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metal films. Thin metal films are widely applied as Ig reflectors, ... Also, metal films are not very resistant to abrasion .... the structure of evaporated films of eight ...

NASA-CR-194632

/_//5

Transparent conductive coatings in the far ultraviolet Jongmin

/,4/

Kim, Muamer Zukic, Jung Ho Park, Michele Charles E. Keffer, and Douglas G. Ton"

Department

of Physics,

Optics

The University

Building,

of Alabama

Suite 300, Huntsville,

- 7_/-

c,'_

/¢1 7o

M. Wilson,

in Huntsville

AL. 35899

ABSTRACT In certain conductive

coating

bombardment conductive material. results

cases

a space-borne

deposited

of ionized

instrument

on the window

particles.

window for the

with a dielectric

to remove

Semiconductor

coatings for the front The theoretical search

for ITO

optical

window

the electrostatic

and

metal

films are

of far ultraviolet camera. semiconductor and metal

requires

charge studied

a transparent

collected for

use

due

to the

as transparent

Cr is found to be the best coating coating materials and experimental

and Cr films are reported. 1. INTRODUCTION

Some

optical

bombardment undesirable

instruments

by ionized effects.

This

for use as a surface

flown

satellites

have

particles,

electrostatic

charge

prompted

investigation

into the properties

layer to remove

Transparent

on orbiting

conductive

the undesirable coatings

accumulates

electrostatic

combine

exterior

high

on

camera

vidicons,

Combining

the

Pokell

properties

of transparency

transmission

transparent properties

semiconductors. than

mechanical

!

thin metal

films.

In general, Also,

cause coating

in the

oxides

with

good

electrical

coatings.

and with very as Ig reflectors,

films are not very

in which

transparency

is much

is usually employed because its absorption edge other classes of devices, in which transparency

etched

may

exhibit

resistant

same

coating

material

is not

thin and very low electrically but are not extensively used as better

electrical

to abrasion

and

and other

optical forms

of

damage.

For applications

oxide

and antistatic

semiconducting

metal

which transparent

to

crystal and gas discharge displays, Ig reflectors, phofoconduCtors in

and conductivity

trivial and is only possible with certain semiconductors resistant metal films. Thin metal films are widely applied

window

Due

charges.

optical

cells for laser Q-switches,

the

windows.

of a conductive

conductivity and have a number of interesting applications : liquid front electrodes for solar cells, heating stages for optical microscopes, television

dielectric

(ITO)

(lnzO3;Sn)

is ordinarily

used

because

more

important

than electrical

conductivity,

SnO 2

occurs further into the UV than other oxide materials. must be sacrificed for maximum conductivity, indium it yields

the highest

conductivity

and because

In tin

it can be

easily. (NASA-CR-19t,632)

CONDUCTIVE ULTRAVIOLET

TRANSPARENT

COATINGS IN THE FAR (Alabama Univ.) 8

N94-16024

p uncl as

G3/T4

0191210

qll

So far

studies

about

transparent

regions. In the far ultraviolet been widely used. Therefore, addressed.

conductive

coatings

(FUV) region, all optical a transparent conductive

Published

materials

reviewed

by Holland

2 in 1958

research

to find FUV

transparent

for transparent and

conductive

image

FUV

imager 3 for the International

four features

(LBH)

bands

system

contains

mirror.

The

of the aurora

between

of the

coatings

These

in the

papers

Terrestrial

short)

devices

entrance

is designed develop

of electrostatic

to work

door

a) substrate

charge

as a broad

a conductive

and

to meet

: 3" diameter,

b) transmittance

:

is to close

the

aperture

IR region

mission

0.125"

10 kD./l'l chemically

for the entire

following

thickness

nm (LBH

door,

instrument

IR

were

point

in our

is designed

long).

during

it should

FUV

The

a filter wheel,

to

conditions

is used at altitudes by ionized particles

fail in the closed

region.

optical

and a folding

non-operating

this instrument to bombardment

Also, in the event

window the

> 50 % for entire

c) resistance : < d) should be stable

and

as a starting

(ISTP)

160 nm -180

: an entrance

is allowed.

passband

coating

visible

served

Physics

(integration, launch, thruster bums), and to protect it. Because between two and nine earth radii (RE), the entrance door is exposed and no buildup

and

REQUIREMENTS

Solar

nm (LBH

electro-mechanical

purpose

on the visible

: O I lines at 130.4 nm and 135.6 run and the N 2 Lyman-Birge-Hopfield

140 nm -160

three

only

coatings.

2. INSTRUMENTAL The

focused

materials are absorbing and reflection optics have coating in the FUV region is a new problem to be

Vossen t in 1977.

conductive

have

Our

research

position,

it

goal was

to

specifications.

MgF 2 window

FUV

region

and mechanically

3. SEMICONDUCTOR

MATERIALS

3.1. Conductivity Oxide films, deposited by whatever films from the outset of deposition and do However, achieve

due the

to a smaller same

semiconductor conductivity In203

transparent is required.

incorporation depend

Haas

deposition processes

rate, often

and result

films in the

temperature

annealing

control

purposeful

parameters; temperature

in quite different

to metal

nm

to

visible

400 and

films,

run

IR region. 4

films

or inadvertent.

properties.

films

as continuous in metal films. are

is usually For

required required

an antistatic

film and

film,

6 kD,/l'l with

to for less

31 nm

application.

fabrication and

thicker

thickness

700 D./I'I with 36 nm In203

of semiconductor

either

fabrication

compared 200

et. al. 5 achieved

properties

of impurities, on the

of carriers Therefore,

conductive

+SnO 2 film for a space The electrical

and

number

conductivity.

means, appear to grow on oxide substrates not have the island structure typically found

time.

are

very

dependent

on stoichiometry

Also they are relatively

process,

starting

For nominally

material, equivalent

unstable, substrate materials,

and

the

chemically, temperature, even

similar

3.2. Transmittance Previously reported studies about the optical properties of oxide semiconductor coatings have on the visible and Ig regions where oxides have low absorption. The lowest wavelength

focused reported

was

200

nm.

Hass

et. al. 5 measured

their

InzO 3 and

In203

+SnO 2 coatings

prepared

by

evaporation and sputtering down to this wavelength. They got 25% transmittance for a 36 nm InzO 3 film and 23 % with a 31 nm In203 +SnO 2 film at 200 nm. Their spectral measurement results showed that there

was less transmittance Dobrowlski

at the shorter

et. al. 4 reported

wavelength.

optical

constants

of their

ITO

films (thickness

formed by ion-assisted deposition down to a wavelength of 400 nm. coefficients were 0.04 - 0.05, but increasing with a very steep gradient. These Therefore,

prereponed

we need

3.3. Experimental

results

showed

that oxide

a very thin film to achieve results

antistatic

R.F-sputtering was films in the FUV

lowest

resistance

and

semiconductors

the transmittance

At

are very

184 nm -

400

rim,

absorbing

Before used for testing. found

the

deposition

afterwards

keeping

was used

to measure

we obtained

required

onto

conductivity

valve

could

be

the masked

the same

square

measurements.

Flight

a 6 ° angle

Center

resistance

Our

is explained

of incidence

400

rim.

achieved

with

minimum

coating

and the substrate

masked to measure

This deposition except

and

unmasked

rate was

of reference

1/2" substrates

with a Talystep

used

time.

to control

The unmasked 6.

The

Sciences. The of 5 X 10 .2 ton"

not heated.

Pyi'ex

the thickness

for deposition

by the method

was

thickness.

In a 4 minute

were

profiler

the Pyrex

and

thicknesses substrate

sputtering

time,

which is very similar to Hass et. al.'s 5 result.

32 nm and 64 nm coatings Space

Pyrex

rate to be 8 nm/minl

5.7 kD./rl resistance

for optical

was kept closed

the MgF 2 substrate,

the parameters the

below

used to fabricate ITO coatings to test the possibility of using semiconductor region. ITO was selected as a trial material because it is reported to have the

The oxygen

We used

nm)

on the ITO coating

the

deposition

412

absorption

requirement.

sputtering target material was 99.99% {(In203)91% (SNO2)9%} supplied by Angstrom initial vacuum was 4 - 5 X 10 "5 tOrT and the Ar gas flow inlet was set to reach a vacuum during the sputtering.

the

are deposited optical

measurement

elsewhere,

are shown

on 1/2" diameter system

v The transmittance

in Figure

1.

The reflectance

and 0.125"

which

thickness

is located

MgF 2 substrates

at the NASA

at normal

incidence

remained

around

Marshall

and the reflectance 15% through

at

the entire

FUV region and the transmittance decreased monotonically to the short wavelength side. The absorption loss increases at shorter wavelengths. This result seems consistent with Hass et. al.'s s and Doborowalsky et. al.'s 4 results. FUV

region.

were

abandoned.

It is evident With these

that ITO

disappointing

is not a good results

further

material

for transparent

experiments

to change

conductive

coatings

the deposition

in the

parameters

2O Era

IS

Z .¢

16 _a

14

ae

12

e,a

._ IO

REFLECTANCES

8 6 4 2 .......

[-

.°-"

0 130

120

140

150

160

170

180

WAVELENGTH

Figure

1.

Measured

(dotted

lines)

transmittances

and reflectances

ITO coatings

200

210

220

of 32 run (solid

lines)

and

64 nm

on MgF 2 substrates.

4. METAL

4.1.

190

[nml

COATINGS

Conductivity Metal

films

that have been

studied

Pt, Pd, Ag, Cu, Fe, and Ni. I However, stable and has a very low electrical

nearly

for transparent

Au is predominantly

resistance.

factor

for the conductivity

every

atoms

arrive

metal

coating

applications

include

used. 8 Au is a noble metal that is chemically

with

of a thin metal

energies

greater

film is island formation.

than

kT

(where

T

Since,

is the

temperature), it is found that some of the atoms reevaporate, some are directly reflected surface, and some lose their energy by moving about the substrate surface until a small cluster, is formed

at a site occupied

and a continuous

rather

films than

sufficient

merge

of an island high. it.

and

if the

all other

is better

but

approximately

They

found 18 rim.

to transparent islands

things this

that

become

being

equal,

results

for this purpose

Scott _° observed

microscope. was

As the film gets

structure

Second,

Third,

conductivity,

film material

Sennett electron

is very

transmit electrical

continuous

by a nucleus.

thicker,

there is a coalescence

in

substrate from the or island,

of the islands

film is obtained.

The implication of such

• Au,

9

The most important case,

conductive

in more than those

the structure

conductors quite

large,

a thicker light

of evaporated

obtainable

they

which films

the resistivity incident

be deposited loss.

have

a thin

bulk resistance

different

the aggregation

electron

light,

to obtain

Therefore,

lower

of eight

for which

in their

First,

act to scatter

film must

absorption

materials

for Au and Ag, the thickness

With the resolution

is threefoid.l

microscope,

metals

in an

began

to

a Cr film

asthin as2 nm appearedto be a required conductivity

continuous•

Therefore,

with a minimum

thickness.

to theoretically

estimate

we selected

Cr as the best coating

material

to get

4.2. Transmittance We coating

attempted

materials

calculated thickness

found

in references

11 and

the

12.

transmittance

The transmittance,

by the standard matrix method with known of the film. _3 The calculated transmittances

using

the

optical

reflectance,

constants

of metal

and absorptance

can be

optical constants of the film and substrate and the for 2 run thickness of Pt, Au, Pd, Ag and Cr films

on MgF 2 substrates are shown in Figure 2. We used the optical constants for a MgF z substrate from reference 14. As we can see there is little difference in transmittance for candidate materials in the FUV region.

The optical

region• Therefore, and the absorption coefficients

( ie.

constants

of our

candidate

materials

do not differ very much

the reflectances from the top surface of the film, assuming loss plays the critical role in determining the transmittance. the imaginary

part of optical constant

over

the

entire

FUV

very thick films, are similar Apparently, the extinction

) are all of the same order of magnitude.

90

85 /

8O ° -

°

o

65

120

130

140

150

160

170

WAVELENGTH

Figure

2. Calculated

and dotted

line), Au (solid

Cr (dashed

and double

dotted

line).

effect

between

the interference

light intensity

I to the light intensity

I= Io

line),

metal

Pd (dotted

multiple Io is given

exp(_4.___kd)

190

200

(nm)

for the 2 nm thickness

Pt (dashed

If we neglect the transmitted

transmittances

180

reflected

films on MgF z substrates line),

Ag (dashed

waves

by the relation

line),

" and

in the thin film, the ratio of •

(l)

Because

the values

a wavelength

of k are close to 1, the thickness,

of 120 rim.

4.3. Experimental For chemically. glass,

Au and Ag are not suitable

results

reason

requirement. calibrated

a Pyrex

substrate

The thickness

resistance.

found

for our purposes.

Cr was

resistance, cathodes

selected it has been

in Geiger

layers for the external

that

We monitored

measurement

This

trial

vacuum

counters

material.

Cr

is fairly

evaporated

to form

with chemically

active

mountings

was used

of electron

and deposition thickness

to find the film thickness

inert

mirrors

tubes. 9

rate were

only a 1 nm thickness the change

monitored

also

do not make

means

Cr coating

of the resistance

that

any serious

which

by a Quartz

satisfied crystal

the

conductivity

monitor

which

had

the

coatings

good

conductivity,

of the Cr coating

with

( thicknesses

2.15

used

transmittance.

from the reference

is different

of 1 run and 2 am)

We measured the square resistance once a There has been no change in the resistance

adhere

so well

by four

from that of our coating Cr. are

of

that

the

probes

for

the

resistance

scratches.

The reason can be explained

has different properties than pure bottom surface of the substrate

was

kD./0

Figure 3 shows the transmittance measurements of the 1 nm and 2 nm Cr coatings. transmittance of the 1 nm film satisfies the requirement but has a very low transmittance compared calculated

on

gas fillings,

profiler.

which had been kept on a lab shelf without any special care. week for the first three months and once a month afterward. in 6 months.

as our

Cr by the evaporation method with an e-beam gun in a 2 X 10 .5 ton" vacuum. The 99.9 % granulate type supplied by Balzers. In order to get good adhesion, the to 200 °C and the deposition rate was slow.( 4 nm/min ) Before deposition on the

using the Talystep It was

4.1,

or evaporated

anti-corrosion

We deposited source material was substrate was heated MgF 2 substrate,

in section

of its high corrosion

used for electroplated

and as electrodeposited

than 6 nm to get Ill o > 0.5 for

on the Cr coating

explained

Because

d, should be smaller

Third, the absorption not included in the

possibilities.

material.

First,

Second,

the optical

the coatingis

The to the

constant

we

oxidized

and

of the substrate and the reflection from the calculation. Fourth, scattering loss is not

calculated. 5. SUMMARY ITO ITO

case,

and Cr thin film materials at least

a 32 nm thickness

were

tried as antistatic

was required

to achieve

coatings

for a FUV

the desired

camera

resistance

and

window.In it had

absorption loss so that it could not be used as a transparent conductive window material. films were found to be good for this purpose. Only 1 am film thickness was required conductivity nm.

Our

mechanical

and

the transmittance

Cr coating adherence.

did not

was higher show

signs

than

of aging

50 % between or deterioration

the

wavelengths

for six months

of and

a severe

Instead, Cr to provide

123 nm and also

the

had

220 good

60 55

.c:.5O _a

45 4O 35 3O 25 2O 120

130

140

150

160 WAVE

Figure

3. Measured

transmittances

of

170

LENGTH

1 nm

180

190

200

210

(ran)

(solid

line)

and 2 nm(dotted

line)Cr

coatings

on

MgF z substrates.

6. ACKNOWLEDGMENTS This work

was

Li and G. S. Martin

supported

by NASA

in the Center

contract

for Applied

NAS8-38145.

Optics

The authors

at the University

of Alabama

appreciate

the help of Y.

in Huntsville

(UAH)

using the sputtering system. The first author would like tO thank the Agency for Defense Development Korea and the Physics Department of UAH for the graduate student assistantship received during

!n of this

study. 7. 1. J. L. Vossen,

"Transparent

R. W. Hoffman 2. L Holland, 3. D. G. Tort, ISTP',

Vacuum

Conducting

Films",

in Phys.

Thin

Press,

New

eds. Voi. 9, pp. l, Academic Deposition

M. R. Ton',

of Thin

M. Zukic,

in Instrumentation

Chakrabarti

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