DiamondBased Sub Millimeter Backward Wave Oscillator

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Diamond-Based Sub Millimeter Backward Wave
Oscillator

• Presented at Jet Propulsion Laboratory
• July 7, 2003
• James A. Dayton, Jr.
• Director of Technology
• Teraphysics Corporation

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Whats New

• Novel fabrication method CVD diamond is
  deposited in a lithographically constructed
  silicon mold.
• Novel slow wave circuit The biplanar
  interdigital circuit
• Novel electron gun The gun is manufactured as
  an integral part of the slow wave circuit, not
  attached later.
• Novel application Use of solid state
  fabrication techniques not previously applied to
  vacuum electron devices
• Novel application Use of precision assembly and
  fabrication techniques commonly found in
  fabrication of liquid crystals, but not
  previously applied to vacuum electron devices

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Potential Applications

• Exoatmospheric spectroscopy for the study of sub
  mm radiation sources in space
• Remote sensing of industrial gaseous pollutants
  in smokestacks, tail pipes etc.
• Remote detection of biological warfare agents
• Broadband wireless interconnections

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The Team

• Dr. James Dayton, Teraphysics Director of
  Technology project conception, computer
  modeling
• Dr. Hsiung Chen, Teraphysics Chief Scientist
  diamond fabrication
• Dr. Gerald Mearini Teraphysics CEO, diamond
  technology
• Dr. Carol Kory, consultant computer modeling
• Dr. Christian Zorman, consultant silicon
  fabrication
• Dr. Donald Davis, advisor liquid crystal
  technology

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Diamond structure after removal of silicon mold
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3-D view of biplanar interdigital circuit
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Circuit Schematic and Dimensions


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Dispersion of Biplanar Interdigital Circuit
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Effect of Beam Tunnel Height on Impedance
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Effect of Varying Diamond Height
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Results of Preliminary Parameter Sweep

• All parameters were varied one at a time to
  determine sensitivity.
• Interaction efficiency is most sensitive to the
  beam tunnel height (ygap).
• Reduction in ygap results in increased
  interaction impedance, but may also result in
  increased beam interception.
• Diamond height (diht) determined to be compatible
  with transverse dimension of electron gun,
  eliminating need for an additional masking and
  etching step.

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Fabrication Process I

• SOI wafers are used to provide precise etch
  stops.
• A negative of the desired diamond shape is etched
  into the SOI wafers.
• One half of the structure is formed in a simple
  one etch step the other half utilizes a two
  layer SOI wafer to create the spacing needed to
  form the beam tunnel.
• Diamond is deposited in the silicon molds using
  chemical vapor deposition.
• The back of the diamond is reinforced
  structurally with an epoxy layer the silicon is
  then etched away.
• The diamond surface is masked and a gold layer is
  deposited to form the circuit, the gun electrodes
  and the electrical connections

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Fabrication Process II

• Utilizing micropositioning tools developed in the
  liquid crystal industry, the two halves of the
  BWO are brought together precisely and bonded
  using high tack, low outgassing glue developed
  for the purpose.
• The glue is applied using a silk screen or offset
  printing process.
• The bodies of approximately 40 BWOs can be
  fabricated at one time from a 100 mm wafer.
• The individual BWO bodies are separated from the
  bonded wafer with a laser cutting tool. The edge
  left by the laser does not provide the same
  precision as the lithographic processes, but it
  is not utilized as a reference dimension.

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Fabrication Process III

• A field emission cathode that has been accurately
  diced from its wafer is inserted into the gun end
  of the BWO body.
• The lithographically determined transverse
  dimensions of the BWO body serve to align the
  cathode.
• The focus electrode of the gun makes contact with
  the gate of the field emitter. The back of the
  field emitter is the base connection.
• A POCO graphite collector is mounted on the other
  end of the BWO body. This may consist of two
  POCO plates that are held at 10 of the cathode
  to circuit voltage and also at a slight potential
  difference from each other to suppress secondary
  emission.

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Fabrication Process IV

• RF output is carried by a planar waveguide that
  is formed in the circuit lithographic process.
  This may feed a lens to connect to a quasi
  optical transmission line. This will be designed
  in Phase II.
• The first experimental BWOs will be operated in
  vacuum systems. The goal is to create a vacuum
  tight diamond walled structure.
• The magnetic circuit is expected to be formed by
  two rectangular bar magnets with iron pole pieces
  at each end and held together with an aluminum
  framework. This will be designed in Phase II

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300 GHz Circuit Dimensions

• Parameter
• Vaneridge
• Vanew
• Vanel
• Vaneth
• Diridge
• P
• Xs
• Zs
• Diht
• Ridgeht
• Ygap

• 10 BW 20 BW
• 44.0 44.0
• 17.2 16.4
• 183.4 175.0
• 4.0 4.0
• 87.5 87.5
• 34.4 32.8
• 17.2 16.4
• 22.3 21.3
• 83.5 83.5
• 23.0 23.0
• 25.0 25.0

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Relevant portion of the dispersion diagram
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300 GHz On-Axis Impedance
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Impedance for a 12.5 micron beam averaged over
the beam width for the 10 design
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Field intensity as a function of transverse
position for BetaL 100 degrees
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300 GHz Circuit Attenuation
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Electronic Efficiency Dependence on Circuit
Length
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300 GHz Start Oscillation Current
(Gewartowski/Grow)
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300 GHz Output Power
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300 GHZ Design Study Results

• Reasonably uniform output power predicted over
  both 10 and 20 bandwidths
• Field configuration favorable for application of
  sheet electron beam
• Higher output power and efficiency can be
  obtained with shorter circuit, but this requires
  higher start oscillation current
• Higher interaction impedance and higher
  attenuation compete at high end of frequency band

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Typical Power Balance

• Power output 25 mW
• RF circuit losses 50 mW
• Beam interception 1
• 27 mW for 1.8 kV case
• 90 mW for 6 kV case
• Collector Dissipation (90 efficiency)
• 260 mW for 1.8 kV case
• 883 mW for 6 kV case
• Total power dissipated
• 337 mW for 1.8 kV case
• 1.023 W 6 kV case

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300 GHz Electron Gun, Magnetic Focusing and
Collector, 1.8 kV (low frequency) Case
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300 GHz Electron Gun, Magnetic Focusing and
Collector, 6 kV ( high frequency) Case
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Electron Gun Design Parameters

• Beam transmitted through 25 micron beam tunnel
• Start oscillation current requirements for 5 mm
  circuit length achieved
• Spindt type field emission cathode 2X50 tip
  array on 1.5 micron centers, 15 µA/tip, 1.5 mA
  total, VGB 112 V
• Transverse energy distribution based on
  Jensen/Whaley model, Gaussian distribution with
  FWHM of 8.84 V
• Beam envelope contains 99 of current
• Beam current can be doubled by increasing beam
  width without increasing current density or
  magnetic field
• Surface electric field in gun 8 kV/cm
• First anode 1.8 kV. Second anode sweeps 1.8 to
  6.0 kV

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Magnetic Circuit Design Parameters

• Uniform magnetic field of 5000 Gauss
• Magnetic circuit as modeled weighs 64 gm
• Magnetic circuit to be redesigned in Phase II
  with 3-D code
• Sheet beam is order of magnitude below threshold
  for dicotron instability (Basten, private
  communication)

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Effect of Axial Misalignment
 

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Effect of Rotational Misalignment
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600 GHz Circuit Dimensions

• Parameter
• Vaneridge
• Vanew
• Vanel
• Vaneth
• Diridge
• P
• Xs
• Zs
• Diht
• Ridgeht
• Ygap

• 10 BW 20 BW
• 22.0 21.9
• 8.6 8.2
• 91.7 87.5
• 2.0 2.0
• 43.8 43.8
• 17.2 16.4
• 8.6 8.2
• 11.2 10.7
• 41.8 41.8
• 11.5 11.5
• 12.5 12.5

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Dispersion, Phase Velocity, Attenuation and
Impedance for the 600 GHz Cases
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Start Oscillation Current at 600 GHz
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600 GHz Electronic Efficiency
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600 GHz Output Power
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600 GHz Electron Gun, 6.4 kV Case (high
frequency)
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600 GHz Electron Gun, 1.8 kV Case (low
frequency)
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Results of 600 GHz Modeling

• 600 GHz case is readily scalable from 300 GHz
  design.
• Dimensions of circuit are no more difficult to
  achieve than 300 GHz.
• Electron gun and focusing system can achieve
  required 2X start oscillation current.
• No margin for higher beam current without
  increase in cathode current density or reduction
  in transverse velocities.
• Magnetic focusing field much higher than 300 GHz
  case, but is achievable.
• More accurate fabrication and positioning
  tolerances required.