CHAPTER 1 Introduction
2.3 Results and Discussions
2.3.2 The Effect of Pattern Dimension
Fig.2-18 shows the electric field distribution on densely packed CNTs. As the figure demonstrated, we found only the CNTs in the edge easily emitted electron because of the strong electric field. Different pattern dimension arranged in a 1mm2 square were used to increase the total emitting area. Fig.2-19 shows the 1000µm, 100µm and 10µm pattern size and its number in a square were 1, 25, 2500, respectively. The field emission characteristics were showed in Fig.2-20. The turn on field decreased from 0.9 V/µm to 0.48 V/µm. The center of packed CNTs were affected by weaker electric field distribution and the electron would be screened out. The large, medium and small patterns have 4000µm, 10000µm, 100000µm edge length, respectively. The increase of total edge length improve the field emission characteristics. It is the reason that nearly CNTs in the edge would be thought the efficient emitting area. In this method, we can lower the turn-on voltage without extra process like plasma post treatment. By proper configuration designs, the turn-on voltage and current density would be improved.
2.4 Conclusions
The modification of surface morphology of CNTs has been achieved by O2 + Ar plasma post treatments. The SEM micrographs revealed the surface distribution of CNTs after plasma post treatments. For the generated plasma power of 200W and 300W, parts of CNTs were shortened and the remaining CNTs protruded from the surface of CNTs films. When the generated plasma power increased to 400W, most of the CNTs were shortened and less protruding CNTs were observed. The field emission characteristics confirmed the improvement of field emission properties under suitable PPT conditions, the field emission current density increased to 2.38 mA/cm2 at the electric field of 0.8 V/µm and the turn-on electric field decreased from 0.9 V/µm untreated to 0.19V/µm for PPT conditions of generated power of 300Wand etching time of 60s . The experimental results reveal that improved emission properties can be achieved by optimizing the density and the length variation of CNTs under proper plasma treatment conditions.
The center of packed CNTs were affected by weaker electric field distribution and the electron would be screened out. The increase of total edge length improve the field emission characteristics. It is the reason that nearly CNTs in the edge would be thought the efficient emitting area. In this method, we can lower the turn-on voltage without extra process like plasma post treatment. By proper configuration designs, the turn-on voltage and current density would be improved.
Chapter 3
Fabrication and Characterization of Insulated Gate Structure Field Emission Device Based on Carbon Nanotubes
In this chapter, a insulated gate structure field emission device based on carbon nanotubes (CNTs) is proposed and the experimental results are reported. The distance between polysilicon gate and the CNTs cathode was determined by the wet etching process. Thus, the interelectrode gap is easily formed in good uniformity and reproducibility with dimensions below 1 micrometer. The CNTs were selectively grown
by microwave plasma enhanced chemical vapor deposition system (MPECVD).
The effect of gate-to-emitter gap and length of carbon nanotubes were controlled to investigate the relation between the field emission current and applied voltage .The turn–on voltage of the fabricated device with interelectrode gap of 1 um is 18 volt, and the emission current density is as high as 0.1 mA/cm2 at gate voltage 136 volt. The emission current fluctuation is about ± 5.5% for 1500sec.
3.1 Introduction
CNTs have attracted increasing attention owing to the promising applications in vacuum microelectronics. Several groups [3.1-3.5] have demonstrated the low turn on electric field properties and extremely large emission current of the CNTs field emission diodes. However, the driving voltage of the diode type field emission devices is still too
high. To lower the driving voltage is crucial for the applications of field emission devices such as field emission displays to cost down the driving circuits. Triode types of field emission devices employing arc-produced CNTs were therefore demonstrated to reduce the driving voltage. Lee, et al [3.6] have fabricated a gated CNT-FED by employing a metal mesh as the gate electrode and achieved the turn-on voltage of 100 V. Wang, el al [3.7] have demonstrated a triode type field emission display with a specific filling
method to reduce the operation voltage; the low turn-on voltage of about 25 volt was achieved and the emission current reaches 0.3 µA as the gate voltage was 50 volt. To increase the gate induced electric field by decreasing the gate aperture is essential to lower the gate voltage. However, reduction of the gate aperture is difficult using conventional arc-produced CNTs. In this work, CNTs field emission triodes were fabricated by IC fabrication process and selective growth of CNTs via MPCVD. The fabricated device accomplished the low turn-on voltage of 18 volt and the extremely high emission current of 5 µA as the gate voltage reaches 136 volt.
3.2 Experiment Procedures
The fabrication procedure of the carbon nanotubes insulated gate structure field emission device is shown schematically in Fig. 3-1. As shown in Fig. 3(a), initially, the 500-nm-thick thermal oxide was grown on a N-type Si(100) substrate at 1050℃. The 200-nm-thick poly-Si was then deposited on the thermal oxide by low pressure chemical vapor deposition(LPCVD) using pure SiH4 at 590℃. The poly Si was further doped with phosphorous using solid diffusion source(PH-1000N) at 950℃ for 30 minutes and then phosphorous was driven in at 950℃ for 30 minutes. Finally, a 200-nm-thick nitride layer was deposited on poly-Si. The sheet resistance of poly-Si was 50 Ω/□ by 4-point probe. A 1-μm-thick photoresist was spin coated, and patterned by photolithography. First, The nitride, poly-Si and SiO2 layers were dry etched by the ICP-RIE using Cl2, SF6, and O2
mixture or CHF3, Ar mixture. Then, the poly-Si layer was continuously etched by the wet etching solution of poly etcher. Employing the previously patterned photoresist layer as the shadow mask, a thin nickel layer (3.5nm) was deposited directly on the patterned Si substrate as the catalyst metal by electron beam evaporation system, and the catalyst layer was thus formed after the photoresist was removed by the lift-off method as depicted in Fig.3-1(d) and Fig. 3-1(e). Finally, carbon nanotubes were grown selectively on the nickel layers by microwave plasma enhanced chemical vapor deposition. CH4, N2
and H2 were used as the source gases and the flow rates were 20, 80 and 80 sccm, respectively. The microwave power was kept at 1.2kW and the chamber pressure was set at 35 Torr. The substrate temperature was estimated at about 500℃ and the deposition time was 2 minutes.
The growth morphology of CNTs was observed by scanning electron microscopy (SEM) and the field emission properties of CNTs triodes were characterized in a high-vacuum environment with a base pressure of 1.0×10-6 Torr. A schematic diagram of the experimental setup is shown in Fig. 2-6. A glass plate coated with indium-tin-oxide (ITO) and phosphor was used as an anode and positioned 500 µm above the carbon nanotubes. The anode current was measured as a function of the gate voltage and anode voltage using Keithley 237 high voltage units with an IEEE 488 interface controlled by a personal computer.
3.3 Results and Discussion
3.3.1 Comparison Electrical Characteristics between Conventional and Insulated Gate Structure Triode
The SEM micrograph of the conventional CNTs field emission triode is shown in Fig. 3-2. The CNTs were selectively grown on the catalyst metal within the cathode region. It can be seen that CNTs are easily grown over the gate height. Carbon nanotubes bended because of its superior length. CNTs touched the gate and cause the short circuit problem when gate is applied voltage due to the metallic conductivity. Fig.
3-2(b) shows short circuit electrical characteristics. Fig.3-3(a) shows the gate voltage versus the anode voltage. The gate can build a electric field to induce the electron emitting and can be a switch or modulate the gray-scale. In a conventional structure, gate
leakage current would increase as the gate voltage increase [Fig.3-3(b)]. Thus, the emitting electrons to anode plate would decrease at the same time. To solve the problem before, a insulated gate structure triode was demonstrated in Fig.3-4. We control the carbon nanotubes length about 1µm and use the wet etching process to etch the poly gate.
Fig.3-4(a) shows the top view of insulated gate structure and Fig.3-4(b) is the cross section of it. From the figure, the insulated layer avoids the touch between gate and emitters. It successfully solves the short circuit problem by depositing an insulated layer.
Fig. 3-5 indicates emission current-voltage characteristics of insulated gate structure CNTs triode. The emission currents (Ia) were measured as a function of gate voltage (Vg) sweep from 0 to 150. The voltage-current plot shows a good rectifying property. The turn-on voltage (Von) defined at which the anode current was 10µA/cm2. Low turn-on voltage was achieved at 18 V for the insulated gate structure triode with 1.2µm interelectrode. Fig.3-6 reveals the field emission current versus anode voltage under different gate bias. With the increase of gate voltage, the anode turn on voltage would decrease because the larger gate bias can build a greater electric field to induce the electrons. The gate operation voltage might be reduced by decreasing the gate-to-emitter gap and the CNTs length.
3.3.2 Gate Leakage in Conventional and Insulated Gate Structure Triode
In a conventional triode, gate leakage current would increase as the gate voltage
increase. It would cause the anode current decreasing [Fig.3-3]. A measurement of gate voltage and current was established at anode voltage 450 V and gate voltage swept from 0 to 100 V. The result was revealed in Fig.3-7. The gate leakage current almost remains constant and is not a function of gate voltage. Comparison with the conventional structure, it not only avoid the short circuit problem between gate and emitters but also reduce the gate leakage current to improve the anode current performance.
3.3.3 Field Emission Current Stability
A emission current stability test was performed on the insulated gate structure CNT triode. An average emission current (I0) of 2.64 µA was established at the anode voltage of 450 V, gate voltage of 30 V and cathode voltage grounded. The emission current reliability over a short term period of 1500 seconds is shown in Fig.3-8. No obvious degradation of emission current was observed and the fluctuation was about ±5.5%.
3.4 Summary and conclusions
Based on the selective growth of CNTs via the MPECVD, the insulated gate structure was fabricated. The distance between polysilicon gate and the CNTs emitter was determined by the wet etching process. Thus, the interelectrode gap is easily formed in good uniformity and reproducibility with dimensions below 1 µm. The turn–on voltage of the fabricated device with interelectrode gap of 1.2 µm is 18 volt, and the emission current density is 0.1 mA/cm2 at gate voltage 136 volt. The emission current fluctuation
is about ± 5.5% for 1500 seconds.
Chapter 4
Conclusions and Future Prospects
Conclusions
The modification of surface morphology of CNTs has been achieved by O2 + Ar plasma post treatments. The SEM micrographs revealed the surface distribution of CNTs after plasma post treatments. For the generated plasma power of 200W and 300W, parts of CNTs were shortened and the remaining CNTs protruded from the surface of CNTs films. When the generated plasma power increased to 400W, most of the CNTs were shortened and less protruding CNTs were observed. The field emission characteristics confirmed the improvement of field emission properties under suitable PPT conditions, the field emission current density increased to 2.38 mA/cm2 at the electric field of 0.8 V/µm and the turn-on electric field decreased from 0.9 V/µm untreated to 0.19V/µm for PPT conditions of generated power of 300Wand etching time of 60s . The experimental results reveal that improved emission properties can be achieved by optimizing the density and the length variation of CNTs under proper plasma treatment conditions.
The center of packed CNTs were affected by weaker electric field distribution and the electron would be screened out. The increase of total edge length improve the field emission characteristics. It is the reason that nearly CNTs in the edge would be thought the efficient emitting area. In this method, we can lower the turn-on voltage without extra process like plasma post treatment. By proper configuration designs, the turn-on voltage and current density would be improved.
Based on the selective growth of CNTs via the MPECVD, the insulated gate structure was fabricated. The distance between polysilicon gate and the CNTs emitter was determined by the wet etching process. Thus, the interelectrode gap is easily formed in good uniformity and reproducibility with dimensions below 1 µm. The turn–on voltage of the fabricated device with interelectrode gap of 1.2 µm is 18 volt, and the emission current density is 0.1 mA/cm2 at gate voltage 136 volt. The emission current fluctuation is about ± 5.5% for 1500 seconds.
Future Prospects
For the synthesize of carbon nanotubes for field emission devices, the further research topics are proposed as follows:
(1) Low temperature (below 450 oC) growth of CNTs.
(2) Pretreatment of the catalyst for reduced density growth of CNTs.
(3) Post treatment of CNTs such as rapid thermal treatment, plasma treatment or ion bombardment to reduce the density of CNTs.
(4) Surface coating with ultra-thin metals to enhance the field emission property of CNTs.
For the field emission property investigation of CNTs (1) The long-term reliability should be investigated.
(2) The field emission behavior in different ambient (e.g. different gas, different pressure
conditions) should be discussed.
For the CNT triodes, the further research topics are proposed as follows:
(1) The gate-to-emitter gap can be further reduced to lower the turn-on voltage.
(2) Optimal gate structure or insulated gate surface for the CNT triodes should be developed to reduce the gate current.
(3) The focus gate can be applied to the CNT triodes to improve the emission characteristics.
(4) To demonstrate a prototype of CNT FED.
Finally, for the applications of CNTs in vacuum microelectronics, the further research topics are proposed as follows:
(1) Fabrication of CNTs lateral field emission device for high frequency and high power circuit applications.
(2) Fabrication of vacuum sensors or gas sensors based on CNTs
References Chapter 1
[1.1] S. M. Sze, “Physics of semiconductor devices”, 2nd ed., John-Wiley & Sons pulisher, New York, p. 648, 1991.
[1.2] R. H. Fowler and L. W. Nordheim, “Electron emission in intense field,” Proc.
R. SOC. A229, p. 173, 1928.
[1.3] C. A. Spindt, I. Brodie, L. Humpnrey, and E. R. Westerberg, “Electrical properties of thin-film field emission cathodes with molybdenum cones,” J.
Appl. Phys., Vol. 47, p. 5248, 1976.
[1.4] R. Meyer, “Recent development on microtips display at LETI,” IVMC’91 Technical Digest, p. 6, 1991.
[1.5] N. E. McGruer and K. Warner, “Oxidation-sharpened gated field emitter array process,” IEEE Trans. Electron Devices, Vol. 38, No. 10, p. 488, 1991.
[1.6] S. E. Huq and L. Chen, “Fabrication of sub-10 nm silicon tips: a new approach,” J. Vac. Sci. & Technol. B, Vol. 13(6), p. 2718, 1995.
[1.7] D. W. Branston and D. Stephani, “Field emission from metal-coated Silicon tips,” IEEE Trans. Electron Devices, Vol. 38, No. 10, p. 2329, 1991.
[1.8] V. V. Zhirnov and E. I. Givargizov, “Field emission from silicon spikes with diamond coating,” J. Vac. Sci. & Technol. B, Vol. 13(2), p. 418, 1995.
[1.9] J. H. Jung and B. K. Ju, “Enhancement of electron emission efficiency and stability of molybdenum field emitter array by diamond-like carbon coating,” IEEE IEDM’96, p. 293, 1996.
[1.10] R. E. Burgess, H. Kroemer, and J. M. Honston, “Corrected value of Fowler-Norheim field emission function v(y) and s(y),” Phys. Rev., Vol. 1, No. 4, p. 515, May, 1953.
[1.11] R. B. Marcus, T. S. Ravi, T. Gmitter, H. H. Busta, J. T. Niccum, K. K. Chin, and D. Liu, “Atomically sharp silicon and metal field emitters,” IEEE Trans.
Electron Devices, Vol. 38, p. 2289, 1991.
[1.12] P. Vaudaine and R. Meyer, “Microtips fluorescent display,” IEEE IEDM’91, p.
197, 1991.
[1.13] C. Curtin, “The field emission display,” International Display Research Conference p. 12, 1991.
[1.14] C. A. Spindt, C. E. Holland, I. Brodie, J. B. Mooney, and E. R. Westerberg,
“Field-emitter array applied to vacuum fluorescent displays,” IEEE Trans.
Electron Devices, Vol. 36, No. 1, p. 225, 1989.
[1.15] David A. Cathey, “Field emission displays,” Information Display, p. 16, Oct., 1995.
[1.16] “Pixtech to produce color FEDs from November,” News reported in Nikkei
Electronics ASIA, p. 42, Nov., 1995.
[1.17] H. G. Kosmahl, “A wide-bandwidth high-gain small size distributed amplifier with field-emission triodes (FETRODE’s) for the 10 to 300 GHz frequency range,” IEEE Trans. Electron Devices, Vol. 36, No.11, p. 2715, 1989.
[1.18] P. M. Larry, E. A. Netteshiem, Y. Goren, C. A. Spindt, and A. Rosengreen,
“10 GHz turned amplifier based on the SRI thin film field emission cathode,” IEEE IEDM’88, p. 522, 1988.
[1.19] C. A. Spindt, C. E. Hollard, A. Rosengreen, and I. Brodie, “Field emitter array development for high frequency operation,” J. Vac. Sci. & Technol. B, Vol. 11, p. 486, Mar./Apr., 1993.
[1.20] C. A. Spindt, “Microfabricated field emission and field ionization sources,”
Surface Science, Vol. 266, p. 145, 1992.
[1.21] T. H. P. Chang, D. P. Kern, et al., ”A scanning tunneling microscope controlled field emission micro probe system,” J. Vac. Sci. & Technol. B, Vol. 9, p. 438, Mar./Apr., 1991.
[1.22] H. H. Busta, J. E. Pogemiller, and B. J. Zimmerman, “The field emission triode as a displacement/process sensor,” J. Micromech. Microeng., p. 45, 1993.
[1.23] H. C. Lee and R. S. Huang, “A novel field emission array pressure sensor,”
IEEE Transducers- International Solid-State Sensors and Actuators, p. 126, 1991.
[1.24] D. G. Fink and D. Christiansen, Electronic Engineering Handbook, Mcgraw-Hill, New York, 1989.
[1.25] H. Imura, S. Tsuida, M. Takahasi, A. Okamoto, H. Makishima, and S.
Miyano, “Electron gun design for traveling wave tubes (TWTs) using a field emitter array (FEA) cathode,” IEEE IEDM’97, p. 721, 1997.
[1.26] S. Itoh, T. Watanabe, T. Yamaura, and K. Yano, “A challenge to field emission displays,” in Proc. Asia Display, Oct. 1995, pp. 617–620.
[1.27] C. A. Spindt, I. Brodie, L. Humphrey, and E. R. Westerberg,“Physical properties of thin-film field emission cathodes with molybdenum cones,” J.
Appl. Phys., vol. 47, no. 12, pp. 5248–5263, 1976.
[1.28] R. Meyer, A. Ghis, P. Rambaud, and F. Muller, “Microtips fluores-cent display,” in Proc. Japan Display, Sept./Oct. 1986, pp. 512–515.
[1.29] S. Itoh AND M. Tanaka, “Current Status of Field-Emission Displays”, PROCEEDINGS OF THE IEEE, VOL. 90, NO. 4, APRIL 2002
[1.30] M. Ding, H. Kim, and A. I. Akinwande” Highly Uniform and Low Turn-On Voltage Si Field Emitter Arrays Fabricated Using Chemical Mechanical Polishing”, IEEE ELECTRON DEVICE LETTERS, VOL. 21, NO. 2,
FEBRUARY 2000
[1.31] X. Xu and G. R. Brandes, “A method for fabricating large-area, patterned, carbon nanotube field emitters,” Appl. Phys. Lett., Vol. 74, p. 2549, 1999.
[1.32] A. M. Rao, D. Jacques, and R. C. Haddon, “In situ-grown carbon nanotube arrays with excellent field emission characteristics,” Appl. Phys. Lett., Vol.
76, p. 3813, 2000.
[1.33] H. Murakami, M. Hirakawa, C. Tanaka, and H. Yamakawa, “Field emission from well-aligned, patterned, carbon nanotube emitters,” Appl. Phys. Lett., Vol. 76, p. 1176, 2000.
[1.34] W. B. Choi, D. S. Chung, J. H. Kang, H. Y. Kim, Y. W. Jin, I. T. Han, Y. H.
Lee, J. E. Jung, N. S. Lee, G. S. Park, and J. M. Kim, “Fully sealed, high-brightness carbon-nanotube field-emission display,” Appl. Phys. Lett., Vol. 75, p. 3129, 1999
.
Chapter 2
[2.1] S. Iijima, “Helical microtubules of graphitic carbon,” Nature, Vol. 354, p. 56, 1991.
[2.2] S. Saito, “Carbon nanotubes for next-generation electronics devices,” Science, Vol. 278, p. 77, 1997.
[2.3] S. J. Tans and C. Dekker, “Molecular transistors: potential modulations along carbon nanotubes,” Nature, Vol. 404, p. 834, 2000.
[2.4] Z. H. Yuan, H. Huang, H. Y. Dang, J. E. Cao, B. H. Hu, and S. S. Fan, “Field emission property of highly ordered monodispersed carbon nanotube arrays,”
Appl. Phys. Lett., Vol. 78, p. 3127, 2001.
[2.5] J. M. Lauerhaas, J. Y. Dai, A. A. Setlur, and R. P. H. Chang, “The effect of arc parameters on the growth of carbon nanotubes,” J. Mater. Res., Vol. 12, p.
1536, 1997.
[2.6] P. M. Ajayan, Ph. Redlich, and M. Ruhle, “Balance of graphite deposition and multi-shell carbon nanotube growth in the carbon arc-discharge,” J. Mater.
Res., Vol. 12, p. 244, 1997.
[2.7] A.G. Rinzler , J. Liu , H. Dai , P. Nikolaev , C. B. Huffman , F. J.
Rodriguez-Macias , P. J. Boul , A. H. Lu , D. T. Colbert , R. S. Lee , J. E.
Fischer , A. M. Rao , P. C. Eklund , and R. E. Smalley, “Large-scale
purification of single-wall carbon nanotubes: process, product, and characterization,” Appl. Phys. A, Vol. 67, p. 29, 1998.
[2.8] J. M. Mao, L. F. Sun, L. X. Qian, Z. W. Pan, B. H. Chang, W. Y. Zhou, G.
Wang, and S. S. Xie, “Growth of carbon nanotubes on cobalt disilicide precipitates by chemical vapor deposition,” Appl. Phys. Lett., Vol. 72, p.
3297, 1998.
[2.9] C. J. Lee, K. H. Son, J. Park, J. E. Yoo, Y. Huh, and J. Y. Lee, “Low temperature growth of vertically aligned carbon nanotubes by thermal chemical deposition,” Chem. Phys. Lett., Vol. 338, p. 113, 2001.
[2.10] Y. C. Choi, D. J. Bae, Y. H. Lee, B. S. Lee, G. S. Park, W. B. Choi, N. S. Lee, and J. M. Kim, “Growth of carbon nanotubes by microwave plasma-enhanced chemical vapor deposition at low temperature,” J. Vac. Sci.
& Technol. A, Vol. 18, No. 4, p. 1864, 2000.
[2.11] W. Lei, B. P. Wang, H. C. Yin, Y. X. Wu, and C. Z. Chang, “Influence of the fringe field and the field interaction on the emission performance of a diode emitter array,” Nuclear Ins. and Methods in Phys. Reaerch A, Vol. 451, p.
389, 2000.
[2.12] N. V. Egorov, and A. A. Almazov, “Optimization of multi-tip field emission electron source,” Vacuum, Vol. 52, p. 295, 1999.
[2.13] O. Groning, O. M. Kuttel, C. Emmenegger, P. Groning, and L. Schlapbach,
“Field emission properties of carbon nanotubes,” J. Vac. Sci. & Technol. B, Vol. 18, No. 2, p. 665, 2000.
[2.14] Y. Gao, J. Liu, M. shi, S. H. Elder, and J. W. Virden, “Dense Arrays of Well-Aligned Carbon Nanotubes Completely Filled With Single Crystalline Ttitanium Carbide Wires on Titanium Substrates”,Appl. Phys. Lett. 74, p.
3642, 1999.
Chapter 3
[3.1] P. G. Collins and A. Zettl, “A simple and robust electron beam source from carbon nanotubes” Appl. Phys. Lett. Vol. 69, p.1969, 1996
[3.1] P. G. Collins and A. Zettl, “A simple and robust electron beam source from carbon nanotubes” Appl. Phys. Lett. Vol. 69, p.1969, 1996