Seonho Seok

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.

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.

Inertial-grade out-of-plane and in-plane differential resonant silicon accelerometers (DRXLs)

Inertial-grade vertical-type and lateral-type differential resonant accelerometers (DRXL) are designed, fabricated and tested for navigation application, using one mask process. The accelerometers consist of an out-of-plane (for z-axis) accelerometer and in-plane (for x, y-axes) accelerometers. The sensing principle of the accelerometer is based on gap-sensitive electrostatic stiffness changing effect. It says that the natural frequency of the accelerometer can be changed according to an electrostatic force on the proof mass of the accelerometer. The out-of-plane resonant accelerometer shows bias stability of 2.5 /spl mu/g, sensitivity of 70 Hz/g and bandwidth of 100 Hz at resonant frequency of 12 kHz. The in-plane resonant accelerometer shows bias stability of 5.2 /spl mu/g, sensitivity of 128 Hz/g and bandwidth of 110 Hz at resonant frequency of 23.4 kHz. The measured performance of two accelerometers are suitable for an application of inertial navigation.

A novel linearly tunable MEMS variable capacitor

The linearly tunable microelectromechanical systems (MEMS) capacitor with 608 comb fingers changing the overlap area is developed. Unlike the conventional micromachined capacitor using the gap between the parallel plates, the proposed capacitor adopts the overlap area as the tuning parameter. In addition, the tuning range of the proposed capacitor has large nominal capacitance C0, whereas the parallel plates have a range of C0/3 theoretically. The 6-μm-thick single-crystal silicon MEMS structure is bonded to the pyrex glass substrate using the glass–silicon anodic bonding technique and the chemical mechanical polish (CMP) to make the desired capacitor. Single-crystal silicon was chosen as a capacitor structure material because it has excellent mechanical properties greater than those of polysilicon and aluminium as the structure material, and the pyrex glass is used as a substrate instead of silicon to reduce the RF losses through the substrate over the high-frequency range. The measured capacitor shows a nominal capacitance of 1.4 pF, and 10% tuning range at 8V. The capacitor model is also developed to explain the parasitic effect over the high-frequency range and proved by using the Serenade software of the Ansoft Corporation.

An inertial-grade laterally-driven MEMS differential resonant accelerometer

Inertial-grade laterally-driven differential resonant accelerometer (DRXL), using a gap sensitive electrostatic stiffness change effect, is designed, fabricated and tested using a mixed micromachining process based on (111) single crystal silicon. The resonant accelerometer consists of a double ended tuning fork (DETF) and two proof masses in the same plane, so it can detect in-plane acceleration. Each pair, with proof mass and resonator, is completely isolated, so the coupling of mechanical vibration between them is protected. The fabricated accelerometer shows a sensitivity of 64 Hz/g per resonator at nominal frequency of 24,888 Hz, bandwidth of 110 Hz, and a bias stability of 5.2 /spl mu/g.

A high performance mixed micromachined differential resonant accelerometer

The vacuum packaged differential resonant accelerometer (DRXL) using gap sensitive electrostatic change effect is proposed by using the reverse surface micromachining (RSM) process based on the single crystal silicon. The differential frequency output is achieved by subtracting the resonant frequency between the two independent resonators, which has advantages with the wide input range and the good linearity. But the resonant accelerometer is fabricated using the epi-polysilicon surface micromachining process in the previous work. The residual and shear stress of the polysilicon result in the mismatch of the mechanical resonant frequency. The mismatch of the resonant frequency may demolish the advantages of the differential scheme because of the mechanical resonance characteristics. So, the single-crystal silicon was chosen for the accelerometer structure material because it has better mechanical properties than the polysilicon. The fabricated resonant accelerometer shows the nominal frequency of 6928 Hz and the sensitivity is about 10Hz in the /spl plusmn/10 G range. The bandwidth is more than 100 Hz.

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.

A vertically guided MEMS probe card with deeply recessed trench-type cantilever

We developed a vertically guided MEMS probe card with deeply recessed trench-type cantilever. This probe card was designed to achieve a displacement of 50 /spl mu/m at a force of 1.5 gram. The measured contact resistance was about 0.4 ohm for 50 /spl mu/m displacement. The leakage current between the shortest tips was approximately 10 pA. To implement fine pitch and make a vertically guided structure, a cantilever beam was formed inside the trench after deep RIE silicon etching. This probe card was capable of a 35 /spl mu/m pad pitch.

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.

A Novel MEMS LC Tank for RF Voltage Controlled Oscillator(VCO)

A linearly tunable MEMS capacitor changing the overlap area and a high-Q electroplated solenoid inductor are developed. The single crystal silicon MEMS structure is bonded with the pyrex glass substrate to make the designed capacitor. The pyrex glass is adopted as a substrate instead of the silicon to reduce the RF losses through the substrate for both a capacitor and a inductor. The measured capacitor shows the nominal capacitance of 1.4 pF, 10 % tuning range at 2 GHz and the inductor has inductance between 1 nH and 5 nH, maximum Q factor over 10 at 2 GHz. The models are also developed and proven by Serenade software of Ansoft Corporation respectively.