Abstract
Optical coatings deposited by the dual ion beam sputtering (DIBS) method usually show high compressive stress, which results in severe wavefront deformation of optical elements. Annealing post-treatment has been widely used to control the residual stress of optical coatings. However, the effect of annealing on the stress of Ta2O5 films deposited by the IBS method has not been reported in detail. In this study, different thicknesses of Ta2O5 films were deposited by IBS and annealed at different temperatures from 473 to 973 K in air, and the effect of annealing on the stress of Ta2O5 films was investigated. The as-deposited Ta2O5 films deposited by IBS show high compressive stress, which are about 160 MPa. The compressive stress decreases linearly with the increasing temperature, and the wavefront deformation of Ta2O5 films increases linearly with film thickness (within 20 μm) at the same annealing temperature. When the temperature rises to 591 K, Ta2O5 films with zero-stress can be obtained. Ta2O5 films show tensile stress instead of compressive stress with further increasing annealing temperature, and the tensile stress increases with increasing temperature. Meanwhile, with the increasing annealing temperature, the refractive index of Ta2O5 film decreases, indicating the decreasing packing density. The atomic force microscope (AFM) test results show that surface roughness of Ta2O5 films slowly increases with the increasing of annealing temperature. Moreover, X-ray photoelectron spectroscopy (XPS) analysis shows that the Ta in Ta2O5 films can be further oxidized with increasing annealing temperature, namely, the absorption of Ta2O5 film can be reduced. X-ray diffraction (XRD) analysis shows that the annealing temperature should be below 923 K to maintain the amorphous structure of the Ta2O5 film.
Highlights
The residual stress introduced during optical thin film deposition [1,2,3] has been paid great attention in past decades, and many efforts have already been made on the research of the relationship between the residual stress and process parameters of different deposition methods [4,5,6].The performance of optical coating elements is affected greatly by the residual stress in thin films, such as spectroscopic and mechanical reliability [7,8], and the surface figure
It is well known that dual ion beam sputtering (DIBS) technology is an effective coating deposition method to fabricate optical films with ultra-low loss, high packing density and a high laser-induced damage threshold (LIDT) [4,10,11]
The high-quality optical coating elements fabricated by IBS have been widely applied in many important engineering projects, such as the National Ignition Facility (NIF), Laser Interferometer Gravitational-wave Observatory (LIGO), etc. [12,13,14,15]
Summary
The residual stress introduced during optical thin film deposition [1,2,3] has been paid great attention in past decades, and many efforts have already been made on the research of the relationship between the residual stress and process parameters of different deposition methods [4,5,6]. The performance of optical coating elements is affected greatly by the residual stress in thin films, such as spectroscopic and mechanical reliability [7,8], and the surface figure. The main drawback of IBS technology is the high compressive stress of films generated in the deposition process [15,16], which result in severe surface distortion of optical elements. In order to attain a high quality of optical coating elements with a good surface figure and good optical performance, it is very necessary to study the stress behavior of single layers, including the relationship between the residual stress and process parameters [17]. The effect of annealing treatments on stress behaviors of Ta2 O5 films deposited by IBS technology was systematically investigated. The results obtained in this study can offer technical feasibility for residual stress control and compensation of Ta2 O5 films
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