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Deep fused silica wet etching using an Au-free and stress-reduced sputter-deposited Cr hard mask

A technological process for wet chemical etching of microfluidical/microoptical elements in fused silica using a Cr hard mask is presented. The Cr layer is fabricated using sputter deposition under different Ar pressures to reduce the intrinsic stress in the hard mask. Stress reduction was achieved using an Ar pressure of 4.8 × 10−3 mbar, whereas compressive and tensile stress are produced by lower or higher pressures, respectively. An etch depth of 104 µm was achieved without detachment of the mask. The overall lithography process and etching defects caused by non-optimal fabrication parameters are also discussed. Special attention is given to the pressure during sputtering and the resulting mask quality in different stress regimes.