Patent Publication Number: US-8115207-B2

Title: Vacuum channel transistor and diode emitting thermal cathode electrons, and method of manufacturing the vacuum channel transistor

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATION 
     This application claims the benefit of Korean Patent Application No. 10-2008-0106581, filed on Oct. 29, 2008, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a transistor, and more particularly, to a vacuum channel transistor emitting thermal cathode electrons having a low driving voltage and stably emitting electrons and a method of manufacturing the vacuum channel transistor. 
     2. Description of the Related Art 
     In general, field emission devices apply an electric field to an electrode, that is, a cathode, in vacuum or under a specific gas atmosphere, and electrons are emitted from the cathode. The field emission devices are called cold cathodes and are used as an electron source of micro devices, sensors, and flat panel displays. In such field emission devices, the efficiency of emitting electrons varies according to the structure of the field emission devices, electrode materials, and the shape of the electrode. 
     Conventional field emission devices may be classified as a diode type formed of a cathode and an anode and a triode type formed of a cathode, a gate, and an anode. In the triode-type field emission devices, an electric field is applied to the gate adjacent to the cathode. Thus, the triode-type field emission devices may be driven in a lower voltage than that of the diode-type field emission devices. Also, current emitted to the anode and the gate of the triode-type field emission devices may be easily controlled and thus the triode-type field emission devices are being widely developed. Examples of the electrode materials of the field emission devices may include metal, silicon, and diamond. When silicon is selected as the electrode material, semiconductor processing equipment may be used and the field emission devices may be manufactured by being compatible with a semiconductor integrated circuit process. 
     However, due to a characteristic of the field emission devices in which electrons cut through the sharp surface of the cathode, electrical characteristics thereof are unstable, the uniformity of the electrical characteristics between the anode and the cathode is poor, and damage to the field emission devices due to excessive current may easily occur. For example, since the conventional field emission devices generally employ a sharp cathode tip structure, instability of emitting current, low efficiency, short life time, and low mass production may exist due to degradation of the tip in the cathode tip structure. In addition, the driving voltage for emitting electrons is very high in the conventional field emission devices. 
     SUMMARY OF THE INVENTION 
     The present invention provides a vacuum channel transistor that emits thermal cathode electrons, provides a stable electron emission structure, has a low driving voltage, and improves mass production, a diode, and a method of manufacturing the vacuum channel transistor. 
     According to an aspect of the present invention, there is provided a vacuum channel transistor including: a motherboard; a micro heater member having a thin-film structure formed on the motherboard; a cathode member having a thin-film structure spaced apart from a center part of the micro heater member by a first interval and formed on the micro heater member; a gate member formed on both outer walls of upper parts of the cathode member; and an anode member spaced apart from the cathode member by a second interval through spacers disposed on the gate member, wherein a vacuum electron passing area is formed between the cathode member and the anode member by the second interval. 
     A lower center part of the motherboard may be removed through etching in order for the micro heater member to have a thin-film structure, wherein the micro heater member includes a silicon oxide film formed on the motherboard; a polycrystal silicon film formed on the silicon oxide film on which a center part of the polycrystal silicon film has a smaller thickness than that of an outer wall of the polycrystal silicon film; and a low work function material layer formed on the center part of the polycrystal silicon film, and wherein the micro heater member has a structure where the center part thereof is projected downward, a lower center part of the silicon oxide film is exposed by removing a part of the motherboard, and the center part of the polycrystal silicon film functions as a local micro heater. 
     The cathode member may include: a polycrystal silicon film formed on the micro heater member; and a cathode formed on a center part of the polycrystal silicon film using a low work function material layer; wherein the cathode member is stacked on an upper surface of the micro heater member through a silicon oxide film formed on both outer walls of upper parts of the micro heater member, the micro heater member has a structure where the center part thereof is projected downward, and the low work function material layer of the cathode, which is formed on the micro heater member, and the polycrystal silicon film are spaced apart from each other by the first interval. 
     The gate member may include: a first silicon oxide film formed on both outer walls of the upper parts of the cathode member; a crystal silicon gate formed on the first silicon oxide film; and a second silicon oxide film formed on the crystal silicon gate. 
     The anode member may include: a silicon substrate having a center part thereof projected downward; a silicon oxide film formed on a lower surface of the silicon substrate and not formed on a predetermined part of the center part of the silicon substrate; and an anode formed on a lower surface of the center part of the silicon oxide film and on the predetermined part of the center part of the silicon substrate using a metal layer. 
     The gate member may include the first silicon oxide film, the crystal silicon gate, and the second silicon oxide film that are sequentially formed on both outer walls of the upper parts of the cathode member, the spacers are formed on the second silicon oxide film, the anode member is stacked on the spacers so that the anode of the anode member and the cathode of the cathode member are spaced apart from each other by the second interval, and thus the electron passing area is formed for electrons emitted from the cathode to reach the anode. 
     According to another aspect of the present invention, there is provided a diode having a cathode-anode structure including: a motherboard; a cathode member having a thin-film structure spaced apart from a center part of the motherboard by a first interval and comprising a local micro heater at a center part of the cathode member; an anode member spaced apart from the cathode member by a second interval through spacers disposed on the cathode member; wherein a vacuum electron passing area is formed between the cathode member and the anode member by the second interval. 
     According to another aspect of the present invention, there is provided a method of manufacturing a vacuum channel transistor, the method including: forming a micro heater member on a motherboard; forming a cathode member on the micro heater member; forming a gate member on both outer walls of upper parts of the cathode member; removing predetermined materials layers of a low work function material layer of the micro heater member so as for the cathode member to be spaced apart from the low work function material layer of the micro heater member by a first interval; forming an upper structure in which an anode is formed on a part of a silicon substrate; and combining the upper structure to the gate member by spacers so as to form a vacuum electron passing area with a second interval between the anode and the cathode member. 
     The forming of the micro heater member may include: defining an active area on the motherboard and etching the active area by a predetermined depth so as to form a trench in the motherboard; forming a silicon oxide film on the resultant motherboard; forming a polycrystal silicon film on the silicon oxide film; etching the center part of the polycrystal silicon film by a predetermined depth and forming a trench in the polycrystal silicon film; forming a low work function material layer in the trench of the polycrystal silicon film; and a forming a protective film on the low work function material layer. 
     The forming of the cathode member may include: forming a silicon oxide film on the micro heater member and planarizing the silicon oxide film; forming a polycrystal silicon film on the silicon oxide film; forming a cathode using a low work function material layer on an upper center part of the polycrystal silicon film; and forming a protective film on the cathode. 
     The forming of the gate member may include: forming a first silicon oxide film on the cathode member; forming a polycrystal silicon film on the first silicon oxide film; forming a second silicon oxide film on the polycrystal silicon film; and etching the center part of the second silicon oxide film, the polycrystal silicon film, and the first silicon oxide film using photolithography and exposing the protective film on the cathode of the cathode member. 
     The forming of the upper structure may include: etching both outer walls of the lower surface of the silicon substrate so as to have a structure where the center part thereof is projected downward; forming a silicon oxide film on the entire lower surface of the silicon substrate; etching and removing a predetermined part of the center of the silicon oxide film and exposing the silicon substrate; and forming an anode on the lower surface of the center part of the silicon oxide film using a metal layer, wherein the anode contacts the silicon substrate through the predetermined part of the center of the silicon oxide film. 
     The method may further include, after the removing of the material layer: removing the protective film of the low work function material layer in the micro heater member and the protective film on the cathode of the cathode member; and etching the lower center part of the motherboard and exposing the silicon oxide film of the micro heater member. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other features and advantages of the present invention will become more apparent by describing in detail exemplary embodiments thereof with reference to the attached drawings in which: 
         FIG. 1A  is a cross-sectional view of a cathode-anode device, that is, a diode device, having a bipolar vacuum tube structure, according to an embodiment of the present invention; 
         FIG. 1B  is a graph illustrating the increase of current density according to temperature change in the cathode-anode device of  FIG. 1A ; 
         FIG. 2  is a cross-sectional view of a vacuum channel transistor according to an embodiment of the present invention; 
         FIGS. 3A through 3P  are cross-sectional views for describing a process of manufacturing a lower structure of the vacuum channel transistor of  FIG. 2 ; 
         FIG. 4  is a cross-sectional view of an upper structure of the vacuum channel transistor of  FIG. 2 ; 
         FIG. 5  is a cross-sectional diagram of the vacuum channel transistor of  FIG. 2 , illustrating the combination of the upper structure of  FIG. 4  and the lower structure of  FIG. 3P ; and 
         FIG. 6  is a cross-sectional view and a perspective view respectively illustrating the lower structure of the vacuum channel transistor and a local micro heater, according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention will now be described more fully with reference to the accompanying drawings, in which exemplary embodiments of the invention are shown. In the description, when it is described that an element is disposed on another element, the element may be directly disposed on the other element or a third element may be interposed therebetween. In the drawings, the thicknesses of layers and regions are exaggerated for clarity. Like reference numerals in the drawings denote like elements. The terminology used herein is for the purpose of describing exemplary embodiments only and is not intended to be limiting of example embodiments. In the description, the detailed descriptions of well-known technologies and structures may be omitted so as not to hinder the understanding of the present invention. 
     Thermionic emission is a method of generating free electrons and has been widely studied. According to the classical literature [W. B. Nottingham, Thermionic Emission In “Handbuch der Physik”(S. Flugge ed.), Vol. 21, pp. 1-175, Springer-Verlag. Berlin, 1956.], a thermionic emission state may be represented by a Richardson equation shown in (1).
 
 J ( T )=120.4 A cm −2 K −2   T   2 exp(−φ/κ T )[A/cm 2 ]  (1)
 
     Wherein, T is absolute temperature and k is the Boltzmann constant of 8.6×10 −5  eV/K. A work function, φ, is changed according to temperature. For example, in barium oxide having a low work function of about 1.2 eV, thermal electrons are emitted at a temperature of about 1000 K. 
       FIG. 1A  is a cross-sectional view of a cathode-anode device, that is, a diode device having a bipolar vacuum tube structure, according to an embodiment of the present invention. 
     Referring to  FIG. 1A , a cathode electrode film  120  and an anode electrode film  170  are formed on a motherboard  100  formed of silicon, in the cathode-anode device having a bipolar vacuum tube structure. The cathode electrode film  120  is spaced apart from the motherboard  100  by a predetermined distance though silicon oxide films  110  disposed on both lower parts of the cathode electrode film  120 . The anode electrode film  170  is spaced apart from the cathode electrode film  120  by a predetermined distance though silicon oxide films  150  and spacers  160  sequentially disposed on both upper parts of the cathode electrode film  120 . Here, the cathode electrode film  120  may be formed of a polycrystal silicon film and the anode electrode film  170  may be formed of a metal film such as nickel. In  FIG. 1A , the anode electrode film  170  is a single layer. However, as illustrated in  FIG. 4 , the anode electrode film  170  may be formed by coating a metal layer on a silicon substrate, as will be described more fully in  FIG. 4 . 
     A local micro heater  130  is formed in the cathode electrode film  120  and a low work function material layer  140  is formed on a part of the upper surface of the cathode electrode film  120  so that heat is generated through the local micro heater  130  and electrons are easily emitted from the low work function material layer  140 . 
     The cathode-anode device having a bipolar vacuum tube structure is formed in a high vacuum chamber  180 . Accordingly, the space between the cathode electrode film  120  and the anode electrode film  170  functions as a vacuum electron passing area, that is, a vacuum channel. Also, power sources are connected to the cathode-anode device having a bipolar vacuum tube structure. As illustrated in  FIG. 1A , a first power source  20  that applies a direct current (DC) voltage between an anode and an cathode is connected to the anode electrode film  170 , and a second power source  10  that applies a voltage to the local micro heater  130  is connected to both sides of the cathode electrode film  120 . Here, the motherboard  100  is connected to ground. Also, a protective resistor Rp  60  for protecting the cathode-anode device and a current meter  50  for measuring passing current may be connected to the front end of the first power source  20 . 
     In the cathode-anode device, a plane-type low work function thermal cathode electrode structure is used in order to remove the instability of electric field emission due to a abrasion of the tip in a conventional sharp cold cathode tip structure at a high electric field state and thus the stability of emitted current and operation may be secured. The low work function material layer  140  may include, for example, diamond or Diamond-Like Carbon (DLC) and barium oxide. Also, the low work function material layer  140  may include all materials having low work function characteristics. 
     In addition, the local micro heater  130  included in the cathode electrode film  120  is heated so as to emit thermal electrons from the low work function material layer  140  disposed on the local micro heater  130 , or the cathode electrode film  120  is directly or indirectly heated so as to increase the current density emitted from the cathode electrode film  120 . 
     As the temperature of the low work function material layer  140  disposed on the cathode electrode film  120  increases, electrons in a covalent bond obtain energy to be free electrons. Thus, a large amount of electrons may be emitted from the low work function material layer  140  with a low driving voltage. 
       FIG. 1B  is a graph illustrating the increase of current density according to temperature change in the cathode-anode device of  FIG. 1A . 
     The graph is obtained under the conditions that DLC is used in the low work function material layer  140  and the DLC is deposited on the upper surface of the local micro heater  130  using plasma-enhanced chemical vapor deposition (PECVD) to have a thickness of about 1 um. Then, the deposited DLC is heat treated at about 600° C. In addition, a predetermined voltage is applied to the local micro heater  130  through both sides of the cathode electrode film  120 , thereby generating Joule&#39;s heat in the local micro heater  130  and increasing the temperature of the local micro heater  130 . Thus, current density according to temperature change is measured until a temperature of about 500° C. 
     As illustrated in the graph, the current density exponentially increases according to the temperature change. Although not illustrated in the graph, when an interval between the cathode electrode film  120  and the anode electrode film  170  may be decreased due to thermal electron emission, the driving voltage may be decreased. 
     The cathode-anode bipolar diode device according to the present embodiment of the present invention employs a new plane structure as the cathode and includes the local micro heater  130  in the cathode electrode film  120 , thereby realizing a diode device being operable at low temperature and with a low driving voltage. 
       FIG. 2  is a cross-sectional view of a vacuum channel transistor according to an embodiment of the present invention. 
     Referring to  FIG. 2 , the vacuum channel transistor according to the present embodiment of the present invention includes a silicon motherboard  1001 , a micro heater member, a cathode member, a gate member, and an anode member. 
     The micro heater member is formed on the motherboard  1001  and includes a silicon oxide film  1003 , a polycrystal silicon film  1004 , and a low work function material layer  1006  that are sequentially formed on the motherboard  1001 . The micro heater member is formed as a thin film system having a structure where a center part is projected downward, that is, a membrane structure. The lower center part of the motherboard  1001  is etched and removed so that a lower part of the micro heater member, that is, a portion of the silicon oxide film  1003 , is exposed in the lower center part of the motherboard  1001 . 
     In addition, the center part of the polycrystal silicon film  1004  has a smaller thickness than that of the outer wall of the polycrystal silicon film  1004 , so as to function as a local micro heater. The polycrystal silicon film  1004  may be formed of a doped polysilicon film having a resistance value of about 10 Ω/square. Also, a conductive material such as platinum Pt may be used instead of the polycrystal silicon of the polycrystal silicon film  1004 . The center part of the polycrystal silicon film  1004  functioning as the local micro heater may have a rectangular plane structure or a zigzag-shaped plate structure, as will be described with reference to  FIG. 6 . 
     The low work function material layer  1006  is deposited on the center part, that is, the part having a smaller thickness than that of the outer wall of the polycrystal silicon film  1004 , of the polycrystal silicon film  1004 . The low work function material layer  1006  may be formed of diamond, DLC, or a material film, such as barium oxide, having a low work function. 
     The cathode member is formed on the micro heater member and includes a polycrystal silicon film  1009  and a cathode  1010  of the low work function material layer. The cathode member is stacked on a silicon oxide film  1008  formed on both sides of the micro heater member. The cathode  1010  is formed on the center part of the polycrystal silicon film  1009  by coating a low work function material layer, for example, diamond, DLC, or barium oxide, thereon. 
     Since the center part of the micro heater member is projected downward, the center part of the cathode member is spaced apart from the center part of the cathode member by a predetermined interval. 
     The gate member is formed on both outer walls of the upper parts of the cathode member and includes a first silicon oxide film  1012 , a crystal silicon gate  1013 , and a second silicon oxide film  1014  that are sequentially formed on both outer walls of the upper parts of the cathode member. 
     In addition, the anode member is formed on the gate member and includes an upper silicon substrate  1051 , a silicon oxide film  1052 , and an anode  1053 . The center part of the silicon substrate  1051  is projected downward. The silicon oxide film  1052  is formed on a lower surface of the silicon substrate  1051  but the silicon oxide film  1052  is not formed on a predetermined part of the center part of the silicon substrate  1051 . The anode  1053 , formed of a material layer such as nickel, is formed on the lower surface of the center part of the silicon oxide film  1052  and on the predetermined part of the center part of the silicon substrate  1051 . The anode  1053  is contacted to the silicon substrate  1051  through the part where the silicon oxide film  1052  is not formed. The anode member is stacked on the gate member through spacers  1041 . The spacers  1041  are formed of an insulation material and the height of the spacers  1041  may be adjusted according to the characteristics of the vacuum channel transistor. Since the anode member is spaced apart from the cathode member by a predetermined interval through the spacers  1041 , the space between the cathode member and the anode member functions as a vacuum electron passing area, that is, a vacuum channel area. 
     In addition, a first power source  10 , a second power source  20 , and a third power source  30  are connected to the vacuum channel transistor, wherein the first power source  10  applies a voltage to the local micro heater of the micro heater member, the second power source  20  applies a voltage to the cathode  1010  and the anode  1053 , and the third power source  30  applies a voltage to the cathode  1010  and the crystal silicon gate  1013 . Also, a protective resistor Rp  60  and a current meter  50  may be connected to the front end of the second power source  20  and a signal source  40  may be connected between the cathode  1010  and the crystal silicon gate  1013 . 
     The vacuum channel transistor according to the present embodiment of the present invention is operated as follows. 
     When the first power source  10  applies a voltage to both ends of the polycrystal silicon film  1004  of the micro heater member, the temperature of the micro heater increases and thereby thermal electrons are emitted. Thus, the emitted thermal electrons are collected around a center part of the cathode member, which are disposed on the upper part of the micro heater. Also, when the third power source  30  applies a voltage between the crystal silicon gate  1013  of the gate member and the polycrystal silicon film  1009  of the cathode member, electrons are emitted from the cathode  1010  and the emitted electrons are transmitted to the anode  1053  by a potential difference generated by the voltage applied between the anode  1053  and the polycrystal silicon film  1009 . Here, the gate member includes an electron passing area so as for the electrons emitted from the cathode  1010  to reach the anode  1053 . That is, the electron passing area is defined by the gate member for not preventing the transmission of electrons between the cathode  1010  and the anode  1053 . In order not to prevent electron transmission, the crystal silicon gate  1013  may be applied, for example, positive potential at a part thereof. 
     Here, the electrons may pass through the positive potential area and may be transmitted from the cathode  1010  to the anode  1053 . Also, the crystal silicon gate  1013  may include, for example, one or more control gates. Here, the electrons may be transmitted from the cathode  1010  to the anode  1053  without interference by the crystal silicon gate  1013 . In this case, an area in which electrons may pass through a cross-section of the crystal silicon gate  1013  is referred to as the electron passing area. 
     When a voltage or current is applied to the polycrystal silicon film  1004  of the micro heater member, the temperature of the center part of the polycrystal silicon film  1004 , that is, the local micro heater, increases. Through convection or radiation generated due to the temperature increase, the temperature of the polycrystal silicon film  1009  in the cathode member increases so as to promote electron emission from the cathode  1010 . 
     In addition, the cathode member and the micro heater member are spaced apart from each other by a predetermined interval so that change in all of the electrical characteristics of the vacuum channel transistor due to the electrical characteristics of the local micro heater of the polycrystal silicon film  1004  may be minimized. 
     The vacuum channel transistor according to the present embodiment of the present invention is a vacuum transistor in which a tripod-type vacuum tube arranged in series is embodied on a semiconductor substrate. Accordingly, electron emission from the cathode electrode in vacuum is theoretically considered as follows. 
     Electron emission from a metal to a vacuum is caused by electron movement due to the tunneling effect generated by the decrease of the height and the width of a potential barrier of a metal surface due to a significantly high electric field. The general intensity of the electric field required to emit the electrons from the metal to a vacuum is  109  [V/m] or above. In general, the metal may include pure metals and may have a work function of about 3 to about 5 eV. However, a specific metal compound or diamond or DLC as a non-metal has a low work function and has an emission current having a similar magnitude as a general metal in an electric field of about 107 to about 108 [V/m]. The micro heater member, on which a metal film having a low work function is coated, is heated to about 450 to about 500° C. and is used, and thus, a thermal electron emission-type transistor operable in a low voltage may be manufactured. 
     The current density of the electrons emitted from the metal to a vacuum is obtained by the Fowler-Nordheim equation [R. H. Fowler and L. W. Nordheim, “Electron Emission in Intense Fields,” Proc. R. Soc., London A119, 173, 1928.] defined in (2) below.
 
 J=aV   2 exp(− b/ V)[A/cm 2 ]  (2)
 
     wherein, a is 1.5×10 −6  (A/φ) exp(10.4/φ 1/2 )β, b is 6.44×10 7 φ 3/2 /β, V is a potential difference, A is an emission area (cm 2 ), φ is a potential difference (eV) corresponding to a work function of a metal, and b represents a Geometric Factor dependent upon a structure of an electrode. 
     The magnitude of the current is determined by the electrons emitted from the cathode  1010 . An emission amount of the electrons varies according to the intensity of an electric field at the edge of the cathode electrode adjacent to the gate electrode and a size of a work function of the metal forming the cathode  1010 . Accordingly, in order to increase the current density, a material having a low work function may be used, the radius of curvature of the edge of the cathode electrode may be decreased, the voltage of the cathode-gate may be increased, and the intensity of the electric field may be increased. 
     However, when tungsten W or molybdenum Mo is used as a cathode electrode material, a work function is about 4.5 eV and thus a significantly high voltage is needed for field emission. Accordingly, an electrode having a sharp tip structure is required. When the cathode  1010  is formed of diamond or DLC each having a very low work function, a desired current density may be obtained in the electric field having a very low intensity. Also, the cathode  1010  may be formed of an electric conductor having excellent conductivity, such as platinum Pt, and the low work function material layer  1006  disposed on the cathode  1010  may be used as an electron emitting layer. 
     Materials such as carbon-based diamond or DLC each having a low work function, chemical stability, heat and electrical conductivity, and stability at high temperatures may be coated on the surface of the cathode  1010  so as to improve the stability of electron emission and emission characteristics. 
     Examples of materials having a low work function used in the present embodiment may include carbon-based diamond, DLC, barium oxide, and any material having above-described characteristics. 
     In addition, the cathode  1010  may be directly or indirectly heated so as to increase the current density emitted from the cathode  1010 . As the temperature of the cathode  1010  increases, electrons in a covalent bond obtain energy to be free electrons. Thus, a large amount of electrons may be emitted with a low driving voltage. 
     The vacuum channel transistor described according to the present embodiment of the present invention includes the membrane-form micro heater member on the lower part thereof, wherein the temperature of the vacuum channel transistor increases through the micro heater member so as to easily emit thermal electrons. Also, the low work function material layer  1006  such as DLC is stacked in the micro heater member so that the temperature of thermal electron emission decreases and thus thermal electron emission is facilitated. In addition, the cathode member and the silicon motherboard  1001  may be spaced apart from each other. Accordingly, the local micro heater does not directly transmit heat to a part, except to the cathode member disposed on the upper part thereof and thus the temperature of other elements of the vacuum channel transistor is not affected. 
       FIGS. 3A through 3P  are cross-sectional views for describing a process of manufacturing a lower structure of the vacuum channel transistor of  FIG. 2 . In the entire processes, a pattern mask is used in each operation of the process and a silicon wafer is used as a substrate. 
     Referring to  FIG. 3A , a trench pattern is formed on the surface of a motherboard  1001   a , formed of silicon, using a photosensitive film or photo-resist  1002 . That is, in order to manufacture the lower structure of the vacuum channel transistor of  FIG. 2 , the photosensitive film  1002  is coated on the motherboard  1001   a  and an active area is defined thereon. Then, the active area is dry etched to a thickness of 8 to 10 um, thereby forming the trench pattern. 
     Referring to  FIG. 3B , the remaining photosensitive film  1002  is removed and the motherboard  1001   a  is washed. Then, a silicon oxide film  1003  having a thickness of about 1 um and the polycrystal silicon film  1004  doped with impurities and having a thickness of about 4 to about 5 um are sequentially grown using low pressure chemical vapor deposition (LPCVD) or plasma-enhanced chemical vapor deposition (PECVD) at a temperature of 1100° C. 
     The polycrystal silicon film  1004  functions as a wiring layer of the micro heater member. 
     Referring to  FIG. 3C , a photosensitive film  1005  coated on the polycrystal silicon film  1004  is patterned so as to form a local micro heater  1020 . Then, the resultant structure is heated for about 30 minutes or above in a vacuum furnace at 450° C. or an electric furnace under N 2  atmosphere so as to remove moisture that remained on the surface of the resultant structure and the polycrystal silicon film  1004  is dry etched so as to reduce the thickness of the center part of the polycrystal silicon film  1004  to about 2 to 3 μm. Here, in order for the center part of the polycrystal silicon film  1004  to be used as the local micro heater  1020 , a doped polysilicon film having a resistance value of about 10 Ω/square may be used. Also, other materials such as platinum Pt may be used to form the local micro heater  1020 . 
     Referring to  FIG. 3D , a low work function material layer  1006   a  is deposited on the entire surface of the resultant structure to have a thickness of about 300 to 3000 nm using PECVD. Here, a carbon-based diamond thin film or a DLC thin film having a low work function may be used as the low work function material layer  1006 . 
     Referring to  FIG. 3E , since the carbon-based diamond thin film or the DLC thin film is hardly removed by dry etching or wet etching after patterning using resist, the carbon-based diamond thin film or the DLC thin film is removed using a lift-off patterning. The low work function material layer  1006  may be formed on the local micro heater  1020  using the DLC thin film. 
     Referring to  FIG. 3F , in order to prevent the low work function material layer  1006  from being exposed during the process, a silicon nitride film  1007 , which is a protective film formed of an insulator, is deposited on the low work function material layer  1006  in a single-layer or a multi-layer to have a thickness of about 100 to 200 nm and the silicon nitride film  1007  is removed except for a part of the silicon nitride film  1007  disposed on the low work function material layer  1006  using lithography patterning. 
     Referring to  FIG. 3G , a silicon oxide film  1008   a  is deposited on the resultant structure to have a thickness of about 4 to 5 um using PECVD. 
     Referring to  FIG. 3H , in order to reduce the surface roughness of the silicon oxide film  1008   a , the silicon oxide film  1008   a  is polished and planarized using chemical-mechanical polishing (CMP) so as to form a low-temperature silicon oxide film  1008   b.    
     Referring to  FIG. 3I , an abrasive is removed and the resultant structure is washed. Then, polysilicon is deposited on the entire upper surface of the resultant structure using LPCVD to have a thickness of about 2 to about 3 μm so as to form a doped polycrystal silicon film  1009 . The polycrystal silicon film  1009  functions as a wiring layer of a cathode member. Then, the resultant structure is post-annealed for about 2 hours in an electric furnace under N 2  atmosphere at 500° C. and compressive stress applied to the polycrystal silicon film  1009  is relaxed. 
     Referring to  FIG. 3J , a carbon-based diamond thin film or a DLC thin film each having a low work function is deposited on the entire upper surface of the polycrystal silicon film  1009  of the cathode member to have a thickness of about 300 to 3000 nm using PECVD so as to form the cathode  1010  of a low work function material layer. As described above, since the carbon-based diamond thin film or the DLC thin film is hardly removed by dry etching or wet etching after patterning using resist, a predetermined part of the center part of the carbon-based diamond thin film or the DLC thin film remains and other parts of carbon-based diamond thin film or the DLC thin film are removed using a lift-off patterning. The cathode  1010  of the low work function material layer is formed using the DLC thin film. Then, a silicon nitride film  1011 , which is a protective film formed of an insulator, is deposited on the cathode  1010  in a single-layer or a multi-layer to have a thickness of about 100 to 200 nm and the silicon nitride film  1011  is removed except for a part of the silicon nitride film  1011  disposed on the cathode  1010  using lithography patterning. 
     Referring to  FIG. 3K , a first low-temperature silicon oxide film  1012   a  having a thickness of about 2 to 3 μm is deposited on the resultant structure using LPCVD so as to be used as an insulator film. 
     Referring to  FIG. 3L , a doped polycrystal silicon film  1013   a  having a thickness of about 2 to 3 μm is deposited on the first silicon oxide film  1012   a  using LPCVD. A second silicon oxide film  1014   a , which is an insulator film, is deposited on the polycrystal silicon film  1013   a . Then, a low-temperature silicon nitride film  1015   a  as a protective film is deposited on the upper surface of the second silicon oxide film  1014   a  to have a thickness of about 100 to 200 nm using PECVD. 
     Referring to  FIG. 3M , a photosensitive film is coated on the low-temperature silicon nitride film  1015   a , and is patterned, thereby defining an exposure area. The low-temperature silicon nitride film  1015  is dry etched or wet etched so as to form an exposure area and the photosensitive film is removed. Then, the second silicon oxide film  1014   a , the polycrystal silicon film  1013   a  and the first silicon oxide film  1012   a  are removed using anisotropic dry etching until the protective film of the silicon nitride film  1011  disposed on the upper surface of the cathode  1010  of the low work function material layer is exposed. Here, the polycrystal silicon film  1013   a  remained after etching forms a crystal silicon gate  1013 . Also, the low-temperature silicon nitride film  1015 , the second silicon oxide film  1014 , the polycrystal silicon film  1013 , and the first silicon oxide film  1012  are the layers remained after etching. 
     Referring to  FIG. 3N , in order to form a space between the upper part of the local micro heater  1020  and the lower part of the silicon oxide film  1009  of the cathode member into a cavity, etching of the silicon oxide film  1008   a , etching of an opening, and exposing the protective film of the silicon nitride film  1011  are simultaneously illustrated. The low-temperature silicon oxide film  1008   a  is removed using wet etching or gas phase etching (GPE) so as to form a cavity part in the center and thus the transmission of heat generated from the local micro heater  1020  is blocked. In GPE, the low-temperature silicon oxide film  1008   a  is removed by a HF etching reaction at a gas phase after inserting a silicon wafer into a GPE equipment, adjusting the substrate temperature to about 22 to 35° C. and the pressure of a reactor to about 10 to 100 Torr, and flowing anhydrous HF and CH 3 OH process gas. 
     Referring to  FIG. 3O , a silicon nitride film is deposited on the rear surface of the motherboard  1001  to have a thickness of about 200 nm using CVD, the photosensitive film is coated on the silicon nitride film, which is a protective film, a part to be etched in the silicon nitride film is defined using the photosensitive film patterning, the silicon nitride film is removed until a bulk silicon layer is exposed, and the bulk silicon layer is immersed in a KOH solution and is wet etched. Here, bulk silicon is removed until the silicon oxide film  1003  disposed on the lower part of the local micro heater  1020  is exposed. As such, since the lower surface of the motherboard  1001  is etched, the local micro heater  1020  has a membrane structure. 
     Referring to  FIG. 3P , the silicon nitride films  1007  and  1011 , which are protective films and function as a mask in a prior process, are removed. A phosphoric acid (H 3 PO 4 ) solution is used to remove the silicon nitride films  1007  and  1011  so as to expose the low work function material layers  1006  and  1010  and the resultant structure is washed using de-ionized (DI) water, thereby completing the manufacture of the lower structure of the vacuum channel transistor. 
       FIG. 4  is a cross-sectional view of an upper structure of the vacuum channel transistor of  FIG. 2 . 
     Referring to  FIG. 4 , the upper silicon substrate  1051  as the main element of the upper structure doped with high concentration impurities is etched so as for the center part thereof to be projected downward by a depth of about 5 um and a thermal oxide film or a low-temperature oxide film as the silicon oxide film  1052  is formed on the upper silicon substrate  1051  to have a thickness of about 1 to 2 um. Here, a predetermined part of the center of the silicon oxide film  1052  is removed using a photolithography process. 
     Then, nickel Ni is deposited on the silicon oxide film  1052  to have a thickness of about 2 um using sputtering or evaporation and the predetermined part of the nickel layer is removed using a photolithography process, thereby forming the anode  1053 . Here, the anode  1053  and the upper silicon substrate  1051  are connected through the predetermined part of the center of the silicon oxide film  1052 . 
       FIG. 5  is a cross-sectional view of the vacuum channel transistor of  FIG. 2 , illustrating the combination of the upper structure of  FIG. 4  and the lower structure of  FIG. 3P . In  FIG. 5 , the lower structure of  FIG. 3P  and the upper structure of  FIG. 4  are combined so as to be spaced apart from each other by the spacers  1041 . 
     Referring to  FIG. 5 , the upper structure, in which anode  1053  is formed, is arranged for the center part of the anode  1053  to face the cathode  1010  in the lower structure. Here, the height of the spacers  1041  may be adjusted according to characteristics of the vacuum channel. The spacers  1041  may be formed by electroplating polyimide or nickel Ni spacers which enable insulation. 
     The spacers  1041  are disposed on the upper surface of the lower structure, and the upper structure is stacked on the spacers  1041 , thereby completing the manufacture of the vacuum channel transistor. 
       FIG. 6  is a cross-sectional view and a perspective view respectively illustrating the lower structure of the vacuum channel transistor and the local micro heater  1020 , according to an embodiment of the present invention. 
     Referring to  FIG. 6 , a resistor structure of the local micro heater  1020  according to the present embodiment of the present invention may be a rectangular plane structure as in  FIG. 3C . However, as illustrated in  FIG. 6 , the local micro heater  1020  may also have a zigzag-shaped plate structure. 
     That is, in an etching process for the polycrystal silicon film  1004  of  FIG. 3 , a local micro heater having a zigzag-shaped plate structure as the local micro heater  1020   a  of  FIG. 6  may be formed. The local micro heater  1020   a  may be used to locally heat the low work function material layer  1006  until the range of about 400 to about 600° C. using platinum Pt, which is a conductive electrode material, or a polysilicon film doped to have a resistance value of about 10 Ω/square. Here, since the surroundings of the edge of the local micro heater  1020   a  have a low resistance value and the center part of the local micro heater  1020   a  has high resistance value, heat is slowly transmitted to a silicon bulk part of motherboard  1001  and local heating is facilitated. 
     In a vacuum channel transistor and diode emitting thermal cathode electrons, and a method of manufacturing the vacuum channel transistor according to the present invention, electron emission of a cathode is less affected by a voltage of an anode and the electrons may be emitted from the cathode in a lower driving voltage than that of the conventional vacuum field emission device. Thus, the vacuum channel transistor may be operated in a low voltage and the instability of emission current is removed, thereby securing stability. 
     In addition, the vacuum channel transistor emitting thermal cathode electrons separately includes a local micro heater and the cathode so that electron emission from a cathode source is less affected by a voltage applied to the local micro heater. Furthermore, electrons may be emitted from the cathode source in a lower gate voltage than that of the conventional vacuum field emission device. 
     While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the following claims.