Describe the principle and process of photolithography used in MEMS fabrication.

Question

Accepted Answer

## Photolithography in MEMS fabrication

**Principle:** a light-sensitive polymer (photoresist) is exposed to UV light through a patterned mask. Exposure changes the resist's solubility, so after development the mask pattern remains as a resist stencil on the substrate. The stencil then protects the surface during etching or deposition.

```mermaid
flowchart TD
    A[Clean wafer and dehydration bake] --> B[Spin-coat photoresist]
    B --> C[Soft bake]
    C --> D[Align mask and UV exposure]
    D --> E[Develop]
    E --> F[Hard bake]
    F --> G[Etch or deposit through the resist]
    G --> H[Strip resist]
```

### Process steps
1. **Cleaning:** remove particles and organics, bake off moisture, add an adhesion promoter (HMDS) if needed.
2. **Spin coating:** resist thickness is set by viscosity and spin speed.
3. **Soft bake:** drives off solvent so the film is stable.
4. **Alignment and exposure:** the mask is aligned to wafer marks and UV passes through its clear areas.
5. **Development:** the developer dissolves the soluble regions.
6. **Hard bake:** hardens the resist and improves etch resistance.
7. **Pattern transfer:** etch the exposed film, or deposit and lift off.
8. **Resist strip:** remove the remaining resist.

### Positive and negative resist
| | Positive | Negative |
|---|---|---|
| Exposed area | Becomes soluble | Becomes insoluble (cross-links) |
| Pattern | Same as the mask | Inverse of the mask |
| Resolution | Higher | Lower, but thick films possible |
| Example | AZ and S1800 series | SU-8 |

### Resolution
Minimum feature size R ≈ k₁λ/NA for projection systems, so a shorter wavelength gives finer features.

### Exam tip
Draw the step-by-step cross-section figure and label resist, mask, UV and substrate.

Photolithography in MEMS fabrication

Process steps

Positive and negative resist

Resolution

Exam tip

Similar questions

Coursify Android App