Preventive maintenance (PM) is scheduled maintenance performed to prevent equipment failure, maintain performance, and extend tool lifetime in semiconductor manufacturing. PM types: (1) Time-based PM—fixed intervals (daily, weekly, monthly, quarterly); (2) Usage-based PM—triggered by wafer count, RF hours, or cycle count; (3) Condition-based PM—triggered by sensor data indicating degradation. PM tasks by category: (1) Consumables replacement (O-rings, chamber liners, focus rings, electrodes); (2) Cleaning (chamber clean, viewport polish, exhaust line cleaning); (3) Calibration (sensor calibration, robot teaching, flow controller verification); (4) Inspection (visual inspection, wear measurement, leak checks). PM scheduling: balance between too frequent (reduces uptime) and too infrequent (increases failure risk). PM metrics: MTTR (mean time to repair), PM efficiency (actual vs. planned duration), PM compliance rate. Documentation: PM checklists, parts consumed, measurements taken, issues found. Post-PM: seasoning wafers, qualification run, SPC baseline verification. PM optimization: analyze failure modes, adjust intervals based on reliability data, implement predictive maintenance where feasible. Critical for maintaining high uptime, consistent process performance, and avoiding costly unscheduled downtime.