Abstract:
A microscale vacuum electronic device ( 10 ) provides for a mechanical modulation of cathode ( 12 ) position allowing improved high-frequency modulation of an electron beam ( 24 ) useful for vacuum electronic devices such as klystrons, klystrodes, and high frequency triodes.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. provisional application 60/843,991 filed Sep. 12, 2006 hereby incorporated by reference. 
     
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     BACKGROUND OF THE INVENTION 
       [0002]    The present invention relates generally to high frequency vacuum electronics, including devices such as klystrons, klystrodes, and high frequency triodes and more specifically to a microscale vacuum electronic device employing mechanical modulation. 
         [0003]    High-powered, high-frequency electrical signals may be created and controlled by vacuum electrical devices including vacuum tubes such as triodes, and traveling wave tubes, including generally magnetrons, klystron, klystrodes and the like. 
         [0004]    One such device, the klystron, provides a cathode producing an electron beam directed toward an anode and then into a drift space. A high-frequency signal, for example at microwave frequencies, is introduced into a resonant cavity positioned along the path of the electron beam to velocity modulate the electrons of the beam. The velocity modulation “bunches” the electrons as they travel through the drift space after which they pass by and release energy to a second resonant cavity in amplified form. 
         [0005]    In a conventional vacuum tube triode, a cathode produces an electron beam that is received by an anode after passing through a grid. A high-frequency signal may be applied to the grid to modulate the current emitted from the cathode and thus the current flowing from the cathode. 
         [0006]    In a klystrode design, elements of the klystron and triode are combined so that the electron beam is velocity modulated with a grid and then passed through a drift space. As with the klystron, energy may be extracted from the bunched and accelerated electrons by a downstream resonant cavity. 
         [0007]    The output of any of these devices may be applied as a feedback signal to the modulating grid or cavity to produce a high frequency oscillator. 
         [0008]    Recent developments in such vacuum electrical devices have addressed the possibility of fabricating microscale vacuum electrical devices, using integrated circuit techniques and the like. The small scale of such devices allows extremely high frequency signals to be generated and controlled, but also raises a number of practical problems including tuning the device when used as an oscillator, which may require changing a microscale physical cavity size. Small scale devices also present problems of creating a hot cathode for thermionic emission, and problems inherent in the close spacing of the elements, for example the control grid to the cathode, such as may increase undesired electrical interactions. 
       BRIEF SUMMARY OF THE INVENTION 
       [0009]    The present invention provides a microscale vacuum electrical device that employs mechanical modulation to control an electron beam. Mechanical modulation, as opposed to electrical modulation of a grid or coupled tuned cavity, offers the possibility of simplified device tuning. Further, by providing an electrically isolated modulation path, undesired electrical interactions among device signals can be reduced and circuit designs simplified. 
         [0010]    Specifically, the present invention provides a microscale high-frequency vacuum electrical device having an evacuated housing holding a cathode and an anode. The anode is biased with respect to the cathode to attract an electron beam from the cathode. An actuator receives a first signal to modulate a relative location of a cathode, for example with respect to a grid or the anode, at a frequency greater than 50 kilohertz and for nanoscale devices to frequencies of up to 10 GHz, to modulate the electron beam. 
         [0011]    It is thus one object of at least one embodiment of the invention to provide a vacuum electrical device that is better suited for microscale fabrication. Mechanical modulation makes possible device construction that can eliminate tuned coupling cavities, grid voltage modulation or the like. 
         [0012]    The invention may provide a grid held within the housing between the cathode and anode and electromechanically biased to control the flow of electrons between the cathode and anode. 
         [0013]    It is thus another object of at least one embodiment of the invention to provide a modified triode or klystrode type device. 
         [0014]    The actuator may be a piezoelectric device. 
         [0015]    It is thus an object of at least one embodiment of the invention to provide a simple solid-state actuator compatible with microscale devices and that may operate at high frequency. 
         [0016]    The actuator may receive an electrical modulation signal. 
         [0017]    It is an object of at least one embodiment of the invention to allow conventional electrical control and feedback of the vacuum electrical device. 
         [0018]    The actuator may move the cathode. 
         [0019]    It is thus an object of at least one embodiment of the invention to provide a simple method of modulating the cathode to grid distance by connection to the more accessible cathode structure. 
         [0020]    The modulation of the electron beam may be at a harmonic frequency of the first signal driving the actuator. 
         [0021]    It is thus an object of at least one embodiment of the invention to allow for high-frequency electron beam modulation above that readily obtained through physical motion of the actuator. 
         [0022]    The cathode may include an array of field-emitting pillars extending toward the grid. 
         [0023]    It is thus an object of at least one embodiment of the invention to improve the electron emissivity of the cathode through the use of nanoscale pillars. 
         [0024]    The grid may include apertures aligned with the pillars so that movement of the pillar tips with respect to the apertures provides modulation of the electron beam. 
         [0025]    It is thus another object of at least one embodiment of the invention to provide better electron beam modulation through relative movement of the pillars. 
         [0026]    The pillar tips may move in flexure with respect to the apertures. 
         [0027]    It is thus another object of at least one embodiment of the invention to provide a second resonant structure that may be used to modulate the electron beams. 
         [0028]    The modulation of the electron beam by the pillars may be at a harmonic of a frequency of movement of a membrane forming the cathode. 
         [0029]    It is thus an object of at least one embodiment of the invention to provide for higher frequency modulation than may be obtained by simple movement of the relatively larger cathode membrane. 
         [0030]    The cathode and the pillars may be formed from a doped semiconductor. 
         [0031]    Thus, it is an object of at least one embodiment of the invention to provide a structure that may be readily fabricated by conventional integrated circuit techniques. 
         [0032]    The tips of the pillars may be coated with a material increasing the electron emissions of the pillars. 
         [0033]    It is thus an object of at least one embodiment of the invention to provide for a high emissivity surface using both geometric and physical properties of the pillar material. 
         [0034]    These particular objects and advantages may apply to only some embodiments falling within the claims and thus do not define the scope of the invention. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0035]      FIG. 1  is a side elevational view of a klystrode constructed according to the principles of the present invention, showing a cathode configured for mechanical movement with respect to a grid to provide a traveling wave directed toward an anode; 
           [0036]      FIG. 2  is a simplified diagram of the cathode and anode showing one resonant motion of the cathode when excited by a piezoelectric actuator; 
           [0037]      FIG. 3  is a fragmentary perspective view of the surface of the cathode facing the grid showing fabrication of a plurality of nanoscale pillars on that surface; 
           [0038]      FIG. 4  is an exaggerated cross-sectional fragmentary view of the grid and cathode of  FIGS. 1 and 3 , showing resonant motion of the pillars with movement of the cathode and their changing alignment with regularly spaced apertures within the grid; 
           [0039]      FIG. 5  is a spectrum showing an operating frequency of a mechanical actuator and harmonics thereof which may drive ones of the cathode and the pillars at yet higher frequencies; and 
           [0040]      FIG. 6  is an elevational cross section of the device of  FIG. 1  implemented using integrated circuit techniques. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0041]    Referring now to  FIG. 1 , one embodiment of the invention may provide a klystrode  10  having a conductive cathode  12  opposed with one or more conductive anodes  16 , defining between them a “drift space”  14 , all held within an evacuated housing  20 . The cathode  12  may be biased with respect to the anodes  16  by a DC bias source  22  as is understood in the art. Under the influence of the bias source  22 , electrons are emitted from the cathode  12  and drawn in an electron beam  24  along a z-axis into the drift space  14 . 
         [0042]    The surface of the cathode may be of a type, as will be described below, to promote non-thermionic, low-temperature emission of electrons (field emissions) to provide for “cold cathode” operation. The cold operation of the cathode  12  allows it to be placed close to a grid  26 , positioned between the cathode  12  and anode  16  so that electrons of the electron beam  24  must pass through apertures  28  in the grid before reaching the drift space  14 . 
         [0043]    In one possible operating mode, an RF modulating source  30  may be applied to the conductive grid  26 , either capacitively or inductively, to both directly affect the emission of electrons from the cathode  12  and to promote a velocity difference in those electrons as they form the electron beam  24 . The resulting modulated electron beam  24  is accelerated through the drift space  14  past an output cavity  32  positioned along the path of the electron beam  24 . The output cavity  32  is tuned to a modulation frequency of the electron beam  24  to extract amplified radio frequency energy from the electron beam  24  through output waveguide  34  according to techniques well understood in the art. A portion of the signal on the waveguide  34  may be fed back to drive the grid  26  to produce an oscillator or may be appropriately divided in frequency and used to drive the mechanical resonance. 
         [0044]    As is understood in the art, modulation of the grid  26 , by RF modulating source  30  alters the velocity of the electrons emitted from the cathode  12  so that there is a bunching of electrons as the electrons move through drift space  14 . The bunching is shown by superimposed plot  27 . The modulation voltage on the grid  26  may also affect the emission of electrons from the cathode  12  causing a current modulation. Electron energy recovered from the cavity  32  is thus amplified both by changes in kinetic energy and changes in current flow. 
         [0045]    Referring now to  FIGS. 1 and 3 , in the present invention, the cathode  12  includes a substrate membrane  36  extending generally along an x-y plane orthogonal to the z-axis along which the electron beam  24  travels. The membrane  36  may be supported, for example, at its edges by a collar  38  attached to a piezoelectric actuator  40  parallel to the membrane  36  on the opposite side of the membrane  36  with respect to the anodes  16  and driven by a modulation source  42 . 
         [0046]    The modulation source  42  causes z-axis motion of the membrane  36  at ultrasonic frequencies of 50 kilohertz and above and frequencies up to 10 GHz. The effect of this actuation is to change the spacing between the cathode  12  and the grid  26 , thereby modulating the effect of the electrical field of the grid  26  on the cathode  12  and thus changing the velocity of the electrons emitted therefrom and to some extent the emissions from the cathode  12 . 
         [0047]    Referring now to  FIG. 2 , the membrane  36  as supported at its edges by collar  38  for movement along the z-axis may exhibit resonant behavior defined by its geometry, stiffness and distributed mass. As shown in  FIG. 2 , this resonant motion changes the spacing of the cathode  12  to the grid  26  from a minimum value of  39  to a maximum value  39 ′ that may exceed the actual motion of the actuator  40 . Further, and referring momentarily to  FIG. 5 , this resonant behavior allows, for example, the actuator to operate at a first frequency f 0  and for motion of the membrane  36  to follow a harmonic f 2  and thus to modulate the electron beam at frequencies much exceeding those obtainable by the actuator  40 . 
         [0048]    Referring now to  FIGS. 3 and 4 , the surface of the membrane  36  facing the grid  26  may be populated with a set of pillars  50  extending outward from the surface of the membrane  36 , along the z-axis. The pillars  50  are nanostructures having, for example, diameters less than 1000 nanometers and typically less than tens of nanometers at their tips, and heights many times their diameters. The small size of the tips of the pillars  50  produce field emissions that differ from those predicted by the classical Fowler-Nordheim model, as described in D. V. Scheible et al., Physical Review Letters vol. 93, 186801 (2004) hereby incorporated by reference. 
         [0049]    The membrane  36  and pillars  50  may be fabricated using integrated circuit techniques (e.g. lithography) or growth of nanostructures, for example carbon nanotubes, at catalysts deposited on the membrane  36  at regular locations. Two techniques for fabrication are described in U.S. Pat. Nos. 6,946,693 and 6,858,521 hereby incorporated by reference. A high emissivity capping material  52  may be placed at the tips of the pillars  50 , for example, gold, diamond, or semiconductor materials, to improve their emission qualities. 
         [0050]    Referring now to  FIG. 4 , the pillars  50  may be located to align axially (at rest) with corresponding apertures  28  in the grid  26  so that the grid  26  may pass electrons from the tips of the pillars  50  through the apertures without striking the grid  26  and providing unnecessary heating of the grid  26 . Control of the grid voltage, may nevertheless be used to control the velocity and/or current of the electron beam  24 . 
         [0051]    Referring still to  FIG. 4 , like the membrane  36 , the pillars  50  may exhibit their own resonant behavior, vibrating in one or more modes along the x-y plane, for example between locations  54 . Referring again to  FIG. 5 , the smaller size of the pillars  50  allow them to resonate at a higher harmonic, for example, f 4  of the actuator frequency f 0 , so that frequencies in excess of 100 megahertz and as much as several terahertz may be obtained. 
         [0052]    The motion of the pillars  50  changes their alignment with respect to the apertures  28  in the grid  26  and the relative field strength of the grid field on their tips. This change in field strength also modulates the electron velocity and/or current from the pillars  50  and thus the motion of the tips of the pillars  50  with respect to the apertures provides additional modulation or the principal modulation of the electron beam. 
         [0053]    Referring now to  FIG. 6 , the present device is well adapted to fabrication using integrated circuit techniques. In such an integrated device, the cathode  12  may be fabricated of a doped semiconductor substrate with pillars  50  formed by lithographic techniques and the actuator  40  bonded to the bottom surface of the substrate. An insulating spacer layer  62  may be bonded to the upper surface of the substrate of the cathode  12  and used to space a grid  26  from the cathode  12 , the latter which may be etched to form apertures  28  aligned with the pillars  50  and then metallized or doped to provide conductivity. A second spacer layer  60  may then be used to create the drift space  14  and to support a conductive anode  16 . A cavity  32  etched in the spacer layer  60  provides an output for the klystrode  10 . 
         [0054]    In an alternative embodiment, the pillars  50  may incorporate multiple quantum wells, for example, by layering materials along the axis of the pillars  50 , to produce a quantum resonant tunneling device in which extremely low field emissions occur at non-resonant voltages and large field emissions occur at resonant voltages. These selective emissions characteristics could enable ultra low noise field emission currents by setting the DC electric field between the tips of the pillars  50  and grid  26  (when the pillars  50  are at rest) just below a resonant voltage thereby producing a very low “dark” current. Ultrasonic excitation would then move the tips of the pillar  50  into a field that provides a resonant voltage allowing precisely modulated field emissions with low noise. 
         [0055]    Another possibility is that of using phonon or photon assisted tunneling (PAT) through the quantum wells of the pillars  50  as controlled by a coupled piezoelectric actuator  40  or a stimulating light source. This mechanism as detected in quantum dots is described in H. Qin et al., Physical Review B vol. 64, R241302 (2001) hereby incorporated by reference. 
         [0056]    An individual piezoelectric actuator  40  could be associated with each pillar  50  or each small group of pillars  50  in order to provide individual control of the field emissions of the pillars or groups, for example, to realize uniform field emission across the cathode area. In one embodiment the pillars  50  may be placed on top of a piezoelectric substrate such as quartz or the piezoelectric substrate may be etched or formed directly to produce the pillars  50 . 
         [0057]    It will be understood that these techniques may be used with other traveling wave type tubes such as klystrons and, in fact, with other vacuum tube-type devices such as triodes in which directed mechanical modulation may be practical for nanoscale-sized structures. In the klystrode and triode, the grid may be held at a constant voltage or modulated to augment the mechanical modulation of the cathode. Clearly in these devices, the grids could also be mechanically modulated or another field generating structure could be modulated including the anode. Modulation of the pillars may be used alone and promoted by an actuator connection providing movement not in the z-axis but in the x or y-axis. 
         [0058]    It is specifically intended that the present invention not be limited to the embodiments and illustrations contained herein, but include modified forms of those embodiments including portions of the embodiments and combinations of elements of different embodiments as come within the scope of the following claims.