Development of the Microplasticity Theory of Internal Friction and its Application to the Evaluation of Thin-Film Mechanical Properties

用弯曲振动自由衰减法测量了硅基上铝、铜和金薄膜在不同温度下的内耗随应变振幅的变化。根据本构方程，膜中的内摩擦力可以从复合材料系统的测量阻尼单独评估。铝和铜薄膜的振幅依赖性被发现在应变范围内的一个或两个数量级高于散装。根据细观塑性理论，将与振幅有关的内耗转化为位错运动时有效应力的函数，即塑性应变。由此获得的铝和铜膜的应力-应变响应表明，10^量级的塑性应变<-9>随应力的增加而非线性地增加。随着薄膜厚度的减小和温度的降低，曲线倾向于向更高的应力移动，这两者都意味着微塑性变形的抑制。在恒定的塑性应变水平下的微流应力在所有温度下与膜厚度成反比。铝膜和铜膜的膜厚效应可以用位错弯曲模型来解释。当膜厚小于0.5 μm时，金膜的微流应力随着膜厚的减小而减小，这可能是由于表面-晶界扩散引起的应力松弛所致。结果表明，对于钛膜覆盖的金膜，虽然应力水平低于铝膜，但其微流应力与膜厚成反比。

Internal friction in aluminum, copper and gold thin films on silicon substrates has been measured as a function of strain amplitude at various temperatures by means of a free-decay method of flexural vibration. According to the constitutive equation, the internal friction in the film can be evaluated separately from the measured damping of the composite system. The amplitude dependence for the aluminum and copper films is found in the strain range one or two orders of magnitude higher than that for the bulk. On the basis of the microplasticity theory, the amplitude-dependent internal friction can be converted into the plastic strain as a function of effective stress on dislocation motion. The stress-strain responses thus obtained for the aluminum and copper films show that the plastic strain of the order of 10^<-9> increases nonlinearly with increasing stress. The curves tend to shift to a higher stress with decreasing film thickness and also with decreasing temperature, both of which means a suppression of the microplastic deformation. The microflow stress at a constant level of the plastic strain varies inversely with the film thickness at all temperatures examined. The film thickness effect for the aluminum and copper films can be explained on the basis of a dislocation-bowing model. In contrast, the microflow stress for the gold films decreases with decreasing film thickness when the film thickness is less than 0.5 μm, which could be due to the stress relaxation caused by surface-grain boundary diffusion. It is found that, for the gold films covered by titanium films, the microflow stress varies inversely with the film thickness although the stress level is lower than that for the aluminum films.