Patent Publication Number: US-7714240-B1

Title: Microfabricated triggered vacuum switch

Description:
GOVERNMENT RIGHTS 
     This invention was made with Government support under Contract No. DE-AC04-94AL85000 awarded by the U.S. Department of Energy. The Government has certain rights in the invention. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates in general to triggerable high-voltage vacuum switches, and in particular to a microfabricated triggered vacuum switch which can be used to switch high voltages up to several kiloVolts or more, and which can operate repeatedly. 
     BACKGROUND OF THE INVENTION 
     High-voltage switches with a high peak current capability and precise, repeatable performance are needed for operating capacitive discharge units (CDUs) for many applications including the initiation of explosives, the triggering of airbags and camera flash units, etc. Current high-voltage vacuum switches require piece-part assembly which makes them relatively expensive for many applications. Additionally, piece-part assembly results in variations in assembly which can affect the operating characteristics of the devices. What is needed is a way of batch fabricating high-voltage vacuum switches to reduce the cost and improve the reliability of electrical vacuum switches. 
     The present invention addresses this need for batch fabricating high-voltage vacuum switches by providing an electrical vacuum switch apparatus that comprises an anode, a cathode and a trigger electrode which can all be microfabricated on the same substrate. A completed vacuum switch can then be formed according to the present invention by attaching a cover over the substrate under vacuum to provide a vacuum environment wherein the anode, cathode and trigger electrode are located. 
     An advantage of the electrical vacuum switch apparatus of the present invention is that a relatively large number (up to hundreds or more) of individual devices can be batch fabricated on a common substrate without piece part assembly. 
     Another advantage of the electrical vacuum switch apparatus of the present invention is that various types of carbon materials can be used in the trigger electrode to provide electron emission for initiating a vacuum arc therein including graphitic carbon, diamond-like materials, and carbon nanotubes. 
     Yet another advantage of the electrical vacuum switch apparatus of the present invention is that one or more channels can be formed extending below a surface of the substrate whereon the anode and cathode are located to prevent surface breakdown on the substrate during operation of the device. 
     Still another advantage is that a metal cover can be used to trigger the vacuum arc in the electrical vacuum switch apparatus of the present invention, and to channel at least a portion of the arc. 
     These and other advantages of the present invention will become evident to those skilled in the art. 
     SUMMARY OF THE INVENTION 
     The present invention relates to an electrical vacuum switch apparatus (also referred to herein as a vacuum switch) which comprises a substrate; an anode and a cathode spaced apart on a surface of the substrate; a trigger electrode disposed between the anode and the cathode; and a cover sealed over the substrate to provide an evacuated region wherein the anode, the cathode and the trigger electrode are exposed to a vacuum environment. 
     In certain embodiments of the present invention, the apparatus can further comprise one or more channels extending below the surface of the substrate between the anode and the cathode. Such channels can extend partway or all the way around the anode, or cathode, or both to prevent surface breakdown in the device. Generally, each channel has a high aspect ratio, with the channel depth being greater than the width thereof. 
     The substrate can comprise an electrically-insulating material such as glass, silica, quartz, diamond, alumina or ceramic. In some embodiments of the present invention, the substrate can comprise silicon, with an electrically-insulating layer being provided over an upper surface of the silicon substrate beneath the anode and cathode. The anode and the cathode can comprise a metal or metal alloy (e.g. comprising niobium, molybdenum, copper, tungsten, aluminum, etc.). 
     The trigger electrode is preferably repeatedly pulsable to provide an electrical conduction path (also termed a vacuum arc, or a current discharge path) which can occur at least partially in an evacuated region and in some instances partially in a metal cover between the anode and the cathode. The trigger electrode can comprise a resistive material such as carbon which can generate sparks (i.e. a plasma or plasma discharge) in response to an electrical current flowing therethrough. Alternately, the trigger electrode can comprise a spark gap. In some embodiments of the present invention, the trigger electrode can comprise a plurality of carbon nanotubes or a diamond-like material, both of which are efficient electron emitters. The trigger electrode can be formed with a notched shape to localize the production of sparks or electrons used to trigger the device. The anode and the cathode can also each have a notched shape on a side thereof proximate to the trigger electrode. 
     A plurality of electrical vias can be provided in the vacuum switch, with the vias extending through the substrate to connect the anode, the cathode and the trigger electrode on one side of the substrate to electrical contacts on an opposite side of the substrate. This is useful so that the vacuum switch can be surface mounted (e.g. on an electrical circuit board, or on a CDU). 
     In certain embodiments of the present invention, the cover can comprise a metal. This can be advantageous since the metal cover can form at least a part of a the electrical conduction path between the anode and the cathode. 
     The present invention further relates to a vacuum switch which comprises an electrically-insulating substrate; an anode and a cathode spaced apart on a top side of the substrate; a trigger electrode disposed on the top side of the substrate between the anode and the cathode; a plurality of channels extending into the substrate on the top side thereof between the anode and the trigger electrode, and between the cathode and the trigger electrode, with the channels at least partially surrounding the anode and the cathode; and a cover sealed over the top side of the substrate to provide an evacuated region wherein the anode, the cathode and the trigger electrode are located. The electrical vacuum switch apparatus can further comprise a plurality of electrically-conducting vias formed through the substrate to electrically connect the anode, the cathode and the trigger electrode to contacts formed on a bottom side of the substrate. 
     The substrate can comprise an electrically-insulating material selected from the group consisting of glass, silica, quartz, diamond, alumina and ceramic. In some embodiments of the present invention, the substrate can comprise silicon, with an electrically-insulating layer being provided over an upper surface of the silicon substrate beneath the anode and cathode. The anode and the cathode can comprise a metal or metal alloy (e.g. comprising niobium, molybdenum, copper, tungsten, aluminum, etc.). 
     For certain embodiments of the present invention, the trigger electrode can comprise carbon which can be in the form of graphitic carbon, a diamond-like material, or a plurality of carbon nanotubes. In yet other embodiments of the present invention, the trigger electrode can comprise a spark gap. 
     The trigger electrode can comprise a plurality of notches on two sides thereof. The anode and the cathode can each comprise a plurality of notches on a side thereof facing the trigger electrode. 
     As described previously, in certain embodiments of the present invention, the cover can comprise a metal, and can be used to form a part of the conduction path between the anode and the cathode. 
     The present invention also relates to an electrical vacuum switch apparatus which comprises a substrate; an anode and a cathode spaced apart on a surface of the substrate; and a metal cover sealed over the substrate to provide an evacuated region wherein the anode and the cathode are contained in a vacuum environment, with the metal cover forming a trigger electrode to initiate an electrical discharge between the anode and the cathode. The metal cover can also form part of a conduction path for the discharge between the anode and the cathode. A plurality of carbon nanotubes, or alternately a diamond-like material, can be provided between the anode and the cathode to provide an electron emission to assist in initiating the electrical discharge between the anode and the cathode. 
     The substrate can comprise an electrically-insulating material such as glass, silica, quartz, diamond, alumina or a ceramic. Alternately, the substrate can comprise silicon, with an electrically-insulating layer provided over an upper surface of the silicon substrate beneath the anode and the cathode. One or more channels can be optionally provided extending below the surface of the substrate between the anode and the cathode to mitigate against surface breakdown in the device. 
     The anode and cathode can comprise a metal such as niobium, molybdenum, copper, tungsten, aluminum or an alloy thereof. The metal cover can also comprise niobium, molybdenum, copper, tungsten, aluminum or an alloy thereof. 
     Additional advantages and novel features of the invention will become apparent to those skilled in the art upon examination of the following detailed description thereof when considered in conjunction with the accompanying drawings. The advantages of the invention can be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are incorporated into and form a part of the specification, illustrate several aspects of the present invention and, together with the description, serve to explain the principles of the invention. The drawings are only for the purpose of illustrating preferred embodiments of the invention and are not to be construed as limiting the invention. In the drawings: 
         FIG. 1  shows a schematic plan view a first example of the vacuum switch of the present invention. 
         FIG. 2  shows a schematic cross-section view of the device of  FIG. 1  along the section line  1 - 1  in  FIG. 1 . 
         FIG. 3  shows a schematic plan view of a second example of the vacuum switch of the present invention. 
         FIG. 4  shows a schematic cross-section view of the device of  FIG. 3  along the section line  2 - 2  in  FIG. 3 . 
         FIG. 5  shows a schematic plan view of a third example of the vacuum switch of the present invention. 
         FIG. 6  shows a schematic cross-section view of the device of  FIG. 5  along the section line  3 - 3  in  FIG. 5 . 
         FIG. 7  shows a schematic plan view of a fourth example of the vacuum switch of the present invention. 
         FIG. 8  shows a schematic cross-section view of the device of  FIG. 7  along the section line  4 - 4  in  FIG. 7 . 
         FIG. 9  shows a schematic plan view of a fifth example of the vacuum switch of the present invention. 
         FIGS. 10A and 10B  show schematic cross-section views of the device of  FIG. 9  along the section line  5 - 5  in  FIG. 9  together with an example of a capacitive discharge circuit with which the vacuum switch can be used.  FIG. 10A  shows the device prior to triggering thereof with trigger switch S 1  open.  FIG. 10B  shows formation of a current discharge path (indicated by the curved line with arrows) which occurs upon closing trigger switch S 1 , with the current discharge path extending from the anode through the cover, which acts as the trigger electrode, to the cathode. 
         FIG. 11  shows a schematic plan view of a sixth example of the vacuum switch of the present invention. 
         FIGS. 12A and 12B  show schematic cross-section views of the device of  FIG. 11  along the section line  6 - 6  in  FIG. 11  together with an example of a capacitive discharge circuit with which the vacuum switch can be used.  FIG. 12A  shows the device prior to triggering thereof with trigger switch S 1  connected to ground.  FIG. 12B  shows the trigger switch S 1  connected to supply a trigger voltage V T  which initiates a vacuum arc in the device by increasing an electric field between the anode and cathode and also by providing a field emission of electrons from a plurality of carbon nanotubes located between the anode and cathode. 
         FIG. 12C  shows a schematic cross-section view of the device of  FIG. 11  with a diamond-like material substituted for the carbon nanotubes therein. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to  FIG. 1 , there is shown schematically in plan view a first example of the electrical vacuum switch apparatus  10  of the present invention. The apparatus  10  comprises a substrate  12 , with an anode  14  and a cathode  16  being spaced apart on an upper surface  18  of the substrate  12 , and with a trigger electrode  20  being located between the anode  14  and the cathode  16  on the same surface  18 . A cover  22  is permanently sealed over the upper surface  18  of the substrate  12  to provide an evacuated region  24  wherein the anode  14 , cathode  16  and trigger electrode  20  are all exposed to a vacuum environment. This is shown in  FIG. 2  which represents a schematic cross-section view of the device  10  of  FIG. 1 . In  FIG. 1 , and in the schematic plan views of each other example of the present invention described herein, it is assumed that the cover  22  is transparent so that the structure beneath the cover  22  can be seen. 
     Electrical connections can be provided from the anode  14 , cathode  16  and trigger electrode  20  to electrical contacts  26  located on a lower surface  28  of the substrate  12  to form a surface-mount package for the device  10 . This is done by providing a plurality of electrically-conducting vias  30  through the substrate  12  which is generally electrically insulating. One or more vias  30  can be provided to each of the anode  14 , cathode  16 , and trigger electrode  20 . The vias  30  can be, for example, 100-250 μm in diameter. 
     Those skilled in the art will understand that there are other ways of making the electrical contacts  26  to the anode  14 , cathode  16  and trigger electrode  20  located in the evacuated region  24 . As an example, one or more of the electrical contacts  26  can extend out from at least one edge of the substrate  12  (e.g. when the substrate  12  comprises a ceramic). 
     The substrate  12  can comprise an electrically-insulating material such as glass, silica, quartz, diamond, alumina or ceramic. Additionally, the substrate  12  can comprise silicon (i.e. a portion of a monocrystalline silicon wafer which is commonly used for forming integrated circuits) with an electrically-insulating oxide layer (i.e. SiO 2 ) being formed over all exposed surfaces of the substrate  12  and over sidewalls of openings formed through the silicon substrate  12  where the vias  30  are formed. Such a silicon substrate  12  covered with an electrically-insulating layer can be referred to as an “electrically-insulating substrate” since the silicon material forming the substrate  12  is electrically insulated from other elements formed thereon or therein including the anode  14 , the cathode  16 , the trigger electrode  20 , the vias  30  and the contacts  26 . The oxide layer, which can be on the order of a micron or more thick, can be formed by a thermal oxidation process or a high-pressure oxidation process whereby a portion of the silicon substrate is oxidized at a high temperature or high pressure or both, and thereby converted to silicon dioxide (SiO 2 ). 
     Other types of electrically-insulating layers can be formed over the substrate  12  in place of the oxide layer. As an example, silicon nitride or silicon oxynitride can be deposited over the surface  12  to form an electrically-insulating layer thereon using chemical vapor deposition. The silicon nitride or silicon oxynitride electrically-insulating layers can be about the same thickness as the oxide layer. 
     A plurality of devices  10 , each having lateral dimensions of 5-7 millimeters (mm) and a thickness of 1-2 mm, for example, can be batch fabricated on a much larger common substrate  12  (e.g. with lateral dimensions of 2-8 inches) and then separated once fabrication is completed. In this way, up to hundreds of individual devices  10  can be batch fabricated at the same time with substantially identical operating characteristics. 
     In the example of  FIGS. 1 and 2 , the cover  22  can be formed from an electrically-insulating material as described above, from silicon (with or without an oxide layer formed thereon), or from a metal such as molybdenum, niobium, copper, tungsten, aluminum or an alloy thereof. The use of a metal cover  22  can be advantageous since the presence of the metal can serve to draw a high-current arc between the anode  14  and the cathode  16  upward into the metal cover  22  and away from the substrate  12  after initial formation of the arc in the evacuated region  24  between the anode  14  and cathode  16 . 
     The metal cover  22  provides a high-current-carrying capacity and can form at least a part of the current discharge path (also termed herein an electrical conduction path) between the anode  14  and the cathode  16 . Additionally, any material which has been evaporated or sputtered from the anode  14 , cathode  16  or trigger electrode  20  by the high-current arc will be drawn to where the arc enters or exits the metal cover  22  and deposited there. This prevents deposition of the evaporated or sputtered material on the upper surface  18  of the substrate  12  where the material could possibly lead to surface breakdown upon repeated operation of the vacuum switch  10 . 
     The cover  22  can be etched to form a recess therein which defines the shape of the evacuated region  24 . When the cover  22  comprises metal, the metal cover  22  can be shaped by etching, stamping, molding, plating over a mandrel, etc. For batch fabrication of the vacuum switch  10 , a plurality of covers  22  can be formed as a single cover plate which can be sealed over a common substrate containing a plurality of anodes  14 , cathodes  16 , trigger electrodes  20 , etc., to form a plurality of devices  10  which can then be separated by sawing or laser cutting once fabrication is completed. 
     The cover  22  can be sealed to the substrate  12  under a high vacuum using a conventional wafer bonding method as known to the art. This can be done by eutectic bonding (e.g. Au/Si eutectic bonding when the substrate  12  and cover  22  comprise silicon with a layer of gold deposited on one or both of the substrate  12  and cover  22 ), or by diffusion bonding (also termed anodic bonding). 
     Alternately, the cover  22  can be sealed to the substrate  12  under high vacuum by brazing using a filler metal  32  (see  FIG. 4 ) which can comprise gold. The filler material can be provided as a preform, or deposited over the substrate  12 , or cover  22 , or both by evaporation, sputtering or plating. As an example, layers of gold and nickel can be evaporated or plated to a layer thickness of up to a few microns to form the filler metal  32 . The gold and nickel upon heating to a brazing temperature of about 1000° C. under vacuum will form an 82% gold/18% nickel alloy (also termed Nioro) which will permanently attach the cover to the substrate  12 . Those skilled in the art will understand that other filler metals can be used to join the materials used for the substrate  12  and cover  22  for the vacuum switch  10  of the present invention, with the exact filler metals being selected depending upon thermal considerations of the various materials used for the vacuum switch  10 . 
     In the example of  FIGS. 1 and 2 , the anode  14  and cathode  16  can comprise metals such as molybdenum, niobium, copper, tungsten, aluminum, or an alloy of one or more of these metals. The above metals and metal alloys can also be made gas-free to prevent an accumulation of gases within the evacuated region  24  that might otherwise emanate from the anode  14  and cathode  16  during operation of the device  10 . Additionally, these metals, and especially niobium, molybdenum and tungsten, have relatively high melting points which minimizes evaporation of the metals during discharge to prevent the upper surface  18  within the evacuated region  24  from being coated with the metals and eventually short-circuiting the device  10 , or reducing the number of times the device  10  can be switched. 
     The metals used for the anode  14  and cathode  16  can be deposited over the substrate  12  to a layer thickness of up to a few microns or more by evaporation, sputtering or plating. If needed, a thin layer of titanium can be provided beneath the metals used for the anode  14  and cathode  16  to promote a better adhesion of these metals to the substrate  12 . 
     The trigger electrode  20  in the example of  FIGS. 1 and 2  can comprise carbon deposited to a layer thickness of up to a few microns. If needed, a thin titanium layer can be deposited and patterned on the substrate  12  to improve adhesion of the carbon used to form the trigger electrode  20 . 
     The carbon used for the trigger electrode  20  can be in several different forms with different electron emission characteristics. The carbon can comprise graphitic carbon which can be deposited, for example, by chemical vapor deposition or sputtering, and which emits electrons due to plasma emission, thermionic emission, thermo-field emission, or a combination of these types of emission. 
     Alternately, the carbon used for the trigger electrode  20  can comprise a diamond-like material (e.g. diamond, diamond-like carbon, or amorphous diamond) which can be deposited by chemical vapor deposition or pulsed laser deposition. The diamond-like material can have a negative electron affinity which allows electrons to be readily emitted under the influence of a voltage applied across the trigger electrode  20 . An electron accelerated by this applied trigger voltage will tend to skip across the surface of the diamond-like material knocking out further electrons through secondary emission, resulting in an avalanche of electrons on the surface of the diamond-like material to initiate the vacuum arc between the anode  14  and cathode  16 . The diamond-like material can be doped (e.g. with boron) to further reduce the work function at the surface of this material and thereby enhance electron emission. The diamond-like material also provides a high stability with little, if any, of the diamond-like material being expected to be dislodged or eroded from the trigger electrode  20  during repeated operation of the vacuum switch  10 . Furthermore, the diamond-like material provides a high thermal conductivity so that any surface heating of the trigger electrode  20  can be conducted away into the substrate  12 , thereby improving the lifetime of the vacuum switch  10  for repeated operation. Further details of diamond-like materials can be found in M. R Siegal et al.,  Diamond and Diamond - Like Carbon Films for Advanced Electronic Applications  (Sandia National Laboratories Report No. SAND96-0516, March 1996, available from National Technical Information Service, U.S. Department of Commerce), which is incorporated herein by reference. 
     In  FIG. 1 , the trigger electrode  20  has a notched shape comprising a plurality of necked-down regions  34  spaced about the length of the electrode  20 , with the necked-down regions  34  being separated by outward-extending teeth  36  on each side of the electrode  20  to form a notched shape for the trigger electrode  20 . The carbon used for the trigger electrode  20  can be deposited along the entire length of the trigger electrode  20 , or alternately only in the necked-down regions  34 , with the remainder of the trigger electrode  20  comprising a metal or metal alloy (e.g. comprising niobium, molybdenum, copper, tungsten, aluminum etc.). The anode  14  and cathode  16  can also each have a notched shape on a side thereof facing the trigger electrode  20 . Teeth  36  on the anode  14  and cathode  16  can be aligned with the necked-down regions  34  to provide a localized higher electric field at these locations so that a plurality of vacuum arcs will be initiated at these locations upon triggering of the vacuum switch  10 . 
     A high-voltage of up to several kiloVolts from an external power source (e.g. a CDU) can be applied between the anode  14  and cathode  16  for switching by the device  10 . The separation of the anode  14  and cathode  16 , which can be on the order of a few hundred microns or more, is sufficient to stand off the applied high voltage and prevent conduction between the anode  14  and cathode  16  in the absence of a trigger signal applied along the length of the trigger electrode  20  via the pair of contacts  26 . However, once the trigger signal, which can be on the order of 100-200 volts or less, is applied, one or more sparks comprising electrons and ions are generated by the trigger electrode; and this initiates a vacuum arc between the anode  14  and cathode  16 . The current in the vacuum arc can be up to a kiloAmpere or more. 
     When the trigger signal is applied suddenly, an electrical current pulse is conducted along the length of the trigger electrode  20 . In the necked-down regions  34 , a cross-sectional area of the carbon forming the trigger electrode  20  is significantly reduced. This leads to a relatively large localized increase in current density at these regions  34  which produces localized heating accompanied by the generation of a plurality of sparks. The exact mechanism for generation of the sparks is not well understood, although it may include plasma emission, thermionic emission, thermo-field emission, or a combination of these types of emission. The sparks then act to trigger the vacuum arc between the cathode  16  and anode  14  as described above. The notched shapes of the anode  14  and cathode  16  serve to concentrate an electric field between the anode  14  and cathode  16  at the locations of the necked-down regions  34  where the sparks are generated so that a plurality of arcs are formed which are stable, and which are expected to be uniformly distributed across the width of the anode  14  and cathode  16 . 
     The term “notched shape” as used herein refers to a shape which comprises a plurality of V-shaped, U-shaped or arbitrary-shaped indentations which are arranged side-by-side along at least one side of an element of the apparatus  10 . 
     Returning to  FIGS. 1 and 2 , openings through the substrate  12  where the vias  30  are to be formed can be etched or drilled (e.g. mechanically, or with a laser). Metal (e.g. tungsten, or copper, or both) can be deposited or plated to fill in the openings and form the vias  30  which can have a diameter of, for example, 100-250 μm. Alternately, a metal, metal alloy or combination of metals can be deposited into the via openings by screen printing followed by sintering. 
     The contacts  26  can be formed from a metal or metal alloy which is solderable (e.g. copper, nickel, tin or a combination thereof). The contacts  26  can be formed by metal evaporation or plating, or alternately by screen printing and sintering. 
       FIGS. 3 and 4  show schematic plan and cross-section views, respectively, of a second example of the vacuum switch  10  of the present invention. In this second example, one or more channels  38  can be formed in the substrate  12 . This can be done by etching through a photolithographically-defined etch mask provided over the substrate  12  with openings in the etch mask to expose portions of the substrate  12  where the channels  38  are to be etched. The etching can be performed using well-known wet etchants, or alternately by plasma etching (e.g. reactive ion etching). 
     Each channel  38  can extend partway or entirely around the anode  14 , the cathode  16 , or both. Each channel  38  is also generally formed with a high aspect ratio so that the depth of the channel  38  is larger than the width thereof. As an example, each channel  38  can be 10-20 μm wide and 50-100 μm deep. The sidewalls of each channel can be substantially straight as shown in  FIG. 4 , or tapered. When the substrate  12  comprises silicon, the channels  38  can be formed prior to oxidation of the substrate so that the oxide layer completely blankets the interior of each channel  38 . 
     The purpose of the channels  38  in the example of  FIGS. 3 and 4  is to provide an increased surface path length between the anode  14  and the cathode  16  so that any electrical breakdown between the anode  14  and cathode  16  does not occur on the upper surface  18  (i.e. surface breakdown), but instead occurs above the surface  18  in the evacuated region  24  (i.e. vacuum breakdown). Generally, surface breakdown occurs over a given electrode spacing at a lower voltage than the voltage required for vacuum breakdown over the same spacing. The voltage at which surface breakdown occurs can also be significantly lowered due to contaminants on the upper surface  18  of the substrate  12  (e.g. due to metal or carbon electrode material evaporated or sputtered from the anode  14 , cathode  16  and trigger electrode  20 ). The provision of one or more high-aspect-ratio channels  38  in the device  10  of  FIGS. 3 and 4  has an additional advantage that a portion of the interior surface of each channel  38  will be shadowed from any emitted electrode material. Thus, the channels  38  will act to interrupt any current discharge path through the emitted electrode material to help to prevent surface breakdown in the device  10  from this emitted electrode material even after multiple operations of the vacuum switch  10 . 
     The provision of channels  38  in the device  10  of  FIGS. 3 and 4  is more critical at the anode  14  since evaporation or sputtering of the electrode material is generally greater at the anode  14  than at the cathode  16 . As a result, a larger number of channels  38  can be provided about the anode  14  according to the present invention. A separation between the anode  14  and the trigger electrode  20  can be made larger than the separation between the cathode  16  and the trigger electrode  20  to accommodate the larger number of channels  38 , and since the voltage between the trigger electrode  20  and cathode  16  is generally much smaller than the voltage between the trigger electrode  20  and the anode  14 . Additionally, since the anode  14  is generally maintained at a higher electrical potential than the cathode  16  prior to switching of the device  10 , the channels  38  can extend completely around the anode  14  to prevent the possibility of surface breakdown between the anode  14  and the filler metal  32 , or the cover  22 . 
     A third example of the electrical vacuum switch apparatus  10  of the present invention is schematically illustrated in plan view in  FIG. 5 , and in cross-section view in  FIG. 6 . The third example of the present invention is similar to the second example of  FIGS. 3 and 4  except that the trigger electrode  20  comprises a plurality of carbon nanotubes  40 . The carbon nanotubes  40 , which are essentially single- or multi-walled tubes having a wall thickness on an atomic scale, can have a relatively low effective work function due to field enhancement at their tips, thereby making them efficient electron emitters. Electron emission in the carbon nanotubes  40  can be produced thermionically (i.e. thermionic emission) by resistive heating produced by the trigger signal conducted across the trigger electrode  20 . This thermionic emission can then act to trigger a vacuum arc between the anode  14  and cathode  16 , thereby switching the device  10  from a nonconducting state to a conducting state. Field emission in the trigger electrode  20  can also be produced in the carbon nanotubes  40  to initiate the vacuum arc. 
     The carbon nanotubes  40  can be vertically oriented as shown in  FIG. 6 , or horizontally oriented, or even randomly oriented. The carbon nanotubes  40 , which can be single-walled or multi-walled, are located proximate to the necked-down regions  34  of the trigger electrode  20 . Although  FIGS. 6 and 7  show only a small number of carbon nanotubes  40  located in the necked-down regions  34  for clarity, the actual number of carbon nanotubes  40  in each necked-down region  40  can be up to thousands or more of carbon nanotubes  40  since each single-wall carbon nanotube  40  has a typical diameter on the order of 1-2 nanometers (nm), with multi-wall carbon nanotubes  40  having diameters of up to 50-100 nm. 
     There are many ways of fabricating the carbon nanotubes  40  in the necked-down regions  34  of the trigger electrode  20  as will be described hereinafter. In the example of  FIGS. 5 and 6 , the carbon nanotubes  40  are located in a shaped trench having the notched shape of the trigger electrode  20 . This can help to prevent erosion of the carbon nanotubes  40  by the vacuum arc by locating the carbon nanotubes  40  below the upper surface  18  of the substrate  12 . In other embodiments of the present invention, the carbon nanotubes  40  can extend above the upper surface  18 , or be entirely located above the upper surface  18  of the substrate  12 . 
     In preparation for forming the carbon nanotubes  40  in the example of  FIGS. 5 and 6 , the shaped trench having the notched shape of the trigger electrode  20  can be etched into the substrate  12 . This can be done at the same time the channels  38  are formed. When the substrate  12  comprises silicon, this is preferably done prior to oxidation of the substrate  12  so that the oxide layer formed by oxidizing the substrate  12  also covers the sidewalls and bottom of the shaped trench. The shaped trench can be, for example, 20-100 μm deep. 
     An electrically-conductive layer  42  can be provided in the shaped trench to make electrical contact with the vias  30  and to conduct the trigger signal between the necked-down regions  34 . The electrically-conductive layer  42  can comprise a metal or metal alloy (e.g. comprising molybdenum, niobium, copper, tungsten, titanium, aluminum, etc.,) or alternately carbon (e.g. graphitic carbon or a diamond-like material), and can have a layer thickness of up to about one micron. If needed, a thin (about 10-20 nm thick) layer of titanium can be used to promote adhesion of the electrically-conductive layer  42  to the substrate  12 . 
     The carbon nanotubes  40  can be grown directly in the necked-down regions  34 . This can be done by depositing a transition metal catalyst (e.g. iron or nickel deposited by evaporation or sputtering) in the necked-down regions  34 . The transition metal catalyzes growth of the carbon nanotubes  40  during deposition by chemical vapor deposition (CVD) at an elevated temperature (e.g. 650-700° C.) using a hydrocarbon feed gas (e.g. acetylene or methane). Additional hydrogen can be added to the feed gas to prevent deposition of carbon on the substrate  12  during formation of the carbon nanotubes. 
     The fabrication of carbon nanotubes by CVD is well known to the art. See e.g. Y. Tzeng et al., “Fabrication and Characterization of Non-Planar High-Current-Density Carbon-Nanotube Coated Cold Cathodes,”  Diamond and Related Materials , vol. 12, pp. 442-445 (2003); and Y. Y. Wei et al., “Direct Fabrication of Carbon Nanotube Circuits by Selective Area Chemical Vapor Deposition on Pre-Patterned Structures,”  Nanotechnology , vol. 11, pp. 61-64 (2000), both of which are incorporated herein by reference. 
     In some embodiments of the present invention, the carbon nanotubes  40  can be grown directly onto a silicon dioxide layer (e.g. the oxide layer formed in the shaped trench when a silicon substrate  12  is used) using CVD without a transition metal catalyst. In these embodiments of the present invention, the electrically-conductive layer  42  can be used to conduct the trigger signal between the necked-down regions  34 , with the conduction of the trigger signal through the necked-down regions  34  being provided by the carbon nanotubes  40  which can be closely packed together. A plurality of horizontally-oriented carbon nanotubes can also be grown in place between portions of the electrically-conductive layer  42  to bridge across each necked down region  34 . 
     In other embodiments of the present invention, the carbon nanotubes  40  can be provided along a majority of the length of the trigger electrode  20 . This can be done, for example, by dispersing the carbon nanotubes into a plating solution and co-precipitated them out with a metal plating (e.g. comprising copper) which can be used to form the electrically-conductive layer  42 . By providing an electric field having field lines oriented substantially perpendicular to the upper surface  18  of the substrate  12  during the plating process, the carbon nanotubes  40  can be oriented vertically. In the absence of an electric field, the carbon nanotubes  40  will generally be randomly oriented. Further details of forming the carbon nanotubes  40  by co-precipitation during metal plating can be found, for example, in U.S. Pat. Nos. 6,796,870 and 6,891,320 to Nakamoto, which are incorporated herein by reference. 
     Yet another way of forming the carbon nanotubes  40 , is to mix a plurality of pre-formed and commercially available carbon nanotubes  40  into a metal paste (e.g. comprising tungsten, copper, molybdenum, niobium, tungsten, aluminum or combinations thereof) which can be deposited in the necked-down regions  34 , or alternately along the entire length of the trigger electrode  20 . This can be done by screen printing or ink-jet deposition. The metal paste containing the carbon nanotubes  40  can then be sintered. This generally results in randomly oriented carbon nanotubes  40 . 
       FIGS. 7 and 8  show schematic plan and cross-section views, respectively, of a fourth example of the vacuum switch  10  of the present invention. In the example of  FIGS. 7 and 8 , the trigger electrode  20  comprises a pair of electrically-conductive strip electrodes  44  separated by a narrow spark gap  46 . The strip electrodes  44  can comprise the same metals used to form the anode  14  and cathode  16  (e.g. molybdenum, niobium, copper, tungsten, aluminum and alloys thereof). The exact width of the spark gap  46  between the strip electrodes  44  will depend upon a predetermined voltage for the trigger signal which is used to generate a spark across the gap  46  and thereby trigger electrical breakdown (i.e. the vacuum arc) between the anode  14  and cathode  16  during operation of the device  10 . The width of the spark gap  46  will generally be on the order of one-tenth of the separation between the anode  14  and cathode  16  or less, and can be, for example, 2-40 μm. 
     An optional channel (not shown) can be etched into the substrate  12  beneath the spark gap  46  to prevent surface breakdown between the strip electrodes  44  during application of the trigger signal. This channel can be, for example, 50-100 μm deep and at least as long and wide as the spark gap  46 . Additionally, this channel can extend for a distance of a few microns or more beneath the ends of the strip electrodes  44  proximate to the spark gap  46  when the channel is formed using an isotropic etchant. By forming a channel beneath the spark gap  46  as described above, the trigger signal will produce a spark in the evacuated region  24  between the strip electrodes  44  (i.e. vacuum breakdown) rather than possibly occurring due to surface breakdown on the upper surface  18  of the substrate  12 . When the substrate  12  comprises silicon, the channel beneath the spark gap  46  can be formed prior to oxidation of the substrate  12  so that the sidewalls and bottom of the channel will be covered by the oxide layer. If needed to provide additional protection against surface breakdown between the anode  14  and the cathode  16  in the example of  FIGS. 7 and 8 , an additional plurality of channels  38  can also be formed partially or entirely around the anode  14  and cathode  16  as described previously with reference to  FIGS. 3 and 4 . 
       FIG. 9  shows a schematic plan view of a fifth example of the vacuum switch  10  of the present invention. In this example of the vacuum switch  10 , a metal cover  22  is used which also functions as a trigger electrode. The metal cover  22  can be electrically connected to a contact  26  on the lower surface  28  of the substrate  12  using a via  30  and the filler metal  32  which attaches the cover  22  to the substrate  12  (see  FIG. 10A ). 
       FIGS. 10A and 10B  show schematic cross-section views of the vacuum switch  10  of  FIG. 9  along the section line  5 - 5  in  FIG. 9  to illustrate operation of this example of the vacuum switch  10 . In  FIG. 10A , the device  10  is shown connected to a capacitive discharge circuit which comprises resistors R 1  and R 2 , capacitor C 1 , and switch S 1  (which can be a mechanical switch or alternately a thyristor or a transistor) to form a CDU. A voltage source, V, can be used to charge the capacitor C 1  to a high direct-current (dc) voltage of up to several thousand volts. With switch S 1  open as shown in  FIG. 10A , the metal cover  22  is electrically floating. However, when S 1  is closed as shown in  FIG. 10B , the metal cover  22  is pulled to ground through resistor R 2 . In other embodiments of the present invention, the metal cover  22  can be connected through resistor R 2  to a negative bias source to negatively bias the cover  22  at a voltage of up to a few hundred volts when switch S 1  is closed. In either case, switching the metal cover  22  to ground or to a negative voltage can initiate a vacuum arc between the anode  14  and cover  22 . The initiation of the vacuum arc very quickly raises the voltage on the metal cover  22  to about the same voltage on the anode  14  and produces a second vacuum arc between the cover  22  and the cathode  16 , thereby discharging the majority of the electrical energy stored in the capacitor C 1  into the load (which can be an explosive initiator, flashlamp, etc.) with a peak current of up to one kiloAmpere or more, with a relatively small amount of energy being dissipated by resistor R 2 . 
     In this example of the present invention, the anode  14  and cathode  16  can be of arbitrary shape since a current discharge path is not formed directly between the anode  14  and cathode  16  in the evacuated region  24 , but instead flows from the anode  14  through the metal cover  22  and back to the cathode  16  as indicated by the curved lines with arrows in  FIG. 10B . As previously mentioned, this can help to prevent any accumulation of contaminants on the upper surface  18  of the substrate  12  due to metal evaporated or sputtered from the anode  14  and cathode  16 . One or more channels  38  can also be optionally formed in the substrate  12  as previously described to further prevent the possibility of surface breakdown during repeated operation of the vacuum switch  10 . 
       FIG. 11  shows a schematic plan view of a sixth example of the vacuum switch of the present invention. This example of the vacuum switch  10  also uses a metal cover  22  which functions as a trigger electrode, with the metal cover  22  being electrically connected through the filler metal  32  and a via  30  to a contact  26  on the lower surface  28  of the substrate  12 . In the device of  FIG. 11 , the metal cover  22  can be used to control and switch an electric field between the anode  14  and cathode  16  and thereby initiate the vacuum arc. 
       FIGS. 12A and 12B  show schematic cross-section views of the vacuum switch  10  of  FIG. 11  along the section line  6 - 6  in  FIG. 11  to illustrate operation of this example of the vacuum switch  10 . In  FIG. 12A , the vacuum switch  10  is shown connected to a capacitive discharge circuit which comprises resistors R 1 , R 2  and R 3 , capacitor C 1 , and switch S 1  (which can be a mechanical switch or alternately a thyristor or a transistor) to form a CDU. A voltage source, V, can be used to charge the capacitor C 1  to a high dc voltage of up to several thousand volts. With switch S 1  connected to ground as shown in  FIG. 12A , an electric field produced by the high dc voltage of up to several thousand volts is concentrated between the anode  14  and the metal cover  22  (i.e. the grounded trigger electrode). A much smaller electric field is produced between the anode  14  and cathode  16  since the spacing between these elements is substantially larger than that between the anode  14  and cover  22 . In  FIG. 12A , the relatively large (i.e. concentrated) electric field between the anode  14  and metal cover  22  is indicated by the plurality of vertical arrows which have a relatively long length indicative of the magnitude of the electric field. The relatively small electric field between the anode  14  and cathode  16  is indicated by the horizontal arrow which is shorter to indicate the smaller magnitude of the electric field directed towards the cathode  16 . The exact electric field profile can be adjusted using the shape of the metal cover  22 . In  FIG. 12A , it is important that the spacing between the anode  14  and the metal cover  22  be sufficiently large so that no vacuum arc is initiated when the cover  22  is electrically grounded. 
     In  FIG. 12B , to initiate a vacuum arc between the anode  14  and cathode  16 , switch S 1  can be connected to a trigger voltage, V T , having the same polarity as V, with the exact trigger voltage generally being on the order of V or less. This substantially reduces a voltage difference between the anode  14  and cover  22 , thereby substantially reducing the electric field between the anode  14  and cover  22 . At the same time, the electric field is substantially increased between the anode  14  and cathode  16  to initiate the vacuum arc. 
     Additionally, the trigger voltage, V T , provides an electric field bias between the metal cover  22  and a plurality of carbon nanotubes  40  which are electrically grounded through resistor R 3 . In  FIG. 12A , the cover  22  and carbon nanotubes  40  were both electrically grounded so that no electron emission from the carbon nanotubes  40  was stimulated by the metal cover  22 . Once the cover  22  is electrically biased to V T , an electric field emission of electrons from the carbon nanotubes  40  is stimulated, with these electrons being emitted into the evacuated region  24  to assist in initiation of the vacuum arc. Once the vacuum arc is initiated between the anode  14  and cathode  16 , the electrical energy stored in the capacitor C 1  is discharged into the load. A portion of the electrical current in the vacuum arc can flow through the metal cover  22 ; and this can be advantageous to prevent surface breakdown upon repeated operation of the vacuum switch  10 . 
     In other embodiments of the present invention, the switch S 1  can be used to provide an open circuit to trigger initiation of the vacuum arc. In these embodiments, opening the switch S 1  allows the metal cover  22  to electrically float, thereby reducing the electric field between the anode  14  and the metal cover  22 , and increasing the electric field between the anode  14  and cathode  16  to initiate the vacuum arc. The increased electric field between the anode  14  and cathode  16  can further generate a field emission of electrons from the carbon nanotubes  40  into the evacuated region  24  to assist in initiation of the vacuum arc. One or more channels  38  can also be optionally provided in the device  10  of  FIGS. 11 and 12A ,  12 B between the anode  14  and the cathode  16  to reduce the possibility for surface breakdown. 
     Although the carbon nanotubes  40  in  FIGS. 12A and 12B  are shown vertically oriented, the carbon nanotubes  40  can be horizontally or randomly oriented in other embodiments of the present invention. Additionally, a layer  48  of a diamond-like material as previously described can be substituted for the carbon nanotubes to provide the electron emission to assist in initiating the vacuum arc. This is schematically illustrated in the cross-section view of  FIG. 12C  taken along the section line  6 - 6  in  FIG. 11  with the layer  48  of the diamond-like material being substituted for the carbon nanotubes  40 . The diamond-like material can be either deposited directly on the substrate  12  as shown in  FIG. 12C  with a rectangular or arbitrary shape. Alternately the diamond-like material can be deposited over an electrically-conductive layer  42  (e.g. comprising carbon) with substantially the same size as the layer  42  or as localized portions aligned between the teeth  36  on the anode  14  and cathode  16  similar to the localization of the carbon nanotubes  40  shown in  FIG. 11 . The layer  48  of the diamond-like material can be, for example, on the order of 1 μm thick; and can be doped (e.g. with boron) to provide a negative electron affinity. In other embodiments of the present invention, the carbon nanotubes  40  (or the diamond-like material substituted for the carbon nanotubes  40 ) can be directly connected to the cathode  16 , or can be provided directly on the cathode  16 . 
     The vacuum environment in the device  10  of the present invention allows the device  10  to be used for certain applications where ionizing radiation may be present. For such applications, the provision of a fill gas in the region  24  is not suitable since the fill gas could be ionized by the radiation, thereby leading to a premature switching of the device  10 . 
     The matter set forth in the foregoing description and accompanying drawings is offered by way of illustration only and not as a limitation. The actual scope of the invention is intended to be defined in the following claims when viewed in their proper perspective based on the prior art.