Akiko Yuzawa

Fabrication of 5 Tdot/in.2 bit patterned media with servo pattern using directed self-assembly

The fabrication of an etching template for 5 Td/in.2 bit patterned media using a self-organization material, namely, poly(styrene)-poly(dimethylsiloxane) (PS-PDMS), was investigated. The molecular weight of the PS-PDMS for forming the areal density of 5 Td/in.2 dot pattern was estimated from the polymerization index related to the Flory–Huggins interaction parameter. Annealing was carried out to obtain a fine-order dot pattern. PS-PDMS films were subjected to thermal treatment or solvent annealing. The ordering of the dot array in these films was evaluated by using Voronoi diagrams. The results indicate that the film annealed in N-methylpyrrolidone (NMP) vapor showed finer ordering than did the thermally treated film. This seemed to be attributable to the high solubility parameter of NMP. The soaking of NMP into the PS phase slightly shifted the phase separation energy of the polymer matrix. The lattice spacing of the obtained hexagonal pattern was 11 nm. By using low-molecular-weight PS-PDMS with solvent...

Nanoimprint Mold for 2.5 Tbit/in.2 Directed Self-Assembly Bit Patterned Media with Phase Servo Pattern

We demonstrate the mold fabrication and replication process for the production of 0.8 and 2.5 Tbit/in.2 directed self-assembly bit patterned media (DSA-BPM). These devices are fabricated with 33 and 17 nm dot pitch patterns using the microphase segregation structure of polystyrene–poly(dimethylsiloxane) as an etching mask template. The self-assembled dot arrays are simultaneously ordered on both the circular tracks for the data area and the arbitrary marks for the servo area by DSA using groove guides. We fabricated the Si mold with dot pillars of 19.3 nm height for the 2.5 Tbit/in.2 DSA-BPM from the poly(dimethylsiloxane) dot mask. We also demonstrated the nickel mold replication of the 0.8 Tbit/in.2 DSA-BPM by electroforming from the Si mold.

Evaluation of ordering of directed self-assembly of block copolymers with pre-patterned guides for bit patterned media

Bit patterned media (BPM) is a promising candidate for next-generation magnetic recording media beyond 2.5 Tb/in2. To realize such high-density patterned media, directed self-assembling (DSA) technology is a possible solution to form fine dots. In order to read and write magnetic signals on a magnetic dot of magnetic media, the position of magnetic dots must be controlled. We examined ordering of directed self-assembly of diblock copolymer dots with a variety of prepatterned guides in some conditions and evaluated the ordering of the dots by using Delaunay triangulation and Voronoi diagram. Applying the optimized conditions, we obtained highly controlled dot pattern suitable for magnetic recording media.

Nanoimprint process for 2.5Tb/in2 bit patterned media fabricated by self-assembling method

Bit patterned media (BPM) is a promising candidate for high-density magnetic recording media beyond 2.5 Tb/in2. To realize such a high-density BPM, directed self-assembling (DSA) technology is a possible solution. On the other hand, from the viewpoint of low-cost production, nanoimprint lithography is a promising process for the mass-production of such a high-density BPM. We examine the replication of the BPM etching mask by UV nanoimprint process. At first, the BPM silicon master mold consisting of servo pattern with dot array is made by the DSA method using PS-PDMS. For the 30-nm pitch corresponding to the density of 2.5 Tb/in2, the nickel stamper is replicated from the silicon master mold by electroplating. The etching mask is transcribed by the UV nanoimprint process with the transparent mold replicated from the nickel mother stamper. On the other hand, as for the DSA-BPM pattern of 17-nm pitch corresponding to the density of 2.5 Tb/in2, we adopt an alternative process and confirm the replication possibility.

Highly sensitive spintronic strain-gauge sensor based on a MgO magnetic tunnel junction with an amorphous CoFeB sensing layer

We investigated spintronic strain-gauge sensors (Spin-SGSs) based on magnetic tunnel junctions (MTJs). To enhance the strain sensitivity of Spin-SGSs, which is defined as the gauge factor = (ΔR/R)/Δe, we investigated MgO-MTJs with an amorphous CoFeB sensing layer that exhibits high magnetostriction and soft magnetic properties. To maintain the amorphous structure of the CoFeB sensing layer even after post annealing, we applied a MgO capping layer (MgO-cap) to the CoFeB sensing layer and compared it with a Ta capping layer (Ta-cap). After post annealing at 320 °C, the CoFeB sensing layer with a MgO-cap maintained a low coercivity of 3 Oe, whereas that with a Ta-cap exhibited a high coercivity of 25 Oe. Microstructure analysis revealed that the CoFeB sensing layer with the MgO-cap has an amorphous structure because boron remains in the CoFeB sensing layer even after post annealing. The gauge factor for the Spin-SGS with the MgO-cap was 4016, which was four times larger than 942 for the Spin-SGS with the Ta-cap.

Spin-MEMS microphone based on highly sensitive spintronic strain-gauge sensors

We report a novel spintronic MEMS (Spin-MEMS) microphone, which is a new type of resistive microphone. For this microphone, spintronic strain-gauge sensors (Spin-SGSs) are integrated on a bulk micromachined diaphragm. The Spin-SGSs are based on magnetic tunnel junctions (MTJs) similar to those used as magnetic sensors in hard disk drives. In work to date, we have experimentally confirmed that the Spin-SGS exhibits a high gauge factor in excess of 5000, which is 100-fold that for a conventional poly-Si piezoresistor, by adopting a novel amorphous Fe-B-based sensing layer with high magnetostriction and low coercivity. Thanks to the high strain sensitivity of the Spin-SGSs, the Spin-MEMS microphone exhibits a signal-to-noise ratio (SNR) of 57 dB(A). A Spin-MEMS microphone with a first resonance frequency of over 70 kHz was also fabricated that exhibits an SNR of 49 dB(A), which is promising for acoustic health monitoring. In this study, we compared the operation sounds of defective and normal bearings using the Spin-MEMS microphone. The Spin-MEMS microphone detected differences in the operation sounds between the defective and normal bearings in the high-frequency range of 10 kHz to 50 kHz.We report a novel spintronic MEMS (Spin-MEMS) microphone, which is a new type of resistive microphone. For this microphone, spintronic strain-gauge sensors (Spin-SGSs) are integrated on a bulk micromachined diaphragm. The Spin-SGSs are based on magnetic tunnel junctions (MTJs) similar to those used as magnetic sensors in hard disk drives. In work to date, we have experimentally confirmed that the Spin-SGS exhibits a high gauge factor in excess of 5000, which is 100-fold that for a conventional poly-Si piezoresistor, by adopting a novel amorphous Fe-B-based sensing layer with high magnetostriction and low coercivity. Thanks to the high strain sensitivity of the Spin-SGSs, the Spin-MEMS microphone exhibits a signal-to-noise ratio (SNR) of 57 dB(A). A Spin-MEMS microphone with a first resonance frequency of over 70 kHz was also fabricated that exhibits an SNR of 49 dB(A), which is promising for acoustic health monitoring. In this study, we compared the operation sounds of defective and normal bearings using ...

Spin-MEMS microphone integrating a series of magnetic tunnel junctions on a rectangular diaphragm

We investigate the enhancement of the signal-to-noise ratio (SNR) of spintronic micro-electro mechanical-system (Spin-MEMS) microphones in which spintronic strain-gauge sensors (Spin-SGSs) are integrated on a micro-electro mechanical-system (MEMS) diaphragm by using a large array of N Spin-SGSs connected in series similar to that in a previous report on magnetic tunnel junction magnetic sensors. Since the strain-gauge properties of Spin-SGSs strongly depend on the angle between the applied uniaxial strain and the magnetization direction of the reference layer, in order to obtain the same signals from each Spin-SGS in an array, it is necessary to locate the Spin-SGS array in a region where the uniaxial strain occurs uniformly on the MEMS diaphragm. We theoretically and experimentally investigate the effect of the diaphragm shape on uniaxial strain on the diaphragm surface. As a result, it is found that a rectangular-shaped diaphragm provides a larger region in which a uniform uniaxial strain is applied to the Spin-SGS array compared with the generic circular diaphragm. Finally, an SNR enhancement of 18 dB by connecting N = 62 Spin-SGSs in series is successfully confirmed in a Spin-MEMS microphone with a rectangular diaphragm.We investigate the enhancement of the signal-to-noise ratio (SNR) of spintronic micro-electro mechanical-system (Spin-MEMS) microphones in which spintronic strain-gauge sensors (Spin-SGSs) are integrated on a micro-electro mechanical-system (MEMS) diaphragm by using a large array of N Spin-SGSs connected in series similar to that in a previous report on magnetic tunnel junction magnetic sensors. Since the strain-gauge properties of Spin-SGSs strongly depend on the angle between the applied uniaxial strain and the magnetization direction of the reference layer, in order to obtain the same signals from each Spin-SGS in an array, it is necessary to locate the Spin-SGS array in a region where the uniaxial strain occurs uniformly on the MEMS diaphragm. We theoretically and experimentally investigate the effect of the diaphragm shape on uniaxial strain on the diaphragm surface. As a result, it is found that a rectangular-shaped diaphragm provides a larger region in which a uniform uniaxial strain is applied to ...