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Bai Jinlin, Jiang Yugang, Wang Lishuan, Li Ziyang, He Jiahuan, Liu Huasong, Su Jianzhong. Study on the design and preparation technology of ultra-low profile wideband high reflection thin films[J]. Infrared and Laser Engineering, 2021, 50(2): 20200413. doi: 10.3788/IRLA20200413
Citation: Bai Jinlin, Jiang Yugang, Wang Lishuan, Li Ziyang, He Jiahuan, Liu Huasong, Su Jianzhong. Study on the design and preparation technology of ultra-low profile wideband high reflection thin films[J]. Infrared and Laser Engineering, 2021, 50(2): 20200413. doi: 10.3788/IRLA20200413

Study on the design and preparation technology of ultra-low profile wideband high reflection thin films

doi: 10.3788/IRLA20200413
  • Received Date: 2020-10-26
  • Rev Recd Date: 2021-01-02
  • Available Online: 2021-02-07
  • Publish Date: 2021-02-07
  • The surface shape deviation of the optical thin film element will cause the wavefront distortion of the transmitted beam in the high-precision laser system, which will seriously affect the performance of the optical equipment. The traditional surface profile deviation control technology uses double-sided coating, but it is necessary to repeatedly polish the substrate to obtain a high-precision surface profile, which will greatly increase the development cost and limit the use of this method. Based on ion beam sputtering deposition technology, a stress-deformation model was used to predict the shape change after coating, and then the coating surface of the component to be plated was pre-processed into a surface shape opposite to the deformation direction, compensating for the deformation of thin film components caused by the stress of the film after coating. Finally, an ultra-low-profile broadband high-reflection film was prepared on the pre-processed substrate to achieve reflectivity R≥99.5% and PV≤0.15λ@632.8 nm at the working wavelength of 550-750 nm. Through calibrating the mechanical parameters of thin film materials, this technology predicts the surface shape changes of any multilayer film under the same process conditions, realizes the introduction of mechanical synchronization design while designing the ultra-wide spectrums, and prepares high-quality optical films that meet the dual indicators of light and force.
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    [2] Poirié T, Schmitt T, Bousser E, et al. Influence of internal stress in optical thin films on their failure modes assessed by in situ real-time scratch analysis [J]. Tribology International, 2017, 109: 355-366.
    [3] Xue Qingsheng, Chen Wei. Design of modified Czerny-Turner spectral imaging system with wide spectral region[J]. Optics and Precision Engineering, 2012, 20(2): 233-240. (in Chinese)
    [4] Li Zhenyu. Application of tunable lasers in optical communication networks[J]. Communication Technology, 2009, 42(8): 4-5. (in Chinese)
    [5] Dong L, Wang Q, Yao Y, et al. Wide wavelength tuning on 4F 3/2 → 4I 13/2 transitions from Nd: Scysio 5 laser [J]. Optics and Laser Technology, 2019(111): 770-774.
    [6] Gao Chunxue. Study on stress distribution and control of optical thin films[D]. Nanjing: Southeast University, 2015. (in Chinese)
    [7] Suhir E. Predicted thermally induced stresses in, and the bow of, a circular substrate/thin-film structure [J]. Journal of Applied Physics, 2000, 88(5): 2363-2370. doi:  10.1063/1.1286096
    [8] Klein C A, Miller R P. Strains and stresses in multilayered elastic structures: The case of chemically vapor-deposited ZnS/ZnSe laminates [J]. Journal of Applied Physics, 2000, 87(5): 2265-2272.
    [9] Liu Huasong, Wang Lishuan, Jiang Yugang, et al. Refractive index and stress adjustment of SiO2 thin Films prepared by ion beam sputtering[J]. Optics and Precision Engineering, 2013, 21(9): 2238-2243. (in Chinese)
    [10] Eda C, Bill B, Oleg Z, et al. Mechanical and thermoelastic characteristics of optical thin films deposited by dual ion beam sputtering [J]. Applied Optics, 2009, 48(23): 4536-4544.
    [11] Stenzel O, Wilbrandt S, Kaiser N, et al. The correlation between mechanical stress, thermal shift and refractive index in HfO2, Nb2O5, Ta2O5 and SiO2 layers and its relation to the layer porosity [J]. Thin Solid Films, 2009, 517(21): 6058-6068.
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Study on the design and preparation technology of ultra-low profile wideband high reflection thin films

doi: 10.3788/IRLA20200413
  • 1. Tianjin Key Laboratory of Optical Thin Film, Tianjin Jinhang Institute of Technical Physics, Tianjin 300308, China
  • 2. Joint Laboratory of Optoelectronic Materials and Intelligent Surface Structures, Tianjin 300308, China

Abstract: The surface shape deviation of the optical thin film element will cause the wavefront distortion of the transmitted beam in the high-precision laser system, which will seriously affect the performance of the optical equipment. The traditional surface profile deviation control technology uses double-sided coating, but it is necessary to repeatedly polish the substrate to obtain a high-precision surface profile, which will greatly increase the development cost and limit the use of this method. Based on ion beam sputtering deposition technology, a stress-deformation model was used to predict the shape change after coating, and then the coating surface of the component to be plated was pre-processed into a surface shape opposite to the deformation direction, compensating for the deformation of thin film components caused by the stress of the film after coating. Finally, an ultra-low-profile broadband high-reflection film was prepared on the pre-processed substrate to achieve reflectivity R≥99.5% and PV≤0.15λ@632.8 nm at the working wavelength of 550-750 nm. Through calibrating the mechanical parameters of thin film materials, this technology predicts the surface shape changes of any multilayer film under the same process conditions, realizes the introduction of mechanical synchronization design while designing the ultra-wide spectrums, and prepares high-quality optical films that meet the dual indicators of light and force.

    • 随着现代的大科学装置和航空航天光电设备的发展,对光学系统提出了愈来愈高的要求,工作谱段由单波长转向宽谱段,因此对光学薄膜元件的需求也从单波长发展到宽谱段。由于宽谱段光学性能的需求,导致膜层的数量和厚度增加,引起较高的薄膜应力,造成光学薄膜面形偏差问题,因此光学薄膜面形偏差问题是光学薄膜元件设计和制造面临的核心问题[1-3]

      可调谐激光器的工作谱段几乎覆盖整个可见光谱段,要实现宽谱段的高反射薄膜制备,所需要的膜层厚度高达10~20 μm[4-5]。这么厚的膜层产生的薄膜应力会导致宽带激光系统中传输光束发生波前畸变,严重影响激光设备的性能。为了获得宽带激光系统所需的能量密度,保持传播光束足够高的光束质量,制备超低面形的宽带高反射光学薄膜元件迫在眉睫。传统的薄膜元件面形精度一般要求小于λ/4 (λ=632.8 nm),文中创新性地实现了在光谱宽度为200 nm的波段范围内面形精度小于λ/6,并且达到反射率99.5%以上。

    • (1)热应力:在真空镀膜的过程中,薄膜和基底都处于较高的温度,当镀膜结束后,整个薄膜系统冷却至室温,由于材料间热膨胀系数的不同,导致薄膜和基底之间因收缩不匹配而产生应变,从而产生热应力[6]。如图1所示,在光学薄膜系统中,假定基底与薄膜结构不发生塑性变形,由于温度变化而引起的薄膜热应力为[7-8]

      Figure 1.  Optical multilayer film system

      可以看出,薄膜热应力发生弯曲变化与温度变化量正相关,并且与薄膜材料的力学性能息息相关。

      (2)本征应力:一般来说,薄膜的本征应力与薄膜本身的结构有关,薄膜的结构又依赖于沉积工艺条件[9]。在薄膜生长过程中,薄膜中的晶粒、晶界、杂质、空位、表面应力以及薄膜与基底界面间的晶格失配等因素导致了本征应力的产生[10- 11]。相对于热应力来说,薄膜本征应力的成因非常复杂,它与薄膜的微观结构密切相关,受到制备材料和沉积工艺参数的影响,是多种物理机制综合作用的结果,因此很难用理论公式定量计算,所以薄膜的本征应力只能通过实验进行标定。

    • 文中应用一个热应力+本征应力模型来预测光学薄膜的应力和形变水平,假设基底呈等厚圆柱的形式,并且对薄膜应力的影响占主导地位,光学薄膜的面形变化是由薄膜与基底的热膨胀系数差异决定的,通过薄膜材料的杨氏模量可以将其转化为双轴应力。这种双轴应力与光学薄膜在沉积期间就与已经存在的本征应力共同作用,以类似于表面张力或表面压力的方式作用在基底上,使光学薄膜元件的表面发生形变或者分层、断裂,如图2所示。

      Figure 2.  Optical thin film stress diagram

      首先,根据薄膜应力热模型计算光学元件产生的形变为:

      其中,$f = 3\dfrac{{E_i^{'}}}{{E_S^{'}}}\dfrac{{{a^2}}}{{t_S^2}}{h_i}\Delta \alpha \Delta T$。此时光学元件最大挠度发生在基板中心(r=0)处,为:

      由于薄膜热应力为${\sigma _t} = E_i^{'}\Delta \alpha \Delta T$,通过和公式(3)对比可知,薄膜热应力是导致基板变形的重要原因。根据2.1节中薄膜应力产生机制可知,热应力和本征应力都是薄膜中所固有的,两种应力共同作用使基板面形发生变化,所以对于光学单层膜,薄膜应力引起的变形为:

      该公式用来预测单层膜的应力变形情况,在多层光学薄膜应力计算时,每层膜都被认为是相互独立的,所以光学多层膜的形变可认为是单层膜形变量的累加和,所以多层膜系统中总变形为:

    • 在多层膜应力形变计算时,每层膜都被认为是独立的,所以要对多层膜系统中单层的力学参数进行标定。该实验选用Ф25×1的熔融石英作为薄膜材料本征应力标定的基片,Ф25×5的熔融石英作为薄膜材料弹性模量/硬度标定的基片,使用离子束溅射镀膜机分别制备膜厚约150 nm的Ta2O5和SiO2单层膜,采用ZYGO激光干涉仪分别测量薄膜的残余应力,采用Nano Indenter G200型纳米压痕仪测量薄膜的弹性模量/硬度。

    • 文中采用纳米压痕仪连续刚度测量模块可获得薄膜材料杨氏模量/硬度随压入深度的关系,并且可以有效避免基底效应,准确获得膜层的力学参数。根据薄膜材料的杨氏模量/硬度与压入深度的曲线图可知,曲线在压入深度为25 nm附近出现一个“峰值”,之后随着压入深度的持续增加,杨氏模量逐渐减小,这说明己经表现出熔融石英基底的特性,所以此“峰值”可以作为薄膜的压入硬度或杨氏模量的最大估计值。同时为消除系统误差和随机误差,准确获得薄膜材料的力学参数,对同一样品进行多次测量,对测量值进行平均化处理。图3图4是对SiO2薄膜进行多次测量的结果,黑色曲线是10次测量的平均值,其中黄色区域是多次测试后误差带,可以看出,多点测量曲线具有较好的一致性,因此通过连续刚度法可以准确地确定SiO2薄膜的弹性模量为64 GPa,硬度为8.7 GPa。同理,Ta2O5薄膜和熔融石英基底的杨氏模量和硬度如表1所示。

      Figure 3.  Young's modulus of SiO2 thin film is plotted along with the pressure depth

      Figure 4.  Young's hardness of SiO2 thin film is plotted along with the pressure depth

      MaterialsModulus/GPaHardness/GPaPoisson’s ratio/GPa
      SiO2648.70.14
      Ta2O51169.60.29
      Fused silica699.70.17

      Table 1.  Mechanical parameters for common thin film by ion beam sputtering

    • 膜层应力是薄膜的重要力学特性,也是薄膜制备非平衡过程的代表性参数。大量研究表明,薄膜应力主要由热应力、本征应力组成,两者的量级和权重与具体的沉积方式和薄膜制备工艺参数相关。而且薄膜本征应力是薄膜制备过程中的固有特性,不随外界环境的变化而被影响,通过薄膜本征应力的标定,可以预知同种制备工艺下不同制备参数的薄膜应力变化情况,为后续高反射多层膜制备提供有力支持。

      文中基于离子束溅射制备技术,选用Ф25×1的熔融石英作为标定基片,分别制备约150 nm厚度的SiO2和Ta2O5单层膜进行薄膜材料本征应力标定。薄膜残余应力采用ZYGO激光干涉仪进行测量,从测量结果图可直接获得基片的面形值,镀膜后基底面形测量图如图5和图6所示。矢高Power用来表征基底的总体弯曲方向和弯曲程度,通过基底的矢高Power和曲率半径R的关系:

      Figure 5.  SiO2 film surface profile (after coating)

      Figure 6.  Ta2O5 film surface profile (after coating)

      式中:Ds为基底的直径。进一步可得到:

      式中:ΔPower为镀膜前后Power的差值。把上式代入Stoney公式,便可得到薄膜残余应力为:

      根据应力形变模型分析可知,薄膜中热应力同样引起薄膜面形的变化,这在薄膜整体面形变化中是不可忽略的一部分,所以薄膜热应力可由2.1节中的热应力公式(1)进行计算,热应力计算结果如表2所示。薄膜本征应力是薄膜制备过程中的固有特性,不随外界环境的变化而被影响。由2.1节中薄膜应力产生机制可知,薄膜中残余应力的构成分为本征应力和热应力,所以通过计算薄膜残余应力和热应力的差值,就可以进行薄膜本征应力的标定,常用的离子束溅射两种薄膜材料的应力分布如表2所示。

      MaterialsResidual stress/GPaThermal stress/GPaIntrinsic stress/GPa
      SiO2−0.26−4.61E-04−0.262
      Ta2O5−0.226−0.83E-03−0.224

      Table 2.  Stress distribution for common thin film by ion beam sputtering

    • 薄膜材料本征应力的修正通过选择四组不同径厚比的基底进行多层膜镀制,其径厚比分别为5、7、25和40,然后通过ZYGO激光干涉仪对镀膜前后的基片进行测量,从测量结果图可以得到不同径厚比基底的实际面形变化值。然后将标定后的本征应力大小代入应力变形模型,发现理论计算出的镀膜后面形变化量与实验所测量的面形偏差有很大的差距,所以要对光学薄膜材料的本征应力进行修正。通过对光学单层膜的本征应力乘以修正因子,再代入应力形变模型进行镀膜后面形变化的输出,与实验中面形变化作对比,最终实现了理论计算多层膜镀膜面形变化值与实际测试面形变化值误差小于3%,如图7所示。

      Figure 7.  Comparison diagram of theoretical and experimental surface changes under different substrates

    • 首先采用Ta2O5和SiO2作为高、低折射率材料设计宽带高反射膜系,膜系结构为:基底/[1.365H 1.365L]16 [1.17H 1.17L]16 [H L]16/空气,设计波长λ0为600 nm,上述膜系中膜层总厚度为9.79 μm。高、低折射率材料的总物理厚度分别为4.047 μm和5.743 μm,其比值为0.705,此膜系结构实现在45°工作角度入射,550~750 nm的工作波长下反射率R≥99.5%;然后将第2节中标定后的薄膜材料力学参数和宽带反射镜膜系结构带入薄膜应力变形模型进行计算,理论计算宽带高反射薄膜镀膜后面形值,得到理论面形变化量为−1.233λλ=632.8 nm);接着将基片待镀膜面预加工出与变形方向相反的面形,来补偿镀膜后膜层应力造成的薄膜元件变形,因此采用光学加工技术加工面形值为1.233λ左右的待镀膜基片,最终实际的基片面形值为1.2λ,如图8所示;最后采用离子束溅射沉积技术镀制宽带高反射薄膜,使用ZYGO激光干涉仪对镀膜后的光学薄膜元件进行测量,面形值为Power=−0.012λ,如图9所示;采用分光光度计对光学薄膜元件光谱进行测量,在550~750 nm的工作波长下反射率R≥99.5%,宽带高反射薄膜元件反射率测试曲线如图10所示。

      Figure 8.  Surface profile of the coated substrate after preprocessing

      Figure 9.  Wideband high reflection thin film element surface diagram

      Figure 10.  Reflectance test curve for wideband high reflective thin film elements

    • 可见光宽带高反膜通常采用离子束溅射沉积技术将高纯度薄膜靶材料沉积到基底上,所得到的薄膜是致密的,并且薄膜内存在高达几个GPa的残余应力。这些应力导致光学元件镜面发生变形,影响薄膜器件的性能、稳定性和使用寿命。文中基于应力预补偿制备工艺,通过理论计算光学元件镀膜后面形变化,对待镀膜面预加工出与变形方向相反的面形,采用离子束溅射沉积技术制备超低面形光学薄膜。此方法根据软件可以准确预估宽带反射镜镀膜后的面形变化,因此在产品研制过程中,避免了对宽带反射镜基片的反复抛光,提高了成品率,降低了研制成本,且是一种控制薄膜应力的新方法,无需对现行镀膜工艺参数进行调整,而且仅对基片单面镀膜,便可对介质反射膜层应力造成的基片形变进行补偿,进而实现对宽带反射镜面形精确控制,具有很高的灵活性和适应能力。

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