武梅妤，王 靜，李斌成

偏振光腔衰蕩技術(shù)測(cè)量單層SiO2薄膜特性

武梅妤，王 靜*，李斌成

電子科技大學(xué)光電科學(xué)與工程學(xué)院，四川 成都 610054

為了探究特定沉積工藝參數(shù)下，不同沉積角度對(duì)SiO2光學(xué)薄膜損耗及應(yīng)力雙折射的影響，本文采用一種高靈敏探測(cè)方法?偏振光腔衰蕩技術(shù)表征單層SiO2光學(xué)薄膜。該技術(shù)基于測(cè)量光學(xué)諧振腔內(nèi)偏振光來(lái)回反射累積后的衰蕩時(shí)間特性及產(chǎn)生的相位差振蕩頻率，實(shí)現(xiàn)光學(xué)元件的光學(xué)損耗和殘余應(yīng)力的同點(diǎn)、同時(shí)絕對(duì)測(cè)量。實(shí)驗(yàn)對(duì)60°、70°和80°沉積角度條件下制備的單層SiO2薄膜樣品進(jìn)行了應(yīng)力和光學(xué)損耗的測(cè)量分析。結(jié)果顯示了不同沉積角度條件下制備的SiO2薄膜表面粗糙程度和致密性變化對(duì)薄膜損耗和應(yīng)力雙折射效應(yīng)的影響，該結(jié)果對(duì)制備低光學(xué)損耗、低應(yīng)力SiO2光學(xué)薄膜提供了技術(shù)指導(dǎo)。

偏振光腔衰蕩；光學(xué)損耗；應(yīng)力雙折射；SiO2光學(xué)薄膜

1 引 言

隨著高精密光學(xué)系統(tǒng)及高功率激光技術(shù)的快速發(fā)展，對(duì)光學(xué)或激光系統(tǒng)中光學(xué)元件的薄膜性能要求日益提高，薄膜光學(xué)損耗(包括吸收和散射損耗)是重要的性能參數(shù)之一[1-2]。薄膜在滿(mǎn)足超低吸收和散射損耗并能被準(zhǔn)確測(cè)量的同時(shí)，還必須控制鍍膜后的殘余應(yīng)力大小[3-4]。若薄膜存在過(guò)大的殘余應(yīng)力，會(huì)導(dǎo)致基板面形彎曲、膜層破裂和脫落[5]，很大程度上限制了其光學(xué)性能，較為嚴(yán)重的殘余應(yīng)力，還會(huì)使入射到薄膜上的反射光波前產(chǎn)生很大的畸變，偏離光學(xué)系統(tǒng)路徑而無(wú)法正常工作[6]。因此對(duì)薄膜光學(xué)元件的光學(xué)損耗和應(yīng)力特性的準(zhǔn)確測(cè)量是制備低損耗、應(yīng)力可控薄膜的前提。目前，針對(duì)薄膜光學(xué)元件應(yīng)力無(wú)損測(cè)量技術(shù)，根據(jù)測(cè)量依據(jù)大致可以分為兩類(lèi)，一類(lèi)是使用較為廣泛的基于應(yīng)力形變的測(cè)量方法[7]，如Stoney曲率法、X射線(xiàn)衍射法(X-ray diffraction)、顯微拉曼光譜法等；另一類(lèi)是基于雙折射效應(yīng)的測(cè)量方法[8]，包括數(shù)字光彈法、光彈調(diào)制器法、偏振光腔衰蕩技術(shù)等，其中偏振光腔衰蕩技術(shù)通過(guò)測(cè)量諧振腔內(nèi)偏振光來(lái)回反射累積產(chǎn)生的應(yīng)力雙折射振蕩頻率和衰蕩時(shí)間，可以同時(shí)計(jì)算出光學(xué)元件的雙折射相位差和光學(xué)損耗，其優(yōu)勢(shì)是可以實(shí)現(xiàn)光學(xué)元件殘余應(yīng)力和光學(xué)損耗的同點(diǎn)、同時(shí)測(cè)量，并且測(cè)量時(shí)不受光強(qiáng)波動(dòng)的影響，具有良好的抗噪聲能力，是測(cè)量精度最高的方法[9-13]。

光學(xué)薄膜領(lǐng)域中，SiO2因其具有吸收率小、耐腐蝕、硬度高等特性，是制備光學(xué)薄膜的優(yōu)選材料[14-17]，如在高性能光學(xué)薄膜元件和高功率激光薄膜元件中是最常用的低折射率材料，因而研究SiO2薄膜的光學(xué)損耗和應(yīng)力特性顯得尤為重要。相關(guān)文獻(xiàn)[18-21]報(bào)道的應(yīng)力數(shù)據(jù)表明，薄膜特性與制備薄膜時(shí)的沉積條件密不可分。例如采用傾斜沉積技術(shù)制備薄膜時(shí)，有文獻(xiàn)[22-23]報(bào)道了改變沉積通量相對(duì)于基底的角度，會(huì)影響薄膜柱狀微結(jié)構(gòu)，進(jìn)而影響薄膜的光學(xué)、力學(xué)特性。

為了探究不同沉積角度對(duì)SiO2薄膜特性的影響，本文分別對(duì)沉積角度為60°、70°和80°的SiO2單層膜樣品的光學(xué)損耗及殘余應(yīng)力雙折射進(jìn)行了測(cè)量，并結(jié)合總積分散射儀測(cè)量的結(jié)果進(jìn)行分析，得出了隨沉積角度的變化以及兩者的變化趨勢(shì)，對(duì)膜層光學(xué)損耗和殘余應(yīng)力的控制提供了一定的參考價(jià)值。

2 應(yīng)力、損耗測(cè)量原理

偏振光腔衰蕩技術(shù)測(cè)量偏振光在諧振腔內(nèi)來(lái)回反射后光腔輸出的光能量衰減到原來(lái)1/e處所用的時(shí)間。當(dāng)諧振腔內(nèi)未插入被測(cè)樣品時(shí)，腔長(zhǎng)為0，腔鏡M1、M2反射率為1、2，如圖1所示，激光進(jìn)入諧振腔后，光腔輸出信號(hào)可表示為

圖1 光腔衰蕩技術(shù)的初始腔光路(a)及對(duì)應(yīng)的衰蕩曲線(xiàn)(b)

將待測(cè)薄膜樣品垂直插入諧振腔內(nèi)，測(cè)量點(diǎn)處的應(yīng)力雙折射視為相位差、快軸角度為角的波片。光腔出射光經(jīng)過(guò)沃拉斯頓分光棱鏡后得到雙折射衰蕩信號(hào)，如圖2所示，輸出信號(hào)表示為

其中：表示諧振腔內(nèi)插入被測(cè)樣品之后的腔長(zhǎng)，表示真空光速，為探測(cè)激光波長(zhǎng)。

3 實(shí)驗(yàn)及結(jié)果分析

3.1 實(shí)驗(yàn)裝置

圖2 測(cè)試腔光路(a)及對(duì)應(yīng)的振蕩衰蕩曲線(xiàn)(b)

圖3 偏振光腔衰蕩技術(shù)測(cè)量光學(xué)元件光學(xué)損耗和應(yīng)力雙折射實(shí)驗(yàn)裝置。ISO：光隔離器；AOM：聲光調(diào)制器；PH：小孔光闌；HR1、HR2：高反射鏡R1、R2；GLP：Glan偏振棱鏡；QWP：1/4波片；M1、M2：諧振腔鏡；M：待測(cè)光學(xué)元件；PBS：偏振分光棱鏡；PD1、PD2：光電探測(cè)器1、2；Z：光傳播方向

3.2 測(cè)量結(jié)果

基于上述實(shí)驗(yàn)裝置和衰蕩信號(hào)擬合公式，對(duì)特定沉積工藝參數(shù)下(沉積速率1.6 A/S)，采用離子束濺射(ion beam sputtering, IBS)技術(shù)[24]和SiO2靶材(純度99.99%)制備沉積角度分別為60°、70°、80°，直徑25.4 mm的SiO2單層膜樣品進(jìn)行測(cè)量。其中，沉積角度是指鍍膜基片所在的斜面與水平方向的傾斜角度(假設(shè)離子束垂直濺射)，如圖4所示，制備薄膜時(shí)將基底固定在加裝好的傾斜柱的斜面上，使濺射沉積通量與基底表面呈不同的角度，從而獲得不同沉積角度的SiO2單層膜。利用分光光度計(jì)測(cè)試樣品透過(guò)率、反射率光譜，通過(guò)Optilayer軟件擬合光譜曲線(xiàn)獲得薄膜厚度。表1所示是不同沉積角度的單層膜樣品對(duì)應(yīng)的薄膜厚度。

圖4 傾斜沉積技術(shù)制備單層膜樣品

表1 不同沉積角度的待測(cè)單層膜樣品 對(duì)應(yīng)的薄膜厚度

在插入待測(cè)單層膜樣品前后，諧振腔腔長(zhǎng)0、分別是0.531 m和0.533 m，初始腔衰蕩時(shí)間0為37.51 μs。對(duì)三塊待測(cè)單層膜樣品上的一點(diǎn)進(jìn)行重復(fù)多次測(cè)量，根據(jù)擬合得到的測(cè)試腔衰蕩時(shí)間和應(yīng)力雙折射振蕩頻率，利用式(4)、式(5)，得到不同單層膜樣品的損耗和應(yīng)力雙折射相位差，如圖5、圖6所示。其中，每次擬合是十次衰蕩信號(hào)的平均，可以看出高斯函數(shù)擬合可以很好地描述統(tǒng)計(jì)分布的結(jié)果。

所測(cè)得的損耗測(cè)量精度均在ppm量級(jí)，其中，沉積角度60°的單層膜損耗擬合平均值22.9 ppm，統(tǒng)計(jì)標(biāo)準(zhǔn)差2.2 ppm，沉積角度70°的單層膜損耗擬合平均值36.4 ppm，統(tǒng)計(jì)標(biāo)準(zhǔn)差2.1 ppm，沉積角度80°的單層膜損耗擬合平均值52.7 ppm，統(tǒng)計(jì)標(biāo)準(zhǔn)差3.5 ppm。與傳統(tǒng)的各種損耗分別測(cè)量(采用不同測(cè)量?jī)x器分別對(duì)吸收、散射損耗進(jìn)行測(cè)量，最后疊加計(jì)算得到總損耗)相比，該技術(shù)測(cè)量效率高且不存在測(cè)量點(diǎn)偏差，因而可以很好地對(duì)測(cè)量點(diǎn)進(jìn)行評(píng)估。

需要指出的是，雖然上述測(cè)量得到的應(yīng)力雙折射相位差是腔內(nèi)所有雙折射相位差的疊加，包括腔鏡多層膜的殘余應(yīng)力等，但由于實(shí)驗(yàn)裝置空腔測(cè)量結(jié)果比測(cè)試腔雙折射相位差小一個(gè)數(shù)量級(jí)，因此，空腔雙折射值不影響待測(cè)樣品測(cè)量精度，認(rèn)為上述結(jié)果可以很好地描述待測(cè)薄膜樣品殘余應(yīng)力雙折射的大小：

沉積角度60°的單層膜應(yīng)力雙折射相位差擬合平均值6.0×10-4rad(OPD 0.0607 nm)，統(tǒng)計(jì)標(biāo)準(zhǔn)差5.7×10-6rad(OPD7.6×10-4nm)；

沉積角度70°的單層膜應(yīng)力雙折射相位差擬合平均值4.4×10-4rad(OPD0.0441 nm)，統(tǒng)計(jì)標(biāo)準(zhǔn)差7.2×10-6rad(OPD7.2×10-4nm)；

沉積角度80°的單層膜應(yīng)力雙折射相位差擬合平均值2.8×10-4rad(OPD 0.0284 nm)，統(tǒng)計(jì)標(biāo)準(zhǔn)差5.0×10-6rad(OPD 5.1×10-4nm)，與光彈調(diào)制器法(相位差測(cè)量精度5×10-5rad)相比[25]，偏振光腔衰蕩法重復(fù)性測(cè)量精度提高了一個(gè)數(shù)量級(jí)，測(cè)量準(zhǔn)確性更高。

圖5 不同沉積角度制備的單層膜樣品光學(xué)損耗統(tǒng)計(jì)分布及對(duì)應(yīng)高斯擬合。(a) 60°；(b) 70°；(c) 80°

圖6 不同沉積角度制備的單層膜樣品應(yīng)力雙折射相位差統(tǒng)計(jì)分布及對(duì)應(yīng)高斯擬合。(a) 60°；(b) 70°；(c) 80°

表2 不同沉積角度單層膜樣品損耗、殘余應(yīng)力測(cè)量結(jié)果

3.3 分析和討論

相關(guān)文獻(xiàn)[26-27]表明，在濺射沉積初期薄膜形貌近似為孤立的島狀結(jié)構(gòu)，隨著膜厚的增加，晶粒度和島狀結(jié)構(gòu)不斷增加，散射隨之增大。但當(dāng)膜厚增加到一定程度后，島之間相互結(jié)合形成走向隨機(jī)的通道，進(jìn)而生長(zhǎng)成網(wǎng)狀結(jié)構(gòu)，使得散射迅速減少。隨著膜厚進(jìn)一步增加(>100 nm)，在薄膜生長(zhǎng)后期薄膜形貌進(jìn)入連續(xù)膜階段，單位體積內(nèi)晶界隨著晶粒度的增加逐漸減少，進(jìn)而對(duì)光的表面散射隨之減少并逐漸趨于一個(gè)穩(wěn)定值。因而，對(duì)于SiO2連續(xù)薄膜材料來(lái)說(shuō)，膜層厚度對(duì)表面散射的影響可忽略不計(jì)，對(duì)此僅考慮沉積角度對(duì)表面散射的影響。

同時(shí)使用總積分散射(total integrated scattering, TIS)儀器測(cè)量了單層膜散射隨沉積角度的變化規(guī)律，測(cè)量結(jié)果如表3所示。沉積角度60°、70°和80°的單層膜前、后散射之和分別為45.8 ppm、84.7 ppm和100 ppm?？梢钥闯觯S著沉積角度的增大，散射損耗呈逐漸增大的趨勢(shì)。根據(jù)表面微粗糙度(root mean square, RMS)和表面散射之間的關(guān)系[28-30]，總積分散射主要受RMS粗糙度的影響，上述測(cè)量結(jié)果表明，隨著沉積角度的增加，單層膜表面RMS粗糙度增大，造成表面散射增強(qiáng)，光學(xué)損耗增加。

需要說(shuō)明的是，由于薄膜存在一定的非均勻性，并且TIS儀器由于標(biāo)定誤差、測(cè)量點(diǎn)偏差等因素，測(cè)量的散射損耗明顯高于基于光腔衰蕩技術(shù)測(cè)量的總光學(xué)損耗(近似為2倍關(guān)系)，但兩者測(cè)量結(jié)果的趨勢(shì)非常一致。根據(jù)總積分散射的定義：

其中：是散射光能量，0是鏡面反射率，0是入射光能量。

測(cè)量結(jié)果顯示：被測(cè)元件的光學(xué)損耗主要為散射損耗，吸收損耗相對(duì)較低。由于光腔衰蕩技術(shù)為絕對(duì)測(cè)量技術(shù)，不需要標(biāo)定，且不存在測(cè)量點(diǎn)偏差，可以認(rèn)為采用偏振光腔衰蕩技術(shù)測(cè)量的高透薄膜光學(xué)元件的光學(xué)損耗更準(zhǔn)確。

表3 待測(cè)不同沉積角度的單層膜樣品的散射測(cè)量結(jié)果

圖7 沉積角度對(duì)單層膜光學(xué)損耗、殘余應(yīng)力影響

4 結(jié) 論

本文采用的偏振光腔衰蕩技術(shù)，將應(yīng)力雙折射測(cè)量轉(zhuǎn)為對(duì)振蕩頻率的測(cè)量，同時(shí)將損耗測(cè)量轉(zhuǎn)為對(duì)衰蕩時(shí)間的測(cè)量，可以實(shí)現(xiàn)薄膜光學(xué)元件光學(xué)損耗和應(yīng)力的同點(diǎn)、同時(shí)測(cè)量。實(shí)驗(yàn)對(duì)不同沉積角度的SiO2單層膜樣品光學(xué)損耗和應(yīng)力特性進(jìn)行了測(cè)量。分析表明，隨著沉積角度增加，薄膜粗糙度增大，致密性越來(lái)越疏松。對(duì)應(yīng)地，薄膜光學(xué)損耗呈逐漸增大的趨勢(shì)，應(yīng)力雙折射呈逐漸減小的趨勢(shì)，這一趨勢(shì)與總積分散射測(cè)量元件表面微粗糙度具有一致性。該實(shí)驗(yàn)裝置的應(yīng)力雙折射重復(fù)性測(cè)量精度可達(dá)5.0×10-6rad，損耗測(cè)量精度達(dá)到ppm量級(jí)，證實(shí)了偏振光腔衰蕩技術(shù)較目前使用的光彈調(diào)制器法具有更高的測(cè)量精度，可以很好地應(yīng)用在薄膜光學(xué)特性參數(shù)測(cè)量中，具有廣闊的應(yīng)用前景。

致 謝

本文工作得到了同濟(jì)大學(xué)張錦龍老師的幫助，感謝張老師提供的三塊不同沉積角度的SiO2單層膜樣品。

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Polarized cavity ring-down technique for characterization of single-layer SiO2films

Wu Meiyu, Wang Jing*, Li Bincheng

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

Experimental arrangement of polarized cavity ring-down to measure the optical loss and stress induced birefringence of optical components

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 SiO2film 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 SiO2film samples prepared with Ion-Beam Sputtering (IBS) coating technique, three single-layer SiO2film 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 5.0×10-6rad for the stress refringence. The measured optical losses were 22.9 ppm, 36.4 ppm, and 52.7 ppm, and the stress birefringence were 5.99×10-4rad, 4.38×10-4rad, and 2.80×10-4rad 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 SiO2film 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 SiO2film 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 5×10-5rad), but also were helpful to the preparation of high-performance SiO2films with low optical loss and low residual stress.

Wu M Y, Wang J, Li B CPolarized cavity ring-down technique for characterization of single-layer SiO2films[J]., 2021, 48(11): 210270; DOI:10.12086/oee.2021.210270

Polarized cavity ring-down technique for characterization of single-layer SiO2films

Wu Meiyu, Wang Jing*, Li Bincheng

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

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 SiO2films 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 SiO2films. In the experiment, the optical losses and stress-induced birefringence of the single-layer SiO2film 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 SiO2films,respectively. These results are helpful to the preparation of high-performance SiO2films with low optical loss and low residual stress.

polarized cavity ring-down; optical loss; stress induced birefringence; SiO2films

10.12086/oee.2021.210270

O484.4

A

* E-mail: jingwang1230@uestc.edu.cn

武梅妤，王靜，李斌成. 偏振光腔衰蕩技術(shù)測(cè)量單層SiO2薄膜特性[J]. 光電工程，2021，48(11): 210270

Wu M Y, Wang J, Li B CPolarized cavity ring-down technique for characterization of single-layer SiO2films[J]., 2021, 48(11): 210270

2021-08-20；

2021-11-11

武梅妤(1996-)，女，碩士研究生，主要從事偏振光腔衰蕩技術(shù)的研究。E-mail：201921050434@std.uestc.edu.cn

王靜(1985-)，女，博士，副教授，主要從事光學(xué)檢測(cè)技術(shù)、光腔衰蕩技術(shù)方面的研究。E-mail：jingwang1230@uestc.edu.cn