DOI: 10.14457/tu.the.2017.1387
title: Field enhancement of near field transducer using metamaterial for heat assisted magnetic recording

description: As an analyst from International Data Corp. and Western Digital Corp, the storage demands, hard disk drives (HDDs), is increasing. Unfortunately, the conventional magnetic recording technology is not able to increase storage capacity more than the superparamagnetism limit because the stability problem of the recording medium. The heat-assisted magnetic recording (HAMR) is a new technique which is introduced to overcome the data density limitation by increasing the areal density, expressed in bits per square inch by using a laser to temporary heat a recording medium to reduce the magnetic coercivity before writing process, which is then quickly cooled back to its high-anisotropy state at ambient temperature to stabilize it. The heart of HAMR is a light delivery system called the near field transducer (NFT), to focus light onto a nano-sub-diffraction-limit spot or nano-focusing, in the recording medium. The NFT is made from plasmonic devices which supports the Surface Plasmon Polaritons (SPPs), the main mechanism for overcoming the diffraction limit and helps to enhance the field and coupling efficiency of the NFT. Therefore, the smallest nano-focusing with the high efficiency are required. Therefore, in this dissertation, the investigation of the field enhancement of NFT using metamaterial is proposed, the NFT based on 3D MIM structure for confining light on the deep sub-wavelength scale with input and output part made from metamaterial for optical experiment evaluating. The fabrication of NFT associated with the mass-productive manufacturing using the simple process capability are also demonstrated. The fabrication of a metal-insulator-metal plasmon waveguide with a linear taper is reported. Highly efficient nano-focusing of light with a Au-SiO2-Au waveguide with a three-dimensional taper had been demonstrated. However, conventional vertical taper structures were fabricated with a low-throughput process based on electron beam scanning. An efficient, mass-productive fabrication process using a standard dry etching technique is proposed. A key improvement is the employment of a mixed gas of CHF3 and O2. By optimizing the gas composition and the cooling process of the substrate, a SiO2 vertical taper with an angle of 19 deg., which is very close to the optimum 20 deg. was successfully produced. At the tip section, an ultra-thin waveguide as thin as 5.6 nm, only one third of the conventional demonstration, is reproducibly realized by the employment of an atomic layer deposition of Al2O3. Coupling efficiency as high as 72% was numerically. To practical applications, it very difficult to do alignment for the light source to directly incident light to the bottom of the MIM waveguide which is a nano scale. Therefore, the metamaterial was integrated to the MIM waveguide as the input part and output part. The new structure required the longer body part causing the coupling efficiency was extremely reduced from 79% to 3.65%. However, this output efficiency can be observed in characterization. Furthermore, it is found that the optimized taper angle is not 19 deg. as the feasibility study but it is 30 deg. The characterization feasibility study was found that the propagating length of the 5 nm-Al2O3 is too short, so it is extended to 40 nm-Al2O3. For fabrication process and to remove the SPPs outer surface, both required Ti for adhesion and preventing the SPPs outer surface. But Ti’s effect seriously reduced the efficiency. Another big problem is that the Gaussian beam from the light source is too large, the possible to observe the output light, the 4.22 μm-distance between the grating and slit long is required. Finally, the final specimen of structure which can be observed in the characterization is composed of 300nm-Au/3nm-Ti/200nm- SiO2/40nm-Al2O3/80nm-Au/20nm-Ti. The optimum input structures are the 4 holes of grating, period 520 nm and hole width 110 nm and the optimized taper angle is 30 deg. With all modifications above, the final specimen efficiency as reduces from 3.65% to 0.1% was calculated. The fabrication results, The 8 typical taper angles (10 – 50 deg.) of final specimens are obtained as A-H specimens with good linearity and controllability of taper angles using the oxide dry etching machine which is the embedded cooling system. But it also produced the 20-30 nm-vertical step length in dry etching process. Fabrication yield is about 1.2%.The characterization, the 830 nm-wavelength of Gaussian light source with 620 nm-beam waists was operated. As the characterization results, the efficiency of A-B and E-H specimens were slightly different from the simulation results. But C and D specimens were large different from the simulation results. The re-simulation with the actual structures shown that the presence or absence of the vertical step and the 3 nm-thickness of Ti at the tip area are the main factor for the discrepancy between the simulation results and the experiment results. Finally, the highest efficiency 0.045%-0.05% are obtained with the 512 nm x 853 nm spot size. The efficiency is dropped from the simulation forecasting because the vertical step appearing in the fabrication process and the Gaussian beam waist reducing to 620 nm. Lastly, the future work target is to improve the coupling efficiency by removing disadvantages above, other parameters which relate to the coupling efficiency will be investigated, and the new design of the input and output part should be also necessary to improve the coupling efficiency to 72% as closed as the ideal concept design.