(530d) High-Pressure Chemical Deposition of Silicon In Extreme Aspect-Ratio Micro-Capillaries

Semiconductor waveguides of nanoscale to microscale dimensions have recently been made by chemical vapor deposition within extreme aspect-ratio microcapillaries under high pressure flow of a silane/helium mixture. The microcapillary is heated in a furnace, causing silane to decompose at the capillary wall to produce silicon and hydrogen. Experimental observations show that the thickness of the deposited silicon layer varies greatly with axial position within the microcapillary. As the annular deposited film grows thicker, the central hole formed at the cross-section with the highest film growth rate becomes smaller until it is completely plugged. Complete filling of the plugged central channel over lengths of centimeters or more is challenging in view of its extreme aspect ratio. Assuming silane decomposition to be a first-order reaction, we use a compressible flow model to examine the observed non-uniformity in the thickness of the deposited semiconductor film. The process to completely fill the pore consists of three phases. In phase I, there is little or no constriction of the pore, and semiconductor deposition has little impact on fluid flow. In phase II, the narrowing of the pore cross-section produces supersonic flow beyond the constriction at the high operating pressures of about 35 MPa.  The coupling between flow and temperature-dependence of the rate of deposition gives rise to an instability that eventually leads to plugging of the pore. In phase III, the pore is largely or completely closed, and a large axial concentration gradient arises within the pore as silica selectively filters silane from helium and hydrogen, due to its permeability to the latter components at high temperature.  The precursor filtering effect favors deposition several centimeters upstream of the closed end of the pore, leading to complete filling of the large aspect-ratio pore.