Integration of Materials and Device Research Enabled by Wafer Bonding and Layer Transfer
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INTEGRATION OF MATERIALS AND DEVICE RESEARCH ENABLED BY WAFER BONDING AND LAYER TRANSFER Q.-Y. Tong Wafer Bonding Lab., Research Triangle Institute and Ziptronix, RTP, NC 27709
Wafer bonding and layer transfer technology has emerged as a versatile approach for integrated research and development of materials and devices. It does not only provide a flexible way to prepare integrated materials but also breaks the barrier between materials science and device engineering since it appears to be one of the fundamental technologies for 3-D device fabrication and for integration of partially or fully processed dissimilar functional layers. The device performance can be significantly enhanced when both sides of device layers can be processed such as in the double gate CMOS and in HBTs. The processed device layers can be considered as unique materials layers and can also be integrated. The integration of dissimilar integrated circuits provides a promising solution to realize micro-systems or system on a 3-D chip in which multi-functional layers are interconnected (3-D SOC). The main issues are to develop methods that form strong bond between required materials at low temperatures and to cut the device layer without compromising the device integrity by using VLSI compatible processes. The advances in low temperature bonding of hydrophilic and hydrophobic silicon, compound semiconductors and other materials in ambient are reviewed, and application examples are discussed.
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Introduction Wafer direct bonding is a technology that allows wafers to be bonded at room temperature without using any adhesive or external forces, therefore is not prone to introduce stress and inhomogeneity. Wafer direct bonding is also different from the popular anodic bonding that employs heating and electric field and requires at least one of the bonding wafers to be a glass wafer or a wafer covered by a glass layer containing mobile ions. Wafer direct bonding technology is based on understanding the question of why broken pieces of any solid material usually can not be reversibly re-joined at room temperature in ambient even if the mating surfaces are perfectly complementary. It was understood that the main factors that prevent reversible rejoining appear to be the changes of the surfaces immediately after separation including surface reconstruction, roughening, adsorption, oxidation and contamination that reduce the surface energy significantly or prevent the surfaces from coming in close proximity. Based on this understanding, wafer bonding technology was introduced such that two wafers with surfaces that are sufficiently smooth, flat and clean can bond to each other without any adhesive or external forces at room temperature in ambient air [1, 2]. However, it was found that the standard wafer direct bonding was attributed to relatively weak van der Waals forces and subsequent annealing at high temperature is required to achieve a strong bond. To monolithically combine a variety of materials to form integrated materials for integrated circui
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