Fabrication of a hard mask for InP based photonic crystals: Increasing the plasma-etch selectivity of poly(methyl methacrylate) versus SiO2 and SiNx

We introduce cyclic reactive ion etching processes for SiO2 and SiNx hard masks where the fluorine-based etch steps are interleaved with N2 flushing steps in order to improve the selectivity to electron-beam resists. For SiO2 etching an etch-step duration of 30s resulted in a doubled selectivity of almost 4:1 between SiO2 and poly(methyl methacrylate) (PMMA) due to a reduced thermal load. We established the pattern transfer from a 200nm thick PMMA resist into a 600nm thick SiO2 layer for 200nm diameter holes. For SiNx etching we demonstrate improved sidewall verticality, an enhanced etch rate, and suppressed redeposition of etch byproducts for a cyclic process. With the use of an additional 30nm titanium intermediate layer we show an excellent overall selectivity between SiNx and PMMA of almost 5:1. This process is applied to the fabrication of planar photonic-crystal devices with 3.5μm deep holes in an InP based slab waveguide with an initial PMMA layer thickness of 220nm.