Donghun Kwak

An x-axis single-crystalline silicon microgyroscope fabricated by the extended SBM process

A high-aspect ratio, single-crystal line silicon x-axis microgyroscope is fabricated using the extended sacrificial bulk micromachining (SBM) process. The x-axis microgyroscope in this paper uses vertically offset combs to resonate the proof mass in the vertical plane, and lateral combs to sense the Coriolis force in the horizontal plane. This requires fabricating vertically and horizontally moving structures for actuation and sensing, respectively, which is very difficult to achieve in single-crystalline silicon. However, single-crystalline silicon high-aspect ratio structures are preferred for high performance. The extended SRN/I process is a two-mask process, but all structural parts and combs are defined in one mask level. Thus, there is no misalignment in any structural parts or comb fingers. In this extended SBM process, all vertical dimensions of the structure, including the comb height, vertical comb offset and sacrificial gap, can be defined arbitrarily (up to a few tens of micrometers). For electrical isolation, silicon-on-insulator (SOI) wafers are used, but the inherent footing phenomenon in the SOI deep etching is eliminated and smooth structural shapes are obtained, because the SBM process is used. In the fabricated x-axis microgyroscope, the lower combs used to vibrate the proof mass are vertically offset 12 /spl mu/m from the upper combs. The fabricated x-axis microgyroscope can resolve 0.1 deg/s angular rate, and the measured bandwidth is 100 Hz. The reported work represents the first x-axis single-crystalline silicon microgyroscope fabricated using only one wafer without wafer bonding. We have previously reported several versions of z-axis microgyroscopes and x-, y-, and z-axis accelerometers, using the SBM process. The results or this paper allow integrating x-, y-, and z-axis microgyroscopes as well as x-, y-, and z-axis microaccelerometers in one wafer, using the same mask and the same process.

Robust SOI process without footing for ultra high-performance microgyroscopes

A microgyroscope with flat bottom surfaces is fabricated by combining the SOI (silicon on insulator) method with the SBM (sacrificial bulk micromachining) process [1/spl sim/3]. Roughened bottom surfaces and loose silicon fragments are common problems in deep silicon RIE (Reactive Ion Etching) using SOI wafers. In this paper, the silicon fragments are removed and the roughened bottom surfaces are smoothed by the SBM process to achieve robust performance. A gyroscope is fabricated by the proposed method. The measured noise equivalent resolution is 0.0044/spl deg//sec, and the measured bandwidth is 12.8 Hz. The linearity of output is within 7.4% for /spl plusmn/50/spl deg//sec range.

A planar, x-axis, single-crystalline silicon gyroscope fabricated using the extended SBM process

A planar, x-axis, single-crystalline silicon gyroscope is fabricated using one (111) SOI wafer using the extended SBM (sacrificial bulk micromachining) process. The gyroscope uses vertically offset combs to resonate the proof mass in the vertical plane, and lateral combs to sense the Coriolis force in the horizontal plane. The extended SBM process is a simple two-mask process, and because all structural parts and combs are defined in one mask level, there is no misalignment in any structural parts or comb fingers. Furthermore, all vertical dimensions of the structure, including the comb height, comb offset and sacrificial gap, can be defined arbitrarily. In addition, the inherent footing phenomenon in the SOI deep etching is completely eliminated and smooth structural shapes are obtained. The fabricated x-axis gyroscope can resolve 0.1 deg/sec angular rate, and the measured bandwidth is 100 Hz. The reported work represents the first x-axis single-crystalline silicon gyroscope fabricated using only one wafer without wafer bonding. In this paper, SOI wafer was used for electrical isolation, but the same device can be fabricated using other available electrical isolation techniques using only one ordinary (111) wafer, albeit fabrication processes are more complicated.

Why Is (111) Silicon a Better Mechanical Material for MEMS: Torsion Case

This paper shows that using the Finite Element Method (FEM), the torsional stiffness of silicon varies by the least amount on silicon (111) with respect to crystallographic directions, when compared to silicon (100) and (110). The used simulator is ANSYS 5.7 with the element type of Solid 64. As a simulation model, we use a simple torsion system, in which a rotational inertia is attached to the center of clamped-clamped beam with a rectangular cross-section. From the results of the modal analysis, the torsional stiffness is derived using the formula between the natural frequency and the torsional stiffness. Simulation results show that the maximum variations of the torsional stiffness on silicon (111), (100) and (110) are 2.3%, 26.5%, and 31.2%, respectively. This implies that on and silicon wafers, substantially different physical dimensions are necessary for devices with the same torsional characteristics, but with different orientations. Therefore, silicon wafers represent a more suitable substrate to design and fabricate torsional micro and nano systems.Copyright

A new isolation method for single crystal silicon MEMS and its application to z-axis microgyroscope

This paper presents a new method for electrically isolating the released high-aspect ratio single crystal silicon MEMS structures. In this method, horizontal dielectric layers are implanted at arbitrary depths in any desired region of a wafer, using the Sacrificial Bulk Micromachining (SBM) process. A z-axis microgyroscope is fabricated by the proposed method. The measured noise-equivalent angular rate resolution is 0.0074/spl deg//sec, the input range is larger than /spl plusmn/ 50/spl deg//sec, and the measured bandwidth is 7.3 Hz. The proposed method achieves electrical isolation with excellent mechanical stability, and is free from the footing phenomenon.

A NOVEL Z-AXIS ACCELEROMETER WITH PERFECTLY-ALIGNED, FULLY-OFFSET VERTICAL COMBS FABRICATED USING THE EXTENDED SACRIFICIAL BULK MICROMACHINING PROCESS

This paper presents a novel z-axis accelerometer with perfectly aligned vertical combs fabricated using the Extended Sacrificial Bulk Micromachining (ESBM) process. The z-axis accelerometer is fabricated using only one (111) SOI wafer and two photo masks without wafer bonding or CMP processes as used by other research efforts that involve vertical combs. In our process, there is no misalignment in lateral gap between the upper and lower comb electrodes, because all critical dimensions including lateral gaps are defined using only one masks. The fabricated accelerometer has the structure thickness of 40 μm, the vertical offset of 15 μm, and lateral gap between electrodes of 4 μm. Torsional springs and asymmetric proof mass produce a vertical displacement when an external z-axis acceleration is applied, and capacitance change due to the vertical displacement of the comb is detected by charge-to-voltage converter. The signal-to-noise ratio of the modulated and demodulated output signal is 80 dB and 76.5 dB, respectively. The noise equivalent input acceleration resolution of the modulated and demodulated output signal is calculated to be 500 μg and 748 μg. The scale factor and linearity of the accelerometer are measured to be 1.1 mV/g and 1.18 % FSO, respectively. The measured bandwidth is more than 100 Hz.

A Novel z-axis Accelerometer Fabricated on a Single Silicon Substrate Using the Extended SBM Process

This paper presents a novel z-axis accelerometer with perfectly aligned vertical combs fabricated using the extended sacrificial bulk micromachining (extended SBM) process. The z-axis accelerometer is fabricated using only one (111) SOI wafer and two photo masks without wafer bonding or CMP processes as used by other research efforts that involve vertical combs. In our process, there is no misalignment in lateral gap between the upper and lower comb electrodes, because all critical dimensions including lateral gaps are defined using only one mask. The fabricated accelerometer has the structure thickness of , the vertical offset of , and lateral gap between electrodes of . Torsional springs and asymmetric proof mass produce a vertical displacement when an external z-axis acceleration is applied, and capacitance change due to the vertical displacement of the comb is detected by charge-to-voltage converter. The signal-to-noise ratio of the modulated and demodulated output signal is 80 dB and 76.5 dB, respectively. The noise equivalent input acceleration resolution of the modulated and demodulated output signal is calculated to be and . The scale factor and linearity of the accelerometer are measured to be 1.1 mV/g and 1.18% FSO, respectively.

Feedback Control for Expanding Range and Improving Linearity of Microaccelerometers

This paer presents a feedback-controlled, MEMS-fabricated microaccelerometer(XL). The XL has received much commercial attraction, but its performance is generally limited. To improve the open-loop performance, a feedback controller is designed and experimentally evaluated. The feedback controller is applied to the x/y-axis XL fabricated by sacrificial bulk micromachining(SBM) process. Even though the resolution of the closed-loop system is slightly worse than open-loop system, the bandwidth, linearity, and bias stability are stability are significantly improved. The noise equivalent resolution of open-loop system is 0.615 mg and that of closed-loop system is 0.864 mg. The bandwidths of open-loop and closed-loop system are over 100Hz. The input range, non-linearity and bias stability are improved from , from 11.1%FSO to 0.86%FSO, and from 0.221 mg to 0.128 mg by feedback control, respectively.

Feedback Control of MEMS Gyroscope to Achieve the Tactical-Grade Specifications

Abstract This paper presents a feedback-controlled, MEMS-fabricated microgyroscope. The microgyroscope is basically a high Q system, and thus, the bandwidth is limited to be narrow. To overcome the open-loop performance limitations, a feedback controller is designed to improve the resolution, bandwidth, linearity, and bias stability of the microgyroscope. The feedback controller is applied to the z-axis microgyroscope fabricated by SBM process. The resolution, bandwidth, input range, and bias stability of closed-loop system are improved from 0.0021 deg/sec to 0.0013 deg/sec, from 14.8 Hz to 115 Hz, from ±50 deg/sec to ±200 deg/sec, and from 0.0249 deg/sec to 0.0028 deg/sec, respectively.