Etching and boron diffusion of high aspect ratio Si trenches for released resonators

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This article discusses etching and boron diffusion in Si for the purpose of fabricating micromachined devices with high aspect ratio features. The etch rate of Si under various doping conditions in a Cl2 plasma generated by an electron cyclotron resonance (ECR) source was measured. It was found that lightly boron and phosphorus doped Si were etched at rates of 0.17 μm/min, whereas heavily boron doped Si had a similar etch rate of 0.16 μm/min. Si doped with high phosphorus concentrations had a faster etch rate of 0.31 μm/min. These etch rates were measured for Si etched at 100 W microwave power, 100 W rf power, 3 mTorr, with 20 sccm of Cl2 flow, and an ECR source to sample distance of 8 cm. The difference between the p++ and n++Si etch rates was more significant when etched at higher microwave power, higher rf power, or higher temperature. The depth of a heavily doped boron diffusion layer was measured for different feature sizes, trench openings, and aspect ratios. The diffusion layer thickness was found to decrease from 2.9 μm at the bottom of 50-μm-wide trenches, to 1.5 μm at the bottom of 2-μm-wide trenches. Diffusion thickness at the bottom of trenches near narrow features was found to increase to 3 μm compared to 2.5 μm for trenches near large features. The diffusion layer on the sides of the trenches for a 30 min boron diffusion at 1175 °C was 3.25 μm thick and was found to be independent of the trench opening and the trench aspect ratio. A deep etch-shallow diffusion process was used to fabricate thick released resonators with a short boron diffusion time. The second dry etch in the deep etch-shallow diffusion process, which is used to etch the p++ layer at the bottom of adjacent features, was studied. It was found that this etch successfully removed the p++ layer at the bottom of Si trenches in addition to widening the slightly tapered profile near the bottom of the trenches. This process has been applied to the fabrication of micromachined comb drive resonators and micromirrors successfully.