27. Hydrocarbon contamination in vacuum dependent scientific instruments

27. Hydrocarbon contamination dependent scientific instruments U R Bance. I W Drummond, Barton

Dock

Road,

Urmston.

D Finbow, Manchester

E H Harden M37

2LD.

and P Kenway.

in vacuum Kratos

Ltd.

AN

Scientific

Instruments.

UK

Hydrocarbon contamination affects the performance of most vacuum dependent scientific apparatus. In the electron microscope contamination of the specimen due to polymerisation of backstreaming pump fluids and other system hydrocarbons, contributes to poor resolution. In surface analysis equipment (e.g. XPS, AES and UPS) hydrocarbon contamination may interfere with the analysis and in the mass spectrometer it can adversely affect resolving power ahd complicate interpretation of results. A combination of pumps is used to generate the vacuum required in scientific instruments. Pumping systems normally employ one or more of the following: rotary, diffusion, turbomolecular, ion, sublimation and cryo pumps. The main sources of contamination are directly attributable to these pumps, the construction materials used in the equipment and the processing of these materials. In an ultra-clean system the sample itself may introduce hydrocarbon contamination. Reduction of hydrocarbon contamination involves the use of traps (either cooled or absorbent), selection of special pump fluids, special processing techniques in the case of ion pumps and selection of system components, including resilient seals, which can be suitably processed to reduce contamination. Special techniques may be used to clean samples in situ. Experiments are described which show the sources of system contamination and remedies are suggested based on practical experience gained in design and operation of vacuum systems for electron microscopes, electron spectrometers, scanning transmission electron microscopes and mass spectrometers.

Introduction Vacuum dependent scientific instruments include transmission electron microscopes (TEM), scanning electron microscopes (SEM), scanning transmission electron microscopes (STEM), mass spectrometers, electron spectrometers (ESCA), particle accelerators and many others. In the electron optical instruments there is overwhelming evidence that *-” contamination of the specimen is due to polymerization of stray hydrocarbon molecules by the electron beam, resulting in effective thickening of the specimen by contaminants, thus, obscuring the finer details and reducing the available resolution. Continuous efforts have been made to reduce the residual hydrocarbon level in the vacuum system of commercial microscopes since they were first constructed in Great Britain by Metrovick Electrical Co Ltd. but there is still room for improvement. Most forms of commercially available spectrometers are devoted to surface analytical techniques, that is to say, they are all providing chemical and physical information concerning the top few atomic layers of a solid sample. It is obviously important that the surface is not covered with a relatively thick layer of hydrocarbon contamination. For this reason all the techniques, i.e. X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS) and Auger electron spectroscopy (AES) are generally carried out in clean uhv conditions, i.e. ultimate vacuum in the 10-‘“-lO-” torr range. The energies of the electrons emitted in XPS, UPS and AES typically lie in the range O-5000 eV. Operation of ion optical equipment, such as mass spectroVacuum/volume @ Pergamon

28/number

Press LtdJPrinted

1 O/l 1. in Great

0042-207X/78/1 Britain

meters, is based on three main functions, first the production of ions, second their separation into discrete beams and third the detection of these discrete beams. All three functions are affected by contamination of the system. In the ion production area, molecules of the sample are bombarded by electrons to produce sample ions. Contaminant molecules are also bombarded and spurious ions can be produced. Additionally the bombardment of contamination increases the build up of insulating polymerized films on critical electrodes, resulting ultimately in the failure of the ion source. Separation of the ions into beams requires accurately maintained ion optical properties, utilizing accurate potentials on electrodes and clean narrow slits. Polymerized films on these critical apertures result in scattering of the beam by charged coated surfaces with consequent loss of resolving power and sensitivity. The scattered ions together with the spurious ions produced in the source increase the difficulties in interpreting the output spectra. Contamination of the detectors themselves (electron multipliers and similar devices) can reduce sensitivity to a point where these expensive components have to be replaced. In mass spectrometers, therefore, particularly in high resolution mass spectrometry. a vacuum system free from mechanical vibration and capable of producing a pressure in the lo-’ torr range is considered essential if resolution is to be maintained and background interference with spectra reduced to low level. In this paper it is proposed to identify the main sources of hydrocarbon contamination and, in the light of practical experience, to discuss contamination problems and to suggest methods

OOl-0489$02.00/O

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U R Bance et

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Hydrocarbon contamination in vacuum dependent instruments

of reduction to acceptable levels. It must be stressed, however, that the main object here is to discuss hydrocarbon contamination not the total ultimate pressure, it is quite common nowadays to reach pressures in the IO-” torr range by using combinations of different pumps. Sources of hydrocarbon

contamination

All vacuum systemsemploy one or more of the following pumps,rotary, diffusion, sorption, ion, sublimation and cryo. They incorporate vapour traps of various types and are constructed of metals,plastics,rubbers and various greases.Most of thesecomponentscontribute either directly or indirectly to the total hydrocarbon contamination of the vacuum system. The mechanismof the contamination and its reduction differ for the various components,the major contamination sources will be discussed separately. Rotary pumps

be 1O-3 I torr s, which is equivalent to several microgrammes per second.For a systemof 1000cm’, oil from the rotary pump will be depositedat the rate of 0. I monolayer/s.It is a wasteof effort to design diffusion pumps with lower backstreaming characteristicsif no precautionsare taken to reduce the backstreaming from the rotary pumps which are, of course, an essentialpart of the diffusion pumpedsystem. Compare the reported backstreamingratei from a 3 in. diffusion pump fitted with a cold cap of 6.8 x 10e6cm3s (say 5 x 10m6I torr s) with the above figure of 10m3I torr s for the rotary pump. Reduction of the rotary pump backstreamingis usually achieved by meansof a sorption trapi in the rotary pump pipework. Such traps can reduce the hydrocarbon contamination by a factor of lo’, but only if the trap is maintained in an operationally satisfactory condition. This necessitates regular replacementof the sorption charge, sincethe baking of thesetraps in situ is not recommended. The worst backstreamingfrom rotary pumps occurs when continuously running near the ultimate pressure for long periods.

In normal operation the ultimate pressureattainable by a two stage rotary pump is around 10m3torr (measuredby total Diffusion pumps pressuregauge). The vapour pressureof rotary pump oil at The diffusion pump is still the most widely usedhigh vacuum 50°C is 10e5 torr (Figure I) and, basedon this figure, one might expect a lower ultimate pressure.On a practical system, pump in industry for the production of high and ultra-high vacuum. The first fluid to be usedin thesepumpswasmercury however, somecontamination of the oil is always presentand with a vapour pressureat room temperatureof about 10e3torr degassingof the systemcomponentsalso contributes to the ultimate. Publishedfigures showing lower ultimate pressures and at -50°C of 10e6 torr. It was C R Birch of AEI (then are measuredby meansof McLeod gauges,which only indicate Metro Vickers) who produced the first low vapour pressure oils, the Apiezon range of fluids. Later Hickman in the USA permanentgasesand not condensiblevapours. If weassumethe conductancefor the rotary pump oil vapours producedpumpingfluids with even lower vapour pressures,the polyphenyl ether oils. The vapour pressureof polyphenyl ether at the high vacuum sideof the systemis 1 I s, then the backoils at room temperature is about 10m9torr, so for most streamingrate from the rotary pump at lo-” torr ultimate will applicationsliquid nitrogen cold traps have been replaced by peltier or water cooled baffles. For uhv applications such as electron spectroscopy, it is essentialto usea liquid nitrogen trap of efficient design,however, it doesnot follow that the useof low vapour pressureoils hasresultedin an end to oil backstreaming. In fact a lower vapour pressurefluid will causefaster system contamination than a higher vapour pressureoil if the backstreamingrate is the samein both casess5.To keep the accumulation of contaminant low at a given backstreamingrate requiresa higher systemtemperature, but to keep the partial pressureof the contamination low requiresa low systemtemperature. If the systembecomescontaminatedwith low vapour pressureoils, it is possibleto drive this out of the systemby baking. Temperaturesof 300°Care required and it is essential that there are no cold spotsin the systemwhere the vapours could condense. In most vacuum systemsexcept that of ESCA it is more or lessimpossibleto maintain such a high uniform temperature over the entire vacuumsystemincludingall internal components. Dismantling and mechanicalcleaning seemsto be the only effective method of cleaningthe internal surfacescontaminated -80 -50 0 50 loo I50 200 300 with the very low vapour pressureoils. Temperature *C Within the last 10years the designof commercially available 6. Sillcons 704 I.Mercury 7. Aplezon C EArochlor andapiezon AP201 rotary and diffusion pumps has been greatly improved, but 3.Aplezon A and silkone 702 6. Scntovac 5 and BL IO scientific instrumentsusing such componentsstill suffer to a 705 4.4.k~~ ; and rotary pump oil 9. Silkone degree from oil migration into their systems.To investigate someof the causesof this oil creepage,the following experiFigure 1. Vapourpressures of vacuumpumpfluids. mental arrangementwasconstructed. 490

U R Bance et

a/:

Hydrocarbon contamination in vacuum dependent instruments

The system (Figure 2) consisted of a 3 in. dia diffusion pump surmounted by a cold trap, MS10 mass spectrometer and nude ion gauge. The backing line was fitted with a thermocouple gauge, isolation valve and rotary pump. The fine side of the system was baked out overnight at 250°C and a system pressure of
0 05

otary pump

Figure 2. Experimentalarrangement for backstreaming tests.

Heater \ on

‘diffusion pump heateroff

I IO

I 20

I 30

Time

I 40

I 50

I 60

I 70

min

Figure 2a. Back migrationof light hydrocarbons (diffusionpump

heateroff).

01

0 I5

02

Fore-linepressure torr Figure 2b. Migration of light hydrocarbons from the backinglineof the diffusionpump. measuredby ,,,/e 43 peak34 occurred within a few minutes, reaching 70 times the original in 9+ min. Heater power was then restored,but a further period of 1 h wasrequiredto reduce the contaminationlevel to its original value. Later experiments (Table 1) confirmed that this increasewasdue to back migration of cracked products from the backing line. A secondexperimentFigure 2b showedthe effect of increasing the hydrocarbon pressurein the diffusion pump backing line by isolatingthe rotary pump. This five timesincreasein the hydrocarbon contamination in the fine sidepressurewascaused by a riseof only 0.25torr in the backingpressure,i.e. below the critical backing pressureof 0.3 torr. Confirmation that this back migration is due to cracking products was obtained by repeating the experiment, but this time raising the backing pressureto 0.25 torr by admitting atmosphericair to the backing line with the rotary pump running. In this casethe fine side partial pressuredue to hydrocarbonsactually reducedby 11%. Two further experimentswere completedin the first the diffusion pump cooling water temperaturewas raisedfrom 10 to 35°C as a result the fine sidehydrocarbon pressureincreased by 10%. In the secondexperiment the backing line pressure was quickly increasedto a point exceeding the CBP, then allowed to reduce. The experiment was repeated 100 times. The diffusion pump stalled as was expected and oil drops appearedin the fine sideof the equipment. The Table 1 summarizesthe results of these experiments, showingthat increaseof cooling water temperature,temporarily switchingoff the pumpheaterand risein pressurein the backing line all contribute to increasein backstreamingof the pumping fluid or their crackedby-products into the high vacuumsystem. Thesecracked products are produced due to thermal cracking or oxidation of the pumpingfluid. Sometimesthere are impurities in the oil itself, which crack into lighter fractions and, thus, show up in the high vacuum side. 491

lJ R Bance et

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Hydrocarbon

contamination

Table 1. Hydrocarbon vapour migration diffusion pumping set

in vacuum dependent instruments

into the fine side from a Peak height (I unit= FSDrange IO

Experiment

Results

Diffusion pump heater switched

Hydrocarbon partial pressure increased by 70 times

50

Rotary pump isolated from diffusion pump till pressure on backing line rises to 0.25 torr

Hydrocarbon partial pressure increased by 5 times

20

Air continuously pumped by rotary pump at 0.25 torr at the backing line

Hydrocarbon partial pressure reduced by I1 %

Cooling water temperature increased from 10 to 35°C

Hydrocarbon partial pressure increased by 10%

02

Gas admitted to backing line to exceed CBP Experiment repeated 100 times

Oildrops on the fine side

61

off for IO min

r

Peak after

height 3h

l-r

Peok after

height 140h

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IO 05

005

0 02

Methods of removing these products by use of an auxiliary series diffusion pump backing the main pump”‘“, or by use of a foreline condenser, improvements to the diffusion pumpI and reduction of the back migration by efficient degassing of the diffusion pump fluid “*‘a, have all been reported. In another series of experiments the partial pressures of various residual gases in a baked metal vacuum system were studied35. The vacuum system in this case was similar to that in Figure 2, but various traps and baffles could replace the cold trap. One of the experimental results using a thermoelectric baffle cooled to -20°C is shown in Figure 3. This shows the large rise in hydrocarbon residual pressure G/e 41 and 43 peaks), which occurred in a 6-day period after bake out. The hydrocarbon background increased by a factor of 100 in this period. A similar experiment using cold traps at -80°C showed a hydrocarbon rise of only 2: 1 in the same period. These experiments were carried out on a medium vapour pressure oil (Apiezon BW) and show that, whilst for many applications a -20°C baffle gives a worthwhile reduction in hydrocarbon residual with minimum maintainance, for mass spectrometers it can be an advantage to use lower temperature traps. However, with this knowledge it was still felt that the effects of hydrocarbon contamination could be further reduced. Recently new fluids called Fomblin (perfluoropoly-etherfluid) have appeared on the market and have been used successfully on diffusion pumps used in some of our mass spectrometers and electron microscopes. The main characteristics of these fluids as claimed by the manufacturer are their resistance to polymerization when exposed to electron or ion beam bombardment (polyphenyl ether and Apiezon fluids do polymerize), resistance to oxidation and resistance to formation of insulating or conducting layers if decomposed. Some of these claims especially its resistance to polymerization have been tested and found to be true*~19-26. I ncidently it is possible to dispense with the rotary pump sorption trap in a diffusion pump system in which the rotary and diffusion pumps use oils of a similar chemical nature. Some of the other precautions which can be taken to reduce contamination are prevention of absorption of hydrocarbon 492

2

I5

20

25

30

35

40

45

m/e

Figure 3. Residual mass peak height 3 h and 140 h after cooling20°C therm0 electric baffle (3OO’C bake for I6 h). vapours from rotary pumps on traps, prevention of migration of oil vapours into the system during bakeout and ensuring that the diffusion pump is isolated during its starting and stopping periods. Turbomolecular

pumps

Hydrocarbon contamination from turbomolecular pumps (using oil bearings) though small has been reportedz7 and though these pumps are inherently self protecting in case of vacuum accident, it is essential to follow the manufacturers instructions regarding venting of the pumps to air when stationary if system contamination by sucked back bearing oil is to be prevented. Titanium

sublimation

pumps

These pumps are now considered an essential part of uhv ion or diffusion pump systems for scientific instruments. Light hydrocarbons are pumped by surface adsorption’“. These adsorbed hydrocarbons on the evaporated titanium layers and on the titanium evaporator itself must be removed during the baking cycle. Ion pumps Ion pumps (diode or triode) are uhv pumps which do not need oil in operation. They are self protected, can be used in any position and can be baked at temperatures in excess of 3OO”C, either into the pump itself or into an auxiliary pump. Hydrocarbon contaminants in these pumps are adsorbed or absorbed during fabrication and assembly of the components parts. Even the best clean room facilities cannot guarantee

U R Bance

et a/: Hydrocarbon

contamination

in vacuum

dependent

instruments

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Figure 4. Percentage

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Totat

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carbons

in concentration

of some

me

lne

pressure

gas constituents

after

processing

HI car

of the Z I s Appendage

Ion Pump.

5 compared with that in an unprocessed pump. Unlike diffusion pumps the ion pumps themselves reduce the total concentration of the hydrocarbons in the system with time and this is due to the decomposition of the hydrocarbons into carbon and hydrogen in the electrical discharge’O.

absolute freedom from hydrocarbons. Although stringent precautions are taken in the production of ion pump modules, sufficient contaminants exist to affect the pumped vacuum system. Reduction of contaminants can be affected, however, by conduction heating bakeout, ion bombardment and electron bombardment. Luckily all these three processes are at work in an ion pump operating at 10e4 torr and use is made of this during the final manufacturing stages of ion pumps. The term ‘processing’ is given to this high pressure, high temperature treatment during the manufacture of ion pumps. The effect of processing on a 2 1 s appendage ion pump is shownzg in Figure 4. It is seen here that after processing the partial pressure due to hydrocarbons is reduced by a factor of

Vacuum system constructional

materials

As well as the hydrocarbon contributions from the system pumps, discussed above, the materials used in constructing the vacuum system itself also contribute to the contamination. These include metals, ceramics, rubbers and plastics, and the choice of processing of these must be carefully considered when

Peak height ,I,

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II

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15

20

25

30

35

40

45

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60

65

70

75

80

100

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Figure 5. Comparative

residual

mass spectra

of a vacuum

system

with

Buna

N rubber

and Viton

gaskets. 493

U R Bance et a/: Hydrocarbon contamination in vacuum dependent instruments designing the system. Figure 5 shows how the system ultimate can be improved by reducing the contamination from the sealing rings used in the equipment. Although these seals form only a small part of the system, change from Buna-N to Viton enabled a reduction of ten times in total system pressure and, of coutse, an equivalent reduction in system contamination. Stainless steel is considered to be one of the best constructional materials for high vacuum systems and there are numerous techniques described in literature to reduce its total degassing rate. The use of materials such as zinc, soft solder and phosphor bronze should be avoided. Phosphor bronze can be porous and can absorb large quantities of oil during machining. ‘0’ ring retaining grooves must be designed to avoid trapped volumes and ‘0’ rings must be dry and if possible outgassed under vacuum before use. For lubrication purposes only Fomblin grease should be used and then only if strictly necessary. Atmospheric hydrocarbons can be prevented from entering the system during shut down by venting with a clean dry gas. Having discussed in general the sources of hydrocarbon contamination in vacuum dependent scientific instruments, it is instructive to consider in more detail one particular class of instrument which can be badly affected by contamination, viz. the electron optical instrument in which high voltage electron beams are used to examine a transmission specimen, i.e. the transmission electron microscope (TEM). Specimen contamination

in electron microscopes

During a workshop3held at Cornell University on 3-6 August 1976on ‘Analytical Electron Microscopes’it was agreedthat

there was a pressingneed for a thorough examination of the problem of specimencontamination in electron microscopes. Interaction betweenthe electron beamsand the residual contaminants produces a contamination layer of moleculesof unknowncompositionswhich hampersand even preventsquantitative analysis.Becauseof the complexity of the problem and becausethe specimenenvironment in no two instruments is identical, neither the exact nature of the contamination nor the detailed mechanismwhich causeit are completely understood. Common techniquesto reducespecimencontamination are useof liquid nitrogen cooledanti-contaminators,heatingof the specimenand reduction of the total hydrocarbon pressurefrom all possiblesources.The principle of hydrocarbon contamination in electron microscopesis thought to be due to the field polymerization of hydrocarbon vapours in the presenceof an electron cloud. Heavy moleculesare decomposedinto light and simple moleculesand again these moleculesare ionized. As morepositive ionsare formed more free electronsare produced. It has also been reported’ that when pumping gasessuch as acetylene, benzeneor toluene a carbonaceouslayer is formed on the insideof the anodecellsof a triode ion pump (Figure 6) and similar carbon depositshave beennoted on the anodeof an orbitron pump. This electroncloud cleansthe surfacesnearthe electronbeam by electron impact desorption.Thus, the centre of an irradiated area of a specimenis cleanedfirst and the hydrocarbonsfrom this are evaporatedand condenseon the periphery of this area. As the partial pressureof hydrocarbon increases,the contamination or build up of the carbon layer advances.X-rays from thecarbon build up alsoincreasethe processof desorption. As explained earlier, heavy hydrocarbon molecules are decomposedin the mild dischargeinto simplemoleculescalled monomersand it is thesewhich are joined together chemically to form the long chain-like macromolecules.Materials containing the macromoleculesmadeup of a long sequenceof repeated structural units are called polymers and the processby which they are produced is called polymerization. The conditions favourable to polymerization are a pressure below 3 x lob2 tar?‘, presenceof hydrocarbon vapours and electron beam or electron cloud. During irradiation by the electron beamthree processes, i.e. crosslinking, polymerization and chain scission,may all be taking place simultaneously. Free radicals which are produced at the solid surface without polymerization of the monomercover the surfaceof the specimen. Free radicalsmay composemore than 90% of the gasin the vicinity of the electron beam.The theory of contamination by polymerization, outlined above, can now be applied to test resultsobtained in equipmentusingan electron beam. (1) Contamination is proportional to the concentration of the partial pressureof the hydrocarbons in the system. The effect of different levels of hydrocarbon contamination on polymerization is shown in Figure 7. These results were taken by introducing contaminantto keepa constanthydrocarbon pressurein an experimentalelectron microscope.It isseenherethat reduction of contaminant pressureby about ten times(from 6.5 x 10m6torr to 5.5 x lo-’ torr) results in a fall of contamination rate by a factor of 10’. It is also shown here that at hydrocarbon pressurehigher than 10e6 torr the contamination rate increasesconsiderably.

Figure 6. Electrodes after pumpingacetylenefor 100h at 10m5torr.

494

(2) The useof an anti-contaminator (liquid Nz cooled probe)4 reduces the level of contamination due to the drop in

U R Bance

et a/: Hydrocarbon

contamination

in vacuum

dependent

instruments

IO'

IO2

Contamination Figure partial

7. Contamination pressure readings

partial pressure specimen.

rate at 80 kV and 6 wrn dia spot size. at specimen stage level using MSlbS control

of the

hydrocarbons

in the

vicinity

of the

(3)

Increasing the total pressure I in the vicinity of the specimen by the introduction of nitrogen reduces the rate of contamination build up since the y0 level of the hydrocarbon present is correspondingly reduced.

(4)

Heating33 the specimen itself to about 250°C reduces the rate of contamination. Again a net reduction in hydrocarbon level is obtained by evaporation of the hydrocarbons from the specimen region and subsequent removal by the pumps. The free radicals on the hot surface are converted back to simple hydrocarbon molecules and pumped away without condensing.

(5)

Flooding as wide an area of the specimen by a beam of electrons for about 5 min before beginning small probe work reduced the rate of contamination build UP“*~~. In this case the specimen and its holder are cleaned with electron impact desorption, but the beam is not intense enough to polymerize the contaminant on the surface.

(6)

Small probes produce intense polymerization because the increased intensity increases the rate of polymerization to a greater extent than that of desorption. Small probes should only be used under uhv conditions.

(7)

The use of Fomblin oil in rotary pumps and diffusion pumps reduces contamination’g-26 since the vapours migrating into the system are fragmented in an electron beam (but not polymerized) and pumped away.

All these results show that a major reduction in contamination in electron optical devices can be achieved if the level of hydrocarbons from all possible sources can be reduced.

Conclusion In this paper the evidence has been presented, both from previous references and from our own work, that a major improvement in hydrocarbon contamination in vacuum based scientific instruments can be achieved by producing ‘clean’ vacua.

rate

Diffusion

A/s pump

EO2

with

Fomblin

oil

(type

AP

300).

Hydrocarbon

The biggest source of hydrocarbons in a well designed vacuum system is that due to backstreaming from diffusion pumps, particularly during starting and stopping periods or as a result of sudden pressure increases in the backing lines, rise in cooling water temperatures and to inefficient trapping or removal of cracked products in the backing lines. Efficient liquid Nz cold traps are essential when using diffusion pumps for uhv applications. The use of backing line sorption traps is highly recommended as is the use of Fomblin oils for both rotary and diffusion pumps. Other vacuum components, however, must be carefully chosen and operated, for instance ion pumps must be cleared of hydrocarbons by approved techniques, electron optical devices must be fitted with anti-contaminant devices and arrangements made for electron beam cleaning. The ultimate aim for future vacuum dependant scientific apparatus must be for really clean hydrocarbon free vacuum systems.

References ’ E K Brandis, F W Anderson and R Hoover, Scotmittg Electron p. 505 (1971). *T Mulvey, Electron Microscopy present and future. Laborarory Equipment Digest, p. 53, Nov. (1977). ’ M S lsaacson and J Silcox, Report of a workshop ‘On Analytical Electron Microscopy’, held at Cornell University, New York, Aug 3-6 (I 976). * G Lehmpfuhl and P J Smith, Reduction of Specimen Contamination in Electron Beam instruments. 34th Annual EMSA Meeting, p. 576 (1976). 5 A E Ennos, Br J appl Phys, 4, 1953, 101. 6 K H Muller, Proc. 7th Internal Congr Electron Microscopy, Genoble I, p. 183 (1970) also K H Muller. Oprik, 33 1971, 296 and 331. ’ (a) A J Tousimis, Biomed Sci Ittstrrrttt, 1, 1963, 249. (b) A J Tousimis, X-ray and Electron Prohe Analysis in Biomedical Reseurch. Plenum Press, New York, p. 87 (1967). (c) S H Moll and G W Bruno, 2nd Nat Conf on Electron Probe Analysis, p. 57, Boston, Mass (1967). B W R Bottoms, Contamination in the SEM, p. 182. Proceedings of Scanning Electron Microscopy, Chicago Illinois, April 23-27 (1973).

Microscopy.

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U R Bance et a/: Hydrocarbon contamination

in vacuum dependent instruments

g R L Stewart, P/rys Reu, 45, 1934,488. I0 P H Carr, Phys Rev, 33, 1929, 1068. Rev scienr Instrum, 1,1930,7 I I. i ’ J J Hillier, J uppl Phys. 19, 1948, 226. I2 C S Zhdonov and V W Vertsner, Sovief Phys Dokl, 8, 1966, 000. i3 P A Redhead and E V Kornelson, The Physical Basis of Ultra High Vacuum. Chapman & Hall, p. 134 (1968). iaM Baker and L -Laurenson, V&urn, 16, 1966, 633. Is G Rattinehaus and W K Huber. Vucuum. 24. 1974. 249. isa J Hengeioss and W K Huber, i’acuum, i3, i963, i. I6 M H Hablanian and A A Landfors, J Vat Sci Technol, 13, 1976, 494. I7 M. H. Hablanian, Vacuum Congress, Kyto, Japan (1974). Is H. G. Noeller, G Reich and W Baechler, Trans of AVS, Pergamon Press (1958). i9 A Luches and M R Perrone, J Vat Sci Technol, 13, 1976, 1097. 20 M. A. Baker. Vacuum, 21, 1971, 479. 2* L Holland, Nafrcre, Land, 238, 1972, 36. z2 L Holland and L Laurenson. Vucuum. 23. 1973. 139. 23 L Holland, Vacaam, 22, 1972, 315. ’ ’ 24 H W Conru and P C Laberge, J Phys E, Scienf Ins/ram, 8, 1975, 138.

496

25 B K Ambrose, L Holland and L Laurenson. J Mcrosc, 96, 1972, 389. 26 L Holland, L Laurenson, R E Hurley and K Williams, Nuclear fnsrrumenrs or Methods. II, North Holland, Amsterdam, p 555 (I 973). 27 L de Chernatony, Perturbational Limitations to the Attainment of UHV. 4th International Symposium on the Residual Gases and Electron Tubes, arranged by the Italian Association of Physical Chemistry. Florence. Italy. 14-16 April (1971). 28 S Stantier, J Vuc &i T&hnof, 8, 1971; 239.. XJ U R Bance, Residual Gases in Systems Pumped by Appendage Pumps. Paper presented at the f day meeting arranged by the Vacuum Group of the Institute of Physics, Imperial College, London, 23 Nov (I 977). JO U R Bance and R D Craig, Vucrtum, 18, 1968, 39 I. 31 U R Bance and R D Craig, Vacwm, 16, 1966, 647, a’ I K Openshaw AEI Departmental Engineering Report DER/504/ 99. 33 H B Heide, Z angew Phys, 15, 1963, I 16. 34 R D Craig and E H Harden, Vacuum, 16, 1966, 67. 35 D Allenden and E H Harden, Vide, 123, 1966, 198. 56 G M Rackham and J A Eades, Optik, 47, 1977. 227.