Nonlinear conduction, dielectric breakdown and electrical aging of silicon nitride films : application to high voltage galvanic isolation technology

Overview & Synopsis

Silicon nitride (SiNx) thin films are widely used in modern technologies due to their tunable electrical, optical, and mechanical properties, including band gap, refractive index, dielectric constant, and internal stress. These properties are closely tied to the film's chemical composition, which depends on the deposition conditions. As such, SiNx is a key material in various applications, including anti-reflective coatings for solar cells, optical waveguides, encapsulation layers for flexible electronics, oxidation masks, and dielectric layers for thin-film transistors. In this thesis, special focus is given to the application of SiNx as a dielectric barrier layer in polyimide-based digital isolators (DIs). These multilayer stacks are used in galvanic isolation systems for high-voltage environments such as electric vehicles and power electronics. The insertion of a thin SiNx layer between the metal electrode and the polyimide enhances insulation reliability by acting as a charge-blocking barrier and field grading layer, limiting leakage currents and improving long-term dielectric strength. To enhance the reliability of next-generation DIs, it is crucial to understand the conduction and degradation mechanisms of SiNx under high electric field stress and over time. This thesis addresses this challenge by investigating how the composition of PECVD SiNx films, controlled via the [N]/[Si] ratio, affects their electrical properties, degradation behavior, and dielectric performance. To investigate the electrical performance of SiNx in such configurations, a series of thin films with varying [N]/[Si] atomic ratios were deposited by plasma-enhanced chemical vapor deposition (PECVD). Ten blanket layers were fabricated and thoroughly analyzed using a combination of XPS, FTIR, SIMS, GDOES, and electron microprobe analysis. These techniques provided insights into film composition, hydrogen and oxygen content, and bonding structure, particularly the evolution of Si-H, N-H, and Si-N bonding configurations as a function of the [N]/[Si] ratio. The electrical characterization of these SiNx layers was performed on metal-insulator-metal (MIM) structures. Using high-voltage dielectric spectroscopy (HVBDS) and DC polarization/depolarization protocols, the dielectric constant, AC conductivity, dielectric loss, and leakage current were extracted over a wide range of frequencies and electric fields. Si-rich films exhibited higher permittivity and conductivity, along with earlier onset of non-linear conduction. To interpret the transport behavior, the Poole-Frenkel conduction model was applied, allowing extraction of barrier heights and insight into defect-assisted charge transport mechanisms. The study also included electrical aging tests under strong electric field conditions to simulate long-term stress and evaluate the degradation behavior of the different compositions. The evolution of leakage current and conduction mechanisms over time was analyzed to identify failure modes and the influence of chemical composition on insulation stability. Finally, representative PI/SiNx-based multilayer stacks were fabricated to emulate real-world digital isolator structures. These were tested under electrical stress to evaluate the impact of the SiNx layer on the overall insulation performance. The results confirmed that the integration of an optimized SiNx barrier significantly improves dielectric robustness and reliability the dependence of the performance on the composition. In summary, this work establishes strong correlations between the chemical structure and electrical behavior of PECVD SiNx films, and demonstrates their potential to enhance the performance and lifetime of next-generation high-voltage digital isolators.