Motivation

For technological commercialization of products today, the cost is the most important issue. The main concern for manufactures is cost down. It is worth noted that lower cost will provide more advantages for a product. In our research of CNT field-emission backlight unit (FE-BLU), we ensure that if the cost of field-emission backlight unit (FE-BLU) continuously decreases, it can even replace the traditional light bulbs or light-emitting diode (LED) in the future. So the following motivation will focus on decreasing the cost and increasing the quality of CNT FE-BLU.

Nowadays, the backlight unit of liquid crystal display (LCD) is CCFL, but CCFL backlight system contains several components for providing planar and uniformity illumination. The following are designs of LED-BLU:

1. Reflection sheet is used to reflect the light beam of wrong direction from the lamp.

2. Light guide panel is used to transform spot or linear light source to planar light source.

3. The prism sheet and diffusion sheet are playing the crucial roles in spreading the brightness.

Fig. 1-10 shows the complex system of LED backlight unit on LCD below [1.55] [1.56].

The uniformity become better due to the uniformity-assisted layer of backlight system, but thickness and cost increase obviously. For a LCD, the cost of backlight system on the total cost is about 14% for 17inch TFT-LCD, and 21% for 32inch TFT-LCD, which are shown in Fig. 1-11 [1.57]. As this result, we could easily to make a prediction that we will require more cost on backlight system for a larger size TFT-LCD.

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(a)

(b)

(c) (d)

Figure 1-10 The profile of LED backlight system (a) shows names of every sheet.

[1.55] (b) direction of light beams in backlight system. (c) bottom lighting type of backlight system. (d) edge lighting type of backlight system. [1.56]

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(a)

(b)

Figure 1-11 The cost of the overall (a) 17inch (b) 32inch TFT-LCD [1.57].

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Instead of LED-BLU, FE-BLU is another aspect for illumination. Comparing between LED and FE backlight system, we could observe easily few advantages of FE-BLU shown below:

1. The efficiency of FE-BLU is not affected by thermal-effect. Since it is a proto-type of vacuum microelectronic. But thermal-effect give rise to degradation of the brightness or efficiency of LED decrease obviously.

2. FE-BLU is a planar light source showing a better feasibility for larger scale as compare with LED spot light source.

3. For TFT-LCD, using FE-BLU will reduce cost of backlight system and decrease the thickness of display as well.

In our research, we have chose the carbon nanotubes as the emitters of FE-BLU because of some superior properties of CNTs for field-emission. One of the superior properties is low work function ( ~5eV ), high conductivity, small tip of radius curvature, low turn-on electric-field, and high emission current density. Another properties are high chemical stability and high mechanical strength, so CNTs emitter is still stable under high electric-field and current density.

Although CNTs have superior field emission characteristics, there are still some drawbacks needed to be overcome, i.e. uniformity and reliability. The degradation of field emission current and brightness due to week adhesion between CNTs and substrates will result in poor reliability. Problem of uniformity is caused by screening-effect, which is determined by dense of emission emitters. The screening-effect is shown in the Fig. 1-12, indicating that the effective field is affected by the height and density of emitter.[1.58]

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(a) (b)

Figure 1-12 The screening-effect occurred (a) because of height (b) because of distance between emitter sites [1.58].

(a)

(b)

Figure 1-13 Improvement of Luminescent Uniformity via Synthesizing the Carbon Nanotubes on an Fe–Ti Co-deposited Catalytic Layer (a) conventional catalyst (b) co-deposited catalyst [1.62].

Under 7.7 V/μm Under 7.7 V/μm

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There are many ways to solve problem of reliability. One is mechanical coating on grown CNTs, such like spin-on-glass (SOG) coating and polymethyl methacrylate (PMMA) coating [1.59][1.60]. Another is CNT printing with Zinc powder mixture [1-61]. Although two method improve the adhesion of CNTs on substrate, it is also increase the cost due to a complex step in the procedure. The way provides with both increasing reliability and keeping low cost is co-deposition of catalyst and buffer layer. Co-deposition means that we produce a solid-solution of catalyst and one buffer metal. In our group, we already got a result of co-deposited catalyst and it really improved the reliability shown in Fig. 1-13 [1-62].

Uniformity is also a very important issue of CNT FE-BLU. There were a lot of ways to enhance uniformity, such as (a) growing CNTs on AAO[1.63], (b) plasma post-treatment[1.64], (c) elastomer or biasing printing[1.65][1.66]… and so on. Here we use the patterned pillar-like CNTs improving the uniformity of emitters. CNT pillar arrays have well-control density and morphology of CNTs, and, moreover, it has been reported that the screening-effect of CNTs can be effectively reduced by the density control of the pillars.

Therefore, not only the field emission characteristics can be enhanced from the compromise of screening-effect and emitter sites, but also the uniformity will slightly be enhanced from the decreasing of screening-effect. By using pillar-like CNTs as a light source for BLU, high brightness and excellent uniformity could be achieved by easy and cheap process.

In the growth reaction of CNTs, the diffusion of carbon in the catalyst metal has been believed to be the rate-determining step. The growth rate of CNTs can be described by an Arrhenius equation that the activation energy is the diffusion energy of carbon in the metal [1.67]. Plasma-enhance chemical vapor deposition (PECVD) is more suitable method for CNT synthesis because PECVD has the much lower activation energy compared to thermal CVD [1.68]. However, PECVD has some drawbacks, such like poor plasma uniformity, and hard to fabricate large panel display or large-size BLU.

For the reasons of cost down and fabrication of large-size BLU, we have utilized the

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thermal CVD (TCVD) in our study. It has been investigated that the nano-size catalyst could enhance CNTs growth at low temperature because nano-size catalyst particles are more active compared to bulk catalyst and the melting point of particles decrease as their sizes decrease [1.69][1.70]. The following Fig. 1-14 shows surface diffusion or surface pre-melting of nano-size particles takes place when the temperature is raised above 500ْ C whose temperature is close to 0.4Tm [1.70].

A multilayer catalyst, Co/Ti/Al, was successfully employed to synthesize CNTs at 550℃

and 500℃ by thermal CVD previously. Following this result, we could fabricate CNTs on the glass coated with electrode or indium tin oxide (ITO), for large area application.

Figure 1-14 In-situ TEM images recorded from a region of capped Pt nano-crystals at various specimen temperatures. Surface diffusion or surface pre-melting of nano-size particles takes place when temperature is raised about 0.4Tm.

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The normal triode gate structure is shown in Fig. 1-15[1.71][1.72].The phosphors provide high efficiency into light by bombardments of electrons at enough high voltages of the anode plate. For field emission display (FED), there is a problem of triode gate structure, i.e. cross-talk noise, which is due to the electron beam spreading caused by gate electrode.

Cross-talk noise is a drawback for FED, but it has merits for FE-BLU since the electron spreading would result in large area of beam overlapping, which could improve the uniformity of photo-luminescent images on anode plates. Therefore, uniformity could be enhanced just because of enlargement of cross-talk noise.

(a)

(b)

Figure 1-15 Examples of triode gate structure

(a) planar gate [1.71] (b) mesh gate. [1.72].

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