Two inch GaN wafers were fabricated on (0001) sapphire substrates by HVPE and separated by the laser lift-off (LLO) process. The bow of as-grown GaN wafer after LLO process was ranged from -300 to -100 um, which was determined by the residual thermal stress. The GaN wafer bow is changed during the wafering process, especially the lapping process. From the Stoney’s formula, it is notable that the thickness of substrate (tS) and sub-surface damaged layer (tSSD) can change the wafer bow.
Fig. 1 shows the wafer bow (black triangle), residual stress (blue square) of experimented GaN wafer and wafer bow calculated by the Stoney’s formula (red triangle). It was confirmed that the wafer bow was reduced as the thickness of GaN wafer reduced. In particular, the bow was inverted from concave to convex at the thickness of around 180 um. As the lapping process progressed, tS is reduced and tSSD is generated, which led to the change of bow to convex.
Fig. 2 shows the sub-surface damaged layer of the GaN wafer after lapping with 2-um diamond abrasives. It exhibits that the crystalline GaN layer was damaged by the diamond abrasives and changed to polycrystalline phase at the top surface. In addition, it indicates that the microcracks propagated into the wafer under the polycrystalline layer. Total thickness of polycrystalline layer and microcracks, tSSD, was around 1.09 um. Thus, the tSSD should be removed for achieving the damage-free GaN wafer.
In our presentation, we report how we achieved zero-bow FS-GaN wafer shown in Fig. 3, and how to control residual stress and optimize wafering process.