Photo-Polymer Wafer Bonding for Transfer of GaN Heterostructures and Devices from Sapphire to Dissimilar Substrates

Vorrada Loryuenyong
(Professor Nathan W. Cheung)
(NSF) DMI-0088145 and (UC-SMART) SM-97-01

A polymer wafer bonding and laser lift-off process has been developed and implemented to integrate GaN heterostructures and functional devices with dissimilar substrates. The integration process takes advantage of the high quality of GaN growth on sapphire and the complementary functionality of the receptor substrates. The process developed in this study uses an SU-8 photo-polymer both as a bonding material and as a delamination material for layer transfer. SU-8 was spun onto an optically transparent handle wafer (e.g., quartz or glass) and then bonded to the GaN-on-sapphire donor wafer. The pre-exposure bake (PEB) temperature has an effect on the Tg or the maximum working temperature of the resist. In this case, 200-300 C PEB was chosen to achieve good SU-8 bonding above 160 C. UV curing of the SU-8 through the backside of the handle wafer could be performed after PEB to yield higher bonding strength at low temperatures. However, the bonding strength of UV-cured samples degraded very quickly at temperatures above 170-200 C. This was expected to be a result of crosslinking, which prevented the relaxation of the resist from residue internal thermal stresses. Laser lift-off of the GaN from the sapphire substrate was then performed with a 38 ns-KrF laser pulse (248 nm wavelength) transmitted through the backside of the sapphire with a fluence of 500-600 mJ/cm2, yielding a GaN/SU-8/silica glass final structure. For the double-transfer experiments, the GaN/SU-8/silica glass structure obtained was then bonded to a Si substrate using the Pd/In transient-liquid-phase method. Laser ablation or heat treatment above Tg of the UV-uncured resist was used to separate the GaN film from the silica-glass substrate.

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