1. Diamond Field Emitter Arrays on Micromachined Silicon
Dr. Wehai Fu
Dr. Sacharia Albin
Nano Science & Engineering Lab
ECE
Old Dominion University
Norfolk, Virginia 23529

2. Outline I Introduction II Field emission III Experiments IV
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

3. Electron emission mechanisms
I. Introduction
Electron emission mechanisms
Surface barrier
Surface barrier bending
due to applied field 
Thermal excitation
Photo excitation
Tunneling
Advantages of field emission
Low energy consumption
High current density
Small device size (sharp tips)

4. Field Emission Applications
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

5. Requirements for applications:
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

6. Diamond Field Emitter Properties of diamond
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

7. Research Developments
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)

8. Design, fabricate, and characterize diamond
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

9. Surface Energy Barrier#
II. Field Emission
Surface Energy Barrier#
Image Charge
Applied Field
Combined effect
Pe(x)
Pext(x)
P(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).

10. Fowler-Nordheim Equation#
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#
J emission current density
e electron charge
h Planck’s constant
F electric field at cathode
m mass of electron
 work function of the cathode
y function of F and 
t(y), v(y) approximated as constants
# R. H. Fowler and L. W. Nordheim, Proc. R. Soc. London A119, 173 (1928).

11. F-N Plot Simplified F-N Equation# II. Field Emission or Where:
a emitting area
f emitter work function
 field enhancement factor
I-V Plot
F-N Plot
2 eV
3 eV
4 eV
5.5 eV
3 eV
2 eV
5 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).

12. Field Enhancement Factor
II. Field Emission
Field Enhancement Factor
 = F / V
Anode
F: electric field at emitter tip
V: voltage between anode and cathode r d
Spacer h
Field enhancement factor for typical emitters#
Cathode r
# H. G. Kosmahl, IEEE Trans. Electron Devices 38 (6), 1534 (1991)

13. Emitter Structure Effect Simulation
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

14. Silicon Tip Array Fabrication
III. Experiments
Silicon Tip Array Fabrication
(a) Thermal oxidation
(b) Photolithography
(c) Silicon dioxide etching
(d) Silicon etching
(e) Tip sharpening
(f) Silicon nano tips

15. Diamond Emitter Fabrication
III. Experiments
Diamond Emitter Fabrication
Waveguide H CH
Microwave generator 2 4
Quartz window
Pressure Control
Plasma
Gas flow meter
Substrate height
control
Substrate
Gauge
Microwave
power control
Exhaust
Valve
Substrate
heating control
Motor
Pumping System

16. Emitter with Self-aligned Gate
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
4. Photoresist
etchback
5. Expose silicon tip
for seeding
6. Diamond deposition

17. 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

18. Silicon Tip Array Wet Etching
IV. Results and Discussion
Silicon Tip Array Wet Etching
Silicon tip array etched at 90oC with various
tetramethylammonium hydroxide (TMAH) concentrations
10% TMAH
25% TMAH
40% 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

19. Optimized Etching Results
IV. Results and Discussion
Optimized Etching Results
Silicon tip array etched using:
10 m square SiO2 mask
40% TMAH at 90oC
2.2 m high and 1.44 m wide
Aspect ratio of 1.53
1.4% non-uniformity
Array
Close-up

20. Effect of Etchant Temperature
IV. Results and Discussion
Effect of Etchant Temperature
Aspect Ratio = h/d
Silicon tip array etched in 40% TMAH
at various temperatures d h
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

21. Effect of Oxidation Sharpening
IV. Results and Discussion
Effect of Oxidation Sharpening
Short time oxidation
(<60 min.)
30 min.
60 min.
Reduces tip radius from
128 nm to 23 nm in 60 min.
No significant height
change
120 min.
240 min.
Extended oxidation
(>60 min.)
No appreciable change
in tip radius
Tip height decreases

22. Diamond Film Growth Optimized process conditions: Array Close-up
IV. Results and Discussion
Diamond Film Growth
Optimized process conditions:
Nanocrystal diamond slurry seeding
0.5-2% CH4 in H2
1 kW microwave power
35 Torr chamber pressure
600oC substrate temperature
5-30 minutes growth time
Array
Close-up

23. IV. Results and Discussion
Diamond film grown for 30 minutes without bias using various methane (CH4) concentrations
0.5% CH4
1% CH4
2% CH4

24. 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

25. Diamond Growth Rate IV. Results and Discussion Without bias
High CH4 concentration increases
diamond growth rate
Negative bias reduces diamond
growth rate
With bias

26. Raman Spectroscopy IV. Results and Discussion
Diamond film grown without
bias shows a diamond peak
at 1332 cm-1 and sp2 non-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

27. Diamond/Graphite Ratio
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

28. Optical Emission Spectroscopy
IV. Results and Discussion
Optical Emission Spectroscopy H2
CH4
Complex diamond growth process
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.

29. IV. Results and Discussion
CH Emission Spectra
CH emission intensity increases with methane concentration
Without bias
Negative 150 V bias
CH emission peak
(431 nm)
0.5% 1% 2%
CH emission peak
(431 nm)
0.5% 1% 2%

30. 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%

31. Emission Intensity Variation
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
CH intensity variation
C2 intensity variation
Negative bias
Without bias
Without bias
Negative bias

32. Field Emission Characteristics
IV. Results and Discussion
Field Emission Characteristics
Diamond coated silicon tip array
5 minute film growth
Without bias
I-V plot
F-N plot
0.5% 1% 2% 2% 1%
0.5%

33. Field Emission Characteristics
IV. Results and Discussion
Field Emission Characteristics
Diamond coated silicon tip array
10 minute film growth
Without bias
I-V plot
F-N plot 2% 1%
0.5% 2% 1%
0.5%

34. Field Emission Characteristics
IV. Results and Discussion
Field Emission Characteristics
Diamond coated silicon tip array
20 minute film growth
Without bias
F-N plot
I-V plot 2% 1%
0.5% 2% 1%
0.5%

35. Field Emission Characteristics
IV. Results and Discussion
Field Emission Characteristics
Diamond coated silicon tip array
30 minute film growth
Without bias
I-V plot
F-N plot 2% 1%
0.5% 2% 1%
0.5%

36. Field Emission Characteristics
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%
20 min
2% CH4, 5 min
2% CH4, 10 min
2% CH4, 20 min 2%
10 min
1% CH4, 5 min 2%
5 min 1%
5 min

37. Effective Work Function
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

38. Effect of Diamond Film Thickness
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

39. Effect of Negative Bias
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

40. Diamond Emitters with Self-aligned Gate
IV. Results and Discussion
Diamond Emitters with Self-aligned Gate
Process conditions:
Nanocrystal diamond slurry seeding
2% CH4 in H2
1 kW microwave power
35 Torr chamber pressure
600oC substrate temperature
5 minute growth time
Emitter array
Close-up

41. I-V Characteristics of Gated Emitter Array
IV. Results and Discussion
I-V Characteristics of Gated Emitter Array
Process conditions:
I-V characteristics:
5 minute growth using 2% CH4
1.5 m gate aperture
20 nm tip radius
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
F-N plot
I-V measurement

42. Anode and Gate Current I-V measurement F-N plot
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

43. 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 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

44. Field emission characteristics
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

45. Publications from this research work
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