Projection-patterned etching of silicon in chlorine atmosphere with a KrF excimer laser

The formation of relief features in silicon by a one-step process that avoids resist patterning has been achieved by laser-projection-patterned etching in a chlorine atmosphere. Etching is performed with a pulsed KrF excimer laser (λ=248 nm, τ=15 ns) and deep UV projection optics having an optical resolution of 2 μm. Etching takes place in two steps. Between laser pulses, the silicon surface is covered with a monolayer of chemisorbed chlorine atoms (one Cl per Si). During the laser pulse, surface transient heating at temperatures in excess of 1250 K results in the desorption of the reaction products (mainly SiCl₂). At laser energy densities that induce surface melting, this desorption results in a saturated etch. rate of 0.06 nm per pulse, corresponding to the removal of about 0.5 Si monolayer per pulse. At densities below the melting threshold, reduced thermal and possibly a small amount of photochemical etching result in lower etch rates. Projection of a resolution test photomask onto the silicon surface shows that the size of etched features differs from the size of the projected features and strongly depends on the laser energy density. As a result of the heat spread in silicon and of the highly nonlinear character of the etching reaction, etched features smaller than the irradiated area are obtained at all fluences in the range 350–700 mJ/cm². Etched lines having a width down to about 1.3 μm were produced. Proximity effects due to heat spread were also evidenced for small projected features (<4 μm). The characteristics of the etched patterns are compared with those obtained for GaAs etching in chlorinated gases with the same experimental set-up. Significant differences in pattern resolution for Si and GaAs etching are observed. This variation in resolution is believed to result from the fact that Si has a greater thermal diffusivity than GaAs.