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<http://code.google.com/p/homecmos/wiki/Si2DLithoProcess>
# 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.
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