Preparation and Properties of Electrodeposited ...

Preparation and Properties of Electrodeposited Cylindrical Magnetic Films M . W . Sagal

Bell Telephone Laboratories, Incorporated, Murray Hill, New Jersey ABSTRACT A process is described for the electrodeposition of cylindrical p e r m a l l o y films w i t h n e a r zero magnetostriction. The design of a plating cell intended to achieve good u n i f o r m i t y of electrolyte agitation over the plating region is given. The steps used in the p r e p a r a t i o n of the w i r e substrate are discussed. Results showing the d e p e n d e n c e of a v e r a g e alloy composition on electrolyte flow r a t e and pH are given, along with typical m a g n e t i c characteristics of the plated wire. Most of the w o r k described in the l i t e r a t u r e on the p r e p a r a t i o n and characteristics of m a g n e t i c thin films for m e m o r y applications has been concerned with films e v a p o r a t e d or electrodeposited on flat insulating or m e t a l l i c substrates (1, 2). A n a l t e r n a t i v e configuration, an anisotropic film deposited on the surface of a small d i a m e t e r wire, offers a n u m b e r of advantages f r o m the device point of v i e w (3). A process for the fabrication of this m a t e r i a l by a continuous electrodeposition process using high agitation conditions for achieving high c u r r e n t densities and compositional u n i f o r m i t y has been described p r e v i o u s l y (4, 5). In this earlier w o r k the i m p o r t a n c e of proper substrate preparation, including the use of a copper preplate, was pointed out. This paper describes an a d v a n c e d design of a continuous plating system in some detail, including a sequence of substrate cleaning and p r e plate steps which has been found to give a satisfactory surface for the subsequent plating of a n i c k e l - i r o n film with desirable magnetic properties. F u r t h e r , the degree of composition control that can be r o u t i n e l y achieved by proper r e g u l a t i o n of the plating p a r a m e ters is discussed, along w i t h typical data showing the d e p e n d e n c e of a v e r a g e film composition on electrolyte flow rate and pH. Finally, values of magnetic p a r a m e ters closely related to the u l t i m a t e p e r f o r m a n c e of the m a t e r i a l as a m e m o r y device are given. The process is shown schematically in Fig. 1. The substrate is a 0.005 in. d i a m e t e r h a r d e n e d B e - C u w i r e which is c a r e f u l l y d r a w n in order to m i n i m i z e scratches caused by dirt particles collecting on the dies. It is pulled t h r o u g h the apparatus at 6 in./min. The w i r e is protected f r o m the l a b o r a t o r y a t m o s p h e r e by a nitrogen blanket b e t w e e n each process step. A n electrolytic alkaline cleaner r e m o v e s organic soil and loose dirt, and an acid dip r e m o v e s surface oxide. The w i r e surface at this state, although superior to that of o r d i n a r y c o m m e r c i a l l y d r a w n wire, still shows (Fig. 2a) a large n u m b e r of pits and small scratches under high magnification. In general, 1~ magnetic films plated onto this surface with no f u r t h e r p r e p a r a t i o n do not h a v e satisfactory properties. The w i r e is then plated w i t h 1~ of copper f r o m a c o m m e r c i a l plating bath. 1 F i g u r e 2b shows the a p p e a r a n c e of the w i r e surface after copper plating. This copper plate m a y serve to 1 Unichrome

copper plating process

Fig. 2. Photomicrographs of Be-Cu wire surface with Nomarski type interference microscope (560X): (a) (top) after cleaning and acid dip; (h) (bottom) after copper plating. smooth the surface although it clearly does not e l i m inate the surface imperfections completely. The omission of the copper plating step in the process has the effect of raising the values of dispersion and m i n i m u m bit w r i t i n g c u r r e n t (see discussion of magnetic p r o p erties for definitions) by a factor of f r o m 2 to 4, depending on the local surface roughness of the substrate. The w i r e is rinsed in ultrasonically agitated deionized w a t e r b e t w e e n each of these p r e p a r a t o r y steps. Electrical contact is m a d e to the w i r e by means of a gold plated brass w h e e l following the last rinse tank to p e r m i t the introduction of direct c u r r e n t into the w i r e w h i l e the p e r m a l l o y film is being deposited. T h e c i r c u m f e r e n t i a l m a g n e t i c field f r o m this current results in a c i r c u m f e r e n t i a l easy axis in the film w h e r e the application r e q u i r e s it. A l t e r n a t i v e l y , if an easy axis along the wire l e n g t h is desired, a field is applied by means of Helmholtz coils m o u n t e d outside the p e r m a l loy plating chamber. In the high c u r r e n t d e n s i t y - h i g h agitation p e r m a l l o y plating process used here, the composition of the plated

of NI a n d T C h e m i c a l s , I n c .

WIRE SPEED 6"/MINUTE COPPERPLATING: I PERMALLOYPLATING:290 MA/CM2~ILL

Iooo ~/SEC] F FIELDCURRENT

ELECTROLYTIC RINSE ACID RINSE COPPER RINSEPERMALLOY TEST PULL CUT ALKALINE DIP PLATING PLATING CLEANER

Fig. 1. Schematic view of plating process 174

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Vol. 112, No. 2

ELECTRODEPOSITED MAGNETIC FILMS ,~ /

411 ~

4.0

ELECTROLYTEtN

i.o4 ~ ,-o~9s2 I I I / / / -Fe2*=O'954G/L (NI2+=71G/L)

P 3.2

OF HOLES

175

IE

8 z4

IN

GAS~AND ELECTROLYTE OUT

/GASBLES SIDE VIEW

a8

EXIT END VIEW

Fig. 3. Plating cell

~ o m N

~ o~ u.

Fe

RICH

g alloy is a sensitive function of the e l e c t r o l y t e agitation at the w i r e surface. T h e p e r m a l l o y plating c h a m b e r is designed to give u n i f o r m l y high agitation along the entire l e n g t h of w i r e being plated in order to r e d u c e the possibility of composition gradients in the film. A c e n t ri fu g al p u m p forces the e l e c t r o l y t e t h r o u g h the cell. Provision is m a d e for r e g u l a t i n g the e lect r o l y t e t e m p e r a t u r e , pI-I, and flow rate. The h e a r t of the p l a t ing c h a m b e r is a L u c i t e t u b e w i t h t h r e e s ta g g e r ed r o w s oZ holes drilled t h r o u g h the wall (shown s c h e mat i cal l y in Fig. 3). The e l e c tr o ly te is forced t h r o u g h these holes and impinges on the surface of th e m o v i n g w i r e at high velocity. It is r e m o v e d t h r o u g h low i m p e d a n c e exit ports at both ends of the chamber. T h e w i r e passes in and o u t of this L u c i t e tube t h r o u g h Teflon plugs wh i ch e x t e n d into th e region of high e lect r o l y t e agitation. N i t r o g e n gas is b u b b l e d t h r o u g h the Teflon plugs to establish a plating region whose l e n g t h is det e r m i n e d by the m a x i m u m p e n e t r a t i o n of gas bubbles into the Lucite tube. The bubbles p e n e t r a t e past the Teflon tips by about one bubble diameter, w h i c h is a p p r o x i m a t e l y equal to the hole d i a m e t e r of the Teflon plugs. T h e gas is swept out t h r o u g h the l o w im p ed an ce exit ports at each end by the high velocity electrolyte. This bubble seals serves two purposes. It p r e v e n t s l eak ag e of the e l e c t r o l y t e f r o m the p la ti n g c h a m b e r into the rest of the apparatus, and it insures that the electrolyte agitation at the e n t r a n c e and exit points of the w i r e is v e r y n e a r l y the same as the agitation in the central portion of the plating cell. Th e nitrogen gas swept out f r o m the plating region leaves the system t h r o u g h a tube in the outer p a r t of the plating chamber. Since the e le c t r o l y t e is not sealed off f r o m air, a g r a d u a l drop of F e z+ c o n c e n t r a t i o n is observed. This oxidation p r o b l e m has not been serious since a change in e l e c tr o ly te composition can be c o m pensated by an a d j u s t m e n t in a n o t h e r process variable. The i m p o r t a n c e of a n e a r - z e r o magnetostriction composition in u n i a x i a l magnetic films has been gene r a l l y recognized (1). The stresses in e v a p o r a t e d or electrodeposited films cause s e v e r e degradation in u n i axial properties if the film is v e r y f a r f r o m th e zero magnetostriction composition. F o r the plated wire, the composition r e q u i r e m e n t s are e v e n m o r e severe b ecause of the b en d in g and twisting of the small d i a m e ter w i r e substrate w h i c h occurs d u r i n g handling. It has been found that the a v e r a g e composition of the film should be held to w i t h i n about • 0.2% of t h e zero magnetostriction composition to avoid difficulties f r o m s t r a i n - i n d u c e d skew d u r i n g m e m o r y assembly. In contrast, a detectable increase in dispersion of the film in an unstressed w i r e (resulting f r o m i n t e r n a l stresses in the film) occurs w h e n the film composition deviates f r o m the zero m a g n e t o s t r i c t i o n composition by m o r e than about • 1%. Th e r e q u i r e d control of composition can be obtained by p r o p e r control of plating parameters. The composition of t h e plated alloy depends on b u l k bath composition, t e m p e r a t u r e , pH, agitation, and plating c u r r e n t density. E l e c t r o l y t e agitation affects the composition of the plated film t h r o u g h its effects on depletion l a y e r

~ 2.4

0

600

800 1000 1200 1400 1600 1800 2000 ELECTROLYTEFLOW RATE,CM3/MIN

Fig. 4. Dependence of film composition on electrolyte flow rate (sulfamate bath). The zero on the ordinate corresponds to about 2 0 % Fe. A composition "0.8% Fe rich" is therefore 20.8% Fe. @

s.o

~ 4.0 a.

Ni

RICH

/

~ 2.0

z

Na F@ RICH ~ 4,0

/

a 6.0

pH

Fig. 5. Dependence of film composition on electrolyte pH (sulfatechloride bath).

thickness and the composition of the electrolyte n e x t to the substrate (which is in general different f r o m the b u l k composition). Fo r a fixed plating cell geometry, agitation is d e p e n d e n t on the el ect r o l y t e flow rate. F i g u r e 4 shows h o w the composition of the film, d e t e r m i n e d by x - r a y fluorescence, depends on el ec t r ol yt e flow r a t e w i t h o t h er p a r a m e t e r s held constant. The probable effect of a change in p l at i n g cell g e o m e t r y will be to m o v e t h e curves of Fig. 4 along t h e flow r a t e axis. The increase in Fe content of the film w i t h increasing flow r a t e (i.e., increasing agitation) is in accord w i t h the e x p e c t e d effect of decreased depletion l a y e r thickness on the deposition rate of the m o r e r ead i l y deposited m e t a l (6) (Fe in this case). F i g u r e 5 illustrates the d e p e n d e n c e of film composition on bath pH w i t h other p a r a m e t e r s held constant. T h e increase in F e content b el o w pI-I = 4 m a y be associated w i t h a slight drop in cathode efficiency at low pH. This w o u l d increase the proportion of the m o r e r e a d i l y deposited m e t a l in the alloy in a w a y similar to a decrease in c u r r e n t density (6). In r o u t i n e operation, the composition is m e a s u r e d indirectly by a m a g n e t o s t r i c t i o n m e a s u r e m e n t . One end of a plated w i r e is c l a m p e d and the other e nd t w i st ed t h r o u g h a k n o w n angle. Th e r e s u l t a n t helical stress in the film introduces an easy axis skew, w hi c h is m e a s u r e d by the m e t h o d of Belson (7). Th e sense and m a g n i t u d e of the induced skew is a m e a s u r e of


JOURNAL

176

OF

THE

ELECTROCHEMICAL

Table I. Magnetic characteristics

F e b r u a r y i965

SOCIETY 9

zoo MA

I CLEAR

L, _ 3 # SEC ~ lbMAXj m a IbMIN, m a

(with H~

= 0)

EASY DIRECTION --._J (BIT)

H~_WRIT E = 9 6 4 3

oe: oe: oe: oe:

4 8 14 23

Belson Belson HK

o~o skew

(loop

tracer)

~__J

I

I'-

t ~ J BIT WRITE READ

Fig. 6. Pulse program ferential easy axis.

= 3 oe

The most significant magnetic characteristics of the plated wire, as d e t e r m i n e d by an o n - l in e pulse test, are g i v en in Table I. T h e o n - l i n e test permits continuous m o n i t o r i n g of m e m o r y characteristics as the w i r e is produced. Only properties of films with the easy axis around the w i r e c i r c u m f e r e n c e are described here, since this type of film has been most t h o r o u g h l y studied. Some of the characteristics of these films h a v e been p r e v i o u s l y g iv e n (3). A 1/2 in. long coil w h i c h supplies a hard direction field is m o u n t e d at the end of the plating apparatus. This coil defines the length of w i r e u n d e r test, and is long e n o u g h to effectively e li m in a t e the g e o m e t r y d e p e n d e n t d em ag n e ti z i n g field in the h a r d direction. The pulse p r o g r a m is shown in Fig. 6. The p a r a m e t e r s of p r i m a r y interest are:

1. Ibm,n, the bit w r i t i n g c u r r e n t

(i.e., the c u r r e n t p ro d u ci n g the easy direction field) needed to switch 90% of the available flux at a p a r t i c u l a r v a l u e of h a r d direction w r i t i n g field H • ; and

2. Ib.... the bit disturb c u r r e n t

(with H L ----- 0) wh i ch reduces the 90% w r i t t e n - i n bit to 81% of full flux on the application of 100 disturbs. (For a 0.005 in. d i a m e t e r wire, 1 m a - 0.032 oe.) to T. R. Long

----"l

HARD DIRECTION (WORD)

= 1.2 ~ = 0.3 ~

Magnetic Properties

is d u e

0.1~SEC

I00 ~ BIT DISTURBS . .

SEI~

35 38 48 57

the deviation of the a v e r a g e film composition f r o m the zero m a g n e t o s t r i c t i o n alloy. 2 The plating conditions are t h e n adjusted until the m a g n e t o s t r i c t i o n is as close to zero as possible (corresponding to a composition of about 20% Fe, balance Ni). F o r example, the electrolyte flow r a t e or pH can be changed to m o d i f y the alloy composition, as indicated in Figs. 4 and 5. The a v e r a g e film composition can be controlled to ___ 0.1%. X - r a y fluorescence composition m e a s u r e m e n t s are based on an a v e r a g e over 3 feet of plated wire. Th e magnetostriction test provides a m e a s u r e m e n t of a v e r age composition o v e r 1 in. lengths of wire. M e a s u r e me n t s of m a g n e t o s t r i c t i o n over a series of adjacent 1 in. segments give the same result w h i c h shows that any composition v a r i a t i o n s along th e w i r e length must be on a scale < < 1 in. F u r t h e r , t h e absence of bad bits r es u l t i n g f r o m m a g n e t o s t r i c t i o n effects w h e n the w i r e is i n c o r p o r a t e d into e x p e r i m e n t a l m e m o r y planes w i t h a p p r o x i m a t e l y 0.05 in. bit centers (3) indicates that significant compositional variations along the w i r e are absent. A l t h o u g h the type of plating cell described h e r e is designed to m i n i m i z e agitation inhomogeneities along the p l at i n g length, e x p e r i m e n t s h a v e not y et been carried out to m e a s u r e composition gradients t h r o u g h the film thickness. In principle, one source of composition g r a d i e n t often found in plated films because of the b u i l d - u p of a depletion l a y e r (8) should be absent here, since w i r e e n t e r i n g the plating cell encounters a steady state depletion layer. Both s u l f a t e - c h l o r i d e (2) and s u lf a m a t e (4) baths h a v e been used successfully.

2 This technique

0.15

[~

(to be published).

for

on-line

test

of

wire

with

circum-

Values of Ibm,, and Ibm~ for a typical film are given in Table I. Since variations of about --+20% in bit curr e n t a m p l i t u d e and pulse w i d t h should be anticipated in actual m e m o r y operation, a good criterion for acceptable film quality is Ibmax ~ 1.5 Ibm, n. In addition, in order to be conservative, the m e a s u r e m e n t s are made using a disturb pulse w i d t h 1.5 times the bit w r i t e pulse w i d t h since the disturb effect increases with pulse w i d t h (3). The increase of Ib.... w i t h an increase in Ibm,, is a general p h e n o m e n o n in these films. This result implies that the specification of an u n d i s t u r b e d read signal equal to 90% of av ai l ab l e flux is not a complete description of the state of the film before the application of disturb pulses. The p a r t i c u l a r combination of H and bit w r i t i n g c u r r e n t w h i ch is chosen to ~-WRITE

switch 90% of av ai l ab l e flux affects the resistance to domain w a l l motion w h e n easy axis disturb pulses are s u b s e q u e n t l y applied. The effects of a larger n u m b e r of disturb pulses and the simultaneous application of easy and hard direction disturb pulses are m e a s u r e d off-line, and have been discussed e l s e w h e r e (3). Table I also contains values of a90 (dispersion) and skew as m e a s u r e d by the Belson t ech n i q u e (7), as well as the anisotropy field HK m e a s u r e d by a loop tracer method. These m e a s u r e m e n t s are of some use in qua l i t a t i v e l y e v a l u a t i n g film samples, but are a poor means of q u a n t i t a t i v e l y predicting p e r f o r m a n c e under pulse switching conditions. Acknowledgments It is a pleasure to ack n o w l ed g e the contributions ef J. T. Chang, I. Danylchuk, U. F. Gianola, C. E. J o h n son, T. R. Long, and A. J. Perneski. T h e author would also like to t h an k K. M. Olsen and R. F. J a c k for samples of B e - C u w i r e and Miss S. M. V i n cen t and J. E. Kessler for x - r a y fluorescence analysis of the films.

Manuscript received J u n e 30, 1964; r ev i sed m a n u script r e c e i v e d October 1, 1964. A n y discussion of this p ap er will appear in a Discussion Section to be published in the D e c e m b e r 1965 JOURNAL.

REFERENCES 1. E. W. Pugh, "Magnetic Films of N i c k e l - I r o n , " in "Physics of Thin Films," Vol. 1, George Hass, Editor; Academic Press, N e w York (1963). 2. I. W. Wolf, J. Appl. Phys., 33, 1152 (1962). 3. I. Danylchuk, A. J. Perneski, and M. W. Sagal, Proc. of the 1964 Conference on N o n l i n ear Magnetics, Washington, D. C. 4. T. R. Long, J. Appl. Phys., 31, 123S (1960). 5. T. R. Long, P a p e r p r e s e n t e d at the Detroit Meeting of the Society, Oct. 1-5, 1961. 6. "Electrodeposition of Alloys," Vol. I, Chap. 6 and 11, A b n e r Brenner, Academic Press, New York (1963). 7. H. Belson, Proc. of the 1963 Conference on Nonlinear Magnetics, Washington, D. C. 8. G. H. Cockett and E. S. S p e n s e r - T i m m s , This Journal, 108, 906 (1961).
