Kyu Dong Jung

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.

A de-coupled vibratory gyroscope using a mixed micro-machining technology

This paper presents the highly reliable and wafer level vacuum packaged vertical micro-gyroscope with decoupled vibrating structures. In order to increase the sensitivity by exact tuning the resonant modes without instability problem, we devised two torsion springs for sensing mode in addition to four driving springs attached to outer gimbal. For the fabrication of the proposed gyroscope with high aspect ratio and wafer level vacuum packaging, we used the mixed micro-machining process which has the merits of a conventional surface and bulk micro-machining process. The measured vacuum level of the fabricated gyroscope was about 100 mTorr and the driving and sensing mode frequencies were tuned in 5 Hz. The gyroscope has the sensitivity of 0.013 deg./sec and bandwidth of 60 Hz. The output non-linearity is below than 2% in +100 deg./sec full scale.

Quality factor measurement of micro gyroscope structure according to vacuum level and desired Q-factor range package method

A micro-machined gyro chip of gyroscope is normally packaged in specific vacuum level to get the specific quality factor(Q-factor). If the Q-factor is too high, frequency tuning and the approximate matching between driving and sensing comb structure become difficult, and if the Q-factor is too low, its sensitivity decreases. The optimum Q-factor of our gyro chip design is 4000 range. To get this range, we measured the drive mode Q-factor as vacuum level of our gyro chip and we found that the vacuum level of the desired Q-factor 4000 is in the range of 740 mTorr. Based on this data, we fabricate the wafer level package gyro chip of the desired Q-factor by controlled the basic pressure of package bonding chamber just prior to the bonding process. After wafer level package process, we measured Q-factor of whole samples. Among 804 samples, 502 packaged gyro chips are worked and the Q-factor of 67% samples is between 3500 and 4500 range.

Extension of Surface/Bulk Micromachining: One-Mask Fabrication Technology Enabling the Integration of 6-DoF Inertial Sensors on a Single Wafer

In this paper, a new technology that enables the fabrication of single-crystal silicon microstructures capable of detecting the vertical motion without a bottom electrode is presented. The vertical-motion sensing is realized by fabricating a quasi-vertical comb electrode. The quasi-vertical comb electrode is fabricated by utilizing the RIE lag phenomenon. This vertical sensing scheme allows the integration of 6-DoF inertial sensors on a single wafer. The developed process for vertical electrodes is very simple and needs only one mask. Currently work is underway to test the performance of the fabricated inertial sensor.