in an independent gan film manufactured by creating a gan layer on a base heterosubstrate using vapor - phase deposition and then removing the base substrate , owing to layer - base discrepancy in thermal expansion coefficient and lattice constant , bow will be a large ± 40 μm to ± 100 μm . since with that bow device fabrication by photolithography is challenging , reducing the bow to + 30 μm to − 20 μm is the goal . the surface deflected concavely is ground to impart to it a damaged layer that has a stretching effect , making the surface become convex . the damaged layer on the surface having become convex is removed by etching , which curtails the bow . alternatively , the convex surface on the side opposite the surface having become convex is ground to generate a damaged layer . with the concave surface having become convex due to the damaged layer , suitably etching off the damaged layer curtails the bow .