Yap et al. Nanoscale Research Letters 2012, 7:105 http://www.nanoscalereslett.com/content/7/1/105 Page 3 of 8 Figure 2 SEM of a CNT bump growth using the PECVD approach. The grown CNT was vertically aligned, and the length of the CNT was 20 μm. The top right (inset) shows the low magnification view of the CNT bumps formed on the Au metal lines. CNT structure, which affected the electrical characteris- tics of CNTs [15,16]. Improving the quality of the CNTs at low temperature is still under investigation, and the variation of metal resistance before and after the CNT growth process will be taken into consideration. In our previous work, the change in Au metal line resistance was found to be insignificant as the Au resistance is smaller than the measured CNT bump resistance, as reported in the latter part of this report [12]. Results and discussion To demonstrate the feasibility of using PECVD approaches to achieve fine pitch CNT bumps, three dif- ferent sets of test structures comprising (structure 1) 170 × 150 μm, (structure 2) 120 × 100 μm, and (structure 3) 70 × 50 μm CNT bump sizes were designed (Figure 3). The scanning electron microscopy [SEM] images in Figure 3 show the CNT bumps grown using the PECVD approach which allowed pitches to downsize to 80 μm in our experiment. A homogenous CNT bump height can also be observed throughout the carrier and die (dummy and CNT bumps on electrodes). Larger catalyst pattern geometry would result in longer CNT length due to the differences in partial pressure of carbon feedstock gas [17]. The effect of catalyst pattern geometry was not sig- nificant in this experiment, and the height of all CNT bumps was assumed to be 20 μm regardless of the bumps’ dimensions. For the first time, CNT interconnection bump joining methodology for fine pitch bump had been achieved, as depicted in Figure 4. In Figure 4a, the flip chip test struc- ture was observed at an angle of 75° under the SEM. A gap of approximately 20 μm, which is equivalent to the CNT height, can be observed in Figure 4b. The bonding load of 0.5 kg was sufficient to cause the CNTs from the bottom carrier to ‘insert’ and touch the top die, as observed in Figure 4c. The load of 0.5 kg, which is equivalent to 4.5 N or 3.125 kg/cm2, is much lower than those applied in previous reported flip chip experiments [4-6]. In this joining methodology, the CNT bumps are ‘inserted’ through the air gaps of opposite CNT bumps thus requiring smaller force. Due to equipment limita- tions, microphotographs of the CNT bumps during the bonding and release processes could not be captured to demonstrate the mechanical flexibility of CNT bumps as observed by Soga et al. [5]. However, based on the SEM images in Figure 4b, the vertical alignment of the CNT bumps can still be observed, which is likely due to the mechanical flexibility of the CNT bumps. In this experiment, structures 1 and 2 were tilted to 75° in the SEM to observe the gap between test structures. This is remarkable as no bonding temperature or adhesive was used during this bonding process to mechanically bond the die to the carrier. However, for structure 3, the die tended to slide off from the carrier during movement of the SEM’s stage. The dummy bumps in structures