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Polarized cavity ring-down technique for characterization of single-layer SiO2 films
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摘要:
为了探究特定沉积工艺参数下，不同沉积角度对SiO2光学薄膜损耗及应力双折射的影响，本文采用一种高灵敏探测方法⎯偏振光腔衰荡技术表征单层SiO2光学薄膜。该技术基于测量光学谐振腔内偏振光来回反射累积后的衰荡时间特性及产生的相位差振荡频率，实现光学元件的光学损耗和残余应力的同点、同时绝对测量。实验对60°、70°和80°沉积角度条件下制备的单层SiO2薄膜样品进行了应力和光学损耗的测量分析。结果显示了不同沉积角度条件下制备的SiO2薄膜表面粗糙程度和致密性变化对薄膜损耗和应力双折射效应的影响，该结果对制备低光学损耗、低应力SiO2光学薄膜提供了技术指导。
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.
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Key words:
- polarized cavity ring-down /
- optical loss /
- stress induced birefringence /
- SiO2 films
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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.
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图 1 光腔衰荡技术的初始腔光路(a)及对应的衰荡曲线(b)
Figure 1. The initial cavity configuration (a) and corresponding ring-down curve (b) for a cavity ring-down technique
图 2 测试腔光路(a)及对应的振荡衰荡曲线(b)
Figure 2. Test cavity configuration (a) and the corresponding oscillation ring-down curve (b)
图 3 偏振光腔衰荡技术测量光学元件光学损耗和应力双折射实验装置。
Figure 3. Experimental arrangement of polarized cavity ring-down to measure the optical loss and stress-induced birefringence of optical components.
图 4 倾斜沉积技术制备单层膜样品
Figure 4. Oblique deposition technique to prepare single-layer film samples
图 5 不同沉积角度制备的单层膜样品光学损耗统计分布及对应高斯拟合。(a) 60°；(b) 70°；(c) 80°
Figure 5. Optical loss statistical distribution and corresponding Gaussian fitting of single-layer film samples prepared at different deposition angles. (a) 60°; (b) 70°; (c) 80°
图 6 不同沉积角度制备的单层膜样品应力双折射相位差统计分布及对应高斯拟合。(a) 60°；(b) 70°；(c) 80°
Figure 6. Statistical distribution of stress birefringence phase difference and corresponding Gaussian fitting of single-layer film samples prepared at different deposition angles. (a) 60°; (b) 70°; (c) 80°
图 7 沉积角度对单层膜光学损耗、残余应力影响
Figure 7. The influence of deposition angle on optical loss and residual stress of single-layer film samples
表 1 不同沉积角度的待测单层膜样品对应的薄膜厚度
Table 1. The film thickness corresponding to the single-layer film samples with different deposition angles
Deposition angle/(°) 60 70 80 Film thickness/nm 170 148 137 
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表 2 不同沉积角度单层膜样品损耗、残余应力测量结果
Table 2. Measurement results of optical loss and residual stress of single-layer film samples at different deposition angles
Deposition angle/(°) 60 70 80 αLoss/ppm 22.9±2.2 36.4±2.1 52.7±3.5 δ/(10-4·rad) 5.987±0.057 4.379±0.072 2.796±0.05
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表 3 待测不同沉积角度的单层膜样品的散射测量结果
Table 3. Scattering measurement results of single-layer film samples with different deposition angles to be measured
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