particle counting on surfaces

**Particle Counting on Surfaces** is the **automated, full-wafer laser scanning inspection technique that detects, localizes, and sizes individual particle defects on bare silicon wafer surfaces** — generating the Light Point Defect (LPD) map that serves as the primary tool qualification metric, incoming wafer quality check, and process contamination monitor throughout semiconductor manufacturing. **Detection Principle** A tightly focused laser beam (typically 488 nm Ar-ion or 355 nm UV) scans across the spinning wafer in a spiral pattern, covering the full 300 mm surface in 1–3 minutes. A smooth, atomically flat silicon surface reflects the beam specularly — no signal at the detectors. When the beam encounters a particle, scratch, or surface irregularity, photons scatter in all directions. High-angle dark-field detectors positioned around the wafer collect this scattered light, with signal intensity proportional to the particle's scattering cross-section, which scales with particle size. **Calibration and Size Bins** Tools are calibrated using PSL (polystyrene latex) sphere standards of known diameter deposited on bare silicon. The relationship between scatter intensity and PSL equivalent sphere diameter establishes the size response curve, enabling conversion of raw scatter signal to reported LPD size. Modern tools (KLA SP7, Hitachi LS9300) report LPDs down to 17–26 nm PSL equivalent. **Key Metrics** **LPD Count at Threshold**: "3 LPDs ≥ 26 nm" — the count of particles above the specified detection threshold. Tool qualification typically requires LPD addition (wafer processed through tool minus blank wafer baseline) < 0.03 particles/cm². **PWP (Particles With Process)**: The primary tool qualification metric — bare wafers processed through a tool compared to pre-process count. PWP below specified adder confirms tool cleanliness. **Spatial Distribution**: The wafer map of LPD positions reveals process signatures — edge-concentrated particles indicate robot handling or chemical non-uniformity; clustered particles indicate slurry agglomerates or contamination events; random distribution indicates general background. **Haze Background**: The tool simultaneously measures background scatter (haze) correlating with surface roughness, used to detect epitaxial surface defects and copper precipitation. **Production Integration**: Every bare wafer entering the fab is scanned (incoming quality control). Process tools run PWP monitors weekly or after maintenance. A sudden LPD count increase triggers immediate tool lock and investigation. **Particle Counting on Surfaces** is **the daily census of contamination** — the automated, full-wafer particle audit that determines whether a surface is clean enough for the next process step or whether an invisible contamination event has occurred.