Low Temperature Si Direct Bonding by Plasma Activation
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Low Temperature Si Direct Bonding By Plasma Activation Yonah Cho1 and Nathan W. Cheung2 Department of Materials Science and Engineering, University of California, Berkeley, CA 94720 2 Department of Electrical Engineering and Computer Sciences, University of California, Berkeley, CA 94720 1
ABSTRACT Chemical and physical effects of plasma exposed Si wafer pairs were investigated on Si wafer bonding. Oxygen plasma treated Si wafer pairs bonded more strongly at room temperature compared to chemically cleaned, hydrophilic and hydrophobic Si. After 50 hours of annealing at 105 oC, the surface energy of the bonded Si pair reached the surface energy of bulk Si (100). Xray photoemission spectroscopy (XPS) measurements indicated that the exposure of both hydrophilic and hydrophobic Si to oxygen plasma increased a SiO2 like state in the surface layer to a depth of 1.5 nm. Atomic force microscopy (AFM) study showed that plasma irradiation at 300 watts up to 30 seconds did not change surface roughness below 0.5 nm. Exposure to He plasma or N2 did result in enhanced bonding after annealing at 165 oC, however, over smaller areas. INTRODUCTION Since the first report by Lasky et al. [1] and Shimbo et al. [2], Si direct bonding has been an area of great research and development. Direct bonding allows the most intimate integration of two entities without external forces such as electric field or an adhesive layer. This advantage has rendered its applications in SOI [3] technology and Si-based pressure sensors [4]. One of the drawbacks of direct bonding has been high temperature annealing above 900 oC after initial room temperature bonding in order to obtain a sufficiently strong interfacial bonding strength. Due to thermal stresses rising from a thermal expansion mismatch and metal diffusion into device layers, high temperature processes are detrimental and not desirable. For these obvious reasons, development of a low temperature bonding method is desirable. Tong et al. [5] initially reported that a stronger bonding was obtained between hydrophilic surfaces than between hydrophobic surfaces after annealing below 600 oC. More recent studies have suggested that exposure to oxygen plasma prior to bonding further improves adhesion and requires lower annealing temperatures [6,7,8]. Various plasma gases including O2, N2 and NH3 were also reported to result in similar bonding performance due to plasma-induced chemical reactivity [9]. Despite the practical use of plasma activated bonding in MEMS applications [10,11], a fundamental understanding of plasma activated bonding is lacking. Especially, bonding kinetics, chemical effect, mechanical effect and surface charge effect are not well understood. In an attempt to contribute to a better understanding of plasma activated bonding, low temperature kinetics, chemical changes, and morphological changes were investigated and addressed in this paper.
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EXPERIMENTAL For the study of bonding kinetics, p-type (ρ=5-10 Ω-cm), 500 µm thick, 4" Si (100) wafers were used. The test wafers were fi
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