H.O. Pierson. Handbook of carbon, graphite, diamond and fullerenes. Properties, processing and applications. 1993

316

Carbon, Graphite,

Diamond, and Fullerenes

The plasma jet can be cooled rapidly just priorto

coming

in contact with

the

substrate by using

a blast

of cold

inert

gas fed

into an annular fixture.

Gaseous boron or phosphorus

compounds

can be introduced into the gas

feed

for the deposition

of doped-semiconductor

diamond.[28]

The sudden expansion

of the gases

as they

are

heated

in the

arc

plasma causes

the

formation

of a high-speed

arc jet

so that

the atomic

hydrogen and the reactive carbon species

are transported almost

instantly

to the deposition

surface

and the chances

of hydrogen

recombination

and

of vapor-phase

reactions

are minimized.

The substrate

may

be heated

to unacceptable

levels by

the

high

temperature

of the gases

and cooling

is usually

necessary.

Temperature

control and substrate

cooling

remain

a problem in arc-plasma

systems.

However, deposition

is rapid and efficient,

high rates of deposition

(80 J1m/

h or higher)

are possible, and thick deposits

are routinely

produced.[29]

The

availability

of free-standing

shapes, 15 cm in diameter and 1 mm thick,

has

recently been announced.[30][31] Typical shapes

are shown

in Fig. 13.5.

Figure 13.5. Typical

free-standing

CVD diamond shapes.

(Photograph

courtesy

GE Superabrasives,

Worthington,

OH.)

318

Carbon,

Graphite,

Diamond,

and

Fullerenes

The

metal

temperature is

maintained

at

2000°C

or

slightly

higher.

Atomic

hydrogen

isformed and the carbon

species

become

activated

in the

vicinity

of

the

hot

metal.

The

deposition

rate

and

the

composition

and

morphology

of the deposit

are functions of the temperature

and the distance

between

the

hot

metal

and the

substrate.

This

distance

is usually

1 cm or

less.

Much

beyond that,

most

of the

atomic

hydrogen

recombines

and no

diamond

is formed.

The

substrate

temperature

should

be kept between

800 and

1000°C

and cooling

may

be necessary.

Gas

composition

and

other

deposition

parameters

are

similar

to

those

used

in

a

microwave-plasma

system.

Deposition

rate is low,

reported

as 0.5 to 1 pm/h.

A disadvantage

of the hot-

filament

process

is the

short

life

of

the

metallic heater, which tends to

carburize,

distort,

and

embrittle

rapidly.

In this

respect, tantalum

performs

better

than

tungsten

with

an

estimated

life of 600 hrs (vs. 100 hrs for

tungsten)

PI

The

heated

metal

may

also

evaporate

and contaminate

the

diamond

film.

Furthermore,

it is not advisable

to add

oxygen or an oxygen

compound

as mentioned

in Sec. 2.4 since,

at these

temperatures,

tungsten

(or most

other

refractory

metals)

would

oxidize

rapidly.

However,

the

equipment

is

relatively

inexpensive

and

experiments

are

readily

carried

out. Other

heating-element

materials

such

as graphite

or rhenium

are being

Diamond

is

grown

in air with

a

simple

unmodified

oxy-acetylene

brazing/welding

torch.t35)f361

Substrates

such

as

silicon

can

be

rapidly

coated

when exposed

to the reducing

portion

of the

flame

but

uniformity

in

structure

and

composition

is not readily

achieved.

The

high

gas temperature

(>2000°C)

makes it mandatory

to cool

the

substrate.

As

a result,

large

thermal

gradients

are

produced

which

are

difficult

to

control.

The

deposition

efficiency

is

extremely

low

with

a

nucleation

rate of

l/l

06.

This

means

high

gas

consumption,

high

energy

requirements,

and

high

cost.

The

deposition

mechanism

is

not

clearly

understood

at this

time.

3.7 Diamond

from

l*C

Isotope

CVD

diamond

has been produced

from the single

carbon

isotope,

‘*C.

As a reminder,

normal

diamond

is composed

of 98.89%

l*C

and 1.I 08% 13C

CVD Diamond

319

(see

Ch. 2, Sec.

2.1).

The deposition

of 12C diamond

is accomplished

by

the decomposition

in a microwave

plasma

of methane

in which the

carbon

atom

is

99.97%

12C.

The

resulting

polycrystalline

deposit

is

crushed,

processed

to

a

single crystal with molten iron and

aluminum

at high

temperature

and

pressure,

and

recovered

by leaching

out the

metalst3fi[38]

(See

Ch. 12, Sec.

3.0).

In diamond,

thermal conductivity

occurs by a flow

of phonons

(see Ch.

11, Sec.

5.2).

These

phonons

are scattered

by imperfections

such

as

isotopic

imperfections

and

the scattering

varies

as the

fourth

power

of the

phonon frequency.

Thus,

the

exclusion

of 13C should

result in a significant

increase

in thermal

conductivity.

This

increase

was confirmed

experimen-

tally

as 12C diamond is reported

to have

a 50%

higher

thermal

conductivity

than

natural

diamond.t37)t38j

It is

also

reported

to be

harder

than

natural

diamond

by a few

percent

as determined

by the

relation

between

hardness

and the

elastic

coefficients

(see

Ch. 11,

Sec. 10.2).

CVD diamond

is a polycrystalline

material

with

grain

size ranging

from

a few

nanometers

to a few

micrometers.

The

polycrystallinity

results

from

the formation

of separate

nuclei on the

deposition

surface.

As

deposition

proceeds, these

nuclei

eventually

coalescetoform

acontinuousfilm.

Some

grow

in size while

other

are crowded

out

and the

average

crystal

size

often

becomes

larger as thickness

increases.

Some

degree

of crystal

orientation

(texture)

can be achieved.

A cross-section

of a coating

is shown in Fig. 13.7.

As

mentioned

in Ch. 11, the

most

prevalent

crystal

surfaces

in CVD

diamond

are

the

(111)

octahedral

and

the

(100)

cubic.

Cubo-octahedral

crystals

combining

both

of these

surfaces

are

also

common.

Twinning

occurs

frequently

at the

(111) surface.

The

6H

structure

may

also

be

present.

3.9

Substrate

Preparation

and

Adhesion

The

nature

of

the

substrate

and

its

predeposition

treatment

play

a

major

role in determining

the surface

nucleation

rate

but not necessarily

the

rate of

subsequent

growth

(after

the immediate

surface

layer

is

depos-

ited) .t13)t3QlThe most widely

used substrate

is still silicon, but other materials

perform

successfully

such

as the

refractory

metals

(W, Ta,

MO), carbides

Figure

13.7.

Cross-section

of diamond

coating

(Photograph courtesy of

Diamonex,

Allentown,

PA.)

(WC, SiC) , and other metals such as Cu, Au and Ni and their alloys.[40]

Of

particular

industrial importance

is the deposition

on tool-steel substrate

(see

Sec.5.0).

Cubic boron nitride (c-BN)

is also a suitable substrate

with

good

lattice matching

but is only available

as small

single

crystals.

The

nucleation

rate and

the

adhesion

vary

with the nature

of

the

substrate

and appear

to be related

to the ability

of the substrate

material

to

form an

intermediate

carbide.

Surface

treatments

such as

etching

or

mechanical working (scratching

with a diamond

powder or diamond

polish)

help promote adhesion.

3.10 Oriented

and Epitaxial Growth

The

properties

of

diamond

are

affected

by

impurities

and

lattice

defects.

In addition,

they

are influenced

by

crystal

boundaries,

especially

Figure 13.8. Scanning electron micrographofCVD-diamond

coating. (Photograph

CVD

Single-Crystal

Diamond

Diamond

Density,

g/cm3

3.51

3.515

Thermal

conductivity

at 25”C, W/m*K

2100

2200

Thermal

expansion

coefficient

x 1OS/C

@ 25 - 200°C

2.0

1.5 - 4.8

Bangap,

eV

5.45

5.45

Index

of refraction

at 10 pm

2.34

- 2.42

2.40

Electrical

resistivity,

ohmcm

lo’*-

10’6

10’6

Dielectric

constant

(45 MHz - 20 GHz)

5.6

5.70

Dielectric

strength,

V/cm

106

106

Loss tangent

(45 MHz - 20 GHz)

<0.0001

Saturated

electron

velocity

2.7

2.7

Carrier

mobilities

(cm*/Vs)

electron

(n)

1350

- 1500

2200

positive

hole

(p)

480

1600

Vickers

hardness

range*, kg/mm*

5000

- 10000

5700

- 10400

Coefficient of friction

0.05

- 0.15

0.05

- 0.15

* Varies

with

crystal

orientation

In many optical,electronic,

and opto-electronic

applications,

the

main

emphasis

is

not

so

much

on thickness

since

the

required thickness is

submicron

or

at the

most

a few

pm, but

more

on

the

deposition

of

pure

diamond

crystals

without carbon,

graphite,

or metallic

impurities.

This

has

324

Carbon,

Graphite,

Diamond,

and

Fullerenes

yet

to

be

achieved and

present

materials

are opaque

or

translucent

and

certainly

not

on

par with

gem-quality

materialt3Q) (see

Ch. 11, Sec.

6.0).

4.4

Electronic

and

Semiconductor

Properties

The

semiconductor

characteristics

of CVD

diamond

are

similar

to

those

of the

single crystal

(see Ch. 11, Sec. 9.3).

It is an indirect-bandgap,

high-temperaturesemiconductor

which

can be changed

from

an intrinsic

to

an

extrinsic

semiconductor

at

room

temperature

by

doping

with

other

elements

such

as boron

or phosphorus.

f6)f2s) This

doping

is accomplished

during

deposition

by introducing

diborane

(B2Hs) or phosphine (PHs)

in the

deposition

chamber.

The

semiconductor

properties of CVD

diamond

are

similar

to those

of the

single crystal.

The

hardness

of

single-crystal

diamond

varies

as a function

of the

crystal

orientation

by

almost

a factor

of two.

This

is

also

true of

CVD

diamond,

and

the

hardness

values

depend

on which

crystal

face

is in

contact

with the

indenter of the testing

device.

Wear

resistance

of CVD

diamond

is generally

superior

to that

of the

single-crystal

material

since the wear of diamond occurs

by chipping.

Since

CVD diamond

is

a polycrystalline

material,

chipping

stops

at

the

grain

boundary

while,

in a single-crystal,

the

entire

crystal

is sheared

off.

The chemical properties of CVD

diamond

are

similar to those of the

single-crystal material

reviewed

in Ch.

11, Sec.

11

.O.

5.0 APPLICATIONS

OF CVD

DIAMOND

CVD

Diamond

325

high cost

of the

crystals.

CVD diamond,

on the

other

hand,

offers a broader

potential

since

size,

and

eventually

cost,

are

less

of

a limitation.

It also

opensthe

door

to applications

that

would

take full advantage

of the

intrinsic

properties

of diamond

in such

areas assemiconductors,

lasers,

optics,

opto-

electronics,

wear

and

corrosion

coatings,

and

others.

To estimate

a market

would

be no more than a guessing

game

since

few applications

at this time

(1993)

have

reached the commercial

stage and

there

is no historical

background

in patterns

of growth

and

market

size

to

buttress

any predictions.

Some

actual

and potential

applications

of CVD diamond

are

listed

in

Table 13.7.tsc)-tZ)

In the

following

sections,

several

typical

applications

are

described.

Table

13.7.

Actual and

Potential

Applications

of CVD

Diamond

Grinding,

cutting:

Inserts

Oil

drilling

tools

Twist

drills

Slitter

blades

Whetstones

Surgical

scalpels

Industrial

knives

Saws

Circuit-board

drills

Wear

parts:

Bearings

Engine

parts

Jet-nozzle

coatings

Medical

implants

Slurry

valves

Ball bearings

Extrusion

dies

Drawing

dies

Abrasive

pump

seals

Textile

machinery

Computer

disk

coatings

Acoustical:

Speaker

diaphragms

Diffusion,

corrosion:

Crucibles

Fiber

coatings

Ion

barriers

(sodium)

Reaction

vessels

Optical

coatings:

Laser

protection

Antireflection

Fiber

optics

UV to IR windows

X-ray

windows

Radomes

Photonic

devices:

Radiation

detectors

Switches

Thermal

management:

Heat-sink

diodes

Thermal

printers

Heat-sink

PC

boards

Target

heat-sinks

Semiconductor:

High-power

transistors

Field-effect

transistors

High-power

microwave

UV sensors

Photovoltaic

elements