RF-sputtered Co-Cr layers for perpendicular magnetic recording. 1 ...

Thin S o I i d F i l m s , 101 ( 1 9 8 3 ) 61 7 3 PREPARATION

AND

6!

CHARACTERIZATION

R.F.-SPUTTERED Co-Cr LAYERS FOR PERPENDICULAR RECORDING I: S T R U C T U R A L P R O P E R T I E S J. C. LODDER,

T.'WIEt~INGA

AND

MAGNETIC

J. WORST

Department 0 / Electrical Engineering, Twente University O/ Technology, 7500 A E Ensehede (The Netherlands )

(Received July 12, 1982; accepted November 9, 1982)

C o - C r layers for the p e r p e n d i c u l a r r e c o r d i n g m o d e were d e p o s i t e d by m e a n s o f r.f. s p u t t e r i n g u n d e r suitable c o n d i t i o n s in a n a r g o n plasma. T h e films were c h a r a c t e r i z e d s t r u c t u r a l l y by X - r a y a n d e l e c t r o n m i c r o s c o p y a n d the m a g n e t i c p r o p e r t i e s were d e t e r m i n e d with a v i b r a t i n g s a m p l e m a g n e t o m e t e r a n d b y t o r q u e measurements. T h e influence of s o m e s p u t t e r p a r a m e t e r s o n the s t r u c t u r a l a n d m a g n e t i c p r o p e r t i e s are discussed. E v e n very thin layers s h o w a high h.c.p. [001 ] o r i e n t a t i o n o n several substrates. T h e i n - p l a n e m a g n e t i c r e m a n e n c e Sil ( = M r / M ~ ) is very small ( a b o u t 4~/u). As will be s h o w n in P a r t II the m a g n e t i c a n i s o t r o p y is c a u s e d by crystal a n i s o t r o p y . T h e c o r r e l a t i o n b e t w e e n the h.c.p, c axis o r i e n t a t i o n a n d S~l s u p p o r t s this c o n c l u s i o n . T h e g r o w t h m e c h a n i s m o f the layer is discussed.

1.

INTRODUCTION

T h i n f e r r o m a g n e t i c films with p e r p e n d i c u l a r m a g n e t i z a t i o n h a v e been p r o p o s e d for high d e n s i t y r e c o r d i n g a n d are c o n s i d e r e d to be s u p e r i o r to films m a g n e t i z e d in the l o n g i t u d i n a I m o d e t-3. C o b a l t has a large m a g n e t o c r y s t a l l i n e uniaxial a n i s o t r o p y in the d i r e c t i o n o f the h.c.p, c axis. In the c o u r s e o f d e v e l o p i n g a m e d i u m with p e r p e n d i c u l a r m a g n e t i c a n i s o t r o p y several c o b a l t m e t a l alloys h a v e been d e v e l o p e d 4" ~. U p to n o w s p u t t e r e d C o - C r films h a v e been a c c e p t e d as the alloy films with the best p r o p e r t i e s for high d e n s i t y r e c o r d i n g . T h e s e properties, o f course, d e p e n d o n the r.f. s p u t t e r i n g c o n d i t i o n s as has been discussed by several r e s e a r c h e r s 6 8. N e w results f r o m standstill r e c o r d i n g e x p e r i m e n t s o n o u r s p u t t e r e d C o C r films h a v e recently been p u b l i s h e d 9. In this p a p e r ( P a r t I) we describe the s t r u c t u r a l p r o p e r t i e s o f o u r films w h i c h are m a d e b y using o p t i m i z e d s p u t t e r p a r a m e t e r s with v a r i o u s substrates. T h e typical d e v e l o p m e n t o f the s t r u c t u r e of the C o - C r layers was studied in detail b y X - r a y d i f f r a c t i o n a n d X - r a y r o c k i n g c u r v e techniques, special a t t e n t i o n b e i n g paid to the initial layer (25 n m o r less). F u r t h e r m o r e t r a n s m i s s i o n electron m i c r o s c o p y ( T E M ) a n d s c a n n i n g e l e c t r o n m i c r o s c o p y ( S E M ) t e c h n i q u e s Were used for the s t u d y 0 f b o t h the m o r p h o l o g y a n d the g r o w t h processes o f the crystallites. 0040-6090/83/0000-0000/~03.00

~ Elsevier Sequoia/Printed in The Netherlands

62 2.

J.C.

PREPARATION

LODDER,

T. W I E L I N G A ,

J. W O R S T

OF THE FILMS

A L e y b o l d - H e r a e u s Z 4 0 0 c o m m e r c i a l r.f. s p u t t e r i n g s y s t e m was used. T h e m o s t i m p o r t a n t c o n d i t i o n s for r e p r o d u c i b l e d e p o s i t i o n o f films are listed in T a b l e I. T w o types of t a r g e t were used for p r o d u c i n g C o - C r layers, n a m e l y a m u l t i t a r g e t ( c o b a l t t a r g e t c o v e r e d with 13 r o u n d c h r o m i u m pieces) a n d a n a l l o y e d target ( M a t e r i a l s R e s e a r c h C o m p a n y ) . U s i n g the s t a n d a r d c o n d i t i o n s m e n t i o n e d in T a b l e I we o b t a i n e d layers with a c h e m i c a l c o m p o s i t i o n of 15 at.~/o C r for the m u l t i t a r g e t a n d 19 at.~/o C r for the a l l o y e d target. T h e c h e m i c a l c o m p o s i t i o n a n d the film thickness were d e t e r m i n e d b y X - r a y f l u o r e s c e n c e 1°. N o significant v a r i a t i o n in the c h e m i c a l c o m p o s i t i o n was m e a s u r e d o v e r a n a r e a o f 5 c m 2 in the centre of the s u b s t r a t e holder. T--his is reflected b y the c o n s t a n t value o f the s a t u r a t i o n m a g n e t i z a t i o n M~ w h i c h is s t r o n g l y d e p e n d e n t o n the c o m p o s i t i o n . TABLE

I SPUTTER CONDITIONS Background pressure Ar pressure R,f. v o l t a g e R.f. c u r r e n t Deposition rate Target-substrate distance Target diameter Substrate holder temperature

< 10 7 m b a r 4 × 10- a mbar 1.6 k V 225 mA 2.2 ~t s 1 50 mm 100 mm Ambient (water cooled)

I n o r d e r t o o b t a i n r e p r o d u c i b l e layers of high q u a l i t y the following p r o c e d u r e is used. (i) T h e bell-jar is b a k e d for 10 h at 1 0 0 ° C until a b a c k pressure of less t h a n 1.5 x 1 0 - 7 m b a r is reached. By m e a n s o f a liquid N 2 M e i s s n e r t r a p the pressure is f u r t h e r d e c r e a s e d to b e l o w 1 0 - 7 m b a r . (ii) T h e t a r g e t is c l e a n e d before d e p o s i t i o n by p r e s p u t t e r i n g (15 m i n at 1.6 kV). (iii) T h e s u b s t r a t e s are c l e a n e d b y g l o w d i s c h a r g e ( 1 0 m i n at 5 0 0 V ) before deposition. T h e s u b s t r a t e s are p l a c e d o n a w a t e r - c o o l e d s u b s t r a t e holder. D u r i n g s p u t t e r i n g the s u b s t r a t e h o l d e r a t t a i n s an e q u i l i b r i u m t e m p e r a t u r e o f 150 °C after a b o u t 1 h. This t e m p e r a t u r e rise is c a u s e d m a i n l y b y e l e c t r o n h e a t i n g f r o m the plasma. D u r i n g g r o w t h a so-called h o t a b s o r p t i o n layer is p r e s e n t 11 just a b o v e the c o n d e n s i n g surface w h e r e the s p u t t e r a t o m s a n d the electrons f r o m the p l a s m a r e a c h the surface. T h e r e is evidence t h a t the surface t e m p e r a t u r e m a y be m u c h h i g h e r t h a n the m e a s u r e d s u b s t r a t e h o l d e r t e m p e r a t u r e 12. T h e g r o w t h p r o c e s s is m a i n l y d e t e r m i n e d b y the surface t e m p e r a t u r e o f the layer (at a p p r o x i m a t e l y 500 °C 11. t 3). S t a n d a r d size 10 m m x 10 m m s u b s t r a t e s of C o r n i n g glass 7059, silicon wafers ([ 100] oriented) a n d for T E M e x a m i n a t i o n freshly cleaved m i c a sheets c o v e r e d with a thin e v a p o r a t e d a m o r p h o u s c a r b o n film are used. In o r d e r to o b t a i n a g o o d h.c.p. s t r u c t u r e with [ 0 0 1 ] p e r p e n d i c u l a r to the s u b s t r a t e a very low b a c k pressure (less t h a n 10 7 m b a r ) is n e c e s s a r y 6-8. T h e a r g o n gas used m u s t be of a very high p u r i t y (99.998~0).

R.F.-SPUTTERED

Co-Or

FOR MAGNETIC

RECORDING.

63

I

I m p u r i t i e s c a u s e a d e c r e a s e in t h e c a x i s o r i e n t a t i o n a n d a r e d i r e c t l y n o t i c e a b l e in t h e m a g n e t i c p r o p e r t i e s , e s p e c i a l l y Sii. U s e o f t h e M e i s s n e r t r a p g a v e Sil -- 0.03 a n d w i t h o u t t h e t r a p w e o b t a i n e d Sll -~ 0.06. I n a d d i t i o n it h a s b e e n s h o w n ~3 t h a t a s m a l l a m o u n t o f r e s i d u a l N z g a s in t h e r.f. s p u t t e r c h a m b e r p r o m o t e s t h e f o r m a t i o n o f t h e f.c.c, p h a s e in C o - C r f i l m s w h i c h a l s o d e s t r o y s t h e w e l l - o r i e n t e d h.c.p. structure. 3.

INFLUENCE

OF THE SUBSTRATE

During every run we sputtered Co-Cr films on silicon, glass and mica covered w i t h c a r b o n . T h e i n f l u e n c e o f t h e d i f f e r e n t s u b s t r a t e s o n t h e c a x i s o r i e n t a t i o n is s h o w n in F i g . 1. I n t h i s f i g u r e A05o is g i v e n a s a f u n c t i o n o f t h e f i l m t h i c k n e s s for t h e m u l t i t a r g e t . T h e d i f f e r e n c e b q t w e e n g l a s s a n d s i l i c o n is m o r e p r o n o u n c e d f o r t h i n l a y e r s . T h i s d i f f e r e n c e m a y b e c a u s e d b y t h e effect o f e l e c t r i c a l c h a r g e o n t h e electrically insulating glass substrate from the sputter plasma. Electrically conductive C/mica substrates show approximately the same d e p e n d e n c e a s t h e s i l i c o n s u b s t r a t e a l t h o u g h t h e h e a t c o n d u c t i v i t y o f C / m i c a is j u s t a s l o w a s t h a t o f g l a s s . T h e i n f l u e n c e o f t h e s u b s t r a t e is less a p p a r e n t for t h i c k e r l a y e r s . T h e s e r e s u l t s a r e s u p p o r t e d b y t h e r e s u l t s o f Sll m e a s u r e m e n t s o n t h e s a m e l a y e r s ( F i g . 2). F r o m o u r o t h e r m a g n e t i c m e a s u r e m e n t s a l s o w e c a n c o n c l u d e t h a t the properties of Co-Cr sputtered on C/mica and silicon are practically the same. I t w a s f o u n d t h a t g l o w d i s c h a r g e c l e a n i n g o f t h e s u b s t r a t e is e s s e n t i a l in o r d e r t o o b t a i n a w e l l - o r i e n t e d l a y e r d i r e c t l y f r o m t h e s t a r t o f g r o w t h . M e a s u r e m e n t s o f A05o f o r a C o - C r l a y e r 25 n m t h i c k o n a c l e a n e d a n d a n o r d i n a r y s i l i c o n s u b s t r a t e y i e l d e d v a l u e s o f 1.5 ° a n d 5.5 ° r e s p e c t i v e l y . W i t h s e c o n d a r y i o n m a s s s p e c t r o m e t r y it is c l e a r t h a t t h e i n f l u e n c e o f g l o w d i s c h a r g i n g is m a i n l y t h e r e m o v a l o f i m p u r i t i e s s u c h a s H , H z O a n d 0 2 f r o m t h e s u r f a c e . I t is u n d e r s t a n d a b l e t h a t t h e s e i m p u r i t i e s i n f l u e n c e t h e g r o w t h o f t h e l a y e r . F r o m t h e l i t e r a t u r e it is k n o w n t h a t r e s i d u a l g a s e s c a n h a v e a s t r o n g i n f l u e n c e o n t h e c r y s t a l h a b i t ~3 a n d in t h e c a s e o f C o - C r l a y e r s a n f.c.c. s t r u c t u r e 14 h a s a l s o b e e n o b s e r v e d . 0.8

12

0.6

8

0.,4

yo

T

0,2

2

-

0

n J

0

I

2

I

I

4

J

I

6

h (10~ n m )

-" I

I

8

I

I

10

i

I

J

I

2

1

4 ~

,

i B

? 10

h (10= n m )

F i g . 1. T h e r e l a t i o n b e t w e e n t h e h a l f w i d t h a n g l e A 0 5 0 o f t h e X - r a y r o c k i n g c u r v e s f o r v a r i o u s s u b s t r a t e s : A g l a s s ; [S], s i l i c o n ; O , C / m i c a . F i g . 2. T h e i n - p l a n e r e m a n e n c e in F i g . 1).

I 6

and the layer thickness

SII a s a f u n c t i o n o f t h e f i l m t h i c k n e s s h f o r v a r i o u s s u b s t r a t e s ( s y m b o l s a s

64

4.

J.C.

LODDER,

T. W I E L I N G A ,

J. W O R S T

FILM STRUCTURE

4.1. X-ray diffraction Several C o - C r layers (15 at.~o Cr) 2~tm thick s p u t t e r e d o n C / m i c a were r e m o v e d f r o m their s u b s t r a t e s a n d p o w d e r e d in an a g a t e m o r t a r . F r o m this p o w d e r a n X - r a y d i f f r a c t i o n s p e c t r u m was o b t a i n e d at 35 k V a n d 4 0 m A with C u K r a d i a t i o n . O n l y the h.c.p, reflections were p r e s e n t in the C o C r s p e c t r u m m e a s u r e d . T h e X - r a y reflections p r e s e n t are given in T a b l e II. A K C I p o w d e r was used as c a l i b r a t i n g m a t e r i a l is. T h e lattice p a r a m e t e r s a = 2.52/~ a n d c = 4.06/~ were c a l c u l a t e d f r o m the d a t a given in T a b l e II. T h e s e give a c/a r a t i o o f 1.62 w h i c h is a l m o s t the s a m e as the r a t i o for h.c.p. ~ - C o t6. T h e 20 values for o u r C o - C r s h o w a small s y s t e m a t i c shift with respect t o t h o s e for c~-Co. T A B L E II MEASURED X-RAY DATA FROM POWDERED SPUTTERED C o - C F

R e s u l t s f o r p o w d e r e d Co~-Cr

LAYERS COMPARED WITH DATA FOR n - C o a

D a t a f o r c~-Co

C o r r e c t e d 20()~c~ ) ( d e g )

Calculated d(~)

d (~k)

hkil

41.46 44.65 47.36 62.53 75.58 84.19

2.175 2.030 1.919 1.485 1.258 1.150

92.23 94.18 98.78

1.069 1.052 1.015

2.165 2.023 1.910 1.48 1.252 1.149 1.083 1 _066 1.047 1.015

10:f0 0002 1011 1012 11~0 10i3 20~0 11 ~ 2 2021 0004

" From

ref. 16.

4.2. Rocking curve method T h e preferred c r y s t a l l o g r a p h i c o r i e n t a t i o n (texture) was e x a m i n e d b y m e a s u r ing the layers with a Philips t e x t u r e g o n i o m e t e r using C o K0t r a d i a t i o n . T h e {0002} a n d { 1 0 i l } p o l e d i s t r i b u t i o n s were m e a s u r e d with a p i t c h of 2.5 ° b y m e a n s of the S c h u l t z m e t h o d 17. T h e {0002} pole figure s h o w s o n l y a very n a r r o w d i s t r i b u t i o n in the c e n t r e of the plot a n d t h a t for {1011} (Fig. 3) consists of a n a r r o w circular d i s t r i b u t i o n w h i c h c a n be assigned to the [ 0 0 1 ] 1° o r i e n t a t i o n . B e c a u s e these pole figures s h o w e d a high d e g r e e o f r o t a t i o n a l s y m m e t r y we d e c i d e d to e x a m i n e all o t h e r layers b y the so-called r o c k i n g c u r v e m e t h o d w h e r e the s p e c i m e n is s c a n n e d in a c u r v e d line i n s t e a d of spirally as in the S c h u l t z m e t h o d . I n Fig. 4 the {0002} r o c k i n g c u r v e s are given for the initial layer (Fig. 4(a)) a n d for thick layers s p u t t e r e d o n silicon (Fig. 4(b)) a n d glass (Fig. 4(c)). F r o m these c u r v e s the h a l f w i d t h angle A05o w a s d e t e r m i n e d . This angle r e p r e s e n t s the d i s p e r s i o n o f the [ 0 0 1 ] d i r e c t i o n (c axis) of the h.c.p, s t r u c t u r e a r o u n d the n o r m a l to the film plane. A 0 5 0 , f o r very thin layers (25 nm) is small (3 °) w h e n the layers have been s p u t t e r e d o n silicon s u b s t r a t e s (Fig. 4(a)). This m e a n s t h a t the initial g r o w t h of the c axis is a l r e a d y h i g h l y oriented. I n o t h e r w o r d s the initial l a y e r g r o w s with the {0002} p l a n e parallel to the s u b s t r a t e . T h e r e l a t i o n b e t w e e n A0so a n d the film t h i c k n e s s for

R.F.-SPUTTERED

Co-Cr

FOR MAGNETIC

RECORDING.

65

I

v a r i o u s s u b s t r a t e s is g i v e n in F i g . 1. F r o m this it c a n be seen t h a t A050 i m p r o v e s d u r i n g g r o w t h o f t h e film. F o r a t h i c k n e s s o f r n o r e t h a n 0.1 lain t h e c a x i s o r i e n t a t i o n f o r C o - C r films o n s i l i c o n is a l m o s t c o n s t a n t (1.7°). F i l m s s p u t t e r e d o n C / m i c a s h o w a s i m i l a r b e h a v i o u r , b u t t h o s e o n g l a s s h a v e a m u c h w o r s e o r i e n t a t i o n at t h e s t a r t a n d A0so is n o t c o n s t a n t f o r t h i c k n e s s e s o f u p t o 1.5 lam.

AO~O

A~50.

('0002

i

-20

(a)

I (ooo2

[ (0002)

0

20

-20

0

20

-20

0

20

(b)

Fig. 3. T h e { 10TI ) pole figure for C o - C r film (15 at.% Cr) 800 n m thick s p u t t e r e d on silicon: IS], 2 0 ; [ ] , 30%. T h e centre o f / h e plot indicates the film n o r m a l . Fig. 4. T h e {0002} X-ray r o c k i n g c u r v e s for s p u t t e r e d C o - C r layers: (a) on silicon (25 n m ; A0so = 2.8'i, (b) on silicon (800 rim; A0+0 = 1+6°); (c) on glass (800 n m ; A0so = 5.06 ).

O t h e r e x p e r i m e n t s w e r e c a r r i e d o u t t o i n v e s t i g a t e w h e t h e r the i n i t i a l l a y e r a l w a y s s t a r t s g r o w i n g w i t h t h e b a s a l {0002} h.c.p, p l a n e s p a r a l l e l to t h e s u b s t r a t e . A s i l i c o n s u b s t r a t e w a s p l a c e d at a n a n g l e o f 45 ° t o t h e s u b s t r a t e h o l d e r n e x t to a n o r m a l l y p o s i t i o n e d s u b s t r a t e . T h e r e w a s o n l y a s l i g h t d i f f e r e n c e b e t w e e n t h e ~two s p u t t e r e d l a y e r s o f t h i c k n e s s 25 n m . T h e d i s p e r s i o n o f t h e c a x i s o r i e n t a t i o n w a s 5.5 ° a n d 3.0 ° r e s p e c t i v e l y . T h i s d e f i n i t e l y s h o w s t h a t for s p u t t e r e d C o C r l a y e r s the {0002} h.c.p, p l a n e g r o w s p a r a l l e l t o the s u b s t r a t e .

4.3. Electron m i c r o s c o p y T h e c r y s t a l d i a m e t e r w a s d e t e r m i n e d b y m e a n s of e l e c t r o n m i c r o s c o p y . R e l a t i v e l y t h i n films u p to 200 n m w e r e e x a m i n e d w i t h a J E O L 200 C X m i c r o s c o p e (200 kV). F i l m s o f t h i c k n e s s u p to a p p r o x i m a t e l y 600 n m w e r e s t u d i e d w i t h t h e h i g h v o l t a g e (1000 k V ) T N O e l e c t r o n m i c r o s c o p e . ( T h i s i n s t r u m e n t , d e s i g n e d a n d c o n s t r u c t e d a t t h e I n s t i t u t e of A p p l i e d P h y s i c s T N O - T H Delft, h a s b e e n i n s t a l l e d at the M e t a l Research Institute T N O , A p e l d o o r n . ) T h e c r y s t a l d i a m e t e r o n t h e s u r f a c e w a s d e t e r m i n e d b y a r e p l i c a t e c h n i q u e as well as f r o m i m a g e s m a d e w i t h the J E O L 200 C X m i c r o s c o p e in the S E M m o d e . T h e films m e a s u r e d b y t r a n s m i s s i o n m i c r o s c o p y w e r e s p u t t e r e d o n f r e s h l y c l e a v e d m i c a substrates covered with a thin amorphous evaporated layer of carbon (thickness, a p p r o x i m a t e l y 30 nm). A f t e r t h e l a y e r h a d b e e n c u t i n t o p i e c e s t h e y w e r e d i p p e d in w a t e r f o r s e p a r a t i o n f r o m t h e m i c a a n d m o u n t e d o n g r i d s . In F i g . 5 t h e b r i g h t field a n d e l e c t r o n d i f f r a c t i o n i m a g e s f r o m t h e i n i t i a l l a y e r a n d r e l a t i v e l y t h i c k l a y e r s

66

(a)

J . C . L O D D E R , T. W I E L I N G A , J. W O R S T

(b)

(c) [d) Fig. 5. T r a n s m i s s i o n electron m i c r o g r a p h s (high voltage electron m i c r o s c o p e ) and the c o r r e s p o n d i n g diffraction p a t t e r n s for s p u t t e r e d C o Cr layers of v a r i o u s thicknesses : (a) 25 n m ; (b) 200 rim; (c~ 400 n m ; (d) 600 nm.

R.F.-SPUTTERED

Co-Cr

FOR

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67

I

are s h o w n . F r o m the d i f f r a c t i o n p a t t e r n s ' i t c a n be seen t h a t o n l y h.c.p, reflections were present. T h e c r y s t a l d i a m e t e r was d e t e r m i n e d as a f u n c t i o n of the l a y e r t h i c k n e s s b y m i c r o s c o p i c e x a m i n a t i o n . T h e crystal d i a m e t e r a p p e a r s to increase with the film t h i c k n e s s (Fig. 6). A t w o - s t a g e replica t e c h n i q u e for i m a g i n g the surface s t r u c t u r e of C o - C r layers was used for very thick layers ( a b o v e 0.5 p.m). First a n e g a t i v e replica is m a d e with T e c h n o v i t e (type 4071) a n d f r o m this the final positive replica is p r e p a r e d u s i n g c a r b o n a n d p l a t i n u m s h a d o w i n g . After the T e c h n o v i t e has been dissolved the final replica c a n be studied. A typical e l e c t r o n m i c r o g r a p h is s h o w n in Fig. 7. ,2o lOO 8o

~

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h: Q, multitarget; O , a l l o y e d t a r g e t .

C r o s s - s e c t i o n a l structures_can be o b s e r v e d with the J E O L 200 C X m i c r o s c o p e in the S E M m o d e . T h e s e s a m p l e s are p r e p a r e d b y c o o l i n g the s p e c i m e n in liquid N 2 before b r e a k i n g it to o b t a i n a brittle fracture. F i g u r e 8 s h o w s the c r o s s - s e c t i o n a l s t r u c t u r e o f a C o - C r film 2 jam t h i c k s p u t t e r e d o n silicon. S u c h m i c r o g r a p h s d o n o t s h o w a c o l u m n a r - l i k e m o r p h o l o g y as c a n m a i n l y be seen in other, relatively thick, s p u t t e r e d layers (see for i n s t a n c e ref. 18). F r o m these a n d the T E M e x a m i n a t i o n we c o n c l u d e d t h a t the crystals t e n d to be c o n i c a l l y s h a p e d (Section 5). 5.

DISCUSSION

OF THE

FILM

MORPHOLOGY

AND

STRUCTURE

T h e film s t r u c t u r e is usually d e s c r i b e d b y its t w o m o s t i m p o r t a n t p r o p e r t i e s : the c r y s t a l l o g r a p h i c (preferred) o r i e n t a t i o n (or texture) a n d the m o r p h o l o g y (crystal size a n d shape). T h e s e t w o features o f thin films are often d e s c r i b e d i n d e p e n d e n t l y I 8-21 b u t s o m e t i m e s t h e r e is a s t r o n g r e l a t i o n b e t w e e n them. E s p e c i a l l y in the case o f the " s u r v i v a l o f the fastest" m o d e l 19 the m o r p h o l o g y is s t r o n g l y d e t e r m i n e d b y the c r y s t a l l o g r a p h i c o r i e n t a t i o n ; i.e. the film o r i e n t a t i o n is finally given b y the c r y s t a l l o g r a p h i c axis w h i c h g r o w s the fastest. In the h.c.p, s t r u c t u r e this is the [ 0 0 1 ] axis. W i t h p h y s i c a l v a p o u r d e p o s i t i o n t e c h n i q u e s these p r o p e r t i e s are m o r e o r less d e t e r m i n e d b y the p r o c e s s involved, i.e. in s p u t t e r i n g b y the p r o c e s s p a r a m e t e r s : s p u t t e r rate, s u b s t r a t e t e m p e r a t u r e , c o n t a m i n a t i o n a n d a r g o n pressure. H e r e it is a s s u m e d t h a t the a r g o n p r e s s u r e used d o e s n o t influence the film properties. At h i g h e r pressures the c axis o r i e n t a t i o n is d i s t u r b e d 8. It is also a s s u m e d t h a t the rate

68

Fig. 7. A n i n d i r e c t r e p l i c a o f a C o

j . (4. L O D D E R ,

Crlayer

T. W I E L I N G A ,

J. W O R S T

1 IJmthick.

o f d e p o s i t i o n b y e v a p o r a t i o n 22 o r b y s e v e r a l s p u t t e r i n g m e t h o d s 6 8,23 d o e s n o t effect t h e p e r p e n d i c u l a r c a x i s o r i e n t a t i o n . F r o m t h i s it c a n b e s e e n t h a t a p e r p e n d i c u l a r c a x i s o r i e n t a t i o n is O b t a i n e d a t r a t e s b e t w e e n 0.2 a n d 6 0 0 n m s ~. i t is n o t y e t c l e a r w h e t h e r t h e d e p o s i t i o n r a t e i n f l u e n c e s t h e f i l m m o r p h o l o g y o f C o - C r films, H o w e v e r , it h a s b e e n s h o w n t 8 t h a t t h e m o r p h o l o g y o f o n e - p h a s e l a y e r s d o e s n o t c h a n g e in a w i d e r a n g e o f d e p o s i t i o n r a t e s . The most important parameter, but unfortunately also the most elusive p a r a m e t e r in t h e c a s e o f s p u t t e r i n g , is t h e t e m p e r a t u r e . I n r.f. s p u t t e r i n g , t h c s u r f a c e t e m p e r a t u r e , i.e. a t w h i c h t h e f i l m f o r m a t i o n t a k e s p l a c e , is p r e d o m i n a n t l y d e t e r m i n e d b y t h r e e f a c t o r s w h i c h a r e o f t h e s a m e o r d e r o f m a g n i t u d e : (i) t h e t e m p e r a t u r e o f t h e s u b s t r a t e h o l d e r , (ii) t h e t e m p e r a t u r e p r o d u c e d b y h e a t f r o m t h e c o n d e n s i n g a t o m s a n d (iii) t h e t e m p e r a t u r e p r o d u c e d b y h e a t i n g f r o m t h e s p u t t e r p l a s m a , m a i n l y b y t h e e l e c t r o n s . T h i s c o m p l e x h e a t m e c h a n i s m n o t o n l y m a k e s it difficult to determine an accurate surface temperature (whatever this may mean) but a l s o i m p l i e s t h a t t h i s t e m p e r a t u r e will n o t b e s t a t i o n a r y d u r i n g t h e s p u t t e r r u n . T h e

R.F.-SPLITTERED

Co-Cr

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MAGNETIC

RECORDING.

I

69

surface t e m p e r a t u r e in r.f. s p u t t e r i n g was d e t e r m i n e d b y m e a s u r i n g the h.c.p.-f.c.c. t r a n s i t i o n t e m p e r a t u r e a n d c o m p a r i n g this with bulk d a t a for C o - C r a n d was f o u n d to be 550 °C h i g h e r t h a n the m e a s u r e d s u b s t r a t e t e m p e r a t u r e 12. H o w e v e r , surface f o r m a t i o n s h o u l d n o t be c o m p a r e d directly with bulk f o r m a t i o n . Besides the u n k n o w n d e p o s i t i o n t e m p e r a t u r e there is also s o m e a m b i g u i t y in the literature a b o u t the d e p e n d e n c e o f the c axis o r i e n t a t i o n o n the s u b s t r a t e t e m p e r a t u r e v" s. D e s p i t e this u n c e r t a i n t y a b o u t the t e m p e r a t u r e the overall picture o f o u r films is quite c o n s i s t e n t a n d s h o w s clear features o f the film g r o w t h . First, the c axis

F i g . 8. C r o s s - s e c t i o n a l

SUM image ofa Co-Cr

layer 2 pm lhick spultered on silicon.

70

J.C.

LODDER,

T. WIELINGA,

J. W O R S T

o r i e n t a t i o n is excellent, b e i n g i n d e p e n d e n t of the film thickness f r o m very thin films up to a film t h i c k n e s s o f a few m i c r o n s . S e c o n d l y , the g r a i n size increases c o n t i n u o u s l y with the film thickness as m e a s u r e d for films in the r a n g e 2 5 - 2 0 0 0 nm. After 1 h a c o n s t a n t s u b s t r a t e t e m p e r a t u r e of 150 °C is r e a c h e d with a s p u t t e r rate of 0.2 n m s 1. T h e t h i c k n e s s r a n g e e x a m i n e d far exceeds the value m e a s u r e d w h e n the t e m p e r a t u r e b e c a m e s t a t i o n a r y . T h e r e f o r e it is a s s u m e d t h a t d u r i n g film g r o w t h the t surface t e m p e r a t u r e c a n be c o n s i d e r e d as c o n s t a n t ; i.e. at least t e m p e r a t u r e v a r i a t i o n s are t o o small to c a u s e different g r o w t h characteristics. T h e relative surface t e m p e r a t u r e (relative to the m e l t i n g point) was e v a l u a t e d as a b o u t 0.3 0.5. T h e excellent c axis o r i e n t a t i o n of the nuclei p r o b a b l y o r i g i n a t e s f r o m the t e n d e n c y to c o n d e n s e with the m o s t densely p o p u l a t e d c r y s t a l planes, the {0002} planes, parallel to the substrate. This is s u p p o r t e d by the excellent c axis o r i e n t a t i o n (A0so = 5 °) of a C o - C r film 25 n m thick, s p u t t e r e d o n a silicon s u b s t r a t e w h i c h was p l a c e d at an angle o f 45 ° t o the s u b s t r a t e holder. T h e preferred o r i e n t a t i o n is m a i n t a i n e d as the film t h i c k n e s s is increased. This c o u l d be c a u s e d b y a m e c h a n i s m o f self-epitaxy. I n this case, h o w e v e r , the o r i e n t a t i o n w o u l d d e t e r i o r a t e with i n c r e a s i n g film t h i c k n e s s ~9. It is m o r e likely t h a t this n o r m a l c axis o r i e n t a t i o n is m a i n t a i n e d b e c a u s e it is also the fastest g r o w i n g c r y s t a l l o g r a p h i c o r i e n t a t i o n . It has been s h o w n 19 t h a t a c e r t a i n c r y s t a l l o g r a p h i c o r i e n t a t i o n c a n arise f r o m a m e c h a n i s m d e s c r i b e d as "'the survival of the fastest", i.e. the crystallites with the fastest g r o w i n g axis ( p e r p e n d i c u l a r to the s u b s t r a t e ) survive, s u p p r e s s i n g crystallites with o t h e r o r i e n t a t i o n s . This t y p e of layer is t h e r e f o r e r e c o g n i z a b l e b y a p o o r l y o r i e n t e d initial layer, as is the case w h e n glass s u b s t r a t e s are used for C o - C r (see Fig. 1). H o w e v e r , w h e n silicon o r C / m i c a s u b s t r a t e s are used the first nuclei are i m m e d i a t e l y p r o p e r l y o r i e n t e d w h i c h m e a n s t h a t the film g r o w s a l o n g its fastest d i r e c t i o n f r o m the very b e g i n n i n g , N o w the q u e s t i o n r e m a i n s : W h y does the crystal d i a m e t e r increase with the film t h i c k n e s s ? I n the case o f the " s u r v i v a l of the fastest" there is a simple m e c h a n i s m w h i c h e x p l a i n s this feature. In o u r case there has to be a different cause. A f o r m a l r e a s o n c o u l d be the crystal b o u n d a r y e n e r g y ; i.e. b y i n c r e a s i n g the crystal d i a m e t e r the e n e r g y d e n s i t y is decreased. This, h o w e v e r , d o e s n o t explain h o w s o m e crystals g r o w at the expense of others. F r o m o u r S E M a n d replica m i c r o g r a p h s it c a n be c o n c l u d e d t h a t there is a large statistical v a r i a t i o n in the crystal d i a m e t e r . F u r t h e r m o r e the crystal b o u n d a r i e s d o n o t s h o w a n y i n t e r m e d i a t e layer o r s e p a r a t i o n , the crystallites b e i n g very close. A simple e x p l a n a t i o n c o u l d t h e r e f o r e be t h a t the a v e r a g e increase in the crystal d i a m e t e r is just g o v e r n e d b y statistical rules. T h e relative surface t e m p e r a t u r e , o f the o r d e r o f 0.4, is in the r a n g e w h e r e surface diffusion is believed to be the d o m i n a n t f a c t o r ~a. T h e r e f o r e a statistical v a r i a t i o n in the local flux at the i n d i v i d u a l crystallites m a y result in the increase of the "'highest" at the e x p e n s e of the "'lowest" crystals w h i c h is m a d e possible by the surface diffusion; i.e. the crystallites are c a p a b l e o f g r o w i n g s i d e w a y s u n h i n d e r e d b y i n t e r m e d i a t e crystallite layers. C l e a r l y this effect is self-enhancing. 6.

RELATION

TO THE MAGNETIC

Perpendicular

magnetic

PROPERTIES

recording

necessitates

a perpendicular

magnetic

R.F.-SPUTTERED

Co-CF

FOR MAGNETIC

RECORDING.

I

71

a n i s o t r o p y o f the layers. O u r C o - C r films h a d a large m a g n e t o c r y s t a l l i n e a n i s o t r o p y with the c axis of the h.c.p, s t r u c t u r e o r i e n t e d p e r p e n d i c u l a r l y t o the film. W e m e a s u r e d Ms a n d the uniaxial m a g n e t i c a n i s o t r o p y c o n s t a n t s K1 a n d Kz with a v i b r a t i n g s a m p l e m a g n e t o m e t e r ( V S M ) a n d a t o r q u e m e t e r respectively (see P a r t II). T h e i n - p l a n e a n d p e r p e n d i c u l a r M - H l o o p s m e a s u r e d with the V S M are given in Fig. 9. O w i n g to the excellent preferred c axis o r i e n t a t i o n the i n - p l a n e hysteresis l o o p is m a r k e d by a small i n - p l a n e r e m a n e n c e r a t i o Sit = 0.04 a n d a s t r o n g a n i s o t r o p y field H k. This field d e p e n d s o n the film c o m p o s i t i o n a n d varied for o u r films b e t w e e n 3 and 7 kOe. T h e relation b e t w e e n the i n - p l a n e r e m a n e n c e a n d the film thickness for three different t y p e s of s u b s t r a t e is given in Fig. 2. In p a r t i c u l a r the initial layer s p u t t e r e d o n silicon s h o w s a small i n - p l a n e r e m a n e n c e c a u s e d by the p e r p e n d i c u l a r o r i e n t a t i o n o f the a n i s o t r o p y axis. Films s p u t t e r e d o n C / m i c a exhibit a p p r o x i m a t e l y the s a m e b e h a v i o u r as t h o s e d e p o s i t e d o n t o silicon. T h e m i n i m u m in-plane r e m a n e n c e is o b t a i n e d for films s p u t t e r e d o n silicon to a t h i c k n e s s of 300 n m o r m o r e , while for C / m i c a s u b s t r a t e s the o p t i m u m film thickness is 600 n m o r m o r e . A b o v e these t h i c k n e s s e s there is n o c h a n g e o b s e r v e d u p to 2 Itm. U s i n g a glass s u b s t r a t e the best r e m a n e n c e is a c h i e v e d at a thickness of a p p r o x i m a t e l y 800 nm. W e have a l r e a d y s h o w n b y X - r a y diffraction that the q u a l i t y o f c axis o r i e n t a t i o n (A05o) was e x t r e m e l y g o o d even for thin layers (see Fig. 1). I n Fig. 10 the in-plane r e m a n e n c e is given as a f u n c t i o n of the d i s p e r s i o n A0s0 of the h.c.p, c axis o r i e n t a t i o n . T h e d a t a p l o t t e d in this figure c o r r e s p o n d to films w h i c h were s p u t t e r e d at v a r i o u s values o f p a r a m e t e r s such as the thickness, a r g o n pressure a n d s p u t t e r voltages. T h e c o r r e l a t i o n b e t w e e n A05o a n d Sii is manifest a n d this s u p p o r t s the -~

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l o o p s m e a s u r e d w i t h t h e V S M f o r l a y e r s (a) 25 n m a n d (b) 6 0 0 n m t h i c k s p u t t e r e d o n s i l i c o n .

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72

J.c.

L O D D E R , T. W I E L I N G A , J. W O R S T

c o n c l u s i o n d r a w n from o u r t o r q u e m e a s u r e m e n t s (see Part II) that the a n i s o t r o p y of C o - C r films originates from crystal a n i s o t r o p y . 7.

CONCLUSIONS

I n d e p e n d e n t l y o f the layer thickness our sputtered C o - C r layers o n silicon substrates h a v e an h.c.p, structure with the [ 0 0 1 ] c axis p e r p e n d i c u l a r to the substrate. In order to start directly with a correct o r i e n t a t i o n it is essential to clean the substrates by the g l o w discharge m e t h o d . T h e c/a ratio calculated from X-ray d a t a is 1.62. This is a l m o s t the s a m e as the ratio for h.c.p, or-Co. T h e type of substrate influences the g r o w t h of the initial layer. T h e g r o w t h m o d e l for o u r layers is b a s e d o n the t e n d e n c y of the a t o m s in C o - C r to c o n d e n s e preferentially with the m o s t d e n s e l y p a c k e d crystal plane, the { 0 0 0 2 } plane, parallel to the substrate, w h i c h also c o r r e s p o n d s to the direction o f fastest growth. T h e f o r m a t i o n of the layer takes place in the range where surface diffusion is the d o m i n a n t factor. T h e crystal size increases a l m o s t linearly with the film thickness. W e p r o p o s e the m e c h a n i s m for crystal g r o w t h to be a statistical variation in the local flux at the individual crystallites, but it m a y also be influenced by a l o w e r i n g o f the grain b o u n d a r y energy. T h e m o r p h o l o g y o f o u r layers s h o w s that they consist of c o n i c a l l y s h a p e d crystals. T h e m a g n e t i c a n i s o t r o p y axis c o i n c i d e s with the c axis o f the crystals. ACKNOWLEDGMENTS

W e s h o u l d like to express our a p p r e c i a t i o n to Professor Dr. G. H. J o n k e r and P r o f e s s o r Dr. J. H. J. F l u i t m a n for their s t i m u l a t i n g discussions. REFERENCES I 2 3 4 5 6 7 8 9 10 I1 12 13 14 15 16 17 18 19

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R.F.-SPUTTERED

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C o - C r FOR MAGNETIC RECORDING. I

73

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