diamond field emitter arrays on micromachined silicon l.
Download
Skip this Video
Loading SlideShow in 5 Seconds..
Diamond Field Emitter Arrays on Micromachined Silicon PowerPoint Presentation
Download Presentation
Diamond Field Emitter Arrays on Micromachined Silicon

Loading in 2 Seconds...

play fullscreen
1 / 45

Diamond Field Emitter Arrays on Micromachined Silicon - PowerPoint PPT Presentation


  • 233 Views
  • Uploaded on

Diamond Field Emitter Arrays on Micromachined Silicon. Dr. Wehai Fu Dr. Sacharia Albin. Nano Science & Engineering Lab ECE Old Dominion University Norfolk, Virginia 23529. Outline . I. Introduction. Field emission and applications Advantages of diamond field emitters Project goal.

loader
I am the owner, or an agent authorized to act on behalf of the owner, of the copyrighted work described.
capcha
Download Presentation

PowerPoint Slideshow about 'Diamond Field Emitter Arrays on Micromachined Silicon' - jana


Download Now An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.


- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript
diamond field emitter arrays on micromachined silicon

Diamond Field Emitter Arrayson Micromachined Silicon

Dr. Wehai Fu

Dr. Sacharia Albin

Nano Science & Engineering Lab

ECE

Old Dominion University

Norfolk, Virginia 23529

slide2

Outline

I

Introduction

Field emission and applications

Advantages of diamond field emitters

Project goal

Field emission

II

Fowler-Nordheim field emission

Field emission enhancement

III

Experiments

Silicon tip array fabrication

Diamond emitter fabrication

Results and discussion

IV

Silicon tip array wet etching

Diamond film characterization

Diamond emitter I-V characteristics

Summary

V

slide3

I. Introduction

Electron emission mechanisms

Surface barrier bending

due to applied field

Surface barrier

Thermal excitation

Photo excitation

Tunneling

Advantages of field emission

Low energy consumption

High current density

Small device size (sharp tips)

slide4

I. Introduction

Field Emission Applications

Vacuum microelectronic device

Flat panel display

Conductive

coating

Anode

Phosphor

5 mm

Gate

Probe

Gate

Emitter

Emitter array

Scanning probe microscope

for surface imaging

Pressure

E-beam

Focusing

electrode

Extracting

gate

d

Emitter array

Microsensor

Cold cathode

slide5

I. Introduction

Requirements for applications:

Low turn-on voltage

Large emission current

Uniform over the array

Stable emission

Drawbacks of metal and silicon emitters:

Anode

High extracting field

Sputtering damage

Surface adsorption

Emitter overheating

Emitter

slide6

I. Introduction

Diamond Field Emitter

Properties of diamond

Advantages for field emission

Hardest known material

10,400 kg/cm2

Resistance to sputtering damage by residual gas

Highest thermal conductivity

20 W/cm-oC

5 times larger than copper

Efficient and fast thermal dissipation

Robust performance in harsh environment

Chemically inert

Small effective work function

Low threshold voltage for electron emission

slide7

I. Introduction

Research Developments

Year

Highlights

1928 Field emission theory proposed (Fowler and Nordheim)

1968 First metal field emitter with self-aligned gate demonstrated (Spindt)

1976 Enhanced chemical vapor deposition (CVD) diamond developed (Deryagin)

1979 Negative electron affinity (NEA) of diamond discovered (Himpsel)

1990 Silicon field emitter array with self-aligned gate developed (Betsui)

1994 Mold-filled CVD diamond tip array fabricated (Okano)

1997 Diamond film coated silicon sharp tips demonstrated (Zhirnov)

1997 Diamond tips for tunneling microscopy developed (Albin)

1999 Diamond field emitter arrays with self-aligned gate succeeded (Albin)

slide8

I. Introduction

Project Goal

Design, fabricate, and characterize diamond

field emitter arrays using:

Silicon surface micro-machining

Plasma enhanced CVD diamond

Scanning electron microscopy

Raman spectroscopy

Optical emission spectroscopy

I-V measurement

slide9

II. Field Emission

Surface Energy Barrier#

Combined effect

Applied Field

Image Charge

P(x)

Pe(x)

Pext(x)

Vacuum

Level

Vacuum

Level

Vacuum

Level

x

x

x



Schottky barrier

reduction

f

f

f

Feff

- exF

Ef

Ef

Ef

d

Surface barrier

thinning

#S. O. Kasap, Principles of Electrical Engineering Materials and Devices (McGraw-Hill, 1997).

slide10

II. Field Emission

Field Emission Current Density

T: temperature

F: applied field

: work function

N(T,S): electron density

D(F,s,): tunneling probability

s: kinetic energy

Fowler-Nordheim Equation#

  • m mass of electron
  •  work function of the cathode
  • y function of F and 
  • t(y), v(y) approximated as constants
  • J emission current density
  • e electron charge
  • h Planck’s constant
  • F electric field at cathode

# R. H. Fowler and L. W. Nordheim, Proc. R. Soc. London A119, 173 (1928).

slide11

2 eV

3 eV

4 eV

5 eV

5.5 eV

II. Field Emission

F-N Plot

Simplified F-N Equation#

or

Where:

aemitting area

f emitter work function

 field enhancement factor

I-V Plot

F-N Plot

2 eV

3 eV

4 eV

5 eV

5.5 eV

(400 tip array with a tip radius of 20 nm and a field enhancement factor of 105 cm-1)

#C. A. Spindt, I. Brodie, L. Humphrey, and E. R.Westerberg, J. Appl. Phys.47, 5248 (1976).

slide12

II. Field Emission

Field Enhancement Factor

Anode

= F/ V

r

F: electric field at emitter tip

V:voltage between anode and cathode

d

Spacer

h

Field enhancement factor for typical emitters#

Cathode

r

#H. G. Kosmahl, IEEE Trans. Electron Devices 38 (6), 1534 (1991)

slide13

II. Field Emission

Emitter Structure Effect Simulation

Effect of emitter height

Effect of tip radius

Tip radius (top to bottom)

10 nm

20 nm

40 nm

60 nm

80 nm

100 nm

Tip height (top to bottom)

4 mm

3 mm

2 mm

1 mm

0.5 mm

slide14

III. Experiments

Silicon Tip Array Fabrication

(a) Thermal oxidation

(b) Photolithography

(c) Silicon dioxide etching

(f) Silicon nano tips

(d) Silicon etching

(e) Tip sharpening

slide15

III. Experiments

Diamond Emitter Fabrication

Waveguide

H

CH

2

4

Microwave generator

Quartz window

Pressure Control

Plasma

Gas flow meter

Substrate height

control

Substrate

Gauge

Exhaust

Microwave

power control

Valve

Substrate

heating control

Motor

Pumping System

slide16

III. Experiments

Emitter with Self-aligned Gate

Photoresist

SiO2

Si

Metal

SiO2

1. Thermal oxidation

and patterning

2. Silicon tip etching

3. Photoresist planarization

for oxide and metal layer

5. Expose silicon tip

for seeding

6. Diamond deposition

4. Photoresist

etchback

slide17

IV. Results and Discussion

Orientation Dependent Etching

Some alkaline etchants etch various crystal planes of silicon at different etch rates

<100>

SiO2

<100>

SiO2

<111>

Silicon

Silicon

slide18

IV. Results and Discussion

Silicon Tip Array Wet Etching

Silicon tip array etched at 90oC with various

tetramethylammonium hydroxide (TMAH) concentrations

40% TMAH

25% TMAH

10% TMAH

15 minute etching

Extremely non-uniform

Hillocks on substrate

4 minute pinching under mask

Hillock-free but non-uniform

Small tip aspect ratio

10 minute pinching under mask

Hillock-free and uniform

Larger tip aspect ratio

slide19

IV. Results and Discussion

Optimized Etching Results

2.2 m high and 1.44 m wide

Aspect ratio of 1.53

1.4% non-uniformity

Silicon tip array etched using:

10 m square SiO2 mask

40% TMAH at 90oC

Close-up

Array

slide20

h

d

IV. Results and Discussion

Effect of Etchant Temperature

Aspect Ratio = h/d

Silicon tip array etched in 40% TMAH

at various temperatures

Height h

Silicon tip arrays etched at various

temperatures show similar appearance

Aspect Ratio

Etch rate increases with temperature

Base width d

The slight decrease of aspect ratio shows

the etching selectivity between side planes

and the base decrease with temperature

slide21

IV. Results and Discussion

Effect of Oxidation Sharpening

Short time oxidation

(<60 min.)

60 min.

30 min.

Reduces tip radius from

128 nm to 23 nm in 60 min.

No significant height

change

240 min.

120 min.

Extended oxidation

(>60 min.)

No appreciable change

in tip radius

Tip height decreases

slide22

IV. Results and Discussion

Diamond Film Growth

Optimized process conditions:

35 Torr chamber pressure

600oC substrate temperature

5-30 minutes growth time

Nanocrystal diamond slurry seeding

0.5-2% CH4 in H2

1 kW microwave power

Close-up

Array

slide23

IV. Results and Discussion

Diamond film grown for 30 minutes without bias using various methane (CH4) concentrations

0.5% CH4

1% CH4

2% CH4

slide24

IV. Results and Discussion

Diamond film grown for 30 minutes with -150 V bias using various methane (CH4) concentrations

1% CH4

2% CH4

0.5% CH4

slide25

IV. Results and Discussion

Diamond Growth Rate

Without bias

High CH4 concentration increases

diamond growth rate

Negative bias reduces diamond

growth rate

With bias

slide26

IV. Results and Discussion

Raman Spectroscopy

Diamond film grown without

bias shows a diamond peak

at 1332 cm-1 and sp2non-diamond

carbon around 1500 cm-1

Diamond film grown with

-150 V bias shows no diamond

peak but a broad band amorphous

carbon signal

1332 cm-1

2% CH4

2% CH4

1% CH4

1% CH4

0.5% CH4

0.5% CH4

slide27

IV. Results and Discussion

Diamond/Graphite Ratio

Diamond film grown for 30

minutes without negative bias

D/G ratio decreases as CH4 increases

D/G ratio saturates after 1% CH4

slide28

IV. Results and Discussion

Optical Emission Spectroscopy

H2

CH4

CH2, CH3, CH, C2H2,

Complex diamond growth process

H, C, C2, …

sp3

sp2

Emission intensities of CH (431 nm) and C2 (517 nm)

are directly related to diamond film growth#

Effects of changing methane concentration and negative

bias on diamond film growth can be studied

through CH and C2 emission intensity variations

#M. Marinelli, E. Milani, M. Montuori, A. Paoletti, A. Tebano, G. Balestrino, and P. Paroli

J. Appl. Phys., 76 (1994) 5702.

slide29

CH emission peak

(431 nm)

CH emission peak

(431 nm)

0.5%

0.5%

1%

1%

2%

2%

IV. Results and Discussion

CH Emission Spectra

CH emission intensity increases with methane concentration

Without bias

Negative 150 V bias

slide30

IV. Results and Discussion

C2 Emission Spectra

C2 emission intensity increases with methane concentration

Without bias

Negative 150 V bias

C2 emission peak

(517 nm)

C2 emission peak

(517 nm)

0.5%

1%

0.5%

2%

1%

2%

slide31

Negative bias

Without bias

Without bias

Negative bias

IV. Results and Discussion

Emission Intensity Variation

CH intensity increases significantly with negative bias

Negative bias has no effect on C2 intensity

Change in CH intensity is correlated with Raman signal

for biased growth

C2 intensity variation

CH intensity variation

slide32

2%

1%

0.5%

IV. Results and Discussion

Field Emission Characteristics

Diamond coated silicon tip array

5 minute film growth

Without bias

F-N plot

I-V plot

2%

1%

0.5%

slide33

2%

1%

0.5%

2%

1%

0.5%

IV. Results and Discussion

Field Emission Characteristics

Diamond coated silicon tip array

10 minute film growth

Without bias

I-V plot

F-N plot

slide34

0.5%

2%

1%

2%

0.5%

1%

IV. Results and Discussion

Field Emission Characteristics

Diamond coated silicon tip array

20 minute film growth

Without bias

F-N plot

I-V plot

slide35

2%

1%

0.5%

2%

1%

0.5%

IV. Results and Discussion

Field Emission Characteristics

Diamond coated silicon tip array

30 minute film growth

Without bias

I-V plot

F-N plot

slide36

IV. Results and Discussion

Field Emission Characteristics

Diamond coated silicon tip array

5-30 minute film growth

With -150 V bias

I-V plot

F-N plot

2% CH4, 5 min

2%

20 min

2% CH4, 10 min

2% CH4, 20 min

2%

10 min

1% CH4, 5 min

2%

5 min

1%

5 min

slide37

IV. Results and Discussion

Effective Work Function

F-N slope

Field enhancement

factor

d = 25 m

h = 2 m

r = 20 nm (5 min. growth)

Estimated effective work function for diamond

films grown under various conditions:

2% CH4 grown 5 minutes: feff = 0.87 eV

0.5% CH4 grown 5 minutes: feff = 2.24 eV

2% CH4 grown 5 minutes

with -150 V bias: feff= 2.25 eV

Lower CH4 concentration

and negative bias increase

the effective work function

of diamond film

slide38

IV. Results and Discussion

Effect of Diamond Film Thickness

Diamond deposition for longer time increases film thickness and the tip radius, consequently reduces field enhancement factor

Diamond film grown from 5 to 30 minutes at 2% CH4 increases film thickness from 20 to 120 nm, decreasing field enhancement factor by 32%

Diamond grown using lower CH4 concentration, although reducing film thickness, increases the effective work function

0.5% CH4

1% CH4

2% CH4

slide39

IV. Results and Discussion

Effect of Negative Bias

F-N slopes of negative biased emitters

Only emitters with either short growth time or high methane concentration have measurable emission current

Negative bias reduces electron emission

The I-V characteristics follow the same

pattern as those without bias

slide40

IV. Results and Discussion

Diamond Emitters with Self-aligned Gate

Process conditions:

35 Torr chamber pressure

600oC substrate temperature

5 minute growth time

Nanocrystal diamond slurry seeding

2% CH4 in H2

1 kW microwave power

Emitter array

Close-up

slide41

IV. Results and Discussion

I-V Characteristics of Gated Emitter Array

Process conditions:

I-V characteristics:

200 V anode voltage and 800 m spacer

Onset emission at Vg = 40 V

Emission current reaches 96 A at Vg= 80 V

0.87 eV effective work function

5 minute growth using 2% CH4

1.5 m gate aperture

20 nm tip radius

F-N plot

I-V measurement

slide42

IV. Results and Discussion

Anode and Gate Current

Gate current is less than 1% of anode current

Same slopes for gate and anode current F-N plots

Gate current is also due to field emission

I-V measurement

F-N plot

Ianode

I anode

Igate

Igate

slide43

V. Summary

Diamond field emitter arrays on micromachined silicon

are fabricated and characterized

Effects of CH4 concentration

(0.5-2%)

Effects of negative bias

(-150 V)

Increases diamond growth rate

Reduces diamond growth rate

Increases D/G ratio

No diamond Raman signal found

Increases CH emission intensity

but has no effect on C2 intensity

Increases CH and C2 optical

emission intensity

Enhances electron emission by

reducing the effective work function

Reduces electron emission by

increasing the effective work function

slide44

V. Summary

Field emission characteristics

0.87 eV effective work function obtained for 5 minute growth

using 2% CH4 without bias

32% decrease in field enhancement factor due to thicker film grown for 30 minutes

Onset gate voltage of 40 V and 96 A emission current

at gate voltage of 80 V obtained for gated emitters

Less than 1% of the total emission current is collected by the gate

slide45

V. Summary

Publications from this research work

Journal papers:

“Diamond coated silicon field emitter array”

S. Albin, W. Fu, A. Varghese, A. C. Lavarias, and G. R. Myneni,

J. Vac. Sci. Technol. A, 17, 2104 (1999).

“Microwave plasma chemical vapor deposited diamond tips

for scanning tunneling microscopy”

S. Albin, J. Zheng, J. B. Cooper, W. Fu, and A. C. Lavarias,

Appl. Phys. Lett. 71, 2848(1997)

Conference papers:

“Plasma Emission Spectroscopic Study of CVD Diamond Growth”

W. Fu, A. Lavarias, and S. Albin, presented at 52nd Annual

Gaseous Electronics Conference, October 5-8, 1999, Norfolk, Virginia

“Field Enhancement in Silicon Nanotip Emitter Array”

W. Fu, A. Varghese, presented at AVS Mid-Atlantic Chapter 1999

Spring Program Student Poster Paper Competition, May 10-12, 1999

Newport News, Virginia. (Second Prize Winner)

“Diamond coated silicon field emitter array”

S. Albin, W. Fu, A. Varghese, A. C. Lavarias, and G. R. Myneni,

45th AVS Internal Symposium, Baltimore, MD November 2-6, 1998