Natsuhiko Mizutani

Fabrication of half-pitch 32nm resist patterns using near-field lithography with a-Si mask

We evaluated the performances of Cr and amorphous Si (a-Si) films as light absorber materials for photomasks of near-field lithography on the basis of numerical studies using finite-difference time domain method and experimental results of fabrication process. With exposure experiments using both a Cr mask and an a-Si mask, fine performance of an a-Si near-field mask was demonstrated with respect to resolution. A half-pitch 32nm resist pattern of 120nm high was fabricated through the near-field lithography using the a-Si mask and a subsequent trilayer resist process.

Development of a Highly Sensitive Optically Pumped Atomic Magnetometer for Biomagnetic Field Measurements: A Phantom Study

We developed a highly sensitive optically pumped atomic magnetometer for measuring biomagnetic fields from the human body noninvasively. The sensor head was a cubic Pyrex glass cell containing potassium metal and buffer gases. A pump laser beam for spin-polarizing potassium atoms and a probe laser beam for detecting magneto-optical rotation crossed at right angles in the cell, which was heated in an oven to vaporize potassium atoms. The sensitivity of the magnetometer reached to 10-100 fT/Hz1/2 at frequencies below several hundred hertz. To test system performance, we made a phantom which models the human brain, taking into account the contribution of distributed electric currents. First, we tested the phantom by a 306-channel whole-head MEG system and confirmed good agreement of the measured field distributions with theoretical calculations. Subsequently, we measured magnetic field distribution with the phantom scanning two-dimensionally above the oven. The signal source location was estimated by least squares fitting to the measured distribution. The goodness of fit value between the measured and the theoretical distributions was 97.9%.

Human magnetoencephalogram measurements using newly developed compact module of high-sensitivity atomic magnetometer

In the field of biomagnetic measurements, optically pumped atomic magnetometers (OPAMs) are expected to be alternative sensors to magnetometers based on superconducting quantum interference devices (SQUIDs). In addition, miniaturized OPAMs are required for practical use. To address this issue, we developed a compact module of a high-sensitivity OPAM with a pump–probe arrangement and potassium used as the sensing atom. Because the noise spectrum density of the OPAM reached 21 fTrms/Hz1/2 at 10 Hz, we attempted to use it to measure human magnetocardiograms (MEGs). Compared with the results obtained with SQUID-based magnetometers, we could successfully observe distinct features of event-related desynchronization in the 8–13 Hz band associated with eye opening. The results demonstrate the feasibility of using the OPAM module for neuromagnetic field measurements.

Noise reduction and signal-to-noise ratio improvement of atomic magnetometers with optical gradiometer configurations.

In the field of biomagnetic measurement, optically-pumped atomic magnetometers (OPAMs) have attracted significant attention. With the improvement of signal response and the reduction of sensor noise, the sensitivity of OPAMs is limited mainly by environmental magnetic noise. To reduce this magnetic noise, we developed the optical gradiometer, in which the differential output of two distinct measurement areas inside a glass cell was obtained directly via the magneto-optical rotation of one probe beam. When operating in appropriate conditions, the sensitivity was improved by the differential measurement of the optical gradiometer. In addition, measurements of the pseudo-magnetic noise and signal showed the improvement of the signal-to-noise ratio. These results demonstrate the feasibility of our optical gradiometer as an efficient method for reducing the magnetic noise.

On-chip fabrication of alkali-metal vapor cells utilizing an alkali-metal source tablet

We describe a novel on-chip microfabrication technique for the alkali-metal vapor cell of an optically pumped atomic magnetometer (OPAM), utilizing an alkali-metal source tablet (AMST). The newly proposed AMST is a millimeter-sized piece of porous alumina whose considerable surface area holds deposited alkali-metal chloride (KCl) and barium azide (BaN6), source materials that effectively produce alkali-metal vapor at less than 400 °C. Our experiments indicated that the most effective pore size of the AMST is between 60 and 170 µm. The thickness of an insulating glass spacer holding the AMST was designed to confine generated alkali metal to the interior of the vapor cell during its production, and an integrated silicon heater was designed to seal the device using a glass frit, melted at an optimum temperature range of 460–490 °C that was determined by finite element method thermal simulation. The proposed design and AMST were used to successfully fabricate a K cell that was then operated as an OPAM with a measured sensitivity of 50 pT. These results demonstrate that the proposed concept for on-chip microfabrication of alkali-metal vapor cells may lead to effective replacement of conventional glassworking approaches.

Threshold Current Density of GaAs/AlGaAs Single-Quantum-Well Lasers Grown by Molecular Beam Epitaxy

We report on the effect of substrate temperature on the threshold current density in GaAs/AlGaAs single-quantum-well lasers grown by molecular beam epitaxy under various flux ratios. It is found that the threshold current density has W-shape dependence on the substrate temperature and exhibits minima of 600 A/cm2 and 400 A/cm2 at the substrate temperatures of 375°C and 650°C, respectively.

Development of high-sensitivity portable optically pumped atomic magnetometer with orthogonal pump and probe laser beams

Optically pumped atomic magnetometers (OPAMs) have been demonstrated to show sensitivities better than the order of a few femtoteslas. In this study, we developed a portable potassium OPAM module using an electrically heated oven and orthogonal pump and probe beams coupled from polarization-maintaining optical fibers. The OPAM module shared a volume as small as 700 cm3, and its footprint was as small as 60 × 60 mm2; it operated with a single-channel sensitivity of 14 fTrms/Hz1/2 at 100 Hz when the 2-cm cubic potassium cell was heated to 180 °C. In our module, the optical beam path was folded along the magnetic field to be measured. The development of a magnetic sensor device comprising an array of OPAM modules will be an important step toward realizing a biomagnetic imaging system based on OPAMs. The orthogonal two-beam OPAMs will be beneficial for realizing a flexible placement of the array.

Magnetic Field Vector Detection in Frequency Domain with an Optically Pumped Atomic Magnetometer

The motion of spin polarization in a high-sensitivity optically pumped atomic magnetometer is described by the Bloch equation. We propose a magnetometer having two nonparallel probe beams to measure both the x and y components of spin polarization, while a pumping beam propagates in the z direction. When a bias magnetic field is applied parallel to the pumping beam, the magnetic fields Bx and By both affect the x and y components of the spin polarization. The relation between the magnetic field and the magnetometer signal is correctly expressed in matrix form for a simple oscillating magnetic field. The relation enables us to solve the Bloch equation in the frequency domain and retrieve the original waveform of the time-dependent magnetic field waveform. This magnetometer also compensates for distortion of the waveform caused by the resonant nature of the Bloch equation. The magnetic field signal recovery is demonstrated by numerical simulation and the preferred parameter range for the proposed dual-probe magnetometer is shown. As a result, the proposed magnetometer will expand the possibility for weak magnetic field measurements, including biomagnetic fields.

Photo-deprotection resist based on photolysis of o-nitrobenzyl phenol ether; challenge to half-pitch 22 nm using near-field lithography

We propose a non chemically-amplified positive-tone photoresist based on photolysis of o-nitrobenzyl phenol ether (NBP). The increase in the amount of the phenolic hydroxyl group just after the exposure to the i-line propagation light is observed via IR spectroscopy. Using near-field lithography (NFL) combined with the NBP, we form half-pitch (hp) 32 nm line and space (L/S) patterns with lower line edge roughness (LER) than those of a chemically amplified resist (CAR). The high-resolution feature of the NBP is attributed to the photoreaction system without the acid diffusion, which is inherently involved in CARs, although the NBP requires six times as much exposure dose as the CAR does. A Hp 32 nm L/S patterns with 10 nm depths are successfully transferred to the 100 nm thick bottom-layer resist through the tri-layer resist process. Hp 22 nm L/S patterns with 10 nm depths are also fabricated on the top portion of a single-layer of NBP.

A plateau in the sensitivity of a compact optically pumped atomic magnetometer

In a compact optically pumped atomic magnetometer (OPAM), there is a plateau in the sensitivity where the dependence of the sensitivity on pumping power is small compared with that predicted by the uniform polarization model. The mechanism that generates this plateau was explained by numerical analysis. The distribution of spin polarization in the alkali metal cell of an OPAM was modeled using the Bloch equation incorporating a diffusion term and an equation for the attenuation of the pump beam. The model was well-fitted to the experimental results for a module with a cubic cell with 20 mm sides and pump and probe beams with 8 mm diameter. On the plateau, strong magnetic response was generated at the regions that were not illuminated directly by the intense pump beam, while at the same time spin polarization as large as 0.5 was maintained due to diffusion of the spin-polarized atoms. Thus, the sensitivity of the magnetometer monitored with a probe beam decreases only slightly with increasing pump beam int...