RF SPUTTERED CoCr ALLOY FOR PERPENDICULAR RECORDING ON RIGID DISC

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RF SPUTTERED CoCr ALLOY FOR

PERPENDICULAR RECORDING ON RIGID DISC

D. Jeanniot, J. Desserre

To cite this version:

D. Jeanniot, J. Desserre. RF SPUTTERED CoCr ALLOY FOR PERPENDICULAR RECORD- ING ON RIGID DISC. Journal de Physique Colloques, 1985, 46 (C6), pp.C6-105-C6-109.

�10.1051/jphyscol:1985619�. �jpa-00224866�

Colloque C6, supplement au n09, T o m e 46, septembre 1985 page C6-105

RF SPUTTEREO C o C r A L L O Y FOR P E R P E N D I C U L A R RECORDING ON R I G I D D I S C

D. Jeanniot and J. ~esserre*

BULL, DRTG/RTSA, rue Jean Jaurds, B.P. 5 3 , 78340 Les CZayes sous Bois, France Resum'e

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Abstract

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I. INTRODUCTION

Since the introduction of perpendicular magnetic recording (PMR) by IWASAKI in 1977 111 on COCK alloys. this medium has been studied by many workers. Some problems still ceiain regarding- the obtention of a product using PMR s"ch as the electionic

writelread channel, the transducer design and the medium elaboration mode. The clas- sical electro or chemical plating processes are not studied for CoCr alloy, even though if some laboratories are working on pure electrodeposited Co [5]. To obtain CoCr thin films, three vacuum deposition methods are mainly used : e-beam evaporation, RF sput- tering and magnetron sputtering. The goal of this research is to get a higher deposi- tion rate allied with a convenient perpendicular magnetic anisotropy ( P M A ) . In our - - l a b o r a t o r i e s , w e h a ~ e o b t a i n e d r e S u l t S w i t h C o C r d e p o s i t e d b y m e a n s o f e - b e a m e v a ~ o r a t i o n ~ 6 ~ ~ Others workers have achieved good PMA using the same process 171, but with a deposi- tion efficiency in the order of 50 %. The RF sputtering method will be discussed af- ter. This deposition mode is used by most researchers to study the CoCr alloy. The results obtained are quite satisfactory with regard to PMA, but the deposition rate R is low (in the order of 10 A/s). Other methods, derived from the RF diode sputte- ring technique, have been explored; the triode DC sputtering, which leads to R=33 A/s

181 and the magnetron sputtering, which allows R up to 120 A/s 191. The latter is a very promising method, because it increases the deposition rate very significantly, but these are only the first results. Unfortunately, progress might be slow because of the difficulty of this method : the path of the electrons is increased between the cathode and the anode by the application of a magnetic field during the deposition.

The fact that the involved materials (i.e. COCK and FeNi) are magnetic would compli- cate the manipulation of the deposition parameters.

* J. Desserre is now with ERNERTEC / SCHLUMBERGER, 1 rue, Nieuport, B.P.54, 78141 Vglizy

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Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1985619

C6-106 JOURNAL DE PHYSIQUE

I I. EXPERIMENTAL PROCEDURE

The diameter of the tarnet was 20.3 cm. Its purity was 99.99 - % and its com~osition was 17 % At. Cr. The cathode-anode spacing was 4,5 cm and the layers were deposited onto glass discs (13 cm in diameter). For recording tests, the COCK layer was over- coated with a 300 A thick Si02 protective layer. No changes in the magnetic proper- ties occur when this layer was deposited. The substrate holder was water-cooled. The residual pressure was better than 2.10-7 Torr. High purity argon was first introduced in the chamber. Prior to deposition, the CoCr target was sputter-etched for 30'. To evaluate the PMA, the longitudinal and the perpendicular hysteresis B(H) loops were traced with a VSM. In fact, only the longitudinal loop was used to evaluate the PMA.

To avoid the errors derived from a partial saturation of the samples, the inner tangent slope at the origin of the axis (tga//) was generally used, except for the bias tension study.

111. MEDIA PREPARATION

The methodology of this optimization is summari- zed in table I." a" is the changing parameter;

fixed parameters are listed on line " b

".

TABLE I

/ and I100/1002 on the thickness of the COCK layer is presented on fig. 1. I100/1002 corresponds to the ratio of the intensities of the diffraction peaks (100) and (002). For the magnetic and structural orientation, the optimal thickness is about 1 p. This evolution of 1100/1002 with the thickness has already been observed [lo]. Two phenomenons occur when the film grows. As it can be seen by the evolution of 1100/1002, the orientation of the very thin layers is not perpendicular. It corresponds to the transition layer, which is magneticallly soft and longitudinal 1111; its structure is hcp, but its c axis is parallel to the plane of the layer [12]. When the thickness increases, this orientation improves because the (002) planes are the denser in the hcp structure. Thus, they tend to be parallel to the plane of the film. This phenomenon is called the " self epitaxy

".

other hand, the diameter of the columns increases with the thickness; so they have a conic shape, and not a cylindrical one [13]. For the layers thicker than 1 pm, the columns are less well defined and the structure becomes disoriented.

1 ^LQ,,

I , 5 3 . d 7 .o

I

I

I I I

0 I

1 2 3 4 t ~ l - , ~ 1 1.5 2 Vk (LVJ

Figure 1 : Evolution of t g a l f and I l 0 0 / I 0 0 2 Figure 2 : Dependence o f I1001002 and t g a / / on with the CoCr thickness. the cathode v o l t a g e .

Cathode voltage - ^The

represented on fig .2.

instable 1141 and the

dependance of tga// and I100/1002 on the cathode voltage are For the low values of V ( < 1.2 kV), the plasma is generally deposition rate is low 1151. An optimal value of ~k appears to be about i.8-k~ for the magnetic properties; for the values greater than 1.6 kV, cristallographic orientation improves. Similar variations have already been observed 1-16, 171. Such variations can mainly be attributed to changes in the temperature of the substrate. This temperature is monotically increasing with Vk 1161.

Bias voltage

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argon incorporation (maximum 2 %); on the other hand, the increase of the difference of potential in the dark space near the substrate can have an influence. The strong decrease of the deposition rate for the higher values of Vb ran be attributed to a selective resputtering of Co, although the sputtering yieldofco is lower thanthat for Cr [18]. This tends to confirm that Cr is bonded in the layer with elements such as oxygen, which decrease the magnetization. Inner stresses variations and temperature of the substrate changes, are certainly important in explaining the observed varia- tions of the properties. The columnar microstructure is also very affected by bias tension variations 11 9 : .

I I

600 760 R (f;minn)

Figure 4 : Evolution of t g a and 110011002 with

t h e deposition rate for dxfLerent PA=.

lFigure 3 : Variation of S / / and 1100/1002 with -5b - ~ 6 0 -140 - 2 6 0 "81VI the bias tension.

Argon pressure - The argon pressure (PAr) action on tga// and 1100/1002 is represen- ted on fig.4. An optimal value of R is observed at 700 A/mn, which corresponds to PAr

= 10 mTorr. In fact, as in the case of Vk, these variations may be attributed to changes of the temperature of the substrate 1161; this latter presents a minima for a given pressure. This phenomenon is probably due to the heating of the substrate by the kinetic energy of the incident atoms and of the secondary electrons coming from the target. For the low PA,, the heating due to the electrons is low, but the mean free path of the particles of the plasma is high. On the other hand, the orientation of the c axis improves at lower pressures 20, 211; for the higher values of PA,, an in-plane oriented phase is observed [11,16\. At the same time, the intercolumnar mat- ter becomes denser [21]. The hysteresis loops of the optimized layer are shown of fin.5. -

IV. RECORDING RESULTS

It is obvious that the most attractive recording system would be the one that allows - . the use of just one R/W sensor on a CrCo single layer. At low densities, a ferrite head with a 1.2 p gap length did not record a pure perpendicular transition (fig.6a). When a DC in-plane field was applied, the vertical transition appeared (fig.6b). Using a 0.6 p gap length head, it was not possible to write on the same media. The use of a thin film head with the same gap is more appropriate. This confirms that thin film heads with anisotropic pole pieces write sharper vertical transitions. Here, the best result was obtained with a large gap (e.g. 2.5 pm) as shown on fig.7. This large gap reduces the longitudinal component of the field but is not adapted for the reading process as show on fig.8.

V. CONCLUSIONS

The influences of the thickness of the layer, the cathode and bias voltages and the argon pressure on the properties of the COCK alloy have been investigated. An optimal thickness appeared around 1 p ; this is mainly due to a self epitaxial phenomenon as- sociated with the growing of the columns. The action of the cathode voltage can be attributed to changes substrate temperature during the deposition. An optimal value is situed around 1.8 kV. The optimal bias voltage has been observed around-60 V; the high sensitivity of the properties for this deposition parameter can be attributed to several phenomena. We have remarked upon the temperature of the substrate and the in- ner stresses. We have related the shape anisotropy and the magnetostriction to Vb. An optimal argon pressure of 10 mTorr has been defined : as for Vk, the observed varia- tions can be attributed to changes of the temperature of the substrate. This optimi- zation is good only for this deposition configuration. For instance, if the Cr concentration is increased to improve the PMA of the layer, another similar study

C6-108 JOURNAL DE PHYSIQUE

will be necessary for the new target. When some differences appear between magnetic and structural properties, we have attributed these differences to an imperfect co- lumnar microstructure or a thicker transition layer. Lastly, it can be noted that the optimal layer for magnetic properties is not necessarily optimal for the PMR.

Although the PMA of this CoCr alloy can still easily be improved, recording tests have nevertheless been carried out. When a conventionnal ferrite head was used, the perpendicular mode appeared when the longitudinal component of the magnetization was erased by an in-plane DC field. Another way to obtain the perpendicular mode was to use a large gap thin film head (2g>2.5 pm). A maximal recording density of 39 KFCI was obtained in these conditions.

Aknowledgements The authors thank J.C. Bouchand for preparing the media, Y. Gaillard for Xrays measurements, N. Pichereau for V.S.M. measurements and H. Lebreton for re- cording tests.

-. Figure 5 : Perpendicular and longitudinal loops

of the optimized CoCr layer.

Figure 6 : Read out signal obtained aC 240 PC1 with a CrCo medium and a 1.2 pm gap length fer- rite head. a) without and b) with a 60 oe in- plane polarization field.

Figure 7 : Read out signal obtained at 240 FCI with a CrCo medium and a 2.5 pm gap length thin film head wirhouc polarization field.

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