Chang-Hoon Han

Metal-oxide-semiconductor field effect transistor humidity sensor using surface conductance

This letter presents a metal-oxide-semiconductor field effect transistor based humidity sensor which does not use any specific materials to sense the relative humidity. We simply make use of the low pressure chemical vapor deposited (LPCVD) silicon dioxide’s surface conductance change. When the gate is biased and then floated, the electrical charge in the gate is dissipated through the LPCVD silicon dioxide’s surface to the surrounding ground with a time constant depending on the surface conductance which, in turn, varies with humidity. With this method, extremely high sensitivity was achieved—the charge dissipation speed increased thousand times as the relative humidity increased.

An Electrostatically Actuated Stacked-Electrode MEMS Relay With a Levering and Torsional Spring for Power Applications

This paper reports on a novel electrostatically actuated microelectromechanical systems (MEMS) relay for use in power-switching applications. It features a levering and torsional spring to enhance the stand-off voltage and contact endurance by means of an active-opening scheme. The proposed relay is based on a unique stacked-electrode structure and a soft insulating layer under the contact material that make it possible to obtain extremely low contact resistance, resulting in high current driving capability and reliable contact endurance. The fabricated relay demonstrated actuation voltages under 40 V, a switching time of 230 μs, and a maximum stand-off voltage of 360 V, which is the highest level among electrostatically actuated MEMS relays reported to date. The contact resistance was under 5 mΩ at 40 V of applied voltage, and more than 1 A could be carried. The contact reliability in a hot-switching condition was investigated for various dc current levels. At a current of 10 mA, the relay operated for more than 107 cycles before the test was stopped. In addition, the permanent contact stiction during switching operation at a 200-mA current level was overcome with a pull-off (active-opening) voltage of 90 V by the levering and torsional spring. Using this healing process, a device that failed at about 104 switching cycles in the 200-mA hot-switching mode was revived and reoperated with negligible contact resistance variation, lasting up to 4.9 ×105 cycles, constituting an order-of-magnitude enhancement in the lifetime even after failure.

Parallel-Plate MEMS Variable Capacitor With Superior Linearity and Large Tuning Ratio Using a Levering Structure

An innovative and simple method is proposed to achieve ultralinear behavior in a capacitance-versus-voltage response and to obtain a large capacitance tuning ratio in a parallel-plate microelectromechanical systems (MEMS) variable capacitor by moving the plate to an increasing-gap direction. By adopting a levering structure, the common closing-gap motion of the electrostatic actuator was transformed into an increasing-gap movement in order to decrease the capacitance as the actuation voltage was increased. By balancing out the rate that the plate moves up as the actuation voltage increased and the rate that the capacitance decreases as the plate moves up, high linearity was achieved. The proposed MEMS variable capacitor, which was fabricated via metal surface micromachining, showed an excellent linearity factor (LF) of 99.5% in the C-V response, and a capacitance tuning ratio of 134% was achieved in the actual usage range (10-45 V) at a low frequency. When it was operated at 1 GHz, the proposed device demonstrated an LF of 99.5% and a capacitance tuning ratio of 125%.

MEMS variable capacitor with superior linearity and large tuning ratio by moving the plate to the increasing-gap direction

This paper reports an innovative and simple method to achieve highly linear capacitance vs. voltage response and large tuning ratio in a parallel-plate MEMS variable capacitor by moving the plate to the increasing-gap direction. The proposed structure adopted a levering actuator to achieve the opposite moving direction and large displacement of the top plate. The proposed MEMS variable capacitor, which was made by metal surface micromachining, showed excellent linearity factor of 99.1% in C-V response and the capacitance tuning ratio of 178% were achieved at 2GHz.

A Highly Reliable MEMS Relay With Two-Step Spring System and Heat Sink Insulator for High-Power Switching Applications

This paper reports a highly reliable electrostatic microelectromechanical systems (MEMS) relay for high-power switching applications. The main proposal to elevate reliability is to reduce thermal damage in the contact area. Since a contact resistance is the key parameter determining the amount of Joule-heating and the corresponding thermal damage, we devised a unique spring structure to maximize the contact force (resulting in a low contact resistance) using a reasonable actuation voltage named a two-step spring system. Another important feature was applied to alleviate Joule-heating, which is to use an insulator having high thermal conductivity to dissipate the generated heat efficiently, named a heat sink insulator. The fabricated MEMS relay exhibited 2 mΩ in contact resistance, which is the lowest level reported so far with an actuation voltage of 45 V. Reliability was remarkably enhanced over ten times by the heat sink insulator. Consequently, by applying these two approaches simultaneously, the fabricated MEMS relay was successfully operated up to the 5.3 ×106 cycles at 1 V/200 mA in ambient air and hot switching condition, which is the highest reliability reported at that power level.

Electrostatic micro-actuator with a pre-charged series capacitor: modeling, design, and demonstration

As a powerful method to reduce actuation voltage in an electrostatic micro-actuator, we propose and investigate an electrostatic micro-actuator with a pre-charged series capacitor. In contrast to a conventional electrostatic actuator, the injected pre-charges into the series capacitor can freely modulate the pull-in voltage of the proposed actuator even after the completion of fabrication. The static characteristics of the proposed actuator were investigated by first developing analytical models based on a parallel-plate capacitor model. We then successfully designed and demonstrated a micro-switch with a pre-charged series capacitor. The pull-in voltage of the fabricated micro-switch was reduced from 65.4 to 0.6 V when pre-charged with 46.3 V. The on-resistance of the fabricated micro-switch was almost the same as the initial one, even when the device was pre-charged, which was demonstrated for the first time. All results from the analytical models, finite element method simulations, and measurements were in good agreement with deviations of less than 10%. This work can be favorably adapted to electrostatic micro-switches which need a low actuation voltage without noticeable degradation of performance.

High-performance MEMS relay using a stacked-electrode structure and a levering and torsional spring for power applications

This paper presents a novel MEMS relay suitable for power switching, which is based on a unique stacked-electrode structure for very low contact resistance and a levering and torsional spring for enhancing a stand-off voltage (maximum drain voltage to withstand in the off-state) and contact endurance. The fabricated MEMS relay showed the contact resistance of less than 5 MΩ at 35 V of applied voltage, stand-off voltage of 360 V, which is the highest level among the electrostatically-actuated MEMS relays. Also, it was able to operate for 4.9 × 105 cycles at 200 mA current level, and demonstrated the possibility of “resurrection of the relay from the stiction failure” for the first time.

Voltage-Controlled

In a conventional parallel plate MEMS variable capacitor, the capacitance versus voltage response (C-V response) has been deterministic. In this work, the C-V response is tuned versatilely through the application of a control voltage to an additional electrode in order that the initial gap between the parallel capacitor plates is set by the control voltage. Then, the capacitor plates are lifted (capacitance decreases) as the actuation voltage applied to the levering actuator increases. In this manner, the shape of the C-V response can be controlled even after the device is fabricated. At a zero control voltage, the fabricated MEMS variable capacitor exhibited a convex shape in the C-V response (i.e., the capacitance decreases slowly in the low actuation voltage region and rapidly in the high actuation voltage region). When 3 V was applied to the control voltage, the capacitor exhibited an almost linear C-V response with a linearity factor of 0.999. At 5 V of control voltage, the C-V response changed to a relatively concave shape (i.e., the capacitance decreases rapidly in the low actuation voltage region and slowly in the high actuation voltage region). The capacitance tuning ratio of the fabricated device exceeded 120% at all control voltages. The proposed C-V response tuning capability is vital and amenable to various circuit demands.

C{-}V

This paper reports an electrostatic energy-harvester exploiting water-layer formed by respiration as a variable-area electrode. We discover that electrically conductive water-layer is instantly formed on a silicon-dioxide surface by exhaled-breath. We adopt this layer as a variable capacitive electrode for electrostatic energy-harvesting. The capacitance change was anticipated using a theoretical modeling and finite-element-method (FEM) simulation, and theoretical power-generation was estimated (~2 μW/cm2 at 1 V). We then fabricated the prototype device and verified the capacitance change experimentally. Finally, the prototype showed charging and discharging characteristics by respiration successfully for being used as an energy-harvester driven by human-breath solely.

Response Tuning in a Parallel Plate MEMS Variable Capacitor

This paper reports an innovative and simple three-dimensional (3-D) reshaping (plastic deformation) technique in MEMS devices by solely electrical control with ultrafine tuning resolution. The proposed plastic deformation technique adopted the creep phenomenon to achieve the desired plastic deformation. While voltage input induced stress on the device to start the creep phenomenon, Joule heating was applied to accelerate the creep phenomenon. Then, plastic deformation was successfully demonstrated with solely electrical control, where the tuning resolution was demonstrated at a sub-100nm level.