This invention relates to slicing a thin semiconductor substrate from side wall into two substrates of half thickness. The substrate slicing process involves a laser irradiation step. The substrate slicing process can also involve a mechanical cleaving process after the laser irradiation step. The apparatus for substrate slicing comprises two opposite-facing substrate chucks, with a gap in between for the substrate to pass through. One portion of the two substrate chucks are in parallel to each other to center the substrate sidewall. The gap can increase between the second portion of the two substrate chucks after the location for substrate separation, to spread out the resulting two substrates after the slicing and to facilitate the continuous substrate separation process. The present invention is further directed to methods and apparatus of separating a continuous thin layer of materials from side wall of a rotating ingot. It can be accomplished by a laser irradiation on the ingot side wall from a tangential direction for materials ablation. In an alternative method, a high-intensity short-pulse laser irradiates on the ingot side wall from a radial direction and converges under the ingot sidewall surface to form a plane of modified region inside the substrate, and a thin layer of materials is separated from the ingot side wall along the plane of modified region in a subsequent mechanical cleaving step. A film can be deposited/bonded to the ingot side wall prior to the separation of the thin film layers. The resulting thin layer of materials can be pulled away from the ingot by a substrate chuck.