THE PLANAR DEFLECTRON

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THE PLANAR DEFLECTRON

• Geoffrey H. Grayer BSc PhD
• A micro-engineered amplifier and logic device.

2
Contents

• Introduction - what is planar deflectron?
• Principles of operation Switch and Amplifier
• Evolution
• Planar Construction and Fabrication
• Unique special features
• IPR position
• Seed funding objectives
• Summary

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Introduction

• The Deflectron idea is being developed at the
  Rutherford Appleton Laboratory by a collaboration
  between the Detector Group of the Instrumentation
  Department and the Central Microstructure
  Facility.
• I was the instigator of the idea, but it is of
  course a team effort, with particular thanks to
  Bob Stevens, Zheng Cui and Ejaz Huq.

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What it is -

• A vacuum device (like a sub-miniature tube/valve)
  which acts as an amplifier or logic element
• It uses field emission instead of a heated
  cathode
• It deflects the electron path rather than trying
  to modulate the current (like a television tube)
  Deflectron
• It is constructed in one plane for simplicity and
  connectivity Planar

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Principle of Operation

• Extractor produces high electric field on cathode
• Hence cathode emits electrons according to the
  Field Effect
• A triplet of focus electrodes alter final spot
  size
• Signal applied to deflection plates move beam
• Split anode collects electrons
• Conventional construction would work from the tip
  up, using cylindrical symmetry
  
  
  

Model by Z. Cui showing electron tracks for one
set of parameters.
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How it evolved

• For at least 20 years people have been trying to
  realise a field-effect amplifier
• Usually an attempt has been made to produce the
  analogy of a triode vacuum tube with a 3-D
  cylindrical shape (see fig)
• They have not worked because the potential of
  electrons produced by a field emitter is
  typically x100 that of a thermionic emitter,
  reducing gain by that amount. Hence typical
  valve stage gain 30 becomes 0.3 !

A scanning electron microscope picture of a
conventional 2- gate field emitter. This
represents state of the art (photo CLRC)
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The Noise Issue

• The variation in emission current is typically
  10 from a field emitter. This is noise
  superposed on the signal, and would make any
  amplifying application untenable
  
  
• My solution to this is to create 100 feedback by
  adding a suitable resistor in the cathode circuit
  (RdVg/dIa)
• Having fixed the current one cannot amplitude
  modulate the electron beam. The solution is to
  deflect it - hence the name Deflectron!
  
  
  

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Conventional 3D Cylindrical Construction

• 3D device fab would be extremely complex,
  requiring many processes to build up the
  structure
• This gets even more difficult when the focus and
  deflection electrodes are added, breaking
  cylindrical symmetry.
• When anodes added on top, closed volume is almost
  impossible to evacuate.
• They are difficult to connect with electrodes all
  at different levels

A scanning electron microscope picture of a
conventional 2- gate field emitter. This
represents state of the art (CLRC device)
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Planar Construction

• To see how the planar device evolved, imagine a
  slice taken through the middle of the cylindrical
  device.
• Construction is then simple lithography of metal,
  plus the tip fabrication.

Simplified layout of Deflectron
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Fabrication issues

• Single layer device -planar
• Critical process is production of pointed
  cathode emitter in plane
• Principle yield defining step
• Direct EBL defines point in plane
• Plasma etch defines point at right angles to
  plane
• All other fabrication processes are
  conventional VLSI patterning in refractory
  metal

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Schematic Diagram

• A1, A2 anodes
• D1, D2 deflector
• F1 focus H
• F2 focus V
• F3focus H
• E extractor
• K field emitting cathode

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Characteristics of Planar Deflectron

• Can work as amplifier or logic element
• Amplifier has gain/dynamic range trade-off, set
  by voltage parameters
• Small size means very fast (fo 110GHz)
• Complementary signals in/out with high common
  mode rejection - benefit is external noise/spike
  rejection
• Coupling can be DC as only reacts to voltage
  difference benefit - broadband operation to10GHz
• Power consumption very low and independent of
  speed or state
• Simple few-step fabrication

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The Deflectron as Logic Element

• Focused to a spot, the current rapidly changes
  from one anode to another, producing the ideal
  binary switch
• These switches may be connected in series to
  carry out the usual Boolean logic functions
• A tri-state device may be produced by introducing
  a third central anode which is connected straight
  to V. Thus devices can be multiplexed to the
  same bus.

Tri-state device
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The Deflectron as Amplifier

• Defocused at the anode, the current varies slowly
  as the beam is deflected, resulting in a analogue
  amplifier
• By varying the spot size, gain is traded against
  dynamic range, according to application

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Gain of the DeflectronThe Gain of the
Deflectron depends on

• 1/ extractor voltage (stiffness)
• integrated deflecting E-field
• Length of deflector plates
• 1/ Spacing of deflector plates
• drift length (lever arm)
• spot size (adjustable by focus)
• (In principle, there is no limit on the drift
  length.)

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Unique Special Features

• This is not expected to replace the ubiquitous
  transistor, but to go where no transistor dares!
• It will work at very high temperatures - eg NASA
  Venus lander ? (max. is melting point!)
• It will work at temperatures approaching absolute
  zero - eg quantum microdot readout
• (This could be important commercially)
• It will work in very high radioactivity - eg
  particle physics experiments, nuclear industry
• It has high noise rejection because of
  complimentary in/out signals

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IPR The idea of deflecting electrons is not new

• It looks like a miniature electrostatic CRT, with
  the screen replaced by anodes
• Deflection - modulated electronic tubes (valves)
  were made and patented by RCA in the 1960s. Their
  high dynamic range made them excellent mixers.
• Its use as a vacuum micro-electronics device for
  generating microwaves has been suggested
• However, this particular application and the
  planar construction are novel

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IPR position

• No filings submitted to-date
• Public disclosure in scientific
  publication Technical Digest, 10th
  International Vacuum Microelectronics Conference,
  August 17-21,1997, Kyongju, S. Korea, p.206,
  G.H.Grayer, S.E.Huq, Z.Cui, P.D.Prewett
• If go-ahead, need urgent review of
• What may be protected
• Search of existing IPR in the field

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Seed funding objectives

• Identify complete PCT patent applications to
  secure IPR of embodiment fabrication processes
  
• Define characterise pointed cathode process
• Demonstrate field emission from pointed cathode
  incorporation in planar device structure
• Production of planar deflectron demonstrator
• Characterisation of complete fab process steps
• Electrical characterisation of
  proof-of-principle demonstrator

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Seed funding required to produce
proof-of-principle demonstrator

• Estimated 70k

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Summary

• The Deflectron is
• A very wide band amplifier or switch (0 to many
  GHz)
• Capable of operating over extremes of temperature
  (cryogenic to high temperatures)
• Extremely radiation hard
• Extremely resistant to EMI
• Very rugged
• Simple and hence cheap to construct ( high
  yield)
• Could be realised with a relatively low capital
  investment