Tunneling Magnetoresistive Heads Beyond 150 Gb/in - IEEE Xplore

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Nurul Amin, Paul Kolbo, Pu-Ling Lu, Phil Steiner, Yong Chang Feng, Nan-Hsiung ... S. Mao, J. Nowak, E. Linville, B. Karr, S. Chen, P. Anderson, M. Ostrowski,.

IEEE TRANSACTIONS ON MAGNETICS, VOL. 40, NO. 1, JANUARY 2004

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Tunneling Magnetoresistive Heads Beyond 150 Gb/in2 Sining Mao, Eric Linville, Janusz Nowak, Zhenyong Zhang, Shawn Chen, Brian Karr, Paul Anderson, Mark Ostrowski, Tom Boonstra, Haeseok Cho, Olle Heinonen, Mark Kief, Song Xue, James Price, Alex Shukh, Nurul Amin, Paul Kolbo, Pu-Ling Lu, Phil Steiner, Yong Chang Feng, Nan-Hsiung Yeh, Bob Swanson, and Pat Ryan

Abstract—Tunneling magnetoresistive (TMR) readers capable of 150 Gb/in2 of areal density magnetic recording for hard disk drive were demonstrated with bit-error-rate performance. The head design used is basically a bottom type stack of Ta/PtMn/CoFe/ Ru/CoFe/oxide barrier/CoFe/NiFe/Ta cap with abutted hard bias to reach stabilization. The electrical reader width is about 4 a very high track density and shield-to-shield spacing is about 700 for high linear density. On-track bit error floor is better than 10 5 at a linear density of 900 KBPI and the recording system noise is dominated by the media. The best areal density achieved (using 4 OTC reference level) is 143 Gb/in2 using symmetric squeeze and 152 Gb/in2 using asymmetric squeeze method, respectively. It was found that the TMR head has several decibels more signal-to-noise ratio gain over spin valve readers at 150 Gb/in2 and beyond. The TMR head is also suitable for perpendicular recording application.

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Index Terms—Areal density, current-perpendicular-to-plane (CPP) geometry, perpendicular recording, signal-to-noise ratio (SNR), tunneling magnetoresistive (TMR) heads.

I. INTRODUCTION

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LTRAHIGH-DENSITY magnetic recording technology needs a high sensitivity and stable recording head for digital information processing. The technologies used during the past 40 years have evolved from thin film heads to anisotropic magnetoresistive (AMR) heads and giant magnetoresistive (GMR) heads. It has been a considerable effort to extend the GMR/spin valve head technology beyond 100G and the technical challenges of such scaling are significant [1], [2]. Alternatively, several potential current-perpendicular-to-plane (CPP) readback heads (CPP-SV, CPP-GMR, and CPP-TMR) are proposed for 100G and beyond [3]–[6]. Among them, the spin tunneling head or tunneling magnetoresistive (TMR) or tunneling giant magnetoresistive (TGMR) head has been demonstrated at spin-stand test by the current authors for the first time in the industry with areal density beyond 100 Gb/in [3], [7]–[9]. The structure of a TMR head is quite different from a conventional spin valve head. First of all, the nonmagnetic Cu spacer is replaced by an insulating tunnel barrier in the sensor Manuscript received July 1, 2003. S. Mao, J. Nowak, E. Linville, B. Karr, S. Chen, P. Anderson, M. Ostrowski, T. Boonstra, M. Kief, S. Xue, J. Price, H. Cho, O. Heinonen, A. Shukl, P. Kolbo, P. L. Lu, and P. Ryan, are with Seagate Technology, Recording Head Operations, Bloomington, MN 55435 USA (e-mail: [email protected]). Z. Zhang, P. Steiner, Y. C. Feng, N. Yeh, and B. Swanson are with Seagate Technology, Recording Subsystem Operation, Fremont, CA 12345 USA (e-mail: [email protected]). Digital Object Identifier 10.1109/TMAG.2003.821167

stack which requires quite different processing optimization. [10]–[16]. The tunnel barrier has a thickness much less than a typical Cu spacer layer in order to achieve the low RA product which is critical for a recording head application. Another major difference between these two head designs is that the sense current in a TMR head junction is applied perpendicular to the plane of the film (so-called CPP geometry). Although CPP head layout requires much improved junction fabrication to avoid any shorting around the sensor, the design is simpler than a current-in-plane (CIP) spin valve because the MR ratio is not directly related to lead resistance, head geometry, and film thickness [17]–[19]. It also has more advantages when scaling to higher areal density. The TMR head technology also utilizes spin-dependent conduction similar to a spin valve head, but the underlying physics is from different origins. There is no spin scattering involved in TMR; instead, conduction relies on an elastic tunneling effect between majority and minority spins. This gives rise to some unique head electrical characteristics like negative temperature coefficient of resistance and bias voltage dependence [20]–[22]. Furthermore, TMR heads can be easily applied to perpendicular recording due to the narrower shield-to-shield spacing and better resolution which are highly preferred for ultrahigh linear density [9], [23]. The objective of this work is to provide existence proof on this new technology for achieving recording performance greater than 100 Gb/in . To the best of our knowledge, this is the first recording demonstration of nonspin-valve head technology beyond 100 Gb/in . We will cover both the head designs and particularly focus on the recording testing with all the data recovery and error detection. Such a recording demonstration is critical for guiding the industry to develop next-generation transducer technologies. The volume manufacturing of this new technology presents more engineering challenges which must be resolved before a product is realized [3], [6]. As a new technology matures, the hurdles appearing in the early development phase may no longer be a critical issue. For example, a better deposition system can improve the uniformity and reproducibility of very thin oxide barriers [24] and an improved junction definition process can boost the yield significantly [25]. It is the purpose of this paper to provide more detailed technical information for further understanding this technology and comparing it with current CIP spin valve technology. It is crucial to identify any fatal flaws in order to move the technology to next development phase, which is out of the scope of this paper and will be presented in future publications.

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Fig. 1. ABS view of a 80G TMR head with permanent magnet longitudinal hard bias layer which is separated from the free layer by an oxide material.

II. HEAD DESIGN AND FABRICATION The tunneling MR heads discussed in this paper utilize improved design and testing methods based on our previous 10–20G prototypes of TMR heads [18], [19]. Changes were made to accommodate the top and bottom lead electrodes made out of the shields. This reduces the shield-to-shield spacing which is key for very high linear density. There is no observable magnetic interaction between the magnetic shields and the head. Fabrication processes have been advanced recently for low resistance-area product (R A) junctions [3], [11]–[16], [23], [24]. This low RA junction process enables the TMR head be more extendible for ultrahigh areal density head application but it is not the focus of this paper. While much effort is focused A product and many additional on further lowering the R questions remain on the shot noise and reliability of thin tunnel barriers, we explore the possibility of implementing a TMR head to the current spin valve head scheme beyond 100 Gb/in without adding system level complexity. It was proven again in this work that the TMR head may be used in place of the spin valve head to give better performance. The TMR head stack is made of Ta/PtMn/CoFe/Ru/CoFe/ oxide barrier/CoFe/NiFe/Ta and stabilized by a permanent magnet abutted to the TMR junction. The reader processing begins with ion milling of the TMR stack multilayer, namely the free layer, the barrier, and the pinned layer to form the bottom electrode. Then, a junction is defined by timed ion mill stopping in the pinning layer. Then, a hard bias layer of CoCrPt was deposited beside the tunneling junction which is laterally insulated by an insulation layer. Finally, we use a self-aligning process to lift off the insulating layer to isolate the top electrode from the bottom. Fig. 1 illustrates a 80G design TMR head ABS TEM image. Fig. 2 is a 130G TMR head design ABS view and Fig. 3 is the same head cross-sectional view. It is evident that the 130G design has better junction definition and PM layer alignment. Unlike the 80G TMR reader experimentation, which focused more heavily on building and testing the reader portion [26], the

Fig. 2. ABS view of a 130G TMR head with permanent magnet longitudinal hard bias layer which is separated from the free layer by an oxide. Note the reduced reader critical dimension and improved PM layout.

Fig. 3. Cross section of a merged TMR head for 130G demonstration [3]. The top reader shield is separated from the bottom writer pole by an insulation layer. A six-turn inductive writer is built on top of the TMR reader element.

current work has focused on the merged head which has a Seagate inductive writer built on top of the reader element (Figs. 3 and 4). An insulation layer is used to electrically separate the TMR reader and the inductive writer. Different reader and writer widths were used to optimize the recording performance, which will be discussed in a later section. It is worth mentioning that the merged head is fabricated in a production environment primarily using existing tools for spin valve head production. Modified and optimized stack deposition, junction formation, PM layout, and eventually lapping process enabled us to produce all the desired experimental parts. Recent advancements in nanotechnology allow sub-tenth-micron head fabrication processes improved significantly. The typical reader widths in this work

MAO et al.: TMR HEADS BEYOND 150 Gb/in

Fig. 4. ABS view of the merged TMR head with PM bias layer. This is the most standard type of head used in this paper.

varies from 80 to 150 nm and reader shield-to-shield spacing is between 600 to 700 . The TMR ratio is from 10% to 20% and m . The writer top pole width is RA product is from 2 to 5 from 150 to 200 nm and writer gap is from 75 to 100 nm. The reader stripe height and writer breakpoints are determined by optimizing process for the best electrical performances.

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Fig. 5. Correlation between TMR ratio and resistance area product RA for a typical TMR wafer. Linear extrapolation to origin indicates that the parallel nonspin-dependent conduction path.

III. WAFER CHARACTERIZATION A. Pinhole Physics and Breakdown Voltage It is well know that the ultralow RA tunneling stack materials have a parallel resistance channel which shunts the signal [3], [6], [10]–[16]. Reduction of RA is caused by thinning the barrier thickness via metal deposition and optimum oxidation. However, if the electronic transport is still controlled by the elastic spin-dependent tunneling process, the TMR ratio should be independent on the RA product down to two monolayers [10], [27]. The nearly linear TMR dependence on the RA product (Fig. 5) can be described well with a two-channel model where a spin-dependent tunneling path and a parallel spin-independent conduction path coexist. The spin-independent conduction path is typically formed by the barrier inhomogeneity, weak links between grain boundaries, diffusion regions along the ferromagnetic and insulating layers. All those probable diffusive conduction paths have been loosely defined as pinholes in the magnetic tunneling junction research [16], [17], [28]. The existence of such pinholes manifests some basic transport properties of the TMR junctions. One of them is the breakdown characteristic. The electric breakdown is a very important aspect of a TMR stack which impacts the reliability and ESD sensitivity dramatically [29]. Depending upon the barrier RA details, it was found that there are two distinctive behaviors when applied electrical stress is present (Fig. 6). For lower RA stack, the resistance drop starts at lower voltage threshold with a gradual increase. One the other hand, for higher RA TMR stack, the breakdown happens more abruptly at relatively higher voltage threshold. This difference can be attributed to pinhole nucleation for the high RA stack and pinhole growth in the lower RA stack [28].

Fig. 6. Breakdown testing for the wafer showing different types of characteristics. Abrupt and gradual drops of resistance.

B. Head Quasi-Static Transfer Curves Transfer curves were routinely tested on wafer and sliders and finished head for magnetic and electrical characterization as we did before [18]. A smooth transition between 300 and 300 Oe is seen and it corresponds to an effective anisotropy Oe and there is no offset field present, indicating of a good bias. Moreover, the high field scan up to 2000 Oe shows that the pinned layer remains fixed. It should be mentioned that the apparent TMR ratio in the finished head is not as high as the test features (or the intrinsic TMR response). This is primarily caused by the fact that in the test feature both the top and bottom electrode shield can be saturated by the applied field, but this is not the case in the finished head where flux efficiency is reduced due to the shielding effect.

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(a) Fig. 8. 747 curves of a merged TMR head. Read width is 3.76  , Writer width is 6.27  , media coercivity is 4600 Oe, and mrt is 0.307 memu/cc. Data rate is 296 Mb/s. Symmetric testing is used with and without squeeze.

(b) Fig. 7. Testing conditions: symmetric versus asymmetric squeeze methods.

IV. SPIN-STAND TESTS A. Symmetric and Asymmetric Squeeze Test Using a standard inductive writer, we write various tracks with different track widths and different bit density on media with different mrt of 0.3–0.36 memu/cm with coercivity of 4300–5600 Oe at a radius of about 0.9 . Both AFC and conventional longitudinal media were used for head evaluation. A spin-stand testing system consists of a TI preamplifier and Diamondback channel with EPRML architecture. Typical head (fly height around less media spacing (HMS) is about 0.4 0.25 ). Isolated pulses for a typical TGMR head show large signal of about 4.5 mV peak to peak. There is nearly no base line shifting in the head, indicating a good magnetic stability. Based on the optimized writing conditions for bit-error rate (BER), two different off-track capability tests were conducted. As shown in Fig. 7, we used symmetric squeeze testing for regular 747 curve test where the data track was encroached from both sides with another similar data track. To mimic the track misregistration effect in a drive, an empirical squeeze factor was considered that compromises the track density a little bit. Another way to test the TMR head reading process and capability is to use an asymmetric squeeze test [Fig. 7(b)]. One can write a series of tracks along one direction (i.e., from ID to OD or from OD to ID) with optimum write current and overshoot for the best on-track BER. This is to mimic the ideal case if a good bit pattern is written on the media how capable is the reader of reading the bits back. This is basically a reader areal density test [3], [7]. B. Areal Density Capability BER testing was used for areal density evaluation in combination with the various squeeze methods mentioned above. We

have seen many parts behave in a similar way on different type of media, so here we just give some examples for demonstration. In all the tests, we have used OTC reference level of 10 , which is widely accepted in the industry [2]. Fig. 8 shows a 747 curve of a TMR merged head tested at 876 KBPI with on-track BER of 10 and track density at 155 KTPI with 5% symmetric squeeze. An areal density of 135G is obtained. If one can ignore the track misregistration due to the mechanical variation, the upper bound of this head can reach 143G capacity. Fig. 9 is another test which used both symmetric and asymmetric squeeze methods. The two tests showed different track density capability due to the writeability and side writing effect [6]. If a symmetric squeeze method is used, the track density is about 158 KTPI (this is partly due to the wider writer on this part) and the areal density is about 141 Gb/in . If an asymmetric squeeze method is used, the track density improved to 171 KTPI and thus the areal density reaches 152 Gb/in . In other words, TMR head can read back bits of information beyond 150 Gb/in for longitudinal recording if a writer and media combination can record such high areal density information. C. SNR Analysis: SV Versus TMR SNR is a key figure of merit for a recording head since it is related to the BER. A complete SNR analysis was conducted as a function of linear density and a rolloff curve was generated [8]. It is clear that (Fig. 10) for 100 Gb/in and beyond the media noise is dominant and both head and media SNR decrease with increase of linear density. It is observed that the SNR rolloff rate for media contribution is slower than the one for head. This revealed that it is still critical to increase the head SNR even in a media dominated recording system since at very high linear density as well as high data rate the improvement of head SNR will benefit the system BER performance. This is a top requirement for a selection of next generation head. Based on the performance criteria, we found that the TMR head can surpass the spin valve head which is discussed next. Since the purpose of this work is to explore the technology feasibility beyond 100 Gb/in it is worthwhile to compare TMR head with existing spin valve GMR head. The spin valve heads used for this comparison are of seed/PtMn/CoFe/Ru/ CoFe/Cu/CoFe/NiFe/cap layer. The shield-to-shield spacing for spin valve head is typically around 800 , similar to the

MAO et al.: TMR HEADS BEYOND 150 Gb/in

Fig. 9. 747 curves of a merged TMR head with symmetric (two side squeeze) and asymmetric (one side squeeze) testing. The track density capability is different due to the track edge effect in two cases.

Fig. 10.

Total SNR breakdown to head and media portions up to 1000 KBPI.

previous design [2]. It has been observed among a handful number of parts that the TMR head has a little bit higher resistance than that of GMR but the amplitude can be two or three times more than what GMR gets. This results into a better SNR for TMR heads. It is very striking to observe that the TMR head SNR can be much better than spin valve GMR (2–3 dB for the same data rate and areal density) since many publications based on theory or simple testing predict that the TMR head should be performing much worse than the spin valve due to the shot noise, 1/f noise, and other high resistance related issues [30]–[35]. It is our current understanding that the contributions that improved the SNR for TMR are a combination of less noise than theoretical prediction and more amplitude from signal. Due to the existence of pinholes as we discussed above the shot noise is suppressed and only shows a fraction of the predicted value [36], [37]. The magnetic noise can be improved by better stabilization [37]. D. TMR Perpendicular Recording Head Finally, we applied the TMR head to perpendicular recording technology. Perpendicular recoding is considered for next-generation recording due to much improved thermal stability and writeability [38]. TMR technology is ideal for perpendicular recording due to the narrower shield-to-shield spacing and high resolution [3], [23]. A single pole writer is built on top of TMR

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Fig. 11. Perpendicular testing results of a merged TMR/single pole writer. The plot shows 747 curve with 5% adjacent track squeeze from both sides.

reader in a merged head for perpendicular recoding evaluation. The writer design is based on our previous publications [39], [40]. Fig. 11 shows the 747 curves with symmetric squeeze methods (0% squeeze used). The media used is a Seagate RMO granular recipe. The linear density is at 884 KBPI. If a 5% track misregistration is used the track pitch, track density, and areal density are 6.37 , 156.8 KTPI, and 138.6 Gb/in , respectively. If a 0% track misregistration is used the track pitch, track density, and areal density are 6.05 , 165.3 KTPI, and 146 Gb/in , respectively. Optimized TMR perpendicular head can reach 170 Gb/in (194 KTPI and 876 KBPI at OTC of 4) [41]. There are no fundamental issues between TMR and perpendicular recording. V. CONCLUSION We have designed, fabricated and demonstrated next-generation TMR heads for 150 Gb/in recording and beyond. It can provide both high amplitude and SNR for a recording system. This technology is applicable for both longitudinal and perpendicular recording. ACKNOWLEDGMENT The authors would like to thank many dedicated and talented colleagues at Seagate across different sites and locations who have been directly and indirectly involved in making this technology a reality. Special thanks go to Z. Murdock at RHO for inspiring discussions on TMR head technology. The authors also thank S. Harkness, S. Wu, R. Ranjan, E. Girt, M. Munteanu, and S. Hwang of Seagate media team at Fremont for excellent perpendicular and longitudinal media samples. REFERENCES [1] S. Mao et al., “Materials properties of spin valve stacks for 100 Gbit/in head applications,” presented at the MRS 2001 Spring Meeting, San Francisco, CA, Apr. 16–20, Paper T1.1. [2] Z. Zhang et al., “Magnetic recording demonstration over 100 Gbit/in ,” IEEE Tran. Magn., vol. 38, p. 1861, 2002. [3] S. Mao et al., “Comparison of GMR and TMR heads above 100 Gbit/in ,” in Abstracts MMM 2002, Tampa, FL, Nov. 11–15, 2002, Paper DC-01.

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[4] K. Nagasaka et al., “CPP head sensor using spin valve film with partially oxidized magnetic layer,” in Abstracts MMM 2002, Tampa, FL, Nov. 11–15, 2002, Paper CA-04. [5] M. A. Seigler et al., “Characterization of magnetic field sensor and magnetic recording heads that utilize a CPP CoFe/Cu multilayer,” in Abstracts MMM 2002, Tampa, FL, Nov. 11–15, 2002, Paper CA-05. [6] S. Araki et al., “Perspectives on CPP tunnel heads for magnetic recording,” in Abstracts MMM 2002, Tampa, FL, Nov. 11–15, 2002, Paper CA-02. [7] B. E. Swanson et al., “System implications of TMR and GMR heads in high density longitudinal recording,” in Abstracts MMM 2002, Tampa, FL, Nov. 11–15, 2002, Paper DC-03. [8] N. Yeh et al., “Advancing areal density beyond 100 Gbit/in ,” in Dig. TMRC 2002, Santa Clara, CA, Aug. 26–28, 2002, Paper B2. [9] S. Mao, “TMR heads for next generation,” in Diskcon Asia-Pacific, Singapore, Mar. 20, 2003. [10] S. S. P. Parkin et al., “Low-field magnetoresistance in magnetic tunnel junctions prepared by contact masks and lithography,” J. Appl. Phys., vol. 81, p. 5521, 1997. [11] R. C. Sousa et al., “Large tunneling magnetoresistance enhancement by thermal anneal,” Appl. Phys. Lett., vol. 73, pp. 3288–3290, 1998. [12] H. Tsuge, T. Mitsuzuka, A. Kamijo, and K. Matusda, “Magnetic tunnel junctions with low resistance, high current density and good uniformity,” in MRS Spring Meeting Proc., vol. 87, 1998, p. 517. [13] J. J. Sun et al., “Magnetic tunnel junctions on magnetic shield smoothed by gas cluster ion beam,” J. Appl. Phys., vol. 89, p. 6653, 2001. [14] J. Fujikata et al., “Low resistance magnetic tunnel junctions and their interface structures,” J. Appl. Phys., vol. 89, p. 7558, 2001. [15] J. R. Childress et al., “MgAlO barriers for low-resistance tunnel valve sensors,” in Dig. Intermag 2003, Boston, MA, March 28–April 1 2003, Paper GD-09. [16] J. Nowak, Invited talk, American Physical Society, Seattle, WA, Mar. 2001. [17] S. Araki et al., “Fabrication and electric properties of lapped type of TMR heads for 50 Gb/in and beyond,” IEEE Trans. Magn., vol. 38, pp. 72–77, 2002. [18] S. Mao et al., “Spin tunneling head above 20 Gb/in and beyond,” IEEE Trans. Magn., vol. 38, pp. 78–83, 2002. [19] D. Song et al., “Demonstrating a tunneling magneto-resistive read head,” IEEE Trans. Magn., vol. 36, pp. 2545–2548, 2000. [20] J. S. Moodera, L. R. Kinder, T. M. Wong, and R. Meservey, “Large magnetoresistance at room temperature in ferromagnetic thin film tunnel junctions,” Phys. Rev. Lett., vol. 74, pp. 3273–3276, 1995. [21] J. Hayakawa et al., “Spin dependent transport phenomenon in the NiFe/AlO/Cu/AiO/NiFe double tunnel junctions,” presented at the Intermag Conf., Boston, MA, Mar. 28–Apr. 1 2003, Paper GD-07.



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