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Zhu Yu, Yan Shihan, Zang Ziyi, Song Shengxing, Wang Jie, Ru Zhanqiang, Cui Hongliang, Song Helun. Application of terahertz mapping in high throughput measurement of the electrical conductance of Cu alloy thin films (Invited)[J]. Infrared and Laser Engineering, 2022, 51(4): 20210942. doi: 10.3788/IRLA20210942
Citation: Zhu Yu, Yan Shihan, Zang Ziyi, Song Shengxing, Wang Jie, Ru Zhanqiang, Cui Hongliang, Song Helun. Application of terahertz mapping in high throughput measurement of the electrical conductance of Cu alloy thin films (Invited)[J]. Infrared and Laser Engineering, 2022, 51(4): 20210942. doi: 10.3788/IRLA20210942

Application of terahertz mapping in high throughput measurement of the electrical conductance of Cu alloy thin films (Invited)

doi: 10.3788/IRLA20210942
Funds:  Chinese Academy of Sciences Science and Technology Service Network Program(KFJ-STS-QYZX-061);"Six Talent Peaks" High-level Talent Project of Jiangsu Province(XYDXX-211)
  • Received Date: 2021-12-09
  • Rev Recd Date: 2022-03-10
  • Accepted Date: 2022-03-10
  • Publish Date: 2022-05-06
  • High throughput materials fabrication and characterization techniques are key to the transformation from materials genome approach to advanced materials R&D and manufacturing practice. Here the authors reported the application of the terahertz (THz-TDS) mapping of the electrical conductance of Cu alloy sample matrix fabricated using high throughput solid state synthesis method. Fast detection of 144 Cu alloy thin films on a single materials library chip were achieved. The mapping technique was based on an algorithm derived from Tinkham equations and Fresnel formula. The THz conductance acquired from alloy thin films of either uniform or continuous gradient thickness agreed semi-quantitatively with those determined with four-probe method. Distinctive conductance differences among the 144 Cu alloy samples were obtained within one batch. Further analysis of the changing trends in the microstructures and semi-quantitative compositions of representative samples against their electrical conductance revealed a clear composition-structural-function relationship. This study demonstrated that THz-TDS may be a powerful tool for fast high throughput screening of Cu alloy materials library chip, and facilitate the R&D procedure substantially.
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    [10] Ferguson B, Zhang X C. Materials for terahertz science and technology [J]. Nature Materials, 2002, 1(1): 26-33. doi:  10.1038/nmat708
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    [14] Lloyd-Hughes J, Jeon Tae-In. A review of the terahertz conductivity of bulk and nano-materials [J]. J Infrared Milli Terahertz Waves, 2012, 33: 871-925. doi:  10.1007/s10762-012-9905-y
    [15] Guo Lantao, Mu Kaijun, Deng Chao, et al. Terahertz spectroscopy and imaging [J]. Infrared and Laser Engineering, 2013, 42(1): 51-56. (in Chinese)
    [16] Yan S H, Wei D S, Tang M J, et al. Determination of critical micelle concentrations of surfactants by terahertz time-domain spectroscopy [J]. IEEE Transactions on Terahertz Science and Technology, 2016, 6(4): 532-540. doi:  10.1109/TTHZ.2016.2575450
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    [18] Li Zaoxia, Yan Shihan, Zang Ziyi, et al. Single cell imaging with near-field terahertz scanning microscopy [J]. Cell Proliferation, 2020, 53(4): e12788.
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Application of terahertz mapping in high throughput measurement of the electrical conductance of Cu alloy thin films (Invited)

doi: 10.3788/IRLA20210942
  • 1. Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences, Suzhou 215123, China
  • 2. Chongqing Institute of Green and Intelligent Technology, Chinese Academy of Sciences, Chongqing 400714, China
  • 3. Chongqing School, University of Chinese Academy of Sciences, Chongqing 400714, China
  • 4. College of Instrumentation and Electrical Engineering, Jilin University, Changchun 130061, China
Fund Project:  Chinese Academy of Sciences Science and Technology Service Network Program(KFJ-STS-QYZX-061);"Six Talent Peaks" High-level Talent Project of Jiangsu Province(XYDXX-211)

Abstract: High throughput materials fabrication and characterization techniques are key to the transformation from materials genome approach to advanced materials R&D and manufacturing practice. Here the authors reported the application of the terahertz (THz-TDS) mapping of the electrical conductance of Cu alloy sample matrix fabricated using high throughput solid state synthesis method. Fast detection of 144 Cu alloy thin films on a single materials library chip were achieved. The mapping technique was based on an algorithm derived from Tinkham equations and Fresnel formula. The THz conductance acquired from alloy thin films of either uniform or continuous gradient thickness agreed semi-quantitatively with those determined with four-probe method. Distinctive conductance differences among the 144 Cu alloy samples were obtained within one batch. Further analysis of the changing trends in the microstructures and semi-quantitative compositions of representative samples against their electrical conductance revealed a clear composition-structural-function relationship. This study demonstrated that THz-TDS may be a powerful tool for fast high throughput screening of Cu alloy materials library chip, and facilitate the R&D procedure substantially.

    • 20世纪90年代中期,Xiang和Schultz等受小分子药物筛选和基因生物芯片技术的启发,开发出基于薄膜固态化学合成[1]的复杂多元氧化物超导材料的高通量并行制备组合材料芯片技术,并通过高通量表征实现了对超导材料的性能优化和快速筛选,建立了材料基因工程高通量实验技术的基本模式[2]。这种并行式高通量组合材料合成与表征技术将提高发现与合成新材料的速度提高数倍乃至数量级别,并可以同时进行新材料组分-结构-功能系统性研究和优化确定工艺线路,在储氢材料[3]、半导体栅极材料、记忆合金、磁性半导体材料[4]等先进功能材料研发领域应用日益广泛,正逐步成为实现新材料设计研发的重要手段。

      铜合金是新一代信息技术、先进轨道交通设备、航空航天和新能源等领域的关键基础材料。我国铜材产量和消费量均居世界首位,但对各种高强高导、高电磁屏蔽性能,以及薄、细异型材等高性能先进铜合金材料严重依赖进口,自主研发能力不足[5]。目前,高性能铜合金材料研发仍然以传统的单配方实验-试错方法为主,研发周期长、成本高,无法满足国家经济发展与战略需求。基于材料基因工程的高通量制备与表征实验方法有望为新型高性能铜合金的快速研发提供新的解决路径。

      导电性能是铜合金最基本和最重要的性能参数,常规电导测量方法主要用于单个体块材料样品。对于如文中所述的高通量铜合金材料芯片,由于样品密度高(>100/片),样品点尺寸小(≤2 mm),以及阵列式分布,限制了传统四探针法、范德堡技术[6]、扩展电阻探针[7]和涡流电导[8]测试的使用。尽管有报道使用微纳加工技术制备的微型四探针(M4PP)可以无损表征硅工业薄膜材料片的电导[9],其分辨率达到了微米甚至纳米级,但对于铜合金薄膜,在较高温度进行热处理后,通常产生相析出、组织结构缺陷等,具有非均匀性,使用M4PP法进行高通量、微区电导测量的准确性降低,测试时间冗长。文中介绍了利用太赫兹时域光谱扫描技术(terahertz time-domain spectroscopy,THz-TDS)的电导高通量表征方法,特别适用于样品密度大,以及微区测量的高通量铜合金材料芯片。

      太赫兹(THz,0.1~10 THz)辐射与物质的相互作用为接近平衡态的系统的电子性质提供了重要的低能探针[10]。Laman和Grischkowsky在2008年利用太赫兹技术研究了Au膜等的电导特性[11]。2012年,Buron等人利用THz-TDS研究了石墨烯薄膜电导,通过对比四探针法及拉曼光谱法结果,证明了太赫兹法具有快速测量的潜力,作为一种非接触式测量方法可用于绘制晶圆级尺寸石墨烯的电导空间分布图[12]。2015年,Bøggild研究组报道了基于THz-TDS成像方法研究标定石墨烯载流子迁移率和化学掺杂水平的方法[13]。THz-TDS还被用在皮秒到纳秒的时间尺度上跟踪包括块体和无机半导体薄膜、有机材料薄膜和金属薄膜等材料的动态光电导 [14],有望成为无损检测领域研究和表征材料导电性能的常规研究工具[15]

      文中针对使用电子束逐层沉积制备的样品密度为144个/批次的高通量铜合金材料阵列芯片,开展了THz-TDS快速扫描成像高通量表征技术验证研究。基于Tinkham薄膜透射方程及Fresnel公式,采用优化算法对太赫兹数据中电导特性的空间变异性分布进行了定性定量分析,并对照四探针法验证了其可靠性,实现了高通量太赫兹扫描成像表征。

    • 使用定制电子束蒸发镀膜设备制备铜合金薄膜高通量样品。该设备具有样品旋转定位、线性掩模移动定位,以及高温热处理等程序控制功能。使用对太赫兹波段透明的高阻硅(>10 kΩ)为基底,分别制备了原位分立和连续梯度形式的高通铜合金材料芯片。为确保太赫兹波可以透过铜合金薄膜在探测器端得到足够强度的信号,多层膜沉积总厚度不超过200 nm。在原位分立模式下(图1(a)、(b)),通过三级掩模图案线性定位移动(P1→P2→P3),配合样品台在原位依次旋转90º,在相应掩模图案位置(P1/P2/P3)分别沉积不同厚度(含量)的组分薄膜,形成12×12多组分多浓度组合的样品阵列。在连续梯度模式下(图1(d),通过控制掩模盲板线性移动速率与电子束蒸镀速率,配合样品台在原位依次旋转120º,分别形成厚度呈楔形梯度变化的组分薄膜。薄膜沉积后,在样品台原位对样品芯片进行高真空热处理,在410 ℃和680 ℃两个温度阶梯分别恒温3 h和2 h,以促进各膜层组分之间进行充分扩散并固溶形成合金(图1(e))。

      Figure 1.  Illustration of the high throughput materials chip fabrication method with electron beam deposition. (a) 12×12 mask of 3 patterns; (b) Illustration of the in site discrete mode process; (c) System setup for e-beam source, mask and sample stage; (d) Illustration of the ternary continuous gradient process; (e) Illustration of heat treatment process

    • 薄膜电导和透射率之间的关系可以用Tinkham薄膜透射方程进行定量表述[12]。该方程描述了平面波通过电导为$ {\tilde{\sigma }}_{s}\left(\omega \right) $的导电薄膜时的传播和折射情况,指出空气和大面积连续介质边界上电磁波的透射与折射情况与薄膜电导有关,可以用于对超导材料、无机半导体材料、石墨烯和金属薄膜等的太赫兹电导率的测量[14]。导电薄膜覆于折射率为$ {\tilde{n}}_{sub}\left(\omega \right) $的大块基底之上时,其透射率$ {t}_{film} $与折射率$ {n}_{film} $满足条件$ {t}_{film}\le \lambda /{n}_{film} $(或$ {n}_{film}\omega {t}_{film}/c\le 1 $)时,其中λ为入射波长,c为光速, 认为空气-薄膜-基底界面所有的内反射平均分布且可以被忽略。由Tinkham方程得到与薄膜电导相关的空气-薄膜-基底界面的透射率如下:

      式中:Z0=337 Ω,为真空阻抗。

      根据光波在介质中的传播理论,空气-薄膜-基底界面的透射率${\tilde{t}}_{air\to {film}\to sub}$可以通过Fresnel公式进行计算:

      当太赫兹波垂直入射并在光厚介质(${n}_{sub}^{\text{%}} \Delta L\geqslant $$ \lambda$)中穿过距离ΔL后,经样品内部多次内反射产生周期性时域信号,即图2(b)中的E(0)E(1)、和E(2)。样品与太赫兹波的作用越多,衰减越大。选择信噪比最佳的主透射信号E(0)进行电导分析,从而可以忽略样品内部的多次反射,则主透射信号可以表示为:

      Figure 2.  (a) Illustration of the THz mapping detection system in transmission mode; (b) THz time-domain spectra of the periodic temporal signals from multiple internal reflections; (c) Experimental setup of the THz emitter, the sample stage and the photoconductive detector

      其中,${\tilde{P} }$代表附加传输因子,${P}={\mathrm{e}}^{{i}\frac{{w}}{{c}}{{n}}_{{s}{u}{b}}^{\text{%}} \Delta {L}}$

      在太赫兹电导测量方法中,分别记录空白基底的参考波$ {\tilde{E}}_{ref} $与样品波$ {\tilde{E}}_{sam} $,得到主透射信号对应的传递函数:

      比较公式(1)和(5),将公式(3)~(5)代入公式(1)中,得到与薄膜电导相关的传递函数$ {\tilde{T}}_{film}\left(\omega \right) $在THz域的表达式:

      对公式(6)进行整理,即推得直接透射波的薄膜电导表达式:

    • 太赫兹扫描与数据采集采用太赫兹时域光谱成像检测系统(T-Ray 5000, Advanced Photonix, Inc., MI, USA),飞秒脉冲激光中心波长为1064 nm,有效频率带宽为0.1~3.0 THz,频谱分辨率为12.5 GHz,快速扫描范围为80 ps,时间分辨率为0.1 ps[16]。测试环境温度约20 ℃,湿度约50%。文中采用透射光路点扫描模式进行电导测试(图2),根据样片尺寸设置扫描范围为5 cm × 5 cm(XY方向),样品台为步进电机控制,移动步径为0.25 mm,扫描速度为75 mm/s。通过逐点扫描获取每个样品点上的太赫兹时域波形。对每一点的太赫兹时域波形进行分析,获得对应坐标的电导特征。对所有样品点的电导值根据扫描坐标进行组合,进行图像化处理后即获得对应样品的太赫兹电导二维图像。

    • 使用四探针法(4PP)在大块样品上进行了电导值对比测试(四探针测试仪SZT-2 A,同创电子,苏州)。进行方法验证时,随机选择10个位置点进行测试,测试结果取平均值得到四探针检测结果。根据样品性质选择恒流源电流输出10 mA进行测试。

    • 使用FEI Quanta FEG 250场发射环境扫描电子显微镜进行SEM和EDX表征,电子束加速电压20 kV,束斑4.5,工作距离10 mm,放大倍数1000×~6000×。EDX 能谱扫描时长10 s,元素定量分析仅针对样品组分,不包含基底Si元素。

    • 为验证太赫兹法检测薄膜电导的准确性,分别对25 mm×25 mm大尺寸金属薄膜进行太赫兹光谱和四探针(探针间距1 mm)电导检测,对结果进行比较。验证试样分别沉积了Ti 56 nm(A)、Ti 56 nm+Zr 30 nm(B),及Ti 56 nm+Zr 30 nm+Al 24 nm(C),并在沉积后于420 ℃进行原位高真空热处理12 h (图3(a)~(c))。

      Figure 3.  (a)-(c) Optical images of the samples; (d)-(f) THz conductance imaging of the corresponding samples; (g) Comparison of metal film conductance obtained by different test methods

      图3(d)~(f)为太赫兹电导测试的二维图像。图3(d)、(e)显示A、B样品薄膜电导连续均匀;C样品由于添加Al导致太赫兹电导分布不均匀(图3(g)),不连续。A样品太赫兹电导与四探针法的相对误差约为2.4%;B样品约为11%;C样品约为18% (表1)。由相图可知,Ti-Zr在600 ℃以下可以完全互溶,而Al则分别和Ti、Zr形成合金。由于四探针法检测的是薄膜二维平面的电导,具有一定空间分辨率限制,薄膜电导非均匀的分布通常无法用四探针法准确测得[17]。而采用透射模式的太赫兹时域光谱成像检测能给出薄膜厚度方向电导空间分布的细节信息,空间分辨率更高。两种方法对样品C检测结果的较大差异即可能由于二者在这种空间尺度的探测差异造成的。

      Sample constituentsConductance/SRD (THz-4PP)/4PP
      THz4PP
      ATi 56 nm0.02140.02092.39%
      BTi 56 nm+Zr 30 nm0.01920.021510.7%
      CTi 56 nm+Zr 30 nm+Al 24 nm 0.1460.17817.8%

      Table 1.  Conductance value of metal films

      为进一步验证太赫兹成像检测薄膜电导的空间分辨率及重现性,制备了12 × 12纯Cu薄膜阵列样品,并于400 ℃进行高真空热处理4 h。铜膜厚度为60 nm,每个样品点尺寸为2 mm × 2 mm,间隔1 mm(图4(a))。样品的太赫兹电导成像如图4(b)所示,不同样品点间的电导差异在±2.5%以内(图4(c))。图3(d)和(e)分别显示了图4(a)、(b)中箭头所指的样品横纵轴代表性样块的光谱信号变化。定义样品边界信号变化强度在10%~90%之间的空间距离为太赫兹成像空间分辨率[18],对样品点边缘的THz信号进行放大比较(图3(d)和(e)太赫兹信号灰色条带所对应的内插图),从中可得太赫兹电导成像的分辨率≤0.82 mm,比通常的四探针法(针尖距离1 mm)的空间分辨率好。

      Figure 4.  (a) Cu thin film sample photo on high resistivity Si substrate; (b) THz conductance mapping image; (c) THz conductance variations of the sample spots; (d) Anisotropic variations along the horizontal direction of Fig.(a) and (b); (e) Anisotropic variations along the vertical direction of Fig.(a) and (b)

      基于以上对太赫兹扫描测量电导的准确性、重现性和空间分辨率的验证,笔者进行了铜合金材料芯片电导的高通量太赫兹扫描表征测试。图5 (a)是使用连续梯度模式制备的Al2O3、Ag和Mg的微量添加的三元相图形式的铜合金材料芯片(虚线箭头指示组分梯度增加方向),具体薄膜沉积参数见表2。铜合金薄膜由Cu、Al2O3、Ag和Mg组成,其中,Ag、Mg和Al2O3为厚度梯度分别为0→12.7 nm和0→25.4 nm、依次旋转120º的类三元相图分布。为减少Cu对硅基底的扩散化合的影响,底层Cu膜厚度较厚(80 nm),并在其与硅基底之间蒸镀了5 nm的Ti作为隔离层。图5(b)为样品的太赫兹电导分布二维图像,显示在Ag以及Ag-Al2O3梯度含量最高一侧,电导强度最高,而随着Mg梯度含量升高,电导强度下降。对太赫兹扫描和四探针方法沿各组分梯度方向的15个点(图5(b)中黑点)检测获得的电导值进行比较(图(d)),可以看出,太赫兹扫描得到的测量数据与使用经典四探针法测得的数据保持了一致的趋势,从而验证说明了太赫兹扫描用于高通量铜合金样品电导测量的可行性和有效性。另一方面,如表3所示,与四探针法比较,太赫兹法测量数据存在平均28%的系统负偏差,其中,点4和点7由于不均匀性引起的偏差较大。如前所述,采用透射模式的THz-TDS可以探测具有纵向深度的三维空间。首先,高阻硅基底与Ti/Cu基薄膜界面在原位热处理后,可能产生一定的扩散化合,对高阻硅基底而言,类似于引入轻微掺杂杂质,造成对太赫兹波的吸收损耗。其次,金属薄膜中存在晶界和缺陷等也会导致太赫兹电导率比块体直流电导率低 [8],从而导致该系统负偏差。因此,文中使用太赫兹扫描法得到的结果为表观电导。此外,鉴于使用高通量制备方法得到的各样品点均基于同一基底和制备工艺条件,使用太赫兹扫描方法测量电导具有一致的比较基准,故可以在单个批次相同实验环境下进行多个组分配方的电导的半定量相对比较,有利于进行多组分配方的快速筛选。

      Figure 5.  Cu-Al2O3/Ag/Mg alloy thin film sample fabricated with the continuous gradient mode. (a) Optical image of the sample; (b) THz mapping imaging, scale bar=4 mm; (c) Comparison between the conductance trend of the 15 points marked in Fig.(b) obtained from THz and 4-point probe respectively

      Layer indexMaterialsThickness/nm
      1 Ti 5
      2 Cu 80
      3 Ag 0→ 12.7
      4 Cu 10
      5 Al2O3 0→ 12.7
      6 Cu 10
      7 Mg 0→ 25.4
      8 Cu 10

      Table 2.  Constituents of the sample fabricated using the continuous gradient mode

      12345
      4PP1.141.151.320.980.97
      THz0.760.860.910.590.66
      RD−0.33%−0.25%−0.31%−0.40%−0.32%
      678910
      4PP1.542.152.891.010.88
      THz1.061.472.240.770.64
      RD−0.31%−0.32%−0.22%−0.24%−0.27%
      1112131415
      4PP0.650.720.810.90.84
      THz0.510.530.590.70.6
      RD−0.22%−0.26%−0.27%−0.22%−0.29%

      Table 3.  Conductance of the selected 15 points along the thickness gradients of Ag, Mg and Al2O3 using 4-point probe and THz methods respectively

    • 笔者进一步对使用原位分立模式制备的12×12铜合金薄膜样品阵列进行了太赫兹电导高通量表征。图6(a)为样品密度为144个/片的Cu-Al2O3/Ag/Mg薄膜合金样品芯片,其中r0→r1→r2→r3为样品台(以P1图案为例)的旋转方向。图6(b)为太赫兹扫描成像结果。为方便对各样品点的电导和组分含量及微观组织形貌进行一一对应分析,对组分的沉积厚度与位置组合做了模拟图(图6(c)),并对样品点按照区-行-列的规则自下而上、从左到右进行分区编号(图6(d))。图6(e)为多层膜沉积参数,其中P1、P2和P3分别对应图1(a)中的掩模图案。与图5连续梯度样品略有不同的,图6样品在Si基底上直接沉积Al2O3,既为薄膜合金组分,又作为Cu与Si基底之间的隔离层,从而避免了添加Ti对合金薄膜体系的影响。表4为与图6(b)对应的太赫兹电导数据表。图7为对应的铜合金样品阵列的SEM表征图像。

      Figure 6.  Cu-Al2O3/Ag/Mg alloy thin film sample fabricated with the in site discrete mode. (a) Optical image of the sample; (b) THz mapping imaging; (c) Simulated combination of the thickness and positions of the constituents; (d) Index table of the sample spots; (e) Deposition parameters of multi-layer film

      0.175 60.101 80.175 60.179 80.180 10.178 10.159 10.173 40.151 00.127 00.172 10.195 6
      0.082 50.060 60.188 00.203 70.202 10.188 00.172 00.193 30.183 80.151 90.195 70.211 3
      0.192 10.180 70.194 90.210 00.203 30.204 80.153 70.167 70.190 40.171 90.176 10.209 0
      0.212 50.207 90.223 00.271 70.216 50.215 50.173 40.197 70.214 80.218 40.243 90.248 5
      0.078 90.076 90.185 20.223 40.223 90.231 50.178 10.194 40.215 40.222 90.256 10.220 7
      0.079 20.077 70.184 00.210 70.225 70.218 70.172 20.200 90.240 60.222 10.234 20.223 9
      0.198 50.203 50.168 50.216 80.231 40.223 00.190 20.194 40.276 30.264 50.262 10.266 8
      0.188 60.186 70.154 30.206 50.191 70.197 80.189 10.201 30.238 90.238 70.250 10.268 3
      0.033 80.040 60.101 50.181 30.072 50.077 60.177 30.193 60.165 30.162 80.251 20.268 7
      0.030 50.042 40.111 40.175 80.083 20.097 40.179 60.189 60.177 30.148 50.237 70.246 1
      0.161 60.159 50.141 70.213 60.179 10.183 20.165 40.179 80.244 80.222 80.248 90.258 8
      0.173 60.168 90.144 90.224 30.181 80.187 60.155 90.175 10.226 50.216 70.242 60.250 5

      Table 4.  THz conductance value of the in site discrete 12×12 Cu-Al2O3/Ag/Mg alloy sample library

      Figure 7.  SEM images of the 12×12 Cu-Al2O3/Ag/Mg Cu alloy thin film samples array, scale bar=100 μm

      图6(b)可以定性看出,电导最强的样品点集中在#263-#266和#253-#256区域,电导中强的样品点如#143、#233和#234,电导较弱的样品点如#336和#333,其对应电导值见表4。对表4统计得到电导最大值为0.2763(#263),最小值为0.0305(#331),中值为0.1904和0.1917(#143和 #355)。将图6(b)与图6(c)对照,显示电导值最小的样品点位置分布看似与Mg的掩模沉积位置相关。进一步与图7对照,可以看出这些样品点的微观组织相析出程度大大高于其他样品点。根据EDX组分含量测试,提取了具有代表性的样品点的组分百分含量。表5列出了上述几个具有代表性的样品点的太赫兹电导值以及氧含量(wt%)。从中可以看出,导电性与氧含量负相关,随氧含量从5.05%增加至25.1%,电导值从0.2763下降至0.0974。

      Sample indexTHz conductance/SO/wt%
      2630.27635.05
      2640.26456.76
      1430.19047.36
      2330.17738.03
      2340.148516.24
      3330.111411.46
      3360.097425.1

      Table 5.  Comparison of THz conductance and the weight percentage of oxygen of representative sample spots

      在电导普遍较强的区域,对单列样品点#164-#214进行电导与各组分含量的变化趋势分析(表6)(图8(a))。可以看出,首先,O和Al的百分含量变化趋势较为一致,但O:Al比例显示O含量高于Al2O3的化学计量比例,即还存在其他形式的氧组分。 Al(Al2O3)含量超过9%时(#234和#244),Cu合金电导显著降低。其次,Ag含量高时对应的Cu合金电导也较高(#114-#134,#224和#214)。但Mg含量与Cu合金电导的关系则不明确。此外,从SEM图像看(图8(b)),经逐层沉积的组分薄膜在经过热处理后得到充分扩散与化合,不再为多层膜状态,样品点合金薄膜中晶粒和缺陷相析出分布分为三种情况:高电导的样品点(#263)仅存在少量相析出,并且相析出点很小,组织形貌整体均匀;而电导接近中值的样品点(#234),局部有较大范围的相析出及大颗粒的Cu析出,并伴随较多的氧含量≥30%的缺陷结构(图8(b)红色箭头所指),晶粒和缺陷尺寸<5 μm。电导接近最小值的样品点(#336),微观组织基本由析出的Cu颗粒及缺陷构成,总体分布均匀。综合而言,太赫兹扫描得到的电导与样品组分含量和组织形貌具有明确的相关对应关系,为进行Cu合金组分-性能关系表征提供了判据。

      Sample
      index
      THz
      conductance/S
      O/wt%Al/wt%Ag/wt%Mg/wt%
      1640.1278.737.795.835.58
      1540.151 94.948.194.996.51
      1440.171 97.195.755.382.13
      1340.218 46.277.4710.153.79
      1240.222 96.654.6510.385.11
      1140.222 18.056.939.764.27
      2640.264 56.766.396.213.65
      2540.238 77.875.985.463.56
      2440.162 812.919.67.744.26
      2340.148 516.2411.8510.774.61
      2240.222 85.886.5511.124.23
      2140.216 76.76.0310.252.8

      Table 6.  THz conductance and weight percentages of the constituents of sample spots in column #164-#214

      Figure 8.  (a) Relationship between the THz mapping conductance and that of the weight percentage of the samples constituents from column 164-214; (b) SEM microstructure morphology of representative sample spots with different conductance

    • 文中演示了一种基于Tinkham薄膜透射方程及Fresnel公式的太赫兹光谱成像技术进行铜合金材料芯片导电性能高通量、快速表征的方法。对不同组分含量铜合金薄膜的太赫兹测量结果与标准四探针数据比较具有一致的趋势,通过太赫兹成像可以进行同一基底上 144个高通量组合铜合金样品点电导的半定量比较。代表性样品点的电导变化趋势与合金组分含量变化趋势、微观组织形貌的对应比较呈现相关对应关系。表7比较了太赫兹扫描相对于四探针法用于电导测量的优势。从材料基因高通量实验方法角度看,太赫兹电导成像检测方法简单、快速、无损,适用于样品量巨大、微区检测的高通量表征需求,在单次实验环境下获得多配方的相对比较值,可以有针对性地减少和优化其他耗时长、速度慢、需要特殊制样的性能测试(如硬度测试、TEM等)所需的样品和数量,从而进一步提高研发效率、降低研发成本,作为初筛手段用于对铜合金样品配方快速筛选潜力巨大。

      Measurement of electrical conductance/conductivityTerahertz mappingFour point probe
      Detection modeContactlessContact with sample
      Applications[12-14]Suitable for large samples or micro sampling(<2 mm), uniform or non-uniform
      Sample area ≥10 mm2, uniform
      Conductivity mapping of graphene and carbon nanotubes, measurements of semiconductors, metallic thin films, superconductors, and metal oxides, etc.
      High throughput mapping of Conductance of Cu alloy thin films in the present study
      Bulk or thin plates of conducting metals or semiconductors
      Efficiency of
      measurement
      High throughput
      144 samples/compositions per chip/batch in the present study
      Low throughput
      Information extractedDiscrete value and mapping of conductivity, complex conductivity, charge/carrier mobility; electron scattering, etc.Discrete value of DC conductivity
      Detection resolutionThree dimensionalTwo dimensional
      Overall resultsComparison of multiple samples with various compositions prepared in one batch under same conditions in the present studyOne singular sample prepared each time

      Table 7.  Advantages of THz scanning method vs. 4-point probe

      需要指出的是,文中使用透射模式进行太赫兹扫描高通量电导测试,铜合金薄膜沉积总厚度≤200 nm,与太赫兹波在金属表面的趋肤深度(skin depth)70~100 nm相近,由此产生相应的吸收损耗(α=Im(k)=1/δ=$\sqrt{\text{π} f {\mu}_{0} \sigma_{\mathrm{dc}}}$,其中δ为入射深度,f为太赫兹频率,μ0为真空中的磁导率,σdc为金属的直流电导率),根据Kirley和Booske对铜膜的表观粗糙度对电导率的损耗的模型研究[19],认为当太赫兹频率≤1 THz时,不论是镜面反射还是漫散射,由趋肤效应导致的电导率异常降低可以忽略不计。另一方面,目前仍然缺乏可以全面较好描述金属薄膜太赫兹电导率函数的定量分析模型。由于文中旨在演示太赫兹成像在铜合金薄膜高通量表征方法中的实践应用,在此不做深入的理论建模分析。笔者将在未来的工作中深入研究铜合金薄膜受相析出程度(包括晶界和缺陷的表观粗糙度差异),以及铜基体与微量添加组分扩散化合等因素影响的太赫兹电导率变化的定量分析理论模型。

      此外,文中使用高阻硅基底,虽然最底层Al2O3组分可以较好地阻碍Si与Cu基的扩散化合,同时底层Cu膜厚度较厚,可以认为基底硅对铜合金薄膜组分间扩散与化合的影响在总体上是一致的,但其对于铜合金微观结构的影响较难量化比较。因此,未来有必要发展基于反射模式的太赫兹扫描高通量电导测量方法及金属基底的高通量制备方法,有望将基于薄膜固态合成的高通量实验方法适用性扩展到块体金属合金研究中。

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