Thermal annealing of thin PECVD silicon-oxide films for airgap-based optical filters

This paper investigates the mechanical and optical properties of thin PECVD silicon-oxide layers for optical applications. The different deposition parameters in PECVD provide a promising tool to manipulate and control the film structure. Membranes for use in optical filters typically are of similar to lambda/4n thickness and should be slightly tensile for remaining flat, thus avoiding scattering. The effect of the thermal budget of the process on the mechanical characteristics of the deposited films was studied. Films with compressive stress ranging from -100 to 0 MPa were deposited. Multiple thermal annealing cycles were applied to wafers and the in situ residual stress and ex situ optical properties were measured. The residual stress in the films was found to be highly temperature dependent. Annealing during the subsequent process steps results in tensile stress from 100 to 300 MPa in sub-micron thick PECVD silicon-oxide films. However, sub-100nm thick PECVD silicon-oxide layers exhibit a lower dependence on the thermal annealing cycles, resulting in lower stress variations in films after the annealing. It is also shown that the coefficient of thermal expansion, hence the residual stress in layers, varies with the thickness. Finally, several free-standing membranes were fabricated and the results are compared.