Polarized cavity ring-down technique for characterization of single-layer SiO<sub>2</sub> films

Wu Meiyu; Wang Jing; Li Bincheng

doi:10.12086/oee.2021.210270

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Wu M Y, Wang J, Li B C. Polarized cavity ring-down technique for characterization of single-layer SiO2 films[J]. Opto-Electron Eng, 2021, 48(11): 210270. doi: 10.12086/oee.2021.210270

Citation:

Wu M Y, Wang J, Li B C. Polarized cavity ring-down technique for characterization of single-layer SiO₂ films[J]. Opto-Electron Eng, 2021, 48(11): 210270. doi: 10.12086/oee.2021.210270

Polarized cavity ring-down technique for characterization of single-layer SiO₂ films

School of Optoelectronic Science and Engineering, University of Electronic Science and Technology of China, Chengdu, Sichuan 610054, China

More Information

^*Corresponding author: Wang Jing, E-mail: jingwang1230@uestc.edu.cn

Received Date 20 August 2021

Revised Date 11 November 2021

Published Date 30 November 2021

Abstract

Abstract

In this paper, a highly sensitive detection method - polarized cavity ring-down (P-CRD) technique - is employed to investigate the influence of deposition angle on the optical loss and stress-induced birefringence of single-layer SiO₂ films prepared with specific deposition process parameters. The P-CRD technique is based on measuring the decay behavior of accumulated polarized light reflecting back and forth inside a resonant cavity. The decay time and oscillating frequency of resulted phase difference of the CRD signal are applied to measure simultaneously the absolute values of the optical loss and residual stress-induced birefringence at the same measurement point of single-layer SiO₂ films. In the experiment, the optical losses and stress-induced birefringence of the single-layer SiO₂ film samples prepared under different deposition angles of 60°, 70°, and 80° are measured and analyzed. The results revealed the effects of the changes of surface roughness and film compact density caused by the different deposition angles on the optical loss and stress-induced birefringence of the single-layer SiO₂ films, respectively. These results are helpful to the preparation of high-performance SiO₂ films with low optical loss and low residual stress.
- polarized cavity ring-down /
- optical loss /
- stress induced birefringence /
- SiO₂ films

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References

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Overview

Overview

Overview: Silicon dioxide (SiO₂) is a preferred low index of refraction material for preparing high-performance optical films because of its low absorption coefficient, high corrosion resistance, high hardness, and so on. During the preparation of optical thin films, the residual stress inside the films needs to be well controlled; otherwise, it may cause surface deformation and refractive index anisotropy of corresponding optical components. There are many methods for measuring residual stress inside optical components that have limited measurement accuracy, such as the Stoney curvature method, X-ray diffraction (XRD) method, photoelastic modulator (PEM) method, and so on. In this paper, the stress birefringence measurement method based on polarized cavity ring-down (P-CRD) is adopted to measure simultaneously the residual stress-induced birefringence and optical loss of single-layer SiO₂ film samples. In P-CRD, the measurement of stress birefringence and optical loss is not affected by the fluctuation of light intensity as instead a delay time is measured. The measurement accuracy of the stress birefringence is significantly improved due to the cumulative effect of the polarization phase difference by multiple back and forth reflections inside the ring-down cavity. In order to explore the influence of deposition angle on the optical loss and stress birefringence of single-layer SiO₂ film samples prepared with Ion-Beam Sputtering (IBS) coating technique, three single-layer SiO₂ film samples with deposition angles of 60°, 70° and 80° were measured with P-CRD. The achieved measurement precisions were less than 3.5 ppm for the optical loss and 5.0×10^-6 rad for the stress refringence. The measured optical losses were 22.9 ppm, 36.4 ppm, and 52.7 ppm, and the stress birefringence were 5.99×10^-4 rad, 4.38×10^-4 rad, and 2.80×10^-4 rad for the samples prepared with deposition angles of 60°, 70°, and 80°, respectively. Clearly, as the deposition angle increases, the optical loss increases and the stress birefringence decreases.
The scattering losses of the single-layer SiO₂ film samples were also measured with a Total Integrated Scattering (TIS) instrument. The scattering measurement results showed that as the deposition angle increases, the surface roughness of the single-layer SiO₂ film gradually increases, resulting in increased surface scattering, which in turn increases the optical loss measured by P-CRD. In addition, the increase in surface roughness makes the film more prone to a loose and porous structure. Since the residual stress has a strong correlation with the packing density of the film, a loose structure indicates a reduced packing density, which causes the residual stress (and the stress-induced birefringence) of the film sample to decrease gradually with the increasing deposition angle.
These results not only confirmed that the polarization cavity ring-down technique has higher stress birefringence measurement accuracy than the currently most sensitive instrument based on PEM (with phase difference measurement accuracy of 5×10^-5 rad), but also were helpful to the preparation of high-performance SiO₂ films with low optical loss and low residual stress.