Stacking different semiconductor materials together is one way to build better devices. This form of integration can be applied to solar cells, biosensing and RF chips, power and photonic devices, and it can lead to increases in efficiency and reliability while trimming size, weight and cost. The phrase ‘more than Moore’ has been coined for such improvements, because they are not directly related to lithographic scaling and they can unleash an ever-increasing array of electronic devices.
Heterogeneous integration can take many forms, including the marriage of mature silicon technology with compound semiconductors sporting superior properties. Significant performance increases can result, alongside novel capabilities at comparatively low costs. Note, however, that this demand for integration is not limited to CMOS wafers, and it can be applied to any form of technically or economically preferred substrate.
Growth of III-Vs on silicon is a common approach for material integration. Much progress has been made in this area, but there are still several weaknesses associated with this technology: there is a high defect density at the growth interface; deposition rates for compound semiconductors are not that fast; and epitaxial equipment is an expensive purchase.
An alternative approach that addresses these concerns is plasma-activated direct bonding of different materials. Compared to epitaxial growth, this technology offers greater freedom for device design and process implementation, and its widespread use in recent times has demonstrated that most compound semiconductors can be directly bonded on different substrates.
Direct wafer bonding has in recent years grown from a process used primarily for the manufacture of SOI wafers to a production-proven process for heterogeneous integration of compound semiconductors. Recent technology advances have overcome the limitations of classical direct bonding and epitaxial growth of compound semiconductor layers on heterogeneous substrates. In particular, the ComBond process with its minimum impact on the crystal structure and ability to provide electrical conductivity via the interface can be key to enabling several applications. Heterogeneous integration using this new wafer bonding process will pave the way for innovative device structures with novel functionalities and increased performance.
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List of several compound semiconductor materials that can be bonded by low-temperature plasma-activated wafer bonding which are already in production (click thumbnail to enlarge)
TEM analysis of a GaAs/InP oxide-free covalent bonded interface. The high- resolution image clearly shows that the underlying crystal is not affected at all and the interface between the materials is below 2 nm. (click thumbnail to enlarge)
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