High Resolution Double-Crystal X-ray Diffraction Imaging for Interfacial Defect Detection in Bonded Wafers

Abstract

Wafer bonding is used for the integration of heterogeneous semiconductor materials, without the constraints of lattice constant or crystal structure mismatch. The most important aspect of such bonding is the surface flatness and the absence of particles in the bonded interface. It is very important to identify these particles, which can further result in poor bonding. The most common method for identifying has been infra-red (IR) imaging, but it is limited to materials which are transparent in the operating region of the CCD camera. Also, the resolution of IR image is low. X-ray topography or X-ray diffraction imaging (XRDI) has been identified as a very sensitive and high resolution imaging technique, capable of resolving variations due to very small particles within both the bonded and the unbonded areas. It also allows for imaging of samples which are opaque to IR, like heavily doped wafers that are used for solar cell applications. Wafer bondingDirect wafer bonding allows for the integration of heterogeneous semiconductor materials, without the constraints of lattice constant or crystal structure mismatch. Direct wafer bonding is based on intermolecular interactions -including Van der Waals forces, hydrogen bonds and strong covalent bonds [1]. Main applications are for bonding of Si substrates for heat sinking of GaN HEMTs, creation of multi-junction solar cells (GaAs/InGaAs etc), creation of SOI wafers (Smartcut, ELTRAN, Nanocleave), and production of microelectronic (MEMS, NEMS) devices [1]. Wafer Bonding RequirementsWafer bonding between two substrate materials relies [2] on the following parameters: (a) the materials being bonded (mismatch of coefficient of thermal expansion (CTE)), (b) surface conditions (flatness, smoothness and cleanliness), and (c) bonding environment (bonding temperature, and applied force or pressure). The creation of a bonded interface between the wafers is usually performed at room temperature [3], with further multi-stage annealing steps performed to increase the bond strength, and make the interface more conductive [4].Particles present on the surface cause problems for the fusion bonding process. Even sub-micron size particles can create voids which prevent bond from occurring in that local region. However, such voids can be reduced in size after annealing the wafers. The voids created due to trapped gas cannot be removed even after a long annealing process. 10.1149/05007.0091ecst ©The Electrochemical Society ECS Transactions, 50 (7) 91-96 (2012) 91 ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 131.215.225.9 Downloaded on 2015-07-12 to IP