Yap et al. Nanoscale Research Letters 2012, 7:105 http://www.nanoscalereslett.com/content/7/1/105 Page 4 of 8 Figure 3 Illustration of CNT bumps on the Au electrode with three different pitches. The smallest designed pitch is 80 μm in this work. The dimensions of each CNT bump are (a) 170 × 150 μm for structure 1, (b) 120 × 100 μm for structure 2, and (c) 70 × 50 μm for structure 3. The scale bar at the bottom right of each image represents 100 μm. 1 and 2 were effective in holding the weight of the die (0.033 g), but not in structure 3. Assuming a perfect align- ment, the area occupied by a total of 74 CNT bumps on structure 2 is 0.74 mm2 out of 16 mm2 (die area). On the other hand, the area occupied by CNT bumps in structure 3 is 0.185 mm2 out of 16 mm2. This is logical because the dummy bumps in structure 3 were smaller as compared with those in structures 1 and 2. Based on structure 3, future designs using the die area will require more than 1.2% to be converted for the implementation of the CNT bump area for sufficient mechanical support. This is to ensure that the bonding forces present within the CNT interconnection bumps is greater than the die’s weight to ensure mechanical stability. A study based on total energy and molecular dynamic calculation on molecular CNT to CNT joining methodology shows that the type of bonding present between CNTs is very strong, and a force larger than 3 nN will be required to disengage the joining struc- ture [18]. Yung et al. suggested the van der Waals forces between CNTs to be the bonding mechanism when CNTs were inserted into each other [11]. In this work, only the dimensions of bumps (area) were varied; the influence of densities and diameter of CNTs within a bump may also be an important consideration to CNT bonding mechanisms. The attached die was subsequently removed from the carrier using tweezers to observe the effect of the CNT bumps after bonding. The carrier and die were then loaded into the SEM chamber with the same orientation to observe the conditions of CNT bumps that were in contact with each other, as shown in Figure 5. A portion of the CNT bump appeared to be smeared as seen from Figure 5a, but a high percentage of CNT bumps retained their original structure. This is similar to the observations of Yung et al. for large-scale CNT to CNT interconnect structure which demonstrated that the bonding process is reworkable [11]. To verify the CNT interconnection bundle resistances as well as the concept of reworkable capabilities of the joining methodology, the die of test structure 1 was removed and was bonded to the same carrier again to measure the I-V characteristics. Using a two-point probe, the DC measurement was performed across the input and output nodes (two CNT interconnection bumps) as described in Figure 1. I-V measurements in Figure 6 displayed a similar DC behavior which demon- strated the concept of a reworkable process. The slight deviation of the second attempt could be caused by the differences in die placement due to the limited align- ment accuracy of the bonding machine. The current increased linearly with the voltage applied, indicating that ohmic contact has been achieved between CNTs and metallization [19]. The line of the ‘first attempt’ was fitted linearly to obtain a slope of 4.20 × 10-4 A/V. This