high density perpendicular recording with wrap-around shielded writer

722 IEEE TRANSACTIONS ON MAGNETICS, VOL. 46, NO. 3, MARCH 2010

High Density Perpendicular Recording With Wrap-Around Shielded WriterDaniel Z. Bai, Yan Wu, Moris Dovek, Yue Liu, Xiaofeng Zhang, Kowang Liu, Kenichi Takano, and Yuchen Zhou

Headway Technologies, Inc., Milpitas, CA 95035 USA

Wrap-around shielded writers combining both trailing shield and side shield have been studied both by modeling and by experi-ments. WAS design is demonstrated to have superior performance over its predecessor trailing shielded writer in perpendicular mag-netic recording systems. While maintaining the down-track performance, the side shield significantly reduces the side fringing field, thusenabling high track densities. In this paper, various aspects of WAS design are investigated. Issues unique to side shielded design such asnear and far track erasure are also discussed in detail. An outlook of future PMR writer and recording systems design is presented atthe end.

Index Terms—Adjacent track erasure, far track erasure, perpendicular recording, side shield, wrap-around shield.

I. INTRODUCTION

W RAP-AROUND SHIELDED (WAS) writer combininga trailing shield with side shields (SS) has been pro-

posed and studied previously [1], [2] and is now utilized inproducts to continue the areal recording density growth of harddisk drives. As expected, WAS design provides significant trackdensity (TPI) capability gain over a non-WAS design thanks tothe much suppressed side field and thus less adjacent track era-sure/interference (ATE/ATI). On the other hand, the introduc-tion of SS also brings additional issues that have to be dealt withproperly. First, SS will cause extra flux shunting from the mainpole thus reducing the write field. At present recording densities,such field loss will usually cause field gradient degradation, thushurting on-track linear density (BPI) performance. Therefore,features such as tapered main pole [3], [4] are highly desired inorder to compensate the field loss and maintain the on-track per-formance while enjoying the higher TPI enabled by the SS. Inaddition, geometry and materials properties of the SS also needto be carefully optimized, in order to avoid either local satura-tion of the shields or excessive return field at the SS edges andcorners. The former will compromise the effectiveness of sideshielding, whereas the latter has similar effects to the return fieldinduced partial erasure (RFPE) from the trailing shield [5], yetit is much less tolerant than trailing shield RFPE as it occurs onthe side tracks that are subject to multiple writes as opposed tothe on-track RFPE that is substantially one-time effect.

Although the SS is meant to reduce side fringing field, ironi-cally, SS itself could be a source of erasure field. SS related era-sure may also occur both at near tracks and at very far tracks [6],[7]. The physical origins of these phenomena are also different,hence requiring different fixes. Near-track erasure due to SS isusually related to the local SS geometry and typically is of thenature of RFPE, unless SS saturation occurs due to too thin SSthroat height or too low SS . On the other hand, far-track era-sure in the presence of SS is usually caused by the domain wallactivities in the shields, which could result from any structural,

Manuscript received August 23, 2009; revised September 29, 2009; acceptedOctober 05, 2009. Current version published February 18, 2010. Correspondingauthor: D. Z. Bai (e-mail: [email protected]).

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/TMAG.2009.2034753

geometrical, and materials property mismatches, defects or im-perfections. In this case, the root cause of the far-track erasurecould be similar to the side track erasure for a non-SS writer[8], yet the SS added additional complexity to the head struc-ture and to the fabrication processes, which in turn could causeSS-unique far-track erasure problem.

In this paper, we will present the recent studies of WAS PMRwriter both by modeling and by experiments. In the modelingsection, various aspects of WAS design including geometricand materials optimization will be discussed, in the context ofon-track and off-track performance. In the experimental datawe will also compare the on-track and off-track performance ofvarious designs. The aforementioned SS-unique issues such asnear- and far-track erasure will also be discussed in detail. Anoutlook of future PMR design is provided at the end, with majorchallenges highlighted and several possible solutions proposed.

II. MODELING

A. Finite Element Model

As shown in [3] and [4], a trailing tapered main pole (MP)is essential for flux concentration and maintaining write fieldat small track widths. This is particularly important for WASdesign where SS causes additional flux shunting. Therefore, allheads discussed throughout this paper are with a trailing taperedMP. An air bearing surface (ABS) view of the head modeled isshown in Fig. 1. Unless otherwise noted, the major parametersused in the finite element modeling (FEM) are: MP width 50 nm,MP bevel angle (BA) 11 degree, SS bevel angle 11 degree(conformal to the main pole side wall), write gap (WG) 30 nm,side gap (SG) 100 nm, side shield 19 kG, side shield throatheight 0.5 um, main pole 23 kG, soft underlayer (SUL)16 kG, distance between ABS and the SUL 50 nm, SUL thick-ness 50 nm. A write current of 30 mA is used to energize a 44 turn coil. The field is caluclated in a plane 20 nm below ABS.

B. Modeling Results

In this section, we will compare the fields for various designparameters. The down-track field profile is plotted along the MPcenter. For the cross-track profile, each data point at a cross-track position is the magnitude of the maximum perpendicularfield from a line sweep along the down-track direction.

1) Side Gap: Fig. 2 shows the FEM result of the perpen-dicular field along down-track and cross-track directions fordifferent side gaps, compared to a reference case of trailing

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BAI et al.: HIGH DENSITY PERPENDICULAR RECORDING WITH WRAP-AROUND SHIELDED WRITER 723

Fig. 1. ABS drawing and parameter definitions of the head studied in the FEMmodel.

Fig. 2. FEM calculated perpendicular field along a) down-track and b) cross-track directions for different side gap sizes compared to a non-SS design.

shielded writer without SS. The SS shielding effect on the sidefield is obvious, confirming that narrow side gap is essential forachieving high TPI capability. On the other hand, the on-trackfield also diminishes rapidly as the side gap continues shrinking.This is an extra challenge added to the existing field loss due tothe normal scaling down of the write pole dimension. It is alsonoted from Fig. 2(a) that the return field at the trailing shieldis significantly reduced for SS case compared to non-SS case,therefore the trailing shield RFPE is less of an issue for WAS

Fig. 3. FEM calculated perpendicular field for different SS bevel angles.

design thanks to the SS that helps shunt MP fulx, although theSS edge/corner could become a hot spot instead, which will bediscussed later in detail.

2) SS Bevel Angle and Conformality to MP: Fig. 3 showsthe effect of the SS bevel angle. A bevel angle of 11 degreeis corresponding to a SS conformal to the MP, whereas 0 de-gree and 11 degree cases are non-conformal. The side gap sizeat the top is 100 nm for all cases. As the side gap opens upat the bottom, on-track field increases due to the reduced SSflux shunting. The penalty on increased side field also becomessignficant if the non-conformality is too large. For comparisonpurpose, an ATI field ratio is defined as the ratio of the max-imum off-track field magnitude at 80 nm off track position tothe maximum on-track field. This ratio is 22.6%, 26.1% and34.5%, for SS bevel angle of 11 degree, 0 degree, and 11 de-gree, respectively. The skew performance is also degraded fornon-conformal SS because the write bubble is not as well con-tained at the main pole bottom as for conformal SS case. This isevidenced in Fig. 4, where the normalized erase width is definedas the erase width at each skew angle divided by the erase widthat zero skew. The erase width is calculated from the contour lineof field at 5000 Oe. Higher normailzed erase width meanspoorer TPI capability at skew. Clearly, in terms of TPI and skewperformance, non-conformal SS is not as desirable as conformal


Fig. 4. Normalized erase width vs. skew angle for different SS bevel angledesigns. Insets are the 5000 Oe Hy contour lines, from inside out: �11 degree,0 degree, 11 degree.

SS. Although non-conformal SS has its own advantages, such asprocess simplicity, and the aforementioned shortcomings in TPIand skew performance might be overcome to a certain extent byother means such as a higher MP bevel angle, going forward,non-conformal SS design still is inferior due to its limitation inthe extendibility to higher TPI and narrow side gap designs.

3) SS Height: Depending on the processes used to fabricatethe WAS head, the height of the SS on the ABS may be different.In particular, some methods may have intrinsic limitations interms of where the SS bottom can be. For a given MP geometry,the SS height can be translated into a more relevant parameter,i.e., the position of the SS bottom relative to the MP bottom. Asshown in Fig. 5(a), the on-track maximum field does not changesignificantly unless the SS bottom is above the MP bottom byas much as 100 nm, meaning nearly half of the MP thicknessis not covered by the SS. However, the off-track field gradientdegradation is already appreciable when the SS bottom is 50 nmabove MP bottom (Fig. 5(b)). Once again, the non-shielded MPbottom will lead to larger field bubble at the leading side, effec-tively decreasing the magnetic bevel angle of the write bubble,thus degrading skew performance, as shown in Fig. 6. There-fore, it is important to have sufficient shielding near the mainpole bottom area.

4) SS Throat Height: It is well known that the trailing shieldthroat height is a critical parameter. In this section, we evaluatethe effect of SS throat height. As shown in Fig. 7, the on-trackand off-track field is not very sensitive to SS throat height whenit is 100 nm or more. Only when SS throat height is 50 nmdoes the field increase significantly both on- and off-track. Anexamination of the FEM result indicates that for SS throat height50 nm, the SS near the MP starts to become saturated, which inturn translates into the high on-track and off-track field.

5) SS : Fig. 8 shows the effect of SS . Similar to the SSthroat height effect, there is essentially no difference in on-trackand off-track field for SS in the range of 15 kG to 23 kG.The effect is only significant when SS is as low as 10 kG,in which case, similar to very thin SS throat height case, SSsaturation occurs near the MP, contributing to higher field on-and off-track.

Fig. 5. FEM calculated perpendicular field along a) down-track and b) cross-track directions for different SS bottom positions with respect to MP bottom.

Fig. 6. Normalized erase width vs. skew angle for different SS bottom positionwith respect to MP bottom. Insets are the 5000 Oe Hy field contour lines, frominside out: 100 nm below, 50 nm below, 0 nm, 50 nm above, 100 nm above.Both the field contour and the EW vs. skew curves are overlapped for the casesof 100 nm below and 50 nm below.

6) On-Track Field and SS Corner Field: Recent studies [6],[7] have shown that the inner edges of the SS that are facing the


Fig. 7. FEM calculated perpendicular field along a) down-track and b) cross-track directions for different SS throat heights.

MP can actually produce a return field that is significant enoughto cause erasure on the media upon multiple writes. The abso-lute magnitude of this return field is much lower than that of thereturn field at the trailing shield of a non-SS head. However, be-cause of the thermal effects associated with the multiple writeson the side tracks, this field could cause significant bit error rate(BER) degradation on the track that is at the cross-track loca-tion of the SS edge. Depending on the side gap thickness, theaffected track could be either the immediate adjacent track orthe second or even third adjacent track. In the latter cases, theerasure will manifests itself as a well isolated erasure peak eventhough the track closer to center track is intact. What is addedon top of this effect, as demonstrated in [6], is that the readerhard bias initialization field will cause an asymmetric additionalfield, which, at any MP writing polarity, always enhances the SSedge/corner return field on one side and weakens it on the other.However, typical FEM models, including the one used in thispaper, only have half of the head built and assume symmetryacross the center plane at the track center of the MP. As a result,FEM results do not include the aforementioned asymmetric ef-fect, and hence will underestimate the magnitude of the worstcase SS edge/corner erasure field.

Fig. 8. FEM calculated perpendicular field along a) down-track and b) cross-track directions for different SS Bs. SS throat height is 0.2 ��, and SS bottomis at same level as MP bottom.

Nevertheless, Fig. 9 summarizes FEM results of the effect ofthe SS parameters discussed above, in the context of maximumon-track field and minimum SS corner field (which is largest SScorner return field magnitude). For SS throat height and SS ,because of the SS saturation discussed above, at the low andlow throat height end, a change of SS corner field from negativeto positive is clearly visible. In the regions where no SS satura-tion occurs, the on-track maximum field and SS return fields arenot very sensitive to SS throat height and . On the other hand,for side gap, SS bevel angle, and SS bottom position, the trendfor on-track field and SS return field is the same, namely, anychanges that give rise to on-track field increase, such as widerside gap, non-conformal SS, or less shielding of SS bottom, willalso increase the magnitude of the SS return field, as shown inFig. 10.

It is well known that the Stoner-Wohlfarth fielddescribes the switching field and its anglular dependce ofsingle domain particles. Yet the actual PMR recording mediaswitching field angular dependence is in between Stoner-Wohl-farth field and pure perpendicular field. Nevertheless, allon-track and SS corner return field discussed above have been


Fig. 9. Maximum on-track field Hy and minimum SS corner field Hy versusdifferent parameters: a) SS throat height, b) SS Bs.

checked with , and the relative ranking between designsremains the same.

III. EXPERIMENT

A. Experiment Setup

The experimental measurements of the heads are carried outon a spin stand, as detailed in [6]. Dynamic fly height controlis employed on every head using a touch-down-and-back-offmethod to ensure substantially same head-media spacing for dif-ferent heads during the test.

B. Results and Discussions

1) Dynamic Performance vs. Design Parameters: The pri-mary benefit that the WAS head is designed for is the reducedside field thus better ATE BER performance. This has beenachieved, as shown in Fig. 11. At same erase width of 100 nm,under the same test condition of 250 kTPI, for the two non-SSheads BER degrades at a faster rate vs. the number of writes atthe adjacent track, compared to the WAS design. This advantagefor WAS heads is translated into higher TPI capability.

As shown earlier by modeling, the write field is sensitive tothe side gap. This is evident in the reverse overwrite (15T over2T) comparison in Fig. 12 for three different side gaps of 120nm, 90 nm, and 70 nm, respectively.

SS throat height or could be effective in suppressing thereturn field at SS edges and corners, when sufficiently thin and

Fig. 10. Maximum on-track field Hy and minimum SS corner field Hy versusdifferent parameters: a) side gap, b) SS bevel angle, c) SS bottom position withrespect to MP bottom, dH (positive means SS bottom is above MP bottom, neg-ative means SS bottom is below MP bottom).

low, as discussed earlier in modeling results. Spin stand levelDC noise power mapping has been demonstrated an effectivetechnique for characterizing the erasure field. The details of thistechnique have been discussed in [6]. Fig. 13 shows the resultfor two heads with high and low SS . For high SS, the twocurves corresponding to “DC+ on DC+” and “DC- on DC-” havetwo clear peaks on either side of the center track, signifying thenegative field at the SS edges. The reason that the peaks only arevisible on one side and switch side upon switching of writingand DC background polarity is the hard bias initialization in-duced asymmetry discussed earlier. For low case, the noise


Fig. 11. Residual BER vs. number of rewrites for different heads. All headsare WAS design with SS except the two heads denoted by the arrows which arenon-SS design.

Fig. 12. Reverse overwrite (ReOW) vs. erase width (EW) for different side gapdesigns: 120 nm (dots), 90 nm (crosses), 70 nm (squares).

peaks associated with negative fields indeed disappeared. Thetwo peaks for the case of “DC+ on DC-” are due to the writtentrack edge noise and are not dependent on SS , as expected.If one continues to push toward further lower SS BS, or overlythin SS throat height, eventually SS will be saturated near theMP, losing the shielding effect. In such case ATI will deterio-rate again, as shown in Fig. 14 for nominal and thin SS throatheights.

2) SS Corner/Edge Erasure: The SS corner/edge erasure hasbeen reported in [6]. Clear dependence of the erasure peak lo-cation relative to MP, left or right, has been demonstrated byDC write stress polarity, magnetic force microscopy (MFM),and DC noise measurements for hard bias initialization (HMI)and reverse hard bias initialization (RMI) cases. These resultsproved that the SS magnetization was pinned along the HMI di-rection at the SS edge/corner by the HMI field during the headfabrication process.

In order to characterize the strength of this pinning field, aseries of DC noise measurements have been performed on thesame heads. The head is first HMI initialized, followed by a DC

Fig. 13. Noise power vs. cross-track position under different DC backgroundand DC write conditions for two head designs with (a) high SS Bs and (b) LowSS � .

Fig. 14. ATI comparison for two heads (a) nominal design with clean ATI and(b) very thin SS throat height that has poor ATI. The legends represent writecurrent and overshoot levels, e.g., “20 0” means write current 20 mA, overshootat level 0.

noise mapping measurement. In each subsequent step, the headis subjected to a field that is along the RMI direction followedby the DC noise mapping test. The reverse field strength is pro-gressively increased at each step. At a certain reverse field level,the DC noise peak switches to the other side of the MP, indi-cating that the SS corner magnetization is switched, as shown


Fig. 15. DC noise mapping for three heads that went through differentHMI/RMI conditions before each mapping test. From top row to bottom row:HMI, 500 Oe RMI, 1000 Oe RMI, 1500 Oe RMI.

in Fig. 15 for three different heads, A, B, and C. The switchingfield is increasing from head A, to head B, to head C, fromseveral hundred Oe to over one thousand Oe. For head B, at500 Oe RMI, the noise peaks show up symmetrically at bothsides of MP, meaning the asymmetric SS corner magnetizationis removed, which suggests that the SS magnetization is in themiddle of reversing from HMI state to RMI state. Similar toFig. 13, the three sets of curves in each subplot of Fig. 15 corre-spond to DC- write on DC- DC+ write on DC+, and DC+ writeon DC-. Although only the DC- write on DC- case is labeled bythe arrows to show the switching of SS corner magnetization,the effect of the switching is obvious for each head on the DC+on DC+ peaks, which switches side at the same field where theDC- on DC- peaks switches side. This switching field is a mea-surement of how persistent the SS edge/corner magnetizationpinning is, which is dependent upon the SS design, materials,and process conditions such as field annealing strength. In gen-eral, lower SS corner pinning field is preferred. Among multiplefactors, reduction of the anisotropy field for the SS, which isusually along the cross-track direction induced during plating,would help, although the intrinsic shape anisotropy in SS couldbe fairly strong. SS geometry change may also help, althoughit may have consequences on other aspects such as the shielddomain configurations.

The SS corner magnetization pinning has also been observedby dynamic micromagnetic modeling in [7], where the SScorner field does not reverse until a certain time after the mainpole field reversal. Similar phenomenon also exists for thetrailing shield [9]. This phase lag is essentially a confirmationof the SS corner pinning field discussed above, which, duringthe dynamic reversal process, manifests itself as a temporaldelay of the SS corner field reversal. The reason that this delayis much more significant on SS corner field than on the trailingshield is because 1) the coupling between MP and trailing shield

is much stronger than that between MP and SS as trailing gapis much narrower than side gap, 2) the SS corner magnetizationpinning is much stronger than the trailing shield because of thereader hard bias initialization.

3) Far-Track Erasure: Far track erasure has previously beenreported on single pole heads with a return shield and on atrailing shielded write head [8]. In both cases, the erasure loca-tions can be up to several micrometers away from the MP, whichare associated with the shields. The introduction of SS addedadditional complexity to the head structure and the fabricationprocesses, which in turn increases the possibility of far trackerasure. The intrinsic geometries of the shields already producefairly complicated domain structures even without any defects.Any structural or magnetic defects will increase the possibili-ties of domain wall pinning or collision, which in turn createhot spots that could be far away from the MP, causing far trackerasure. In today’s hard disk drives, the so-called “ATI refresh”techniques have been widely used to deal with ATI. The essenceof the technique is to keep track of number of writes at each trackand rewrite the data of any track before certain number of adja-cent writes thus non-recoverable ATI BER degradation occurs.These techniques typically are most effective and efficient fornear track ATI events that are within several tracks. At present,the far track erasure events that are ten or more tracks away arestill considered beyond the working range of these techniques.Therefore, it is critical to eliminate the far track erasure.

Similar to the write current dependence of near track ATI inFig. 14(b), higher write stress typically worsens the far track era-sure situation. Although in practice it is possible to apply a capon the write current and overshoot amplitude in order to miti-gate the far track erasure, in the mean time it will also limit theoperating window within which the write current and overshootare to be optimized, which in turn may hurt the nominal BERperformance in drive. This, once again, requires fix of far trackerasure from head side.

IV. CHALLENGES OF FUTURE PMR

Several generations of products have been shipped since thefirst PMR drive was commercially introduced. There have beengreat efforts on head, media, channels, and drive integrationto keep the areal density growing at aggressive speed. Fromwriter’s standpoint, it also evolved from single pole, to trailingshielded, to WAS, along with other key enablers such as taperedwriter gap [3].

As the track width continues to shrink, it is becoming increas-ingly challenging to maintain the write field level as in currentand previous generations PMR products. Head-media spacingreduction played a crucial role in the past years thanks to newtechnologies such as the dynamic fly height (DFH) control [10]as well as the continuously shrinking thickness of the head andmedia overcoat and lubricant. Nevertheless, in PMR, not onlythe head-media spacing, but the head-SUL spacing also is crit-ical for the achievable write field for a given writer. Yet thescaling of head-SUL spacing is not fully at the speed at whichthe write pole lateral dimensions are shrinking. FEM showedthat as the write pole width is reduced from 60 nm to 40 nm,the perpendicular write field decreases by as much as 1200 Oe.On the other hand, as shown in Fig. 16, for the same head, adecrease of head-SUL spacing from 50 nm to 40 nm could re-cover as much as 900 Oe. Among the layers that are in between


Fig. 16. Maximum perpendicular field vs. ABS-SUL distance for a WAS headwith physical write pole width of 40 nm.

the head and the SUL, the interlayer between the SUL and therecording layer, which currently is non-magnetic, accounts forone third to half of the head-SUL spacing. On this front, ag-gressive interlayer thickness reduction, or alternatively makinginterlayer (or at least part of it) magnetic [11], could be key en-ablers for maintaining field level for future PMR.

Within the conventional PMR scheme, it is most likely thatthe approaches of boosting write field from head will result instronger fringing. More ATI tolerant environment in drive viamore powerful ATI refresh or similar techniques will be es-sential. Continued media improvements toward less write fielddemanding direction while maintaining all other performanceswill be also critical.

Other challenges include the dimension control during headmanufacturing both at wafer level and at back end lapping.On the one hand, more and more design features in the headare added in order to compensate the field loss at narrow trackwidth. Just to name a few, the tapered write gap, shorter neckheight, and shorter trailing shield throat height. Every one ofthese approaches will increase the sensitivity of magnetic writewidth to physical variations while the nominal dimension isshrinking, essentially double hitting the relative variations. Yetthe overall design and performance margin keeps shrinking,which poses even more demanding requirements on wafer andback end processes. In this regard, shingled writing [12] couldbe one of the key enablers that can break the fundamental wallof the ever tightened requirements on write head fabrication.Due to its corner writing nature, the write track width is nolonger a critical dimension, greatly relieving both fundamentalchallenges for the writer, i.e., field loss at narrow track width,and critical dimension control.

V. CONCLUSION

WAS design is a key enabler for the latest growth in PMRrecording areal density thanks to the TPI capability boost. On

the other hand, it does reduce the on-track field, which in turnrequires additional features to maintain the on-track overwriteand BER performance. Although WAS design is meant to sup-press side fields, it does introduce additional sources of erasure,both near-track and far-track. The near-track erasure is typicallyassociated with the SS edge/corner return field, or MP fringingin cases of too weak SS. The far-track erasure, on the other hand,is mostly related to hot spots due to domain wall pinning orcollision.

Future non-assisted PMR is becoming very challengingfrom the perspectives of writer design and fabrication. Fieldloss at narrow track is the top issue, among others such ascritical dimension control during fabrication. More aggressiveapproaches in drive and media development are essential forcontinuing conventional PMR areal density growth.

ACKNOWLEDGMENT

The authors are grateful for the tremendous help and supportfrom many of our colleagues at Headway, SAE and TDK.

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