温度变化对多层铜/铝纳米薄膜拉伸和疲劳性能影响的研究

Multilayered metal nanofilms, particularly those made of Cu/Al, are widely used in large-scale integrated circuits and microelectrical and mechanical system devices where the films are under either constant uniaxial tension or cyclic loading. Thus, in order to increase the fatigue life and reduce the cost of the metal films, we propose the molecular dynamics study of the tensile and fatigue characteristics of Cu/Al multilayered nanofilms. The embedded-atom method potentials will be utilized to describe the interaction between atoms in the simulation process. We will simulate the tensile and fatigue properties of the films under different temperatures and analyze their deformation properties, which will be then used to quantitatively reveal their tensile deformation and fatigue mechanisms. Molecular dynamics simulations will be performed on Cu/Al thin films under uniaxial tensile and cyclic loading in order to identify the critical temperature for the transition of deformation mechanism. The deformation will then be analyzed by examining the variation of the atomic structure of the emerging dislocation. Thus, the mechanisms of the temperature dependence of tensile and cyclic deformation will be explored and investigated. By the thermal activation theory and diffusion theory,we will also investigate whether for FCC lattice metal films there exist a few temperature regions, which correspond to different thermal activation mechanisms of the dislocation motion. We will finally perform molecular dynamics and first-principles density functional theory calculations of the electronic structure and properties of various kinds of metal/metal (Cu/Al) interfaces in the layered nanofilms so that we can construct good electronic, thermal and mechanical metal/metal (Cu/Al) multilayers. These findings will be helpful in the design of nanoscale multilayers and in the use of multilayered metal nanofilms.

铜/铝等多层金属纳米薄膜现正广泛应用于大规模集成电路及微电机设备中。由于这些薄膜在不同环境温度下承受各种荷载作用，其可靠性研究至关重要。因此,本研究将用嵌入原子势的分子动力学模拟方法研究温度对铜/铝多层纳米薄膜的拉伸和疲劳性能的影响，并用热激活成核理论和扩散理论对模拟结果进行定量研究：给出热激活参数，结合观测到的位错原子结构，分析探讨出各种塑性形变机制。并将进一步揭示具有面心立晶格结构金属薄膜的拉伸塑性和循环形变机理及其临界温度效应的一般规律。我们还将用基于密度泛函理论的第一性原理方法研究铜/铝等面心立方晶格结构的金属纳米多层膜的电学性能、热学性能和力学性能，以期为设计出综合性能优异的金属纳米多层薄膜、更好地使用金属纳米多层薄膜、延长其使用寿命提供理论依据。

用嵌入原子势的分子动力学模拟方法来研究不同温度环境中拉伸和循环荷载下薄膜材料的力学性能，对保证微纳米器件的功能性和可靠性至关重要。近年来，金属层状结构复合材料尤其铜/铝多层膜的设计、研发和应用倍受关注。本课题采用基于大规模分子动力学方法，研究了纳米铜/铝双层膜和多层膜力学性能的尺寸效应和温度效应。首先研究了层厚对纳米单晶铜/铝双层膜的拉伸力学性能影响，结果发现了铜/铝双层膜的屈服强度存在有临界层厚（7nm）：屈服强度在大于或小于或等于临界层厚范围内分别具有三种变化特征。进一步，与其相应的三种形变机制，可以通过拉伸过程中观察到的原子级位错演化及定量计算出的压杆位错密度而得到充分解释；其机理是在临界层厚处形成的复杂堆垛层错结构可以促进压杆位错产生、进而提高铜/铝薄膜的屈服强度。接着研究了铜/铝双层膜的疲劳力学性能，结果发现：在疲劳循环加载过程中，铜侧中都有层错与微孪晶；铝侧中只有层错；而且随循环次数增加铝侧层错数目逐渐增加而出现循环软化现象。在此基础上，构建出了铜/铝多层膜模型，研究了层厚对铜/铝多层膜的拉伸力学性能影响，也发现铜/铝多层膜在层厚大于或小于或等于10nm范围内屈服强度分别具有三种变化特征，所以10nm为临界层厚，而且取临界层厚的铜/铝多层膜拉伸形变也形成了奇异的位错结构，并且定量计算出了屈服形变时压杆位错数目，正是因为这种位错数在层厚10nm处取得极大值，才使得层厚10nm的铜/铝多层膜具有较大屈服强度；由此开展研究了层厚为10nm铜/铝多层膜拉伸性能的温度效应。另外构建出了多晶铜/铝双层膜模型，用分子动力学方法研究了室温下多晶铜/铝双层膜的拉伸性能。以上结果可为铜/铝双层膜或多层膜的设计提供了理论参考。譬如，参考模拟出的具有临界层厚结果，在设计和制备铜/铝双层膜或多层膜，考虑将层厚分别设计为7nm或10nm左右，就可能会制备出较好力学性能的铜/铝双层膜或多层膜。