Dry etching and boron diffusion of heavily doped high-aspect ratio Si trenches

The deep etch-shallow diffusion process has been applied to the fabrication of comb drive resonators and micromirrors successfully. Etch rate of Si with various doping concentrations in a Cl ₂ plasma generated by an electron cyclotron resonance source and B diffusion in high aspect ratio Si trenches were characterized. It was found that lightly B and P doped Si were etched at similar rates of 0.17 micrometers/min, whereas heavily B doped p ⁺⁺Si had a slower etch rate of 0.16 micrometers/min and heavily P doped n ⁺⁺Si had faster etch rate of 0.31 micrometers/min. Typical etch conditions are 100 W microwave power and 100 W rf power at 3 mTorr, with 20 sccm of Cl ₂ flow and a source to sample distance of 8 cm. The difference between the p ⁺⁺ and n ⁺⁺Si rate was more significant when etched at higher microwave power, higher rf power, or higher temperature. The depth of a heavily B doped Si layer was measured for different feature sizes, trench openings, and aspect wide trenches to 1.5 micrometers at the bottom of 2 micrometers wide trenches. The diffusion layer on the sides of the trenches for a 30 min B diffusion was 3.25 micrometer thick and it is independent of the trench opening and the trench aspect ratio.