Residual stress in SiO₂ thin films on silicon

Silicon dioxide is common low-refractive index material used for example in optical interference coatings, in active devices as dielectrics or in microelectromechanical systems (MEMS) as sacrificial layers [1]. Depending on device and substrate limitations different process temperatures for film formation are encountered. In addition, required film thicknesses can vary from sub-nanometer to micrometer range thus limiting the applicable deposition methods. The residual stress of conventional silicon dioxide films deposited using thermal and chemical vapour deposition (CVD) has been widely studied, but only a scattered information about the residual stress in silicon dioxide films made by atomic layer deposition (ALD) is available [1, 2]. In this study a matrix of silicon dioxides grown onto 150 mm silicon wafers using thermal oxidation, low-pressure (LP) CVD, plasmaenhanced (PE) CVD and ALD was investigated for residual stress. Thermal oxide was grown from O₂. LPCVD TEOS, low temperature oxide (LTO) and PECVD oxide were deposited using tetraethylorthosilicate, O₂/SiH₄ and N₂O/SiH₄ as precursors, respectively. ALD SiO₂ was grown using AP-LTO-330 as a precursor. The process temperatures varied from 200 to 1050 °C depending on deposition method. For all oxides targeted film thickness was 100 nm. Wafer curvature measurement was used to determine the stress and its stability. Supporting information on thickness was obtained for the same samples by spectroscopic reflectometry. Residual stress varied from tensile to compressive depending on the deposition method. Large differences in stress stability were found for different deposition methods.