This paper proposes and verifies a structure for the isolation of packaging stress in silicon-on-insulator-based microelectromechanical systems devices. The packaging-stress isolation structure resides on the handle layer and consists of a circular disk, eight elastic beams, and a support frame. The disk is located in the center of the die and occupies less than 5% of the handle-layer area; this can reduce packaging stress and avoid uneven stress distribution. The elastic beams are L-shaped and symmetrically distributed to decouple the deformation from the disk to the frame and suppress the stress evenly. The in-plane and out-of-plane deformation induced by packaging stress was modeled and experimentally measured. The comparison results demonstrate that the packaging stress was successfully isolated.

Design and Verification of a Structure for Isolating Packaging Stress in SOI MEMS Devices

This paper describes the design, simulation, and fabrication of a dual-axis gyroscope, whose working principle is based on the thermal convective and thermoresistive effects in lightly doped p-type silicon. The sensor configuration consists of a piezoelectric pump and a microthermal sensing element that is packaged in an aluminum case with a diameter of 14 mm and a length of 25 mm. The novel structure of the sensing element reduces the thermal-induced stress up to 89% as compared with the previous design. The sensor has been fabricated by micro- electromechanical systems technology, and completely packaged and characterized. The measured sensitivities of the gyroscope for the X-axis and Y-axis were 0.082 and 0.078 mV/deg/s, respectively. The cross sensitivities between the two input axes were less than 0.26%, and the nonlinearity was smaller than 0.5% full scale in the range of plusmn200deg/s. The resolution was 0.2deg/s at a measurement frequency of 1 Hz. The noise equivalent rate was 0.18deg/s/radicHz, which is equivalent to an angle random walk of 10.8deg/radich in a 65-Hz bandwidth. The offset drift was 360deg/h in 12-h measurement.

Development of a Dual-Axis Convective Gyroscope With Low Thermal-Induced Stress Sensing Element

In this paper, we present the simulation and fabrication of the gas gyroscope. The gas flow inside the hermetically packed sensor is simulated by utilizing 3-D transient compressible flow analysis. The pump working principle and the effect of the Coriolis acceleration on the laminar jet are validated by both analytical formulas and experiments. The sensor utilizes a new sensing element consisting of a thermistor heated by an interior heater, which is independently power-supplied. The sensor performance can be adjusted by the applied voltage on the heater. Both heater and thermistor are optimized in terms of thermal stress. The effect of thermal stress in a p-type silicon thermistor reduces the performance of sensor by 9.5%. The sensor has been calibrated and the role of the heater is verified.

Simulation and Fabrication of a Convective Gyroscope

MEMS (Micro-Electro-Mechanical Systems) have been used widely in almost all the major industries, while few companies have commercial products in MEMS gyroscope. Besides, there are not many papers on the sensor design, and the papers regarding the packaging for gyroscopes are few, partly due to its highly proprietary nature [1]. The fabrication imperfections are unavoidable during MEMS gyroscope manufacture process. In this paper, Monte Carlo method is used for the simulation of the width of the driving spring defects in vibratory MEMS gyroscope. The yield of MEMS gyroscope is estimated based on the simulation results. For a specific structure of gyroscope, the optimal value of the width of the driving spring to maintain high yield and high performance was found through the yield simulation analysis and structural optimization. There are so many factors that may cause a low yield, and it is critical to determine which factors are most critical. Sensitivity has been analyzed for the driving comb structure by Monte Carlo method. The simulation result shows that gap is the significant factor effecting driving force and the finger tilting angle is the most important factor producing the “redundancy force”, which is undesirable in industry. The two factors must be well designed and processed to improve the yield of the MEMS gyroscope.

MEMS gyroscope yield simulation based on Monte Carlo method

This paper describes design guidance for a compensation method of spring width loss during silicon deep RIE (reactive ion etch) process. We found that a compensation factor (CF), defined by circumference (C) of unit etch hole divided by its area (A), is directly related to the variation of resonant frequencies of silicon MEMS resonators. We proposed 5 models with different etch hole shapes and investigated the effect of etch holes on resonant frequencies. The model with proposed etch holes showed three fold smaller variation of resonant frequencies than other models with conventional etch holes.

Engineering design guide for etch holes to compensate spring width loss for reliable resonant frequencies

MEMS devices such as a vibratory gyroscope often suffer from a lower yield rate due to fabrication errors and the 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 SOI (Silicon-On-Insulator) wafer and packaged using the 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 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.

Lee Moon Chul

Papers

A High Yield Rate MEMS Gyroscope with a Packaged SiOG Process