A study on the effect of the laser-scribing on spalling of electrodeposit-assisted stripping(EAS) process in nickel citrate bath

Abstract

The silicon itself comprises a large proportion of the solar cell overall cost. Therefore, lowering its cost is directly related to the cost-reduction in the silicon solar cell manufacture. In the last few decades, there were a number of attempts to reduce the production cost of the silicon by reducing the thickness of the silicon wafer such as a sawing technique, a high-temperature induced spalling process and etc. Among these techniques, a novel method called Electrodeposit Assisted Stripping(EAS) process is one of the most effective and inexpensive kerf-free methods for acquiring a high quality thin-single crystalline silicon film. The mechanism of this process is a ‘spalling’ where the maximum shear stress takes place not on the surface, but just below the surface, causing the spall off. During the EAS process, a metal stress layer of a certain thickness is electrochemically deposited onto the bulk silicon substrate inducing a thin silicon film to be lifted-off. The advantage of the EAS process is that this can be repeatedly carried out from the bulk silicon as long as it is worn out. In order to repeat this process, the initiation of the crack should be controlled to take place from the side of the bulk silicon. During spalling, however, the crack initiates from the inner surface on top of the bulk silicon leaving silicon residues at the edge of bulk silicon which should be removed to repeat the EAS process. To repeat the EAS process, these residues should be polished away, or the quality of a thin silicon film will be degraded as the steps of this process are carried out. In this work, pre-crack is created by a laser scribing at the side of the bulk silicon just below the top surface. This pre-crack is to directly initiate the crack propagation parallel to the top surface from the notch point. The effect of the laser scribing on silicon, such as river-like patterns due to the stresses involved in the EAS process, is also investigated using different laser parameters.