Formation threshold and structural evolution of molybdenum nanocrystals with sputtering pressure
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C. L. Chien Department of Physics and Astronomy, Johns Hopkins University, Baltimore, Maryland 21218
A. S. Edelstein Naval Research Laboratory, Washington, DC 20375 (Received 9 July 1990; accepted 11 September 1990)
The evolution of the size and shape of molybdenum nanocrystals fabricated by sputtering in a thermal gradient has been studied as a function of the argon gas pressure,/?. For 4 < p < 100 mTorr, continuous Mo films are deposited. At p = 150 mTorr, isolated and well-faceted Mo nanocrystals of two sizes (20 and 5 nm average size) are formed. For 200 =S p ^ 400 mTorr, the particle size decreases with increasing pressure and is about 7 nm at 400 mTorr. On increasing/? further, larger particles start to form and aXp = 700 mTorr, particle agglomerates are observed. Possible mechanisms leading to these results are suggested.
The sputtering gas pressure utilized in conventional sputter deposition of thin films and coatings is usually only a few mTorr. Studies by Thornton have provided the most detailed systematic investigation in the low pressure range.1 It has been reported recently that sputtering at higher pressures produces powder materials which are formed in the vapor.2 Using this new technique, we have developed a new method for fabricating nanocomposite materials, specifically Mo particles in an Al matrix, by alternately sputtering Mo at high pressures (200 to 600 mTorr) and Al at 2 mTorr.3 To date, there has not been a systematic study of sputtering at high pressures. In this communication, we report some unexpected effects that we observed upon varying the Ar sputtering pressure, p, for 4 mTorr < p =S 700 mTorr in a triode magnetron sputtering system, with the other deposition parameters kept constant. With increasingp we have observed the following: (1) a threshold for particle formation occurs at p ~ 150 mTorr; (2) at the threshold two sizes of well-faceted nanocrystals are formed (20 and 5 nm); (3) for 200 ^ p ^ 400 mTorr, the particle size decreases with increasing pressure; (4) particle agglomeration occurs at p > 600 mTorr. Thus we have followed the evolution of particle formation in the vapor as a function of p, from the threshold for particle formation to particle agglomeration at high pressure. These results demonstrate that by varying sputtering pressure one can control the morphologies of the particles to fabricate many new potentially useful materials. "'Research Associate of the National Research Council, National Academy of Sciences. 8
J. Mater. Res., Vol. 6, No. 1, Jan 1991
Molybdenum was dc sputtered for 5 min using a 2.25 inch L.M. Simard triode-magnetron source at a sputtering voltage of 300 V (the equivalent power is 150 W). The substrates, carbon coated transmission electron microscopy (TEM) grids, were on a liquid nitrogen-cooled table located 10 cm above the sputtering target. The thermal gradient between the substrate and the target enhances the deposition rate of particles formed in the vapor.3 The base pressure was 10 7 Torr. The deposits were analyzed by TEM. Though the t
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