The substrates are offered by Qromis as commercial 200mm QST substrates as part of their patented product portfolio.
GaN-on-Si technology is the standard platform for commercial GaN power switching devices for wafer diameters up to 150mm/6 inch. Imec has pioneered the development of GaN-on-Si power technology for 200mm/8 inch wafers and qualified enhancement mode HEMT and Schottky diode power devices for 100V, 200V and 650V operating voltage ranges, in a move that could pave the way to high volume manufacturing applications.
For applications beyond 650V, such as electric cars and renewable energy, it's proved difficult to further increase the buffer thickness on 200mm wafers to the levels required for higher breakdown and low leakage levels, because of the mismatch in coefficient of thermal expansion (CTE) between the GaN/AlGaN epitaxial layers and the silicon substrate. While it's possible to use thicker Si substrates to keep wafer warp and bow under control for 900V and 1200V applications, the mechanical strength is a concern in high volume manufacturing, and the ever thicker wafers can cause compatibility issues in wafer handling in some processing tools.
However, by careful engineering and using CMOS fab-friendly QST substrates with a CTE-matched core having a thermal expansion that very closely matches the thermal expansion of the GaN/AlGaN epitaxial layers, it is possible to move to 900V-1200V buffers and beyond, on a standard semi-spec thickness 200mm substrate. Moreover, QST substrates open perspectives for very thick GaN buffers, including realisation of free-standing and very low dislocation density GaN substrates by >100 micron thick fast-growth epitaxial layers.
These features are expected to help open up opportunities for long awaited commercial vertical GaN power switches and rectifiers suitable for high voltage and high current applications presently dominated by Si IGBTs and SiC power FETs and diodes.
In this collaboration, imec and Qromis have developed enhancement mode p-GaN power device specific GaN epitaxial layers on 200mm QST substrates, with buffers grown in AIXTRON’s G5+ C 200mm high volume manufacturing MOCVD system.
Imec then ported its p-GaN enhancement mode power device technology to the 200mm GaN-on- QST substrates in their silicon pilotline and demonstrated high performance power devices with threshold voltage of 2.8 Volt.
“The engineered QST substrates from Qromis facilitated a seamless porting of our process of reference from thick GaN-on-Si substrates to standard thickness GaN-on- QST substrates using the AIX G5+ C system, in a joint effort of imec, Qromis and AIXTRON,” explained Stefaan Decoutere, program director for GaN power technology at imec. The careful selection of the material for the core of the substrates, and the development of the light-blocking wrapping layers resulted in fab-compatible standard thickness substrates and first-time-right processing of the power devices.