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.