Pierson Hugh O. Handbook of carbon, graphite, diamond and fullerene. New Jersey / carbon

C

C

H. + C-H

*

H,

+ ‘C.

I

I

In the second step, the activated

surface-carbon

radical

reacts with the

carbon-hydrogen

species

(acetylene as a monomer

unit)

in the

gas

phase

to become

the

site for carbon

addition:

C

C

H

H

-\

-\

C.+C,H,

+,C-

L

=C.

C’

C

The

model

is consistent

with

experimental

observations

and

should

provide a useful

guideline

for future experiments.

A similar

model has been

proposed

that

is

based on the addition

of

a methyl

group to

one

of the

carbonsfollowed

by atomic

hydrogen

abstractionfromthe

methyl group.t121t13]

Hydrogen,

in

normal

conditions,

is a

diatomic

molecule

(H2) which

dissociates

at high

temperature

(i.e.,

>2000°C)

or in a high current-density

arc to form atomic

hydrogen.

The dissociation

reaction

is highly

endothermic

(AH = 434.1 kJmol-l).

The

rate of dissociation

is a function of temperature,

increasing

rapidly

above

2000°C.

It also

increases

with

decreasing

pressure.tlO)

The

rate of

recombination

(i.e., the formation

of the

molecule)

is rapid

since

the

mean-

free-path

dependent

half-life

of atomic

hydrogen

is only

0.3

s.

As

shown

in the models

reviewed

above,

atomic

hydrogen

plays

an

essential

role in the

surface

and

plasma

chemistry

of diamond

deposition

as it contributes

to the

stabilization

of the sp3 dangling

bonds

found

on the

diamond

surface

plane

(Fig.

13.1).t4]

Without

this

stabilizing

effect,

these

bonds

would

not be maintained

and the diamond

(111)

plane would collapse

(flatten

out)

to the

graphite

structure.

The

other

function

of atomic

hydrogen

is to

remove

graphite

selec-

tively.

In contrast

with

molecular

hydrogen,

atomic hydrogen

is extremely

reactive.

It etches

graphite

twenty

times as fast as it etches diamond

and

even

faster

in

the presence of

oxygen.

This etching ability

is

important

since,

when

graphite and

diamond

are deposited

simultaneously,

graphite

is preferentially

removed

while

most of the diamond

remains.n51

Thesetwo

effects of atomic

hydrogen,

graphite

removal

and sp3 bond

stabilization,

are believed

essential

to the

growth

of CVD diamond.

Beside

the need for

atomic

hydrogen,

the

presence

of oxygen or an

oxygen

compound

such

as H,O,

CO, methanol,

ethanol,

or acetone,

can be

an

important

contributor

to diamond

film

formation.

A

small

amount

of

oxygen

added

to methane

and

hydrogen tends

to suppress

the

deposition

of graphite

by

reducing

the acetylene

concentration

as well

as

increasing

the

diamond

growth rate.t61t16] The addition

of water

to hydrogen

appears

to

increase

the formation

of

atomic

hydrogen

which

would

explain

the

observed

increased

deposition

rate.

Carbon,

Graphite, Diamond,

and Fullerenes

2.5

Halogen-Based

Deposition

It was recently found that diamond growth also occurs in several

halogen-based reactions. These reactions

proceed at lower temperature

(250 - 750°C) than those based on the methyl-radical

mechanism

reviewed

above.

The

halogen

reaction mechanism

is still

controversial

and the

optimum

precursor species are yet to be determined.t’q

To proceed, the reactions must be highly favored thermodynamically.

This

is achieved

when

the reaction

products are solid carbon and stable

gaseous

fluorides

or chlorides (HF,

HCI, SFA.

Typical

reactions

and their

free energy at 1000

K are shown in Table 13.3.

These

free-energy values

are for the formation

of graphite and not diamond.

However the diamond

kJ/mol) .

The deposition

reaction

is carried out in a simple

flow tube.

The

amount of carbon-containing

gas is maintained

at ~5% of the overall

gas

composition to retard formation of non-diamond

carbon.

The addition of

oxygen or oxygen compounds

(air, H,O, CO,) enhandes

growth.

Diamond growth with the halogen

reactions

has also been observed

with the hot-filament,

RF-discharge,

and microwave

processes.t18]-t20)

These processes are described in the following section.

AG” at 1000 K

Reaction

(kJ/mol)

CH,+C+2H2

-4.5

(for comparison)

CH,,t2F2+Ct4HF

-1126.5

CCI,F,

t 2H,

+

C t

2HCI

t 2HF

-407.6

CH,OH

t

F2 +

C t

H,O

t

2HF

-725.9

CH,CH,OH

t

2F2 --+ 2C

t

H,O t 4HF

-1358.7

CH$H

t 5l=, -+ C t 4HF

t SF,

-2614.4

CS2 t

6F2 *

C t 2SFe

-3407.2

As mentioned above, the carbon species

must be

activated

since

at

low

pressure,

graphite

is thermodynamically

stable,

and without

activation,

only

graphite

would be formed (see Ch. 11, Sec. 3.2).

Activation

is obtained

bytwo

basic

methods:

high temperature

and plasma,

both of which

requiring

a great

deal

of energy.

Several

CVD processes

based on these

two methods

are presently

in

use. These

processes

are continuously

being

expanded

and improved

and

new

ones

are regularly

proposed.

The

four most important

at this

time

are:

high-frequency

(glow)

plasma,

plasma

arc, thermal

CVD, and

combustion

synthesis

(oxy-acetylene torch).

Their

major

characteristics

are

summa-

rized in Table

13.4.L6]

Substrate

Activation

Deposition

Temperature

Main

Method

Process

Rate

Control

Product

Glow-discharge

Microwave

Low

Good

Coating

plasma

RF

(0.1

- lOpm/h)

Arc plasma

DC Arc

High

Poor

Coating

RF Arc

(50 - 1000 pm/h)

Plates

Thermal

Hot-filament

Low

Good

Coating

(0.1

- 10 pm/h)

Plates

Combustion

Torch

High

Poor

Coating

Powder

CVD Diamond

311

3.3 Glow-Discharge

(Microwave)

Plasma

Deposition

A glow-discharge

(non-isothermal)

plasma

is generated

in a gas

by a

high-frequency

electric

field

such

as microwave

at relatively

low

pressure.

In such

a plasma,

the

following

events

occur:

.

In the

high-frequency

electric field, the gases are ionized

into

electrons

and

ions.

The

electrons,

with

their

very

small

mass,

are quickly accelerated

to high energy

levels

corresponding

to 5000

K or higher.

n

The

heavier

ions with their greater

inertia

cannot respond

to the

rapid

changes

in field direction,

As

a result,

their

temperature

and

that

of

the

plasma

remain

low,

as

opposed

to the electron

temperature

(hence

the

name

non-isothermal

plasma).

. The

high-energy

electrons

collide

with the gas molecules

with

resulting

dissociation

and

generation

of reactive

chemical species and the initiation

of the chemical

reaction.

The most common frequencies

in diamond

deposition

arethe

microwave

(MW) frequency

at 2.45 GHz and, to a lesser

degree, radio frequency

(RF) at

13.45MHz(theuseofthesefrequenciesmustcomplywithfederalregulations).

Deposition

Process.

A typical

microwave

plasma

for

diamond

deposition

has an electron

density

of approximately

1Go electrons/m3,

and

sufficient

energy

to dissociate

hydrogen.

A microwave-deposition

reactor

is shown

schematically

in Fig. 13.2.t1slt22] The

substrate

(typically

a silicon

wafer) is positioned

at the lower end

of the

plasma.

Gases are introduced

at the top of the

reactor,

flow

around

and

react

at the substrate,

and

the

gaseous

by-products

are removed

into the exhaust.

The substrate

must

be

heated

to

800

- 1000°C

for

diamond

to form.

This

can be done

by

the

interaction

with

the

plasma

and

microwave

power

but

this

is

difficult

to

regulate

and,

more

commonly,

the substrate

is heated

directly

by radiant

or

resistance

heaters

which

provide

more

accurate

temperature

control.

Typical

microwave

deposition

conditions

are

the following:

Incident

Power:

600

W

Substrate

Temp.:

800 - 1000°C

Gas

mixture

HdCH,:

50/l

to 200/l

Pressure:

10 to 5000

Pa

Total

gas flow:

20 - 200

scm3/min

at the deposition surface. Deposition

rate is low, averaging

1 - 3pmlh. This

may be due to the limited amount

of atomic

hydrogen

available

in the

deposition zone (estimated at 5%).

An advantage of microwave

plasma is

its stability which

allows

uninterrupted deposition lasting for days if necessary.

Howeverthe

plasma

can be easily disturbed by the addition of oxygenated

compounds.

Microwave

Magnetron

Microwave

Short

Network

Feed

Circuiting

SUPPlY +

/

/Slide

I

Carrier

Figure 13.3.

Schematic of electron-cyclotron-resonance

(ECR) apparatus for the

deposition of

diamond.[23]

314

Carbon,

Graphite,

Diamond,

and

Fullerenes

An

ECR

plasma

has

two

basic

advantages

for

the

deposition

of

diamond:

(a) itminimizes

the potential

substrate

damage

caused

by high-

intensity

ion

bombardment,

usually found

in an

standard

high-frequency

plasma where the ion energy

may

reach

100 eV,

(b)

it minimizes

the risk of

damaging

heat-sensitive

substrates

since

it operates

at

a

relatively

low

temperature.

Disadvantages

are

a more

difficult

process

control

and more costly

equipment

due

to the

added

complication

of the

magnetic

field.

RF

Plasma.

The

activation

of

the

reaction

and

the

generation

of

atomic hydrogen can also

be obtained

with

an RF plasma

(13.56

MHz)

but

this

process

is more likely

to produce

diamond-like

carbon

(DLC)

and

not

pure

diamond

(see

Ch.

14).

In addition

to microwave

deposition,

another common

plasma

deposi-

tion

system

for

diamond

coatings

is based

on

plasma-arc.

Plasma-arc

deposition

is

usually

obtained

in a high-intensity,

low-frequency

arc,

generated

between

two

electrodes

by either

direct

or

alternating

current.

The

process

requires

a

large

amount

of

power

and

the

equipment

is

Costly.t*41

In a low-frequency

plasma,

both

electrons

and

ions respond

to

the

constantly,

but

relatively

slowly,

changing

field direction,

as

opposed

to a

high-frequency

plasma

where only

the

electrons

respond.

Both

electrons

and

ions

acquire

energy

and

their

temperature

is

raised

more

or

less

equally.

The

plasma

is in equilibrium

(isothermal)

as opposed

to the

non-

isothermal

condition

found

in a high-frequency

plasma.

Isothermal

plas-

mas for diamond

deposition

are generated

at a higher

pressure

than

glow-

discharge

plasmas

(0.15 to 1 atm).

At such

pressure,

the average

distance

traveled

by the species

between

collisions

(mean

free

path) is reduced

and,

as a

result,

molecules

and

ions

collide

more frequently

and

heat

more

readily.

By

increasing

the

electrical

energy

in

a

fixed

amount

of

gas,

the

temperature

is

raised

and

may

reach

5000°C

or

higher.t*‘]

Such

high

temperatures

produce

an almost

complete

dissociation

of the

hydrogen

molecules,

the

CH

radicals,

and

other

active

carbon

species.

From

this

standpoint,

arc-plasma

systems

have an advantage

over glow-discharge

or

thermal CVD

since

these

produce

a far

smaller

ratio

of atomic

hydrogen.

DC Plasma System. Typical direct-current

(DC)

plasma

deposition

systems

are

shown

schematically

in Fig. 13.4.[25j[26j

Electrodes

usually

consist

of a water-cooled

copper

anode and a tungsten

cathode.

Several

gas-jet

nozzles can be operated simultaneously

and many design

variations

are possible,

including

separate

input nozzles

for

hydrogen and

methane

(the latter mixed with

argon) and the feeding of these gases in a coaxial feed

electrode.

Water Cooled

4To Vacuum Pump

Figure

13.4.

Schematic o‘f arc-discharge

apparatus

for the deposition

of dia-

mond .~1

Another

system incorporates

the

interaction

of a solenoid

magnetic

field to give the arc a helical shape.

This

stabilizes

and increases

the

length

of the

arc.t27)