STEP 1 • THE WAFER

Start with silicon

Every PolyMUMPs run starts from the same wafer specification, so every customer's design shares the same electrical starting point.

150 mm
n-type (100)
1–2 Ω·cm

The wafer is 150 mm across, n-type silicon with a (100) crystal orientation, and 1–2 Ohm-cm resistivity.

STEP 2 • DOPE & INSULATE

Doping and the nitride layer

The wafer surface is first doped with phosphorous using standard diffusion. This reduces charge feed-through to the substrate from electrostatic devices built on the surface.

Next, a 0.6 µm low-stress silicon nitride layer is deposited by LPCVD for electrical insulation.

SiN — 0.6 µm
━━━━━━━━━━━
Si (doped)

Silicon nitride keeps your structures electrically separate from the substrate underneath them.

STEP 3 • POLY 0

Poly 0: the ground plane

A 0.5 µm polysilicon film, called Poly 0, is deposited by LPCVD and patterned with the first mask. It's commonly used for wiring and as stationary electrodes underneath moving structures.

Poly0 — 0.5 µm
━━━━━━━━━━━
SiN — 0.6 µm

Poly 0 doesn't move. It's the fixed electrode layer that later moving parts will act against.

STEP 4 • FIRST OXIDE

PSG1: the first sacrificial layer

A 2.0 µm PSG (phosphosilicate glass) layer — called First Oxide — is deposited by LPCVD and annealed at 1050°C for 1 hour in argon. It's removed at the very end of the process to free the first mechanical layer.

DIMPLES mask

Etches shallow dimples (about 750 nm deep) into the PSG, creating small standoffs so beams don't stick flat against Poly 0.

ANCHOR1 mask

Etches holes through the PSG down to Poly 0, later filled by Poly 1 to anchor it in place.

Everything about this oxide is temporary — it just holds the shape until release.

STEP 5 • POLY 1

Poly 1: the first moving layer

A 2.0 µm polysilicon layer is deposited and capped with a thin 200 nm PSG layer. The wafer is annealed at 1050°C for 1 hour — doping Poly 1 with phosphorous from the PSG above and below, and relieving internal stress at the same time.

The PSG cap then acts as a hard mask, giving a cleaner pattern transfer than photoresist alone when Poly 1 is etched.

Poly1 — 2.0 µm
PSG1 (sacrificial) — 2.0 µm
Poly0 — 0.5 µm
SiN — 0.6 µm

Poly 1 fills the ANCHOR1 holes from the last step, locking it to Poly 0 wherever you designed an anchor.

STEP 6 • SECOND OXIDE

PSG2: connecting Poly 1 to Poly 2

A second, 0.75 µm PSG layer — Second Oxide — is deposited and annealed. Two separate masks pattern it, each with a different job.

POLY1_POLY2_VIA

Etches holes down to Poly 1, creating a mechanical and electrical connection between Poly 1 and Poly 2.

ANCHOR2

Etches through both First and Second Oxide in one step, so Poly 2 can anchor straight down without misalignment.

Cutting ANCHOR2 through both oxides in a single etch avoids misalignment between two separately aligned holes.

UNIT 14 STUDY COMPLETE

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Ready for the Fab Challenges?

You've covered Building the Base & Sacrificial Layers.