Explain the basic principle of lithography.

Question

Accepted Answer

## Basic principle of lithography

Lithography (Greek: lithos = stone, graphein = to write) transfers a designed pattern from a master (mask or beam) into a radiation-sensitive layer, the **resist**, on a substrate. The resist is then a temporary stencil for etching, deposition or doping.

```mermaid
flowchart LR
    A[Pattern design] --> B[Mask or direct write]
    B --> C[Expose resist to radiation]
    C --> D[Develop]
    D --> E[Resist stencil on substrate]
    E --> F[Etch, deposit or implant]
```

### Working idea
Radiation (UV, electron beam, X-ray) changes the chemistry of the resist.
- **Positive resist:** the exposed region becomes soluble and is washed away.
- **Negative resist:** the exposed region cross-links and stays.

### Types of lithography
| Type | Radiation | Feature |
|---|---|---|
| Photolithography | UV light (g-line, i-line, DUV) | Workhorse of MEMS; low cost |
| Electron-beam | Focused electrons | Nanometre resolution; serial, slow, no mask |
| X-ray (LIGA) | Synchrotron X-rays | Very tall, high-aspect-ratio structures |
| Soft lithography | Elastomer stamp (PDMS) | Cheap microfluidics, replica moulding |
| Nanoimprint | Mechanical embossing | High-resolution replication |

### Key parameters
Resolution R ≈ k₁λ/NA, depth of focus, alignment accuracy, resist sensitivity and contrast.

### Exam tip
Define lithography, state the positive/negative idea, then list the types in a table.

Basic principle of lithography

Working idea

Types of lithography

Key parameters

Exam tip

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