1. Field of the Invention
The present invention relates to a field emission-type electron source using semiconductor materials, for emitting electron beams by means of electrical field emission, and to a manufacturing method thereof, the electron source being applied to a plane light-emitting apparatus, a display, a solid vacuum device and so on.
2. Description of the Prior Art
As a field emission-type electron source, conventionally, there has been known a Spindt-type electrode which is disclosed, for example, in the U.S. Pat. No. 3,665,241 and so on. The Spindt-type electrode is provided with a substrate on which many emitter chips of triangular pyramid shape are disposed, and a gate layer having emitting holes for exposing apexes of the emitter chips to the outside, the gate layer being disposed so as to be insulated to the emitter chips. The Spindt-type electrode can emit electron beams from the apexes of the emitter chips to the outside through the emitting holes by applying high voltage between the emitter chips and the gate layer under a vacuum atmosphere in such a manner that the emitter chips act as negative electrodes against the gate layer.
However, in the Spindt-type electrode, there exists such a problem described below. That is, the manufacturing process of the electrode is complicated, and further it is difficult to construct many emitter chips of triangular shape with higher efficiency. In consequence, if it is applied, for example, to a plane light-emitting apparatus, a display and so on, it may be difficult to enlarge the area of the electron-emitting surface.
Meanwhile, in the Spindt-type electrode, there exists also such another problem described below. That is, in the Spindt-type electrode, the electrical field converges to the apexes of the emitter chips. In consequence, if the degree of vacuum around the apexes of the emitter chips is lower so that residual gas exists thereabout, a part of the residual gas is ionized by the emitted electron beams to become positive ions. Because the positive ions collide to the apexes of the emitter chips, the apexes of the emitter chips suffer damages (for example, damages due to ion impacts). Therefore, the current density and efficiency of the emitted electrons may become unstable, or the lives of the emitter chips may be shortened.
In order to prevent the above-mentioned disadvantages, the Spindt-type electrode is required to use under a higher vacuum atmosphere (about 10xe2x88x925 Pa to about 10xe2x88x926 Pa. In consequence, there may occur such a disadvantage that the cost for sealing it with higher vacuum or for maintaining the higher vacuum, may be increased.
In order to improve the above-mentioned disadvantages, a field emission-type electron source of MIM (Metal Insulator Metal) type or MIS (Metal Insulator Semiconductor) type has been proposed. The former is a flat field emission-type electron source having a laminated structure of (metal)xe2x80x94(insulator film)xe2x80x94(metal) The latter is a flat field emission-type electron source having a laminated structure of (metal)xe2x80x94(insulator film)xe2x80x94(semiconductor). In order to elevate the electron-emitting efficiency of the above-mentioned type of field emission-type electron source (namely, in order to emit more electrons), it is necessary to decrease the thickness of the insulator film. However, if the insulator film becomes thinner to excess, it is feared that dielectric breakdown is caused when voltage is applied between the upper and lower electrodes of the laminated structure. Because there is a certain restriction on decreasing the thickness of the insulator film as described above, there may exist such a problem that its electron-emitting efficiency (electron extracting efficiency) can not be elevated so much.
Moreover, in recent years, as disclosed in the Japanese Laid-Open Patent Publication No. 8-250766, there has been proposed another field emission-type electron source (semiconductor element for emitting cold electrons), in which a porous semiconductor layer (porous silicon layer) is formed by performing anodic oxidation to a surface of a monocrystalline semiconductor substrate such as a silicon substrate or the like, and further a thin metal film is formed on the porous semiconductor layer. In the field emission-type electron source, electrons are emitted by applying voltage between the semiconductor substrate and the thin metal film.
However, in the field emission-type electron source described in the Japanese Laid-Open Patent Publication No. 8-250766, it is difficult to enlarge the area of the electron-emitting surface, because the monocrystalline semiconductor substrate is an essential constructive element. In consequence, it may not be suitable for applying to an apparatus which requests an electron source having a large electron-emitting surface area, such as a flat display apparatus. Meanwhile, in the Japanese Laid-Open Patent Publication No. 9-259795, there is disclosed a construction for achieving a flat type display based on the invention disclosed in the Japanese Laid-Open Patent Publication No. 8-250766.
In each of the above-mentioned field emission-type electron sources, electrons are emitted due to the electrical field produced by applying voltage to the both surfaces of the porous semiconductor layer. In this case, the porous semiconductor layer is composed of many fine pores and the remaining silicon particles while differing from the above-mentioned MIM or MIS. Hereupon, the porosity is 10 to 80%, while the inner diameter of each of the fine pores is 2 to several nm. In the above-mentioned Publication, there is such a description that because the number of atoms in the remaining silicon particles is several tens to several hundreds, an electron emitting phenomenon may be expected due to the quantum size effect. Meanwhile, in the Japanese Laid-Open Patent Publication No. 9-259795, there is such a description that because the electron emission occurs at a position which is very near with the surface of the porous semiconductor layer, it is desirable that the thickness is thinner, in consequence the practically usable range of the thickness may be 0.1 to 50 xcexcm.
However, in the field emission-type electron source described in the Japanese Laid-Open Patent Publication No. 8-250766 or Japanese Laid-Open Patent Publication No. 9-259795, an electron popping phenomenon may be easily caused and further the amount of electrons emitted within the same plane may be easily dispersed. In consequence, if the electron source is applied to a plane light-emitting device or a display, There may occur such a disadvantage that the brightness dispersion or flicker of the screen may grow larger. Meanwhile, when the electron source is applied to the plane light-emitting device or the display, it is necessary to increase the amount of the emitted electrons. Hereupon, if it is intended to increase the amount of the emitted electrons by decreasing the thickness of the porous semiconductor layer, the above-mentioned disadvantages may grow much larger.
The present invention, which has been performed to solve the conventional problems described above, has an object to provide a field emission-type electron source, in which the popping phenomenon or in-plane dispersion of the emitted electrons hardly occurs while the amount of the emitted electrons and the electron-emitting efficiency are higher together, and to provide a manufacturing method of the electron source.
A field emission-type electron source according to the present invention which is preformed to achieve the above-mentioned object, includes (i) a conductive substrate, (ii) a semiconductor layer formed on a surface of the conductive substrate, at least a part of the layer being made porous, and (iii) a conductive thin film formed on the semiconductor layer, wherein (iv) electrons injected into the conductive substrate are emitted from the conductive thin film through the semiconductor layer by applying voltage between the conductive thin film and the conductive substrate in such a manner that the conductive thin film acts as a positive electrode against the conductive substrate. The field emission-type electron source is characterized in that (v) the semiconductor layer includes a porous semiconductor layer in which columnar structures and porous structures composed of fine semiconductor crystals of nanometer scale coexist, the surface of each of the structures being covered with an insulating layer, and (vi) the average dimension of each of the porous structures in the thickness direction of the semiconductor layer is smaller than or equal to 2 xcexcm.
Because the thickness of each of the porous structures is smaller than or equal to 2 xcexcm in the field emission-type electron source, the amount of emitted electrons may be highly stabilized, and further the amount of the emitted electrons may be increased. In consequence, the popping phenomenon, which is a large time-depending fluctuation of the amount of the emitted electrons, may not occur. Moreover, the in-plane dispersion of the amount of the emitted electrons may be reduced.
In order to apply the field emission-type electron source in various ways, it is essential to increase the amount of the emitted electrons. The present inventors have found that it is desirable that the thickness of each of the porous structures is smaller than or equal to 2 xcexcm, in view of the amount of the emitted electrons of the field emission-type electron source.
Hereupon, the conductive substrate according to the present invention includes a semiconductor substrate in which a conductive region is formed by doping impurities into the substrate, and a substrate in which a thin metal film (bottom electrode) is formed on a surface of an insulating substrate such as glass plate.
In the above-mentioned field emission-type electron source, it is preferable that the thin film side end portions of the columnar structures and the thin film side end portions of the porous structures are located on the same position in the thickness direction of the semiconductor layer (namely, the height of the columnar structures being equal to the height of the porous structures at the conductive thin film side). In this case, because comparatively larger irregularity (namely, convex portions and concave portions) is not formed on the surface of the porous semiconductor layer, the conductive thin film of very small thickness formed on the surface of the porous semiconductor layer may cover the porous semiconductor layer with very high covering ratio in an electrically communicated state. In consequence, the porous semiconductor layer and the conductive thin film are electrically connected to each other. Further, because the covering ratio for the porous structures is higher, the necessary electrical field may be effectively applied to the porous structures. In consequence, its properties such as the amount of the emitted electrons or the electron-emitting efficiency may be efficiently improved.
In the above-mentioned field emission-type electron source, it is preferable that the porous semiconductor layer is composed of porous polycrystalline silicon formed by the anodic oxidation process. In this case, the columnar structures and the porous structures can be formed by a single step by performing the anodic oxidation to the polycrystalline silicon. Therefore, the manufacturing process may be simplified. Further, the formation or anodic oxidation of the polycrystalline silicon layer is advantageous for enlarging the area of the electron-emitting surface. Particularly, if there exist grains (columnar structures) which have grown to the columnar shapes, the pore formations progress along the grains. In this case, the angles of the porous structures in their depth direction become approximately perpendicular to the conductive substrate. In consequence, the electrical field in the porous structures becomes approximately perpendicular to the substrate. Because the emission of electrons is dominated by the electrical field in the porous structures, the electrons are emitted in the direction perpendicular to the substrate in this case. In consequence, the dispersion of the emitting-angles of the electrons is decreased so that a higher definition can be achieved when it is applied to a display or the like.
In the above-mentioned field emission-type electron source, it is preferable that the difference between the maximum dimension and minimum dimension of the porous structures in the thickness direction of the semiconductor layer is smaller than or equal to 0.5 xcexcm (namely, the thickness dispersion of the porous structures being smaller than or equal to 0.5 xcexcm). If the thickness dispersion of the porous structures is smaller, the electrical field applied to the porous structures is uniformed so that the in-plane distribution of the amount of the emitted electrons may be restrained. Particularly, when the columnar structures and the porous structures are provided to prevent occurrence of the popping phenomenon etc. and the thickness of each of the porous structures is smaller than or equal to 2 xcexcm, an extreme dispersion of the intensity of the electrical field does not occur if the thickness dispersion of the porous structures is smaller than or equal to 0.5 xcexcm. In consequence, the amount of the emitted electrons within the same plane may be comparatively uniformed.
In the above-mentioned field emission-type electron source, it is preferable that the thickness of the porous semiconductor layer is approximately equal to the thickness of the semiconductor layer disposed between the conductive thin film and the conductive substrate. In this case, the voltage loss does not occur in a portion which is not made porous. In consequence, the amount of the emitted electrons is increased when the same voltage is applied so that the electron-emitting efficiency may be elevated. Therefore, when the field emission-type electron source is applied to a display or the like, the electrical demand of the display may be reduced.
In the above-mentioned field emission-type electron source, it is preferable that an anticorrosive conductive layer, which has an anticorrosion against an electrolyte for the anodic oxidation process used for making the semiconductor layer porous, is provided on the surface of the conductive substrate at the semiconductor layer side. In this case, the conductive substrate (substrate itself or bottom electrode) may not corroded by the electrolyte. In consequence, the electrical field is effectively applied to the porous structures so that the amount of the emitted electrons may not decreased. Further, it is prevented that an inferior element is produced due to a snap of the bottom electrode.
In the above-mentioned field emission-type electron source, it is preferable that a low-resistance layer of predetermined thickness is provided on the thin film side end portion of the porous semiconductor layer in the thickness direction of the semiconductor layer, the low-resistance layer having a lower resistance in comparison with other parts of the porous semiconductor layer. In this case, the low-resistance layer provided in the surface portion of the porous semiconductor layer acts as a mimic electrode. Therefore, even if the porous semiconductor layer does not partially contact to the conductive thin film, the electrical potential in the whole surface portion of the porous semiconductor layer is uniformed in the same plane. In this case, because the electrical field is uniformly applied into the porous semiconductor layer at the same plane, the dispersion of the amount of the emitted electrons at the same plane may be restrained. Therefore, when the field emission-type electron source is applied to a display, the bright dispersion on the screen may become smaller.
In the above-mentioned field emission-type electron source, it is preferable that the thickness of the low-resistance layer is smaller than the mean free path of the electrons in the semiconductor forming the low-resistance layer. In this case, the deterioration of the electron-emitting efficiency due to the low-resistance layer may be restrained.
In the above mentioned field emission-type electron source, the low-resistance layer may be composed of a low-porosity layer having a smaller porosity in comparison with other parts of the porous semiconductor layer. In this case, because the irregularity of the surface of the porous semiconductor layer becomes less, it may be restrained that the electrical field converges at the apexes of the convex portions or the bottom of the concave portions on the surface of the porous semiconductor layer. In consequence, when the field emission-type electron source is applied to a display etc., it may be prevented that only specific spots become bright. Further, the brightness dispersion in the same plane may become smaller.
In the above-mentioned field emission-type electron source, the low-resistance layer may be composed of a re-crystallized layer which is formed by re-crystallizing a surface portion of the porous semiconductor layer. In this case, the irregularity of the surface of the porous semiconductor layer becomes less so that it may be restrained that the electrical field converges at the apexes of the convex portions or the bottom of the concave portions on the surface of the porous semiconductor layer. In consequence, when the field emission-type electron source is applied to a display etc., it may be prevented that only specific spots become bright. Further, the brightness dispersion in the same plane may become smaller.
In the above-mentioned field emission-type electron source, the low-resistance layer may be composed of an impurity-implanted layer which is formed by implanting impurity ions into the porous semiconductor layer through a surface of the porous semiconductor layer. In this case, it may be easy to control the concentration or distribution of the impurity in the low-resistance layer.
In the above-mentioned field emission-type electron source, the low-resistance layer may be composed of an impurity-diffused layer which is formed by diffusing an impurity into the porous semiconductor layer through a surface of the porous semiconductor layer. In this case, it may be easy to enlarge the area of the electron-emitting surface in comparison with the case that the impurity is implanted by the ion implantation process.
In the above-mentioned field emission-type electron source, it is preferable that the thin film side surface of each of the porous structures is parallel to the surface of the conductive substrate. In this case, the electrical field in the porous structures is applied to the conductive substrate in the direction perpendicular to the substrate. In consequence, the electrons, which are emitted in the direction approximately perpendicular to the surface of the porous structure, are also emitted in the direction approximately perpendicular to the surface of the conductive substrate. As the result, the in-plane distribution of the emitting-angles of the electrons is further decreased. That is, the directions of the emitted electrons uniformly become perpendicular to that. In consequence, higher definition can be achieved when the field emission-type electron source is applied to a display or the like.
A method of manufacturing a field emission-type electron source according to the present invention is a process for producing the field emission-type electron source including (i) a conductive substrate, (ii) a semiconductor layer formed on a surface of the conductive substrate, which includes a porous semiconductor layer in which columnar structures and porous structures composed of fine semiconductor crystals of nanometer scale coexist, a surface of each of the structures being covered with an insulating film; and the average thickness of the porous structures being smaller than or equal to 2 xcexcm; and (iii) a conductive thin film formed on the semiconductor layer, wherein (iv) electrons injected into the conductive substrate are emitted from the conductive thin film through the semiconductor layer by applying a voltage between the conductive thin film and the conductive substrate in such a manner that the conductive thin film acts as a positive electrode against the conductive substrate. The manufacturing method is characterized in that it includes the step of (v) making the semiconductor layer porous by means of an anodic oxidation process to form the porous semiconductor layer, wherein (vi) the thickness of the porous semiconductor layer is controlled by adjusting the depth of the semiconductor layer to be made porous by means of an amount of electric charges during a period that the semiconductor layer acts as a positive electrode in the step. According to the manufacturing method, the thickness of the porous semiconductor layer may be easily controlled by means of the electric charges during the anodic oxidation process so as to become a predetermined value.
In the above-mentioned method of manufacturing the field emission-type electron source, it is preferable that pulse current or pulse voltage is applied between the counter electrode and the conductive substrate on which the semiconductor layer to be made porous is formed in such a manner that a period that the conductive substrate acts as a positive electrode and a period that the current or voltage is off state are mutually set, while the thickness of the porous semiconductor layer is controlled by changing the amount of electric charges during the period that the semiconductor layer acts as the positive electrode. In this case, the thickness of the porous semiconductor layer may be easily controlled so as to become the predetermined value. Further, because the anodic oxidation process can be intermittently performed by the pulse treatment when the porous semiconductor layer of predetermined porosity is formed with larger current density, the rate of progressing the anodic oxidation can be set to a comparatively smaller value. Therefore, the thickness of the porous semiconductor layer may easily controlled in comparison with the case that the current is continuously fed.
In the above-mentioned method of manufacturing the field emission-type electron source, pulse current or pulse voltage may be applied between the counter electrode and the conductive substrate on which the semiconductor layer to be made porous is formed in such a manner that a period that the conductive substrate acts as a positive electrode and a period that the conductive substrate acts as a negative electrode are mutually set, while the thickness of the porous semiconductor layer may be controlled by changing the amount of electric charges per one pulse during the period that the semiconductor layer acts as the negative electrode. In this case, the pore formation in the semiconductor layer progresses when the semiconductor layer acts as the positive electrode, and while the condition of the pore formation disperses in accordance with the surface shape or state of the semiconductor layer. When the polarity is reversed then so that the semiconductor layer acts as the negative electrode, the electrical field converges at the portion where the pore formation has rapidly progressed so that carriers converge at the same portion. In consequence, a large amount of gas is produced at the portion by the electrolysis process. Thus, at the portion where the gas has been produced, the contact between the portion and the electrolyte is snapped so that the pore formation does not progress when the semiconductor layer acts as the positive electrode next. The above-mentioned steps are repeated so that the thickness of the porous structures is uniformed all over the plane. The degree of the uniformity can be controlled by means of the amount of the electric charges during the period that the semiconductor layer acts as the negative electrode. If the thickness of the porous structures is uniformed, the electron source, in which the in-plane distribution of the amount of the emitted electrons is extremely small, may be achieved.
Another method of manufacturing a field emission-type electron source according to the present invention is a process for producing the field emission-type electron source including (i) a conductive substrate, (ii) a semiconductor layer formed on a surface of the conductive substrate, which includes a porous semiconductor layer in which columnar structures and porous structures composed of fine semiconductor crystals of nanometer scale coexist, a surface of each of the structures being covered with an insulating film; and the average thickness of the porous structures being smaller than or equal to 2 xcexcm; and (iii) a conductive thin film formed on the semiconductor layer, wherein (iv) electrons injected into the conductive substrate are emitted from the conductive thin film through the semiconductor layer by applying a voltage between the conductive thin film and the conductive substrate in such a manner that the conductive thin film acts as a positive electrode against the conductive substrate. In the manufacturing method, (v) a low-resistance layer of predetermined thickness is provided on a thin film side end portion of the porous semiconductor layer in the thickness direction of the semiconductor layer, the low-resistance layer having a smaller porosity and a lower resistance in comparison with other parts of the porous semiconductor layer. Further, the manufacturing method is characterized in that it includes the steps of (vi) forming the porous semiconductor layer by decreasing the porosity of a surface portion of the semiconductor layer in comparison with the porosity of other parts of the semiconductor layer, after the semiconductor layer has been formed on the conductive substrate, (vii) forming the porous semiconductor layer including the low-resistance layer by oxidizing or nitrifying the porous semiconductor layer, and (viii) forming the conductive thin film on the porous semiconductor layer. According to the manufacturing method, the low-resistance layer may be provided without independently adding another step for forming the, low-resistance layer. Moreover, the field emission-type electron source, in which the in-plane dispersion of the emitted electrons is smaller, can be achieved with a lower cost.
If the semiconductor layer is made porous by means of the anodic oxidation process in the above-mentioned method of manufacturing the field emission-type electron source, it is preferable that the current density is set to a smaller value during a predetermined initial period of the anodic oxidation process, and then the current density is increased after the predetermined initial period. During the anodic oxidation process, there exist an interrelation between the current density and the porosity. Meanwhile, the resistance changes in accordance with the degree of the porosity. Therefore, the resistance of the low-resistance layer can be controlled by controlling the current density.
When the semiconductor layer is made porous by means of the anodic oxidation process in the above-mentioned method of manufacturing the field emission-type electron source, the light power applied to the surface of the semiconductor layer may be smaller during a predetermined initial period of the anodic oxidation process, and then the light power may be increased after the predetermined initial period. During the anodic oxidation process, there exist an interrelation between the light power and the porosity. On the other hand, the resistance changes in accordance with the degree of the porosity. Therefore, the resistance of the low-resistance layer can be controlled by controlling the light power applied to the surface of the semiconductor layer.
If the low-resistance layer is composed of a re-crystallized layer formed by re-crystallizing a surface portion of the porous semiconductor layer in the above-mentioned method of manufacturing the field emission-type electron source, there may be provided a step for forming the porous semiconductor layer by making the semiconductor layer porous after the semiconductor layer has been formed on the conductive substrate, and another step for re-crystallizing the surface portion of the porous semiconductor layer by means of the laser anneal process, instead of the above-mentioned step of (vi) forming the porous semiconductor layer by decreasing the porosity of the surface portion of the semiconductor layer in comparison with the porosity of the other parts of the semiconductor layer after the semiconductor layer has been formed on the conductive substrate. In this case, the low-resistance layer can be easily provided, comparatively. Meanwhile, the field emission-type electron source, in which the in-plane dispersion of the emitted electrons is smaller, can be achieved with a lower cost.
Meanwhile, if the low-resistance layer is composed of an impurity-implanted layer formed by implanting impurity ions into the porous semiconductor layer in the above-mentioned method of manufacturing the field emission-type electron source, there may be provided a step for forming the porous semiconductor layer by making the semiconductor layer porous after the semiconductor layer has been formed on the conductive substrate, and another step for implanting the impurity ions into the porous semiconductor layer from the surface side of the porous semiconductor layer by means of the ion implantation process. In this case, the low-resistance layer can be formed with a good controllability. Meanwhile, the field emission-type electron source, in which the in-plane dispersion of the emitted electrons is smaller, can be achieved with a lower cost.
Moreover, if the low-resistance layer is composed of an impurity-diffused layer formed by diffusing an impurity into the porous semiconductor layer in the above-mentioned method of manufacturing the field emission-type electron source, there may be provided a step for forming the porous semiconductor layer by making the semiconductor layer porous after the semiconductor layer has been formed on the conductive substrate, and another step for diffusing the impurity into the porous semiconductor layer from the surface of the porous semiconductor layer by means of the thermal diffusion process. In this case, the low-resistance layer can be easily provided comparatively, while enlarging the area of the electron-emitting surface. Meanwhile, the field emission-type electron source, in which the in-plane dispersion of the emitted electrons is smaller, can be achieved with a lower cost.
A further method of manufacturing a field emission-type electron source according to the present invention is a process for producing the field emission-type electron source including (i) a conductive substrate, (ii) a semiconductor layer formed on a surface of the conductive substrate, which includes a porous semiconductor layer in which columnar structures and porous structures composed of fine semiconductor crystals of nanometer scale coexist, a surface of each of the structures being covered with an insulating film; and the average thickness of the porous structures being smaller than or equal to 2 xcexcm; and (iii) a conductive thin film formed on the semiconductor layer, wherein (iv) electrons injected into the conductive substrate are emitted from the conductive thin film through the semiconductor layer by applying a voltage between the conductive thin film and the conductive substrate in such a manner that the conductive thin film acts as a positive electrode against the conductive substrate. The manufacturing method is characterized in that it includes the step of forming the porous semiconductor layer in which the surface of each of the porous structures is parallel to the surface of the conductive substrate by performing the anodic oxidation process to the semiconductor layer after the surface of the semiconductor layer has been smoothed. According to the manufacturing method, the surface of the porous semiconductor layer is smoothed. In consequence, the surface potential is approximately uniformed during the anodic oxidation process so that the rate of progressing the anodic oxidation process is also approximately uniformed. In consequence, the depth of the porous structures becomes approximately uniform so that the electrical field is applied approximately uniformly. Therefore, the field emission-type electron source, in which the in-plane dispersion of the emitted electrons is smaller, can be achieved with a lower cost. Meanwhile, the direction of progress of the anodic oxidation becomes approximately perpendicular to the conductive substrate so that the electrical field in the porous structures becomes approximately perpendicular to the conductive substrate. In consequence, the progressing directions of the emitted electrons become uniformly perpendicular to that. Further, because the distribution of the emitting angles of the electrons is very small, a display with high definition may be achieved.
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