Wafer Bonding and Layer Transfer For Thin Film Ferroelectrics
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Wafer Bonding and Layer Transfer For Thin Film Ferroelectrics Jennifer L. Ruglovskya and Young-Bae Parka, Cecily A. Ryanb and Harry A. Atwatera,b a Gordon-McKay Laboratory, Applied Physics Department, Harvard University, 9 Oxford, Cambridge, MA 02138, b Thomas J. Watson Laboratory of Applied Physics, California Institute of Technology, Pasadena, CA 91125 ABSTRACT We report on the layer transfer of thin ferroelectric materials onto silicon substrates. H+ and He+ ion implantation created a buried sacrificial layer in the c-cut BaTiO3 and LiNbO3 single crystals. Bubble formation and thermodynamics of cavity at the bonding interface have been investigated, and single crystal thin film layers were transferred onto crystalline silicon substrates. We have found that defects generated by ion implantation in ferroelectric materials can be significantly recovered with the subsequent annealing for layer splitting. INTRODUCTION In recent decades, ferroelectric materials have been extensively studied for application in dynamic random access memories and non-volatile random access memories (NV-RAM) [1]. Recently, these materials have been applied to MEMS and electro-optic device applications including microfluidic pump/actuators and electro-optic modulators [2]. In most cases, ferroelectric thin films have polycrystalline microstructures which for ferroelectric device applications like NVRAM and MEMS may affect charge retention, time-dependent fatigue, piezoelectric coefficients, etc. Although ferroelectric thin films offer several advantages over bulk materials, no thin film devices superior to bulk devices exist to date. Therefore, a number of different methods to fabricate functional oxide films have been studied to obtain a high quality ferroelectric active layer formation. The growth of epitaxial thin films at relatively high temperature (> 450oC) have been reported using metal organic CVD (MOCVD), molecular beam epitaxy (MBE), sputtering, evaporation, atomic layer deposition (ALD), pulsed laser deposition (PLD), chemical solution deposition (CSD) and sol-gel processes in which alternate substrates hold promise for the growth of high-quality lattice-matched epitaxial films [3]. Unfortunately, most of these technologies have been plagued by difficulties in reproducibility of results. There is considerable interest in a recent layer transfer and wafer bonding process that produces "Smart-Cut" or "ion-sliced" single crystal thin film layers. Light elements such as hydrogen and helium are implanted into a wafer to a specified projected range and a film of equivalent thickness is exfoliated [4-6]. So far, layer transfer has been successfully performed for Si, Ge, SiC, diamond, and InP[7-10]. In this study, we have performed H+ and He+ ion implantation on single crystal LiNbO3 and BaTiO3 and bonded to SiO2/Si, a dissimilar substrate. The main purpose of this study is the investigation of the ion-induced layer transfer mechanism as a pathway toward high quality single crystal ferro/piezoelectric and electro-optic thin film
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