Particles in Ulsi Grade Chemicals and Their Adhesion to Silicon Surfaces

Abstract

Despite the advancement of dry processing in ULSI manufacturing, wet chemical cleaning remains the only reliable method for controlling particulate contamination on semiconductor wafer surfaces.

It is necessary to accurately measure the number of particles in all liquid chemicals and understand the mechanisms of particulate adhesion so that this form of contamination may be minimized on silicon surfaces. We believe that the particle concentration and zeta potential, and the condition of the wafer surface are all factors which determine the number of particles which deposit on surfaces immersed in liquids.

A new laser particle counter, the Horiba PLCA 520, with specifications for detecting particles to 0.2 μm in chemicals was used in this study. This particle counter is unique in its ability to use either pressure or suction fluid sampling. Thus, the question of whether particle counts are inflated through the inclusion of bubbles during suction sampling may be addressed. No appreciable difference between suction and pressure sampling was found, with two exceptions: nitric acid and ammonium hydroxide. Calibration tests, as well as pressure/suction sampling characterizations of common ULSI-grade processing chemicals are presented.

We have demonstrated that particles adhere to bare silicon wafers in proportion to their zeta potential and concentration in liquids. Furthermore, it is shown that the number of particles deposited in acidic and alkaline solutions are quite different, in keeping with published data on zeta potentials as a function of solution pH.