Anisotropic Delamination Energy of Bonded Rippled Silicon Surfaces Created by Ar + Bombardment
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Anisotropic delamination energy of bonded rippled silicon surfaces created by Ar+ bombardment Z.X. Liu and N.W. Cheung Plasma Assisted Materials Processing Lab., UC Berkeley, CA 94720, U.S.A.
ABSTRACT The surface topography of Si(100) modified by low energy Ar+ bombardment was characterized by Atomic Force Microscopy (AFM). AFM images show that ripples can be formed by 500eV Ar+ at incidence angle 40°. The spacing wavelength of ripples is around 70nm with wave vector parallel to the projected direction of ion beam. Direct bonding and mechanical delamination of Si wafer pairs with ripples are investigated. Delamination energy measured by crack-opening method along the direction perpendicular to the wave vector,γ⊥, is always smaller than that along the wave vector direction,γ//; Both γ⊥ and γ// are found to decrease with sputtering time. The AFM images after delamination indicate that the bonding and delamination process do not eliminate the ripples on the wafer surface. INTRODUCTION Direct wafer bonding is an important technology for materials integration in microelectronics, optoelectronics, and MEM systems [1]. Although most of works focus on forming a permanent bond between two wafers, a growing aspect of bonding involves the use of temporary or controlled bonding. Surface texturing could be a way to engineer the bonding to a prescribed strength, and delamination energy being dependent on the surface roughness is already demonstrated [2]. If the anisotropic delamination energy can be made anisotropic with respect to the cleaving directions, it can be used to delaminate multi-layer stack structures with interesting combinations. For example, consider a simple three-layer stack ABC (Fig.1). The edge-initiated crack can choose different interfaces (between A and B or between B and C) to propagate. One can get either the combination of AB and C or the combination of A and BC by delaminating the three-layer stack along two different delamination directions. The reason is that if the delamination energy between A and B (γAB) is anisotropic, γAB could be larger than the delamination energy between B and C (γBC) along one direction while γAB is smaller than γBC along the orthogonal direction. A potential application of anisotropic delamination energy will be in a double layer transfer process (Fig.2). One always desires the delamination energy between the handle wafer and the transfer layer as large as possible for the first delamination to detach the donor substrate. For the second delamination, one desires very small delamination energy to detach the handle wafer. Having an anisotropic delamination energy between the handle wafer and transfer layer is an elegant approach to facilitate a dual-function interface which can be used as a bonding layer as well as a delamination layer.
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γ⊥AB γ//AB A B C
Figure 1. The schematic of two different combination of the simplest three-layer stack ABC after mechanical delamination if the delamination energy γAB is anisotropic. Here we assume γ//AB> γBC > γ⊥AB where t
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