Heterogeneous Integration Schemes of Compound Semiconductors for Advanced CMOS and More-than-Moore applications

The continuation of Moore’s law by conventional complementary metal oxide semiconductor (CMOS) scaling is becoming more and more challenging, requiring huge capital investments. On proposed scenario is the implementation of compound semiconductors as parts of advanced CMOS devices for More-than-Moore integration. The continuation of improved performance characteristics in CMOS manufacturing is coming to a critical point, where electrical properties of silicon introducing a hard stop. One discussed route to increase performance is the heterogeneous integration of compound semiconductors into state of the art CMOS circuits. While growth of III-Vs on silicon shows limited success till now, as well as growing III-Vs in small trench structures shows to be challenging, direct wafer bonding overcomes these challenges. Wafer bonding is the enabling process technology to make this happen. Two of the key wafer bonding techniques – low temperature fusion bonding as well as temporary bonding and de-bonding are the major subject of this contribution, introducing basic process flows and working principles for their CMOS integration. HETEROGENEOUS INTEGRATION SCHEMES INTRODUCTION Most of today‘s applications rely on silicon. However, silicon has its limits. Especially when it comes to optoelectronics or high frequency applications, silicon has inherent material restrictions. Researchers came up with great new materials and device combinations. Major obstacles remain, though, how to facilitate this technology on low enough cost to everybody. Here, silicon technology, again, plays a major role, where yields are high and manufacturing is greatly optimized. One solution is to enable functions already at a substrate level by material engineering. These so-called engineered substrates enable new functionality and heterogeneous integration by novel materials or the combination of different materials for optimized device performance. Fusion of different functionalities on one chip is a central topic of engineered substrates. As an example, nitride-based compound semiconductor devices show fast progress for light emitting diodes (LED), laser diodes, high-frequency transistors, power electronics and solar cells. The gain in efficiency and/or speed of these devices has mostly been enabled by recent advances in material design and growth technology. However, many applications are still suffering of reduced yield, a fact that is mostly stemming from reduced material quality and homogeneity. While high-quality substrates for homoepitaxy are still expensive, alternative growth substrates show an offset in material properties such as lattice matching and coefficient of thermal expansion. As a result, the epitaxial film quality suffers a high dislocation density, resulting in reduced electrical and optical quality of the lateron devices. Direct wafer bonding is a technology to join two substrate materials with different structural properties. Additionally, plasma activation of both wafer surfaces can be used to change the surface chemistry of both materials and therefore reducing the bonding temperature. In this way, materials supporting a high crystal quality of compound semiconductors can be joined with a carrier that accounts for differences in thermal expansion. In this way, it is possible to have Si-based transistors side by side with high electron mobility transistors (HEMTs) made from GaN as an example. Similar process flows with different compound semiconductors are feasible as well. While silicon is the most versatile material for producing highly dense, high-frequency logic circuits, GaN is attractive for high-frequency power handling in analog/mixed-signal circuits and for its green-to-ultraviolet optoelectronic properties. Two bonding techniques offer essential benefits for the heterogeneous integration of compound semiconductors with silicon devices in a More-than-Moore approach, namely temporary bonding as well as direct wafer bonding. In the case of temporary bonding, thin layers can reliably be handled and transferred onto another substrate. On the other hand, direct wafer bonding allows a permanent layer transfer, for low defect compound semiconductor layer. Both these techniques are introduced in the following. 12 403 CS MANTECH Conference, May 13th 16th, 2013, New Orleans, Louisiana, USA