Moon Chul Lee

A high yield rate MEMS gyroscope with a packaged SiOG process

MEMS devices such as a vibratory gyroscope often suffer from a lower yield rate due to fabrication errors and external stress. In the decoupled vibratory gyroscope, the main factor that determines the yield rate is the frequency difference between the sensing and driving modes. The gyroscope, fabricated with a SOI (silicon-on-insulator) wafer and packaged using anodic bonding, has a large wafer bowing caused by thermal expansion mismatch as well as non-uniform surfaces of the structures caused by the notching effect. These effects result in a large distribution in the frequency difference, and thereby a lower yield rate. To improve the yield rate we propose a packaged SiOG (silicon-on-glass) technology. It uses a silicon wafer and two glass wafers to minimize the wafer bowing and a metallic membrane to avoid the notching. In the packaged SiOG gyroscope, the notching effect is eliminated and the warpage of the wafer is greatly reduced. Consequently, the frequency difference is more uniformly distributed and its variation is greatly improved. Therefore, we can achieve a more robust vibratory MEMS gyroscope with a higher yield rate.

Effects of step conditions on the properties of YBCO step-edge Josephson junctions

We have studied transition properties of YBa/sub 2/Cu/sub 3/O/sub 7/ step-edge junctions as a function of the angle of the step-line with respect to the major axes of the SrTiO/sub 3/ substrate. Substrate steps were prepared by Ar ion milling with photoresist mask, and the YBa/sub 2/Cu/sub 3/O/sub 7/ film was deposited by pulsed laser ablation and patterned by ion milling to form junctions. The critical temperature of the junctions was independent of the angle. However, the critical current of the junctions showed a modulation with the angle. A maximum was observed for the step-line oriented parallel to the major axes of the substrate and a minimum for the step-line oriented 45/spl deg/ against the axes. The modulation of the critical current is believed to be caused by the difference in the microscopic structure of the epitaxially grown YBa/sub 2/Cu/sub 3/O/sub 7/ film at the step and also symmetry of the high Tc superconductor.

Hybrid Bulk Acoustic Wave Structure for Temperature Stability in LTE Applications

A novel structure of hybrid bulk acoustic wave (BAW) resonator with low temperature coefficient of frequency (TCF) is presented. Implemented with a pair of quarter-wave reflectors on air-cavity reflector, the hybrid BAW resonator exhibits a TCF of as low as -11.5 ppm/°C and an effective electro-mechanical coupling coefficient (kt2) of 5.1% at the frequency of 2.5 GHz. The hybrid BAW resonator also shows excellent Q-factors of 1627 and 1842 at resonance and anti-resonance frequency, respectively. RF duplexer configured with the hybrid BAW resonators is provided as well. The fabricated RF duplexer for long term evolution (LTE) band-7, which should be designed to prevent the co-existence problem with WiFi having extremely narrow fractional band-gap of 0.7%, achieves an attenuation of 36.8 dB and an insertion loss of 2.4 dB.

Characteristics of bi-crystal grain boundary junctions with different tilt angles for digital circuit applications

We have characterized bi-crystal grain boundary junctions with various tilt angles, i.e., 24/spl deg/, 30/spl deg/, 36.8/spl deg/, and 45/spl deg/ for digital applications. A simple rapid single-flux-quantum (RSFQ) logic circuit has been fabricated using a single layer high-T/sub c/ superconducting YBa/sub 2/Cu/sub 3/O/sub 7-x/ (YBCO) thin film structure with 4 bi-crystal grain boundary Josephson junctions. The circuit includes one readout superconducting quantum interference devices (SQUID) and one storage SQUID. We have focussed upon the readout SQUID properties, including J/sub c/, R/sub n/A, and I/sub c/R/sub n/, as a function of the tilt angles of the grain boundary junctions. The 24/spl deg/-tilt grain boundary junctions were the best candidates for the simple digital circuits among the bi-crystal grain boundary junctions of the tilt angle from 24/spl deg/ through 45/spl deg/. The response of the readout SQUID was demonstrated by an input signal through the storage SQUID at 82 K.

Bulk acoustic wave resonator with suppressed energy loss using improved lateral structure

A bulk acoustic wave (BAW) resonator with suppressed energy loss is proposed. Implemented with air edge reflector and current spreading electrode along the perimeter region of the BAW, the resonator successfully suppresses the acoustic wave leakage and the electric energy loss as well. As results of optimized lateral structure, the Q-factor at anti-resonance frequency (Qa) is significantly improved to over 2100. Furthermore, the peak insertion loss exhibits as low as 0.04 dB and effective electro-mechanical coupling coefficient (kt), which is essential to achieve wide band-width of RF filters, is 6.54%.

Effects of Packaging Induced Stress on MEMS Devices and Its Improvements

In MEMS, packaging induced stress or stress induced structure deformation becomes increasing concerns since it directly affects the performance of the device. The conventional MEMS SOI (silicon-on-insulator) gyroscope, packaged using the anodic bonding at the wafer level and EMC (epoxy molding compound) molding, has a deformation of MEMS structure caused by thermal expansion mismatch. Therefore we propose a packaged SiOG (Silicon On Glass) process technology and more robust spring design.