sufficient ozone for effective
surface cleaning. Approx-imately 5% of the output of these lamps is
at 184.9 nm. Medium and high-pressure UV lamps 7 generally have a
much higher output in the short wavelength UV range. These lamps
also emit a variety of additional wavelengths below 253.7 nm, which
may enhance their cleaning action. However, they operate at high
temperatures (the envelopes are near red hot), have a shorter
lifetime, a higher cost, and present a greater safety hazard. The
mercury tubes can be fabricated in a variety of shapes to fit
different applications. In addition to mercury arc lamps,
microwave-powered mer-cury vapor UV lamps are also available.
Other good sources of short wavelength UV, such as xe-non lamps
and deuterium lamps, are also available. These lamps must also be
in an envelope transparent to short wave-length UV, such as quartz.
In setting up a UV cleaning facili-ty, one should choose a UV
source which will generate enough UV lozone to allow for rapid
photosensitized oxida-tion of contaminants; however, too high an
output at the ozone generating wavelengths can be counterproductive
be-cause a high concentration of ozone will absorb most of the UV
light before it reaches the samples. The samples should be placed
as close to the UV source as possible to maximize the intensity
reaching the samples. In the UV cleaning box I of Vig et al., the
parts to be cleaned are placed on an Alzak stand, the height of
which can be adjusted to bring the parts close to the UV lamp. The
parts to be cleaned can also be placed directly onto the tube if
the box is built so that the tube is on the bottom of the
box.43
VII. APPLICATIONS The UV lozone cleaning procedure has found
numerous
applications during the past several years. A major use is
substrate cleaning prior to thin-film deposition as is widely used
in the quartz crystal industry during the manufacture of quartz
crystal resonators. There is probably no other de-vice of which the
performance is so critically dependent on surface cleanliness. For
example, the aging requirement for a 5 MHz resonator is that the
frequency change no more than two parts on 1010 per week, whereas
adsorption or desorp-tion of a monolayer of contamination from such
a device changes the frequency by about one part in 106. The
surface
J. Vac. Sci. Technol. A, Vol. 3, No.3, May/Jun 1985
1032
cleanliness must therefore be such that the rate of transfer of
contamination within the (hermetically sealed) resonator en-closure
is less than 10-4 mono layers per week! In the auth-or's quartz
resonator fabrication laboratory, UV lozone is used at several
points during the fabrication sequence, such as for cleaning and
storing metal tools, masks, resonator parts, and storage
containers.
The process is also being applied in a hermetic sealing method
which relies on the adhesion between clean surfaces in an ultrahigh
vacuum. 14,43-46 It has been shown that metal surfaces will weld
together under near-zero forces if the sur-faces are atomically
clean. A gold gasket between gold met-allized (UV lozone cleaned)
aluminum oxide sealing surfaces is currently providing excellent
hermetic seals in the produc-tion of ceramic flatpack -enclosed
quartz resonators. The fea-sibility of achieving good hermetic
seals by pressing a clean aluminum gasket between two clean,
unmetallized alumi-num oxide ceramic surfaces has also been
shown.44-46
The same adhesion phenomenon between clean (UV I ozone cleaned)
gold surfaces has been applied to the con-struction of a novel
surface contaminant detector.47.48 The rate of decrease in the
coefficient of adhesion between freshly cleaned gold contacts is
used as a measure of the gaseous condensable contaminant level in
the atmosphere.
The process has also been applied to improve the reliabil-ity of
wire bonds, especially at reduced temperatures. It has been
shown,49.5o for example, that the thermocompression bonding process
is highly temperature dependent when or-ganic contaminants are
present pn the bonding surfaces. The temperature dependence can be
eliminated by UV lozone cleaning of the surfaces just prior to
bonding. In a study of the effects of cleaning methods on gold ball
bond shear strength, UV lozone cleaning was found to be the most
effec-tive method of cleaning contaminants from gold surfaces.51 UV
lozone is also being used for cleaning alumina substrate surfaces
during the processing of thin-film hybrid circuits.52
When the nonuniform appearance of thermal/flash pro-tective
electro-optic goggles was traced to organic contamin-ants on the
electro-optic wafers, a number of cleaning meth-ods were tested. UV
lozone proved to be the most effective method for removing these
contaminants, and thus it was chosen for use in the production of
the goggles. 53
Other applications which have been described are photo-resist
removal, 1.5.13 the cleaning of: vacuum chamber walls,2
photomasks,54 silicon wafers (for enhancing photoresist ad-hesion),
54 lenses, 53 mirrors, 54 solar panels, 54 sapphire (before the
deposition of HgCdTe),54 cold-rolled steel (for paint and zinc
coating adhesion improvement),54 surface acoustic wave and other
fine linewidth devices,54.55 inertial guidance subcomponents
(glass, chromium-oxide surfaced gas bear-ings, and beryllium),55
and gallium arsenide wafers.57 Since short wavelength UV can
generate radicals and ions, a side benefit ofUV I ozone cleaning of
insulator surfaces can be the neutralization of static charges.
58
UV lozone cleaning of silicon substrates in silicon molecu-lar
beam epitaxy (MBE) has been found to be effective in producing
near-defect-free MBE films. 59 By using UV I ozone cleaning, the
above 1200 C temperatures required for removing surface carbon in
the conventional method can be
Redistribution subject to AVS license or copyright; see
http://scitation.aip.org/termsconditions. Download to IP:
128.114.34.22 On: Mon, 01 Dec 2014 15:02:16
-
1033 John R. Vlg: UV/ozone cleaning of surfaces
lowered to below 1000 0c. The slip lines resulting from ther-mal
stresses and thermal pits that are often produced by the high
temperature treatment are minimized in the lower tem-perature
processing. Impurity redistribution in the substrate is also
reduced.
VIII. EFFECTS OTHER THAN CLEANING Short wavelength UV, ozone,
and the combination of the
two, can have effects other than surface cleaning. Among the
more significant of these effects are the following:
A. Oxidation Ozone's oxidation power is second only to that of
fluorine.
Ozone can oxidize most inorganic compounds to their final
oxidative state.42 For most substrates, UV lozone cleaning for the
minimum time necessary to obtain a clean surface will not cause a
significant amount of oxidation. However, extended storage under UV
lozone may be detrimental for some oxidizable surfaces. In some
cases, the enhanced oxide formation may be beneficial. For example,
whereas the "na-tive" oxide on GaAs is only about 30 A thick, UV
lozone produces an oxide layer that is 100 to 300 A thick,60 i.e.,
UV I ozone can produce a clean, enhanced oxide "passivated"
surface. 10 min of UV lozone cleaning increased the oxide thickness
on silicon substrates from 0.9 to 1.2 nm.59 Similar-ly, the native
UV lozone produced oxide layer at the inter-face of HgCdTe-Si02 has
been found to enhance the inter-face properties.61 Solar radiation
and atmospheric ozone have been found to markedly enhance the
sulfidation of cop-per.62 Extended exposure to UV lozone has been
found to significantly increase the oxide layer thickness on
aluminum surfaces.63 Whereas the oxide thickness on air exposed
alu-minum surfaces is normally limited to about 50 A, after 10 min
ofUV lozone exposure, the oxide thickness was found to be 90 A, and
after 60 min, 200 A.
B. UV-enhanced outgassing Short wavelength UV has been found to
enhance the out-
gassing of glasses. 64 The UV light produced evolution of
sig-nificant quantities of hydrogen, and also water, carbon
diox-ide, and carbon monoxide. The hydrogen evolution was
proportional to the amount of radiation incident onto the samples.
For UV -opaque glasses, the evolution occurred from the side
exposed to the UV; for high transmission sam-ples, the gas evolved
from both sides.
C. Other surface/lnterface effects Energetic radiation such as
UV and y-radiation has been
reported to produce dehydration and the formation of free
radicals on silica surfaces.65 However, dehydrated (or
silox-inated) silica surfaces are hydrophobic,66.67 whereas UV I
ozone cleaned silica (quartz) surfaces exhibit a very low (less
than 4) contact angle, thus indicating that the UV lozone cleaning
does not dehydrate the surfaces, nor does it modify surface silanol
groups the way high-temperature vacuum baking does.68 Short
wavelength UV has also been found to produce a bleaching effect in
Si-Si3 interfaces with thin ox-ides,69 and has also been found to
produce yellowing (color
J. Vac. Sci. Technol. A, Vol. 3, No.3, May/Jun 1985
1033
centers) during the cleaning of aluminum oxide ceramics. The
yellowing can be readily bleached by heating the sam-ple.
D. Etching Short wavelength (193 nm) UV laser irradiation
ofbiologi-
cal and polymeric materials has been shown to be able to etch
the materials with great precision, via "ablative
photo-decomposition," and without significant heating of the
sam-ples. Linewidths 5 pm wide have been etChed onto a plastic film
to demonstrate the capability of this technique. 70 Oxy-gen does
not appear to have the same significance in this process as it does
in UV lozone cleaning. The etch depth versus fluence in vacuum and
in air were found to be the same. 71 The UV lozone has been found
to etch Teflon33.34 and Vi ton, 33 and wil1likely etch other
organic materials as well.72.73
IX. SUMMARY AND CONCLUSIONS The UV lozone-cleaning procedure has
been shown to be
a highly effective method of removing a variety of contam-inants
from surfaces. It is a simple-to-use dry process which is
inexpensive to set up and operate. It can produce clean surfaces at
room temperature, either in a room atmosphere or in a controlled
atmosphere.
The variables of the UV cleaning procedure are: the
con-taminants initially present, the precleaning procedure, the
wavelengths emitted by the UV source, the atmosphere between the
source and sample, the distance between the source and sample, and
the time of exposure. For surfaces which are properly precleaned
and placed within a few milli-meters of an ozone-producing UV
source, the process can produce a clean surface in less than 1 min.
The combination of short wavelength UV light plus ozone produces a
clean surface substantially faster than either short wavelength UV
light without ozone or ozone without UV light. Clean sur-faces will
remain clean indefinitely during storage under UV lozone, but
prolonged exposure of oxide-forming metals to UV lozone in room air
can produce rapid corrosion.
The cleaning mechanism seems to be a photosensitized oxidation
process in which the contaminant molecules are excited and/or
dissociated by the absorption of short wave-length UV light.
Simultaneously, atomic oxygen is generated when molecular oxygen is
dissociated and when ozone is dissociated by the absorption of both
short and long wave-lengths of radiation. The products of the
excitation of con-taminant molecules react with atomic oxygen to
form simpler molecules, such as CO2 and H20, which desorb from the
surfaces.
'D. A. Bolon and C. O. Kunz, Polym. Eng. Sci. 12,109 (1972);
also D. A. Bolon, U. S. Patent No.3 890 176, June 17, 1975.
2R. R. Sowell, R. E. Cuthrell, D. M. Mattox, and R. D. Bland, J.
Vac. Sci. Techno!. 11,474 (1974).
'J. R. Vig, C. F. Cook, Jr., K.Schwidtal, J. W. LeBus, and E.
Hafner, in Proceedings of the 28th Annual Symposium Frequency
Control (US Army Electrons Command, Ft. Monmouth, NJ, 1974), ADA
011113, pp 96-108. Article reprinted as ECOM Technical Report 4251,
AD 785513, 1984.
4J. R. Vig, J. W. LeBus, and R. L. Filler, in Proceedings of the
29th Annual
Redistribution subject to AVS license or copyright; see
http://scitation.aip.org/termsconditions. Download to IP:
128.114.34.22 On: Mon, 01 Dec 2014 15:02:16
-
1034 John R. Vig: UV I ozone cleaning of surfaces
Symposium on Frequency Control, pp. 220-229, 1975. Copies
available from the National Technical Information Service,
Springfield, V A, ADA017466.
5J. R. Vig and J. W. LeBus, UV /Ozone Cleaning afSurfaces (IEEE
Trans. Parts, Hybrids and Packag., 1976), Vol. PHP-12, pp.
365-370.
6Alzak is an aluminum reflector material with a corrosion
resistant oxide coating. The Alzak process is licensed to several
manufacturers by the Aluminum Co. of America, Pittsburgh, PA
15219.
7J. G. Clavert and J. N. Pitts, Jr., Photochemistry (Wiley, New
York, 1966), pp.205-209,687-705.
"V. S. Fikhtengolt's, R. V. Zolotareva, and Yu A. L'vov,
Ultraviolet Spec-trum of Elastomers and Rubber Chemicals (Plenum,
New York, 1966).
L.Lang, Absorption Spectra in the Ultraviolet and Visible Region
(Aca-demic, New York, 1965).
IOModel No. R-52 Mineralight Lamp, Ultraviolet Products, Inc.,
San Ga-briel, CA 91778.
"See, e.g., Encyclopaedic Dictionary of Physics (Pergamon, New
York, 1962), Vol. 5, p. 275.
12M. E. Schrader, in Surface Contamination: Its Genesis,
Detection and Con-trol, edited by K. L. Mittal (Plenum, New York,
1979), Vol. 2, pp. 541-555.
"P. H. Hollaway and D. W. Bushmire, Proceedings of the 12th
Annual Reliability Physics Symposium (Institute of Electrical and
Electronic En-gineers, Piscataway, NJ, 1974), pp. 180-186.
I4R. D. Peters, Proceedings of the 30th Annual Symposium on
Frequency Control, pp. 224-231, 1976. Copies available from the
National Technical Information Service, Springfield, V A,
ADA046089.
15e. E. Bryson and L. 1. Sharpen, in Surface Contamination: Its
Genesis, Detection and Control, edited by K. L. Mittal (Plenum, New
York, 1979), Vol.2, pp. 687-696.
16W. L. Baun, Surf. Sci. 6, 39-45 (1980). 17J. R. McNesby and H.
Okabe, "Oxygen and Ozone," in Advances in Pho-
tochemistry, edited by W. A. Noyes, lr., G. S. Hammond, and J.
N. Pitts (Interscience, New York, 1964). Vol. 3, pp. 166--174.
'"D. H. Volman, in Advances in Photochemistry, edited by W. A.
Noyes. G. S. Hammond, and 1. N. Pitts (Interscience, New York,
1963), Vol. I, pp. 43-82.
lOp. R. Hoffman Co., Carlisle, PA. 2John Crane Lapping Vehicle
3M, Crane Packing Co., Morton Grove, IL
60053. "Welch Duo-Seal, Sargent-Welch Scientific Co., Skokie, IL
60076. "Dow Corning Corp., Midland, MI 48640. "Dutch Boy No. 205,
National Lead Co., New York, NY 10006 2'Eastman Kodak Co.,
Rochester, NY 14650. "H. W. Prengle, e. E. Mauk, R. W. Legan, and
e. G. Hewes, Hydrocarbon
Process. 54, 82 (1975). 26H. W. Prengle Jr., e. E. Mauk, and J.
E. Payne, Forum on Ozone Disin-
fection, 1976; International Ozone Institute, Warren Bldg.,
Suite 206, 14805 Detroit Ave., Lakewood, OH 44107
27H. W. Prengle, Jr., and e. E. Mauk, Workshop on Ozone/Chlorine
Diox-ide Oxidation Products of Organic Materials, EPA/International
Ozone Institute Warren Bldg., Suite 206, 14805 Detroit Ave.,
Lakewood, OH 44107, Nov. 1976.
28H. W. Prengle, Jr., in Proceedings of the lnternational Ozone
Institute Ozone Symposium, Warren Bldg., Suite 206, 14805 Detroit
Ave., Lake-wood, OH 44107,1978.
20J. D. Zeff, R. R. Barton, B. Smiley and E. Alhadeff, US Army
Medical Research and Development Command, Final Report, Contract
No. DADA 17073-C-3138, Sept. 1974. Copies available from NTIS, AD
A004205.
30H. V. Boenig, Structure and Properties of Polymers (Wiley, New
York, 1973), p. 246.
3IV. N. Filimonov, in Elementary Photoprecesses in Molecules,
edited by B. S. Neporent (Consultants Bureau, New York, 1968), pp.
248-259.
"R. L. Filler, J. M. Frank, R. D. Peters, and J. R. Vig,
Proceedings of the 32nd Annual Symposium on Frequency Control, pp.
290-298, 1978. Copies available from Electronics Industries Assoc.,
2001 Eye St., NW, Washington, D. e. 20006.
33J. W. LeBus and J. R. Vig (unpublished). 34J. Kusters
(personal communication). 35E. Lasky (personal communication).
J. Vac. Sci. Technol. A, Vol. 3, No.3, May/Jun 1985
1034
36L. Zafonte and R. Chiu, Technical Report on UV-Ozone Resist
Strip Feasibility Study, UVP, Inc., 5100 Walnut Grove Avenue, San
Gabriel, CA 91778, September, 1983; to be presented at the SPIE
Santa Clara Conference on Microlithography in March 1984.
"D. M. Mattox, Thin Solid Films 53,81 (1978). '""Occupational
Safety and Health Standards", Vol. I, General Industry
Standards and Interpretations, Oct. 1972, Pt. 1910, 1000, Table
Z-I, Air Contaminants, P. 642,4 as per change 10, June 26,
1975.
39e. W. Chen and R. G.Struss, Cryogenics 9,131 (1969). 4J.R. Vig
and 1. W. LeBus, U. S. Patent NO.4 028135, June 7,1977. 4ID. A.
Ehlers, Report No. PT81-004, General Electric Neutron Devices
Dept., St. Petersburg, FL, 1981. 42Matheson Gas Data Book, 6th
ed. (Matheson Gas Products Co., East
Rutherford, NJ, 1980), pp. 574-577. 43R. D. Peters (personal
communication). 44J. R. Vig and E. Hafner, Technical Report
ECOM-4134, US Army Elec-
tronics R&D Command, Fort Monmouth, NJ, July 1973. Copies
avail-able from NTIS, AD 763215.
"E. Hafner and 1. R. Vig, U. S. Patent No.3 914 836, Oct. 28,
1975. 46p. D. Wilcox, G. S. Snow, E. Hafner, and 1. R. Vig,
Proceedings of the
29th Annual Symposium on Frequency Control, pp. 202-210,1975.
See Ref. No.4 above for availability information.
47R. E. Cuthrell and D. W. Tipping, Rev. Sci. Instrum. 47, 555
(1976). 48R. E. Cuthrell, in Surface Contamination: Its Genesis,
Detection and Con-
trol, edited by K. L. Mittal (Plenum, New York, 1979), Vol. 2,
pp. 831-841.
49J. L. Jellison, IEEE Trans. Parts Hybrids Packag. PHpn, 206
(1975). 51. L. lellison, in Surface Contamination: Its Genesis,
Detection and Con-
trol, edited by K. L. Mittal (Plenum, New York, 1979), Vol. 2,
pp. 899-923.
"J. A. Weiner, et al., Proceedings of the Electronic Components
Confer-ence,pp.208-220,1983.
"R. Tramposch, Circuits Manufacturing 23, 30 (1983). 53J. A.
Wagner, in Surface Contamination: Its Genesis, Detection and
Con-
trol, edited by K. L. Mittal (Plenum, New York, 1979), Vol. 2,
pp. 769-783.
54E. Lasky (personal communcation). "H. I. Smith (unpublished
and personal communications). 561. R. Stemniski and R. L. King,
Jr., in Adhesivesfor Industry (Technology
Conferences, EI Segundo, CA, in cooperation with the So. Calif.
Section, Soc. of Plastics Engineers, 1980), pp. 212-228.
571. A. McClintock, R. A. Wilson, and N. E. Byer, J. Vac. Sci.
Technol. 20, 241 (1982).
'"D. H. Baird, Final Technical Report No. TR 76-807.1, Dec.
1976. Copies available from NTIS, AD A037463.
59M. Tabe, Appl. Phys. Lett. 45,1073 (1984). 6OJ. A. McClintock
(personal communication). 61B. K. Janousek, R. e. Carscallen, and
P. A. Bertrand, J. Vac. Sci. Tech-
nol. (to be published). 6'T. E. Graedel et al., Science 224, 599
(1984). 6'G. V. Clatterbaugh et al., Proceedings of the 34th
Electronic Components
Conference, pp. 21-30,1984. MV. O. Altemose, in Vacuum Physics
and Technology, Vol. 14 of Methods
of Experimental Physics, edited by G. L. Weissler and R. W.
Carlson (Academic, New York, 1979), Chap. 7, pp. 329-333.
"'M. M. Tagieva and V. F. Kiseler, Russ. J. Phys. Chern. 35, 680
(1961). MM. L. Hair, Proceedings of the 27th Annual Symposium on
Frequency
Control, AD771042, pp. 73-78, 1973. 67M. L. White, Proceedings
of the 27th Annual Symposium on Frequency
Control, AD77I042, pp. 79-88,1973; also in Clean Surfaces: Their
Prep-aration and Characterization for Interfacial Studies, edited
by G. Gold-finger (Marcel Dekker, New York, 1970), pp. 361-373.
bRR. N. Lamb and D. N. Furlong, J. Chern. Soc. Faraday Trans. I
78,61 (1982).
bOp. J. Caplan, E. H. Poindexter, and S. R. Morrison, J. Appl.
Phys. 53, 541 (1982).
7R. Srinivasan, Science News 123, 396 (1983). 71R. Srinivasan
and B. Braren, 1. Polym. Sci.: Polym. Chern. edition 22,
2601 (1984). 72G. S. Alberts, U. S. Patent No.3 767490, Oct.
23,1973. 73A. N. Wright, U. S. Patent No.3 664 899, May
23,1972.
Redistribution subject to AVS license or copyright; see
http://scitation.aip.org/termsconditions. Download to IP:
128.114.34.22 On: Mon, 01 Dec 2014 15:02:16