Femtosecond laser pulses are irradiated on a single-side polished single-crystalline silicon wafer. Metamorphic zone appears around the ablated zone and the morphology changes gradually in the metamorphic zone. Typical phenomena of thermal ablation such as melt-resolidification and subtransparent glassy materials occur in the marginal area of the ablated zone. No apparent changes are found among microscopic morphologies of the ablated, metamorphic and unirradiated zones. There are flaws and spallations on the smooth back surface of the wafer, which are caused by the stress inside the sample. Nanomechanical properties of the sample surface hardly change in the small scope of the backside around the ablated zone. While regular changes occur in large scope, which is the conjunct result of such stresses caused by many different collateral damages.