Experimental Study of Particle Deposition on Semiconductor Wafers

A sensitive method for detecting particle deposition on semiconductor wafers has been developed. The method consisted of generating a monodisperse fluorescent aerosol, depositing the known-size monodisperse aerosol on a wafer in a laminar flow chamber, and analyzing the deposited particles using a fluorometric technique. For aerosol particles in the size range of 0.1–1.0 μm, the mobility classification-inertial impaction technique developed by Romay-Novas and Pui (1988) was used to generate the monodisperse test aerosols. Above a particle diameter of 1.0 μm, monodisperse uranine-tagged oleic acid aerosols were generated by a vibrating-orifice generator. The test wafer was a 3.8-cm diameter silicon wafer placed horizontally in a vertical laminar flow chamber which was maintained at a free stream velocity of 20 cm/s. A condensation nucleus counter and an optical particle counter were used to obtain the particle concentration profile in the test cross section and to monitor the stability of aerosol concentration during the experiment. The results show that the measured particle deposition velocities on the wafers agree well with the theory of Liu and Ahn (1987) in the particle size range between 0.15 and 8.0 μm. The deposition velocity shows a minimum around 0.25 μm in particle diameter and increases with both smaller and larger particle size owing to diffusional deposition and gravitational settling, respectively.