High-definition displays and technology trends in TFT-LCDs
`
`I-Wei Wu*
`
`Abstract— The current status of flat—paneI-display (FPD) technologies is outlined, with emphasis on
`liquid-crystal displays (LCDs). Recently announced high-density LCDs, their structures, and their un-
`derlying technologies are summarized and compared. Technology trends in thin-fiIm-transistor (TFT)
`LCDs are discussed. Advancements in a-Si TFT technology are reviewed; in particular, the geometry,
`structure, and materials of TFTs, gate bus-line metallization, storage capacitors, self-aligned doping
`methods, viewing angle, aperture ratio, and power consumption are discussed. Emerging poly-Si TFT-
`LCD technology is presented. Its advantages in integrating peripheral driver circuits, improving pixel
`density, and image quality as compared to the a-Si TFT counterparts are reported. Projection high-
`definition liquid-crystal modules fabricated by using a-Si and poly-Si TFTs are compared. Scaling up
`of the substrate size and scaling down TFT geometry are the two main directions for the poly-Si TFT;
`this should enable large-size high-definition direct—view panel fabrication and will enhance the image
`quality of these displays.
`
`Keywords — AMLCDs, aperture ratio, a-Si TFTs, color displays, CRT-addressed liquid-crystal light
`valves, direct-view displays, flat-panel displays, gray scale, laser-addressed liquid-crystal light valves,
`liquid—crystal displays, liquid-crystal light valves, poly-Si TFTs, projection displays.
`
`question may be no longer whether but how soon the LCD
`displaces the CRT.3
`The demand for higher and higher resolution in dis-
`plays is very similar to the increasing integration in VLSI
`chips. Users of VLSI chips have always found ways to utilize
`the new capabilities offered by the next-generation prod-
`ucts instead of staying with the mature and cheaper devices.
`The VLSI manufacturers that could constantly produce new
`and improved products have flourished while those that
`could not have perished. For color displays for television
`(TV) and computer screens, typical sizes and resolutions are
`listed in Table 1. High—definition-television (HDTV) displays
`call for 1125 lines, each with about 1920 color pixels. This is
`more than 2M pixels, about 10 times the pixel count of the
`current NTSC TVs. The aspect ratio will also change from 4:3
`to 16:9, coupled with a significant increase in size. For com-
`puter applications, the pixel density of displays increases
`from today’s video graphic array (VGA) to extended graphic
`array (XGA) to engineering workstation (EWS) and perhaps,
`in the near future, to the super-EWS format. The aspect ratio
`will likely be a constant of 4:3 with a moderate size increase,
`
` 1
`
`Introduction
`
`The rapid development of flat-panel-display (FPD) technol-
`ogy in the last few years has attracted attention throughout
`the electronics industry. In particular, liquid-crystal displays
`(LCDs) have greatly expanded in physical size and in com-
`plexity, resulting in completely new kinds of electronic
`products. The most prominent of these is the notebook
`computer, whose existence would not be possible without
`IOchost LCDs. Other kinds of applications are emerging and
`are the subject of active development around the world.
`Active-matrix LCDs (AMLCDs) driven by thin-film transistors
`(TFTs) have the best performance of all FPDs, equal to or
`better than that of the most-prevalent display device, the
`CRT.1'2 The strong advantages ofTFT-LCDs, compared to CRT
`or other FPD technologies, are low power consumption, light
`weight, flickerless image, thin profile, low-voltage operation,
`large dynamic range of screen luminance, high contrast ratio,
`full color, video rates, and IC process compatibility. There are
`still many hurdles to overcome, especially the higher produc-
`tion cost, before the LCD displaces the CRT. However, the
`
` Table 1. Formats of typical color displays for television and computers
`Video
`Extended
`Engineering
`graphic
`graphic
`NTSC
`array
`array
`workstation
`Super-EWS
`
`TV
`(VGA)
`(XGA)
`(EWS)
`HDTV
`(forecast)
`
`
`
`Color pixels per line
`Lines
`
`440
`485
`
`640
`480
`
`1024
`780
`
`1280
`1024
`
`1920
`1125
`
`2560
`2048
`
`4:3
`4:3
`Aspect ratio
`8.4-13
`
`Size (in.) typical: 27
`
`4:3
`10.4—17
`
`4:3
`16:9
`4:3
`32—200
`13-19
`
`
`*Member, SID.
`Received 4/22/93; accepted 10/15/93.
`The author is with Xerox PARC, 3333 Coyote Hill Road, Palo Alto, CA 94304; telephone 415/812-4540; fax -4502.
`© Copyright 1994 Society for Information Display 1071 -0922/93/0201-001 $1 .00
`
`Journal of the SID, 2/1, 1994
`
`1
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`

`Table 2. FPD technologies and the major corporations
`Comments
`Major Corporations
`Display Technology
`
`AMLCD
`
`a-Si TFT
`
`Poly-Si TFT
`
`CdSe TFT
`
`Sharp, Toshiba, NEC, Hitachi, ADI,
`Matsushita, Hosiden, Fujitsu, etc.
`Seiko-Epson, Matsushita, Sony,
`Sharp, HDTEC, Xerox, CEC, etc,
`Litton
`
`a-Si diode (DZR)
`
`Philips
`
`MIM diode
`Seiko-Epson, Stanley, etc.
`Tektronix
`Plasma addressd
`
`Passive LCD
`
`Twisted nematic
`
`Supertwisted nematic
`
`Ferroelectric
`
`Many small companies
`Sharp, Toshiba, NEC, Hitachi,
`Seiko-Epsom, In Focus, Asahi, etc.
`Canon
`
`Polymer dispersive
`
`Asahi, Raychem, Fujitsu, etc.
`
`Leading technology, sizes from 3 to 17 in.
`
`Emerging technology: projection, viewfinder,
`cockpit displays
`First TFT-LCD, now less attention
`
`Small reduction in both cost and performance
`compared to a-Si TFT
`Lower cost, less performance
`
`Maybe lower in cost and density
`
`Color recently added
`Color at video rate recently added
`
`High density demonstrated
`Projection and wide-viewing-angle direct view,
`also in AMLCD
`
`
`Nematic cholesteric phase Fujitsu, Stanley, etc.
`transition
`
`Very high density
`
`Other LCDs
`
`Laser addressed
`Very high density, no video rate
`Greyhawk, Fujitsu, etc.
`
`CRT addressed
`Projection HDTV, no video rate
`Hughes-jVC
`
`Self—luminescent
`Field emission
`Vacuum fluorescent
`Plasma
`
`Electroluminescent
`
`Single-Crystal Si
`Digital mirror
`
`LETI, Mitsubishi, Coloray
`Futaba, ISE, NEC
`
`Fujitsu, Sharp, Matsushita, Oki,
`Photonics, etc.
`Planar, Komatsu, NSC, Matsushita,
`Sham
`
`6 in. demonstrated
`
`For instrument panels mainly
`Largest size (>40 in.), recently contrast and
`brightness improved
`Blue color difficult
`
`Texas Instruments
`
`SRAM + micromachining reflected projector,
`VGA demonstrated
`
`CMOS process, transfer process cost is high
`Kopin
`SOI substrate transfer
`
`compared to that of TVs. AMLCDs will likely compete with
`CRTs for all applications. In the HDTV case, the projection
`TFT-LCD may be the technology of choice because of size
`limitations of direct-view panels.
`The estimated investment in LCD—module production
`in japan between 1989 and 1993 is plotted in Fig. 1. These
`data were gathered through various news releases and pub-
`lished articles. The investment of each individual company
`may not be exact, but this plot does give an idea of the rela-
`tive investment of the major corporations. It is noteworthy
`that almost all of the major companies in the japanese elec-
`tronics industry were participating in LCD activity. A total of
`$4.4B is shown to have been spent in LCD-module fabrica-
`tion. This amount of investment roughly represents about
`85% of the world total during this period. Volume produc-
`tion and progression on the yield learning curve have al-
`ready developed great momentum.
`
`The purpose of this paper is to review the current
`status of FPD technology, especially LCD technology, in the
`hope of elucidating the market and technology trends. The
`progress of major companies in the field of FPDs are out-
`lined. The limitations and strong points of different FPD
`technologies are briefly discussed and evaluated for future
`technology trends. Performance and advancement of re-
`cently announced high-density LCDs are described. The
`state-of-the-art technology and the most prominent issues
`facing the amorphous-silicon (a-Si) TFT-LCD today are ana—
`lyzed. Finally, polycrystalline—silicon (poly-Si) technology is
`described, especially for application in high-definition pro-
`jection and direct-view displays.
`
`2
`
`Overview of FPD technologies
`
`A survey of the available FPD technologies is given in
`Table 2. Two major FPD categories that exist today are
`
`2
`
`Wu/High-definition displays and technology trends in TFT-LCDs
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`80
`
`70
`
`60
`
`50
`
`
`
`
`
`
`
`30
`
`Mitsubishi
`Asahi
`ADI
`
`Fujitsu
`5pm. .
`San o
`
`—
`
`——
`
`—
`
`
`
`
`Estimated Investment in LCD Production in Japan
`3 ¥
`100
`
`
`
`
`Other known players (R a D only or unknown investment):
`Casio, Citizen Watch, Kyocera, IBM Japan, NTT;
`two consortia: GTC and HDTEC, and
`universities such as: Tohoku, Hiroshima, Yokyo Inst. Tech, and more
`
`20
`
`
`Seiko
`10'
`Epson
`Stanley
`___..
`Sony Alps Seiko
`Canon
`Inst.
`
`
`Total investment: ¥4503 ~$4.4B
`
`FIGURE 1 — Estimated investment by Japanese corporations in the
`production of LCD modules between 1989 and 1993.
`
`LCD and non-LCD. There are three types of LCDs being
`actively pursued: active matrix, passive matrix, and laser—
`or CRT-addressed liquid-crystal modules. For the non-
`LCD FPDs, there are the self-luminescent and single-
`crystal silicon technologies.
`
`2.1
`
`LCDs
`
`2.1.1
`
`AMLCDs
`
`The leading technology in this category is a-Si TFT in terms
`of research, development, and investment.4 Poly-Si TFT is
`next, with small LCD panels dominant and with the scaling.
`up of substrate size being actively investigated by almost all
`companiess‘7 These technologies will be discussed further
`in the following sections. The third technology is the CdSe
`TFT, which was the first proposed TFT material.8 However,
`it has not been successfully introduced into the market and
`has been largely displaced by silicon TFTs.9 Relatively small
`efforts are still being made to compete with silicon, notably
`by Litton Systems in Canada.
`There are two other AMLCD technologies using diodes
`instead of transistors. The a-Si p/n-junction diode array re-
`ferred to as diode-diode-reset (D2R)10 is being pursued by
`Philips in the Netherlands. Although it requires an extra
`reset line for every two pixel columns, a high ON/OFF switch—
`ing ratio similar to that of TFTs can be achieved. The process
`is quite complex, however, so the reduction in production
`cost may be limited. Also, the DZR array needs custom VLSI
`driving chips, which will cost more. Another diode array,
`metal-insulator—metal (MIM),11 can be made with a simpler
`process compared to TFTs (about 50% in array fabrication
`cost), resulting in a 20% reduction of the overall display
`module cost. However, the image quality of the MIM is infe-
`rior to that of TFTs. At this moment, only Seiko-Epsom of
`Japan is producing MIM in volume.
`
`Plasma-addressed AMLCDs were proposed and devel—
`oped by T. Buzak of Tektronix in 1990,12 and a l6-in.-diago-
`nal VGA panel was recently demonstrated.13 This technology
`uses plasma conduction in a strobe—line to load data volt-
`ages, as opposed to the use of a scan bus—line to turn the gate
`electrodes of pixel transistors in a TFT-LCD on and off. The
`manufacturing process is more similar to the common
`plasma displays than to the TFT-AMLCD. An advantage may
`be in the larger pixel size, but disadvantages are lower pixel
`density and contrast ratio, compared to the a-Si TFT-AMLCD.
`
`2.1.2
`
`Passive LCDs
`
`There are four major technologies today in the passive-
`matrix-addressed display category. The first uses the
`twisted-nematic liquid crystal (TN-LC), which can be found
`in a wide variety of consumer products such as calculators,
`watches, and hand-held video games. There are numerous
`companies producing TN displays. The most notable recent
`advance is the introduction of color. Because of its low cost,
`
`the TN-LCD may be here to stay for the low-end market.
`The next technology is the super-twisted-nematic
`(STN), which is an enabling technology for notebook com—
`puters. STN is the dominant FPD technology in the market
`today, with its manufacturing yield reaching maturity. Re-
`cently, there were efforts to improve STN in viewing angle,
`gray-scale presentation, and contrast ratio. Video frame
`rates with eight colors have been demonstrated using novel
`driving methods.14 Even though the display quality still does
`not rival that of TFT-LCDs, the lower projected cost is attrac-
`tive.
`
`A third technology uses the bistability of the surface-
`stabilized ferroelectric liquid crystal (FLC), which can retain
`its ON or OFF state between refreshing addresses in a frame
`period.15 This property alleviates the driving constraints of
`the matrix-addressed LCDs so that higher density and larger
`display size can be realized. Indeed, Canon has developed a
`14-in.-diagonal FLCD with 1024 X 1280 pixels and digital
`color (no gray scale). Production cost should be lower than
`that of TFT-LCDs, and Canon is said to be establishing mass
`production. On the down side of FLC technology, gray scale
`is limited because of the bistability. Also, production yield is
`in doubt because of the l—2-ttm LC cell gap required. The
`best achieved contrast ratio is about 40:1, still inferior to the
`TFT-LCD, in which a ratio of more than 100:1 is typical.
`Another important technology is the polymer-dis—
`persed liquid-crystal (PDLC) display. Since the PDLC relies
`on light scattering controlled by an electric field instead of
`changing the polarization as in the TN case, PDLC displays
`do not require polarizing films. The resulting enhancement
`in screen brightness can be as large as five times for projec—
`tion displays made by PDLC versus TN. PDLC has also been
`incorporated into TFT panels, which significantly increases
`pixel density as well as other performance. Because of the
`light-scattering mechanism, it is more difficult to achieve a
`low OFF-state transmission, so the contrast ratio is typically
`about 40:1. Driving voltage (about 15 V) for PDLC is still
`
`Journal of the SID, 2/1, 1994
`
`3
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`

`
`Table 3. Recently published high-density (EWS, VGA, etc.) LCDs
`Diagonal
`size (in.)
`13.8
`
`Aperture
`Contrast
`
`Pixel count
`Gray levels
`ratio (%)
`ratio (%)
`Viewing angle
`Publication date
`1152 X 3 X 900
`16
`30
`>100:1
`—
`SID ’92
`
`Company
`Toshiba
`
`Matsushita
`
`15
`
`1152 X 3 )< 900
`
`256
`
`34
`
`—~
`
`>200:l
`
`>100:1
`
`>100:l
`
`i45°H
`+30, —20°V
`i45°H
`+20°, —25°V
`
`:30°H
`+23°, —15°V
`
`SID ’92
`
`SID ’92
`
`Japan DisPlay ’92
`
`NEC
`
`Hitachi
`
`12.9
`
`1280 X 3 x 1024
`
`16
`
`11
`1120 x 3 x 780
`16
`29
`
`
`SID '93
`—
`100:1
`——
`8
`1280 X 3 X 1024
`17
`Sharp
`Xerox
`13.2
`3072 x 2048
`2
`26
`100:1
`—
`SID ’93
`
`HDTEC (poly)
`3.7
`1440 x 1024
`64
`70
`>300:1
`Projection
`IDRC ’93
`
`Seiko~Epson (poly)
`0.7
`420 x 3 x 220
`32
`27
`>200:1
`Viewfinder
`SID ’93
`
`Sharp (poly)
`1.9
`1472 X 1024
`32
`23
`>200:l
`Projection
`SID ’93
`
`Toshiba (poly)
`3.3
`1840 x 1035
`32
`36
`>200zl
`Projection
`ISSCC ’94
`
`NEC (a-Si TFT)
`4.2
`1280 x 1024
`16
`35
`>150:1
`Projection
`SID ’93
`CNET
`10
`640 x 3 X 480
`2
`—
`150:1
`i45°H
`51D ’92
`
`(28 a-Si TFT)
`+10°, —20°V
`
`Canon (FLC)
`15
`1280 x 1024
`2
`—
`40:1
`i70°H, i55°V
`japan Display ’92
`
`Stanley (CSH)
`9.4
`640 x 3 X 480
`16
`—
`20:1
`i60°H, 150°V
`japan Display ’92
`
`In Focus (STN)
`4.5
`(240 X 240) x 3
`4
`—
`40:1
`Projection
`SID ’92
`
`
`Asahi (STN)
`5.7
`320 X 240 X 3
`2
`—-
`20:1
`—
`SID ’92
`
`considerably higher than that of TN (about 5 V).16 PDLC is
`mainly used in projection displays because of its high screen
`brightness. With improvements in material properties, it
`may emerge as the LC material for direct-view panels be-
`cause of benefits such as a wider viewing angle.17
`The last type of passive LCD uses nematic-cholesteric
`phase-transition liquid crystals. This material can achieve
`very—high-density displays, but at present cannot display at
`video rates. Its application is aimed at still pictures such as
`those used in overhead projectors.“
`
`
`2.1.3 Other LCDs
`
`Displays without any matrix addressing, either active or pas-
`sive, comprise the final category of LCDs. These displays use
`a scanning laser beam or a CRT to address the LCD.19 The
`most noteworthy development is the joint venture between
`Hughes and jVC to develop a CRT-addressed LCD projec-
`tion HDTV system. The response of the LC is fast enough to
`follow video rates. On the other hand, a light valve ad-
`dressed by a laser beam generally cannot achieve video rates
`due to scanning limitations, so its application is mainly in
`image storage. Compared to the nematic-cholesteric phase-
`transition liquid crystal mentioned above, laser—addressed
`LC light valves may have an advantage in density.
`
`
`2.2
`
`Non-liquid-crystal displays
`
`There are many non-liquid-crystal FPD technologies, each
`of which represents a tremendous effort in research and de-
`
`velopment. Only a very limited description and commentary
`are given here.
`
`
`2.2.1
`
`Self-luminescent FPDs
`
`There are four major self-luminescent FPDs: vacuum-fluo-
`rescent displays (VFDs), field—emission displays (FEDs),20
`plasma displays (PDs),21 and electroluminescent (EL) dis-
`plays.22 The largest FPD with full color at video rate re-
`ported thus far is the plasma display, with a 40-in.-diagonal
`screen.23 The FED has attracted much interest as an alterna-
`tive to the LCD for FPDs in the 19805. For example, LETI of
`France has demonstrated a 6-in.-diagonal FED panel with
`color. While niche markets may exist for these self-lumines-
`cent FPDs, utilizing their respective strong points such as
`larger panel size, LCD panels can cover the entire market
`spectrum much better. This viewpoint is reflected in the
`much larger investment in LCD technologies.
`There is yet another type of self-luminescent display
`not included in Table 2: light-emitting-diode (LED) arrays.
`These displays generally have a much lower pixel density
`(<0.1 pixel/mmz) than all the other FPDs (typically >100 pix-
`els/mmz) discussed here and were used mainly in the spe-
`cialized application of outdoor long-distance viewing.
`
`
`2.2.2
`
`Single-crystal silicon-related FPDs
`
`There are two proposed display technologies related to sin-
`gle-crystalline-silicon VLSI processing: deformable mirror
`and silicon-on-insulator (SOI) wafer-to—glass transfer. The
`deformable mirrorz’4 (later named digital mirror),25 pro-
`
`4
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`Wu /High-definition displays and technology trends in TFT-LCDs
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`Cross-Section of AMLCD Module
`
`
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`
`posed and developed by Texas Instruments, uses the emerg-
`ing micro-machining technology and is targeted at reflec-
`tion-type projection displays. In this technology, bistable
`pixel mirrors can be addressed with transition times ofabout
`10 us, fast enough for color-sequential TV with gray scale.
`The digital-mirror device should be capable of producing
`high-resolution and high-quality images. If all technical and
`manufacturing hurdles can be overcome, this technology
`can be a strong competitor to TFT projection displays.
`Alternatively, the SOI wafer-to-glass transfer method
`adds a significant cost to the processing and materials.26 The
`argument that using a VLSI fabrication facility will increase
`yield as compared to a TFT fabrication line seems irrelevant
`because the same facility can certainly be used to produce
`poly-Si TFr-LCDs with the same functionality (i.e. , with inte-
`grated peripheral driver circuits) at a much lower cost. It
`remains to be seen if any additional benefit derived from
`better transistor performance can outweigh the steep cost
`increase.
`
`3
`
`Recent high-density LCDs
`
`The size, resolution, and several performance parameters of
`recently announced high-density LCDs that are compatible
`with video-rate applications are listed in Table 3. The most
`noteworthy are the engineering workstations reported by six
`different companies using a-Si TFT-AMLCDs. The sizes range
`from 11 to 17 in. in diagonal. Pixel counts of 1120 x 3 X 780
`(Hitachi),27 1152 x 3 x 900 (Toshiba),28 1280 x 3 x 1024
`(NE029 and Sharp”), and 3072 x 2048 (Xerox)31 have been
`achieved. Impressive gray-level ranges, viewing angles, and
`contrast ratios have been reported. More than 4000 to more
`than 16 million colors at video rates have been achieved.
`
`Typical aperture ratios (the fraction of panel area that trans-
`mits light) for these LCD panels are 26—34%.
`Several poly-Si and a-Si TFT-AMLCDs are included for
`comparison. These displays have very high densities and
`resolutions from 420 x 3 x 220 for viewfinder display mod-
`ules32 to 1840 x 1035 for projection modules33‘3“ (approach-
`ing the required resolution of about 1920 x 1125 for HDTV),
`with 32-64 levels of gray and more than 200:1 contrast ratio.
`For projection modules, three LCDs, each controlling one
`primary color, are used to form full-color projected images.
`The aperture ratio of a poly-Si AMLCD is also larger than the
`best obtained by an a-Si TFT,35 a point to be discussed in the
`next section. Polysilicon projection panels are much smaller,
`less than 2 in. in diagonal,“ which means less-expensive op-
`tical components can be used. Another important point of
`this poly-Si TFT technology is that the peripheral driving
`circuits are fully integrated onto the TFT substrate, so that
`no separate VLSI driver chips and bonding processes are
`necessary. At high resolution and small panel size, the bond-
`ing of driver VLSI chips to LCD panels becomes impractical
`because of the close spacing of the bonding pads. This is
`why the a—Si TFT projection light valves are usually lower in
`resolution and/or larger in size. The larger the light-valve
`size, the more expensive and more bulky the projection sys-
`
`FIGURE 2 — Cross-sectional view of an assembled AMLCD module.
`
`tem. Because of the integration of driver circuitry, it is be-
`lieved at present that the poly-Si AMLCD will be dominant
`for veryohigh-density small-size projection light valves,
`while the a-Si AMLCD will be more suitable for direct-view
`
`applications.
`High—density LCDs using technologies other than a~Si
`and poly-Si TFrs are also listed in Table 3. Several new tech-
`nologies are worth noting, such as the simplified TFT proc-
`essing by CNET known as the 2S-“l'1-‘T,36 the FLCD by
`Canon,15 the color super homeotropic LCD by Stanley,37 and
`the color and video frame~rate STN by both In Focusl4 and
`Asahi.” In general, these LCDs cannot rival a-Si or poly-Si
`TFT-LCD; in image quality or screen size, although each has
`its own particular strong point. At this moment, the conven-
`tional a-Si and poly-Si TFT-LCD: remain the dominant tech-
`nologies; it will be interesting to see if any of the newly
`developed technologies can dethrone them.
`
`Technology trends for TFT-lCDs
`4
`AMLCDs are superior to other LCDs in image quality when
`using the 90° TN-LC mode. Several characteristics of the TN
`mode make it ideal for the AMLCD: (1) high ON/OFF trans-
`mission ratio, resulting in a high contrast ratio, (2) LC re-
`sponse time fast enough for video frame rates, (3) high
`resistivity for pixel-charge holding, and (4) capability of par-
`tial light transmission (about 2—5 V) without hysteresis for
`unlimited gray scales. Future improvements desired in the
`properties of the TN material to enhance TFT-LCD panels
`are: (1) increased resistivity for better charge holding, (2)
`lower temperature dependence, (3) lower threshold and
`saturation voltages to decrease the data-driving voltage, (4)
`faster response for higher frame rates, (5) thinner LC cell
`gap for better viewing angle, and (6) improved linearity for
`gray-scale control.
`The bottom-gate inverted structure is the typical a—Si
`'I'FI‘ configuration used in LCDs.1-2 Alternatively, a top-gated
`staggered structure39 has also been used. A clean interface
`
`Journal of the SID, 2/1, 1994
`
`5
`
`SAMSUNG EX. 1010
`
`

`

`Pixel Circuit Operation
`
`Data
`Line
`
`Gate Line
`
`
`
`Common
`Electrode
`
`CST/Gate
`Line
`
`VP
`
`AVp
`
`FIGURE 5 — Circuit operation at the pixel level of TFT-LCDs.
`
`Based on such TF'I‘ structures, the TFr-LCD module is
`constructed with driver chip connections and backlighting,
`as shown in Fig. 2. The LG material is sealed between the
`TFI‘ plate and the color-filter plate. The color-filter plate
`contains the black matrix, pixel color filters, a passivation
`layer, and the transparent conducting indium-tin-oxide (1T0)
`film common electrode. LC alignment is generally achieved
`by rubbing a thin polyimide layer on both plates. The LC cell
`gap is controlled by dispensing spacer balls.
`A schematic of an AMLCD-TFT array is shown in Fig. 3.
`TFTs, acting as switches controlled by the gate electrodes,
`connect the data lines to the pixel electrodes. For high-den-
`sity displays, the row-driver chips for the TFI' gate lines are
`usually on one side, while the data-driver chips alternate
`between upper and lower edges. This design is used be-
`cause the bonding pad spacings are approaching bonding-
`technology limits.
`The pixel layout is shown in Fig. 4. A black-matrix
`layer is generally formed on the color-filter plate to block
`the light to the TFT and to shield the space between the ITO
`pixel electrodes. The aperture ratio, which strongly affects
`display clarity and power consumption, is defined by the
`black-matrix layer. Recently, integration of the black matrix
`onto the TF1" plate has been shown to dramatically increase
`the aperture ratio of the display.33-4° The LC and TFI‘ gate
`capacitances (CLC and Cg? in Fig. 5) must be supplemented
`by a storage capacitor (C5,) to maintain the pixel voltage dur-
`
`TFT Array Circuit for LCD
`UPPER COLUMN DRIVER
`
`LOWER COLUMN DRIVER
`
`ROW DRIVER
`
`FIGURE 3 — Schematic of an AMLCD matrix layout showing the display
`cell, driver circuits, and their connections.
`
`between the a-Si and the gate dielectric, achieved by se-
`quential deposition without air exposure, is the key to ob-
`taining good TFT characteristics and stability. Other critical
`processing modules include: step height control or tapered
`metal gates; proper plasma~enhanced chemical-vapor depo-
`sition (PECVD) conditions for SiNx gate dielectric, a-Si, and
`etch-stop top SiN,l layers; a mobile-ion barrier; and an am-
`bient-moisture-barn'er passivation layer.
`
`Pixel Layout in a-Sizl-l TFT LCD
`
`om
`
`rrr
`
`sfi' firerrwrwtromm;
`(’55:;
`I (III?
`5! Gate Line
`,
`I f ‘E
`““IIIII$
`
`Pixel
`ITO
`
`nu u .
`
`
`
`q
`s\\\\\\fi\k\\r\x\
`
`a
`
`. («ctcttrctcc-zv;l
`
`FIGURE 4 —— An example of pixel layout for TFT—LCDs.
`
`6
`
`Wu/High-definition displays and technology trends in TFT-LCDs
`
`SAMSUNG EX. 1010
`
`

`

`Table 4. Various structures and technologies used in large-area a-Si TFT-LCDs
`TFT
`Gate-line
`Gate
`Data-line
`Display
`Publication
`structure
`materials
`dielectric
`materials
`size (in.)
`date
`
`Company
`
`Etch-stop
`
`Toshiba
`
`Matsushita
`
`NEC
`
`Hitachi
`
`Sharp
`
`Sanyo
`
`Mitsubishi
`
`Hosiden
`
`Al/MoTa
`
`SiN/SIO,
`
`Al/Mo
`
`Cr/Al
`
`SiNflaO,
`
`Al/Ti
`
`Cr/Al
`
`smgsm,
`
`Al/Cr
`
`SiN,
`
`SiNx
`
`5m,
`
`Al
`
`SileAlzos
`
`Al/Cr
`
`None
`
`Ta/Al
`
`SIN/Tagos
`
`ITO/Ti
`
`Sim/11205
`
`Al/Mo
`
`Mo—Ta
`alloy
`Cr
`
`Al
`
`Inverted
`stagered
`Inverted
`stagered
`Inverted
`staggered
`Inverted
`staggered
`Inverted
`staggered
`Inverted
`staggered
`Inverted
`stagered
`Top-gate
`staggered
`SID '93
`13.2
`SiN,
`Al/Cr
`SIN,r
`NA
`Inverted
`Xerox
`staggered
`
`13.8
`
`15
`
`12.9
`
`11
`
`17
`
`10.4
`
`10.3
`
`9.5
`
`SID ’92
`
`SID '92
`
`SID '92
`
`japan Display ’92
`
`SID '93
`
`SID '91
`__
`SID ‘91
`
`[BBC ’91
`
`SiN,
`
`SIN,
`
`SiN,
`
`Al/Cr
`
`SiN‘
`
`SiNx
`
`Al/Cr/ITO
`
`None
`
`ing the frame time. The other electrode of the C,, is con-
`nected either to the previous gate-line (Cn on gate) or to an
`independent metal line (0,, on ground) to which the appro-
`priate dc voltage is applied. The former design can realize a
`higher aperture ratio, but a lower-resistivity gate-line is nec-
`essary. A lowcresistance gate-line is desired to minimize ad-
`verse effects on gray-scale uniformity and image sticking
`from the gate-pulse distortion caused by the RC delay,
`which is very severe for high—definition displays. The re-
`quirement for the scan~bus~line resistivity is less stringent in
`the latter case, since the scan-line capacitance is reduced.
`Notice that the C3, does not contribute directly to the scan-
`line capacitance because it is serially contacted to ground
`through Cw. CLC dominates in the contribution to the scan-
`line capacitance since it typically has a smaller capacitance
`than C“.
`The parasitic gate-to—pixel capacitance (Ca, in Fig. 5)
`is the major issue in pixel design for displays having gray
`scale.5-41 Cgp can be modeled approximately as being com-
`posed of two components: the gate-tom“ diffusion overlap
`capacitance (COL), which is always present, and a fraction of
`the channel capacitance (fCCH, f = 1/2), which is present
`only when the TFI‘ is on; that is, when the gate voltage ex-
`ceeds the data voltage by at least the TFT threshold. Thus,
`
`Ca, 8 COL = LOLWEOEaa/tor when Vgate—Vdata S Vth, (l)
`
`C87 S COL + m6" — (L01: + WIJWEIfia/tox '
`
`wmnnm-nmfla.
`
`a)
`
`where W, L, and L01. are the TFI‘channelwidthJength,
`andgateoverlap,respectively.Thetermse0 and so, denote
`the permittivities of free space and Si02, respectively, while
`
`to, is the thickness of the SiOz. The feedthrough voltage
`AVp can be expressed as:
`
`Cgpwdm) AVgate
`= ..__.___.__..__.__ _
`AVPde) Cngd...) + Csr'l' C“Wang, )
`
`(3)
`
`LCD Driving Methods “$370739:
`
`999999
`First
`9999992»
`Frame
`
`999999mm
`£939.39
`'00'0906
`nnnnnn
`
`(a). Frame Inversion
`o Flicker is over
`entire display
`
`(b). Gate Line Inversion
`o Data line
`crosstalk reduced
`
`
`
`060060
`
`
`nnnnnn
`
`
`nnnnnn
`
`
`nnnnnn
`nnnnnn
`
`
`nnnnnn
`_ Flicker Dispersed by Spatial Averaging —
`
`
`fiflflflfifi
`
`
`QEEEEE
`
`
`DQBQBE
`
`
`HEEEEE
`
`
`BDQGDE
`
`
`HEEEEQ
`
`999999
`999999
`9.9.9.939.
`nnnnnn
`nnnnnn
`
`(c). Column Inversion
`0 Power consumption
`
`nnnnnn
`nnnnnn
`nnnnnn
`nnnnnn
`nnnnnn
`nnnnnn
`
`nnnnnn
`nnnnnn
`nnnnnn
`nnnnnn
`nnnnnn
`nnnnnn
`
`'00'00'60
`060663
`(d). Dot In ersion
`o Data‘t’o pier capacitive EQEBED @3030“
`coupling reduced
`DEBEOE
`HDEDEB
`fifimMWM" nnnnnn.
`nnnnnn
`0 Image sticking concern Banana
`EDEGED
`reduced
`606006
`006000
`
`FIGURE 6 — Driving methods to provide alternating drive polarities for
`TFT-LCDs.
`
`Journal of the SID, 2/1, 1994
`
`7
`
`SAMSUNG EX. 1010
`
`reduced _
`
`

`

`A low-resistivity Al film is used by many groups to achieve
`low gate bus-line resistance. PECVD—deposited silicon ni—
`tride (SiNx) is the most common gate dielectric. Recently, to
`enhance yield and reliability, double-layer dielectric films
`have become very popular. These films include: anodic
`A1203, anodic Ta205, and atmospheric—pressure CVD SiOz.
`However, PECVD SiNx is always used for the interface layer
`because it produces a low—threshold TFT. Double- or triple-
`layered data bus—lines were used by all groups to avoid bus—
`line defects and diffusion of Al.
`
`Self—alignment fabrication of a-Si TFT is one of the
`most important procedures used to reduce the parasitic ca-
`pacitance, Cgp, of the bus line. This structure is achieved by
`using back-side flood exposure and patterning of the etch-
`stop layer on top of the channel a-Si with respect to the gate
`electrode. The self-alignment reduces the overlap capaci-
`tance [COL in Eqs. ( 1) and (2)] between the gate and heavily
`doped source/drain regions. Except for Hitachi in Table 4,
`all companies using the inverted-staggered structure are us—
`ing SiNx as etch—stop. Hitachi relies on the independent
`metallization line to the pixel C3, to tolerate a larger Cgp.
`To further illustrate the importance of self-alignment
`(SA), the aperture ratio as a function of field-effect mobility
`is plotted for three different TFT structures in Fig. 7.2343
`Here, non-SA refers to a TFT structure with its geometries
`(W and L) defined by photolithographic masks without self-
`alignment. Semi-SA refers to a structure using etch-stop to
`define the channel L by self-aligned back-side exposure
`with respect to the gate metal; an n’“ a—Si film deposited on
`top of the etch—stop, overlapping the gate, was used to make
`source and drain contacts. In the fully SA case, ion implan-
`tation of phosphorus to dope the source and drain regions is
`
`Aperture Ratio, %
`so
`
`Simulated for a-Si: H TFT (after Tanaka et. al., sun 92)
`
`‘° ‘ “N FE=0.7cm2/Vs
`\.
`_
`N"'\.
`L -‘
`
`\N.
`‘\~\~
`Pra=06cm2/Vs\ \
`'
`
`PFE=0.5cm2/Vs
`
`"R
`‘\
`
`\\\
`
`\-
`
`\~
`
`\
`
`\ \
`
`\
`
`\
`
`\
`
`\
`
`‘
`J
`
`A]
`_
`
`fl
`
`.
`
`—
`
`
`
`-
`
`
`
`30
`
`_
`
`20 —
`
`‘
`
`'
`1o -
`
`f
`
`Aperture Ratio. %
`
`45 r—‘r—r—r—I—‘rfi'fi—‘r—r—I—fl—r—‘v—l—r—r—r—I—j—r—w—r—r—
`Simulated for u-Si:H TFT (after Tanaka et. al., SID 92
`and lbaraki, Japan Display 93L
`
`4° —
`
`,,--
`
`I
`
`.
`
`.
`
`Semi-SA TFT
`r
`/'1
`‘/
`
`
`
`
`.25
`
`.5
`
`.75
`
`1
`
`1.25
`
`1.5
`
`Field Effect MOblllty,cm2/Vs
`
`FIGURE 7 — The effects of TFT structures and field-effect mobility on
`the aperture ratio of a simulated high-density large-area LCD.
`
`The Cgp causes a feedthrough voltage shift (AVP in Fig. 5)
`from the gate to the pixel when the TFT turns off, VP being
`the pixel voltage. Since the LC capacitances in the ON and
`OFF states are not the same, different feedthrough voltages
`between AVP(ON) and AVp(OFF) cause image sticking and
`other crosstalk behavior due to a residual dc component in
`the LC. The storage capacitor reduces this behavi