Thermal stress modelling of diamond on GaN/III-Nitride membranes

TL;DR

This paper investigates thermal stresses in diamond-on-GaN/III-N membranes during CVD growth, using models and experiments, and proposes pre-stressing to reduce bow for bonding-free device fabrication.

Contribution

It introduces an analytical and numerical approach to model thermal stresses in diamond on GaN/III-N membranes and suggests pre-stressing techniques to mitigate membrane bow.

Findings

Thermal stresses are mainly due to CTE mismatch between materials.

Numerical models accurately predict membrane bow and stresses.

Pre-stressing the membrane reduces bow and facilitates bonding-free diamond growth.

Abstract

Diamond heat-spreaders for gallium nitride (GaN) devices currently depend upon a robust wafer bonding process. Bonding-free membrane methods demonstrate potential, however, chemical vapour deposition (CVD) of diamond directly onto a III-nitride (III-N) heterostructure membrane induces significant thermal stresses. In this work, these thermal stresses are investigated using an analytical approach, a numerical model and experimental validation. The thermal stresses are caused by the mismatch in the coefficient of thermal expansion (CTE) between the GaN/III-N stack, silicon (Si) and the diamond from room temperature to CVD growth temperatures. Simplified analytical wafer bow models underestimate the membrane bow for small sizes while numerical models replicate the stresses and bows with increased accuracy using temperature gradients. The largest tensile stress measured using Raman spectroscopy at room temperature was approximately 1.0 $\pm0.2$ GPa while surface profilometry shows membrane bows as large as \SI{58}{\micro\metre}. This large bow is caused by additional stresses from the Si frame in the initial heating phase which are held in place by the diamond and highlights challenges for any device fabrication using contact lithography. However, the bow can be reduced if the membrane is pre-stressed to become flat at CVD temperatures. In this way, a sufficient platform to grow diamond on GaN/III-N structures without wafer bonding can be realised.