Panel discussions on all topics.
and L. Westerberg
B. Hjörvarsson, B. Kasemo, A. Kleyn, and L. Westerberg.
Final Report on the 16th IUVSTA workshop
Outgassing properties of materials:
The kinetics and thermodynamics of adsorption, desorption and passivation
Gräftåvallen, near Östersund, Sweden 6-10 April 1997
Fred Dylla1, Björgvin Hjörvarsson2, Aart W.
Kleyn3 and Lars Westerberg4
1Jefferson Lab, 12000 Jefferson Avenue, Newport News,
VA 23606, USA
2Department of Physics, Box 530, S-751 21 Uppsala,
3FOM-Instituut voor Atoom- en Molecuulfysica, Kruislaan
407, 1098 SJ AMSTERDAM, NL
4The Svedberg Laboratory, Box 533, S-751 21 Uppsala,
This workshop was the 16th in series sponsored by various Divisions
of the International Union of Vacuum Science, techniques and Applications
(IUVSTA). It was initiated by the Vacuum Science Division and co-sponsored
by the Surface Science Division of IUVSTA in order to strengthen
the connection between researchers working on applied problems and
those studying the corresponding phenomena at a basic level. The
scope of this meeting was to obtain a coherent picture of outgassing
properties of materials in vacuum by bringing together scientists
interested in vacuum, surface and bulk material properties. The
workshop was sponsored and hosted by the Swedish Vacuum Society.
Financial support from IUVSTA, The Royal Swedish Academy of Sciences
through its Nobel Committees for Chemistry and Physics, Swedish
Natural Research Foundation and Swedish Research Council for Technical
Sciences is greatfully acknowledged. Thanks to this support, full
financing was obtained for the invited speakers and supports could
be given to 4 participants from Eastern Europe.
The meeting was held at Gräftåvallen, near Östersund, Sweden, in
a remote mountain location. In addition to the scientific programme
held in the mornings, late afternoons and evenings, time was available
for various outdoors activities, often related to snow, and a concert.
The local organisers were Lars Westerberg, Hjörgvin Hjörvarsson
with the help of conference secretary Inger Ericson (all at Uppsala
University). The other members of the organising committee were
Bengt Kasemo (Chalmers university of Technology, Sweden), Joe Greene
(University of Illinois at Urbana, USA) and Aart Kleyn. There were
42 participants from 14 countries, with appreciable participation
from Germany, Italy, Japan, the Netherlands, Russia, Sweden, and
Fifteen invited talks were presented at the meeting. F Dylla, A
Kleyn and U Gelius were invited to be responsible for the summary
session. In addition to this, 14 contributed talks were presented
at the workshop that added supporting material to the invited talks.
E. Karlsson (Uppsala, Sweden) gave a late evening talk on Classical
and Quantum Transports in Solids. Here follows a list of the invited
speakers and their titles:
- F. Dylla (Thomas Jefferson National Accelerator Facility, Newport
News, USA): Summary of outgassing measurements.
- R. P. Redhead (NRC Canada): Modelling of Outgassing andthe Limits
of Sustainable Vacuum (XHV-UHV).
- A Wildes (Australia, pres. at IFM Linköping, Sweden): Passivation
- D. Manos (Williamsburg, VA, USA): Outgassing from Polymers.
- E. Murad (Philips Labs, MA, USA): Adsorption, Desorption and
Outgassing on Spacecraft.
- I. Zoric (Chalmers, Sweden): Dynamics of Adsorption and Collision
Induced Molecular Processes on Surfaces.
- W. Lanford (SUNY Albany, USA): Hydrogen and the Weldingof High
- R. E. Pedder (ABB Extrel, Pittsburgh PA, USA): State of the
art Residual Gas Analysis.
- T. Dickinson (Washington State University, Pullman, USA): Desorption
and Photo-Desorption from ionic surfaces, Role of defects.
- R. Kirchheim (Univ. Göttingen, Germany): Interstitial Diffusivity
in Defected and Amorphous Materials; Hydrogen Diffusion along
- M. Sancrotti (Lab. Nationale TASC-INFM, Trieste, Italy): Gettering
Materials Investigated by Means of Surface Sensitive Techniques.
- Y. Ishikawa (Hitachi, Ibaraki, Japan): Outgassing, Hydrogen
Diffusion Behaviour and Surface Characterization of Stainless
- F. Dylla for R. Weiss (MIT, MA, USA): Outgassing treatments
andanalysis for the Laser Interferometer Gravity Observatory (LIGO).
- N. Hilleret (CERN, Geneva, CH) Outgassing in large accelerators:
An operationnal experience. IUVSTA Workshop Summary
From the lively discussions at the workshop and these summaries
one can definitely conclude that the topic of outgassing from materials
is far from beeing sufficiently understood and relevant experimental
data are available to vacuum users. It has already been discussed
in the IUVSTA divisions to arrange another meeting in this field
in about three years time. After this follows some detailed comments
by Kleyn, Dylla and Hjörvarsson:
Comments by Aart Kleyn:
Recurrent topics seen at the workshop were:
- the nature of the surface including questions as how many atomic
layer before vacuum are relevant to the outgassing process; what
is the nature of capping oxide of a degassing material.
- Sticking probabilities with questions such as why do first
principles calculations not reproduce experimentally observed
activation barriers for O2/Al(111) and O2/Ag(111), and why are
sticking probabilities found in applied work lower than anticipated
from surface science work.
- Adsorption isotherms, such as Henri's law, BET, Freundlich,
DRK, Temkin appeared to have very limited value even though in
some cases using the Clausius-Clapeyron equation 'surface science'
adsorption energies could be recovered.
- Desorption mechanisms: thermal desorption is most relevant
in most practical cases. A very important question here is what
are active sites for recombinative desorption, and how can they
be passivated. In the accelerator community photon stimulated
desorption (synchrotrons!), electron or ion induced desorption
were very relevant. In case of spacecraft orbiting in high vacuum
reactive desorption, possibly caused by direct, Elea Rideal reactions
and leading to 'shuttle glow' is very relevant.
- Diffusion: this determines outgassing from the bulk of materials.
It is most relevant for hydrogen. This brought in some solid state
physics into the meeting. Important issues are diffusion barriers,
mobile and immobile hydrogen and crack diffusion.
- Residual Gas Analysis: in general good measurements can be
carried out with the possible exception of measurements on spacecraft
yielding atmospheric composition data, due to surface reactions
in the ioniser.
- Engineering design could be formulated, especially for the
design for small systems. For large vacuum systems dedicated research
seems necessary. Especially in case the size and the handling
of the system makes bake-outs unpractical. Lots of empirical data
is compiled in 'phone books'.
- Particle of the meeting was without any doubt hydrogen. This
atom is hardest to remove in vacuum systems because it is very
mobile and can be dissolved in bulk materials. It strongly influences
materials properties. There were common detection problems: in
welding studies hydrogen bubbles with the sample under oil were
used for detection. Nuclear reaction analysis works very well
and provides depth resolution, but is not easily accessible. Thermal
desorption spectroscopy in vacuum is presumably the best practical
method to detect hydrogen in the near surface region.
Comments by Fred Dylla:
To summarize the technical program of the workshop, comments are
given below on the topics of (1) outgassing measurements; (2) the
relationship between surface conditions and outgassing; (3) the
status of modelling of outgassing phenomena; and (4) suggestions
for future work to simulate progress in this field.
There are two methods which have been used to measure outgassing
rates: (1) the throughput method and (2) the rate-of-rise method.
The throughput method relies on sampling the outgassing from a well
defined area and volume of sample through a well defined pumping
aperture. The rate-of-rise method relies on isolation of the test
volume from all pumps followed by measurement of the pressure rise
per unit time under zero pumping speed conditions. Both methods
have their limitations which became topics of considerable discussion
at the workshop. Recent theoretical analysis by Redhead and measurements
by Akaishi and Edleman have reemphasized earlier work in the literature
(which is often overlooked) that enumerates under what conditions
the throughput method can lead to outgassing rates that are dependent
on the pumping speed and a sizable underestimate of the true outgassing
rate of the material under question. The relevant system parameter
is the area ratio divided by the low coverage sticking coefficient.
For the system of most practical interest : water outgassing from
technical surfaces (stainless steel and aluminum), many of the literature
measurements may fall into this regime making a comparison of literature
values more difficult. Key questions that developed from this discussion
were the value of the sticking coefficient of water on stainless
steel, and the nature of the adsorption sites and kinetics (i.e.,
appropriate isotherm) for this important system.
The rate of rise method avoids the above difficulty at first glance;
however if the method is pursued over many orders of magnitude in
pressure rise than the adsorption/desorption characteristics of
the outgassing molecule may change with pressure. The desorption
of hydrogen from stainless steel should be a relatively simple system
to measure, analyze and understand. The diffusion coefficient of
hydrogen is large in steels and a large data base is evident in
the literature. Measurements of outgassing of hydrogen from stainless
steel given by Jousten at the workshop show a linear slope for 6
orders of magnitude in pressure. Hydrogen is thus acting as a non-adsorbable
gas in this circumstance with a vanishingly small sticking coefficient.
Relationship between surface conditions and outgassing
A significant topic of discussion at the workshop was the relationship
between the surface structure and chemistry of a material and the
observed outgassing rate. Very little data have been gathered on
this topic for practical materials, although there is a rich database
on well characterized model systems (often single crystal) in the
surface science literature. For the case of stainless steel, the
composition of this oxide has been characterized in several studies
by conventional sputter profile techniques. Correlation of these
data with subsequent outgassing behavior has not proven useful.
For example, a large selection of recent data on stainless steel
outgassing was shown by Ishikawa that included a variations in the
oxide layer thickness and chromium content as the bakeout temperature
was changed from 100oC to 450oC. When the data is displayed from
a large set of measurements, no trend is evident. However, when
one data set is selected which is constrained to variation of only
one parameter on one sample type (i.e., systematically growing the
oxide layer thickness or enriching the near surface chromium content,
then a more systematic trend is observed. A related topic from this
discussion is the choice of bakeout temperature to minimize the
outgassing from stainless steel. There is at present no clear answer
to this question. In fact, the present situation may be even more
confused by recent data presented at the workshop. For a variety
of treatments described by Ishikawa, all resulted in very low outgassing
rates (<<10-11 Pa.m2/s), and there was no clear advantage
to the higher temperature bakeouts. Jousten also showed a study
that indicated somewhat lower outgassing rates with lower temperature
(100-200oC) treatments. Questions were raised concerning more detailed
characterization of the passivation oxide layer. Ishikawa presented
a beautiful series of micrographs and depth profiles of stainless
steel taken with a field ionization atom probe. The micrographs
allowed full 3D elemental analysis with atomic resolution. One example
showed the changes in the oxide layer as a steel sample was heated
and oxidized in pure oxygen.
Modelling of Outgassing
Traditionally, modelling of outgassing phenomena has involved
two types of models where either diffusion of the outgassed species
(or a precursor) or desorption of the outgassed species was considered
as the rate-limited step. Redhead presented his recent analysis
of the outgassed of water from stainless steel as being desorption
limited and described with a modified Tempkin isotherm. The model
fits well with the data of Li and Dylla, who used a diffusion limted
model to describe the data. Hence, another of the thought provoking
questions posed at the workshop was: what is the rate limiting process
for the adsorption/desorption of water on steel surfaces? An understanding
of this simple question has significant practical significance considering
the number of vacuum systems in use for scientific and industrial
applications that are exposed to ambient conditions and not baked.
For this case, it is likely the kinetics involves an interlinkage
of several processes: diffusion of water or water precursors (H,
OH) through the oxide layer to the surface (perhaps through pores
or grain boundaries connected to the surface); recombination on
the surface; and finally, desorption from the adsorption or recombination
site to the gas phase.
The case of hydrogen diffusion may be more simple to understand
and model. Redhead presented a model that has hydrogen diffusing
from the bulk steel, recombining on the surface at a site that is
not accessible to surface adsorption of hydrogen. These later sites
need a surface density less than 1011 cm2 to account for the low
sticking coefficients that the rate-of rise outgassing measurements
(noted above) imply.
Suggestions for Future Work
- The above discussions on the state-of-the-art of the measurements
and theoretical understanding of outgassing phenomena generated
a number of interesting suggestions for future work to stimulate
progress in the field:
- Better correlation between outgassing measurements and the
morphology and chemical structure of the surface and near surface
- Identification of the chemical state of H2O on or near the
- Measurement of the sticking coefficient of H2O on practical
surfaces (steel and aluminum) as a function of surface conditions.
- Influence of deposited passivation layers (such as TiN, diamond-like
carbon, etc.) on outgassing.
- Measurements and corresponding modelling of outgassing from
model systems (simple metals, oxides, and polymers) that may contribute
to the understanding of outgassing behavior observed with practical
materials used in vacuum systems.
- Experiments that could identify and quantify the role of diffusion
in the near-surface region of vacuum materials (what is the diffusant
- H, CH, OH - and what are appropriate values for the defusion
Thermal desorption spectroscopy measurements of H2 and
H2O from model systems.
Clearly, more than enough work was identified by the workshop
participants to keep research in this field quite active. The talks
presented on large scale applications of vacuum in both basic science
and industry provided significant impetus for advancing the state
of our understanding of outgassing phenomena from a variety of practical
Workshop participants are indebted to the members of the Swedish
Vacuum Society who planned and hosted the workshop. In addition
to the experience in the workshop conference room, our hosts at
the Gräftåvallen Conference Center gave us the opportunity to participate
in a number of unique experiments on stick-and-slip friction on
frozen water surfaces in the slightly rarefied but most beautiful
atmosphere of Gräftåvallen.
Additional comments by Björgvin
The influence of the overlayer (metal-oxide) on the hydrogen uptake
and release is nontrivial. The presence of an oxide layer can hinder
the uptake as well as the release of hydrogen, which was illustrated
by the deterioration of single crystal Yttrium films. By covering
the surface by Gold, the catalytic splitting of water was enhanced
and the Yttrium films demolished rapidly. Without any Au capping,
this effect was negligible. These results have implications on the
role of Cr-oxides on the surface of stainless steel, as the Cr oxides
are catalytically inactive. The recombination and the dissociation
of molecules is restricted to areas where the metal is exposed to
the ambient. This has implications on the sticking coefficient,
the dissociation as well as the recombination rate of hydrogen on