PVD导热涂层的研究综述

余斌，孙德恩,1b，Yongda Zhen



PVD导热涂层的研究综述

余斌1a，孙德恩1a,1b，Yongda Zhen2

（1.重庆大学 a. 材料科学与工程学院 b. 机械传动国家重点实验室， 重庆 400030；2. Singapore Polytechnic, Singapore 139651）

首先从导热涂层的应用背景出发，分析了导热涂层研究的必要性，其次探讨了导热涂层的导热机理和影响涂层导热的宏观和微观因素。在此基础上，阐述了PVD导热涂层的研究现状，重点分析了SiC、AlN、DLC三种常见的具有较大应用潜力的PVD导热涂层。声子散射是影响涂层热导率的直接原因，涂层内部同位素、杂质、缺陷及晶界等均会引起声子发生散射，而界面声子散射引起的界面热阻对涂层导热性能影响巨大，通过合理选择制备技术和精确控制工艺参数，在一定程度上能改善涂层的导热性能，提高热导率。在此基础上，笔者提出了离子源辅助高功率脉冲磁控溅射（HiPIMS）的工艺配合，提高涂层质量和致密度，优化界面结构，降低界面热阻，以期实现涂层的高导热性能。

导热涂层；热导率；热阻；声子散射；界面结构

随着科技的迅速发展，轻量化和集成化成为现代及未来电子设备及电子电路的发展潮流。越来越复杂的电路，越来越小的电路板面积要求，导致微电子设备及集成电路的缩小化，元器件密度和功率不断增加，热拥挤现象越来越严重，大量材料界面的热电阻成为限制电路或电子设备高效散热的重要因素[1]。统计数据表明，电子元器件的温度每升高2 ℃，可靠性下降10%，温度为50 ℃时，电子设备的寿命只有25 ℃时的1/6[2]。高功率发光二极管LED，因其高的发光效率、较长的使用寿命及节能等优点，在汽车照明、家用照明等领域应用广泛[3]。LED灯的发光效率为20%左右，剩余70%～80%的能量转化为热量，热量使得大功率LED灯具的温度升高。如果不及时高效散热，将降低元器件的可靠性，甚至损坏电路及电子设备。LED灯具产生故障大约有70%来由散热不及时导致的芯片温度过高[4-5]。散热问题成为提高产品功率，发展先进LED产品的最大障碍，解决LED散热问题的途径之一是应用高导热、绝缘和透过率的基板材料，将热量有效地传出去，但基板上的热量往往不能有效地传导至外壳表面[6-7]。因此，导热涂层在芯片封装设计及散热方面极为重要[8]。

导热涂层的研发和应用对于提高大功率集成化电子设备外壳及封装的有效导热率起着重要的贡献。PVD导热涂层由于PVD技术的优势，能在低沉积温度下实现高沉积效率，得到表面形态较好、内应力低的厚膜，实现涂层的高热导性能[9-10]。在集成电路系统中加入导热涂层，能显著提高其散热效率，并且高热导率涂层的散热效果远远优于低热导率涂层[11-12]。目前，研究较多较为成熟的PVD导热涂层主要有SiC[13-15]、AlN[16-18]、DLC[19-21]等。这些涂层都在一定程度上缓解了元器件的热拥挤问题，即便随着元器件功率的提高，产热过多，高热导率涂层也能将热能及时高效地传导出去，保证器件或电路在额定温度下使用，提高了大功率微电子设备的使用性能和寿命。Horng[22-23]等人在LED封装基板表面沉积DLC涂层，在1400 mA注入电流下的热阻比无涂层的要低34%，并且能在2000 mA的注入电流及620 mW的输出功率下正常工作，极大地提高了LED的光性能和寿命。PVD导热涂层的发展，对于大功率微电子器件的发展有着重要的意义，并且在高导热的同时，一定程度上也提高了设备的耐蚀抗磨等性能。

1 PVD涂层导热的机理及影响因素

导热涂层实现热量的快速传导，保证电路设备安全可靠的基础是涂层材料具有较高的热导率。与块状材料不同，薄膜材料表现出来的热导率往往比同种的块状材料要低得多。目前已经有研究表明，同种材料块状和薄膜形式的热导率不同，是由于导热机理的差异和影响热导率的因素差异导致的。高的热导率往往得益于材料的高导热及传输过程的低损耗，因此需要对涂层导热的机理和影响因素有清晰的认识，以便能够对涂层的导热进行调控，降低传输过程中的热量损耗，实现热量地高效传导。

1.1 导热机理

根据固体传热理论，材料热导率主要由两部分组成，即热传导是由自由电子和声子两种载体传递热量的综合（=p+e）[24]。对于金属而言，主要依靠材料内部的自由电子传输能量；非金属主要是通过晶格振动（声子导热）来传递能量，实现热传导。涂层与块状材料导热的基本原理大致相同，不同的是涂层本身是一个微尺度量，不像块状材料尺度很大。涂层厚度一般在几纳米到几百纳米之间，尺度与声子或电子平均自由程量级相当，涂层与衬底间的界面热阻会超过涂层本身固有热阻，大大降低热导率。常见的PVD导热涂层多为陶瓷非金属（SiC、AlN等），广大学者研究了从基体金属到非金属涂层的传热途径。Shukla[25-27]等人在文献中提出了金属-非金属热流传导模型，有两种方式：热量直接从金属中的自由电子转移到非金属的晶格振动；可以首先从金属中的自由电子转移为金属晶格振动，然后通过界面上声子-声子的耦合实现金属-非金属之间的热流传导。

1.2 影响因素

涂层的热导率是实现高效导热的基础，在传导热量的过程中，任何能使声子产生散射，降低平均自由程的因素都会直接或间接地对涂层的热导率产生影响，降低导热性能。在涂层结构内部，主要存在散射机制有[28]：声子-声子、声子-同位素、声子-杂质或缺陷及声子-晶界散射[29]等，膜基界面与声子间的散射机制对涂层热导率也有显著影响。完美单晶结构中只有声子-声子散射，随温度的下降，其热导率升高。源于晶格中声子数量减少，散射概率降低，声子的平均自由程增加[30]，而实际涂层会存在各种各样的散射阻碍。因此，要提高PVD涂层的热导率，必须从制备工艺和涂层结构入手，优化工艺参数，减少引起声子散射的因素。经过众多学者的深入研究，发现诸如沉积温度[13, 31-32]、涂层厚度[33-38]、缺陷[39-40]及界面结构[41-45]等对涂层热导率影响很大，需要对工艺和参数进行精确控制[46]。

涂层的沉积温度对于涂层组织结构及热导率是一个关键因素。沉积温度表示涂层制备过程中能量的多少，对于涂层结构产生重要的影响，比如提高涂层的致密度，对声子散射会有一定的减小。对界面结构而言，提高溅射温度可以增强涂层与基底的结合，减小界面无定形层的热阻影响[47]。对于晶体和非晶涂层，二者的热导率具有不同的温度依赖性。非晶涂层对温度的依赖性类似于块状材料；而晶体涂层厚度与平均自由程相近时，热导率峰值随温度变化，较厚涂层的热导率在低温下达到峰值[48]。

研究发现，涂层厚度对热导率的影响很大。随着涂层厚度的增加，热阻下降，热导率升高。DLC涂层的热导率及热阻随温度的变化曲线如图1所示，实线源于热导率及热阻与涂层厚度之间的函数公式[45,49]：f=i/(1+ik/f)及f/f=f/i+k。式中：f为涂层的有效热导率；i为涂层材料的体热导率；ik/f表示涂层的边界效应。随着涂层厚度的增加，边界效应减小，有效热导率f逐渐趋于材料的体热导率i，微尺度效应减小。对于较薄涂层，厚度因素对涂层的热边界阻碍更大，这就是图1刚开始变化趋势较大的原因。当涂层厚度的增量远大于声子波长及声子平均自由程时，涂层热导率逐渐接近于材料体热导率，涂层散热高效。

图1 DLC涂层热导率及热阻随厚度变化[43]

由于空气的热导率很低，涂层内部的孔隙不仅增加了界面，而且孔隙中的空气也阻碍了热传导，严重影响了涂层的导热性能及综合性能[50]，较大的致密度对于实现高导热涂层也有一定的积极影响。除此之外，多层膜结构由于界面增加，界面阻碍效应严重，声子散射增强，对热导率产生不利影响。Samani等人[51]通过对多层膜导热性能的研究，发现随着层数的增加，涂层系统的热导率显著下降。认为其原因是多层膜会打断柱状结构，柱状结构的分裂导致声子散射增加，择优取向降低，涂层尺寸减小，产生严重的微尺度效应，以及界面的位错等缺陷增多。这些均会使声子散射强烈增加，平均自由程大大缩短，涂层热导率下降。

2 PVD导热涂层的研究现状

国内外研究学者对PVD导热涂层已经进行了很多研究，由于PVD制备工艺的优越性，PVD导热涂层的潜能也逐渐被进一步挖掘，实现了涂层的高热导率及各种防护性能。导热涂层要求具有较高的热导率和低的热膨胀系数，高热导率能保证热量高效传导，低热膨胀系数可以使得涂层和基底之间实现良好的结合，常见的几种高热导低热膨胀涂层材料如图2所示。左上角DLC材料具有极高的热导率和低的热膨胀系数，热导率是铜的1.5倍，而热膨胀系数与Mo相当，展现出巨大的导热潜能。此外，发明专利（ZL201420603314.6）[52]提出了一种电子元件导热涂层结构，是以连接层结合于电子元件的表层，接触层与外部接触，可以提高位于其二端面间的导热性，快速地协助电子元件散热。

图2 不同材料的热导率及热膨胀系数[23]

目前，该领域对高热导率及低热膨胀系数的DLC、AlN、SiC三种材料用作导热涂层展开了广泛的研究，研究对象主要针对涂层沉积技术、沉积温度、涂层厚度及界面结构优化等。制备方法常采用磁控溅射[53]及高功率脉冲磁控溅射[54]等PVD技术，也有人通过等离子喷涂制备导热涂层[55-56]。研究结果表明，在不同制备方法和工艺参数下获得的涂层，实现了较高的热导率，提高了衬底的散热效率，保证了密集电路和设备正常运行。

2.1 SiC涂层

目前对于SiC涂层的导热性能研究，发现SiC涂层的热导率远小于SiC块状材料。文献[57] 对比了SiC块状和薄膜形式的组织结构和导热性能，通过不同方法制备得到的SiC块状材料结构为晶体，而厚度在几百个纳米的SiC涂层表现为非晶结构。相比于SiC块状晶体材料，薄膜内部非晶结构的紊乱是导致SiC涂层热导率较低的原因。刘霞等人[14]的研究结果也得到了相同的结论。Wang等人[58]通过射频磁控溅射PVD技术在镁合金表面沉积了单层及复合SiC涂层，结果显示，复合SiC涂层的热导率随温度不同而改变。经过腐蚀后，复合涂层在25 ℃和100 ℃下的热导率分别为90.1 W/(m·K)和108.4 W/(m·K)，表明SiC涂层能在腐蚀环境下保持高热导性能。

2.2 AlN涂层

单晶AlN在室温下的热导率为320W/(m·K)，平均自由程为100 nm，使得AlN涂层在高功率高温电子设备和集成电路中有巨大的应用潜力[59-60]。大量的报道[16,61-63]表明，在几百纳米或几个微米范围厚的AlN涂层仍保持晶体结构，有(002)的择优取向，并且随着晶粒尺寸的增加，涂层的成形性越好，热导率越高。Duquenne等人[64]通过平衡磁控溅射和非平衡磁控溅射技术在Si基体表面沉积了AlN涂层，同时比较了涂层厚度、气体含量等参数对热导率的影响。结果表明，热导率取决于涂层晶型质量、致密度等微观结构，相比平衡磁控溅射，非平衡磁控溅射沉积的涂层的晶型更完整、组织更致密、厚度更大、界面结合更好（见图3），获得了更大的热导率。此外，元素C[65]、Si[66]、B[67]等的掺杂，在某一方面可以降低边界热阻，提高导热性能。

2.3 DLC涂层

与SiC涂层相似，DLC涂层也是一种非晶结构。DLC涂层是一种由sp3和sp2杂化碳键组成的性质介于金刚石和石墨之间的碳膜。金刚石具有最高的热导率2000 W/(m·K)，石墨由于具有与金属相似的性质，体内含有大量的自由电子，导热性能也很好。因此，DLC也具有优异的导热性能，热导率高达600 W/(m·K)，是一种非常具有潜力的导热涂层。与其他涂层略有差异的是，结构独特的DLC涂层的导热兼顾金属的自由电子导热和非金属的声子导热（晶格振动），以及可以将表面热能转换为红外线的电磁波（原子振动），以黑体辐射的形式加速散热[68]，而热辐射散热效率主要取决于红外发射率[69]。大量的研究[70-72]表明，DLC涂层的热导率与sp3杂化键的数量和结构有序度有关，取决于涂层密度、杨氏模量及sp3的含量。涂层热导率随厚度及温度等的变化归因于这些因素对涂层密度、杨氏模量及sp3的含量的影响，并且影响涂层与基体界面结构，引起界面热阻的变化。在高注入电流下，LED封装基底上沉积有DLC涂层的光输出功率和EQE都优于无DLC涂层[73]。此外，研究显示，旌宇显卡散热器在沉积DLC涂层后，热阻能降低到大约0.05，温度下降5 ℃左右。表明DLC涂层在提高各种器件散热性能上有较大的应用潜力[74]。

图3 不同方法制备的AlN涂层截面SEM图[64]

2.4 涂层热导率的测量及界面结构的优化

与结构材料不同，涂层尺度在纳米或微米量级，由于涂层的厚度与界面层厚度相当，纳米尺度上测量的涂层导热性可能受到界面层热阻的影响[75]。因此，常规热导率测量方法并不适用与涂层材料。目前用于薄膜热导率的测试技术主要有3ω技术[76-79]、Raman光谱法[80]、瞬态热带技术[62-64]、TDTR法[37,81]等，各种测试技术都有各自的优势和应用。3ω技术是目前薄膜热导率最常用的测试技术之一，因为其对辐射不敏感，测量条件简单方便，非常适用于薄膜纵向热导率的测量[82]。3ω技术的热穿透深度大于薄膜的厚度，所测得的热导率受界面热阻的影响，TDTR法则没有（见图4）[81]。

图4 3ω技术和TDTR法的热流传输[81]

界面散射造成的界面热阻对热导率的影响很大，甚至阻隔大量热传输，大大降低涂层的导热性能。优化界面结构，减小界面散射是提高涂层导热性能的关键。研究[47,83-84]发现，在界面处生成的无定形结构扩散层大大增强了对声子的散射，并随着扩散层厚度的增加，散射增强，热导率下降。界面无定形扩散层与涂层有效热导率的关系为：tot/eff=p/p+a/a，其中eff、p、a分别为有效热导率、薄膜热导率及界面扩散层热导率；tot、p及a分别为薄膜厚度、薄膜顶层厚度及界面扩散层厚度。同时研究发现，随着沉积温度的升高，界面扩散层的厚度会减小，涂层热导率增加（见图5），界面扩散层厚度的降低可能与温度升高致使能量增加有关。与此同时，Aissa等人[85]通过直流磁控溅射（dcMS）和高功率脉冲磁控溅射（HiPIMS）沉积AlN涂层（如图6所示），dcMS沉积涂层有明显的界面无定形扩散层，而HiPIMS制备的涂层没有或不明显，这可能与HiPIMS技术的高离化率有关。

在此对导热涂层的分析基础上，笔者提出用离子源辅助HiPIMS技术制备DLC涂层。DLC涂层的热导率和热膨胀系数对于作为导热材料来说是非常有导热潜力的一方面，但是由于现有的制备技术和工艺未能发挥DLC更大的导热潜能，进一步研究和提高现有DLC涂层的热导率具有重要意义。其次，离子源辅助HiPIMS技术能够在沉积过程中实现高的离化率，在高沉积能量下获得光滑致密的涂层，有利于减小涂层的内部缺陷，提高致密度，实现高的热导率。再者，用离子源轰击过渡层和基材，去除表面缺陷或不稳定结构，预期实现优化界面结构，达到界面良好结合，减小界面无定形扩散层厚度，提高导热性能的效果。

图5 不同沉积温度下界面的HRTEM图[47]

图6 不同制备技术的界面HRTEM图[85]

3 结语

PVD导热涂层对于提高微电子元器件及集成电路散热性能、缓解热拥挤具有重要的意义。可以通过合理选择制备技术和精确控制工艺参数提高涂层质量，优化界面结构，减小声子散射，实现涂层导热性能的进一步提升。

目前对于导热涂层的研究不足，很多理论和机制尚不清楚，比如晶体涂层和非晶涂层导热机理的差异。如何进一步降低界面热阻等都大大限制了导热涂层的实际应用，这将是接下来重要的研究方向。此外，导热涂层的发展也需要更科学精确的测量技术来支撑。在此基础上，笔者认为影响声子散射及热导率的因素，反过来也可以作为提高涂层导热性能研究的切入点，进一步挖掘，实现导热涂层研究现状的突破。这需要广大科学研究者的不懈努力，未来定可以突破导热涂层的现有研究局限，实现PVD导热涂层的大发展。

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Thermal Conductive Coatings by PVD Technology

1a,1a,1b,2

(1.a. School of Materials Science and Engineering, b. State Key Laboratory of Mechanical Transmission, Chongqing University, Chongqing 400030, China; 2. Singapore Polytechnic, Singapore 139651, Singapore)

Firstly, the necessity of thermal conductive coating research was analyzed based on the application background of thermal conductive coating. Secondly, the thermal conduction mechanism of the coating and the macroscopical and microcosmic factors affecting the thermal conductivity of the coating were discussed. On this basis, the research status of PVD thermal conductive coatings was described, and three common PVD thermal conductive coatings, namely SiC, AlN and DLC, were emphatically analyzed.Phonon scattering was the direct factor affecting the thermal conductivity of the coating, and phonon scattering could be caused by some factors, such as coating internal isotope, impurities, defects and grain boundary. The interfacial thermal resistance caused by phonon scattering had great influence on the thermal conductivity of the coating. Thermal conductivity of the coating could be improved to a certain extent by reasonably selecting preparation technology and accurately controlling process parameters.On this basis, the technological cooperation of ion source assisted high-power pulsed magnetron sputtering (HiPIMS) is proposed to improve coating quality and density, optimize interface structure and reduce interface thermal resistance, in order to achieve the high thermal conductivity of the coating.

thermal conductive coatings; thermal conductivity; thermal resistance; phonon scattering; interface structure

2018-11-29；

2019-01-14

YU Bin (1996—), Male, Master, Research focus: hard and functional film.

孙德恩（1974—），男，博士，教授，主要研究方向为硬质及功能薄膜。邮箱：deen_sun@cqu.edu.cn

TG174.444

A

1001-3660(2019)06-0158-09

10.16490/j.cnki.issn.1001-3660.2019.06.018

2018-11-29；

2019-01-14

国家自然科学基金（51771037）；材料腐蚀与防护四川省重点实验室开放基金（2016CL13）；重庆市基础与前沿研究计划项目（cstc2015jcyjA70005）

Support by National Natural Science Foundation of China(51771037); Material Corrosion and Protection in Sichuan Province Key Laboratory of Open Fund(2016CL13); The Basic and Frontier Research Project of Chongqing (cstc2015jcyjA70005)

余斌（1996—），男，硕士，主要研究硬质及功能薄膜。

SUN De-en (1974—), Male, Doctor, Professor, Research focus: hard and functional film. E-mail: deen_sun@cqu.edu.cn