CNT(INEC)
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Investigation into the Performance of CNT-Interconnects by Spin Coating Technique Wei-Chih Chiu and Bing-Yue Tsui Department of Electronics Engineering and Institute of Electronics National Chiao-Tung University, Hsinchu, Taiwan, R.O.C. [email protected] Abstract In this work, we proposed a simple fabrication process, spin coating and dry etching, to construct CNT- interconnects. The CNT-interconnects formed by the slow rate spin coating method have conductivity of more than 10 2 (S/cm) and the CNT-interconnects with 10 2 square numbers possess about 30% conductive probability. In addition, we inserted metal bridges into 1000-μm-long CNT-interconnects to effectively facilitate the performance. Introduction Since the late 1990s, copper has been adopted to be the main material for interconnect in VLSI technology due to the exceptional conductivity, 5.88x10 5 (S/cm). However, with the scaling of integrated circuit, the occurrence of size effect and electromigration has been deeply influencing the performance of Cu-interconnect [1-2]. Besides, the fabrication difficulty of Cu-interconnects constructed by electroplating becomes more and more challenging [3]. New interconnect technology should be developed. Since 1991, the discovery of carbon nanotubes revolutionizes the post-Si technology. Carbon nanotubes have been adopted for various applications due to the excellent electrical properties. In theory, the current capability of a single metallic CNT with diameter 1nm, 2.4x10 8 (A/cm 2 ) of current density, is one thousand times greater than that of copper without any adverse effects [4]. In this study, we used spin coating and dry etching to fabricate various sizes of CNT-interconnects and investigated the characteristics of the CNT-interconnects. Fabrication Two sets of CNT-interconnects were designed for investigation. The width varying set has the width ranging from 5μm to 500μm with fixed length at 100μm, and the length varying set has the length ranging from 5μm to 1000μm with fixed width at 5μm as illustrated in Fig.1. For better performance of the 1000-μm-long CNT-interconnects, Pd/Ti metal bridges were inserted at every specified distance. The conductance of each CNT-interconnect is measured by the four-point bridge resistors as shown in Fig.2. The starting material, silicon wafer, was first capped by a 200-nm-thick wet oxide grown in high temperature furnace and by a 1-nm-thick Al 2 O 3 by an atomic-layer deposition system (ALD). The CNT solution was prepared by dissolving 2mg arc-discharge grown CNTs provided by Carbolex in dimethylformamide (DMF) and then uniformly dispersing by 24hr sonication. The first type of CNT film is formed by total 400 drops of the CNT solution with spin speed 500 rpm for 30 sec per cycle. This sample is named the normal rate (NR) sample. The other type is formed by 100 rpm for 10 sec per cycle and is named the slow rate (SR) sample. A 120 o C baking after each cycle was adopted for the entire evaporation of the DMF solution. Next, the Pd/Ti metal was deposited by a sputtering system with the ratio 8/1 and patterned by a lift off process. Finally, the CNT-interconnects were patterned by O 2 plasma etching. Results and Discussions Figure 3 shows the conductance distribution of the width varying CNT-interconnect set. From the statistical results, the NR samples with less than 6 square numbers are probable to be conductive while every CNT-interconnect of the SR sample has at least 70% conductive probability. As for the CNT-interconnect conductance distribution of the length varying set shown in Fig.4, the CNT- interconnects of the SR samples also demonstrate higher conductivity and conductive probability than the NR sample. The results interpret that slow rate spin coating and heating process could effectively keep most of the well suspended CNTs deposited on the wafer and possibly form more conductive paths within the interconnect regime. In addition, the size of 5μmx500μm CNT -interconnects has about 30% conductive probability. According to percolation theory, the conductivity of randomly distributed CNT sticks, as shown in Fig.5, has a power law dependent relation with the density. For a two dimensional conductive plane, the exponent of the power function can be theoretically calculated as 1.33 [5]. In this study, we interpret the size variation of CNT- interconnects under the same CNT density as CNT density varying at a designated interconnect regime. Figures 6, 7 show the characteristics and fitting curves of the average conductance as a function of the square numbers of the CNT-interconnects. The high exponents of the power fits for both NR samples, about 1.5, means that there are only few CNT connected path within the interconnect regime and these interconnects are sorted into percolation region in this study. The SR samples in width varying set have the exponent 1.45 close to 1.33 implies that part of CNT-interconnects had two dimensional CNT connected conductive planes, while in the length varying set, all of the CNT-interconnects are classified into power region based on 1.34 of the exponent. The size effect occurred in CNT- interconnect conductance shown in Fig.8 arises from the bundling effect between CNT sticks during baking process. Figure 9 shows the conductance distribution of the 5μmx1000μm CNT-interconnects with metal bridges crossed at every specified distance. The result shows that inserting metal bridges within the CNT-interconnects is indeed able to improve the conductive probability of long interconnect. However, too many bridge metals may 240 978-1-4673-4842-3/13/$31.00 c 2013 IEEE
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Transcript of CNT(INEC)
Investigation into the Performance of CNT-Interconnects by Spin Coating Technique
Wei-Chih Chiu and Bing-Yue Tsui
Department of Electronics Engineering and Institute of Electronics National Chiao-Tung University, Hsinchu, Taiwan, R.O.C.
Abstract In this work, we proposed a simple fabrication process,
spin coating and dry etching, to construct CNT- interconnects. The CNT-interconnects formed by the slow rate spin coating method have conductivity of more than 102 (S/cm) and the CNT-interconnects with 102 square numbers possess about 30% conductive probability. In addition, we inserted metal bridges into 1000-μm-long CNT-interconnects to effectively facilitate the performance.
Introduction Since the late 1990s, copper has been adopted to be the
main material for interconnect in VLSI technology due to the exceptional conductivity, 5.88x105 (S/cm). However, with the scaling of integrated circuit, the occurrence of size effect and electromigration has been deeply influencing the performance of Cu-interconnect [1-2]. Besides, the fabrication difficulty of Cu-interconnects constructed by electroplating becomes more and more challenging [3]. New interconnect technology should be developed.
Since 1991, the discovery of carbon nanotubes revolutionizes the post-Si technology. Carbon nanotubes have been adopted for various applications due to the excellent electrical properties. In theory, the current capability of a single metallic CNT with diameter 1nm, 2.4x108 (A/cm2) of current density, is one thousand times greater than that of copper without any adverse effects [4]. In this study, we used spin coating and dry etching to fabricate various sizes of CNT-interconnects and investigated the characteristics of the CNT-interconnects.
Fabrication
Two sets of CNT-interconnects were designed for investigation. The width varying set has the width ranging from 5μm to 500μm with fixed length at 100μm, and the length varying set has the length ranging from 5μm to 1000μm with fixed width at 5μm as illustrated in Fig.1. For better performance of the 1000-μm-long CNT-interconnects, Pd/Ti metal bridges were inserted at every specified distance. The conductance of each CNT-interconnect is measured by the four-point bridge resistors as shown in Fig.2. The starting material, silicon wafer, was first capped by a 200-nm-thick wet oxide grown in high temperature furnace and by a 1-nm-thick Al2O3 by an atomic-layer deposition system (ALD). The CNT solution was prepared by dissolving 2mg arc-discharge grown CNTs provided by Carbolex in dimethylformamide (DMF) and then uniformly dispersing by 24hr sonication. The first type of CNT film is formed by total 400 drops of the CNT solution with spin speed 500 rpm for 30 sec per cycle. This sample is named the normal rate (NR) sample. The other type is formed by
100 rpm for 10 sec per cycle and is named the slow rate (SR) sample. A 120oC baking after each cycle was adopted for the entire evaporation of the DMF solution. Next, the Pd/Ti metal was deposited by a sputtering system with the ratio 8/1 and patterned by a lift off process. Finally, the CNT-interconnects were patterned by O2 plasma etching.
Results and Discussions
Figure 3 shows the conductance distribution of the width varying CNT-interconnect set. From the statistical results, the NR samples with less than 6 square numbers are probable to be conductive while every CNT-interconnect of the SR sample has at least 70% conductive probability. As for the CNT-interconnect conductance distribution of the length varying set shown in Fig.4, the CNT- interconnects of the SR samples also demonstrate higher conductivity and conductive probability than the NR sample. The results interpret that slow rate spin coating and heating process could effectively keep most of the well suspended CNTs deposited on the wafer and possibly form more conductive paths within the interconnect regime. In addition,
the size of 5μmx500μm CNT -interconnects has about 30% conductive probability.
According to percolation theory, the conductivity of randomly distributed CNT sticks, as shown in Fig.5, has a power law dependent relation with the density. For a two dimensional conductive plane, the exponent of the power function can be theoretically calculated as 1.33 [5]. In this study, we interpret the size variation of CNT- interconnects under the same CNT density as CNT density varying at a designated interconnect regime. Figures 6, 7 show the characteristics and fitting curves of the average conductance as a function of the square numbers of the CNT-interconnects. The high exponents of the power fits for both NR samples, about 1.5, means that there are only few CNT connected path within the interconnect regime and these interconnects are sorted into percolation region in this study. The SR samples in width varying set have the exponent 1.45 close to 1.33 implies that part of CNT-interconnects had two dimensional CNT connected conductive planes, while in the length varying set, all of the CNT-interconnects are classified into power region based on 1.34 of the exponent. The size effect occurred in CNT- interconnect conductance shown in Fig.8 arises from the bundling effect between CNT sticks during baking process.
Figure 9 shows the conductance distribution of the 5μmx1000μm CNT-interconnects with metal bridges crossed at every specified distance. The result shows that inserting metal bridges within the CNT-interconnects is indeed able to improve the conductive probability of long interconnect. However, too many bridge metals may
240978-1-4673-4842-3/13/$31.00 c©2013 IEEE
conversely exacerbate the conductivity owing to the contact resistance from Pd/Ti to CNTs.
Summary In summary, the investigation into performance of
CNT-interconnects by spin coating technique has been completed. Through slow rate spin coating method, the conductivity as well as conductive probability obtained great enhancement. Next, the size-dependently percolative characteristics of the CNT-interconnect conductance were demonstrated. At final, the conduction of 1000-μm-long CNT-interconnects with the metal bridges inserted approves the potential for future VLSI interconnects.
Acknowledgement This work was supported in part by the Ministry of Education
in Taiwan under ATU Program, and was supported in part by the National Science Council, Taiwan, R.O.C. under the contract No. NSC 100-2221-E-009- 010-MY2. References
S. M. Rossnagel, et al., J.Vac.Sci.Technol., B22 (2004) 240. Y. Chai, et al.,Electro.Dev.Lett., 29 (2008) 1001 P. C. Andricacos, et al., Soc.Interface, (1999) 32 B. Q. Wei, et al., Appl.Phys.Lett., 79 (2001) 1172 L. Hu, et al., Nano Lett., 4 (2004) 2513.
Fig.1 Schematic width and length varying sets of CNT-interconnects with four-probe bridge resistors designed for this work.
Fig.4 Conductance distribution of the length varying set of CNT-interconnects (5μm in width). The SR samples have higher conductivity and conductive probability.
Fig.7 Power fit for average conductance and square number of the length varying set of CNT-interconnects (5μm in width) All NR samples locate in percolation region, while all SR samples are in power region.
Fig.2 Schematic layout of the four-probe bridge resistor with metal bridges. The length and width of the metal bridge is 7μm and 4μm, respectively. The distance between metal bridges ranges from 25μm to 500μm. The 5μm space between two probes is for precise measurement of CNT -interconnect conductance.
Fig.5 Top-view SEM image of CNT networks formed by 200 drops CNT solution and spin coating at 500rpm for 30sec. The 1-nm-thick ALD Al2O3 layer is used to improve the uniformity.
Fig.8 The distribution of the average conductance of width and length varying sets of the SR samples within the same range of square number. The difference of conductance results from the size effect caused by bundling phenomena.
Fig.3 Conductance distribution of the width varying set of CNT-interconnects (100μm in length). The SR samples have higher conductivity and conductive probability.
Fig.6 Power fit for average conductance and square number of the width varying set of CNT-interconnects (100μm in length). The entire NR samples belong to percolation region, while the SR samples with less than 15 square numbers are around power region.
Fig.9 Conductance distribution of CNT-interconnects (1000μm in length) made by slow rate spin coating with various square numbers between Pd/Ti metal bridges.
2013 IEEE 5th International Nanoelectronics Conference (INEC) 241
