A monitor wafer is a dedicated wafer processed through specific tools to check equipment performance, cleanliness, particle levels, and process quality. Purpose: Verify that individual process tools are performing within specification before committing product wafers. Early warning system for tool problems. Types: Particle monitor: Bare wafer processed through tool, then scanned for particle adders. Verifies tool cleanliness. Film monitor: Wafer with deposited film measured for thickness, uniformity, and properties. Verifies deposition performance. Etch monitor: Patterned wafer etched to verify CD, profile, and selectivity. Contamination monitor: Wafer processed and analyzed by TXRF or SIMS for metallic contamination levels. Frequency: Daily, weekly, or after PM events depending on tool criticality and fab practice. Specifications: Each monitor type has acceptance criteria (e.g., <20 particles >45nm for particle monitor, thickness uniformity <1%). Qualification gate: Tool cannot process product until monitor wafers pass acceptance criteria. Especially after maintenance or tool recovery. Data tracking: Monitor results tracked over time in SPC charts. Trends indicate degrading tool health. Cost: Monitor wafer consumption is significant fab cost. Balance monitoring frequency with cost. Automation: Monitor wafer runs often automated - scheduled, processed, and measured with minimal operator intervention. Action on failure: Failed monitor triggers tool hold, investigation, additional PM, or re-qualification before product release.