Ag-Pd纳米合金低温连接及其抗电化学迁移性能 
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Objective There is an increasing demand for die attach materials with the rapid development of SiC devices, which can be bonded at low-temperature and function at high temperature. Nano-Ag sintering has been extensively investigated for application in high-temperature power electronics. However, the electrochemical-migration of Ag ions is the main drawback. Pd is famous for its chemical stability, and various studies have focused on the influence of Pd content on the effectiveness and its mechanism. Recently, researchers have been trying to mix Pd and Ag nanoparticles (NPs) to improve the resistance to electrochemical-migration of the sintered layer. However, Pd has a melting point higher than that of Ag, whereas the alloying process needs high temperature (~850 ℃) to form Ag-Pd alloy. Pulsed laser deposition (PLD) is a physical method feasible for fabricationg Ag-Pd nanoalloy without using organic additives such as polyvinylpyrrolidone, which is required in the chemical method. In this work, Ag-10%Pd nanoalloy was fabricated by the PLD method, which can be used to connect SiC and Ag-coated direct bonding copper (DBC) substrates. The sintered layer enhances resistance to electrochemical-migration with low-temperature bonding characteristics. The microstructure of the bonding, shear properties, and its electrochemical-migration resistance are studied.
Methods Ag-10%Pd NPs were fabricated using PLD with a pressure of 750 Pa of Ar atmosphere. The Ag-Pd target was fabricated by powder sintering with weight ratio of 90∶10. A picosecond laser with a pulse width of 10 ps was employed to ablate the target. Ag-Pd NPs were deposited on the back side of SiC chip (G.P.Tech, Ti/Ni/Ag metallization), then the SiC chip was removed from the substrate and placed on the Ag-coated DBC (HuaSemi Electronics, Ni/Au metallization). The interconnecting process is performed at a temperature range of 200 ℃-350 ℃ assisted with a pressure of 5 MPa for 30 min in air. The shear test is conducted using Dage 4000. The electrochemical-migration test is conducted using a water drop test.
Results and Discussions The microstructure of as-deposited Ag-Pd film comprises various NPs with diameters less than 1 μm (Fig. 3). Element results indicate that these deposited NPs are in alloy state with a uniform composition distribution. The sintered joint comprises SiC chip, bondline and Ag-coated substrates (Fig. 4). The bondline thickness is about 27 μm, which is only 31.6% of the as-deposited state. Thus, the Ag-Pd film had excellent deformability. The bondline exhibited Ag-9.57%Pd alloy microstructure without obvious element segregation. The sintered joint achieved a shear strength of 21.89 MPa at the sintering temperature of 250 ℃, which is higher than the US military standard MIL-STD-883K(7.8 MPa). Therefore, Ag-Pd nanoalloy film can be used as die attach material for low-temperature bonding. The sintering temperature provides the driving force for sintering process, as a denser bondline is achieved when the temperature is increased to 300 ℃ (Fig. 6). Fracture surface reveals that the failure mainly occurred at the bondline, indicating that high bonding quality interface is realized (Fig. 7). Compared with pure Ag, Ag-Pd nanoalloy exhibited a more than quadruple resistance to electrochemical-migration during the water drop test (Fig. 8). For pure Ag electrode, the current reached 1 mA with only 81.4 s, while the Ag-Pd electrode required 349.7 s for the short-circuit process. The dissolution of Ag ion was blocked by PdO formation on the anode, which played a paramount role in extending the short-circuit time, whereas the migration product was cloud-like instead of dendritic growth. This work proposed a method for fabricating Ag-Pd nanoalloy films as die attach material without the high alloying temperature. It should be noted that, Pd has a higher melting point (1554 ℃) than Ag (961.7 ℃), and Ag-Pd nanoalloy sintering requires higher sintering temperature than pure Ag NPs. Moreover, adding Pd is costly. Consequently, the sintering temperature, demand of electrochemical-migration resistance and its cost should be balanced when applying Ag-Pd nanoalloy in electronic packaging.
Conclusions Ag-10%Pd nanoalloy was successfully fabricated as die attach material using PLD. The sintered joint achieved a shear strength of 21.89 MPa at the sintering temperature of 250 ℃, which was higher than the US military standard MIL-STD-883K (7.8 MPa). Compared with pure Ag, Ag-Pd nanoalloy exhibited a more than quadruple electrochemical-migration resistance. The dissolution of Ag ion was blocked by PdO formation on the anode with obviously extended short-circuit time, whereas the migration product was cloud-like. Compared with conventional direct sintering of Ag and Pb nanoparticles, pulsed laser deposited Ag-Pd nanoalloy sintering avoids high-temperature alloying process (850 ℃), which is promising for Ag-Pd low-temperature bonding and is expected to provide a solution for the high-reliability power electronic packaging.
1 引言
随着第三代功率半导体器件的发展,以SiC为代表的宽禁带半导体芯片在大功率电力电子器件中扮演着越来越重要的角色,并逐渐替代了传统的硅芯片。然而,与传统硅芯片配套的芯片封装材料却难以在高温下持久服役,成为SiC功率电子器件应用的短板[1-3]。人们迫切需要一种能够低温连接、高温服役的封装互连材料,因而纳米颗粒焊膏烧结技术应运而生。纳米银焊膏因连接温度低和具有优异的导电、导热性能,已成为宽禁带半导体芯片封装领域的研究热点[4-5]。然而,银是最容易发生电化学迁移的金属之一,尤其是在高温、高湿环境下,阳极失去电子后,银离子迁移至阴极并不断沉积,最终导致器件短路失效[6-7]。功率电子器件的功率密度越来越高,工作温度也越来越高,尤其是一些高功率电子器件,其工作环境更为恶劣,极大地限制了烧结银在功率电子器件中的可靠应用。
钯被认为是改善银抗电化学迁移能力最有效的金属之一[8]。目前,一些国外公司(例如Panasonic、ADATA)已经采用钯来提高含银电阻的抗电化学迁移能力。早在1979年,Naguib等[9]就采用水滴试验研究了添加不同含量的钯对银抗电化学迁移能力的影响,结果表明,当钯的质量分数达到19%时,银的迁移速率降低为原来的1/100。Lin等[10]研究了钯抑制银迁移的机理,并指出PdO的生成抑制了银在阳极的溶解。天津大学的王迪[11]向纳米银焊膏中添加了质量分数为15%的纳米钯颗粒(粒径为50~100 nm),结果发现,烧结层抗电化学迁移的能力由于钯的氧化而得到了明显改善。目前,添加钯的形式大多为银、钯颗粒直接混合,这种方法后续需要高达850 ℃的合金化温度。如果二者的合金化不够充分,就会影响烧结层的致密化,同时也会带来较高的电阻等一系列问题[12]。然而,如此高的合金化温度用于功率电子器件的封装显然是不能被接受的[13-14]。如果能直接制备Ag-Pd纳米合金颗粒,就可以避免烧结过程所需要的高温合金化过程,从而就可以利用纳米颗粒的尺寸效应实现低温连接。
本文采用脉冲激光沉积技术,避开了传统化学法所需的有机物,成功制备了Ag-10%Pd纳米合金(合金中钯的质量分数为10%),并将其用于SiC芯片的封装互连。本文研究了沉积态和烧结态Ag-10%Pd纳米合金的微观组织,分析了烧结温度对接头剪切性能的影响。此外,还讨论了Ag-10%Pd纳米合金的抗电化学迁移过程,并将其与纯银进行了对比分析。
2 试验方法
采用脉冲激光沉积技术制备Ag-10%Pd纳米合金,如
图 1. 纳米合金的制备及接头截面示意图。(a)脉冲激光沉积制备纳米合金;(b)SiC与DBC基板封装接头截面
Fig. 1. Schematic illustration of nanoalloy preparation and joint cross-section. (a) Pulsed laser deposition of nanoalloy; (b) cross-section of SiC and DBC substrate joint
电化学迁移测试方法采用经典的水滴试验。电极所在基板为Al2O3,在距离1 mm的两电极之间滴加去离子水,并加载5 V电压,以短路电流达到1 mA所用时间作为两片电极被迁移产物短路的时间。
3 试验结果与讨论
3.1 沉积态Ag-10%Pd纳米合金的表征
沉积态Ag-10%Pd纳米合金薄膜如
图 2. 沉积态Ag-10%Pd纳米合金薄膜。(a)Ag-10%Pd纳米合金薄膜;(b)Ag-10%Pd纳米合金的高倍形貌;(c)合金颗粒中银元素的分布;(d)合金颗粒中钯元素的分布
Fig. 2. As-deposited Ag-10%Pd nanoalloy film. (a) Ag-10%Pd nanoalloy film; (b) high magnification of Ag-10%Pd nanoalloy; (c) element distribution of Ag in nanoalloy; (d) element distribution of Pd in nanoalloy
沉积态Ag-10%Pd纳米合金薄膜由无数纳米颗粒组成。不同粒径颗粒的数量分布及体积比如
图 3. Ag-10%Pd纳米合金颗粒的尺寸分布。(a)数量分布;(b)体积比
Fig. 3. Size distribution of Ag-10%Pd nanoalloy particles. (a) Number distribution; (b) volume ratio
3.2 Ag-10%Pd纳米合金烧结接头的微观组织
将沉积态Ag-10%Pd纳米合金薄膜用于SiC芯片与镀银DBC的互连,典型的烧结接头(烧结温度为300 ℃)截面及元素分布如
图 4. Ag-10%Pd纳米合金烧结接头及接头中的元素分布(烧结温度为300 ℃)。(a)接头的宏观形貌;(b)元素线扫描
Fig. 4. Sintered joint using Ag-10%Pd nanoalloy and elements distribution in the joint (sintering temperature of 300 ℃). (a) Macro morphology of joint; (b) elements distribution by line scanning
典型Ag-10%Pd纳米合金烧结层(烧结温度为300 ℃)的微观组织及元素分布如
图 5. 典型Ag-10%Pd纳米合金烧结层的微观组织及元素分布(烧结温度为300 ℃)。(a)烧结层的微观组织;(b)银元素分布;(c)钯元素分布
Fig. 5. Typical microstructures and element distribution of Ag-10%Pd sintered layer (sintering temperature of 300 ℃). (a) Microstructure of Ag-Pd sintered layer; (b) Ag distribution; (c) Pd distribution
3.3 Ag-10%Pd纳米合金烧结接头的剪切强度
烧结温度是影响烧结程度以及为烧结提供驱动力的最重要参数之一。本文研究了烧结温度对烧结接头剪切强度的影响。如
图 7. 典型纳米合金烧结接头的断口形貌(烧结温度为300 ℃)。(a)断口的宏观形貌;(b)断口局部放大
Fig. 7. Typical fracture surface of nanoalloy sintered joint (sintering temperature of 300 ℃). (a) Macro morphology of fracture surface; (b) high magnification of fracture surface
3.4 Ag-10%Pd纳米合金的抗电化学迁移能力
在5 V/mm的电场作用下,对Ag-10%Pd纳米合金及纯银的抗电化学迁移能力进行测试,并实时记录电流以及电极间的形貌随通电时间的变化。如
图 8. Ag-10%Pd纳米合金与纯银的短路电流随时间的变化
Fig. 8. Short-circuit current variation with time for Ag-10%Pd nanoalloy and pure Ag
纯银电极间的电流达到1 mA时(81.4 s)的电化学迁移产物如
图 9. 电化学迁移产物。(a)纯银;(b)Ag-10%Pd
Fig. 9. Electrochemical-migration product. (a) Pure Ag; (b) Ag-10%Pd
需要指出的是,块体材料钯的熔点高达1554 ℃,远高于银的熔点(961.7 ℃)。钯的添加提高了银合金的熔点,使得Ag-Pd纳米合金的烧结相比纯银需要更高的温度。而对于功率电子器件的封装来说,烧结温度不能过高。另一方面,钯的成本也非常高。本研究提出的以脉冲激光沉积为手段制备Ag-Pd纳米合金以避免高温合金化过程从而降低烧结温度的思路,在实际应用中还应根据具体的应用场景,综合考虑烧结温度、抗电化学迁移要求及成本,选择具有合适成分比例的Ag-Pd纳米合金进行芯片的封装。
4 结论
本文采用脉冲激光沉积成功制备了完全互溶的Ag-10%Pd纳米合金,其烧结接头组织为元素均匀分布的Ag-Pd固溶体。相比于银、钯纳米颗粒直接混合烧结,脉冲激光沉积Ag-Pd纳米合金的烧结避免了高温合金化过程,实现了不高于300 ℃的低温封装互连。
Ag-10%Pd纳米合金作为封装互连材料,在250 ℃的烧结连接条件下可以获得剪切强度为21.89 MPa的接头,超过了美国军标MIL-STD-883K的要求(7.8 MPa);进一步提高烧结温度到300 ℃,接头的剪切强度可达到56.85 MPa。接头的失效发生在烧结层,说明Ag-10%Pd纳米合金烧结实现了高质量的界面连接。
Ag-10%Pd纳米合金的抗电化学迁移能力是同等条件下纯银的4.3倍。Ag-10%Pd纳米合金表面的PdO阻碍了银离子的析出,迁移产物的生长受到抑制并呈云雾状分布,有效延长了电极的短路时间。
致谢 本研究得到了清华大学机械工程系邹贵生教授的大力支持和指导。
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Article Outline
贾强, 王文淦, 阿占文, 邓钟炀, 冯斌, 刘磊. Ag-Pd纳米合金低温连接及其抗电化学迁移性能[J]. 中国激光, 2021, 48(8): 0802014. Qiang Jia, Wengan Wang, Zhanwen A, Zhongyang Deng, Bin Feng, Lei Liu. Low-Temperature Bonding of Ag-Pd Nanoalloy and Its Resistance to Electrochemical-Migration[J]. Chinese Journal of Lasers, 2021, 48(8): 0802014.
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