PCM (Process Control Monitor) uses dedicated test structures or wafers to monitor the manufacturing process independently from product wafers, ensuring process stability and specification compliance. Test structures: Standard set of devices (transistors, resistors, capacitors, diodes, chains) designed to be sensitive to process variations. Located in scribe lines or on dedicated test wafers. Scribe line PCM: Test structures placed between product dies in scribe lines. Measured during WAT. Lost when wafer is diced (scribe line cut away). Dedicated test wafers: Full wafers with arrays of test structures. Used for detailed process characterization and tool qualification. Parameters monitored: Transistor Vt, Idsat, Ioff, gate oxide properties, sheet resistance, contact resistance, metal resistance, junction characteristics, capacitance. Frequency: PCM measured on production lots at defined intervals (every lot, every nth lot, or periodic). SPC tracking: PCM results plotted on control charts. Statistical limits define normal variation. Out-of-control triggers investigation. Trend detection: PCM detects gradual process drift before it reaches specification limits. Enables proactive correction. Tool monitoring: PCM wafers run on specific tools to monitor individual tool performance and detect chamber-specific issues. Process development: PCM data essential during process development for optimizing parameters and establishing baselines. Design: PCM test structure design is specialized skill. Structures must be sensitive, robust, and compact.