超快激光纳米连接及其在微纳器件制造中的应用 
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Significance With the increasing demand for the miniaturization and multifunctioning of micro/nanoelectronics and optical-electromechanical devices or systems, high-performance integration of materials at small scales is urgently needed. Nanojoining technology, which involves interconnection among low-dimension materials and even cross-dimensional nano-micro-macro materials, has shown great potential in manufacturing and developing micro-nano devices/systems.
Conventional welding/joining technologies such as ultrasonic welding, cold-pressure welding, and laser welding can be employed for the integration of diverse nanomaterials. However, to achieve high-performance and low-damage nanojoints, high-precision manipulation of input energy sources (e.g., thermal energy or laser beam) or mechanical tools (e.g., ultrasonic head or indent) is crucial in the nanojoining process, which poses great technical difficulties, thereby limiting the mass production of nanojoints and adaptability for multimaterial nanojoining. Ultrafast laser (UFL), with an extremely high pulse energy and short pulse duration, has been widely used in the precise manufacturing of a broad range of materials. Notably, a plasmonic effect, arising at the metal-dielectric interface, is a new approach to redistribute the incident optical (e.g., laser) energy with a “self-limited” effect, which is used for low-damage nanojoining. Therefore, nanojoining with UFL is promising for high-precision nanojoint formation, not only in homogeneous metal-metal structures but also in heterogeneous metal-oxide-semiconductor combinations.
Progress Local energy input within nanostructures can be redistributed because of incident laser-induced plasmonic effects (Figs.1--4). By constructing a metal-dielectric structure and simultaneously adjusting incident-laser parameters, spatial energy can be confined at local positions without the need to accurately control the spot size and spatial location of the incident laser beams. The self-limited effect of input energy contributes to the highly efficient formation of nanojoints with low damage, which permits the fabrication of complex micro-nano structures.
The interface structure of a nanojoint determines the electronic band diagram and thereby affects the electron transportation at the junction (Figs. 5 and 6). As indicated, a highly defective region with a large number of oxygen vacancies will be formed at a metal-oxide heterojunction after UFL nanojoining, which can reduce the Schottky barrier, thereby facilitating the origin and conduction of electrons at the junction. Based on this, high-performance heterojunctions with a variety of material combinations have been successfully achieved using UFL nanojoining technology. By regulating the interface structures, our team has developed several electronic nanodevices, which shows that UFL is promising for high-precise nanodevice fabrication and performance modification.
In addition, pulsed laser deposition (PLD) based on UFL developed by our group provides an alternative approach for low-temperature nanojoining (Figs.7--11). Nanoparticles fabricated via PLD possess high surface energy and fast diffusivity, which will be combined to form a connection layer composed of submicron particles. Therefore, the melting point will be close to that of bulk silver, which could be used for “low-temperature nanojoining and high-temperature service”. These UFL-deposited metal/alloy nanoparticle films have been successfully used for large-scale packaging of SiC wafers, which is expected to be widely used in the packaging of third-generation semiconductor devices that require high-temperature service and high reliability.
Accordingly, functional micro-nano devices can be fabricated based on the UFL nanojoining technology (Figs.12--19). We have developed several micro-nano devices by nanojoining nanomaterials at diverse scales, which will be used in devices such as optical waveguides, multilevel memristors, rectifier units, field-effect transistors, p-n junction units, supercapacitors, and flexible pressure sensors. This implies that the UFL nanojoining is promising for constructing basic micro-nano device units and is conducive to realizing multifunction device miniaturization.
Conclusion and Prospect UFL nanojoining is a vital way for micro-nano device fabrication. The local energy distribution within nanostructures can be well controlled, and the energy band diagram of the heterogeneous interface can be modified by interface metallurgy. In addition, low-temperature nanojoining can be realized using PLD based on UFL. The UFL nanojoining technology can be used to fabricate many micro-nano devices and has been proven to achieve excellent performance. Further research in the aspects of accurate positioning of three-dimensional structures, uniform fabrication of micro-nano patterns, common mechanism and quality control of heterogeneous interface formation, mass production, reliable performance evaluation, special equipment for nanojoining, and engineering applications will enrich and perfect nanojoining theory and technology, thereby permitting a broad range of applications in advanced micro-nano fabrications in various fields.
1 引言
根据被连接材料即母材、连接材料以及所形成焊点的尺度分类,连接包括宏观尺度连接、微米尺度连接(简称“微连接”)、纳米尺度连接(简称“纳米连接”或“纳连接”)[1-2]。纳米连接是焊接与连接学科领域的新兴方向,涉及纳米尺度材料-纳米尺度材料、纳-微-跨尺度(包括纳与微、纳与宏、纳与微及宏尺度组合)材料的连接。伴随纳米技术及相关学科的高速发展,电子元器件及其系统、光机电系统等也迅速向尺寸小型化、结构复杂化、多材料应用、多功能方向发展,纳米连接在其制造与快速研发中发挥着越来越重要的作用[3]。纳米连接除满足结构完整和力学性能等一般要求外,往往更需要接头具备光、声、电、磁等某种或多种功能特性,因此,纳米连接的发展更需要物理、化学、电子、材料、环境、力学、机械等多学科的交叉。经过近20年发展,借鉴传统或常规焊接与连接工艺方法,并基于纳米连接自身特点和要求进行创新,形成了2个大类方向的系列纳米连接工艺方法[1-3]。
第一大类方向:纳米尺度材料自身及其与其他材料的连接,主要包括:1)高能束连接,如早期的聚焦电子束和离子束熔化焊同质单壁碳纳米管(CNTs)、纳米线(Au、Si、Cu、Ni、Sn)[4-6]以及异质Si与金属纳米线等[7],以及近10年迅猛发展的激光辐照钎焊或熔化焊各种同质和异质金属纳米线或颗粒(Ag、Cu、Pt等)、半导体纳米线(TiO2、ZnO、SiC等)[3, 8-12];2)超声波加压固态连接,如采用专用超声波加压连接装置连接CNTs、SiC、Si纳米线与Pd、Ti、Au、Al金属电极[13-16];3)高温钎焊和低温钎焊,如采用Ag-Cu-Ti合金在高真空中辐射加热高温钎焊CNTs束 [17],采用Sn-Pb、Sn、Zn等纳米金属在热台上加热软钎焊Au、Ni和Pt纳米线,采用绿光辐照加热钎焊连接Ag纳米线等[3, 6];4)冷压连接,如Au、Ag纳米线的冷压大塑性变形连接[18-19];5)电阻加热连接,如Pt纳米线与表面镀碳的锗金属纳米线之间的连接[6];6)物理连接,如采用薄膜卷覆法连接双壁CNTs,并进一步采用环氧树脂复合强化或激光强化等获得良好电性的CNTs宏观体高强接头[20]。通过上述纳米连接,学者们研制了系列原理性纳电子组元或微纳元器件,如多级记忆电阻、整流单元、微型超级电容、神经网络单元、场效应晶体管单元和微太阳能电池等[1-3, 8-11];
第二大类方向:采用具有纳米特征的材料连接其他材料,主要包括:1)采用纳米金属颗粒及其复合材料作为连接材料的低温烧结连接,如Ag、Cu纳米颗粒及其机械混合物,Ag-Cu和Ag-Pd合金颗粒,Sn或者Ag包覆Cu复合纳米颗粒等焊膏作为连接材料的低温烧结连接[1-3, 21-23];微米尺度Ag2O和CuO等氧化物颗粒前驱粉焊膏作为连接材料的原位生成纳米颗粒低温烧结连接[1-3];Ag及其合金Ag-Cu、Ag-Pd等不包含有机物的纳米颗粒薄膜的低温烧结连接[24-26];Ni等纳米金属颗粒焊膏作为连接材料的钎焊[3];2)纳米多孔结构如Ag薄片作为连接材料的低温烧结连接[3];3)采用数百层纳米结构复合膜的纳米连接(包括钎焊、扩散焊、过渡液相扩散连接),复合膜体系目前主要有Ag/AlN、Cu/AlN、Ag-Cu/AlN、Al-Si/AlN、Cu/W、Ag-Cu/W和Ni-Ti/Ti等,被连接的母材包括陶瓷、不锈钢、钛合金和铝合金[3];4)采用纳米金属间化合物(IMCs)如Cu6Sn5、Cu3Sn、Ag3Sn、Ni3Sn4与Fe、A12O3、TiO2、CNTs等纳米颗粒强化复合焊膏的软钎焊[27]。其中,低温烧结连接主要用于功率器件低温连接、高温服役封装,用Ni纳米颗粒焊膏和纳米结构复合膜连接主要是降低多材料复杂结构的连接温度,用纳米颗粒强化焊膏钎焊主要是为了减缓接头界面脆性IMCs的生长以提高可靠性[3]。
上述纳米连接工艺方法均有各自应用的适应性和局限性,如电阻加热需要被连接材料具备导电性、高真空环境难以适应批量制造、加压对纳米器件可能导致损伤、纳米尺度材料的连接与性能测试操控困难、有些纳米材料连接是随机的即只证明了连接原理的可行性、纳米颗粒焊膏因含有机物难以应用于大面积封装、物理连接可靠性有待提高等。此外,理想的纳米材料接头对纳米尺度材料的操作提出了更高的要求。现有纳米材料操作方法常是基于扫描探针显微镜或是电子显微镜进行操作,但是前者难以获得及时、准确的视觉信息,后者能操作的材料受到真空和高能电子束的限制且成像精度在一定程度上会被操作系统影响,且两种纳米操作通常是针对选定的纳米材料进行操作,难以实现对纳米材料的大面积、图形化操控[28]。总的来讲,纳米连接目前的主要挑战和前沿发展方向包括:纳米尺度材料的操作特别是三维复杂操作与定位、纳米尺度能量源获取及其精确控制、纳米尺寸效应对连接的影响机制、纳米连接与检测一体化、纳米连接材料/纳米结构特别是图形化制备与调控、纳米尺度连接界面调控及可靠性评测、纳米连接精密专用装备与高效连接等[3]。
超快激光一般指脉宽小于10 ps的脉冲激光,包括皮秒、飞秒、阿秒激光,其具有峰值功率密度极高、多材料适用、加工热影响区极小等显著优势。因此,超快激光在微纳制造中发挥着越来越重要且广泛的作用。同样,基于超快激光制造的纳米连接是该领域最有前景的发展方向,并已成为国际研究热点。
2 超快激光纳米材料连接局域能量调控
纳米连接中的被连接对象即母材为纳米尺度的材料时,由于其表面原子拥有较高的活性,进而纳米材料的组织结构极易受到输入能量的影响。为达到低损伤高效连接/互连的目标,连接过程中需要对输入能量进行精细化调控。
已知偏振光激励下,金属与介电材料的界面处会出现共振电子,而当振荡强度处于最大值时将获得表面等离子激元共振。当光激励处于无限大平整的金属-介电界面处时,等离子激元将周期性间隔分布并沿着界面向远侧传播,而强度则呈指数式衰减。从而,受界面处等离子效应的影响,空间能量的输入也将呈现周期性非均匀分布的特征。
当光辐照对象为纳米尺度结构如纳米颗粒时,远场入射的光将受纳米材料自身形貌影响,产生差异性的光散射特征即近场光学特征,进而影响光能在结构中的输入。同时,纳米结构中由表面等离子激元效应产生的振荡电子会受到结构的影响产生极化,通过调整入射偏振光源的参数,可获得多种模态的电子振荡及空间分布,进而得到可控的空间能量输入,如
图 1. 金属纳米颗粒对中的等离子激元效应[29]。(a)、(c)、(e)入射光偏振方向平行于纳米颗粒对轴向时,纳米颗粒对中的等离子激元效应;(b)、(d)、(f)入射光偏振方向垂直于纳米颗粒对轴向时,纳米颗粒对中的等离子激元效应
Fig. 1. Plasmonic effect in metal nanoparticles pair[29]. (a), (c), (e) Plasmonic effect in nanoparticles pair when incident laser polarization direction is parallel to the nanoparticles pair axis;(b), (d), (f) plasmonic effect in nanoparticles pair when incident laser polarization direction is perpendicular to the nanoparticles pair axis
已知激光辐照下瞬态热传输方程为
式中,Cs为热容,ρ为材料密度,k为热传导率,T为温度,单位体积热源Qs可表示为
式中,ε0为真空中介电常数,ε(ω)为与频率ω相关的材料介电常数,E为电场。
由上述表达式可知,当入射偏振激光辐照纳米材料时,结构周围的温度分布将由光激励产生的电磁场决定,即表面等离子激元效应将影响结构中的热输入特性。因此,当调整入射光源参数,如激光功率、激光波长、脉冲宽度、脉冲频率和偏振方向等,可对等离子激元的分布进行定点调控,从而可获得纳米尺度下可控的空间能量输入。特别是,当纳米材料之间距离足够接近时,各自结构上收集电子共振产生的电磁场将产生相干作用,从而进一步影响复合结构中的能量分布。由表面等离子激元产生的原理及相干特性可知,通过构造金属-介电环境且同时调控输入光参数,便可实现局域定点的能量聚集输入,从而无需对输入光源的大小及空间定位进行精确操控。
图 2. 偏振光激励下Au纳米线结构中能量分布特征[30-31]。上:实验结果;下:模拟结果;波长为532 nm
Fig. 2. Charateristics of energy distribution in Au nanowires structure under polarized laser excitation[30-31]. Up: experimental results;down: simulation results;Wavelength is 532 nm
结合以上对纳米结构中的能量调控的策略,当光辐照金属纳米颗粒结构时,可选择性地将“热点”即输入能量效率最高的点约束在颗粒对的间隙处,局域熔融的金属材料将获得较高的流动性进而可以填充间隙,以起到连接纳米颗粒的“桥梁”作用。研究显示,当飞秒激光辐照在Pt与Ag的纳米颗粒对上,Pt颗粒之间的Ag会被聚集输入的光能熔化,而两侧的Pt纳米颗粒由于输入能量较低,从而可以保持结构的相对完整[32]。同样地,当金属纳米线处于强激光场的辐照下,由于表面等离子激元效应在一维结构中的分布,纳米线结构出现损伤的区域将最先发生在纳米线的末端,如
图 3. 飞秒激光辐照后Ag纳米线的形貌[33]。(a)低激光能量输入;(b)持续激光能量输入
Fig. 3. Morphology of Ag nanowire after femtosecond laser irradiation[33]. (a)Low laser energy input; (b) continuous laser energy input
根据上述对纳米结构中的能量调控,可实现空间能量的精确输入,同时简化对光源的操控。材料之间的高效低损伤互连不仅可在同/异种纳米材料中实现,在多维度的结构中也能快速获得,从而推动了纳米材料在复合微纳结构的构建以及微纳光电子器件中的广泛应用。超快激光辐照下,不同形式材料的纳米连接结构如
图 4. 飞秒激光纳米连接结构[34-37]。(a)Ag纳米颗粒对;(b)Ag纳米线分支
Fig. 4. Femtosecond laser nanojoined structures[34-37]. (a) Ag nanoparticles pair; (b) Ag nanowires branch
3 超快激光异质纳米连接界面冶金与界面能带调控
金属纳米材料由于表面原子极高的活性,通常在低于材料块体熔点的温度下也能发生熔化。超快激光脉冲宽度极短,在相对较低的激光重复频率下,金属纳米材料可出现快速熔化-冷凝过程。由第2节可知,材料在激光作用下最易发生变化的部位会出现在纳米结构间隙/接头处,从而形成的接头连接界面会与传统熔焊接头界面出现较大的差异。此外,研究表明超快激光的引入增强了材料对光子的吸收,使得材料修饰/改性能够在多种材料体系上发生,进而使得纳米尺度下异质金属-氧化物/半导体的连接成为可能。另外,由于超快激光与材料作用过程中的“非热效应”,得到的异质接头界面可在较大范围内进行操控,进而可实现对界面电学能带结构的有效调控。
在可互相固溶的金属(如Ag与Pt)纳米颗粒之间的超快激光纳米连接中,间隙处的金属材料由于聚集的能量输入而出现熔化,在保持固相的金属颗粒表面进行铺展,急冷后可形成分离的富Pt与富Ag区域,且界面处可获得较好的 (111)Ag和 (111)Pt晶格匹配,如
图 5. 飞秒激光纳米连接接头透射电镜照片[38]。(a)、(b)固溶金属纳米颗粒对;(c)、(d)非固溶金属纳米颗粒对
Fig. 5. Transmission electron microscope images of femtosecond laser nanojoined joint[38]. (a), (b) Miscible metal nanoparticles pair; (c), (d) immiscible metal nanoparticles pair
金属与氧化物/半导体之间超快激光异质互连时,在金属材料熔融与凝固的同时,氧化物/半导体材料会由于对光子的非线性吸收而发生变化(包括相变、相分离及烧蚀等),异质材料之间也会因为氧化物/半导体的变化而获得冶金结合。
图 6. 飞秒激光纳米连接的金属-氧化物纳米线异质接头[40]。(a)、(b)纳米连接异质接头的强度测试前后的扫描电镜(SEM)图;(c)、(d)纳米连接异质接头的透射电镜图
Fig. 6. Femtosecond laser nanojoined metal-oxide nanowire heterogeneous joint[40]. (a), (b) Scanning electron microscope (SEM) images of nanojoined heterogeneous joint before and after strength test; (c), (d) transmission electron microscope images of nanojoined heterogeneous joint
值得一提的是,异质接头的形成极大地改善了金属与氧化物/半导体之间的接触状态,进而影响了一定偏压下电子在材料之间的传递。超快激光纳米连接得到的金属-氧化物异质接头处将出现极窄的过渡区,该区域中出现的晶体缺陷以及氧空位结构等都能增强电子的传输,从而相比于金属与氧化物的直接接触,可极大地降低过渡区的肖特基势垒[44-46]。同样地,在多介电层的金属-氧化物-半导体结构中,超快激光纳米连接过程中将会增强金属与氧化物之间的结合,同时可精细地减薄氧化物插入层,进而可实现对纳米连接接头处能带结构的调控。而在氧化物进一步削减直至完全移除后,接头将呈现出金属-半导体的接触特性,这使得基于纳米连接的异质接头能带调控与电学性能优化成为可能,为纳米器件的开发提供了新的思路[8,10]。
4 基于超快激光纳米颗粒薄膜沉积的低温连接新技术
随着SiC、GaN等第三代宽禁带半导体材料发展,电子器件的功率密度和工作温度显著提高[47]。然而,受限于目前的互连封装技术,器件在高温条件下长期服役仍然面临挑战,尤其是散热和可靠性方面的问题亟待解决[48]。典型功率器件互连封装结构示意图如
图 7. 典型互连封装结构示意图。TIM为热界面材料
Fig. 7. Schematic diagram of typical interconnection packaging structure. TIM represents thermal interface materials
纳米Ag颗粒烧结技术的发展为功率器件的高温应用提供了一种理想的封装替代方案。由于纳米尺寸效应,纳米级的Ag颗粒因其极高的表面能而拥有远低于块体Ag的熔点和很强的互扩散能力,纳米Ag颗粒膏/纳米Ag颗粒薄层在低温固态烧结连接后会形成具有亚微米尺度孔洞结构的连接层,其熔点接近块体Ag的熔点,进而可实现“低温连接、高温服役”[47, 49-50]。另外,与传统的钎料相比,烧结Ag层拥有更优异的导电导热能力以及更高的连接强度和可靠性[51]。
目前最常见的纳米Ag连接材料是基于Ag纳米颗粒为主而配制的焊膏,为防止Ag纳米颗粒团聚,焊膏中往往需要添加多种有机物组分。该类焊膏烧结连接存在的问题是:一方面有机物在大面积烧结连接的应用场合中会导致有机物残留以及空洞、裂纹、分层等缺陷,大大降低器件的可靠性[52];另一方面,有机物的存在也使得高质量的超低温连接(低于180 ℃)难以实现,限制了纳米Ag在诸如柔性电子以及系统多级封装工艺等超低温烧结连接需求场合中的应用。另外,有机物的蒸发和分解需要多梯度烧结工艺,这就增加了烧结工艺的复杂性。
针对常规纳米Ag焊膏烧结技术存在的瓶颈问题,清华大学邹贵生、刘磊团队提出了基于超快激光纳米颗粒薄膜沉积的低温连接新原理、新技术,并将其用于功率芯片的耐高温封装连接。其特点为,利用脉冲激光沉积(PLD)技术,研究中使用的是皮秒和飞秒激光,在被连接材料(芯片或DBC或者两者)表面绿色高效沉积制备出不含有机物的包含大量纳米颗粒的薄膜。用该薄膜作为中间层连接材料可避免纳米金属颗粒焊膏中有机组分所带来的诸多问题,进而可实现超低温、大面积的高可靠互连封装。超快激光沉积制备互连纳米连接材料的另一个显著优点是,其能够高效制备常规方法难以获得的颗粒成分均匀的纳米合金颗粒薄膜,例如,超快激光沉积能够制备难溶体系合金(如Ag-Cu)成分均匀的超饱和固溶体颗粒。
王文淦等[25]用皮秒激光在氩气中沉积不包含有机物的Ag纳米颗粒薄膜,并将其应用于SiC芯片与DBC基板的封装连接,如
图 8. 皮秒激光沉积Ag纳米薄膜及其在低温封装连接中的应用[25]。(a)PLD系统及沉积过程;(b)薄膜中纳米颗粒结构的SEM图;(c)Ag薄膜中纳米薄膜颗粒结构示意图;(d)Ag纳米颗粒薄膜封装应用示意图;(e)低温烧结接头横截面SEM图
Fig. 8. Nano-Ag film deposited via picosecond laser deposition and its application in low-temperature packaging and connection[25]. (a) PLD system and deposition process; (b) SEM image of nanoparticle structure in film; (c) schematic diagram of nanoparticle structure in Ag films; (d) schematic diagram of Ag nanoparticle film packaging application; (e) SEM image of low-temperature sintered joint cross section
控制PLD的环境气压可调控薄膜的结构,进而可控制薄膜的性能并获得更为优异的低温烧结连接特性。冯斌等[24]利用超快脉冲激光沉积制备了一种新型晶格无序的纳米颗粒双层协同结构(CBLDN),这种双层结构由低气压下沉积的致密结构Ag颗粒薄膜和高气压下沉积的疏松结构Ag颗粒薄膜复合而成,如
图 9. 超快PLD制备的新型CBLDN[24]。(a)CBLDN的沉积原理示意图及SEM图;(b)疏松结构SEM图;(c)致密结构SEM图;(d)低温烧结过程中疏松结构和致密结构的协同效应;(e)低温烧结接头剪切强度;(f)低温烧结薄膜电阻率;(g)CBLDN储存4个月前后的互连性能对比
Fig. 9. New CBLDN fabricated by ultrafast PLD[24]. (a) Schematic diagram of deposition mechanism and SEM image of CBLDN; (b) SEM image of loose structure; (c) SEM image of compact structure; (d) cooperative effect of the loose and the compact layers in low-temperature sintered process; (e) shear strength of low-temperature sintered joint; (f) resistivity of low-temperature sintered film; (g) comparison of the interconnecting performances of CBLDN before and after 4-month storage
除了低温连接应用,超快PLD制备的无有机物纳米结构颗粒薄膜在大面积芯片封装方面也具有独特优势。Zubir等[53]利用皮秒激光沉积的无有机物Ag纳米颗粒薄膜成功实现了面积大于100 mm2的大芯片连接。由于连接层中没有有机组分的存在,从而有效避免了大面积烧结过程中由于有机物无法充分蒸发逸出所带来的残留问题,整个连接层组织中未观察到空洞、裂纹和分层等缺陷的出现,展现了无有机组分薄膜在大面积连接中的优势。
除Ag纳米颗粒薄膜外,超快PLD制备的纳米合金同样在功率模块的封装中展现出了优势。贾强等[26]利用超快PLD制备了一种成分均匀的超饱和Ag-Cu纳米合金颗粒薄膜,如
图 10. 超快PLD制备超饱和Ag-Cu纳米合金及其烧结原理[26]
Fig. 10. Fabrication of supersaturated Ag-Cu nanoalloy by ultrafast PLD and its sintered mechanism[26]
另外,贾强等[54]还利用超快PLD制备了互溶体系Ag-Pd纳米合金颗粒薄膜,并成功应用于SiC芯片低温封装连接,如
图 11. Ag-Pd纳米合金连接层的表征及其抗电化学迁移性能[54]。(a)~(c)250 ℃下的烧结接头;(d)~(f)400 ℃下的烧结接头;(g)烧结接头的剪切强度和孔隙率;(h)水滴实验中的电流-时间关系;(i)电化学迁移实验
Fig. 11. Characterization of Ag-Pd nanoalloy sintered layer and its resistance to electrochemical migration[54]. (a)--(c) Sintered joint at 250 ℃; (d)--(f) sintered joint at 400 ℃; (g) shear strength and porosity of sintered joint; (h) current-time relationship in water drop experiment; (i) electrochemical migration experiment
综上,相比于纳米颗粒焊膏,基于超快PLD制备的纳米颗粒薄膜对超低温、大面积芯片互连封装具有独特的优势,并在纳米合金的制备方面具有很大的应用潜力。这种基于超快PLD制造的纳米连接新技术有望在需要高温、高可靠性的第三代半导体器件封装中发挥不可替代的作用。
5 基于超快激光纳米连接新型高性能微纳器件的制造
如上所述,基于超快激光的显著特性可有效实现特定材料之间的纳米连接,进而可快速研发新型高性能微纳器件。根据使用材料的类型可主要分为基于金属-金属、金属-半导体、半导体-半导体纳米连接的新型微纳器件,其主要应用于电学、光学、传感等领域。
5.1 基于金属-金属纳米材料互连的新型高性能纳米器件
合理调控激光能量场能够实现纳米材料之间的有效连接,进而制备基于纳米材料互连的纳米器件。林路禅等[36]通过控制飞秒激光脉冲辐照参数,在无钎料情况下实现了原本空间分离的Ag纳米线的原位连接,如
图 12. 飞秒激光纳米连接Ag纳米线用于光学波导[36]。(a)在激光功率密度为90 mJ/cm2条件下辐照10 s;(b)纳米连接前器件的归一化电场分布;(c)纳米连接后器件的归一化电场分布;(d)纳米连接前后传输比对比
Fig. 12. Nanojoined Ag nanowires for optical waveguides by femtosecond laser[36]. (a) Irradiation for 10 s at the laser power density of 90 mJ/cm2; (b) normalized electric field distribution of the device before nanojoining; (c) normalized electric field distribution of the device after nanojoining; (d) comparison of the transmission ratio before and after nanojoining
5.2 基于金属-半导体纳米材料互连的新型高性能纳米器件
由于等离子激元效应,超快激光对金属-半导体纳米连接具有独特的优势。林路禅等[44]发现Au与TiO2纳米线接触时局部区域会在强光场作用下基于等离子激元效应形成晶格缺陷相TiO2-x,该晶格缺陷相有利于界面处形成有效的纳米连接,如
图 13. 飞秒激光纳米连接金属与TiO2纳米线及其在纳米器件制造中的应用[40, 44-45]。(a)Au电极与TiO2纳米线互连示意图;(b)基于Au -TiO2互连的多级可控存储器件;(c)基于Ag-TiO2互连的整流单元;(d) Pt电极与TiO2纳米线互连; (e)纳米连接后Pt-TiO2单元的RS行为
Fig. 13. Nanojoining of metal and TiO2 nanowires by femtosecond laser and its application in manufacture of nano-devices[40, 44-45]. (a) Schematic diagram of interconnection between Au electrodes and TiO2 nanowire; (b) controllable multi-level memory device based on Au-TiO2 interconnection; (c) rectification units based on Ag-TiO2 interconnection; (d) interconnection of Pt electrode and TiO2 nanowire; (e) RS behavior of Pt-TiO2 cell after nanojoining
此外,邢松龄等[45]验证了Pt电极与TiO2纳米线之间可用能量密度为5.02 mJ/cm2的飞秒激光辐照10 s以实现互连,如
SiC芯片作为功率电子器件中至关重要的半导体材料,其性能一直受到广泛的关注,而SiC纳米线的互连工艺为开发基于SiC纳米线的小型器件提供了基础。邢松龄等[41]利用飞秒激光的局域能量场特性,在能量密度为72.9 mJ/cm2的激光作用下,通过不同的辐照时间对不同厚度的SiC/SiO2核壳纳米线外层SiO2进行减薄,并实现了其与Au电极之间的纳米连接,如
图 14. 飞秒激光纳米连接金属电极与SiC/SiO2纳米线[41, 43]。(a)激光对SiC/SiO2纳米线壳SiO2部分减薄后实现纳米连接;(b)基于Au-SiC/SiO2的场效应管;(c)纳米连接的场效应管电学特性;(d)基于Au-SiC/SiO2的记忆单元;(e)纳米连接前后记忆单元的性能
Fig. 14. Nanojoining of metal electrode and SiC/SiO2 nanowire by femtosecond laser[41, 43]. (a) Partial thinning of SiO2 shell of SiC/SiO2 nanowire by laser to achieve nanojoining; (b) field effect transistor based on Au-SiC/SiO2; (c) electrical properties of nanojoined transistor; (d) memory unit based on Au-SiC/SiO2; (e) performance of memory unit before and after nanojoining
冯斌等[55]通过皮秒激光沉积制备了Cu纳米颗粒层与FeOx纳米颗粒层组合的复合结构,且逐层沉积时就实现了Cu纳米颗粒层与FeOx纳米颗粒层之间的良好连接,据此成功制备了可设计的成分梯度柔性压力传感器,如
图 15. 皮秒激光沉积异质纳米颗粒层的纳米连接复合结构及其应用[55]。(a)Cu-FeOx纳米连接复合层结构;(b)纳米连接结构用于压力传感时的性能;(c)纳米连接结构用于检测气流;(d)纳米连接结构用于检测脉搏;(e)纳米连接结构用于检测手部动作
Fig. 15. Nanojoined composite structure of heterogeneous nanoparticle layer by picosecond laser deposition and its applications[55]. (a) Cu-FeOx nanojoined composite structure; (b) performance of nanojoined structures used for pressure sensing; (c) nanojoined structure used for airflow detection; (d) nanojoined structure used for pulse detection; (e) nanojoined structure used for detecting hand movements
5.3 基于半导体-半导体纳米材料互连的新型高性能纳米器件
半导体纳米材料通常具有较高的熔点和烧蚀阈值,飞秒激光由于具有高峰值功率密度的特性从而在对该类材料进行纳米连接时具有独特的优势。邢松龄等[42]基于ZnO纳米线对飞秒激光的多光子吸收效应,采用脉冲能量密度为77.6 mJ/cm2的飞秒激光脉冲辐照30 s,实现了ZnO纳米线的同质纳米连接,如
图 16. 飞秒激光纳米连接ZnO纳米线[42]。(a)同质ZnO纳米连接结构的制备;(b)纳米连接前后器件的电学响应;(c)纳米连接器件在594 nm光激励下的响应;(d)纳米连接器件在543 nm光激励下的响应
Fig. 16. Nanojoined ZnO nanowires by femtosecond laser[42]. (a) Fabrication of homogeneous ZnO nanojoined structure; (b) electrical response of devices before and after nanojoining; (c) response of nanojoined device under photoexcitation at 594 nm; (d) response of nanojoined device under photoexcitation at 543 nm
除激光直接纳米连接两种材料的方式外,另一种互连形式是直接对材料本身进行局域改性继而实现良好的界面接触,进而达到良好的纳米连接效果。霍金鹏等[37]采用能量密度为1 J/cm2的飞秒激光辐照5 s,实现了对SiC纳米线的掺杂和纳米连接,如
图 17. 基于飞秒激光的SiC纳米线选区掺杂及界面修饰[37]。(a)选区掺杂结构的制备;(b)激光辐照前器件的电学响应;(c)激光辐照后器件的电学响应;(d)激光局部辐照后制备的p-n结
Fig. 17. Selective doping and interface modification of SiC nanowire based on femtosecond laser[37]. (a) Fabrication of selective doping structure; (b) electrical response of device before laser irradiation; (c) electrical response of device after laser irradiation; (d) p-n junction fabricated after laser local irradiation
5.4 基于其他纳米材料互连的新型高性能纳米器件
除了上述方法外,基于激光诱导转移(LIFT)技术的纳米连接工艺扩展了纳米连接器件的材料体系。沈道智等[56]利用飞秒激光LIFT技术成功实现了还原氧化石墨烯(rGO)和聚乙烯醇-硫酸(PVA-H2SO4)的纳米连接,如
图 18. 基于飞秒激光LIFT技术的超级电容器[56]。(a)使用LIFT制备超级电容器;(b)器件的能量存储能力及其与同类产品的对比;(c)基于该结构的不同微型滤波器设计;(d)滤波器的输入输出信号
Fig. 18. Supercapacitor based on femtosecond laser LIFT technology[56]. (a) Fabrication of supercapacitor using LIFT; (b) energy storage capacity of device and its comparison with similar products; (c) designs of different micro-filters based on this structure; (d) input and output signals of filter
5.5 基于超快激光纳米颗粒薄膜沉积与低温连接的第三代半导体耐高温功率器件
如第4节所述,超快激光可在被连接材料如芯片和基板表面分别沉积不同组织结构特征的纳米颗粒薄膜或复合薄膜,将该薄膜作为互连材料,可在低温下实现芯片与基板的烧结连接,进而达到第三代半导体功率器件“低温连接,高温服役”的封装目的。任辉等[57]设计了一种新型无引线封装结构,并利用皮秒激光沉积纳米颗粒薄膜技术实现了该结构的封装连接,据此制备的SiC二极管器件展现出了低电感、高导热性能和优异的高温可靠性。如
图 19. SiC二极管连接的新型无引线封装设计及其高温可靠性[57]。(a)无引线封装结构示意图;(b)无引线封装结构制造工艺示意图;(c)高温条件下无引线封装结构功率循环结果;(d)无引线封装结构功率循环结果与参考文献的数据对比;(e)红外热像仪的测温结果
Fig. 19. New wireless packaging design of SiC diode connection and its high temperature reliability[57]. (a) Schematic diagram of wireless packaging structure; (b) schematic diagram of fabricated process of wireless packaging structure; (c) power cycling result of wireless packaging structure under high temperature; (d) comparison of power cycling results of wireless packaging structure with that of references; (e) temperature measurement results by thermal infrared imager
6 总结与展望
基于纳米尺度下材料的互连特征,并且其结合本团队在超快激光纳米连接原理与技术的研究基础,对已研发纳米连接主要工艺方法及其存在的局限性、面临的主要挑战进行了阐述,并指明了超快激光纳米连接在能量操控、材料高性能互连上的巨大优势与潜力,并且其将成为该领域的重要发展方向。同时,对超快激光纳米材料连接的局域能量及其连接界面调控、基于超快激光沉积纳米颗粒薄膜的低温连接新原理和新技术及其在微纳器件制造中的应用进行了重点阐述。
超快激光具有峰值功率密度极高、多材料加工适用、热影响区极小等特点,其用于纳米尺度的同质和异质材料甚至多材料连接时,将在材料表面产生等离子激元效应。通过构造纳米结构如纳米线与纳米线、纳米线与薄膜电极等并同时调控光参数,可实现局域定点的能量聚集输入,结构中仅间隙处或接触部位的材料会发生变化如熔化、凝固,进而可实现低损伤互连。同时,超快激光辐照会增强纳米结构中材料对光子的吸收,从而修饰/改性多种材料体系,可实现金属-氧化物/半导体的异质连接以及连接界面的有效调控。而基于纳米材料的互连,可实现基于金属-金属、金属与半导体、半导体-半导体纳米结构构建的新型高性能器件的快速研制,如光学波导器件、多级忆阻器、整流单元、场效应管、p-n结电学基本单元、微型超级电容和柔性压力传感器等。超快激光用于纳米连接材料沉积制备时,除适合纯金属外,还适合难溶体系合金如Ag-Cu和互溶体系合金如Ag-Pd;通过控制激光参数和环境气压等,可获得成分均匀、致密与疏松形貌可调及厚度可控的纳米颗粒薄膜;用该类薄膜可实现材料之间低温甚至室温互连,用于以SiC为代表的第三代半导体功率器件封装中可实现大面积(100 mm2)、无有机物残留的低温连接(≤250 ℃)和高温服役(≥300 ℃);基于超快激光沉积Ag纳米颗粒薄膜实现了SiC二极管耐高温、高可靠性的封装,基于超快激光沉积Cu-FeOx复合层并结合烧蚀雕刻以形成垄式复合结构,实现了高性能柔性压力传感器的制备。
超快激光纳米连接技术在微纳电子器件的封装制造和研发中将发挥越来越重要的作用。目前,相关技术及原理处于研究的起步阶段,从而其在三维结构精确定位操作、微纳尺度图形化均匀制备、异质界面形成的共性机制与质量调控、高效批量连接、性能可靠评价、纳米连接精密专用装备和工程化应用等方面还有待深入的研究,这些方面也是其未来的发展方向。
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Article Outline
邹贵生, 林路禅, 肖宇, 邓钟炀, 贾强, 冯斌, 王文淦, 邢松龄, 任辉, 沈道智, 刘磊. 超快激光纳米连接及其在微纳器件制造中的应用[J]. 中国激光, 2021, 48(15): 1502001. Guisheng Zou, Luchan Lin, Yu Xiao, Zhongyang Deng, Qiang Jia, Bin Feng, Wengan Wang, Songling Xing, Hui Ren, Daozhi Shen, Lei Liu. Ultrafast Laser Nanojoining and Its Applications in the Manufacturing of Micro-Nano Devices[J]. Chinese Journal of Lasers, 2021, 48(15): 1502001.
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