Surface Characterization Driven CMP Optimization for GaN

Abstract

III-V nitride semiconductors are widely used for optoelectronic devices such as blue lasers diodes (LDs), light-emitting diodes (LEDs), and high power, high frequency field effect transistor (FET) devices [1]. Among these, GaN has exceptional characteristics such as providing direct and wide band-gap energy and high electron mobility and hence is typically used for the high power applications and LEDs [2]. The performance and quality of the optical devices directly correlate to the level of surface defectivity and planarization. Consequently, application of Chemical Mechanical Planarization (CMP) with minimal defectivity is required. However, introducing CMP on a film that is poorly grown due to epitaxial challenges such as GaN [3], in addition to GaN being a very robust film to effectively remove material limits the CMP performance. In this study, a systematic experimental approach is followed to determine the conditions to enhance material removal rate while controlling surface defectivity for GaN CMP. Two different crystallographic orientations of the GaN are characterized and compared to a commercial 2” GaN wafer to optimize the CMP material removal rate performances on the basis of the surface electrical, topographic and crystallographic properties. Silica based slurries were used as a function of pH, type of polishing pad and conditioning. CMP responses of Ga reach orientation (Face A) and N reach orientation (Face B) were different while the commercial 2” GaN wafer with unknown surface crystallographic structure was observed to have similar behavior with the Face A bulk wafer. The optimized CMP conditions as a function of surface characteristics helped in increasing the very limited material removal rates of GaN while minimizing defect formation and enhancing the selectivity against silica.