Aluminum Chemical Vapor Deposition Using Triisobutylaluminum: Mechanism, Kinetics, and Deposition Rates at Steady State
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ALUMINUM CHEMICAL VAPOR DEPOSITION USING TRIISOBUTYLALUMINUM: MECHANISM, KINETICS, AND DEPOSITION RATES AT STEADY STATE BRIAN E. BENT*, LAWRENCE II. DUBOIS**, AND RALPH G. NUZZO** *Current address: Department of Chemistry, Columbia University, New York, NY 10027. **AT&T Bell Laboratories, Murray Hill, New Jersey 07974. ABSTRACT An important step in the chemical vapor deposition (CVD) of aluminum from triisobutylaluminum (TIBA) is the reaction between TIBA (adsorbed from the gas phase) and the growing aluminum surface. We have studied this chemistry by impinging TIBA under collisionless conditions in an ultra-high vacuum system onto single crystal Al( 11) and Al(100) substrates. We find that when TIBA (340K) collides with an aluminum surface heated to between 500 and 600K, the aluminum atom is cleanly abstracted from this precursor with near unit reaction probability to deposit, epitaxially, carbon-free aluminum films. The gas phase products are isobutylene and hydrogen. From monolayer thermal desorption experiments, we have determined the kinetic parameters for the rate-determining step, a 03-hydride elimination reaction by surface bound isobutyl ligands. Using these kinetic parameters and a Langmuir absorption model, we can predict the rate of aluminum deposition at pressures ranging from 10-6 to 1 Torr. INTRODUCTION In the late 1950's, Ziegler and coworkers found that aluminum alkyl compounds, particularly triisobutylaluminum (AI(C 4 H 9 )3 , TIBA) and diisobutylaluminum hydride (Al(C 4 Hq) 2 H, DIBAH) could be decomposed thermally to deposit high purity (>99 atom %) aluminum films [1]. The overall chemistry, as determined by analysis of the gas phase products, is as described in the following scheme:
TIBA
Al
/,
U H DIBAH
Al
/.H2
'Al SUBSTRATE (500 - 600K)
+
=
There are three significant features to note about this reaction. First, the overall process is reversible, which lead Ziegler et al. to suggest the potential of this system for refining aluminum. Second, the conversion of the isobutyl ligands into isobutylene can occur at least in part in the gas phase. Third, the reaction mechanism for conversion of the aluminum isobutyl moieties to isobutylene and hydrogen was judged to be a 03-hydride elimination reaction; trimethylaluminum, which lacks 13-hydrogens, decomposes thermally to produce aluminum carbide. Mat. Res. Soc. Symp. Proc. Vol. 131. t1989 Materials Research Society
328
This aluminum deposition system was largely ignored [2] although interest was revived somewhat in 1982 when Cooke et al. demonstrated its potential to deposit conformal aluminum films for conductive contacts on silicon-based microelectronics devices [3]. Subsequent studies showed that the nucleation of these CVD aluminum films was strongly influenced by chemical pretreatments of the silicon substrates [4]. In order to examine the role of DIBAH and TIBA in this deposition, determine the relative importance of gas phase vs. surface chemistry, and characterize the surface reactions involved in the nucleation of film growth on
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