Wafer bonding of gallium arsenide on sapphire

Three-inch (100) gallium arsenide wafers were bonded to (1102) sapphire in a micro-cleanroom at room temperature under hydrophilic or hydrophobic surface conditions. Subsequent heating up to 500 ◦C increased the bond energy of the GaAs-on-sapphire (GOS) wafer pair close to the fracture energy of the bulk material. The bond energy was measured as a function of the temperature. Since the thermal expansion coefficients of GaAs and sapphire are close to each other, the bonded wafer pair is stable against thermal treatment and quenching in liquid nitrogen. During heating in different gas atmospheres, macroscopic interface bubbles and microscopic imperfections were formed within the bonding interface, which were analysed by transmission electron microscopy (TEM). These interface bubbles can be prevented by hydrophobic bonding in a hydrogen atmosphere. PACS: 68.35; 81.40; 85.30 Direct wafer bonding (DWB) refers to the adhesion of mirrorpolished and flat surfaces of wafers of various materials to each other at room temperature. Its main application area has been silicon wafer bonding for silicon-on-insulator (SOI) materials fabrication, power devices, and silicon based sensors and actuators [1–5]. Roomtemperature bonding occurs because of attractive Van der Waals forces or hydrogen bridge bonds between a few monolayers of water molecules on the surface of the wafers. The resulting bonding energy is typically weak compared to bond energies of covalently bonded materials. Therefore, after wafer bonding at room temperature, heat treatment at elevated temperatures is required to increase the bonding energy across the bonding interface. DWB can also be demonstrated for a variety of other materials besides silicon, such as Si/sapphire, Si/GaAs, and other combinations [6–9]. In contrast to heteroepitaxy, DWB works independently of the crystal structure, lattice mismatch, and even crystallinity of the materials to be bonded and allows combinations of materials without the introduction of dislocations due to a difference in the lattice constants. A disadvantage of the wafer bonding approach to the combination of different materials is the presence of different thermal expansion coefficients. Thermally induced mechanical stress may cause cracking and debonding of the bonded wafers in the heat treatment required to increase the bonding energy. A few material combinations of technological interest exist for which this problem is negligible or at least tolerable, for example silicon carbide/silicon [10], germanium/sapphire, and gallium arsenide/sapphire. This article deals with DWB of gallium arsenide on sapphire. Packaging of gallium arsenide and sapphire is of technological interest for producing integrated high-frequency devices, e.g. those based on GaAs amplifiers and microwave filters made from high-temperature superconductor films epitaxially grown on (1120)-oriented sapphire substrates [11], which offer very low dielectric loss [12]. Such devices operating at liquid nitrogen temperatures may find applications in the field of satellite and cellular phone communications. Because of the different lattice constants of (1102) sapphire and (100) GaAs it is not possible to grow GaAs of high quality on sapphire. Hence the DWB of GaAs-on-sapphire, optionally with subsequent thinning of the GaAs wafer, offers an attractive way to fabricate GaAs layers on sapphire. In this paper, we will deal with only the issue of wafer bonding. 1 Experimental procedures and results Three-inch wafers of (100) GaAs and (1102) sapphire with thicknesses ranging from 400 to 500 μm are positioned, in a micro-cleanroom setup, with their polished sides face to face [13]. The wafers are separated by three small spacers. The versatility of the DWB technique, means it is not necessary to take into account the relative crystallographic orientation of the wafers. Usually, the wafers are placed with their flats lining up. The mean roughness of the surfaces is lower than 0.5 nm, measured with atomic force microscopy Fig. 1. The wafers have a negligibly low waviness. A jet of purified water between the two wafers removes dust particles from the surfaces. Then, the micro-cleanroom is closed, and the wafers are dried by being rotated at 3300 r.p.m. for 5−7 minutes. During the rotation, the wafers are illuminated by an IR lamp, which heats the wafers to nearly 100 ◦C to speed up the drying process. After this final cleaning procedure, the surfaces remain