Wafer bonding of wide bandgap materials

Abstract

Wafer bonding is a powerful technique for integration of materials. It enables creation of junctions and structures not attainable by the epitaxial growth due to lattice mismatch. Wafer bonding may involve no intermediate layer and allow the joined wafers to be stable at elevated temperatures.

Atomically smooth and flat wafers of almost any material spontaneously bond to each other even at room temperature, although further annealing might be required to increase the strength of bonding. High values of surface roughness make the bonding process more challenging. In this case, high temperature combined with applied stress is an effective route for a successful process.

The goal of the current work was to assess the potential of pressure assisted wafer bonding technique applied to diamond/silicon and silicon carbide/silicon carbide systems, where the wafer surface smoothness was limited.

Polished and unpolished (100) highly oriented diamond (HOD) films with an RMS roughness of 5 nm and 150 nm, respectively, as well as polished, polycrystalline diamond films with an RMS roughness of 15 nm were bonded to single-side polished silicon in ultra high vacuum at 32 MPa of applied uniaxial pressure. Successful fusion of unpolished HOD and polished polycrystalline diamond was observed at temperatures as low as 950°C while bonding of polished HOD to silicon was achieved at 850°C.

Fusion of polished polycrystalline diamond to silicon resulted in the formation of a non-uniform bonded interface. An abrupt boundary between the two wafers existed only in some regions of the interface, while other regions contained an up to 40 nm thick amorphous interlayer consisting of C, Si and O. A local phase transformation of diamond to graphite near the diamond surface asperities followed by inter-diffusion of C and Si has been suggested.

Fusion of polished HOD to Si resulted in the formation of the abrupt interface between the wafers, in the areas away from the diamond grain boundaries. Voids, partially filled with amorphous material, were observed at the fused interface near the diamond grain boundaries. Diamond polishing defects, potential out-diffusion of hydrogen from diamond and oxygen from Si are believed to have contributed to the observed non-uniformity of the bonded interface.

SiC wafers with an RMS roughness of 2 nm were successfully bonded at temperatures as low as 800°C. Cross-section transmission electron microscopy (XTEM) of specimens bonded at 1100°C revealed an atomically abrupt interface between the bonded wafers without any intermediate layer between them. The bonded SiC retained its high crystalline quality; no extended defects emanating from the interface were observed within the sampling region. Electrical measurements showed that azimuthal orientation of the bonded couple significantly influences the electrical character of the junction. A low resistance Ohmic interface can be created by high temperature fusion of aligned 6H-SiC/6H-SiC wafers.