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Effect of annealing temperature on structure and stress properties of Ta2O5/SiO2 multilayer reflective coatings

Liu Bao-Jian Duan Wei-Bo Li Da-Qi Yu De-Ming Chen Gang Wang Tian-Hong Liu Ding-Quan

Effect of annealing temperature on structure and stress properties of Ta2O5/SiO2 multilayer reflective coatings

Liu Bao-Jian, Duan Wei-Bo, Li Da-Qi, Yu De-Ming, Chen Gang, Wang Tian-Hong, Liu Ding-Quan
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  • In the optical system of spaceborne laser altimeter, dielectric mirror is an indispensable optical film element. Its surface shape quality directly affects the resolution and accuracy of distance measurement of the detection system. It is pressing and necessary to carry out research on the surface shape control technology of dielectric mirror to eliminate or reduce the effect of film stress on surface shape. The Ta2O5/SiO2 multilayer reflective coatings are deposited on quartz substrates by using the ion beam assisted electron beam evaporation (IBE), and then annealed in air in a temperature range from 200 to 600 ℃. The effect of annealing temperature on the structure, optical and stress properties of Ta2O5/SiO2 multilayer reflective coatings are systemically investigated by using x-ray diffraction, atomic force microscope, spectrophotometer and laser interferometer. The results show that all the Ta2O5/SiO2 multilayer reflective coatings, after being annealed, are amorphous in structure. The annealing temperature has a great influence on the surface roughness of reflective coating. With the increase of annealing temperature, the surface roughness of reflective coating first decreases and then gradually increases, but is still smaller than that of as-deposited sample. After being annealed, the reflectance spectrum of reflective coating shifts slightly toward the long-wave direction, and the reflectivity increases a little. When being annealed at 500-600 ℃, the compressive stress of reflective coating could be transformed into tensile stress, and the surface is changed from convex to concave shape. It can be concluded that annealing at an appropriate temperature can effectively release residual stress of Ta2O5/SiO2 multilayer reflective coating and eliminate the deformation of substrate caused by film stress, and thus improving the surface shape quality of dielectric mirror., After being annealed, the reflective coating still possesses the stable structure and spectral properties, so that dielectric mirror can meet the application requirements of spaceborne laser altimeter. In this paper, the experimental results are of great significance for applying the annealing technology to the surface shape control technology of dielectric mirrors.
      Corresponding author: Duan Wei-Bo, duanweibo@mail.sitp.ac.cn
    [1]

    唐新明, 谢俊峰, 付兴科, 莫凡, 李少宁, 窦显辉 2017 测绘学报 46 714

    Tang X M, Xie J F, Fu X K, Mo F, Li S N, Dou X H 2017 Acta Geod. Cartogr. Sin. 46 714

    [2]

    刘斌, 张军, 鲁敏, 滕书华, 马燕新, 张文广 2015 激光与红外 54 5104

    Liu B, Zhang J, Lu M, Teng S H, Ma Y X, Zhang W G 2015 Laser Infr. 54 5104

    [3]

    刘豪, 舒嵘, 洪光烈, 郑龙, 葛烨, 胡以华 2014 物理学报 63 104214

    Liu H, Shu R, Hong G L, Zheng L, Ge Y, Hu Y H 2014 Acta Phys. Sin. 63 104214

    [4]

    Schiltz D, Patel D, Baumgarten C, Reagan B A, Rocca J J, Menoni C S 2017 Appl. Opt. 5 6

    [5]

    Kumar S, Shankar A, Kishore N, Mukherjee C, Kamparath R, Thakur S 2019 Optik 176 438

    [6]

    Qiao Z, Pu Y T, Liu H, Luo K, Wang G, Liu Z C, Ma P 2015 Thin Solid Films 592 221

    [7]

    Ailloud Q, Zerrad M, Amra C 2018 Opt. Express 26 13264

    [8]

    马跃, 阳凡林, 易洪, 李松 2015 红外与激光工程 44 2401

    Ma Y, Yang F L, Yi H, Li S 2015 Infrar. Laser Eng. 44 2401

    [9]

    庞志海, 樊学武, 陈钦芳, 马臻, 邹刚毅 2013 光学学报 33 186

    Pang Z H, Fan X W, Chen Q F, Ma Z, Zou G Y 2013 Acta Opt. Sin. 33 186

    [10]

    Wang L S, Liu H S, Jiang Y G, Yang X, Liu D D, Ji Y Q, Zhang F, Chen D Y 2017 Optik 142 33

    [11]

    Sertel T, Sonmez N A, Cetin S S, Ozcelik S 2019 Ceram. Int. 45 11

    [12]

    Li S D, Liu H S, Jiang Y G, He J H, Wang L S, Ji Y Q 2019 Optik 181 695

    [13]

    Bischoff M, Nowitzki T, Voß O, Wilbrandt S, Stenzel O 2014 Appl. Opt. 5 3

    [14]

    Jena S, Tokas R B, Rao K D, Thakur S, Sahoo N K 2016 Appl. Opt. 55 6108

    [15]

    Çetinörgü-Goldenberg E, Klemberg-Sapieha J E, Martinu L 2012 Appl. Opt. 51 6498

    [16]

    季一勤, 姜玉刚, 刘华松, 王利栓, 刘丹丹, 姜承慧, 羊亚平, 樊荣伟, 陈德应 2013 红外与激光工程 42 418

    Ji Y Q, Jiang Y G, Liu H S, Wang L S, Liu D D, Jiang C H, Yang Y P, Fan R W, Chen D Y 2013 Infrar. Laser Eng. 42 418

    [17]

    冷健, 季一勤, 刘华松, 庄克文, 刘丹丹 2018 红外与激光工程 47 196

    Leng J, Ji Y Q, Liu H S, Zhuang K W, Liu D D 2018 Infrar. Laser Eng. 47 196

    [18]

    Shen Y M, Han Z X, Shao J D, Shao S Y, He H B 2008 Chin. Opt. Lett. 6 225

    [19]

    Stoney G G 1909 Proc. R. Soc. London Ser. A 82 172

    [20]

    黄才华, 薛亦渝, 彭桦, 夏志林, 郭培涛 2009 中国激光 36 364

    Huang C H, Xue Y Y, Peng H, Xia Z L, Guo P T 2009 Chin. J. Lasers 36 364

    [21]

    申雁鸣2008 博士学位论文(上海: 中国科学院上海光学精密机械研究所)

    Shen Y M 2008 Ph. D. Dissertation (Shanghai: Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences) (in Chinese)

  • 图 1  Ta2O5/SiO2多层反射膜在不同温度下退火后的XRD谱图

    Figure 1.  XRD patterns of Ta2O5/SiO2 multilayer reflective coatings annealed at different temperatures.

    图 2  经不同温度退火后Ta2O5/SiO2多层反射膜表面形貌AFM测试图 (a) 沉积态; (b) 200 ℃; (c) 300 ℃; (d) 400 ℃; (e) 500 ℃; (f) 600 ℃

    Figure 2.  AFM images of Ta2O5/SiO2 multilayer reflective coatings annealed at different temperatures: (a) As-deposited; (b) 200 ℃; (c) 300 ℃; (d) 400 ℃; (e) 500 ℃; (f) 600 ℃.

    图 3  经不同温度退火后Ta2O5/SiO2多层反射膜表面RMS

    Figure 3.  Surface RMS of Ta2O5/SiO2 multilayer reflective coatings annealed at different temperatures.

    图 4  经不同温度退火后Ta2O5/SiO2多层反射膜反射率光谱曲线

    Figure 4.  Reflectance spectra of Ta2O5/SiO2 multilayer reflective coatings annealed at different temperatures.

    图 5  经不同温度退火后介质膜反射镜面形图 (a)沉积态; (b) 200 ℃; (c) 300 ℃; (d) 400 ℃; (e) 500 ℃; (f) 600 ℃

    Figure 5.  Surface figures of dielectric mirrors annealed at different temperatures: (a) As-deposited; (b) 200 ℃; (c) 300 ℃; (d) 400 ℃; (e) 500 ℃; (f) 600 ℃.

    图 6  经500—600 ℃退火后介质膜反射镜面形图 (a) 525 ℃; (b) 550 ℃; (c) 575 ℃

    Figure 6.  Surface figures of dielectric mirrors annealed at the temperature range from 500 to 600 ℃: (a) 525 ℃; (b) 550 ℃; (c) 575 ℃.

    表 1  经不同温度退火后Ta2O5/SiO2多层反射膜基片变形和残余应力值

    Table 1.  Substrate deflection and residual stress of annealed Ta2O5/SiO2 multilayer reflective coatings.

    Annealing temperature/℃Substrate deflection/nmResidual stress/MPa
    Before coatingAfter coatingAfter annealingAfter coatingAfter annealing
    As-deposited31.6–282.8–282.8–90.9–90.9
    20029.5–281.6–375.3–89.9–117.0
    30031.4–278.4–363.9–89.5–114.3
    40030.2–282.8–320.2–90.5–101.3
    50031.8–284.1–67.1–92.3–28.6
    60030.5–280.3213.3–89.852.9
    DownLoad: CSV

    表 2  Ta2O5/SiO2多层反射膜在500—600 ℃退火后基片变形和残余应力值

    Table 2.  Substrate deflection and residual stress of Ta2O5/SiO2 multilayer reflective coatings annealed at the temperature range from 500 to 600 ℃

    Annealing temperature/℃Substrate deflection/nmResidual stress/MPa
    Before coatingAfter coatingAfter annealingAfter coatingAfter annealing
    52531.2–279.816.4–89.9–4.3
    55029.6–283.660.1–90.58.8
    57530.8–281.5112.6–90.323.6
    DownLoad: CSV
  • [1]

    唐新明, 谢俊峰, 付兴科, 莫凡, 李少宁, 窦显辉 2017 测绘学报 46 714

    Tang X M, Xie J F, Fu X K, Mo F, Li S N, Dou X H 2017 Acta Geod. Cartogr. Sin. 46 714

    [2]

    刘斌, 张军, 鲁敏, 滕书华, 马燕新, 张文广 2015 激光与红外 54 5104

    Liu B, Zhang J, Lu M, Teng S H, Ma Y X, Zhang W G 2015 Laser Infr. 54 5104

    [3]

    刘豪, 舒嵘, 洪光烈, 郑龙, 葛烨, 胡以华 2014 物理学报 63 104214

    Liu H, Shu R, Hong G L, Zheng L, Ge Y, Hu Y H 2014 Acta Phys. Sin. 63 104214

    [4]

    Schiltz D, Patel D, Baumgarten C, Reagan B A, Rocca J J, Menoni C S 2017 Appl. Opt. 5 6

    [5]

    Kumar S, Shankar A, Kishore N, Mukherjee C, Kamparath R, Thakur S 2019 Optik 176 438

    [6]

    Qiao Z, Pu Y T, Liu H, Luo K, Wang G, Liu Z C, Ma P 2015 Thin Solid Films 592 221

    [7]

    Ailloud Q, Zerrad M, Amra C 2018 Opt. Express 26 13264

    [8]

    马跃, 阳凡林, 易洪, 李松 2015 红外与激光工程 44 2401

    Ma Y, Yang F L, Yi H, Li S 2015 Infrar. Laser Eng. 44 2401

    [9]

    庞志海, 樊学武, 陈钦芳, 马臻, 邹刚毅 2013 光学学报 33 186

    Pang Z H, Fan X W, Chen Q F, Ma Z, Zou G Y 2013 Acta Opt. Sin. 33 186

    [10]

    Wang L S, Liu H S, Jiang Y G, Yang X, Liu D D, Ji Y Q, Zhang F, Chen D Y 2017 Optik 142 33

    [11]

    Sertel T, Sonmez N A, Cetin S S, Ozcelik S 2019 Ceram. Int. 45 11

    [12]

    Li S D, Liu H S, Jiang Y G, He J H, Wang L S, Ji Y Q 2019 Optik 181 695

    [13]

    Bischoff M, Nowitzki T, Voß O, Wilbrandt S, Stenzel O 2014 Appl. Opt. 5 3

    [14]

    Jena S, Tokas R B, Rao K D, Thakur S, Sahoo N K 2016 Appl. Opt. 55 6108

    [15]

    Çetinörgü-Goldenberg E, Klemberg-Sapieha J E, Martinu L 2012 Appl. Opt. 51 6498

    [16]

    季一勤, 姜玉刚, 刘华松, 王利栓, 刘丹丹, 姜承慧, 羊亚平, 樊荣伟, 陈德应 2013 红外与激光工程 42 418

    Ji Y Q, Jiang Y G, Liu H S, Wang L S, Liu D D, Jiang C H, Yang Y P, Fan R W, Chen D Y 2013 Infrar. Laser Eng. 42 418

    [17]

    冷健, 季一勤, 刘华松, 庄克文, 刘丹丹 2018 红外与激光工程 47 196

    Leng J, Ji Y Q, Liu H S, Zhuang K W, Liu D D 2018 Infrar. Laser Eng. 47 196

    [18]

    Shen Y M, Han Z X, Shao J D, Shao S Y, He H B 2008 Chin. Opt. Lett. 6 225

    [19]

    Stoney G G 1909 Proc. R. Soc. London Ser. A 82 172

    [20]

    黄才华, 薛亦渝, 彭桦, 夏志林, 郭培涛 2009 中国激光 36 364

    Huang C H, Xue Y Y, Peng H, Xia Z L, Guo P T 2009 Chin. J. Lasers 36 364

    [21]

    申雁鸣2008 博士学位论文(上海: 中国科学院上海光学精密机械研究所)

    Shen Y M 2008 Ph. D. Dissertation (Shanghai: Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences) (in Chinese)

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  • Received Date:  21 December 2018
  • Accepted Date:  08 April 2019
  • Available Online:  01 June 2019
  • Published Online:  05 June 2019

Effect of annealing temperature on structure and stress properties of Ta2O5/SiO2 multilayer reflective coatings

    Corresponding author: Duan Wei-Bo, duanweibo@mail.sitp.ac.cn
  • Shanghai Institute of Technical Physics, Chinese Academy of Sciences, Shanghai 200083, China

Abstract: In the optical system of spaceborne laser altimeter, dielectric mirror is an indispensable optical film element. Its surface shape quality directly affects the resolution and accuracy of distance measurement of the detection system. It is pressing and necessary to carry out research on the surface shape control technology of dielectric mirror to eliminate or reduce the effect of film stress on surface shape. The Ta2O5/SiO2 multilayer reflective coatings are deposited on quartz substrates by using the ion beam assisted electron beam evaporation (IBE), and then annealed in air in a temperature range from 200 to 600 ℃. The effect of annealing temperature on the structure, optical and stress properties of Ta2O5/SiO2 multilayer reflective coatings are systemically investigated by using x-ray diffraction, atomic force microscope, spectrophotometer and laser interferometer. The results show that all the Ta2O5/SiO2 multilayer reflective coatings, after being annealed, are amorphous in structure. The annealing temperature has a great influence on the surface roughness of reflective coating. With the increase of annealing temperature, the surface roughness of reflective coating first decreases and then gradually increases, but is still smaller than that of as-deposited sample. After being annealed, the reflectance spectrum of reflective coating shifts slightly toward the long-wave direction, and the reflectivity increases a little. When being annealed at 500-600 ℃, the compressive stress of reflective coating could be transformed into tensile stress, and the surface is changed from convex to concave shape. It can be concluded that annealing at an appropriate temperature can effectively release residual stress of Ta2O5/SiO2 multilayer reflective coating and eliminate the deformation of substrate caused by film stress, and thus improving the surface shape quality of dielectric mirror., After being annealed, the reflective coating still possesses the stable structure and spectral properties, so that dielectric mirror can meet the application requirements of spaceborne laser altimeter. In this paper, the experimental results are of great significance for applying the annealing technology to the surface shape control technology of dielectric mirrors.

    • 随着航天遥感技术的不断发展, 星载激光测高仪以其测量分辨率高、多功能的优势, 在对地观测、深空探测以及大气探测等领域获得了广泛的应用[1-3]. 介质膜反射镜是激光测高仪光学系统中不可缺少的光学薄膜元件, 主要功能是实现光信号高效传递, 是系统中需要做精确控制的环节之一. 五氧化二钽(Ta2O5)和二氧化硅(SiO2)是介质反射膜镀制时两种常用的高、低折射率材料. Ta2O5光谱透过范围宽(0.3—10.0 μm), 具有较低的光学吸收, 同时具有较高的折射率, 被广泛地应用于各种光学器件[4-7]. 一般情况下, 介质反射膜要达到反射率99%以上的要求, 高、低折射率材料往往需要镀制十多层甚至几十层, 膜层中的应力会使基底产生弯曲, 进而影响反射镜面形. 当反射镜的面形精度较低时, 将会使传输的波前畸变, 导致输出光束质量显著下降,直接影响探测系统测速测距的分辨率和精度[8,9]. 因此, 开展介质膜反射镜面形控制相关技术的研究, 以消除或减小膜层应力对镜面面形的影响显得迫切和必要.

      热处理是最常用的薄膜后处理工艺之一, 薄膜在退火后可以改变其微观结构、光学性能和应力特性. 当前, 己有不少学者对光学薄膜热退火处理进行了研究[10-15]. 季一勤等[16,17]分析了热处理对离子束溅射SiO2薄膜结构与力学特性的影响, 结果表明当退火温度为550 ℃, 离子束溅射SiO2薄膜的短程有序范围最大, 结构稳定, 并且热处理有助于改善SiO2薄膜内应力. Shen等[18]研究了退火对电子束蒸发制备HfO2/SiO2多层膜残余应力的影响, 结果表明当退火温度为200 ℃时, HfO2/SiO2多层膜由压应力转变为张应力, 并且随着退火温度的升高而增大. 但是, 目前的研究主要集中于退火对光学材料单层膜以及HfO2/SiO2多层膜的影响研究. 而针对不同退火温度对Ta2O5/SiO2多层反射膜应力特性以及反射镜面形影响的研究, 尚未见相关的报道. 反射膜膜层结构和应力特性是影响介质膜反射镜面形状态的关键因素. 因此, 有必要研究不同退火温度对Ta2O5/SiO2多层反射膜结构和应力特性的影响.

      本文通过离子束辅助电子束蒸发工艺在石英基底上沉积了Ta2O5/SiO2多层反射膜, 然后对薄膜样品在200—600 ℃的温度下进行了退火处理, 研究了不同退火温度对Ta2O5/SiO2多层反射膜结构、光学性能和应力特性的影响. 本文的研究内容对退火工艺在Ta2O5/SiO2介质膜反射镜面形控制技术方面的应用具有重要意义.

    2.   实 验
    • Ta2O5/SiO2多层反射膜膜系结构为(HL)12H, 其中, H为光学厚度为λ/4高折射率材料Ta2O5膜层, L为光学厚度为λ/4低折射率材料SiO2膜层, 工作角度为17°, 设计波长λ为1100 nm. 基底为Φ30 mm × 3 mm的JGS-1石英基片. 基片经过抛光, 面形精度均方根粗糙度(RMS)优于1/(50λ)(λ = 632.8 nm). Ta2O5/SiO2多层反射膜的制备在德国Leybold公司生产的LAB900型真空镀膜机上进行. 该设备的极限真空可达5 × 10–5 Pa. 蒸发源为: 两把e型电子枪, 高压均为10 kV,可根据需要自由选择环形或多穴坩埚. 膜厚监控采用4探头石英晶振 + OMS5100光学自动控制系统. 辅助源为: RF射频离子源, 栅网口径12 cm, 最大束流可达500 mA. 当真空室真空达到1 × 10–3 Pa时, 打开工件转动, 转速设定为50 r/min, 打开离子源对基片进行清洁5 min. Ta2O5, SiO2膜层的沉积采用电子束蒸发加离子辅助的方式沉积, 离子源通入28 sccm O2和3 sccm Ar (1 sccm = 1 mL/mim), 真空室压强约为2.8 × 10–2 Pa, 离子源的束流设置为250 mA, Ta2O5和SiO2两种材料的沉积速率分别为0.20 nm/s和0.8 nm/ s. 沉积结束后, 将薄膜样品放入自动控温石英管式炉中, 分别在200, 300, 400, 500和600 ℃的空气中退火, 保温时间2 h, 保温时间达到后随炉冷却, 保温时的温度误差最大为 ± 1 ℃.

    • 采用德国布鲁克公司的X射线衍射仪(XRD, Cu-Kα)研究样品微观结构, 测量角度为20°—80°, 采用的步进尺度(2θ)为0.02°. 采用Veeco公司的Nanoscope Multimode IV型原子力显微镜(AFM)测量样品的表面形貌并测量反射膜表面粗糙度. 为了减少测量误差, 每个样品选取3个不同区域进行测量, 计算平均值作为样品的最终表面粗糙度. 通过使用美国Perkin Elmer公司的Lambda900型分光光度计测量薄膜样品的反射率(扫描波长范围为800—1400 nm). 通过使用ZYGO公司的GPI-HS型激光干涉仪分别测量镀膜前后基片的曲率半径, 计算出基片曲率的改变量, 并由Stoney公式[19]可得到Ta2O5/SiO2多层反射膜的残余应力(σ)

      其中Es为基片杨氏模量, υs为基片泊松比, tf为薄膜厚度, ts为基片厚度, R1R2分别为镀膜前后基片的曲率半径. 石英基片的杨氏模量和泊松比分别为72 GPa和0.17.

    3.   结果与讨论
    • 为了研究退火温度对Ta2O5/SiO2多层反射膜结构的影响, 对所得样品进行了XRD测试. Ta2O5/SiO2多层反射膜在不同温度退火后的XRD衍射谱如图1所示, 可以看出, 图谱中没有出现尖锐的衍射峰, 说明所制备的薄膜并没有结晶, 同时, 在衍射谱2θ为27°附近存在一定宽度的弥散峰, 说明膜层结构存在短程有序. 由此表明, 经过200 ℃到600 ℃退火处理, Ta2O5/SiO2多层反射膜结构没有发生明显改变, 均为非晶态结构.

      Figure 1.  XRD patterns of Ta2O5/SiO2 multilayer reflective coatings annealed at different temperatures.

      薄膜的表面形貌是光学薄膜的重要特性之一, 粗糙的表面会使薄膜表面的散射变大, 从而影响光学损耗. 图2为原子力显微镜(AFM)观测样品的表面形貌测量结果, 扫描范围为5 μm × 5 μm. 通过对测量的表面形貌图进行分析, 获得了不同温度退火后Ta2O5/SiO2多层反射膜的RMS如图3所示. 从图中可以看出, 沉积态的Ta2O5/SiO2多层反射膜表面粗糙度较大, RMS为0.916 nm. 随着退火温度的升高, 表面形貌得到很大程度的改善, 样品的表面颗粒尺寸逐渐减小, 膜层变得致密、平整, 当退火温度为400 ℃时RMS值达到最小, 为0.368 nm; 当继续增加退火温度, Ta2O5/SiO2多层反射膜的表面粗糙度逐渐变大, 同时可以看到样品表面存在较大型颗粒的聚集, 导致表面粗糙度变大, 但是仍比沉积态的薄膜表面粗糙度小.

      Figure 2.  AFM images of Ta2O5/SiO2 multilayer reflective coatings annealed at different temperatures: (a) As-deposited; (b) 200 ℃; (c) 300 ℃; (d) 400 ℃; (e) 500 ℃; (f) 600 ℃.

      Figure 3.  Surface RMS of Ta2O5/SiO2 multilayer reflective coatings annealed at different temperatures.

    • 退火温度对Ta2O5/SiO2多层反射膜反射率的影响如图4所示, 可以看出随着退火温度的升高, 反射率曲线整体向长波方向移动. 当退火温度达到600 ℃时, 反射率光谱大约往长波方向移动了12 nm. 退火后Ta2O5/SiO2多层反射膜光谱漂移的原因与薄膜光学厚度的变化有关. 膜层光学厚度为薄膜物理厚度与折射率的乘积, 退火后薄膜的聚集密度和折射率均发生了变化. 若膜层光学厚度大于退火前, 则光谱曲线向长波方向漂移, 反之则向短波方向漂移. 同时, 退火后Ta2O5/SiO2多层反射膜在1064 nm处的反射率, 相比沉积态均有略微升高, 反射率由沉积态时的99.5%变为300 ℃退火后的99.8%. 反射率变化的原因一方面是由于在大气中退火可以有效消除薄膜中存在的氧空位, 降低膜层的吸收; 另一方面, 退火后Ta2O5膜层折射率变大[20], 进而Ta2O5/SiO2折射率比值增大, 使得多层反射膜反射率升高.

      Figure 4.  Reflectance spectra of Ta2O5/SiO2 multilayer reflective coatings annealed at different temperatures.

    • 表1给出了Ta2O5/SiO2多层反射膜经不同温度退火后基片变形以及膜层残余应力值. 表中应力值为正时, 表示薄膜受到张应力作用, 基片变形为正值, 镀膜面呈凹形; 反之, 若应力值为负时, 表示薄膜受到压应力作用, 基片变形为负值, 镀膜面呈凸形. 从表中可以看出, 退火前Ta2O5/SiO2多层反射膜具有较高的压应力, 其值约为–90 MPa, 由膜层应力引起的基片变形在–282 nm左右. 随着退火温度的增加, 压应力呈现出先增大后减小的趋势. 反射膜经200 ℃退火后, 膜层压应力最大, 其值高达–117.0 MPa, 基片变形为–375.3 nm. 退火温度在200—400 ℃时, 膜层压应力有轻微下降, 但变化不明显. 当退火温度为500 ℃时, 反射膜残余应力有了明显释放, 降低到了–28.6 MPa, 基片变形变为–67.1 nm. 当退火温度进一步升高到600 ℃时, 反射膜残余应力转变为张应力, 其值为52.9 MPa, 基片变形变为213.3 nm. Ta2O5/SiO2多层反射膜样品在高温退火后, 残余应力呈现出由压应力向张应力转变的趋势, 使基片变形由上凸向下凹方向转变. 可能的原因是, 退火消除了薄膜中存在的微孔和缺陷, 薄膜体积发生收缩, 从而使反射膜残余应力向张应力转变[21]. 另外, 退火排除了薄膜中的一些吸附水, 也有利于残余应力向张应力发展.

      Annealing temperature/℃Substrate deflection/nmResidual stress/MPa
      Before coatingAfter coatingAfter annealingAfter coatingAfter annealing
      As-deposited31.6–282.8–282.8–90.9–90.9
      20029.5–281.6–375.3–89.9–117.0
      30031.4–278.4–363.9–89.5–114.3
      40030.2–282.8–320.2–90.5–101.3
      50031.8–284.1–67.1–92.3–28.6
      60030.5–280.3213.3–89.852.9

      Table 1.  Substrate deflection and residual stress of annealed Ta2O5/SiO2 multilayer reflective coatings.

      Ta2O5/SiO2多层反射膜退火后, 由反射膜与基底构成的介质膜反射镜面形状态如图5所示. 沉积态的介质膜反射镜由于膜层压应力的作用, 表面呈凸形, 镜面面形参数峰谷(PV)值为0.470λ (λ = 632.8 nm); 当退火温度低于400 ℃时, 介质膜反射镜面形PV值略有升高, 最高值达到了0.731λ; 当退火温度继续升高到500 ℃, 介质膜反射镜镜面面形有了明显改善, PV值减小到了0.177λ; 当退火温度为600 ℃时, 反射膜应力由于转变为了张应力, 反射镜面形呈现了凹形, PV值增大到了0.372λ.

      Figure 5.  Surface figures of dielectric mirrors annealed at different temperatures: (a) As-deposited; (b) 200 ℃; (c) 300 ℃; (d) 400 ℃; (e) 500 ℃; (f) 600 ℃.

      为了进一步研究Ta2O5/SiO2多层反射膜在500—600 ℃退火温度范围内膜层应力的转变过程, 分别对反射膜在525, 550 ℃及575 ℃温度下进行了退火处理. 表2给出了Ta2O5/SiO2多层反射膜在500—600 ℃退火后基片变形以及膜层残余应力值, 图6给出了Ta2O5/SiO2多层反射膜在500—600 ℃退火后介质膜反射镜面形状态. 实验结果表明, 当退火温度在525 ℃时, 反射膜残余应力得到了有效释放, 应力值降到了最小, 为–4.3 MPa. 同时, 膜层应力导致的反射镜面形变化得到了有效改善, 使介质膜反射镜具有了较好的面形状态, PV值优于0.1λ, RMS值优于0.02λ.

      Annealing temperature/℃Substrate deflection/nmResidual stress/MPa
      Before coatingAfter coatingAfter annealingAfter coatingAfter annealing
      52531.2–279.816.4–89.9–4.3
      55029.6–283.660.1–90.58.8
      57530.8–281.5112.6–90.323.6

      Table 2.  Substrate deflection and residual stress of Ta2O5/SiO2 multilayer reflective coatings annealed at the temperature range from 500 to 600 ℃

      Figure 6.  Surface figures of dielectric mirrors annealed at the temperature range from 500 to 600 ℃: (a) 525 ℃; (b) 550 ℃; (c) 575 ℃.

    4.   结 论
    • 采用离子束辅助电子束蒸发工艺在石英基底上沉积Ta2O5/SiO2多层反射膜, 并在200—600 ℃的空气中做退火处理, 系统研究了退火温度对Ta2O5/SiO2多层反射膜结构、光学性能以及应力特性的影响. 研究结果如下: Ta2O5/SiO2多层反射膜在200—600 ℃范围退火后, 膜层均为非晶态结构; 退火温度对Ta2O5/SiO2多层反射膜表面粗糙度影响较大, 随着退火温度的升高, 膜层表面粗糙度先减小后逐渐增大, 但均小于沉积态; Ta2O5/SiO2多层反射膜退火后, 反射光谱略向长波方向漂移, 反射率略微升高, 光谱性能整体稳定; 反射镜在500—600 ℃退火后, 残余应力由压应力向张应力转变, 镜面面形由凸形向凹形转变. 结果表明: 采用合适的温度退火可以有效释放Ta2O5/SiO2薄膜的残余应力, 消除膜层应力造成的基片变形, 进而改善介质膜反射镜面形状态, 同时退火后反射膜层结构与光谱性能稳定, 可使介质膜反射镜能够满足星载激光测高仪应用要求.

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