Amorphous film growth of electroless osmium deposition on silicon single crystal studied by an analytical scanning transmission electron microscope

Materials

Chemistry

AMORPHOUS

and Physics,

FILM GROWTH

(1989) 13 1~ 145

24

OF ELECTROLESS

SINGLE CRYSTAL STUDIED ______..__ ELECTRON MICROSCOPE

Yee-Shyi

CHANG

Department

Received

BY

14,1989,

DEPOSITION

ON SILICON TRANSMISSION

CHOW

Science

Hsin-chu

March

OSMIUM

AN ANALYTICAL .____ __- SCANNING

and Mei-Ling

of Materials

Hua University,

131

and Engineering,

National

Tsing-

(Taiwan)

accepted

April

21,1989

ABSTRACT Amorphous

osmi.um

thin

film has been

autocatalytically

on silicon

bath

time.

for the first

electron used

microscope

to identify

(STEM) with

film microstructure. identified

cross-sectional

From

TEM method

proposed

with

partial

reactions

relationship

the

effect

analysis.

The

the uniformity between

mechanism

of the electrochemical

and cathodic

the deposition

to study

and

and

the film applicat-

and

and discussed.

the measurement

of anodic

was

and film were

patterns

as the interface growth

10 i

of the agglomerates

of agglomerates

was used

Tentative

substrate.

transmission

size of less than

by diffraction

of the film as well

and silicon

probe

deposited

hypophosphite-based

scanning

fine details

The structure

to be amorphous

morphology

ions were

An analytical

extremely

successfully

from a special

were

among

of the pH value the microstructure

reactions identified

those

potentials

on the mixed

potentials

rest

and mixed

potentials,

to be electroless

and The

was discussed.

potentials

of the OS film were

and also

correlation investigated.

INTRODUCTION Electroless excellent

plating

uniformity,

has many precise

ability

to plate

plating

to the discrete

0254-0584/89/$3.50

on irregular

attractive

control

features

such as

of thickness,

economy,

or complex

geometry

shapes

the

and selective

of the substrate

surface

which

0 Elsevier Sequoia/Printed inThe Netherlands

132

is more

convenient

On the other namely,

better

comparisons nickel

electroless

adhesion

plating

recently

electronic

industry.

the electronic

devices, have

chips

storage

can

through-hole

printed

The great

(Osram) [61. been

hardness

In addition,

overcoating metal

Osmium

little

expense

and also

metal.

Thus,

electroless

powerful

and unique

it shall

be of great

of osmium

use for the tips light

catalyst

of organic

reflectance,

Shuttle

chiefly

interest

than synthesis has

for faras

the

[71. Osmium

has still owing

difficulty

thin

filaments

osmium

coating

properties

industrially

as

electroless

in suh applications

to the considerable

plating

such

.

optical

thin film on the Space

and

multilayer

multilayer

However,

in the field

of its higher

attention

discs

[3,41 or

and electric

is a more

films

cobalt

memory

copper/nickel

[51.

need

semiconductor

of applications

as a reflective

advantages

on

has led its alloy

and to be attractive

many

of

investigated

styli

oxidation

because

optical

with

attracted

record

to be useful

UV instruments

boards

of osmium

gramophone

has a growing

(EMI) shielding

circuit

for organic

found

range

bonding,

in the micro-

electroless

industry

has not even been

in the past.

ruthenium

in a wide

interference

of osmium

of pen nibs,

gold plating

Thin

in

approach

for wire

such as fabrication

the electroless

Moreover,

connections

as a practical

in fabrication

121.

advantages,

use of the electroless

commercially

Electroless industry

be useful

electromagnetic

plating

introduced

in the electronics

devices.

coating

is recognized

and ceramics

been used

electrical

Thus,the

and metallization

area deposition.

has some basic

and no external

technique

die-attachment

for selective

plating

to electroplating[l!.

and has been

in

than evaporation

hand

to its

working

of

develop

to

film for a wide

the

the

range

of

applications. Epitaxial their

metal

potential

ficant

applications

understanding

of the silicide/Si such as Schottky years

silicides

[8-181.

platinum-group thin films silicon

barrier

However,

evaporation.

as well height

novel

generally

been

deposition

Recently

prepared

a new approach

and signi-

with

properties in recent

the silicides hand,

of the

metallic

on chemically

such as. electron has been

for

of the growth

resistivity

about

On the other

method

attention

devices

mechanisms

and sheet

limited.

much

as correlations

the literature

is fairly

have

in various

in the fundamental system

by a vapor

have attracted

cleaned gun

successfully

133 developed

where

chemically

the metallic

electroless

to form an epitaxial

thin

immersion silicide

film

[8].

alternative

to grow epitaxial

advantages

such as fast processing

wafer,

mass

production,

installation metal very

pure

cheaper

raw chemicals

metals

used

air before plating,

deposition

the deposited

not produce

In order

of transition

to visualize metal

since

and rate of mass

the species

interdiffusion film

structure

microstructure

of newly

to be studied

fruitful and bright electron

field

thin

virtually

direct

which

structure metal

and devices

of

since

with

plated

specified

OS on Si

is

pattern

transmission

on the growth,

of the initial prior

This stage

potential

and behaviour

correlated

great

evidence range

to be investigated, for whether

to be

and quality

of

growth

continuous

a

and structure The electro-

film. with

with

the micro-

the effect

since

they

it is electroless

for a deposited

and to

is concerned

of formation,

of both

film with

of

aid to

appears

structure,

study

are

interest

properties

to the formation

and the evolution

and growth

there

chemical

working

by the

the

that can ultimately

OS metallic

identification

as

growth,

scanning

to continuous

a suitable

Moreover,

well

area diffraction

from discontinuous

of

to

are affected

electroless

deposition.

exists

film deposit

atoms

down

its uniformity

as

Unfortunately,

available

osmium

observation

tens of pumping

for epitaxial

of the formation

is important

usefulness.

no data

of electroless

of the

transport

techniques

understanding

films

form new materials have practical

layer between

(STEM).

The fundamental metallic

oxide

mechanism

by the selected

imaging

microscopy

for electroless

for silicide

developed

noble

time to

it is beneficial

and silicon

annealing

for

is enormously

the film and the substrate.

metal

of

during

and

group

the exposure

of the film material,

between

large

for pure

but it is many

a kinetic

and the interface

for material

plating

than that

because

silicides,

the structure

investigate

method

apparent

that the cost

an intervening

film and the substrate,

time.

low

of platinum

in electroless

In addition,

for the evaporation

growth

and

is just a few seconds

does

attractive of many

feasibility

profitable

of magnitude

in evaporation.

which

minutes

rate,

For the formation

used

and annealed

on account

apparatus,

it is particularly

at least one order

by using

technology

It is a fairly

silicides

simple

[9,10].

cost

silicides,

is deposited

plating

good

of pH

provide

or not and quality.

134

EXPERIMENTAL 3-5 ohm cm, n-type,(lll) Si wafers of 15 mils thickness substrate were firstly cleaned chemically in trichloroethelene, acetone and the solution A( H202:HC1 = 1~1) followed by a deionized water in a ultrasonic cleaner.

The wafers were then

immersed in the solution B (HN03:H2S04 = 1:l ) for ten minutes followed by rinsing and etching in dilute HF solution C (HF : HZ0 = 1:50). less

The etched

wafers were then immersed into the electro-

plating bath at 85°C to deposit osmium thin film. The main

composition of plating bath and operating conditions are summarized in Table I.

Table I. The main composition and operating conditions of OS bath. Osmium tetraoxide

O.OlM

Sodium Hypophosphite di-~monium

0.05M

citrate

C1.003M

Glycine

O.OlM

Ammonia

small amount

NaOH

small amount

addition agent

small amount

Alkalinity

pH=lO

Temperature

T=85'C

In order to avoid the contamination complication of Sn and Pd, neither conventional senstization nor activation were used.

The

thickness of the thin films were was controlled by the immersion time and measured by the cross-sectional TEM method.

To prevent

contamination from impurities, the ulta-high purity electroless deposition bath was prepared by using reagents of analytical gsade and high purity deionized water produced from reverse osmosis deionization (18 Mohm-cm) followed by two-stage quartz distillation. To foil

elucidate TEM

the initial stage of the film

formation,

specimens of the Si single crystal with a

very

thin small

hole were first prepared by appropriate etching so that the electron beam of TEM can penetrate through the_ fairly large area of the Si specimen to meet the requirement for excellent

thin

135

visibility

of its bright

specimens

were

times

to deposit

was examined Thus

image.

OS metal

with

various

in the TEM directly changes

immersion

times

The TEM specimens

of Si were

from the unpolished nitric

acid

Transmission

Electron

charge

cooled dark

field

diffraction

growth

the microstructure

with

or

image

over

the substrate are too small

transmission

size of less than

the smallest

electron

microcrystalline

since TEM

5500

many

stage.

the smallest

over

Since

selected

by

the Si

the

area

pattern

aperture

of the small

the

with

in Fiq.1.

clumps

formed

diffraction

a

analytical very

small

It is unique

the film or agglomerates

is of the order

dark

were

the structure

microscope shown

electroless

dispersed

to be amorphous

10 i as

whether

reveals

no electron

TEM. Nevertheless,

of

agglomerates

in the initial

identified

was

of the conventional

widely

the

in size,

even by using

to determine

shape

that

energy

in the hypophosphite-based

for 3 seconds

irregular

the

[191. whether

of Tracer-Northern

micrography

Si substrate

indicates

and Sheng

The

microscopes.

electron

immersion

with

This

identified

(EDAX),

electron

OS on an etched bath after

in the conventional

way

both

field

can be observed

scanning

bath.

by

for X-rays

used

formation

the

deposition

were

a film deposited,

of agglomerates

probe

were

specimens,

bright

agglomerates

TEM method

and

electron

(OS) on immersed

AND DISCUSSION

randomly

field

selected-area

during

type

a nitrogen

was that of Marcus

RESULTS

agglomerates

from the

with

The bright

with

evolution

film

Generators

followed

analysis

substrate.

Vacuum

elements

linked

plating

combined

and

by a Scanning

photographed

(CCD) camera.

of

of 30 ml

of Si before

(STEM), were

TEM.

etching

solution acid

examined

and the cross-section

system,

deposited

by chemically

metallic

with

dispersive

were

during

procedure

In addition,

The

device

techniques

of the OS thin

preparation

or not

times

specimen

afterwards.

by analytical

side in a mixed

patterns

analysis

to investigate and

prepared

Microscope

couple

imaging

studied

short

Each

etching

The specimens

for short

Microdiffraction

HB5.

thicknesses.

without

acid of 50 ml, hydrofluoric

of 30 ml.

deposition

well-prepared

for various

of the film formation

concentrated

after

these

bath

can be clearly

and thinning

and acetic

Then

in the plating

the progressive

different

field

immersed

are

amorphous

electron-beam-probe of 1000 ii.

size

136

Fig.

7.

Analytical scanning transmission

electron

diffraction

pattern of as-deposited OS on Si after 3 seconds immersion. The typical diffuse halo ring shows the amorphous structure of as deposited OS film.

The advantages of using our new method in TEM sample preparation for observation of the initial deposition as described are distinct. The conventional

method is that a deposited specimen on Si is

etched from the Si side to the interface of Si and film to expose the

film, provided that the film side was sealed by a wax

etching. initial

before

Thus it easily resulted in the risk of over-etching the deposit

with

its very small amount

of

metal

with

a

misleading ambiguity of whether the initial deposit was etched off, so affecting the understanding of film growth process. Consequently the extent of etching in the conventianal method

of

Si specimen preparation for TEM observation is very difficult

to

control optimumly and straightforwardly.

In contrast, our new

method to prepare deposited TEN specimens is proven to be neat and informative. The failure probability of the well-prepared deposited specimen ready for scrutiny by analytical STEM is reduced to about zero by the new method and wastage in experimental time

and

effort can also be reduced.

Furthermore,

the

direct

observation of the initial stage of deposition by TEM is realistic since there is no risk of overetching the extremely small amount of deposit as in the conventional Probable

method of etching after deposition.

misinterpretation of the analysis of TEM micrography

taken by the conventional method of etching process after deposition can be avoided.

137

Fig. 2. Bright field (BF) image of as deposited OS on

Si(ll1)

from hypophosphite bath taken from conventional TEM after 30 seconds immersion.

Figuse 2 reveals the bright field image which corresponds to the OS deposit after 30 seconds immersion. The dark grain-like regions, whose image was obtained by the diffraction contrast method, correspond to deposited materials. The increment in the

coverage

of

of

the

agglormerate

aggregrated into 0s fil.m with

regions

was observed

larger grains.

and

some

them

The structure of the as-deposited

50-60'8 surface coverage is

amorphous,

from

the

observation of the electron diffraction pattern. As above reported observations at the initial deposition stages are very significant since fairly limited information concerning deposition

phenomena

without conventional activation treatment for the initial few seconds

appears in the literature.

A query about whether the

Osmium is deposited or not by the oxidation of hypophosphite taking place at the initial immersion or not was this

clarified

by

direct evidence with excellent spatial resolution by

though it did not form a film.

STEM,

The so-called incubation time for

electroless plating, in our case of OS deposited autocatalytically on Si was found to be nearly zero. For the

immersion time up to 1 minute, the deposit showed

better continuity laterly, due to the larger aggregate formation from

the incorporation

of smaller clumps of grains and the

reduction of the additional OS atoms from the solution. A of

aggregated

regions gave about

uniformly over the whole Si surface.

60-70%

coverage

number

distributed

A fairly diffuse halo

ring

(a)

(b) 3.(a) BF image

Fig. after

5 minutes

immersion;

observed

from

regions

indicates

For thin

The structure before.

each

other.

minutes,a

nearly

on etched

Si substrate

The deposited

film appeared

of the deposited

amorphous.

Furthermore,

islands.

these

laterly

to be of very dense

were

,

found

as

coalesced

and

impinged

immersed

of OS metallic

as indicated

OS

in Fig.3(a). amorphous

to be

specimen

film was still there

of

a continuous

aggregated

, grow

coverage

was observed

structure

among

identified

islands

surface

pattern

was observed

agglomerates

In the case of the full

immersion.

for 5 minutes,

coverage

of the OS film was

into the larger

bath

to be amorphous.

immersion

surface

stage many

10 minutes

area diffraction

regions

after

80-90%

At this

subsequently on

these

OS from hypophosphite

(b) after

the selected

the sample

film with

of as-deposited

for

thin

10

film

in Fig.3(b). appearance.

The

to be persistently

some channel-like

spaces

left

139

Figure

4 shows

15 minutes clearly,

the bright

immersion.

that the film became

agglomerates

distributed

probable

that

existing

grains

observed.

rather

to be' renucleated the neighbored A typical thin

Fig.5. were be

islands

layer

observed.

The surface

important

information

envolved

with

structure

substrate,

with

new phase

ions. On the other morphology important

can

hand,

also be clearly

of the thin electroless with

vacuum

than some of those In order

Fig.

4.

bath

in

or porosity

epitaxy

single

phase

transformatand

interface

In addition,

techniques,

since

and

of

crystal

it

is

the quality

film of such thickness

deposition

which

film

from the

of surface

application,

to

YL It provides

state

and parent

whether

developed

the osmium

solution

TEM BF image of as deposited

hypophosphite

shown

OS film appears

the

observed.

deposited

an

even

is better

techniques.

to identify

Si from the newly

and

is

of

pinholes

30

interdiffusion

formation

and then bridged

crystal

within

the fine detail

found

micrography

of the thin metallic

for the microelectronic

comparable

Si single

identified

were

were

film.

for the study of solid

just

on the

on Si.

islands

spaces

and no localized

the complicated

amorphous

and coarse

electron

the fluctuation

It is

preferentially

to form a continuous

of cross-section

flat with

space.

lots of aggregates

the channel-like

interface

and lots of small

new OS grains

spaces

of 140 h on n-type Os/Si

dense

proceed

immersion,

within

example

A sharp

rather

deposition

after

can be observed

the channel-like

than by forming

longer

of the OS deposit

extremely

some channel-like

With

image

phenomenon

within

subsequent

Consequently,

OS

field

The above

after

15 minutes

is

deposition

displacement

reaction

OS on Si (111) from immersion.

on

or electro-

the

Fig.

5.

TEM cross-sectional

after

3 minutes

less,

the observation

firstly

immersion

visually without

whether

change

reducing

agent.

electron

microscopy

immersing for 10 and confirmed

was

diffraction rays

(EDAX).

between

30 minutes by

the

pattern

with

as shown Both

hand,

potentials. partial

measured,

field

reactions

as well

This

imaging,

analysis to

it is important

The natural

TEM specimen

of the reducing

dispersive

reactions

scanning

deposited.

bright

no

of X-

distinguish

by the measurement

rest potentials

and the

mixed

as the relationships

of

potentials among them,

in Fig.6. the potential

potential

were

for 6 minutes. to be very plating

found

It is significant

stable

bath,

of the cathodic to be steady

with

which

time

of displacement

increases

remarkably

the potential

characteristics

with that

after

of displacement

reaction after

the mixed

noble

more

potential

insertion was

immersion

the mixed noble

metal.

behaviour

and the mixed

sample

found

in the

from the typical

in which

towards

of the more

time

different

reaction

at first

partial

the Si specimen

is completely

behaviour

attain

of

and energy

not

is

in a solution

by

was

or

There

silicon

in the absence

and displacement

and cathodic time were

observation

no osmium

techniques

On the other

of electrochemical anodic

bath

place

immersed

of the etched

showed

analysis

electroless

is taking

hypophosphite.

Further

in the osmium

OS film on Si

interface

image of TEM was used

on the Si specimen

the

after

field

agent,

transmission

agent

Si/Os

any reaction

of the reducing

apparent

a sharp

from the bright

to examined

in the absence

image of as deposited

shows

potential

and continuously

to

The potential-time

indicate

that the reaction

141

o hYP0

0 mix A OS -5d

I 12

I 6

0

I 18

I 24

Time Fig.

6. The relationship

cathodic calomel

partial electrode

of the nobler

should

increase

less galvanic


Furthermore,

that

those

that

in which

The

important Little curious

rather

which

relationship

should

behaviour

due to

as it is Therefore

in osmium

deposition

was found

potentials,

of

become

to be

indicating reaction

the same as the more

metal.

is well

affects paid

of the mixed

area

potential

and cathodic

for us to investigate

electrochemical

process

substrate

rapidly

potential.

on Si is not a displacement

of the bath

was

saturated

than by the Si substrate.

potential

factors

attention

noble

reduction

of the displacing

value

decrease

substrate

or more

of the mixed

deposition

the rest

pH

the main

of anodic,

against

by the less noble

and then

less exposed

of the anodic

potential

potentials

potential

displaced

metal

the value

the osmium

noble

from

I

of time.

at first

deposited

is by hypophosphite

between

cations

current

by more

the rest

and mixed

enormously

I 42

I 36

( mins 1

as a function

amount

covered

among

reactions

I 30

the

known

be

electrochemical

to the osmium the effect electroless

potential

to

system

one

the

behaviour.

so that

it

is

on

the

deposition.

The

of the pH value osmium

of

of the electroless

osmium

it

142

PH (Value) 7. The relationship

Fig.

a function

solution mixed

withthe

variation

potential

was found

against

to vary

of the pH value. from As

the

Some

black

was above

active

calomel

S.C.E.

as

that the coverages increase

with

the mixed

potential

of an increase

11,

the increment of experiments toward

in the deposition

the

structures

of the OS films,

bath

at various

pH values

This

reflects

from

immersed

studies

image

rate.

of

of TEM Si

minutes.

is an indi-

On the other deposited found

of

in the range

for 10

direction

electroless

range

bath

OS films on etched

9 to 11, were

that pH in a certain

fast

aggregrated

Further

of pH value

the active

very

11.

the immersion then

field

of the deposited

to

decomposition.

after which

by the bright

abruptly

10

become

to spontaneous

of the container.

observation

from

electroless

the

immediately

(S.C.E.)

the increment

dropped

changed

the deposition

easily

in Fig.7.The

electrode with

potential

evolution,

was above

at the bottom

9 to 11 for a series

cation

11,

occured

the pH value

deposition

substrates

against

was shown

direction

the mixed

bubble

and moved

suspension

Si when

reveal

Thus

the saturated

to the more

hydrogen

unstable

into sediment

of

of the pH value

Moreover,

pH value

vigorous

became

the

potential

-200 mV to -700 mV as the pH value

with

the

of the mixed

of pH value.

from

hand, the

to be amorphous.

has no significant

effect

143

on

the

though

amorphous

state

microstructure

it has an apparent

The immediate conventional

effect

deposition

sensitization

supplied

oxidation ensures quite

to reduce

large

OS cations

amounts

activation between

agents

sectional

TEM micrography opportunity

via various

with

formed

atoms

as shown

annealing

annealing

formation

really

and

scheme

The

5. Thus

taking

it provides

place

an

Schottky

a new family

technique

can be achieved

reactions

from the cross-

silicide/Si

Furthermore

epitaxy

and

contact

direct

was observed

in Fig.

state

Si and OS without

from sensitizing

epitaxial

schemes.

Si immedi-

This process

between

used.

to fabricate

from the solid

a suitable

and compound

exists

to

from hyposphosphite

to form OS metal.

of Sn and Pd aggregates

any

is considered

of n-type

formation

as conventionally

OS and Si actually

devices

electrons

potential.

Si without

process

of the interface

excellent diodes

donor

hydrogen

film,

deposited

on the electrochemical

and activation

for atomic

the cleanliness

the

of OS on semiconductor

be due to the role of excess ately

of

of

of silicides

due to interfacial

as both

Si and metal

contact.

CONCLUSIONS

1.

An osmium

single bath

Growth

Initially

phenomena

film were of being The

coalesce

instead

was

of a

potential

observed

were

found

into larger

identified

with

time,

by plane-view

islands

to be so reaction

and TEM.

TEM as follows.

to distribute

to be amorphous

on silicon

displacement

randomly,

and then

The structures

full coverage.

identified

grow

form a continu-

of agglomerates

for the entire

process

and instead

microcrystalline. pH effect

deposition

was

active

active

on the electrochemical

found,

in that

direction

TEM observation more

were

deposited

the hypophosphite-based

The deposition

plating,

agglomerates

laterally,

from

of electrochemical

ous film with

more

time.

electroless

via studies

3.

film was successfully

autocatalytically

for the first

called

2.

thin

crystal

revealed

mixed

in the deposition

with

that

potential rate.

the mixed

increasing surface

which

behaviour

of osmium

potential

decreased

pH. Further

studies

coverage

increased

was an indication

in the from

with

the

of an increase

144 4.

The structures

11 were

of OS films

identified

a certain

range

has no significant

formation,

though

5.

the cross-section

From

contact

with

interface

sharp

effect

This

or porosity.

cations

in microelectronics.

advantage

9 to

that pH in

on an amorphous

state

on the potential.

an intervening

and flat without

pH from

reflects

TEM, OS film was

without

pinhole

of various

This

effect

it has apparent

silicon

was

from baths

to be all amorphous.

found

oxide

the occurence can provide

to have direct

layer,

and the OsfSi

of localized

attractive

appli-

ACKNOWLEDGEMENT The authors

wish

of the University the analytical supported

to express

of Illinois

their

for her assistance This

STEM experiments.

by the National

appreciation

Science

research

Council,

to M.E. Mochel

in carrying

out

was partially

ROC.

REFERENCES

1.

B. L. Syharma and their

, Metal-Semiconductor

, Plenum.Press,

Applications

2.

K. Wong,

3.

J. Hajdu

4.

A. Krulik,

Met. Finish., Jr.,

(1988)

Schottky

K. Chi and A. Rangappan,

Barrier

New York,

Plat.

Surf.

and G. Krulik,

Surf. Finish., -70 (1983) 82(1984) 75. Surf. Finish., -71 (1984) 92.

R. Shipley

W. P. Griffith

I.

T. R. Gull,

Howard

Applied

,24, 16 (1985) 2660 . and J. Y. Lee, Proc. International

(Os,Ru,Ir

Plat.

Plat.

5.

, The Chemistry

and Rh)

, Wiley, Herzig,

of the Rarer

1967,

pp.8-9

10. 11.

1984,

Y. S. Chang

and J. J. Chu, Materials

Y. S. Chang

,I. J. Hsieh

Coulman

Platinum

42.

Metals

and A. R. Toft,

Optics

Y. S. Chang

&(1989)

75

.

J. F. Osantowski

,

9.

1984, p.140.

Finish.,

70.

6.

8.

Junctiorjs

154

and

Haydn

Electronic

p.491.

Lett., Chen,

5 (1987)

J. Appl.

phys.,

.

and Chen,

J. Appl.

Phys.,

59(10)

(1986)

67.

3467.

145

12.

N. M. Davey (1985)

and R. J. Seymour,

13.

S. P. Murarka

14.

Z. Zhongde

15.

P. S. Ho

(1986) 16.

Metals

Rev.,

29

J. Vat.

Sci. Technol.,

and N. W. Cheung

J. Vat.

B4

(1986)

1325.

Sci. Technol.,

B4(6)

1398. J. Vat.

R. T. Tung, 150 (1983)

Sci. Technol.,

J. M. Gibson

429

Ii. T. Tung,

18.

M. Liehr,

19.

Lett., - 54 (1985) 2139 . _-I?. B. Marcus and T. T. Microscopy

, pp.

Phys.

Rev.

(2) (1983)

and J. M. Poate,

Lett.,

P. E. Schmid,

of

Al

Phys.

745. Rev.

Lett.,

.

17.

1983

Platinum

2.

silicon

22-26.

52 (1984)

F. K. Legous

Sheng VLSI

461.

and P. S. Ho

Transmission

circuits

,Phys. Rev.

Electron

and structures

, Wiley,