Excimer laser cleaning of silicon wafer backside metallic particles

Abstract

An excimer laser cleaning system operating at 248 nm was developed to remove micrometer-sized metallic particles from the backside of silicon wafers. Deliberate iron contamination has been performed using iron-oxide particles having a diameter of 0.5–2 μm. The surface photovoltage (SPV) method was used to characterize the cleaning efficiency through the change in diffusion length and iron concentration in the silicon bulk. Following a rapid thermal annealing at 1050 °C for 4 min, the SPV measures diffusion lengths down to 40 μm for the iron-contaminated wafer, corresponding to an iron concentration up to 1.55×1013 cm−3. The minimum diffusion length increases to 130 μm after two steam laser cleanings done at a laser fluence below the silicon threshold damage of 200 mJ/cm2. The iron concentration measured in the bulk of the iron-contaminated wafers is reduced by more than 91%, to a concentration of 1.4×1012 cm−3.