Layer Transfer by Bonding and Laser Lift-Off
Monolithic integration by sequential deposition, lithographic patterning, and etching of metal, dielectric and semiconductor thin films has been the dominant manufacturing scheme throughout the history of the integrated circuit. This manufacturing paradig
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11.1 Introduction
11.1.1
Overview
Monolithic integration by sequential deposition, lithographic patterning, and etching of metal, dielectric and semiconductor thin films has been the dominant manufacturing scheme throughout the history of the integrated circuit. This manufacturing paradigm has been adapted to the fabrication of Micro-ElectroMechanical Systems (MEMS), active-matrix displays, read/write heads for disk drives and optoelectronic devices. Although this single-substrate approach is scalable and inherently cost-effective, there are some materials systems, functionalities and device designs that cannot be realized in this manner. The ideal growth substrate for a specific thin-film heterostructure may not be the ideal substrate from the standpoint of heat extraction, mechanical properties, thermomechanical behavior, optical transparency, electrical conductivity or chemical compatibility. In these cases, the growth substrate must be removed, although it is not always practical to do so by mechanical or chemical means alone. In the longer term, it may be desirable to integrate thin-film heterostructures grown separately on several growth substrates onto a single platform. As IC scaling approaches an era of saturation, the focus is gradually turning to the challenge of intimately integrating a broad spectrum of high-performance materials to enhance the functionality of microsystems. The ability to transfer thin-film heterostructures from one substrate to another is central to both advanced packaging and heterogeneous integration. In this Chapter, the application of pulsed lasers to the separation of nitride and oxide heterostructures from their growth substrates is described. The basic process as illustrated in Fig. 11.1 involves the irradiation of an interface between an absorbing film and a transparent substrate using a short laser pulse, usually of nanosecond duration, directed through the transparent substrate. The laser wavelength must be chosen so that the photon energy is between the band gap energies of the substrate and of the interfacial thin film. This technique, often referred to as "laser lift-off' (LLO), was first demonstrated in the GaN/sapphire materials system by Kelly and coworkers [1] using the third harmonic of a NdYAG laser at a wavelength of 355 nm. Subsequent development of the technique with more powerful lasers at shorter wavelengths (e.g. KrF excimer lasers at 248 nm) revealed the utility of this method for the rapid separation and transfer of
M. Alexe et al. (eds.), Wafer Bonding © Springer-Verlag Berlin Heidelberg 2004
378 T.D. Sands et al. (In,Ga)N device heterostructures without sacrificing device performance [2- 8] ; in some cases, the device performance was shown to be enhanced [9- 10]. As unsupported thin-film membranes are generally not sufficiently robust to permit handling, the transfer of heterostructures from one substrate to another necessitates a bonding step prior to the separation process. This chapter thus considers the entire "paste-and-cut" process. l aser
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