compositional depth profile analysis of coatings on hard ... jx_sct2003.pdf · compositional depth...

Surface and Coatings Technology 176(2003) 93–102

0257-8972/03/$ - see front matter� 2003 Elsevier B.V. All rights reserved.doi:10.1016/S0257-8972(03)00020-3

Compositional depth profile analysis of coatings on hard disks by X-rayphotoelectron spectroscopy and imaging

Jianxia Gao*, Erjia Liu, David Lee Butler, Aiping Zeng

Centre for Mechanics of Microsystems, School of Mechanical and Production Engineering, Nanyang Technological University,50 Nanyang Avenue, Singapore 639798, Singapore

Received 19 September 2002; accepted in revised form 10 January 2003

Abstract

A hard disk medium is typically composed of several layers including the magnetic recording layer, a buffer layer, as well asa wear protective layer. In the work presented here, the hard disks analysed have a total of five layers with the uppermost layerbeing the lubricant. The second layer is diamond like coating and this is followed by the magnetic layer consisting of an alloy ofcobalt and other elements. The fourth layer is a buffer composed of an alloy of chromium, vanadium and molybdenum with thefinal layer being a nickel transition layer doped with phosphorus. These multilayers were subjected to numerous etchings byargon ions. The chemical structures of these layers were analysed with an X-ray photoelectron spectroscope(XPS) after eachetching. Combining the XPS spectra with XPS imaging it is possible to determine the depth distribution of elements in the harddisk coating. In addition, it is also shown that XPS imaging can be employed to monitor the thickness of all multilayers.� 2003 Elsevier B.V. All rights reserved.

PACS: 33.60.Fy; 73.21.Ac; 73.90.qf; 75.5.Cc

Keywords: Hard disk; Multilayers; X-ray photoelectron spectroscopy; Image

1. Introduction

X-ray photoelectron spectroscopy(XPS) uses highlyfocused monochromatised X-rays to probe the materialof interest w1,2x. The energy of the photo emittedelectrons ejected by the X-rays are specific to thechemical state of the elements and compounds present,i.e. bound-state or multivalent states of individual ele-ments can be differentiated. Specifically, the small-spotXPS technique can provide the following informationabout samples:(1) elemental identification and quanti-fication; (2) chemical functional group identificationand quantification;(3) layer-by-layer depth profiling. Inaddition, using the parallel XPS imaging technique thesechemical state images are acquired in only a few minuteswith a resolution of approximately 3mm w3,4x. Withparallel imaging, photoelectrons are collected from thewhole field of view simultaneously. A combination of

*Corresponding author. Tel.:q65-67906874; fax:q65-67904674.E-mail address: [email protected](J. Gao).

lenses before and after the energy analyzer focuses thephotoelectrons on a channel plate detector. The hemi-spherical analyzer permits electrons with only a narrowband of energy to reach the detector.In the field of hard disk technology, a hard disk

medium is normally composed of a lubricant layer, anovercoat layer, a magnetic recording layer, transitionlayers and substratew5,6x. Currently the liquid monolay-er organic films Z-dol (X–CF –O–(CF –CF –O)2 2 2 p

–(CF –O) –CF –X) or perfluoropolyether are used as2 q 2

lubricants of hard disks to reduce friction while amor-phous hydrogenated diamond-like carbon(DLC) hasbeen the choice of overcoats over the past 10 yearsw6,7x. DLC coatings combine the advantages of highhardness with low friction and low wear rates. That is,an extremely thin DLC film being deposited on magneticlayer is mainly used in a wear protective role.It is well known that magnetic layers of hard disk are

composed of several kinds of elements such as cobalt(Co), platinum(Pt), chromium(Cr) and so on. In orderto obtain hard disks that exhibit a long life, it is

94 J. Gao et al. / Surface and Coatings Technology 176 (2003) 93–102

necessary to investigate the elemental distribution andinterface properties. However, to the best of authors’knowledge, no papers discuss the usage of XPS imagein researching the depth profile of coating of hard disks.In our work, argon ions beam was used as etching

beam, the XPS image and electronic profile obtained byXPS method were used to determine the depth infor-mation of coating of a hard disk.

2. Experimental

Nickel film, as a transition layer, was deposited on ametal substrate(alloy of aluminium and magnesium) bya radio frequency(RF) magnetron sputtering systemequipped with a cryogenic pump. The base vacuumpressure was less than 2.0=10 Torr. The sputteringy7

was conducted with a sintered nickel target that was 3inches in diameter and 99.99% in purity. The depositionswere carried out in pure Ar 99.99%. The deposition ratewas measured using quartz crystal of 0.01 nmys accu-racy. The applied RF powers were in the range of 75–200 W. The substrate was initially at room temperatureand no effort was made to control its temperature duringdeposition.The chromium(Cr), molybdenum(Mo) and vanadi-

um (V) were co-sputtered on the nickel layer by thesame magnetron sputtering instrument and form thebuffer layer. Similarly, the cobalt(Co), platinum (Pt),tantalum(Ta) and chromium were co-sputtered on thebuffer layer by the same instrument and form themagnetic layer. The diamond-like carbon(DLC) filmwas then synthesized on the magnetic layer using themagnetron sputtering system. Finally, Z-dol lubricantfilm was dip-coated on the DLC layer, and its thicknesswas controlled by adjusting the concentrations of thelubricant or by adjusting the removal velocity from thesolution. So, the coating of the hard disk contains severallayer of films such as Z-dol lubricant, DLC overcoat,magnetic, buffer, and the transition layer.These coating had been etched many times by argon

ions beam and analysed by XPS method after eachetching. The distributions and chemical state of elementsin the coating were also measured by XPS image methodwithin a few minutes after each etching.The etching and analysis process were performed in

an ultrahigh vacuum chamber of Kratos Ultra XPSsystem(Kratos company, UK). The base pressure of thechambers is 2=10 Torr. This chamber mainly con-y9

tained a dual anode(Al yMg Ka ) and a monochro-1,2

matised Al Ka X-ray source, and a hemispherical1,2

electron energy analyzer. During etching, the argonpressure in the chamber is approximately 1=10 Torr.y7

Then the hard disk was exposed to the X-ray irradiationfrom a monochromated Al Ka X-ray source(hns1,2

1486.6 eV) operated at 150 W(15 kV, 10 mA). Thephotoelectrons induced by X-ray were filtered by hem-

ispherical analyzer, and finally recorded by the eightchanneltron multi-detector(for XPS spectrum) or bythe microchannel plate and CCD camera detector(forimaging).The set of etching time to each layer depends on the

brightness of the imaging of an element and the peakheight of the element in the XPS spectrum for this layer.For example, the fluorine only exist in the lubricantlayer, the XPS imaging and XPS spectrum of fluorine(F 1s) can be obtained after etching this layer for 2 s.This process can be repeated for several times till thatits XPS imaging became dark while the peak height ofF 1s in the spectrum became very low. Then a relationbetween etching time and peak height of F 1s can beobtained and the thickness of the lubricant can bedetermined. It should be mentioned that carbon existsin both the lubricant layer and DLC layer, however, thecontent of carbon is higher in the DLC layer than in thelubricant layer. If its XPS image(C 1s) becomes brighterwhile its peak height in the spectrum also becomeshigher, it means that the lubricant layer is alreadyremoved and the DLC layer was revealed, so themeasurement of exact thickness of the lubricant dependson XPS imaging and spectrum of both fluorine andcarbon.

3. Results and discussion

3.1. Etching and separation of multilayer

The multilayers on a hard disk were etched gradually,and the composition in each layer was obtained by XPStechnique. The first layer is Z-dol lubricant and the XPSspectrum in Fig. 1a shows that the lubricant is composedof carbon, oxygen and fluorine before etching. The peakposition of carbon(C 1s), fluorine (F 1s) and oxygen(O 1s) are 284.42, 687.94 and 534.65 eV, respectively.Fig. 1b is the XPS spectra of fluorine(F 1s) beforeetching, and after etching for 6 s.The second layer is DLC overcoat layer and the

imaging of the layer whose binding energy is 284.4 eVis shown in Fig. 2. Fig. 2a–c shows the carbon layerbefore etching, and after etching for 35 and 40 s byargon beam, respectively. With increasing etching time,the depth, which was etched, also increased. Afterapproximately 40 s, the DLC layer was removed andthe XPS image of carbon 1s peak was generated(asshown in Fig. 2). The centre region in Fig. 2b changedcolour from bright to gray, it shows the area of the DLClayer was nearly removed after etching for 35 s. Thedark colour in the centre region in Fig. 2c means theDLC film was removed completely after etching for 40s. The brightness in this XPS image depends on thecontent of carbon; for the higher content of carbon, thebrightness of the area which contained carbon in thisimage is stronger. In detail, the colour scale on the right

95J. Gao et al. / Surface and Coatings Technology 176 (2003) 93–102

Fig. 1. The XPS spectra of lubricant of a hard disk.(a) Obtainedbefore etching. C 1s: carbon, 284.42 eV; F 1s: fluorine, 687.94 eV;the O 1s: oxygen, 534.65 eV.(b) The XPS spectra of fluorine beforeetching, and after etching for 6 s.

hand side of the image represents an intensity gradienton the XPS image. When in the imaging mode the Ultrauses a microchannel plate and a phosphorus screen todetect the photoelectrons. The photoelectrons hit thephosphor screen causing visible flashes of light whichare recorded by a CCD camera, then the lateral distri-bution and number of light flashes are stored by thecomputer. At the end of the experiment the image isdisplayed with an intensity scale which is equal to thepixel intensity(the number of light flashes recorded perpixel). This is then converted to a colour scale with lowpixel intensities dark and high intensities bright. Thecolour is therefore a qualitative indication of the amountof an element present on the sample surface.

It should be noted that, although the density of theargon beam used as etchant is highest in the centre ofthe argon beam, the focus of the argon beam is notideal. This results in an asymmetric lateral distributionin those centres of all images.The third layer is the magnetic layer used as recording

media, that is an alloy of several metals, the XPSspectrum in Fig. 3 shows that layer is composed ofcobalt(Co), platinum(Pt), tantalum(Ta) and chromium(Cr). Fig. 4 is the XPS image of Pt 4f peak whosebinding energy is 71.3 eV. The magnetic layer was justdisclosed in the centre of Fig. 4a. The Fig. 4b and cshow the magnetic layer has been etched for 15 and 25s, respectively. The increasing area of light region meansthe revealed area of the magnetic layer was increased inFig. 4b. Clearly, with increasing etching time, this layeralso became thinner and thinner, and finally, afterapproximately 25 s, this layer was removed when a darkregion appeared in the centre area of the image showedin Fig. 4c.The XPS image of the Co 3p peak and Ta 4f peak

are similar to that of Pt 4f. However, the chromium iscontained both in the magnetic layer and in the bufferlayer, its image is different from that of Pt 4f, and itwill be discussed later.The fourth layer, as a buffer layer, is alloy of chro-

mium, molybdenum(Mo) and vanadium(V). Fig. 5 isthe image of Mo 3d peak whose binding energy is 227.8eV. Fig. 5a shows the buffer layer was just disclosed inthe centre, Fig. 5b and c indicate the buffer layer hadbeen etched for 30 and 60 s, respectively. The image ofV 2s peak is similar to that of Mo 3d. Fig. 5b indicatesthe light region was enlarged and part of light regionhas changed into grey colour near the centre of theimage. Thus, more area of the buffer layer was revealedand part of this layer was nearly removed near thecentre. The dark region in the centre of Fig. 5c meansthat the fourth layer was removed in the area by argonetching for 60 s while the lighter region was still thepresent layer.In addition, the image of Cr 2p(binding energy:

573.81 eV) is shown in Fig. 6. It confirmed the elementchromium existed both in the magnetic layer and in thebuffer layer. It should be mentioned that the imagebrightness of chromium in Fig. 6a–c was weaker thanthat of Fig. 6d–f. It means Fig. 6a–c belong to themagnetic layer and Fig. 6d–f belong to the buffer layer,and the content of chromium in the magnetic layer islower than that of the buffer layer.The light region in Fig. 6a means the magnetic layer

was just disclosed in the centre, Fig. 6b and c showsthat the lighter region became larger after the magneticlayer had been etched for 15 and 25 s, respectively.Thus the disclosed area of magnetic layer increased withincreasing etching time. The light region in Fig. 6dmeans the buffer layer was just disclosed in the centre.


Fig. 2. The imaging of carbon overcoat layer whose binding energy is 284.4 eV.(a) The carbon layer before etching by argon beam.(b) and(c)The carbon layer after etching for 35 and 40 s, respectively.

Fig. 6e and f shows the buffer layer had been etchedfor 45 and 60 s, respectively. Compared with Fig. 6d,the light region was enlarged and part of the regionbecame grey again in the centre of Fig. 6e, this impliesthat the disclosed area of buffer layer increased and partof buffer layer was nearly removed. With increasingetching time, more area of light region changed into

gray and even dark colour in the centre of Fig. 6f, thebuffer layer had therefore begun to be removed in thecentre of the image.The fifth layer is nickel transition layer. The XPS

images of Auger peak of nickel(Ni LMMa) are shownin Fig. 7. The lighter region in Fig. 7a is small and issurrounded by large black area, it means the Ni layer is


Fig. 3. XPS spectrum of magnetic layer used as recording media, andit is composed of Co, Pt, Ta and Cr.

just disclosed in the lighter region by etching beam andthe dark area still belongs to the fourth layer. Fig. 7band c shows the nickel layer was etched 1000 and 2240s, respectively. They indicate that, during the etchingprocess, the lighter region became brighter as the regionexpands. So the disclosed area of nickel layer wasenlarged gradually.The stack of those layers was fabricated on the

substrate of aluminium and magnesium alloy, and theetching depth had not yet reached to the substrate.

3.2. Thickness measurement

From the above discussion, each layer thickness canbe obtained by measuring the depth distribution of anelement, which only exists in that specific layer. Thefirst, the etching rate for each layer can be calculatedby the transport of ions in matter(TRIM) simulationw8,9x. The second, the etching time for each layer canbe calculated and finally the thickness of layers can bemeasured. TRIM is the most comprehensive programand will accept complex targets made of compoundmaterials with up to eight different layers. It can beused to calculate both the final 3D distribution of theions and also all kinetic phenomena associated with theion’s energy loss: target damage, sputtering, ionization,and phonon production.The fluorine element only exists in the lubricant layer;

it can be used to measure the thickness of lubricant.From the peaks F 1s at 687.94 eV in Fig. 1b, it isestimated that the lubricant layer was completelyremoved after etching it for 6 s, the etching ratecalculated by TRIM simulation is 0.83 nmys, so thethickness of the lubricant is approximately 5 nm. Itshould be noted that standard binding energy of F 1s

peak is 685.70 eV and this is related to the fluorine–carbon bond. The energy shift of the experimental valueis due to two reasons, the first is the charge accumulationof hard disk during XPS measurement; the second isthe binding energy of fluorine–carbon atoms in Z-dollubricant is higher than that of fluorine–fluorine atoms(standard binding energy of F 1s).The carbon elements exists both in lubricant and DLC

layer, so the thickness of DLC layer can be measuredby the difference between the depth of fluorine and thatof carbon elements. Fig. 2a is the image of the carbonlayer after the lubricant layer is just removed. Thebinding energy is 284.4 eV. The centre region in Fig.2c changed colour from light to dark, it means the DLClayer was removed completely in the centre after thislayer had been etched for 40 s. The etching rate of thelayer calculated by TRIM is 0.33 nmys, thus thethickness of DLC layer can be determined is 13.2 nm.Since the platinum element only exists in the magnetic

layer, it was used to trace the thickness of the layer.The result shown in Fig. 4a is the chemical state imageof element Pt 4f component whose binding energy is71.3 eV, this suggests that DLC layer was just removedand the magnetic layer was disclosed in the centre. Thebright area in the picture contains high content ofplatinum while the gray area in the picture has lowcontent of platinum. The black area means there is noplatinum in the area. With increment of etching time,the third layer has been removed gradually. Fig. 4c isalso the XPS image of Pt 4f component, it shows thecentre region in the image became black, so the platinumelement on the region of the layer was removed whilethe magnetic layer was just removed. From Fig. 4a toc, the etching time is 25 s. The etching rate is 0.99 nmys calculated by TRIM simulation according to thecomposition of this layer, so the thickness is 24.8 nmaccording to the etching time.Molybdenum element is only contained in the buffer

layer of the hard disk, so it can be used to trace thedepth of the layer and the results are shown in Fig. 5.The binding energy of Mo 3d is 227.8 eV, and it is usedas the detecting energy of the XPS image. The brightarea in Fig. 5a indicates that the magnetic layer wasremoved and the buffer layer was shown in the centralarea of the image. Similar to the magnetic layer, theargon ion beam etched the buffer layer until a blackregion appeared in the centre of the layer. The imagingin Fig. 5c shows a dark region in the centre of theimage and it means that the centre of the layer wasremoved. The etching rate of the layer is 0.595 nmyscalculated by TRIM simulation. The etching time is 60s, so the thickness of the layer is 35.7 nm.The fifth layer, nickel as a transition layer between

the substrate and the buffer layer, is a thick film. Thepeak energy of Ni LMMa component used in Fig. 7 isthe 640.7 eV, the bright area means high content nickel


Fig. 4. The XPS image of Pt 4f peak whose binding energy is 71.3 eV.(a) The magnetic layer was just disclosed in the centre;(b) after etchingthe magnetic layer for 15 s;(c) after etching the magnetic layer for 25 s.

while the grey area around the bright area means it isat the edge of the nickel layer which was disclosed. Theetching rate for this layer is 4.07 nmys obtained byTRIM simulation. From Fig. 7a to c, it had been etchedfor 2240 s and the depth, which was etched, is 9111

nm. Fig. 7c also shows high content nickel, so the layerhad not been etched completely.The calibration of these thickness values obtained by

the above method can be performed by high resolutionscanning electron microscopy(HRSEM) technique. That


Fig. 5. The XPS the images of Mo 3d peak whose binding energy is 227.8 eV.(a) The buffer layer was just disclosed in the centre;(b) and(c)the buffer layer was etched for 30 and 60 s, respectively.

is, as an XPS imaging of an element belonging to alayer become dark while its XPS spectrum become flat,then the etching region of the layer can be measured byHRSEM and its thickness can be obtained. Comparingthis value with that value obtained from XPS imaging

and spectrum, the latter can be calibrated by HRSEMtechnique.

4. Conclusions

XPS spectrum and imaging techniques are effective


Fig. 6. The image of Cr 2p whose binding energy is 573.81 eV.(a) Magnetic layer was just disclosed in the centre;(b) and(c) after etching themagnetic layer for 15 and 25 s, respectively.(d) The buffer layer was just disclosed in the centre;(e) and(f) the buffer layer was etched for 45and 60 s, respectively.


Fig. 7. The image of Ni LMMa whose binding energy is 640.7 eV.(a) The transition layer was just disclosed in the centre;(b) and (c) afteretching the transition layer for 1000 and 2240 s, respectively.

to be used to characterize the depth distribution of theelements in the coating of hard disks. For the coatingin this study, the thickness of Z-dol lubricant layer wasapproximately 5 nm, that of DLC hard coat, CoPtCrTarecording layer, and CrMoV buffer layer were 13.2,24.8 and 35.7 nm, respectively, and the Ni transitionlayer was thicker than 9111 nm.

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