Patent Publication Number: US-7911142-B2

Title: Electron emission thin-film, plasma display panel and methods for manufacturing

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of U.S. application Ser. No. 10/275,795, filed on Jun. 2, 2003, now U.S. Pat. No. 7,161,297. 
    
    
     TECHNICAL FIELD 
     The present invention relates to an electron emission thin-film used as a protective layer in a plasma display panel and the like, and in particular to a technique for improving electron emission characteristics of the electron emission thin-film. 
     BACKGROUND ART 
     In recent years, among color display devices used for image displays in computers and televisions, field emission display panels and plasma display panels (hereafter simply, “PDPs”) have received special attention as display devices that can realize slim-type panels. Particularly, PDPs are advantageous in their rapid responses and wide viewing angles, and so companies and research institutions are engaged in active developments to make PDPs widely available. 
     A PDP has the following construction. A front glass substrate on which a plurality of line-shaped electrodes are arranged in parallel, and a back glass substrate on which a plurality of line-shaped electrodes are arranged in parallel are arranged opposed to each other with gap members interposed between them, in such a manner that the electrodes on the front panel and the electrodes on the back panel are perpendicular. A discharge gas is enclosed in a space formed between the front and back glass substrates. On the surface of the front glass substrate opposing to the back glass substrate, a dielectric layer is formed to cover the electrodes arranged on the front glass substrate. Further, a protective layer, which is an electron emission thin-film, is formed on the dielectric layer. 
     The PDP is driven in the following way. An address discharge is performed successively between the electrodes on the front glass substrate and the electrodes on the back glass substrate, generating charge on the protective layer surface of cells in which light emission is intended. Then, a sustained discharge is performed between adjacent electrodes on the front glass substrate relating to the cells in which the charge has been generated. 
     The protective layer on which charge is generated by an address discharge mainly has two functions. The one function is to protect the dielectric layer and the electrodes against ion bombardment (spattering) occurring at the time of address discharge. The other function is a so-called memory function to retain charge by emitting secondary electrons at the time of address discharge. To realize these functions, magnesium oxide (MgO) that excels in resistance to spattering and in secondary electron emission characteristics is commonly used as a material for the protective layer. 
     In the field of display devices, demands for higher-definition screens have emerged recently. To meet the demands, higher-definition screens are realized by increasing the number of electrodes per unit area of each substrate and thereby increasing the number of cells. 
     However, the address time to be spent on one cell becomes shorter as a larger number of electrodes are provided to increase the number of cells. The number of secondary electrons emitted from the protective layer at the time of address discharge decreases accordingly, causing the above-described memory function to be degraded. As a result, such a PDP may suffer from erroneous light emission easily occurring along with generation of an erroneous address discharge. With this background, a technique for improving secondary electron emission characteristics of an MgO thin-film is presently being called for. 
     DISCLOSURE OF THE INVENTION 
     In view of the above problems, the present invention aims to provide a PDP that includes a protective layer with improved secondary electron emission characteristics and that is less likely to cause erroneous light emission as compared with conventional ones, and to provide a manufacturing method for the PDP. The present invention also aims to provide an electron emission thin-film suitable for the PDP, and a manufacturing method for the electron emission thin-film. 
     To achieve the above aims, the electron emission thin-film of the present invention is an electron emission thin-film that is formed on a substrate by densely arranging a plurality of columnar crystals so as to extend from the substrate, the columnar crystals being composed of an electron emission material, wherein at a surface of the thin-film, an exposed end of at least one of the columnar crystals has a flat plane that is inclined with respect to the surface. 
     This electron emission thin-film emits a larger number of secondary electrons than conventional ones. The reason for this can be considered that the columnar crystals constituting the thin-film have higher single-crystallinity than conventional ones. 
     It is particularly preferable that the flat plane of the at least one of the columnar crystals is inclined at an angle of 5 to 70° with respect to the surface of the thin-film. This is because secondary electron emission characteristics of such columnar crystals are better than those of conventional ones, and so secondary electron emission characteristics of the thin-film are improved. 
     Also, when the flat planes of the columnar crystals are equivalent to (100) plane of crystal orientation, the columnar crystals emit a larger number of secondary electrons than when the flat planes of the columnar crystals are equivalent to other planes of crystal orientation, such as (110) plane. 
     Also, the extending direction of each of the columnar crystals is equivalent to &lt;211&gt; direction of crystal orientation. 
     When the width of each of the columnar crystals is in a range of 100 to 500 nm, the columnar crystals are considered to have high single-crystallinity, and accordingly to have improved secondary electron emission characteristics. 
     To be more specific, using columnar crystals composed of magnesium oxide enables the electron emission thin-film that excels in secondary electron emission characteristics as well as in resistance to spattering to be formed. 
     The above thin-film that excels in secondary electron emission characteristics can be formed by depositing a material for forming the thin-film on a substrate in such a manner that an angle at which the material is incident on the substrate is exclusively in a range of 30 to 80°. According to this method, the electron emission thin-film made up of columnar crystals that excel in single-crystallinity can be formed, and therefore, the number of secondary electrons emitted from the electron emission thin-film can be increased. 
     To be more specific, magnesium oxide can be used as the material for forming the thin-film. 
     A vacuum deposition method can be employed as a method for forming the electron emission thin-film, thereby enabling the thin-film that excels in secondary electron emission characteristics to be formed in a short time period. 
     Also, the plasma display panel of the present invention is a plasma display panel that includes a front panel on which first electrodes and a dielectric glass layer that covers the first electrodes are arranged, and a second panel on which second electrodes are arranged, the first panel and the second panel being arranged in such a manner that the dielectric glass layer and the second electrodes are opposed to each other with gap members being interposed therebetween, an address discharge being performed between the first electrodes and the second electrodes to realize addressing, the plasma display panel characterized in that the dielectric glass layer is covered by a protective layer that protects the dielectric glass layer against spattering occurring at the address discharge, the protective layer is formed by a plurality of columnar crystals composed of an electron emission material, and at a surface of the protective layer, exposed ends of the columnar crystals each have a flat plane that is inclined with respect to the surface of the protective layer. 
     In this plasma display panel, the protective layer excels in secondary electron emission characteristics. Therefore, even if the address time is shortened to deal with demands for higher-definition, generation of erroneous light emission occurring along with an erroneous address discharge can be reduced. 
     It is particularly preferable that the flat planes of the columnar crystals are inclined at an angle of 5 to 70° with respect to the surface of the protective layer. This is because secondary electron emission characteristics of such columnar crystals are improved in this case, and accordingly, secondary electron emission characteristics of the protective layer are improved. 
     Here, when the flat planes of the columnar crystals are equivalent to (100) plane of crystal orientation, the columnar crystals emit a larger number of secondary electrons than when the flat planes of the columnar crystals are equivalent to other planes of crystal orientation, such as (110) plane. 
     To be more specific, the extending direction of each of the columnar crystals is equivalent to &lt;211&gt; direction of crystal orientation. 
     Also, when the width of each of the columnar crystals is in a range of 100 to 500 nm, the columnar crystals have even higher single-crystallinity, and therefore, the protective layer has improved secondary electron emission characteristics. 
     Magnesium oxide can be used as a material for forming the protective layer. In this case, the protective layer excels in secondary electron emission characteristics, and also in resistance to spattering at the time of address discharge. 
     Also, the plasma display panel manufacturing method of the present invention may include a protective layer formation step of forming a protective layer on a dielectric glass layer formed on a substrate, wherein in the protective layer formation step, a material for the protective layer is deposited on the substrate in a reduced-pressure atmosphere, in such a manner that an angel at which the material is incident on the substrate is exclusively in a range of 30 to 80°. 
     According to this manufacturing method, the protective layer excels in secondary electron emission characteristics. Therefore, the plasma display panel with reduced generation of erroneous light emission occurring along with an erroneous address discharge can be manufactured. 
     Also, magnesium oxide can be used as the material for forming the protective layer in the protective layer formation step. In this case, the plasma display panel that excels in secondary electron emission characteristics as well as in resistance to spattering at the time of address discharge can be manufactured. 
     Also, a vacuum deposition method can be used as a method for forming the protective layer in the protective layer formation step. By doing so, the protective layer that excels in secondary electron emission characteristics can be formed in a short time period. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other objects, advantages and features of the invention will become apparent from the following description thereof taken in conjunction with the accompanying drawings that illustrate a specific embodiment of the invention. In the drawings: 
         FIG. 1  is a sectional perspective view schematically showing a part of a PDP relating to a preferred embodiment of the present invention; 
         FIG. 2  is an enlarged sectional view showing the part of the PDP as viewed from y-axis direction in  FIG. 1 ; 
         FIG. 3  is a sectional view of the PDP taken along line b-b′ in  FIG. 2 ; 
         FIG. 4A  is a scanning electron micrograph of a section of a protective layer used in the PDP; 
         FIG. 4B  is a scanning electron micrograph of a plane of the protective layer used in the PDP; 
         FIG. 5A  is a pattern diagram showing columnar crystals in  FIG. 4A ; 
         FIG. 5B  is a pattern diagram showing a columnar crystal in  FIG. 4B ; 
         FIG. 5C  is a pattern diagram showing columnar crystals formed using a conventional technique; 
         FIG. 6  shows a state where the protective layer is formed on a dielectric layer on a front glass substrate, using a vacuum deposition system; 
         FIG. 7  is a graph showing a secondary electron emissivity of the protective layer plotted for an angle at which a protective layer forming material is incident on a substrate; and 
         FIG. 8  is a graph showing a secondary electron emissivity of the protective layer plotted for an angle that a flat plane of a columnar crystal in the protective layer forms with a surface of the protective layer. 
     
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     The following describes a PDP to which the present invention is applied, with reference to the drawings. 
     &lt;Overall Construction of the PDP&gt; 
       FIG. 1  is a sectional perspective view schematically showing the essential components of the PDP of alternating current surface discharge type, as one application example of the present invention.  FIG. 2  is a sectional view of the PDP as viewed from y-axis direction in  FIG. 1 .  FIG. 3  is a sectional view of the PDP taken along line b-b′ in  FIG. 2 . 
     In each figure, z-axis direction corresponds to the thickness direction of the PDP, and x-y plane corresponds to a plane parallel to the panel surface of the PDP. 
     As  FIG. 1  shows, the PDP is roughly composed of a front panel  10  and a back panel  20  that are arranged opposed to each other. 
     The front panel  10  includes a front glass substrate  11 , display electrodes  12  and  13 , a dielectric layer  14 , and a protective layer  15 . As  FIG. 3  shows, on the opposing surface of the front glass substrate  11 , a plurality of pairs of display electrodes  12  and  13  are alternately arranged in parallel. The dielectric layer  14  is arranged to cover surfaces of the electrodes  12  and  13 , and the protective layer  15  is arranged to cover a surface of the dielectric layer  14 . 
     The front glass substrate  11  is a flat-plate substrate made of a sodium borosilicate glass material, and is arranged at the display direction side. 
     The display electrodes  12  and  13  each have a three-layer structure in which a Cr-layer, a Cu-layer, and a Cr-layer are laminated in the stated order. The display electrodes  12  and  13  each have a thickness of about 2 μm. As these display electrodes, metals such as Ag, Au, Ni, and Pt may be used. Further, to provide a large discharge area within each cell, electrodes each formed by combining a narrow Ag electrode onto a wide transparent electrode made of conductive metal oxide, such as ITO (Indium Tin Oxide), SnO 2 , and ZnO, may be used as the display electrodes. 
     The dielectric layer  14  is formed to cover the display electrodes  12  and  13  (with a thickness of about 20 μm). As one example, the dielectric layer  14  may be made of a low-melting glass element, such as lead oxide glass and bismuth oxide glass. Lead oxide glass may be made of a mixture of lead oxide, boron oxide, silicon oxide, and aluminum oxide, whereas bismuth oxide glass may be made of a mixture of bismuth oxide, zinc oxide, boron oxide, silicon oxide, and calcium oxide. The dielectric layer  14  has the function of insulating the display electrodes  12  and  13 . 
     The protective layer  15  is formed to cover the surface of the dielectric layer  14 . The protective layer  15  microscopically is a dense layer of columnar crystals that are composed of MgO. The structure of the protective layer  15  is described later in this specification. 
     Referring back to  FIG. 1 , the back panel  20  includes a back glass substrate  21 , address electrodes  22 , a dielectric layer  23 , barrier ribs  24 , and phosphor layers  25 R,  25 G, and  25 B. 
     The back glass substrate  21  is, as the front glass substrate  11 , a flat-plate substrate made of a sodium borosilicate glass material. On the opposing surface of the back glass substrate  21  the address electrodes  22  are arranged in parallel stripes as  FIG. 2  shows. 
     The address electrodes  22  have, as the display electrodes  12  and  13 , a three-layer structure in which a Cr-layer, a Cu-layer, and a Cr-layer are laminated in the stated order. The dielectric layer  23  is formed to cover the address electrodes  22 . 
     The dielectric layer  23  is a dielectric glass layer containing the same glass element as in the dielectric layer  14  in the front panel  10 . The dielectric layer  23  insulates the address electrodes  22 . 
     The barrier ribs  24  are arranged parallel with the address electrodes  22  on the surface of the dielectric layer  23 . Between every adjacent barrier ribs  24 , phosphor layers  25 R,  25 G, and  25 B that respectively emit red, green, and blue light are arranged in the stated order. 
     The phosphor layers  25 R,  25 G, and  25 B are each formed by bonding phosphor particles emitting the corresponding one of R, G, and B light. 
     The PDP has the following construction. The front panel  10  and the back panel  20  are arranged opposed to each other, and peripheral parts of the front panel  10  and the back panel  20  are sealed by a sealing layer made of a glass frit (not shown). Within a discharge space  26  formed between the front panel  10  and the back panel  20 , a discharge gas (e.g., a mixture gas of neon 95 vol % and xenon 5 vol %) is enclosed at a predetermined pressure (e.g., about 66.5 to 106 kPa). 
     &lt;Construction of the Protective Layer  15 &gt; 
       FIG. 4A  is a scanning electron micrograph of the protective layer  15  as viewed from the side surface of the front panel  10 .  FIG. 4B  is a scanning electron micrograph of the protective layer  15  in  FIG. 4A  as viewed from the above. Note here that X, Y, and Z axis directions are shown beside each micrograph for ease of explanation. The dielectric layer  14  is formed in the negative direction of Y axis. In  FIGS. 4A and 4B , the axis shown by a black point that is an intersection of the X, Y, and Z axes indicates the direction orthogonal to the paper surface. 
     As  FIG. 4A  shows, the protective layer  15  is a dense layer of a plurality of MgO columnar crystals that all extend into one direction. One end of each columnar crystal is exposed. 
     As  FIG. 4B  shows, each of the columnar crystals appears to be substantially triangular as viewed from the above. 
       FIG. 5A  is a pattern diagram showing the columnar crystals in the protective layer shown in  FIG. 4A .  FIG. 5B  is a pattern diagram showing one of the columnar crystals in the protective layer viewed from the above in  FIG. 4B .  FIG. 5C  is a pattern diagram showing columnar crystals in a conventional protective layer. 
     As  FIG. 5A  shows, a plurality of columnar crystals  31  extend from the dielectric layer  14  in the front panel  10 , and a horizontal plane that includes the exposed ends of the columnar crystals constitutes a surface  33  of the protective layer  15 . 
     Each columnar crystal  31  has, at its exposed end, a flat plane  32  that forms angle a with the surface  33 . According to an analysis of crystal orientation using an X-ray diffraction method, the flat plane  32  is equivalent to (100) plane of crystal orientation. Therefore, the columnar crystals  31  are considered to have high single-crystallinity. 
     A conventional protective layer is commonly formed by a vacuum deposition method in such a manner that MgO is incident on the substrate substantially at an angle of 90°. As  FIG. 5C  shows, in such a conventional protective layer, the above-mentioned flat planes are not formed at exposed ends  42  of columnar crystals  41 . This can be considered because the columnar crystals  41  are not constructed by single crystals but are constructed by polycrystals that each are oriented in a different direction. 
     The reason for the fact that the columnar crystals  41  constructed by polycrystals are inferior in secondary electron emission characteristics can be considered as follows. The columnar crystals  41  have low single-crystallinity, and so have a number of defects. Therefore, valence electrons flicked out of the columnar crystals  41  when primary electrons are incident on the columnar crystals  41  are less likely to be subject to Bragg reflection caused by a crystal lattice. 
     On the other hand, the columnar crystals  31  in the present embodiment are constructed by single crystals, and therefore, the columnar crystals  31  have the flat planes  32  that are equivalent to (100) plane. The columnar crystals  31  that are constructed by single crystals are considered to have high crystallinity and a uniform crystal lattice. Therefore, valence electrons flicked out of the columnar crystals  31  are easily subject to Bragg reflection caused by a crystal lattice. Accordingly, a larger number of secondary electrons are emitted from the columnar crystals  31  due to Bragg reflection than from the conventional columnar crystals. 
     The flat planes  32  of the columnar crystals  31  may be made as equivalent to (110) plane or (100) plane, by changing a temperature of the substrate, a pressure, etc., at the time of deposition. Particularly, it is experimentally verified that the flat planes  32  being made as equivalent to (100) plane have the best secondary electron emission characteristics. It should be noted here that the flat planes  32  may be made as equivalent to (111) plane. However, the flat planes  32  made as equivalent to (111) plane are not flat, and are inferior to the flat planes  32  equivalent to (110) plane, in secondary electron emission characteristics. 
     It is preferable to set the angle α that each flat plane  32  forms with the surface  33  in a range of 5 to 70°, where the number of emitted secondary electrons is larger than conventional cases. It is more preferable to set the angle α in a range of 5 to 55°, and still more preferable in a range of 10 to 40°. The reason for this can be considered as follows. With the angle α being in a range of 5 to 70°, the experimental results of the practical examples show that the number of emitted secondary electrons is larger than conventional cases for some reasons. With the angle α being in a range of 5 to 55°, or further in a range of 10 to 40°, the number of emitted secondary electrons is still larger. 
     Here, it is preferable that the size of the columnar crystals  31  is larger. To be more specific, it is preferable that the width w being the widest part of each columnar crystal  31  (see  FIG. 5B ) is in a range of 100 to 500 nm. This range is determined based on the following consideration. A columnar crystal with the width w being less than 100 nm has low single-crystallinity, and emits a smaller number of secondary electrons. On the other hand, a columnar crystal with the width w being 500 nm or more is difficult to form. 
     The protective layer  15  that is made up of the above-described columnar crystals is a thin-film that excels in secondary electron emission characteristics. In such a PDP, therefore, an address discharge can be performed in a preferable manner even with short address time, and further, generation of erroneous light emission can be reduced. 
     &lt;Manufacturing Method for the PDP&gt; 
     The following describes a method for manufacturing the PDP. The PDP is manufactured by first forming the front panel  10  and the back panel  20 , and then bonding the front panel  10  and the back panel  20  together. 
     1. Forming the Front Panel  10   
     The front panel  10  is formed as follows. The display electrodes  12  and  13  are formed on the front glass substrate  11 , and the dielectric layer  14  is formed to cover the display electrodes  12  and  13 . Then, the protective layer  15  is formed on the surface of the dielectric layer  14 . 
     The display electrodes  12  and  13  each have a three-layer structure of a Cr-layer, a Cu-layer, and a Cr-layer, and each are formed by continuously sputtering Cr, Cu, and Cr in the stated order. 
     The dielectric layer  14  is formed to have a thickness of about 20 μm by applying a paste of a mixture of, for example, PbO 70 wt %, B 2 O 3  14 wt %, SiO 2  10 wt %, Al 2 O 3  5 wt %, and an organic binder (α-terpineol in which 10% of ethyl cellulose is dissolved) by screen printing, and then baking the paste at 520° for 20 minutes. 
     The protective layer  15  is made of MgO. The protective layer  15  may be formed by sputtering, but here, it is formed by a vacuum deposition method using MgO as a target. A method for forming the protective layer  15  is described in detail later in this specification. 
     2. Forming the Back Panel  20   
     The back panel  20  is formed as follows. The address electrodes  22  are formed on the back glass substrate  21  by continuously forming layers of Cr, Cu, and Cr in the stated order in the same manner as that for the display electrodes  12  and  13 . 
     Following this, the dielectric layer  23  is formed by applying a paste containing a lead glass material by screen printing, and baking the applied paste in the same manner as that for the dielectric layer  14 . Here, a lead glass material paste into which TiO 2  particles are added may be used, for the purpose of reflecting visible light emitted by the phosphor layers  25 R,  25 G, and  25 B. 
     The barrier ribs  24  are formed by repeatedly applying a barrier rib paste containing a glass material using screen printing, and then baking the paste. 
     Following this, the phosphor layers  25 R,  25 G, and  25 B are formed by applying phosphor ink in every groove formed between adjacent barrier ribs  24 , for example, by an ink jet method. 
     3. Completing the PDP by Bonding the Panels Together 
     Following this, peripheral parts of the front panel  10  and the back panel  20  formed in the above-described way are bonded together using a glass material for a sealing layer. Then, the discharge space  26  divided by the barrier ribs  24  is exhausted to create a high vacuum (e.g., 8*10 −7  Torr), and a discharge gas (e.g., an He—Xe inert gas or an Ne—Xe inert gas) is enclosed in the discharge space  26  at a predetermined pressure (e.g., 66.5 kPa to 106 kPa), to complete the PDP. 
     When the PDP is driven to perform display, a driving circuit (not shown) is mounted on the electrodes  12 ,  13 , and  21 . An address discharge is performed between display electrodes  12 ( 13 ) and address electrodes  21  in cells in which light emission is intended, to generate wall, charge in the intended cells. Then, a sustained discharge is performed by applying a pulse voltage between the display electrodes  12  and  13 , to drive the PDP so as to perform display. 
     □ Method for Forming the Protective Layer  15   
     The protective layer  15  is formed using the vacuum deposition method that is characterized by high-speed film formation and relatively easy deposition even for a large substrate. 
       FIG. 6  shows a schematic construction of a vacuum deposition system  50 . 
     As the figure shows, the vacuum deposition system  50  includes a chamber  51  that is a closed chamber, a vacuum pump for depressurizing the inner space of the chamber  51 , a heater (not shown) for heating a target  52  that is composed of MgO, and a heater (not shown) for heating the front glass substrate  53 . 
     Within the chamber  51 , the front glass substrate  53  on which the dielectric layer  14  is formed, and the target  52  that is composed of MgO are fixed by holders (not shown). The front glass substrate  53  and the target  52  are fixed in such a manner that the dielectric layer  14  on the front glass substrate  53  forms a predetermined angle with the target  52 . 
     By setting this angle in a predetermined range described later, the protective layer that is made up of columnar crystals constructed by single crystals described above can be formed. The central point of the target  52  is referred to as point P 0 , the central point of the dielectric layer  54  on the front glass substrate  53  is referred to as point P 1 , and both ends of the dielectric layer  54  on the front glass substrate  53  are referred to as points P 2  and P 3 . 
     Angles that straight lines linking point P 0  and each of points P 1 , P 2 , and P 3  form with the surface of the dielectric layer  54  are respectively referred to as angles β 1 , β 2 , and β 3 . It is preferable that the target  52  and the front glass substrate  53  are fixed in such a manner that the angles β 1 , β 2 , and β 3  are each exclusively within a range of 30 to 80°, and that the target material is not incident on the substrate at any angle out of this range. By doing so, the above-described angle that the flat plane  32  forms with the surface  33  can be fallen within a range of 5 to 70°, although it may depend on temperature conditions. More preferably, each of the angles β 1  , β 2 , and β 3  is in a range of 45 to 80°, and still more preferably, in a range of 50 to 70°. By doing so, the single-crystallinity of the formed protective layer is considered to be improved for some reasons, resulting in secondary electron emission characteristics of the protective layer being improved remarkably. The deposition of the target  52  at such angles results in the protective layer  15  that excels in the secondary electron emission characteristics. 
     It should be noted here that the inner space of the chamber  51  is depressurized to about 1*10 −2  Pa by the vacuum pump at the time of deposition. By heating the target  52  to a temperature of 2000° or higher with the use of the heater, MgO deposits on the dielectric layer  54  on the front glass substrate  53 , thereby forming the protective layer. Also, it is preferable to heat the front glass substrate  53  to approximately 150 to 300°, and more preferably to approximately 200°. This is because experimental results verify that beyond this temperature range columnar crystals are formed to have low single-crystallinity. Also, when the front glass substrate  53  is small or when the distance between the target  52  and the front glass substrate  53  is large, the angles β 1 , β 2 , and β 3  may be regarded as substantially the same. 
     &lt;Effects&gt; 
     As described above, the vacuum deposition that makes the target material incident on the substrate at a predetermined angle enables the protective layer that excels in secondary electron emission characteristics to be formed in a relatively short time period (about 5 minutes). 
     To be more specific, the protective layer formed in this way is a dense layer of columnar crystals that excel in single-crystallinity. Each columnar crystal has high single-crystallinity, and further, has, at its exposed end, a flat plane equivalent to (100) plane that forms a predetermined angle with the surface of the protective layer. This protective layer, therefore, has remarkably improved secondary electron emission characteristics as compared with a conventional protective layer. 
     In the PDP including such a protective layer, an address discharge can be performed in a preferable manner even with short address time, and generation of erroneous light emission can be reduced as compared with conventional cases. 
     PRACTICAL EXAMPLES 
     (1) Samples of Practical Examples 
     Samples S 1  to S 6  of Practical Examples 
     For samples S 1  to S 6  of practical examples, protective layers made of MgO were formed on glass substrates using the vacuum deposition method described in the above embodiment, each varying in the angle β 1  that the straight line linking the central point of the target (MgO) and the central point of the glass substrate forms with the glass substrate at the time of vacuum deposition. For samples S 1  to S 6 , the angle β 1  was respectively set at 80°, 70°, 60°, 50°, 40°, and 30°. 
     Samples S 7  to S 14  of Practical Examples 
     For samples S 7  to S 14  of practical examples, protective layers made of MgO were formed on glass substrates using the vacuum deposition method described in the above embodiment, each varying in the angel a that the flat plane of the columnar crystal forms with the surface of the protective layer. For samples S 7  to S 14 , the angle β 1  that the target (MgO) forms with the glass substrate was adjusted at the time of vacuum deposition in such a manner that the angel a was respectively set at 5°, 10°, 20°, 30°, 40°, 50°, 60°and 70°. 
     (2) Samples of Comparative Examples 
     Sample R 1  of Comparative Example 
     For sample R 1  of a comparative example, a protective layer was formed on a glass substrate using the same method as that for samples S 1  to S 6  of the practical examples. Note here that this sample of the comparative example differs from the samples of the practical examples in that the angle β 1  was set at 90° at the time of vacuum deposition. 
     Sample R 2  of Comparative Example 
     For sample R 2  of a comparative example, a protective layer was formed on a glass substrate using the same method as that for samples S 7  to S 14  of the practical examples. Note here that this sample of the comparative example differs from the samples of the practical examples in that the angle β 1  formed by the glass substrate with the target was adjusted at the time of vacuum deposition in such a manner that the angle α was set at 0°. 
     It should be noted that at the time of vacuum deposition of the protective layer for each of the samples of the practical examples and the samples of the comparative examples, the pressure within the vacuum deposition system was set at 1*10 −2  Pa, and the glass substrate was heated to 200°. 
     (3) Experiments 
     1. Experimental Method 
     For the samples of the practical examples and the samples of the comparative examples, the number of emitted secondary electrons was measured. The measured numbers of emitted secondary electrons were compared and examined, for various values of the angle β 1  at which the target material was incident on the glass substrate, and for various values of the angle α that the flat plane of the columnar crystal formed with the surface of the protective layer. 
     2. Experimental Conditions 
     Irradiation Ion: Ne ion 
     Acceleration Voltage: 500V 
     The above acceleration voltage was applied to accelerate irradiation of the protective layer with Ne ions, and the number of secondary electrons emitted from the protective layer was detected by a collector. 
     (4) Results and Considerations 
       FIGS. 7 and 8  show the experimental results. 
       FIG. 7  shows the experimental results relating to samples S 1  to S 6  of the practical examples and sample R 1  of the comparative example. The figure shows a secondary electron emissivity plotted for the angle β 1  at which the target material is incident on the glass substrate. It should be noted here that the “secondary electron emissivity” is a ratio of the number of secondary electrons emitted from each sample with respect to the number of secondary electrons emitted from sample R 1  of the comparative example. 
     As the figure shows, when the angle of incidence β 1  at the time of vacuum deposition is in a range of 30 to 80°, the protective layer emits a larger number of secondary electrons than the protective layer of sample R 1  of the comparative example (90°) that corresponds to a conventional technique. In particular, when the angle of incidence β 1  is in a range of 45 to 80°, the number of emitted secondary electrons is twice or more of that of the comparative example. Further, when the angle of incidence β 1  is in a range of 50 to 70°, the number of emitted secondary electrons is 2.2 times or more of that of the comparative example. This range of 50 to 70°, therefore, is considered the most preferable in view of increasing the number of secondary electrons to be emitted. 
       FIG. 8  shows the experimental results relating to samples S 7  to S 14  of the practical examples and sample R 2  of the comparative example. The figure shows a secondary electron emissivity plotted for the angle α 1  that the flat plane of the columnar crystal forms with the surface of the protective layer. It should be noted here that the “secondary electron emissivity” is a ratio of the number of secondary electrons emitted from each sample with respect to the number of secondary electrons emitted from sample R 2  of the comparative example. 
     As the figure shows, when the angle of incidence β 1  is in a range of 5 to 70°, the protective layer emits a larger number of secondary electrons than the protective layer of sample R 2  of the comparative example. In particular, when the angle of incidence β 1  is in a range of 5 to 55°, the number of emitted secondary electrons is twice or more of that of the comparative example. Further, the angle of incidence β 1  being in a range of 10 to 40° is considered the most preferable because the number of emitted secondary in this range is 2.3 times or more of that of the comparative example. 
     It should be noted here that little difference was observed in resistance against spattering for the samples of the practical examples and the comparative examples. 
     &lt;Modifications&gt; 
     1. Although the above embodiment describes the case where a layer made of MgO is used as a protective layer, the same effect of the present invention can be obtained when a layer made of a material having a face-centered cubic lattice crystal structure, such as beryllium oxide, calcium oxide, strontium oxide, and barium oxide, is used. 
     2. The above embodiment describes the case where the protective layer is formed using a vacuum deposition method. An electron beam (EB) deposition method may be used as this vacuum deposition method. Further, the same effect of the present invention can be obtained when sputtering is used instead of the vacuum deposition method. 
     3. Although the above embodiment describes the case where a thin-film that excels in secondary electron emission characteristics is used as a protective layer of a PDP, the present invention should not be limited to such. The present invention can be applied to a thin-film used in a cathode of a field emission display panel for which improved electron emission characteristics is desired. 
     INDUSTRIAL APPLICATION 
     A display panel such as a PDP that is manufactured using the electron emission thin-film of the present invention is effective as a display panel for use in a computer, a television, and the like, and is particularly effective as a display panel for which high definition is required.