Wafer Bonding Involving Complex Oxides
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Abstract The present paper proposes a simple method which may be able to provide true single-crystal films of complex oxides on large substrates including semiconductors like silicon or gallium arsenide. The method describes a layer transfer process using layer splitting by hydrogen implantation and direct wafer bonding (DWB) to obtain single-crystal oxide films on different substrates. Alternatively, a fabrication process of ferroelectric-semiconductor heterostructures based on direct wafer bonding and layer transfer is also described. This process is an alternative method to the direct deposition of oxides films (ferroelectric, high-k) on silicon and allows fabrication of metal oxide-silicon heterostructures with an interface having a good structural quality as well as a low trap density.
INTRODUCTION Many applications which involve oxide thin films demand high quality films and a device using a single crystalline film is always highly desirable. Nowadays major effort is put into achieving epitaxial oxide thin films on different substrates including single crystalline silicon. However, even very good epitaxial ferroelectric films exhibit internal grain boundaries and high defect densities. It will be either very difficult or very expensive to achieve true single crystalline ferroelectric thin films on large area. Therefore, it is useful to develop a simple method able to provide true single crystalline films on large single crystalline semiconductor substrates. We propose a layer transfer process using layer splitting by hydrogen implantation and direct wafer bonding to obtain single crystalline oxide thin films on appropriate substrates. As the last decade has seen a surge of interest for ferroelectric thin films due to the wide range of their potential applications in microelectronics, especially in non-volatile memories, we recently proposed a novel fabrication process of ferroelectric-semiconductor heterostructures based on direct wafer bonding and layer transfer. This process is an alternative to direct deposition for the integration of ferroelectric oxides on silicon and allows fabrication of ferroelectric film-silicon heterostructures with an interface having a sufficient structural quality as well as a low trap density. Layer transfer processes have as a core step direct wafer bonding (DWB). DWB is colloquially known as "gluing without glue" and refers to the phenomenon wherein mirror-polished, flat, and clean wafers of almost any material, when brought into contact at room temperature, are locally attracted to each other by van-der-Waals forces and adhere or bond. The bonding at room temperature is usually relatively weak compared with that of covalently or ionically bonded solids. Therefore. the room temperature bonded wafers have to undergo a heat treatment to strengthen the bonds across the interface.' 285 Mat. Res. Soc. Symp. Proc. Vol. 574 © 1999 Materials Research Society
In most cases, the wafers involved in actual applications are semiconductor wafers consisting of single-crystal materials suc
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