Chapter 1: Introductions
1.5 Thesis Organizations
The overview of vacuum microelectronics, field emission display and basic principles of field emission theory was described in chapter 1.
The experimental procedures were revealed in chapter 2. First, we utilize three types of different inter-pillar distance patterns, such as 80, 150 and 250 μm then use them on silicon substrate to compare their morphology and discussion their uniformity. We discussed the effect of R/H ratios of CNTs pillars. In the part, we investigated and found the relations of R/H ratios (inter-pillar spacing (R)/pillar height (H) ratios) to obtain the optimization and perimeter of the field emission characteristics. We also post-treat CNT pillar array by plasma bombardment and find the optimal condition of plasma. Finally, try to enhance emission electron from triode-pillar array by gate voltage.
Results and discussion were summarized in chapter 3. then, we accomplished many important results including, (1) SEM images, (2) TEM images, (3) AFM images, (4) Raman analysis, (5) EDS analysis, and (7) Field Emission Measurement.
Finally, the summaries and conclusions are provided in chapter 4.
Table 1-1
Comparison between vacuum microelectronics and solid-state electronics.
Items
Structure solid/solid interface solid/vacuum interface
Electron Transport in solid in vacuum
Electron Velocity 3107 (cm/sec) 31010 (cm/sec) Flicker Noise due to interface due to emission Thermal & Short Noise comparable comparable
Electron Energy < 0.3 eV a few to 1000 eV
Cut-off Frequency < 20 GHz (Si) &
100 GHz (GaAs)
< 100 – 1000 GHz
Power small – medium medium – large
Radiation Hardness poor excellent
Temperature Effect -30 – 50 C < 500 C
Fabrication & Materials well established (Si) &
fairly well (GaAs)
not well established
Figure 1-1 The SEM micrograph of (a) Spindt type triodes array, (b) Spindt type
field emission triode, (c) Emitting way of spindt type triode. [1.5]
(a)
(b)
Figure 1-3 The schematic diagram of (a) conventional CRT and (b) comparison between CRT and FED. [1.24].
(a) (b)
(c) (d)
(e) (f)
Figure 1-4 The full color FED products: (a) Motorola 5.6” color FED based on Spindt-type , (b) Pixtech 5.6” color FED based on Spindt-type, (c) Futaba 7” color FED based on Spindt-type, (d) Sony/Candescent 13.2” color FED based on Spindt-type, (e) Samsung 32“ under-gate CNT-FED, and (f) Canon-Toshiba 36”
SED-TV.
(a)
(b)
Figure 1-5 (a) Si tip formed by isotropic etching and (b) Si tip field emission triodes array formed by CMP [1.28] [1.29]
(a)
t
(b)
(c)
Figure 1-6 (a) The structure of SED, (b) SEM image of SCE cathode array, and (c) A 36-inch prototype of surface conduction electron emitter display. [1.30]
[1.31]
(a)
(b)
Figure 1-7 High-resolution transmission electron microscopy images of (a) SWNTs, and (b) MWNTs. Every layer in the image (fringe) corresponds to the edges of each cylinder in the nanotube assembly [1.42].
(a)
(b)
Figure 1-8 Molecular models of SWNTs with (a) chiral vector (b) the categories of the configuration [1.45] [1.46].
(a)
(b)
(b)
Figure 1-9 (a) Schematic structure of the fully sealed 128 lines matrix-addressable CNT-FED. (b) Cross section SEM image of CNT cathode from Samsung’s FED.(c) A 4.5-inch FED from Samsung, the emitting image of fully sealed SWNT-FED at color mode with red, green, and blue phosphor columns. (d) A prototype of 5” CNT flat panel display by Samsung. [1.54]
(a)
(c) (d)
(b)
(c) (d)
Fig 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]
(a)
(b)
Fig 1.11 The cost of the overall (a) 17inch (b) 32inch TFT-LCD [1.57].
Chapter 2
Experimental Procedures
2.1 Introduction
Carbon nanotubes (CNTs) are one of the field-emission materials because of their high geometric aspect ratio, small tip radius of curvature, high electrical conductivity, high mechanical strength, and chemical stability[2.1].So far, the usefulness of electron sources using a CNT emitter as a cold cathode has been demonstrated for vacuum electronic devices such as field-emission displays[2.2][2.3], backlight sources[2.4], and x-ray tubes[2.5]. For such device applications, CNT emitters are required to possess a low driving voltage and durability for a long lifetime.In recent years, CNT emitters have been fabricated by two main methods: chemical vapor deposition (CVD)[2.6] and screen printing[2.4]. In both methods, it is important to control the length and the inter-tube distance of CNTs to reduce the screening effect in adjacent CNTs[2.7] shown in Fig.2.1.
It has been reported that field emission can effectively enhanced for aligned CNTs as field emitters when the ratio of the inter-tube distance to the height of each CNT is about 2[2.8], but, in our research, the ratio of the inter-tube distance to the height of each CNT is not approximately 2. However, the reduction of the field screening effect, and the optimal ratio of the inter-tube distance to the height which are components of an efficient field emitter, has not been sufficiently investigated.
Reports have stated that field emission is dependent on the direct parameters of CNTs such as the number of walls, the shape and structure of the tips, and indirect parameters such as surface treatment and CNT-alignment methods on a substrate. Recent works on CNT field emission are focused on several post-treatment methods and new growth method of CNTs to improve the uniformity and density of the electron emission site. Plasma surface treatment has been used as one of the post-treatment methods to improve the field emission properties of carbon-based materials. For example, hydrogen, oxygen, and argon plasma are all found to improve the field emission properties of carbon-based material films by changing the atomic configurations on the surface.
In our research, we used a method of transforming the CNT pillar arrays into a ring
edge shaped emitter array by plasma of O2 and Cl2 gases. The pillar arrays have been fabricated on a patterned silicon wafer by thermal chemical vapor deposition technique (T-CVD). The ring edge formed after the plasma treatment by high density plasma reactive ion etching system (HDP-RIE) shown in Fig.2.2 [2.9], of almost perfect shape, are found to be excellent field emitters. The improved emission current density is attributed to enhancement in the field at the tip of the emitter, reduction in screening effect, and increase in the aspect ratio.
Triode type CNT field emitters have some advantages such as field emission at a lower electric field, the uniformity and stability of field emission, easy adjustment, and high-quality screen, compared with diode type CNT emitters in display applications of CNTs. In order to fabricate the triode type CNT field emitters, it is desirable to selectively synthesize vertically aligned CNTs on substrates with patterned trench structure. Vertical alignment of CNTs on the substrate is crucial for the practical Fig.2-3 [2.10] including light source, reflector, light guide, diffuser, and brightness enhancement film (BEF). The light source can be an incandescent light bulb, light emitting diodes (LED), cold cathode fluorescent lamp (CCFL), hot cathode fluorescent lamp (HCFL). All the backlights employ a diffuser and a BEF. The diffuser posited between the light source and the display panel is used to scatter the light for display uniformity. The BEF is used to enhance display brightness. The cost structure of materials for TFT-LCDs is described in Fig. 1-11 [1.57].
If we success to solve the reliability and uniformity problems, CNT-BLU will replace the traditional backlight system of LCD, it will be ensure to decrease amount of cost.
2.2 Experimental Procedures
150μm and 250μm respectively in experiments (A) (B) (C).In the field emission measurement, n-type silicon (100) wafer was used as its multilayer catalysts could be sequentially deposited without breaking the vacuum environment and co-deposited uniformly.
2.2.2 CNTs Synthesis
In our researches, an atmospheric pressure thermal chemical vapor deposition (T-CVD) system consists of a 2-inch-diameter horizontal quartz tube, an electric heating system, reaction gas supply, and related mass flow controllers shown in Fig.2.4 is used for CNTs’ synthesis. In order to prevent the nanoparticles are wrapped up by carbon graphite quickly. We reduce carbon source (C2H4) from 135sccm to 60sccm. Therefore, nanoparticles obtain suitable carbon source flowing and vertically aligned CNTs with long length and high density are obtained.
First, Samples loaded into the quartz tube are heated to the predetermined temperatures (700℃) in nitrogen at 1000 sccm to avoid catalyst being reacted during steps of heating. Secondary, hydrogen at 50 sccm is fed into quartz tube about 5 minutes to reduce the catalyst metal to the metallic phase, and then transforming into
nano-particles. Third, CNTs are grown at 700℃ with the flow rate of hydrogen is 10 sccm, 1000 sccm for nitrogen and 60 sccm for ethylene [2.11]. Finally, samples are furnace-cooled to room temperature in nitrogen at 3000 sccm. The schematic of growth process is shown below in Fig 2-5.
2.2.3 Analysis
The morphologies of CNTs’ samples are characterized by scanning electron microscopy (SEM), the morphologies of pre-treatment catalyst are observed by atomic force microscopy (AFM). The finer internal structures of interface of CNTs and nano-sized catalytic materials were examined by high-resolution transmission electron microscopy (HRTEM), JEOL JEM-2000EX and X-ray energy dispersive spectroscopy (EDS) respectively.
Field emission characteristics of CNTs are measured with a parallel diode-type configuration in a high-vacuum chamber with the pressure of 5×10-6 torr. A glass substrate coated with indium tin oxide (ITO) and P22 phosphor (ZnS: Cu, Al) was used as the anode plate, and the distance between the cathode and the anode plate was set to be 150 μm. The emitting area was variable, which was determined by pillar-spacing and pattern-area.
Anode voltages is sweep-type from 0 V to 1000 V which are applied at intervals of 10 V by a source measure unit (Keithley 237) shown in Fig. 2-6 for the verification of field emission characteristics while the cathode was biased at 0 V.
2.3 Experiment Procedures
2.3.1 Experiment A: Optimum R/H Ratio of Different Inter-Pillar Spacing
We investigated field emission properties from a pillar arrays of aligned carbon nanotube (CNT) bundles, which are fabricated on a Si substrate by thermal chemical
vapor deposition (T-CVD). To explore the influence of the pillar arrays’ arrangement on its field emission, the ratio of inter-pillar distance (R) to pillar height (H), R/H shown in Fig.2-7[2.12], was investigated by changing H while maintaining R at 80 μm, 150 μm and 250 μm, respectively.
A circle arrays with 50 μm in diameter with three different inter-distances (80μm, 150μm and 250μm, respectively) using Co-Ti (40 Å )/Ti(10 Å ) /Al (100 Å ) as multilayer catalyst shown in Fig.2.8 are made on a Si substrate (1cm×1cm) by photolithography and magnetron sputtering. To form catalyst nanoparticles, the catalyst was pre-treated at 700
°C for 5 min in hydrogen at 50sccm. Subsequently, thermal CVD was carried out at a growth temperature of 700 °C maintained by flowing 60sccm ethylene (C2H4) gas diluted with nitrogen (N2) gas as carrier gas. The total experimental process of profile was shown schematically in Fig. 2-9.
Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) are used to characterize the synthesized pillar array of aligned CNT bundles. The field emission property is measured using a parallel diode type configuration at a pressure of
~10−6 torr. The gap between the anode plate and the CNT emitter was set at 150μm or 300μm.
2.3.2 Experiment B: Two-Step Growing Method for CNTs’ synthesis
In this section, circle patterns with 50μm in diameter using Co-Ti(40 Å )/Ti(10 Å ) /Al (100 Å ) multilayer catalyst and inter-pillar distance is 80μm shown in Fig.2.8-(a). is made on a Si substrate (1cm×1cm) by photolithography and magnetron sputtering. To form catalyst nanoparticles, the catalyst pattern was pre-treated at 700 °C for 5 min in hydrogen at 50sccm. Subsequently, thermal CVD was carried out at a growth temperature of 700 °C maintained by flowing 60sccm ethylene (C2H4) gas diluted with nitrogen (N2) gas as carrier gas and maintain 20mins for first step growth. Than flowed 1000sccm nitrogen (N2) to perch space, and pretreated again for holding on second step CNTs growth. The total experimental process of profile was shown schematically shown in Fig. 2-10.
2.3.3 Experiment C: Plasma Post-treatment for CNTs’ Pillar arrays
Aligned CNT pillar array samples are synthesized by thermal CVD (T-CVD) on the patterned silicon wafer. The patterns for circular of 50μm diameter are fabricated silicon substrate.
In this section, sample with pillar height about 30μm and inter-pillar distance about 80μm is post-treated by High Density Plasma Reactive Ion Etching System (HDP-RIE).
The plasma treatment of as grown CNT pillar array was carried out in a plasma of a mixture of O2 and Cl2 gases generated by microwave power. The flow rates and treating time of O2 and Cl2 were shown in Table 2-1. The operating temperature, pressure, and process duration in the plasma treatment are room temperature , 10 mtorr, and 2、3、5、
10 min, respectively. The structural and chemical modifications of CNTs arrays before and after the plasma treatment are examined by a Hitachi model S-4700I scanning electron microscope (SEM) and Jobin Yvon model HR800 micro-Raman spectrometer, EDS, and TEM respectively. The field emission characteristics of the sample before and after plasma treatment were measured in a high vacuum chamber with a parallel diode-type configuration at a base pressure of ~10-6 torr.
2.3.4 Experiment D: Triode Structure for CNT Field Emission
Schematically illustrates the fabrication procedures of triode structure for CNT field emission. Heavily doped N-type silicon wafers are used as the substrate. A 5000 –Å -thick silicon dioxide is thermally grown on the silicon substrate at 1050oC. A 2000 –Å -thick Poly-Si layer is then deposited on the thermal oxide by pressure chemical vapor deposition (LPCVD) using a pure SiH4 gas at 620oC. The Poly-Si layer was further doped by POCl3 at 950oC for 20 min. An array of 100×100 circle patterns with the diameter of 6μm and inter-circle distance of 9μm are defined by photolithography and etching by Die-electric Material RIE 200L shown in Fig 2-12[2.13]. Then, the Poly-silicon layer is etched laterally by Die-electric Material RIE 200L to create a gap between the Poly-silicon gate and the CNTs shown in Fig 2-13. A catalytic layer of 50 Å cobalt-titanium in thickness is deposited on the 100 Å aluminum layer on the sample by sputtering. For deposition of the catalytic layer, the base and operating pressure of the sputter chamber are kept below 2×10-6 and 7.6×10-3 torr, respectively. The input RF power is 60W and the deposition rate was 0.1 Å /S for 100 Å aluminum layer. Then, the
input RF power for cobalt-titanium co-sputtering was kept 70W and 100W, respectively for 40 Å catalyst layer. Utilizing the original photoresist as a mask and lift-off technique, the circle pattern is transferred to the silicon substrate. Finally, CNTs are grown selectively on the iron layer by thermal chemical vapor deposition (T-CVD) system shown in Fig 2-12. The growth morphology of CNTs was observed by scanning electron microscopy (SEM).
Field emission properties of triode structure are measured in a high-vacuum environment with a base pressure of 1.0×10-6 torr. A glass plate coated with indium-tin-oxide (ITO) and phosphor is used as an anode and located 150μm above the sample surface. During the measurement, the device is in a common emitter configuration with the emitter grounded. The gate is applied with a voltage swept from 0 V to 80 V to obtain the current-voltage characteristics of the CNT triodes. The anode is applied with the constant voltage 800 V and the cathode maintain grounded, configuration shown in Fig 2-14.[2.14]
Fig.2.1 Simulation of the equipotential lines of the electrostatic field for tubes of different distances between tubes.
Fig.2.2 High Density Plasma Reactive Ion Etching System, HDP-RIE.[2.9]
Fig.2.3 schematic of a typical backlight unit.
Fig. 2-4 (a) Schematic picture and (b) photograph of thermal CVD.
Fig. 2-5 Process of CNTs synthesis (an example of CNTs growing 30min at 700℃.
Fig. 2-6 High vacuum measurement system.
Fig.2-7(a) SEM image of pillars of aligned CNT bundles grown by thermal CVD. (b) Cross-sectional SEM image of the pillar, showing that it is aligned perpendicular to the substrate surface and consists of high density CNTs[2.12]
50μm 80μm
1cm
1cm
(a)
50μm 150μm
1cm
1cm
(b)
50μm 250μm
1cm
1cm
(c)
Fig.2.8 Masks design: (a)-(c) show array of three different inter-distance for 80 μm、150 μm and 250 μm defined in 1 cm×1 cm area, respectively.
Photoresistance Silicon substrate
Mask(pillar-like pattern)
PR development
Spatter catalyst
Lift off
Exposure
Pretreatment
CNTs growth (e)
(f)
(g)
(h)
(j)
Fig.2.9 Fabrication flow diagrams (a) ~ (j). (f) Co-Ti / Al (2nm-3nm/10nm) catalyst by sputtering system, (h) pretreatment with H2 (50 sccm), and (j) CNTs
(1)
Fig.2-10 (1) Process of two steps growing for CNTs’ synthesis.(2) Fabrication flow diagrams (a) ~ (e). (d) pretreatment in H2 at 50 sccm again , and (e)
second step CNTs growth method under C2H4 atmosphere.
Silicon substrate
PR development
Spatter catalyst
Lift off by acetone
CNTs growth
CNTs treated after high density plasma (a)
(b)
(c)
(d)
(e)
(f)
Fig.2.11 Fabrication procedures for CNTs treated by plasma etching.
Table 2-1 Sample1 and sample2 represent the post-treatment process in oxygen and chlorine plasma. (R.T=room temperature)
Gas flow rate ICP power BIAS power O2 Cl2 Time Substrate temperature Pressure (W) (W) (sccm) (sccm) (min) (oC) (mtorr) Sample1 600 20 40 0 2 R.T 10 600 20 40 0 10 R.T 10 Sample2 600 20 30 10 2 R.T 10 600 20 30 10 3 R.T 10 600 20 30 10 5 R.T 10 600 20 30 10 10 R.T 10
Photoresist
Carbon Nanotubes N+ Poly-Si Gate
Thermal Oxide
Si(100)
Photoresist Mask
Catalytic Metal Layer (a)
(b)
(c)
(d)
(e)
(f)
(g)
Fig 2-12 Schematic representation of fabrication procedures of triod structure.
Fig 2-13The under etching condition of N-type Poly-silicon by Die-electric Material RIE 200L[2.13]
Fig 2-14 The aligned nanotubes in the bunch have variable heights and may protrude through the gate opening, as shown in the figure, or lie beneath the gate electrode plane.[2.14]
Va＞ Vg
Vg＞ 0
VC =0 Anode
Gate
Cathode
Chapter 3:
Results and Discussion
3.1 Analysis of Catalyst
3.1 Analysis of Catalyst
Prior to CNTs’ growing, the choose of catalyst is very important. According to our group’s researches, there are many advantages of CNT field emission characteristics by using novel co-deposition of catalyst. The most obvious characteristic is substantial increment of reliability.
In the Fig 3.1.7, better uniform roughness and nanoparticle diameter are obtained in cobalt-titanium co-deposition case.[3.1] We have speculated that this phenomenon is caused by decreasing the diameter of nano-sized particles, therefore, finer particle provide higher activity and lower melting temperature. The AFM top view is the easiest method for making a comparison and it will clearly show the particle size and roughness, as shown in Fig 3.1.7.
We compare catalysts of tri-components with catalysts of bi-components, with or without aluminum between electrode and other catalyst layers. Subsequently, we can easily find out the differences. The fluctuation in catalyst surface shows that simple and repeated nano-sized particle when pretreatment of bi-components catalyst without 100 Å Al buffer film, on the other hand, the fluctuation in catalyst surface is complex and additional curvature under those nano-sized particles when using the tri-components catalyst with 100 Å Al layer. As this result, we knew that Al plays a role of providing an additional curvature for surface of catalyst and quite increasing roughness mean square (RMS). The advantage of increasing RMS is raising the density of catalyst particle on the same top view area as shown in Fig 3.1.2, and then we can obtain better density of CNTs’
pillar after the growing step.
Furthermore, we focus the advantages of titanium (Ti) element in CNTs’ growth. To
verify the effect of the Ti layer, because Co and Ti surface energy are very nearly [3.2], so that cobalt film turns into nanoparticles with uniform diameter during pre-treatment process. This phenomenon is shown in Fig 3.1.3. Adhesion is evidently improved is also found in our experiment. As for CNT films on Ti, in the high temperature due to the formation of conductive TiC , a barrier on the CNT–Ti junction is removed (as exhibited in Fig 3.1.5). Electrons can pass through this junction without obstacle. So, during the whole field emission, electrons need to overcome CNT-vacuum barrier only, as shown in Fig 3.1.6. Therefore, a very small voltage will result a considerable electron emission. So, titanium (Ti) layer can improve uniformity and strengthen adherence between CNTs and substrate for CNTs’ growth [3.3]. The evident is shown in Fig 3.1.4.
3.2 Finding Optimum R/H Ratio of Different Pillar Spacing
3.2.1 Effect of carbon source flow rates on the morphology of CNTs’
growth
It is indispensable to reduce the turn on field and threshold field to achieve practically applicable field electron emitters that operate at lower power consumption. It has been reported that field emission can effectively enhanced for aligned CNTs as field emitters when the ratio of distance between neighboring nanotubes to the height of each individual CNT is about 2 [3.4]. However, the optimum rule (R/H=2) for CNTs as electron field emitters has not been realized.
In our researches, different R/H ratios have been designed to study the relation between R/H ratios and the field emission characteristics, and it is essential to grow longer aligned CNTs’ pillar with larger inter- pillar distance to meet the R/H ratios we need. However, growth of longer CNTs beyond 100μm is a little bit difficult. Therefore, it is very important to study the relation between the length of CNTs′ pillar and the growth condition including growth time, flow rate of carbon source, etc.
The length of the aligned CNTs pillar as a function of growth time is shown in Fig 3.2.4(a). It is observed that there is a CNTs′ length limit of approximately to 130μm. In
this case of the aligned CNTs′ pillars grown on the aluminum substrate, the aligned CNTs can′t be lengthened even if prolongs the growth time beyond 90 min. When we increase the flow rate of C2H4 to 135sccm, its growth rate is about 2μ m/min faster than that at 60sccm at 700oC in growth process. But the length of CNTs′ pillar does not match our prediction. Fig 3.2.4 (b) shows the longest CNTs′ pillar is about 50μ m , even if prolongs
this case of the aligned CNTs′ pillars grown on the aluminum substrate, the aligned CNTs can′t be lengthened even if prolongs the growth time beyond 90 min. When we increase the flow rate of C2H4 to 135sccm, its growth rate is about 2μ m/min faster than that at 60sccm at 700oC in growth process. But the length of CNTs′ pillar does not match our prediction. Fig 3.2.4 (b) shows the longest CNTs′ pillar is about 50μ m , even if prolongs