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.


Under active development, no good yields yet. Check lab notes for latest status.


  • 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


  • 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.