Excimer Laser Induced Photochemistry of Silane-Ammonia Mixtures
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EXCIMER LASER INDUCED PHOTOCHEMISTRY OF SILANE-AMMONIA MIXTURES J. M. JASINSKI, D. B. BEACH and R. D. ESTES IBM Research Division, Thomas J. Watson Research Center, Yorktown Heights, NY 10598
ABSTRACT Steady-state and time resolved molecular beam sampling mass spectrometry have been used to study the ArF excimer laser induced photochemistry of silaneammonia mixtures at 193 nm. Under both steady state and single laser shot conditions, products as complicated as tetraaminosilane are formed promptly. A mechanism which accounts for the formation of all observed products is proposed and evaluated.
INTRODUCTION Argon fluoride excimer laser photolysis of silane-ammonia mixtures at 193 nm has been demonstrated as a possible low temperature large area photo-CVD technique for the deposition of amorphous hydrogenated silicon nitride thin films [1]. Since silane is for all practical purposes transparent at 193 nm [2], the photodeposition process is initiated by the well known photodissociation of ammonia [3]. The major photoproducts are hydrogen atoms and amidogen radicals, eq. 1. NH 3
-•
NH 2 + H
(1)
Hydrogen atoms are known to react with silane to produce silyl radicals [3], eq 2., while NH 2 is less reactive with silane [5]. SiH 4 + H -o
SiH3 + H 2
(2)
Eqs. 1 and 2 result in the formation of reactive gas phase species containing both silicon and nitrogen, and it is possible that film growth proceeds directly from the impingement of these species on surfaces. It is also possible that these radicals lead to gas phase chemistry which produces molecules containing silicon-nitrogen bonds, and that those species are the immediate film growth precursors. In this paper we report preliminary results of a mass spectrometric study of the 193 nm photochemistry of silane-ammonia mixtures. We find that the gas phase radical chemistry following photolysis is complicated and leads, unexpectedly, to the formation of aminosilane molecules as large as Si(NH 2 )4 . Mass spectrometric studies of the glow discharge chemistry [6] and of the mercury sensitized photochemistry [7] of silane-ammonia mixtures have recently been reported. Our results for direct photolysis are strikingly similar to the glow discharge results and markedly different form the mercury sensitized photochemical results. Mat. Res. Soc. Symp. Proc. Vol. 131. 91989 Materials Research Society
502
EXPERIMENTAL A schematic of the excimer laser photolysis/molecular beam sampling mass spectrometer is shown in Figure 1. In typical experiments, silane and ammonia, 1-25 mTorr each, dilute in helium buffer gas at total pressures of 0.5-1.0 Torr and ambient temperature are photolyzed in the flow cell with the unfocused output of an ArF excimer laser operating at repetition rates of 1-30 s-1. The flow cell is operated at constant total gas flow and constant total pressure. Gas from the flow cell is formed into a molecular beam by a sampling orifice, and the beam is directed into the ionizer region of a differentially pumped quadrupole mass spectrometer. For steady state studies
