Appl Phys A (2013) 113:385–388DOI 10.1007/s00339-013-7921-y

Effect of etching on the electrical and magnetic propertiesof writer shield material

Hongmei Han · Yang Li · Wencheng Su · Lifan Chen ·Jinqiu Zhang

Received: 23 May 2013 / Accepted: 10 August 2013 / Published online: 25 August 2013© Springer-Verlag Berlin Heidelberg 2013

Abstract Magnetron sputtered permalloy films are treatedby wet chemical etchings with acid etchant as well as flu-orine based reactive ion etch (RIE). Upon these treatments,the resistivity and coercivity of the permalloy film increaseis within 10 %. No significant increase observed with pro-longed etching time. The effective magnetization changeof the permalloy films are within 5 % post the treatments.Atomic force microscope (AFM) and transmission electronmicroscope (TEM) are used to study the surface and inter-face evolution of permalloy film upon etching. The smallimpact on the electrical and magnetic properties of permal-loy films can be correlated with the surface oxide protect-ing layer formation during the etch. Consequently, sputteredNiFe is a safe material to expose to these etching processesfor write pole shield application.

1 Introduction

Complex etching processes become essential to magneticdevice fabrication on a submicron scale, as the rapid devel-opment of magnetic recording [1] highly demands miniatur-ized device with complicated structures. Wet etching, beingwidely used to etch silicon, has been used in semiconduc-tor manufacturing for many years [2]. Dry etching has beenused in both semiconductor and magnetic industries with avariety of applications; particularly, plasma-based reactiveion etching (RIE), has artificially tunable anisotropy, whichcan precisely control the geometries as device scales down.

H. Han (B) · Y. Li · W. Su · L. Chen · J. ZhangWestern Digital Corporation, 44100 Osgood, Fremont, CA 94539,USAe-mail: hongmei.han@wdc.com

In order to successfully apply these etching techniques inhard disk drive write head fabrication, it is imperative to un-derstand the change of the electrical and magnetic propertiesof wet/dry etched magnetic thin films. NiFe is especially in-teresting due to its near zero magnetostriction with high per-meability and low coercivity, it has many industrial appli-cations such as magentoresistive sensor and magnetic headshield. When NiFe is used as the writer shield material, itmay be exposed to various etching processes. Any degrada-tion of the electrical and magnetic properties of NiFe is notfavorable after encounter with these etchings. Therefore, westudy and compare the different etching effects on permal-loy thin films for magnetic recording by wet etching with aweak acid etchant and by fluorine based RIE.

2 Experiment

The etched films (Ni81Fe19, Ru, Cr, Ta, and Al2O3) on Al-TiC substrates used in this work were prepared by mag-netron sputtering. In wet etching, the etching rates were ob-tained after five minutes etching in a commercially avail-able chemical TFN (Transene Inc.) etchant, which is basedon ceric ammonium nitrate (10–20 weight percent) and ni-tric acid (5–6 weight percent) with pH value less than 1.Among them, the 100 nm thick NiFe thin films were furtherstudied with sequential etching of 3, 6, 9, and 12 minutes.All wet etchings were conducted at room temperature. Inthe dry etching, fluorine based (SF6/O2 = 5/1) RIE wereused to treat the NiFe thin films with 20 and 60 seconds in aPlasma Therm etching system. The film thickness was ana-lyzed using X-ray fluorescence (XRF). Electrical propertieswere characterized with a four-point probe station. B-H looptracer was used to measure the magnetic properties at roomtemperature. The microstructure structures of the studied

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films were examined by transmission electron microscope(TEM). Atomic force microscope (AFM) was used to probethe surface morphology of the studied films.

3 Results and discussion

Etching selectivity indicates the ability to etch only the de-sired materials relative to the etching of mask and other ma-terials in contact; it is an important factor for pattern trans-fer. Thus, the relative wet etching rates of the studied thinfilms with respect to the NiFe film upon etching in the TFNtank at room temperature are compared in Table 1. It is seenthat the different films are subject to quite different etchingrates due to the nature of chemical selectivity in wet etching.TFN exerts a slow etching process (<0.1 Å/sec) on the sput-tered NiFe thin film. Nitride acid has been reported to etchNiFe [3, 4]. In the current study, the ceric ammonium nitrateinside TFN could help passivate the film surface to preventthe etching of NiFe by the nitride acid component insideTFN. The Cr film has the fastest etching rate among the stud-ied thin films. The Ta film thickness remains the same afterthe wet etching. Etching rates of the spin coated photo resist(I051); sputtered Ru and Al2O3 films are also exhibited asreferences.

When the NiFe thin films are treated with different etch-ing processes, the etching rate could change due to the sur-face effects of the thin films. Changes in the magnetic prop-erties of permalloy thin films can occur, which will be com-pared in the percentage of change in coercivity and effec-tive magnetization. The percentage of change is calculated

by the change of coercivity (etc.) divided by its initial value.Figure 1 displays the evolution of the etching rate and the re-sistivity percentage change of the magnetic NiFe thin filmswith the etching time. In both the wet etching and the RIEdry etching, the etching rate slightly decreases as the etchingproceeds. For the resistivity percentage change, it decreasesas the time of RIE treatment increases, reaching a maximumof 3 % (in average) for the 20 seconds RIE treatment. Underthe wet etching, the resistivity change increases from 4 %to 8 % (in average) with prolonging the etching time from3 minutes to 6 minutes. Then it decreases to 6 % (in av-erage) as the etching time extends to 12 minutes. Figure 2shows the change of the NiFe films’ magnetic propertieswith respect of etching time. Upon etching, both the effec-tive magnetization and coercivity of the magnetic NiFe thinfilm are changed by less than 10 %. These results indicatethat the RIE dry etching with fluorine based gases and thewet etching with TFN have small impacts on the NiFe thinfilm.

High resolution cross-section TEM has been used tocharacterize the pre-etched and the post-etched NiFe thinfilms. As shown in Fig. 3, the bright field TEM image ofNiFe thin film in the as-sputtered state shows crystallineNiFe with lattice image both inside and on the surface of thefilm; the dry etch treated sample under RIE for 60 secondsshows a clear interface between the inside and the surfacelayer, both show clear lattice images; the wet etch treatedsamples for 6 min and 12 min in a TFN tank also show athin surface layer with lattice images inside and on the sur-face layers. The clear lattice images of the etching treated

Table 1 Relative wet etchingrate of TFN for Ru, Cr, Ta,Al2O3, and photo resist I051films with respect to NiFe film

Material NiFe Ru Cr Ta Al2O3 Photo resist (I051)

Relative etch rate 1.0 41.1 >76.9 0 0.5 60.6

Fig. 1 Impact of etching timeon the resistivity of the NiFefilms. RIE were processed for20 and 60 seconds with fluorinebased gases. Wet etchings wereprocessed for 3, 6, 9, and12 minutes in a TFN tank

Effect of etching on the electrical and magnetic properties of writer shield material 387

Fig. 2 Effect of etching time onthe effective magnetization (a)and coercivity (b) of the NiFefilms. RIE were processed for20 and 60 seconds with fluorinebased gases. Wet etchings wereprocessed for 3, 6, 9, and12 minutes in a TFN tank

Fig. 3 Cross sectional TEM image of the NiFe film in as-sputteredstate (a); post RIE treatment for 60 seconds with fluorine basedgases (b); post wet etching of 6 min (c), and 12 min (d) in a TFNtank

samples indicate there is no large crystallographic deterio-ration post the treatment. EDX measurements indicate thepresence of an oxide layer in the surface of all the sam-ples. The surface layer in the plasma treated NiFe thin filmhas an oxidization layer around 3 nm, the wet etch treatedNiFe thin film has a thinner layer of oxidization, especiallyon the grain boundaries. The thin surface oxide layer dueto etching protects the degradation of magnetic flux of theNiFe thin films [5]. As the wet etching time increases, thepossibility of diffusion from film surface along the grain

Fig. 4 AFM characterization of NiFe film surface: (a) as-sputteredstate; (b) etched under RIE for 60 seconds with fluorine based gases;Wet etched of 6 min (c), and 12 min (d) in a TFN tank

boundary is increasing. If any diffusion happens, it couldmodify the surface grain structures. When there is any anti-ferromagnetic phase formation, such as NiO forms, the ex-change coupling between the antiferromagnetic phase andthe ferromagnetic phase could increase the coercivity of thefilm. These may related to the higher coercivity increase ob-served for the wet etching (6 min and 12 min) treated sam-ples [6, 7].

As coercivity has high correlation on the surface rough-ness in thin films [8], AFM has been used to characterizethe pre-etched and the post-etched NiFe thin films. Fig-ure 4 compares the AFM images of NiFe thin films in theas-sputtered state and the dry etched state, as well as post

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the wet etching treatments. The 12 min wet etching treatedNiFe film has a RMS roughness of 2.1 nm, which is largercompared to the dry etch treated one with a RMS rough-ness of 1.3 nm and 1.1 nm of the as-sputtered film. Thisis consistent with the larger increase of the coercivity ofthe NiFe films post wet etching treatments as compared inFig. 2. The small film roughness change due to etching cor-relates well to the low impact on the coercivity of NiFe thinfilms.

4 Conclusion

No significant etching related damages are observed in theNiFe thin films post treatments with wet etching and fluo-rine based RIE. The effective magnetization change of thepermalloy film is within 5 % post treatments. The small im-pact on the electrical and magnetic properties of the permal-loy films can be correlated with the surface oxide protect-

ing layer formation during the etch. Consequently, sputteredNiFe is a safe material to expose to several important etchingprocesses for write pole shield application.

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