Patent Publication Number: US-9853235-B2

Title: Organic electroluminescent display device

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a semiconductor device having a circuit based on insulated-gate field effect transistors in which a single-crystal semiconductor is used for an active layer, and a method of fabricating the semiconductor device. More particularly, the present invention is well suited for applications to an electrooptic device which is typified by an organic electroluminescent display device wherein the same substrate is overlaid with a pixel unit and driver circuits disposed around the pixel unit, and to an electronic apparatus in which the electrooptic device is installed. Incidentally, here in this specification, the expression “semiconductor device” is intended to signify general devices which function by utilizing semiconductor properties, and it shall cover within its category the electrooptic device and the electronic equipment including this electrooptic device. 
     Description of the Related Art 
     In the field of flat display devices (flat panel displays) typified by liquid-crystal display devices, organic EL (electroluminescent) display devices, etc., there has been known technology wherein a display device of active matrix type is fabricated by employing insulated-gate field effect transistors (hereinbelow, the “field effect transistors” shall be abbreviated to “FETs”) formed on a single-crystal semiconductor substrate. Unlike in a case where a display device of active matrix type is fabricated by forming thin-film transistors (hereinbelow, abbreviated to “TFTs”) on a glass substrate or a quartz substrate, the technology has had the advantage that techniques fostered in the field of large-scale integrated circuits (LSIs) are applicable as they are, and that the FETs of high performance which are capable of low-voltage drive at high speed can be integrated and formed at a high density on the substrate. On the other hand, however, it has been considered the disadvantage of the technology that the display device is restricted to one of reflection type or spontaneous luminescence type because the substrate is opaque to visible light, or that the single-crystal semiconductor substrate is restricted to sizes available on the market. 
     In the technological trends toward higher image quality and full digitization in the field of the display devices, the enhancements of performances required of the active matrix type display device have inevitably heightened. The active matrix type display device is so constructed that transistors (such as TFTs or FETs) in the number of several tens to several millions are arranged in a pixel unit for displaying an image, and that pixel electrodes are respectively connected to the transistors. In operation, the image is displayed in such a way that voltages to be applied to respective pixels are controlled by the switching functions of the corresponding transistors, whereby some of EL elements are caused to luminesce. In the organic EL display device, when the switching transistors disposed in the respective pixels are turned ON, currents are caused to flow through current controlling transistors by signals generated in accordance with image data, whereby the EL elements luminesce spontaneously. 
     However, an organic EL layer which serves as the basic portion of the organic EL display device is very liable to oxidize, and it easily deteriorates in the presence of a slight amount of oxygen. Besides, it has a low becomes a cause for resistance to heat, and this also is a factor promoting oxidation. The liability to oxidize is the cause of a short lifetime of the organic EL element, and has formed a serious obstacle in putting this element into practical use. 
     SUMMARY OF THE INVENTION 
     The object of the present invention is to overcome the problem as stated above, and to provide an organic EL display device of high reliability. 
     Another object of the present invention is to provide an electron device whose display unit is highly reliable, by adopting such an organic EL display device for the display unit. 
     The construction of the present invention for accomplishing the objects consists in an organic EL display device of active matrix type wherein insulated-gate field effect transistors formed on a single-crystal semiconductor substrate are overlaid with an organic EL layer; characterized in that the single-crystal semiconductor substrate is held in a vacant space which is defined by a bed plate and a cover plate formed of an insulating material and a packing material for bonding the bed and cover plates; and that the vacant space is filled with an inert gas and a drying agent. 
     Also, the construction of the present invention consists in an organic EL display device of active matrix type having a pixel unit wherein insulated-gate field effect transistors formed on a single-crystal semiconductor substrate are overlaid with an organic EL layer; characterized in that the single-crystal semiconductor substrate is held in a vacant space which is defined by a bed plate and a cover plate formed of an insulating material, and a packing material for bonding the bed and cover plates; that the cover plate is formed of a transparent member in its area which lies over the pixel unit; and that the vacant space is filled with an inert gas and a drying agent. 
     A single-crystal silicon substrate can be favorably employed as the single-crystal semiconductor substrate. Besides, the vacant space should preferably be filled with an inert gas selected from the group consisting of helium, argon, krypton, xenon and nitrogen, and a drying agent selected from the group consisting of barium oxide and silica gel. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a sectional view of an organic EL display device of active matrix type; 
         FIGS. 2(A) and 2(B)  are diagrams showing the top plan structure and circuit arrangement of a pixel unit in the organic EL display device, respectively; 
         FIG. 3  is a top plan view of the active matrix type organic EL display device; 
         FIG. 4  is a sectional view showing the internal construction of the organic EL display device; 
         FIG. 5  is a perspective view showing the construction of a goggle type display device in which the organic EL display devices are installed; and 
         FIGS. 6(A) and 6(B)  are sectional views of the goggle type display device in which the organic EL display devices are installed. 
     
    
    
     PREFERRED EMBODIMENTS OF THE INVENTION 
     First, an organic EL display device according to the present invention will be described with reference to  FIG. 1 . The organic EL display device according to the present invention has such a structure that a pixel unit and driver circuits around the pixel unit are disposed using field effect transistors (FETs) of insulated gate type which are formed on a single-crystal semiconductor substrate (for example, single-crystal silicon substrate). 
     A substrate  101  is made of single-crystal silicon having a comparatively high resistance (for example, one of n-type at about 10 [Ωcm]), and a p-well  102  and n-wells  103 ˜ 105  are formed in self-alignment therein. Adjacent FETs are isolated by a field oxide film  106 . In forming the field oxide film  106 , channel stoppers may be formed by introducing boron (B) into the selected parts of the substrate  101  in accordance with ion implantation. 
     Gate insulating films  110 ,  116 ,  122  and  128  are formed by thermal oxidation. Gates  111 ,  117 ,  123  and  129  consist of polycrystalline silicon layers  111   a ,  117   a ,  123   a  and  129   a  formed from a polycrystalline silicon film deposited to a thickness of 100˜300 [nm] by CVD, and silicide layers  111   b ,  117   b ,  123   b  and  129   b  formed thereon to a thickness of 50˜300 [nm], respectively. The polycrystalline silicon layers may be doped with phosphorus (P) at a concentration of 10 21  [/cm 3 ] or so beforehand in order to lower the resistance thereof, or an n-type impurity at a high concentration may be diffused after the polycrystalline silicon film has been formed. Applicable as the material of the silicide layers is any of molybdenum silicide (MoSi x ), tungsten silicide (WSi x ) , tantalum silicide (TaSi x ), titanium silicide (TiSi x ), etc., and the silicide layers may well be formed in accordance with a known method. 
     The lightly-doped drain (LDD) regions  107  of a p-channel FET  201  are doped with boron (B) at a dose of 1×10 12 ˜1×10 14  [/cm 2 ] as an impurity element which bestows the conductivity type of p-type. On the other hand, the LDD regions  113  of an n-channel FET  202 , and those  119  and  125  of a switching FET  203  and a current controlling FET  204  made up of n-channel FETs are doped with phosphorus (P) or arsenic (As) as the impurity element which bestows n-type conductivity, at a dose similar to that of p-type. These LDD regions are respectively formed in self-alignment in accordance with ion implantation or ion doping by employing the corresponding gates as masks. 
     Side wall spacers  112 ,  118 ,  124  and  130  are formed in such away that, after the formation of the LDD regions, an insulating film such as silicon oxide film or silicon nitride film is formed on the whole surface of the resulting substrate by CVD, and that the insulating film is uniformly etched over the whole area thereof by anisotropic dry etching, so as to be left behind on the side walls of the corresponding gates. The source regions and drain regions of the respective FETs are formed by employing the corresponding side wall spacers as masks. More specifically, the source region  108  and drain region  109  of the p-channel FET  201  is formed by ion-implanting boron (B) at a dose of 5×10 14 ˜1×10 16  [/cm 2 ]. The n-channel FET  202 , and the switching FET  203  and current controlling FET  204  made up of these n-channel FETs are respectively formed with the source regions  114 ,  120  and  126  and drain regions  115 ,  121  and  127  by ion-implanting arsenic (As) at a dose of 5×10 14 ˜1×10 16  [/cm 2 ]. 
     A first interlayer insulating film  131  is formed to a thickness of 100˜2000 nm out of a silicon oxide film, an oxidized silicon nitride film or the like which should preferably be prepared by plasma CVD or low-pressure CVD. Further, the first interlayer insulating film  131  is overlaid with a second interlayer insulating film  132  which is made of phosphosilicate glass (PSG), borosilicate glass (BSG) or phosphoborosilicate glass (PBSG). The second interlayer insulating film  132  is prepared by spin coating or normal-pressure CVD. The prepared film is caused to reflow by a treatment of thermal activation at 700˜900 [° C.], which is carried out after the preparation and which serves also as a heat treatment, whereby the surface of the second interlayer insulating film  132  is flattened. 
     Source wiring lines  133 ,  135 ,  137  and  139  and drain wiring lines  134 ,  136 ,  138  and  140  are respectively formed after contact holes reaching the source regions and drain regions of the corresponding FETs are formed in the first interlayer insulating film  131  and the flattened film  132 . Aluminum (Al) which is usually and often used as a low-resistance material, may be employed for the wiring lines. Alternatively, a multilayer structure consisting of an Al layer and a titanium (Ti) layer may be employed for each of the wiring lines. 
     A passivation film  141  is formed of a silicon nitride film, a silicon oxide film or a nitrified silicon oxide film by plasma CVD. Further, a third interlayer insulating film  142  is formed of an organic resin material to a thickness of 1 [μm]˜2 [μm]. Any of the following: a polyimide resin, a polyamide resin, an acrylic resin, benzo-cyclo-butene (BCB), etc. can be used as the organic resin material. The merits of the use of the organic resin material are that a method of forming the film is simple, that parasitic capacitance can be lowered owing to a low relative dielectric constant, that the material is suited to be flattened, and so forth. Of course, any organic resin film other than mentioned above may be employed. Here, the polyimide resin of the type which is applied on the resulting substrate and then is treated by thermal polymerization is employed, and it is baked at 300 [° C.] in a clean oven. 
     A pixel electrode  143  is connected to the drain wiring line of the current controlling FET  204 . The pixel electrode  143  is formed of a low-resistivity material typified by Al. An Al film can be readily formed by a known method of forming the film, for example, vacuum deposition or sputtering. In order to improve the contrast , the surface of the pixel electrode  143  may be roughened into a diffusing reflective surface. 
     After the formation of the pixel electrodes  143 , cathode layers  144  containing a metal of low work function are formed on all the pixel electrodes. Since the cathode layer  144  is as thin as a few nm or so, whether a true layer is formed or it exists sporadically in the shape of islands is unclear, and hence, its contour is indicated by a broken line. 
     A material which is usable as the cathode layer  144  containing the metal of low work function, is lithium fluoride (LiF), lithium oxide (Li 2 O), barium fluoride (BaF 2 ), barium oxide (BaO), calcium fluoride (CaF 2 ), calcium oxide (CaO), strontium oxide (SrO) or cesium oxide (Cs 2 O). Since the material is insulating, the short-circuiting between the pixel electrodes is not incurred even when the cathode layer  144  is a connecting layer. Of course , a cathode layer made of a known material having conductivity, such as MgAg electrode, can be used as the cathode layer. It is necessary, however, to form cathodes themselves selectively or to perform patterning, so as to prevent the pixel electrodes from short-circuiting. 
     An organic EL (electroluminescent) layer  145  is formed on the cathode layer  144  containing the metal of low work function. Although a known material or structure can be employed for the organic EL layer  145 , a material capable of white luminescence is used in the present invention. Structurally, the organic EL layer  145  may be formed of nothing but a luminescent layer which offers a site for recombination. If necessary, it is also allowed to stack on this an electron injection layer, an electron transport layer, a hole transport layer, an electron blocking layer, a hole blocking layer or a hole injection layer. Here in this specification, all the layers where carriers are injected, transported or recombined shall be comprehensively called the “organic EL layer”. 
     In addition, the organic EL material used for the organic EL layer  145  is a high-molecular one based on a polymer . For example, the organic EL layer  145  is formed in such a way that PVK (polyvinyl carbazole), Bu-PBD (2-(4′-tert-butylphenyl)-5-(4″-biphenyl)-1,3,4-oxadiazle), coumarin 6, DCM 1 (4-dicyanomethylene-2-methyl-6-p-dimethylaminostyryl-4H-pyran), TPB (tetraphenylbutadiene) and Nile red are dissolved in 1,2-dichloromethane or chloroform, and that a solution thus obtained is applied by spin coating. The substrate structure coated with the solution is rotated at a rotational frequency of about 500˜1000 [rpm], for 20˜60 [seconds], whereby a uniform coating film is formed. 
     Of course, the coating film is formed after the organic EL material is refined (typically, by dialyzing) at least 3 times, preferably 5 times or more, thereby to lower the sodium content of this material to 0.1 [ppm] or less (preferably, 0.01 [ppm] or less). Thus, the sodium content of the organic EL layer  349  becomes 0.1 [ppm] or less (preferably, 0.01 [ppm] or less), and the volume resistance thereof becomes 1×10 11 ˜1×10 12  [Ωcm] (preferably 1×10 12 ˜1×10 13  [Ωcm]). 
     The organic EL layer  145  formed in this way is overlaid with a transparent conductive film as an anode layer  146 . Usable for the transparent conductive film is a compound (called “ITO”) produced from indium oxide and tin oxide, a compound produced from indium oxide and zinc oxide, tin oxide (SnO 2 ), zinc oxide (ZnO), or the like. 
     Besides, the anode layer  146  is overlaid with an insulating film as a passivation film  147 . The passivation film  147  should preferably be a silicon nitride film or a nitrified silicon oxide film (expressed by “SiO x N y ”). 
     The substrate structure completed up to this point in this specification shall be called “active matrix substrate”. That is, the “active matrix substrate” is the substrate which is formed with the FETs, the pixel electrodes electrically connected to the FETs, and organic EL elements including pixel electrodes as the cathodes (capacitors consisting of the cathode layers, the organic EL layer and the anodes). 
       FIG. 2(A)  is a top plan view of the pixel unit of the active matrix substrate, while  FIG. 2(B)  is a connection diagram of the circuit arrangement of the pixel unit. In actuality, the pixel unit (image display unit) is so constructed that a plurality of pixels are arrayed in the shape of a matrix. Incidentally, a sectional view taken along A-A′ in  FIG. 2(A)  corresponds to the sectional view of the pixel unit in  FIG. 1 . Accordingly, common reference numerals are indicated in  FIG. 1  and  FIG. 2(A) , both of which may be referred to on occasion. Besides, two pixels are illustrated in the top plan view of  FIG. 2(A) , and they have the same structure. As shown in  FIG. 2(B) , two FETs per pixel are disposed for the organic EL element  205 . Both the FETs are of n-channel type, and they function as the switching FET  203  and the current controlling FET  204 . 
     In the above way, upon the single-crystal silicon substrate can be formed driver circuits each of which is based on a CMOS circuit configured with a p-channel FET  201  and a n-channel FET  202 , and pixel units each of which includes switching FET  203  and current controlling FET  204  formed of n-channel FETs. The driver circuits based on the CMOS circuits can form, for example, a shift register circuit, a buffer circuit, a sampling circuit , a D/A converter, and a latch circuit. Since such circuits are constructed of the insulated-gate FETs whose active layers are made of single-crystal silicon, they are capable of high-speed operations, and a lower power consumption can be achieved by setting their drive voltages at 3˜5 [V]. By the way, the structures of the FETs explained in this embodiment are nothing more than mere examples, and the FETs need not be restricted to the structures shown in  FIG. 1 . 
       FIG. 3  is a top plan view showing an active matrix substrate. Referring to the figure, the active matrix substrate includes a substrate  1000 , a pixel unit  1001 , data line side driver circuits  1003 , and scanning line side driver circuits  1002 . Input terminals for the respective driver circuits are pads  1006  for wire bonding which are disposed near the edges of the substrate  1000 , and they are connected to the driver circuits via leads  1004 ˜ 1005 . The pixel unit having a size of from 0.5-inch class to 2.5-inch class is well suited for fabrication. 
     The active matrix substrate formed with the organic EL layer is sealed in a package in order to be cut off from external shocks, and ambient conditions such as dust and humidity. The shape and scheme of the package are exemplified in  FIG. 4 . A bed plate  401  is formed of an insulating material such as ceramics, and the active matrix substrate  413  formed with the organic EL layer is fixed thereon by a low-melting glass or metallized layer  402 . The active matrix substrate  413  is connected with an external circuit by a lead frame  404 , which is connected with the active matrix substrate  413  by wire pieces  412  of gold (Au) through pads  410  for wire bonding. 
     The active matrix substrate  413  is sealed with a cover ceramic plate  405 . The ceramic cover plate  405  is bonded with the bed plate  401  by a binder layer  404 . Pyroceram cement, bismuth oxide-based glass, lead oxide-based glass, or the like is used for the binder layer  404 . A window member  406  made of a transparent quartz plate, a transparent glass plate or the like is mounted and fixed with adhesives  407  in an area where the cover plate  405  formed of the ceramics or the like insulating material similarly to the bed plate  401  lies over the pixel unit of the active matrix substrate  413 . In this way, the active matrix substrate  413  formed with the organic EL layer is enclosed, and a vacant space  414  is formed. Further, the vacant space  414  should desirably be filled with an inert gas (such as argon, helium, krypton, xenon or nitrogen) or have a drying agent (such as barium oxide) put therein. In this way it is possible to suppress the deterioration of the EL element attributed to moisture etc. 
     Although not shown in this embodiment, a color display can also be constructed by disposing color filters or black matrix layers (light intercepting layers) on the organic EL layer in correspondence with the individual pixels formed from the organic EL layer of the active matrix substrate. Alternatively, color filters may well be disposed at the window  406  shown in  FIG. 4 . 
     In the state shown in  FIG. 4  as described above, the lead frame  403  is connected to the terminals of the external equipment, whereby an image signal etc. can be inputted to display an image on the pixel unit. Here in this specification, an article which is brought into a state where it is capable of displaying an image, by attaching a lead frame to an external circuit, is defined as an “organic EL display device”. 
     Now, there will be described a practicable example in which an organic EL display device of active matrix type is applied to a goggle type display device.  FIG. 5  shows a schematic view of the goggle type display device in this example. The goggle type display device main body  3600  is furnished with two, right and left display units, which are constructed of organic EL display devices  3602 R,  3602 L, circuit boards  3603 R,  3603 L and lenses  3601 R,  3601 L. 
       FIG. 6(A)  shows a sectional view of part A indicated in  FIG. 5 , while  FIG. 6(B)  shows an enlarged view of part B indicated in  FIG. 6(A) . As shown in  FIGS. 6(A) and 6(B) , in this example, in the goggle type display device  3600  the organic EL display device  3602 R mounted on the lens  3601 R is connected to the circuit board  3603 R equipped with a signal control circuit etc., through a lead frame  3606 R. Light luminescing from the organic EL display device  3602 R arrives at the eyeball  3604 R of a user via an optical path indicated by arrows in  FIG. 6(A) , whereby the user can recognize an image. 
     The organic EL display device has a wide view angle owing to its spontaneous luminescence. When applied to the goggle type display device, the organic EL display is not spoilt even if the relative positions of this display device and an observer&#39;s eye have deviated. 
     The present invention brings forth an effect as stated below: 
     A single-crystal semiconductor substrate which is formed with insulated-gate field effect transistors and an EL layer, is held in a vacant space which is defined by a bed plate and a cover plate formed of an insulating material, and a packing material for bonding the bed and cover plates, and the vacant space is filled with an inert gas and a drying agent, whereby the oxidation of the EL layer can be prevented to provide an organic EL display device of high reliability.