# purpose This process is used for patterning a silicon substrate by wet etching with KOH or TMAH. Instead of LPCVD Si3N4 the hardmask is Ta2O5 deposited by a sol-gel process. This hardmask is patterned by wet etching with HF through a copper hardmask. Sidewalls of the etch are vertical for <110> and approximately 54.7 degrees to the surface for <100>. There is no significant etching of <111> planes. # status Under active development, no good yields yet. Check lab notes for latest status. # materials * Suitable substrate (<100> or <110> Si) * Tantalumfilm (16% wt. Ta2Cl10 in ethanol/methanol) * Distilled water (DI preferred) * KOH flakes (Other literature suggests TMAH may be substituted if K+ contamination is a concern, such as for CMOS. We currently have little information on the properties of TMAH, details are welcome.) * 3% HF * Copper wire * Chromium wire * Filament evaporator * Acetone * Compressed air, N2, duster spray, or equivalent * Lab oven * Lint-free swabs * Hot plate * Spin coater * Plastic centrifuge tubes * Concentrated HCl * 3% hydrogen peroxide # process * Thoroughly clean substrate. A full RCA clean is not necessary however it is critical it be as free from particulates as possible. * Place substrate on spin coater. Apply one drop of Tantalumfilm and immediately spin (optimal parameters still being determined). * Bake in air at 200C for 60 minutes. * Evaporate 5-10nm Cr adhesion layer followed by 100-200nm of Cu. * Spin coat with 50% dilution of SP24 photoresist in acetone, soft bake * Expose positive mask using standard photolithography process, develop * Prepare dilute SC2 etch: 0.1 ml conc. HCl, 0.5 ml 3% H2O2, 5 ml water. * Wet etch metal layers in dilute SC2 until Ta2O5 is exposed. * Place substrate in centrifuge tube, add 1 ml 3% HF. * Place tube in water bath at 60C and etch 5 mins or until Ta2O5 layer is penetrated * Remove tube from heat, suction HF. * Distilled water rinse. * Prepare solution of 30% by weight KOH in distilled water. * Heat KOH solution to 85C on hot plate. * Etch substrate in KOH until desired etch depth is achieved (TODO: link to etch rate chart). * Optional dilute HCl rinse to ensure non-alkaline pH. * Distilled water rinse.