Improving Polyimide Isolation Performance by Tailoring Interfaces with Nitride Layers for Digital Isolator Application

This paper reports on the beneficial effects of adding silicon nitride (SiN) thin layers to protect polyimide (PI) insulation used in new, high voltage digital isolator devices. The larger bandgap of SiN enables an increased barrier height at the interfaces between metal and PI thus limiting charge injection and mitigating space charge formation in PI bulk. Wafer-level electrical tests on PI, PI-SiN, SiN-PI and SiN-PI-SiN multilayer capacitor structures are presented and show reduced leakage currents, lower HV dielectric losses and higher breakdown with SiN barriers. This is confirmed at device level with superior high voltage endurance and surge for SiN-PI-SiN insulated devices. These deliver a high working voltage of 900 V
_rms
(750 V
_rms
after derating) for single die and 1500 V
_rms
for back-to-back device with > 25 kV surge rating.