Effect of internally focused laser processing of sapphire substrate on bowing management for III-nitride epitaxy

We review a new approach to the bowing management of sapphire substrates for III-nitride epitaxy based on the internally focused laser processing. The laser process modifies the phase of the sapphire, inducing a volume expansion effect that enables the bow to be managed. Bowing control is required in two main areas: 1) control of the initial bow of the sapphire substrate, and 2) reduction in the bow after the epitaxy. The initial bow control was demonstrated for ~250 μm pre-bowed convex and concave sapphire substrates. The effect of the pre-bowed substrate on III-nitride epitaxy was also experimentally verified with an in situ curvature monitoring system during the III-nitride epitaxy; the possibility to accommodate substrate bowing to any target values by applying the initial offset to the substrate was also confirmed. When applied to the substrate after the epitaxy, the same technique was successful in flattening of the substrate for a subsequent chip-fabrication process. This new approach provides wide flexibility in the design engineering of epitaxial and device fabrication processes. Thus, it accelerates the realization of larger diameter device processes with III-nitride/sapphire.