MAGNETIC RECORDING TECHNOLOGY

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MAGNETIC RECORDING TECHNOLOGY

J. Desserre

To cite this version:

J. Desserre. MAGNETIC RECORDING TECHNOLOGY. Journal de Physique Colloques, 1985, 46

(C6), pp.C6-85-C6-92. �10.1051/jphyscol:1985615�. �jpa-00224853�

Colloque C6, suppl6rnent a u n09, Tome 46, septernbre 1985 page C6-85

MAGNETIC RECORDING TECHNOLOGY

J . R . D e s s e r r e

BULL,

B.P.

RCsumC-L'introduction des technologies couches minces dans le domaine de I'enregistrement magnetique permet d'envisager un accroissement de performances. Cependant, l e chemin a parcourir entre la "fabrication" du premier prototype et la mise sur le marche du produit correspondant est long. Les exemples sont nombreux : les tCtes integrees, les media en couches minces pour l'enregistrement longitudinal ou perpendiculaire ... . Malgre cela de nouveaux materiaux et de nouveaux modes d'enregistrement sont aujourd'hui proposes

I'enregistrement

alliage de terre rares-metaux de transition [ 11, l e mode transversal 121. Des densites areolaires de 625 MBits /pouceX*2 sont esperees. L'objectif de cette communication est de donner et rappeler certaines donnees concernant les technologies couches minces pour l'enregistrement magnetique. Les points critiques de ces "nouvelles technologies" vues sous l'aspect industriel seront abordes A la question :"est ce que les technologies couches minces representent une opportunite ou une necessite pour l'accroissement des performances des peri heriques magnetiques

il sera repondu positivement comme cela le f i t pour les semi-conducteurs,

Abstract -Since the introduction of thin f i l m technology a large improvement of the magnetic recording capacity has been expected. As usual there i s a long time between the f i r s t prototype and the product. For example, we could notice thin f i l m heads and longitudinal and vertical recording on thin media. New materials and mode are now suggested

magnetooptic [ I ] and transversal recording [2]. B i t densities of 625 MBits per inch **2 or more are predicted. The aim of this paper i s t o bring some data about the thin f i l m technology for magnetic recording.

For instance, the following question w i l l be discussed

"Do thin f i l m devices and

or vertical recording have t o be substituted for particulated media and longitudinal recording ?". The crucial points i n these new technologies are very important i n spite of the fact that some optimistic announcements are made. Then it i s possible from the available results t o ask "Is thin f i l m technology an opportunity or a necessity to increase the capability of the peripheral magnetic memories ?". As for semi-conductors, the reply w i l l be

YES because of the physical properties of the f i l m s and the product quality we need.

*now w i t h ENERTEC SCHLUMBERGER, I r u e N i e u p o r t , 78140 Vglizy, F r a n c e

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1985615

C6-86 JOURNAL DE PHYSIQUE

When we have t o design a product, the needs of the market must be considered, not only for a business point of view but also for the technical requirements that it imposes. For instance, because of the extension of the micro computers, removable media must be developped. So, their sizes are reduced while the capacity i s increased. This w i l l be obtained by an improvement of the linear b i t density (using vertical recording) or the track density or both. But, f i r s t of a l l we have t o consider the environment of the recorders itself, that i s to say the external magnetic field, the temperature, the pressure, the atmosphere and the local cleanness. Most of the time, these problems have to be solved for an office environment and constitute a limitation. For example, to record data w i t h a small access time and a high linear b i t density , a low flying height w i l l be required. Then, dust w i l l have to be avoided t o prevent a crash on the disc (fig.1 1 or

l l ~ ~ y l l r ~ l C

s u

e

.i

q

layer.

f i g - 1 Head t-o medium interface

some damage or errors (e.9. drop outs

on a tape. I f a clean atmosphere can be expected on a fixed disc drive, it w i l l be more d i f f i c u l t for a cartridge unit. In that way, the thin f i l m technologies w i l l allow the deposition of dry and hard protective or lubrlficant layers. Then a head-medium contact only w i l l be responsible for errors, but the medium, and therefore the data, w i 11 not be destroyed

Floppy disc units and tape recorders are less sensitive to thls problem because of the speed of the media and the relatively low track densities

on f l e x i - d i s c s and 9 to 18 tracks on a 1/2

tape), but the head to medium interface become very crucial. As an example, the wear of heads and media i s strongly dependent on both the relative humidity RH and the temperature [31 (fig.2). I f these two physical data can be controled in a computer room it w i l l not be the case i n an office a plane or a satellite.

80 petcent RH. ~ n p e x 80 pe#ccnl R h . A r n ~ c l 70 peccen! RH: A r n ~ e . 6 0 oc-cenl RH: A r n e % 4 0 PelCrnl RH. A . r ~ e i 30 P e r C I r l RH. & r r . a e ~

especially f o r the thin media because of t h e i r thicknesses

1 p m (vs 4 p m or more f o r tapes).

Capacity, access t i m e and data transfer r a t e (DTR) constitute an other group of requirements. Vertical recording could be a reply. As an example the magnetooptic devices are looked as the future of the mass storage technology. I t has been shown by P BERNSTEIN and C.GUEUGNON that 625 MBits/ inch**2 could be available on a quadrilayer structure

1 I, [41). A magnetic head o r a laser beam record the data (vertical recording) w h i l e Kerr and Faraday e f f e c t s are used t o read back. Prototypes (1.05 Gbytes, 43750 tracks/ side, DTR

350 Kbytes/sec) were described by KDD [51 and SONY 161. I n the same t i m e a Dso of 230 K f c i was observed on vertical recording on a floppy disc [71. But it would be unreasonable t o consider these new memories as the announcement of the end of the tapes

f o r the capacity) and the discs

f o r the access time) neither as some available products. The choice between the capacity plus the DTR and the access t i m e (including the seek time) w i l l be imposed This may be obvious but the users are generally disappointed when they compare what

was expected from the scientific announcements t o the corresponding product. The new technologies are very promising but they cannot solve a l l the problems a t the same time.

I I -WI FXIO& ON THF MANUFACTURFRS' REQUIREMENTS

As every t i m e a new technology i s introduced by the scientists, the "old processes" are improved. This effect can be explained by the cost of a new industrial process compared to the relatively low outlay required to improve the conventional devices. This can be observed w i t h the development of the thin f i l m media f o r high density recording. New pigments are developped. Metal 1 i c and Co-6Fe20 , particles are now available and the,new longitudinal media are on contest w i t h the plated magnetic layers (CoNi, COP ..., w e t process). On the other hand, isotropic particles (181,[91) are being studied f o r high linear b i t densities on tapes while barium f e r r i t e pigments [lo]

are being developed f o r vertical recording and compared t o the CrCo sputtered films.

Despite the performances of the thin f i l m media, and because of the cost of equipment and the l o w price expected f o r these products, we cannot neglect the challenge w i t h the particulate media. The gap between the t w o magnetic layers i s not yet large and can be easily f i l l e d up by the use of new codes ,,heads and / o r electronic preamp1 i f iers.

The same effect i s observed f o r the heads. Since the introduction of the thin f i l m heads (TFH

1 1 I, a f e w companies have developed these sensors. But there was a long way t o make a product from a prototype w i t h a good y i e l d and a l o w cost. Consequently, new f e r r i t e s and new head designs were studied. For instance , a minicomposite head w i t h a Mn Zn f e r r i t e core can be now used t o record data a t 13 Kfci, as w e l l as a TFH. The cost and the traditional knowledge are yet stonger than the s c i e n t i f i c performances i n t h i s case. This can be explained by the f a c t we did not need TFH t o improve the products.

But, new high coercivity media are now used; a new eighteen track standard i s introduced

on tapes

121; vertical recording could be on the market before the nineties. Thus

something w i l l have t o change

the objectives or the head technologies. Therefore, the

thin f i l m processes could be the good reply f o r an evolution of the magnetic recording. To

introduce them on a production line, we w i l l have t o be realistic. A manufacturer w i l l

never produce a device f o r the beauty of the process or the stucture, but t o satisfy the

C6-88 JOURNAL DE PHYSIQUE

needs of the market and t o make some business. Consequently, t o be on the field of the mass storage technology, it seems crucial t o start from the present status of the international market and to avoid the traps of the short-lived ideas.

111-1 DEPOSlTlON PROCESS The results obtained from the manufacturing of

longitudinal platted discs [ 131, can make this method very attractive. However, i n spite of the fact that a growth of 0.2 pm/ mm can be achieved for the cobalt f i l m s [141, their magnetic properties are not good enough compared t o those of sputter deposited or evaporated media. They vary w i t h the homogenity and the temperature of the chemical bath. Moreover a 250 A thick layer i s considered as the lower l i m i t to acheive a continuous medium. This w i l l be a real difficulty t o increase the b i t density. Therefore, i f the wet process i s usable for a mass production, the specifications cannot be easily guaranteed. On the contrary, it i s easier to control the physical parameters by using sputtering or LPVD. The advantages of these technologies are

- the minimum thickness for a continuous f i l m

150 A, - the wide variety of elements, componds or alloys available,

- the possibility to develop multilayer structures and consequently, t o deposit a suitable overlayer t o l i m i t the wear and the corrosion,

-

the adhesion of the layer on the substrate,

- the i n s i t u processing.

Despite these advantages, the vacuum processes are a crucial point f o r the futur of magnetic media. New equipments w i l l be necessary. Moreover, although some very high deposition rates

1 pm / s) can be hoped f o r w i t h some materials like Co and Cr coevaporated w l t h t w o e-beams, the production rate i s limited by the capacity and the environment of such equipements, which are more adapted for the tape and the floppy disc manufacturing

151 or the research

161. Using a RF sputtering configuration, a low deposition rate i s acheived

600 A/ mn 1. Even by taking into account the quality of the layer, this method cannnot be used for a mass production. S t i l l another, and probably the best process would be the magnetron sputtering. High deposition rates

2 pm / mm) can be obtained. But, because of the energy of the atoms removed from the target, stresses are induced i n the layer and the magnetic properties of the medium are changed (fig.3).

Consequently a reduction of the depositon rate and/or the increasing of the temperature of the substrate during the growth of the layer would have to be introduced. This could be a d i f f i c u l t y for tapes or floppy discs.

f

ig-3 BltlllaaenaaCrComedium: ^a/under stresses. b/without stress

As shown from this discussion, the vacuum processes are not yet available for

mass production. To make them a reality, the manufacturing cost, included the costs of

the knowledge and the equipments, w i l l have to be compared to the performances, the

1 1 1-2 WOICF QF THF SUBSTRATF Anodized aluminium substrates are generally used for conventional discs. I f they are of a great interest for electrodeposition or sputtering processes, they need a special surface treatment. Before deposition they are overlayed w i t h a plated Ni f i l m and polished. These substrates are expensive compared to the plastic or glass discs (carbon discs were also suggested for high RPM). The choice of substrate i s very important because of the preparation of i t s surfaces (laping, polishing, cleaning process, etching.. 1. They play a role on the adhesion of the layers, on the nucleation and the growth of the thin f i l m s (structure, thickness ...I and, finally on their magnetic properties and the error rate. By the way, the glass substrates seem t o minimize the riscs and the cost.

The same problems occur to make tapes and flexible discs (thickness of the magnetic 1ayer:sl pm). The wear and/or the scratches due t o the head medium contact w i l l be strongly dependent on the i n i t i a l state of the surface deposition. Moreover, the kapton or the ribbon used as support w i l l have to allow vacuum processes and temperature deposition as high as 250°C.

It i s clear that the thin f i l m technologies w i l l impose new constraints to the manufacturers. The smoothness of the media, the number of defects, the magnetic properties of the layers w i l l be a consequence of the substrate and i t s preparation. The present status i s unsatisfactory. Here i s a challenge.

111-3 JHF COMPOSITION HOMOGENEITY This point contributes to the signal modulation. Throughout the thickness of the f i l m s

1 pm), the atomic concentration of the alloys i s generally kept constant for a CrCo medium as well as a FeTbGd as shown on fig.4 and fig.5. Along a radius or along a track, the result depends on the process and the

Fig.4 - m m e d i u m fig.5 aFeTbGd ~ + S i n n ~ v e r ~

nature of the elements. For instance the CrCo media present a low variation of the concentration and properties, but i t i s not the case for an amorphous alloy . The anisotropy of an aFeTbGd layer i s a function of the location of the studied sector. A shift of 5 At% of the rare earth concentration on the surface of the disc, due to the random evolution of the RE-TM plasma, could explaine this result. A better homogeneity i s acheived by e-beam evaporation [ 141.

Some alterations were also observed on a multilayer structure

Co (=200A) on

Cr (=~oooA). An atomic diffusion: Cr <==>Co, occurs w i t h the time and modifies both

C6-90 J O U R N A L

magnetic thickness and properties of the layer (a vertical component appears in this case). This could be a problem f o r magnetooptic recording.

11 1-4 Thickness and_m)stallograDhlc properties ~ L U ~ m e d i a When studying the magnetic properties of a layer, the thickness and the crystallographic characteristics of the f i l m s are considered as two major paramaters. As an example, we can trace the evolution of Ms, MR, Hc ... w i t h respect to orientation ratio ( 1 100/1002 for CrCo.) I f some workers [I61 have shown that the same set of data used on t w o deposition units do not lead to the same results, it was generally observed that the magnetic properties were strongly dependent on the deposition paramaters -e.g. the bias voltage

and the thickness: 1 j.tm i s an optimum [ 171. To explain the scattering of the results, it w i l l be consider that the crystallographic parameters, e.g. do,, and AQ,, , are very sensitive to the depositon configuration and the nature of the underlayer. This last point was proved by M. FUTAMOTO and al. 171. They found that a germanium f i l m was the best underlayer to improve the c-axis orientation

"

LAYER MATERIAL

When the study i s performed w i t h respect to the thickness,

defined f o r 0.8 t o 1 bm. Such a thickness minimizes the demagnetizing fields and i s suitable for the growth of the columns.

Despites these results a thinner f i l m i s used f o r vertical recording -except may be f o r floppy discs. This can be explained by the poor anisotropy of the CrCo layers

1 91 which imposes a high field gradient for the head and / or a high coercive force for the medium, even w i t h a soft underlayer.

These results show that the study of the intrinsec properties of materlal or a stucture i s not sufficient to define a product. Some external data as the. peak shift, the overwriting ... must be added t o the physical evaluations. They constitute a limit, but are the minimum requirement to make a product.

1 1 1-5 I_tlLAGING ^EFFECT This phenomenon essentialy affects the amorphous recording

layer. For instance, i t i s observed on a-FeTb an a-FeTbGd. Without any protective layer an

oxidation of the rare earth occurs

181. The vertical anisotropy disappears after a few

hundred minutes. When the medium i s protected, this phenomenon i s limited to the

interface (fig.5) and affect only the f i r s t 10 t o 30 A. Starting w i t h nominal composition

of the medium, the aging effect leads to a thin layer w i t h a high coercive force broken

into small domains. Then the SNR i s reduced. This problem i s very crucial for the

magnetooptic applications. With a multilayer structure, this effect could be avoid 141.

I t seems that the manufacturing head problems are more d i f f i c u l t t o solve than those described for the thin media. In spite of the fact thin f i l m heads (TFH) are now used on some commercial drives and though some companies are able to produce them, the corresponding processes (thin f i l m and photolithographic techniques, lapping and polishing) are not completely under control. To have a better understanding of the problem, i n the field of vertical recording, four trends must be considered

1 -ring heads on a monolayer medium,

2-probe heads on double layer medium for flexible discs or tapes, 3-hybrid or thin f i l m heads

probe head like) for rigid discs, 4-exotic structures.

When studying the recording process , we have to consider the head field orientation, the head field gradient and the orientation of the the medium in term of anisotropy (squareness S, S*; Hk, Hc, Ms). The soft underlayer i s also to introduce as a part of the head. I t i s obvious that the most attractive recording system would be the one that allows the use of a RH or a TFH on a single CrCo medium. Because of the poor anisotropy of these medium the head field orientation i s very important during the recording process. Such a structure was developped by POTTER ( w i t h LANX). He obtained 20,000 bpi 0n.a r i g i d disc. Because of the low coercive force of the layer

250 Oe and the poor squarness of the B(H) loop, this way seems to me limited for high recording densities. As it has been shown i n 1983 [18], a better result was obtained on a RE-TM alloy. Because of the strong vertical anisotropy of these material

the squarness of the B(H) loop i s S=Sg= I) and the low demagnetizing fields (4TlMs=1000 G

a RH or a TFH can be used.

Despite this result, we can assert that the CrCo based alloys w i l l be used for future products, except for the magnetooptic devices. They w i l l be designed as a double medium, that i s to say w i t h a soft under layer of FeNi or CoNb f o r example. In this case, the head field orientation and the head field gradient a r e t h e o r e t i c a l l y not very important during the recording process. However the performances of the HMS are very sensitive to the longitudinal component of the field (fig.7). Then the anisotropy of medium w i l l have to be improved, and the orientat ion of the head increased.

fig.7 BeabnutsianalmaCrComediumm_the_n_anhL-

L U r n r n L c o n o o n e n t

This whs obtained by both Dr TSUl and Dr HAMILTON [201 and presented during I NTERMAG 1985. There results are of a great interest and can be summarized as following

I -the SNR i s greater than 28 dB

at 30000 f c i 1,

2-the resolution i s 70

C6-92 JOURNAL

DE

3-the soft underlayer i s very efficient,

4-the R/W sensor i s designed as an hybrid head,

5-the manufacturing process i s the same that the one developped for the TFH on longitudinal recording.

I f the f i r s t point i s crucial for the signal processing, the two last ones are very important. Many exotic structure were or w i l l be suggested. It i s obvious that they are worthwhile about the design, the performance, the possibility t o extend them more and more. But generally they are not suitable for an industrial process. It w i l l be an error to design a head and ignoring at the same time the manufacturers' and environment's requirements. This assertion does not have to be considered as a l i m i t to the research which give the understanding, but must be taken as a goal to acheive to make a product.

W L U S I O N Thin f i l m technologies are very promising t o improve magnetic data storage. They w i l l allow a reduction of the cost per b i t while the linear b i t density, w i t h the vertical mode, w i l l rise. A l o t

results are now available. We have t o learn from them that there exist now some solutions f o r each specific problem. Moreover we can consider they are known. Because of the properties of the thin f i l m s : thickness, crystal structure, composition ... we are able t o optimize each parameter. What it was only possible to study i n the great research center of the manufacturer of components, i n the past, i s now available i n small laboratories or start up companies. But a result or a prototype are not a product. I f we want to be on the magnetic recording market we need t o make some products. 257 Kfci on a floppy disc i s a scientific result, not a product. The same comment can be done for a pattern or a prototype.

As a conclusion, I should reply to the i n i t i a l question as following

Yes, the thin f i l m technology i s suitable to improve the capacity

Yes it i s a necessity, step by step. We have the knowledge. Then we have to learn the industrial processes and the laws of the users' and environment's requirements. Here i s the challenge of the magnetic technology.

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