Che-heung Kim

Prevention method of a notching caused by surface charging in silicon reactive ion etching

This paper proposes a method to prevent silicon from a notching in a reactive ion etching (RIE) process by introducing a self-aligned metal interlayer to a silicon/glass bonded fixture. A metal interlayer prevents a charge buildup at the bottom of a silicon trench, therefore silicon structures do not suffer from charge-induced local damage. A self-aligning process of Ti/Au interlayer is accomplished by a lift-off process using a photoresist etch mask as a sacrificial layer. Titanium is a diffusion barrier of gold to silicon and gold is preferred due to its chemical stability in a pre-cleaning step. The effectiveness of the proposed method is verified by applying it to the fabrication of a simple trench and comb electrode. The applicability of the proposed methodology is examined by fabricating an optical knife edge device.

MEMS variable optical attenuator using a translation motion of 45? tilted vertical mirror

A reflection-type variable optical attenuator (VOA) using the translation of a 45° tilted vertical mirror is proposed. The translation of the mirror results in an optical axis offset between transmitting and receiving optical signal, which attenuates the transmitted optical power. Single- and dual-mirror-type VOAs are implemented and compared. A conic-cladding fiber and collimating lensed fiber are applied for low insertion loss. Optical attenuation performances are predicted using the CODE V optical simulator. An electrical spring method is studied for mode separation of the mirror actuator. The self-aligned metal interlayer process is applied for achieving a notching-free mirror surface. Performances of the fabricated VOAs are measured and discussed. An attenuation range is achieved from −0.9 dB (0 V) to −35 dB (10 V). The wavelength-dependent loss (WDL) is as small as 0.4 dB even at −20 dB attenuation level. Excellent polarization-dependent loss (PDL) less than 0.24 dB is achieved in the whole attenuation range.

A Wafer-Level Micro Mechanical Global Shutter for a Micro Camera

A novel wafer-level manufactured micro mechanical global shutter utilizing thin-film roll actuators is presented for a micro mobile camera. The aperture of the shutter is 2.2 mm in diameter and covered with 36 triangular roll actuators whose radius of curvature is designed to 235 ¿m. The stress induced rolling of thin composite layers and their pull-in behaviors are analyzed and experimented. A 0.8 mm-thick wafer-level shutter array is successfully implemented using batch processes. The fabricated shutter can follows 500 Hz-square wave signal of 30 V

Mechanically Coupled Low-Voltage Electrostatic Resistive RF Multithrow Switch

This paper proposes an electromechanically actuated single-pole multithrow RF switch utilizing an axial force coupling mechanism among switching membranes. All switches are evenly distributed and coupled to each other with a pair of microbeams. When one membrane is switching on, the others become stiffer than before by the increased axial force. This enables the individual switch to achieve low switching-on voltage and high power handling capability at an off state simultaneously. The analytic estimation presents a 36% reduced stiffness at switching- on and a 34% increased stiffness in sustaining an off state. The topologies for both of the single pole double throws (SPDT) and single pole triple throws (SP3T) using this technique are designed and implemented. Actuation voltages are as low as 15 V for an SPDT and 20 V for an SP3T. The insertion loss and isolation among throws of the SP3T are measured to be 0.25 and 51.5 dB at 2 GHz, respectively. Significant improvement of isolation (from 37 to 45.7 dB) in the high-frequency band (at 10 GHz) proves the benefit of the proposed concept. Typical switching time of the SP3T is measured to be 7.4 μs for on switching and 3.5 μs for off switching at 20 V. The number of switching cycles has reached 1 × 107 cycles.

A novel and simple fabrication technology for high power module with enhanced thermal performance

A novel packaging method for high power modules based on one-step sintering process using a proper jig and nano silver paste is described. First, using Ansoft Q3D Extractor, electromagnetic simulation is carried out to design the best chip array and DC bus connection, which giving the lowest stray inductance. A die-bonder is used for a precise die attachment, which provides high placement accuracy (<; 25 μm). One step sintering process between Si chip to DBC and DBC to baseplate was established under a low temperature (<; 260 °C) and low pressure (<; 10 MPa). In addition, the relation of the porosity and pressure on the adhesion of sintered silver layers was investigated. Finally, thermal performance of the proposed package and cooling is then evaluated with both FEA (finite element analysis) simulation and experiments. The simulation and experimental results, which show the lowest value (<;0.09 °C/W) of thermal resistance of junction to fluid, agree well.

Force Coupled Electrostatic RF MEMS SP3T Switch

An electrostatic RF MEMS SP3T switch using an axial force coupling is proposed. Topologically equivalent three switching membranes are coupled through flexible beams so that the deflection of one switching membrane develops mutual axial load and moment to the others (one in already ON state and the other in idle state) and helps them easily be OFF. A built-in silicon stopper amplifies an axial force and moment by restricting the displacement of coupling beams. With maintaining a low driving stiffness, about 34 % increase in the restoring stiffness was examined by the simulation. Symmetric tri-layer film stack (sandwich like) is preferred to minimize the warpage of large membrane for low driving voltage. A RF characteristics of unit switch is measured to 0.25dB-insertion loss and 51.5 dB isolation at 2 GHz with 20 V r iving voltage. Especially high RF isolation over 45 dB is achieved even at 10 GHz by using the proposed force coupled driving concept. Switching time is measured to 7.5 mus for ON switching and 3.2 mus for OFF switching

Study on the reliability of the mechanical shutter utilizing roll actuators

This paper handles a single approach to solve two major reliability issues of the previously proposed micro shutter using roll actuators. Material and structural change from Al/SiNx to Mo/Mo improves the temperature stability of an aperture size by matching CTEs of bi-layer ideally. It is also effective to extend the rated lifetime of the shutter by eliminating a dielectric charging layer, SiNx. The measured temperature-invariant diameter of the Mo/Mo roll from 25°C to 80°C is compared with a dramatic change of the former Al/SiNx roll, which proves our approach effective. The rated lifetime reaches 200,000 (5 times higher than the previous work) just after applying Mo/Mo roll and 350,000 (1.7 times more) when dimples are added on it.