Study of the origin mechanism and controlling method of stress in thin films

薄膜应力是薄膜生产、制备中的一种普遍现象，几乎所有的薄膜都处于一定的应力状态。薄膜应力是影响光学元器件甚至整个光学系统性能及可靠性的重要因素，过大的张应力或压应力都会引起薄膜表面破损，使生产成本不断提高，极大地限制了光学薄膜的使用性能。同时应用于各种光学系统中的薄膜元件的基底必须保持绝对平整，这一点对于基底面形变化会导致反射入射波前发生畸变的光学仪器来说更是非常关键的。而实际上，光学薄膜中的残余应力常常是引起其基底面形发生弯曲导致其光学性能下降的主要原因。通过对薄膜应力的研究，可以了解薄膜的破坏机理，进而达到改善薄膜抗损伤性能的目的。本文采用宏观应力测量与微观结构检测相结合的方法对ZrO2、SiO2单层膜及ZrO2／SiO2组合多层膜中的应力进行了实验研究，对应力随沉积条件的变化规律进行了深入研究，从微观结构的角度对应力的产生机理进行了分析，并在实验基础上建立起了多层膜应力的发展模型。研究了ZrO2单层膜应力随沉积温度、沉积速率的发展规律，结果表明在所选实验条件下，室温沉积的ZrO2薄膜为非晶结构，应力性质为张应力；当沉积温度在室温至350℃范围内变化时，ZrO2薄膜中的残余应力有一个从张应力向压应力的变化过程，应力值变化范围为64-150MPa，沉积温度为190℃，沉积速率为0.6nm／s时，21.02薄膜中应力接近零值；沉积速率在0.2～0.6nm/s范围内变化时，ZrO2薄膜中应力值变化范围为122-46MPa，沉积速率为0.4nm／s，沉积温度为260℃时，薄膜中应力接近零值；对ZrO2薄膜样品微结构的X射线衍射分析表明，随着沉积条件的改变，样品结构也发生了相应的演变，说明薄膜中的残余应力与薄膜沉积过程中微结构的变化有密切关系。因此，选择合适的沉积参量可控制薄膜中残余应力的发展。SiO2单层膜应力实验结果表明，SiO2薄膜中残余应力的性质和大小可以通过改变沉积时的氧分压及沉积温度来进行调整，这对于要求基底最小变形的多层膜光学元件来说是非常有意义的；电子束蒸发SiO2薄膜的内应力与其堆积密度之间有非常密切的关系，当薄膜的堆积密度增大时，薄膜中的压应力随之增大：用蒸发方法沉积薄膜的多孔结构会导致其在环境变化时的力学性能不稳定性；当SiO2薄膜被储存在干净的室温环境中时，其中的残余应力随着存放时间的延长发生了从压应力到张应力的转变，这主要是由于吸附的水分子与SiO2薄膜发生了复杂的反应引起的。在单层膜应力实验基础上，对不同膜厚组合周期数的ZrO2／SiO2多层膜中的应力进行了研究。ZrO2／5102多层膜由相同沉积条件下的电子束蒸发方法制备而成，通过改变多层膜中高（ZrO2）、低（SiO2）折射率材料膜厚组合周期数的方法，研究了沉积在熔石英和BK7玻璃基底上多层膜中残余应力的变化。用ZYGO光学干涉仪测量了基底镀膜前后曲率半径的变化，并确定了薄膜中的残余应力。结果发现：该多层膜中的残余应力为压应力，随着薄膜中膜厚组合周期数的增加，压应力值逐渐减小。而且在相同条件下，石英基底上所沉积多层膜中的压应力值要小于BK7玻璃基底上所沉积多层膜中的压应力值。用X射线衍射技术测量分析了膜厚组合周期数不同的ZrO2／SiO2多层膜的微结构，发现随着周期数增加，多层膜的结晶程度增强。同时多层膜的微结构应变表现出了与所测应力不一致的变化趋势，这主要是由于多层膜中，膜层界面之间复杂的相互作用引起的。在对单层膜及多层膜应力的形成机理及发展规律有了深入犷解之后，根据平面应力的作用原理提出了一种多层膜中应力的计算模型，通过该模型可以对由不同材料组成的多层膜中的应力进行计算。对模型的实验验证表明，该模型的理论计算结果与实验结果相吻合。Residual stress in thin films can cause severe problems for a number of applications in advanced technology; for instance, excessive residual stress can limit the reliability and function of thin films-based structures due to peeling, cracking and curling. So the control of the residual stress is very crucial in almost all coatings. The residual stress in Z1O2 films prepared by Electron beam evaporation was measured by viewing the substrate deflection using an optical interference method. The influence of deposition temperatures and deposition rates on the residual stress was studied. The results show that residual stress in ZrC>2 films changes from tensile to compressive with the increase of deposition temperature and deposition rate and the value of the compressive stress increase with the increase of deposition temperature. At the same time, the microstructure of the ZrC>2 films was inspected by X-ray diffraction (XRD). The relationship between the residual stress and the microstructure was also discussed. The residual stress in the SiC^ films prepared by electron beam evaporation was measured by viewing the substrate deflection using ZYGO optical interferometer. The influences of deposition temperatures, oxygen partial pressure and aging on the residual stress had been studied. The results showed that the compressive residual stress in S1O2 films increased with the decrease of the oxygen pressure and increase of the deposition temperature. And the residual stress varied from -223.5 MPa to 20.4 MPa when the oxygen partial pressure increased from 3.0X 10"3 Pa to 13.0X 10"3 Pa, which may be attributed to the variation of the microstructure concluded from the variation of the refractive index. An evolution of residual stress from compressive to tensile with time of sample storage in a conventional clean-room environment was measured for all evaporated SiCh films investigated in this study. The effects of the repeating thickness periods on the stress is studied in multilayers deposited by electron beam evaporation on BK7 glass and fused silica substrates, respectively. The results show that the residual stress was compressive and decrease with the increase of the periods of repeating thickness in the ZrCVSiC^MLs deposited on both BK7 and fused silica substrates. At the same time, the residual stress in multilayers deposited on BK7 glass substrates is less than that of samples deposited on fused silica substrates. The variation of the microstructure examined by the X-ray diffraction shows that the microscopic deformation does not corresponds to the macroscopic stress, which may be due to the variation of the interface stress. A calculation model is developed to calculate the stress in multilayers. This model accounts for the effects of interface stress and thermal expansion coefficients on the total stress of multilayers. The multilayer structures are build-up by alternative layers with high and low refractivity. A formulation based on the composite shell theory is implemented for numerical analysis of the stress in multilayers. The advantages of the model proposed are its flexibility and estimated performance. The stress in multilayers can be calculated by this approach before they are deposited