【作者】 廖宁波；

【导师】 杨平；

【作者基本信息】 江苏大学 ， 机械电子工程， 2009， 博士

【摘要】 多层结构和多个界面是电子器件本身以及器件互连和封装中普遍存在的现象,界面分层失效成为产品性能和可靠性方面关心的重要问题。国外研究者通过大量实验发现界面是微系统制造和运行中的关键部分,很多破坏和缺陷都发生在界面附近。目前,微电子器件有不断向微型化发展的趋势,最小芯片的特征尺寸已经达到微米量级,界面效应越来越明显,对微观界面相关物理规律的研究势在必行。IC朝微小化发展意味着对传热的要求越来越高,一些现象如空位、填隙原子、孔洞、杂质,晶格应力、界面效应等对传热都有较大影响,其中界面热阻对微观传热影响较大,而宏观方法已经不再适用。通过微观途径,可以建立起对材料行为的基本认识,它正逐渐成为发展新材料和高性能器件的不可或缺的重要手段。分子动力学(MD)是微观建模的主要手段。许多在实验中无法获得的微观细节,在分子动力学模拟中都可以方便地观察到。这些优点使分子动力学在物理、化学、材料科学等领域研究中显得非常有吸引力。本文就是在这样的背景下,采用分子动力学方法对界面传热过程中的物理特征进行了研究,在通过实验进行模型研究的基础上,进一步研究了纳米级缺陷在传热过程中的演变及对热参数的影响,最后基于前面的MD模型构建了MD/FE多尺度模型框架。主要工作如下:1.界面的形成及特征分析:研究温度对界面区域的影响,模拟拉伸过程研究界面的结合特性。这里采用施加高温高压的方法,模拟扩散键合形成界面的过程。仿真结果表明界面层厚度有随温度增加而增加的趋势,但温度过高会导致不稳定状态的出现。通过对底面和顶面三层原子的位移进行控制,对模型进行拉伸加载,根据得到的应力—应变曲线,应力在达到最大值后突然下降,键合Cu—Al界面的曲线比理想Cu—Al界面更平稳,其拉伸力可以达到理想界面的82%,证明了扩散键合过程对在不同材料间成键的有效性。2.基于实验的模型验证:以石英玻璃为基板,采用磁控溅射法在基板上制备两层厚度为300-500nm的金属薄膜,作为界面试样。对不同界面试样进行热循环加载,然后用划痕仪(日本Rhesca公司)和纳米压痕仪(美国HYSITRON公司)对加载前后的试样进行测试分析。通过对比划痕实验的界面关键载荷结果和MD仿真得到的界面断裂能量结果,可以看出MD计算的结果与实验是相符的,即Al-W界面的结合力比Cr-W界面更大。从热加载的角度分析,热载荷对Al-W界面的影响比Cr-W界面小。根据纳米压痕测试结果,界面结构在经过热循环后弹性模量和硬度均呈现减小的趋势,MD模型得到的弹性模量与实验值有一定差距,但从仿真中也反映了界面结构经过热载荷后弹性模量有所减少的现象,并且对于不同的界面组合而言,仿真结果定性地体现出了弹性模量受热加载影响的程度。3.面向封装界面物理特征的微/纳观研究:用分子动力学描述界面组织演化和材料缺陷特征,计算界面传热的温度分布,热流等参数的变化规律,观察微观界面尺度材料组织的演变,分析热缺陷的产生机制。根据本文的仿真,在尚未达到稳定状态时,在界面附近存在高温区域,而巨大的温差会最终导致应力和裂纹。界面附近出现的高温度和高应力都是由界面处材料的不同造成的,并对界面破坏有重要影响。而对Cu-Al界面和Cu-Cu界面进行的对比研究为了验证了以上结论。探索材料中裂纹对界面热力特性的影响,为封装材料界面破坏机制提供理论依据。4.通过移除一定数量的原子,纳米尺度的裂纹被加入到仿真材料中。从仿真结果可以观察到热流条件下裂纹的扩展过程:在2000时间步时,加入的两条裂纹有扩大的趋势;在4000时间步时,原始裂纹附近出现了位错,在界面处出现了一条新的裂纹;在8000时间步时,原始裂纹和新产生的裂纹不断扩大最终合并,界面出现了分层破坏。在20000时间步时,分层继续扩大。根据裂纹附近的温度分布,表明裂纹的扩展会导致在裂纹扩展方向形成高温区域。另外研究了裂纹长度对热流的影响,在4000时间步前,裂纹越长则对应的热流也较大,而较靠近热源的裂纹对热流有更大的影响。5.界面传热的多尺度模型:构建了针对界面传热的MD-FE多尺度模型。根据仿真结果,多尺度区域的温度分布图可以看出原子区域描述的界面相互作用对有限元区域的影响,并展现出合理的温度梯度,这体现了FE区域和MD区域的相互作用。将多尺度模型计算得到界面热阻与MD模型进行了对比,多尺度模型得到热阻随时间的变化表现出与MD模型相似的趋势;多尺度模型计算得到的热阻更大,这与MD区域和FE区域的耦合机制有关。而在热源温度增加后,两种模型的计算结果有更接近的趋势。通过仿真结果的对比表明,在提高计算效率的同时,在这里建立的多尺度模型可以获得与MD模型相近的精确度。这里的工作对界面传热问题的多尺度建模进行了有益的探索。更多还原

【Abstract】 The interfaces of dissimilar materials suffer high stress gradients due to the presence of thermal and stiffness mismatches of bonded materials. Interfaces of dissimilar materials are prone to crack initiations,leading to delaminations.Reliable strength predictions for regions involving an interface cannot be made unless the critical values of fracture parameters are available.Heat transfer across interfaces is a critical consideration in a wide variety of scientific and engineering applications.When the system becomes extremely small,the atomistic effects have to be taken into account correctly and continuum method such as FEM is not capable of capturing all the information.At the nanoscopic length scale the molecular dynamics(MD) method takes correctly into account these effects,when the potential is chosen correctly.In this paper,atomic simulations of interfacial thermal conductance under the effect of nanocracks are proposed.The simulations results show the propagation mechanisms of cracks and the corresponding change of temperature distribution in the heat transfer process.The main works include:1.To construct a non-ideal interface,high pressure and high temperature conditions are applied to the system.Because temperature and pressure play important roles in interface forming.The simulation results showed that the thickness of interfacial region increases with the temperature increases.To effect the tensile deformation,the displacements of three layers of boundary atoms are controlled by time steps.Basing on the stress- strain curves,after reaching the maximum stress,the stress drops precipitously and plastic flow occurs.For the case of the constructed copper-aluminum interface,the curve is flatter than that of the ideal-contact case.The tensile strength of the constructed interface can reach 82%of that for the ideal-contact interface,demonstrating the effectiveness of diffusion-bonding process in creating bonds between the two materials.2.Model verification by experiments:firstly,to make the sample by depositing two metal file on the glass plate,with thickness of 300-500nm for per film.After applying thermal cycle conditions to the sample to mimic thermal loading,use scratch test and nano-indention test to get the results of interface critical force and materials properties. By the result of scratch test,the Al-W interface is stronger than Cr-W interface,and the effects of thermal cycling loading on critical force is less for Al-W interface,which is according with the MD results of interface fracture energy.And from nano-scratch test,MD results also show similar trend on Young’s module by comparing with the experimental results.It means MD simulation can be used to analysis the interface properties qualitatively.3.The physical properties and failure mechanism on interface of dissimilar materials under thermal flux conditions.Before a steady state is reached, the temperature distributions near the interface and there is a jump in temperature.The temperature near the interface increases at first and then decreases severely,and the highest temperature lies in the interfacial region of Al block.It means high temperature region formed near the interface before the system reach steady state,and the large temperature difference will generate stress and cracks eventually.It is concluded that the characteristic of atoms’ moving along the interface is caused by dissimilarity of material.And this conclusion is proved by a comparison study on Cu-Al interface and Cu-Cu interface.4.The nano-level cracks were added to the simulation materials by removing some atoms from the perfect lattice structure,and three types of cracks are studied.The propagation of cracks can be observed:at 2000 time steps,the movement of the atoms tend to enlarge the size of the original cracks;at 4000 time steps,some dislocations were emitted near the original cracks and a crack generated at the interface;at 8000 time steps,failure by coalescence of the crack at interface and original cracks,the delamination is generated;and at 20000 time steps,the delamination continue to expand.According to the results of temperature distribution,the propagation of crack will lead to formation of high temperature area along the direction of crack propagating,which is consisted with our results.According to the results of thermal flux for different crack models,the thermal flux increase with the crack length increase before 4000 time steps,and the crack close to the heat source has larger effect on the thermal flux.5.A multi-scale frame is proposed for interface heat transfer by coupling MD model and finite elements model concurrently.From the temperature distribution and thermal resistance results,the multi-scale model gives a similar trend and reasonable value on thermal resistance comparing with MD model,while with less computing time because of reduction of freedom by combining FE model.The multi-scale frame proposed here is meaningful for future multi-scale modeling of interfacial heat transfer.更多还原