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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。
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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和图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。
Materials Modulus/GPa Hardness/GPa Poisson’s ratio/GPa SiO2 64 8.7 0.14 Ta2O5 116 9.6 0.29 Fused silica 69 9.7 0.17 Table 1. Mechanical parameters for common thin film by ion beam sputtering
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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:
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。
Materials Residual stress/GPa Thermal stress/GPa Intrinsic 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
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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。
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
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Key words:
- thin film element surface shape /
- stress pre-compensation technology /
- wide spectrum /
- optical processing
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.