Chandradip Patel

Performance and reliability of mems gyroscopes at high temperatures

This paper reports the variation in the noise and signal output of a 3-axis MEMS gyroscope that results from physical degradation of the MEMS gyroscope sensor or its packaging caused by long time exposure to wide temperature cycling to high temperatures. Three different 3-axis MEMS gyroscopes were used for this analysis. This study was accomplished by measuring the signal and noise variation at stationary and rotary conditions. Both stationary and rotary tests were conducted at room temperature, and angular velocity was measured in °/s. For the rotary test measurement, the 3-axis MEMS gyroscope was placed on a precise rotary table and rotated at 60°/s (10 rpm), 120°/s (20 rpm) and 240°/s (40 rpm) for five minutes each. These MEMS gyroscopes were then subjected to thermal cycling from −25°C to 125°C for 100 hours (100 cycles).This process was repeated five times for a total of 500 hours of thermal cycling. The same stationary and rotary test measurements were conducted after every 100 hours of thermal cycling exposure to 500 hours. A permanent change in the signal was measured, which is discussed in this paper.

Modeling and simulation of the MEMS vibratory gyroscope

With advances in fabrication technologies lowering cost, MEMS gyroscopes are being used in an ever wider variety of applications, including those requiring operation at/or beyond the manufacturers recommended temperature range. In these high temperature applications, such as deep water energy exploration and down-hole drilling, extensive lab testing is required to assess the effects of temperature on the response of a MEMS gyroscope. The objective of this paper is to develop a method to simulate the behavior of a MEMS vibratory gyroscope at various temperature conditions. The MEMS vibratory gyroscope is a two degree-of-freedom spring-mass-damper system. With known values of mass, spring stiffness and damping coefficient in the drive and sense direction, the characteristic equations of motion of the MEMS vibratory gyroscope can be solved using the first order approach developed in this paper. It is also shown, by comparing simulation results with experimental results, that this approach can accurately simulate the temperature-dependent characteristics of a MEMS vibratory gyroscope.

Decapsulation of Plastic Encapsulated Microelectronics with Copper Wire Bonds

Abstract A novel method for copper wire bond decapsulation was developed, which involves conducting a sequential process to ensure minimal corrosion or, in some cases, no corrosion/degradation of the copper wire. It requires a pre-treatment which involves mechanical milling or acid etching; the main decapsulation process, utilizing a mixture of nitric and sulfuric acid under electrically biased conditions; and a post decapsulation procedure of encapsulant debris removal using a solvent. Previous studies use decapsulation methods that reveal either the ball bond or the wedge bond, separately, and thus cannot be used to perform wire pull tests. Exposing both the ball and wedge bond is not trivial, as the wire tends to break at the loop due to the long exposure time. The three-step process, when properly executed, ensures that the entire wire along with both ball and wedge bonds are exposed with minimal damage. Five parameters were found to affect the decapsulation process. These are as follows: acid ratio, ...

Performance Degradation of the MEMS Vibratory Gyroscope in Harsh Environments

The use of MEMS gyroscopes in a wide range of applications requiring then to function from medium to harsh environments make it necessary to evaluate the performance of MEMS gyroscopes under those conditions. This paper focuses on the effects of elevated temperature and humidity on the performance of MEMS vibratory gyroscopes. Performance of the MEMS gyroscope was evaluated by conducting Highly Accelerated Stress Testing (HAST) on a COTS (commercial-off-the-shelf) single axis MEMS vibratory gyroscope having an operating temperature range from −40C to +105C. The gyroscope sensors were exposed to 130°C and 85% relative humidity with a pressure of 33.3 psia or 230 kPa for 96 hours. Pre-baking and post-baking tests were conducted before and after HAST at 125C for 24 hours respectively. Also, stationary baseline testing (SBT) and rotary baseline testing (RBT) were performed before and after the pre-baking, HAST and post-baking tests to measure any permanent shift during the respective test. A preliminary result shows that the MEMS gyroscope output degraded in the pre-baking test and HAST; while it showed a recovery in post-baking test. After completing the entire test procedure, it was observed that MEMS gyroscope output didn’t come back to the original position, and resulted in a permanent output shift of 1.85deg/s.Copyright

Combined Temperature and Humidity Effects on MEMS Vibratory Gyroscope Sensor

Reliability and long term stability are the greatest challenges for commercialization of MEMS gyroscopes. Their vast use in different applications that required MEMS gyroscopes to function from medium to harsh environments make necessary to evaluate the performance of MEMS gyroscope under those conditions. This paper focuses on the combined long term effects of temperature and humidity on the performance of MEMS vibratory gyroscope. Performance of the MEMS gyroscope was evaluated over time by conducting temperature humidity bias (THB) test on a COTS (commercial off-the-shelf) single axis MEMS vibratory gyroscope having an operating temperature range from −40°C to +85°C. The gyroscope sensors were exposed to 60°C and 90%RH (Relative Humidity) for 500 hours. Six single axis gyroscopes were tested, three with in-situ device calibration and three without in-situ device calibration. Out of three MEMS vibratory gyroscopes tested without in-situ device calibration, it was observed that samples had minimum and maximum in-situ zero rate output (ZRO) drift of 1.3°/s and 2.2°/s respectively over 500 hours. These drifts were disappeared when gyroscope sensors were tested after six months by keeping at room condition. Other three single axis gyroscopes were tested in the same chamber with in-situ device calibration which didn’t show any major performance ZRO drift.Copyright