evaporation

Evaporation is a PVD technique that heats source material until it vaporizes, with atoms traveling through vacuum to condense on the wafer surface. **Methods**: **E-beam evaporation**: Electron beam heats source material in crucible. Can evaporate high-melting-point metals. **Thermal evaporation**: Resistive heating of boat or filament containing source material. Simpler, lower cost. **Vacuum**: Requires high vacuum (<10^-6 Torr) so evaporated atoms travel without gas collisions (long mean free path). **Directionality**: Highly directional, line-of-sight deposition. Creates shadowing effects on topography. **Step coverage**: Poor - films thin dramatically on sidewalls and bottom of features. Bottom coverage drops rapidly with AR. **Applications**: Historically used for aluminum metallization. Now less common in advanced semiconductor manufacturing. Still used for lift-off patterning, MEMS, research, and packaging. **Alloy deposition**: Co-evaporation from multiple sources for alloy films. Composition control can be challenging. **Rate**: Can achieve very high deposition rates (>1 um/min). **Film quality**: Very pure films (no gas incorporation). Low stress. **Planetary system**: Wafers mounted on rotating dome above source for improved uniformity. **Comparison to sputtering**: Sputtering preferred for semiconductor manufacturing due to better adhesion, uniformity, and alloy control.