THE PLANAR DEFLECTRON

Geoffrey H. Grayer BSc PhD

A micro-engineered amplifier and logic device.

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

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.

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

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.

How it evolved

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

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 !

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 (R=dVg/dIa)

Having fixed the current one cannot amplitude modulate the electron beam. The solution is to deflect it - hence the name Deflectron!

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)

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

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

Schematic Diagram

A1, A2: anodes D1, D2: deflector F1: focus H F2: focus V F3:focus H E: extractor K: field emitting

cathode

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 = 1~10GHz) 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

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

Deflectron as Logic Switch

0

1

2

3

4

5

6

-10 -5 0 5 10

Deflecting Potential

An

od

e C

urr

en

t

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

Deflectron as Amplifier

0

1

2

3

4

5

6

-20 -10 0 10 20

Deflector Volts

Gain of the Deflectron

The 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.)

Unique Special FeaturesThis 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

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

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

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

Seed funding required to produce proof-of-principle demonstrator

Estimated £70k

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