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APPENDIX B    EQUIPMENT AND PROCESS DETAILS

B.1    Wafer and Photomask Details

The four-inch SOI wafers used for 3DMEMS and thermal actuator fabrication were procured from SEH America, Inc. The wafers with the 50 mm active layer had an orientation of <100> and were p-type doped with boron to a resistivity of 0.0133 W-cm.

The photomasks are five-inch low-expansion glass plates with chromium patterns printed on the surface. Separate light-field photomasks were produced for the sld, oop, cap, and thermal actuator layers by DuPont Photomasks, Inc. from AutoCAD drawings.

B.1    Process Details

This section consists of detailed information used for 3DMEMS and thermal actuator fabrication.

Photolithography

Chemicals: Acetone, methanol, DI water, HDMS adhesion promoter, AZ9245 photoresist, AZ400K developer

Procedure: Clean wafer with acetone, methanol, DI water; spin dry.
Dehydrate wafer at 150 °C for 30 minutes.
Cover wafer with HDMS; let wafer sit for 1 minute; spin off.
Cover wafer with AZ9245; spin at 4000 RPM for 30 seconds (4–5 mm layer typical).
Bake wafer at 110 °C for 2 minutes.
Align if necessary; expose wafer at 10 mW/cm² for 110 seconds (1100 mJ/cm²).
Develop in diluted AZ400K (1 part AZ400K, 4 parts DI water) for 3–5 minutes.

Remarks: HDMS layers will build up. Do not apply HDMS more than once to a wafer without an intermediate acid clean.

RCA1 cleaning

Chemicals: DI water, NH₄OH, 30% H₂O₂

Procedure: Heat 5 parts DI water and 1 part NH₄OH to 70 °C.
Add 1 part H₂O₂ and immediately immerse wafer for 5 minutes.
Quench wafer in DI water.

Remarks: RCA1 dissolves metal contaminants (group Ib, IIb, Au, Ag, Cu, Ni, Cd, Co, and Cr). H₂O₂ reduces roughening of the silicon surface from the NH₄OH by continuously creating an oxide layer. Do not exceed 10 minutes or 70 °C to prevent dissociation of the H₂O₂.

Reference: [19]

RCA2 cleaning

Chemicals: DI water, HCl, 30% H₂O₂

Procedure: Heat 5 parts DI water and 1 part HCl to 70 °C.
Add 1 part H₂O₂ and immerse wafer for 5 minutes.
Quench wafer in DI water.

Remarks: RCA2 removes alkali ions, NH₄OH-insoluble hydroxides, and residual trace metals. Chemical proportions, cleaning time, and temperature can be varied much more with RCA2 than with RCA1. RCA2 does not etch silicon or oxide.

Reference: [19]

Piranha cleaning

Chemicals: 98% H₂SO₄, 30% H₂O₂, DI water

Procedure: Heat 3 parts H₂SO₄ to 130 °C.
Add 1 part H₂O₂ and immerse wafer for 10 minutes.
Vigorously rinse in DI water to remove viscous liquid.

Remarks: Can use 2–4 parts H₂SO₄. Piranha removes heavy organic contaminants such as photoresist but does not desorb inorganic materials such as metals. The remaining impurity-containing oxide film can be removed by a brief dip in diluted HF; however, an HF solution that is not ultra-pure will contaminate the reactive bare silicon surface.

Reference: [19]

Thermal evaporation

Equipment: Cooke thermal evaporator

Parameters: 1–2 mTorr, 5–20 Å/sec deposition rate

Reactive ion etching

Equipment: Trion Minilock RIE

Parameters: 100 W, 300 mTorr, 50 sccm SF₆

Remarks: 5000–8000 Å/min etch rate observed

Deep reactive ion etching

Equipment: PlasmaTherm DRIE

Remarks: Bosch process [10], 0.4–0.5 mm/loop

HF release

Chemicals: 49% HF, DI water, isopropyl alcohol (IPA)

Procedure: Immerse wafer in HF until desired structures are released.
Immerse in DI water for 10 minutes; repeat with fresh DI water.
Immerse in IPA for 10 minutes; repeat with fresh IPA.
Dry on hot plate >90 °C for 2 minutes.

Remarks: Approximate 1 mm/min HF etch rate of oxide observed.

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