Slim-Cut: A Kerf-Loss-Free Method for Wafering 50-µm-Thick Crystalline Si Wafers Based on Stress-Induced Lift-Off

Abstract

In order to tackle the increasing demand of thin wafers, we have developed a new wafering technique for the production of ultra-thin (< 100 μm) crystalline silicon wafers. This new lift-off process, named SLIM-cut (for Stress-induced LIft-off Method), requires only the use of a screen-printer and a belt furnace; no ion-implanted, porous layer or additional thickening by epitaxy is needed to obtain high quality wafers in the thickness range of 50 μm without kerf loss. We deposit on a thick substrate a layer with mismatched coefficient of thermal expansion with respect to the substrate. During cooling, the differential contraction induces a large stress field, which is released by the initiation and the propagation of a crack parallel to the surface. The first unoptimized solar cell device with a very simple process (no back-surface field, no intentional texturing, heterojunction emitter) showed an energy conversion efficiency of 10.0% (1 cm2). This proves the conservation of the material quality during the process. The potential of the method is estimated by the development of a custom-made numerical model. The model uses a 2D finite element method and is able to propagate a crack in a multi-layered structure. The model proposes a concentric re-meshing in the region of the crack tip at every increment and a crack initiation and propagation based on maximum stress criteria. The numerical results are compatible with experimental observations.