Fabrication of triode-type field emission displays with high-density carbon-nanotube emitter arrays

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1 Physica B 323 (2002) Fabrication of triode-type field emission displays with high-density carbon-nanotube emitter arrays J.E. Jung a,c,f, Y.W. Jin a,c, J.H. Choi a, Y.J. Park a,d, T.Y. Ko b, D.S. Chung a, J.W. Kim a, J.E. Jang a, S.N. Cha a, W.K. Yi a, S.H. Cho b, M.J. Yoon b, C.G. Lee b, J.H. You b, N.S. Lee e, J.B. Yoo c, J.M. Kim a,b,f, * a Samsung Advanced Institute of Technology, FED Project, P.O. Box 111, Suwon , South Korea b Samsung SDI, 575 Shin-Dong, Paldal-Gu, Suwon , South Korea c Department of Materials Engineering, Sungkyunkwan University, 300, Chunchun-Dong, Jangan-Gu, Suwon , South Korea d Department of Vacuum Science and Technology,Sungkyunkwan University, 300, Chunchun-Dong, Jangan-Gu, Suwon , South Korea e Department of Advanced Materials Engineering, Sejong University, 98 Gunja-Dong, Gwangjin-Gu, Seoul , South Korea f The National Creative Research Initiatives Center for Electron Emission Source, South Korea Abstract Triode-type carbon-nanotube field emission displays (CNT-FEDs) with the diagonal were fabricated using screen printing as well as thin film processing. We developed a photosensitive paste including single walled CNTs for screen printing. CNT emitter dots with the diameter of 5 mm were defined inside gate holes with a diameter of 10 mm by screen printingthe CNT paste and a subsequent backside photolithography. CNTs were exposed to erect on the surface by development, servingas electron emitters. A thick Ni wall structure was electroplated just above gate electrodes to suppress diode emission caused by an anode potential and to improve focusing of electron beams. An onset gate voltage for emission was as low as V. Under an optimum design of a FED panel, R, G, B colors were satisfactorily separated. Our vacuum-sealed CNT-FED demonstrated full color moving images with high brightness and good color purity at a video-speed operation. r 2002 Elsevier Science B.V. All rights reserved. PACS: Kt; Fd Keywords: Carbon nanotubes; Field emission display; Triode-type structure; Screen printing; Backside lithography; Diode emission 1. Introduction Carbon nanotubes (CNTs) have remarkable mechanical, electronic, and magnetic properties *Correspondingauthor. SamsungAdvanced Institute of Technology, FED Project, P.O. Box 111, Suwon , South Korea. Tel.: ; fax: address: jongkim@sait.samsung.co.kr (J.M. Kim). that can be tailored, in principle, by varying diameters and chirality of CNTs and the number of concentric shells [1]. CNTs with extremely small radii of curvature, hollowness, and chemical and mechanical strengths have provided a vast range of applications of CNTs such as electron field emitters [2], room-temperature transistors [3], vehicles for hydrogen storage [4], etc. In particular, the high aspect ratio and high chemical stability of /02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S (02)

2 72 J.E. Jung et al. / Physica B 323 (2002) CNTs have enabled them to be used as promising electron emitters. There have been tremendous efforts in developingfield emission displays using CNTs. De Heer et al. [2] has shown the feasibility of CNTs as field emitters usinga ceramic filter drawn through a solution containing CNTs. Bonard et al. [5] has demonstrated electron field emission from CNTs by drawinga CNT suspension through a 0.2 mm pore ceramic filter and transferringfilms to copper or brass covered with a Teflon film. Uemura et al. [6,7] fabricated a cathode ray tube lighting element using bundles of multi-walled CNTs that was operated at an anode voltage of 10 kv. Wang et al. [8] has developed a row-column matrix addressable CNT emitter display usingphotosensitive glass. Our previous studies have not only demonstrated 5 00,9 00, and fully vacuum-sealed CNT-FEDs with a diode structure [9 11], but also 5 00 and triode-type CNT-FEDs [11 13]. Amongseveral triode-type emitter structures [11 13] that we have developed, includingnormal gate, undergate, and remote gate structures, the normal-gate triode structure CNT-FEDs seems to be one of the most promisingfor an application of CNTs to field emitters in terms of operation voltages as well as FED device performance [12,13]. This structure is similar to the typical Spindt-type FED, but larger gate hole diameters due to excellent field emission characteristics of CNTs. On the field emitter array (FEA) templates fabricated by the same processes as those of the Spindt-type FEAs, CNT emitter dots were formed inside gate holes by screen printing of a photosensitive paste containingsingle-walled CNTs and subsequent self-aligned backside exposure of an ultra-violet (UV) light [12]. Our previous report realized the CNT emitter dots with the diameter of 20 mm inside the gate holes with the diameter of 30 mm [12,13]. For this emitter structure, an onset gate bias for electron emission was about 60 V. In this paper, thereafter, we reduced the CNT dot and gate hole diameters down to 5 and 10 mm, respectively. The size reduction of the gate holes as well as the emitter dots enables us to increase the density of emitter dots as many as approximately 3 times, which undoubtedly contributes to improvingemission uniformity over a large area, lowering operating gate voltages, increasing emission current density at the same level of gate voltages, and enhancingdisplay resolutions. For CNT-FEAs with extremely low onset electric fields, diode emission from CNT emitters can occur by anode voltages. A thick Ni wall structure (NWS) physically and electrically in contact with gate electrodes was engaged to suppress the diode emission and to enhance electron beam focusing by electroplating[13]. Our fully vacuum-sealed CNT-FEDs demonstrated full-color moving images with high brightness and good color separation operated at a video speed. 2. Experimental A schematic cross section of triode-type CNT field emitters is shown in Fig. 1. In this structure, a 5 mm diameter dot of CNT emitters is defined inside a 10 mm diameter electron extraction gate hole. The 1.5 mm thick SiO 2, deposited using plasma enhanced chemical vapor deposition (PECVD), was used as an insulator between the indium tin oxide (ITO) cathode and 3500 ( A thick Ni gate electrodes. The 5 mm diameter dot of CNT emitters was produced usingthe exposure of a UV light from the backside of the substrate. An amorphous Si (a-si) layer with a thickness of 2500 ( A, grown by PECVD, was used as an embedded photo-lithographic mask for backside photolithography of CNT dots, because the doped a-si layer did not transmit a UV light. The Fig. 1. A schematic cross section of a triode-type CNT field emitter with the 20 mm thick NWS.

3 J.E. Jung et al. / Physica B 323 (2002) amorphous silicon also played a role of a resistive ballast layer in the electron emission. For such a cathode template prepared, the photosensitive CNT paste was screen-printed over a whole area. The CNT emitter dot was formed inside a gate hole by the self-aligned backside exposure of a UV light with the 310 nm main peak and with the energy density of 2000 mj/cm 2 through the holes of the a-si mask layer underneath the SiO 2 insulator, subsequent dryingat 901C, and development with a distilled water solution containing 0.4% Na 2 CO 3. Then, the CNT dots were fired at 4501C inann 2 ambient. Single-walled CNT powders prepared by arc discharge were mixed with frit and other inorganic and organic vehicles to be a printable paste. While those materials were well mixed together using ball mill and/or 3-roll mill, appropriate organic vehicles or solvents were added to control viscosity that played a very important role in screen printing processes. The CNT paste was then finally filtered through a high mesh to remove large particles. The prepared paste consisted of CNTs for electron emission, frits for concrete cohesion between several solid particles in the paste, organic vehicles (polyvinyl alcohol and photo-initiators) for good printingquality as well as photosensitivity, fillers for better printingshape, etc. The CNT paste affects critically the emission capability of CNT emitters and so the display characteristics of a CNT-FED device. Selections of ingredients and their quantities in the paste, therefore, must be the most important key factors. Followingthe fabrication of CNT-FEAs, the NWS with a thickness of 20 mm a width of 10 mm was constructed in contact with the Ni gate electrode by electroplating, as shown in Fig. 1. Photoresist (PR) with a thickness of 20 mm was patterned to protect the CNT emitter dots from electroplatingas well as to define the regions where Ni would be deposited. The Ni electroplatingwas carried out in a dippingbath under the voltage of 3 V and the current density of 3A/cm 2 between a bath electrode and the gate electrodes of the CNT- FEAs, which corresponded to a deposition rate of 0.6 mm/min. Ni was deposited through the PR patterned areas. PR strippingproduced the final CNT-FEA structure with the NWS. The deposition time was varied to obtain different thickness of the NWS layer. The fabrication processes of the CNT-FEA template and electroplated NWS were described elsewhere in detail [12,13]. For the anode plate, the low-voltage phosphors such as Y 2 O 2 S:Eu, ZnS:Cu,Al, and ZnS:Ag,Cl for red, green, and blue colors, respectively, were screened as thick as mm on ITO-coated soda-lime glass by a slurry method [14]. The cathode and anode plates were assembled with 500 mm thick spacers in-between and were then sealed at 4151C usingfrit glass in an ambient of argon and hydrogen gases. The panel was evacuated down to the pressure of Torr at 3301C. Nonevaporable getters of a Ti Zr V Fe alloy were activated duringthe final heat-exhaustingprocess, finally leadingto the complete fabrication of fully sealed CNT-FEDs. Our 5 00 diagonal (89 mm 77 mm) CNT-FED device with the resolution of 240 lines (data) 120 lines (scan) was video-run for full colors in a pulse width modulation mode with a matrix addressable, current-limited circuit. 3. Results and discussion Fig. 2 presents the triode-type CNT emitter array structure observed by optical and scanning electron microscopy. The 25 CNT dots comprise an emitter array, three of which again constitute a color subpixel. The CNT emitter dot is well defined inside the gate hole, revealing the emitter dot and gate hole diameters and thickness of 5, 10, and 0.25 mm, respectively. Screen-printingtechnologies to prepare thick films are usually limited in terms of line-and-space resolution and thickness. When usinga front side UV light exposure of the printed photosensitive paste, the minimum feature size of a dot pattern after firingis about mm in line and space and 3 4 mm in thickness. For the backside exposure through the embedded a-si photolithographic mask in contact with the CNT paste, we have achieved the dot dimension down to the diameter of 5 mm and thickness of 0.25 mm. The smaller and thinner dimension of CNT emitter dots by the backside lithography seems to be attributed to cross-linkingof a photosensitive

4 74 J.E. Jung et al. / Physica B 323 (2002) Fig. 2. Triode-type CNT emitter array structure: (a) low-magnification and (b) high-magnification images, observed by optical electron microscopy, (c) tilted image around a CNT dot and (d) erect CNT emitters on the surface, observed by scanning electron microscopy. substance of the CNT paste from the bottom and less spreadingof a UV light due to a shorter time of exposure. The backside exposure was discussed elsewhere in detail [12]. In Fig. 2(d), CNT emitters rooted to the dot erect on the surface as high as nm. Only CNTs revealed on the surface can play a role as emitters. The backside exposure easily makes CNTs stand out of the surface since the photosensitive CNT paste is removed from the top by the development due to less UV light illumination at the heights further from the bottom. Duringthe operation of such a triode structure, there are two problems to be solved to realize normally operated FED devices. One is to suppress a diode emission from the CNT emitters which is induced by anode voltages. The electron emission from emitters has to be controlled only by gate voltages, instead CNT emitters with extremely low onset electric fields can emit electrons by anode voltages exceeding a certain level. If high anode voltages beyond this level are required for sufficient luminance from phosphors, such a diode emission phenomenon cannot be avoided. The other problem is to focus electron beams onto their designated phosphor subpixels. For this structure with gate hole diameters much larger than those of the Spindt-type FEAs,

5 J.E. Jung et al. / Physica B 323 (2002) electrons launched from CNTs would spread in wide angles. To suppress the diode emission as well as to focus electron beams duringthe triode operation of the FEAs, electroplated NWS was applied to CNT-FEAs. The NWS, electrically as well as physically in contact with the gate electrode, is expected to modify electrical potential distributions around CNT emitter dots towards decreasingthe electric field strengths of anode voltages as well as reducing the divergence of electron beams. Fig. 3 shows a typical structure of the CNT-FEAs with the 20 mm high and 10 mm wide NWS, observed by scanningelectron microscopy. The NWS width increases with higher elevations due to an inclined wall profile of the PR pattern. The rough NWS surface is characteristic of electroplatinga thick Ni layer. Each set of five dots of CNT emitters is surrounded by the NWS and each body of NWS enclose five sets of (a) the CNT dots. A discrete distribution of the NWS is expected to relieve a stress which might be generated by the thick NWS layer. Such a design of the NWS was optimized by consideringphysical and electrical characteristics of a device as well as compatibility of fabrication processes. In our previous study [13], a role of the NWS was investigated in terms of diode emission and electron beam focusingby simulatingelectrical potential distributions and electron beam trajectories usingopera-3d (Vector Fields, Ltd.). The 25 mm thick NWS reduced the electric field strengths of anode voltages inside the gate hole by an order of magnitude compared to the FEAs without the NWS. The diode emission from the CNT dots caused by anode voltages was more effectively suppressed as the NWS thickness increased. In addition, the NWS modified the electrical potential distributions around the gate holes so that the electron beams were less spread out. Field emission characteristics of the triode-type CNT-FEAs were measured under a vacuum of Torr and in a spacingof 1.1 mm between anode and cathode plates. Fig. 4 presents anode currents as a function of gate voltages for anode biases of V. Measurements were carried out with a duty ratio of 1/120 and a frequency of 100 Hz. The onset gate electrode voltages for electron emission are V for an anode biases 50 µm (b) Current (ua) V 500V 600V 700V 800V 10 µm Fig. 3. Typical CNT-FEA structure with the 20 mm high and 10 mm wide NWS: (a) low magnification and (b) high magnification, observed by scanning electron microscopy Gate Voltage (V) Fig. 4. Anode currents as a function of gate voltages for anode biases of V, which were measured with a duty ratio of 1/120 and a frequency of 100 Hz.

6 76 J.E. Jung et al. / Physica B 323 (2002) of V. In the previous report [12], on the other hand, the FEAs with the CNT dot and gate hole diameters of 20 and 30 mm, respectively, exhibited the onset voltage of V for the anode biases of kv. In comparinganode currents measured at the same gate voltage of 70 V, the FEAs with the smaller diameters of CNT dots and gate holes produced as high currents as 20 times. Consideringa larger transmittance of emitted electrons to the anode at a higher anode voltage, the difference of emission currents would become larger if they were measured at the same level of anode biases. The reduction of the gate hole diameters and resultantly the higher density of emitter dots seems to considerably contribute to the decrease of onset gate voltages and the increase of emission currents. Magnified pixel images observed in a vacuumsealed CNT-FED panel with the 20 mm thick NWS are shown in Fig. 5. Very uniformly emitting pixels of red, green and blue colors are seen without cross talk between subpixels. This images were taken at the gate and anode electrode biases of 70 and 1 kv, respectively, with a duty ratio of 1/120 and a frequency of 60 Hz. By adoptingthe NWS, the red, green, and blue colors are successfully separated to achieve satisfactory color purity in runningmoving pictures in our device. Fig. 6 gives a video-running, full-color movingimage of our 5 00 diagonal CNT- FED with the NWS. This image was observed under the same condition as that of Fig. 5. The CNT-FED device was electrically operated by current-limited, matrix-addressable circuitry with a 256 (8 bits) gray scale for each color. 4. Conclusions Triode-type CNT-FEDs were fabricated usingscreen printingand self-aligned backside (a) (b) (c) (d) Fig. 5. Magnified color pixel images observed in a vacuum-sealed CNT-FED panel with the 20 mm thick NWS: (a) white, (b) red, (c) green, and (d) blue colors. Images were taken at the gate and anode electrode biases of 70 V and 1 kv, respectively, with a duty ratio of 1/120 and a frequency of 60 Hz.

7 J.E. Jung et al. / Physica B 323 (2002) operation with satisfied color separation as well as with the suppression of diode emission. References Fig. 6. Video-running, full-color moving image of the 5 00 diagonal CNT-FED with the NWS, which were taken at the gate and anode electrode biases of 70 V and 1 kv, respectively, with a duty ratio of 1/120 and a frequency of 60 Hz. photolithography of the photosensitive CNT paste. CNT emitter dots with the 5 mm diameter were well formed inside the gate holes with the diameter of 10 mm. The 20 mm thick NWS was electroplated above gate electrodes to suppress diode emission induced by strongelectric field strengths of an anode potential and to focus electron beams to their destined color subpixels. Our CNT-FEAs with the NWS revealed the onset gate voltages as low as V. Our fully sealed CNT-FEDs with the NWS demonstrated fullcolor movingimages under the video speed [1] T.W. Ebbesen, Carbon Nanotubes, CRC Press, New York, 1997 (Chapter 9). [2] W.A. de Heer, A. Chatelain, D. Ugarte, Science 270 (1995) [3] S.J. Tans, R.M. Verschueren, C. Dekker, Nature 393 (1998) 49. [4] D.S. Bethune, C.H. Kiang, M.S. de Vries, G. Gorman, R. Savoy, J. Vazquez, R. Beyers, Nature 363 (1993) 605. [5] J.M. Bonard, J.P. Salvetat, T. St.ockli, W.A. de Heer, L. Forr!o, A. Ch#aelain, Appl. Phys. Lett. 73 (1998) 918. [6] S. Uemura, T. Nagasako, J. Yotani, T. Shimojo, Y. Saito, Technical Digest of SID 98, Society for Information Display, Anaheim, CA, USA, 1998, p [7] Y. Saito, S. Uemura, Carbon 38 (2000) 169. [8] Q.H. Wang, M. Tan, R.P.H. Chang, Appl. Phys. Lett. 78 (2001) [9] 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, J.M. Kim, Appl. Phys. Lett. 75 (1999) [10] W.B. Choi, Y.H. Lee, N.S. Lee, J.H. Kang, S.H. Park, H.Y. Kim, D.S. Chung, S.M. Lee, J.M. Kim, Jpn. J. Appl. Phys. 39 (2000) [11] N.S. Lee, D.S. Chung, J.H. Kang, H.Y. Kim, S.H. Park, Y.W. Jin, Y.S. Choi, I.T. Han, N.S. Park, M.J. Yun, J.E. Jung, C.J. Lee, J.H. You, S.H. Jo, C.G. Lee, J.M. Kim, Jpn. J. Appl. Phys. 39 (2000) [12] D.S. Chung, S.H. Park, T.Y. Ko, S.H. Cho, H.W. Lee, Y.W. Jin, Y.J. Park, J.W. Kim, S.N. Cha, J.E. Jang, B.G. Song, M.J. Yoon, J.S. Lee, S.Y. Whang, J.H. Yoo, N.S. Lee, J.E. Jung, J.M. Kim, Appl. Phys. Lett., 2001, submitted for publication. [13] J.E. Jung, J.H. Choi, Y.J. Park, H.W. Lee, Y.W. Jin, D.S. Chung, S.H. Park, J.E. Jang, S.Y. Hwang, T.Y. Ko, Y.S. Choi, S.H. Cho, C.G. Lee, J.H. You, N.S. Lee, J.B. Yoo, J.M. Kim, J. Vac. Sci. Technol. B, 2001, submitted for publication. [14] L. Ozawa, Application of Cathode Luminescence to Display Devices, Kodansha, Tokyo, Japan, 1994, p. 32.