PECVD Low Stress Silicon Nitride Analysis and Optimization for the Fabrication of CMUT Devices

An analysis of two technological options to achieve a high deposition rate, low stress PECVD silicon nitride to be used in Capacitive Micromachined Ultrasonic Transducers (CMUTs) fabrication is presented. A tradeoff between deposition rate, residual stress and electrical properties is showed. A double layer of silicon nitride with a deposition rate of ~100 nm/min and low compressive residual stress is obtained, which is suitable for the fabrication of the thick nitride layer used as mechanical support. A mixed frequency nitride with compliant insulating performance is analyzed. A complete characterization is reported in terms of interface density influence on residual stress, refractive index, deposition rate, and thickness variation. A complete device was fabricated using the optimized nitrides and its performance was tested, demonstrating full functionality in ultrasound imaging applications.