Chapter 7

Flat Panel Displays

RESEARCH OPPORTUNITIES

Introduction

Vision is one of man's most highly developed senses. Thus it is no surprise that display technology is playing an ever-increasing role as we move deeper into the information age. Performance requirements for displays depend on the application; critical attributes include:

  • Resolution;

  • Color;

  • Fidelity;

  • Cost;

  • Energy consumption;

  • Efficiency;

  • Brightness;

  • Ruggedness, reliability, lifetime, and mean time between failure;

  • Weight;

  • Volume;

  • Ease of interface;

  • Size; and

  • Availability/sourcing.

Of particular interest are the flat display technologies that lend themselves to portability and low included volume. These are the display technologies that will drive the ubiquitous spread of display technology. The impact of these displays will span consumer, commercial, industrial, and military applications.

Today the cathode ray tube (CRT) display is the benchmark against which all other displays are measured. CRTs are manufactured globally, and although the technology is quite mature, they continue to dominate in unit volume, revenue, and breadth of applications. The newer technologies are driving into niche markets—personal television sets, portable computers, rugged industrial equipment markets, and military applications. Table 1 lists these newer display technologies. The dominant new display technology is the liquid crystal display (LCD). Production of these displays is not global but is concentrated in Japan.

That the dominant center of LCD technology and its concomitant technological infrastructure lie outside the United States is of concern industrially, politically, and militarily. Indeed, the Executive Branch has taken a strong stand on the urgent need for the United States to become competitive in the world market in flat panel displays. The nation is growing more aware that its security depends on its industrial technology strength. In peacetime, military applications alone cannot support a major technology and certainly cannot compete when a civilian application is driving a technology. Herein lies both the opportunity and the difficulty for NRL's plasma processing research. The opportunity is to provide a U.S. competitive technological advantage; the difficulty is that the issues are more than technological, and entrenched positions will be hard to overcome. However, the promise of a large commitment of funds for this field opens the way for new participants in this arena.

The technological research and development opportunities for plasma processing in display technology are generally of two types: (1) precision processing of the type that also impacts the semiconductor industry (VLSI, submicrometer processing); and (2) large-area processing.

Projection displays are expected to play major roles in applications for very large displays (high-definition television [HDTV], conference, and group displays) and for very small personal displays (virtual reality and head-mounted displays). Here the critical issue is high performance in small sizes. Precision of



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Plasma Processing and Processing Science Chapter 7 Flat Panel Displays RESEARCH OPPORTUNITIES Introduction Vision is one of man's most highly developed senses. Thus it is no surprise that display technology is playing an ever-increasing role as we move deeper into the information age. Performance requirements for displays depend on the application; critical attributes include: Resolution; Color; Fidelity; Cost; Energy consumption; Efficiency; Brightness; Ruggedness, reliability, lifetime, and mean time between failure; Weight; Volume; Ease of interface; Size; and Availability/sourcing. Of particular interest are the flat display technologies that lend themselves to portability and low included volume. These are the display technologies that will drive the ubiquitous spread of display technology. The impact of these displays will span consumer, commercial, industrial, and military applications. Today the cathode ray tube (CRT) display is the benchmark against which all other displays are measured. CRTs are manufactured globally, and although the technology is quite mature, they continue to dominate in unit volume, revenue, and breadth of applications. The newer technologies are driving into niche markets—personal television sets, portable computers, rugged industrial equipment markets, and military applications. Table 1 lists these newer display technologies. The dominant new display technology is the liquid crystal display (LCD). Production of these displays is not global but is concentrated in Japan. That the dominant center of LCD technology and its concomitant technological infrastructure lie outside the United States is of concern industrially, politically, and militarily. Indeed, the Executive Branch has taken a strong stand on the urgent need for the United States to become competitive in the world market in flat panel displays. The nation is growing more aware that its security depends on its industrial technology strength. In peacetime, military applications alone cannot support a major technology and certainly cannot compete when a civilian application is driving a technology. Herein lies both the opportunity and the difficulty for NRL's plasma processing research. The opportunity is to provide a U.S. competitive technological advantage; the difficulty is that the issues are more than technological, and entrenched positions will be hard to overcome. However, the promise of a large commitment of funds for this field opens the way for new participants in this arena. The technological research and development opportunities for plasma processing in display technology are generally of two types: (1) precision processing of the type that also impacts the semiconductor industry (VLSI, submicrometer processing); and (2) large-area processing. Projection displays are expected to play major roles in applications for very large displays (high-definition television [HDTV], conference, and group displays) and for very small personal displays (virtual reality and head-mounted displays). Here the critical issue is high performance in small sizes. Precision of

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Plasma Processing and Processing Science production is more important than cost per unit area. The expectation is that display (die) sizes can be made smaller to reduce cost while still increasing performance functionality, as in VLSI experience. Table 1. New Display Technologies TECHNOLOGY SIZE • Passive matrix LCD Simple multiplex Active addressing Large/small • Active matrix LCD Amorphous silicon Polycrystalline silicon Transfer silicon Large/small • Thin film electro-luminescent displays Passive matrix Active matrix Large Small • Digital micromirror devices Small • Plasma displays Large • Field emission displays Large Most of the interest in flat displays is for direct view displays. Thus cost per unit of display area is critical for a practicable process. Typical display sizes are about 10 inches diagonally for video graphics array (VGA) displays with 480 × 640 resolution. The market desires larger, 15-inch diagonal, and higher-resolution, 768 × 1024 and 1024 × 1280, displays but as yet few applications can afford them because of high manufacturing costs. The remainder of this section identifies, for each of the display technologies listed in Table 1, the current and potential future roles of plasma processing. Passive Matrix Liquid Crystal Display There are two basic types of LCD configurations: passive matrix and active matrix. Low-cost large and small LCDs are typically of the passive matrix type. Passive matrix LCDs consist simply of two transparent substrates having transparent electrodes. The substrates are separated by a gap of about 10 µm filled with the liquid crystal material. One substrate contains column electrodes, and the other contains the row electrodes. Picture elements (pixels) are defined by the intersection of these orthogonal electrodes. Today such LCDs are produced without any plasma processing. Potentially, plasma dry etching could replace the wet etching techniques currently used to define the electrode structures, but this is not currently pursued because of the higher cost of plasma processes. However, the environmental impact of liquid waste may eventually change the cost balance. Active Matrix Liquid Crystal Display The active matrix liquid crystal display (AMLCD) is the flat display technology of choice for high-performance applications—both large-area, direct-view displays and small-area, projection displays. In active matrix LCDs, one substrate contains the active matrix and the other acts as a ground plane and is electrically featureless. The active matrix consists of an array of isolated pixels connected to a matrix addressing structure via thin film, field-effect transistors. Gates of these thin film transistors (TFTs) are organized as the matrix rows, and the sources are organized as the matrix columns. Although the geometry of these TFTs is large compared with current VLSI chips, these 2- to 10-mm geometries do warrant precision processing. Plasma dry etching techniques similar to those used in the semiconductor industry are commonly used where wet chemical etching does not suffice. An R&D opportunity exists for increasing the etch rates and uniformity of large-area etching processes. Silicon is the material from which these TFTs are fabricated. Two silicon forms are typically used: hydrogenated amorphous silicon (aSi) and high-temperature polycrystalline silicon (pSi). For special applications, transfer crystalline silicon (xSi) is also being developed.

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Plasma Processing and Processing Science Amorphous Silicon Hydrogenated amorphous silicon is used for large-area, direct-view displays. These displays are formed on glass substrates that cannot tolerate very high temperatures. A plasma-enhanced chemical vapor deposition (PECVD) process is used to deposit the aSi on the glass substrates at temperatures of about 300° C. Current PECVD techniques are relatively slow and therefore costly. Deposition rates have to be low in order to form electrically high-quality material and to avoid gas phase nucleation and particulates that substantially reduce the active matrix yields. An additional problem is that the equipment requires frequent, time-wasting cleaning in order to maintain the high yields needed for operating displays. A clear opportunity exists for an R&D contribution if better understanding can result in high deposition rates and reduced equipment downtime required for cleaning. Polycrystalline Silicon High-temperature polysilicon is currently deposited on quartz substrates by chemical vapor deposition at temperatures of about 600° C. Lower-temperature PECVD processes could offer improved economics by reducing the heat-cooling cycle times while increasing the deposition rate of high-quality silicon. Hydrogen passivation of the polysilicon is typically part of the active matrix process in order to reduce the leakage currents in the pSi TFTs to a useful level. This passivation is achieved by plasma processing of the polysilicon TFT. Here, too, there is an opportunity to increase the passivation efficacy and overall processing rates. Many experts are convinced that large-area pSi active matrix LCDs on glass substrates will be the next-generation AMLCD technology. It is anticipated that plasma processes will play a major role in this development by enabling the large-area, low-temperature processing that will be required for fabrication on glass substrates. Specific processes that need to be developed include PECVD of silicon, anisotropic plasma etching, hydrogen plasma passivation, and perhaps large-area ion doping. Transfer Silicon Kopin, a small U.S. company, is currently commercializing another type of silicon active matrix LCD. In the Kopin technology, the active matrix is formed in a thin silicon layer on top of a thickly oxidized silicon substrate. After processing, this thin silicon layer is transferred to a glass substrate. At this point it can be handled in much the same way as an aSi active matrix substrate to fabricate a complete active matrix LCD. The exciting aspect of the Kopin technology is that it results in excellent display performance because the thin silicon layer is of crystalline quality, and furthermore the active matrix is processed exactly like it is for fabricating VLSI circuits. Here the role of plasma processing is identical to that used in VLSI technology. Thin Film Electroluminescent Displays Currently commercial thin film electroluminescent (TFEL) displays are only of the passive matrix type. They consist of (1) a glass substrate on which is deposited a transparent conducting film that is patterned to form vertical stripe electrodes, (2) an insulating dielectric layer, (3) a thin film electroluminescent phosphor, (4) another insulating dielectric layer, and (5) a deposited film of aluminum that is patterned to form horizontal stripes. The first and last layers constitute the electrodes of the passive matrix. At this time no plasma processes are used in the fabrication. Under development but not yet commercialized is an active matrix TFEL display technology. These displays are fabricated by depositing the TFEL material onto a silicon-active matrix fabricated on a dielectrically isolated silicon wafer. Plasma processing is used for etching and deposition as with other VLSI silicon devices.

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Plasma Processing and Processing Science Digital Micromirror Devices Digital micromirror devices (DMDs) are currently being commercialized by Texas Instruments. The DMD consists of an array of mirrors, each about 15 × 15 µm, fabricated on a silicon wafer. Each mirror is capable of a torsional deflection caused by an electrostatic drive circuit built below the mirror on the silicon wafer. The array of mirrors forms an image-generating light valve. By means of a Schlieren optical system, a very efficient, high-resolution projection display system can be built. The DMD itself is fabricated using “standard” silicon VLSI processes. Accordingly, any plasma processing enhancements that serve the semiconductor industry will have a synergistic impact on DMD displays. As with other displays, cost reductions are particularly important. Plasma Displays Plasma displays are receiving an increased amount of attention because they seem to be particularly suited to consumer on-the-wall HDTV applications. These are relatively low-resolution displays—one or two pixels per millimeter—but they need to be of high reliability and performance and cost-competitive with CRTs. Basically, the display consists of an array of gaseous glow discharge cells. To achieve color, each cell also contains a phosphor that is excited by ultraviolet emission from the glow discharge. Fabrication of these displays is achieved primarily by means of various thick film processes. These processes are adequate for the resolutions required for HDTV and enable application and patterning of the relatively large volumes of materials needed to form the individual pixel discharge cells. At this time no plasma processes are used in the fabrication. Field Emission Displays Significant R&D effort is being applied to the field emission display (FED) technologies. These devices are basically low-voltage cathodoluminescent displays formed by a matrix of field emitters separated by a narrow gap from a phosphor-coated faceplate. They are vacuum devices like CRTs. The field emitters are organized as an x-y matrix and use row-at-a-time addressing similar to electroluminescent and LCD displays. Major anticipated advantages of the FED devices are high brightness, self-emissivity, low cost, low factory startup cost, and scalability to large areas. Several approaches are being developed for the field emission cathodes. These include amorphic diamond films and metal microtips. At this time no plasma processes are used in the fabrication of either type of FED. Although currently experimental devices are fabricated using shadowmask depositions, plasma etching of the amorphic diamond films into the individual pixel areas may be advantageous in the future. Additionally, growth of amorphic diamond films with the appropriate properties by plasma-enhanced chemical vapor deposition may be more cost-effective than the current technology of laser ablation. A ROLE FOR NRL The challenge for introducing plasma processing into display applications is to maintain process performance while increasing process areas and rates and decreasing process equipment capital costs and operation and maintenance costs. To achieve this, an improved understanding of plasmas and plasma processes will be necessary. Particular emphasis should be placed on “linear” plasma processing geometries (line sources), that is, systems that scale easily in one dimension and achieve scaling in the other direction by substrate motion on a continuous moving belt. There is a growing trend for the semiconductor industry to migrate toward larger and larger silicon wafer sizes. Wafer diameters of 12 to 16 inches are likely to be in use by the year 2000. An ancillary benefit of developing improved plasma deposition, etching, and passivation processing techniques for silicon-based AMLCDs is that these processes are likely to be

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Plasma Processing and Processing Science applicable in VLSI fabrication as the industry migrates to larger and larger wafer sizes. As with VLSI processes, one can expect that, in the various display industries, there will be a growing concern with the environmentally benign disposal of wet etchant wastes. Gaseous processes, such as plasma etching, generally result in fewer waste disposal problems and effluent volumes than, say, wet etching. Accordingly, although the required breakthroughs are challenging, so are the opportunities for large-area, low-cost plasma processes. There are more similarities than differences among the technology requirements for VLSI and silicon-active matrix LCDs, although VLSI geometries are almost an order of magnitude smaller. Thus, as an example, anisotropic plasma etching is required to maintain critical dimensions in small etching geometries for VLSI, whereas in displays, anisotropic etching is required to maintain process tolerance over larger areas with minimum dimensions. Overetching in depth can be tolerated, whereas lateral etch nonuniformities cannot. A second example can be drawn from the need for faster processing. Single wafer processing in VLSI needs faster process rates to achieve competitive economics. Displays also need faster process rates because display areas are large, yet their marketable costs per unit area are low. Other problems encountered in VLSI fabrication will also become important in dry processing of displays: cleaning of substrates, control of particulates, and charge-induced damage in the etching process. To have an impact in display applications, NRL needs to proactively form teaming relationships and cooperative joint R&D programs and sponsor programs with display producers and would-be producers, researchers, and infrastructure suppliers. Specific contacts should be made with the MCC FED Consortium, the newly formed U.S. Display Consortium (USDC), and the North American Flat Panel Division of SEMI, as well as key U.S. display manufacturers. The corporate members of the USDC would be a good start: AT&T, Electro-Plasma, Magnascreen, Optical Imaging Systems, Photonics Imaging, Planar Systems, Plasmaco, Standish Industries, Tektronix, and Xerox, plus Micron Display, Sarnoff, SI Diamond Technology, MRS, Applied Materials, and so on. NRL expertise in FEDs is an internal resource that could be leveraged to acquire visibility in the display community.