light scattering particle detection
**Light Scattering Particle Detection** is the **fundamental optical physics underlying all laser-based wafer surface inspection systems**, exploiting the phenomenon that particles and surface irregularities scatter incident photons out of the specular reflection angle — with scattering intensity and angular distribution depending on particle size relative to wavelength, governing the detection limits, wavelength selection, and optical design of tools like the KLA Surfscan SP7 and Hitachi LS9300.
**Two Scattering Regimes**
The relationship between particle size (d) and incident wavelength (λ) determines which physical model applies:
**Rayleigh Scattering (d << λ, typically d < λ/10)**
Scattering intensity scales as I ∝ d⁶/λ⁴ — the sixth power of diameter and inverse fourth power of wavelength. This extreme size dependence creates the fundamental challenge of sub-20 nm particle detection: halving the particle diameter reduces scattered signal by 64× (2⁶). Simultaneously, the λ⁴ dependence means halving the wavelength (488 nm → 244 nm) increases signal by 16× — the primary driver of the fab industry's push to deep ultraviolet (DUV) and vacuum ultraviolet (VUV) inspection lasers.
**Mie Scattering (d ≈ λ, typically λ/10 < d < 10λ)**
When particle size approaches the wavelength, the simple Rayleigh approximation breaks down and exact Mie theory must be applied. Scattering patterns become complex, with strong forward lobes and interference fringes. Signal is still a strong function of size but with oscillations — a 200 nm particle on a 488 nm tool may scatter more or less than a 180 nm particle depending on refractive index and exact geometry.
**Geometric Optics (d >> λ)**
Large particles (>1 µm) scatter geometrically — signal scales approximately with cross-sectional area (d²), making large defects easy to detect but providing less size discrimination.
**Tool Design Implications**
**Wavelength Selection**: KLA SP7 uses 355 nm UV laser; advanced systems push to 193 nm ArF to access the deep Rayleigh regime for sub-20 nm particles. Shorter wavelength yields lower detection limits but requires more expensive optics and introduces surface sensitivity to atomic-scale roughness.
**Collection Angle**: Dark-field detectors positioned at high angles from specular collect predominantly scattered light from small features. Multiple detector channels at different angles provide angular distribution data that aids defect type classification.
**Signal-to-Noise**: The silicon substrate itself scatters weakly at smooth surfaces — this establishes the noise floor (haze) above which discrete LPDs must be detected. Surface roughness directly limits the minimum detectable particle size for a given laser power and collection solid angle.
**PSL Calibration**: Polystyrene latex (PSL) spheres of known diameter calibrate the response curve, converting raw scattered intensity to a reported "PSL equivalent sphere diameter" — enabling cross-tool and cross-site comparison.
**Light Scattering Particle Detection** is **radar for nanoscale debris** — using the deflection of photons to locate and size particles that are 10–1,000× smaller than the wavelength of visible light, with detection physics that drive every design choice from laser wavelength to detector geometry.