Joo-Young Jin

MEMS micromirror characterization in space environments

This paper describes MEMS micromirror characterization in space environments associated with our space applications in earth observation from the International Space Station and earths orbit satellite. The performance of the micromirror was tested for shock and vibration, stiction, outgassing from depressurization and heating, and electrostatic charging effects. We demonstrated that there is no degradation of the micromirror performance after the space environment tests. A test bed instrument equipped with the micromirrors was delivered and tested in the ISS. The results demonstrate that the proposed micromirrors are suitable for optical space systems.

Extra-gonadal sites of estrogen biosynthesis and function

Estrogens are the key hormones regulating the development and function of reproductive organs in all vertebrates. Recent evidence indicates that estrogens play important roles in the immune system, cancer development, and other critical biological processes related to human well-being. Obviously, the gonads (ovary and testis) are the primary sites of estrogen synthesis, but estrogens synthesized in extra- gonadal sites play an equally important role in controlling biological activities. Understanding non-gonadal sites of estrogen synthesis and function is crucial and will lead to therapeutic interventions targeting estrogen signaling in disease prevention and treatment. Developing a rationale targeting strategy remains challenging because knowledge of extra-gonadal biosynthesis of estrogens, and the mechanism by which estrogen activity is exerted, is very limited. In this review, we will summarize recent discoveries of extra-gonadal sites of estrogen biosynthesis and their local functions and discuss the significance of the most recent novel discovery of intestinal estrogen biosynthesis. [BMB Reports 2016; 49(9): 488-496]

Deep wet etching of borosilicate glass and fused silica with dehydrated AZ4330 and a Cr/Au mask

This research highlights a superior glass-wet-etch technique which enables a glass wafer to be etched for more than 20 h in 49 wt% hydrofluoric acid (HF) only with Cr/Au film and a common positive photoresist, AZ4330. We demonstrated that pits on the wet-etched glass wafer were generated not only due to HF diffusion through the Cr/Au film but also due to pinholes on the Cr/Au films created by the diffusion of the Cr/Au etchant through a photoresist etching-mask during the Cr/Au wet etching process. These two types of diffusion, HF diffusion and Cr/Au etchant diffusion, were eliminated by the thermal curing of a photoresist (PR), AZ4330, before the Cr/Au wet etching process. The curing process allowed the PR to dehydrate, increased the hydrophobicity, and prevented the diffusion of the hydrophilic HF and Cr/Au etchant. Optimization of the curing process was performed, showing that curing at 130 °C for 20 min was the proper condition. With the optimized process, a 525 µm thick borosilicate glass wafer was penetrated with 49%wt HF. A fused silica wafer 525 µm thick was also wet-etched and penetrated with 49 wt% HF at 10 h. Moreover, no pits were found in wet etching of the fused silica for 20 h in 49 wt% HF. These findings demonstrate that the proposed technique allows the wet etching of a glass wafer for more than 20 h in 49%wt HF, the best result thus far. We fabricated a glass substrate with a 217.0 µm deep cavity and a penetrating through-via using the proposed technique, proving the feasibility of the product as an optical component with a surface roughness of 45.5 A in the cavity.

Numerical analysis and demonstration of a 2-DOF large-size micromirror with sloped electrodes

We report on the numerical analysis and demonstration of a two-degree-of-freedom (2-DOF) micromirror with a large mirror plate and a reduced actuation voltage. The micromirror consists of two layers, namely, a top layer and a bottom layer. In the top layer, the flat reflection surface is comprised of single crystalline silicon, while the bottom layer has sloped electrodes allowing for a decrease in the actuation voltage. The sloped electrodes are fabricated by time-delayed electroplating using nickel. Two different types of electrodes, a cone-type electrode and a wedge-type electrode, are designed for the mirror plate actuation and for the frame actuation, respectively. The mirror size is 1.5 mm × 1.5 mm, and the distance between the top and bottom layers is 65 µm. The slope angle of the bottom electrode is 10.3° for the cone-type electrode and 11.3° for the wedge-type electrode. The mechanical maximum tilt angles are measured at 2.7° with 127 V and 3.1° with 120 V for the cone-type electrode and wedge-type electrode, respectively. The cone-type electrode and the wedge-type electrode provide decreased actuation voltages of 49.3% and 62.1%, respectively, compared to a parallel-type electrode. This design is useful for building large-size mirrors actuated by electrostatic forces.

UV Radiation from the Night-Time Atmosphere seen from the “Universitetsky-Tatiana” Satellite

Detectors on the “Universitetsky‐Tatiana” satellite measured a smoothly varying intensity of UV radiation from the night‐time atmosphere in the nadir direction and the intensity of the energetic electron flux at the orbit. At high latitudes the UV intensity in the auroral oval is interpreted as being due to electrons penetrating into the atmosphere. At middle latitudes the UV intensity is an order of magnitude less and more data are needed to reveal the origin of this radiation. Millisecond flashes of UV radiation were observed. The flashes’ energy, temporal profile and global distribution are similar to these parameters for Transient Luminous Events (TLEs). These studies will be continued aboard the next satellite “Tatiana‐2”.

A Novel Telescope with Micromirror for Observation of Transient Luminous Events from Space

A novel type of telescope, a pinhole-like camera with a micromirror array, is introduced for space observation of Transient Luminous Events (TLEs) like gigantic lightning occurring at upper atmosphere currently under question or investigation. The presented telescope has a unique feature of wide field of view (FOV) of surveillance, fast zoom-in and tracking. A high-fill factor, two-axis rotational micromirror array is proposed for the key component of the telescope. To obtain a large static deflection of a mirror, the self-aligned vertical comb-drive actuator is used. Multi-layered structures of a micromirror are fabricated using bulk micromachining techniques, such as deep reactive ion etching and wafer bonding on the full wafer-scale.

Design and fabrication of a self-aligned parallel-plate-type silicon micromirror minimizing the effect of misalignment

This paper describes a self-alignment method whereby a mirror actuation voltage, corresponding to a specific tilting angle, is unvarying in terms of misalignment during fabrication. A deep silicon etching process is proposed to penetrate the top silicon layer (the micromirror layer) and an amorphous silicon layer (the addressing electrode layer) together, through an aluminum mask pattern, in order to minimize the misalignment effect on the micromirror actuation. The size of a fabricated mirror plate is 250 ? 250 ? 4 ?m3. A pair of amorphous silicon electrodes under the mirror plate is about half the size of the mirror plate individually. Numerical analysis associated with calculating the pull-in voltage and the bonding misalignment is performed to verify the self-alignment concepts focused upon in this paper. Curves of the applied voltage versus the tilt angle of the self-aligned micromirror are observed using a position sensing detector in order to compare the measurement results with MATLAB analysis of the expected static deflections. Although a 3.7 ?m misalignment is found between the mirror plate and the electrodes, in the direction perpendicular to the shallow trench of the electrodes, before the self-alignment process, the measured pull-in voltage has been found to be 103.4 V on average; this differs from the pull-in voltage of a perfectly aligned micromirror by only 0.67%. Regardless of the unpredictable misalignments in repetitive photolithography and bonding, the tilting angles corresponding to the driving voltages are proved to be uniform along the single axis as well as conform to the results of analytical analysis.

Silicon-rim-reinforced silicon nitride microscanner with vertical comb actuator and wafer-level vacuum packaging

This research proposes a silicon rim for an optically flat mirror and a vertical comb actuator in a silicon nitride (SiN) microscanner. A wafer-level vacuum packaging was also successfully implemented for large angular deflections with low driving voltage. Radius of curvature (ROC) of a SiN mirror of 1 μm-thickness and 1 mm-diameter was measured to be 0.357 m. Optical experiment showed optical tilt angle of 22° at driving voltage of 53 Vrms and resonant frequency of 14.8 kHz, respectively.

2-D forward optical scanner with glass microlens and isolation blocks using thermal reflow

We present a novel fabrication method of a two-dimensional forward optical scanner with integrated microlens. Glass microlens and electrical isolation blocks were integrated with silicon XY-stage using thermal reflow process. Measured scan angles in X and Y directions at resonance were ±4.9° and ±5.0°, respectively, at atmospheric pressure.

Reliability-based characterization of single crystalline silicon micromirrors for space applications

We focus on reliability-based tests of one-axis single crystalline silicon micromirrors and performance in space environments. Reliability testing inhere shows how to deal with fabrication misalignment, charging effect, settling time reduction, shock and vibration in space, stiction in humidity, and reflectivity degradation related to outgassing. The micromirror in the international space station (ISS) actuated successfully under non-gravity condition as on earth.