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What is Nano-Twin Cu? |
Nano-twin copper refers to the (111) single-direction columnar crystalline copper microstructure, where there is a high density of copper twin-crystal stacks in the columnar grains and the twin-crystal grain boundary spacing ranges between a few nanometers to a few hundred nanometers. The nano-twin copper microstructure with the preferred (111) orientation was published by Professor Zhi Chen of Chiao Tung University’s Department of Materials Science and Engineering in 2012. This patented microstructure is formed by depositing the copper film via electrochemical plating using a copper-plating solution containing special additives [1][2]. The structure of this nano-twin microstructure is shown in Figure 1. It can be seen in the figure that the copper columnar grains contain a high density of copper-twin crystal stacks.
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Figure 1. Nano-Twin Copper Crystal Structure (Ion-Beam Imaging) |
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What are the characteristics and applications of nano-twin copper? |
Nano-twin copper’s greatest characteristic is its ability to greatly improve mechanical strength while maintaining similar levels of electrical conductivity. In addition, nano-twin copper has high thermal stability, excellent resistance to electro-migration, and excellent resistance to the Kirkendall effect. Since Professor Chen discovered in 2012 that nano-twin copper can be prepared via DC electroplating [3], this technology has become widely researched and applied.
Moore’s Law predicts that the number of transistors per unit area inside a chip will double every 18 months. This prediction would mean reaching mass production of the 3 nanometer node in 2023. It will subsequently become necessary to develop mass production processes for 2nm/1nm nodes, which will significantly increase production costs and difficulty. As such, some experts have predicted that Moore’s Law will be curbed by physical limitations in the future or become too difficult to continue due to cost considerations. This has led to the idea of the More Than Moore era. In this post Moore era, the Heterogenous Integration and Chiplet technologies have garnered the most attention. Heterogenous integration advanced packaging has become another important trend in the development of technology for realizing function integration and component size reduction [4]. Because heterogenous integration promotes the development of IC 2.5D & 3D packaging, they too are an inevitable trend. The most famous 2.5D/3D packaging technologies include Chip on Wafer on Substrate (CoWoS) and System on Integrated Chips (SoIC).
Many areas of 2.5D/3D packaging, including the Through-Silicon Via (TSV), bumps and micro-bumps, Wafer Redistribution Layers (RDL) and Wafer-on-Wafer (WoW), etc., require the use of high performance copper connections. The use of nano-twin copper in these processes can not only increase the performance of connections but also improve the reliability of 2.5D/3D packaging.
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Detection of Crystal Orientation |
The large-area crystallographic orientation of a material is typically determined using X-ray diffraction analysis (XRD), Electron Back Scatter Diffraction (EBSD) and TEM electron diffraction. XRD has the largest detection range, followed by EBSD. The TEM has the smallest electron diffraction range. TEM electron diffraction will be explained in the next chapter.
The advantage of EBSD is that it can be used to analyze the grain orientation distribution on both a sample’s surface and cross-section. In the EBSD surface analysis shown in Figure 2, the sample’s vertical surface is the Z axis. Using the Inverse Pole Figure (IPF), it can be determined that almost all the orientations on the vertical axis are the preferred 111 orientation. In the Figure 3 EBSD cross-section analysis, the Inverse Pole Figure also shows that almost all the orientations on the vertical surface are the preferred 111 orientation.
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Figure 2. Sample Surface EBSD; The Z axis is perpendicular to the sample surface; It can be determined via the Inverse Pole Figure that almost all the orientations on the perpendicular surface are the preferred 111 orientation
Figure 3. Sample Cross-Section EBSD; It can be determined via the Inverse Pole Figure that almost all the orientations along the vertical axis are the preferred 111 orientation |
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Observation of Nano-Twin Copper |
The nano-twin copper samples used in this article were all provided by Professor Zhi Chen of Chiao Tung University.
The observation of nano-twin copper can be conducted using a variety of methods. These include ion-beam imaging, the Transmission Electron Microscope (TEM), and Scanning Transmission Electron Microscopy (STEM), etc..
Ion-beam imaging refers to the obtaining of an image by applying a Focused Ion Beam (FIB) to a sample’s surface, as shown in Figure 1. When ions hit a sample’s surface, differences in the crystallization direction on the sample result in differing contrasts in the image. The nano-twin crystal structure can be determined based on these differences. Figure 1 shows the nano-twin copper crystals were stacked within the columnar grains. The direction of thickness for nano-twin crystals is approximately the same as the direction of columnar crystals. By comparing EBSD results, it can be seen that the nano-twin crystal direction of thickness is roughly equivalent to the lengthwise direction of the columnar crystals along the 111 orientation of copper.
For the observation of nano-twin crystals, the TEM uses a 200KV electron beam to penetrate a sample with a thickness of less than 100nm to produce thickness-mass contrasts or diffraction contrasts that reveal the different crystallographic directions, as shown in the TEM photos in Figure 4. The image on the left in Figure 4 is a TEM bright field image, and the image on the right is a TEM dark field image. The images show that the minimum thickness of the nano-twin crystals measured is about 6nm, while the maximum thickness is 125nm. The STEM method is similar to that of TEM. STEM is a scanning method that relies on irradiating the test piece, as shown in Figure 5. In Figure 5, it can be clearly observed that there is a high density of copper twin stacks inside the copper columnar grains.
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Figure 5. STEM Image; The direction of thickness of the nano-twin crystals is roughly the same as the lengthwise direction of the columnar crystals |
圖4. TEM照片，Figure 4. TEM Images; Bright Field Image on the Left; Dark Field Image on the Right
In addition to observing their structure, the TEM can also use electron diffraction technology to determine the crystalline orientation of grains. Its electron diffraction range is relatively small compared to that of an X-Ray or EBSD. It can go so far as to identify the crystal orientations of nanometer single crystal grains. Figure 6 shows the electron diffraction pattern of the nano-twin crystals of the sample cross-section. It can be seen in the figure that the result is the same as those obtained via EBSD. The thickness direction of the nano-twin crystal is approximately the same as the lengthwise direction of the columnar crystals, that being the 111 orientation of copper.
In addition to observing the structure of the nano-twin crystals, we can also observe the structure of the defects in the cross-section. Figure 7 shows the cross-sectional structure of the sample. In it, we can observe a longitudinal misalignment. The longitudinal misalignment lies roughly along the 111 direction. Note that it is not a straight line. It is curved and irregular.
Figure 6. Nano-Twin Electron Diffraction Pattern |
Figure 7. Observation of Longitudinal Misalignment Under Different STEM Conditions |
Since it was proposed by Professor Zhi Chen’s team in 2012, nano-twin copper has become an indispensable part of advanced 2.5D/3D packaging technologies such as CoWoS and SoIC. Extensive nano-twin copper research and technological development have expanded its applications. The copper connection technology required by the WoW, RDL, u-bump and other technologies essential to future advanced packaging means that more nano-twin copper is bound to be needed, so research into nano-twin copper too will doubtless continue to increase. To enhance MA-tek’s analytical capabilities in regards to nano-twin copper, this article used ion imaging via EBSD, FIB and TEM/STEM to observe nano-twin copper crystal structures and determine crystal orientations. The results showed that the direction of thickness of nano-twin copper is roughly the same as the lengthwise direction of columnar crystals, both of which lie along the 111 orientation of copper. TEM technology also enabled the observation that the nano-twin copper sample has a misalignment along the 111 direction. This misalignment is not straight but curved and irregular.
Reference:
[1] 添鴻科技, https://www.chemleader.com.tw/autopage_detail/3/nt-cu
[2] HSIANG-YAO HSIAO, CHIEN-MIN LIU, HAN-WEN LIN, TAO-CHI LIU, CHIA-LING LU, YI-SA HUANG, CHIH CHEN , AND K. N. TU,SCIENCE, 25 May 2012, Vol 336, Issue 6084, pp. 1007-1010.
[3] Liu, Tao-Chi, et al. "Fabrication and characterization of (111)-oriented and nanotwinned Cu by DC electrodeposition." Crystal Growth & Design 12.10 (2012): 5012-5016.
[4] 異質整合先進封裝設計趨勢, 洪志斌博士‒日月光研發中心副總經理
