Implementations of CVD diamond on gan device layers: Optical characterization of wafer-scale stress uniformity

The use of CVD diamond films for electronic thermal management is quickly being realized as a viable application in high-power and high-temperature electronics, owing to the exceptional thermal properties of diamond and the recent advances in diamond CVD growth technology. In this study, a thick (~100 μm) CVD diamond film was grown on a 75-mm wafer consisting of AlGaN/GaN high electron mobility transistor (HEMT) device layers. By removing the original growth substrate and transition nitride layers, the CVD diamond can be deposited on the back side of the wafer and used as a heat-spreading substrate in close proximity to the active region of the device layers. Ultraviolet (UV) and visible micro-Raman spectroscopy, as well as UV photoluminescence (PL) were used to characterize the stress distributions within the (~1 μm-thick) GaN active layer across the entire wafer. The shallow optical penetration depth of the UV excitation allows for measurements to be accomplished within the first ~100 nm of the material; measuring from both sides of the wafer yields separate stress information for the top and bottom regions of the GaN. Examinations of the stress profile across the wafer reveal an average tensile stress of ~ 0.86 GPa from the top side of the GaN layer (near the AlGaN/GaN interface) and a lower tensile stress of ~0.23 GPa from the bottom side (near the GaN/Diamond interface), resulting in a significant stress gradient along the growth direction of the material which is otherwise unexpected for a uniform layer. Factors influencing the stress and stress gradient will be presented and discussed, supported by results from finite element simulations of thermal growth stresses and electron microscopy of the device layers and interfaces.