feature-scale simulation
**Feature-scale simulation** models the **evolution of individual device features** (trenches, vias, lines, contact holes) during fabrication processes — capturing the detailed geometry development that determines device dimensions, profiles, and structural characteristics at the nanometer scale.
**What Feature-Scale Simulation Covers**
- **Etch Profile Evolution**: How a trench or via shape develops during reactive ion etching — sidewall angle, bottom rounding, notching, bowing, micro-trenching, and ARDE (aspect-ratio dependent etch).
- **Deposition Conformality**: How thin films deposit inside high-aspect-ratio structures — step coverage, void formation, seam issues, overhang, and keyhole development.
- **Lithography/Patterning**: How resist profiles develop during exposure and development — footing, rounding, scumming.
- **CMP Surface Evolution**: How planarization evolves across feature topography — dishing in wide trenches, erosion of dense arrays.
**Physics Involved**
- **Ion Transport**: In plasma etch, ions travel through the sheath and arrive at the wafer surface with angular and energy distributions. Feature walls shadow ions, creating directional effects.
- **Neutral Transport**: Reactive neutrals (radicals) enter features through random walk / Knudsen transport — aspect ratio affects how many neutrals reach the bottom.
- **Surface Chemistry**: Etch rates, deposition rates, and selectivity depend on local flux of ions, neutrals, surface temperature, and surface composition.
- **Redeposition**: Etch byproducts can redeposit on feature sidewalls — affecting profile shape and CD.
**Simulation Methods**
- **Level Set Method**: Tracks the evolving surface as the zero-contour of a higher-dimensional function. Handles topological changes (merging, splitting) naturally. Widely used in commercial tools.
- **String/Segment Method**: Represents the surface as connected segments that move according to local etch/deposition rates. Simple and fast for 2D.
- **Monte Carlo (Particle Tracking)**: Simulates individual ion and neutral trajectories — captures angular distributions and multiple reflections inside features. Most physically accurate but computationally expensive.
- **Cell-Based (Voxel)**: Divides space into cells and evolves each based on local conditions. Good for 3D simulations.
**Applications**
- **High-Aspect-Ratio Etch**: Predict profile shape for deep trenches (capacitor trenches in DRAM, TSVs, deep STI) — identify conditions that prevent bowing, twisting, or non-opening.
- **Contact/Via Fill**: Simulate metal fill of high-AR contact holes — predict void-free fill conditions.
- **Gate Spacer**: Model spacer deposition and etch to predict final spacer width and shape.
- **Dual Damascene**: Simulate the trench-via integration sequence.
Feature-scale simulation is **essential for process development** at advanced nodes — it predicts whether a process recipe will produce acceptable feature profiles before committing expensive silicon experiments.