GaN-on-diamond - Robust mechanical and thermal properties

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Metadata	Details
Publication Date	2016-05-27
Authors	Huarui Sun, Dong Liu, James W. Pomeroy, Daniel Francis, Firooz Faili
Institutions	University of Bristol, Institute of Solid Mechanics
Citations	3

Abstract

Achieving robust mechanical and thermal properties in GaN-on-diamond is critically important for reliable next-generation high-power electronics based on this material system. The work described here demonstrates that excellent stress management and interfacial strength has been achieved, as well as homogeneous interfacial thermal properties across the wafer for the latest GaNon-diamond technology. INTRODUCTION Over the past decade significant effort and progress has been made to integrate diamond with GaN-based electronics for improved thermal management. To date, GaN-ondiamond HEMTs have been demonstrated with outstanding performance for high-power microwave device applications [1,2]. Of the various GaN-diamond integration technologies, the Element-Six approach considered in this work involves removing the substrate from a standard GaN-on-Si wafer, followed by the deposition of a thin dielectric layer and the subsequent growth of CVD diamond on the exposed GaN surface. Stress management in the GaN and the mechanical stability of the GaN-diamond interface, owing to the large thermal expansion mismatch between these materials, is naturally a concern which arises. Moreover, the wafer-level homogeneity of mechanical and thermal properties is yet to be assessed. In this paper, we use Raman spectroscopy to characterize the GaN layer stress in the manufactured GaN-on-diamond wafers, showing that this stress has nearly been eliminated through successive generations of wafer development. Using a micro-pillar based fracture test, we illustrate that the GaNdiamond interface has excellent mechanical stability. We further demonstrate a nearly homogeneous distribution of the effective thermal boundary resistance (TBReff) between GaN and diamond across a three-inch wafer using transient thermoreflectance mapping. MEASUREMENT TECHNIQUE Raman spectroscopy measurements were performed using a Renishaw InVia system with a 488 nm Argon laser. The E2 peak with stress coefficient -2.7 cm/GPa [3] was used to determine the inbuilt stress in the GaN layer, referenced to a stress-free value of 567 cm measured for bulk GaN (consistent with previously reported values). To assess the GaN-diamond interfacial strength, micropillars comprising the GaN/dielectric/diamond layer structure were created using focused Ga beam milling. A force measurement silicon probe mounted on a micromanipulator was used to apply controlled displacement or load to the GaN layer from the side of the micro-pillar. The force was measured by a piezoelectric sensor with a resolution of 0.1 N. The deformation and the fracture process were monitored in situ in a scanning electron microscope (SEM). The TBReff at the GaN-diamond interface is a lumped thermal resistance with contributions from the amorphous dielectric layer with its lower thermal conductivity and the diamond nucleation layer formed in the initial phases of the diamond growth. To measure the TBReff, a contactless laserbased transient thermoreflectance technique was employed. A 355 nm pulsed laser is used to heat the GaN surface, inducing a rapid temperature increase, which subsequently relaxes due to heat diffusion into the layers. A 532 nm CW laser is used to monitor the surface reflectance change caused by the temperature rise as a function of time. The TBReff between GaN and diamond is then extracted by fitting the thermoreflectance transient with a finite element thermal simulation. The laser measurement system is -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 Wafer S tr e s s ( G P a ) 568.0 567.5 567.0 566.5 566.0 565.5 565.0

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