Anomalous surface transformations in silicate glasses and a-Si induced by high-power lases pulses

Journal of Non-Crystalline Solids 95 & 96 (1987) 1103 - 1110 North-Holland, Amsterdam

1103

ANOMALOUS SURFACE TRANSFORMATIONSIN SILICATE GLASSES AND a-Si INDUCED BY HIGH-POWER LASER PULSES Yoshihiko KANEMITSU and Yuichi TANAKA* Department of Image Science and Technology, Faculty of Engineering, Chiba University, Yayoi-cho, Chiba-shi, Chiba 260, Japan *The I n s t i t u t e f o r Solid State Physics, The University of Tokyo, Roppongi, Minato-ku, Tokyo 106, Japan Long-range surface transformations in s i l i c a t e glasses and a-Si were observed a f t e r high-power laser pulse i r r a d i a t i o n . Under laser e x c i t a t i o n , intense shock waves propagated hemispherically i n t o the bulk. In s i l i c a t e glasses, cracks and l i q u i d drops appeared outside the l a s e r - i r r a d i a t e d region over a distance of many millimeters. Such a long-range crack formation became v i s i b l e at 12 hours a f t e r laser i r r a d i a t i o n . Long-range and delayed crack formation is caused by chemical reactions at microcracks in the surface induced by l a s e r - d r i v e n shock. In a-Si, high-power laser i r r a d i a t i o n caused ring-shaped c r y s t a l l i z a t i o n in non-irradiated regions. I t was pointed out that l a s e r - d r i v e n shock waves play an important r o l e in surface transformations during high-power laser pulse i r r a d i a t i o n . I. INTRODUCTION Interactions of laser pulses with s o l i d surfaces are of intense i n t e r e s t in such of

f i e l d s as laser annealing of amorphous semiconductors and laser optical

materials.

Focused laser pulses can cause localized

vaporization of solid surfaces.

Moreover,

and dense plasmas at s o l i d surfaces. by

the

High amplitude stress waves are generated As

hot

laser-produced

intense shock waves propagate i n t o solid materials.

High

high temperatures associated with laser i r r a d i a t i o n can i n i t i a t e

and

physical

addition, other

such as s t r u c t u r a l changes in solid

plasmas pressures

and

processes

and

high-power laser pulses produce hot

formation of laser-produced plasmas.

expand,

damaging

melting

chemical

surfaces. 1

In

l a s e r - d r i v e n shock generation has a much higher r e p e t i t i o n rate than

schemes.

means to

study

It

is expected that high-power laser i r r a d i a t i o n is a unique

new laser processing of

solid

surfaces

chemical and physical e f f e c t s . We report here the observation of long-range surface

and

shock-induced

transformations

in

s i l i c a t e glasses and a-Si a f t e r high-power laser pulse i r r a d i a t i o n . In s i l i c a t e glasses,

the formation of cracks accompanied by the appearance of l i q u i d drops

occurred

over

a distance of many millimeters

outside

the

laser-irradiated

region. Such a long-range crack formation became c l e a r l y v i s i b l e at 12 hours or more a f t e r single shot i r r a d i a t i o n . caused

by

Long-range and delayed crack formation

chemical reactions on the glass surface i n i t i a t e d by

0022-3093/87/$03.50 © Elsevier Science Publishers B.V. (North-Holiand Physics Publishing Division)

is

laser-driven

1104

Y. Kanemitsu, Y. Tanaka / Transformations in silicate glasses and a-Si

shock.

Moreover,

crystallization

in in

a-Si,

high-power

laser i r r a d i a t i o n caused ring-shaped

non-irradiated regions.

Laser-driven shock waves play

an

explosive role in c r y s t a l l i z a t i o n in non-irradiated regions. 2. EXPERIMENTAL PROCEDURES The samples used in this work were a soda-lime glass, glass(BK7),

a ~aetal-coated BK7,

a borosilicate crown

a fused s i l i c a glass (fused quartz),

amorphous s i l i c o n . generated by

and an

A picosecond-pulse t r a i n was

an actively mode-locked

Nd-doped

i

I

>SOLID

y t t r i u m - l i t h i u m - f l u o r i d e laser system.

A single

pulse was switched out using a Pockels c e l l , was successively

amplified

by

and

an eight-stage

amplifier of Nd-doped phosphate glass. The output average intensity of the laser beam was 3 J. The output single pulse at 1.05 ~m had a duration lO0 ps. surface (lO -3

of

The laser beam was focused on the sample with

a diameter of

200 ~m in a vacuum

Torr) in order to avoid laser-induced

air

breakdown. Focused laser pulses produced plasmas at the sample surface, waves were generated. samples, to

be

probing

In

hot and dense

and intense shock

transparent

monitored

by

technique.

used as

probe

observed camera.

using

a transmissive

these probe pulses the

optical

was

and a SIT time

delay

we measured

propagation of shock waves as a function

w e r e allowed

temperature.

to

stand

in

Sample surfaces were

morphologically also characterized

single air

laser

at

room

investigated

using a Nomarski microscope and

an optical microscope. changes of

of

laser i r r a d i a t i o n .

The samples irradiated with a shot

FIGURE 1 The propagation of shock waves as a function of delay time:(a) 42.8 ns, (b) 81.6 ns and (c) 120.5 ns. The marker A shows laser-produced plasmas and B shows shock waves.

were

pulses and the shock f r o n t

By changing

time a f t e r

500~m

optical

Second harmonic pulses

between excitation and probe pulses, the

material

the propagation of shock waves was able

The surface of a-Si

using RHEED. Moreover,

surface transformations

in

was time

glasses

were observed with an optical microscope composed of a TV camera and a video tape recorder.

Y. Kanemitsr

3.

EXPERIMENTAL 3.1.

of

RESULTS

Long-range

First,

after

laser-produced the our

the

surface

be

about

shock

1 Mbar

waves

anomalous shown

using

was

able

surface

Figure

(

A

2

of

of

the

shock

be observed

in

shock

all

were

glasses

are

were

waves

pressure

transformations

shows

is

a

in

Fig.21

typical

1105

shock

waves

front

velocity.

( B 1

Fig.1.

at

the

were

When glass

observed.

hemispherically was crudely

from estimated

to

The propagation

used in

in

observed

waves

glasses

as a function

shown

propagated

observed

photograph

at

outside

the

accompanied of

0 hour

both

to

the liqiud

central by

damage the

of

in

this

specific

the

work.

of However,

glasses

delay.

laser

spot

as will

region. also

1 C 1 were

region.

These of

liquid

The

be

and

BK7,

but

In

laser

Fig.Z(b))

became at

on the

12

sample

structural

hours surface

transformations

clearly not

damage

after

Fig.Z(cl,

observed

were

Figure laser

At 4 hours ( B in

were

surface

irradiation.

localized

size.

delayed drops

delayed

laser

transformations

la&r-irradiated

glass

and

after

drops

appearance

soda-lime

long-range time

time

structural

outside small

of

a function

corresponded

far delay,

results

BK7 as

long-range

observable

surface

and a-.%

glasses in

results

of shock

The

silicate waves

Fig.l(b))

propagation

values to

in

irradiation,

time

glasses

below.

transformations 2(a)

A in

conditions, bulk.

shock

Typical

(denoted and

in

of

irradiation.

generation

the

in silicofe

formation

propagation

laser

experimental into

crack

the

plasmas

surface, Under

delayed

measured

time

/ TransJormcrrions

AND DISCUSSION

and

we

delay

Y. Tannko

observed seen

FIGURE 2 Long-range and delayed morphological changes in function of delay time after laser The i;;t;i;t;ms D hour, (b) 4 hours, and (cl 12 hours. laser-irradiated region, B shows the outer ring of changes and C shows small liquid drops.

in

on fused

BK7

silica

as

a :,a:

structural

the

Long-range

glass. structural in

transformations

the

glass

surface

chemical laser

of

soda-lime

glass

surface

irradiated

500 vrn

Cracks

around

vapor

interactions is promoted

also

stimulated the

accompanied

are

at the

liquid

laser

by the

the

conditions, by

associated

with form

irradiated

to the

of

atmospheric

water

of liquid

shown

the

propagation

The the

and the drops

in

formed

with

and that

the

hand,

on metalthin

metal

water glass

strongly

glass

200 pm,

drops These

of complex waves

vapor

surface.

suggest

that

atmospheric growth

water

of cracks

in

glasses

vapor.

This were

position spirally,

at

High

is

observed

of cracks as

pressures

surface

known in the formed mentioned

found

far

glass spirally above.

temperatures

are considered the

promotes

alkalies) because

propagate

outside

micrqcracks

glasses

to be generated

high

waves

of

containing coincided

to laser-

chemical

the

glass

long-range

In

5 mm

experimental

waves

and

shock

cracks of

silicate our

Shock

regions

(especially is

in

were

of

a distance

Under

plasmas.

Fig.1.

formation over

transfo?mations

at non-irradiated stress

the

occurred

processes.

of laser-driven

components

transformations

alkali

shock

as

region.

findings

formed

other

the

attaching

an

of cracks

formed

from

laser-produced

microcracks

interactions

structural

of

were where

atmospheric

was about

of liquid region.

amplitude

formation

hemispherically

initially

size

appearance

to be composed high

the

drops that

prevents

of cracks,

nor spirally region.

On the

drops

was

interactions.

spot

laser-irradiated

considered

circle appeared

with

glass,

of the

nonlaser-

shape

film

components

position

by chemical

Although

outside

of the

d

the

of liquid

spirally.*

coated

These chemical

around

laser-irradiated

coincided

formed no

the

also

position

by after

clearly

on

concentric

the

spirally

FIGURE 3 morphology of the lasersoda-lime glass by chemical etching. are formed spirally the laser-irradiated

Surface irradiated treated Cracks outside region.

a

ellipse,

The

hours

region;their

neither

laser-

treated

Cracks

spirally

irradiated

shows

the

at 48

irradiation.

appeared

amount

Fig.3

morphology

etching

found

a large

Furthermore,

irradiated

delayed were

containing

of alkali. the

and

and a large with

addition,

with delayed amount

that delayed

of

Y. Kanetnitsu.

structural

changes

is

against

strong

contain

were

and

silica

glass.

in

silicate

the

glass

and

delayed

has

Explosive is

are

thermal

caused

by

Fused

to

delayed

it 4

be observed surface

interactions

at microcracks

in

accepted a-Si

not

Thus,

long-

in

fused

transformations

of in

glass does

the

the

components

surface

of

induced

by

morphology

does of

will

a-Si

shown

a-Si

that

occurs

crystallization

not after

below,

during

via

the

occur

in

picosecond and

pulsed

liquid

the

the

laser

phase solid

and phase.

high-power mechanisms

irradiation

purely 5 laser

of

the

laser

the

irradiation

is

crystallization

E/E,1

FIGURE 5 Square of the diameters L of central crystalline region (A the amorphous ring (0) and crystalline ring (0) outside amorphous ring as a function laser energy density. crystallization threshold EC1 below the amorphization threshold and the other crystallization Ea* threshold &is above Ea.

100 pm FIGURE 4 recrystallization a-Si after at 0.93 J/cm?

thermally However,

%I Ea Ec2

A multiannular pattern of irradiation

silica

because

resistance.

1107

waves.

in

as

chemical

and a-3

glass.

not

and

glarsm

shock,

shock

considered

vapor

crystallization

crystallization

silica

thermal

long-range

water

widely

complicated

a greater are

in silicate

fused and

that

atmospheric shock

It

in

reactions

transformations

glasses with

3.2.

surface

it

/ Truns/ormotions

observed

We conclude

laser-driven

activated

not

chemical

alkalies

range

Y. Tanaka

the ), the the of One is

process

are

energy

density

still

not

multiannular

of

because

it

optical

microscope

is

diameters

of

From below

the

0.4 amorphization

simple

melting

liquid

be explained homogeneous the

by

ultra-rapid

are

near

melting

*inhomogeneous either

solid

picosecond

time

melting

cause

threshold

crystallization

observed.6

melting

model

This 5

liquid-solid

size

pulses

in implies

Shock

because Clusters of

of

degrees

the

sense or

scale.6

of

the

negative

of

c-Si

are

a-Si,

that

the

crystal

liquid

surface

that

shock due

that

crystallization

existence

not

melted

the

melting of

studied is

generated heating

and

of

clusters volume

at

point

so

c-Si

below

is

melting

is

rapidly

indistinguishable as well

the

intensities of

The

c-Si

for

laser

occurs

intensities

the hand,

ultra-rapid

a-Si.

stress

no

of

other

morphology

activation

nearly

shock at

by The

cannot

stress

to

nucleation is

was

and

On the

the

that

phase

We believe

anomalous

because than

be

preceded

on the

transition,

higher

explained

crystallization

based

stress

amorphous-crystalline

J/cm2)

be

expected,

interface.

dependence

laser

to

transition.

was

planar

for

found

(0.6 can

transition

processes.7

phase

was

amorphous-liquid-crystalline in

The

density

thresholds

been

threshold

in the

this

the

amorphous-liquid-amorphous

crystallization and

the

(the

energy

J/cm2)

crystallization

spot

rapidly,

hundreds

threshold other

two

(0.25

have

30 ps

very

determine

of

that

and

the

a function

sense

the

heating

the

easily

by

by RHEED.

would

and

and

respectively, as

The

microscope, observed

the

process.

several

plotted

a

Fig.4.

optical

melt

conventional

crystallization

are

in

region

any

by the

enhances

formed

a-Si,

anomalous

the

fs

and

an

the

J/cm'),

shown

bright

above

(0.25

previously

during

400

processes

of is

pulse-duration using

as c-Si

just

a-Si as

thresholds

The

which

melting

by

the

can

of

using

and

threshold model

in

phase

we

intensities

observed

observed

amorphization

crystallization

amorphization

was

crystallization

J/cm2).

the

former

of

laser melting

region

patterns

Fig.5,

the

surface

easily

dark

identified

One

transition,

the

multiannular

Fig.5.

clearly

was

that is

At

the

pattern

a-Si

known

crystallization.

above

understood. for

recrystallization

crystallization

in

well

requried

that on

as the

the

inhomogeneous amorphization

threshold. Laser-driven

stress

intensities

just

amplitude during

stress

driven

shock

are

laser-irradiated

region. oblique

incidence

by the

6 shows

Hence,

Ring-shaped shape the

is

the

surface

the energy

central profile

even

mentioned of

above.

laser-produced

we demonstrate explosively morphology

crystallization of

crystallization as

formation

crystallization

The

in

threshold,

irradiation.

Figure

10 kJ/cm?

role

melting

generated

pulse

irradiation

waves. at

an important

the

laser

laser

irradiation

ellipse;at

waves

high-power

high-power

plays

above

a-Si

was observed laser-irradiated deposited

low High

plasmas that

caused of

at

during by

after

laserlaser

around

the

region

was

on the

surface

Y. Kanemirsu.

Y. Tanaka

/ Trans/ormations

in silicafe

glasses

1109

and o-S

depended

on the

angle

of

the

pulse.

However,

a

crystalline

region

formed

Ploywas

circulaly

the

around

laser-irradiated

region

even

at

incidence did

an oblique

and not

laser

its

shape

depend

on

the

angle

of

the

incident

pulse.

Ripple

structures

were

formed

also

concentrically,

and

the

ripple

decreased

with

distance

from

spacing increasing the

spot

The

center.

500pm’

incident

laser

where

ripple

were

region

structures formed

was

recrystallized.* FIGURE 6 morphology of a-Si at 10 kJ/cm2.

Surface irradiation

The front

crystallization after

laser

seems

to

outward

crystallization shock

process. waves

incidence. of

and

ripple

magnitude neighboring strongly explosive responsible

propagate

like

in

are

shock

ring

kick

with

the

by in

experimental

conditions, waves

The laser-driven

narrower

than

the

even

at and

shock

ripple

spacing

shock

waves

crystallization.

spot to

the because

an the

waves

oblique formation

playing

around

waves.

region, the

the

for

be a

to

The because

center. possible

an spot

the

corresponds

shock

from

from

a circle,

regions

previous

distance by

as

surface

laser-driven

by

ring-shaped

formed

our

propagate

bulk

trigger

crystallization

provided

driven observed

is

the

the

external

non-irradiated

8,9

increasing

crystallization for

explained

dominoes.

becomes

decrease

on ring-shaped

crystallization

a game of of

of

an

region

concentrically

structures role

as

crystallized

The characteristics

explosive center

The

operate

spot

Under

shock

hemispherically

the

center.

into surface

propagate

from

the

succeeding shock We

waves

consider mechanism

1110

Y. Kanemiau,

4.

Y. Ta~rolu

/ Trans/ormations

in silicurc

glasses

and o-Si

SUMMARY We

glass

have

shown

surfaces

that

are

transformations

glasses

components

are

of inouced

glass laser-driven

explosive

laser-driven

shock

waves

in

High

pressures

surface

and

high new

during

it

for

chemical

and

the

was

silicon

physical

that

initiated

by shock

pulse

with

study

the

out

laser-driven

laser

irradiation.

high-power

of

the

in

pointed is

of

associated

possibilities

in of

at microcracks

importance

high-power

temperatures

shock-induced

vapor

amorphous the

the

interactions

Moreover,

in

We stressed

outside transformations

chemical

water

Structural

millimeters surface

by

waves.

crystallization

open

materials

shock

transformations

many

be caused

in

irradiation.

delayed

atmospheric

transformations

pulse

of

and to

with

waves.

and

irradiation

a distance

considered

by

structural

laser

Long-range

the

ring-,shaped

delayed

high-power

over

region.

silicate

and

by

occurred

laser-irradiated

surface

long-range

caused

shock

laser

processing

of

effects.

ACKNOWLEDGEMENTS The

authors

Shionoya,

would

Prof.

encouragement

H.

like

to

Kuroda

throughout

express

and

this

their

Prof.

5.

deep

appreciation

Imamura

for

to

helpful

Prof.

S.

discussions

and

work.

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