Contaminant particles peculiar to a type of equipment emanate inside and around a semiconductor, or thin-film-transistor–liquid-crystal-display device fabrication front-end equipment. To analyze such deposition phenomena and implement contamination control measures, it is necessary to observein situdeposition behavior over a long period. This article describes a measuring instrument for this purpose. Deposition and tearing off of particles or mist take place on the surface of a transparent plate (of glass or quartz) and Mie scattering occurs when a laser beam is irradiated onto the particles or mist from below. The scattered light is detectedin situwhile the detection signals are accumulated and kept track of. In this instrument, a semiconductor laser with beam focusing optics and an ellipsoidal mirror are integrated into a small, portable unit. The laser beam is scanned onto the glass surface over a maximum area of 250 by 250 mm in one or two dimensions. It is possible to adjust the scanned area, scanning speed, and laser beam diameter. One-spot, fixed irradiation is also possible for detection of particle flow or tearing on the surface. The minimum detectable particle size was about 0.3–0.4 &mgr;m for polystyrene latex sphere (PSL) particles deposited on the glass surface with a beam diameter of 35 &mgr;m and a scanning speed of 0.3 m/s (0.14 &mgr;m for a silicon wafer surface). Two kinds of experiments were carried out by the use of one glass with PSL particles deposited on the surface and another glass on whose surface PSL particles were spouted so they would roll without deposition. By using this instrument, it became clear that the response curve for the rolling of the particles agreed well with the Mie equation, whereas the curve for the deposited particles deviated from the equation, but only in the range of minute particle sizes. Another phenomenon clarified throughin situmeasurement was that once deposited, real-world particles were subjected to the influence of the down flow of clean air (0.3 m/s), and some were torn off in a time period on the order of hours. A comparison of the real-world particle deposition rate was made between the grounded and ungrounded states using a glass plate with a transparent conductive thin film. An increase in the deposition rate was observed for the ungrounded state, while a remarkable tearing-off rate was seen for the grounded state. ©1997 American Institute of Physics.