resist profile simulation
**Resist profile simulation** is the computational prediction of the **3D shape of photoresist** after exposure, bake, and development steps in lithography. It models how the resist responds to the aerial image, chemical reactions during baking, and the dissolution process during development to predict the final resist cross-sectional profile.
**Why Resist Profile Matters**
- The resist profile — its **sidewall angle, top rounding, footing, undercut**, and residual thickness — directly determines how well the pattern transfers during subsequent etch.
- A perfectly vertical, rectangular resist profile is ideal. In practice, resist profiles have sloped sidewalls, rounded tops, and other deviations that affect etch fidelity.
- Resist profile simulation helps predict and optimize these characteristics before expensive wafer processing.
**Simulation Components**
- **Exposure Model**: Calculates how the aerial image (light intensity distribution) is absorbed in the resist. Models **standing wave effects** (interference between incident and reflected light creating periodic intensity variations through the resist thickness), **bulk absorption**, and **photoactive compound decomposition**.
- **Post-Exposure Bake (PEB) Model**: During PEB, photoacid generated by exposure **diffuses** and catalyzes chemical reactions (deprotection in chemically amplified resists). The simulation models acid diffusion, reaction kinetics, and the resulting solubility distribution.
- **Development Model**: Models how the resist dissolves in the developer solution as a function of local chemical composition. The dissolution rate varies with depth and position, creating the 3D resist profile.
**Key Physical Effects**
- **Standing Waves**: Vertical ripples on resist sidewalls caused by optical interference. PEB smooths these by acid diffusion.
- **Top Loss**: Resist surface exposed to developer dissolves faster, rounding the resist top.
- **Footing**: Resist at the bottom may be under-developed due to optical absorption or substrate reflection, leaving unwanted material ("foot") at the base.
- **Dark Erosion**: Even unexposed resist dissolves slightly during development, reducing resist thickness.
**Simulation Software**
- **Prolith** (KLA): Industry-standard lithography simulator with comprehensive resist models.
- **Sentaurus Lithography** (Synopsys): Part of the TCAD suite for process simulation.
- **HyperLith**: Academic/research lithography simulator.
**Applications**
- **Process Optimization**: Determine optimal exposure dose, focus, PEB temperature, and development time.
- **Defect Prediction**: Identify conditions where resist collapse, bridging, or scumming might occur.
- **OPC Validation**: Verify that OPC corrections produce acceptable resist profiles, not just acceptable aerial images.
Resist profile simulation bridges the gap between **optical image calculation** and **actual wafer results** — it transforms the aerial image into a physical prediction of what the fab will produce.