Wafer Bowing Causes And Mathematical Model

The stress in the films often results from epitaxy in the growth processes and from thermal expansion coefficient (TEC) mismatch in the postgrowth processes of both metal-organic chemical vapor deposition and hydride vapor phase epitaxy. For example, the stress typically observed is dominated by thermal stress in the post-growth processes, which is usually considered to bring defects or damage to the films, such as, dislocation, buckling, and cracking. Cracks normally extend to the principal tensile stresses within the thin brittle layers and interact with the interfaces of GaN/sapphire. Due to the mismatch of TECs between the GaN film and sapphire, wafer bowing will occur when the GaN film is mechanically constrained by the sapphire after cooling. Thermal stress, wafer bending, and cracking are the main drawbacks that hamper the production of large-area GaN substrates and the application of GaN films. Wafer bowing and stress concentration significantly influence not only the devices’ mechanical performance but also their optical, electrical, and magnetic properties. Therefore, reducing wafer bowing and stress concentration in GaN films is very important. To the best of our knowledge, there are no feasible methods for reducing GaN wafer bowing and few papers on wafer bowing have been published.

When GaN is grown on the sapphire substrate using MOCVD, bowing will happen in epitaxial wafers. There are three major factors for the wafer bowing. One is the temperature difference between the upper and lower interfaces of epitaxial wafers. The lower surface temperature is higher than that of the upper surface, so that the expansion of the lower surface can be larger than that of the upper surface, causing the epitaxial wafers concave. Another is the lattice mismatch between sapphire substrate and epitaxial material, which makes the sapphire suffer from tensile and compressive stress and produces concave or convex bowing. The third is thermal mismatch between epitaxial material and the sapphire substrate, which occurs mainly in the case of a sudden jump or a sudden drop of the outside temperature.


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