© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Mater. 2010, 22, 5039–5042 5039 www.advmat.de www.MaterialsViews.com COMMUNICATION wileyonlinelibrary.com By Yifeng Fu, Yiheng Qin, Teng Wang, Si Chen, and Johan Liu* Ultrafast Transfer of Metal-Enhanced Carbon Nanotubes at Low Temperature for Large-Scale Electronics Assembly Vertically aligned carbon nanotubes (CNTs) have been proposed for many applications in electronics, such as thermal interface materials (TIMs), [1,2] field emission (FE) devices, [3,4] vertical interconnects, [5,6] gas sensors, [7] electrical brush contacts, [8] etc. However, the high CNT synthesis temperature during chem- ical vapor deposition (CVD) [9–12] and poor adhesion between synthesized CNTs and supporting substrates resulting from the weak van der Waals force [13,14] have greatly restricted the application of CNTs in electronics. [15,16] Thereby, CNT transfer (or printing in some literature) techniques were proposed as a solution to these problems. [13,14,17–24] Nevertheless, drawbacks such as transfer tools limitation, [17,18] receptor patterning dif- ficulty, [13,14,19,22] time-consuming, [20,23,24] high electrical resist- ance, [21] etc. still exist in these methods. Furthermore, some methods introduced very complex processes which will lower transfer repeatability and scalability. Other researches [25–30] on CNT transfer are also performed through a wet or chemical approach for various purposes, in which the transferred CNT structures are either difficult to position or not vertically aligned. In this study, we present a very timesaving and reliable method for CNT structure transfer at low temperature to address those problems. The CNTs are enhanced by metal coating as a novel and efficient solution to improve their electrical performance. The adhesion between CNTs and substrates is also greatly pro- moted after the transfer process. The experiment process is illustrated in Figure 1 . First of all, 10 nm thick Al 2 O 3 and 1 nm thick Fe are sequentially evaporated onto a silicon substrate as catalyst for CNT growth (Figure 1a). The patterning of this catalyst layer is obtained through standard photolithography and lift-off processes. The Al 2 O 3 / Fe catalytic chips are then put into a commercial CNT growth system (Black Magic II, Aixtron) for CNT synthesis (Figure 1b) where acetylene is utilized as carbon precursor. The CNTs height is controlled by varying the growth time and after growth the chips are cooled down to room temperature in a nitrogen (N 2 ) flow. [6,31] In order to transfer as-grown CNT structures, indium (In) is evaporated onto both unpatterned and patterned Ti- and Au- coated silicon substrate (Figure 1d,e). Ti acts as adhesion promoter and Au was demonstrated to show very good interac- tion behavior with In. [32] The transfer process is carried out by using a flip-chip bonder, in which the pressure, heating time, and temperature on both the CNT growth chip and target sub- strate can be controlled. Prior to contact with In on the target substrate, the as-grown CNT structures are coated with a Ti/Au layer (Figure 1c) to improve electrical performance and interac- tion on the interface. [33] Subsequently, the In is melted at 170 °C and aligned CNT structures are contacted and pressed onto the In layer with a typical pressure of 6.4 × 10 6 Pa to start transfer (Figure 1d,e). The transfer process lasts for only 2 minutes and then both the target substrate and the CNTs growth chip are cooled down in air naturally. Afterwards, the growth chip is separated manually and CNT structures are attached onto the target position (Figure 1f,g). Figure 2 a,b show the CNT struc- tures before and after transfer onto unpatterned substrate. Figure 2c,d show the transferred CNTs on the target substrate and the cavities (after removing CNT structures) and residues (some CNTs and Ti/Au coating) left on the growth substrate, respectively. From Figure 2a–d, it can be clearly observed that the CNT transfer is realized in good quality at a large scale. In order to verify the repeatability of this method, another three CNT samples with a 100 × 100 CNT bundle array are transferred under the same processing conditions. Results indicate that an average yield rate of more than 90% is achieved. The thickness of the In layer evaporated on the target sub- strate is 1 μm in this work. The Ti/Au layer coated onto the CNTs is 20/100 nm thick, which not only serves as an electrical enhancer to decrease the resistance of the CNTs but also estab- lishes efficient metallic contact between CNTs and the substrate. Transfer of CNTs without the Ti/Au enhancer is also tried but the yield rate is lowered to about 30%. Electrical characterization is performed on as-transferred CNTs with and without the Ti/Au enhancer. A Keithley 4200 multimeter equipped with a probe station is utilized to examine the current-voltage ( I–V) response of the CNT bundles. One probe contacts the In film on the substrate near the bundle to be measured and the other probe is placed on the top of the transferred CNT bundle. The measurement results are shown in Figure 3 . For the transferred CNT bundles 20 μm in DOI: 10.1002/adma.201002415 [∗] Y. Fu, Y. Qin, T. Wang, S. Chen, Prof. J. Liu Department of Microtechnology and Nanoscience Chalmers University of Technology SE-412 96 Göteborg (Sweden) E-mail: johan.liu@chalmers.se Y. Fu FOAB Elektronik AB S:t Jörgens väg 8, SE-422 49 Hisings Backa (Sweden) Y. Qin SHT Smart High Tech AB Fysikgränd 3, SE-412 96 Göteborg (Sweden) S. Chen, Prof. J. Liu Key Laboratory of Advanced Display and System Applications and SMIT Center School of Automation and Mechanical Engineering Box 282, No. 149 Yan Chang Road Yan Chang Campus Shanghai University Shanghai, 200072 (P. R. China)