Plasma-enhanced chemical vapor deposition of silicon oxynitride for micromachined millimeter-wave devices

Silicon oxynitride films were deposited by plasma-enhanced chemical vapor deposition at low temperature and frequency using SiH4 + NH3 + N2O gas mixtures. The process is optimized in order to deposit film with low tensile stress and high resistance during KOH etching. By increasing the gas flow of nitrous oxide (N2O), the film tends to be oxygen rich and the usual as-deposited high compressive stress is reduced to its lowest state at O/Si = 0.74. Annealing films above 480 degrees C generates low tensile stress suitable for membrane fabrication, and further infrared spectroscopy analysis shows that the shrinking of Si-O and Si-N bonds seems to be the cause of reversing the stress's nature. Young's modulus of the optimized layer is characterized by indentation. In application, 75 Omega coplanar waveguides (CPW) were fabricated on the top of an oxynitride membrane and characterized in term of insertion loss and effective permittivity. The results were compared to those obtained with the well-controlled bilayer silicon oxide-nitride membrane technology. The obtained losses are lower than 0.2 dB at 30 GHz with a free-space propagation signal.