Epitaxial Oxide Growth on Si(001) for Floating Epitaxy, a Novel Process for Silicon-on-Insulator Wafer Production

Abstract

As scaling continues in the semiconductor industry, silicon-on-insulator (SOI) wafers are increasingly becoming the substrate of choice, due to higher channel mobility, effective device isolation, reduced short channel effects, minimized parasitic capacitance, and therefore higher speed, compared to a regular silicon wafer. Current methods of SOI wafer production, however, will have difficulty achieving the desired silicon device layer and buried oxide insulator layer thicknesses and eliminating interface roughness as scaling proceeds. We propose "Floating Epitaxy SOI" as a novel method of SOI production utilizing an all in-situ growth process. Floating Epitaxy SOI involves Molecular Beam Epitaxy deposition of an epitaxial template oxide, oxidizing through the epitaxial template layer to establish the insulation layer, and silicon growth on top of the epitaxial template oxide layer (which is now "floating" on top of an amorphous oxide layer). The key to this process is the epitaxial oxide template layer, which must deposit on the silicon substrate as an atomically smooth film with a lattice parameter close to that of silicon and must be sufficiently stable in both an oxygen an in vacuum annealing to relatively high temperature to achieve Floating Epitaxy SOI. Although many researchers have examined epitaxial oxides on silicon, this study focuses on epitaxial films over large area substrates, while virtually all other studies report on growth on small substrate sizes. Also, the oxide stability limits on silicon in vacuum have not been thoroughly established by previous work, and are investigated here. The growth and thermal stability of this epitaxial oxide template layer are discussed, as well as brief results for through-oxidation "floating" of the template oxide layer and silicon growth experiments.

BaO, SrO, CaO, Ba[subscript 1-x]Sr[subscript x]O, SrTiO₃, CaTiO₃, and Ca[subscript 1-x]Sr[subscript x]TiO₃ were successfully epitaxially deposited on Si(001) substrates. A 64:36 Ba:Sr ratio was used for the solid solution of Ba[subscript 1-x]Sr[subscript x]O, in order to achieve close lattice matching with silicon; a 50:50 Ca:Sr ratio was used initially for the Ca[subscript 1-x]Sr[subscript x]TiO₃ solid solution, an attempt to mediate SrTiO₃'s 2% lattice mismatch with silicon and CaTiO₃'s orthorhombic structure. Alloying SrTiO₃ with calcium to alter the lattice parameter has not been studied much to this point in thin films, and this is the first demonstration of Ca[subscript 1-x]Sr[subscript x]TiO₃ and CaTiO₃ thin films grown directly on silicon. Reflection High Energy Electron Diffraction patterns of both Ba[subscript 1-x]Sr[subscript x]O and Ca[subscript 1-x]Sr[subscript x]TiO₃ indicated high quality 2D epitaxial films. A thin (3 monolayer) film of Ba[subscript 1-x]Sr[subscript x]O is stable on silicon to 535°C in vacuum, while a 5 monolayer Ca[subscript 1-x]Sr[subscript x]TiO₃ film survives to 740°C in vacuum, but roughens from a 2D toward a 3D surface above ˜650°C. Of the epitaxial oxides studied, the solid solution Ca[subscript 1-x}Sr[subscript x]TiO₃ would be the best choice for Floating Epitaxy SOI, based on epitaxial growth quality and stability.

High-resolution TEM indicates the presence of an amorphous interfacial layer at the SrTiO₃Si interface, as grown. X-ray diffraction confirms an epitaxial film, with a lattice parameter larger than that of bulk SrTiO₃, likely due to oxygen deficiency in the film. Annealing 17.5nm SrTiO₃Si(001) at 800°C in 5.5 Torr of oxygen for 30 minutes results in an equivalent oxide thickness of 10.3nm, sufficient for scaling to 2020. X-ray diffraction after annealing reveals a still-epitaxial SrTiO₃ film, with sharper 2θ and χ peaks and a lattice parameter closer to that of bulk SrTiO₃. These results validate the "floating" epitaxy approach: an epitaxial film remains on top of an amorphous insulator, after through-oxidation of the substrate. Direct deposition of epitaxial silicon on Ca[subscript 1-x]Sr[subscript x]TiO₃ and solid-phase epitaxy of silicon on a CaTiO₃ film are promising, but interface engineering or a surfactant may be required to achieve a high quality, single crystal silicon layer.