Abstract:
An electron emitter that includes a metal film having a set of layers that are selected and arranged to adhere the metal film to a remainder of a structure of the electron emitter while avoiding electron loss in the metal film. A multiple layer metal film according to the present techniques enables a balance among adhesion properties, metal diffusion, and oxide properties that might otherwise hinder the performance of an electron emitter.

Description:
BACKGROUND  
       [0001]     Ballistic electron emitters may be employed in a variety of applications. For example, ballistic electron emitters may be used in lithography applications, display applications, and in storage devices.  
         [0002]     A ballistic electron emitter may include an emitter that is embedded in a dielectric material and may further include a metal film formed on the surface of the dielectric material. An electric field may be applied across the emitter and the metal film to cause the emitter to emit electrons. The emitted electrons may accelerate through the dielectric material to the metal film under the influence of the applied electric field. The accelerated electrons may pass through the metal film and emerge as ballistic electrons.  
         [0003]     The metal film in a prior ballistic electron emitter may be a single layer of a precious metal, e.g. gold or platinum. Unfortunately, a single metal film may not adequately adhere to a dielectric material. For example, a gold film may not maintain adequate adhesion to an oxide structure. In addition, a single precious metal film may have a relatively high resistivity and/or a relatively high work function. Unfortunately, a relatively high resistivity and/or a relatively high work function may cause a leakage current in a metal film and thereby reduce the efficiency of ballistic electrons emission from the metal film.  
       SUMMARY OF THE INVENTION  
       [0004]     An electron emitter is disclosed that includes a metal film having a set of layers that are selected and arranged to adhere the metal film to a remainder of a structure of the electron emitter while avoiding electron loss in the metal film. A multiple layer metal film according to the present techniques enables a balance among adhesion properties, metal diffusion, and oxide properties that might otherwise hinder the performance of an electron emitter.  
         [0005]     Other features and advantages of the present invention will be apparent from the detailed description that follows.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0006]     The present invention is described with respect to particular exemplary embodiments thereof and reference is accordingly made to the drawings in which:  
         [0007]      FIG. 1  shows an electron emitter according to the present teachings;  
         [0008]      FIG. 2  shows one embodiment of a thin metal film that includes a top layer and a bottom layer;  
         [0009]      FIG. 3  shows an embodiment of a thin metal film that includes a diffusion barrier between a top layer and a bottom layer;  
         [0010]      FIG. 4  shows an embodiment of an emitter structure that includes a metal film on a substrate;  
         [0011]      FIG. 5  shows an embodiment of an emitter structure that is a semiconductor substrate.  
     
    
     DETAILED DESCRIPTION  
       [0012]      FIG. 1  shows an electron emitter  10  according to the present teachings. The electron emitter  10  includes a thin metal film  12 , an intervening structure  14 , and an emitter structure  16 . The emitter structure  16  emits electrons when an electrical potential is applied across the metal film  12  and the emitter structure  16 . The emitted electrons from the emitter structure  16  accelerate through the intervening structure  14  to the metal film  12  and emerge from the metal film  12  as ballistic electrons.  
         [0013]     The metal film  12  includes a set of layers of materials that are selected and arranged to adhere the metal film  12  to the intervening structure  14 . The layers of materials in the metal film  12  may also be selected and arranged to avoid oxidation and to avoid diffusion among the metal layers in the metal film  12 .  
         [0014]     The metal film  12  has a total thickness that is selected to avoid electron loss in the metal film  12 . For example, the thickness of the metal film  12  may be selected to avoid electron loss caused by scattering as the accelerated electrons from the emitter structure  16  move through the metal film  12 . In one embodiment, the metal film  12  has a total thickness less than 10 nanometers.  
         [0015]     The intervening layer  14  may be a dielectric material. Examples dielectric materials include silicon dioxide and aluminum oxide. Alternatively, the intervening layer  14  may be a semiconductor material.  
         [0016]      FIG. 2  shows one embodiment of the metal film  12  that includes a top layer  20  and a bottom layer  22 . The materials for the top and bottom layers  20  and  22  are selected to facilitate adhesion of the metal film  12  to the intervening structure  14  and to avoid metal diffusion between the top and bottom layers  20  and  22  and to avoid oxidation.  
         [0017]     In one embodiment, the top layer  20  is gold because gold does not readily oxidize. Other materials that may be selected for the top layer  20  because they do not readily oxidize include silver, platinum, iridium, rhodium, and palladium.  
         [0018]     The material for the bottom layer  22  may be selected because it adheres well to the intervening structure  14  and does not react with the material of the top layer  20 . In one embodiment, the bottom layer  22  is molybdenum because molybdenum adheres well to a dielectric material or a semiconductor material that may be used in the intervening structure  14  and because molybdenum is immisible with the gold material of the top layer  20 . For example, molybdenum does not form inter-metallic compounds with gold.  
         [0019]     Other materials that may be selected for the bottom layer  22  because they adhere well to silicon and oxides of silicon and because they do not form inter-metallic compounds with the materials that may be used in the top layer  20  include cobalt, nickel, rhenium, and rhodium. Chromium may be used for silver and gold top layer  20  metals as may molybdenum and tungsten. Chromium, molybdenum, and tungsten may be problematic with other top layer  20  metals due to inter-metallic compound formation that would enhance scattering and hence electron loss.  
         [0020]     The total thickness of the top and bottom layers  20  and  22  is selected to minimize the loss of the accelerated electrons that move through the top and bottom layers  20  and  22 . In one embodiment, the top and bottom layers  20  and  22  have a total thickness less than 10 nanometers.  
         [0021]      FIG. 3  shows an embodiment of the metal film  12  that includes a diffusion barrier  24  between the top layer  20  and the bottom layer  22 . The diffusion barrier  24  is a metal or conducive oxide, nitride, or carbide of a metal that prevents reactions between the metals in the top and bottom layers  20  and  22  or when the metals in the top and bottom layers  20  and  22  are miscible. The diffusion barrier  24 , for example titanium nitride, may be used if the top layer  20  is gold and the bottom layer  22  is aluminum.  
         [0022]     The top layer  20  and the bottom layer  22  and diffusion barrier  24  have a total thickness that is selected to minimize electron loss caused by scattering as the accelerated electrons from the emitter structure  16  move through. In one embodiment, top layer  20  and the bottom layer  22  and the diffusion barrier  24  have a total thickness of less than 10 nanometers.  
         [0023]      FIG. 4  shows an embodiment of the electron emitter  10  in which the emitter structure  16  includes a metal film  30  on a substrate  32 . Ballistic electrons in this embodiment are generated by applying an electrical potential across the metal film  30  and the metal film  12  which causes the metal film  30  to emit electrons that accelerate through the intervening layer  14  and emerge from the metal film  12 . This embodiment of the electron emitter  10  may be referred to as a metal-insulator-metal (MIM) structure.  
         [0024]      FIG. 5  shows an embodiment of the electron emitter  10  in which the emitter structure  16  is a semiconductor substrate  34 . Ballistic electrons in this embodiment are generated by applying an electrical potential across the semiconductor substrate  34  and the metal film  12  which causes the semiconductor substrate  34  to emit electrons that accelerate through the intervening layer  14  and emerge from the metal film  12 . This embodiment of the electron emitter  10  may be referred to as a metal-insulator-semiconductor (MIS) structure.  
         [0025]     The layers of the metal film  12  may be deposited by sputtering. For example, the kinetic energy of material deposition provided by sputtering may increase the adhesion of the metal film  12  to the intervening layer  14 . Alternatively, the layers of the metal film  12  may be deposited using evaporation or chemical vapor deposition or other such means.  
         [0026]     The foregoing detailed description of the present invention is provided for the purposes of illustration and is not intended to be exhaustive or to limit the invention to the precise embodiment disclosed. Accordingly, the scope of the present invention is defined by the appended claims.