A charge trapping layer to suppress RF losses using silicon nitride on high resistivity silicon substrates

Charge trapping layers suppress parasitic surface conduction on high-resistivity Silicon substrates for radiofrequency (RF) applications. This work investigates Silicon Nitride (SixNy) as a charge trapping layer, using variants deposited by physical vapor deposition (PVD) and plasma-enhanced chemical vapor deposition (PECVD). Coplanar waveguide (CPWG) insertion loss is measured to assess RF suitability. Layers are characterized by spectroscopic ellipsometry, Time-of-Flight Secondary-Ion-Mass-Spectroscopy, and Fourier transform infrared spectroscopy. Insertion loss is low for wafers with PVD SixNy and high for PECVD SixNy. CPW measurements at different temperatures show insertion loss increases for PVD SixNy but decreases for PECVD SixNy. Chemical and physical analysis reveals negligible hydrogen in PVD SixNy, while PECVD layers contain hydrogen at least an order of magnitude higher. Finite Element Method (FEM) simulations calculate insertion loss for high and low parasitic surface conductivity scenarios. Hydrogen presence appears to dominate insertion loss differences: traps in SixNy saturate with hydrogen, preventing suppression of parasitic surface conduction. The hypothesized mechanism involves changes in sheet conductivity in the Si carrier’s space charge region. Simulations using varying conductivity values show 450 S/m matches PECVD SixNy measurements, while 2 S/m aligns with PVD SixNy. Variations in SixNy thickness do not significantly affect CPWG insertion loss in simulations or measurements. Overall, hydrogen-free PVD SixNy is well suited as a charge trapping layer to reduce parasitic surface conduction on high-resistivity Si substrates.