PARTICLE REMOVAL FROM SILICON-WAFER SURFACE IN WET CLEANING PROCESS

Particle removal from silicon wafer surfaces was studied using acid and alkaline solutions employed in wet cleaning processes found in semiconductor manufacturing. It has been demonstrated that the alkaline solutions are superior to the acid solutions in terms of particle removal efficiency. The particle removal mechanism in the alkaline solutions has been confirmed as follows: the solutions etch the wafer surfaces to lift off particles, and the particles are then electrically repelled from the wafer surfaces. It has been determined experimentally that an etch rate of 0.25 nm/min or more is required to lift off the particles adsorbed on the wafer surfaces. The etch rate of NH4OH-H2O2-H2O solution is 0.3 nm/min when the mixing ratio is set at 0.05:1:5 (NH4OH:H2O2:H2O). With this mixing ratio, the surface smoothness of the wafers is maintained at initial level. It is therefore possible to reduce the NH4OH content in NH4OH-H2O2-H2O solution to 1/20 of the conventional level. In addition, it has been confirmed that when the pH value of NH4OH-H2O2-H2O solution becomes higher, polystyrene latex spheres and natural organic particles are oxidized with their surface turning into a gel and their shape changing. The particle removal efficiency has been demonstrated to be degraded by the oxidation of organic particles. The oxidation of organic particles becomes significant as the NH4OH content in NH4OH-H2O2-H2O solution rises above 0.1:1:5 (pH is over 9.1). These experimental results suggest that the mixing ratio of NH4OH-H2O2-H2O solution should be set at 0.05:1:5. This mixing ratio is effective in maintaining both particle removal efficiency and surface smoothness of the wafers.