The Formation and Behavior of Particles in Silane Discharges
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Mat. Res. Soc. Symp. Proc. Vol. 557 © 1999 Materials Research Society
nm radius escape the plasma and incorporate into the films in copious quantities.2 These particles constituted 10-'-10' of the film volume, and their surface bonds to the surrounding film exceed the electronic defect density of a-Si:H by many orders of magnitude. Here I will report that those observations were just the tip of the iceberg; when smaller particles not visible in that experiment are included, particles probably constitute 1-10% of the film volume, and their bonds to the surrounding film are a significant fraction of all Si-Si bonds within the film. Thus, these particles have become a major concern, as it appears likely that voids and strained bonds occur at particle-film interfaces, particularly below the particles where subsequent growth cannot easily penetrate. Here I will describe what is now understood about this growth and incorporation of particles into the growing a-Si:H film, and mitigation methods that are suggested by this understanding.
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rf electrode Fig. 1. Diagram of forces on negatively charged particles in a RF, capacitively coupled discharge. Electrostatic forces due to the sheath fields are indicated by large arrows, as these are the strongest forces for small particles. Ion drag results from cation-particle scattering while cations drift toward the electrodes. The gas-drag force increases as R6, 2 whereas the other forces have slower R5 dependence. Thus, large particles are carried away to the pump. MEASUREMENTS OF PARTICLES IN SILANE PLASMAS
Since the original detection of particles in silane plasmas by Roth et al., 3 the most common method of measuring particle suspended in plasmas is still light scattering. However, important insights have also been gained from measurements of negative ions escaping the plasma afterglow, 4 and from scanning tunneling microscope2 and transmission electron microscope (TEM) measurements of particles collected below the electrodes.5 Unfortunately, most of these measurements were not carried out in pure Sill4 or SiH 4 -H2, or for conditions that yield device-quality a-Si:H films. Thus, to understand particle growth under device production conditions, I have utilized primarily the insights, not the particle data, from these experiments. In my laboratory we have concentrated on the conditions and gases used to produce devices, so I will primarily discuss our data. Initially, we studied the accumulation of larger particles (R, >50 nm
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