Growth and Characterization of Spinel Single Crystals for Substrate Use in Integrated Electronics

An extensive study has been made on the growth and characterization of single‐crystal magnesium aluminate spinel. This material has received attention as a superior substrate for epitaxial silicon‐integrated electronic devices. Large spinel single crystals within the compositional range MgO:Al₂O₃–MgO:3Al₂O₃, have been successfully grown by flame fusion using a modern three‐tube post‐mix‐type Verneuil burner. Substrate wafers of (111), (100), and (110) orientations were prepared for silicon epitaxy. Surface preparation of the substrates was studied, including mechanical polishing, hydrogen annealing, and chemical etching. The composition, lattice parameter, crystalline perfection, mechanical hardness, thermal stability, and dielectric properties of the spinel single crystals have been characterized by various techniques in an effort to obtain a basic understanding for substrate use of this material system. Some trends in the dependence of these properties on composition were observed. In particular, the thermal stability increases with decreasing Al content in the spinel system. It was found that the spinel crystals with alumina content up to MgO:2.5Al₂O₃ are thermally stable and exhibit no cracking and/or exsolution behavior to at least 1200°C, which is the temperature commonly used for fabricating many silicon devices. However, the ease of growth and the mechanical stability decrease with decreasing Al content in spinel. A post‐growth annealing treatment enhances the mechanical stability and prevents formation of cracks during cutting. Substrates within the compositional range of MgO:1.5Al₂O₃–MgO:2.5Al₂O₃ are suitable for silicon epitaxy, and the choice of the optimum substrate composition depends on the specific conditions for processing of the silicon devices.