`CHAPTER “90: DISPLAY MHIIHICTIIRING
`
`LIQUID CRYSTAL FLAT PANEL DISPLAYS
`
`is either being designed or is in prototype form. Not all the test and repair
`equipment needed for manufacturing has be invented yet, and a continuous flow
`of new products is expected for the next decade. Multiple items of each category
`may be required for manufacturing, including optical or voltage imaging for defects.
`
`Table 2-18 Iii—Process Inspection and Repair Equipment List
`
`Equipment
`
`Remarks
`
`Electrical Parametric Test
`Substrate Flatness
`
`Sheet Resistivity Monitor
`
`Critical Dimension Measurement
`Particle Monitors
`Optical Microscope Inspection
`
`DigitaUAnalog Optical Inspection
`Voltage Imaging or
`other TFT imaging
`
`Laser Cutting
`
`Laser Deposition
`
`Design based on IC test equipment
`Similar to IC equipment
`
`ITO, metal line monitor, off-line and
`in-situ
`
`Similar to IC equipment
`Similar to IC equipment
`Similar to IC equipment
`
`Based on maskfwafer inSpection of ICs
`Specific design for TFT arrays
`
`Cut shorted metal lines -combine
`
`operation with laser deposition
`
`Metal deposition to repair Opens
`
`Table 2-19 is a list of assembly equipment with pricing for each item. The price
`includes semi~automatic cassette to cassette handlers, but does not include
`
`transport equipment from equipment item to equipment item. Equipment for
`assembly includes the actual joining and sealing of the substrates, separation and
`final testing, and die attach equipment for placing driver circuits on the completed
`panel or on flexible circuit boards which are attached to the panel.
`
`2.5.2 MANUFACTURING COST AND YIELD
`
`Yield vs ASP
`
`Yield considerations will be even more important for AMLCD manufacturing
`than for integrated circuit production. Figure 2-21 shows a simulated yield chart
`based on differing levels of defect densities and display sizes. This chart,
`developed by the process consultant N. Yamamura and published in the August,
`1990 issue of Nikkei Mierodevices, shows the dramatic effect of defect density
`
`on yield and cost of color TFT displays.
`
`In”!
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`
`

`

`CHAPTER TWO: DISPLAY HAN UFAC'I'IIRIHG
`
`LIQUID CRYSTAL FLAT PANEL DISPLAYS
`
`Table 2-19 Assembly and Die Attach Equipment
`
`
`PricelRemarks
`Equipment
`————-—-—_______.._________.____
`
`Orientation Film Printer
`
`Orientation Film Rubbing
`Substrate Cleaning
`Spacer Spraying
`Seal Printing
`AlignmentISealing
`
`Liquid Crystal Injection
`Scribe/Break
`
`Die Attach Equipment
`
`$180K
`
`$80K
`$120K
`$35
`$120K.
`
`$560K includes cassettefcassette han-
`
`dling
`$40K
`$65
`
`$500K for both inner and outer lead
`
`TAB bonder
`
`
`Figure 2-21 Simulated yieid curves for various defect densities in TFT display
`manufacturing
`
`Yield (as;
`100 -
`
`Source: Mikkel Mfcrodevices
`
`80
`
`so
`
`4o
`
`20
`
`
`
`Display Size (Inches)
`
`104
`
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`
`

`

`
`CIIM'TER mo: DISH“ MANUFACTURING
`
`LIQUID CRYSTAL FLAT PANEL DISPLAYS
`
`On the left hand side of the illustration is the calculated yield vs display size line
`corresponding to 0.1 defectSICmi. This solid line is about the defect density for
`small LCD displays, like those used for calculators and watches. It is impossible
`to build large TFT displays at all with this defectdensity. The nextlevel of defects,
`0.01 defects/c1112, corresponds approximately with the current manufacturing
`practice in Japan. At this defect level, shown by the dashed line. a 10" diSplay
`yield is about 20% at best, and corresponding display pricing is $2000 each. A
`much reduced defect level, 0.003 defectsfcmg. results in a higher yield curve
`shown by the dotted line. At this defect density, the next step for TFT manufac-
`turing, a 60% yield can be obtained. This yield might be expected in the mid
`19905, and will result in an average selling price of $800.
`
`A much lower defect density, 0.001 defectsfcmz, will lead to yields in the range
`of 80% for displays, and a corresponding selling price of $500 each or less. This
`level of defects, a total of 10 defects per square meter of substrate, is a lower level
`that is currently achieved in semiconductor manufacturing. Of course, the size of
`the critical dimension in TFT manufacturing is larger, but absolute defect levels
`are much more important for displays than for integrated circuits where many
`chips are manufactured at the same time on the silicon wafer.
`
`Manufacturing Cost Model
`A manufacturing cost analysis of a high definition TV factory based on AMLCD
`displays has been presented recently. According to this analysis, substantial
`differences exist between flat panel display and IC manufacturing. A written
`presentation by G. Resor, entitled “The Surprising Economics of Flat-Panel
`Production”, was published in the Society for Information Display Technical
`Digest, p186, 1990. One of the principal differences between AMLCD and IC
`manufacturing is the optimum factory size, which is relatively small for displays
`at about 500,000 starts per year. Another difference concerns the relative
`importance of the cost of capital and materials. For integrated circuits, especially
`cost competitive products such as DRAMs, the initial investment dominates all
`other costs. For displays, the situation is quite different, and materials costs are
`dominant. Figure 2-22 shows the cost breakdown for a “minifab” with 500,000
`display starts per year. The factory productin this case is a completed 14 inch high
`definition television. The cost analysis indicates that material costs account for
`more that 50% of the total, with depreciation and R&D costs at less than 10%
`each. The factory price of $862 per completed TV applies to the fifth year after
`the start of the project, and the factory is still ramping up production. After ten
`
`10”
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`

`

`
`CHIPTER TWO: DISPLAY HAHIIFACTIJRING
`
`LIQUID CRYSTAL FLAT PANEL DISPLAYS
`
`years of operation, production costs have declined to levels competitive with
`CRT-base TV sets, but materials costs continue to dominate.
`
`
`
`
`
`HDTV “Minifab” Manufactur-
`
`Figure 2-22 Cost components of
`flat panel display production.
`
`ing Costs (5th Year of Project)
`
`
`
`Material
`
`Labor
`
`RSID
`
`Gross Profit
`
`Depreciation
`
`Other
`
`
`$862 ASP (1989 DOLLARS)
`
`Manufacturing Yield Model
`A comparison of TFT—LCD manufacturing to DRAM manufacturing reveals
`significant differences. (See R. R. Troutman, “Forecasting Array Yields for
`Large—Area TFT—LCDS”, Society for Information Display Technical Digest,
`1990. p197) Partially good displays are not acceptable, and the probability of a
`fault must be small overa large area. A Poisson distribution is used to analyze and
`predict the yield in this situation.
`
`The most common single cell fault is a pinhole short in a storage capacitor because
`of the large area of the capacitor. The resulting high leakage path produces a fixed
`ON or OFF condition, depending on voltage and polarizer settings. Another
`single cell fault is a source to drain short in the data metallization, which prevents
`charge transfer. A Poisson distribution of faults is constructed, and it is assumed
`that a few single cell faults are tolerable, as long as they are not clustered. If 10
`single cell faults are allowed, then the yield is 100% at 10 faults, about 50% at 12
`faults, and 20% at 15 faults. However. to ensure 99% yield at 10 allowable faults,
`the average number of faults has to be fewer than 5 per array.
`
`Faults other than single cell faults are intolerable, and must be repaired. If a gate
`
`”"5
`
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`
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`
`

`

`(IMPTER TWO: DISPLAY ”HUFACTURIHG
`
`LIQUID CRYSTAL FLAT PANEL DISPLAYS
`
`or data line is open, it is easily detected by an electrical continuity check. Repair
`consists of laser welding a spare line driver to the initially undriven end of the line.
`
`An interlevel short is repaired by laser Cutting either the gate line or the data line
`
`and then treating the cut line as an Open line. Two types of repairability are
`described.
`
`Type 1 repairability is the case where gate line repairs are made only for open gate
`lines. In displays using the inverted staggered TF1" structure, the gate is buried
`
`under several thin film layers, and laser scribing these lines might cause a yield
`loss by itself. Data lines are more accessible, and data line repair options cover
`
`both open lines and interlevel shorts.
`
`Type 2 repairability assumes a high yielding technique for scribing gate lines in
`
`the array. Type 2 repairability is significantly better than Type 1 only when there
`are many Spare gate lines available to repair interlevel shorts.
`
`Total array yield is the product of single cell fault yield and repairable fault yield.
`lezYchrr. If the single cell fault yield can be raised to 99%, then the overall yield
`is determined by the repairable fault yield. Table 2-20 shows the relation between
`
`yield, defect size, and defect density without repairability. The calculation is
`performed for a 640x480x3 array, and assumes an average defect size of 1pm.
`
`Table 2-20 TFTArray Yt'eld Summary (No Repair)
`
`Yield
`
`10%
`
`50%
`
`90%
`
`FaultsIDisplay
`
`Faultsllcm2
`
`-
`
`2.3
`
`0.68
`
`0.10
`
`0.24
`
`0.072
`
`0.01
`
`Consider a leit DRAM line operating at 50% yield. For a chip area of 0.50m2,
`the allowable fault density is 1.5/cm2. Compare this to the value of 0.072 from the
`table, and one sees that without repair, the fault densities for a TPT array must be
`
`21 times lower thap er Qurrent DRAM mangfagtigijng.
`
`Repajrability of the data lines and gate lines can significantly improve the yields. By
`providing 5-10 extra lines for repair, the major challenge to achieving high yields is
`interlevel (crossover) shorts. This fault type must then be maintained below 1 faultfcmz.
`
`1m
`TCL 1017, Page 123
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`
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`
`

`

`
`CHAPTER TWO: BISPLAY MAHIJFRCTURIHG
`
`LIQUID CRYSTAL FLAT PANEL DlSPLAYS
`
`AMLCD Factory
`The NBC Kagoshima plant in Kyushu is manufacturing 10" flat panel TFT displays,
`and has achieved 50% manufacturing yields, according to an article in the August
`1991 issue of Nikkei Microdevices. The yield might go as high as 70% in certain
`instances. The plant is inputting 5000-7500 substrates per month, which, 0n the
`300x350mm substrate, comes to a potential 10,000- 15,000 10" displays per month.
`Factoring in the yield, output is 6000-9000 displays per month.
`
`The manufacturing equipment is interconnected only where this is easy to do.
`Substrates are transported around the factory by automatic ground vehicles.
`These vehicles, which are provided with HEPA filters, are used to transport the
`heavy 20-substrate carriers, and to avoid particulate contamination which would
`OCCur if an Operator performed the transport. Operators are used to load carriers
`onto the equipment, which may be an in-line series of individual processes.
`Equipment such as for photolithography is not fully interconnected. Computers
`are used to control the operation and collect information regarding machine
`performance.
`
`NBC has made a high yield process using their LSI experience, and finds the
`process quite similar to LSI. They don‘t understand the cause of the yield
`problems in TFI‘ fabrication reported at other companies. In NEC’s opinion, the
`cell assembly processis a much stricter and more difficult to control process. The
`TFI‘ manufacturing equipment is very like the LSI processing equipment already
`in use at most companies who are making TFT‘s, and the equipment is made by
`large, reliable companies. In contrast, the equipment for cell assembly is made by
`small and medium sized companies, and may not be available.
`
`Figure 2-23 shows the floor plan of the factory. The upper floor contains the TFI‘
`panel fabrication area. This areais composed of lithography, cleaning, sputtering,
`CVD and so forth. An elevator connects this floor to the assembly area below. The
`floorspace for the clean room is 40mx90m, and the total area is 4000 m2 per floor.
`20% of the clean room is class 100, 20% is class 1000, with the rest class 10,000.
`For the lithography area, class 10 is used. In the cell assembly anea, the equipment
`and processes that generate particles, such as the rubbing equipment, are enclosed.
`
`Right now, completed TFT panels are sent downstairs, and purchased color filter
`panels are started into the cell assembly process. Module assembly, with back-
`light occupies 2000 m2 in another building.
`
`108
`
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`
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`
`

`

`
`CHAPTER TWO: DISPLAY MHUHCIURIHG
`
`LIQUID CRYSTAL FLAT PANEL DISPLAYS
`
`Figure 2-23 NEC two story AMLCD fabrication facility layout
`
`Plasma CVD
`
`Sputten‘ng
`
`DryEtching
`
`Expansion Area
`Cleaning
`
`Inspection
`
`Seal P rinting
`
`Spacer Spraying
`
`Orientation
`FilmPnntm-
`
`Orientation
`Fllm
`Pnntin-
`
`.
`
`Assembly
`
`163.14%
`Injection
`
`mgegtion
`Sealing
`
`Panel
`
`Polarizer
`Attach
`
`Color Filter
`(Purchased)
`
`109
`
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`
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`

`

`
`CHIPTEII TWO: DISPlAY MHUFAC'I'URIHG
`
`LIQUID CRYSTAL FLAT PANEL DISPLAYS
`
`The factory is Operated with 250 people on three shifts. In addition to the
`Operators, there are 100 additional staff, and of these, 70 are technicians. 25-26
`
`are LCD specialists, and the others are from the semiconductor process area. New
`technology transfer takes 3-6 months.
`
`The TFT area has room for expansion to 10,000 substrates per month. It has been
`converted for 10 inch panel manufacturing- New equipment was installed and
`pilot manufacturing was initiated. The biggest problem was with the lithography
`and plasma CVD equipment. Then, Nikon stepper equipment was adopted. Each
`panel requires 4 exposures. Stitching accuracy is 0.22pm. Defect inspection
`equipment has not been chosen yet. The operation rate of the equipment has
`reached LSI levels. 4 stepper lines are in operation.
`
`Anelva supplied the plasma CVD equipment for 3 lines. Due to particle buildup,
`thorough periodic maintenance is required. Uptime of 50% is achieved. There are
`3 sputtering lines and 5 dry etching lines, also from Anelva. Resist coating lines
`come from Dainippon Screen. Particle inspection equipment is from Hitachi
`Engineering.
`
`The 4096 color diSplay produced at this factory has a diagoual measurement of
`9.8", with 640xRGBx400 lines. Each color pixel measures 0.11x0.33mm2.
`Contrastis 80:1. A 20:1 contrast ratio or greater is maintained for a viewing angle
`of $20” vertical, :350 horizontal. Response speed is 20ms. Brightness is 800d!m3.
`Backlight power consumption is 12 W. TFT design employs a bottom gate
`construction, with a 2-layer, ITO/Crconstruction. Gate insulator is SiO2 plus Si3N4
`deposited by plasma CVD. Gate line and data line metal is chromium, and
`
`transparent electrode is ITO. The gate electrode, channel, dielectrics and so forth
`are dry etched. Defective pixels number 10 or less in completed displays.
`Defective blue pixels are most easily detected, and two defective blue pixels can
`reject the entire display. As the level of cleanliness of the clean room improves,
`the defect level is expected to decrease.
`
`110
`
`TCL 1017, Page 126
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`
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`
`

`

`
`CHAPTER TWO: DISPLAY MNUFICTURING
`
`LIQUID CRYSTAL FLAT PANEL DISPLAYS
`
`REFERENCES
`
`
`1 . F. Okamoto, “Glass S ubstrates forLiquid Crystal Displays”, Technical Proceedings, SEMICON
`Kansai 1990. SEMI, 1990, p380.
`
`2. J. Makino, "Glass Substrates for Flat Panel Display," Technical Proceedings, SEMICON
`Kansai 1991, SEMI, 1991. p111.
`
`3. From "Liquid Crystal Displays, 1990", Nikkei Business Publications, Tokyo, 1990.
`
`4. A. Tarnada, "West Process Equipment for LCD", Technical Proceedings, SEMICONI’Kansai
`1991. SEMI Japan. 1991. p460.
`
`S. Tamada, ibid, p465,
`
`6. RJ. Saia, RF. Kwasnick, and CY. Wei, "Selective Reactive Ion Etching of ITO in a
`Hydrocarbon Gas Mixture", J. Electrochem. Soc. 138. 493 (1991).
`
`7. S. lshibashi et a1, "1.0X10-4Q-cm ITO Films by Magnetron Sputtering", presented at the first
`International Conference on Sputtering and Plasma Processing, (Tokyo) Japan, February, 1991.
`
`8. S. Takaki et 31., "Preparation of Highly Conducting ITO Electrodes on Color Filters by Highly
`Dense Plasma-Assisted EB Evaporation", Society for Informatioa Display Technical Digest,
`1990, p?6.
`
`9. WJ. Latham and D.W. Hawley, "Color Filters from Dyed Polyimidcs", Solid State Technol-
`ogy, May 1988.
`
`10. S. Nemoto, "Color Filters for Liquid Crystal Display". Technical Proceedings, Semieom'
`Kansai 91 FPD Seminar, p162, 1991.
`
`11. Michael Goldowsky, Economical Color Filter Fabrication for LCDs by EIectro-Mist Depo-
`sition, Society for Information Display Technical Digest, 1990, p80.
`
`12. A. Tamada, op. cit. p460.
`
`13. Y. Oana, "Technical Developments and Trends in A-Si I'l1'1'-LCDs" Flat Panel Display
`Process Tutorial, Semicon West. May 23, 1991.
`
`14. El . Henley and G. Addiego, "In-line Functional Inspection and Repair Methodology During
`LCD Panel Fabrication". Society for Information Display Technical Digest, 1991, p686.
`
`15. J. Vanney, "Automated Equipment Considerations for Liquid Crystal Display Production",
`Electronic Imaging. Science and Technology: Feb. 11-16, 1990, Santa Clara, CA, USA.
`
`16. K. Adachi, "Packaging Technology for LCD", SEMICONI’Kansai Technical Proceedings
`1991, Semi Japan, 1991, p119.
`
`17. K.J . Hathaway et. al., “New Backlighting Technologies for LCDs", Society for Information
`Display Technical Digest 1991, p751.
`
`lll
`TCL 1017, Page 127
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`
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`
`

`

`CHAPTER THREE; PRODUCT HATERIAIS
`
`LIQUID CRYSTAL FLAT PANEL DISPLAYS
`
`Materials for Flat Panel Displays
`
`The particulars of materials for flat panel displays are presented in this part of the
`book, emphasizing the unique requirements for active matrix displays. In many
`cases, the ultimate combination of materials performance, quality and price has
`
`not yet been achieved. Particularly for color filters, significant cost reduction is
`essential for high volume manufacturing. For other materials, cost effectiveness
`
`is determined primarily by beneficial effects on transistor manufacturing yield.
`
`Glass Substrates
`
`m
`
`Glass substrates of various types are used for liquid crystal displays. Substrate
`types were discussed in Part 2, with emphasis on fusion glass as an active matrix
`substrate. This section describes float glass manufacturing and ITO deposition.
`
`Advantages of float glass material were pointed out in a recent article by Pilkington
`
`Micronics [1]. Flat glass was made by drawing processes until the advent of the
`
`float process, in which molten glass is allowed to settle onto a bath of molten tin.
`The glass achieves a uniform thickness with smooth surfaces on both sides.
`
`Stringent glass specifications are required for LCD applications in spite of the fact
`
`that this constitutes less than 1% of the world-wide flat glass market.
`
`Some difficulty in obtaining the tolerances on glass properties, especially surface
`
`smoothness, arose from processing float glass for LCDs in the same furnaces used
`for architectural and other purposes. These furnaces might have a production
`capacity of 5000 tons per week, enough to satisfy the world market for LCD
`applications in a few days. By moving the float glass manufacturing for LCDs to
`
`a small dedicated float line, materials specifications were improved.
`
`Float lines can produceB meterwide continuous ribbons with thickness variations
`of i0. lum, negligible warp, and microcorrugation controlled to averages of less
`than 0.1mm. On-line CVD processes are also used by this glass manufacturer to
`provide SiO2 coated surfaces. Characterization of microroughness will allow
`classification of substrates, and reduced polishing of selected classes of glass can
`be achieved for STN applications. Eventually, float glass suppliers hope to
`
`eliminate the polishing process completely.
`
`..
`
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`
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`
`

`

`CHIPTER THREE: PRODUCT HATEIIMIS
`
`LIQUID CRYSRAL FLAT PANEL DISPLAYS
`
`Processing the base glass substrate into a form useful to the display manufacturer
`is a multi—step process, with several Japanese firms offering services ateach step.
`Table 3-1 shows the types of operations for LCD glass and suppliers at each step.
`Processes for SiO2 and ITO coating are indicated. Various glass fabricators
`supply material to several firms for cutting and polishing. SiO2 coating may be
`done be another firm, and ITO by a fourth.
`
`Table 3-1 LCD Glass Supplier Matrix in Japan
`
`
`
`
`
` Bare Glass Cutting/Polishing SiO2 Coating ITO Coating
`
`Asahi Glass
`
`Kuramoto
`
`Matsuzaki
`
`Matsuzaki
`
`NSG
`
`Fujimi
`
`Central Glass
`
`Mitsum
`
`(dipping)
`Tokyo Ohka
`(dipping)
`Kuramoto
`
`(sputter & evap.)
`Asahi Glass
`(sputter & cvap.)
`Sanyo Shinku‘
`
`N.E.G.
`
`N.E.G.
`
`Asahi
`
`(BLC & OAZ types)
`
`Corning
`
`I'I'O Sputtering
`
`(sputter)
`NSG
`
`CVD Central
`
`Seiko Epson
`
`Sharp
`
`Optrex
`
`Sputter
`
`* 100% Sharp subsidiary
`
`ITO sputtering is a crucial and ubiquitous part of the flat panel display industry.
`The material is deposited from a sintered powder source to a thickness of
`1500-3500A. Important deposited film properties are transparency, resistivity,
`and ease of patterning. Transmittance of 90% and resistivity of 1-3ttohm~cm are
`normal values for these properties. These are a sensitive function of the oxygen
`content of the film, and oxygen is often added in the gas stream during sputtering,
`to make up for any loss of oxygen during transport from the ITO target to the glass
`substrate. Deposition temperature is a critical parameter of the process.
`
`The higher the deposition temperature, the lower the resistivity of the film.
`
`Previously, a glass substrate temperature of 300°C was common for ITO depo-
`
`114
`
`TCL 1017, Page 129
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`
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`
`

`

`
`CHAPTER THREE: PRODUCT HITERIILS
`
`LIQUID CRYSTAL FLAT PANEL DISPLAYS
`
`sition, but 200°C is necessary for color LCD displays, where the polymer color
`filters cannot withstand the higher temperature. Lower temperature deposition
`has been made possible by the new, higher density targets, which can provide
`uniform deposition at higher magnetic fields than the previous model, which
`allows for lower deposition temperatures. Table 3-2 shows film properties of
`deposited films. Ulvac has described a process for low temperature sputter
`deposition which provides resistivities as low as 5 QIsquare.
`
`Table 3-2 ITO Film Properties
`
`
`
`
` Deposition Conditions ResistivityfResistance Application
`
`350°C, 1000A, e-beam
`
`1.5x10-4 ohmicm
`
`131w STN ~20 ohm!
`
`or sputtering
`200°C, 1000A. e-beam
`sputter
`
`2x104 ohmicm
`
`square
`Color Filter (requires or
`2000A for 10 ohm;
`
`square)
`As low as 5 ohm/square
`Ulvac - new system
`is possible at low temp-
`200°C sputter
`erature for color filter
`
`1.2x“ ohmlcm
`
`ITO targets are usually prepared in the form of rectangles about 15"x5" in size,
`about 6 mm thick. The material is usually cold-pressed, then sintered after
`isostatic pressing. Usual composition of the target is 10 wgt% SnOz. After ma-
`chining to the correct final size and shape, the ITO material is bonded to a backing
`plate using some form of indium or indium alloy bonding preform. Table 3-3
`shows the currently available target densities available from suppliers. The higher
`density material is more difficult to prepare, but provides longer target life and
`fewer deposited InO black particles.
`
`Table 3-3 Characteristics ofITO Targets
`
`Target Life
`InO
`(using high density life
`black particles
`Density
`as ~100%)
`
`
`Low (70%)
`
`Medium (80%)
`
`High (95%)
`
`most
`
`some
`
`fewest
`
`70%
`
`80%
`
`~100%
`
`11?
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`
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`
`

`

`
`CHAPTER THREE: PRODUCT EMERIILS
`
`LIQUID CRYSRAL FLAT PANEL DISPLAYS
`
`Sputtering processes consist of bombarding a material such as ITO with energetic
`argon ions. These ions dislodge atoms from the target, and these atoms travel to
`the substrate under the influence of electric and magnetic fields. If the composi-
`tion of the target is non-uniform, less desirable results can occur. These include
`
`non-uniform sputtering across the target, which shortens its useful life. In addition,
`non-uniform density or composition can lead to absorbing particle formation.
`
`The rectangular target is used in large batch or in-Iine sputtering equipment made
`by a variety of suppliers in Japan, including Ulvac, the largest commercial
`supplier, Anelva, Asahi Glass (makes equipment for their own use), and Shinku
`Kikai. For very large systems, the targets can be used two at a time, forming a one
`meter long sputtering source. Glass panels have a wide range of sizes, both
`because many products may be prepared in a single factory, and also because no
`standard sizes have been developed by the manufacturers.
`
`Erosion of the target by the sputtering process is not uniform, and an elliptical
`cavity is etched out more or less in the center of the rectangle. Because a lot of ITO
`is left in an unusable form at the end of life of the target, and because the material
`is somewhat expensive, there is some interest in reclaiming the targets.
`
`Film properties may be enhanced by annealing in air after deposition. S ubsequent
`patterning and etching forms the transparent electrodes whose function was
`discussed in the previous section.
`
`ITO Sputtering is mostly done by glass coaters, primarily Matsuzaki Shinku and
`Asahi Glass. Glass can be supplied in coated form, except for the panels used for
`the active matrix TFT backplane. These are ITO coated after transistor fabrica-
`tion, and this will be done internally at the display manufacturer.
`
`Requirements for glass type, ITO thickness and other parameters vary according
`to the type of display being produced. Table 3-4 is a summary of the types of
`displays, ITO resistivity, deposition methods, and type of substrate.
`
`3.2.1 ITO POWDER AND THIN FILM PROPERTIES
`
`ITO targets depend on powder formulations of consistent particle size and shape,
`as well as adequate purity. Powder sources are Nippon Mining and Mitsui Metal
`Mining, with some material produced by Dowa Mining and Osaka Asahi Metals,
`and perhaps Sumitomo Metal Mining for internal use. Nippon Mining and
`
`116
`
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`
`LOWES 1017, Page 131
`
`

`

`
`CHAPTER THREE: PRODUCT MATERIALS
`
`LIQUID CRYSTAL FLAT PANEL DISPLAYS
`
`Table 3-4 Glass Substrates for Liquid Ciystal Displays
`
`
`Application
`
`ITO Deposition
`Resistivity
`(ohm-cm)
`Method
`
`
`Substrate Type
`
`100-200
`
`15-100
`
`10
`
`10
`
`Soda-lim e, SiO2
`coated
`Soda-lime, SiO2
`coated
`Soda—lime, SiO2
`coated
`Low-expansion
`
`TN
`(twisted nematic)
`STN
`(supertwist)
`STN (newer type)
`
`Active Matrix
`
`Electron Beam
`(200°C)
`Electron Beam
`(300°C)
`Sputter
`(300°C)
`Spotter
`(300°C)
`Low-expansion
`Spotter
`10
`Color filter
`(<200°C)
`
`MitsuiMining may sell a portion of their powder production for outside use, and
`Tosoh is currently buying powder from one of them. But each of these is concentrat-
`ing on the target market.
`
`Typical properties of ITO target material are shown in Table 3-5.
`
`Matsuzaki Shinku is the primary glass coater in Japan, and uses equipment made
`by Shinku Kikai. Asahi Glass makes their own equipment- Neither Shinku Kikai
`nor Asahi attempts to make the target material. This is in contrast to the older
`technology of sputtering ITO from indium-tin metal alloy targets, using an
`oxygen ambient as the source of oxygen in the film. In that case, the user could
`make his own target in a crucible by melting the proper amounts of metal. The
`other supplier of equipment of note is Ulvac. with a captive target supplier, VMC.
`VMC is the largest supplier of targets for the semiconductor industry in Japan, but
`is just beginning ITO target development.
`
`Other Sputtering Materials
`
`E
`
`Metals used for TFI‘ fabrication are deposited by sputtering, using similar or
`identical equipment as for ITO. Chromium is used for black matrix formation on
`the color filter portion of the display in most Current processes. For TFT
`
`117
`
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`
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`
`

`

`
`CHAPTER THREE: PRODUCT HATERIMS
`
`LIQUID CRYSRAL FLAT PANEL DISPLAYS
`
`Table 3-5 Specifications for High Density ITO Target
`
`
`
`
` Property Value
`
`Composition
`Actual Density
`Percent Theoretical Density
`Bulk Resistivity
`Thermal Conductivity
`Tensile Strength
`Bonding Material
`Bond Void Ratio
`
`Deposited Film Properties
`Thickness
`
`10 Wgt% SnO2
`6.07 gmm’cm3
`95% ,
`0.22 mohm-crn
`19.55 mcall’cm-sec°C
`12.8 lrg/mm2
`In, InISn, Im‘Bi
`<2.0%
`
`1500-2500013
`
`<20 ohm/square
`Resistivity
`
`Optical Transmission >85%
`
`manufacturing, the materials choices are aluminum, tantalum, molybdenum,
`tungsten or other refractory metals for the gate lines. Seurce and drain metalliza-
`tion is primarily aluminum, with underlayers of molybdenum or other refractory
`for diffusion barriers. Themetallurgy of thin film transistors was described in Part
`2. Other than the form factor of the sputtering target, requirements for these metals
`appear to be identical to those for semiconductor applications. Also mentioned in
`Part 2 is research to develop sputterwdeposition of silicon for the thin film
`transistor material itself.
`
`Color Filters
`
`Color filter applications began with manufacture of pocket sized TV which
`appeared on the market in the mid-1980’s. The filter was made by the dye method,
`and its spectral properties equalled those of the CRT [2]. This method and its
`companion, pigment dispersion, were scaled to meet the size requirements of
`notebook computers. Required properties of color filter material include those
`shownin Table 3-6. Color filter materials such as dyes and pigments are supplied
`by companies such as Dainippon Paint, Dainippon Ink and so forth. Specialized
`formulations of polyimide with dissolved colorants are available from Toray and
`Brewer Science.
`
`118
`
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`
`LOWES 1017, Page 133
`
`

`

`
`CHAPTER THREE: PRODUCT HATERIILS
`
`LIQUID CRYSTAL FLAT PANEL DISPLAYS
`
`Table 3-6 Properties of Color Filters
`
`
`
` Property Comment
`
`Spectral property
`
`Colors of red, blue and green filters
`
`should be
`
`Contrast ratio
`Uniformity
`
`Flatness
`
`Defects
`
`close to CIE chromaticity of CRT
`
`Loss of light should be minimal
`Spectral properties should be uniform across the
`
`Surface of the array
`No foreign objects or projections. (most
`severe restriction on STN filters, i0.lttm)
`No foreign object, pin-hole, crack or dirt
`
`Dimensional accuracy
`Thermal stability
`
`Sufficient to align substrates easily
`Resistant to cell sealing and polarirer application
`
`Chemical resistance
`
`process (250°C, 1hr target)
`No discoloration, swelling, separation or creasing
`during cleaning or ITO etching processes.
`
`Reliability
`
`Temperature extremes and thermal shock
`
`(80°C to -30°C). and temperaturefhumidity lesis
`
`tance (40°C + 95% RH)
`
`Light stability
`
`Resistant to bleaching in ambient light
`
`Test conditions for color filters are summarized in Table 3-1
`
`3.4.1 DYE METHOD
`
`Presently the dyeing method is the most widely used technique. Its materials and
`processes are both well established, with reproducible spectral properties. How-
`
`ever, the use of dyes and water soluble polymers as binders pose some problems,
`resulting in poor resistance to heat, light, and chemicals. The way around this
`results in increased process complexity. Patterning and dyeing of each color plus
`other steps such as substrate cleaning and drying result in a process with a total
`
`of 40-50 steps of various types. For example, the water soluble polymer such as
`gelatin is made photosensitive, exposed with UV light, and then the pattern is
`
`developed. This sequence is accomplished using equipment similar to the pho-
`tolithography equipment in the semiconductor industry. Next, the relief pattern
`
`is dyed by acid or reactive dyes. Because the optical density of the dye is
`influenced by the degnee of polymerization of the resin, and because the film
`
`thickness varies depending on the dyeing process, control is necessary for each
`
`11
`TCL 1017, Page 134
`LOWES 1017, Page 134
`
`LOWES 1017, Page 134
`
`

`

`
`CHAPTER THREE: PRODUCT HATERIMS
`
`LIQUID CRYSRAL FLAT PANEL DISPLAYS
`
`Table 3-7 Test Conditions for Color Filters
`Test m Treatment Condition
`
`Oven
`
`ISO-200°C, 1hr
`80°C. 500hr
`
`Thermal stability
`Thermal stability
`
`(long term)
`
`Thermal Shock
`
`Light stability
`
`Chemical resistance
`
`Environmental
`
`80°C, 50hr
`
`-30-80°C, .5hr 20 cycles
`
`Testing Machine
`
`40°C, 95%RH, 420m:
`
`IPA
`
`Xylene
`
`Butyl Acetate
`
`Butyl Lactate
`
`NaOH (5%)
`
`H2804 (5%)
`H2O
`IP
`3:-
`
`H20
`"U
`
`I F
`
`luorocarbon
`
`200-500 hr
`
`Dipping
`10—30min
`
`
`
`60°C. 30min
`
`Supersonic
`Wave 5min
`
`Vapor
`5min
`
`dye lot, its stability over time, and its optical density. Some investigation is being
`made to replace the water soluble resin with acrylic resin. In order to prevent the
`mixing of colors, it is possible to use an intermediate hardening step after each
`filter is formed