Seung-Deok Ko

3-terminal nanoelectromechanical switching device in insulating liquid media for low voltage operation and reliability improvement

A nanoelectromechanical (NEM) switching device is developed with a new technique involving a liquid medium. Operation voltage is reduced by about 40% and the number of switching cycles with reliable device performance is improved dramatically, more than 5-fold. The device has a 50 nm thick TiN cantilever with a 40 nm air-gap. A CMOS compatible process is employed.

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.

Complementary Dual-Contact Switch Using Soft and Hard Contact Materials for Achieving Low Contact Resistance and High Reliability Simultaneously

This paper reports a dual-contact microelectromechanical switch, which consists of two contacts in a single switch: one with a soft contact material and the other with a hard contact material to achieve low contact resistance and high reliability at the same time under hot switching conditions. In a single switching operation, the proposed dual-contact switch makes contact twice in sequence, where the first contact is made with a hard contact material (Pt-to-Pt) that can withstand an abrupt hot switching condition (high electric field or micro-arcing). The second contact is then accomplished with the soft contact material (Au-to-Au) that has low-contact resistance, through which most of the current flows. In contrast, when the switch releases contact, the Au-to-Au contact is initially detached, and this is followed by the release of the Pt-to-Pt contact. In this way, the dual-contact switch showed longer lifetime than that of a single Au-to-Au contact-only switch by up to fortyfold, and even better lifetime than that of a single Pt-to-Pt contact-only switch by more than two times in open laboratory environments (unpackaged). At the same time, contact resistance of the dual-contact switch was under 0.3 Ω at 50 V of the gate voltage, which is more than seven times smaller than that of the single Pt-to-Pt contact-only switch (2.2 Ω), due to the Au-to-Au contact sub-switch (the contact resistance of the single Au-to-Au contact-only switch was 2.2 Ω).

High-Performance Hybrid Complementary Logic Inverter through Monolithic Integration of a MEMS Switch and an Oxide TFT

A hybrid complementary logic inverter consisting of a microelectromechanical system switch as a promising alternative for the p-type oxide thin film transistor (TFT) and an n-type oxide TFT is presented for ultralow power integrated circuits. These heterogeneous microdevices are monolithically integrated. The resulting logic device shows a distinctive voltage transfer characteristic curve, very low static leakage, zero-short circuit current, and exceedingly high voltage gain.

An insulating liquid environment for reducing adhesion in a microelectromechanical system

Stiction has been one of the major failure problems in microelectromechanical systems (MEMS). As a solution for stiction failure, we investigated an insulating liquid environment for MEMS to eliminate adhesion force. We speculated that three forces—capillary, solid-solid contact, and van der Waals (vdW) forces decrease when the devices are operated in an insulating liquid environment. In the experiment, the adhesion force of the devices was measured to be 42.8 μN on average in air, whereas it decreased to 2.52 μN on average in the insulating liquid, corresponding to a remarkable 94.1% decrement.

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.

A Highly Reliable Two-Axis MEMS Relay Demonstrating a Novel Contact Refresh Method

This paper reports on a two-axis actuated microelectromechanical systems (MEMS) relay to realize a unique contact-refresh concept. In comparison with all other conventional MEMS relays utilizing only several designated contact spots during their whole lifetime, the proposed concept can change the real contact spots (asperities) by altering the lateral position of contact asperities, thus providing highly reliable contact endurance. In addition, it can enhance lifetime of the switches that fail by contact resistance increase, and potentially even for switches that fail by contact stiction if the contact position is changed before a critical number of switching cycles is reached; however, the device inevitably has a relatively large device area and additional control circuitry in this stage of development. The fabricated relays showed vertical actuation voltages under 40 V, a switching delay of 190 μs, and a maximum lateral displacement of 10 μm. Owing to the suggested contact-refresh scheme, the total contact endurance in one switching device was dramatically increased, and the sum of dozens of lifetimes measured at the selected lateral positions reached 6 × 107 cycles at 100 mA in hot switching conditions (Au-to-Au contact), which is nearly 50 times higher than the average value of the measured lifetimes in a designated contact spot.

Efforts toward ideal microelectromechanical switches

Since a microelectromechanical (MEM) switch with an electrostatically actuated cantilever was first demonstrated by Petersen in 1978 [1], MEM switches have actively been researched by many research groups. However, comparing with the conventional metal-oxide-semiconductor field-effect transistor (MOSFET), MEM switches are still suffering from their high actuation voltage and insufficient operational reliability, which still remain as a difficult challenge to many MEMS researchers and hinder commercialization of the MEM switches. In this work, we look at what lies behind these difficulties in MEM switches and illustrate bright ideas that have been sought to enhance the actuation voltage and switch endurance (lifetime) problems.

Annealing effect on contact characteristics in TiN based 3-terminal NEM relays

Thermal annealing process has been accommodated to improve contact characteristics in TiN based NEM relays. The annealing process was performed after TiN electrode formation and before HF release. As a result, abrupt I-V characteristic and current stability have been attained. In cyclic test, the device with thermal treatment showed slow contact degradation. These results suggest that the thermal annealing is effective for improving the device performance and for obtaining a reliable operation in TiN based NEM relays.

>1000-Fold Lifetime Extension of a Nickel Electromechanical Contact Device via Graphene

Micro-/nano-electromechanical (M/NEM) switches have received significant attention as promising switching devices for a wide range of applications such as computing, radio frequency communication, and power gating devices. However, M/NEM switches still suffer from unacceptably low reliability because of irreversible degradation at the contacting interfaces, hindering adoption in practical applications and further development. Here, we evaluate and verify graphene as a contact material for reliability-enhanced M/NEM switching devices. Atomic force microscopy experiments and quantum mechanics calculations reveal that energy-efficient mechanical contact-separation characteristics are achieved when a few layers of graphene are used as a contact material on a nickel surface, reducing the energy dissipation by 96.6% relative to that of a bare nickel surface. Importantly, graphene displays almost elastic contact-separation, indicating that little atomic-scale wear, including plastic deformation, fracture, and atomic attrition, is generated. We also develop a feasible fabrication method to demonstrate a MEM switch, which has high-quality graphene as the contact material, and verify that the devices with graphene show mechanically stable and elastic-like contact properties, consistent with our nanoscale contact experiment. The graphene coating extends the switch lifetime >103 times under hot switching conditions.