The lift-off step of a layer transfer process for the production of thin-film solar cells using a double porous layer is studied in detail. The goal is to locally weaken the thin film to improve the detachment. A UV nanosecond pulse laser is used to induce the weakening by scribing trenches. A relation between the pulse fluence, the number of pulses per point and the ablated area is found and used to predict the ablation depth. The grooving step is applied at different points in the process chain. The most suitable grooves are obtained when the scribing process is done after front-side processing of the attached thin film and before the bonding step. A yield of around 80% is obtained for samples that went through the process chain up to the detachment step, by using the optimized process conditions.
SEM of the mother substrate (right) and the remaining of the thin film (left) after detachment of the useful foil area. The white points on the substrate are the leftover of the pillars from the weak layer.