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.

I. Introduction

Digital isolators are electronic devices that carry digital signals to and from transceiver die whilst also providing galvanic isolation thereby delivering safe voltage levels for user interfaces and low voltage circuitry. They are employed in a wide range of applications from isolated-gate drivers for electric vehicles to medical electronics, each application requiring a specific minimum level of isolation. In general, isolation can be provided by optical, capacitive or magnetic coupling [1]. One of the most efficient technologies available on the market is based on a magnetic coupling through a micro-transformer where coils are separated by a polyimide isolation barrier m thick (cf. Fig. 1) thus enabling a high data transmission rate Mb/s [2]. A digital isolator must pass several regulatory standards before it can be released to the marketplace. These include reliability tests such as withstand and surge voltage as well as high voltage endurance (HVE) test protocol to determine the lifetime [3], [4].