High-yield indium-based wafer bonding for large-area multi-pixel optoelectronic probes for neuroscientific research

Abstract

This paper reports on the optimization of wafer-scale bonding of micro-light-emitting diodes (µLEDs) on four-inch sapphire wafers onto silicon (Si) substrates using an indium–gold (In–Au) reflow process. The bonding yield was optimized from initial values below 10% to 100% by applying a systematic variation of the two most relevant bonding parameters, namely the indium (In) bonding pad height t2 and the relative bonding area Q. The fabrication process is designed to reliably transfer mLEDs from sapphire to Si substrates. The process is demonstrated by realizing optoelectronic probes carrying arrays of mLEDs dedicated for applications in neuroscientific research applying optogenetic methods. The 6 m thick mLEDs comprise of a gallium nitride layer epitaxially grown on sapphire wafers and lithographically patterned into mLEDs with lateral dimensions down to using reactive ion etching. The mLEDs are transferred onto Si substrates carrying interconnecting tracks and bonding pads made of gold (Au). Two technological challenges were successfully addressed. The first is to pattern thick In layers with lateral dimensions down to on top of the 6 m high mLED mesas. The second results from residual thermomechanical stress after wafer bonding due to the thermal expansion mismatch of sapphire and Si. It is demonstrated that the stress induced rupture of the In–Au bond interface between sapphire and Si is completely suppressed for t2, Q combinations with m and . These results enable fully functional two-dimensional arrays with mLEDs to be realized on 65 m thin Si substrates.