Applied Mechanics and Materials Online: 2013-01-25 ISSN: 1662-7482, Vols. 284-287, pp 468-472 doi:10.4028/www.scientific.net/amm.284-287.468 2013 Trans Tech Publications, Switzerland Experimental Study of Dual Heater Slider Touchdown Detectability Dapeng Zhao 1, a, Xiongfei ei 1, b, Yu ang 2, c, Bin Liu 1, d, Shengxiang Chen 1, e and Zhaoqing Xu 1, f 1 SAE Technologies Development (Dongguan) Co. Ltd. innerway Industrial Area, DongGuan City, Guangdong Province, China 2 Center for Prognostics and System Health Management, City University of Hong Kong, Hong Kong a dp_zhao@sae.com.hk, b xf_wei@sae.com.hk, c ywang95@student.cityu.edu.hk, d lbbin_liu@sae.com.hk, e shengxiang_chen@sae.com.hk, f zhaoqing_xu@sae.com.hk Keywords: Dual heater, Detectability, Touchdown, Slider Abstract. ith the increase of areal density in hard disk drive (HDD), the mechanical spacing between magnetic head and disk has decreased to sub~1nm. Under such low spacing, accurate spacing control becomes more important for mitigating the risk of head-disk interference and getting good bit per inch (BPI) performance. Dual-heater slider technology has been considered as a most useful solution. This paper introduces the advantages of dual heater in reading/writing mode. To utilize these advantages, precise spacing control with good touchdown detectability is necessary. Therefore, the touchdown detectability of dual-heater slider was investigated using three TD detection methods under writing and reading mode. esult shows that a single method is difficult to get the best TD detectability under both writing and reading mode. Hybrid method may be a feasible solution. Introduction To achieve the TB/in 2 level of areal recording density of hard disk drive (HDD), the mechanical spacing between magnetic head and disk has decreased to sub~1nm. Under such ultra-low spacing, the spacing control needs higher accuracy for mitigating the head-disk-interface (HDI) risk and getting better BPI performance. Currently the dynamic flying-height control (DFH) architecture has been widely used in HDD manufacturers, utilizing a thermal-protrusion heater embedded in slider. ith applying electric power on the heater, it will thermally protrude, which makes the connected read/write element protrude towards the disk. To get relative spacing between head and disk, it needs a touchdown (TD) process to find out zero-spacing point where head touches disk [1 3]. For single heater technology, it always uses writer elements as head-disk contact point to determine spacing in both writing and reading mode. Therefore, it is difficult to control the spacing between reader and disk precisely. The dual heater slider technology has been considered as one of the most useful solutions for accurate reader-disk spacing control. Compared to single heater slider, dual heater design has two heaters instead of single heater. One heater (reader heater) is close to reader and the other (writer heater) is close to writer. This design aims at providing accurate reading and writing spacing control, respectively. Under writing mode, only writer-disk spacing needs to be calibrated and only writer heater works, while, under reading mode, only reader heater works. TD detectability is extremely critical for precise spacing control with application of dual heater. In this paper, dual heater advantage for reader/writer performance and TD detectability are discussed. In TD detectability study, three TD detection methods are investigated in writing and reading mode. Finally, feasible TD method is proposed for dual heater TD detection. All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications, www.ttp.net. (#69692969, Pennsylvania State University, University Park, USA-12/09/16,12:30:55)
Applied Mechanics and Materials Vols. 284-287 469 Dual heater advantage Dual heater and single heater structure comparison The structure difference between single heater and dual heater design are illustrated in Fig. 1 and Fig. 2. Single heater design has only one heater for writing and reading spacing control, while dual heater design has two heaters to control writing and reading spacing independently. Air Bearing Surface riter eader Heater Air Bearing Surface riter eader riter heater eader heater Fig. 1 Schematic of single heater Fig. 2 Schematic of dual heater Dual heater advantage in operation stage During normal operation state of HDD, head flies on a disk for the data writing and reading. The disk is made of aluminum or glass substrate, covered with three layers: magnetic layer for data storing, diamond-like carbon () layer for anti-corrosion and lubricant () layer for hard contact prevention. Under writing mode of dual heater, writer-disk spacing control requires solely applying electric power to writer heater. Thus, the writer becomes minimum flying point during operation. This mechanism is same with single heater design. The difference is that dual heater provides larger reader-disk clearance than that of single heater. This is very helpful to decrease reader-disk contact and improve reader reliability under writing mode. The detailed protrusion sketches for two designs are shown in Fig. 3 and Fig. 4. Single heater eader heater riter heater Disk Fig. 3 Sketch of single heater writing eader clearance difference Fig. 4 Sketch of dual heater writing During reading mode of dual heater design, only reader heater works. Thus, accurately reader-disk spacing control can be easily realized by adjusting the power applied on read heater. As a result, the reading performance can be improved. For single heater design, it is difficult to control the reader-disk spacing precisely because writer is always the minimum flying part no matter under reading mode or writing mode. The protrusion sketches of the reading modes for two designs are illustrated in Fig. 5 and Fig. 6.
470 Innovation for Applied Science and Technology eader is minimum fly point Fig. 5 Sketch of single heater reading Fig. 6 Sketch of dual heater reading Table 1 is BPI comparison between dual heater and single heater design at similar commercial platform. The comparison result shows dual heater design has about 11% BPI improvement compared to single heater. Table 1 BPI comparison between dual heater and single heater Dual heater Single heater Improvement Kilo BPI ~2000 ~1800 ~11% Based on above introduction, dual heater design shows better BPI performance and reader reliability compared to single heater design under both reading and writing mode. In order to make use of the advantages of dual heater, good TD detectability of reading and writing mode is extremely critical. Therefore, TD detectability of dual heater is investigated in the next section. Experimental setup In order to investigate the dual heater TD detectability, a drive-based-tester (DBT) is developed. The schematic of this platform is shown in Fig. 7. A Head-Gimbal-Assembly (HGA) is flying on the media under 7200 r/min. Under different operation modes, a stair-like DFH power pulse (Fig. 8) is applied to the different heaters. Two laser beams from two laser Doppler vibrometers (LDVs) shined on the back side and lateral side of slider for vertical and lateral vibration measurement, respectively. At the same time, an on-board sensor thermal asperity (TA) sensor is also used as a TD detection instrument via monitoring head vibration. The output of the TA sensor reflects integrated result of vibration and cooling effect due to spacing fluctuation [4]. hen DFH power increases, all signals including lateral/vertical LDV, TA sensor signal, and DFH power will be captured and processed. LDV and TA sensor signals will demonstrate obvious change at a certain power level, which indicates the head-disk contact, namely touchdown (TD). To further confirm the contact, scanning electron microscopy (SEM) is also used for head wear inspection. DFH power [m] Fig. 7 Schematic of experimental setup Time [ms] Fig. 8 DFH power increase with time
Applied Mechanics and Materials Vols. 284-287 471 Experimental results and discussion In the following dual heater TD detectability study, three TD detection methods are investigated by using above testing system under reading and writing modes. A feasible TD detection solution is proposed. TD detectability study in writing mode Under writing mode, DFH power is applied on writer heater step by step, then the head protrudes to the disk and eventually make the writer touch the disk. The signals of TA sensor, vertical LDV and lateral LDV during TD process are shown in the Fig. 9. It shows that the vertical LDV and lateral LDV output have a significant increase when power is bigger than 80m. However, TA sensor s output is always weak and does not show obvious change trend with the increasing of power. Generally, TD appears when LDV signal reaches preset threshold, illustrated in Fig. 9. To further confirm whether TD really happened at the marked power, HGA flies on the spinning disk for 10 minutes wearing test under TD power. After wearing test, SEM is used to check whether the contact happens or not. The SEM image is shown in Fig. 10. The writer elements show some wear, which reveals that TD happened at the marked TD power. From above discussion, TA sensor method has not enough TD detection sensitivity and cannot accurately detect TD during writing mode. The reason is that TA sensor is far away from writer. Under writing mode, writer is the TD point, a far distance between writer and TA sensor results in a big spacing between TA sensor and disk. The bigger spacing between TA sensor and disk, the weaker output of TA sensor is. Vertical/Lateral LDV [nm] TD power TA sensor Output [V] Slight wear in writer TA sensor DFH Power [m] Fig. 9 LDV and TA sensor output during writing mode TD Fig. 10 SEM image for wearing test during writing mode TD detectability study in reading mode Under reading mode, DFH power is applied to reader heater step by step to make reader touch with disk. TA sensor signals, vertical LDV and lateral LDV during TD process are shown in the Fig. 11. As seen from this figure, when TA sensor output reaches the maximum value, vertical/lateral LDV changed slowly. Through the similar wearing testing at the marked power and SEM inspection (show in Fig. 12), reader element shows some wear, which confirms that TD happens. TA sensor method accurately detects TD during reading mode, but lateral and vertical LDV show TD delay. In contrast with the situation of writing mode, the TA sensor output has significant increase. It is because that the position of TA sensor is close to reader element and reader is TD contact point under reading mode. Small TA sensor-disk spacing results in sensitive output of TA sensor under TD. Thus, TA sensor method is helpful for TD detection under reading mode.
472 Innovation for Applied Science and Technology Based on above investigations of TD detectability, vertical/lateral LDV methods show more sensitive TD detectability than TA sensor method under writing mode TD, but TA sensor method shows more sensitive TD detectability under reading mode TD. The main reason for the difference of TA sensor TD detectability is caused by different TA sensor-disk spacing. Bigger TA sensor-disk spacing results in weaker TA sensor output and weaker TD detectability. Conversely, small TA sensor-disk spacing results in obvious TA sensor output and accurate TD detectability. Vertical/Lateral LDV [nm] TD power TA sensor Output [V] TA sensor Slight wear in reader DFH Power [m] Fig. 11 LDV and TA sensor output during reading mode TD Fig. 12 SEM image for wearing test during reading mode Table 2 summarizes the TD results under reading and writing mode. It shows that it is difficult to use one single method for dual heater TD detection. A hybrid method may be a better solution for dual heater TD detection, i.e. lateral method for writing mode TD, TA sensor method for reading mode TD. Table 2 Dual heater TD detectability result summary Vertical LDV Lateral LDV TAs riting mode TD OK OK Poor eading mode TD Delay Delay OK Conclusions As a next-generation technology, dual heater shows better reader reliability in writing mode and better BPI performance. This study also investigates dual heater TD detectability by three different TD detection methods under writing and reading mode. It reveals that a single method cannot have good detectability on both writing and reading mode. Hybrid method may be a promising way to realize good TD detectability. eferences [1] N. Li; L. Zheng; Y. Meng and D. B. Bogy; IEEE Trans. Magn. 10, 45 (2009). [2] N. Li; L. Zheng; D. B. Bogy and Y. Meng; Tribol. Trans. 2, 53 (2010). [3] Z. H. Lin; T. Lam; X. D. Che; A. Shteyn and. D. Huang; IEEE Trans. Magn.6, 43 (2007). [4]. Sundaram;. Yao;. Ku and D. Kuo; IEEE Trans. Magn. 5, 35 (1999).