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Recent advances in large-screen Carbon Nanotube Field Emisson Displays By R.L. Fink, of Applied Nanotech, Inc.
Abstract Advances have been made in the development of carbon nanotube field emission displays. We fabricated a 25 inch diagonal, full color RGB, video display using standard screen printing techniques. The pixel pitch is compatible with an approximately 90 inch diagonal HDTV display and the fabrication processes are scalable to large area displays. Introduction The development of carbon nanotube field emission displays (CNT FED) is progressing. The target market for these displays is the large, 60 to 100 inch diagonal, television. For these displays to be cost competitive with existing technologies, inexpensive materials and processes must be used in their fabrication.
Several companies and research groups are pursuing CNT FEDs, although each is taking a different approach. Some companies are using photolithographic patterning techniques to make the gated emitter structure and then growing the CNT emitters directly on the glass using CVD techniques. For example, Motorola demonstrated a 4.6 inch diagonal, full-color display with pixel dimensions representative of a 42 inch diagonal 16:9 HDTV [1]. Using a similar approach, LETI demonstrated a 6 inch diagonal monochrome CNT FED in video mode with 320 x 240 pixels with a size of 350 ¼m. They achieved a luminous level of 800 Cd/m2 with a 50% aperture black matrix and pixel to pixel uniformity within 5% [2]. These direct CVD growth approaches will be difficult and expensive to scale to large area in high volume manufacturing since large sheets of glass will need to be heated very uniformly to high temperatures (600°C or higher). Thus, there is no manufacturing advantage over LCD TVs. Other companies are dispensing CNT emitters using various printing techniques. Samsung SDI has developed several approaches. One recent approach uses standard lithography techniques to form the emitter structures that have both the emitter extraction gate and focus gate electrodes and incorporating an a-Si resistive layer in the structure to help control uniformity [3]. The unique Samsung approach includes printing the CNT emitter using photosensitive paste and then using backside illumination to expose the paste to form the CNT emitter patterns. Thus, the features are self-aligned and the structure is easier to manufacture. Mitsubishi Electric Corp., as part of an effort funded by the Japanese Government, has developed an approach in which the CNT emitter material is printed early in the fabrication process and then a SiO-based polymer is printed and patterned to form the gate dielectric [4]. Various activation processes have been
developed to improve the emission performance of dispensed CNT films [5]. In previous efforts, Applied Nanotech, Inc. has demonstrated a 14 inch diagonal monochrome CNT FED [6] and a 14 inch diagonal color CNT FED. These display panels were made using inexpensive screen printing and frit sealing techniques. Here we present a 25 inch diagonal, full color CNT FED with a pixel pitch compatible with a 60 inch diagonal ATV. This is a direct scale - up of our 14 inch diagonal color display with a pixel pitch of 1.575mm and a color subpixel pitch of 0.525mm. This larger panel demonstrates the scalability of this technology using printing techniques and shows the potential of smaller capital investment in the manufacturing process in the future. Display Fabrication A 25 inch diagonal, full RGB color, CNT video display was fabricated and demonstrated. The methods used to produce the display were inexpensive, standard screen printing and common glass frit sealing techniques. The processes used are all scaleable to large area displays. The display is made from two primary components: the cathode substrate and the anode substrate. The two components are then assembled together and evacuated, resulting in the sealed display. The cathode substrate is sodalime glass. Conducting feedlines are printed onto the glass using commercial screen printing methods. Next, an insulating overlayer is printed on top of the feedlines, but leaving open windows on the conducting material for the emitting pixels. This insulator becomes a spacer for the extraction grid. CNT is dispensed into the open windows. ANI has developed several CNT-based inks and pastes for different dispensing techniques including printing.
Figure 1-a. Schematic of activation by particle blasting process.
Figure 1-b. Image of emission from CNT cathode with top half activated by tape process and bottom half activated by particle blasting process. Both halves are operated at the same electric field.
Printed CNT emitters generally require an activation step to improve their field emission quality. Typically, the activation process is a mechanical modification of the emitter surface. Watanabe, et al, [5] describes an electrical pulse technique and references various techniques developed by others. In order to be compatible with scalability to large area processes, we have developed bead-blast activation processes [7]. Basically, the activation process thins out the CNT emitters so that they do not electrically screen each other and frees up one end of the CNT emitter such that it is easily aligned by the electron extraction field (see Figure 1). A mechanical extraction grid is placed on top of the insulator. This grid is manufactured using standard CRT shadow mask techniques. The grid is attached using a special high temperature, low outgassing adhesive material that is applied to the grid/substrate using a screen printing method. The cathode processes are diagrammed in Figure 2.
Figure 2. Cathode printing process schematic. Post spacers were used in the display. These were attached to the cathode substrate before assembly with the anode
screen. The spacers are sodalime glass with a special surface treatment. The surface treatment is to prevent charging of the spacers and the subsequent high voltage arcing during display operation. Although there is improvement to the stability of the display over the use of non-treated spacers, further work is needed to optimize the surface treatment in order to achieve optimal anode voltage. The anode screen was also made by screen printing the various layers onto a glass substrate. Several printings were needed to print conductors, insulators, black matrix and RGB phosphors. Sidewalls and getters are attached to the anode screen before assembling with the cathode.
The cathode and anode substrates are assembled into the display panel (Figure 3). The final frit sealing of the display must be done in an inert gas atmosphere, to prevent degradation of the CNT emitters.
Figure 3. Diagram showing the basic flow of display fabrication. Display Operation A cross section of a display panel during operation is diagrammed in Figure 4. The display is matrix addressed with the grids as the rows and the cathode lines as the columns. The rows are rastered while the cathode data lines are pulse width modulated to achieve gray scale. The panel consists of 200 rows and 280 RGB columns. The rows are scanned providing a duty cycle of 1/200 with a refresh rate of about 16 msec. The drivers allow for a gray scale of 256 levels per color with gamma correction. The cathode lines are operated at -50V and the grid lines are operated at 100V. These voltages are compatible with standard driver electronics. Lower driver voltages are expected with optimized emitter structure design and the use of higher quality manufacturing equipment.
Figure 4. Cartoon showing a cross section of the display panel illustrating display operation.
The anode faceplate was operated at 4kV DC during demonstration. We used standard P22 phosphor material and we are working to increase this voltage to 10kV in order to increase the brightness. Improvement of the spacers is needed to prevent arcing due to spacer charging. Figure 5 shows images captured from the display in operation in video mode. No motion artifacts are seen, as expected. Since the light is emitted in a Lambertian distribution from the phosphor there are no viewing angle issues. Some of the non-uniformity seen in the display is a result of the non-uniform grid attachment to the cathode plate. Furthermore, the displays were made using ordinary manual equipment, with components made in both the US and Japan, and assembled in our facility without the benefit of a clean room. The mechanical assembly and uniformity issues are expected to be solved when fabrication is performed in a dedicated pilot line brought under one roof [8]. The cathode for this display was made without including a resistive layer to help control emission uniformity from the CNT emitters. The drive electronics also did not include uniformity correction. Adding a resistive layer in future designs is expected to improve the inherent emission uniformity from the cathode, as seen in other efforts [2]. More recent CNT paste formulations and improved activation process techniques have yielded improved cathode uniformity on smaller CNT-based lamp test structures (see Figure 6). We have improved the uniformity of the field emission to a standard deviation
of 0.042. This compares to an LCD panel standard deviation of 0.04 for the same area measured.
Figure 5. Images of the display in operation.
Summary The market for large color TVs continues to grow. The CNT FED is suited to fill this need for the reasons that they will be relatively inexpensive while possessing a high picture quality. Using industry standard screen printing and vacuum sealing techniques, the price per panel will be dramatically reduced. Others have focused on in situ CNT growth [1,2,9,10] but we believe that for commercializing these large display, screen printing and dispensing is preferred. The display is operated with low voltage drivers, so the higher price of non-standard high voltage electronics is avoided as well. The display presented here was made using only printing and dispensing techniques. The printing technology for CNT TVs, such as that being developed at Applied Nanotech, Inc., will be advantageous for scaling to large displays in volume manufacturing. In situ growth of CNT on display substrates will likely become impractical for large-size color TVs, and therefore unable to compete with existing technologie s.
Figure 6. Field emission image from a 3 x 3 cm area. The brightness has a standard deviation of 0.042. The
operating field is 2.3 V/micron and the total current is 30 mA.
Acknowledgements We thank our Japanese partners who helped us achieve this proof-of-concept display. "Permission for Reprint, courtesy Society for Information Display" References 1. K. A. Dean, et al., "Color Field Emission Display for Large Area HDTV", SID 2005 International Symposium Digest of Technical Papers, pp. 1936-1939, May 2005. 2. J. Dijon, et al., "6-in. Video CNT-FED with Improved Uniformity", International Display Workshop Digest of Technical Papers, pp. 1636 " 1638, Takamatsu, Japan, December 2005. 3. Deuk-Seok Chung, et al., "Carbon nanotube electron emitters with a gated structure using backside exposure", Appl. Phys. Lett., Vol. 80, p. 4045, (27 May, 2002). 4. K. Nishimura, et al., "Fabrication of CNT Emitter Arrays with Polymer Insulator," SID 2005 International Symposium Digest of Technical Papers, pp. 1612-1615 (2005). 5. S. Watanabe, et al., "Improvement of Emission Distribution by Electrical Activating the Cathode Surface for CNTFED," Proceedings of the 12th International Display Workshop, pp. 1643 " 1646, Takamatsu, Japan, December 2005. 6. L.H. Thuesen, D.S. Mao, V. Ginsberg, M. Yang, Y.J. Li, R.L. Fink, Z. Yaniv, "CNT Field Emission Video Display" Proceedings of the 10th International Display Workshops, pp. 1191 " 1193, 2003. 7. R.L. Fink, Y.J. Li, M. Yang, Z. Yaniv, Sho Katanahara, "Sandblast Activation of Carbon Nanotube Cathodes" Proceedings of the 11th International Display Workshops, pp. 1251 " 1254, 2004. 8. Z. Yaniv, "Nanomanufacturing as Applied to Large Area CNT TVs" Proceedings of The 2nd International Symposium on Nanomanufacturing, pp. 704 " 709, 2004. 9. J. Yotani, et al., "High-Resolution CNT-FED for Graphic Displays" SID Symposium Digest, Vol. 36. pp. 1720 "1723, 2005. 10. S. Kang, et al., "Improved Brightness of a FED with Low-Temperature Carbon Nanotubes" SID Symposium Digest, Vol. 36. pp. 1940 " 1943, 2005.
About the author
Richard Fink is currently VP, Engineering at Applied Nanotech Inc. in Austin TX. He has been working in field emission applications of carbon films for over 10 years. Dr. Fink received his PhD in Physics from the University of Illinois at Urbana-Champaign. He is also active in the Society for Information Display and serves as the Chairman of the SID Texas Chapter. He can be reached at [email protected]
Abstract Advances have been made in the development of carbon nanotube field emission displays. We fabricated a 25 inch diagonal, full color RGB, video display using standard screen printing techniques. The pixel pitch is compatible with an approximately 90 inch diagonal HDTV display and the fabrication processes are scalable to large area displays. Introduction The development of carbon nanotube field emission displays (CNT FED) is progressing. The target market for these displays is the large, 60 to 100 inch diagonal, television. For these displays to be cost competitive with existing technologies, inexpensive materials and processes must be used in their fabrication.
Several companies and research groups are pursuing CNT FEDs, although each is taking a different approach. Some companies are using photolithographic patterning techniques to make the gated emitter structure and then growing the CNT emitters directly on the glass using CVD techniques. For example, Motorola demonstrated a 4.6 inch diagonal, full-color display with pixel dimensions representative of a 42 inch diagonal 16:9 HDTV [1]. Using a similar approach, LETI demonstrated a 6 inch diagonal monochrome CNT FED in video mode with 320 x 240 pixels with a size of 350 ¼m. They achieved a luminous level of 800 Cd/m2 with a 50% aperture black matrix and pixel to pixel uniformity within 5% [2]. These direct CVD growth approaches will be difficult and expensive to scale to large area in high volume manufacturing since large sheets of glass will need to be heated very uniformly to high temperatures (600°C or higher). Thus, there is no manufacturing advantage over LCD TVs. Other companies are dispensing CNT emitters using various printing techniques. Samsung SDI has developed several approaches. One recent approach uses standard lithography techniques to form the emitter structures that have both the emitter extraction gate and focus gate electrodes and incorporating an a-Si resistive layer in the structure to help control uniformity [3]. The unique Samsung approach includes printing the CNT emitter using photosensitive paste and then using backside illumination to expose the paste to form the CNT emitter patterns. Thus, the features are self-aligned and the structure is easier to manufacture. Mitsubishi Electric Corp., as part of an effort funded by the Japanese Government, has developed an approach in which the CNT emitter material is printed early in the fabrication process and then a SiO-based polymer is printed and patterned to form the gate dielectric [4]. Various activation processes have been
developed to improve the emission performance of dispensed CNT films [5]. In previous efforts, Applied Nanotech, Inc. has demonstrated a 14 inch diagonal monochrome CNT FED [6] and a 14 inch diagonal color CNT FED. These display panels were made using inexpensive screen printing and frit sealing techniques. Here we present a 25 inch diagonal, full color CNT FED with a pixel pitch compatible with a 60 inch diagonal ATV. This is a direct scale - up of our 14 inch diagonal color display with a pixel pitch of 1.575mm and a color subpixel pitch of 0.525mm. This larger panel demonstrates the scalability of this technology using printing techniques and shows the potential of smaller capital investment in the manufacturing process in the future. Display Fabrication A 25 inch diagonal, full RGB color, CNT video display was fabricated and demonstrated. The methods used to produce the display were inexpensive, standard screen printing and common glass frit sealing techniques. The processes used are all scaleable to large area displays. The display is made from two primary components: the cathode substrate and the anode substrate. The two components are then assembled together and evacuated, resulting in the sealed display. The cathode substrate is sodalime glass. Conducting feedlines are printed onto the glass using commercial screen printing methods. Next, an insulating overlayer is printed on top of the feedlines, but leaving open windows on the conducting material for the emitting pixels. This insulator becomes a spacer for the extraction grid. CNT is dispensed into the open windows. ANI has developed several CNT-based inks and pastes for different dispensing techniques including printing.
Figure 1-a. Schematic of activation by particle blasting process.
Figure 1-b. Image of emission from CNT cathode with top half activated by tape process and bottom half activated by particle blasting process. Both halves are operated at the same electric field.
Printed CNT emitters generally require an activation step to improve their field emission quality. Typically, the activation process is a mechanical modification of the emitter surface. Watanabe, et al, [5] describes an electrical pulse technique and references various techniques developed by others. In order to be compatible with scalability to large area processes, we have developed bead-blast activation processes [7]. Basically, the activation process thins out the CNT emitters so that they do not electrically screen each other and frees up one end of the CNT emitter such that it is easily aligned by the electron extraction field (see Figure 1). A mechanical extraction grid is placed on top of the insulator. This grid is manufactured using standard CRT shadow mask techniques. The grid is attached using a special high temperature, low outgassing adhesive material that is applied to the grid/substrate using a screen printing method. The cathode processes are diagrammed in Figure 2.
Figure 2. Cathode printing process schematic. Post spacers were used in the display. These were attached to the cathode substrate before assembly with the anode
screen. The spacers are sodalime glass with a special surface treatment. The surface treatment is to prevent charging of the spacers and the subsequent high voltage arcing during display operation. Although there is improvement to the stability of the display over the use of non-treated spacers, further work is needed to optimize the surface treatment in order to achieve optimal anode voltage. The anode screen was also made by screen printing the various layers onto a glass substrate. Several printings were needed to print conductors, insulators, black matrix and RGB phosphors. Sidewalls and getters are attached to the anode screen before assembling with the cathode.
The cathode and anode substrates are assembled into the display panel (Figure 3). The final frit sealing of the display must be done in an inert gas atmosphere, to prevent degradation of the CNT emitters.
Figure 3. Diagram showing the basic flow of display fabrication. Display Operation A cross section of a display panel during operation is diagrammed in Figure 4. The display is matrix addressed with the grids as the rows and the cathode lines as the columns. The rows are rastered while the cathode data lines are pulse width modulated to achieve gray scale. The panel consists of 200 rows and 280 RGB columns. The rows are scanned providing a duty cycle of 1/200 with a refresh rate of about 16 msec. The drivers allow for a gray scale of 256 levels per color with gamma correction. The cathode lines are operated at -50V and the grid lines are operated at 100V. These voltages are compatible with standard driver electronics. Lower driver voltages are expected with optimized emitter structure design and the use of higher quality manufacturing equipment.
Figure 4. Cartoon showing a cross section of the display panel illustrating display operation.
The anode faceplate was operated at 4kV DC during demonstration. We used standard P22 phosphor material and we are working to increase this voltage to 10kV in order to increase the brightness. Improvement of the spacers is needed to prevent arcing due to spacer charging. Figure 5 shows images captured from the display in operation in video mode. No motion artifacts are seen, as expected. Since the light is emitted in a Lambertian distribution from the phosphor there are no viewing angle issues. Some of the non-uniformity seen in the display is a result of the non-uniform grid attachment to the cathode plate. Furthermore, the displays were made using ordinary manual equipment, with components made in both the US and Japan, and assembled in our facility without the benefit of a clean room. The mechanical assembly and uniformity issues are expected to be solved when fabrication is performed in a dedicated pilot line brought under one roof [8]. The cathode for this display was made without including a resistive layer to help control emission uniformity from the CNT emitters. The drive electronics also did not include uniformity correction. Adding a resistive layer in future designs is expected to improve the inherent emission uniformity from the cathode, as seen in other efforts [2]. More recent CNT paste formulations and improved activation process techniques have yielded improved cathode uniformity on smaller CNT-based lamp test structures (see Figure 6). We have improved the uniformity of the field emission to a standard deviation
of 0.042. This compares to an LCD panel standard deviation of 0.04 for the same area measured.
Figure 5. Images of the display in operation.
Summary The market for large color TVs continues to grow. The CNT FED is suited to fill this need for the reasons that they will be relatively inexpensive while possessing a high picture quality. Using industry standard screen printing and vacuum sealing techniques, the price per panel will be dramatically reduced. Others have focused on in situ CNT growth [1,2,9,10] but we believe that for commercializing these large display, screen printing and dispensing is preferred. The display is operated with low voltage drivers, so the higher price of non-standard high voltage electronics is avoided as well. The display presented here was made using only printing and dispensing techniques. The printing technology for CNT TVs, such as that being developed at Applied Nanotech, Inc., will be advantageous for scaling to large displays in volume manufacturing. In situ growth of CNT on display substrates will likely become impractical for large-size color TVs, and therefore unable to compete with existing technologie s.
Figure 6. Field emission image from a 3 x 3 cm area. The brightness has a standard deviation of 0.042. The
operating field is 2.3 V/micron and the total current is 30 mA.
Acknowledgements We thank our Japanese partners who helped us achieve this proof-of-concept display. "Permission for Reprint, courtesy Society for Information Display" References 1. K. A. Dean, et al., "Color Field Emission Display for Large Area HDTV", SID 2005 International Symposium Digest of Technical Papers, pp. 1936-1939, May 2005. 2. J. Dijon, et al., "6-in. Video CNT-FED with Improved Uniformity", International Display Workshop Digest of Technical Papers, pp. 1636 " 1638, Takamatsu, Japan, December 2005. 3. Deuk-Seok Chung, et al., "Carbon nanotube electron emitters with a gated structure using backside exposure", Appl. Phys. Lett., Vol. 80, p. 4045, (27 May, 2002). 4. K. Nishimura, et al., "Fabrication of CNT Emitter Arrays with Polymer Insulator," SID 2005 International Symposium Digest of Technical Papers, pp. 1612-1615 (2005). 5. S. Watanabe, et al., "Improvement of Emission Distribution by Electrical Activating the Cathode Surface for CNTFED," Proceedings of the 12th International Display Workshop, pp. 1643 " 1646, Takamatsu, Japan, December 2005. 6. L.H. Thuesen, D.S. Mao, V. Ginsberg, M. Yang, Y.J. Li, R.L. Fink, Z. Yaniv, "CNT Field Emission Video Display" Proceedings of the 10th International Display Workshops, pp. 1191 " 1193, 2003. 7. R.L. Fink, Y.J. Li, M. Yang, Z. Yaniv, Sho Katanahara, "Sandblast Activation of Carbon Nanotube Cathodes" Proceedings of the 11th International Display Workshops, pp. 1251 " 1254, 2004. 8. Z. Yaniv, "Nanomanufacturing as Applied to Large Area CNT TVs" Proceedings of The 2nd International Symposium on Nanomanufacturing, pp. 704 " 709, 2004. 9. J. Yotani, et al., "High-Resolution CNT-FED for Graphic Displays" SID Symposium Digest, Vol. 36. pp. 1720 "1723, 2005. 10. S. Kang, et al., "Improved Brightness of a FED with Low-Temperature Carbon Nanotubes" SID Symposium Digest, Vol. 36. pp. 1940 " 1943, 2005.
About the author
Richard Fink is currently VP, Engineering at Applied Nanotech Inc. in Austin TX. He has been working in field emission applications of carbon films for over 10 years. Dr. Fink received his PhD in Physics from the University of Illinois at Urbana-Champaign. He is also active in the Society for Information Display and serves as the Chairman of the SID Texas Chapter. He can be reached at [email protected]
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