Byeoung-ju Ha

Electrostatic actuation of surface/bulk micromachined single-crystal silicon microresonators

In fabricating microelectromechanical systems (MEMS), bulk micromachining using [100] and [110] single crystal silicon and surface micromachining using polycrystalline silicon are used. However, both micromachining methods have drawbacks, and micromachining actuating or sensing MEMS using single crystal silicon has been an active research topic in resent years. This paper presents electrostatic actuation of a resonator fabricated by the SBM (surface/bulk micromachining) process. The SBM process allows fabricating released structures in single crystal silicon. To fabricate electrodes and to electrically isolate them, a junction isolation method using reverse-biased diodes is developed. The breakdown voltage of this isolation method is measured to be larger than 150 volts. A SBM processed microresonator is actuated at 36 kHz. A displacement of several /spl mu/m is achieved in atmosphere with a 20 volts peak-to-peak supply.

Capacitive type surface-micromachined silicon accelerometer with stiffness tuning capability

Abstract A surface-micromachined silicon accelerometer with a novel concept, which has a stiffness tuning capability to improve the sensor resolution, is developed. Imposing an electrostatic force to the electrodes reduces the stiffness of the sensor structure. By adopting the stiffness tuning, the initially stiff structure guarantees the stability of fabrication, and the reduced stiffness, only along the sensing direction, produces the improved resolution. One of the major improvements in the developed accelerometer is the branched comb-finger type electrode which senses the relative position between the mass and the electrode. Maintaining the same capacitance variation, such electrodes allow a larger initial gap between the mass and the electrode, so that the clash problem can be easily eliminated. The accelerometer was successfully fabricated with the active size of 650×530 μm 2 , the 7-μm thick polysilicon structure, and a proof mass of about 1 μg. Experimental results show that the equivalent noise level of the accelerometer is improved by 30 dB through the stiffness tuning. The accelerometer has the bandwidth of 350 Hz, linearity of 0.3% FS, and sensing range of 50 g.

A low temperature, hermetic wafer level packaging method for RF MEMS switch

In this paper, a low temperature hermetic wafer level packaging (WLP) scheme for RF-MEMS devices such as micro-switches is presented. The real component with size 1mm/spl times/1mm is composed of two parts: cap substrate and device substrate, cap substrate has a via-in-cavity structure with cavity depth of 20/spl mu/m. High aspect ratio via hole is fabricated by inductive coupled plasma-reactive ion etching (ICP-RIE) and electroplated with Cu for electrical feed-through. Eutectic bonding is still the most commonly used packaging technology at present. For the purpose of hermetic sealing, Au-Sn multilayer metallization with a close square loop of 100/spl mu/m width have been sputtered onto cap wafer surface as soldering system. Deposition of cap wafer metallization should be finished in one high vacuum chamber process in order to prevent oxidation of Sn layer during producing process. And Ti-Ni-Au combination structure is deposited and patterned on device wafer in accordance with the sealing and interconnection areas in cap wafer. Bonding is performed in wafer level using eutectic bonder (TPS-2000A, BNP science) at a relative low temperature of 280/spl deg/C for heating in static N/sub 2/ ambience for a period of time. As-bonded wafers are then diced into pieces and subjected to a series of performance test for evaluation. Shear strength of two bonded interfaces are measured for sample cells by shear tester ROYCE 552 100K to evaluate mechanical property. RF characteristics insertion loss at 2GHz has measured by HP 8510C network analyzer probe station, a total packaging insertion loss less than 0.05DB could be achieved. For hermeticity test, specific test vehicles which have a large cavity of 0.5/spl times/0.5/spl times/0.05cm/sup 3/ are designed for helium leak test based on M1T-STD-883F since real device cavity has a tiny volume of only 600/spl times/600/spl times/30/spl mu/m/sup 3/, test vehicles indicate a maximum equivalent leak rate in air of 1.6/spl times/10/sup -8/ mbar.l/sec. Also residual gas analysis (RGA) test is performed for bonded device sample. Reliability tests like thermal shock and high temperature, high humidity storage test are also performed according to MIL-STD-883F. For samples before and after reliability tests, measurements also have been made for comparison to evaluate the quality and reliability of packaging structure.

Micromachined inertial sensors

The emergence of micro sensors and actuators is pushing forward the revolution of intelligent systems technology. Intelligent systems technology allows a dramatic change in the way that machines, tools, and systems serve humans. This paper reviews recent R&D and commercial status of inertial sensors using silicon micromachining technologies and discusses the requirement for commercialization of the research results.

Dynamically tuned vibratory micromechanical gyroscope accelerometer

A comb driving vibratory micro-gyroscope, which utilizes the dynamically tunable resonant modes for a higher rate- sensitivity without an accelerational error, has been developed and analyzed. The surface micromachining technology is used to fabricate the gyroscope having a vibrating part of 400 X 600 micrometers with 6 mask process, and the poly-silicon structural layer is deposited by LPCVD at 625 degrees C. The gyroscope and the interface electronics housed in a hermetically sealed vacuum package for low vibrational damping condition. This gyroscope is designed to be driven in parallel to the substrate by electrostatic forces and subject to coriolis forces along vertically, with a folded beam structure. In this scheme, the resonant frequency of the driving mode is located below than that of the sensing mode, so it is possible to adjust the sensing mode with a negative stiffness effect by applying inter-plate voltage to tune the vibration modes for a higher rate-sensitivity. Unfortunately, this micromechanical vibratory gyroscope is also sensitive to vertical acceleration force, especially in the case of a low stiffness of the vibrating structure for detecting a very small coriolis force. In this study, we distinguished the rate output and the accelerational error by phase sensitivity synchronous demodulator and devised a feedback loop to maintain resonant frequency of the vertical sensing mode by varying the inter-plate tuning voltage according to the accelerational output. Therefore, this gyroscope has a high rate-sensitivity without an acceleration error, and also can be used for a resonant accelerometer. This gyroscope was tested on the rotational rate table at the separation of 50(Hz) resonant frequencies by dynamically tuning feedback loop. Also self-sustained oscillating loop is used to apply dc 2(V) + ac 30(mVpk) driving voltage to the drive electrodes. The characteristics of the gyroscope at 0.1 (deg/sec) resolution, 50 (Hz) bandwidth, and 1.3 (mV/deg/sec) sensitivity.

Area-variable capacitive microaccelerometer with force-balancing electrodes

A surface micromachined accelerometer which senses an inertial motion with an area variation and a force balancing electrodes is developed. The grid-type planar mass of a 7 micrometers thick polysilicon is supported by four thin beams and suspended above a silicon substrate with a 1.5 micrometers air gap. The motion sensing electrodes are formed on the substrate. The sensor is designed as an interdigital rib structure that has a differential capacitor arrangement. The moveable electrodes are mounted on the mass and the pairs of the stationary electrodes are patterned on the substrate. In the accelerometer that has comb-type movable electrodes, the mechanical stress and the electrical pulling effects between a moveable electrodes and the fixed electrodes occur. However this grid-type structure can have a large area variation in a small area relatively without stress and pulling, high sensitivity can be achieved. In order to improve the dynamic rang and a linearity, a pair of comb shape force-balancing electrodes are implemented on both sides of the mass. The force-balancing electrodes are made of the same layer as the mass and anchored on a silicon substrate. When acceleration is applied in the lateral direction, the difference of capacitance results from the area variation between the two capacitors and is measured using a charge amplifier. As AC coupled complimentary pick- off signals are applied in paris of stationary electrodes, the undesirable effects due to temperature and electrical noise are reduced effectively. The accelerometer has a sensitivity of 28mV/g and a bandwidth of DC-120Hz. A resolution of 3mg and a non-linearity of 1.3 percent is achieved for a measurement range of +/- 9 g.

Ultra-miniature monolithic FBAR filters for wireless applications

This paper presents ultra-miniature monolithic transmit (Tx) filters for 1900 MHz PCS using thin film bulk acoustic resonators (FBAR) technology. Conventional FBAR filters often include multiple tuning inductors, which have prevented the miniaturization and the monolithic integration of the filters. However, the developed filter has a novel circuit configuration using a single integrated inductor to minimize the size. The FBAR block consists of seven FBARs electrically coupled in a ladder topology. A meander type inductor is used for simplicity of fabrication processes and is optimized to increase the receive (Rx) band rejection performance. The filters are hermetically packaged at a wafer-level, which enables us to achieve a size of 1.0 mm /spl times/ 1.0 mm /spl times/ 0.7 mm. The packaged filters showed high performance with insertion loss less than 3.3 dB and Rx band rejection over 35 dB.

A area variable capacitive microaccelerometer with force-balancing electrodes

A surface micromachined accelerometer which senses a inertial motion with an area variation is developed. The grid-type planar mass of 7 /spl mu/m thick polysilicon is supported by four thin beams and suspended above a Si substrate with a 1.5 /spl mu/m air gap. The motion sensing electrodes are formed on the substrate. The accelerometer is designed as an interdigital rib structure that has a differential capacitor arrangement. The movable electrodes are mounted on the mass and the pairs of the stationary electrodes are patterned on the substrate. In the accelerometer that has comb-type movable electrodes, the mechanical stress and the electrical pulling effects between a movable electrodes and the fixed electrodes occur. However this grid-type structure can have a large area variation in a small area relatively without stress and pulling, high sensitivity can be achieved. In order to improve the dynamic range and a linearity, a pair of comb shape force-balancing electrodes are implemented on both sides of the mass. The force-balancing electrodes are made of the same layer as the mass and anchored on a Si substrate. When acceleration is applied in the lateral direction, the difference of capacitance results from the area variation between the two capacitors and is measured using a charge amplifier. The accelerometer has a sensitivity of 95 mV/g and a bandwidth of DC/spl sim/1 kHz. A resolution of 3 mg and a non-linearity of 0.1%(F.S) is achieved for a measurement range of /spl plusmn/5 g.