Abstract:
The present invention is a process for making a matching pair of surfaces, which involves creating a network of channels on one surface of two substrate. The substrates are then coated with one or more layers of materials, the coating extending over the regions between the channels and also partially into the channels. The two coated surfaces are then contacted and pressure is applied, which causes the coatings to be pressed into the network of channels, and surface features on one of the layers of material creates matching surface features in the other, and vice versa. It also results in the formation of a composite. In a final step, the composite is separated, forming a matching pair of surfaces.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims the benefit of U.K. Provisional Application No. GB0423534.7, filed Oct. 25, 2004. This application is a continuation-in-part of U.S. patent application Ser. No. 10/234,498, filed 2 Sep. 2002, which claims the benefit of U.S. Provisional Application No. 60/316,918, filed 2 Sep. 2001. This application is a Continuation-in-Part of U.S. patent application Ser. No. 10/507,273, which is the U.S. national stage application of International Application PCT/US03/07015, filed Mar. 6, 2003, which international application was published on Oct. 30, 2003, as International Publication WO03090245 in the English language. The International Application claims the benefit of U.S. Provisional Application No. 60/362,494, filed Mar. 6, 2002, and U.S. Provisional Application No. 60/373,508, filed Apr. 17, 2002. This application is a Continuation-in-Part of U.S. patent application Ser. No. 10/823,483, filed 12 Apr. 2004, which is a Continuation-in-Part of U.S. patent application Ser. No. 09/481,803, filed 31 Aug. 1998, U.S. Pat. No. 6,720,704, which is a Continuation-in-Part of U.S. patent application Ser. No. 08/924,910, filed 8 Sep. 1997, abandoned. The above-mentioned patent applications are assigned to the assignee of the present application and are herein incorporated in their entirety by reference. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     This invention relates to a method for making electrode pairs.  
         [0003]     The use of individual actuating devices to control the separation of electrodes in a gap diode is disclosed in U.S. Pat. No. 6,720,704.  
         [0004]     The use of composite materials as matching electrode pair precursors is disclosed in US2003/0068431. The approach comprises the steps of fabricating a first electrode with a substantially flat surface; placing over the first electrode a second material that comprises a material that is suitable for use as a second electrode, and separating the composite so formed along the boundary of the two layers into two matched electrodes. The separation step involves the use of an electrical current, thermal stresses, or mechanical force. A similar approach is also disclosed in US2004/0195934.  
       BRIEF SUMMARY OF THE INVENTION  
       [0005]     From the foregoing, it may be appreciated that a need has arisen for a simpler, more direct approach for manufacturing matched pairs of surfaces.  
         [0006]     The present invention is a process for making a matching pair of surfaces, which involves creating a network of channels on one surface of two substrate. The substrates are then coated with one or more layers of materials, the coating extending over the regions between the channels and also partially into the channels. The two coated surfaces are then contacted and pressure is applied, which causes the coatings to be pressed into the network of channels, and surface features on one of the layers of material creates matching surface features in the other, and vice versa. It also results in the formation of a composite. In a final step, the composite is separated, forming a matching pair of surfaces. 
     
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING  
       [0007]     For a more complete explanation of the present invention and the technical advantages thereof, reference is now made to the following description and the accompanying drawing in which:  
         [0008]      FIG. 1  shows a diagrammatic overview of the process of the present invention.  
         [0009]      FIG. 2  is a schematic showing a process for the manufacture of a diode device having a tubular housing/actuator. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0010]     In the disclosure which follows, when surface features of two facing surfaces of electrodes are described as “matching” it means that where one surface has an indentation, the other surface has a protrusion and vice versa. Thus when “matched” the two surfaces are substantially equidistant from each other throughout their operating range.  
         [0011]     Embodiments of the present invention and their technical advantages may be better understood by referring to  FIG. 1 , in which a first substrate  102  is provided. Preferably the substrate comprises silicon, though other materials commonly used, such as without limitation glass, silica or molybdenum may be utilized.  
         [0012]     In a first step  100 , a network of channels  104  is created in the surface of the substrate. The channels may be formed by any conventional method, including but not limited to photolithography and ion beam milling. Typically the channels have a depth of 100 nm, and the spacing between the channels is typically 500 μm. Other depths and spacings may be conveniently employed, the key feature of this part of the invention is that the channels are of sufficient depth and spacing to accommodate material pushed laterally in step  150  below. In a preferred embodiment the channels are arranged in a grid-like formation as shown in the plan view  110 . However, other arrangements are possible; the key feature of this part of the invention is that the channels are interconnected into a network of channels.  
         [0013]     In a second step  120 , a first material  122  is deposited on a surface of the substrate. The first material comprises material that is suitable for use as an electrode. Preferably, the first material comprises silver. Other materials include gold, platinum, palladium, tungsten or chromium. Whilst step  120  is shown as a single step, it may comprise multiple steps. For example, in a preferred embodiment, a layer of silver is first deposited. Then, the surface of the layer of silver is oxidized to form a layer of silver oxide. Subsequently the layer of silver oxide is caesiated to form a layer of AgCsO on the surface of the first material. The scope of the invention is not limited to the use of these materials, and the use of other materials commonly employed in wafer applications are encompassed within the present invention.  
         [0014]     In a third step  130 , a second substrate  132  is provided, and in a step analogous to step  100 , a network of channels is created in the surface of the substrate. Preferably the channels have a depth of 100 nm, and the spacing between the channels is typically 500 μm. Other depths and spacings may be conveniently employed, the key feature of this part of the invention is that the channels are of sufficient depth and spacing to accommodate material pushed laterally in step  150  below. In a preferred embodiment the channels are arranged in a grid-like formation as shown in the plan view  110 . However, other arrangements are possible; the key feature of this part of the invention is that the channels are interconnected into a network of channels.  
         [0015]     In a fourth step  140 , a second material  142  is deposited on a surface of the substrate. The second material comprises material that is suitable for use as an electrode. Preferably, the second material comprises silver. Other materials include gold, platinum, palladium, tungsten or chromium. Whilst step  140  is shown as a single step, it may comprise multiple steps. For example, in a preferred embodiment, a layer of silver is first deposited. Then, a layer of an insulator material, as disclosed in WO04049379, such as C 3 N 4  or Al 4 Si 3  may be formed on the layer of silver. The scope of the invention is not limited to the use of these materials, and the use of other materials commonly employed in wafer applications are encompassed within the present invention.  
         [0016]     In a fifth step  150 , the first substrate and the one or more layers deposited thereon, and the second substrate and the one or more layers deposited thereon are pressed together with sufficient force that surface features on material  122  are ‘matched’ on surface material  142 , and surface features on material  142  are ‘matched’ on surface material  122 .  
         [0017]     During the pressing process, material displaced is able to squeezed into the network of channels. Without the network of channels, the surface replication step will not work, as there is nowhere for displaced material to be squeezed.  
         [0018]     Depending on the nature of the layers deposited on the two substrates, the two substrates may need to be heated (to reduce the hardness of the layers) or cooled (to increase the hardness of the layers). For example, in the embodiment described above, substrate  102  and its layer of AgCsO would need to be cooled to harden the layer of AgCsO prior to pressing.  
         [0019]     Preferably, all the steps above are performed in a substantially evacuated atmosphere.  
         [0020]     In a sixth step  160 , the composite is split between layers  122  and  142  to form two electrodes in which surface features of one are reflected in the other; thus where layer  122  has a protruding feature, layer  142  has a matching indented feature, and vice versa. This relationship, of course, does not hold in the regions of the channels. The separation step may be achieved, for example and without limitation, by applying an electrical current through the materials to separate the electrodes along the boundary of two layers; by cooling or heating the materials, so that the differential in the Thermal Coefficient of Expansion (TCE) between two materials breaks the adhesive bond between the two materials; by forcible separation of the two materials to break the adhesion between the two materials; or by the addition or removal of energy, for example by means of an ultrasonic treatment step.  
         [0021]     In a preferred embodiment the force with which the two substrates are pressed together in step  150  is sufficient that the two substrates and the one or more layers deposited thereupon form a single composite  152 . According to this embodiment, during a sixth step  160 , the temperature of the composite is altered such that the composite splits between layers  122  and  142  to form two electrodes in which surface features of one are reflected in the other; thus where layer  122  has a protruding feature, layer  142  has a matching indented feature, and vice versa. For example without limitation, a composite formed from the materials described above (Ag/AgO/AgCsO on substrate  102  and insulator/Ag on substrate  122 ) is cooled further, which causes the composite to split into two halves along the junction between the AgCsO layer and the insulator layer.  
         [0022]     Thus two matching electrodes are formed, which may be utilized in devices requiring close-spaced electrodes, such as the tunnelling devices described in U.S. Pat. No. 6,720,704.  
         [0023]     For example and without limitation, the composite may be housed in the device described in WO03090245, as shown in  FIG. 2  and as disclosed below. Referring now to  FIG. 2 , composite  78  is composite  152  depicted in  FIG. 1  having a further layer of copper  76  grown electrochemically by conventional processes on substrate  132 . In step  500  a first substrate  502  is brought into contact with a polished end of a quartz tube  90 . Substrate  502  is any material which may be bonded to quartz, and which has a similar thermal expansion coefficient to quartz. Preferably substrate  502  is molybdenum, or silicon doped to render at least a portion of it electrically conductive. Substrate  502  has a depression  504  across part of its surface. Substrate  502  also has a locating hole  506  in its surface. In step  510 , liquid metal  512 , is introduced into depression  502 . The liquid metal is a metal having a high temperature of vaporization, and which is liquid under the conditions of operation of the device. The high temperature of vaporization ensures that the vapor from the liquid does not degrade the vacuum within the finished device. Preferably the liquid metal is a mixture of Indium and Gallium. Composite  78  is positioned so that alignment pin  514  is positioned above locating hole  506 . Alignment pin  514 , which is pre-machined, is placed on the composite near the end of the electrolytic growth phase; this results in its attachment to the layer of copper  76 . The diameter of the alignment pin is the same as the diameter of the locating hole. In step  520 , the polished silicon periphery of the composite  78  is contacted with the other polished end of the quartz tube  90 ; at the same time, the attachment pin seats in locating hole. During this step, substrate  502  is heated so that locating hole expands; when the assemblage is subsequently cooled, there is a tight fit between the alignment pin and the locating hole. High pressure is applied to this assemblage, which accelerates the chemical reaction between the polished silicon periphery of the composites and the polished ends of the quartz tube, bonding the polished surfaces to form the assemblage depicted in step  520 . In step  530 , the assemblage is heated, and a signal applied to the quartz tube to cause the composite to open as shown, forming two electrodes,  72  and  74 . This is analogous to step  160  and the electrode composite opens as shown, forming a pair of matching electrodes,  72  and  74 . During the opening process, the tight fit between the alignment pin and the locating hole ensures that the electrodes  72  and  74  do not slide relative to one another.  
         [0024]     Other housing designs and integration approaches may be adopted, and the scope of the present invention is not limited by the housing and integration example disclosed above.  
         [0025]     Although the above specification contains many specificities, these should not be construed as limiting the scope of the invention but as merely providing illustrations of some of the presently preferred embodiments of this invention.  
         [0026]     Devices made according to the present invention may be used in diode devices, vacuum diode devices, heat pumps, any other devices that are based on tunneling effects, and the like.  
         [0027]     While this invention has been described with reference to numerous embodiments, it is to be understood that this description is not intended to be construed in a limiting sense. Various modifications and combinations of the illustrative embodiments will be apparent to persons skilled in the art upon reference to this description. It is to be further understood, therefore, that numerous changes in the details of the embodiments of the present invention and additional embodiments of the present invention will be apparent to, and may be made by, persons of ordinary skill in the art having reference to this description. It is contemplated that all such changes and additional embodiments are within the spirit and true scope of the invention as claimed below.  
         [0028]     All publications and patent applications mentioned in this specification are indicative of the level of skill of those skilled in the art to which this invention pertains. All publications and patent applications are herein incorporated by reference to the same extent as if each individual publication or patent application was specifically and individually indicated to be incorporated by reference.