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Conversion of vacuum coating units for sputter coating
Micron, 1976, Vol. 7: 141-144. Pergamon Press. Printed in Great Britain.
Conversion of vacuum coating units for sputter coating
T. D. ALLEN E.M. Unit, Paterson Laboratories, Christie Hospital and Holt Radium Institute, Manchester M20 9BX and S. C. SIMMENS Shirley Institute, Didsbury, Manchester M20 8RX, England, U.K. Manuscript received December 1, 1975; revised January 19, 1976 Sputter coating for the preparation of biological and textile materials for scanning electron microscopy appears to be superior to conventional thermal evaporation in both ease of operation and results. The commercial units for sputter coating are pumped by rotary pumps alone and are thus often inferior in the cleanliness of vacuum that can be achieved compared to the vacuum coating equipment which is standard in most electron microscopy laboratories. The present paper shows how standard type vacuum coating units can be concertedfor sputter coating (without detriment to other functions) at a cost which is considerably less than that of commercial sputter units.
INTRODUCTION The process of deposition of metallic thin films by sputtering is by no means new, having been formulated first in 1852 and used commercially as early as 1928 for the manufacture of phonograph records. T h e basic process of sputtering is the ejection of material from a source (the target cathode) by b o m b a r d m e n t of the cathode surface with gas ions accelerated by a high voltage. Particles of atomic dimensions are ejected from the cathode surface as a result of m o m e n t u m transfer between incident ions and the target. These particles traverse the vacuum chamber and are deposited on the substrate (specimen) as a thin film. During the actual process of sputtering a characteristic bluish glow occurs in the argon. T h e discharge is sustained by the ion bombardment of the cathode which gives rise to electrons which are ejected into the argon. Under the influence of the applied voltage the electrons accelerate towards the positive anode gaining energy from the electric field. As each electron travels toward the positive electrode it m a y collide with a gas molecule giving up part of its energy, leaving behind an ion and an extra free electron. T h e cumulative effect of this phenomenon is a self-sustaining glow discharge (this, in fact, is identical to the process which occurs in a fluorescent tube where mercury vapour is ionised). T h e area close to the
target (usually 0.5-1.0cm) does not glow, and is referred to as the 'dark space'. Whilst several commercial sputter coating units are available, purchase of such a unit involves duplication of a large amount of vacuum equipment which is pre-existing in most electron microscopy laboratories in the form of the standard vacuum coating unit. Also, the purchase of a commercial sputtering unit m a y often necessitate the further purchase of a rotary p u m p as this is not necessarily included with m a n y of the commercial units. T h e vacuum requirements for sputtering are easily met in terms of pressure alone by a rotary pumped system. However, unless the vacuum is free of hydrocarbons, reactions can occur during the discharge with any hydrocarbons that are present causing problems with the coating (Simmens, 1975). Thus although only a low vacuum is required, it must be free of hydrocarbons, one source of which is oil vapour backstreaming from a rotary pump. Consequently a conventional coating unit with diffusion p u m p (and often cold trap) can provide a much cleaner vacuum environment for sputtering than the commercial units. Hence a relatively small amount of money spent converting a pre-existing coating unit may well produce better results than obtained with a commercial sputter unit at a considerably greater cost. The purpose of this paper is to show how 141
I42
I . 1). Altcll and S. (:. Simmcn~
standard type vacuum coating units can be converted for sputter coating at a comparatively low cost. As such a conversion makes it necessar~ to take into account some of the basic parameters of sputtering, these will be mentioned where relevant. Some of the advantages of sputter coating as opposed to normal thermal evaporation have already been reported (DeNee and Walker, 1975), namely better penetration into cryptic areas of the specimen and better uniformity of coating. Furthermore, some of the original criticisms have also been shown to be unfounded (e.g. specimen heating, decoration artefacts and surface etching (Echlin, 1975)). Both authors of the present paper have also found the process quick, easy to use, and entirely satisfactory over a wide range of biological, textile, and other specimens, some of which were considered impossible to coat by thermal evaporation. CONVERSION
Apparatus requiredfor conversion oJ coating units Two coating units are referred to, an AEI Metrovac Type 12 coating unit (AEI Scientific Apparatus Ltd., Barton Dock Rd,, Urmston, Manchester M31 2LD, England, U.K.), and the Edwards E12E4 coating unit (Edwards High Vacuum, Manor Royal, Crawley, W. Sussex RH10 2LW, England, U.K.). In the case of the AEI coating unit, a complete sputtering kit was purchased from AEI consisting of power pack, baseplate terminal for the H T supply, cathode stand, baseplate and cathode. This kit is no longer available. With the Edwards EI2E4 coating unit, however, the pre-existing H T supply (supplied as alternating current HT) needs only to be rectified for use as a sputtering power source. Thus the Edwards kit for conversion of the E 12E4 coating unit consists of a rectifying kit for the H T supply and a copper cathode which can be supported with the standard jig assembly in the bell jar.
Power supply A d.c. power pack to supply 1.5kV and current in the range of 75mA is necessary to provide the energy for ionisation. In the converted AEI coating unit, the values used are 40-50mA and 750-1000V, and 15-25mA and 750--1000V for the converted Edwards EI2E4 coating unit. In some other coating units besides the Edwards E12E4 the pre-existing
AC H'F supply can be rectified, e.g. NGN Type 12 SG, Millitor MT12. Ill the absence of such power supplies, an unregulated supply, ccmsisting of a step up transtbrmer, rectifier, capacitor and ballast resistor can be assembled at a reasonable cost in a competent electronics workshop. The power supply may be wired through the existing system (Regavolt) t,~ allow variation of the input energy. Current and voltage can be monitored by the existing metering (in the case of Edwards or NGN coating units) or the permanent installation of suitable meters. More simply still, two 'Aw)meters' can be used temporarily during the setting up procedure and the settings noted and used subsequently.
Target cathode and anode (specimen holder) The cathode is supplied as a standard accessory fi'om Edwards (a copper disc 10cm in dia and 0.7cm thick), or easily produced by a competent mechanical workshop. The cathode must be coated with the material to be sputtered (in this case, gold). Simple thermal evaporation of gold wire on to the copper cathode will provide sufficient gold for several sputter coatings. In place of wire, a gold plated cathode can be used. Commercially, the cost of electrolytically depositing a 10~tm layer of 22 carat gold over the entire cathode (by N. T. Frost Ltd., Great Hampton Street, Birimingham B18 6AX) is approximately £30. In this case, the total cathode life gives 7hr sputtering before the copper surface becomes apparent. This, in fact, is between 100-180 separate coating--each of which can accommodate up to 25 stubs, at a cost for gold of less than 2p per stub. The cathode itself must be mounted in the vacuum chamber. In the case of the Edwards coating unit the necessary parts are supplied with the cathode as part of the sputtering conversion kit. In the AEI sputtering conversion kit (no longer available) the cathode is supported on a simple tripod assembly (see diagram) which would serve as a suitable model for construction. In both cases, the cathode is insulated from the remainder of the bell jar with an earthed backing plate sufficiently close to limit ionisation to the surface of the cathode directly opposite the specimen holder(anode). This arrangement and the necessary insulation is shown schematically in Fig. 1. The anode is also a 10cm copper disc, approximately lcm thick. It is not connected
Conversion of Vacuum Coating Units for Sputter Coating other than by contact to earth via the earthed base plate. Thus the anode may be simply removed, cooled if necessary (cold tap water or brief immersion in liquid nitrogen) loaded with stubs and placed in position. Up to 25 specimen stubs may be coated at the same time. The rise in temperature that occurs during the sputter coating itself may be monitored simply by a thermocouple mounted on a specimen stub with adhesive tape, leaving the actual junction exposed (Fig. 1).
Argon A cylinder of 'Puragon' (Air Products Ltd., St. Georges Square, New Malden, Surrey K T 3 4HH) is used. The pressure is regulated to approximately 51b/sq.in, and the gas bled into the bell jar via a needle valve. The argon must be bled in at a rate appropriate for ionisation to occur, which can be found experimentally in the first instance, for the discharge will not occur at pressures that are either too high or too low. The pumping speed can be varied
143
to achieve the necessary vacuum by manipulation of the baffle valve (Edwards coating unit) or the needle valve on the argon inlet (AEI coating unit). In the latter case, pumping is through a cold-trapped back diffusion .pump with the baffle valve fully open. Whilst the presence of a cold trap between the chamber and diffusion pump is the best possible guarantee of the absence ofoil molecules in the working chamber, an untrapped diffusion pump is still probably a better safeguard against rotary oil backstreaming than a vacuum system with rotary pump alone. PROCEDURE 1.
2.
Specimens are inserted into the working chamber which is then pumped out to a relatively high vacuum (10- 5 Torr) to reduce the possibility of reactive contaminants (Simmens, 1975). The Penning gauge is switched off and the low vacuum gauge (Pirani or thermo-
r ¸
Fig. 1. Schematic drawing of the AEI sputter coating jig, consisting of a tripod supporting an upper insulated lead through to the cathode (upper insert) with a specimen table beneath (variable height possible by collars on legs) on which is placed a copper disc (anode) to accommodate the specimens. Specimen temperature monitoring is achieved by a thermocouple head sellotaped in position to a specimen stub (lower insert).
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T.D. Allen and S. C. Simmens
couple) switched on with the baffle valve open and rotary pump set to backing. 3. Argaon is bled in first to completely flush the vacuum system and then to achieve the predetermined vacuum (between 0.3-0.1 Torr). 4. The H T is switched on and the glow discharge occurs. Coating then proceeds for the required length of time (2-3min). The parameters of each coating cycle are logged on a sheet (Table I) allowing the total input energy and other parameters to be recorded. Typical readings on a coating run on the converted A E I unit are given below. Table 1. Typical parameters of sputter coating in the converted AEI Coating Unit showing variation of temperature with time and changes of voltage and current with vacuum. Time Temp Vacuum Voltage Current (min) (0°C) (Torr) (V) (mA) 0 15 0.115 1200 33 1 19 0.120 1100 36 2 23 0.125 1000 40 3 24 0.130 900 45 4 25 0.135 800 50
Input (W) 39"6 39.6 40.0 40.5 40.0
The temperature column shows how the temperalure increases over a 4min coating run. Variations in the vacuum may occur during coating but these are usually small. The vacuum readings quoted here show the overall variation which may occur over a series of coatings and are quoted to illustrate that a wide variation in vacuum readings may be tolerated without causing a significant change in the cathode wattage. Temperature The largest rise in temperature occurs over the first 2-3min (which may be the total running time) and may be of the order of 6-10°C. From an ambient of 22°C, however, this rise is not considered likely to cause difficulties. If the anode is cooled this rise in temperature can be further limited and routinely we briefly immerse the anode in liquid nitrogen to produce a starting temperature of 12°-15°C. Too severe cooling of the anode produces problems due to condensation. Vacuum and cathode wattage The control of pressure whilst bleeding the argon is not always exactly constant, and small variations produce changes in cathode voltage and amperage. The applied wattage to the
cathode, however, stays fairly constant (Table 1!. Recording of these parameters during the coating process allows the use of a formula quoted by Echlin (1975) to determine the thickness of the coating tbr a distance of 2.5cm between specimen surface and cathode. The formula is : 7 = W :< t . K/10 where T is the thickness of the coating in nanometres; W the average wattage applied to the cathode; t, time of discharge (min); K a constant dependent on the metal and gas (tbr gold/argon :-~ 5). The above equation was derived using a quartz crystal monitor to determine the rate of metal deposition. With an input of 40W the coating rate is 20nm/min. This is somewhat at variance with physical measurements made by by a stylus (Rank Talystep 1) from coatings made with the AEI unit where the coating rate at 40W input is 6nm/min. This discrepancy may arise possibly from the way in which input energy to the cathode is measured, but may also serve to point out the difficulty in attaining accurate absolute thickness measurements of thin films. It seems likely however that the rate of deposition is in the region of 10-20nm/min. Each operator can quite easily produce the required thickness of coating by trial and error over different time periods and subsequent observation in the scanning microscope. Acknowledgements--The work of one of us (T.D.A.) was supported by the Medical Research Council and the Cancer Research Campaign. The authors are also grateful to Mr. G. R. Bennion for technical assistance and Mr. H. D. Clark of Plessey Semi Conductors who measured the thickness of the coatings. REFERENCES DeNee, P. B. and Walker, E. R., 1975. Spccimcn coating technique for the SEM. A comparative study. In: Scanning Electron Microscopy 1975, Proc. 8th Ann. Symp. SEM, Johari, O. (ed.), IIT Research Institute, Chicago, Illinois, U.S.A., 225-232. Echlin, P., 1975. Sputter coating techniques tbr scanning electron microscopy. In: Scanning Electron Microscopy 1975. Proc. 8th Ann. Syrup. SEM, Johari, O. (ed.), IIT Research Institute, Chicago, Illinois, U.S.A., 233-239. Simmens, S. C., 1975. An observation on the metallizing of specimens for scanning electron microscopy using cathode sputtering. J. Micrscopy, 105~ 233-234.
Conversion of vacuum coating units for sputter coating
T. D. ALLEN E.M. Unit, Paterson Laboratories, Christie Hospital and Holt Radium Institute, Manchester M20 9BX and S. C. SIMMENS Shirley Institute, Didsbury, Manchester M20 8RX, England, U.K. Manuscript received December 1, 1975; revised January 19, 1976 Sputter coating for the preparation of biological and textile materials for scanning electron microscopy appears to be superior to conventional thermal evaporation in both ease of operation and results. The commercial units for sputter coating are pumped by rotary pumps alone and are thus often inferior in the cleanliness of vacuum that can be achieved compared to the vacuum coating equipment which is standard in most electron microscopy laboratories. The present paper shows how standard type vacuum coating units can be concertedfor sputter coating (without detriment to other functions) at a cost which is considerably less than that of commercial sputter units.
INTRODUCTION The process of deposition of metallic thin films by sputtering is by no means new, having been formulated first in 1852 and used commercially as early as 1928 for the manufacture of phonograph records. T h e basic process of sputtering is the ejection of material from a source (the target cathode) by b o m b a r d m e n t of the cathode surface with gas ions accelerated by a high voltage. Particles of atomic dimensions are ejected from the cathode surface as a result of m o m e n t u m transfer between incident ions and the target. These particles traverse the vacuum chamber and are deposited on the substrate (specimen) as a thin film. During the actual process of sputtering a characteristic bluish glow occurs in the argon. T h e discharge is sustained by the ion bombardment of the cathode which gives rise to electrons which are ejected into the argon. Under the influence of the applied voltage the electrons accelerate towards the positive anode gaining energy from the electric field. As each electron travels toward the positive electrode it m a y collide with a gas molecule giving up part of its energy, leaving behind an ion and an extra free electron. T h e cumulative effect of this phenomenon is a self-sustaining glow discharge (this, in fact, is identical to the process which occurs in a fluorescent tube where mercury vapour is ionised). T h e area close to the
target (usually 0.5-1.0cm) does not glow, and is referred to as the 'dark space'. Whilst several commercial sputter coating units are available, purchase of such a unit involves duplication of a large amount of vacuum equipment which is pre-existing in most electron microscopy laboratories in the form of the standard vacuum coating unit. Also, the purchase of a commercial sputtering unit m a y often necessitate the further purchase of a rotary p u m p as this is not necessarily included with m a n y of the commercial units. T h e vacuum requirements for sputtering are easily met in terms of pressure alone by a rotary pumped system. However, unless the vacuum is free of hydrocarbons, reactions can occur during the discharge with any hydrocarbons that are present causing problems with the coating (Simmens, 1975). Thus although only a low vacuum is required, it must be free of hydrocarbons, one source of which is oil vapour backstreaming from a rotary pump. Consequently a conventional coating unit with diffusion p u m p (and often cold trap) can provide a much cleaner vacuum environment for sputtering than the commercial units. Hence a relatively small amount of money spent converting a pre-existing coating unit may well produce better results than obtained with a commercial sputter unit at a considerably greater cost. The purpose of this paper is to show how 141
I42
I . 1). Altcll and S. (:. Simmcn~
standard type vacuum coating units can be converted for sputter coating at a comparatively low cost. As such a conversion makes it necessar~ to take into account some of the basic parameters of sputtering, these will be mentioned where relevant. Some of the advantages of sputter coating as opposed to normal thermal evaporation have already been reported (DeNee and Walker, 1975), namely better penetration into cryptic areas of the specimen and better uniformity of coating. Furthermore, some of the original criticisms have also been shown to be unfounded (e.g. specimen heating, decoration artefacts and surface etching (Echlin, 1975)). Both authors of the present paper have also found the process quick, easy to use, and entirely satisfactory over a wide range of biological, textile, and other specimens, some of which were considered impossible to coat by thermal evaporation. CONVERSION
Apparatus requiredfor conversion oJ coating units Two coating units are referred to, an AEI Metrovac Type 12 coating unit (AEI Scientific Apparatus Ltd., Barton Dock Rd,, Urmston, Manchester M31 2LD, England, U.K.), and the Edwards E12E4 coating unit (Edwards High Vacuum, Manor Royal, Crawley, W. Sussex RH10 2LW, England, U.K.). In the case of the AEI coating unit, a complete sputtering kit was purchased from AEI consisting of power pack, baseplate terminal for the H T supply, cathode stand, baseplate and cathode. This kit is no longer available. With the Edwards EI2E4 coating unit, however, the pre-existing H T supply (supplied as alternating current HT) needs only to be rectified for use as a sputtering power source. Thus the Edwards kit for conversion of the E 12E4 coating unit consists of a rectifying kit for the H T supply and a copper cathode which can be supported with the standard jig assembly in the bell jar.
Power supply A d.c. power pack to supply 1.5kV and current in the range of 75mA is necessary to provide the energy for ionisation. In the converted AEI coating unit, the values used are 40-50mA and 750-1000V, and 15-25mA and 750--1000V for the converted Edwards EI2E4 coating unit. In some other coating units besides the Edwards E12E4 the pre-existing
AC H'F supply can be rectified, e.g. NGN Type 12 SG, Millitor MT12. Ill the absence of such power supplies, an unregulated supply, ccmsisting of a step up transtbrmer, rectifier, capacitor and ballast resistor can be assembled at a reasonable cost in a competent electronics workshop. The power supply may be wired through the existing system (Regavolt) t,~ allow variation of the input energy. Current and voltage can be monitored by the existing metering (in the case of Edwards or NGN coating units) or the permanent installation of suitable meters. More simply still, two 'Aw)meters' can be used temporarily during the setting up procedure and the settings noted and used subsequently.
Target cathode and anode (specimen holder) The cathode is supplied as a standard accessory fi'om Edwards (a copper disc 10cm in dia and 0.7cm thick), or easily produced by a competent mechanical workshop. The cathode must be coated with the material to be sputtered (in this case, gold). Simple thermal evaporation of gold wire on to the copper cathode will provide sufficient gold for several sputter coatings. In place of wire, a gold plated cathode can be used. Commercially, the cost of electrolytically depositing a 10~tm layer of 22 carat gold over the entire cathode (by N. T. Frost Ltd., Great Hampton Street, Birimingham B18 6AX) is approximately £30. In this case, the total cathode life gives 7hr sputtering before the copper surface becomes apparent. This, in fact, is between 100-180 separate coating--each of which can accommodate up to 25 stubs, at a cost for gold of less than 2p per stub. The cathode itself must be mounted in the vacuum chamber. In the case of the Edwards coating unit the necessary parts are supplied with the cathode as part of the sputtering conversion kit. In the AEI sputtering conversion kit (no longer available) the cathode is supported on a simple tripod assembly (see diagram) which would serve as a suitable model for construction. In both cases, the cathode is insulated from the remainder of the bell jar with an earthed backing plate sufficiently close to limit ionisation to the surface of the cathode directly opposite the specimen holder(anode). This arrangement and the necessary insulation is shown schematically in Fig. 1. The anode is also a 10cm copper disc, approximately lcm thick. It is not connected
Conversion of Vacuum Coating Units for Sputter Coating other than by contact to earth via the earthed base plate. Thus the anode may be simply removed, cooled if necessary (cold tap water or brief immersion in liquid nitrogen) loaded with stubs and placed in position. Up to 25 specimen stubs may be coated at the same time. The rise in temperature that occurs during the sputter coating itself may be monitored simply by a thermocouple mounted on a specimen stub with adhesive tape, leaving the actual junction exposed (Fig. 1).
Argon A cylinder of 'Puragon' (Air Products Ltd., St. Georges Square, New Malden, Surrey K T 3 4HH) is used. The pressure is regulated to approximately 51b/sq.in, and the gas bled into the bell jar via a needle valve. The argon must be bled in at a rate appropriate for ionisation to occur, which can be found experimentally in the first instance, for the discharge will not occur at pressures that are either too high or too low. The pumping speed can be varied
143
to achieve the necessary vacuum by manipulation of the baffle valve (Edwards coating unit) or the needle valve on the argon inlet (AEI coating unit). In the latter case, pumping is through a cold-trapped back diffusion .pump with the baffle valve fully open. Whilst the presence of a cold trap between the chamber and diffusion pump is the best possible guarantee of the absence ofoil molecules in the working chamber, an untrapped diffusion pump is still probably a better safeguard against rotary oil backstreaming than a vacuum system with rotary pump alone. PROCEDURE 1.
2.
Specimens are inserted into the working chamber which is then pumped out to a relatively high vacuum (10- 5 Torr) to reduce the possibility of reactive contaminants (Simmens, 1975). The Penning gauge is switched off and the low vacuum gauge (Pirani or thermo-
r ¸
Fig. 1. Schematic drawing of the AEI sputter coating jig, consisting of a tripod supporting an upper insulated lead through to the cathode (upper insert) with a specimen table beneath (variable height possible by collars on legs) on which is placed a copper disc (anode) to accommodate the specimens. Specimen temperature monitoring is achieved by a thermocouple head sellotaped in position to a specimen stub (lower insert).
144
T.D. Allen and S. C. Simmens
couple) switched on with the baffle valve open and rotary pump set to backing. 3. Argaon is bled in first to completely flush the vacuum system and then to achieve the predetermined vacuum (between 0.3-0.1 Torr). 4. The H T is switched on and the glow discharge occurs. Coating then proceeds for the required length of time (2-3min). The parameters of each coating cycle are logged on a sheet (Table I) allowing the total input energy and other parameters to be recorded. Typical readings on a coating run on the converted A E I unit are given below. Table 1. Typical parameters of sputter coating in the converted AEI Coating Unit showing variation of temperature with time and changes of voltage and current with vacuum. Time Temp Vacuum Voltage Current (min) (0°C) (Torr) (V) (mA) 0 15 0.115 1200 33 1 19 0.120 1100 36 2 23 0.125 1000 40 3 24 0.130 900 45 4 25 0.135 800 50
Input (W) 39"6 39.6 40.0 40.5 40.0
The temperature column shows how the temperalure increases over a 4min coating run. Variations in the vacuum may occur during coating but these are usually small. The vacuum readings quoted here show the overall variation which may occur over a series of coatings and are quoted to illustrate that a wide variation in vacuum readings may be tolerated without causing a significant change in the cathode wattage. Temperature The largest rise in temperature occurs over the first 2-3min (which may be the total running time) and may be of the order of 6-10°C. From an ambient of 22°C, however, this rise is not considered likely to cause difficulties. If the anode is cooled this rise in temperature can be further limited and routinely we briefly immerse the anode in liquid nitrogen to produce a starting temperature of 12°-15°C. Too severe cooling of the anode produces problems due to condensation. Vacuum and cathode wattage The control of pressure whilst bleeding the argon is not always exactly constant, and small variations produce changes in cathode voltage and amperage. The applied wattage to the
cathode, however, stays fairly constant (Table 1!. Recording of these parameters during the coating process allows the use of a formula quoted by Echlin (1975) to determine the thickness of the coating tbr a distance of 2.5cm between specimen surface and cathode. The formula is : 7 = W :< t . K/10 where T is the thickness of the coating in nanometres; W the average wattage applied to the cathode; t, time of discharge (min); K a constant dependent on the metal and gas (tbr gold/argon :-~ 5). The above equation was derived using a quartz crystal monitor to determine the rate of metal deposition. With an input of 40W the coating rate is 20nm/min. This is somewhat at variance with physical measurements made by by a stylus (Rank Talystep 1) from coatings made with the AEI unit where the coating rate at 40W input is 6nm/min. This discrepancy may arise possibly from the way in which input energy to the cathode is measured, but may also serve to point out the difficulty in attaining accurate absolute thickness measurements of thin films. It seems likely however that the rate of deposition is in the region of 10-20nm/min. Each operator can quite easily produce the required thickness of coating by trial and error over different time periods and subsequent observation in the scanning microscope. Acknowledgements--The work of one of us (T.D.A.) was supported by the Medical Research Council and the Cancer Research Campaign. The authors are also grateful to Mr. G. R. Bennion for technical assistance and Mr. H. D. Clark of Plessey Semi Conductors who measured the thickness of the coatings. REFERENCES DeNee, P. B. and Walker, E. R., 1975. Spccimcn coating technique for the SEM. A comparative study. In: Scanning Electron Microscopy 1975, Proc. 8th Ann. Symp. SEM, Johari, O. (ed.), IIT Research Institute, Chicago, Illinois, U.S.A., 225-232. Echlin, P., 1975. Sputter coating techniques tbr scanning electron microscopy. In: Scanning Electron Microscopy 1975. Proc. 8th Ann. Syrup. SEM, Johari, O. (ed.), IIT Research Institute, Chicago, Illinois, U.S.A., 233-239. Simmens, S. C., 1975. An observation on the metallizing of specimens for scanning electron microscopy using cathode sputtering. J. Micrscopy, 105~ 233-234.