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Vacuum 84 (2010) 1075–1079

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Vacuum

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On the geometry control of magnetic devices: Impact of photo-resist profile,shadowing effect, and material properties

Xilin Peng a,*,1, Zhongyan Wang a,2, Yongxiong Lu b, Brendan Lafferty b, Thomas McLaughlin b,Mark Ostrowski a

a RHO, Seagate Technology, 7801 Computer Avenue South, Bloomington, Minnesota 55435, USAb RHO, Seagate Technology, 1 Disc Drive, Springtown Ind. Estate, Derry BT48 0BF, Northern Ireland

a r t i c l e i n f o

Article history:Received 10 September 2009Received in revised form10 January 2010Accepted 10 January 2010

Keywords:Magnetic materialsIon beam etchingShadowing effectGeometry control

* Corresponding author.E-mail address: xilin_peng_us@yahoo.com (X. Pen

1 The author is currently working with First Solar I2 Present address: Western Digital, 1250 Reliance W

0042-207X/$ – see front matter � 2010 Elsevier Ltd.doi:10.1016/j.vacuum.2010.01.029

a b s t r a c t

In this paper, we described how to control the magnetic junction critical dimension (CD) and profiledefined by ion beam milling and its implication to device performance. The impact of standing waves onbreaking the photo-resist and on the resultant junction defects have been highlighted. The ideal deviceprofile and geometry are also discussed.

� 2010 Elsevier Ltd. All rights reserved.

1. Introduction

With the ever-increasing requirements to handle a huge amountof data in modern information/communication systems, there is anincreasing demand for higher hard disk drive capacity and betterperformance. The recent significant progress using a MgO tunnel-ling barrier and CoFeB for the magnetic layers has enabled morethan 200% room temperature tunnelling magneto-resistance (TMR)[1–3]. As the areal density approaches w1 TB/in2, the magneticwrite pole width must be decreased to enable the narrow bits to berecorded within the medium. Similarly, reader width needs to bereduced accordingly to enable the written bits information be readback with minimal side reading (which causes written informationdistortion). The geometry control is critical for a writer to minimizethe adjacent track interference (ATI) [4,5], while it is crucial that thereader achieves the desired permanent magnet biasing, readerstability and asymmetry.

To fabricate such magnetic writer and reader devices, a photo-resist (PR) with required width is first printed out and then trans-ferred into the underneath imaging layer, or, directly into themagnetic stack layers via ion beam etching (IBE) or other methods.

g).nc.ay, Fremont, CA 94539, USA

All rights reserved.

The critical dimension (CD) and geometry control of sucha magnetic device defined via ion beam milling is important forhead performance control. Due to the material re-deposition in theion milling process, pattern transfer fidelity will depend on howmuch material is re-deposited on the sidewall of the device. Toensure close to 1–1 pattern transfer fidelity, minimal re-depositionis required. Further, minimum photo-resist thickness is desired forreduced shadowing effect during the ion milling definition and thesubsequent permanent magnet (PM) placement to enable a betterjunction and a flatter PM profile. However, either short PR height orhigh PR height to PR width aspect ratio will cause loss of PRintegrity or mechanical stability/bending. These will result in thejunction defects, such as the formation of a cone-shaped device (oreven a punched through barrier layer), trenching at the bottom ofthe device, and an asymmetrical junction.

2. Experiments

The results reported here are collected using either single layersheet films or multiple layer film stacks. A photo-pattern is printedonto the above test films and is used as a mask to transfer thepattern into the underlying material layers, using conventional ionbeam etching (IBE). Chemically amplified positive photo-resist witha thickness of w1.0 mm is used for photo-patterning. Exposure isdone on a KrF ASML stepper with a numerical aperture (NA) w0.6.

Substrate surface

A

B

h2

h3

δ2

δ2’

h

a

b

1

δ1

δ1’

BARC

δ2

Substrat

h1δ1

δ2’

EtchBARC

h2

Fig. 1. Various possible PR profiles without (a) and with (b) BARC. Once the photo-resistthickness reaches a critical point at the narrowest location (A or B), it can break during thephoto development and/or ion milling process and results in the PR height loss. If breakingat location A, resist height will be h2, while breaking at B, resist height will be h3, both areshorter than targeted height of h1. Consequently, there is not enough photo-resist left overto protect the sensor and part of the sensor cap layer will be milled away. Note: actual TEMimages are shown below the sketch drawings. Atomic layer deposited (ALD) Al2O3 isapplied to cover the PR in figure (b) TEM image to increase the edge contrast.

Fig. 3. TEM images showing the asymmetrical device profiles due to the photo-resistmask bending: (a) tilted towards right hand side and (b) tilted towards the left handside. These two TEMs are cut from two locations separated by w10 mm.

X. Peng et al. / Vacuum 84 (2010) 1075–10791076

Various photo-pattern processes and IBE beam conditions havebeen studied to evaluate the effect of the photo-resist initial profile,aspect ratio, and beam angle on the final pattern transfer. Ion beametching is carried out on VEECO milling equipment with a typicalbeam energy of 250 eV and beam current of 600–700 mA. The

Fig. 2. (a) SEM images for normal straight photo-resist pattern, (b) and (c)

wafer stage is cooled by water to ensure a substrate temperature ofless than 70 �C during the ion beam etching operation.

3. Results and discussion

3.1. Photo-resist initial profile effect

Due to the fact that light reflected from film interfaces interferesconstructively and destructively, a standing wave is formed alongthe photo-resist height direction and longitudinal direction. Thiscauses both photo-resist roughness in the height direction and inthe length direction. The latter is commonly referred to as line edgeroughness (LER). As shown in Fig. 1(a), when a bottom dimensiond10 is printed, the narrowest dimension of the resist will be d1

(where d1< d10). As the bottom CD is reduced to d2

0, the narrowestlocations at A and B will have a width of d2 (where d2< d2

0). Once d2

reaches a critical point, the photo-resist will break at point A or B orboth and result in a photo-resist height reduction from previousvalues h1–h2 (breaking at point A) or h3 (breaking at point B), whereh3< h2< h1. Obviously standing waves will limit the smallest photodimension. To solve this issue, a bottom anti-reflective coating(BARC) layer can be applied and thus minimizes or completelyremoves the formation of such constructive and destructive inter-ference fringes [6–8]. Therefore, a nearly straight PR pattern can beprinted (see Fig. 1(b)). Either organic or inorganic BARC have beenused. An additional etching process to transfer the photo-resistpattern into BARC layer is generally required as shown in Fig. 1(b).

for a bent photo-resist pattern. Scale bar is 200 nm for all the images.

Fig. 4. TEM image of a defective (part of the sensor top layer was milled out) device caused by inadequate PR thickness. (a) Part of the capping layer is milled out; and (b) all of thecapping layer and most of free layer is milled out.

X. Peng et al. / Vacuum 84 (2010) 1075–1079 1077

Maintaining good pattern transfer fidelity from photo-resist toBARC is very important. A resist height reduction from h1 to h2 andwidth reduction from d2 to d2

0 as illustrated in Fig. 1(b) are usuallyexpected because the etching chemistry not only attacks BARC, butalso etches photo-resist both laterally and vertically. Although toomuch PR loss in height direction can be detrimental the lateral PRwidth reduction may be helpful for narrower device photolithog-raphy patterning.

a

c

w1

d1

x1

t1

Etch depth

Re-dep

t3

Etch depth

x3

Re-dep

Fig. 5. Initial mask width (d), height (t), and profile impact (wall angle, q) on the re-deposit(w) on top of photo-resist and the on sidewall (x). (a) Short A/S PR profile. On top of the phodeposition is uniform across the photo-resist height t1. (b) For smaller feature size (d2< d1),deposition layer thickness (x2) on top thicker (i.e. x2 > x1). (c) At large photo-resist height (t3

When the photo-resist profile start with an angle (q), the re-sputtering effect can significan

There is a trade-off between the narrowest PR CD that can bepatterned and its stability/integrity. Generally, for a larger aspect(height/width) ratio (A/S), a PR pattern is more susceptible towardsmechanical instability. Another important aspect to be consideredfor minimum photo CD is that, the PR must be able to survive themilling process without breaking or bending, i.e., to maintainmechanical stability during the whole milling process. To this extent,re-deposited materials around the photo-resist actually provide

b

d

d2

x2

Etch depth

Re-dep

θ

Bombardment

Etch depth

Re-dep

ed materials (assuming normal beam incidence), such as the re-deposition layer widthto-resist, the re-deposition width (w1) is larger than that on sidewall (x1); while the re-re-deposition layer from both sides of the photo-resist coalescences and make the re-> t1), the re-deposited layer on top of the photo-resist is thinner than at the bottom. (d)tly reduce the re-deposition amount.

-0.6

-0.4

-0.2

0

0.20.4

0.6

0.8

1

1.2

0 10 20 30 40 50

Mill angle (degrees)

No

rm

alized

R

ed

ep

th

ickn

ess 245nm resist

122nm resist

82nm resist

Fig. 6. Simulation results for normalized re-deposited layer thickness as a function ofprimary milling angle, to mill through a 30 nm thick material stack with 245 nm,122 nm, and 82 nm thick of photo-resist mask for a magnetic device.

Fig. 8. Device final photo to physical CD bias (¼final TEM physical dimension to initialphoto dimension) as a function of the initial PR feature width for three types of processconditions. All of them show a similar trend.

Primary beam angle

αa

X. Peng et al. / Vacuum 84 (2010) 1075–10791078

a protection to avoid further photo-resist loss, which will eventuallylead to its mechanical failure if not avoided. The PR failure modeincludes breaking, bending, and kinking (see Fig. 2(a)–(c)). Theinability to maintain photo-resist integrity and stability results ineither an asymmetrical device profile (see Fig. 3) or the attack, oreven loss, of part of the whole layer device (see Fig. 4). When the PRheight to the material stack thickness ratio is below a critical value(selectivity), there will be not enough PR left over to protect the top

Fig. 7. (a) Images show that re-deposited material across a wide feature is still sepa-rated (b) while it coalesces for a narrow feature. Scale bar¼ 200 nm. (c) No re-depo-sition (white colour) builds up on sloping PR (grey colour) sides due to beingre-sputtered away.

of the stack during the ion milling. As a result, a part of the TMRdevice will be milled away and a ‘‘cone’’ shaped profile as shown inFig. 4 results.

re-dep

L1

L2

δα1

α2

To be etched film

Photo resist

b

Primary beam

Shadowinglength, L

Fig. 9. Schematic drawings showing the impact of device profile defined by the ionbeam at various primary beam angles due to both shadowing effect and angle-dependent milling rate. Generally, a larger primary beam angle will result in a moresloped device profile due to longer shadowing length. (a) Initial PR profile and the ionbeam impingement direction. (b) Device profile in the middle of the milling process forvarious primary beam conditions.

Table 1Measured actual milling rate (Angstrom/sec) for Ta, NiFe, FeCo, IrMn, and Al2O3 fora fixed testing beam condition (250 eV beam energy, 700 mA beam current, andbeam divergence angle of w5 degrees)

Angle, deg Ta NiFe FeCo IrMn Al2O3

0 2.05 2.04 2.22 3.33 0.7120 2.11 2.33 2.56 3.77 0.9840 2.00 2.56 3.03 3.64 1.4760 1.24 1.60 2.00 2.41 1.5270 0.74 0.91 1.27 1.31 1.13

Fig. 10. Etch rate difference caused geometry variation for a reader device (a) reader width direction (ABS view), (b) strip height direction (perpendicular to ABS view), and (c) thewriter pole (1¼Main pole CoFex; 2¼ Al2O3 gap, 3¼NiFex side shield). The Al2O3 recess at ABS direction could be formed if the milling process is not optimized due to the materialand angle-dependent milling rate difference.

X. Peng et al. / Vacuum 84 (2010) 1075–1079 1079

3.2. Re-deposition dependence on the beam angle

The final device physical CD and profile largely depend on thecompetition process between milling and re-deposition rate. Itconstantly changes during the ion milling process. Initial PR profileshave tremendous impact on the final device CD and profiles [9], asillustrated schematically in Fig. 5. For a short A/S PR profile, re-deposition on the sidewall of the PR is more uniform and may coverthe corner regions on top of the PR as well at a nearly vertical ionmilling angle (see Fig. 5(a)). When the PR dimension gets smaller,re-deposition from one side can start to build up on the other sideand coalesce, causing a ‘‘mushroom’’ type of re-deposition profile(see Fig. 5(b)). Certainly, this will result in a larger photo to physicalCD bias. However, once the A/S is large enough as shown in Fig. 5(c),not all the re-deposited atoms can reach the top of the PR and there-deposition thickness on top of the PR is lower than at bottomfrom our simulation results (not shown here). A sloping PR initialprofile (Fig. 5(d)) has a similar effect as a large milling angle interms of re-deposition build up along the PR. As the re-sputteringrate is close to the re-deposition rate, there is nearly no re-depos-ited build up on a sloping PR.

Assuming a normal cosine distribution of the ejected atomsfrom stack materials during the ion milling, Fig. 6 shows thesimulated re-deposited material thickness on the sidewall ofa junction as a function of the primary milling angle. Abovea certain primary angle, it is obvious that such re-deposition willbecome zero due to the increased ion beam component from thelateral direction and this may be helpful to keep a 1–1 photo tophysical CD transfer. However, once the lateral material removalrate is larger than the re-deposition rate, PR will also be removed bythe lateral ion beam component. Above some critical point, suchlateral PR etching will trigger the bending, deformation, and failureof the PR as a mask for device definition. This will either result in anasymmetrical or ‘‘cone’’ shaped profile as discussed above.

Fig. 7(a) shows the SEM images similar to cases Fig. 5(a) and (c),while Fig. 7(b) shows the re-deposition coalescence for narrow PRwidth, similar to case Fig. 5(b); while Fig. 7(c) mimics the resultsfrom Fig. 5(d). Because of such re-deposition coalescence aroundthe narrow PR, the final photo to physical CD bias is dependent onthe initial PR width as highlighted in Fig. 8. Physical to PR CD bias ismore positive for smaller initial PR width for all three differentprocesses shown here in this article.

3.3. Shadowing effect and re-deposition

In addition to the PR profile, the ion milling angle also playssignificant role in terms of dimension and profile control onfinished devices [10].

At high primary milling angle, the shadowing effect has a hugeimpact on the final device profile. Fig. 9 schematically shows how theprimary milling angle and the PR height can affect the junction

profile evolution due to such shadowing effect. At vertical millingangle, the PR shadowing effect is close to zero (if we ignore the re-deposition effect) and a vertical device profile can be obtained. Oncethe ion beam is tilted to an angle of a, the shadowing length, L, will bedependent on both the angle and the PR height (see Fig. 9(a)) at theinitial milling stage. When the ion beam angle is changed to a1 anda2 (relative to normal), such shadowing effect results in a slopedjunction with a slope length of L1 and L2, respectively.

3.4. Material effect

The different milling rates (ref. to Table 1) for a material stackmake the device profile evaluation even more complex. For example,when multiple materials exist in a reader (and a writer) stack, a roughsidewall profile (both at the air-bearing-surface (ABS) and perpen-dicular to ABS) may be expected (see Fig. 10). Such roughness hassignificant impact on reader performance (such as the head-mediumspace (HMS) (see Fig. 10(a)–(b)). Similarly, the roughness caused bymaterial/angle-dependent rate during writer process, such as therecess of Al2O3 gap (see Fig. 10(c))) has strong impact on writerperformance and needs to be minimized. Varying the mill angleduring the reader width definitionprocess or pre-DLC deposition ABScleaning step to enable the close etching rate for multiple materials inthe reader and writer film stack or by optimization of materialselection can partially avoid this unwanted geometric formation.

4. Conclusions

We have reviewed the principles on how to control devicegeometry. This includes the proper consideration of photo-resistprofile, aspect ratio, ion beam configuration and the materialselection. A careful optimization or trade-off may be required toachieve certain geometries.

Acknowledgement

The authors are grateful to the technical support from Tien Damfor photo-patterning assistance.

References

[1] Parkin SSP, Kaiser C, Panchula A, Rice PM, Hughes B, Samant M, et al. NatMater 2004;3:862.

[2] Yuasa S, Nagahama T, Fukushima A, Suzuki Y, Ando K. Nat Mater 2004;3:868.[3] Yuasa S, Djayaprawira DD. J Phys D Appl Phys 2007;40:R337.[4] Okada T, Kimura H, Nunokawa I, Yoshida N, Etoh K, Fuyama M. IEEE Trans

Magn 2004;40:2329.[5] Larson BM, Lambert SE, Hai N, Nguyen L, Wallash A, Huang T, et al. IEEE Trans

Magn 2001;37:1223.[6] Ogawa T, Gocho T, Nakano H, Tsukamoto M. Jpn J Appl Phys 1997;37:3775.[7] Gocho T, Ogawa T, Muroyama M, Sato JI. Jpn J Appl Phys 1994;33:486.[8] Peng X, Wang Z, Dimtrov D, Boonstra T, Xue S. J Vac Sci Technol 2007;25:1078.[9] Lee RE. J Vac Sci Technol 1979;16:164.

[10] Gokan H, Esho S. J Vac Sci Technol 1981;19:28.