Structure and phase transformations of sputtered NbFe and NbCo films

Journal of Non-Crystalline Solids 117/118 (1990) 199-202 North-Holland

199

STRUCTURE AND PHASE TRANSFORMATIONS OF SPUTTERED Nb-Fe AND Nb-Co FILMS Hans-Ulrich Institut D-3400

KREBS, W o l f g a n g BIEGEL, RUdiger BORMANN, and Ralf BUSCH

fur Metallphysik, Universit~t G 6 t t i n g e n , F. R. G e r m a n y

G6ttingen,

HospitalstraSe

3-5,

N b - F e and N b - C o thin films w e r e p r e p a r e d over a wide c o m p o s i t i o n range by t r i o d e - s p u t t e r i n g at r o o m t e m p e r a t u r e and at 3 0 0 °C, r e s p e c t i v e l y . The f i l m s w e r e i n v e s t i g a t e d by X - r a y m e a s u r e m e n t s and t r a n s m i s s i o n e l e c t r o n m i c r o s c o p y in o r d e r to s t u d y the s t r u c t u r e and the f o r m a t i o n r a n g e of the d i f f e r e n t phases. The r e s u l t s w e r e c o m p a r e d with Gibbs f r e e e n e r g y c u r v e s of the a m o r p h o u s and the e q u i l i b r i u m phases in the N b - C o s y s t e m c a l c u l a t e d by the calphad m e t h o d . C o n t r a r y to the liquid alloys rapidly q u e n c h e d / t h e s o l u b i l i t y of the t e r m i n a l phases p r e p a r e d by s p u t t e r i n g is not enhanced up to t h e i r m a x i m u m value. The e x t e n s i o n of the solid s o l u t i o n N_bbFed e c r e a s e s f o r higher s u b s t r a t e t e m p e r a t u r e s . For N b a o F e 2 o the t r a n s f o r m a t i o n into the m e t a s t a b l e e q u i l i b r i u m b e t w e e n the a m o r p h o u s and the bcc solid s o l u t i o n was m o n i t o r e d by i n - s i t u X - r a y e x p e r i m e n t s . For the d e c r e a s e of the volume f r a c t i o n as well as f o r the F e - c o n c e n t r a t i o n of the N bFe g r a i n s we found s q a r e - r o o t of time l a w s .

I. INTRODUCTION

On some s a m p l e s i n - s i t u X - r a y e x p e r i m e n t s w e r e

A m o r p h o u s binary alloys can be p r e p a r e d by a

u n d e r t a k e n on a hot s t a g e under a vacuum b e t t e r

v a r i e t y of d i f f e r e n t m e t h o d s , f o r e x a m p l e by rapid

than 10 - 5 m b a r using a c o n s t a n t heating r a t e of

quenching f r o m the melt, by v a p o r d e p o s i t i o n or -

1.5 ° C / m i n . X - r a y scans w e r e p e r f o r m e d e v e r y 6

since r e c e n t y e a r s - by s o l i d - s t a t e a m o r p h i z a t i o n

minutes.

first

m e n t s w e r e done at high t e m p e r a t u r e s up to 8 5 0 °C.

reported

by S c h w a r z

and Johnson 1. It

is

In a d d i t i o n , i s o t h e r m a l

X-ray

measure-

i n t e r e s t i n g to s t u d y the r a n g e of f o r m a t i o n of the

For the N b - C o s y s t e m the f r e e e n e r g y of the

d i f f e r e n t phases and t h e i r t r a n s f o r m a t i o n s into the

m e t a s t a b l e and s t a b l e phases and the e q u i l i b r i a

thermodynamic

between

equilibrium.

Here

we

describe

them

were

determined

by the

calphad

r e s u l t s on s p u t t e r e d N b - F e and N b - C o thin films

method ( c a l c u l a t i o n of phase d i a g r a m s ) 2,3. T h e r e b y

and c o m p a r e the f o r m a t i o n r a n g e of the a m o r p h o u s

the t h e r m o d y n a m i c f u n c t i o n s of the m e t a s t a b l e and

and the solid s o l u t i o n phases with t h o s e p r e d i c t e d

s t a b l e phases a r e c a l c u l a t e d by using the e x p e r i -

by c a l c u l a t e d f r e e e n e r g y c u r v e s .

mental t h e r m o d y n a m i c

data of the alloy s y s t e m .

The a m o r p h o u s phase is t r e a t e d as an u n d e r c o o l e d 2. EXPERIMENTAL PROCEDURES

liquid at the glass t r a n s i t i o n .

N b - F e and N b - C o thin films with a t h i c k n e s s of about 1 ~m w e r e d e p o s i t e d on A I 2 0 3 or KBr s u b s t r a t e s by t r i o d e s p u t t e r i n g

in a vacuum s y s t e m

3. RESULTS AND D I S C U S S I O N As

confirmed

by

X-ray

measurements

the

with a base p r e s s u r e of about 10 - 8 mbar. W h e r e -

s p u t t e r e d N b - F e and N b - C o films

as most of the f i l m s w e r e p r e p a r e d at r o o m t e m -

a m o r p h o u s phase and the t e r m i n a l s o l u t i o n s .

p e r a t u r e , some F e - p o o r N b - F e films w e r e s p u t t e -

i n t e r m e t a l l i c c o m p o u n d s could be d e t e c t e d even f o r

r e d at a s u b s t r a t e t e m p e r a t u r e of 3 0 0 °C.

the s u b s t r a t e t e m p e r a t u r e of 3 0 0 °C. Fig. 1 s u m m a -

The c o n c e n t r a t i o n of the f i l m s was m e a s u r e d by

rizes

the r e s u l t s

of

the

c o n s i s t of the

structural

analyses

No

of

e l e c t r o n m i c r o p r o b e analysis. A f t e r d i s s o l v i n g the

N b l o o _ x F e X films s p u t t e r e d at room t e m p e r a t u r e

KBr s u b s t r a t e s

o v e r the c o m p o s i t i o n

transmission

some films w e r e

electron

microscopy

investigated

by

experiments.

0022-3093/90/$03.50 (~) Elsevier Science Publishers B.V. (North-Holland)

range of 8 " x " g 3

at.X

Fe.

Pure N b - f i l m s and films with low Fe c o n c e n t r a t i o n

H.-U. Krebs et al./Sputtered Nb-Fe and Nb-Co films

200

between 77 and 88 at.X Fe. For higher Fe contents I I

44

• crystalline o amorphous

I I

the peak position of the (110) r e f l e x for the bcc-F_eeNb phase linearly increases. The lattice p a r a m e t e r of

I

the pure Fe film (2.864 ~,) agrees with the one

O) t..

~42

observed in bulk material. In o r d e r to study the influence of the s u b s t r a t e

CM

t e m p e r a t u r e on the f o r m a t i o n range of the b c c -

I

N bFe phase, F e - p o o r

films

with

concentrations

between 8 and 28 at.% Fe were also s p u t t e r e d at 3~

NbFe

I

'

IF--~eNb !FeNiE l,,am. - -

amorphous

N__bbFe + am.

I

2'o

I

4

I

I I

60

ment of the surface diffusion (during the p r e p a -

II

B0

lOO

x~bt.%)

Nb

300 °C. At this s u b s t r a t e t e m p e r a t u r e the enhance-

Fe

ration) reduces the extension of the N__bFe singlephase to about 11 at.% Fe. The concentration dependence of the d i f f e r e n t

FIGURE 1 Compositional ranges of the amorphous phase and of the crystalline terminal solutions for s p u t t e r e d NbFe films, determined by the position of the r e s p e c t i v e most intensive X - r a y peak

s t r u c t u r e s for Nb-Co films is similar to the results observed for the Nb-Fe films. As in Nb-Fe a t w o phase

region

between

the

bcc-NbCo

and

the

amorphous phase is observed for c o n c e n t r a t i o n s

are strongly t e x t u r e d and mainly show the (110)

from 16 to 30 at.X Co, However, the limits of the

peak in X - r a y

N b - p o o r t w o - p h a s e region are shifted to higher

position of this

measurements. peak

For pure

corresponds

to

Nb the

a lattice

C o - c o n t e n t (86 and 91 at,X Co, respectively),

O

constant of 3.315 A, which is about 0.15 ~, larger

For some of the samples the s t r u c t u r e and the

than in bulk Nb. Obviously the N b - r i c h thin films

m i c r o s t r u c t u r e of the films was analyzed by TEM

are less dense probably due to the rapid cooling 0

process

during

deposition.

With

increasing

,

.

'

i

.

.

.

.

J

.

.

.

.

+

'

'

'

Fe

c o n c e n t r a t i o n the l a t t i c e constant of the bcc N bFe phase linearly decreases with a rate of 0 . 0 0 4 6 O

A/at.XFe

up to

16 at.X Fe, in agreement

with

V e g a r d ' s law. Between 16 and 30 at.~ Fe a t w o -

q3

~-iO

phase region of the bcc-NbFe and an amorphous phase e x i s t s , indicated by a position of the (110) peak being independent of the c o n c e n t r a t i o n . The

NU-Co <3 -15

T=550° C

~ NbCo

NbC°3 NbCo2

upper limit of the t w o - p h a s e region is d e t e r m i n e d by the vanishing of the intensity of this peak+ Between 30 and 77 at.X Fe the Nb-Fe films are completely

0.25

0.50

0.75

1.00

Co - Concentration

amorphous. The position of the broad amorphous maximum linearly increases up to about 55 at.~ Fe with the same rate as the peak position of the crystalline bcc-NbFe phase. Between 55 and 60 at.% astepwiseincreaseoccurs, followedbyatwo-phase region of the amorphous and the bcc-FeNb phase

FIGURE 2 The free energy of the metastable and equilibrium phases at 650 °C. The f r e e energies of bcc Nb and fcc Co are chosen as energy r e f e r e n c e points. The t r a n s f o r m a t i o n of s u p e r s a t u r a t e d N__bCo into the metastable equilibrium between N bCo and the amorphous phase is marked

H.-U.

Krebs et al./ Sputtered Nb-Fe and Nb-Co films

201

at.X Co) and the a m o r p h o u s phase ( w i t h 42 at.X Co)

ZJ..2

is included. I.I,.C

An e x t e n s i o n of s u p e r s a t u r a t e d N__bbCoup to about

"•39f.

26 at.~ Fe was v e r i f i e d in bulk s a m p l e s p r e p a r e d

22

by a piston and anvil t e c h n i q u e 4, Under our p r e p a -

L cr= (3J

ration

,#

conditions

such a large e x t e n s i o n of

the

17

t e r m i n a l s o l u t i o n s and the a m o r p h o u s region is not 390

r e a c h e d i n d i c a t i n g t h a t the m o b i l i t y of the a t o m s during d e p o s i t i o n was l a r g e enough f o r a d e c o m p o sition into t w o phases. H o w e v e r , even a s u b s t r a t e

38.5

t e m p e r a t u r e of 300 °C is not high enough to a t t a i n the m e t a s t a b l e e q u i l i b r i u m s t a t e . In a d d i t i o n , we p e r f o r m e d s e r i e s of i n - s i t u X - r a y

3BC

e x p e r i m e n t s with a heating r a t e of 1.5 K / m i n

on

s a m p l e s which e x h i b i t an e x t e n d e d t e r m i n a l solution. I

I

200

i

i

400

i

/

i

5(30

I

From

800

T (°C)

the

expects

free the

energy

formation

curves

(see

of

amorphous

an

Fig.

during h e a t i n g , as long as the f o r m a t i o n FIGURE 3 Change of the (110) peak p o s i t i o n with t e m p e r a t u re during annealing of NbFe films f o r d i f f e r e n t c o m p o s i t i o n s at a heating r a t e of 1.5 ° C / r a i n

2)

one

phase of the

c r y s t a l l i n e phases a r e k i n e t i c a l l y s u p p r e s s e d . The actual a v e r a g e c o n c e n t r a t i o n of the bcc NbFe or F eNb phase can be d e t e r m i n e d by the c o n c e n t r a t i o n d e p e n d e n c e of the (110) d i f f r a c t i o n peak ( c o r r e c t e d

i n v e s t i g a t i o n s . They d e m o n s t r a t e t h a t a film of 22

by the t h e r m a l e x p a n s i o n of the l a t t i c e ) . As shown

at.~ Co e x h i b i t s long m i c r o c r y s t a l l i n e NbCo g r a i n s

in Fig. 3 the F e - c o n t e n t in the bcc N bFe d e c r e a s e s

in an a m o r p h o u s m a t r i x and, t h e r e f o r e ,

p r o o v e the

above 5 0 0 °C and r e a c h e s the same value at about

e x i s t a n c e of the t w o - p h a s e r e g i o n on the N b - r i c h

700 °C f o r all N b - r i c h samples. At this t e m p e r a t u r e

side. C o n t r a r y to t h i s , a film with 35 and 50 at.~

the samples a r e in t h e i r m e t a s t a b l e e q u i l i b r i u m . At

Co, r e s p e c t i v e l y ,

low-contrast

higher t e m p e r a t u r e s the a m o r p h o u s phase c r y s t a l -

p i c t u r e and a b r o a d ring in the d i f f r a c t i o n p a t t e r n ,

lizes into the NbFe i n t e r m e t a l l i c compound and the

both t y p i c a l f o r an a m o r p h o u s s t r u c t u r e .

t e r m i n a l s o l u t i o n . Also f o r the F e - r i c h s a m p l e s the

only

showed

the

These r e s u l t s can be c o m p a r e d with f r e e e n e r -

formation

of the a m o r p h o u s

phase is c o n n e c t e d

gy c u r v e s of the m e t a s t a b l e and s t a b l e phases in

with a c o n t i n u o u s r e d u c t i o n of the Nb c o n t e n t

the N b - C o s y s t e m , which w e r e c a l c u l a t e d by the

the FeNb phase i n d i c a t e d by an i n c r e a s e

calphad m e t h o d (Fig. 2) 3 . Due to the s i m i l a r i t y of

peak p o s i t i o n (see Fig. 3).

the phase d i a g r a m s the c a l c u l a t i o n s f o r the N b - F e

In o r d e r to study the t r a n s f o r m a t i o n

in

of the

k i n e t i c s into

phases should be c o m p a r a b l e to the N b - C o s y s t e m .

the a m o r p h o u s phase and the c r y s t a l l i z a t i o n p r o -

The limits of the s i n g l e - and t w o - p h a s e r e g i o n s a r e

cess, isothermalin-situ

given by the common t a n g e n t s applied to the f r e e

performed

e n e r g y c u r v e s of the c o r r e s p o n d i n g

r e s u l t s show that the peak p o s i t i o n as well as the

phases.

For

at

X-ray measurements were

6 5 0 °C on a N b s o F e 2 o

film.

The

i n s t a n c e , in the f i g u r e the t a n g e n t f o r the m e t a -

peak i n t e n s i t y of the (110) r e f l e x f o r the bcc N__bbFe

s t a b l e e q u i l i b r i u m b e t w e e n the b c c - N b C o ( w i t h

phase d e c r e a s e s with time. T h e r e f o r e , both the Fe

3

H.-U. Krebs et al./Sputtered Nb-Fe and Nb-Co films

202

avoid the f o r m a t i o n of i n t e r m e t a l l i c

39.0" ~ F

compounds.

H o w e v e r , s u r f a c e diffusion allows a chemical and

-~3B= .

structural

650°C

decomposition

film

deposition

leading to the f o r m a t i o n of t w o - p h a s e regions for c e r t a i n c o m p o s i t i o n s . With

3BO ~ . . . .

during

-,.

i

decreasing s u b s t r a t e

t e m p e r a t u r e the e x t e n t i o n of the solid solution of the N bFe phase is enhanced. T h e r e f o r e , some of the samples s t a r t in a state, which is e n e r g e t i c a l l y

3?5

above that of the m e t a s t a b l e equilibrium.

During

annealing they can l o w e r the f r e e energy t o w a r d s

i

i

the m e t a s t a b l e equilibrium: The t r a n s f o r m a t i o n of

i

supersaturated

~r (~1

N__bbFe is f i r s t

connected

with

a

continuous r e d u c t i o n of the Fe c o n t e n t in the N__bbFe grains and the f o r m a t i o n of an a m o r p h o u s phase,

FIGURE 4 Plot of the change in the (110) peak position and its i n t e n s i t y f o r the NIoFe solid solution with V~ during i s o t h e r m a l annealing at 6 5 0 °C

l a t e r with the c r y s t a l l i z a t i o n of this phase into the stable t h e r m o d y n a m i c equilibrium s t a t e . In i s o t h e r mal heat t r e a t m e n t s we observed that during this

c o n c e n t r a t i o n in the N bFe phase and its volume

t r a n s f o r m a t i o n the d e c r e a s e of the c o n c e n t r a t i o n

f r a c t i o n a r e reduced due to the f o r m a t i o n of the

as well as the volume f r a c t i o n of the N bFe grains

a m o r p h o u s phase. In Fig. 4 the data a r e p l o t t e d vs.

due to the f o r m a t i o n of the a m o r p h o u s phase obey

square r o o t of time t. Obviously, over a t i m e scale

a ~ - law.

of about 40 hours the peak position obeys a ~ - law, indicating t h a t during the a m o r p h i z a t i o n and

REFERENCES

c r y s t a l l i z a t i o n p r o c e s s e s the c o n c e n t r a t i o n within

1. R. B. S c h w a r z and W. L. Johnson, Phys. Rev. L e t t . 51 (1983) 415.

the N.__bbFegrains linearly d e c r e a s e s with V't. For s h o r t times also the i n t e n s i t y of the (110) r e f l e x and, t h e r e f o r e , the volume f r a c t i o n of the N bFe phase d e c r e a s e s linearly with VT. The amount of the a m o r p h o u s phase f o r m e d is p r o p o r t i o n a l to the diminishing of the N.__bbFe phase. A f t e r

about

27

hours a change of the slope is visible in Fig. 4 originating

from

Fe-poor

N__bbFe f o r m e d

during

c r y s t a l l i z a t i o n of the a m o r p h o u s phase.

4. CONCLUSIONS The r e s u l t s transformations

of the s p u t t e r e d during

an

films

and t h e i r

annealing

treatment

can be explained by the f r e e energy curves d e t e r mined

by

the

calphad

method.

Obviously,

the

quenching r a t e of the s p u t t e r i n g process at subs t r a t e t e m p e r a t u r e s up to 300 °C is high enough to

2. R. Bormann, F. G ~ r t n e r and K. Z ~ l t z e r , J. L e s s Common Met. 145 (1988) 19. 3. R. Bormann and R. Busch, this c o n f e r e n c e . 4. H. S c h l l l t e r , H. C. F r e y h a r d t , H. U. Krebs, and R. Bormann, Zt. Phys. Chem. 157 (1987) 4.07.