Transcript FIELD ENHANCEMENT IN SILICON NANOTIP EMITTER ARRAYS

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
II Field emission
Fowler-Nordheim field emission
Field emission enhancement
III Experiments
Silicon tip array fabrication
Diamond emitter fabrication
IV Results and discussion
Silicon tip array wet etching
Diamond film characterization
Diamond emitter I-V characteristics
V Summary
2
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)
3
I. Introduction
Field Emission Applications
Vacuum microelectronic device
Flat panel display
Anode
Gate
Probe
Conductive
coating
Phosphor
5 mm
Gate
Emitter
Emitter array
Scanning probe microscope
for surface imaging
Pressure
E-beam
d
Microsensor
Extracting
gate
Focusing
electrode
Emitter array
Cold cathode
4
I. Introduction
Requirements for applications:
Low turn-on voltage
Large emission current
Uniform over the array
Stable emission
Drawbacks of metal and silicon emitters:
High extracting field
Sputtering damage
Surface adsorption
Emitter overheating
Anode
Emitter
5
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
Chemically inert
Robust performance in harsh environment
Small effective work function
Low threshold voltage for electron emission
6
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)
7
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
8
II. Field Emission
Surface Energy Barrier#
Pe(x)
Ef
P(x)
Pext(x)
Vacuum
Level

Combined effect
Applied Field
Image Charge
x
e2

16π o x
Vacuum
Level
x

Ef
Vacuum
Level


- exF
Feff
x
Schottky barrier
reduction
Ef
d
Surface barrier
thinning
#
S. O. Kasap, Principles of Electrical Engineering Materials and Devices (McGraw-Hill, 1997).
9
II. Field Emission
Field Emission Current Density
J  e  N (T , s) D( F , s,  )ds
N(T,S):
D(F,s,):
s:
electron density
tunneling probability
kinetic energy
T:
F:
:
temperature
applied field
work function
Fowler-Nordheim Equation#
 8 ( 2m )1 / 2 3 / 2

J
exp  
v ( y )
2
3
heF
8ht ( y )


e 3F 2




#
J
e
h
F
emission current density
electron charge
Planck’s constant
electric field at cathode
mass of electron
 
work function of the cathode
 y
function of F and 
 t(y), v(y)
approximated as constants

m
R. H. Fowler and L. W. Nordheim, Proc. R. Soc. London A119, 173 (1928).
10
II. Field Emission
F-N Plot
 b
I  aV 2 exp  
 V
Simplified F-N Equation#
6
2


Where: a  1.56  10 a exp 10.4 
 1/ 2 
1.1


 I 
1
or ln  2   ln( a)  b 
V 
V 
a


b  6.44  107  3 / 2 / 
emitting area
emitter work function
field enhancement factor
I-V Plot
F-N Plot
-10
1.E-03
5 eV
1.E-05
2
3 eV
4 eV
1.E-01
-20
2
2 eV
1.E+01
-30
ln(I/V ) (A/V )
Emission current (A)
1.E+03
5.5 eV
2 eV
-40
3 eV
1.E-07
5.5 eV
5 eV
4 eV
0.002
0.004
0.006
-50
1.E-09
0
200
400
600
Voltage (v)
800
1000
0
0.008
0.01
1/V (1/V)
(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).
11
II. Field Emission
Field Enhancement Factor
 =F/V
Anode
r
F: electric field at emitter tip
V: voltage between anode and cathode
d
Spacer
h
Field enhancement factor for typical emitters#
Cathode
h
2
1
r


h
d
 
ln 4   2
 r
#
H. G. Kosmahl, IEEE Trans. Electron Devices 38 (6), 1534 (1991)
r
12
II. Field Emission
Emitter Structure Effect Simulation
h
r
 '
 h
ln  4   2
 r
2
Effect of emitter height
Effect of tip radius
160
Field enhancement factor
Field enhancement factor
160
Tip height (top to bottom)
4 mm
3 mm
2 mm
1 mm
0.5 mm
120
80
40
0
Tip radius (top to bottom)
10 nm
20 nm
40 nm
60 nm
80 nm
100 nm
120
80
40
0
0
50
100
Emitter radius (nm)
150
200
0
1
2
3
4
5
Emitter height (mm)
13
III. Experiments
Silicon Tip Array Fabrication
(a) Thermal oxidation
(b) Photolithography
(d) Silicon etching
(e) Tip sharpening
(c) Silicon dioxide etching
(f) Silicon nano tips
14
III. Experiments
Diamond Emitter Fabrication
Waveguide
H
2
Microwave generator
CH
4
Quartz window
Pressure Control
Plasma
Gas flow meter
Substrate height
control
Gauge
Substrate
Exhaust
Microwave
power control
Valve
Substrate
heating control
Motor
Pumping System
15
III. Experiments
Emitter with Self-aligned Gate
SiO2
Si
1. Thermal oxidation
and patterning
4. Photoresist
etchback
Metal
2. Silicon tip etching
5. Expose silicon tip
for seeding
Photoresist
SiO2
3. Photoresist planarization
for oxide and metal layer
6. Diamond deposition
16
IV. Results and Discussion
Orientation Dependent Etching
Some alkaline etchants etch various crystal planes of silicon at different etch rates
<100>
SiO2
SiO2
<100>
<111>
Silicon
Silicon
17
IV. Results and Discussion
Silicon Tip Array Wet Etching
Silicon tip array etched at 90oC with various
tetramethylammonium hydroxide (TMAH) concentrations
10% TMAH
15 minute etching
Extremely non-uniform
Hillocks on substrate
25% TMAH
4 minute pinching under mask
Hillock-free but non-uniform
Small tip aspect ratio
40% TMAH
10 minute pinching under mask
Hillock-free and uniform
Larger tip aspect ratio
18
IV. Results and Discussion
Optimized Etching Results
Silicon tip array etched using:
10 mm square SiO2 mask
40% TMAH at 90oC
Array
2.2 mm high and 1.44 mm wide
Aspect ratio of 1.53
1.4% non-uniformity
Close-up
19
IV. Results and Discussion
Effect of Etchant Temperature
Aspect Ratio = h/d
Silicon tip array etched in 40% TMAH
at various temperatures
h
Emitter Height and Width (m m)
and Aspect Ratio
3
2.5
d
Height h
Silicon tip arrays etched at various
temperatures show similar appearance
2
Aspect Ratio
1.5
Etch rate increases with temperature
Base width d
1
The slight decrease of aspect ratio shows
the etching selectivity between side planes
and the base decrease with temperature
0.5
0
50
60
70
80
90
100
Temperature (oC)
20
IV. Results and Discussion
Effect of Oxidation Sharpening
Short time oxidation
(<60 min.)
30 min.
60 min.
120 min.
240 min.
Reduces tip radius from
128 nm to 23 nm in 60 min.
No significant height
change
Extended oxidation
(>60 min.)
No appreciable change
in tip radius
Tip height decreases
21
IV. Results and Discussion
Diamond Film Growth
Optimized process conditions:
Nanocrystal diamond slurry seeding
0.5-2% CH4 in H2
1 kW microwave power
Array
35 Torr chamber pressure
600oC substrate temperature
5-30 minutes growth time
Close-up
22
IV. Results and Discussion
Diamond film grown for 30 minutes without bias
using various methane (CH4) concentrations
0.5% CH4
1% CH4
2% CH4
23
IV. Results and Discussion
Diamond film grown for 30 minutes with -150 V bias
using various methane (CH4) concentrations
0.5% CH4
1% CH4
2% CH4
24
IV. Results and Discussion
Diamond Growth Rate
Without bias
With bias
High CH4 concentration increases
diamond growth rate
Negative bias reduces diamond
growth rate
25
IV. Results and Discussion
Raman Spectroscopy
Diamond film grown without
bias shows a diamond peak
at 1332 cm-1 and sp2 non-diamond
carbon around 1500 cm-1
22000
1332 cm-1
2% CH4
Intensity (ard.)
Intensity (arb.)
20000
Diamond film grown with
-150 V bias shows no diamond
peak but a broad band amorphous
carbon signal
18000
1% CH4
16000
0.5% CH4
14000
1000
2% CH4
20000
18000
1% CH4
16000
14000
0.5% CH4
12000
1200
1400
-1
Wavenumber (cm )
1600
1000
1200
1400
1600
Wavenumber (cm-1)
26
IV. Results and Discussion
Diamond/Graphite Ratio
4
D/G Intensity Ratio
Diamond film grown for 30
minutes without negative bias
3
D/G ratio decreases as CH4 increases
2
D/G ratio saturates after 1% CH4
1
0
0
0.5
1
1.5
2
2.5
3
Methane Concentration (%)
27
IV. Results and Discussion
Optical Emission Spectroscopy
H2
Complex diamond growth process
CH4
CH2, CH3, CH, C2H2,
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.
28
IV. Results and Discussion
CH Emission Spectra
CH emission intensity increases with methane concentration
Negative 150 V bias
8000
6000
CH emission peak
(431 nm)
4000
0.5%
1%
2%
2000
0
Emission Intensity (arb.)
Emission Intensity (arb.)
Without bias
CH emission peak
(431 nm)
6000
4000
0.5%
1%
2%
2000
0
400
410
420
430
440
Wavelength (nm)
450
460
400
420
440
460
Wavelength (nm)
29
IV. Results and Discussion
C2 Emission Spectra
C2 emission intensity increases with methane concentration
Without bias
Negative 150 V bias
40000
C2 emission peak
(517 nm)
20000
0.5%
1%
2%
10000
0
Emission Intensity (arb.)
Emission Intensity (arb.)
30000
C2 emission peak
(517 nm)
30000
20000
10000
2%
0
480
500
520
Wavelength (nm)
540
480
500
520
0.5%
1%
540
Wavelength (nm)
30
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
400
C2 Emission Intensity (arb.)
CH Emission Intensity (arb.)
CH intensity variation
Negative bias
300
200
Without bias
100
0
0
0.5
1
1.5
CH4 Concentration (%)
2
2.5
3000
2500
Without bias
2000
Negative bias
1500
1000
500
0
0
0.5
1
1.5
2
2.5
CH4 Concentration (%)
31
IV. Results and Discussion
Field Emission Characteristics
Diamond coated silicon tip array
5 minute film growth
Without bias
F-N plot
I-V plot
-20
-25
0.5%
2%
-30
2
1%
ln(I/V )
2%
-35
-40
1%
0.5%
-45
-50
0
0.002
0.004
0.006
0.008
1/V
32
IV. Results and Discussion
Field Emission Characteristics
Diamond coated silicon tip array
10 minute film growth
Without bias
I-V plot
F-N plot
-20
2%
-30
2
ln(I/V )
-25
1%
0.5%
-35
-40
2%
1%
0.5%
-45
-50
0
0.001
0.002
0.003
0.004
1/V
33
IV. Results and Discussion
Field Emission Characteristics
Diamond coated silicon tip array
20 minute film growth
Without bias
F-N plot
I-V plot
-20
2%
-25
1%
ln(I/V2)
0.5%
-30
-35
-40
2%
-45
0.5%
1%
-50
0
0.001
0.002
0.003
0.004
1/V
34
IV. Results and Discussion
Field Emission Characteristics
Diamond coated silicon tip array
30 minute film growth
Without bias
I-V plot
F-N plot
-20
2%
1%
-25
-30
2
1.5
ln(I/V )
Emission Current (m A)
2
1
0.5%
0.5
-35
2%
-40
1%
-45
0
0.5%
-50
0
200
400
600
Voltage (V)
800
1000
1200
0
0.001
0.002
0.003
0.004
0.005
1/V
35
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
-20
2%
20 min
2%
10 min
0.4
2%
5 min
0.2
700
800
-35
1% CH4, 5 min
-40
1%
5 min
0
600
-30
2
0.6
2% CH4, 5
min
2% CH4, 10 min
2% CH4, 20 min
-25
ln(I/V )
Emission Current (m A)
0.8
900
1000
Voltage (V)
1100
1200
-45
-50
0
0.001
0.002
0.003
0.004
1/V
36
IV. Results and Discussion
Effective Work Function
F-N slope b  6.44  107  3 / 2 / 
h
1
r
Field enhancement  

d
 h
factor
ln 4   2
 r
2
d = 25 mm
h = 2 mm
r = 20 nm (5 min. growth)
Estimated effective work function for diamond
films grown under various conditions:
2% CH4 grown 5 minutes:
eff = 0.87 eV
0.5% CH4 grown 5 minutes:
eff = 2.24 eV
2% CH4 grown 5 minutes
with -150 V bias:
eff = 2.25 eV
Lower CH4 concentration
and negative bias increase
the effective work function
of diamond film
37
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
F-N Slope
25000
20000
0.5% CH4
15000
1% CH4
10000
2% CH4
5000
0
0
10
20
30
40
Deposition Time (min.)
38
IV. Results and Discussion
Effect of Negative Bias
F-N slopes of negative biased emitters
CH4
5 min.
10 min.
20 min.
30 min.
2%
10832
12611
21653
N/A
1%
20187
N/A
N/A
N/A
0.5%
N/A
N/A
N/A
N/A
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
39
IV. Results and Discussion
Diamond Emitters with Self-aligned Gate
Process conditions:
Nanocrystal diamond slurry seeding
2% CH4 in H2
1 kW microwave power
Emitter array
35 Torr chamber pressure
600oC substrate temperature
5 minute growth time
Close-up
40
IV. Results and Discussion
I-V Characteristics of Gated Emitter Array
Process conditions:
I-V characteristics:
200 V anode voltage and 800 mm spacer
Onset emission at Vg = 40 V
Emission current reaches 96 mA at Vg= 80 V
0.87 eV effective work function
5 minute growth using 2% CH4
1.5 mm gate aperture
20 nm tip radius
F-N plot
I-V measurement
-15
-17
80
2
ln(I/V )
Anode Current (m A)
120
40
-19
-21
-23
0
0
20
40
60
Gate Voltage (V)
80
100
-25
0
0.01
0.02
0.03
0.04
1/V
41
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
-15
Ianode
6
2
Ln(I/V )
Emission Current (m A)
8
4
-20
I anode
-25
2
Igate
0
0
20
40
Gate Voltage (V)
60
Igate
-30
80
0.01
0.015
0.02
0.025
0.03
1/V
42
V. Summary
Diamond field emitter arrays on micromachined silicon
are fabricated and characterized
Effects of CH4 concentration
Effects of negative bias
(0.5-2%)
Increases diamond growth rate
(-150 V)
Reduces diamond growth rate
Increases D/G ratio
No diamond Raman signal found
Increases CH and C2 optical
emission intensity
Increases CH emission intensity
but has no effect on C2 intensity
Enhances electron emission by
reducing the effective work function
Reduces electron emission by
increasing the effective work function
43
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 mA 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
44
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
45