Abstract:
In order to provide a thin oxysulfide film excellent in crystallinity and suitable for use as a luminescent layer of a thin film EL device and a thin fluorescent film for a CRT, a metal element is evaporated from an evaporation source provided in a chamber in which a sulfur gas and an oxygen gas have been introduced to combine those substances chemically on a substrate provided in the chamber to form a thin oxysulfide film on a surface of the substrate.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method of forming a thin oxysulfide film and more particularly to a method of forming a thin oxysulfide film having a good crystallinity suitable for use as a fluorescent film in thin film EL (electroluminescence) devices and CRTs. 
     2. Description of the Related Art 
     Because of many problems in respect of luminance, a dispersive type EL device which uses zinc sulfide (ZnS) fluorescent powder has been deterred its development as a light source for illumination. In its place, a thin film EL device using a thin film phosphor layer has drawn attention recently because it can generate high luminance. 
     In the thin film EL device, since the luminescent layer is made of a thin transparent film, halation and oozing due to scattering of light incident into the luminescent layer and light generated within the luminescent layer do not occur to any great degree, the thin film EL device exhibits clear and high-contrast display performances. Therefore, the thin film EL device is suitable for display units of vehicle-mounted type and for computer terminals, etc. as well as a light source for illumination. 
     The thin film EL device is generally of a layered structure comprising a transparent substrate, a transparent electrode made of a tin oxide (SnO 2 ) layer, a first dielectric layer, a luminescent layer made of a host material layer with luminescent center impurities being added thereto, a second dielectric layer, and a rear electrode made of an aluminum layer, sequentially laminated in this order. 
     The luminescence process of the thin film EL device is as follows. When a required voltage is applied across the transparent and rear electrodes, an electric field is created within the luminescent layer by which electrons trapped in the interface state are drawn out and accelerated to have sufficient energy and collide with orbital electrons of the luminescent center substance, for example, Eu, to excite the orbital electrons. When the excited luminescent center substance returns to its ground state, it emits light. 
     In a conventional thin film EL device, a luminescent layer comprising, for example, a host material of Y 2  O 2  S containing Eu as a luminescent center impurity (hereinafter expressed as Y 2  O 2  S:Eu) is formed by the process of sputtering or electron beam deposition. 
     In the sputtering process, for example, a sintered pellet made of a mixture of Y 2  O 2  S:Eu fluorescent powder and sulfur is sputtered, thereby to deposit the sputtered mixture on a substrate. 
     According to the conventional method of forming the luminescent layer, when the substrate temperature and the sulfur density are low, Y 2  O 3  :Eu is produced while when the substrate temperature is increased to about 200°-400° C., Y 2  O 2  S:Eu is produced. The resulting Y 2  O 2  S:Eu, however, exhibits a low orientation characteristic and has a granular multi-crystalline structure or a structure containing a so-called dead layer in which many small crystalline grains is produced at the early stage of growth. When the substrate temperature is further increased, orientation characteristics are improved, but sulfur is eliminated, thereby producing Y 2  O 3  :Eu, the undesirable. 
     When a luminescent layer contains a dead layer, electrons in the luminescent layer accelerated by an externally applied electric field are scattered by a crystalline granular interface before they collide with luminescent center impurities so as to emit light. Thus, the externally applied electric field does not contribute effectively to the light emission. 
     For a CRT display, Y 2  O 2  S:Eu is used which is produced by sintering for several hours at a temperature of about 1000° C. However, crystallinity is low unless it is sintered at high temperature and the resulting grain size is large, for example, larger than several μm. Therefore, it is disadvantageous when used as a thin film luminescent material. 
     This problem applies not only to Y 2  O 2  S:Eu, but also to other thin oxysulfide films. 
     As described above, in the conventional method, a film having high crystallinity cannot be obtained at low temperature while if the substrate temperature is increased, sulfur is eliminated, so that a thin excellent oxysulfide film cannot be obtained in this manner either. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide a thin oxysulfide film excellent in crystallinity and suitable for a luminescent layer of thin film EL devices or a fluorescent film for CRTs. 
     According to the present invention, a thin oxysulfide film is formed by evaporating a metal element within a chamber in which an oxygen gas and a sulfur gas are introduced to combine them chemically on a substrate to form a thin metal oxysulfide film on a surface of the substrate. 
     Preferably, at least one of impurities of Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm and Yb is evaporated from an independent evaporation source to be added to the film when the film is formed. 
     According to the method of the present invention, evaporation of a metal element and supplying of sulfur and oxygen are independently controlled such that these substances are chemically combined on a substrate to form a luminescent layer whose composition is stoichiometric. Thus, a thin oxysulfide film having high crystallinity is produced. 
     Even if the temperature of the substrate is raised for the purpose of improving the crystallinity, since sulfur is supplied into the chamber in the form of gas, the sulfur will not be eliminated and the quantity of sulfur is appropriately controlled. Therefore, a thin rare earth oxysulfide film having a good stoichiometric composition can be produced. 
     It is preferable to add at least one of Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb as the luminescent center impurity at the time of the film formation. Then, a thin luminescent layer having uniform distribution of luminescent center impurity and excellent crystallinity can be produced. 
     This is because the process of growth takes place as follows. 
     Assume that substance A is evaporated from an evaporation source, and substances B and C are supplied in a vapor phase to form a substance ABC on the substrate. Let the substances A, B and C be a metal, oxygen and sulfur, respectively. If the metal A is an element in Group IIa of the periodic table such as Ca or Sr or an element in Group IIb such as Zn or Cd, and the temperature Ts of the substrate is relatively low (for example, lower than 600° C.), it is possible to establish the following conditions: P O  &lt;&lt;P A , P O  &lt;&lt;P B  and P O  &lt;&lt;P C  where P O  is degree of vacuum (pressure) of the chamber and P A , P B  and P C  are the vapor pressures of the substances A, B and C, respectively. Under such conditions, the substances A, B and C by themselves do not substantially stick to the substrate, so that the compound ABC alone grows selectively. 
     In the case where the metal A is an element in Group IIIa such as La or Y, P A  does not become very large so that the metal A alone sticks to the substrate. However, it is considered that the compound ABC may also grow selectively probably for the following reason. Taking Y 2  O 2  S for example, the process: 2Y+O 2  +1/2S 2  Y 2  O 2  S is an exothermic reaction and the elements A, B and C are very active on the surface of the thin film. Therefore, these elements, particularly the element A, can have kinetic energy sufficient for settling themselves in the most stable position. As a result, a thin film excellent in flatness and crystallinity will be formed even at a low substrate temperature. 
     This applies to a case where luminescent center impurities are added. As a result, although a thin fluorescent film, for example, of Y 2  O 2  S:Eu, does not generally exhibit sufficient luminous efficiency unless heat treatment is carried out at a temperature above 1000° C., the film forming method according to the present invention provides a thin fluorescent film having a satisfactory high luminous efficiency even at a temperature lower than 600° C. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 schematically illustrates a thin film EL device of an embodiment of the present invention; 
     FIG. 2(a)-(e) illustrates a process for manufacturing the thin film EL device of FIG. 1; 
     FIG. 3 schematically illustrates a device for forming a thin film in carrying out a method according to the present invention; 
     FIG. 4 is a graph illustrating a result of X-ray diffraction of a luminescent layer of a thin film EL device formed by the inventive method; and 
     FIG. 5 shows an X-ray fluorescent plate in a second embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Embodiments of the present invention will be described below in detail with reference to the drawings. 
     Referring to FIG. 1, a thin film EL device of a first embodiment has a double dielectric layer structure in which a luminescent layer 1 is made of a host material of Y 2  O 2  S containing Eu as a luminescent center impurity (Y 2  O 2  S: Eu) and has a thickness of 500 nm. 
     More particularly, the EL device comprises a transparent glass substrate 2 having a thickness of 1 mm, a transparent electrode 3 made of tin oxide (SnO 2 ) having a thickness of 0.3 μm formed on substrate 2, a first dielectric layer 4 made of tantalum oxide (Ta 2  O 5 ) having a thickness of 0.5 μm, the luminescent layer 1 as described above, a second dielectric layer 5 made of tantalum oxide (Ta 2  O 5 ) having a thickness of 0.5 μm, and rear electrodes 6 made of aluminum having a thickness of 0.5 μm, disposed in this order. 
     A method of producing such a thin film EL device will be described by referring to FIGS. 2(a)-(e). 
     First, the transparent electrode 3 made of SnO 2  is formed on a transparent glass substrate 2 by sputtering process (FIG. 2(a)), and the first dielectric layer 4 made of tantalum oxide is formed by sputtering process (FIG. 2(b)). 
     Subsequently, the luminescent layer 1 is formed using a thin film growth device shown in FIG. 3. The device comprises a vacuum chamber 10 in which a crucible 11 for containing yttrium (Y), a crucible 12 for containing luminescent center impurity of Eu, a sulfur gas introducing tube 13 for supplying sulfur gas, an oxygen gas introducing tube 14 for supplying oxygen gas, a substrate support 16 for supporting a substrate and a heater 15 for heating the temperature of the substrate. Temperatures of crucible 11 and 12, quantities of sulfur gas supplied from the sulfur gas introducing tube 13 and oxygen gas supplied from the oxygen gas introducing tube 14 are controlled independently. The sulfur gas is supplied by heating sulfur 18 by means of a heater 17. The supply of the sulfur gas and the oxygen gas is controlled by valves 19a and 19b and a mass-flow controller 20. 
     In the formation of the film, the vapor pressure within the vacuum chamber 10 is first set at 10 -5  Torr. Then, setting the temperature T s  of glass substrate 2 at 565° C., sulfur gas and oxygen gas are supplied while controlling the temperatures of crucible 11 and 12 and, the quantities of supplied sulfur gas and oxygen gas independently such that the composition of the luminescent layer is stoichiometric. 
     The luminescent layer 1 having grown in the above manner is made of a thin Y 2  O 2  S: Eu film having a thickness of 300 nm where the luminescent center impurities of Eu are uniformly distributed and having excellent crystallinity (FIG. 2(c)). The partial pressures of oxygen and sulfur gases are 3.0×10 -4  Torr and 1.5×10 -4  Torr, respectively. 
     FIG. 4 shows the result of X-ray diffraction of Y 2  O 2  S: Eu thus obtained. The result shows that Y 2  O 2  S: Eu has excellent crystallinity and orientation (100). 
     Then, as shown in FIG. 2(d), the second dielectric layer 5 made of a tantalum oxide layer is formed by sputtering process. 
     Finally, as shown in FIG. 2(e), an aluminum film is formed by vacuum deposition and then patterned to form the rear electrode 6 by photolithography process. 
     The thin film EL device is operable by applying an alternating electric field across the transparent and the rear electrodes. The device has a high luminance performance at a low voltage. 
     While the luminescent layer is made of a thin Y 2  O 2  S: Eu film in the above embodiment, the present invention is not limited to this. Same effects can be achieved by using other metal oxysulfide as host material and at least one of Ce, Pr, Nd, Pm, Sm, Gd, Tb, Dy, Ho, Er, Tm and Yb as a luminescent center impurity which is added to the host material when the film is formed. 
     Thin metal oxysulfide film is usable not only in thin film EL devices, but also in fluorescent films for CRTs and X-ray intensifying screens. 
     In addition, the present invention is applicable to the formation of thin ZnO x  S 1-x  films in addition to the formation of thin films of oxysulfides of rare earth elements. 
     A method of making an X-ray fluorescent plate as a second embodiment of the present invention will be described referring to FIG. 5. The X-ray fluorescent plate is characterized by a sensitized fluorescent layer 13 formed by the thin film forming process of the present invention. As shown in FIG. 5, the X-ray fluorescent plate comprises a reflective tungsten layer 22 having a thickness of 0.5 μm formed on a transparent glass substrate 21, a sensitized fluorescent layer 23 made of Gd 2  O 2  S: Tb and having a thickness of 3 μm formed on reflective layer 22, an X-ray film stuck to the layer 23 and a photo-preventive cover which covers the whole of the product thus formed. 
     The process of making the X-ray fluorescent plate is as follows. 
     A thin tungsten film 22 is formed on the glass substrate 21 having a thickness of 1 mm by electron beam vapor deposition. 
     Then, the sensitized fluorescent layer 23 is formed using a thin film growing device shown in FIG. 3 in which the crucible 11 contains gadolinium (Gd) and the crucible 12 contains luminescent center impurity Tb. 
     In the formation of the film, the vapor pressure within the vacuum chamber 10 is first set at 10 -5  Torr. Then, setting the temperature T s  of the glass substrate 21 at 580° C., sulfur gas and oxygen gas are suplied while controlling the temperatures of crucible 11 and 12, the quantities of supplied sulfur gas and oxygen gas independently such that the composition of the sensitized fluorescent layer is stoichiometric. 
     The thin sensitized fluorescent layer 23 having grown in the above manner is made of a thin Gd 2  O 2  S: Tb film having a thickness of 3 μm where the luminescent center impurities of Tb are uniformly distributed and having excellent crystallinity. The partial pressures of oxygen and sulfur gases are 3.0×10 -4  Torr and 1.5×10 -4  Torr, respectively. 
     An X-ray film 24 is stuck to the sensitized fluorescent layer 23 thus obtained, and a cover 25 which prevents the film from being exposed is mounted on the whole of the product thus obtained. 
     The conventional X-ray fluorescent plate has a structure in which a sensitized sheet is stuck to each side of an X-ray film. According to the present invention, the sensitized fluorescent layer 23 is made of Gd 2  O 2  S: Tb with Tb being added uniformly. Since the sensitized fluorescent layer has an excellent performance, it is required to be formed on only one side of the X-ray film. Therefore, the X-ray fluorescent plate can be made with a simple structure and can detect X-rays with very high sensitivity. Therefore, productivity of the plate is improved and the production cost is reduced.