collimation

Collimation uses a physical structure (collimator) between the sputtering target and wafer to filter out off-angle atoms, improving directionality for better step coverage. **Design**: Honeycomb array of tubes or channels placed between target and wafer. Only atoms traveling near-normal to wafer pass through. **Mechanism**: Off-angle atoms are captured on collimator walls. Only near-perpendicular atoms reach wafer. **Benefit**: Improved bottom coverage in features compared to uncollimated sputtering. **Drawback - efficiency**: Most sputtered atoms (70-90%) are captured by collimator. Very low deposition rate. Significant material waste. **Collimator clogging**: Captured material builds up on collimator. Changes effective aspect ratio of collimator channels over time, affecting performance. Requires periodic replacement. **Particle risk**: Material buildup on collimator can flake off, generating particles. **Historical context**: Used in 1990s-early 2000s for barrier and liner deposition before IPVD matured. Largely replaced by IPVD which achieves similar directionality without throughput penalty. **Aspect ratio**: Collimator aspect ratio (channel length/diameter) determines acceptance angle. Higher AR = better directionality but lower throughput. **Current use**: Limited to specialized applications. IPVD and long-throw PVD are preferred modern solutions.