Orientation and Boron Concentration Dependence of Si Layer Transfer by Mechanical Exfoliation
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Orientation and Boron Concentration Dependence of Si Layer Transfer by Mechanical Exfoliation Kimmo Henttinen, Tommi Suni, Arto Nurmela, Veli-Matti Airaksinen1, Ilkka Suni and S.S Lau2 VTT Electronics, FIN-02044, Finland. 1 Okmetic Oyj, FIN-01510 Vantaa, Finland. 2 Department of Electrical and Computer Engineering, University of California, San Diego, La Jolla, CA 92093-0407, U.S.A.
ABSTRACT Mechanical exfoliation strength has been measured in hydrogen implanted , and oriented Si wafers using the crack opening method. The bonding temperature required for exfoliation increases in the order , and . The same method has been applied to study the influence of boron doping on mechanical exfoliation in Si wafers. The required bonding temperature to exfoliate mechanically decreases with increasing doping level independent of the electrical activation of boron. The enhanced crystallization rate of boron doped Si is suggested as a plausible explanation for the result. INTRODUCTION Low-temperature bonding and layer transfer processes are promising techniques for three dimensional integration of electronic, optical and micromechanical devices. Hydrogen implantation induced silicon layer transfer is a well-documented method to form silicon on insulator (SOI) and other device structures [1]. When the H-implanted and bonded wafer is annealed at moderately high temperatures of 400-600oC, the exfoliation of the implanted wafer takes place due to hydrogen pressure built-up in the microcavities located near the projected range of hydrogen. It has been shown that the H-implanted silicon can also be exfoliated by mechanical means after low-temperature bonding and subsequent bond annealing at 150-200 ºC [2,3]. As an implanted and bonded wafer pair is subjected to mechanical splitting forces, fracture takes place along the path of least resistance. If the implanted region is rendered weaker than the bonded interface, the fracture propagates within or near the implanted zone, instead of at the bonded interface. Mechanical exfoliation has the advantage of producing a smooth split surface and an intrinsically low-temperature process for the matching of dissimilar materials. Recently, the mechanical splitting process has been reported for patterned implanted Si wafers [4]. Many physical and chemical phenomena in silicon are dependent on crystal orientation and doping concentration. The objective of this work was to study the effect of silicon crystal orientation and boron concentration on the mechanical exfoliation process. Similar studies have been carried out to investigate blister formation associated with the thermal splitting process [5,6,7]. We have carried out direct measurements on hydrogen induced weakening of the implanted layer using the crack opening method [8]. The implanted and split layers were characterized by Rutherford backscattering spectrometry (RBS) and ion channeling, atomic force microscopy (AFM) and optical methods.
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EXPERIMENTAL In this work , and oriented p-type 100 mm Si wafers with a resistivity of 1-35 Ω
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