A transfer-enhanced semiconductor substrate has been demonstrated, integrating a thin InP layer on a Si handle substrate with an imbedded porous silicon layer. A Si wafer was anodized to create a low-density porous surface layer. A thin layer of InP was transferred to the Si handle substrate through wafer bonding and hydrogen-induced exfoliation. High-resolution X-ray diffraction, atomic force microscopy, and transmission electron microscopy characterization showed the structure to have high surface and crystalline quality for epitaxial deposition after undergoing chemical mechanical polishing. Layer transfer capability was demonstrated with a similar structure by mechanical fracture through the porous Si layer.
Engineered composite substrates for thin film layer transfer applications are fabricated by incorporating the techniques of anodic etching, wafer bonding, and hydrogen-induced exfoliation. Silicon substrates (p/p+) are subjected to anodic electrochemical etching in 25% HF electrolyte to create double layer (40%/60% porosity) structures, which provide the means for subsequent mechanical transfer. Indium phosphide (InP) layers are transferred to the porous Si/Si substrate via silicon nitride interlayer bonding and hydrogen exfoliation. After chemical mechanical polishing, the transferred InP layers have a surface roughness of 0.6 nm and high crystalline quality. Metal-organic chemical vapor deposition on the composite substrate shows that residual ion implantation defects present in the InP template layer do not extend into epilayers, and the substrate maintains its high crystalline quality and mechanical integrity. Transfer of the epitaxial layers from the porous silicon handle wafer to a secondary substrate was achieved via fracture along the double porous layer interface, with no impact on the epilayer strain.
The chemical mechanical polishing of III-V materials including GaAs, InP, InAs, and GaSb is investigated with sodium hypochlorite and citric acid solutions. It is found that the surfaces can be polished to below 0.5 nm RMS surface roughness without the induction of crystalline damage using similar abrasive-free polishing solutions for all the materials with controlled polishing rates of 10 nm / min which is important for touch polishing of exfoliated III-V layers. The optimal composition of the slurry is adjusted for the particular material. A balance between reduced sub-surface mechanical damage and smooth surface morphology is obtained by adjusting the amount of citric acid. However, the damage-free planarization in these cases may be aided by an oxide formation as predicted by Pourbaix diagrams. Additionally, triple axis x-ray diffraction is found to be an extremely sensitive, nondestructive method for evaluating damage induced by the CMP process.
A Cleave-Engineered Layer Transfer (CELT) substrate was fabricated consisting of a thin InP template layer transferred, via wafer bonding and hydrogen exfoliation, onto a silicon handle wafer with a porous layer at its surface. The mechanically weak porous silicon layer enables transfer of subsequent epitaxial growth and device fabrication on the template layer by initiating cleavage through the porous layer. Structural and mechanical property changes in the porous silicon layer after various processing conditions were studied. Characterization included atomic force microscopy, high-resolution x-ray diffraction, scanning electron microscopy, and nanoindentation measurements. Atomic force microscopy and transmission electron microscopy show the template layer has high surface- and crystalline- quality for subsequent epitaxial deposition. Layer transfer capability was demonstrated on a similar structure by fracture through the porous layer.
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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