STEP 7 • POLY 2

Poly 2: the second moving layer

A 1.5 µm polysilicon layer is deposited with another 200 nm PSG cap, annealed for about 1 hour at 1050°C, then patterned with the seventh mask and etched by plasma and RIE.

Poly2 — 1.5 µm
PSG2 (sacrificial) — 0.75 µm
Poly1 — 2.0 µm
PSG1 (sacrificial) — 2.0 µm
Poly0 — 0.5 µm
SiN — 0.6 µm

Two structural layers means you can build things Poly 1 alone can't — like a moving mirror sitting on top of a fixed hinge.

STEP 8 • METAL

The metal layer

A final 0.5 µm metal layer is patterned with the eighth mask and deposited using lift-off. It provides probing pads, wire bonding, electrical routing, and highly reflective mirror surfaces.

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This is the layer that turns a mechanical structure into something you can wire up, test, and — for mirrors — actually see light bounce off.

THE COMPLETE STACK

Seven layers, eight masks

Metal 0.5 µm

Poly2 1.5 µm

PSG2 (sacrificial) 0.75 µm

Poly1 2.0 µm

PSG1 (sacrificial) 2.0 µm

Poly0 0.5 µm

Silicon Nitride 0.6 µm

From substrate up: insulate, wire, hold, build, connect, build again, finish.

STEP 9 • RELEASE

Dissolving the scaffolding

Wafers are diced, sorted, and shipped to the customer unreleased. Release is performed by the customer, dissolving both sacrificial PSG layers.

1. HF bath 49% HF, room temperature, 1.5–2 minutes

2. DI water rinse Several minutes

3. Alcohol rinse Reduces stiction

4. Oven bake 100°C for 10 minutes

The sacrificial oxide's whole job was to disappear at this step. Whatever is left afterward is your device.

UNIT 15 STUDY COMPLETE

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Ready for the Fab Challenges?

You've covered Structural Layers, Metal & Release.