Thin Solid Films, 21 (1974) $23-$26 © Elsevier Sequoia S.A., Lausanne---Printed in Switzerland
Short Communication Chemical vapour deposition of osmium films S. LEHWALD AND H. WAGNER Institut fiir Technische Physik der Kernforschungsanlage Jiilich GmbH, 517 Jiilich (Germany) (Received December 10, 1973; accepted February 5, 1974)
Osmium shows potential for various technical applications because of its high melting point of about 3000 °C, its great hardness and its high electron work function. So far, reports in the literature only describe the preparation of osmium films and coatings by sputtering 1 and electrolytic deposition2' a. With one exception 3, the reported techniques either yielded only thin layers or, for larger coating thicknesses, caused the layers to be disrupted by inclusions. Because of our interest in the eventual use of osmium in thermionic diodes we have investigated the formation of osmium films on molybdenum and tungsten substrates by chemical vapour deposition (CVD).
Evaporation of the starting material Osmium tetrachloride OSC14, a black crystalline powder, was used as the starting material. According to refs. 4-60sC14 starts to dissociate markedly around 300 °C and decomposes to OsC13. However, only above 450 °C does it show an appreciable evaporation rate. A similar behaviour is found with OsC13; it dissociates readily below 500 °C but a marked evaporation only occurs above this temperature. Therefore, only a few grains of OsCI4 at a time were put into a flash evaporator by a shaking device and evaporated at about 550 °C. Under these conditions about half of the OsCl 4 sublimed, the remaining part being already reduced to metallic osmium within the evaporator. This metallic Os, however, can easily be chlorinated to OsC14 at about 800 °C and 1500 Torr C12 pressure and is hence regained.
Substrates Polycrystallinemolybdenum and tungsten discs 16 mm in diameter and singlecrystal tungsten discs and samples coated with tungsten by CVD, of 6 mm diameter and 1.5 mm thickness, were used as substrates. In addition, tungsten wires of 0.125 mm diameter and 150--200 mm length were coated with osmium. The latter samples were used for work function measurements in a caesium atmosphere 7. The substrate surfaces were ground and polished and sometimes etched in order to yield a rougher surface and obtain a better adherence of the osmium coating. The substrate discs were then placed on a molybdenum or a tungsten support, respectively, and heated inductively by a h.f. generator. The tungsten
wires were cleaned for 15 min in boiling concentrated NaOH and subsequently annealed for 2-3 h at 1200 °C in a hydrogen stream. For the osmium deposition three wires were arranged in parallel and heated by resistance heating.
C VD apparatus and technique The CVD apparatus used is shown schematically in Fig. 1. The reaction chamber and the evaporator are made of quartz glass. The reaction chamber has a diameter of 35 mm and is 250 mm long. The narrow portion of the evaporator filled with quartz wool has an inner diameter of 1.5 mm and is heated externally by a heating wire.
[~/-oscq shakin! dewc~~ ~
~.-.~,,,.-monomet er (/) oo,.,...
Fig. 1. Osmium CVD apparatus.
The reaction chamber was somewhat modified for the coating of the tungsten wires. The gas inlet was arranged at the side of the chamber and the three wires were stretched in parallel between two electrodes at the top and bottom. The leak-tested apparatus was evacuated to 10-? Torr. Prior to the deposition the substrates were annealed in a pure hydrogen stream for 20-30 min at 1250 °C. The hydrogen inlet valve was then closed and when a total pressure of 5 x 10-3 Torr was reached in the reaction chamber the evaporation of OsCl~ was started. The osmium chloride vapour was pyrolytically decomposed at the substrate surface, which was held at 1250 °C.
Results Crystalline osmium films and coatings of up to 15 Ixrnthickness were deposited onto the molybdenum and tungsten substrates. The film thickness obtainable does not seem to be limited. An amount of 0.25 g OsC14 powder sufficed for layer thicknesses of 5-10 gra. The osmium layers showed a polycrystaUine grain structure without inclusions, the grain size depending on the size of the underlying substrate grains. The molybdenum substrates showed much larger grain sizes than the tungsten substrates, and so did the osmium films grown upon them. No individual grains could
be distinguished, using light microscopy, on the osmium films grown on tungsten single crystals. These surfaces looked homogeneously rough. For applications where self-supported osmium films are required, e.g. for X-ray, neutron or electron radiation studies, the osmium films can easily be removed from the substrates by dissolving the tungsten or molybdenum supports in a mixture of HF/HNO3 (1:1). The osmium films are not attacked chemically by this reagent. For use as surface coatings the layers were not sufficiently adherent to the chosen substrate materials because they peeled off after heat cycling between 20 ° and 1500 °C or sometimes came off by applying the "tape test". The osmium films grown on polycrystalline substrates were polycrystalline, and showed some preferred orientations. An X-ray inverse pole figure technique 8 applied to the substrates'and to the removed osmium films showed, for example, that in the case of tungsten substrates which were preferentially (100) and (103) oriented the osmium films had grains preferentially with the (100), (110), (120) and (121) planes parallel to the surface. Osmium films grown on tungsten single-crystal substrates with an orientation close to (100) were found to have a strong texture and exhibited only planes of (120) orientation parallel to the surface. Figure 2 shows a transmission Laue photograph of such a film (4 lam thick). It shows asterism--the film is a distorted single-crystal film. The Vickers hardness of the osmium films measured with a Leitz microhardness tester and a load of 25 p gave values around 600 kp/mm 2. In comparison,
Fig. 2. Transmission Laue photograph of an o s m i u m film, 4 lam thick, grown on a tungsten singlecrystal substrate.
m i c r o h a r d n e s s m e a s u r e m e n t s for sintered m a t e r i a l yield 530 (ref. 9) a n d 520 (ref. 10) k p / m m 2. D a t a on the c r y s t a l l o g r a p h y a n d h a r d n e s s o f o s m i u m films p r o d u c e d by the o t h e r techniques m e n t i o n e d in the l i t e r a t u r e 1-3 are n o t available. T h e r e f o r e a c o m p a r i s o n with the p r o p e r t i e s o f the o s m i u m films o b t a i n e d by the d e p o s i t i o n m e t h o d described in this c o m m u n i c a t i o n is n o t possible at this time. In s u m m a r y , starting f r o m OsC14 the C V D process offers a very simple m e t h o d o f p r e p a r i n g high quality crystalline o s m i u m films o r c o a t i n g s o f any desired thickness w i t h o u t inclusions. T h e C V D process is simpler t h a n the o t h e r two d e p o s i t i o n m e t h o d s m e n t i o n e d earlier. S u b s t r a t e s o f any given shape, e.g. discs, cylinders o r wires, can easily be c o a t e d using this process. 1 2 3 4 5 6 7
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