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1. Introduction
UHV has long been an important tool in research and development laboratories for the physical sciences particularly those concerned with the preparation and analysis of materials often in the form of thin films. The need for rapidly and reliably obtained UHV is also a developing need in certain production industries and perhaps particularly those concerned with semiconductor manufacture. Stainless steel has been traditionally used for such systems and remains the most popular material for ease of fabrication and economic reasons, although in recent years there has been some move towards the use of aluminium (Ishimaru et al., 1992). UHV nowadays is standard technology and can be achieved rapidly and reliably providing cost is no object. In this tutorial we will consider the practical realities of the research laboratory which often involve working with existing systems, complex internal fittings for experimental purposes and sometimes the enforced use of non-UHV compatible materials. The aim is to be able to make estimates of what can be expected in less than ideal situations where a compromise may have to be reached using the available equipment.
2. Basic principles
UHV may be roughly defined as vacuum pressures below E-8 mbar. For fusion
or outer-space simulation applications, pressures below E-10 mbar with
low hydrogen content would probably be required, whereas some high rate
deposition processes for example, although sensitive to impurities, would
tolerate E-8 to E-9 mbar and hydrogen may or may not be a problem. Hydrogen
is highlighted for good reason; once surface adsorbed water, carbon oxides
and hydrocarbons are removed by baking and assuming no leaks or build-up
of poorly pumped gases, then hydrogen diffusing from the walls of stainless
steel chambers and from internal fittings is the main source of residual
gas and will be directly related to the wall thickness of the vacuum vessel.
It can be shown experimentally and theoretically from Fick's Law (Jost,
1960), that long-term high-temperature bakeouts are necessary for thick-walled
vessels if these are to be used at elevated (300C) operating temperatures
(Santeler, 1991).
Before examining the significant factors in the attainment of UHV it
is worthwhile to take as a simple example a typical vacuum chamber of say
450 mm diameter and 750 mm length, an area,A, of about E+4 cm2. The pressure,p,
achieved in the chamber is given by the relation,
p = QA/S mbar Equ. 1
where Q is the outgassing rate from the walls of the chamber in mbar
l/ s cm2, and S is the pumping speed in l/s. Outgassing rates from cleaned
and /or low temperature-baked, (<150C) stainless steel have been quoted
in the literature over a wide range from E-8 to E-11 mbar l/s cm2, but
using a commonly quoted (but fairly conservative), value of E-10, (Young,
1969; Sullivan, 1992; Dylla et al.,1993) gives a pressure,p, of E-8 mbar
for a pumping speed of 100 l/s (a small ion-pump or turbo); or E-9 mbar
for a speed of 1000 l/s (a much more expensive pump). Hence to achieve
low pressures in systems containing fittings and fixtures which may
involve a considerable amount of metal area and gas traps (if for example
mechanical parts or manipulators are involved), it is necessary to reduce
the outgassing rate by orders of magnitude if reasonable size pumps are
to used. If costs are no object, pump size may be increased with
advantage but not without limit, since the pump itself adds a considerable
area of metal in its connecting tubulation; the outgassing from this area
must be reduced to the same level as the rest of the system if advantage
is to be taken from the increased pumping capacity. One should note also
that in a laboratory where old equipment may have been pressed into service,
the outgassing rate may well be considerable higher than in the above example
even with rigorous cleaning techniques.
The principal method of reducing outgassing is baking at elevated temperature
although other factors as discussed below can also be significant.
3. Factors involved in the achievement of UHV
3.1 Materials for construction and hardware
Stainless steel is the most frequently used material for fabricating
vacuum chambers and for internal hardware but significant variations of
outgassing rate have been found between different grades as reviewed for
example by Adams (1983). Early systems and components were often made with
304 and later with 316 but considerations of outgassing, fabrication and
quality control have led to the almost standardized use of 316L. Considerable
variations in outgassing performance have been reported even for
this one material leading to the conclusion that the quantity of gas held
trapped in the metal is in many cases uncontrolled in the manufacturing
process. Thus for the highest performance as required for example in fusion
devices, space hardware and particle accelerators, or experiments involving
the elimination of monolayers on surfaces, vacuum-melted 316L is the material
of choice. The outgassing rate for cleaned but unbaked 316L is quoted as
E-9 to E-10 mbar l/s cm2, whereas vacuum induction melted/vacuum
arc remelted (a double vacuum-melting process) has an unbaked outgassing
rate of E-11 mbar l/s cm2, (Sullivan et al., 1992). The use of such a material
is a major advantage and can yield outgassing rates a further two or three
orders of magnitude lower when surface passivation and baking techniques
are subsequently used.
3.2 Surface condition
Desorption of surface gas is a major component of outgassing load and
it is therefore to be expected that the condition and chemistry of internal
surface layers would be of importance in determining achievable vacuum.
Significant factors include: chemical composition, structure and thickness
of oxide layers since these affect adsorption and desorption from the surface
as well as diffusion from underlying metal (Ishikawa, 1995); surface passivation
to produce Cr oxide or fluoride rich or boron nitride layers (Tomari, 1991;
Yoshimura et al., 1991; Tohyama et al., 1991); effect of metal forming
and subsequent machining or surface finishing techniques employed (Young,
1969); polishing, which may be chemical, electro-chemical or mechanical;
and cleaning where a wide range of possible techniques is available.
3.2.1 Surface chemistry
Early work (Huffine and Williams, 1960), suggested that oxidation of
the surface of stainless steel could reduce hydrogen outgassing by a factor
of ten or more. Other workers have since found that outgassing can
be reduced by oxidation. More recently Ishikawa (1995), examined
the effect of a surface oxide layer on outgassing from stainless steel
and found that oxidation in air produced a surface layer of iron oxide
whose effect was to form a diffusion barrier for hydrogen diffusing from
the bulk metal. The improvement is rather less than an order of magnitude
but evidence is given that carbon contaminants also are reduced and hence
carbon containing outgassing species. Young (1969), found evidence
that oxidation of 304 stainless steel in air at 450C reduced outgassing
by depleting hydrogen from the steel rather than any diffusion barrier
effect. According to Sullivan (1992), the important chemical processes
that cause the formation of volatile gases, occur in the near surface layer
which extends through the native oxide and to some extent into the bulk.
These authors conclude that a thin dense and chemically clean surface oxide
layer reduces the probability of chemical surface processes contributing
to outgassing rates.
3.2.2 Surface structure
Some early workers tended to assume that outgassing would be higher
from rough rather than smooth surfaces on the assumption that a rougher
surface would contain more gas traps and have greater surface area. This
view was questioned by Young (1969), who produced evidence with 304 stainless
steel to show that outgassing from smooth electropolished surfaces was
no different from glass bead shot-blasted surfaces, both surfaces having
subsequently been baked at 250C. His results interestingly, agreed quantitatively
with Calder and Lewin (1967) who used U15C material and who did not state
any particular surface treatment. Dylla et al. (1993), examined the effect
of surface roughness on outgassing performance and showed that there was
no effect when the surface roughness was varied by over two orders of magnitude.
It may be stated that there is little if any real evidence to show that
electropolishing together with other forms of fine surface machining reduces
outgassing and while such processes are commonly used as finishing techniques
by manufacturers, their main function would seem to be cosmetic.
3.2.3 Surface Cleaning
It is obvious that rigorous cleaning should be undertaken before exposing
surfaces to vacuum and bakeout. ESCA studies (Sullivan et al., 1992), have
shown that electropolishing alone or in combination with abrasive polishing
does not result in noticeably lower surface carbon content than just trichloroethane
vapor degreasing, and this conclusion applies to most other impurities
examined. The authors criticise the use of electropolishing in manufacturing
practice for quality control reasons, since commercial electropolish suppliers
may not change solutions frequently enough to remove impurities which can
lead to outgassing products as well as particles and corrosion centers.
They conclude that for vacuum component manufacture, an environmentally
safe cleaning process using an alkaline-based detergent, hot deionized
water rinse and drying is an effective technique resulting in surfaces
that desorb water efficiently and also have extremely low particulate generation.
A form of cleaning which has been used successfully in the fusion energy
field is glow discharge cleaning (Santeler, 1991). In this process the
higher energies (compared to thermal motions), of the plasma particles
removes tightly bound carbon oxides and and hydrocarbons which are removed
by the pumps. This is of vital importance when a low surface contamination
is necessary (for example when surfaces are to be subsequently bombarded
during a processing schedule). Plasma cleaning may be combined with simultaneous
baking for maximum effectiveness.
3.3 Baking
Early workers baked systems only to about 200C which was effective
in removing weakly bound surface water and hydrocarbon molecules. It was
not long before it was discovered that baking to higher temperatures was
required to remove more tightly bound species and the hydrogen diffusing
from the bulk, both of which had the effect of limiting the ultimate system
vacuum. The effectiveness of high temperature bakes is amply demonstrated
for example by Calder and Lewin (1967), who showed that outgassing could
be reduced to about E-16 mbar l/s cm2 by baking for 11 days at 300C or
1 hr only at 635C. Barton and Govier (1968), showed that baking new
18/18/1 stainless steel components at 450C in vacuo, with the exception
of traces of hydrogen, successfully removes all gases resulting from the
previous history . Interestingly these workers found that gas adsorbed
at the surface of stainless steel on re-exposure to atmosphere could be
remove by baking at this temperature for 2 to 3 hrs. If pressures only
of the order of E-9 mbar were required then sufficient gas could be removed
by a 12 hr bake at 200 to 300C; for some applications it is thus
worthwhile to use a separate vacuum oven for baking new components at 450C
thus avoiding the need for the main system to withstand high temperatures.
Santeler (1991), quotes early work by Aero Vac Corporation which gives
valuable data on the effect of baking to different temperatures which is
reproduced here below:
Outgassing rates in Torr l/s cm2
BAKING TIME
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The data above bring home clearly the benefits of high baking temperatures and the diminishing returns from increasing baking time much beyond 20 hrs for baking temperatures up to 400C; at 500C the table shows that it is worthwhile to bake for as long as 200 hrs although one might legitimately question the unusually low value of 8.0E-19 torr l/s cm2 that is quoted. Recent work (Ishikawa, 1995; Ishikawa et al 1991; and Ishikawa and Odaka1990), confirms the values above for lower temperatures but indicates that surface treatments and raw material quality can improve these figures substantially, (section 3.2 above).
3.4 Pumping systems
The selection of pumps, whether for research or production, is not
always straightforward and invariably involves cost considerations. All
four main types of high vacuum pump: turbo, cryo, diffusion and ion,
are capable of producing pressures in the UHV region, often with the addition
of a titanium sublimation pump (TSP), to achieve the lowest pressures.
The choice of pump will often be determined by the application. However,
there are characteristics of pumps which limit their ultimate UHV performance
and capability of removing products of outgassing and these are now considered.
3.4.1 Turbomolecular pumps (TMPs)
These are compressors and are limited inherently by the low compression
ratio for hydrogen which is the main residual gas in a UHV system. To achieve
pressures approaching E-10 mbar or less it is advisable to combine this
type of pump with a titanium sublimation pump for the above reason. TMPs
can usually be baked to a maximum of about 120C and for some models less,
hence some thought is necessary when designing connecting manifolds to
avoid overheating the pump and at the same time minimising the area of
metal that is heated only to low temperature during bakeout. Lower cost
TMPs have hydrocarbon oil or greased bearings which is a potential source
of contamination but providing the pump is operated correctly hydrocarbon
molecules will not migrate into the vacuum vessel. Correct venting is particularly
important and the pump must not be stopped under vacuum. Higher cost TMPs
are available with air or magnetic bearings which avoid these possible
problems.
3.4.2 Cryopumps(CPs)
These pumps are limited by their capacity to cryosorb hydrogen onto
cold charcoal. An ultimate system pressure somewhat below E-9 mbar
is typical which can be extended to below 5E-10 mbar with a TSP. The capacity
to adsorb hydrogen will decrease with time once the hydrogen cryosites
are used up and this can result in an increase in system pressure. CRPs
cannot be baked and moreover must be shielded from radiated or conducted
heat caused by baking or processing schedules of the main system. The area
of unbaked shields and tubulation must be minimal if UHV pressures are
to be achieved. Some manufacturers have incorporated additional radiation
cold shields and baffles into their pump designs to reduce the scale of
this problem. Cryopumps have very high pumping speeds for water-vapour,
but in a UHV system water-vapour is removed by baking so that this may
not be such a great advantage. CRPs require periodic regeneration and a
high vacuum isolation valve; for the lowest pressures an all metal bakeable
design is necessary, but many UHV systems operate satisfactorily with a
viton plate seal and copper gasket or wire seals for flanges with a bellows
sealed drive. The viton seal can be baked safely to 120C closed and 150C
in the open position. As stated above the area of metal not heated to the
full baking temperature must be minimised.
3.4.3 Diffusion pumps (DPs)
DPs will pump to about 5E-10 mbar which can be extended to well
below E-10 mbar with a TSP. Current practice is to use low vapour pressure
hydrocarbon based fluids with a liquid nitrogen and/or water or cryo-cooled
baffle. Thorough tests have shown, (for example, Gay et al., 1994;
Dennis et al., 1982) have shown that even with only a water-cooled baffle
hydrocarbon contamination from the pump is at the limits of measurement
and effectively close to zero for all practical purposes. The work by Gay
et al., is particularly illustrative; the experiments were made to confirm
contamination free conditions for a GaAs source which could not tolerate
hydrocarbons or CO and revealed the importance of correct operation not
only of the diffusion pump, but the foreline where trapping and purging
procedures were found to be essential to ensure isolation from rotary pump
backstreaming. Dennis et al., showed that integral diffusion pump/baffle/valve
systems could be cleaner than hydrocarbon lubricated turbopumps.
DPs do carry the risk of gross contamination from the consequences
of accidental air-dumping or partial contamination from faulty operation.
Diffusion pumps, like turbopumps, act by compression and hence their pumping
action in a UHV system is also finally limited by the compression ratio
for hydrogen, but the problem is less severe with a DP compared to a TMP.
A high vacuum valve is normally used to isolate the pump when the system
is vented, but some designs of UHV system would eliminate this to achieve
higher pumping speeds and remove a source of outgassing.
3.4.4 Ion pumps (IPs)
IPs will pump to 5E-10 mbar or less with extension to below E-10 mbar
with a TSP. In many respects the ion pump which has no moving parts or
working fluids is ideal for UHV in cases where there is no subsequent process
gas load. The IP does have certain characteristics which the user needs
to be aware of and may cause problems in some circumstances: (i) Selective
pumping. Pumping is by gettering for chemically active gases and
burial for inert gases, which occurs at a much lower pumping speed. Inert
gases will therefore tend to build up as a residual gas background, the
lighter inerts (He and Ne), are particularly prone since these may diffuse
out again from beneath the surface. Hydrogen readily diffuses into titanium
as a solid solution, but will be released again on heating. The presence
of argon in the residual enhances sputtering and hence the chemical removal
of hydrogen as titanium hydride. Methane is normally present in the residual
since this is generated at titanium surfaces although also removed by dissociation
in the pump discharge. (ii) Memory effects. These are characteristic of
IPs and will affect the residual gas composition, gas ions buried in the
cathodes earlier in the life-cycle are released by subsequent sputtering.
The effect can be particularly bad for inert gases and leads to the phenomenon
of argon instability where significant amounts of released argon lead to
periodic increases of pressure. This has been designed out of some models
of pump by slotted cathodes, triode structures or Ti/Ta cathodes. (iii)
No tolerance to gas load. IPs are not suitable for even small loads and
particularly inert or hydrocarbon gases; once the pressure inside the pump
rises above E-6 to E-5 mbar, the pumping speed falls and pumping may become
unstable. This can eventually lead to heating inside the pump and the additional
release of hydrogen, helium and neon. Undetected leaks during starting
can be a particular problem. (iv) IPs have magnetic fields which can interfere
with processes unless screened which is not always possible.
The ion pump may be operated with sorption pumps (some workers use
a turbopump) for starting, to produce a completely contamination free system.
The whole system including the pump can be baked, the baking temperature
depending on whether the ion-pump magnets are removed. A high vacuum
valve, which increases outgassing load and reduces pumping speed,
is not necessary although this can be useful for isolation for faster starting
during pump-down.
4. Example from a silicon processing research laboratory
A small stainless steel chamber 30 cm diameter by 50 cm long and used
for SIMS analysis is attached through a Viton-sealed gate-valve to the
central distribution chamber of a cluster-tool which processes 8 inch diameter
silicon wafers. The gate-valve is of 'letter-box' design allowing the wafer
to pass back and forth through a slot between the two chambers. The SIMS
chamber is pumped directly by a 150 l/s ion pump with no high vacuum valve,
the chamber and ion-pump being initially roughed from atmosphere through
a small Viton-sealed valve connected via a foreline to a turbopump. A titanium
sublimation pump is available and an additional 50 l/s turbopump is used
to evacuate the SIMS ion-gun, but this pump is ineffective in pumping the
chamber because of the constriction to gas-flow by the ion-optics.
After loading the wafer and closing the gate-valve the system is required
to be pumped to less than E-9 mbar and pressures approaching E-10 mbar
would be preferred. A fast cycle time is important to prevent a bottle-neck
as processed wafers are passed from the other process chambers. The budget
on this project is exhausted and only minor purchases can be considered.
What baking times and temperatures are required and are they consistent
with all the requirements? Would the titanium sublimation pump overcome
any limitations of the baking schedules?
The total area of the chamber with flanges and tubulation is about 6000
cm2. It is difficult to calculate the area of the internal parts which
contain many gas traps in the form of bearings and screw threads of manipulators,
several sets of bellows and in addition the ion optics and detector components
of the surface analysis system. For the sake of the example we will assume
a total area of three times the above, that is 18000 cm2. For a preliminary
estimate, assume a 20 hr bake at 150C which creates no problems for the
Viton seals provided the valves are open (this assumes the distribution
chamber is open during the bake which may not be desirable). Using Equation
1, and Santeler's values above for Q, then:
p = 6.3 * E-11 * 18000/150 Torr = E-8 mbar
This is clearly inadequate for the application and too high to make
worthwhile use of the titanium sublimation pump. To achieve a pressure
just an order of magnitude lower would require baking for 20 hrs at 250C,
so that for the required lower pressures, baking at 300C would probably
suffice. The difficulty is protecting the Viton seals. The options available
are: (i) Cool the seals. This is difficult and may be impossible
to engineer with the geometries and existing construction of the gate-valve
although the small roughing valve could cooled without difficulty. (ii)
Replace the valves with metal sealed versions. In this example the
budget does not allow new capital purchases, moreover, a metal sealed UHV
gate-valve which would pass an 8 inch diameter wafer would be of high cost
although the small roughing valve could possibly be replaced with a small
conflat sealed valve. (iii) Use larger pumps. The example shows
that a pump of considerable more than 1500 l/s would be required and again
this is beyond budget. (iv) Bake for longer times. This is not acceptable,
even 20 hrs is too long for process turn-round requirements. The table
above shows that in any case the gains from longer bakes are only minimal.
One is left with a compromise situation; unless residual vacuum requirements
are relaxed, one must try and bake the chamber at the highest temperature
possible while relying on the large mass of the distribution chamber to
keep the gate-valve at an acceptable temperature to protect the seals.
It is worthwhile to estimate the permitted fraction,x, of the area of the
chamber metal baked at lower temperature in order to achieve required pressures.
The final pressure will then be the sum of two components from metal outgassed
at lower and at higher temperature. For the estimate we will ignore the
temperature gradient and assume average values.
If x not too large then the contribution from baking the main chamber
at 300C for 20 hrs assuming an outgassing rate of E-12 mbar l/s cm2,
is given to an approximation by:
p(300) = E-12 *(18000)/150 mbar Equ. 2
The contribution from baking a smaller area, 6000x cm2, at 150C
for 20 hrs with an outgassing rate of 8*E-11 mbar l/s cm2 is given by:
p(150) = 8*E-11 * 6000x/150 mbar Equ. 3
Adding Equs. 2 and 3 yields:
p(300) + p(150) = 1.2 E-10 + 3.2 E-9x Equ. 4
The second term in Equ. 4 equals the first when x is about 4%, yielding a pressure of 2.4E-10 mbar, if x is allowed to increased to 10%, the total pressure increases only to 4.4E-10 mbar, which is perfectly acceptable for the application. This approach looks feasible, the example illustrates the initially assumed fact that outgassing is dominated by the internal parts and it is clearly important to ensure thorough degassing of the internal mechanisms when the gate-valve is maintained at lower temperature. Once the pressure is below E-9 mbar it is worthwhile to use a titanium sublimation pump for attaining lowest pressures.
5. References
Adams R O, "A review of the stainless steel surface," J Vac Sci
Tech. A1 12 1983 pp. 12-18
Calder R and Lewin G, " Reduction of stainless-steel outgassing in
ultra-high vacuum," Brit J Appl Phys. (1967) pp. 1459-72
Dennis N T M, Colwell B H, Laurenson L and Newton J R H, "Further
developments with single structure vapour pumping groups," Vacuum
28 1978 pp. 551-8
Dylla H F, Manos D M, and LaMarche P H, "Correlation of outgassing
of stainless steel and aluminum with various surface treatments,"
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Gay T J, Brand J A , Fritts M C, Furst J E, Khakoo M A, Mell
E R, Sieger M T, and Wijayaratna W M K P, "Clean ultrahigh vacuum system
with single-structure diffusion pumps," J Vac Sci Tech A 12(5) 1994
pp. 2903-10
Huffine C L and Williams J M, Corrosion 16 1960 p. 430
Ishikawa Y, Koguchi Y and Odaka K, "Outgassing rate of some austenitic
stainless steels," J Vac Sci Tech. A 9 1991 pp. 250-3
Ishikawa Y and Odaka K, "Reduction of outgassing from stainless surfaces
by surface oxidation," Vacuum 41 1990 pp.1995-7
Ishikawa Y and Yoshimura T, "Importance of the surface oxide layer
in reducing outgassing," J Vac Sci Tech. A 13(4) 1995 pp.1847-52
Ishimaru H, Itoh K, Ishigaki T and Furutate M, "Fast pump-down aluminum
ultrahigh vacuum system," J Vac Sci Tech. A 10(3) 1992 pp. 547-52
Jost W, Diffusion in Solids, Liquids, Gases (Academic,
New York, 1960)
Santeler Donald J, "Estimating the gas partial pressure due to diffusive
outgassing," J Vac Sci Tech A 10(4) 1992 pp.1879-83
Sullivan J, Schaffer S and King S, "Contamination control in the design
and manufacture of gas flow components," J Vac Sci Tech A 10(4) 1992 pp.
1869-74
Tohyama et al., J Jpn Inst Met. 54 1990 p. 247
Tomari H, Hamada H, Nakahara Y, Sugiyama K and Ohmi T, "Metal surface
treatment for semiconductor equipment: oxygen passivation, Solid State
Tech. 34 Feb 1991 pp.S1-S5
Yoshimura N, Hirano H, Sato T, Ando I, and Adachi S, "Outgassing characteristics
and microstructure of a 'vacuum fired' (1050C) stainless steel surface,"
J Vac Sci Tech A9 1991 pp. 2326-30
Young J R, "Outgassing characteristics of stainless steel and aluminum
with different surface treatments," J Vac Sci Tech 6(3) 1969 pp. 398-400
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