Explain UV lithography for SU-8. Describe the major steps involved in pattern formation.

Question

Accepted Answer

## UV lithography with SU-8: pattern formation

**SU-8** is an epoxy-based, chemically amplified **negative** photoresist (EPON SU-8 resin with a photoacid generator in a solvent). UV exposure releases acid that triggers cross-linking during the post-exposure bake. It gives thick films and very high-aspect-ratio structures.

```mermaid
flowchart TD
    A[Clean and dehydrate substrate] --> B[Spin-coat SU-8]
    B --> C[Soft bake]
    C --> D[UV exposure through mask]
    D --> E[Post-exposure bake]
    E --> F[Develop in PGMEA]
    F --> G[Rinse with IPA and dry]
    G --> H[Optional hard bake]
```

### Steps
1. **Substrate preparation:** clean and dehydrate; an adhesion layer may be used.
2. **Spin coating:** spin speed and SU-8 grade set the thickness (about 1 µm to several hundred µm).
3. **Soft bake:** evaporates solvent; a slow ramp avoids cracking.
4. **UV exposure:** near-UV (i-line, 365 nm) through a mask; thicker films need a larger dose.
5. **Post-exposure bake (PEB):** acid in the exposed region diffuses and cross-links the epoxy.
6. **Development:** unexposed SU-8 dissolves in a developer such as PGMEA.
7. **Rinse and dry:** isopropanol rinse, nitrogen dry.
8. **Hard bake (optional):** improves stability and reduces cracks.

### Why SU-8 for BioMEMS
- Aspect ratios above 20:1 are achievable.
- Chemically and thermally stable once cured.
- Used as the master mould for PDMS soft lithography and for permanent channels.

### Exam tip
Stress that SU-8 is negative: the exposed region stays and the unexposed region dissolves.

UV lithography with SU-8: pattern formation

Steps

Why SU-8 for BioMEMS

Exam tip

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