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4 April 2007


Hitachi Cable prototypes 3-inch GaN substrate; 4-inch possible

At last week's 54th Symposium of the Japan Society of Applied Physics at Aoyama Gakuin University, Hitachi Cable Ltd said that it has prototyped a highly reproducible 3-inch GaN substrate, according to the publication Nikkei Electronics. Current commercial GaN substrates are 2-inches in diameter. 3-inch GaN substrates have been announced previously, but Hitachi Cable claims that it is first to release photos and tangible data.

Currently, most GaN substrates are produced by hydride vapor-phase epitaxy (HVPE) deposition of a thick film of GaN on a base substrate of sapphire or GaAs, which is then removed by laser, etching or grinding. However, because the type of base substrate materials used have different linear expansion coefficients to GaN and substantial thermal stress accumulates, these methods tend to cause cracks in the GaN thick film during separation, making it difficult to improve the reproducibility.

The 3-inch substrate was made possible by Hitachi Cable’s ‘void-assisted separation’ (VAS) method, which has been used previously to develop 2-inch products (see Japanese Journal of Applied Physics (2003) Pt.2 (1A/B) L1). During HVPE growth at 1050°C of the 600 micron thick GaN film (at 140 microns per hour), microscopic spaces (voids) are formed in openings in a titanium nitride film near the interface between the GaN film and the base substrate, creating a thin sacrificial layer. Because this has low mechanical strength, due to the thermal stress generated by the drop in temperature after HVPE growth the GaN thick film separates from the base substrate more easily and more reproducibly by itself.

Hitachi Cable claims that separation for 3-inch substrates was no more difficult than for their 2-inch products. Also, the radius of curvature is no greater. The carrier mobility of the 3-inch GaN substrate is 340-400cm2/Vs.

In response to the question “How far can it grow in size?”, Hitachi Cable said it believes that 4-inch products can be produced with no difficulty “if the production equipment is arranged accordingly”.