Application of 3D CNT Array





12 13

sin . . T T T

v      _m  . (6-16)

Therefore, the rolling speed could be enhanced by slightly raising the reflowing temperature for a shorter reflow time for the seed aggregation. In comparison with the two morphologies as shown in Figure 6-3(a) and (b), the appearance of large coalescences of seeds in the inverted pyramid confirms the tendency via 100C reflowing temperature increase.

Meanwhile, the radius of seed coalescence (RC) can be calculated using (6-15).

Obviously, this radius simply depends on the bottom edge length of inverted pyramid and the thickness of deposited metal film. As shown in Figure 6-4, the calculation provides a good size prediction suggesting the seed size can be further reduced by reducing the thickness of deposited Co and the size of pyramid. Once the location and edge length of the inverted silicon nano-pyramid, the thickness of deposited Co film, and the temperature of the thermal reflow are all definitely determined, the location, diameter size, and length of the CNTs can be also easily controlled. Therefore, the miniaturization of the microsystem could be realized by means of using the seeding control scheme adequately. In order to reveal the ingenuity of determining the locations of each seed using the scheme under some prescribed limits, for instance, the arrangement of field-ionizer, an application of 3D CNT array is employed as an example in the following section.

6.5.2 Application of 3D CNT Array

Nanotechnology development has been a major research topic in our country. One of research direction of nanotechnology is to find new application, where 3D-CNT array is also a key nanotechnology to advance the nanoelectronics and microsystems. Via achieving a group

of stand-alone and well-aligned semiconducting CNTs on a substrate, the development of 3D CNTFET IC can be further realized. As aforementioned, previous investigations have shown that the synthesis methods, growth conditions, as well as catalytic seed varieties and sizes play important roles in determining the diameter and chirality of a SWCNT, i.e. the semi-conductivity of a SWCNT, and further pointed out that one of the research efforts should move toward precisely catalytic seed sizing and positioning control to pave the way for the future nanoelectronic fabrication of SWCNTs. It is our belief that this technique will be useful for future SWCNT growth control and practical for micro- and nano-electronic CNTFET IC fabrication. In the section, as a suitable example, the foundation and advantages of a 3D-CNT array based field-ionizer will be clarified theoretically and experimentally.

The fundamental of 3D-CNT array based field-ionizer in this case is to provide high energy electron beam to ionize helium atoms to form He^(-) ions that will transfer electrons later to channeltron electron multiplier (CEM) for counting. The typical value of the field strength for the helium ionization is about 1.5~4 V/Å that can be formed on a tungsten tip with the diameter of 50 nm while the tip is applied with 6kV at 295K. Previously, the tungsten tip was fabricated from ordinary tungsten wire of 0.1 mm diameter. A short length of this wire was generally crimped in a small piece of copper tube with 1.5 mm outer diameter, 0.65 mm inner diameter, and 10 mm long. The tips were then electrolytically etched by following fairly standard practices [136]. Specifically, the copper tube was mounted in a holder placed above a glass beaker filled with a 2M NaOH solution. A power supply (2V dc) was connected to the copper tube on the one side and to a platinum electrode in the beaker on the other side. The wire was lowered into the solution and continuously moved up and down over a few millimeters by means of a manual translational drive. This motion distributed the preferential etching of the fluid-air interface that produces a gentle rather than an abrupt taper. The wire was monitored through magnifying glass lenses during this process. For few minutes later, when a visible indentation appeared in the wire, a series of short current pulses would be

applied by flicking the power supply on and off. By flicking bias voltage to finish the etch-through of the wire, a tip with a radius of curvature of typically 10-20 nm can be formed.

Since the process is very complicated and hard for making a dense array, it is a critical research topic in the development of matter-wave microscope by implementing wafer batch process combined with nanofabrication technique to have a cost-effective solution for the fabrication of high performance field-ionizer.

Furthermore, the tips suffered with very high bias voltage, such as 6 KV in the case, will have a large erosion rate i.e. low operation lifetime. In general, the stability as well as the lifetime of the tips is directly and strictly proportional to their own electric conductivity and thermal conductivity. Previously, copper and tungsten are commonly adopted in the tip materials of microscope techniques due to their good thermal and electrical conductivity and high melting point as compared with the other solid metals. Experimental results have shown that the total erosion of copper and tungsten under applied 0.3 KV biased voltage is about 0.1μg/C [137]. In comparison with the thermal and electrical properties of the possible materials as ionizer tip listed in Table 6-1, it seems that the thermal conductivity of CNTs is an order of magnitude larger than that of Cu and W at least. Much longer lifetime could be expected in the operation of processes of the field ionization due to the unusually high thermal conductivity of the CNTs. Thus, the employment of CNT tips could mechanically provide a

Table 6-1

Thermal and electric properties of the possible tip materials Tip material Work function

(eV)

Thermal conductivity (W/m-K) @100K

Electrical Resistivity (-m) @300K

Tungsten 4.5 208 5.7 × 10^-8

Copper 4.5 482 1.7 × 10^-8

Carbon nanotubes 4.5~5.1 [138-141]

6600 [142]

5.1× 10^-8 [143]

very stable and long-term observation and time-depended image trace.

Based on the aforementioned analysis, the proposed 3D-CNT array based field-ionizer requires the following critical feature for high resolution performance of helium detector (1) singular CNT tips in a form of array in an economic way, (2) uniform CNT emission property, (3) each CNT tip can be controlled individually which can further enhanced ionization efficiency. Previous investigation regarding the work function at the tip of individual single-walled CNTs (SWCNTs) and multi-walled CNTs (MWCNTs) has shown no significant relation with tip size as listed in Table 6-1. Experimental results [138,141] indicated there are no significant differences of work function of MWCNTs in the diameter ranging 15~60 nm as listed in Table 6-2, in which the values of work function are 4.5~4.9 eV. Additionally, the work function of the SWCNTs with diameter about 1 nm [139,140] roughly has a fixed value of 4.8~5.1 eV. Thus, it can be safely to assume that there is no significant relation and sufficient expressions between the diameter of a CNT and its own work function while the D is lower than 61 nm. It is noted that these values of a CNT is very close to the work functions of Cu and W. According to Fowler-Nordheim theory (F-N theory) [141], similar emission characteristics can be expected.

On the other hand, the electric field, E, nears the tip should be governed by the tip

Table 6-2

Systematic field emission data [141]

Sample No. l (μm) d (μm) D (nm) φ (eV)

1 0.32 2.16 52.4 4.60

2 3.9 4.3 31.7 4.51

3 3.9 4.3 31.7 4.78

4 11.2 16.9 61.1 4.58

5 6.4 8.2 46.4 4.60

The symbols used above l, d, D, and φ are length of a CNT, distance from substrate to electrode, diameter of a CNT, and the work function, respectively.

morphology and the applied voltage, V. The first approximation is expressed as follows [136,144]:

R E V

 , (6-17)

where R and κ are the tip radius of curvature and the field factor with a value typically of 3~8 that depends on the tip material and the geometry, respectively. For instance, if we apply a voltage of 10 kV on the tip with radius about 50 nm, the strength of the electric field with value of 2~5 V/Å will be induced. The smaller the radius as well as the diameter of the tip is, the stronger the strength of the electric field will be. It is noted that the parameter κ strongly relies on the adopted material and geometry of tip and it could be only alternatively determined by tailor-made measurements or F-N plot [ln(I /V²) vs. 1/V, where I is emission current and V is applied voltage]. Meanwhile, in F-N theory, the field emission current can be determined by β which is called field enhancement factor and the work function φ:

) ) exp(

( ^{2 32}

E B E

I K







 _

 , (6-18)

where K is a constant, B=6.83×10⁹V eV^-3/2 m^-1, and E is electric field. Thus, the field enhancement factor β can be determined by means of the slope of the F-N plot, m, as the follows [141]:

V m E B

2

1 _3



 

 . (6-19)

The measurements are to trace a slope of F-N plot as shown in Figure 6-7 for the case of CNTs. According to the measurements, high field-emission phenomenon as well as high field enhancement factor, β, only happens when the tubes with high aspect ratio (height: diameter

~3: 1) are separated from each other at about the distance that corresponds to their height as shown in Figure 6-8, which is called field-screening effect [145].

Figure 6-8: Enhancement factor vs. the tube height for CNTs whose intertube distances are (a) 104 and (b) 65 nm [145].

Figure 6-7: The corresponding measured F-N plot for CNTs [141].

Thus, the efficiency of the detection will be significantly depending on tip aspect ratio and the corresponding spacing with each other. Previous investigation has shown that catalytic seed varieties and sizes can play an important role in determining the diameter and property of CNT in addition to the synthesis methods and growth conditions. For realizing the research objective, the first technical challenge to fabricate the 3D-CNT array based field-ionizer as well as the 3DICs embedded in a micor- or nano-electronic system is to satisfy the requirement of a particular method useful for 1D material seeding control. Both physically and experimentally, the developed scheme in Chapter 6 should be one of the candidates for achieving the particular requirement.

6.6 Summary

This chapter presents a seeding control scheme by utilizing gravity force to form an agglomeration of molten Co seeds on a patterned inverted silicon nanopyramid. Nanometer sized molten Co seeds formed on a nonwettable inverted pyramid surface can roll along the inclination followed by aggregation to form a singular seed with the size depending on the pyramid size and the thickness of as-deposited Co film inside the pyramid. The proposed scheme allowing the formation of well-aligned catalytic seeds with manipulated size will promise the control growth of 1-D material for practical integrated microelectronic device fabrication.