Chapter 6 STEAM LASER CLEANING OF SILICON SURFACES: LASER-INDUCED GAS BUBBLE NUCLEATION AND EFFICIENCY MEASUREMENTS

The removal of dust particles from semiconductor surfaces requires new cleaning strategies such as Steam Laser Cleaning (SLC). It is based on laser-induced explosive evaporation of a liquid layer applied on the surface. We have investigated the laser-induced nucleation and growth of gas bubbles at silicon/water, silicon/isopropanol and silver-film/water interfaces by light scattering and surface plasmon spectroscopy. The achieved superheating of the liquid before bubble nucleation sets in strongly depends on the substrate roughness. On rough metal films it is only about 30 K in water, compared to about 150 K on smooth silicon surfaces. Isopropanol (IPA) on smooth silicon surfaces could be heated to 116°C, corresponding to a superheating of 36 K. In combination with numerical calculations it was possible to determine the heat transfer coefficients silicon-water (ξ = 3 ·10 W/m K) and silicon – IPA (ξ = 1 ·10 W/m K). Using optical techniques we have measured the pressure wave created by the growing bubbles and the bubble growth velocities. For a quantitative study of the efficiency of SLC we deposited spherical colloidal particles on industrial silicon wafers. We observed a sharp threshold for particle removal at 110 mJ/cm (laser λ = 532 nm, FWHM = 8 ns) which is independent of the size (diameter 800 nm down to 60 nm) and material of the particles and efficiencies above 90% for particle removal. On the basis of our results we discuss the validity of the existing SLC models and the perspective of the application of SLC as an industrial cleaning tool.