Photomask Technology
Photomask Technology
A photomask (reticle) is the master pattern that is projected onto the wafer. Mask quality directly limits chip quality.
- DUV masks: Chrome absorber patterns on quartz glass (transmissive). The pattern is typically 4× larger than the wafer image (the projection optics demagnify by 4×).
- EUV masks: Absorber patterns on Mo/Si multilayer reflective substrates. Must have near-zero defects on the multilayer — even buried defects cause printable errors.
A single advanced mask set (all layers for one chip design) costs $10–30 million and takes 2–4 months to produce.
Key Concept: OPC (Optical Proximity Correction)
At sub-wavelength dimensions, the pattern printed on the wafer doesn't look like the mask pattern — optical diffraction distorts it. OPC pre-distorts the mask pattern using computational models so that the printed result matches the intended design. This is a massive compute problem — OPC for one layer can take thousands of CPU-hours.
Mask Defect Management
Mask Defect Management
A single defect on a mask gets printed on every wafer, every exposure — potentially killing millions of dies. Mask quality control includes:
- Mask inspection: Electron-beam and optical inspection tools scan every pixel of the mask for defects.
- Mask repair: Focused ion beam (FIB) or nanomachining tools can remove or add absorber material to fix defects.
- Pellicles: A thin membrane stretched over the mask surface keeps particles at a distance where they're out of focus and don't print. DUV pellicles are standard; EUV pellicles are only recently becoming available.
Mask shops (Photronics, Toppan, DNP) and captive mask shops at large fabs are critical infrastructure for the industry.
Knowledge Check
Knowledge Check
1 / 2What is OPC (Optical Proximity Correction)?