Low Temperature Processing of Porous Silicon Films for Wafer Bonding-Based Thin-Film Layer Transfer Applications

Abstract

Porous silicon films were fabricated from p + -Si ͑0.001-0.005 ⍀ cm͒ wafers and characterized for their applicability to wafer bonding and layer transfer schemes. Conditions for producing porous silicon films compatible with both ͑i͒ wafer bonding surface smoothness requirements and ͑ii͒ layer transfer mechanical requirements were investigated. Nanoindentation of various films showed the quadratic dependence of Young's modulus on relative density. High porosity films ͑Ͼ90%͒ exhibited prohibitively high surface roughness values for wafer bonding applications, while wafers with lower porosity produced surfaces that were sufficiently smooth; films with midrange Young's modulus were selected for layer transfer applications. Demonstration of wafer bonding of porous silicon surfaces using silicon nitride interlayers was shown without the use of a high temperature anneal step to densify the porous surface or chemical mechanical polishing. Coarsening of the porous structure was observed at temperatures as low as 300°C, and the porous films remained pseudomorphic after annealing at all temperatures, as high as 900°C. Strong bonds were formed at the silicon nitride interfaces after low temperature annealing. After higher temperature annealing, mechanical fracture of the bonded stack occurred in the porous film parallel to the wafer bond interface. A fracture mechanism for these structures is proposed.Thin-film layer transfer represents a promising technique to increase versatility in the fabrication of complex semiconductor devices. Interest in layer transfer techniques to fabricate engineered substrates was largely sparked by the desire for silicon-on-insulator structures. One such technique, ELTRAN ͑Epitaxial Layer TRANsfer͒, 1-5 incorporated a silicon wafer that was electrochemically etched to produce a porous film at the surface; after annealing in a hydrogen atmosphere to sinter pores at the surface to produce a thin, fully dense surface layer, this film is subsequently used as a template for silicon homoepitaxy. This structure is then bonded via silicon dioxide interlayers to a silicon handle wafer, and transfer of the epitaxial Si layer is achieved by use of a high pressure water jet to fracture the stack at the weakest layer, namely the porous silicon. 1,5 The use of porous silicon as an imbedded mechanically weak layer leads to large area yield layer transfer; however, utilization of this and similar methods such as the porous Si process ͑PSI͒ 6 is limited to the transfer of silicon homoepitaxial or pseudomorphic layers.Using porous silicon/Si wafers in conjunction with wafer bonding and hydrogen exfoliation processes may bypass the limitation to homoepitaxial applications to create transfer-capable monolithic heterostructures. 7,8 A variety of composite semiconductor structures has been produced 9-19 using wafer bonding and hydrogen exfoliation techniques; this study focuses on demonstrating the compatibility of porous silicon films with these processes for use in layer transfer applications.Previous studies cond...

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