Kukjin Chun

A study on wafer level vacuum packaging for MEMS devices

A new vacuum packaging process at the wafer level is developed for the surface micromachining devices using glass–silicon anodic bonding technology. The rim for the glass–silicon bonding process which is needed to prevent vacuum leakage is built up simultaneously as the structure is being etched. The mechanical resonator is used as a tool for evaluating the vacuum level of the packaging. The inside pressure of the packaged device was measured indirectly by measuring the quality factor of the mechanical resonator. The measured Q factor was about 5 × 104 and the estimated inner pressure was about 1 mTorr. It is also possible to change the inside pressure of the packaged devices from 2 Torr to 1 mTorr by varying the amount of Ti getter material. The yield of the vacuum packaging process is about 80% and vacuum degradation was not observed after 1000 h had passed. The developed vacuum packaging process is also applied to resonant accelerometers which need a high vacuum environment to implement higher performance.

Polysilicon surface-modification technique to reduce sticking of microstructures

Abstract A new and simple surface-modification technique is proposed to reduce sticking of microstructures fabricated by surface micromachining. This technique realizes a very rugged surface at the polysilicon substrate, resulting in reduced sticking through a decrease of real contact area. The surface, which consists of honeycomb-shaped grain holes at the polysilicon substrate layer, is defined by a two-step dry etch without an additional masking step for photolithography or deposition of thin films. By varying the time for etching the grain holes of the polysilicon substrate, controlled surface roughness can be obtained. Test structures, including polysilicon cantilever beams of various lengths, fabricated by surface micromachining with the proposed surface modification show a doubled detachment length without sticking to the substrate.

A study on resonant frequency and Q factor tunings for MEMS vibratory gyroscopes

A new approach for improving the performance of MEMS vibratory gyroscopes was developed. The methodology suggests a simple way of improving the performance such as the overshoot, settling time and shock immunity by tuning the resonant frequency and the quality factor. The difference in the resonant frequency in two modes (driving and sensing mode) and the quality factors were found to be key factors in determining the dynamics of the gyroscopes. The difference in the frequency could be easily controlled by the electrical stiffness but it was difficult to control the quality factor because it is determined by vacuum level and the shape of structure. An electrostatic feedback technique allowed the control of the quality factor of the micro-gyroscopes. The experimental results show that the magnitude of the resonant peak in the frequency response of the gyroscope is reduced by 58% when the equivalent quality factor of the sensing system is tuned from 264 to 100 at a 100 Hz frequency difference between the driving and sensing modes. The time domain estimation was an approximate 50% reduction in the overshoot and an approximate threefold shortening of the settling time in that case. The estimation in the time domain was based on the simulation because there is no method to measure the transient response of gyroscopes directly.

A new organic modifier for anti-stiction

The chemical and mechanical characteristics of a new surface modifier, dialkyldichloromethylsilane (DDMS, CH/sub 3/)/sub 2/SiCl/sub 2/, for stiction-free polysilicon surfaces are reported. The main strategy is to replace the conventional monoalkyl-trichlorosilane (MTS, RSiCl/sub 3/) such as octadecyltrichlorosilane (ODTS) or 1H,1H,2H,2H-perfluorodecyltrichlorosilane (FDTS) with dichlorodisilane (DDS, R/sub 2/SiCl/sub 2/) with two short chains, especially DDMS. DDMS, with shorter chains in aprotic media, rapidly deposits on the chemically oxidized polysilicon surface at room temperature and successfully prevents long cantilevers 3 mm in length from in-use as well as release stiction. DDMS-modified polysilicon surfaces exhibit satisfactory hydrophobicity, long term stability and thermal stability, which are comparable to those of FDTS. DDMS as an alternative to FDTS and ODTS provides a few valuable advantages; ease in handling and storage of the solution, low temperature-dependence and low cost. In addition to the new modifier molecule, the simplified process of direct release right after washing the modified surface with isooctane was proposed to cut the processing time.

Fabrication of an electrostatic track-following micro actuator for hard disk drives using SOI wafer

This paper presents track-following control using an electrostatic microactuator for super-high density hard disk drives (HDDs). The electrostatic microactuator, a high aspect ratio track-following microactuator (TFMA) which is capable of driving 0.3??g magnetic head for HDDs, is designed and fabricated by a microelectromechanical systems process. It was fabricated on a silicon on insulator wafer with a 20??m thick active silicon layer and a 2??m thick thermally grown silicon dioxide layer; a piggyback electrostatic principle was used for driving the TFMA. The first vibration mode frequency of the TFMA was 18.5?kHz, which is enough for a recording density of higher than 10?Gb?in-2. Its displacement was 1.4??m when a 15?V dc bias plus a 15?V ac sinusoidal driving input was applied and its electrostatic force was 50.4??N when the input voltage was 30.7?V. A track-following feedback controller is designed using a feedback nonlinear compensator, which is derived from the feedforward nonlinear compensator. The fabricated actuator shows 7.51?dB of gain margin and 50.98? of phase margin for a 2.21?kHz servo bandwidth.

A sticking model of suspended polysilicon microstructure including residual stress gradient and postrelease temperature

A sticking (stiction) model for a cantilevered beam is derived. This model includes the effect of the bending moment, which stems from stress gradient along the vertical direction of structural polysilicon, and the temperature during the release process. The bending moment due to the stress gradient will play an important role in evaluating antisticking efficiency since liquid tension and surface energy of microstructures tend to become smaller by newly developed antisticking techniques. The effects of stress gradient and temperature were analyzed and verified with surface-micromachined polysilicon cantilevers. By modifying the substrate polysilicon with grain-hole formation technique, the effects of residual stress gradient in polysilicon on stiction could be observed in the condition of low work of adhesion.

A vacuum packaged differential resonant accelerometer using gap sensitive electrostatic stiffness changing effect

This paper proposes an INS (Inertial Navigation System) grade, surface micro-machined differential resonant accelerometer (DRXL) by using the epitaxially grown thick polysilicon process. This proposed DRXL device produces a differential digital output upon an applied acceleration, and the principle is a gap-dependent electrical stiffness variation of the electrostatic resonator with torsion beam structures. Using this new operating concept, we designed, fabricated and tested the proposed device. The final device was fabricated by using the wafer level vacuum packaging process. The hermetic sealing cap structure was made of Pyrex 7740 glass with Ti layer as gettering material, and this cap wafer was anodically bonded with the polysilicon wafer at vacuum ambience. The measured Q-factor of the vacuum packaged DRXL was about 1/spl times/10/sup 3/ and the estimated inner pressure was about 200[mTorr]. We also achieved 73[Hz] output frequency change per unit G(9.8 m/s/sup c/) input with 12,716[Hz] nominal resonant frequency.

Quantitative evaluation of cardiomyocyte contractility in a 3D microenvironment

Three-dimensional cultures in a microfabricated environment provide in vivo-like conditions for cells, and have been used in a variety of applications in basic and clinical studies. In this study, the contractility of cardiomyocytes in a 3D environment using complex 3D hybrid biopolymer microcantilevers was quantified and compared with that observed in a 2D environment. By measuring the deflections of the microcantilevers with different surfaces and carrying out finite element modeling (FEM) of the focal pressures of the microcantilevers, it was found that the contractile force of high-density cardiomyocytes on 3D grooved surfaces was 65-85% higher than that of cardiomyocytes on flat surfaces. These results were supported by immunostaining, which showed alignment of the cytoskeleton and elongation of the nuclei, as well as by quantitative RT-PCR, which revealed that cells on the grooved surface had experienced sustained stimuli and tighter cell-to-cell interactions.

A novel 3D process for single-crystal silicon micro-probe structures

A new fabrication method for a three-dimensional (3D), single-crystal silicon micro-probe structure is developed. A probe card structure requires tips that are at least 50 μm tall on cantilevers thick enough to withstand a few mN of force as well as 50 μm of tip bending. The cantilever structure also must be able to move at least 50 μm of vertical motion, requiring a large sacrificial gap. The developed 3D fabrication method is based on the surface/bulk micromachining technology, which can fabricate released, high aspect ratio, single-crystal silicon microstructures with high yield using (111) silicon.

A 2x2 MIMO Tri-Band Dual-Mode Direct-Conversion CMOS Transceiver for Worldwide WiMAX/WLAN Applications

This paper describes a fully integrated 130 nm CMOS 2×2 MIMO tri-band dual-mode transceiver for fixed and mobile WiMAX and IEEE 802.11a/b/g/n applications. The proposed transceiver features reduced RF interface (only 4 RF pins) with the wideband circuit topology of the LNA and drive amplifier that minimizes the performance degradation. With carefully chosen LO frequency planning, the transceiver is capable of operating at 2.3-2.7 GHz, 3.3-3.9 GHz, and as well as 5.1-5.9 GHz bands covering whole frequency spectrum of fixed and mobile WiMAX and WLAN. The measured noise figure of the receiver is 3.6-4.2, 4.2-4.7, and 5.4-6.2 dB for each 2/3/5 GHz bands respectively. The measured PLL phase noise from 1 kHz to 10 MHz is 0.5/0.8/0.95 rms degree for 2/3/5 GHz bands respectively. The transceiver ensures low EVM over the wide dynamic range due to linear RX and TX signal paths and low integrated PLL phase noise characteristics.