Root Cause Analysis (RCA) is a systematic investigation methodology to identify the fundamental underlying cause of defects, failures, or process excursions, not just symptoms. Methodology: Multiple structured approaches used: 5 Whys: Repeatedly ask why until root cause reached. Simple but effective for straightforward problems. Fishbone (Ishikawa) diagram: Categorize potential causes by Man, Machine, Material, Method, Measurement, Environment. 8D problem solving: Disciplined 8-step process from team formation through permanent corrective action. Data analysis: Correlate defect data with process parameters, tool history, material lots, time, and operator data to identify patterns. Pareto analysis: Rank potential causes by frequency or impact. Focus on top contributors. DOE: Design of Experiments to systematically test hypotheses about cause-effect relationships. Timeline analysis: Reconstruct sequence of events leading to problem. Identify what changed. Common root causes in fab: Equipment degradation, preventive maintenance gaps, chemical quality variation, recipe errors, environmental excursions, design marginality. Cross-functional: RCA often requires expertise from process, equipment, metrology, yield, and quality teams. Corrective action: Fix the root cause, not just the symptom. Implement preventive measures to avoid recurrence. Verification: Confirm that corrective action resolves the problem. Monitor for recurrence. Documentation: Full RCA report with evidence, analysis, root cause, corrective action, and verification results.