- Email: [email protected]
write testing and scanning probe microscopy
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 303 (2006) e141–e144 www.elsevier.com/locate/jmmm
A study of FIB-patterned discrete track recording media by spinstand read/write testing and scanning probe microscopy Y.J. Chen, S.H. Leong, K.W. Ng, Z.B. Guo, J.Z. Shi, J.M. Zhao, B. Liu Data Storage Institute, 5 Engineering Drive 1, Singapore 117608 Available online 24 February 2006
Abstract We have developed a new technique of quick fabrication and testing of discrete track recording (DTR) media for DTR concept and insight. DTR media by focused ion beam (FIB) patterning have been studied by spinstand read/write testing and scanning probe microscopy. DTR media show smaller magnetic track width and better signal separation between adjacent tracks and therefore higher track density than that of continuous media due to reduced side fringe effect and edge noise. It was also found that at a designed groove depth of 4–8 nm, the shallow FIB etched grooves already provide good isolation between adjacent tracks, indicating the importance of ion beam induced modification of magnetic properties in film media over physical modification of disk surface topography. r 2006 Elsevier B.V. All rights reserved. PACS: 85.70.Kh; 68.37.Rt; 85.40.Hp Keywords: Discrete track recording; Focussed ion beam; Magnetic force microscopy; Magnetic media; Spinstand
1. Introduction
2. Experimental methods
Magnetic patterned media have long been proposed as one of the most promising approaches towards breaking the superparamagnetic limit of conventional continuous thin film media. As one of its simpler and easier versions, discrete track recording (DTR) media are gaining more and more attention [1] due to a multitude of technological advantages over continuous media for high track/linear density recording as well as the possibility of low-cost manufacturing using nanoimprint lithography. Various methods such as conventional photo-lithography [2] and ebeam lithography [3] have been employed to fabricate DTR media. In this study, we have used focused ion beam (FIB) lithography to pattern discrete tracks over a small area of a conventional continuous medium disk and investigated their dynamic recording performance by spinstand read/write testing. With the advantages of it being a clean, quick and flexible plus straightforward onestep process, FIB is also capable of modifying magnetic property as well as physically etching the magnetic film.
A conventional longitudinal thin film media disk (CoCrPtB magnetic layer 20 nm, carbon overcoat 4 nm, lube layer 1.5 nm) with the area density of 40 Gb/in2 and a magnetic head with a magnetic write width of 275 nm (corresponding to a track density of 73 ktpi with an estimated track pitch 1.25 magnetic write width) and a magnetic read width of 150 nm were selected for this study. Fig. 1 shows the discrete track pattern design. A FEI XP200 FIB system was used to do the patterning. Parallel grooves or trenches a few mm long and along the disk circumferential direction were sputter-etched out by focused Ga+ ion beam with ion energy of 30 keV and beam spot size of 10 nm on the media disk. The designed groove depth (Gd) ranged from 4 to 40 nm. Mark and index lines were also patterned by FIB on the disk for the ease of locating the patterned area. The read/write testing was performed by a Guzik spinstand (model 1701A). The readback signals from the magnetic head were also recorded and analyzed by a digital oscilloscope. AFM and MFM observations were carried out on a DI Dimension 3000 SPM system.
Corresponding author. Fax: +677 77 2406.
E-mail address: [email protected] (B. Liu). 0304-8853/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2006.01.121
ARTICLE IN PRESS Y.J. Chen et al. / Journal of Magnetism and Magnetic Materials 303 (2006) e141–e144
e142
N+6 N+5 Groove N+4 Track
N+3 R/W Head N+2 N+1 N N-1 Index
N-2
Fig. 1. Discrete track pattern design.
3. Results and discussion For easy location of the patterned tracks by the spinstand tester for R/W testing, a ‘T’ mark as shown in Fig. 2a was fabricated by FIB patterning on the disk surface. Fig. 2b and c show the readback signals along the tracks at the two typical positions across the mark as indicated by the dashed and dotted lines in Fig. 2a. By first locating the mark lines as shown in Fig. 2a, the small area patterned tracks in its vicinity can be easily found. Fig. 3 shows a typical AFM image (Fig. 3a) and a crosssectional line profile (Fig. 3b) across the patterned area (Gd ¼ 12 nm), a non-patterned area and an index groove as indicated by a dotted line in Fig. 3a. It can be seen that 9 tracks (lands) with a period of 250 nm are isolated by 10 grooves or trenches. The groove (land) width was measured by AFM to be 60–80 nm (170–190 nm), which was greater (smaller) than the designed width of 50 nm (200 nm) due to the finite beam spot size and possible patterning inaccuracy. The depth for the narrower grooves was measured to be 4 nm, which was much smaller than that for the wider index groove of 12 nm (the designed etching depth). This is due to the redeposition of the etched material onto the narrower grooves, which also leads to the bump observed in the narrower grooves. Fig. 4 shows a typical MFM image for a patterned area with nominal groove depth of 8 nm written with a linear density of 160 kfci. To write the signals on the 9 patterned tracks, the magnetic write head was first positioned at the track center of the first track that had been detected by means of the read-back signal on oscilloscope; the other 8 tracks were then recorded with a track pitch of 250 nm (corresponding to 100 ktpi), which is smaller than that for the continuous area. It can be seen that the recorded tracks are well separated on the patterned area. Fig. 5 shows on track readback signals over different patterned areas with groove depth of 4, 8 and 12 nm. For
Fig. 2. OSA image of ‘T’ mark (a) and readback signals (b and c) across typical positions across ‘T’ mark as indicated in (a).
Gd of 4–8 nm, the amplitude of the patterned area decreases only about 10–20%, while the amplitude for Gd ¼ 12 nm drops to about 1/3 of that of the nonpatterned area. The ion beam-induced modification of magnetic properties of the media film is non-linear. It was found that 4–8 nm etching depth is sufficient for track isolation. Fig. 6 shows the track profiles for a patterned (nominal groove depth of 4 and 12 nm) and corresponding unpatterned area. As shown in Fig. 6, the patterned area exhibits better recorded signal separation between adjacent tracks than that of unpatterned areas. 4. Summary A new technique of quick fabrication and testing of DTR media for DTR concept and insight has been developed. DTR media shows smaller magnetic track
ARTICLE IN PRESS Y.J. Chen et al. / Journal of Magnetism and Magnetic Materials 303 (2006) e141–e144
e143
Fig. 5. Readback signals for different patterned areas (upper: 4 nm; middle: 8 nm; lower: 12 nm).
MRW
Fig. 3. AFM image and line profile.
Groove
Land
Readback Signal (mV)
600 500
Non-patterned
400 300 200
Gd=4nm
100 0 -100 -15
Gd=12nm
-10
-5 0 5 10 Cross Track Position (microinch)
15
Fig. 6. Track profiles.
Fig. 4. MFM image.
width and better signal separation between adjacent tracks and therefore higher track density than that of continuous media due to reduced side fringe effect and edge noise.
With the head-media combination used, track density could be increased from 73 to 100 ktpi. It was also found that at a designed groove depth of 4–8 nm, the shallow FIB etched grooves already provide good isolation between adjacent tracks, indicating the importance of ion
ARTICLE IN PRESS e144
Y.J. Chen et al. / Journal of Magnetism and Magnetic Materials 303 (2006) e141–e144
beam-induced modification of magnetic properties in film media over physical modification of disk surface topography. References [1] Komag press release, http://www.komag.com, February 18, 2004.
[2] T. Watanabe, K. Takeda, Okada, H. Takino, IEEE Trans. Magn. 29 (1993) 4030. [3] Y. Soeno, M. Moriya, K. Ito, K. Hattori, A. Kaizu, T. Aoyama, M. Matsuzaki, H. Sakai, IEEE Trans. Magn. 39 (2003) 1967.
Journal of Magnetism and Magnetic Materials 303 (2006) e141–e144 www.elsevier.com/locate/jmmm
A study of FIB-patterned discrete track recording media by spinstand read/write testing and scanning probe microscopy Y.J. Chen, S.H. Leong, K.W. Ng, Z.B. Guo, J.Z. Shi, J.M. Zhao, B. Liu Data Storage Institute, 5 Engineering Drive 1, Singapore 117608 Available online 24 February 2006
Abstract We have developed a new technique of quick fabrication and testing of discrete track recording (DTR) media for DTR concept and insight. DTR media by focused ion beam (FIB) patterning have been studied by spinstand read/write testing and scanning probe microscopy. DTR media show smaller magnetic track width and better signal separation between adjacent tracks and therefore higher track density than that of continuous media due to reduced side fringe effect and edge noise. It was also found that at a designed groove depth of 4–8 nm, the shallow FIB etched grooves already provide good isolation between adjacent tracks, indicating the importance of ion beam induced modification of magnetic properties in film media over physical modification of disk surface topography. r 2006 Elsevier B.V. All rights reserved. PACS: 85.70.Kh; 68.37.Rt; 85.40.Hp Keywords: Discrete track recording; Focussed ion beam; Magnetic force microscopy; Magnetic media; Spinstand
1. Introduction
2. Experimental methods
Magnetic patterned media have long been proposed as one of the most promising approaches towards breaking the superparamagnetic limit of conventional continuous thin film media. As one of its simpler and easier versions, discrete track recording (DTR) media are gaining more and more attention [1] due to a multitude of technological advantages over continuous media for high track/linear density recording as well as the possibility of low-cost manufacturing using nanoimprint lithography. Various methods such as conventional photo-lithography [2] and ebeam lithography [3] have been employed to fabricate DTR media. In this study, we have used focused ion beam (FIB) lithography to pattern discrete tracks over a small area of a conventional continuous medium disk and investigated their dynamic recording performance by spinstand read/write testing. With the advantages of it being a clean, quick and flexible plus straightforward onestep process, FIB is also capable of modifying magnetic property as well as physically etching the magnetic film.
A conventional longitudinal thin film media disk (CoCrPtB magnetic layer 20 nm, carbon overcoat 4 nm, lube layer 1.5 nm) with the area density of 40 Gb/in2 and a magnetic head with a magnetic write width of 275 nm (corresponding to a track density of 73 ktpi with an estimated track pitch 1.25 magnetic write width) and a magnetic read width of 150 nm were selected for this study. Fig. 1 shows the discrete track pattern design. A FEI XP200 FIB system was used to do the patterning. Parallel grooves or trenches a few mm long and along the disk circumferential direction were sputter-etched out by focused Ga+ ion beam with ion energy of 30 keV and beam spot size of 10 nm on the media disk. The designed groove depth (Gd) ranged from 4 to 40 nm. Mark and index lines were also patterned by FIB on the disk for the ease of locating the patterned area. The read/write testing was performed by a Guzik spinstand (model 1701A). The readback signals from the magnetic head were also recorded and analyzed by a digital oscilloscope. AFM and MFM observations were carried out on a DI Dimension 3000 SPM system.
Corresponding author. Fax: +677 77 2406.
E-mail address: [email protected] (B. Liu). 0304-8853/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2006.01.121
ARTICLE IN PRESS Y.J. Chen et al. / Journal of Magnetism and Magnetic Materials 303 (2006) e141–e144
e142
N+6 N+5 Groove N+4 Track
N+3 R/W Head N+2 N+1 N N-1 Index
N-2
Fig. 1. Discrete track pattern design.
3. Results and discussion For easy location of the patterned tracks by the spinstand tester for R/W testing, a ‘T’ mark as shown in Fig. 2a was fabricated by FIB patterning on the disk surface. Fig. 2b and c show the readback signals along the tracks at the two typical positions across the mark as indicated by the dashed and dotted lines in Fig. 2a. By first locating the mark lines as shown in Fig. 2a, the small area patterned tracks in its vicinity can be easily found. Fig. 3 shows a typical AFM image (Fig. 3a) and a crosssectional line profile (Fig. 3b) across the patterned area (Gd ¼ 12 nm), a non-patterned area and an index groove as indicated by a dotted line in Fig. 3a. It can be seen that 9 tracks (lands) with a period of 250 nm are isolated by 10 grooves or trenches. The groove (land) width was measured by AFM to be 60–80 nm (170–190 nm), which was greater (smaller) than the designed width of 50 nm (200 nm) due to the finite beam spot size and possible patterning inaccuracy. The depth for the narrower grooves was measured to be 4 nm, which was much smaller than that for the wider index groove of 12 nm (the designed etching depth). This is due to the redeposition of the etched material onto the narrower grooves, which also leads to the bump observed in the narrower grooves. Fig. 4 shows a typical MFM image for a patterned area with nominal groove depth of 8 nm written with a linear density of 160 kfci. To write the signals on the 9 patterned tracks, the magnetic write head was first positioned at the track center of the first track that had been detected by means of the read-back signal on oscilloscope; the other 8 tracks were then recorded with a track pitch of 250 nm (corresponding to 100 ktpi), which is smaller than that for the continuous area. It can be seen that the recorded tracks are well separated on the patterned area. Fig. 5 shows on track readback signals over different patterned areas with groove depth of 4, 8 and 12 nm. For
Fig. 2. OSA image of ‘T’ mark (a) and readback signals (b and c) across typical positions across ‘T’ mark as indicated in (a).
Gd of 4–8 nm, the amplitude of the patterned area decreases only about 10–20%, while the amplitude for Gd ¼ 12 nm drops to about 1/3 of that of the nonpatterned area. The ion beam-induced modification of magnetic properties of the media film is non-linear. It was found that 4–8 nm etching depth is sufficient for track isolation. Fig. 6 shows the track profiles for a patterned (nominal groove depth of 4 and 12 nm) and corresponding unpatterned area. As shown in Fig. 6, the patterned area exhibits better recorded signal separation between adjacent tracks than that of unpatterned areas. 4. Summary A new technique of quick fabrication and testing of DTR media for DTR concept and insight has been developed. DTR media shows smaller magnetic track
ARTICLE IN PRESS Y.J. Chen et al. / Journal of Magnetism and Magnetic Materials 303 (2006) e141–e144
e143
Fig. 5. Readback signals for different patterned areas (upper: 4 nm; middle: 8 nm; lower: 12 nm).
MRW
Fig. 3. AFM image and line profile.
Groove
Land
Readback Signal (mV)
600 500
Non-patterned
400 300 200
Gd=4nm
100 0 -100 -15
Gd=12nm
-10
-5 0 5 10 Cross Track Position (microinch)
15
Fig. 6. Track profiles.
Fig. 4. MFM image.
width and better signal separation between adjacent tracks and therefore higher track density than that of continuous media due to reduced side fringe effect and edge noise.
With the head-media combination used, track density could be increased from 73 to 100 ktpi. It was also found that at a designed groove depth of 4–8 nm, the shallow FIB etched grooves already provide good isolation between adjacent tracks, indicating the importance of ion
ARTICLE IN PRESS e144
Y.J. Chen et al. / Journal of Magnetism and Magnetic Materials 303 (2006) e141–e144
beam-induced modification of magnetic properties in film media over physical modification of disk surface topography. References [1] Komag press release, http://www.komag.com, February 18, 2004.
[2] T. Watanabe, K. Takeda, Okada, H. Takino, IEEE Trans. Magn. 29 (1993) 4030. [3] Y. Soeno, M. Moriya, K. Ito, K. Hattori, A. Kaizu, T. Aoyama, M. Matsuzaki, H. Sakai, IEEE Trans. Magn. 39 (2003) 1967.