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Preparation and evaluation of epitaxial silicon films prepared by vacuum evaporation
Preparation and Evaluation of Epitaxial Silicon Films prepared by Vacuum Evaporation A. P. HALE Fairchild Semiconductor, a division of Fairchild Camera & Instrument Corporation, Palo Alto, California (Received25 October1962 ; accepted17 January 1963)
Single crystal silicon films of both types have been deposited in a high (10-s torr) vacuum Electron beam heating, as well as resistive heating, were used to heat the sources. system. Low energy argon ion bombardment and thermal etching were used to clean the surfaces of the Single The investigated temperature range of the substrates was IOOO-1300°C. substrates. crystal films by back reflection X-ray were obtained when the substrates were above 1125°C Fast deposition rates (t_lp/min) (14OO”K), which is about 0.8 of the melting point (1700°K). Diodes and transistors were prepared from some of these films to obtain gave the best films. The p-n junctions showed higher an indication of the p-n junction and interface perfection. reverse bias leakage and thus less perfection than that observed with devices in crystals grown by the Czochralski method. I. Introduction
Vacuum evaporation has been used for many years as a method of depositing a film of metal on the same or another metal.1 Vacuum evaporation has also been used in studies Seifert4 refers to Royer’s use of of epitaxial growth.z-4 epitaxy as an “ arrangement on ” and Collins5 refers to on “. Thus, Royer’s use as an “ orderly arrangement epitaxy refers to the laying down of a material on a substrate so that the orderly arrangement, of the substrate is continued into the deposited material. Consequently, the laying down of a material on a substrate of the same material is simply a special case of the general epitaxy. Epitaxy by vacuum evaporation has been reported for Ge on Ge+lo. Kurov, et al.7 report single crystal films when the substrate is over 600°C that are p-type regardless of type evaporated. Daveyg reported epitaxy on substrates heated to only 300°C after a cleaning at 575°C. Weinreich, et al. reported n-type films and p-n junctions by evaporating heavily doped n-type Ge. Silicon films on various substrates have been reportedll-14. Nielsen et al.13 report good epitaxy when pressure is down Handelman and Povilonisl4 to 10-7 torr during evaporation. reported epitaxy and p-n junction formation by sublimation at 1 x 10-S torr. II. Experimental
Sect.“AA”
Wafer
(far
holder gQg
evaporation
SeCtian”AA”
source
double evaporation)
FIG. 1. Schematic
diagram
of the resistive heating
apparatus.
and stainless steel in cooler areas. The schematic diagram for resistive heating is shown in Fig. 1. Crucibles of Al2O3, ThO2 and SiO2 were used. The crucibles were wrapped in 1 mil tantalum foil which was resistively heated. The outside shield was used to conserve power. The wafers were held 2 cm above the source by tungsten clips. The wafers were about 2 mm below the molybdenum wire (l/16 in. dia.) heater. The shield around the wafers and heater reduces the temperature gradient and conserves power. The temperature gradient was about 20°C at 1150°C when measured with an optical pyrometer. Optical pyrometer readings Corrected temperatures given here may be were corrected. in error by f25 “C. Another method of heating the source was employed for
techniques
The vacuum evaporator was a commercially built unit providing an ultimate pressure of 5 x 10-7 torr. The 4 in. diffusion pump was used with a water cooled baffle and a cold finger liquid nitrogen trap. The seals were neoprene and the bell was 18 x 18 in. The jigging in the bell was made of molybdenum, tantalum, and tungsten for the hottest areas and from nickel, copper 93
A. P. HALE
94
Electron part of the experiments to reduce the contamination. beam heating arrangementis* is shown in Figs. 2 and 3. The silicon wafers in the same heater and shield arrangement were 5 cm above the molten silicon. The tungsten wire was held near -3000 V and the silicon was at ground potential. The box around the filament was at the same potential as the filament. The silicon wafers were cut and lapped from single crystals grown by the Czochralski method. The (111) surface was obtained. The wafers were ultrasonically cleaned and washed in D.I. water then etched in CP-8 and washed in D.I. water and finally ultrasonically cleaned. The final ultrasonic cleaning proceeds through D.I. water, acetone, , I
Molten sflicon I
I
i
I
1
Tungsten
1
wire
Cooling water -
FIG. 2. Schematic
diagram
of the electron
beam heating
unit.
trichloroethane, acetone, D.I. water and methanol. Methanol is a good final solvent, since methanol, as well as ethanol and isopropyl alcohol remove the last traces of water and acetone and evaporate without water condensation. The wafers were removed by pouring off most of the solvent to leave a small portion of the wafer above the surface and then carefully and slowly removing the wafers with clean smooth tweezers so that the solvent stays in the container and is not attracted up the tweezers by capillary action. After the bell was pumped to about 5 x lo-6 torr a short degassing was started. At this time the wafers were in the other side of the bell. The degassing was continued for 3&60 min. This degassed the heater and source quite well and reduced the degassing of the other internal surfaces during the subsequent evaporation. The bell was allowed to cool and pump down for about 2&30 min. The argon ion bombardment was used to remove the contamination left by the solvents and picked up in transfer to the bell. The argon was purified by a sequence of Dry Ice, tantalum powder at 1100°C and Dry Ice traps. The argon was bled through a needle valve and Pyrex tube to the top of the bell. The argon pressure was maintained at about 10~~. The wafers were maintained at a negative potential with respect to the grounded collector. The potentials were -400 and - 1000 V. The same control settings corresponded to -800 and -2000 V when the pressure in the bell was 10-s torr, or less. The current was about 1 mA for the -1000 V discharge. The -1000 V discharge was barely visible while the -400 V discharge was not visible. The time of discharge was varied, as was the annealing time following the discharge. After the wafers and the source were at the desired temperatures, or power inputs, the evaporations were started by removing the shutter. The deposition was stopped by replacing the shutter. The cooling rates were also varied. III.
Results
Preparation
FIG. 3. Jigging arrangement in the bell. The silicon wafers are mounted on one of the rotatable rods so that they are on the right during degassing and ion bombardment, and on the left in the substrate heater box during evaporation.
and discussion variables
Many variations of the cleaning procedure were tried by omitting some of the solvents and by changing the ultrasonic times. The sequence of solvents without omissions, as described above, is a natural one, since any solvent used is miscible in the next. The ultrasonic times were best kept short (30 set) to avoid contamination of the solvents by settling dust from the air. The use of a final rinse with D.I. water was not satisfactory due to the particles in the water and the necessity of blotting the water drops. Omission of the blotting step with a final D.I. water rinse left visible imperfections in the films in the spot where the water drops formed. The surface of polished wafers was not wet by the D.I. water so that the polished wafers could usually be pulled from the final D.I. water rinse with no visible water adhering to the polished surface. However, film imperfections were least in number and size when the final rinse was alcohol. According to the manufacturer, the alcohol left about 0.0002 per cent residue on evaporation. Ion bombardment with low energy argon was used to Here the desired end remove last traces of contamination.
Preparation and Evaluation of Epitaxial Silicon Films prepared by Vacuum Evaporation was to remove the remaining contaminates with a minimal damage to the surface. The ion bombardment was done in one side of the bell and the discharge localized to the immediate volume around the wafers. The supports for the The argon flowed past wafers were short legs of tungsten. the wafers minimizing recontamination. The damage to the surface by the ion bombardment was evidenced by the increase in resistivity of p-type surfaces when using the 4-point probe method. Extended (3 h) bombardment with either 400 and 1000 V gave n-type surfaces by the thermal Heavily doped probe method on 1 lncm p-type wafers. p-type wafers did not develop n-type surfaces. The n-type surface annealed out in less than ten minutes at 1150°C and the resistivity by the 4-point probe was the same as The n-type wafers showed no change before the treatment. after bombardment in type or in resistivity. The bombardment caused no visible surface damage with magnifications
FIG. 4. Thermal
etching
at 1125°C. Etching Magnification 500 x .
FIG. 5. Thermal
etching
near 1200°C. Etching Magnification 500 x .
time
time
was
30 min.
was
30 miny
95
up to x 1250. Thus, a ten minute anneal appeared to be a practical compromise between annealing time and thermal etching. The annealing at 1125 “C-1300°C caused some thermal etching.17 At 1125°C (in Fig. 4) the thermal etching leaves an oriented pattern that was partly colored, indicating the presence of some silicon monoxide. Also, note the circular thermal etching at a solvent drop site in Fig. 4. Between 1200°C and 13OO”C, the thermal etching patterns were random with no colored areas, as shown in Fig. 5. Slow rates of deposition and low substrate temperature gave slightly brown film surfaces. Faster rates of +l,u/min and substrate temperatures over 1150 “C usually gave clear films. The use of Al203 and ThOz crucibles with faster rates gave brown films. The use of silica boats did not result in brown films. However, above the softening point of the silica, appreciable evaporation of the boat took place and the surface of the films, though still clear, had a very high resistance even after five minute H.F. (49 per cent) soak and D.I. water wash. The least contamination was obtained with the electron beam heating since the only material in contact with the molten silicon is water cooled. Insufficient water cooling resulted in erosion of the water cooled hearth. In practice a (1 cm dia. x 3 cm high) cylinder of single crystal silicon was melted. The melting had to be done carefully, since the molten silicon would settle to the contour of the hearth. The layer of silicon in contact with the hearth would immediately freeze and sometimes the solidification was half way up the silicon. No attack, or erosion of the hearth was noted when the copper hearth was made as shown in Fig. 2. Increasing the power input lowered the solid-liquid interface in the silicon to give an erratically vibrating sphere. The higher the power input, the quicker the sphere flattened out and the more rapidly and erratically it vibrated. Sometimes a part of molten silicon left the hearth for nearby jigging, especially those parts at a negative potential. However, the molten silicon has been maintained for one hour with the electron beam-water cooled hearth method with a deposition rate of +p/min. Usually, remelting the silicon was not successful once the silicon had been melted down to the copper hearthsilicon interface. In general, this hearth method of electron beam heating gave up to lp/min at 5 cm distance with about l/10 the p-type doping as obtained with the silica boats. The substrate temperature influences the contamination of the fihn, the crystallinity of the film and the diffusion of dopents in the film and substrate. As mentioned above, lower (below 1100 “C) substrate temperatures tended to give slightly brown films. Apparently this is due to the lower volatility of silicon monoxide. It is interesting to note that when the substrate temperature was higher than about 1125”C, the deposited layer was oriented with the substrate. The orientation was determined by X-ray back reflection patterns. Films deposited above 1125°C gave patterns of sharp spots with no arcing or rings. The temperature of 1125 “C is then the epitaxial temperature for silicon on silicon. The epitaxial temperature coincides, or is slightly above the temperature required to eliminate the brown deposits.
96
A. P. HALE o-n
Equal doping
levels
iunction
Lower
lilm
doping
Substrote
Film
Higher
film doping
FIG. 6. Position and shape of p-n junctions with various doping levels in the film with the substrate doping level as referenced.
This suggests the epitaxial temperature may be dependent on the partial pressure of oxygen and water. If this is true, the epitaxial temperature may be lower in well baked ultrahigh vacuum systems. This point has not been investigated. The substrate temperature also influences the diffusion of dopents and the higher the temperature, the faster the diffusion. Thus rapid depositions were necessary to obtain layers or films which had markedly different dopings from the substrates. When different doping types were used, a p-n junction was obtained in addition to the interface between the film and the substrate. Consequently, there are three cases, as shown in Fig. 6. When the doping levels are equal, the p-n junction coincides with the interface. When the doping in the film is lower than the doping in the substrate and of opposite type, the p-n junction is in the film and is dependent on film properties, the interface and the substrate. When doping in the film is higher and of opposite types, the junction is in the substrate so that the properties of the junction are less dependent on the film properties. When the space charge region is completely in the substrate, the properties of the p-n junction are independent of the film properties. IV. Film growth The films were single crystal when the substrate was maintained above 1125°C. Figures 7 and 8 show the oriented triangular deposits on (111) surfaces when the substrates were about 1125 “C. Initially, the silicon deposited on preferential sites and then gradually coalesced. After coalescing the surface of the film was rippled and contained many holes. The films were about 1-2,~ thick at the coalescing stage and about 3,~ thick after coalescing. Some thickness variation was obtained due to the relative positions of the substrate and source and their sizes. At higher substrate temperatures, the orientation and definition of the initial triangular deposits was not observed. However, the surface of the film still contained holes, as shown in Fig. 9. The clearest part is the film and the pitted area is the thermally etched area behind the mask. The holes are more frequent near the edge of the film. Away from the edge of the film there are very few holes and the surface is slightly rippled. The rippling was only visible with oblique lighting. The only surface other than (111) used for substrates was the (115) surface. An example is shown in Fig. 10. This film was deposited on an etched twinned wafer. The film
FIG. 8.
Consolidation
of nucleated Magnification
deposits 1250 x .
from
left
to
right.
was grooved mechanically and Dash etched for 15 min with 3HN03, 1 HF and 10 acetic acid. The step resulting from etching is the twin boundary. The groove is streaked with rows of pits resulting from the Dash etching of scratches caused by the grooving mandrel. The line of black dots (pits) correspond to the p-n junction and interface. The junction coincides with the interface in this case, and only one line is visible. The surfaces of the Iilm in the two differently oriented areas are rippled differently. The back reflection X-ray patterns were sharp spots for both sides of the twin boundary ; the (111) pattern was obtained for the film on the (111) surface, and the (115) pattern was obtained for the film on the (115) surface. A case where the p-n junction does not coincide with the interface is shown in Fig. 11. The edge of the film is horizontal and the edge of the groove is almost at right angles to the film edge. The Dash etching caused a diffuse
Preparation
and Evaluation
of Epitaxial Silicon Films prepared
by Vacuum Evaporation
97
row of pits at the p-n junction which is about l/3 the way through the film. The interface is barely visible.
FIG. 11. Silicon film (8 Qcm p-type) on silicon substrate (0.09 Qcm n-type). Mechanically grooved and Dash etched to reveal the noncoincident
FIG. 9. Film growth at 1200°C. No visible nucleation and smoother surface. Top half was behind the mask. Film is lower half. Magnification 500 X .
interface
and p-n junction.
explanation is that polished wafers can be lifted out of the final D.T. water with no adhering water film, or drops, and etched wafers cannot. After the cleaning procedure was improved, the films on etched wafers improved, while those on polished wafers did not. The problem with polished wafers is the presence of many very fine scratches. The films on polished wafers pitted readily with Dash etch giving a greater density of pits than films on the etched wafers.
: was 1300°C. L..__
ight.
mz1__
Magnification is 200 x .
The Dash etching makes the p-n junction and the interface visible so that the fihn thickness is easily obtained across the films. Dash etching shows up imperfections in or near the surface. The density of pitting was greatest next to the groove when the groove was made. In general, the pitting
varied by several orders of magnitude over the wafers so that any values were almost meaningless. An example of the pitting obtained is shown in Fig. 12. Most of the substrates were etched substrates as opposed to those mechanically polished. At the start of this work, when the cleaning procedure was not as good, polished wafers seemed to give the best fihns. Apparently the
FIG. 12. Characteristic pits obtained by Dash etching deposited silicon films. Magnification 1250x .
vacuum
Masking Mechanical masking is only partially effective. At these high substrate temperatures the vapor pressure and mobility are significant so that a small amount of the evaporated material deposits behind the mask. Consequently, a slight etch has to be used after the evaporation to clean up between the desired major deposits.
98
Oxide masking, since it is in substrate, makes an effective mask. were investigated. Near 1125 “C on the grown SiOz mask, as well as
A. P. direct contact with the Only grown SiO2 masks the deposition occurred on the exposed substrate,
HALE
substrate mask interface. The fringed area is the grown dioxide mask. The thick layer between the dioxide mask and the polycrystalline deposit is brown and is probably SiO. At substrate temperatures over about 1150 “C, erosion of the dioxide mask took place, giving epitaxial films surrounded by the eroded mask, as shown in Fig. 14. The increase in temperature increases the vapor pressure of SiO so that the SiO layer evaporates and the depositing silicon can react with the exposed SiO2 surface. The erosion took place at twice the rate of silicon deposition. Thus, the mask needs to be slightly more than twice as thick as the desired film thickness. With the equipment described, it was not possible to maintain a substrate temperature and deposition rate at the equilibrium between no erosion of mask and no deposition on the mask. The tim quality, as judged by microscopic, X-ray and device properties, was equal to films deposited without an oxide mask. Doping
FIG. 13. Polycrystalline silicon on dioxide mask and epitaxial silicon on exposed wafer surface. Substrate was near 1125 “C during deposition. Mechanically grooved and stained slightly. Lower right is the epitaxial deposit. Arrow indicates top of groove on epitaxial deposit. Magnification 120 x
as shown in Fig. 13. Figure 13 shows a part of a wafer and a grown dioxide mask with rectangular exposed areas covered with a silicon deposit. The deposit on the mask is polycrystalline, while the deposit on the exposed substrate is epitaxial. The HF stain does not show the exposed interface in the groove. The grooved area is streaked ; coming from the bottom of the groove to the top there is a clear narrow band where the stain removed some of the dioxide at the
Doping of the films can be accomplished by eiaporation of doped silicon by evaporation of the dopent from a separate source and by diffusion. Unfortunately, the vapor pressure of the various dopents is different from silicon and each other so that the conditions are particular for each dopent. Examination of the rates of evaporation and vapor pressures in Dushmanls indicate the boron doping loss during evaporation ought to be about 50 per cent. More recent data collected by Honigl9 indicates the doping loss might be about four orders of magnitude. Evaporation of boron doped silicon gave an apparent doping loss as shown in Fig. 15. The doping loss is apparent, since the measurements were 4-point probe measurements only. Since many other imperfections beside boron influence the resistivity, the high resistivity end of the curve may be more dependent on other imperfections than on boron. The doping at the high resistivity low doping end was very much dependent on the
I
FIG. 14. Epitaxial silicon on exposed wafer substrate which was near 1200°C during evaporation. Only several fringes of dioxide mask Arrows indicate top edge of remain between the epitaxial films. Magnification 260 X groove on the epitaxial deposit.
0.01
FIG. 15. Relationship
and boron
IO
04
Source
I
resistivity
between resistivities of boron doped doped films using electron beam heating.
sources
99
Preparation and Evaluation of Epitaxial Silicon Films prepared by Vacuum Evaporation cleanliness of the system. If a crucible was used, the highest reproducible resistivity obtainable was about 1 L&m. When the hearth and electron gun were used, the highest reproducible resistivity was lOR/cm. Evaporation of the other dopents was sometimes difficult and uncertain. The higher vapor pressures at any given temperature cause greater evaporation rates at the source and the substrate. The. rates of evaporation from the substrates are appreciable18 at these substrate temperatures, so that this process is the major source of loss. The change in resistivity with the evaporation of gallium doped silicon was about a factor of 25 on 1250°C substrates. With a clean system, phosphorous doped silicon could be evaporated onto an 1125 “C substrate with no doping loss. When the substrate was heavily p-type and the source was lightly doped with phosphorous, the films were p-type. When the substrates were over 12OO”C, the films were p-type even with highly doped phosphorous sources. The resistivity of the film from the evaporation of phosphorous doped silicon was very dependent on the contamination from parts of the jigging and bell, doping from substrates, as well as the temperature of the substrates. Another approach to the doping loss problem is to evaporate the dopent from a separate boat. Here the temperatures of the boats are adjusted to give the desired doping. This provided a means for reproducible Sb doping levels, even though the amount of Sb re-evaporated was 99 + per cent. V.
doped, the junction is in the film (see Fig. 8), and the best junction obtained did not give sharp breakdowns, as shown in Fig. 17. The lifetime of this junction was 50 n sec.
I Voltage,
20 V/cm
FIG. 17. Relationship between voltage and reverse bias leakage for 7 ficm p-type film on 1 Qcm n-type wafer substrates. The film was epitaxial on the substrate at 1250°C. The data is for 10 mil diameter mesas.
Devices
Diodes can be prepared by depositing one type of silicon on a substrate of the opposite type. More heavily doped films cause the p-n junctions to be in the substrate (see Fig. 8), and the junctions behaved like normal diffused junctions with the same doping gradient. These junctions always had very sharp breakdowns when viewed on the scopes. An example of one of these junctions is shown in Fig. 16. The capacitance varied with voltage to the minus 0.4 power. The forward current was 50 mA at 1.2 V. The lifetime was about 100 n sec. When the films are lightly IO
Voltage,
IOV/cm
FIG. 18. PNP characteristics for an experimental transistor made from a 1 Qcm film on a 10-Z Rem wafer by double diffusion into the film. Base step is 0.05 mA.
FIG. 16. Relationship between voltage and capacitance and voltage and reverse bias leakage. The polycrystalline film was 0.2 Q/cm p-type. The substrate was 1 Q/cm n-type and only 1050°C during deposition. The data is for 10 mil diameter mesas.
These films can also be used to fabricate transistors. For example, a 1 Q/cm p-type film was deposited on a lo-2Q/ cm p-type wafer. The resulting film-wafer was double diffused to provide a base and emitter in the film. The diffusion rates in the films were the same as in the pulled crystals. The results are shown in Fig. 18. Except for soft breakdowns, the parameters of the epitaxial transistors were much the same as the normal transistors, since the
A. P.
100
design was not optimized. However, one large difference was in storage times ; the epitaxial transistors had much lower storage times. Acknowledgments
The author would like to thank Don Ramsay for supplying many films, K. Murray and M. Julian for X-ray orientations, R. Craig for the transistor fabrication, B. Thomas and B. Kinsman for the photographs, and J. Hoerni, S. Roberts and B. D. James for helpful discussions. References 1 L. Holland, Vacuum Deposition of Thin Films, John Wiley & Sons, New York. 1956. 2 H. Levinstein, J. Appl. Phys., 20, (1949) 306-315, (16 references). 3 D. W. Pashley, Advances in Physics 5, 173-240 (April, 1956) (over
100 references). 4 H. Seifert, Structure
and Properties of Solid Surfaces (edited by R. Gomer and C. S. Smith), University of Chicago Press, (1953), pp. 318-383 (about 250 references). s L. E. Collins, Ph.D. Thesis, University of Reading, Reading, England. 6 S. A. Semiletov, Sov. Phys., Crystallography 1, 5, (1956) 542-545.
HALE 7 G. A. Kurov, S. A. Semiletov (Doklady), 1, (1956) 604-606.
and
Z. G. Pinsker,
8 0. Weinreich, G. Dermit and C. Tufts, J. Appt. Phys., 32, (June, 1961), 1170-1171. 9 J. E. Davey, /. Appl. Phys., 33, (March, 1962), 1015-1016. 1” W. Reichelt and G. F. P. Mueller, 1961 Vacuum Symposium Transactions, Pergamon Press, London, (1962), pp. 956-964. 11 G. Hass, & f: Anorg. Chem., 257, (1948), 166-172. 12 F. M. Collins, 1961 Vacuum Symposium Transaction, Pergamon Press, London, (1962), pp. 899-904. 13 S. Nielsen, D. G. Coates and Mrs. J. E. Maines, International Symposium on Condensation and Evaporation of Solids, 12-14 September, (1962), Dayton, Ohio. 14 E. T. Handelman and E. I. Povilonis, 122nd Meeting of the Electrochemical Society, 16-20 September, (1962), Boston, Massachusetts. 1s J. P. Reames, 1960 Vacuum Symposium Transactions, Pergamon Press, London, (1961), pp. 219-223. 16 R. W. Berry, Proceedings of the Third Symposium on Electron Beam Processes, (edited by R. Bakish), Alloyed Electronics Corp., 1961, pp. 358-366. 17 H. E. Farnsworth, R. E. Schlier and J. A. Dillon, Jr., Solid State Physics in Electronics & Telecommunications, Vol. I, Part 1, Semiconductors, (edited by M. Desirant and J. L. Michiels, Academic Press, (1960), London, pp. 602-612. 1s S. Dushman, Scientific Foundations of Vacuum Technique, John Wiley & Sons, New York, (1949), pp. 743-778. 19 R. E. Honig, RCA Rev 18, (June, 1957), 195-204.
Conference Sorption Properties of Vacuum Deposited Metal Films THE
Institute of Physics and The Physical Society in collaboration with the Joint British Committee for Vacuum Science and Technology have arranged a Conference on “ Sorption Properties of Vacuum Deposited Metal Films “, to be held at the University of Liverpool on 17-19 April 1963. Mr. L. Holland, F.Inst.P., of Edwards High Vacuum Ltd., Cramley, is Chairman of the Organizing Committee. The Provisional Programme includes papers by the following : R. W. Roberts (U.S.A.) ; D. G. Brandon, P. Bowden and M. Wald (United Kingdom) ; M. W. Thompson (United Kingdom) ; G. Carter and J. H. Leek (United Kingdom) ; R. M. Oman and J. A. Dillon Jr. (U.S.A.) ; G. Ehrlich (U.S.A.) ; C. Kleint (Germany) ; W. J. M. Rootsaert, L. L. van Reijen and W. M. H. Sachtler (The Netherlands) ; M. W. Roberts (United Kingdom) ; L. Elsworth and L. Holland (United Kingdom) ; D. Brennan, M. J. Graham and F. H. Hayes (United Kingdom) ; P. della Porta, F. Ricca and T. Giorgi (Italy) ; R. W. Roberts (U.S.A.) ; R. Suhrmann (Germany) ; N. Hansen (Germany) ; G. W. Poling and J. Leja (Canada) ; J. P. Hobson (Canada) ; E. E. Donaldson and H. F. Winters (U.S.A.) ; G. Comsa (Roumania) ; R. E. Hayes, R. W. Alsford and D. I. Kennedy (United Kingdom)
; C. Weaver (United Kingdom).
Further details may be obtained from 47 Belgrave Square, London, S. W. 1.
Sov. Phys.
The Institute
of Physics
and the Physical
Society,
Single crystal silicon films of both types have been deposited in a high (10-s torr) vacuum Electron beam heating, as well as resistive heating, were used to heat the sources. system. Low energy argon ion bombardment and thermal etching were used to clean the surfaces of the Single The investigated temperature range of the substrates was IOOO-1300°C. substrates. crystal films by back reflection X-ray were obtained when the substrates were above 1125°C Fast deposition rates (t_lp/min) (14OO”K), which is about 0.8 of the melting point (1700°K). Diodes and transistors were prepared from some of these films to obtain gave the best films. The p-n junctions showed higher an indication of the p-n junction and interface perfection. reverse bias leakage and thus less perfection than that observed with devices in crystals grown by the Czochralski method. I. Introduction
Vacuum evaporation has been used for many years as a method of depositing a film of metal on the same or another metal.1 Vacuum evaporation has also been used in studies Seifert4 refers to Royer’s use of of epitaxial growth.z-4 epitaxy as an “ arrangement on ” and Collins5 refers to on “. Thus, Royer’s use as an “ orderly arrangement epitaxy refers to the laying down of a material on a substrate so that the orderly arrangement, of the substrate is continued into the deposited material. Consequently, the laying down of a material on a substrate of the same material is simply a special case of the general epitaxy. Epitaxy by vacuum evaporation has been reported for Ge on Ge+lo. Kurov, et al.7 report single crystal films when the substrate is over 600°C that are p-type regardless of type evaporated. Daveyg reported epitaxy on substrates heated to only 300°C after a cleaning at 575°C. Weinreich, et al. reported n-type films and p-n junctions by evaporating heavily doped n-type Ge. Silicon films on various substrates have been reportedll-14. Nielsen et al.13 report good epitaxy when pressure is down Handelman and Povilonisl4 to 10-7 torr during evaporation. reported epitaxy and p-n junction formation by sublimation at 1 x 10-S torr. II. Experimental
Sect.“AA”
Wafer
(far
holder gQg
evaporation
SeCtian”AA”
source
double evaporation)
FIG. 1. Schematic
diagram
of the resistive heating
apparatus.
and stainless steel in cooler areas. The schematic diagram for resistive heating is shown in Fig. 1. Crucibles of Al2O3, ThO2 and SiO2 were used. The crucibles were wrapped in 1 mil tantalum foil which was resistively heated. The outside shield was used to conserve power. The wafers were held 2 cm above the source by tungsten clips. The wafers were about 2 mm below the molybdenum wire (l/16 in. dia.) heater. The shield around the wafers and heater reduces the temperature gradient and conserves power. The temperature gradient was about 20°C at 1150°C when measured with an optical pyrometer. Optical pyrometer readings Corrected temperatures given here may be were corrected. in error by f25 “C. Another method of heating the source was employed for
techniques
The vacuum evaporator was a commercially built unit providing an ultimate pressure of 5 x 10-7 torr. The 4 in. diffusion pump was used with a water cooled baffle and a cold finger liquid nitrogen trap. The seals were neoprene and the bell was 18 x 18 in. The jigging in the bell was made of molybdenum, tantalum, and tungsten for the hottest areas and from nickel, copper 93
A. P. HALE
94
Electron part of the experiments to reduce the contamination. beam heating arrangementis* is shown in Figs. 2 and 3. The silicon wafers in the same heater and shield arrangement were 5 cm above the molten silicon. The tungsten wire was held near -3000 V and the silicon was at ground potential. The box around the filament was at the same potential as the filament. The silicon wafers were cut and lapped from single crystals grown by the Czochralski method. The (111) surface was obtained. The wafers were ultrasonically cleaned and washed in D.I. water then etched in CP-8 and washed in D.I. water and finally ultrasonically cleaned. The final ultrasonic cleaning proceeds through D.I. water, acetone, , I
Molten sflicon I
I
i
I
1
Tungsten
1
wire
Cooling water -
FIG. 2. Schematic
diagram
of the electron
beam heating
unit.
trichloroethane, acetone, D.I. water and methanol. Methanol is a good final solvent, since methanol, as well as ethanol and isopropyl alcohol remove the last traces of water and acetone and evaporate without water condensation. The wafers were removed by pouring off most of the solvent to leave a small portion of the wafer above the surface and then carefully and slowly removing the wafers with clean smooth tweezers so that the solvent stays in the container and is not attracted up the tweezers by capillary action. After the bell was pumped to about 5 x lo-6 torr a short degassing was started. At this time the wafers were in the other side of the bell. The degassing was continued for 3&60 min. This degassed the heater and source quite well and reduced the degassing of the other internal surfaces during the subsequent evaporation. The bell was allowed to cool and pump down for about 2&30 min. The argon ion bombardment was used to remove the contamination left by the solvents and picked up in transfer to the bell. The argon was purified by a sequence of Dry Ice, tantalum powder at 1100°C and Dry Ice traps. The argon was bled through a needle valve and Pyrex tube to the top of the bell. The argon pressure was maintained at about 10~~. The wafers were maintained at a negative potential with respect to the grounded collector. The potentials were -400 and - 1000 V. The same control settings corresponded to -800 and -2000 V when the pressure in the bell was 10-s torr, or less. The current was about 1 mA for the -1000 V discharge. The -1000 V discharge was barely visible while the -400 V discharge was not visible. The time of discharge was varied, as was the annealing time following the discharge. After the wafers and the source were at the desired temperatures, or power inputs, the evaporations were started by removing the shutter. The deposition was stopped by replacing the shutter. The cooling rates were also varied. III.
Results
Preparation
FIG. 3. Jigging arrangement in the bell. The silicon wafers are mounted on one of the rotatable rods so that they are on the right during degassing and ion bombardment, and on the left in the substrate heater box during evaporation.
and discussion variables
Many variations of the cleaning procedure were tried by omitting some of the solvents and by changing the ultrasonic times. The sequence of solvents without omissions, as described above, is a natural one, since any solvent used is miscible in the next. The ultrasonic times were best kept short (30 set) to avoid contamination of the solvents by settling dust from the air. The use of a final rinse with D.I. water was not satisfactory due to the particles in the water and the necessity of blotting the water drops. Omission of the blotting step with a final D.I. water rinse left visible imperfections in the films in the spot where the water drops formed. The surface of polished wafers was not wet by the D.I. water so that the polished wafers could usually be pulled from the final D.I. water rinse with no visible water adhering to the polished surface. However, film imperfections were least in number and size when the final rinse was alcohol. According to the manufacturer, the alcohol left about 0.0002 per cent residue on evaporation. Ion bombardment with low energy argon was used to Here the desired end remove last traces of contamination.
Preparation and Evaluation of Epitaxial Silicon Films prepared by Vacuum Evaporation was to remove the remaining contaminates with a minimal damage to the surface. The ion bombardment was done in one side of the bell and the discharge localized to the immediate volume around the wafers. The supports for the The argon flowed past wafers were short legs of tungsten. the wafers minimizing recontamination. The damage to the surface by the ion bombardment was evidenced by the increase in resistivity of p-type surfaces when using the 4-point probe method. Extended (3 h) bombardment with either 400 and 1000 V gave n-type surfaces by the thermal Heavily doped probe method on 1 lncm p-type wafers. p-type wafers did not develop n-type surfaces. The n-type surface annealed out in less than ten minutes at 1150°C and the resistivity by the 4-point probe was the same as The n-type wafers showed no change before the treatment. after bombardment in type or in resistivity. The bombardment caused no visible surface damage with magnifications
FIG. 4. Thermal
etching
at 1125°C. Etching Magnification 500 x .
FIG. 5. Thermal
etching
near 1200°C. Etching Magnification 500 x .
time
time
was
30 min.
was
30 miny
95
up to x 1250. Thus, a ten minute anneal appeared to be a practical compromise between annealing time and thermal etching. The annealing at 1125 “C-1300°C caused some thermal etching.17 At 1125°C (in Fig. 4) the thermal etching leaves an oriented pattern that was partly colored, indicating the presence of some silicon monoxide. Also, note the circular thermal etching at a solvent drop site in Fig. 4. Between 1200°C and 13OO”C, the thermal etching patterns were random with no colored areas, as shown in Fig. 5. Slow rates of deposition and low substrate temperature gave slightly brown film surfaces. Faster rates of +l,u/min and substrate temperatures over 1150 “C usually gave clear films. The use of Al203 and ThOz crucibles with faster rates gave brown films. The use of silica boats did not result in brown films. However, above the softening point of the silica, appreciable evaporation of the boat took place and the surface of the films, though still clear, had a very high resistance even after five minute H.F. (49 per cent) soak and D.I. water wash. The least contamination was obtained with the electron beam heating since the only material in contact with the molten silicon is water cooled. Insufficient water cooling resulted in erosion of the water cooled hearth. In practice a (1 cm dia. x 3 cm high) cylinder of single crystal silicon was melted. The melting had to be done carefully, since the molten silicon would settle to the contour of the hearth. The layer of silicon in contact with the hearth would immediately freeze and sometimes the solidification was half way up the silicon. No attack, or erosion of the hearth was noted when the copper hearth was made as shown in Fig. 2. Increasing the power input lowered the solid-liquid interface in the silicon to give an erratically vibrating sphere. The higher the power input, the quicker the sphere flattened out and the more rapidly and erratically it vibrated. Sometimes a part of molten silicon left the hearth for nearby jigging, especially those parts at a negative potential. However, the molten silicon has been maintained for one hour with the electron beam-water cooled hearth method with a deposition rate of +p/min. Usually, remelting the silicon was not successful once the silicon had been melted down to the copper hearthsilicon interface. In general, this hearth method of electron beam heating gave up to lp/min at 5 cm distance with about l/10 the p-type doping as obtained with the silica boats. The substrate temperature influences the contamination of the fihn, the crystallinity of the film and the diffusion of dopents in the film and substrate. As mentioned above, lower (below 1100 “C) substrate temperatures tended to give slightly brown films. Apparently this is due to the lower volatility of silicon monoxide. It is interesting to note that when the substrate temperature was higher than about 1125”C, the deposited layer was oriented with the substrate. The orientation was determined by X-ray back reflection patterns. Films deposited above 1125°C gave patterns of sharp spots with no arcing or rings. The temperature of 1125 “C is then the epitaxial temperature for silicon on silicon. The epitaxial temperature coincides, or is slightly above the temperature required to eliminate the brown deposits.
96
A. P. HALE o-n
Equal doping
levels
iunction
Lower
lilm
doping
Substrote
Film
Higher
film doping
FIG. 6. Position and shape of p-n junctions with various doping levels in the film with the substrate doping level as referenced.
This suggests the epitaxial temperature may be dependent on the partial pressure of oxygen and water. If this is true, the epitaxial temperature may be lower in well baked ultrahigh vacuum systems. This point has not been investigated. The substrate temperature also influences the diffusion of dopents and the higher the temperature, the faster the diffusion. Thus rapid depositions were necessary to obtain layers or films which had markedly different dopings from the substrates. When different doping types were used, a p-n junction was obtained in addition to the interface between the film and the substrate. Consequently, there are three cases, as shown in Fig. 6. When the doping levels are equal, the p-n junction coincides with the interface. When the doping in the film is lower than the doping in the substrate and of opposite type, the p-n junction is in the film and is dependent on film properties, the interface and the substrate. When doping in the film is higher and of opposite types, the junction is in the substrate so that the properties of the junction are less dependent on the film properties. When the space charge region is completely in the substrate, the properties of the p-n junction are independent of the film properties. IV. Film growth The films were single crystal when the substrate was maintained above 1125°C. Figures 7 and 8 show the oriented triangular deposits on (111) surfaces when the substrates were about 1125 “C. Initially, the silicon deposited on preferential sites and then gradually coalesced. After coalescing the surface of the film was rippled and contained many holes. The films were about 1-2,~ thick at the coalescing stage and about 3,~ thick after coalescing. Some thickness variation was obtained due to the relative positions of the substrate and source and their sizes. At higher substrate temperatures, the orientation and definition of the initial triangular deposits was not observed. However, the surface of the film still contained holes, as shown in Fig. 9. The clearest part is the film and the pitted area is the thermally etched area behind the mask. The holes are more frequent near the edge of the film. Away from the edge of the film there are very few holes and the surface is slightly rippled. The rippling was only visible with oblique lighting. The only surface other than (111) used for substrates was the (115) surface. An example is shown in Fig. 10. This film was deposited on an etched twinned wafer. The film
FIG. 8.
Consolidation
of nucleated Magnification
deposits 1250 x .
from
left
to
right.
was grooved mechanically and Dash etched for 15 min with 3HN03, 1 HF and 10 acetic acid. The step resulting from etching is the twin boundary. The groove is streaked with rows of pits resulting from the Dash etching of scratches caused by the grooving mandrel. The line of black dots (pits) correspond to the p-n junction and interface. The junction coincides with the interface in this case, and only one line is visible. The surfaces of the Iilm in the two differently oriented areas are rippled differently. The back reflection X-ray patterns were sharp spots for both sides of the twin boundary ; the (111) pattern was obtained for the film on the (111) surface, and the (115) pattern was obtained for the film on the (115) surface. A case where the p-n junction does not coincide with the interface is shown in Fig. 11. The edge of the film is horizontal and the edge of the groove is almost at right angles to the film edge. The Dash etching caused a diffuse
Preparation
and Evaluation
of Epitaxial Silicon Films prepared
by Vacuum Evaporation
97
row of pits at the p-n junction which is about l/3 the way through the film. The interface is barely visible.
FIG. 11. Silicon film (8 Qcm p-type) on silicon substrate (0.09 Qcm n-type). Mechanically grooved and Dash etched to reveal the noncoincident
FIG. 9. Film growth at 1200°C. No visible nucleation and smoother surface. Top half was behind the mask. Film is lower half. Magnification 500 X .
interface
and p-n junction.
explanation is that polished wafers can be lifted out of the final D.T. water with no adhering water film, or drops, and etched wafers cannot. After the cleaning procedure was improved, the films on etched wafers improved, while those on polished wafers did not. The problem with polished wafers is the presence of many very fine scratches. The films on polished wafers pitted readily with Dash etch giving a greater density of pits than films on the etched wafers.
: was 1300°C. L..__
ight.
mz1__
Magnification is 200 x .
The Dash etching makes the p-n junction and the interface visible so that the fihn thickness is easily obtained across the films. Dash etching shows up imperfections in or near the surface. The density of pitting was greatest next to the groove when the groove was made. In general, the pitting
varied by several orders of magnitude over the wafers so that any values were almost meaningless. An example of the pitting obtained is shown in Fig. 12. Most of the substrates were etched substrates as opposed to those mechanically polished. At the start of this work, when the cleaning procedure was not as good, polished wafers seemed to give the best fihns. Apparently the
FIG. 12. Characteristic pits obtained by Dash etching deposited silicon films. Magnification 1250x .
vacuum
Masking Mechanical masking is only partially effective. At these high substrate temperatures the vapor pressure and mobility are significant so that a small amount of the evaporated material deposits behind the mask. Consequently, a slight etch has to be used after the evaporation to clean up between the desired major deposits.
98
Oxide masking, since it is in substrate, makes an effective mask. were investigated. Near 1125 “C on the grown SiOz mask, as well as
A. P. direct contact with the Only grown SiO2 masks the deposition occurred on the exposed substrate,
HALE
substrate mask interface. The fringed area is the grown dioxide mask. The thick layer between the dioxide mask and the polycrystalline deposit is brown and is probably SiO. At substrate temperatures over about 1150 “C, erosion of the dioxide mask took place, giving epitaxial films surrounded by the eroded mask, as shown in Fig. 14. The increase in temperature increases the vapor pressure of SiO so that the SiO layer evaporates and the depositing silicon can react with the exposed SiO2 surface. The erosion took place at twice the rate of silicon deposition. Thus, the mask needs to be slightly more than twice as thick as the desired film thickness. With the equipment described, it was not possible to maintain a substrate temperature and deposition rate at the equilibrium between no erosion of mask and no deposition on the mask. The tim quality, as judged by microscopic, X-ray and device properties, was equal to films deposited without an oxide mask. Doping
FIG. 13. Polycrystalline silicon on dioxide mask and epitaxial silicon on exposed wafer surface. Substrate was near 1125 “C during deposition. Mechanically grooved and stained slightly. Lower right is the epitaxial deposit. Arrow indicates top of groove on epitaxial deposit. Magnification 120 x
as shown in Fig. 13. Figure 13 shows a part of a wafer and a grown dioxide mask with rectangular exposed areas covered with a silicon deposit. The deposit on the mask is polycrystalline, while the deposit on the exposed substrate is epitaxial. The HF stain does not show the exposed interface in the groove. The grooved area is streaked ; coming from the bottom of the groove to the top there is a clear narrow band where the stain removed some of the dioxide at the
Doping of the films can be accomplished by eiaporation of doped silicon by evaporation of the dopent from a separate source and by diffusion. Unfortunately, the vapor pressure of the various dopents is different from silicon and each other so that the conditions are particular for each dopent. Examination of the rates of evaporation and vapor pressures in Dushmanls indicate the boron doping loss during evaporation ought to be about 50 per cent. More recent data collected by Honigl9 indicates the doping loss might be about four orders of magnitude. Evaporation of boron doped silicon gave an apparent doping loss as shown in Fig. 15. The doping loss is apparent, since the measurements were 4-point probe measurements only. Since many other imperfections beside boron influence the resistivity, the high resistivity end of the curve may be more dependent on other imperfections than on boron. The doping at the high resistivity low doping end was very much dependent on the
I
FIG. 14. Epitaxial silicon on exposed wafer substrate which was near 1200°C during evaporation. Only several fringes of dioxide mask Arrows indicate top edge of remain between the epitaxial films. Magnification 260 X groove on the epitaxial deposit.
0.01
FIG. 15. Relationship
and boron
IO
04
Source
I
resistivity
between resistivities of boron doped doped films using electron beam heating.
sources
99
Preparation and Evaluation of Epitaxial Silicon Films prepared by Vacuum Evaporation cleanliness of the system. If a crucible was used, the highest reproducible resistivity obtainable was about 1 L&m. When the hearth and electron gun were used, the highest reproducible resistivity was lOR/cm. Evaporation of the other dopents was sometimes difficult and uncertain. The higher vapor pressures at any given temperature cause greater evaporation rates at the source and the substrate. The. rates of evaporation from the substrates are appreciable18 at these substrate temperatures, so that this process is the major source of loss. The change in resistivity with the evaporation of gallium doped silicon was about a factor of 25 on 1250°C substrates. With a clean system, phosphorous doped silicon could be evaporated onto an 1125 “C substrate with no doping loss. When the substrate was heavily p-type and the source was lightly doped with phosphorous, the films were p-type. When the substrates were over 12OO”C, the films were p-type even with highly doped phosphorous sources. The resistivity of the film from the evaporation of phosphorous doped silicon was very dependent on the contamination from parts of the jigging and bell, doping from substrates, as well as the temperature of the substrates. Another approach to the doping loss problem is to evaporate the dopent from a separate boat. Here the temperatures of the boats are adjusted to give the desired doping. This provided a means for reproducible Sb doping levels, even though the amount of Sb re-evaporated was 99 + per cent. V.
doped, the junction is in the film (see Fig. 8), and the best junction obtained did not give sharp breakdowns, as shown in Fig. 17. The lifetime of this junction was 50 n sec.
I Voltage,
20 V/cm
FIG. 17. Relationship between voltage and reverse bias leakage for 7 ficm p-type film on 1 Qcm n-type wafer substrates. The film was epitaxial on the substrate at 1250°C. The data is for 10 mil diameter mesas.
Devices
Diodes can be prepared by depositing one type of silicon on a substrate of the opposite type. More heavily doped films cause the p-n junctions to be in the substrate (see Fig. 8), and the junctions behaved like normal diffused junctions with the same doping gradient. These junctions always had very sharp breakdowns when viewed on the scopes. An example of one of these junctions is shown in Fig. 16. The capacitance varied with voltage to the minus 0.4 power. The forward current was 50 mA at 1.2 V. The lifetime was about 100 n sec. When the films are lightly IO
Voltage,
IOV/cm
FIG. 18. PNP characteristics for an experimental transistor made from a 1 Qcm film on a 10-Z Rem wafer by double diffusion into the film. Base step is 0.05 mA.
FIG. 16. Relationship between voltage and capacitance and voltage and reverse bias leakage. The polycrystalline film was 0.2 Q/cm p-type. The substrate was 1 Q/cm n-type and only 1050°C during deposition. The data is for 10 mil diameter mesas.
These films can also be used to fabricate transistors. For example, a 1 Q/cm p-type film was deposited on a lo-2Q/ cm p-type wafer. The resulting film-wafer was double diffused to provide a base and emitter in the film. The diffusion rates in the films were the same as in the pulled crystals. The results are shown in Fig. 18. Except for soft breakdowns, the parameters of the epitaxial transistors were much the same as the normal transistors, since the
A. P.
100
design was not optimized. However, one large difference was in storage times ; the epitaxial transistors had much lower storage times. Acknowledgments
The author would like to thank Don Ramsay for supplying many films, K. Murray and M. Julian for X-ray orientations, R. Craig for the transistor fabrication, B. Thomas and B. Kinsman for the photographs, and J. Hoerni, S. Roberts and B. D. James for helpful discussions. References 1 L. Holland, Vacuum Deposition of Thin Films, John Wiley & Sons, New York. 1956. 2 H. Levinstein, J. Appl. Phys., 20, (1949) 306-315, (16 references). 3 D. W. Pashley, Advances in Physics 5, 173-240 (April, 1956) (over
100 references). 4 H. Seifert, Structure
and Properties of Solid Surfaces (edited by R. Gomer and C. S. Smith), University of Chicago Press, (1953), pp. 318-383 (about 250 references). s L. E. Collins, Ph.D. Thesis, University of Reading, Reading, England. 6 S. A. Semiletov, Sov. Phys., Crystallography 1, 5, (1956) 542-545.
HALE 7 G. A. Kurov, S. A. Semiletov (Doklady), 1, (1956) 604-606.
and
Z. G. Pinsker,
8 0. Weinreich, G. Dermit and C. Tufts, J. Appt. Phys., 32, (June, 1961), 1170-1171. 9 J. E. Davey, /. Appl. Phys., 33, (March, 1962), 1015-1016. 1” W. Reichelt and G. F. P. Mueller, 1961 Vacuum Symposium Transactions, Pergamon Press, London, (1962), pp. 956-964. 11 G. Hass, & f: Anorg. Chem., 257, (1948), 166-172. 12 F. M. Collins, 1961 Vacuum Symposium Transaction, Pergamon Press, London, (1962), pp. 899-904. 13 S. Nielsen, D. G. Coates and Mrs. J. E. Maines, International Symposium on Condensation and Evaporation of Solids, 12-14 September, (1962), Dayton, Ohio. 14 E. T. Handelman and E. I. Povilonis, 122nd Meeting of the Electrochemical Society, 16-20 September, (1962), Boston, Massachusetts. 1s J. P. Reames, 1960 Vacuum Symposium Transactions, Pergamon Press, London, (1961), pp. 219-223. 16 R. W. Berry, Proceedings of the Third Symposium on Electron Beam Processes, (edited by R. Bakish), Alloyed Electronics Corp., 1961, pp. 358-366. 17 H. E. Farnsworth, R. E. Schlier and J. A. Dillon, Jr., Solid State Physics in Electronics & Telecommunications, Vol. I, Part 1, Semiconductors, (edited by M. Desirant and J. L. Michiels, Academic Press, (1960), London, pp. 602-612. 1s S. Dushman, Scientific Foundations of Vacuum Technique, John Wiley & Sons, New York, (1949), pp. 743-778. 19 R. E. Honig, RCA Rev 18, (June, 1957), 195-204.
Conference Sorption Properties of Vacuum Deposited Metal Films THE
Institute of Physics and The Physical Society in collaboration with the Joint British Committee for Vacuum Science and Technology have arranged a Conference on “ Sorption Properties of Vacuum Deposited Metal Films “, to be held at the University of Liverpool on 17-19 April 1963. Mr. L. Holland, F.Inst.P., of Edwards High Vacuum Ltd., Cramley, is Chairman of the Organizing Committee. The Provisional Programme includes papers by the following : R. W. Roberts (U.S.A.) ; D. G. Brandon, P. Bowden and M. Wald (United Kingdom) ; M. W. Thompson (United Kingdom) ; G. Carter and J. H. Leek (United Kingdom) ; R. M. Oman and J. A. Dillon Jr. (U.S.A.) ; G. Ehrlich (U.S.A.) ; C. Kleint (Germany) ; W. J. M. Rootsaert, L. L. van Reijen and W. M. H. Sachtler (The Netherlands) ; M. W. Roberts (United Kingdom) ; L. Elsworth and L. Holland (United Kingdom) ; D. Brennan, M. J. Graham and F. H. Hayes (United Kingdom) ; P. della Porta, F. Ricca and T. Giorgi (Italy) ; R. W. Roberts (U.S.A.) ; R. Suhrmann (Germany) ; N. Hansen (Germany) ; G. W. Poling and J. Leja (Canada) ; J. P. Hobson (Canada) ; E. E. Donaldson and H. F. Winters (U.S.A.) ; G. Comsa (Roumania) ; R. E. Hayes, R. W. Alsford and D. I. Kennedy (United Kingdom)
; C. Weaver (United Kingdom).
Further details may be obtained from 47 Belgrave Square, London, S. W. 1.
Sov. Phys.
The Institute
of Physics
and the Physical
Society,