Accurate prediction of multilayered residual stress in fabricating a mid-infrared long-wave pass filter with interfacial stress measurements

Abstract

We present an accurate approach to predict the residual stress in a multilayered mid-infrared long-wave pass filter (MIR-LWPF) by using interfacial stress measurements. Magnesium fluoride (MgF₂) and zinc sulfide (ZnS) thin films were used to fabricate 7-layer (MgF₂/ZnS)³/MgF₂ MIR-LWPF devices by electron-beam evaporation with ion-assisted deposition technique. The interfacial stress between the high-index of ZnS and low-index of MgF₂ thin film materials was obtained from the residual stress measurements based on Twyman-Green interferometer and fast Fourier transformation (FFT) method. The modified Ennos formula was used to estimate the residual stress in the (MgF₂/ZnS)³/MgF₂ multilayered thin films. The difference between the predicted stress value and the measured value is 28 MPa by the proposed method. In the MIR-LWPF design of (MgF₂/ZnS)³/MgF₂ multilayer structure, the optical transmittance at a near-infrared wavelength of 1.0 µm to 2.5 µm is less than 10%, and the transmittance at a mid-infrared wavelength of 2.5 µm to 7.5 µm is greater than 93%. The proposed method can accurately evaluate and predict residual stress in fabricating mid-infrared long-wave pass filter device which possesses low residual stress as well as lower surface roughness.