LTCC as substrate - enabling semiconductor and packaging integration

Abstract

Gallium nitride (GaN) is commonly used in high-power and high-frequency devices but one limiting factor for its wide application is the availability of cost-effective substrates. The approach of “engineered substrates” is consequently followed up in this work. Low temperature cofired ceramics are used as carrier substrates in this work. The flexible material system allows the creation of a thermo-mechanically compatible base for GaN components. Sputtered aluminum nitride (AlN) layer form the buffer layer for the GaN deposition. The surface quality of these buffer layers reaches an arithmetic mean height R of 10 nm after chemical mechanical polishing and forms thus a surface quality which is adequate for thin film processing. GaN layers grown on these substrates can have both, tensile and compressive stress levels, while such grown on (100) silicon for comparison purpose show exclusively tensile stress. The growth of the GaN layer and resulting morphology is assumed to be mainly influenced by the process conditions during molecular beam epitaxy. The polycrystalline GaN layers show pronounced c-axis orientation at deposition temperatures of 700°C and 800°C. These first results encourage a further development of this new substrate architecture for integrated GaN systems.