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. 1
Introduction 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 obtained by electrons tunneling through barrier reduced by externally applied electric field on cold metals). 2
Introduction 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\V 2 } vs. 1\V Expected shape of F-N plots: Straight line. The F-N plots were used mainly to derive characteristics from experimental data. 3
Aims of this work 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 = 2 πr 2 . Study the effects of the conditioning procedures, i.e. thermal treatment & relaxation process on the behavior of the field electron emission. 4
Materials & Methods high melting I-V Characteristics Field Emission point of 3650 K, Microscopy 3377˚ C (FEM) Emission current images High mechanical Tungsten and thermal Methods (W) strength Transmission Electron Microscopy (TEM) simplicity of preparing an Scanning Electron emitter Microscopy (SEM) 5
Preparation of sharp tips from tungsten wire (B) (A) The cut-off process The electrolytically etching process 6
Experimental Experimental conditions: Initial thermal treatment (heating) Baking at 200 ˚C overnight. Relaxation process leaving the system under the UHV only overnight. Re-baking at 200 ˚C overnight. Follow up thermal treatment 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). 7
Components of the field emission microscope Turbo pump 8
I-V characteristics & F-N plots 45 45 EMISSION CURRENT I [NA] Relaxation 40 Initial heating EMISSION CURRENT I [NA] 40 35 35 30 30 I-V plot 25 25 20 20 15 15 10 10 5 5 0 0 2000 2500 3000 3500 4000 2500 3000 3500 4000 APPLIED VOLTAGE U [V] APPLIED VOLTAGE U [V] -14 -13 Slope= 14085 Slope= 23067 LOG(I/U^2[NA/V^2]) -15 -14 LOG(I/U^2[NA/V^2]) -15 -16 -16 -17 F-N plot -17 -18 -18 -19 -19 0.2 0.3 0.4 0.5 0.2 0.3 0.4 0.5 1000/U[V -1 ] 1000/U[V -1 ] 9
I-V characteristics & F-N plots 45 45 EMISSION CURRENT I [NA] Relaxation 40 Re-heating EMISSION CURRENT I [NA] 40 35 35 30 I-V plot 30 25 25 20 20 15 15 10 10 5 5 0 0 2000 2500 3000 3500 4000 2600 3100 3600 4100 APPLIED VOLTAGE U [V] APPLIED VOLTAGE U [V] -13 -14 Slope= 14085 Slope= 20544 -14 LOG(I/U^2[NA/V^2]) LOG(I/U^2[NA/V^2]) -15 -15 -16 -16 -17 F-N plot -17 -18 -18 -19 -19 0.2 0.3 0.4 0.5 0.2 0.25 0.3 0.35 0.4 1000/U[V -1 ] 1000/U[V -1 ] 10
Electron Emission Images Typical multi spot Emission image with Low brightness image image with one bright two bright spots at V ~ 3900V spot at V ~ 4050V at V ~ 4050V b) c) a) Bright.1 Bright Not bright Bright.2 Electron emission images taken at same current I ~ 5x10 -8 A, a) Initial heating, b) Relaxation, c) Re-heating 11
Transmission & Scanning Electron Microscopes TEM SEM 12
TEM & SEM Micrographs (a) (b) TEM image at magnification of 30.000X SEM image at magnification of 40.000X 13
TEM Micrograph TEM micrograph at 30.000X Irregular shape Surface contamination 14
SEM Micrograph SEM micrograph at 40.000X Radius ; r SEM 15
Results Calculations 350 r SEM [nm] r FEM [nm] α Tip # 300 1.16 × 10 -4 W.1 93 108 250 1.69 × 10 -4 W.2 100 115 r FEM [nm] 200 5.59 × 10 -4 W.3 139 183 150 6.94 × 10 -4 W.4 145 187 100 1.30 × 10 -3 W.5 191 241 50 2.05 × 10 -3 W.6 215 278 0 0 50 100 150 200 250 300 2.21 × 10 -3 W.7 220 288 r SEM [nm] * We notice a good agreement between both extracted methods (SEM & FEM), and this support the relation 𝒔 𝑮𝑭𝑵 ≈ 𝟐. 𝟒𝟔 × 𝒔 𝑻𝑭𝑵 − 𝟑𝟏 𝒐𝒏. 16
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. 17
Applications Flat panels display, such as TV’s & Computer monitors: 1. Faster response time. 2. Higher contrast. 3. Less power needed. Ele ctron microscopes, such as TEM, SEM & STEM: 1. Higher image resolution. 2. Higher beam density. 18
Future Work Study and analyze the tested samples under different vacuum conditions. Coating the tested samples with epoxy resin, to investigate the effects of 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) 19
Results Presentation 20
Results Presentation 21
Results Presentation 22
Th Than ank yo you for u for at atten tentio tion 23
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