Laser Studies of the SiH Radical/Surface Interaction During Deposition of a thin Film
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LASER STUDIES OF THE SiH RADICAL/SURFACE INTERACTION DURING DEPOSITION OF A THIN FILM PAULINE HO, RICHARD J. BUSS, AND WILLIAM G. BREILAND Sandia National Laboratories, Albuquerque, NM 87185-5800
ABSTRACT This paper presents a new method for studying the interaction of radicals with the surface of a depositing film using a combination of laser spectroscopy and molecular beam techniques. The reactivity of SiH molecules with the surface of a depositing a-Si:H film is measured to be at least 0.95, with no strong dependence on rotational state. INTRODUCTION Understanding the interactions of molecules with surfaces is important for the development and control of many thin-fihn materials processing technologies, including chemical vapor deposition (CVD), plasma-enhanced CVD, and plasma etching. These processes are often controlled by the chemical reactions that occur when gas-phase molecules collide with the substrate. In deposition systems, the starting compounds themselves may be relatively unreactive and the process can depend to a major extent on the generation of gas-phase species that are more reactive with the surface. For example, silane, which is used in both thermal and plasnma-enhanced CVD processes, is relatively inert; reactive sticking coefficients for silane on hot silicon of 10-2-10-5 have been observed [1,2]. However, decomposition of the starting compounds and subsequent gas-plhase reactions produce radical species which can be much more reactive at the surface. Unfortunately, little information is available on the reactivity of such intermediate species with surfaces. In this paper, we introduce a technique that can directly monitor the surface reactivity of an intermediate species from within a complex mixture of molecules and ions colliding with a surface upon which deposition or etching is occuring. The work in this paper will be described in more detail elsewhere [3]. The technique is illustrated by measurements of the interaction of the free radical SiH with the surface of depositing silicon. Using laser-excited fluorescence of molecules in a molecular beam, we are able to measure the reactivity of SiH with the surface of amorphous hydrogenated silicon (a-Si:H) during plasma deposition.
EXPERIMENT Our method for studying the reactivity of a radical with a surface combines um,,ecular beam and laser spectroscopy techniques. An effusive molecular beam produced from a silane plasma impinges on a silicon sample at 30 degrees from the surface nornial. This beam contains virtually all the neutral and ionized species present imithe plasma. A laser beam, directed parallel to the surface and propagating in the principal scattering plane, is tuned to an absorption frequency of a particular molecular species, in this case SiH, and thus selects one chemical component of the beam for study. The laser simultaneously excites both the molecules in the incident beam and molecules scattered from the surface. The spatial dependence of the fluorescence excited by the laser beam is measured by imaging light from the
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