Diamond and carbon-derived vacuum micro- and nano-electronic devices

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Diamond & Related Materi

Diamond and carbon-derived vacuum micro- and nano-electronic devices

W.P. Kanga,T, J.L. Davidsona, A. Wisitsora-ata, Y.M. Wonga, R. Takalkara, K. Subramaniaa,

D.V. Kernsb, W.H. Hofmeisterc

aDepartment of Electrical Engineering and Computer Science, Vanderbilt University, Nashville, TN 37235, USAbOlin College of Engineering, Needham, MA 02492, USA

cDepartment of Chemical Engineering, Vanderbilt University, Nashville, TN 37235, USA

Available online 17 March 2005

Abstract

A bmorphologically engineeredQ fabrication approach has been utilized for the development various configurations of diamond/carbon

field emitter arrays and devices. Based on this approach, we have developed several fabrication processes to micro-pattern diamond and other

forms of carbon structures and achieve topologically managed field emitters that optimize both the geometrical and quantum aspects of the

Fowler–Nordheim governed field emission. We report the fabrication of (a) pyramidal and edge-shaped vertical field emission devices, (b)

lateral diamond field emission devices, (c) nano-patterned nano-scaled diamond field emitter arrays, and (d) gated carbon nanotubes field

emitters. Excellent field emission device structures over a large area have been achieved with these fabrication methods. The interesting field

emission results of these devices suggest the potential of diamond/carbon emission devices beyond conventional vacuum microelectronics is

significant.

D 2005 Elsevier B.V. All rights reserved.

Keywords: Diamond/carbon; Carbon nanotubes; Field emission diode/triode; Vertical and lateral micro-arrays; Micro- and nano-patterning

1. Introduction

Chemical vapor-deposited (CVD) diamonds or related

carbons are excellent cathode materials for vacuum elec-

tronics because of their low electron affinity and excellent

mechanical and chemical properties. However, the bdirectQusage of diamond or other forms of carbon emitters leads to

non-uniform emitter microstructures, uncontrolled randomly

scattered emission sites, and inconsistent emission behav-

iors. Consequently, practical engineering design and control

of emitter structures are needed to fully exploit the use of

diamond/carbon field emitters.

We have developed a molding process for the fabrication

of diamond field emitter cathodes [1] and reported practical

modifications involving the sp2 content, boron doping, and

tip sharpening to further enhance diamond field emission

[2–5]. This paper describes the use of micro- and nano-

fabrication methods to achieve various forms of diamond/

0925-9635/$ - see front matter D 2005 Elsevier B.V. All rights reserved.

doi:10.1016/j.diamond.2005.01.033

T Corresponding author.

E-mail address: [email protected] (W.P. Kang).

carbon bmorphologically engineeredQ structures such as

vertically self-aligned gated micro pyramidal tips and micro

edges, lateral micro emitter structures with built-in gate and

anode, nano-scaled field emitter arrays, and gated carbon

nanotubes (CNTs) arrays, for electron emission modulation

and control.

Vertically self-aligned gated diamond vacuum triodes

were fabricated on a silicon-on-insulator (SOI) mold

utilizing conventional silicon micro-patterning and etching

techniques to define the anode, gate, and cathode. The

fabrication has achieved diamond field emitter triodes over

practical wafer areas. Lateral diamond vacuum devices

were fabricated with a diamond patterning technique

utilizing an oxide mask and lift-off. Nano-scaled diamond

field emitter arrays were fabricated by focus ion beam

(FIB) and nanocrystalline diamond deposition process.

Gated carbon nanotubes field emission devices were

achieved by catalytic-aided thermal CVD of CNTs on

pre-defined cathode area on silicon substrate. Detail

physical structures and field emission characteristics of

these devices are presented.

als 14 (2005) 685–691

SiSiO2

SiO2

SiO2Si

(a)

(b) SiO2Si

SiSiO2

SiO2

(c) SiO2Si

SiSiO2

SiO2

(d)SiO2Si

SiSiO2

SiO2

Diamond

SiO2

SiSiO2

Diamond

SiO2

Si

Diamond

(e)

(f)

Fig. 1. The fabrication scheme of the vertical self-align gated diamond

emitter triode utilizing silicon-on-insulator wafer.

Si-Gate

Diamond Tip

(a)

(c)

Fig. 2. Scanning electron micrographs of: (a) diamond pyramidal tips array; (b) d

emitter structure with pyramidal tips; (d) vertical self-aligned gated diamond emi

W.P. Kang et al. / Diamond & Related Materials 14 (2005) 685–691686

2. Device fabrications, structures, and electron emission

characteristics

2.1. Vertical diamond field emission devices

The vacuum diode and triode in a vertical configuration

are robust electronic devices with high-speed capability,

high temperature, and radiation tolerance. These diamond

emitters have superior transistor performance. However,

there are challenging obstacles to the development of a

diamond field emitter triode due to the difficulty of diamond

processing to achieve the three-terminal structure.

We have developed a practical and well-controlled

micro-fabrication method that utilizes silicon-on-insulator

(SOI) wafer as a substrate with a self-contained basic triode

structures. Vertically self-aligned gated pyramidal-tip and

edge-shaped triode structures were fabricated on a silicon-

on-insulator (SOI) mold. This fabrication method (Fig. 1)

utilizes conventional silicon micro-patterning and etching

techniques to define the anode, gate, and cathode. Details of

the fabrication steps have been described in prior publica-

tions [1,5]. The fabrication has achieved pyramidal-shaped

(Fig. 2(a)) and edge-shaped (Fig. 2(b)) diamond field

emitter cathode arrays and self-aligned gated triodes (Fig.

(d)

(b)

iamond edge-shaped emitters array; (c) vertical self-aligned gated diamond

tter structure with edge-shaped emitters.

4.5

4

3.5

3

2.5

2

1.5

1

0.5

0

Ano

de C

urre

nt (

µA)

0 50 100 150 200 250 300 350

Anode Voltage (V)

Fig. 3. The anode emission current vs. anode voltage (Ia–Va) of a self-

aligned gated diamond emitter triode (with 4 tips).

(a)

W.P. Kang et al. / Diamond & Related Materials 14 (2005) 685–691 687

2(c)–(d)) over practical wafer areas. The SEM of the

fabricated diamond field emission triode structures show

the nano-sharp apex and edges, surrounded by a self-aligned

silicon gate derived from the process.

SiO2

SiO2

SiO2Si

Si

SiO2

SiO2Si

Si

SiO2

Anode pattern

Cathodepattern

SiO2

SiO2

Si

Si

SiO2

Diamond

SiO2

Si

Si

SiO2

Diamond

(a) Starting material: oxidized SOI wafer

(b) Oxide patterning for diamond cathode and anode

(c) PECVD diamond deposition

(d) SiO2 and Si etching to form diamond anode and cathode

Si

SiO2SiSiO2

DiamondAnode Cathode

Fig. 4. Fabrication process for lateral diamond emitter utilizing silicon-on-

insulator wafer.

The field emission of the self-aligned gated triode

exhibits transistor characteristics [5]. Ia–Vg–Va plots (Fig.

3) of emission characteristics from 4 tips show a very low

gate turn-on voltage of 10 V and high emission current of 4

AA at gate voltage of 20 V. Ia–Va plots of emission

characteristics demonstrate the desired saturation behavior

of field emission transistor with a high voltage gain of 250.

The high amplification factor value indicates that the

diamond field emission transistor provides high voltage

gain when operated as an amplifier. The low turn-on gate

voltage and high gain factor are comparable to solid-state

metal-oxide-semiconductor field effect transistor, MOSFET,

devices, confirming the diamond field emission triode has

significant potential for IC-compatible vacuum microelec-

tronic applications.

2.2. Lateral diamond vacuum devices

Field emission device in lateral configurations has

interest to vacuum microelectronics due to simpler fabrica-

tion, design versatility of electrode geometry, and precise

spacing between electrodes by fine photolithography con-

Diamond Anode

Diamond Cathode

SiO2

Diamond gate

Diamond gate

(b)

Diamond Anode

SiO2

Diamond Cathode

Fig. 5. Scanning electron micrographs of lateral diamond emitter structures:

(a) diode configuration and (b) triode configuration.

Fig. 6. The emission current vs. anode voltage of a lateral diamond emitter

diode (with 4 fingers). Insert is the F–N plot of the emission data.


trol. A number of lateral field emitters utilizing materials

other than diamond have been reported [6–12], but their

emission characteristics are marginal.

(e) Si mold removal

(d) Boron-doped Microcrystalline diamond growth us

(a) Starting material: 1 cm2 silicon sample

(b) Mold formation on silicon by high-precision millin

(c) Nanodiamond growth using CH4/H2/N2 PECVD d

Fig. 7. Fabrication process of the nanodiamond vacuum field emission

We have developed a lateral diamond emitter fabrica-

tion technique on a SOI wafer utilizing oxide patterning

and lift-off process. The fabrication steps for the lateral

diamond emitter are shown in Fig. 4. The process begins

with the growth of 1 Am SiO2 on a SOI wafer followed by

patterning the anode and cathode structures on the SiO2

layer. The exposed SiO2 was then etched away, and the

diamond grown on the silicon area by PECVD with bias-

enhanced nucleation. The SiO2 masking layer was etched

away. The patterned diamond anode and cathode served as

a mask for etching silicon to yield isolation between the

electrodes as well as setting the anode–gate–cathode

spacing. The final structure consists of patterned diamond

anode, gate, and cathode, supported by a silicon layer

underneath, sitting on the SiO2 layer on the silicon

substrate.

Fig. 5 shows lateral device structures in a diode and a

triode configuration. However, from the SEM micrograph

observation, it would be seem that the apparently large

radius curvature of the diamond finger cathode would be

further reduced. Application of sub-micron photolithogra-

phy patterning is of interest. More elaborate lateral config-

urations are conceived and under evaluation.

Preliminary emission test (Fig. 6) has shown the lateral

diode exhibits a low turn-on voltage of ~5 V and a high

emission current of 6 AA, from 4 diamond fingers, at an

anode voltage of 25 V. The emission current of the lateral

ing CH4/H2/TMB PECVD deposition technique

Si

g using the FIB

eposition technique

Si

Si

Nanodiamond

Microcrystalline diamond

Si

Nanodiamond

Diamond thin film with high aspect ratio nano-tips

cathode by the focused ion beam nano-mold transfer technique.

(a) (b)

Fig. 8. SEM micrographs of an array of high aspect-ratio diamond nano-tips array (a) low magnification and (b) high magnification.

Fig. 9. The emission current vs. anode voltage of a nano-diamond nano-tips

array.


emitter conforms to F–N behavior (insert of Fig. 6) and thus

confirms that the observed current is electron field emission.

The emission current is stable over time, with current

fluctuation less than 10% over ~3 days, under a fixed anode

voltage. The low turn-on voltage (turn-on field ~3 V/Am)

and high emission characteristics are among the best of

reported lateral field emitter structures. The lateral diamond

field emitter has potential applications in vacuum micro-

electronics, sensors and microelectromechanical systems

(MEMS).

2.3. Nano-patterned nano-scaled diamond field emitter

arrays

We have recently developed a FIB (focused ion beam)

nano-mold transfer technique for the development of nano-

diamond vacuum field emission cathode arrays. The nano-

crystalline diamond grown by CH4/H2/N2 MPCVD was

applied for the fabrication of a field emission nano-cathode

array. The fabrication process steps are outlined in Fig. 7.

An n-type silicon substrate of area 1 cm2 is the starting

material. The nano-mold is then created on the silicon

substrate by high precision milling using a focused ion

beam, whereby a finely focused beam of gallium (Ga+) ions

are rastered on the surface of the silicon, forming arrays of

high aspect-ratio conical cavities of controlled diameter and

depth. Nanocrystalline diamond film is then deposited in

and on the silicon mold by the CH4/H2/N2 CVD process.

The mold-filling is followed by a thicker layer of micro-

crystalline diamond film deposited using CH4/H2/Trime-

thylboron CVD process. Next, a metal layer such as Ti/Ni

(~1 Am) is deposited on the diamond film, which is then

brazed with high-temperature copper alloy [Cu(26.7%)/

Ti(4.5%)/Ag(68.8%)] ~500 Am thick on a Mo substrate in a

vacuum of ~10�6 Torr at 900 8C. The final process step is to

flip the chip and completely etch remove the silicon using

KOH solution, which yields a diamond thin film with high

aspect-ratio nano-tips.

Fig. 8 includes the SEM micrographs displaying an array

of high aspect-ratio diamond nano-tips fabricated by nano-

crystalline diamond growth in and on the inverted silicon

molds. The radius of curvature of the diamond nano-tip

shown in the figure is less than 30 nm. The aspect ratio of

the tips is about 25 and 37 in specific cases. The diamond

nano-tips, as seen in the figure, take the contour of the

cavities created in the silicon mold by the FIB. The diamond

nano-tip array cathode was examined for enhanced electron

field emission. Fig. 9 shows a low turn-on field of ~4 V/Amand a high current density of ~2 A/cm2 have been be

achieved. The novel FIB nano-mold transfer technique is a

step towards the development of vacuum bnanoelectronicQdevices.


2.4. Carbon nanotubes field emission devices

Carbon nanotubes field emission devices are attractive

due to the simplicity of carbon nanotubes synthesis and the

compatibility of using conventional silicon micro-patterning

technique for the fabrication of CNTs vacuum devices. We

have presented the development of thermal CVD grown

carbon nanotubes (CNTs) for cold cathode applications [13].

We have recently developed a fabrication process for gated

CNTs, configured as triode field emitter.

The gated field emitter or triode device was fabricated

utilizing conventional silicon IC fabrication process. The

SiO2

Si

Gate

SiO2

Si

Dry Oxidation

Sputtering of Metal Gate

SiO2

SiO2

SiO2

Si

Gate Gate

PR PR

Patterning & Wet-etching of Metal

SiO2

(a)

(b)

(c)

(d

(e

(f

(g)

Self-aligned Gate

CNTs

Fig. 10. (a)–(f) Schematic diagram of the fabrication process for the single-mask

over-etched gate.

fabrication process is a single-mask, self-aligned integrated

gate electrode triode structure, as illustrated in Fig. 10(a)–

(f). The process begins with thermal oxidation of highly

doped n-type silicon (100) substrate followed by sputter-

deposition of the gate metal (gold or copper). The thickness

of the thermal oxide and the sputtered metal layers was ~1.5

Am and 0.8 Am, respectively. After that, the metal gate and

the oxide were isotropically wet-etched. Next, a thin layer of

Ti and Pd was sputter-deposited in sequence. Following

that, the unwanted catalyst above the gate layer was lifted

off with acetone. Lastly, the mold was transferred to the

CVD chamber for selective growth of CNTs inside the

Gate

SiO2

PR

Gate

SiO2

PR

Si

Sputtering of catalysts

Wet-etching of SiO2 (BOE)

Gate

SiO2

PR

Gate

SiO2

PR

Si

Catalysts Lift-off & Growth of CNTs

Gate

SiO2

Gate

SiO2

Si

)

)

)

self-aligned 3-band triode structure, and (g) SEM of the 3-band triode with

0.0

0.5

1.0

1.5

2.0

2.5

3.0

25 50 75 100 125 150 175 200

Va (V)

I a(µ

A)

Vg = 50 V

Vg = 65 V

Vg = 80 V

Fig. 11. Plot of the measured anode currents, Ia vs. the anode voltage, Va of

the self-aligned CNT-vacuum triode for different applied gate voltage, Vg.


mold, using the method described earlier [13]. Alternatively,

we have also used a silicon-on-insulator (SOI) wafer to

significantly simplify the triode fabrication process. The

SOI wafer consisted of ~1 Am buried oxide (BOX)

intermediate layer and 2 Am active Si layer. The advantage

of a SOI wafer was that thermal oxidation and metal gate

sputtering were not required. The SOI wafer was first

patterned and followed by wet etching of the active Si layer

using isotropic acid mixture (126 HNO3: 60 H2O: 5 NH4F).

The remaining processes were identical to those described

earlier.

SEM micrograph in Fig. 10(g) shows the fabricated CNT

vacuum triode utilizing a 3-band structure mold (10�75

Am2). The multiwalled CNTs were grown selectively inside

the bands on the Si substrate with an over-etched gate to

avoid bshortingQ between the gate and the cathode. Thicker

and a denser layer of CNTs film could be grown on the

over-etched mold, leading to possible higher emission

currents.

The CNT-triode device shows gate-controlled modula-

tion of emission current (Fig. 11) with distinct linear,

saturation, and cut-off regions. The emission characteristics

show a gate-turn-on voltage of ~40 V (gate-turn-on field of

~3.3 V/Am). To reduce the gate turn-on voltage, further

miniaturization of the gate-cathode spacing of the device is

in progress.

3. Conclusions

We have applied a bmorphologically-engineeredQapproach for the development of various configurations of

diamond and other forms of carbon nanostructures for field

emitter array and device applications. Based on this

approach, we have developed various practical and efficient

techniques for fabrication of diamond/carbon vacuum

micro- and nano-electronic devices. Vertically self-aligned

gated diamond vacuum triodes were fabricated on a silicon-

on-insulator (SOI) mold. This fabrication technique utilizes

existing silicon micro-patterning and etching techniques to

define the anode, gate, and cathode early in the SOI mold

fabrication stage prior to diamond deposition. The fabrica-

tion has achieved diamond field emitter diode and triodes

over a large area. The fabrication of lateral diamond emitter

by the diamond patterning techniques is demonstrated. The

processing flow is simple and compatible with integrated

circuit (IC) fabrication process. The fabrication utilizes

oxide patterning and lift-off process on a silicon-on-

insulator (SOI) wafer. The novel FIB nano-mold transfer

technique has shown a positive step towards the develop-

ment of vacuum bnanoelectronicsQ with high aspect-ratio

diamond nano-tip array for enhanced electron field emis-

sion. The gated CNT triode devices have also been

fabricated. The emission characteristics show gate-con-

trolled modulation of emission current. Further miniatur-

ization and optimization on the diode and triode structures

are in order. The excellent emission characteristics of these

devices have also been demonstrated.

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Diamond and carbon-derived vacuum micro- and nano-electronic devices

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