United States Patent
`Tang et al.
`
`115
`
`[54] METHOD OF FABRICATINGA TFT-EL
`PIXEL
`
`[75]
`
`Inventors: Ching W. Tang, Rochester; Biay C.
`Hseih, Pittsford, both of N.Y.
`
`[73] Assignee: Eastman Kodak Company, Rochester,
`N.Y.
`
`[21] Appl. No.: 355,940
`
`[22] Filed:
`
`Dec. 14, 1994
`
`Ean, Cae inneccsesesssssessssssesssssessesecseceecsees HOIL 21/86
`[SD]
`[52] U.S. Ch. oe eceecessceseseeeerees 437/40; 437/21; 437/59;
`437/919; 148/DIG. 150
`.......0.....ccccce 437/40 TF, 41 TF,
`[58] Field of Search.
`437/21, 913, 919, 54, 59; 148/DIG. 150
`
`[56]
`
`References Cited
`U.S. PATENT DOCUMENTS
`
`
`
`NUAAA AA
`
`US005550066A
`
`[11] Patent Number:
`[45] Date of Patent:
`
`5,550,066
`Aug. 27, 1996
`
`5,073,446 12/1991 Scozzafava et al. 0... 313/504
`5,150,006
`9/1992 VanSlykeet al.
`csessssesssscssee 313/504
`5,151,629
`9/1992 VanSlykeet al.
`....
`313/504
`5,235,195
`8/1993 Tranet al...
`437/21
`5,276,380
`1/1994 Tang .sesssccsee
`.. 313/504
`5,294,870
`3/1994 Tang et al. caccsssessssscssssesseseesen 313/504
`OTHER PUBLICATIONS
`
`
`
`Literature
`
`for model
`
`Suzuki et al, “The Fabrication of TFEL Displays Driven by
`a-Si TFTs;”SID 1992 Digest, pp. 344-347.
`K. Ichikawa, et al, SID Digest, 226 (1989).
`M.J. Powell, et al, Proceeding, International Display Con-
`ference, 63, 1987.
`Sharp Corporation Technical
`LQ424A01, 1989.
`D. E. Castleberry, et al, SID Digest, 232 (1988).
`S. Morozumi, Advances in Electronics and Electron Physics,
`Edited by P. W. Hawkes, vol. 77, Academic Press 1990.
`A. G. Fischer, IEEE Trans. Electron Devices, 802 (1971).
`T. P. Brody, et al, IEEE Trans. Electron Devices, 22, 739
`(1975).
`C. W. Tang, et al, Appl. Phys. Lett., 51, 913 (1987).
`C. W. Tang,et al., Appl. Phys., 65, 3610 (1989).
`
`Primary Examiner—Tom Thomas
`Assistant Examiner—Michael Trinh
`Attorney, Agent, or Firm—Raymond L. Owens
`
`4/1974 Fischer 0... cssecsssscssetseneeee 437/59
`3,807,037
`5/1975 Fischer 0...
`cssssseceesssseeesseeeene 345/80
`3,885,196
`3,913,090 10/1975 Fischer...
`ccssesseeteeseneeee 345/80
`4,006,383
`2/1977 Luo etal.
`...
`313/505
`4,042,854
`8/1977 Luoetal.
`...
`313/505
`4,356,429
`10/1982 Tang .ccessseceeens
`313/503
`(37)
`ABSTRACT
`
`4,409,724 10/1983 Tasch, Jr. et al.weessseeseeeee 437/83
`4,523,189
`6/1985 Takahara ou... csesseeseesseeeeeers 345/80
`A method of making a 4-terminal active matrix electrolu-
`
`4,539,507
`9/1985 VanSlyke et al.
`..
`313/504
`minescent device that utilizes an organic material as the
`4,602,192
`7/1986 Nomura etal. .........
`257/300
`electroluminescent medium is described. In this method,
`
`4,720,432
`1/1988 VanSlykeet al.
`..
`313/504
`thin film transistors are formed from polycrystalline silicon
`.....
`4,950,950
`8/1990 Perry etal.
`313/504
`
`at a temperature sufficiently low such that a low temperature,
`9/1991 Nicholas ........
`5,047,360
`437/984
`silica-based glass can be used as the substrate.
`
`9/1991 VanSlykeet al. oo... ee 313/504
`5,047,687
`10/1991 Littman et al. oceans 313/504
`5,059,861
`5,061,569
`10/1991 VanSlyke et al. ose 313/504
`
`10 Claims, 5 Drawing Sheets
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`U.S. Patent
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`Aug. 27, 1996
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`U.S. Patent
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`Aug. 27, 1996
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`Sheet 3 of 5
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`U.S. Patent
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`Aug. 27, 1996
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`Sheet 4 of 5
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`U.S. Patent
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`5,550,066
`
`1
`METHOD OF FABRICATING A TFI-EL
`PIXEL
`
`CROSS REFERENCE TO RELATED
`APPLICATIONS
`
`Reference is made to commonly assigned U.S. Ser. No.
`08/355,742 entitled “TFT-EL Display Panel Using Organic
`Electroluminescent Media” by Tang et al and U.S. Ser. No.
`08/355,786 entitled “An Electroluminescent Device Having
`an Organic Electroluminescent Layer” by Tang et al, both
`filed concurrently herewith, the disclosures of which are
`incorporated herein.
`
`FIELD OF THE INVENTION
`
`The present invention relates to a process for making a
`4-terminal active matrix thin-film-transistor electrolumines-
`cent device that employs organic material as the electrolu-
`minescent media.
`
`INTRODUCTION
`
`Rapid advancesin flat-panel display (FPD) technologies
`have made high quality large-area, full-color, high-resolu-
`tion displays possible. These displays have enabled novel
`applications in electronic products such as lap top computers
`and pocket-TVs. Among these FPD technologies,
`liquid
`crystal display (LCD) has emergedas the display of choice
`in the marketplace. It also sets the technological standard
`against which other FPD technologies are compared.
`Examples of LCD panels include: (1) 14", 16-color LCD
`panel for work stations (IBM and Toshiba, 1989) (see K.
`Ichikawa, S. Suzuki, H. Matino, T. Aoki, T. Higuchi and Y.
`Oano, SID Digest, 226 (1989)), (2) 6", full-color LCD-TV
`(Phillips, 1987) (see M. J. Powell, J. A. Chapman, A. G.
`Knapp, I. D. French, J. R. Hughes, A. D. Pearson, M.
`Allinson, M. J. Edwards, R. A. Ford, M. C. Hemmings, O.
`F. Hill, D. H. Nicholls and N. K. Wright, Proceeding,
`International Display Conference, 63, 1987), (3) 4" full-
`color LCD TV (model LQ424A01, Sharp, 1989) (see Sharp
`Corporation Technical Literature for mode! LQ424A01),
`and (4) 1 megapixel colored TFT-LCD (General Electric)
`(see D. E. Castleberry and G. E. Possin, SID Digest, 232
`(1988)). All references, including patents and publications,
`are incorporated herein as if reproduced in full below.
`A common feature in these LCD panels is the use of
`thin-film-transistors (TFT) in an active-addressing scheme,
`which relaxes the limitations in direct-addressing (see S.
`Morozumi, Advances in Electronics and Electron Physics,
`edited by P. W. Hawkes, Vol. 77, Academic Press 1990). The
`success of LCD technologyis in large part due to the rapid
`progress in the fabrication of large-area TFT (primarily
`amorphoussilicon TFT). The almost ideal match between
`TFT switching characteristics and electrooptic LCD display
`elements also plays a keyrole.
`A major drawback of TFI-LCD panels is they require
`bright backlighting. This is because the transmission factor
`of the TFT-LCD is poor, particularly for colored panels.
`Typically the transmission factor is about 2—3 percent(see S.
`Morozumi, Advancesin Electronics and, Electron Physics,
`edited by P. W. Hawkes, Vol. 77, Academic Press, 1990).
`Power consumption for backlighted TFT-LCD panels is
`considerable and adversely affects portable display applica-
`tions requiring battery operation.
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`The need for backlighting also impairs miniaturization of
`the flat panel. For example, depth of the panel must be
`increased to accommodate the backlight unit. Using a typi-
`cal tubular cold-cathode lamp, the additional depth is about
`% to 1 inch. Backlight also adds extra weight to the FPD.
`An ideal solution to the foregoing limitation would be a
`low power emissive display that eliminates the need for
`backlighting. A particularly attractive candidate is thin-film-
`transistor-electroluminescent (TFT-EL)displays. In TFT-EL
`displays, the individual pixels can be addressed to emitlight
`and auxiliary backlighting is not required. A TFT-EL scheme
`was proposed by Fischer in 1971 (see A. G. Fischer, IEEE
`Trans. Electron Devices, 802 (1971)). In Fischer’s scheme
`powdered ZnS is used as the EL medium.
`In 1975, a successful prototype TFT-EL panel (6") was
`reportedly made by Brody et al. using ZnS as the EL element
`and CdSe as the TFT material (see T. P. Brody, F.C. Luo, A.
`P. Szepesi and D. H. Davies, IEEE Trans. Electron Devices,
`22, 739 (1975)). Because ZnS-EL required a high drive
`voltage of more than a hundred volts, the switching CdSe
`TFT element had to be designed to handle such a high
`voltage swing. The reliability of the high-voltage TFT then
`became suspect. Ultimately, ZnS-based TFI-EL failed to
`successfully compete with TFT-LCD. U.S. Patents describ-
`ing TFT-EL technology include: U.S. Pat. Nos. 3,807,037;
`3,885,196; 3,913,090; 4,006,383; 4,042,854; 4,523,189; and
`4,602,192.
`Recently, organic EL materials have been devised. These
`materials suggest
`themselves as candidates for display
`media in TFI-EL devices (see C. W. Tang and S. A.
`VanSlyke, Appl. Phys. Lett., 51,913 (1987), C. W. Tang,S.
`A. VanSlyke and C. H. Chen, J. Appi. Phys., 65, 3610
`(1989)). Organic EL media have two important advantages:
`they are highly efficient; and they have low voltage require-
`ments. The latter characteristic distinguishes over other
`thin-film emissive devices. Disclosures of TFI-EL devices
`in which EL is an organic material include: U.S. Pat. Nos.
`5,073,446; 5,047,687, 5,059,861; 5,294,870; 5,151,629;
`5,276,380; 5,061,569; 4,720,432; 4,539,507; 5,150,006;
`4,950,950; and 4,356,429.
`Theparticular properties of organic EL material that make
`it ideal for TFT are summarized as follows:
`
`the organic EL cell
`1) Low-voltage drive. Typically,
`requires a voltage in the range of 4 to 10 volts depend-
`ing on the light output level and the cell impedance.
`The voltage required to produce a brightness of about
`20 fL is about 5V. This low voltage is highly attractive
`for a TFT-EL panel, as the need for the high-voltage
`TFT is eliminated. Furthermore, the organic EL cell can
`be driven by DC or AC. Asa result the driver circuity
`is less complicated and less expensive.
`2) High efficiency. The luminousefficiency of the organic
`EL cell is as high as 4 lumens per watt. The current
`density to drive the EL cell to produce a brightness of
`20 fL is about
`1 mA/cm?. Assuming a 100% duty
`excitation, the power needed to drive a 400 cm? full-
`page panelis only about 2.0 watts. The power need will
`certainly meet the portability criteria of the flat panel
`display.
`3) Low temperature fabrication. Organic EL devices can
`be fabricated at about room temperature. This is a
`significant advantage compared with inorganic emis-
`sive devices, which require high-temperature (>300°
`C.)
`processing. The
`high-temperature
`processes
`required to make inorganic EL devices can be incom-
`patible with the TFT.
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`3
`The simplest drive scheme for an organic EL panelis to
`have the organic display medium sandwiched between two
`sets of orthogonal electrodes (rows and columns). Thus, in
`this two-terminal scheme, the EL element serves both the
`display and switching functions. The diode-like nonlinear
`current-voltage characteristic of the organic EL element
`should, in principle, permit a high degree of multiplexing in
`this mode of addressing. However, there are several major
`factors limiting usefulness of the two-terminal scheme in
`connection with organic EL:
`1) Lack of memory. Therise and decay timeof the organic
`ELis very fast, on the order of microseconds, and it
`does not have an intrinsic memory. Thus, using the
`direct addressing method, the EL elements in a selected
`tow would have to be driven to produce an instanta-
`neous brightness proportional to the number of scan
`rowsin the panel. Depending on the size of the panel,
`this
`instantaneous brightness may be difficult
`to
`achieve. For example, consider a panel of 1000 scan
`rows operating at a frame rate of 1/60 seconds. The
`allowable dwell time per row is 17 ps. In order to obtain
`a time-averaged brightness of, for example, 20 Fl, the
`instantaneous brightness during the row dwell time
`would have to be a thousand times higher, i.e., 20,000
`Fl, an extreme brightness that can only be obtained by
`operating the organic EL cell at a high current density
`of about 1 A/cm? and a voltage of about 15-20 volts.
`Thelong-term reliability of a cell operating under these
`extreme drive conditions is doubtful.
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`The present invention provides an active matrix 4-termi-
`nal TFT-EL device in which organic material is used as the
`EL medium. The device comprises two TFTs, a storage
`capacitor and a light emitting organic EL pad arranged on a
`substrate. The EL padis electrically connected to the drain
`of the second TFT. Thefirst TFT is electrically connected to
`the gate electrode of the second TFT which in turn is
`electrically connected to the capacitor so that following an
`excitation signal the second TFT is able to supply a nearly
`constant current to the EL pad between signals. The TFI-EL
`devices ofthe present invention are typically pixels that are
`formed into a flat panel display, preferably a display in
`which the EL cathode is a continuouslayeracross all of the
`pixels.
`The TFT-organic EL device of the present invention are
`2) Uniformity. The current demanded by the EL elements
`formed in a multi-step process as described below:
`is supplied via the row and column buses. Because of
`
`the instantaneous high current, the IR potential dropsAfirst thin-film-transistor (TFT1) is disposed over the top
`along these buses are not insignificant compared with
`surface of the substrate. TFT1 comprises a source electrode,
`the EL drive voltage. Since the brightness-voltage
`a drain electrode, a gate dielectric, and a gate electrode; and
`the gate electrode comprises a portion of a gate bus. The
`characteristic of the EL is nonlinear, any variation in
`source electrode of TFT1 is electrically connected to a
`the potential along the buses will result in a non-
`source bus.
`uniform light output.
`Consider a panel with 1000 rows by 1000 columns with
`a pixel pitch of 200ux200y and an active/actual area ratio of
`0.5. Assuming the column electrode is indium tin oxide
`(ITO) of 10 ohms/square sheet (Q/C) resistance, the resis-
`tance of the entire ITO buslineis at least 10,000 ohms. The
`IR drop alongthis bus line for an instantaneouspixel current
`of 800 "A (2 A/cm?) is more than 8 volts. Unless a constant
`current source is implemented in the drive scheme, such a
`large potential drop along the ITO bus will cause unaccept-
`able non-uniform light emission in the panel. In anycase, the
`resistive powerloss in the bus is wasteful. A similar analysis
`can be performed for the row electrode bus that has the
`additional burden of carrying the total current delivered to
`the entire row of pixels during the dwell time,ie., 0.8 A for
`the 1000-column panel. Assuming a 1 um thick aluminum
`bus bar of sheet resistance about 0.028 ohms/square the
`resultant IR drop is about 11 volts, which is also unaccept-
`able.
`
`5,550,066
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`4
`applying the photoresist from an organic solvent on the
`EL cell deijeteriously affects the soluble organic layer
`underneath the magnesium-based alloy layer. This
`causes delamination of the organic layers from the
`substrate.
`Anotherdifficulty is the extreme sensitivity of the cathode
`to moisture. Thus, even if the photoresist can be successfully
`applied and developed without perturbing the organic layers
`of the EL cell, the process of etching the magnesium-based
`alloy cathode in aqueousacidic solution is likely to oxidize
`the cathode and create dark spots.
`
`SUMMARYOF THE INVENTION
`
`A secondthin-film-transistor (TFT2) is also disposed over
`the top surface of the substrate, and TFT2 also comprises a
`source electrode, a drain electrode, a gate dielectric, and a
`gate electrode. The gate electrode of TFT2 is electrically
`connected to the drain electrode of the first
`thin-film-
`transistor.
`
`A storage capacitor is also disposed over the top surface
`of the substrate. During operation, this capacitor is charged
`from an excitation signal source through TFT1, and dis-
`charges during the dwell time to provide nearly constant
`potential to the gate electrode of TFT2.
`An anode layer is electrically connected to the drain
`electrode of TFT2. In typical applications where light is
`emitted through the substrate, the display is a transparent
`material such as indium tin oxide.
`
`A dielectric passivation layer is deposited over atleast the
`source of TFT1, and preferably over the entire surface of the
`device. The dielectric passivation layer is etched to provide
`an opening over the display anode.
`An organic electroluminescentlayeris positioned directly
`on the top surface of the anode layer. Subsequently, a
`cathode layer is deposited directly on the top surface of the
`organic electroluminescentlayer.
`In preferred embodiments,
`the TFI-EL device of the
`present invention is made by a method using low pressure
`and plasma enhanced chemical vapor deposition combined
`with low temperature (i.e. less than 600° C.) crystallization
`and annealing steps, hydrogen passivation and conventional
`patterning techniques.
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`3) Electrode patterning. One set of the orthogonal elec-
`trodes, the anode-indium tin oxide, can be patterned by
`a conventional photolithographic method. Thepattern-
`ing of the other set of electrodes however, presents a
`major difficulty peculiar to the organic EL device. The
`cathode should be made of a metal having a work
`function lower than 4 eV, and preferably magnesium
`alloyed with another metal such as silver or aluminum
`(see Tang et al., U.S. Pat. No. 4,885,432). The magne-
`sium-based alloy cathode deposited on top of the
`organic layers cannot be easily patterned by any con-
`ventional meansinvolving photoresists. The process of
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`5
`The thin-film-transistors are preferably formed simulta-
`neously by a multi-step process involving:
`the deposition of silicon that is patterned into polycrys-
`talline silicon islands;
`chemical vapor deposition ofa silicon dioxide gate elec-
`trode; and
`deposition of another polycrystalline silicon layer which
`is patterned to form a self-aligned gate electrode so that
`after ion-implantation a source, drain, and gate elec-
`trode are formed on each thin-film-transistor.
`The construction of pixels having thin-film-transistors
`composed of polycrystalline silicon and silicon dioxide
`provides improvements in device performance, stability,
`reproducibility, and process efficiency over other TFTs. In
`comparison, TFTs composed of CdSe and amorphoussilicon
`suffer from low mobility and threshold drift effect.
`There are several important advantagesin the actual panel
`construction and drive arrangement of a TFT-organic EL
`device of the present invention:
`1) Since both the organic EL pad and the cathode are
`continuouslayers, the pixel resolution is defined only
`by the feature size of the TFT and the associated
`display ITO pad and is independent of the organic
`componentor the cathode of the EL cell.
`2) The cathode is continuous and commonto all pixels. It
`requires no patterning for pixel definition. The difii-
`culty of patterning the cathode in the two-terminal
`schemeis therefore eliminated.
`
`3) The numberof scanning rowsis no longer limited by
`the short row dwell time in a frame period, as the
`addressing and excitation signals are decoupled. Each
`scan row is operated at close to 100% duty factor. High
`resolution can be obtained since a large numberof scan
`rows can be incorporated into a display panel while
`maintaining uniform intensity.
`4) Thereliability of the organic EL element is enhanced
`since it operates at a low current density (1 mA/cm”)
`and voltage (SV) in a 100% duty factor.
`5) The IR potential drops along the busesare insignificant
`because of the use of a common cathode and the low
`current density required to drive the EL elements.
`Therefore the panel uniformity is not significantly
`affected by the size of the panel.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`FIG. 1 is a schematic diagram of an active matrix 4-ter-
`minal TFT-EL device. Tl and T2 are thin-film-transistors,
`Cs is a capacitor and EL is an electroluminescentlayer.
`FIG. 2 is a diagrammatic plan view of the 4-terminal
`TFT-ELdevice of the present invention.
`FIG, 3 is a cross-sectional view taken along the line A-A'
`in FIG, 2.
`
`FIG. 4 is a cross-sectional view taken along the line A-A’,
`illustrating the process of forming a self-aligned TFT struc-
`ture for ion implantation.
`FIG. 5 is a cross-sectional view taken along the line A-A',
`illustrating the processing steps of depositing a passivation
`oxide layer and opening contactcuts to the source and drain
`regions of the thin-film-transistor.
`FIG. 6 is a cross-sectional view taken along line A-A’,
`illustrating deposition of an aluminum electrode.
`FIG. 7 is a cross-sectional view taken along line A-A',
`illustrating deposition of the display anode and a passivation
`
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`layer that has been partially etched from the surface of the
`display anode.
`FIG. 8 is a cross-sectional view taken along line A-A’,
`illustrating the steps of depositing an electroluminescent
`layer and a cathode.
`FIG. 9 is a cross-sectional view taken along line B-B' in
`FIG. 2.
`
`DETAILED DESCRIPTION OF THE
`INVENTION
`
`FIG. 1 showsthe schematic of an active matrix 4-terminal
`TFT-EL display device. Each pixel element includes two
`TFTs, a storage capacitor and an EL element. The major
`feature of the 4-terminal schemeis the ability to decouple
`the addressing signal from the EL excitation signal. The EL
`elementis selected via the logic TFT (T1) and the excitation
`power to the EL elementis controlled by the power TFT
`(T2). The storage capacitor enables the excitation power to
`an addressed EL elementto stay on onceit is selected. Thus,
`the circuit provides a memory that allows the EL element to
`operate at a duty cycle close to 100%, regardless of the time
`allotted for addressing.
`The construction of the electroluminescent device of the
`present invention is illustrated in FIGS. 2 and 3. The
`substrate of this device is an insulating and preferably
`transparent material such as quartz or a low temperature
`glass. The term transparent, as it is used in the present
`disclosure, means that the component transmits sufficient
`light for practical use in a display device. For example,
`components transmitting 50% or more oflight in a desired
`frequency range are considered transparent. The term low
`temperature glass refers to glasses that melt or warp at
`temperatures above about 600° C.
`In the TFT-EL device illustrated in FIG. 2, TFT1 is the
`logic transistor with the source bus (column electrode) as the
`data line and the gate bus (row electrode) as the gate line.
`TFT2 is the EL power transistor in series with the EL
`element. The gate line of TFT2 is connected to the drain of
`TFT1. The storage capacitor is in series with TFT1. The
`anodeof the EL elementis connected to the drain of TFT2.
`The construction of the TFT-EL of FIG. 2 is shown in
`cross-sectional view in FIGS. 3-9. The cross-sectional
`views shown in FIGS. 3-8 are taken along section line A-A'
`in FIG. 2. The cross-sectional view in FIG. 9 is taken along
`line B-B' in FIG. 2.
`
`In thefirst processing step, a polysilicon layer is deposited
`over a transparent, insulating substrate and the polysilicon
`layer is patterned into an island (see FIG. 4) by photolithog-
`raphy. The substrate may be crystalline material such as
`quartz, but preferably is a less expensive material such as
`low temperature glass. When a glass substrate is utilized,it
`is preferable that the entire fabrication of the TFI-EL be
`carried out at low processing temperatures to prevent melt-
`ing or warping of the glass and to prevent out-diffusion of
`dopants into the active region. Thus, for glass substrates, all
`fabrication steps should be conducted below 1000° C. and
`preferably below 600° C.
`Next, an insulating gate material 42 is deposited over the
`polysilicon island and over the surface of the insulating
`substrate. Insulating material is preferably silicon dioxide
`that is deposited by a chemical vapor deposition (CVD)
`technique such as plasma enhanced CVD (PECVD)or low
`pressure CVD (LPCVD). Preferably, the gate oxide insulat-
`ing layer is about 1000A in thickness.
`
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`

`5,550,066
`
`7
`In the nextstep, a layer of silicon 44 is deposited over the
`gate insulator layer and patterned by photolithography over
`the polysilicon island such that after ion implantation,
`source and drain regions are formed in the polysilicon
`island. The gate electrode material is preferably polysilicon
`formed from amorphoussilicon. Jon implantation is con-
`ducted with N-type dopants, preferably arsenic. The poly-
`silicon gate electrode also serves as the bottom electrode of
`the capacitor (see FIG. 9).
`In a preferred embodiment of the present invention, the
`thin film transistors do not utilize a double gate structure.
`Thus manufacturing is made less complex and less expen-
`sive.
`
`A gate bus 46 is applied and patterned on the insulating
`layer. The gate bus is preferably a metal silicide such as
`tungsten silicide (WSi,).
`In the next step, an insulating layer, preferably silicon
`dioxide, 52 is applied over the entire surface of the device.
`Contact holes 54 and 56 are cut in the second insulating
`layer (see FIG. 5) and electrode materials are applied to form
`contacts with the thin-film-transistors (see FIGS. 6 and 7).
`The electrode material 62 attached to the source region of
`TFT2 also formsthe top electrode of the capacitor (see FIG.
`9). A source bus and ground bus are also formed over the
`second insulating layer (see FIG. 2). In contact with the
`drain region of TFT2 is a transparent electrode material 72,
`preferably ITO, which serves as the anode for the organic
`electroluminescent material.
`
`wm
`
`20
`
`25
`
`8
`injecting and transporting zone can be formedofa single
`material or multiple materials, and comprises a hole inject-
`ing layer in contact with the anode and a contiguous hole
`transporting layer interposed between the hole injecting
`layer and theelectron injecting and transporting zone. Simi-
`larly, the electron injecting and transporting zone can be
`formed of a single material or multiple materials, and
`comprises an electron injecting layer in contact with the
`cathode and a contiguouselectron transporting layer that is
`interposed between the electron injecting layer and the hole
`injecting and transporting zone. Recombination ofthe holes
`and electrons, and luminescence, occurs within the electron
`injecting and transporting zone adjacent the junction of the
`electron injecting and transporting zone and the hole inject-
`ing and transporting zone. The components making up the
`organic EL layer are typically deposited by vapor deposi-
`tion, but may also be deposited by other conventional
`techniques.
`In a preferred embodiment the organic material compris-
`ing the hole injecting layer has the general formula:
`
`Ty
`
`T fp
`
`Ty
`
`° SZ > 1
`
`N =
`
`N
`
`Q
`
`= N
`
`\
`
`7
`
`M
`/\ A&A
`N
`I
`
`°
`
`Ty
`
`Ty
`
`T2
`
`wherein:
`
`Q is N or C(R)
`M is a metal, metal oxide or metal halide
`R is hydrogen,alkyl, aralkyl, aryl or alkaryl, and
`T, and T, represent hydrogen or together complete an
`unsaturated six membered ring that can include sub-
`Stituents such as alkyl or halogen. Preferred alkyl
`moieties contain from about 1 to 6 carbon atoms while
`phenyl constitutes a preferred ary] moiety.
`In a preferred embodimentthe hole transporting layer is
`an aromatic tertiary amine. A preferred subclass of aromatic
`tertiary amines include tetraaryldiamines having the for-
`mula:
`,
`
`R7
`
`AR
`
`N—Are,—N
`
`Rg
`
`Ro
`
`wherein
`
`Are is an arylene group,
`n is an integer from 1 to 4, and
`Ar, R,, Rg and Rg are independently selected aryl groups.
`In a preferred embodiment,
`the luminescent, electron
`injecting and transporting zone contains a metal oxinoid
`compound. A preferred example of a metal oxinoid com-
`pound hasthe general formula:
`
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`
`35
`
`In the next step, a passivating layer 74 of an insulating
`material, preferably silicon dioxide, is deposited over the
`surface of the device. The passivation layer is etched from
`the ITO anode leaving a tapered edge 76 which serves to
`improve the adhesion of the subsequently applied organic
`electroluminescent layer. A tapered edge is necessary to
`produce reliable devices because the present invention uti-
`lizes relatively thin organic EL layers, typically 150 to 200
`nm thick. The passivation layer is typically about 0.5 to
`about 1 micron thick. Thus, if the edge of the passivation
`layer forms a perpendicular or sharp angle with respect to
`the anode layer, defects are likely to occur due to disconti-
`nuities in the organic EL layer. To prevent defects the
`passivation layer should have a tapered edge. Preferably the
`passivation layer is tapered at an angle of 10 to 30 degrees
`with respect to the anode layer.
`The organic electroluminescent layer 82 is then deposited
`over the passivation layer and the EL anode layer. The
`materials of the organic EL devices of this invention can take
`any of the forms of conventional organic EL devices, such
`as those of Scozzafava EPA 349,265 (1990); Tang U.S. Pat.
`No. 4,356,429; VanSlyke et al. U.S. Pat. No. 4,539,507;
`VanSlykeet al. U.S. Pat. No. 4,720,432; Tanget al. U.S. Pat.
`No. 4,769,292; Tang et al. U.S. Pat. No. 4,885,211; Perry et
`al. U.S. Pat. No. 4,950,950; Littman et al. U.S. Pat. No.
`5,059,861; VanSlyke U.S. Pat. No. 5,047,687; Scozzafava et
`al. U.S. Pat. No. 5,073,446; VanSlyke et al. U.S. Pat. No.
`5,059,862; VanSlyke et
`al. U.S. Pat. No. 5,061,617;
`VanSlyke U.S. Pat. No. 5,151,629; Tang et al..U.S. Pat. No.
`5,294,869; and Tang et al. U.S. Pat. No. 5,294,870, the
`disclosures of which are incorporated by reference. The EL ,
`layer is comprised of an organic hole injecting and trans-
`porting zone in contact with the anode, and an electron
`injecting and transporting zone forming a junction with the
`organic hole injecting and transporting zone. The hole
`
`50
`
`55
`
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`

`Re
`
`Ry
`
`Ra
`
`\_y
`R3
`
`N
`
`Ry
`
`
`Oo
`
`Al
`
`Al
`
`5,550,066
`
`Ry
`
`Re
`
`Rs
`
`Ra
`
`N
`
`\ f
`
`Ro
`
`R3
`
`3
`
`10
`
`In
`represent substitutional possibilities.
`wherein R,-R,
`another preferred embodiment, the metal oxinoid compound
`has the formula:
`
`Rg
`
`Ry
`
`Rs
`
`Oo
`
`Al—O
`
`ly
`
`wherein R,—R, are as defined above and L,-L, collectively
`contain twelve or fewer carbon atoms and each indepen-
`dently represent hydrogen or hydrocarbon groupsof from 1
`to 12 carbon atoms, provided that L, and L, together or Ly
`and L, together can form a fused benzo ring. In another
`preferred embodiment, the metal oxinoid compoundhas the
`formula:
`
`typically deposited by physical vapor deposition, but other
`suitable deposition techniques are applicable. A particularly
`desirable material for the EL cathode has been found to be
`a 10:1 (atomic ratio) magnesium:silver alloy. Preferably, the
`cathode is applied as a continuous layer over the entire
`surface ofthe display panel. In another embodiment, the EL
`cathode is a bilayer composedof a lower layer of alow work
`function metal adjacentto the organic electron injecting and
`transporting zone and, overlying the low work function
`metal, a protecting layer that protects the low work function
`metal from oxygen and humidity. Optionally, a passivation
`layer may be applied over the EL cathode layer.
`Typically, the anode material is transparent and the cath-
`ode material opaqueso thatlight is transmitted through the
`anode material. However, in an alternative embodiment,
`light is emitted through the cathode rather than the anode. In
`this case the cathode must be light transmissive and the
`anode may be opaque. A practical balance light transmission
`and technical conductanceis typically in the thickness range
`of 5~25 nm.
`A preferred method of making a thin-film-transistor
`according to the present invention is described below. In a
`first step, an amorphoussilicon film of 2000+20A thickness
`is deposited at 550° C. in an LPCVD system with silane as
`the reactant gas at a process pressure of 1023 mTorr. This is
`followed by a low temperature anneal at 550° C. for 72
`hours in vacuum to crystallize the amorphoussilicon film
`into a polycrystalline film. Then a polysilicon island is
`formed by etching with a mixture of SF, and Freon 12 ina
`plasma reactor. Onto the polysilicon island active layer is
`deposited a 1000+20A PECVD SiO, gate dielectric layer.
`The gate dielectric layer is deposited from a 5/4 ratio of
`N.O/SiH, in a plasmareactorat a pressure of 0.8 Torr with
`a powerlevel of 200W and a frequency of 45