Packaging Solution For GaN-on-200mm Silicon Power Devices

Imec and UTAC have developed a unique process for the wafer thinning and backside metallization of highly stressed GaN-on-200 mm silicon wafers. This has resulted in a packaged 650 V GaN device – smaller than state-of-the-art. Stefaan Decoutere, Program Director GaN Power Electronics at imec and Nicolo Ronchi, Researcher at imec explains the process.

The wide-bandgap material gallium-nitride (GaN) has many advantages over silicon (Si) when it comes to applications in the area of power electronics. With a higher breakdown strength and lower on-resistance, GaN-based power devices can convert power more efficiently than today’s most advanced Si-based devices. Apart from these assets, GaN-based power devices have a 10 – 100 times faster switching speed than silicon-based devices, which makes them perform significantly better at the system level. With these properties, the first generation of GaN-based power devices have found applications, such as battery chargers for mobile phones and electric cars, point-of-load power systems, industrial power supplies, DC/DC convertors for servers, and invertors for solar panel connections to the grid, to name a few.

Today, GaN is grown on a variety of substrates, including sapphire, silicon carbide (SiC) and silicon (Si). Silicon substrates have become attractive because of the lower cost perspective and the ability to use standard semiconductor processing lines. While industry is mainly working on wafers with diameters of 150 mm, imec has pioneered the development of the technology on 200 mm Si wafers

Fabricating GaN-on-silicon power devices on 200 mm wafers is however challenging: because of the larger diameter and targeted higher voltage range, the wafer bow gets worse. The fundamental difficulty is caused by the mismatch in lattice and especially in thermal expansion coefficient between the GaN/AlGaN layers and the Si substrates. The AlGaN buffer, which is required to ‘bridge’ the Si and GaN lattices, is grown at high temperatures. During cool down, the wafer gets warped, and the highly stressed wafers can easily break or exhibit micro-fractures. By careful stress engineering, imec has enabled a 200 mm CMOS-compatible GaN power device platform. With this platform, enhancement-mode power switches and Schottky diodes can be developed, for 100, 200 and 650V voltage ranges.