Measuring Out-of-Plane Permittivity of Thin Films to Millimeter Wave Frequencies

Modern microchips utilize multilayer stack-ups with many interstitial layers of dielectrics. Optimizing device performance and maximizing yield requires precise measurements of the out-of-plane permittivity of these dielectric layers. At the same time, high-performance microchips are pushing operating frequencies into the millimeter-wave range, requiring precise materials property knowledge at these frequencies to perform as intended. With this context, one outstanding challenge is to accurately measure the out-of-plane permittivity of thin films. Unfortunately, the conventional method to extract this property, the metal-insulator-metal (MIM) capacitor technique, produces inconsistent and therefore unreliable material data for frequencies above a few gigahertz. To address these inconsistencies, we designed an experiment with on-wafer devices of varying topology and varying geometry. We chose to study silicon nitride for this experiment because it is a ubiquitous dielectric in microchips, widely accepted as approximately dispersionless, and available with established processes in our cleanroom. Our experiment resulted in an out-of-plane permittivity of thin film silicon nitride of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${\varepsilon }_{r} = 7.0~\pm ~0.1$</tex-math> </inline-formula> and a loss tangent of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\tan \delta \lt 0.03$</tex-math> </inline-formula> up to 90 GHz. Our key findings about improving on-wafer calibrations and modeling of the MIM devices will help material scientists and microchip designers obtain reliable permittivity data on thin films at millimeter wave frequencies.