Low Temperature Film Growth by Supersonic Jets of CBr 4
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DOUGLAS A. A. OHLBERG, GARRY ROSE, JAMES REN, AND R. STANLEY WILLIAMS Department of Chemistry and Biochemistry, University of California Los Angeles, Los Angeles, CA 90024-1569
ABSTRACT
Pulsed, supersonic jets of CBr4 seeded in a hydrogen bath gas have been used to deposit films on silicon at low temperatures (ca. 1000 C) in a high vacuum chamber. In situ analysis of the films using x-ray photoelectron spectroscopy (XPS) and depth profiling indicate a surface composition of 34% Br and 66 % C and a bulk composition of 88% C and 12% Br. The deposition efficiency of CBr 4 was found to drop dramatically when seeded in bath gases of deuterium, helium, and argon, suggesting that the film growth is an activated process, requiring precursor energies of at least 3.6 eV. 1. INTRODUCTION
A persistant problem with diamond growth through convential CVD methods has been the poor incorporation efficiency of carbon in the growing film. It has been suggested that the efficiency may be improved through the use of supersonic beams. 1 At first glance, the method promises many advantages: the ability to achieve high kinetic energies which can surmount the activation barriers to dissociative chemisorption, increased flux, and the ability to precisely control the amount of material hitting the substrate. Indeed, the method has been attempted for diamond growth 2 , but thus far, results have been discouraging. One possibility for this may be that precursors used were not massive enough to retain the initial kinetic energy they gained during expansion, losing it, instead, through subsequent collisions with bath gas molecules rebounding from the growth substrate to form a boundary layer. Evidence for the efficacy of translational activation in film growth has been seen, however, in the encouraging work of Connolly 4 et al. with chlorinated hydrocarbons. 3 This study, along with others ,5 indicates that the full advantages of supersonic expansion for film growth are best realized by using precursors with large molecular weight and low bond strengths. Thus, CBr 4 is of special interest: the 233 Mat. Res. Soc. Symp. Proc. Vol. 388 01995 Materials Research Society
molecular weight is 331.65 amu and the relatively weak CBr3-Br bonds have been calculated to be 2.44 eV. 6 Although CBr 4 is a solid up to 95 'C, it has a significant vapor pressure over a temperature range easily accessible to a conventional bubbler.
II. EXPERIMENTAL PROCEDURE Figure 1 is a schematic of the apparatus used for the deposition experiments. CBr 4 of 99% purity from Aldrich 7 was used without further purification and packed between glass wool plugs into a small stainless steel container. The quarter inch stainless steel tubing which served as the outlet line for the container led to a pulsed solenoid valve 8 , which was mounted inside a small vacuum chamber (ca. 6 liters) about 1 cm from the substrate. IComputer
3 .olenoid Mechanical Pump
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2. Substrate and sample holder
3. Stainless steel vessel
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