Abstract: A particle sensor for a semiconductor device fabrication tool scans a laser beam through a measurement volume immediately adjacent a wafer during processing and detects light scattered by particles adjacent the wafer. Scanning provides a real-time count of particles without interfering with processing. Detected light can be forward-scattered, side-scattered, or back-scattered depending on available optical access for a detector portion of the sensor. A pulse in the intensity of scattered light results each time a particle is scanned. Because the scanning velocity is high relative to the particle velocity, each particle may be scanned several times while the particle is in the measurement volume. Analysis fits a series of pulses observed for a single particle to a Gaussian distribution to determine a size, position, and velocity for each particle and a time-resolved particle count of the particles.