Transient thermal conductivity in PECVD SiNx at high temperature: The thermal signature of an on-going irreversible modification

PECVD amorphous silicon nitride (a-SiNx) films are largely used into the dielectric stacks of integrated circuits, as passivation or capping layers, and are today an important alternative to silicon for integrated nonlinear optical applications, such as waveguides. In such applications, a-SiNx presents also the advantage of having a very low thermal conductivity, about 0.7 W/m K at room temperature, which is of relevance, as such films contribute to the thermal balance of the devices, and thus play a role in solving the thermal management problem in microelectronics. The deposition parameters, and more specifically the deposition temperature, have been found to be critical in determining its physical properties, like density, elastic properties and intrinsic stress, as they influence the microstructure of the amorphous film. In this work, we report the first investigation of the effect of the temperature deposition on the thermal conductivity in amorphous SiNx films of thicknesses between 200 and 500 nm, as measured by thermoreflectance technique up to 773 K. Surprisingly, for all deposition temperatures between 300 and 573 K, thermal conductivity exhibits a steep decrease above 473 K, decreasing by more than 30% down to a minimum around 673 K, before increasing back to values comparable with the room temperature one. This transient behavior, observed only for a first heating of the sample, is associated to an irreversible modification of the thin film, and may be related to a partial desorption of the hydrogen trapped in a-SiNx during the deposition.