Byeung-leul Lee

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.

Electrostatic 1D microscanner with vertical combs for HD resolution display

An electrostatic 1 dimensionally (1D) scanning mirror for HD resolution display is introduced. Vertical comb drive was used to tilt the micro mirror. To minimize the moment of inertia and maximize the tilting angle of the mirror having the diameter of 1.6 mm, the rib was patterned on the backside of the mirror surface and optimized. Via the finite element simulation, the dynamic deformation of 45nm was achieved within the reflecting area in operating resonant mode thanks to the optimized rib structure. The actuating part of scanner was also optimized manipulating with several design variables to get maximum tilting angle. As the fabrication result, mechanical tilting angle of ±12.0 degree was achieved with the resonant frequency of 24.75kHz and the sinusoidal driving voltage of 280Vpp. For stable resonant motion of the scanner, the feedback control algorithm was realized in the driving circuit. Rigorous reliability characterization was carried out using statistical analysis on the fabricated samples. As a result, HD-resolution image with 720 progressive horizontal lines was demonstrated.

A fabrication process for integrating polysilicon microstructures with post-processed CMOS circuits

A MEMS-first fabrication process for integrating CMOS circuits with polysilicon micromechanical structures is described in detail. The overall process uses 18 masks (22 lithography steps) to merge a p-well LOCOS CMOS process that has one metal and two polysilicon layers with a surface micromachining process that has three layers of polysilicon. The microstructures are formed within recesses on the surface of silicon wafers such that their uppermost surfaces are coplanar with the remainder of the substrate. No special planarization technique, such as chemical-mechanical polishing, is used in the work described here. Special aspects of the process include provisions to improve lithography within the recesses, to protect the microstructures during the circuit fabrication, and to implement an effective lead transfer between the microstructures and the on-chip circuitry. The process is validated using a test vehicle that includes accelerometers and gyroscopes interfaced with sensing circuits. Measured transistor parameters match those obtained in standard CMOS, with NMOS and PMOS thresholds at 0.76 V and −0.96 V, respectively.

A new compensation method for the footing effect in MEMS fabrication

A new compensation pattern method to eliminate the footing effect on MEMS devices was proposed using the buffer structure in silicon deep RIE (reactive ion etching). The buffer structure gets the entire footing effect instead of the desired structure and then it is removed after the deep RIE process. The compensation pattern was devised from the characteristics of the RIE lag and the footing effect. The silicon inside the buffer structure is etched to the sacrificial layer and the footing effect first occurs at that moment. The desired structure is not etched to the bottom layer at that moment because of the RIE lag. The etch process is terminated when the desired structure is patterned. The proposed method makes it possible to remove the footing effect in the deep RIE process by modifying the mask layout without changing the fabrication processes. The sense capacitances were approximately 1.7 times higher than that of conventional devices, which verified the effectiveness of the proposed method.

Development of a gyro test system at Samsung Advanced Institute of Technology

Samsung Advanced Institute of Technology has developed a robust gyroscope based on microelectromechanical systems (MEMS) technology. To assist the gyro development and manufacturing an automated test system was created. The system is low cost one with modular and flexible structure controlled by computer having both basic and advanced features. The first aim was to create flexible system to meet test requirements of different development stages of a gyro and then manufacturing. The further major objectives of the work were accurate measurement of gyro characteristics, comprehensive characterization and performance evaluation, reducing of gyro development cycle time by the test system using suitable hardware and software combination. Methodology of the system is building an automated, modular, scalable, flexible system using of available standalone equipment with PC remote operation capabilities. The test system is hierarchically controlled by host computer with General Purpose Interface Bus (GPIB) interface and special software, and may consist of various required and optional equipment. The software written in Visual Basic has both data acquisition and analysis capabilities providing: a flexible utilization each unit of the equipment, redistribution of computational power between host computer and standalone instruments, data saving in a detailed form for quick and convenient postprocessing and plotting. The software structure is built using a concept of object oriented programming. Latest software allows to carry out calibration and 7 different tests including a frequency response, phase delay, output signal level, signal-to-noise ratio, power spectral density of noise, time domain noise level, and many calculated parameters, such as scale factor, linearity error and others. The test procedure follows as much as possible recommended guidelines of IEEE standards for gyroscope testing. In this paper, principles of the test system, application, results and future demands are discussed.

A new electrical residual stress characterization using bent beam actuators

In this paper we propose a new class of sensitive and compact passive stress measurement pattern that uses the piezoresistivity of polysilicon. The proposed measurement pattern utilizes the resistance change of the beam that connects the structures moving in opposite directions. The unit structure is composed of a pair of bent beam actuators with apexes at their mid-points connected by a link beam. The bent beam actuators are 15 μm wide and 200 μm long, and are bent by 0.1 rad (0.7°). The link beam is 5 μm wide and 264 μm long. The unit structure composed of bent beam actuators amplifies and transforms deformations caused by residual stress into elongation or contraction of the linking beam. Hence, the residual stress can be evaluated by measuring the resistance change of the proposed pattern. It is shown that tensile and compressive residual stress levels of about 10 MPa, corresponding to strains below 6 × 10−5, can be measured by using a 40 μm thick test pattern of polysilicon. The sensitivity of the proposed test pattern is 0.83% for the stress of 10 MPa. Additionally, electrical measurements allow this pattern to be suitable for post-packaging stress monitoring.

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.

Piezo-driven inkjet printhead monitoring system

For the industrial printing applications, the stability of the piezo-driven inkjet printhead is a major requirement. In this paper, we focused on the failure modes of the inkjet printhead and realized a method to detect and repair them at high speed. The printhead monitoring is performed by detecting the residual vibration of the actuating plate using the self- sensing capability of the piezoelectric material. To measure the channel acoustics and to identify the malfunctioning nozzle, we devised the bridge sensing circuitry and failure detection algorithm. The residual vibration signals can be affected by the boundary conditions of the channel acoustics, so it is possible to identify the failure causes by analyzing the monitoring signals. Therefore it is also possible to apply a proper restoring process to the defective printhead. The experimental results show that this method is effective in improving the reliability of the industrial printing.

MEMS-based angular rate sensors

The development of MEMS-based angular rate sensors offers revolutionary improvements in low power, size, and cost. Driven by high-volume commercial market needs, applications continue to grow for modestly performing components at prices below

Force-balanced dual-axis microgyroscope

10/axis. The continued maturation of the technology will enable new applications and markets to be realized. The paper reviews the development of MEMS-based angular rate sensors in Samsung and the applications to consumer electronics.