Ad Advan vanced ced Ti Tip Shap p Shape e An Analy alysis - - PowerPoint PPT Presentation

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Ad Advan vanced ced Ti Tip Shap p Shape e An Analy alysis sis fo for Tu r Tung ngsten sten Mi Microemitt croemitters ers St Stud udent: ent: Ma Mazen n Ad Adel l Ma Mada dana nat t 1 Su Supe pervi rvisor: sor:

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SLIDE 1

Ad Advan vanced ced Ti Tip Shap p Shape e An Analy alysis sis fo for Tu r Tung ngsten sten Mi Microemitt croemitters ers

St Stud udent: ent: Ma Mazen n Ad Adel l Ma Mada dana nat t 1 Su Supe pervi rvisor: sor: Pr Prof.

  • f. Ma

Marwa wan n S.

  • S. Mo

Mous usa a 1 Co Co-Super Supervi visor: sor: Pr Prof.

  • f. An

Anas as N. Al Al-Rabadi abadi 2

  • 1. Mu‘tah University,

, Dept.

  • t. of Ph

f Physics sics.

  • 2.

2. Jo Jordan an Un Univ iversity rsity, , Dept. . of Co f Compu pute ter r Engin ineering. ering.

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Introduction

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 Field electron emission, the phenomena of releasing electrons from material after gaining energy.  Most common methods of electron emission are:

  • 1. photo emission (electrons gaining energy from light).
  • 2. Thermionic emission (electrons gaining energy from heat).
  • 3. Secondary electron emission (electrons gaining kinetic energy from another

primary electron).

  • 4. Cold field electron emission (no energy required because electron emission is
  • btained by electrons tunneling through barrier reduced by externally applied

electric field on cold metals).

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SLIDE 3

Introduction

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 Cold field electron emission (CFE), is the phenomenon of electrons emission from pointed spherical surface into the vacuum by applying intense electrical field to the specially prepared pointed surface.  Most common data representation: Fowler-Nordheim (F-N) plot log {i\V2} vs. 1\V  Expected shape of F-N plots: Straight line.  The F-N plots were used mainly to derive characteristics from experimental data.

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SLIDE 4

Aims of this work

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 Study the relation between two different apex radii extracted methods, namely the scanning electron microscope (SEM) imaging and analyzing F-N plots.  Calculate the area efficiency factor (α), which is the ratio between the actual emission area required to generate the current I according to the F-N theory and the area of the hemispheric emitter model A = 2πr2.  Study the effects of the conditioning procedures, i.e. thermal treatment & relaxation process on the behavior of the field electron emission.

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Materials & Methods

Methods

Field Emission Microscopy (FEM)

I-V Characteristics Emission current images

Transmission Electron Microscopy (TEM) Scanning Electron Microscopy (SEM)

Tungsten (W)

high melting point of 3650 K, 3377˚ C High mechanical and thermal strength simplicity of preparing an emitter

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SLIDE 6

Preparation of sharp tips from tungsten wire

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(B)

The cut-off process

(A)

The electrolytically etching process

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SLIDE 7

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Experimental

Experimental conditions:

 Initial thermal treatment (heating) Baking at 200˚C overnight.  Relaxation process leaving the system under the UHV only overnight.  Follow up thermal treatment Re-baking at 200˚C overnight.  The field emission characterization were recorded, namely the current-voltage (I-V) characteristics, Fowler-Nordheim (FN) plots and the electron emission images after each process under ultra high vacuum (UHV) conditions with pressure of (P ~ 10-9 mbar).

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SLIDE 8

Components of the field emission microscope

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Turbo pump

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0.2 0.3 0.4 0.5

LOG(I/U^2[NA/V^2]) 1000/U[V-1]

Slope= 14085 5 10 15 20 25 30 35 40 45 2500 3000 3500 4000

EMISSION CURRENT I [NA] APPLIED VOLTAGE U [V]

Initial heating

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0.2 0.3 0.4 0.5

LOG(I/U^2[NA/V^2]) 1000/U[V-1]

Slope= 23067 5 10 15 20 25 30 35 40 45 2000 2500 3000 3500 4000

EMISSION CURRENT I [NA] APPLIED VOLTAGE U [V]

Relaxation

I-V characteristics & F-N plots

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I-V plot F-N plot

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SLIDE 10

5 10 15 20 25 30 35 40 45 2600 3100 3600 4100

EMISSION CURRENT I [NA] APPLIED VOLTAGE U [V]

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0.2 0.25 0.3 0.35 0.4

LOG(I/U^2[NA/V^2]) 1000/U[V-1]

Slope= 20544

Re-heating

I-V characteristics & F-N plots

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I-V plot F-N plot

5 10 15 20 25 30 35 40 45 2000 2500 3000 3500 4000

EMISSION CURRENT I [NA] APPLIED VOLTAGE U [V]

Relaxation

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0.2 0.3 0.4 0.5

LOG(I/U^2[NA/V^2]) 1000/U[V-1]

Slope= 14085

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SLIDE 11

Electron Emission Images

Electron emission images taken at same current I ~ 5x10-8 A, a) Initial heating, b) Relaxation, c) Re-heating

Typical multi spot image with one bright spot at V ~ 4050V Emission image with two bright spots at V ~ 4050V Low brightness image at V ~ 3900V

Bright Bright.2 Bright.1

a) c) b)

Not bright

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SLIDE 12

Transmission & Scanning Electron Microscopes

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TEM SEM

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SLIDE 13

(a)

TEM image at magnification of 30.000X

TEM & SEM Micrographs

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SEM image at magnification of 40.000X

(b)

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SLIDE 14

TEM Micrograph

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TEM micrograph at 30.000X

Irregular shape Surface contamination

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SLIDE 15

SEM micrograph at 40.000X

Radius; rSEM

SEM Micrograph

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Results

Calculations

Tip #

rSEM [nm] rFEM [nm] α

W.1 93 108 1.16 × 10-4 W.2 100 115 1.69 × 10-4 W.3 139 183 5.59 × 10-4 W.4 145 187 6.94 × 10-4 W.5 191 241 1.30 × 10-3 W.6 215 278 2.05 × 10-3 W.7 220 288 2.21 × 10-3

* We notice a good agreement between both extracted methods (SEM & FEM), and this support the relation 𝒔𝑮𝑭𝑵 ≈ 𝟐. 𝟒𝟔 × 𝒔𝑻𝑭𝑵 − 𝟑𝟏 𝒐𝒏.

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50 100 150 200 250 300 350 50 100 150 200 250 300 r FEM [nm] r SEM [nm]

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SLIDE 17

Summary

 The apex radii extracted by both SEM & FEM found to be in good agreement.  The calculated area efficiency factor (α) found to be in the expected range ( ~ 10-3).  The performance of the W microemitters was found to be dependent on

the conditioning procedures:

  • Produced good stability in the emission current.
  • Enhanced the electron emission image.

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Applications

 Flat panels display, such as TV’s & Computer monitors:

1. Faster response time. 2. Higher contrast. 3. Less power needed.

 Electron microscopes, such as TEM, SEM & STEM:

1. Higher image resolution. 2. Higher beam density.

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Future Work

 Study and analyze the tested samples under different vacuum conditions.  Coating the tested samples with epoxy resin, to investigate the effects

  • f the coating on the behavior of the emission current.

 Compare the results obtained in this work on tungsten microemitters with results obtained from different emitters materials (Carbon fiber)

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Results Presentation

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Results Presentation

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Results Presentation

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Th Than ank yo you for u for at atten tentio tion

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