Process Monitoring for a-Si:H Materials and Interfaces
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PROCESS MONITORING FOR a-Si:H MATERIALS AND INTERFACES R.W. COLLINS AND J.M. CAVESE BP America Research and Development. 4440 Warrensville Center Road, Cleveland. OH 44128 ABSTRACT Real time ellipsometry results are presented which have been useful in characterizing nucleation, interface formation, and surface modification for hydrogenated amorphous silicon (a-Si:H) thin films and device structures. Earlier studies of the effect of deposition procedure on a-Si:H nucleation are reviewed, and new data on plasma-enhanced CVD p-type a-Si:H are discussed. Changes in the initial microstructural coalescence are observed when gas phase concentrations of 0.2% B2 H6 are added to SiHl4 to obtain the p-type films. When depositing p-type a-Si:H on undoped a-Si:H at 180"C and 250°C. ellipsometry allows detection of film formation at the rate of a few monolayers/minute by CVD (in the absence of plasma excitation) from the doping gas mixture. CVD requires both SiHl4 and B 2H in the gas. Other interesting aspects of this process are presented. Finally. in situ elfipsometry is applied with H2 plasmas and ion beams to study near-surface Si-Si bond breaking and etching. A common effect of many of these treatments is a reduction in the density of Si-Si bonds in the film bulk by 6-8%. INTRODUCTION AND SUMMARY In situ ellipsometry experiments have been effective in characterizing the microstructural evolution of thin film hydrogenated amorphous silicon (a-Si:H) in the initial nucleation process,[I-3] in the later stages of growthJ4] and upon exposure to inert[3.5] and reactive gas plasmas.[3,6] When probing a-Si:H in the near ultraviolet, ellipsometry provides information on the packing density of Si-Si bonds throughout the absorption depth. typically the top few hundred R of the film.[7J Si-Si bonds dominate the contributions to a-Si:H polarizability in this spectral region. As a result, bulk density deficits as a function of preparation or process parameters can be deduced from a-Si:H optical data with the Bruggeman effective medium approximation (EMA)[8] using mixtures of void and reference standard a-Si:H of high density. Similarly. surface roughness, which may arise when initial nuclei merge incomp letely or when the film is exposed to ion bombardment, can also be deduced.191 Thin roughness (< 50 R) is modeled as a single layer with optical properties determined from the EMA assuming - 0.5/0.5 volume fractions of bulk a-Si:H/void. In this paper, in situ ellipsometry results in two areas of recent interest are reported. In the first part, the effect of p-type doping (with B 2 H6 ) on the nucleation and microstructural evolution of plasma-enhanced chemical vapor deposited (PECVD) a-Si:H is discussed. Interesting results on the formation of interfaces between intrinsic and p-type material are also described. In the second part. the surface and bulk modifications resulting when a-Si:H is exposed to H plasmas and ion beams are discussed. These two systems are of interest owing to the quasi-equilibrium in the growth process that determine
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