(19) United States
`(12) Patent Application Publication (10) Pub. No.: US 2002/0009538A1
`(43) Pub. Date:
`Jan. 24, 2002
`Arai
`
`US 2002OOO9538A1
`
`(54) METHOD OF MANUFACTURING A
`LIGHT-EMITTING DEVICE
`
`Yasuyuki Arai, Kanagawa (JP)
`(76) Inventor:
`Correspondence Address:
`NIXON PEABODY, LLP
`8180 GREENSBORO DRIVE
`SUTE 800
`MCLEAN, VA 22102 (US)
`(21) Appl. No.:
`09/847,308
`(22) Filed:
`May 3, 2001
`(30)
`Foreign Application Priority Data
`
`May 12, 2000 (JP)...................................... 2OOO-141005
`
`201
`
`Publication Classification
`
`(51) Int. Cl." ....................................................... B05D 5/12
`(52) U.S. Cl. ................................................................ 427/66
`
`(57)
`
`ABSTRACT
`
`A technique for manufacturing a light-emitting device by
`using a method of forming a thin film having a highly
`uniform thickness with high throughput is provided. The
`technique includes the Steps of filling a Small molecular
`organic electroluminescence material into an evaporation
`cell that has an orifice-like evaporation material ejecting
`port, and heating the Small molecular organic electrolumi
`neScence material in an inert gas atmosphere to form a light
`emitting layer on a Substrate from the Small molecular
`organic electroluminescence material.
`
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`Patent Application Publication
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`US 2002/0009538A1
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`Jan. 24, 2002. Sheet 5 of 8
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`Patent Application Publication
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`Jan. 24, 2002 Sheet 6 of 8
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`US 2002/0009538A1
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`Jan. 24, 2002
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`METHOD OF MANUFACTURING A
`LIGHT-EMITTING DEVICE
`
`BACKGROUND OF THE INVENTION
`0001) 1. Field of the Invention
`0002 The present invention relates to a method of form
`ing a thin film used to manufacture an EL (electrolumines
`cence) element comprised of an anode, a cathode, and a
`luminous material, a light-emitting organic material (here
`inafter referred to as an organic EL material) in particular,
`that is Sandwiched between the anode and the cathode to
`provide electroluminescence.
`0003. The electroluminescence or light emission herein
`refers to either fluorescence from Singlet excitation or phos
`phorescence from triplet excitation, or both.
`0004 2. Description of the Related Art
`0005. In recent years, development is proceeding on
`display devices using an EL element as a light-emitting
`element that emits light through EL phenomenon of an
`organic EL material (hereinafter referred to as EL display
`devices). Being light-emitting, the EL display devices do not
`needbacklight unlike liquid crystal display devices. The EL
`display devices also have a wide Viewing angle, which
`makes them prospective display units for portable equip
`ment used outdoors.
`0006 The EL display devices can be divided into two
`types; passive (passive matrix) EL display devices and
`active (active matrix) EL display devices. Both types are
`Vigorously being developed. Of the two, the active matrix
`EL display devices particularly attract attention at present.
`EL materials for forming a light emitting layer of an EL
`element can be divided into organic materials and inorganic
`materials. The organic materials are classified further into
`Small molecular organic EL materials and polymer organic
`EL materials. They are equally actively researched. The
`Small molecular organic EL materials are deposited mainly
`by evaporation whereas the polymer organic EL materials
`are deposited mainly by coating.
`0007. In order to manufacture a color EL display device,
`EL materials emitting different colors of light have to be
`deposited Separately to form pixels of different colors.
`However, patterning by photolithography is not an option
`because EL materials are generally weak against moisture
`and oxygen. It is thus necessary to deposit the EL materials
`and pattern them at the same time.
`0008. The most common method therefore is to place a
`mask of a metal plate or a glass plate with openings
`(hereinafter referred to as Shadow mask) between an evapo
`ration Source and a Substrate on which the EL materials are
`to be deposited. According to this method, the EL materials
`evaporated from the evaporation Source passes only through
`the openings So that the materials are deposited Selectively.
`The deposition and patterning of the EL layer can thus be
`achieved simultaneously.
`0009 Every conventional evaporation device uses a
`Single evaporation Source and an EL material radially dis
`charged from the Source is deposited on the Substrate to form
`a thin film. For that reason, Suitable arrangement of the
`Substrate has to be thought out in accordance with how far
`the discharged material flies. For instance, fixing the Sub
`
`Strate to a conical Substrate holder has been thought out So
`that the distance from the evaporation Source to the Substrate
`is the same for all the directions.
`0010. However, the above method requires an oversize
`Substrate holder in the case of employing a multi-pattern
`process in which a plurality of panels are formed on a
`large-area Substrate, leading to an increase in size of the film
`forming apparatus itself. On the other hand, a Single wafer
`method has difficulties in forming a film of uniform thick
`neSS because the Substrate is flat to vary the distance from
`the evaporation Source to points within the Substrate Surface.
`0011. The large substrate also requires setting the dis
`tance between the evaporation Source and the Shadow mask
`long in order to disperse the evaporated EL material Suffi
`ciently and form a thin film uniformly over the entire surface
`of the Substrate. Setting this distance long is another factor
`in enlargement of the apparatus.
`
`SUMMARY OF THE INVENTION
`0012. The present invention has been made in view of the
`above problems, and an object of the present invention is
`therefore to provide a technique for manufacturing a light
`emitting device by using a method of forming a thin film
`having a highly uniform thickness with high throughput.
`0013 In order to attain the object above, a structure of the
`present invention is characterized by comprising the Steps
`of:
`0014 filling a Small molecular organic electrolumines
`cence material into an evaporation cell; and
`0015 heating the small molecular organic electrolumi
`neScence material in an inert gas atmosphere to form a light
`emitting layer on a Substrate from the Small molecular
`organic electroluminescence material.
`0016. Another structure of the present invention is char
`acterized by the Steps of:
`0017 placing in a reaction chamber an evaporation
`Source that has an evaporation cell containing a Small
`molecular organic electroluminescence material and placing
`a shutter over an orifice of the evaporation cell;
`0018 heating the small molecular organic electrolumi
`neScence material in an inert gas atmosphere; and
`0019 opening and closing the shutter to form a light
`emitting layer on one Surface of a Substrate from the Small
`molecular organic electroluminescence material, the Sub
`Strate being fixed to a Sample Stage.
`0020. Another structure of the present invention is char
`acterized by comprising the Steps of:
`0021 filling a small molecular organic electrolumines
`cence material into an evaporation cell; and
`0022 heating the small molecular organic electrolumi
`neScence material in an inert gas atmosphere to Selectively
`form a light emitting layer on a Substrate from the Small
`molecular organic electroluminescence material.
`0023. Another structure of the present invention is char
`acterized by comprising the Steps of:
`0024 placing in a reaction chamber an evaporation
`Source that has an evaporation cell containing a Small
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`molecular organic electroluminescence material and placing
`a shutter over an orifice of the evaporation cell;
`0.025
`heating the small molecular organic electrolumi
`neScence material in an inert gas atmosphere; and
`0.026 opening and closing the shutter to selectively form
`a light emitting layer on one Surface of a Substrate from the
`Small molecular organic electroluminescence material, the
`Substrate being fixed to a Sample Stage.
`0027. An evaporation cell having an orifice-like evapo
`ration material ejecting port is used as an evaporation
`Source, which makes it possible to Selectively deposit an
`organic electroluminescence material on a Substrate. In
`order to deposit by evaporation the organic electrolumines
`cence material over a wide region within the Substrate
`Surface, one or both of the Substrate and the evaporation cell
`are moved during the evaporation. The move of the Substrate
`or the evaporation cell, or both, is associated with opening
`and closing of the shutter, which makes it possible to deposit
`the organic electroluminescence material as if to draw a
`pattern with Strokes.
`0028 Adopting the above structures of the present inven
`tion allows the substrate to have a selectively formed layer
`made of an organic electroluminescence material in a given
`region without using a shadow mask or the like. In this
`Specification, an evaporation method as above is referred to
`as gasification evaporation and a device using the evapora
`tion method is referred to as gasification evaporation device.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`In the accompanying drawings:
`0029)
`0030 FIG. 1 is a diagram illustrating the structure of an
`evaporation device;
`0.031
`FIG. 2 is a diagram illustrating an evaporation cell
`and an evaporation method;
`0.032
`FIG. 3 is a diagram illustrating an apparatus used
`in manufacturing a light-emitting device;
`0.033 FIGS. 4A and 4B are diagrams showing the struc
`ture of an EL display device, where FIG. 4A is a top view
`thereof and FIG. 4B is a sectional view thereof;
`0034 FIGS. 5A and 5B are sectional views showing a
`pixel portion of an EL display device;
`0035 FIGS. 6A and 6B are a top view of a pixel portion
`of an EL display device and a circuit diagram thereof,
`respectively;
`0036 FIGS. 7A to 7F are diagrams showing examples of
`a Semiconductor device; and
`0037 FIGS. 8A to 8C are diagrams showing examples of
`the Semiconductor device.
`
`DETAILED DESCRIPTION OF THE
`PREFERRED EMBODIMENTS
`0038 Embodiment 1
`0.039
`FIG. 1 is a diagram illustrating the structure of a
`gasification evaporation device according to the present
`invention. A reaction chamber 101 is an air-tight container
`and the interior thereof is completely shut out from the
`outside air. The reaction chamber 101 is filled with inert gas
`
`(typically, argon) Supplied from gas introducing means 107.
`The gas is kept at the same pressure as the atmospheric
`pressure (1.01x10 Pa). Discharging means 108 is activated
`as the need arises to circulate the inert gas or to discharge the
`gaS.
`0040. One or plural evaporation cells used as evaporation
`Sources are provided depending on the need. In FIG. 1, an
`evaporation cell (1) 109a, an evaporation cell (2) 109b, and
`an evaporation cell (3) 109c, three in total, are provided. The
`temperature of these evaporation cells is controlled by
`heating means 110.
`0041) A substrate 102 is fixed to a sample stage 103. A
`Shutter automatically opening and closing is interposed
`between the Substrate and the evaporation cells. A computer
`111 centrally controls control means 104 for moving the
`Sample Stage 103 in the horizontal direction, control means
`106 for opening and closing the shutter 105, and the heating
`means 110, which operate in association with one another.
`The associated operation makes it possible to form a pre
`programmed evaporation pattern on the Substrate 102 with
`out using a Shadow mask.
`0042 FIG. 2 is a diagram given for a simple explanation
`of this gasification evaporation. Evaporation cells 206 to 208
`contain evaporation materials and are heated to the evapo
`ration temperature. Each evaporation cell is formed of boron
`nitride, alumina, tungsten, or the like, and has its tip formed
`into an orifice with a diameter of Several tens to Several
`hundreds um. When the evaporation cells are heated by a
`heater, the pressure in the evaporation cells is risen to gasify
`the contained materials and the flux distribution of the
`evaporation materials ejected through the orifices gains
`directivity.
`0043. The directivity is determined in accordance with
`the orifice diameter and the thickness thereof. Since the
`deposition takes place under the atmospheric pressure, the
`gasified and evaporation materials have Small mean free
`process and the evaporation materials can be deposited on
`the Substrate while maintaining a relatively high directivity.
`0044) The position of the Substrate and the orifice is
`controlled so that evaporation films 209 to 212 are formed
`at positions that coincide with positions of pixel electrodes
`202 to 204 formed on a substrate 201. Abump 205 is useful
`in Separating adjoining evaporation films.
`0045 With the gasification evaporation device as above,
`a given pattern of evaporation films can be formed on a
`Substrate without using a shadow mask. In this case, the
`pattern can have a width of about 50 to 200 lum. The
`Substrate is placed on a Sample Stage and hence is movable
`in the horizontal direction (direction X-Y). By associating
`the movement of the Substrate with opening and closing of
`the shutter and by using the evaporation cells illustrated in
`FIG. 2, minute patterns can be drawn on a large-area
`Substrate.
`0046. A description given next with reference to FIG. 3
`is an example of a film forming apparatus Suitable for
`manufacturing a light-emitting device. In FIG. 3, reference
`symbol 501 denotes a transfer chamber and the transfer
`chamber 501 is provided with a transferring mechanism 502
`to carry a substrate 503. The interior of the transfer chamber
`501 is set to the atmospheric pressure and is filled with inert
`gas. Gates 500a to 500e each separate one processing
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`chamber from other chambers. The Substrate is carried from
`one processing chamber to another by the transferring
`mechanism 502 when the associated gate is opened.
`0047 Reference symbol 504 denotes a loading chamber
`for Setting the Substrate and the loading chamber also Serves
`as an unloading chamber. The loading chamber 504 is
`connected to the transfer chamber 501 through the gate
`500a, and this is where a carrier (not shown) in which the
`substrate 503 is set is placed. The loading chamber 504 may
`be divided into a room for bringing the Substrate in and a
`room for Sending the Substrate out.
`0.048. The substrate 503 Supplied to the apparatus has
`finished the manufacture process up through formation of a
`transparent conductive film to Serve as an anode of an EL
`element. The Substrate 503 is set in the carrier with its film
`forming Surface facing downward. This is to facilitate a face
`down method (also called deposit up method) at a later Step
`of forming a film by evaporation. The face down method
`refers to a method in which the film is formed while the film
`forming surface of the substrate faces downward. This
`method prevents dusts from Settling onto the film forming
`Surface.
`0049) Next, denoted by 505 is a processing chamber for
`processing the Surface of the anode of the EL element or a
`cathode thereof (hereinafter referred to as pre-processing
`chamber). The pre-processing chamber 505 is connected to
`the transfer chamber 501 through the gate 500b. The pre
`processing chamber can be modified So as to Suit a proceSS
`of manufacturing an EL element, but any modified pre
`processing chamber has to be capable of heating the Sub
`strate at 100 to 120° C. while irradiating the Surface of the
`anode made of a transparent conductive film with ultraViolet
`light in oxygen. Pre-processing as Such is effective in
`treating the Surface of the anode of the EL element.
`0050. The next processing chamber is an evaporation
`chamber denoted by 506. The chamber 506 is for depositing
`an organic EL material through evaporation and is called an
`evaporation chamber (A). The evaporation chamber (A) 506
`is connected to the transfer chamber 501 through the gate
`500c. The evaporation chamber (A) 506 provided here has
`the structure shown in FIG. 1.
`0051). In a film forming unit 507 inside the evaporation
`chamber (A) 506, a hole injection layer is first formed over
`the entire Surface of the Substrate. Subsequently, a light
`emitting layer emitting red light is formed, then a light
`emitting layer emitting green light, and then a light emitting
`layer emitting blue light. The hole injection layer, the light
`emitting layer emitting red light, the light emitting layer
`emitting green light and the light emitting layer emitting
`blue light can be formed from arbitrary materials.
`0052) The structure of the evaporation chamber (A)506
`allows the evaporation Sources to be Switched in accordance
`with the kind of organic materials to be deposited. Specifi
`cally, a preparatory chamber 508 storing plural kinds of
`evaporation cells is connected to the evaporation chamber
`(A) 506 so that its interior transferring mechanism can
`replace an evaporation cell in 506 with an evaporation cell
`in 508. Accordingly, the evaporation cells are switched
`every time the organic EL materials to be deposited change.
`A Shadow mask is moved by a distance corresponding to one
`pixel whenever the organic EL material forming the same
`mask is changed.
`
`0053. The deposition method used in the evaporation
`chamber (A) 506 is the one illustrated in FIGS. 1 and 2.
`0054) Next, reference symbol 509 denotes an evaporation
`chamber for forming by deposition a conductive film (metal
`film Serving as the cathode) serving as the anode or cathode
`of the EL element, and the chamber 509 is called an
`evaporation chamber (B). The evaporation chamber (B) 509
`is connected to the transfer chamber 501 through the gate
`500d. The evaporation chamber (B) 509 provided here has
`the structure shown in FIG. 2. In a film forming unit 510
`inside the evaporation chamber (B) 509, an Al-Li alloy
`film (film of an alloy of aluminum and lithium) is formed as
`a conductive film to serve as the cathode of the EL element.
`The gasification evaporation can also be applied to forma
`tion of an alloy film as this.
`0055. A processing chamber that comes next is a sealing
`chamber (also called an enclosing chamber or a glove box)
`511, which is connected to the loading chamber 504 through
`the gate 500e. In the sealing chamber 511, the final process
`ing of Sealing the EL element in an air-tight Space is
`conducted. This processing is to protect the fabricated EL
`element against oxygen and moisture, and uses methods
`Such as automatic Sealing using a Sealing member or Sealing
`with a thermally-curable resin or a UV-curable resin.
`0056. The sealing member can be formed from a material
`Such as glass, ceramics, plastics and metals, but the material
`has to be light-transmissive if the EL element emits light
`toward the Sealing member Side. The Sealing member is
`bonded to the Substrate on which the EL element is formed
`by curing a thermally-curable resin or a UV-curable resin
`through heat treatment or irradiation of ultraviolet light. The
`air-tight Space is thus formed. It is also effective to place a
`hygroscopic material represented by barium oxide in this
`air-tight Space.
`0057 The space between the sealing member and the
`substrate on which the EL element is formed may be filled
`with a thermally-curable resin or a UV-curable resin. In this
`case, to add a hygroscopic material represented by barium
`oxide in the thermally-curable resin or the UV-curable resin
`is effective.
`0058. In the film forming apparatus shown in FIG. 3, the
`sealing chamber 511 has therein a mechanism 512 for
`irradiating ultraviolet light (hereinafter referred to as ultra
`violet light irradiation mechanism). The ultraviolet light
`irradiation mechanism 512 emits ultraviolet light to cure the
`UV-curable resin. The interior of the sealing chamber 511
`may be set to reduced pressure if a vacuum pump is
`provided. When the above Sealing Step is automated through
`operation of robots, the reduced pressure in the Sealing
`chamber prevents oxygen and moisture from entering the
`chamber. Alternatively, the interior of the Sealing chamber
`511 may be pressurized. In this case, it is pressurized while
`being purged with nitrogen gas or rare gas of high purity to
`prevent oxygen and the like of the outside air from entering
`the chamber.
`0059) Next, a handing-over chamber (pass box) 513 is
`connected to the sealing chamber 511. The handing-over
`chamber 513 is provided with a transferring mechanism (B)
`514, which brings the substrate whose EL element has been
`enclosed through the processing in the Sealing chamber 511
`into the handing-over chamber 513. The interior of the
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`handing-over chamber 513 may also be set to reduced
`preSSure if a vacuum pump is provided. The handing-over
`chamber 513 is provided to avoid direct exposure of the
`sealing chamber 511 to the outside air, and the substrate is
`taken out from the handing-over chamber.
`0060 AS has been described in the above, the film
`forming apparatus shown in FIG. 3 is capable of keeping the
`substrate away from the outside air until after the EL
`element is completely enclosed in an air-tight Space. This
`make the film forming apparatus capable of manufacturing
`an EL display device of higher reliability.
`0061 Now, a description is given on an example of using
`this film forming apparatus to manufacture a light-emitting
`display panel that uses an EL material (hereinafter referred
`to as EL display device). FIG. 4A is a top view of the EL
`display device. In FIG. 4A, reference symbol 10 denotes a
`Substrate, 11, a pixel portion, 12, a Source Side driver circuit,
`and 13, a gate side driver circuit. The driver circuits are
`respectively led to an FPC 17 through wirings 14 to 16 and
`connected to external equipment.
`0.062
`FIG. 4B shows a sectional view taken along the
`line A-A of FIG. 4A. An opposite substrate 80 is provided
`at least above the pixel portion, preferably, above the pixel
`portion and the driver circuits. The opposite substrate 80 is
`bonded, using a Sealing agent 19, to an active matrix
`substrate on which TFTs and a light-emitting layer made of
`an EL material are formed. The sealing agent 19 has a filler
`(not shown) mixed therein, which enables the two substrates
`to be bonded with an almost uniform distance. The exterior
`of the Sealing agent 19 and the top and peripheral Surface of
`the FPC 17 are sealed with an enclosing agent 81. The
`enclosing agent 81 is formed from a Silicone resin, an epoxy
`resin, a phenol resin, butyl rubber, or the like.
`0063) When an active matrix substrate 10 is bonded to the
`opposite Substrate 80 with the Sealing agent 19, a Space is
`formed therebetween. The space is filled with a filling agent
`83. The filling agent 83 also has an effect of adhering the
`opposite substrate 80. As the filling agent 83, PVC (poly
`vinyl chloride), an epoxy resin, a silicone resin, PVB (poly
`vinyl butylal), EVA (ethylene vinylacetate), or the like can
`be used. The light-emitting layer is weak against moisture or
`humidity and is liable to degrade. Therefore it is desirable to
`mix a drying agent Such as barium oxide in the filling agent
`83 to maintain the moisture absorbing effect. A silicon
`nitride film or a silicon oxynitride film is formed as a
`passivation film 82 on the light-emitting layer in order to
`prevent corrosion caused by an alkaline element contained
`in the filling agent 83.
`0064. The opposite substrate 80 may be a glass plate, an
`aluminum plate, a stainless-steel plate, an FRP (fiberglass
`reinforced plastics) plate, a PVF (polyvinyl fluoride) film, a
`Mylar film (trade name for a product of DuPont), a polyester
`film, an acrylic film, an acrylic plate, or the like. A sheet
`consisting of aluminum foil Several tens um in thickness and
`PVF films or Mylar films sandwiching the aluminum foil
`may be used to enhance resistance against moisture. The EL
`element is thus Sealed and shut out from the outside air.
`0065. In FIG. 4B, formed on the substrate 10 and a base
`film 21 are a driver circuit TFT (shown here is a CMOS
`circuit using an n-channel TFT and a p-channel TFT in
`combination) 22 and a pixel portion TFT (only a TFT for
`
`controlling a current to the EL element is shown here) 23.
`The TFTs, the n-channel TFT in particular, have LDD
`regions structured as shown in this embodiment in order to
`prevent a decrease in ON current due to the hot carrier effect
`and to prevent characteristic degradation due to Vth shift and
`bias StreSS.
`0066. Manufacture of the EL display device is continued
`and an interlayer insulating film (leveling film) 26 is formed
`from a resin material on a Source wiring and a drain wiring.
`On the interlayer insulating film 26, a pixel electrode 27
`electrically connected to a drain of the pixel portion TFT23
`is formed from a transparent conductive film. The transpar
`ent conductive film may contain a compound of indium
`oxide and a tin oxide (this compound is called ITO) or a
`compound of indium oxide and Zinc oxide. After forming the
`pixel electrode 27, an insulating film 28 is formed and an
`opening is formed in the insulating film over the pixel
`electrode 27.
`0067. A light-emitting layer 29 is then formed. The
`light-emitting layer 29 may have a single layer Structure or
`a laminate Structure in which known EL materials to form a
`hole injection layer, a hole transportation layer, a light
`emitting layer, an electron transportation layer and an elec
`tron injection layer are layered in an arbitrary combination.
`Whichever structure is to be formed, known techniques can
`be used. The EL materials preferable for the present inven
`tion are Small molecular materials and are deposited by
`gasification evaporation.
`0068. In the light-emitting layer, wavelengths of light
`emitted from light emitting layers (red light emitting layers,
`green light emitting layers and blue light emitting layers) are
`different between different pixels, thereby enabling the
`device to display in color. Other methods to obtain color
`display include combining a color conversion layer (CCM)
`with color filters and combining a white light emitting layer
`with color filters, and either one can be employed. The EL
`display device may of course be a monochrome light emit
`ting display device.
`0069. After forming the light-emitting layer 29, a cathode
`30 is formed thereon. Desirably, moisture and oxygen in the
`interface between the cathode 30 and the light-emitting layer
`29 are removed as much as possible. This requires Some
`cotrivance Such as forming the light-emitting layer 29 and
`the cathode 30 successively in vacuum or forming the
`light-emitting layer 29 in an inert atmosphere to then form
`the cathode 30 in vacuum without exposing it to the air. The
`film formation as above can be carried out by using a film
`forming apparatus of multi-chamber type.
`0070 Y (yttrium) is used for the cathode 30. The cathode
`30 is connected to the wiring 16 in a region denoted by 31.
`The wiring 16 is a power Supply line for applying a given
`voltage to the cathode 30, and is connected to the FPC 17
`through anisotropic conductive paste material 32. A resin
`layer 80 is formed on the FPC 17 to enhance the adhesion
`of this portion.
`0071. In order to electrically connect the cathode 30 with
`the wiring 16 in the region denoted by 31, a contact hole has
`to be formed through the interlayer insulating film 26 and the
`insulating film 28. The contact hole is formed when the
`interlayer insulating film 26 is etched (upon forming a
`contact hole for the pixel electrode) and when the insulating
`
`SAMSUNG EX. 1017 - 13/16
`
`

`

`US 2002/0009538A1
`
`Jan. 24, 2002
`
`film 28 is etched (upon forming the opening prior to the
`formation of the light-emitting layer). Alternatively, the
`contact hole may be formed by etching the insulating film 28
`all the way through the interlayer insulating film 26. In this
`case, the contact hole can be shaped appropriately if the
`interlayer insulating film 26 and the insulating film 28 are
`formed from the same resin material.
`0.072 The wiring 16 passes through a gap between the
`sealing agent 19 and the substrate 10 (the gap is filled with
`the enclosing agent 81) to be electrically connected to the
`FPC 17. Although the description given here is about the
`wiring 16, the other wirings 14 and 15 similarly pass under
`the sealing agent 19 to be electrically connected to the FPC
`17.
`0073 FIGS. 5A and 5B each show a more detailed
`sectional structure of the pixel portion. FIG. 6A and FIG.
`6B show a top Structure of the pixel portion and a circuit
`diagram thereof, respectively. In FIG. 5A, a Switching TFT
`2402 formed on a substrate 2401 has a double gate structure
`in which substantially two TFTs are connected in series. An
`LDD region having an offset region that does not overlap
`with a gate electrode can thus be formed, thereby providing
`an advantage of reduced OFF current value. Although the
`TFThere has a double gate Structure, it may take a triple gate
`Structure or a multi-gate Structure having more than three
`gateS.
`0.074 An n-channel TFT is used for a current controlling
`TFT 2403. This TFT has a structure in which an LDD region
`overlapping with a gate electrode is formed only on the drain
`Side. The Structure reduces the parasitic capacitance between
`the gate and the drain and reduces the Serial resistance,
`thereby increasing a current driving ability. This structure is
`also Significant from another viewpoint. The current con
`trolling TFT is an element for controlling the amount of
`current flowing into the EL element, and hence a large
`amount of current flows through the TFT to increase the risk
`of degradation by heat or by hot carriers. Therefore, the
`degradation of the current controlling TFT is prevented and
`stability in operation of the current controlling TFT can be
`improved by providing it with an LDD region partially
`overlapping with a gate electrode. In this case, a drain 35 of
`the Switching TFT 2402 is electrically connected to a gate
`electrode 37 of the current controlling TFT through a wiring
`36. A wiring denoted by 38 is a gate line for electrically
`connecting gate electrodes 39a and 39b of the Switching
`TFT 2402 with each other.
`0075) The current controlling TFT 2403 shown here has
`a single gate Structure. However, it may take a multi-gate
`structure in which a plurality of TFTs are connected in
`Series. Another structure may be employed in which a
`plurality of TFTs are connected in parallel to substantially
`divide a channel forming region into plural Sections, thereby
`releasing heat with high efficiency. This structure is effective
`as countermeasures against degradation by heat.
`0.076. As shown in FIG. 6A, a wiring serving as the gate
`electrode 37 of the current controlling TFT 2403 overlaps
`with a drain line 40 of the current controlling TFT 2403 in
`a region denoted by 2404 through the insulating film. A
`capacitor is formed in the region denoted by 2404. The
`capacitor 2404 functions as a capacitor for holding the
`Voltage applied to the gate of the current contro