Ar+ beam sputtering on solid surfaces and nematic liquid crystal orientation

HAL Id: jpa-00247950

https://hal.archives-ouvertes.fr/jpa-00247950

Submitted on 1 Jan 1994

HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are pub- lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.

L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Ar+ beam sputtering on solid surfaces and nematic liquid crystal orientation

Z. Sun, J. Engels, I. Dozov, Geoffroy Durand

To cite this version:

Z. Sun, J. Engels, I. Dozov, Geoffroy Durand. Ar+ beam sputtering on solid surfaces and ne- matic liquid crystal orientation. Journal de Physique II, EDP Sciences, 1994, 4 (1), pp.59-73.

�10.1051/jp2:1994115�. �jpa-00247950�

J.

Phys.

II France 4 (1994) 59-73 JANUARY 1994, PAGE 59

Classification Physics Abstracts

61.30 68.10

Ar+ beam sputtering

_on

solid surfaces and nematic liquid crystal orientation

Z. M. Sun

(*),

J. M.

Engels,

I. Dozov

(**)

and G. Durand

Laboratoire de Physique des Solides. Bit. 510, Umversitd de Paris-Sud, 91405 Orsay Cedex, France

(Received 8 July J993, received in final form 20 September J993, accepted 27 September J993)

Rdsumd. Nous _avons dtudid

expdrimentalement

l'orientation moldculaire du ndmatique SCB _sur des substrats solides bombardds obliquement _par un faisceau d'ions Ar+. Le bombardement d'un substrat isotrope donne _une orientation planaire le long de la projection de la direction de bombardement _sur la surface. Un bombardement _{sur un} substrat qui donne ddjh une orientation planaire (selon _un autre m6canisme)

produit

_une orientation intermddiaire _saris inclinaison. Darts le

cas d'un bombardement _{sur un} substrat donnant _une orientation planaire _par dvaporation _{sur une}

sous couche d'ITO, _nous trouvons en

plus

de la torsion _une inclinaison additionnelle. Nos exp6riences montrent que la variation de l'orientation _en surface crdde par le bombardement _est reticle h la profondeur drodde.

Abstract.- The molecular orientation of nematic SCB _on various solid substrates eroded obliquely by _an Ar+ ion-beam has been studied

experimentally.

The sputtering on an isotropic

substrate

(normally evaporated

SiO) results in _a nematic planar orientation along the

projection

of the local

velocity

of the ion direction _on the surface. When using an anisotropic substrate

(obliquely evaporated SiO which gives planar orientation in absence of sputtering) we find _a continuous twist of the

resulting

alignment, from the initial planar orientation towards the beam track projection. Although the geometry ^allows it, _we do _not observe any surface orientation bistability.

1. Introduction.

Molecular orientation of nematic

liquid crystals

_on solid substrates is

important

because of its fundamental interest and for

practical application

in

liquid crystal display

devices. In the last years,

anchoring properties

of

liquid crystals

on surfaces have been

extensively investigated

(*) Present address Laboratory of Solid State Microstructures, Nanjing University, Nanjing, 21008, P-R- of China.

(**) Present address Institut of Solid State Physics. Bvd Lenin, Sofia, Bulgaria.

[1-3].

In

thermotropic nematics,

surface

anchoring

is

usually

described in _terms of the

Rapini- Papoular expression [4].

The _« order electric _»

polarization

associated with the

gradient

of

order parameter ^also

plays

an

important

role in surface

anchoring

_on

rough

surfaces

[5].

Recently

Monkade _et al. described _{an «} order

electrically

_» controlled continuous transition from _a

planar

to an

oblique

nematic orientation _on SiO

obliquely evaporated glass plates [6].

In the transition _{zone, a} twofold

degenerate oblique

surface orientation _was observed

experimen- tally.

This

discovery

allowed the realization of surface transitions between these bistable

anchorings,

driven

by

electric fields of

opposite polarities [7, 8].

It _{seems to} be

possible

to

develop

with these bistable

plates

a new type ^of

display

device

comparable

to that

using

ferroelectric

liquid crystals

in

switching speed.

On the other hand, the transition _zone of

evaporation

_parameters where surface

anchoring bistability

is observed is very narrow and

requires

a delicate control of the process.

Therefore,

it is

important

to seek for _a method

inducing

a twofold

degenerate

surface orientation _more

easily.

In this paper, we

study

the influence of

sputtering

with _an Ar+ ion-beam _on solid

plates,

upon the nematic orientation induced

by

these surfaces. First _we shall start with _a

simple

case

I-e-,

sputtering

of _an

isotropic

substrate, to see the pure effect of the

sputtering

_on the surface

orientation. Then, we

study anisotropic

surfaces _{: we} describe the variation of surface

orientation induced

by oblique

ion

sputtering perpendicular

to the

planar

orientation

produced by oblique

SiO

evaporation.

We observe _a symmetry

breaking

of the surface orientation.

Finally, considering

the need to

apply

_an electric field in

switching

surface orientations, we

investigate

the _same

problem

when the surface is _a

planar

^ITO-coated

plate, looking

^for _a

better

approach

to create a twofold

degenerate

surface orientation.

2.

Experimental.

The

liquid crystal

used in _our

experiment

is the nematic SCB

(pentyl cyanobiphenyl).

The

optical

observation of the surface orientation is

performed

in _{a «}

hybrid

_» cell

consisting

of _a SiO

evaporated plate la

standard

glass plate

or an ITO-coated Baltracon

plate)

^and a silane-

coated

homeotropic aligned glass plate.

The SiO

plates gives «planar» orientation,

I.e.

nematic molecules in the

plane

of the

plate,

and

perpendicular

to the

plane

of

evaporation.

The silane

plane gives

«

homeotropic

_» orientation, I.e. molecules normal _to the

plates.

These

orientations result in _a bent _«

hybrid

» nematic _texture. The

hybrid

geometry ^{is chosen} because the

resulting

orientation of the nematic _texture

depends only

on the

planar

treated substrate

plate.

The

evaporation

_set-up has been

long

_ago described

[9].

The

evaporation

zenithal

angle

with the normal (see

Fig. I)

_a is 0° and

60°,

and the

evaporated

^{SiO nominal thickness} 3 is 300

Ji

_{and 700}

h,

_measured

_normally

_{to the}

_plate

_{from the}

_frequency

shift _{on an}

oscillating

quartz.

Before

preparing

the

hybrid

cell, SiO

evaporated plates

_are

sputtered by

an 8 kev ion-beam

with _a beam diameter of l _mm. In the _case of

oblique sputtering,

_{we want} to

study

the

influence of the

sputtered

thickness _on the surface nematic

orienting

_power. To do _{so, we} vary the

sputtered depth

with the

spatial position along

the

sputtered

track. This _can be done

by

a

step-scan

technique, I.e.,

the

spatial juxtaposition

and

superposition

of

sputtered

_areas with different

depths.

Several methods _can be

independently

used to fulfil this

goal,

_e.g. mechanical small rotation of targets, ^{variation of} sweep ^beam

voltage,

a.s.o. In _our

experiment,

the ion beam is

electrically

scanned _{to create a} line in _one direction. The scan in the

perpendicular

direction is achieved

by

a

five-step

_scan. Each step ^{takes between 10} to 60 _s.

Generally, sputtered

tracks _are about 7 _to 12 _mm

long,

6 to 8 mm wide. The maximum

sputtered depth

can reach 000

h.

_{For the normal} incidence, the substrate is

sputtered

for 4 to 40 _s without steps scan.

N' NEMATIC ORIENTATION ON SPUTTERED GLASS 61

When

sputtering

_an insulator substrate such _as, for

example,

SiO

evaporated glass plates,

a

space

charging phenomenon happens

_so that

sputtering

cannot be

effectively performed.

To

overcome this

difficulty,

_{we use a}

grounded

metal _cover

[10]

with _a

rectangular

window in

contact with the surface of the

plate,

to surround the chosen _zone to be

sputtered.

^When

sputtering

at

grazing

incidence, a breach is _cut

along

the side of the window

facing

the ion-

beam _so that no obstacle shades the ion-beam. When

sputtering

SiO

evaporated glass plates

with

ITO-coating,

_no

grounded

metal _cover needs _to be used, the

conducting

ITO

layer being

in fact

grounded by

the metallic holder itself.

The variation of the eroded

depth

d with

spatial position

inside the track _can be assessed

from the measurement of surface

profiles.

This measurement is made on a surface

profiler

Dektak 3030 with _a vertical resolution of 20

h

_and

a

profile

scan

length

of 10-12 _mm. On the non-treated side of the

plate,

we draw _a series of

points

_at intervals of 2 _mm

along

two directions

perpendicular

to each other in order to locate reference

positions.

On the surface

profiler,

we let the diamond

stylus

_scan

along

different

straight

lines located

by

the marked

points.

We thus obtain _a series of surface

profiles. By comparing

two surface

profiles along

_a

chosen line before and after

sputtering,

we can determine the

spatial

variation of eroded

depth along

this line. The

practical

_accuracy of the measurement is _more related _to the

difficulty

of

finding

back the _same track _on the

glass

than to the resolution. In

practice,

_we loose almost _an order of

magnitude

in accuracy. We _can achieve _an

uncertainty

on d of ± 50

h, _depending

on

the

glass

^surface

roughness.

The thickness of the nematic

hybrid

cell used _to determine the nematic orientation is defined

by mylar

_spacers 8 _~Lm thick. The

experiment requires

a uniform thickness of the cell, which can be controlled _as follows _: let the empty ^{cell be} illuminated

by

_a beam of

Hg

monochromatic green

light.

Interierence

fringes

can be observed in the cell.

By suitably adjusting

three

fixing

screws on the cell holder, we can eliminate all

fringes.

In this way, we obtain _an empty ^cell

with uniform thickness. This uniform thickness. This uniform thickness is _not

exactly

that of the spacers. The real thickness of the nematic

layer

can be determined

by

an

optical

method that _we will describe later. The cell is filled

by capillarity

with nematic SCB

(pentylcyanobi- phenyl)

in the

isotropic

state at 40-50 °C.

Anisotropic

nematic textures are observed at room

temperature

(22

_{°C j} under _a Leitz

polarizing microscope

with crossed

polarizers.

The

sample

is illuminated with

linearly polarized light

^which is

propagating

normal to the

glass plates.

The wave

length

^of

light

is A

=

5

461h.

To

analyze

the effect of the eroded

surface,

_{we must} determine the orientation of the nematic director _{n on} the

sputtered

surface with the geometry ^{defined in}

figure

I. Under

microscope

z

b

a e

n fi

/

~

/

x

Fig.

I. Orientation geometry on the

sputtered plate.

_e, b _are

respectively

the directions of SiO evaporation ^{and Ar~} ~puttering.

with crossed

polarizers,

black _zones characterize

regions

where the horizontal

projection

of

n

is

along

one of the

polarizer

or

analyzer

_{axes. n can} be further determined with _an

optical

compensator. ^{The azimuthal}

angle

_~b ^of n can be

easily

measured for each surface

point by rotating

the

microscope

stage ^and

taking

the

starting planar

direction

imposed by

the SiO

coating

in absence of

sputtering

_{as ~b}

=

0°. The tilt

angle

o of _n with the z-axis normal to the cell

plates

_can, in

principle,

be obtained

by using

the twin domain method

[I II

for which _we need not know the cell thickness. In the _case of absence of twin domains, _we have to use the

relationship

between the tilt

angle

and _some

optical

and elastic parameters

reported

in

reference

[12]

to calculate the

angle

if other parameters are known. However,

just

as

mentioned

above,

_we do not know the real thickness of the cell. Therefore _{we must} first

determine this parameter. ^{In the non-eroded}

region

of SiO

obliquely evaporated

substrate, we

have checked that the molecular orientation is

strictly planar,

I.e., o

=

90° in terms of the twin

domain method

[I I] (note

that in the non-eroded

region

twin domains

always appear).

We

measure the

optical path

difference between the

ordinary

and

extraordinary

waves with _a

compensator ⁱⁿ the non-eroded

region.

We _can then derive the cell real thickness

by

means of _a standard

optical

calculation

(Eq. (7)

in Ref.

[12]),

and the data of elastic _constants and refractive indices of SCB

[13, 14].

Since _we have _a uniform thickness cell, we can deduce the

angle

o in the

sputtering

track with this calibration.

3. Nematic orientation induced

by

the

sputtering.

3.I SPUTTERING _{ON ISOTROPIC} SUBSTRATES.

3. I. I

Single sputtering.

First of all, we would like _to know the pure effect of

sputtering

on an

isotropic

substrate. It is known that

oblique

SiO

evaporation

can

produce

nematic surface

orientation

(planar

or

tilted), depending

_upon the

preparation. Conversely,

from the symmetry,

a SiO normal

evaporation

must result in _{an «}

isotropic

_» texture. Such _{a texture} has been

obtained

by

an SiO normal

evaporation

_on

glass plates la

=

0°,

3

=

700

h).

Observation of the nematic

hybrid

cell does show _a local ordered

planar

orientation but _a random distribution of _n. We _{are not sure} whether _n is

exactly parallel

to the surface _or not, because it is

impossible

to measure the tilt

angle

in such _a

spatially inhomogeneous

texture.

As _a first step, an

isotropic

SiO normal

evaporated glass plate la

=

0° is

sputtered

^with

Ar+ ions at

grazing

incidence

(p

=

80°

).

The nematic cell

(Fig. 2)

shows _a

reasonably good

classical

planar

texture. In the

sputtered

track, limited

by

two

straight boundary

lines induced

by

^the

straight

border of the

grounded

metal _cover, the directors _n orient themselves

everywhere along

the

projection

of the

sputtering

direction _on the surface without

tilting.

For

simplicity,

this

projection

direction is called

b~

^direction

(parallel

to the

y-axis

of

Fig. Ii.

In the

case of nematic surface

anchoring,

the ion

sputtering

at

grazing

incidence

plays

the _same role

as a unidirectional

rubbing,

which also induces

planar

texture. It has been revealed

by

electron

microscopy

that for standard SiO incidence

angle (a

=

60° the SiO

layer

_appears

quite

smooth for small thicknesses 16 100

hi

_{and becomes}

_fully

_{compact above 000}

_{h [15]. Up}

to now no information is available for SiO normal

evaporation.

From _texture observation, _we suppose that ionic

sputtering produces

tracks inside the smoother SiO

layer,

which forces

molecules to

align along _b~.

^Note that, in the ion

sputtering

at

grazing incidence,

_{we were}

unfortunately obliged

to use thin

microscope

cover

glass plates

of lower surface

quality.

It is then difficult _{to measure} the

spatial

variation of eroded

depth

in this _case. We _{are sure} however that the eroded

depth

does increase,

although

we did not measured it

reliably. Experiments

show that

planar

surface orientation induced

by

such _a

sputtering

seems to be

independent

of the eroded

depth, although

its

strength

_{may vary.}

Then, we

change

the zenithal

angle

p of the Ar+

sputtering

at constant azimuth

(Fig.

I). We

study

the influence of p on the nematic surface orientation. The same «

isotropic

_» substrate is

N° I NEMATIC ORIENTATION ON SPUTTERED GLASS ~~

Fig. 2. - of _an

sotropic SiO

sputtered obliquely

with

p

=

40°.

Figure

3 shows the nematic _texture after

oblique sputtering.

We find that the nematic director orients itself

along

the

projection

on the surface of the local

velocity

of ions, without

tilting.

In the central part, ^{the directors}

align along

the

projection

of the

sputtering

direction _on the surface. Close _to the _two lateral boundaries, the nematic

directors deviate of about _± 10°-30°. Such distributions of _n is

probably

related _{to a} surface electric field. SiO is _an insulator. When

sputtering

on such _a substrate with Ar+ ions, there must exist _an electric field between the centre of the ion eroded SiO and the

grounded

metal

Fig.

3. Texture of _an

isotropic

SiO normal

evaporated

(a ₌ 0°, 3

=

7001)

_glass

_plate sputtered

obliquely (p 40° ), (x 10).

cover. Therefore, ions could deviate outwards from the

original

track under the action of this

field,

and _{create a} small twist of _n

compared

to the centre.

Finally.

let _{us see} the effect of normal

sputtering (fl

=

0

).

We expect no

planar

orientation, when

sputtering normally

_on an

isotropic plate.

However, as a result of a surface field

action,

ions should deviate

radially

from their

original

track. We do observe

(Fig. 4)

a « core » in the

centre of the eroded track and four black brushes outside the core. These black brushes remind

us of the texture of _a nematic _« disclination _» with S

= + I. Measurements indicate that

directors

align

more or less

radically

without

tilting.

This

experimental

fact is in

good

agreement ^with the surface electric field

hypothesis.

P A

l'

Fig.

4. Texture of _an isotropic SiO normal

evaporated

(n ₌ 0°, 3 ₌ 700

h) _glass

_{plate sputtered}

normally (p 0° ), (x 10).

In summary,

sputtering

^of an

isotropic

substrate

by

^{Ar+ ions} creates in

general

a

planar

nematic orientation without tilt,

along

the

projection

of the local

velocity

of ions _on the

glass plate.

Normal

sputtering

creates a radial distribution of directors but this is

probably

due to _a surface deviation of the ion from _a radial surface field. When

increasing

the zenithal

angle

p of Ar+

sputtering,

the nematic director tends to

align unidirectionally along

the ion beam

projection

on the

plate. Sputtering

at

grazing

incidence induces the best uniform

planar

orientation.

3.1.2 Double

sputterin~g.

We have been

knowing

so far that

sputtering

at

oblique

incidence

on

isotropic

substrates _{can create a}

planar

orientation without tilt. We would like _{now to} check if double

oblique sputtering along

^different

directions, performed

on the _same

plate,

could

result in two

planar

« bistable

» orientations.

So,

_we let _an Ar+ ion-beam sputter an

isotropic

substrate twice

along

two directions

b~j

^and

b~j. b~j

^makes a 45°

angle

^with respect to

b ~_ When the two associated

sputtering

times ti ^and t~ are the _same

it,

= t~ ²⁰ s), the

n~matic

_{texture induced}

_by

_the treated

plate

is shown in

figure

^5. In the

overlap region

all directors orient

along

b

~ direction. It seems that the second

sputtering destroys completely

^the effect of the first _one. ~~ie _can still find _some very small domains in which directors

keep

the

original

orientation b

j. If the second

sputtering

time is shorter than the first _one, e.g.

N° I NEMATIC ORIENTATION ON SPUTTERED GLASS 65

, ^l'~ bpi

Fig. 5. - of

an isotropic SiO _{normal evaporated} (n = 6 = 700hi glass plate

tj/t2

_~ ⁵ and t~ = 4 _s, the _texture exhibits

differing

features, as shown in

figure

6. We note that the second

sputtering

cannot

destroy completely

the first _one in the

overlap region.

Across the

sputtered

track, the

_molecular

orientation

changes

^from

b~,

^to b~~. Meanwhile, we also observe

a few small double-leaf domains shown in

figure

7. These domains have defined leaf-like

shapes

and do not

change

their

shapes

when

forcing

the nematic to flow

by squeezing

the cell.

The

optical

pattem ^{of the} double-leaf domain shows that surface orientation in each leaf of domain is oriented

along

the

projection

direction of the

corresponding sputtering.

Fig.

6. - of

an isotropic SiO normal _evaporated (a = 0°. ₃ =

001) glass late at

r'

N° I NEMATIC ORIENTATION ON SPUTTERED GLASS 67

the

sputtered depth.

Based _on these ideas,

planar

SiO

obliquely evaporated glass plates

are

prepared (a

= 60°, 3

=

700

hi

_and

_{sputtered obliquely (p}

=

40°

),

in the

plane

of the direction e~ ^{of the SiO}

evaporation,

and _z

(Fig. Ii, I.e., perpendicular

to the

planar

nematic

orientation.

To

study

the

dependence

^{of surface} orientation _on the eroded

depth,

we measure the surface

profiles.

A

typical

surface

profile

is shown in

figure

8. Curves A and B represent

typical

surface

profiles along

the chosen line inside the

sputtered

track after and before

sputtering, respectively.

The vertical distance difference between these _{two curves}

gives

the

spatial

variation of the eroded

depth.

»

a)

'j

' ^{' I} j

' ' ' ~ i

~f'~'j"~,~i ^,'

~~-~~~~~~/,i'i'[i

_ii

~ ~'~ ~~ ~

"'l'ii~l~'l

~~~~~

_~~ '

~'"iii)(, 'j,i'i,1

~~~

^~~~ "i

_~'~~ ~~~~"'l~ _~l' ~',l

If'~

^'°

_'S~~l

^<'

^l'II, ~

~'~

II

⁽

jj p

~~ ~~ ~~

~lj~ll

^{i)('I 1' ~-}

~~ "If _~

~-~ ^{i l'l'/}

^{' '} 'j

y

Z ~) -~~~zj ^I

l'jj(( jj

llli~~l'/,$ $j~i)i~'i~lj')'j~ ^{j'~' e}

~~)~~~~~~~$~j~~~~~ ^~~'j~~

~

~

~lll _~l( ^i)/j'>i _Ii Ii ^"'

^x

' Ii ^' '~l j~j

Ill jl

'~ ' (j

b)

Fig. 9. a) Texture of _a planar SiO oblique evaporated (n ₌ 60°, 3

= 700

hi

_glass

_{plate sputtered}

obliquely (p 40° ). when the optical

eigenaxis

makes the

angle

_~b

=

0°, with the non-eroded

planar

orientation. b) Distribution of the director field

corresponding

to the _texture shown in figure 9a.

We _now observe the

optical

_texture of the

corresponding

nematic

sample

shown in

figure

9a.

The

sample

is

placed

between crossed

polarizers,

and _can be rotated. For each

position,

the black appearance shows the

regions

where the nematic molecules _are

parallel

or

perpendicular

to the

polarizer

_axes.

Figure

^9a shows for instance the

sample

texture when these _{axes are}

parallel

to the

edge

of the

picture.

Both

right

and left parts ^{of the}

picture

_appear to be black.

They

are however

separated by

a defect line

(bright

_on the

picture).

The

analysis

with

compensator ^{shows that} on the

right

the molecules

align vertically, although

on the left

they

are horizontal.

Rotating

the stage, we can follow these orientations

by continuity.

The

corresponding

distribution of the director field is

plotted

ⁱⁿ

figure

9b. The

experiment

shows that the director rotates

parallel

to the surface in _{two senses} and do _not tilt at all. This _can be

seen more

clearly

ⁱⁿ

figure

^10. The

figure plane

is the

projection

_on the _x-y

plane

^{of the unit}

sphere

n~

= I. The initial

planar

orientation is

along

_x.

Increasing

the

depth

of the eroded

region

makes the director _{n rotate} towards b~

along

_y. The

point

P is _a bifurcation of the orientation, similar to the _one described in reference

[17], although

there is _no tilt of _n in _our

case.

_,' ,~

.h' 0 8 _{t 'I,}

"' /,"'

<~ -~-~ ^{i~ f} .,_'~.

,"~,_ ',, _.., ^{0.6 i~} _{_.' ,"} _,~,,

~",

_{'- 1'}

', ~

,'

~

j. 'lo ~

,' '. '- ~i /,' _{_.' ',}

". ~-'i', ~. _{/ "}

,"

l' ^"''~ ,'m' _{_."} I 1'

/ j

'.

,'

~

x

-1

0-(

fi÷O.6.=,0 0 4~ D-E- Q~8

"'[ _'- / / ')"j"

,' ',',"" / _j

.h I ,"' '-"'," -"'-" -", (~, ,'~

,' l -~'~ ,/ j

~' ~-~

'- '~_

~ ~

,' "' ~. ', -', P p

', '.

-' "-

,' )-

~' _' -~

'.,' -' " '" ,' ^'~

,', / ; ~

'/( ["

J "~ ,-j." ,.'"'

/ ,,~,"".

~i

Fig. 10. -Variation of surface orientation in _{two senses,} starting from planar

(nflox)

for _non sputtered SiO coating ^{toward the ion direction} b~ ^{for the} large

depth

eroded region.

With reference to the

graph le.

_g. shown in

Fig. 8)

and the observation of texture

(e.g.

shown in

Fig. 9a),

_{we can} determine the

correspondence

^between the surface

profile

and the different

zones of texture. In this way, the

dependence

of _{~b on} the eroded

depth

d is measured.

Figure

I I shows the _~b

id dependence

observed _{on seven}

samples,

with the associated noise.

~b increases

together

^with

d,

and _{saturates at} 90° when d is _over 200

h.

_As

a matter of fact, the

measurement of d cannot be

accurately

made

(because

^{of various} reasons, such _as surface

roughness,

diffusion of the

fringe etc.).

The measurement error on d is about 20-50

h.

According

to the symmetry, ^{four domains should be observed in the nematic}

sample.

In the

non-sputtered planar region,

twin domains with _{same mean} tilt

angle

and different surface azimuthal

angles

_~b

=

0° and _~b =180° may form in the

sample. They

are

separated by

N° I NEMATIC ORIENTATION ON SPUTTERED GLASS 69

1 (°)

o

«

«

. «

«

«

@ o

i,

/fi$$/(()~(i~~~~~')~i~'~~)'~'jl)~~

~fi~,,"",Qj'~'~<,"

_ijj j<i

jl

Q'~i"'~'),'i),'jlj'~~~[li,jlj('j'jl

/~,'~~~Q'~),/I

'j~ i~~,ji(,j<,jlj',i

/fii~~~/,'jil,'jj('j'l jljjijll,l~j

"-"~-~.m ~"~~~~~~~~)~~~~~)~)jj~l' ~~

~~~~~~~~ll 'jlj lij'l'j l~j

~~-

~-~~'-?~'~ _ljj

~i~- ^~~ _-

till j'jl

ii jjj

j-j~ -~$fllfil <',j _jj ep

/- - i~/ jj

~f~l'~$"' _l'ii ^~l<'~l

~

~-~~/$Sf~~ jl,,"ij<ji X

'-~S~~/~)" j"lj'jl'jjj

~'/~~ll/ <'jj,I'

_"I

^ill ill

$~$f~ lj'jljljljljl

b)

Fig.

12. al Nematic _texture

showing

the disclination lines and points defects _on the boundary between the non-eroded

region

and the sputtered track (x 40 ). b) Distribution of the director field

corresponding

to the texture shown in

figure12a.

After

sputtering,

the _texture is observed and shown in

figure

13a. Under

microscope,

^the

eroded track _can be located

only by

a series of marks made _on the other side of the

plate.

At the

edge

^of the

sample,

the non-eroded

region

shows _a

planar

texture (n

along

the

x-axis). By rotating

the

microscope

stage, ^shadow

fringes

move from the

edge

of the

sample

to the centre,

where the black part shows a

homeotropic alignment corresponding

to the maximum eroded

depth.

This _means that the surface orientation

changes progressively

when

increasing

the

eroded

depth. Although

ITO

plates generally

favour _a

planar

but

degenerate (undefined

WI

N° I NEMATIC ORIENTATION ON SPUTTERED GLASS 71

~~i~/I)l~~)')')'i~')

j(jj~~~i~[~jl~ (~lj)'j

)~)l)j~)I))~j~j))) ^bP

$i~~/i~jj~ l~l~lj'i

jii[~~ jj j1 $

'

~ii(~~~l~~~(' I)' lj~'j

~

i)(l'~)lj/(l~l _j~(j~'j

~

,,

°; ;.j/([,)/~ijjl(I j'jllj~'l

b)

Fig.

13. a) ^Texture of _a planar SiO

oblique

evaporated («

= 60°. 3

=

300 h ) _on ITO coated glass plate sputtered

obliquely

(p

= 40° ). On the

right

side of the photograph the black region shows _a

homeotropic

alignment (x 40). b) The distribution of the director field corresponding to the _texture shown in figure13a.

orientation

[6], they

tend to

align

molecules

homeotropically

after

being

^{eroded. Around} the

homeotropic

part, ^{there is} a narrow transition _zone

(about

100-500 ~Lm). It is worth

noting

that outside the transition _{zone ~b}

changes

a little 20-30° ), but o

hardly

varies

(~

5°

).

However,

in the transition _zone both _~b and o vary

sharply.

The distribution of the director field

corresponding

to the texture shown in

figure

13a is

plotted

in

figure

13b. Since the width of the transition _zone is much

larger

than the thickness of the cell

(~

8 _~Lm

),

it is

impossible

for the

curvature elastic interaction from the

hybrid

texture to tilt molecules. Various measurements of surface orientation _are

plotted

on the unit

sphere

^n~

=

I,

_as shown in

figure

14.

Experimentally

we observe that the azimuthal

angle

_~b can

change

in two senses, clockwise and _coun-

terclockwise. From symmetry, ^surface orientations associated with both rotations of the

director should be

equivalent.

Here _we

only

take the absolute value of _~b in

figure14

to

compare ^the results of measurements. From this

figure,

_{we can see} that _~b

approaches

90° and o goes ^to 0°. The director _n tends _to

change

^its orientation from the non-eroded

planar

texture

(along x-axis)

_to the

plane

of

sputtering (y-z plane)

and normal _to the surface. The

measurements are

reproducible, especially

when both _~b and

(90°-

_{H) are} not

large.

For

larger

values of _~b and

(90°- o),

the

experiments

show _some data

scattering.

In fact such

scattering

is not difficult _to understand. We note that the transition _zone shown in

figure

13a is _narrow and

inhomogeneous.

As _a result it is _not easy ^to measure ~b and o very

accurately. Besides,

the surface

morphology

in different

samples

_may also lead to a different variation of surface

orientation. In

spite

of

this,

_we still have _a

good empirical

law of variation of nematic surface orientation. Since _{~b can} twist in two senses, we suppose that _a bifurcation similar _to that

reported

in reference

[17]

_may

probably

be observed, which

depends

_upon the

preparation

of

samples.

In _case I and

2,

_no tilt

happens

for the

sputtering

on the SiO

evaporated glass plates

without ITO

coating.

The tilt from

sputtered

ITO

layer

is _not yet understood. More work has to be done _to

clarify

these

points.

o

-i

~ ~ -o.5

~

~ ^~p

o $

~p

i i

X y

Fig. 14.

Spatial representation

^of the surface director _on the unit sphere, for the figure ^13b nematic orientation.

Conclusion.

We have shown that _an

oblique sputtering

with Ar+ ion-beam _on solid subsuates has _a strong influence on the nematic surface orientation. For _an

isotropic

SiO substrate, the

sputtering

induces a pure

planar

orientation without tilt

along

the

projection

of the local

velocity

of ions _at the surface.

Sputtering

at

grazing

incidence creates the best uniform

planar

texture. Ion

N° NEMATIC ORIENTATION ON SPUTTERED GLASS 73

sputtering perpendicular

to _an

already planar (SiO oblique evaporated) glass plate

results in _a continuous twist in two _senses without

tilting. Increasing

the eroded

depth

from 0 to 200

h,

we

obtain

symmetrical planar anchoring

with local orientation

going

from the initial

planar

^SiO to the 90° twisted ion

velocity projection

_on the surface. For

planar

^SiO

plates,

coated above

by

an ITO electrode, ^the

sputtering

induces both twist and tilt which also vary when

increasing

the eroded

depth. Although

_we do observe the

symmetrical

left- and

right-handed

twists, we have not been able to demonstrate that these surface orientations _were twofold

degenerate.

Acknowledgments.

The authors wish _to thank M. Boix for

preparation

of the SiO

evaporated plates,

Mr. Rousse for

help

ⁱⁿ

sputtering,

and Prof. Blaise and Prof.

Martinot-Lagarde

for fruitful discussions.

Z. M. Sun _was

supported by

a contract n 179909 from the French C.N.R.S.

References

[1] Blinov L. M., Kats E. I., Sonin A. A., Sol,. Phys. Usp. 30 (1987) 604.

[2] Jdrsme B., Rep. Frog. Phys. 54 (1991) 391.

[3] Cognard J., Mtll. Cryst. Liq. Cr_vst. Siippl. Set 1(1982) 1.

[4]

Rapini

A., Papoular M., J.

Phjs.

Colloq. ^{France 30} (1969) C4.

[5] Barbero G., Dozov I., Palierne J. F., Durand G., Phys. Ret,. Lett. 56 (1986) 2056.

[6] Monkade M., ^Boix M., Durand G.,

Ettrtlphys.

Left. 5(8) (1988) 697.

[7] Barbed R., Durand G., Appl. Phys. Lett. 58 (1991) 2907.

[8] Barben R., Giocondo M., Durand G., Appl. Phys. Left 60 (1992) 1085.

[9]

Janning

J., Appl. Phys. Lett. 21(1972) 173.

[10] Le Gressus C., Blaise G., J. Electron Spectiosc. Relat. Phenom. 59 (1992) 73.

[I ii ^Lelidis I., Gharbi A., Durand G., Mol. Cryst. Liq. Ciyst. 223 (1992) 263.

[12] Barbero G., Madhusudana N. V., ^Durand G., J. Phvs. France Lett. 45 (1984j L 613.

[13] Karat P., Madhusudana N. V., Mol. Cryst. Liq. Ciyst. 36 (1976) 51.

[14] Madhusudana N. V., Pratibha R., Mol. Cryst. Liq. Cryst. 89 (1982) 249.

[15] Monkade M., Durand G., Presented at the 14th International Liquid Crystals Conference, Pisa, Italie (June 21-26, 1992).

[16] Barberi R., Durand G., Appl. Phys. Lett. 55 (1989) 2506.

[17] Petrov M., Braslau A., Levelut A. M., Durand G., J. Phys. II France 2 (1992) l159.