`CHAPTER TWO: DISPLAY MANUFACTURING
`
`LIQUID CRYSTAL FLAT PANEL DISPLAYS
`
`is either being designed or is in prototype form. Notall the test and repair
`equipmentneeded 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 fordefects.
`
`Table 2-18 In-Process Inspection and Repair EquipmentList
`
`Equipment
`
`Remarks
`
`Electrical Parametric Test
`Substrate Flatness
`Sheet Resistivity Monitor
`
`Critical Dimension Measurement
`Particle Monitors
`Optical Microscope Inspection
`Digital/Analog 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 mask/waferinspection 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 equipmentwith 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
`assemblyincludesthe actual joining andsealing of the substrates, separation and
`final testing, and die attach equipmentfor 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. Yamamuraand publishedin the August,
`1990 issue of Nikkei Microdevices, showsthe dramatic effect of defect density
`on yield and cost of color TFT displays.
`
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`
`Table 2-19 Assembly and Die Attach Equipment
`
`
`Price/Remarks
`Equipment
`SSS
`Orientation Film Printer
`$180K
`Orientation Film Rubbing
`$80K
`Substrate Cleaning
`$120K
`Spacer Spraying
`$35
`Seal Printing
`$120K
`Alignment/Sealing
`$560K includes cassette/cassette han-
`dling
`$40K
`$65
`$500K for both inner and outer lead
`TAB bonder
`
`
`Liquid Crystal Injection
`Scribe/Break
`Die Attach Equipment
`
`Yield (%)
`
`Figure 2-21 Simulated yield curvesfor various defect densities in TFT display
`manufacturing
`
`Source: Nikkel Microdevices
`
`Display Size (inches)
`
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`
`Ontheleft handsideofthe illustration is the calculated yield vs display size line
`corresponding to 0.1 defects/cm’. This solid line is about the defect density for
`small LCDdisplays,like those usedfor calculators and watches.It is impossible
`to build large TFT displaysatall with this defect density. The nextlevelof defects,
`0.01 defects/cm’, corresponds approximately with the current manufacturing
`practice in Japan. Atthis 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 defects/cm?, results in a higher yield curve
`shownbythedotted 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
`1990s, and will result in an average selling price of $800.
`
`A muchlowerdefect density, 0.001 defects/cm’,will lead to yields in the range
`of 80% for displays, and a correspondingselling price of $500 eachorless. This
`level of defects, a total of 10 defects per square meterof substrate,is a lowerlevel
`that is currently achieved in semiconductor manufacturing. Of course,the size of
`the critical dimension in TFT manufacturingis larger, but absolute defect levels
`are much more important for displays than for integrated circuits where many
`chips are manufactured at the sametime onthesilicon 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 ofthe principal differences between AMLCD and IC
`manufacturingis the optimum factory size, whichis relatively small for displays
`at about 500,000 starts per year. Another difference concerns the relative
`importanceofthe cost of capital and materials. For integratedcircuits, especially
`cost competitive products such as DRAMs,theinitial investment dominatesall
`other costs. For displays, the situation is quite different, and materials costs are
`dominant. Figure 2-22 showsthe cost breakdownfora “‘minifab” with 500,000
`display starts per year. Thefactory productin this case is acompleted 14 inch high
`definition television. The cost analysis indicates that material costs accountfor
`more that 50% ofthe total, with depreciation and R&Dcosts at less than 10%
`each. The factory price of $862 per completed TV appliesto thefifth yearafter
`the start of the project, and the factory is still ramping up production. After ten
`
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`
`years of operation, production costs have declined to levels competitive with
`CRT-base TVsets, but materials costs continue to dominate.
`
`
`Figure 2-22 Cost components of
`
`flat panel display production.
`
`HDTV “Minifab” Manufactur-
`ing Costs (5th Yearof Project)
`
`
`
`GrossProfit
`
`Material
`
`Labor
`
`R&D
`
`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 yieldin this situation.
`
`The most commonsingle cell fault isa pinhole shortina storage capacitor because
`ofthe large area of the capacitor. The resulting high leakage path producesa fixed
`ON or OFF condition, depending on voltage and polarizer settings. Another
`single cell fault is a sourceto drain short in the data metallization, which prevents
`charge transfer. A Poissondistribution offaults 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 allowablefaults,
`the average numberoffaults has to be fewer than 5 perarray.
`
`Faults other than single cell faults are intolerable, and must be repaired. If a gate
`
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`
`or data line is open,it is easily detected by an electrical continuity check. Repair
`consists of laser welding a spare line driverto theinitially undriven endoftheline.
`An interlevel shortis repaired by laser cutting eitherthe gate line orthe 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 TFT 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 andinterlevel shorts.
`
`Type 2 repairability assumesa high yielding technique for scribing gate lines in
`the array. Type 2 repairability is significantly better than Type 1 only when there
`are manyspare gate lines available to repair interlevel shorts.
`
`Total array yield is the productof single cell fault yield and repairable faultyield.
`Yin=..Y,- ifthe single cell fault yield can be raised to 99%, thenthe overall yield
`is determinedbythe repairable fault yield. Table 2-20 showsthe 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 Im.
`
`Table 2-20 TFT Array Yield Summary (No Repair)
`
`Yield
`
`10%
`50%
`90%
`
`Faults/Display
`
`Faults/cm?
`
`;
`
`2.3
`0.68
`0.10
`
`0.24
`0.072
`0.01
`
`Consider a 1Mbit DRAM line operating at 50% yield. For a chip area of 0.5cm’,
`the allowable fault density is 1.5/em?. Compare this to the value of 0.072 from the
`table, and one sees that without repair, the fault densities fora TFT array must be
`21 times lower than for current DRAM manufacturing.
`
`Repairability 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 fault/cm?.
`
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`
`AMLCDFactory
`The NEC Kagoshimaplant 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, on the
`300x350mm substrate, comesto a potential 10,000-15,000 10" displays per month.
`Factoringin the yield, output is 6000-9000 displays per month.
`
`The manufacturing equipmentis interconnected only where this is easy to do.
`Substrates are transported around the factory by automatic ground vehicles.
`These vehicles, which are provided with HEPAfilters, are used to transport the
`heavy 20-substrate carriers, and to avoid particulate contamination which would
`occurif an operator performed the transport. Operators are usedto load carriers
`onto the equipment, which may be anin-line series of individual processes.
`Equipmentsuch as for photolithographyis notfully interconnected. Computers
`are used to control the operation and collect information regarding machine
`performance.
`
`NEC 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 TFT fabrication reported at other companies. In NEC’s opinion,the
`cell assembly processis a muchstricter and moredifficult to control process. The
`TFT manufacturing equipmentisverylike the LSI processing equipmentalready
`in use at most companies who are making TFT’s, and the equipmentis made by
`large, reliable companies. In contrast, the equipmentfor cell assembly is made by
`small and medium sized companies, and may notbe available.
`
`Figure 2-23 showsthe floorplan of the factory. The upper floor contains the TFT
`panel fabrication area. This areais composedof lithography, cleaning,sputtering,
`CVD andsoforth. An elevator connects this floor to the assembly area below. The
`floor space for the clean room is 40mx90m, andthetotal area is 4000 m? perfloor.
`20% of the clean room is class 100, 20% is class 1000, with the rest class 10,000.
`Forthe lithographyarea, class 10 is used. In the cell assembly area, the equipment
`and processes that generate particles, such as the rubbing equipment, are enclosed.
`
`Right now, completed TFT panels are sent downstairs, and purchasedcolorfilter
`panels are started into the cell assembly process. Module assembly, with back-
`light occupies 2000 m?in anotherbuilding.
`
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`
`LIQUID CRYSTAL FLAT PANEL DISPLAYS
`
`Figure 2-23 NEC two story AMLCDfabricationfacility layout
`
`Plasma CVD
`
`Sputtering
`
`Lithography
`
`Lithography
`
`Inspection
`
`inti
`SealPrinting
`
`Spacer Spraying
`
`Orientation
`Film
`Printing
`
`Orientation
`Film
`Printing
`
`:
`
`Assembly
`
`Liquid
`Crystal
`Injection
`
`Polarizer
`Attach
`
`Color Filter
`(Purchased)
`
`Expansion Area
`o
`
`DryEtching
`
`D&co2O
`
`o@coQ
`
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`
`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 LCDspecialists, and the others are from the semiconductorprocess area. New
`technologytransfer 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 wasinstalled 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 0.2um. Defect inspection
`equipment has not been chosen yet. The operation rate of the equipment has
`reached LSI levels. 4 stepperlines are in operation.
`
`Anelva supplied the plasma CVD equipment for3 lines. Dueto particle buildup,
`thoroughperiodic maintenanceis 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 diagonal measurementof
`9.8", with 640xRGBx400 lines. Each color pixel measures 0.11x0.33mm7?.
`Contrast is 80:1. A 20:1 contrast ratio or greater is maintained for a viewing angle
`of +20°vertical, +35° horizontal. Response speed is 20ms. Brightness is 80cd/m?.
`Backlight power consumption is 12 W. TFT design employs a bottom gate
`construction, with a 2-layer, ITO/Crconstruction. Gate insulatoris SiO, plus Si,N,
`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
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`CHAPTER TWO: DISPLAY MANUFACTURING
`
`LIQUID CRYSTAL FLAT PANEL DISPLAYS
`
`REFERENCES
`
`
`1.F. Okamoto,“Glass Substrates 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. Tamada, "West Process Equipment for LCD", Technical Proceedings, SEMICON/Kansai
`1991, SEMIJapan, 1991, p460.
`
`5. Tamada,ibid, p465,
`
`6. RJ, Saia, R.F. Kwasnick, and C.Y. Wei, "Selective Reactive Ion Etching of ITO in a
`Hydrocarbon Gas Mixture", J. Electrochem. Soc. 138, 493 (1991).
`
`7.8. Ishibashiet al, "1.0X10-4Q-cm ITO Films by Magnetron Sputtering", presented atthe first
`International Conference on Sputtering and Plasma Processing, (Tokyo) Japan, February, 1991.
`
`8.S. Takaki et al., "Preparation of Highly Conducting ITO Electrodes on ColorFilters by Highly
`Dense Plasma-Assisted EB Evaporation", Society for Information Display Technical Digest,
`1990, p76.
`
`9. WJ. Latham and D.W. Hawley, "Color Filters from Dyed Polyimides", Solid State Technol-
`ogy, May 1988.
`
`10. S. Nemoto, "Color Filters for Liquid Crystal Display", Technical Proceedings, Semicon/
`Kansai 91 FPD Seminar, p162, 1991.
`
`11. Michael Goldowsky, Economical ColorFilter Fabrication for LCDs by Electro-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 TFT-LCDs" Flat Panel Display
`Process Tutorial, Semicon West, May 23, 1991.
`
`14. FJ. 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", SEMICON/Kansai Technical Proceedings
`1991, Semi Japan, 1991, p119.
`
`17. K.J. Hathawayet. al., "New Backlighting Technologies for LCDs", Society for Information
`Display Technical Digest 1991, p751.
`
`111
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`CHAPTER THREE: PRODUCT MATERIALS
`LIQUID CRYSTAL FLAT PANEL DISPLAYS
`
`UTCie imey he
`
`Theparticulars of materials for flat panel displays are presentedin 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 colorfilters, significant cost reduction is
`essential for high volume manufacturing. For other materials, cost effectiveness
`is determined primarily by beneficial effects on transistor manufacturing yield.
`
`PTaehss
`
`31
`
`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.
`
`Advantagesoffloat glass material were pointed outin a recentarticle by Pilkington
`Micronics [1]. Flat glass was made by drawing processesuntil the adventof 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 applicationsinspite ofthe fact
`that this constitutes less than 1% of the world-wide flat glass market.
`
`Somedifficulty in obtaining the tolerances on glass properties, especially surface
`smoothness,arose from processingfloat glass for LCDsin 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 movingthefloat glass manufacturing for LCDsto
`a small dedicated float line, materials specifications were improved.
`
`Floatlines can produce 3 meter wide continuousribbonswith thicknessvariations
`of +0. lum, negligible warp, and microcorrugation controlled to averages of less
`than 0.1,1m. On-line CVD processesare also used by this glass manufacturerto
`provide SiO, coated surfaces. Characterization of microroughness will allow
`classification of substrates, and reducedpolishing of selected classes of glass can
`be achieved for STN applications. Eventually, float glass suppliers hope to
`eliminate the polishing process completely.
`
`a=
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`
`LIQUID CRYSRAL FLAT PANEL DISPLAYS
`
`Processing the base glass substrate into a form usefulto the display manufacturer
`is a multi-step process, with several Japanesefirms offering services at each step.
`Table 3-1 showsthe types of operations for LCDglass and suppliers at eachstep.
`Processes for SiO, and ITO coating are indicated. Various glass fabricators
`supply material to several firms for cutting and polishing. SiO, coating may be
`done be anotherfirm, and ITO bya fourth.
`
`Table 3-1 LCD Glass Supplier Matrix in Japan
`
`
`
` Bare Glass Cutting/Polishing SiO,Coating ITO Coating
`
`
`
`Asahi Glass
`
`Kuramoto
`
`NSG
`
`Fujimi
`
`Central Glass
`
`Mitsuru
`
`Matsuzaki
`(dipping)
`Tokyo Ohka
`(dipping)
`Kuramoto
`
`N.E.G.
`
`Asahi
`N.E.G.
`(BLC & OAZtypes)
`
`Corning
`
`ITO Sputtering
`
`Matsuzaki
`(sputter & evap.)
`Asahi Glass
`(sputter & evap.)
`Sanyo Shinku’
`(sputter)
`NSG
`CVD Central
`Seiko Epson
`Sharp
`Optrex
`Sputter
`
`* 100% Sharp subsidiary
`
`ITOsputtering is a crucial and ubiquitous partof 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% andresistivity of 1-340hm-cm are
`normal values for these properties. These are a sensitive function of the oxygen
`contentof the film, and oxygenis often added in the gas stream during sputtering,
`to makeup for any loss of oxygen during transport from the ITOtargetto the glass
`substrate. Deposition temperatureis a critical parameterof 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-
`
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`
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`
`sition, but 200°C is necessary for color LCD displays, where the polymercolor
`filters cannot withstand the higher temperature. Lower temperature deposition
`has been madepossible 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 showsfilm properties of
`deposited films. Ulvac has described a process for low temperature sputter
`deposition which providesresistivities as low as 5 Q/square.
`
`Table 3-2 /TO Film Properties
`
`
`
`
` Deposition Conditions Resistivity/Resistance Application
`
`350°C, 1000A, e-beam
`or sputtering
`200°C, 1000A, e-beam
`sputter
`
`1.5x10* ohm/cm
`
`2x104 ohm/em
`
`Ulvac - new system
`200°C sputter
`
`1.2x* ohm/cm
`
`B/W STN ~20 ohm/
`square
`Color Filter (requires or
`2000A for 10 ohm/
`square)
`As low as 5 ohm/square
`is possible at low temp-
`erature for colorfilter
`
`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 compositionofthe target is 10 wgt% SnO,. After ma-
`chining to the correctfinal size and shape, the ITO material is bondedto a backing
`plate using some form of indium or indium alloy bonding preform. Table 3-3
`showsthe currently available target densities available from suppliers. The higher
`density material is more difficult to prepare, but provides longertargetlife and
`fewer deposited InO black particles.
`
`Table 3-3 Characteristics ofITO Targets
`
`TargetLife
`InO
`(using high density life
`black particles
`Density
`as ~100%)
`
`
`Low (70%)
`Medium (80%)
`High (95%)
`
`most
`some
`fewest
`
`70%
`80%
`~100%
`
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`
`LIQUID CRYSRAL FLAT PANEL DISPLAYS
`
`Sputtering processesconsist of bombarding a material such as ITO with energetic
`argon ions. These ions dislodge atoms from thetarget, and these atomstravel to
`the substrate underthe 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 shortensits usefullife. In addition,
`non-uniform density or composition can lead to absorbingparticle formation.
`
`The rectangulartargetis used in large batchorin-line sputtering equipment made
`by a variety of suppliers in Japan, including Ulvac, the largest commercial
`supplier, Anelva, Asahi Glass (makes equipmentfor their own use), and Shinku
`Kikai. Forvery large systems,the targets can be used twoata time, forming a one
`meter long sputtering source. Glass panels have a wide range of sizes, both
`because many products maybe 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 anelliptical
`cavity is etched out moreorlessin the centerofthe rectangle. Because a lot of ITO
`is left in an unusable form at the end oflife of the target, and because the material
`is somewhatexpensive,there is someinterest in reclaimingthetargets.
`
`Film properties may be enhancedby annealingin air after deposition. Subsequent
`patterning and etching forms the transparent electrodes whose function was
`discussed in the previoussection.
`
`ITO sputtering is mostly done by glass coaters, primarily Matsuzaki Shinku and
`Asahi Glass. Glass can be supplied in coated form, exceptfor the panels used for
`the active matrix TFT backplane. These are ITO coated after transistor fabrica-
`tion, and this will be doneinternally at the display manufacturer.
`
`Requirements for glass type, ITO thickness andother 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 ofconsistentparticle 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 Miningforinternal use. Nippon Mining and
`
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`
`CHAPTER THREE: PRODUCT MATERIALS
`
`LIQUID CRYSTAL FLAT PANEL DISPLAYS
`
`Table 3-4 Glass Substrates for Liquid Crystal Displays
`
`
`Substrate Type
`ITO Deposition
`Resistivity
`Application
`(ohm-cm)
`Method
`
`
`100-200
`
`15-100
`
`10
`
`10
`
`Soda-lime, SiO,
`coated
`Soda-lime, SiO,
`coated
`Soda-lime, SiO,
`coated
`Low-expansion
`
`T™N
`(twisted nematic)
`STN
`(supertwist)
`STN (newer type)
`
`Active Matrix
`
`Electron Beam
`(200°C)
`Electron Beam
`(300°C)
`Sputter
`(300°C)
`Sputter
`(300°C)
`Low-expansion
`Sputter
`10
`Colorfilter
`(<200°C)
`
`MitsuiMining maysell a portionof their powder production for outside use, and
`Tosohis currently buying powderfrom oneof them. But eachof these is concentrat-
`ing on the target market.
`
`Typical properties of ITO target material are shown in Table 3-5.
`
`Matsuzaki Shinkuis the primary glass coater in Japan, and uses equipment made
`by Shinku Kikai. Asahi Glass makestheir own equipment. Neither Shinku Kikai
`nor Asahi attempts to makethe target material. This is in contrast to the older
`technology of sputtering ITO from indium-tin metal alloy targets, using an
`oxygen ambientas the source of oxygenin the film. In that case, the user could
`make his owntarget in a crucible by melting the proper amounts of metal. The
`other supplier of equipmentof note is Ulvac, with a captive target supplier, VMC.
`VMCisthelargest supplier of targets for the semiconductorindustry in Japan, but
`is just beginning ITO target development.
`
`Other Sputtering Materials
`
`Metals used for TFT fabrication are deposited by sputtering, using similar or
`identical equipmentas for ITO. Chromiumis used for black matrix formation on
`the color filter portion of the display in most current processes. For TFT
`
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`
`CHAPTER THREE: PRODUCT MATERIALS
`LIQUID CRYSRAL FLAT PANEL DISPLAYS
`
`Table 3-5 Specifications for High Density ITO Target
`
`
`
` Property Value
`Composition
`10 Wgt% SnO,
`Actual Density
`6.07 gm/cm?
`Percent Theoretical Density
`95% |
`Bulk Resistivity
`0.22 mohm-cm
`Thermal Conductivity
`19.55 mcal/em-sec°C
`Tensile Strength
`12.8 kg/mm?
`Bonding Material
`In, In/Sn, In/Bi
`Bond Void Ratio
`<2.0%
`Deposited Film Properties
`1500-3000A
`Thickness
`<20 ohm/square
`Resistivity
`
`Optical Transmission >85%
`
`manufacturing, the materials choices are aluminum, tantalum, molybdenum,
`tungstenor otherrefractory metals for the gate lines. Source and drain metalliza-
`tion is primarily aluminum, with underlayers of molybdenumorotherrefractory
`fordiffusion barriers. The metallurgyof thin film transistors was describedin Part
`2. Other than the form factorof the sputtering target, requirementsfor these metals
`appearto be identicalto those for semiconductorapplications. Also mentionedin
`Part 2 is research to develop sputter-deposition of silicon for the thin film
`transistor material itself.
`
`reymails:
`
`Color filter applications began with manufacture of pocket sized TV which
`appeared onthe marketin the mid-1980’s. Thefilter was madeby the dye method,
`and its spectral properties equalled those of the CRT [2]. This method andits
`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. Colorfilter materials such as dyes and pigments are supplied
`by companies such as Dainippon Paint, Dainippon Ink andso forth. Specialized
`formulations of polyimide with dissolved colorants are available from Toray and
`Brewer Science.
`
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`
`CHAPTER THREE: PRODUCT MATERIALS
`
`LIQUID CRYSTAL FLAT PANEL DISPLAYS
`
`Table 3-6 Properties of Color Filters
`
`
`
` Property Comment
`
`Spectral property
`
`Contrast ratio
`Uniformity
`
`Flatness
`
`Defects
`Dimensional accuracy
`Thermalstability
`
`Chemical resistance
`
`Reliability
`
`Light stability
`
`should be
`
`Colors of red, blue and green filters
`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, +0.1Lum)
`No foreign object, pin-hole, crack or dirt
`Sufficient to align substrates easily
`Resistantto cell sealing and polarizer application
`process (250°C, Lhr target)
`No discoloration, swelling, separation or creasing
`during cleaning or ITO etching processes.
`Temperature extremes and thermal shock
`(80°C to -30°C), and temperature/humidity resis
`tance (40°C + 95% RH)
`Resistant to bleaching in ambientlight
`
`Test conditions for colorfilters are summarized in Table 3-7.
`
`3.4.1 DYE METHOD
`Presently the dyeing methodis 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 polymersas binders pose someproblems,
`resulting in poor resistance to heat, light, and chemicals. The way aroundthis
`results in increased process complexity. Patterning and dyeing of each colorplus
`other steps such as substrate cleaning and drying result in a process with a total
`of 40-S0 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 equipmentsimilar to the pho-
`tolithography equipmentin the semiconductorindustry. Next, the relief pattern
`is dyed by acid or reactive dyes. Because the optical density of the dye is
`influenced by the degree of polymerization of the resin, and because the film
`thickness varies depending on the dyeing process, control is necessary for each
`
`11
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`

`
`CHAPTER THREE: PRODUCT MATERIALS
`
`LIQUID CRYSRAL FLAT PANEL DISPLAYS
`
`
`
`Table 3-7 Test Conditions for Color Filters
`Test|Equipment—_|Treatment Condition
`
`Thermalstability
`Oven
`180-200°C, hr
`Thermalstability
`80°C, 500hr
`(long term)
`-30°C, 5Ohr
`Environmental
`Thermal Shock
`Testing Machine|40°C, 95%RH, 420hr.
`-30-80°C, .Shr 20 cycles
`200-500 hr
`
`Lightstability
`
`Chemical resistance
`
`Butyl Lactate NaOH (5%)
`Fluorocarbonane
`
`Dipping
`10-30min
`
`IPA
`Xylene
`Butyl Acetate
`
`H,SO,(5%)
`H,O
`
`Wave 5min
`Vapor
`S5min
`
`;
`IPA
`
`dye