PECVD SiNx passivation with more than 8 MV/cm breakdown strength for GaN-on-Si wafer stress management

Abstract

In this work, multi-layer PECVD SiNx/SiNx and SiNx/SiOy passivations are developed featuring very high soft breakdown strength and tunable stress properties, which would allow for stress engineering and wafer bow minimization. AlGaN/GaN-on-Si wafers (150 mm) with very low initial bow (<5 μm) are processed in a CMOS compatible manner. The effect of the major processing steps, namely passivation and metal deposition, on the wafer bow is continuously monitored. In this process aimed at power devices, relatively thick passivation is needed (1.5 μm), which would induce very high stresses on the wafer if a single-layer deposition is applied. Hence, deposition of multiple layers is explored through mechanical modelling and simulation, leading to a stress-free passivation. The optimized multi-layer dielectric consists of two different SiNx single layers (referred to as T40 and R100), which have opposite stress properties, with T40 being tensile and R100 being compressive. By adjusting the thickness ratio of both layers and the number of total layers, mechanical stress within the multi-layer can be neutralized to achieve stress-free deposition. In addition, the optimization of the film properties includes the electrical properties of the passivation, and is designed primarily for high voltage applications. The developed SiNx/SiNx passivation has a soft breakdown strength with more than 8 MV/cm, and leakage currents below 1 nA/mm2 up to soft breakdown. After dielectric development, Schottky and MIS device characteristics with SiNx/SiNx multi-layers are characterized in DC and pulse mode measurements. As measurements suggest, the developed passivation is suitable for GaN-on-Si HEMT applications.

Keywords

• PECVD
• Multi-layer
• Dielectric
• Power
• SiNx
• SiOy