Investigation of Near Field Transducer for HAMR Application

Transcription

1 ADVANCED STORAGE TECHNOLOGY CONSORTIUM RESEARCH PROPOSAL Investigation of Near Field Transducer for HAMR Application Submitted to Topic HAMR No. 5: Near Field Transducer (NFT) for HAMR Application Submitted by Xianfan Xu Professor of Mechanical Engineering Purdue University Phone: (765) April 19,

2 Abstract We propose to investigate near-field transducers (NFT) to produce a thermal spot less than 50 nm in the recording media for heat-assisted magnetic recording (HAMR). This work will be a continuation of the previous projects funded by INSIC and IDEMA. However, the new study will be focused on the following efforts: (1) Manufacturability of the NFT, i.e., we will investigate NFTs which will be possible to be fabricated using top down planar manufacturing processes. (2) We will optimize the efficiency of the NFT, and determine and improve its thermal reliability. (3) We will study the placement of recording pole and its effect on the NFT performance. (4) We will investigate and minimize the thermal protrusion of the NFTs relative to the surrounding material. (5) We will investigate and minimize the sensitivity of the NFT performance to process variations. (6) We will investigate and minimize the sensitivity of the NFT performance to variations of head-media spacing (NMS). The NFT will use materials appropriate for use in recording heads, and will be in the presence of recording media. 2

3 I. Detailed Descriptions of the Proposal The focusing capability of near-field transducer (NFT) is one of the key factors that will determine HAMR technology extendibility. To support the development of the HAMR technology, we will perform combined electromagnetic (EM) and thermal studies of NFT to achieve a sub-50 nm thermal spot in the recording media with high efficiency, reliability, and manufacturability. The NFT will use materials and light sources practical for HDD. The proposed work will be a continuation of the previous projects funded by INSIC and IDEMA. However, this proposal will be focused on the following efforts: (1) Manufacturability of the NFT, i.e., we will study NFTs which will be possible to be fabricated using top down planar manufacturing processes. (2) We will optimize the efficiency of the NFT, and determine and improve its thermal reliability. (3) We will study the placement of recording pole and its effect on the NFT performance. (4) We will investigate and minimize the thermal protrusion of the NFTs relative to the surrounding material. (5) We will investigate and minimize the sensitivity of the NFT performance to process variations. (6) We will investigate and minimize the sensitivity of the NFT performance to variations of head-media spacing (NMS). I.1NFT for manufacturability We will first discuss what have been learned from previous works, and then discuss the new study on NFTs. I.1.a Previous studies of a bowtie or half bowtie aperture as an NFT Our earlier studies funded by INSIC and IDEMA was to investigate the performance of a type of NFT called bowtie aperture antenna. The 4 nm results of this study are briefly discussed here. Metal film 4 nm E These results provide valuble insights and guidance to the study of new NFTs proposed in this work. Our group has extensive experience in the development of bowtie aperture antenna for focusing light far below the diffraction limit with high efficiency. 1-8 Such bowtie antennas have been proposed as NFT for HAMR As shown in Fig. 1, the bowtie aperture is made in a thin metal layer coated on a transparent substrate. The two tips of the antenna are separated by a nanometer size gap d. When illuminated by a polarized laser beam, (a) d E t 2 max = 212 ()A (b) Ag Fig. 1: (a) Illustration of bowtie aperture antenna. (b) Simulation results of a bowtie aperture in silver film at a distance 5 nm below the aperture. The spot size is 12 nm x 16 nm, and its intensity is more than 200 times higher than the incident intensity. 3

4 electric potential and currents are induced in the tips, resulting in a displacement current across the gap that radiates like a Hertzian dipole. The size of the light spot produced by the bowtie is therefore determined by the gap size d. The efficiency of the bowtie antenna is very high (orders of magnitude higher than that of the incoming radiation), which is necessary for providing sufficient heating for HARM without significant laser power or thermal budget. (a) (c) t v 1 m z w e (b) a g r 1 y y b g x u Figure 1b shows our simulation results of the E-field distribution at 5 nm below a bowtie aperture. 2 With proper design, including utilizing plasmonic resonance and localizing surface plasmon, an optical spot as small as 12 nm x 16 nm was obtained, with an intensity more than 200 times higher than the incident laser intensity. Our group has developed many numerical tools (FDTD and FEFD), 1-8 and has been using bowtie antennas for imaging, waveguide coupling, and nano-manufacturing, in which the bowtie antenna is used to produce a nanometer size heat source just as in HAMR. w n Fig. 2: (a) Illustration of bowtie aperture antenna surrounding by grating structure. (b) Top view (c) Cross section. w 0 g p One of the techniques to improve optical efficiency of NFT is to include a grating structure surrounding the bowtie aperture. Figure 2 shows such an arrangement. In Fig. 3, the-near field enhancement of a factor of 10 was measured experimentally. This strategy can be used for many other types of NFT, and will be further discussed in Sec. I.1.b. Fig. 3: field enhancement as a result of surrounding of bowtie aperture with grating. The near field is increased by a factor of 10, the far field is by a factor of 6. Half bowtie aperture as shown in Fig. 4 performs similarly compared with a full bowtie aperture. (The gray area is the area where the metal film is removed.) This has been proven in our earlier studies. Compared with a full bowtie aperture, similar near-field spot size, efficiency, and the effect of including grating structures for improving efficiency have been shown. The optical efficiency produced by a half bowtie aperture with grating can reach 13% (irradiated by a laser spot of 1 m in diameter.) Fig. 4: Illustration of a half bowtie aperture Temperature field in the media stack can be computed with the knowledge of optical and thermal properties by solving the three dimension heat diffusion equation. In Fig. 5, we illustrate the media stack, the thicknesses of each layer, and the optical properties used in our calculations, as well as the calculated transient temperature field in the media. Because of heat diffusion, the heated spot size is increased to about 30 nm FWHM compared to a 15 nm FWHM optical spot size. Lowering thermal conductivity in the in-plane direction can reduce the thermal spot size. 4

5 Fig. 5: The media stack computed and its properties. Right: the transient temperature along the x-direction (the laser scanning direction) and the y-direction. In summary, the following are what we found for full or half bowtie aperture: 1. The smallest possible FWHM optical spot is about 15 nm, assuming a gap between the two tips of bowtie of a few nm. 2. Heat diffusion in the media will increase the heated spot size to about 30 nm FWHM. 3. Half bowtie and full bowtie are both suitable for generating a small spot. 4. A heated spot with a bit-aspect-ratio (BAR) of 2-3:1 (or any ratio) can be generated by changing the geometry of the NFT, by increasing the spot dimension in one direction. 5. With the use of grating structure, the efficiency can be increased, with a total efficiency of about 13% assuming an incident beam of 1 m in diameter. I.1.b Proposed NFT studies The new NFT will be first focused on manufacturability. Based on our previous studies, we will investigate new NFTs using a half bowtie aperture as a basic structure. Another reason for choosing the half bowtie aperture is its ability to spread the heat more efficiently compared with some other structures. There are other NFTs reported in literatures such as a lollipop [17] and a C-shape resonator with a notch [18]. Other variations, including inverted structures (an antenna instead of an antenna aperture), will also be studied with the consideration of the overall (a) (b) (c) Fig. 6: Illustration of a half bowtie aperture design which can be fabricated using top town techniques. Gray: ground plane, Green, filling material, i.e., oxide, Red: magnetic pole, Green: gold, Purple: media stack. 5

6 performance metrics. The NFT based on half bowtie aperture is illustrated in Fig. 6. Such a structure can possibly be fabricated with the use of top-down techniques such as lithography, etching, etc. It preserves the key features of a feasible HAMR head, including light delivery, magnetic pole piece, etc. [19], together with the key features of a half bowtie aperture and the grating to achieve sufficiently small optical spot and high efficiency. Figure 6 shows three different views. Figure 6a shows a dielectric waveguide that guides the incoming light toward the NFT. Figure 6b shows the view seen from the media stack. The green material is the filling material in the bowtie aperture, which can be silicon dioxide or other oxide material. The three gray bars are three grading structure for field enhancement as those used in Fig. 2. Instead of circular grading, straight gratings are used for easy of fabrication. (These gratings are not for coupling external diode laser, but for increasing efficiency of NFT.) The red is the magnetic pole piece. Figure 6c shows the view cutting across the head and the media (purple). We will use gold as the NFT metal material. Information on other materials such as magnetic pole piece, oxide, etc. will be obtained from the member companies. We will also investigate variations of the structure shown in Fig. 6. One possibility is the inverted structure from an aperture, with the rest of the structure similar to what is shown in Fig. 6. We will compare the efficiency of the two structures, as well as other aspects of the metrics as described below. I.2 Optimization of the efficiency and thermal reliability of NFT The NFT shown in Fig. 6 will be optimized for its optical efficiency, such as the geometry of the half bowtie, grating placement, etc. The optimization procedures have been developed and used in our earlier works. The temperature increase in the NFT is likely to be large, and will also be computed. One of the performance metrics is to minimize the temperature increase in NFT (also see I.4). I.3 The effect of the recording pole and on the NFT performance The NFT will consider the effect of the magnetic recording pole as shown in Fig. 6. It is desirable that there is no gap between the magnetic pole and the near-field hot spot so the magnetic field and the temperature field can overlap. On the other hand, the magnetic pole will compromise the performance of NFT due to its undesirable optical properties, and may affect the field localization and enhancement of the NFT. Therefore, for a given distance between the NFT and the magnetic pole as shown in Fig. 6, there will be a tradeoff between this distance and the optical performance. We will investigate such a tradeoff and will produce a correlation between the distance and the optical performance. I.4 Thermal protrusion of the NFTs relative to the surrounding material The thermal protrusion of the NFT due to different thermal expansion coefficient of the NFT relative to that of the surrounding materials will be computed. We will attempt to minimize such a thermal protrusion by examining different heat transfer strategies, including increasing the thermal transport from NFT to the surrounding materials using high thermal conductivity materials. 6

7 I.5 Sensitivity of the NFT s performance to process variations We will investigate the sensitivity of the NFT performance to process variations, including the geometry of the NFT across a wafer. We will obtain these process variations from member companies. The effects of these variations on the NFT performance need to be minimized. Taking the thickness of the NFT for example, it can vary in the lapping process. The optical performance of an NFT is dependent on the thickness due to the Febry-Perot resonance. To minimize the effect of thickness variation, the thickness should be chosen when the optical output reaches the peak (for another obvious reason) where the optical output changes the least with the variation in thickness. The goal is to have an NFT which is least susceptible to this and other variations in the NFT parameters. I.6 Sensitivity of the NFT s performance to variations of head-media spacing (NMS) The variation of NFT performance to the variations of NMS will be computed. We will aim at minimizing the sensitivity of the NFT performance to NMS. This would occur for an NFT with the least amount of near-field divergence. We have shown in our previous work that including the gratings will help to minimize such divergence. I.7 Likely outcome of research The likely outcome of the research is an NFT which is possible to be fabricated using top-down fabrication techniques and materials compatible to HDD, with information including efficiency, thermal reliability, effects of the placement of the recording pole, and with minimized thermal protrusion of NFT and sensitivity to process variations and HMS. II. Resources required to perform project This project will be performed by a graduate student, under the guidance of the PI. Computational tools are already available in the PI s laboratory. III. Resources other than ASTC funding dedicated to perform project The project will leverage a large amount of works (>$5M of total funding) funded by other sources including DARPA, AFOSR, DOE, and NSF. IV. Resources requested from ASTC and how they will be utilized a. Funding: $70,000 total i. Overhead: 54% ii. Direct project cost: $46,000. iii. Facility use fees: None iv. Materials: None v. Student stipends: $22,800 + tuition, fees and health insurance, faculty stipend: ~ $7,000. vi. Travel: $3,000 7

8 b. Expected technical cooperation with sponsor(s): we expect the sponsors to provide the details of the media stack, the optical and thermal properties, process variation range, head-media spacing variation range, and constant feedback. c. Sponsors facility utilization: None d. Expected students internships: none, but can be arranged if necessary. V. Time line The time line of the project is roughly as follows: 1 st 3 rd month: establish the new NFT model 4 th -5 th month: efficiency optimization 6 th month: effect of magnetic pole 7 th month: thermal consideration: thermal reliability and thermal protrusion 8 th month: sensitivity to processing parameters 9 th month: sensitivity to NMS 10 th -12 th month: alternative NFTs and considerations of the effects mentioned above. V.I Home institutions & resources The proposed work will be carried out at Purdue University. The proposed project focuses on numerical studies. We have well-established computational facilities and software developed from past researches on very similar projects. Experimental facilities including NSOM, and NFT fabrication tools (EBL, FIB) are also available at Purdue. VII. Contact information and biographical sketch of researcher Contact: Prof. Xianfan Xu, School of Mechanical Engineering, Phone: (765) xxu@purdue.edu. Prof. Xu is James J. and Carol L. Shuttleworth Professor of Mechanical Engineering at Purdue University. He obtained his M.S. and Ph.D. degrees in Mechanical Engineering in 1991 and 1994, both from the University of California, Berkeley. His research interests include near field nano-optics, developing novel laser processing techniques for laser nano-machining, applying laser nano machining techniques for MEMS and NEMS fabrication, and fundamental studies of laser-matter interaction at nanoscale. He has given over seventy invited talks in academic institutes, technical conferences, government laboratories, and industry, and published over 100 papers in archival journals. He is the recipient of the National Science Foundation Faculty CAREER Award, the Office of Naval Research Young Investigator Award, and the B.F.S. Schaefer Young Faculty Scholar Award of Purdue University. He is a fellow of the American Society of Mechanical Engineers and a Fellow of SPIE. 8

9 References (papers from Prof. Xu s group can be found at 1. Jin, E.X., and Xu, X., 2004, "FDTD Studies on Optical Transmission through Planar Nano- Apertures in a Metal Film," J. Journal Applied Phys., Vol. 43, pp Jin, E.X., and Xu, X., 2005, "Obtaining super resolution light spot using surface plasmon assisted sharp ridge nano-aperture," Appl. Phys. Lett. Vol. 86, pp Jin, E.X., and Xu, X., 2005, "Radiation Transfer through Shaped Nanoscale Apertures," J. of Quantitative Spectroscopy and Radiative Transfer, Vol. 93, pp Jin, E.X., and Xu, X., 2006, Enhanced optical near field from a bowtie aperture, Appl. Phys. Lett. Vol. 88, pp Jin, E.X., and Xu, X., 2006, Plasmonic effects in near-field optical transmission enhancement through a single bowtie-shaped aperture, Appl. Phys. B., Vol. 84, pp Jin, E.X., and Xu, X., 2007, Obtaining subwavelength optical spots using nanoscale ridge apertures, J. Heat Trans., Vol. 129, pp Wang, L., and Xu, X., 2007, Spectral resonance of nanoscale bowtie apertures in visible wavelength, Appl. Phys. A., Vol. 89, pp Jin, X., and Xu, X., 2008, Optical resonance in bowtie-shaped nanoapertures, J. Comp. Theoretical Nanoscience, Vol. 5, pp Challener, W.A., McDaniel, T., Mihalcea, C., Mountfield, K., Pelhos, K., and Sendur, K., (2003) Light Delivery Techniques for Heat-Assisted Magnetic Recording Jpn. J. Appl. Phys. 42, pp Challener, W.A., Gage, E., Itagi, A., and Peng, C., 2006, Optical transducers for near field recording, Jap. J. Appl. Phys., Vol. 45, pp Wang, L., and Xu, X., 2007, High transmission nanoscale bowtie aperture probe for near field optical imaging, Appl. Phys. Lett, Vol. 90, p Wang, L., Jin, E.X., Uppuluri, S.M., and Xu, X., 2006, Contact optical nanolithography using nanoscale C-shaped apertures, Opt. Exp., Vol. 14, pp Xu, X., Jin, E.X., Uppuluri, S.M., Wang, L., 2007, Concentrating light into nanometer domain using nanoscale ridge apertures and its applications in laser-based nanomanufacturing, J. Phys., Vol. 59, pp Wang, L., Uppuluri, S.M.V., Jin, E.X., and Xu, X., 2006, Nanolithography using high transmission nanoscale ridge apertures, Nano Lett. 6, pp N. Murphy-DuBay, L. Wang, E.C. Kinzel, S.M. V. Uppuluri, and X. Xu, 2007, Nanopatterning using NSOM probes integrated with high transmission nanoscale bowtie aperture ; Optics Express, Vol. 16, Issue 4, pp Kinzel, E.,C., and Xu, X., 2009, High efficiency excitation of plasmonic waveguides with vertically integrated resonant bowtie aperture, Opt. Exp. Vol. 17, pp Challener, W.A., Peng, C., Itagi, A.V., Karns, D., Peng. W., Peng, Y., Yang, X., Zhu, X., Gokemeijer, N.J., Hsia, Y.-T., Ju, G., Rottmayer, R.E., Seigler, M.A., Gage, E.C., 2009, Heat-assisted magnetic recording by a near-field transducer with efficient optical energy transfer, Nature Photonics., Vol. 3, pp Stipe, B.C., et al, 2010, Magnetic recording at 1.5 PB m-2 using an integrated plasmonic antenna, Nature Photonics, Vol. 4, pp Seigler, M.A., et al., 2008, Integrated Heat Assisted Magnetic Recording Head: Design and Recording Demonstration, IEEE Trans. Mag., Vol. 44, pp