Nucleation Barriers in Chemical Vapor Deposition of Triisobutylaluminum on Silicon
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NUCLEATION BARRIERS IN CHEMICAL VAPOR DEPOSITION OF TRIISOBUTYLALUMINUM ON SILICON D.A. MANTELL Xerox Webster ResearchCenter, Webster, NY 14580
ABSTRACT The nucleation of chemical vapor deposition (CVD) using triisobutylaluminum (TIBA) on Si (100) surfaces is observed in situ with x-ray photoelectron spectroscopy (XPS). Oxygen from oxide on the silicon inhibits the rate of nucleation by reacting with adsorbed TIBA and forming a thin layer of oxidized organometallic. This layer blocks active adsorption sites and prevents further deposition. On a surface without oxide, the TIBA molecules decompose liberating aluminum that can migrate and nucleate into islands opening sites for further adsorption and film growth. By removing the oxide (native or thermal) in selected areas of the surface, the barrier to nucleation is removed and aluminum deposition can occur in a predetermined pattern. INTRODUCTION When the initial nucleation step of chemical vapor deposition (CVD) film growth is slow, slower than the subsequent film growth, poor film morphologies are usually the result. Differences in the time of the initial nucleation across the surface result in film thickness variations by the rapid film growth. Triisobutylaluminum (TIBA) chemical vapor deposition on silicon surfaces is known to exhibit this behavior[1,2]. In order to avoid poor quality films, the surface can be pretreated in various ways to lower this "nucleation barrier" before CVD[1,2]. In CVD deposition with TIBA on silicon it has been shown that if this pretreatment is done in an area selective manner, patterned CVD growth is possible[2-51. This has been demonstrated with UV lasers which are used to initiate film growth by direct writing[3,4] or projecting[5] a pattern which grows by CVD (as long as the surface temperature is low enough to prevent nucleation in untreated parts of the surface). The initial nucleation of CVD film growth for TIBA/silicon is examined in this paper. There is a vital role played by surface oxygen, from silicon's native oxide, in the adsorption of TIBA, the dissociation of TIBA, and the nucleation of CVD growth. As with0 WF6/silicon[6], TIBA is found to selectively deposit (at low temperature 300 C) on silicon and not to deposit on silicon oxide. This is possible because film nucleation is suppressed by the surface chemical processes that occur at the organometallic silicon interface. In particular, the silicon (native) oxide is reduced by the aluminum[7], leaving an aluminum oxide (and carbon) layer that suppresses adsorption of more TIBA that is necessary for further aluminum deposition. EXPERIMENT The ultrahigh vacuum (UHV) system used for these experiments was pumped by a 500 I/sec ion pump and titanium sublimation pump (base pressure of 1 x 10-10 torr). Since it is impractical to passivate the walls of the UHV chamber with organometallics, a doser introduced the gas so that it hit the surface before the walls of the chamber. The effective pressure at the surface was 1 x 10-7 torr while the chamber pressure remained at 1 x 10-8
