photomask
**Photomask / Reticle** — the master template containing the circuit pattern that is projected onto the wafer during lithography, the most expensive and critical consumable in semiconductor manufacturing.
**What It Is**
- Ultra-flat quartz plate (6" × 6" × 0.25") coated with patterned chrome (or phase-shifting material)
- Pattern is 4x larger than what prints on wafer (4x reduction lithography)
- One mask per layer: A modern chip needs 60–100+ masks (one full "mask set")
**Mask Types**
- **Binary mask**: Chrome on glass (opaque/transparent). Simplest
- **Phase-shift mask (PSM)**: Adds 180° phase shift to improve resolution. Required at advanced nodes
- **EUV mask**: Reflective (not transmissive) — multilayer Mo/Si mirror with absorber pattern
**Mask Cost**
| Node | Mask Set Cost | Masks per Set |
|---|---|---|
| 28nm | $2–5M | 40–50 |
| 7nm | $15–20M | 80+ |
| 3nm | $30–50M | 90–100+ |
| 2nm | $50M+ | 100+ |
**Mask Making**
- Written by electron-beam lithography (extremely slow — hours per mask)
- Inspected for defects at nanometer resolution (KLA, Lasertec)
- Defects repaired by focused ion beam (FIB) or nanomachining
- Stored in ultra-clean, humidity-controlled environments
**Photomasks** are the blueprints of the chip — a single defect on a mask prints on every die across every wafer, making mask quality absolutely critical.