Jay Mitchell

An Improved Performance Poly-Si Pirani Vacuum Gauge Using Heat-Distributing Structural Supports

A new micro-Pirani gauge has been designed and fabricated that employs a ladder-shape structure with two parallel bridges and cross-links in between. This design enhances the physical performance of the gauge by increasing structural rigidity, thus allowing for longer beams and a wider selection of materials and by allowing for better heat distribution across the sensor -therefore improving the full scale range of device functionality. Using a 4 /spl times/ 2 /spl times/ 1000 /spl mu/m polysilicon Pirani gauge with the ladder structure, pressures below 0.01 Torr were resolved. The measurement accuracy was the limiting factor in Pirani gauge performance and was roughly 5% of the measured pressure in the 10 to 0.1 Torr range.

Encapsulation of vacuum sensors in a wafer level package using a gold-silicon eutectic

A vacuum package based on gold-silicon eutectic wafer bonding has been developed and evaluated using high sensitivity poly-Si Pirani vacuum sensors. Encapsulation of the devices was achieved by bonding a silicon cap wafer to a device wafer using a Au-Si eutectic solder at or above 390/spl deg/C in a vacuum bonder. The Au-Si eutectic solder encircled the devices, providing an airtight seal. Strong bonds could be achieved using /spl ges/2.5 /spl mu/m of gold on the cap wafer, to bond to either a poly-Si film of 0.5 /spl mu/m or less, or a gold thin film patterned to the same dimensions as the cap wafer gold. Pirani gauges were encapsulated and tested over several months. Devices packaged without getters produced initial pressures from 2 to 12 torr with initial leak/outgassing rates of -0.073 to 80 torr/year. Devices packaged with Nanogetters/spl trade/ provided by ISSYS produced pressures as low as 5 mtorr with leak/outgassing rates of <10 mtorr/year.

A detailed study of yield and reliability for vacuum packages fabricated in a wafer-level Au-Si eutectic bonding process

An Au-Si eutectic wafer-level bonding process was developed for low-temperature vacuum packaging of MEMS devices. Using Au-Si eutectic bonding, devices were encapsulated by bonding a silicon cap wafer to a device wafer. Micromachined Pirani vacuum sensors were encapsulated in order to characterize the packaged pressures. These packages had cavity dimensions of 2.3×2.3 mm with a depth of 90 µm. Yields of 84.6% and 94.1% were achieved in packages with bond ring widths of 100 and 150 µm. With the use of getters and a pre-bond outgassing step, pressures from ≪3.7 to 23.3 mTorr were achieved. Furthermore, pressures were shown to remain stable to within ±2.5 mTorr for over 4 years of testing.

Wafer-level vacuum/hermetic packaging technologies for MEMS

An overview of wafer-level packaging technologies developed at the University of Michigan is presented. Two sets of packaging technologies are discussed: (i) a low temperature wafer-level packaging processes for vacuum/hermeticity sealing, and (ii) an environmentally resistant packaging (ERP) technology for thermal and mechanical control as well as vacuum packaging. The low temperature wafer-level encapsulation processes are implemented using solder bond rings which are first patterned on a cap wafer and then mated with a device wafer in order to encircle and encapsulate the device at temperatures ranging from 200 to 390 °C. Vacuum levels below 10 mTorr were achieved with yields in an optimized process of better than 90%. Pressures were monitored for more than 4 years yielding important information on reliability and process control. The ERP adopts an environment isolation platform in the packaging substrate. The isolation platform is designed to provide low power oven-control, vibration isolation and shock protection. It involves batch flip-chip assembly of a MEMS device onto the isolation platform wafer. The MEMS device and isolation structure are encapsulated at the wafer-level by another substrate with vertical feedthroughs for vacuum/hermetic sealing and electrical signal connections. This technology was developed for high performance gyroscopes, but can be applied to any type of MEMS device.

An improved performance poly Si Pirani vacuum gauge using heat distributing structural supports

A new micro-Pirani vacuum gauge that employs a ladder-shaped structure with two parallel bridges and crosslinks in between has been designed and fabricated. This design enhances the physical performance of the gauge by increasing structural rigidity, thus allowing for longer beams and a wider selection of materials, and by allowing for better heat distribution across the sensor - therefore improving the full-scale range of sensor response. Furthermore, this Pirani gauge can be fabricated in a one-, two-, or three-mask process without postprocessing steps such as KOH etching. In a CMOS-compatible process, poly-Si 4 times 2 times 250-mum and 4 times 2 times 1000-mum Pirani gauges with the ladder structure were fabricated and tested with pressure ranges from 10-3 to 50 torr (0.133 to 1 times 103 Pa) and 5 times 10-2 to 760 torr (6.67 to 1.01 times 105-Pa atmospheric pressure) and with resolutions of approximately 10-3 and 5 times 10-2 (0.133 to 6.67 Pa), respectively. Constant temperature circuitry and thermoelectric temperature stabilization would further extend the range of operation and the resolution of these devices. Furthermore, these sensors operate at very low powers ranging from 300 to 600 muW depending on their geometry and pressure measurement range.

1.09 – Wafer Bonding

This chapter reviews different wafer bonding techniques and discusses their advantages and disadvantages. After providing a brief review of applications of wafer bonding technologies, requirements and desirable characteristics for these technologies are listed. Wafer bonding technologies are discussed under three main categories: direct wafer bonding (DWB), mediated wafer bonding (MWB), and specialized techniques developed for localized and selective heating of wafers. DWB techniques reviewed in this chapter are field-assisted anodic bonding, fusion bonding (including hydrophobic and hydrophilic, high and low temperature, and vacuum- and plasma-assisted), and chemical assisted bonding of glass surfaces. MWB techniques reviewed are anodic bonding using deposited films, thermocompression, solder and eutectic, bonding using glass films, and polymer-assisted wafer bonding. Finally, selective wafer heating techniques reviewed include localized resistive heating, radio frequency (RF) and microwave and electromagnetic heating, ultrasonic bonding, rapid thermal processing (RTP), and laser-assisted bonding. This chapter compares these technologies and ends with concluding remarks.

±2ppm frequency drift and 300x reduction of bias drift of commercial 6-axis inertial measurement units using a low-power oven-control micro platform

The performance of a commercial 6-axis (3-axis accelerometer and 3-axis gyroscope) MEMS inertial measurement unit (IMU) has been improved by a factor of >300x by utilizing a low-power ovenized microsystem. The IMU is thermally isolated from the external ambient by mounting it on a custom-designed micro-machined glass platform and packaging it in vacuum. In the present study, a microcontroller and voltage-controlled current source are assembled together with the thermally-isolated packaged IMU on a printed circuit board. The entire system is thermal-cycled over a temperature span from -40°C to 85°C. Both temperature control (i.e., ovenization) and compensation are used to reduce bias drift due to temperature change. The measured frequency drift of the IMU is improved by a factor of 950x and stabilized to ±2ppm, and the bias drift of the IMU is reduced to 60 °/hr for one of the gyroscope axes, and 1.7 mg for one of the accelerometer axes.

Miniaturized digital oven-control microsystem with high power efficiency and ±1.8ppm frequency drift

This paper introduces a digital pulse width modulation technique for MEMS oven-control system. The proposed method generates digital, pulse width modulated signals from analog signals produced by a proportional-integral-derivative controller. This technique, therefore, provides good digital properties, such as better design scalability and higher power efficiency. These advantages are confirmed with an oven-control system for a commercial 6-axis inertial measurement unit. As a result, the oven-control system is fabricated on a single printed circuit board measuring 42mm×42mm×10mm and the power efficiency is increased to 94.4%. The entire system shows ±1.8ppm frequency drift during a thermal-cycle test, from -40°C to 85°C with the rate of the change 1°C/min.

Localized back-side heating for low-temperature wafer-level bonding

A new wafer-level method has been developed for localized heating of the bond region between two wafers. Using this method, one of the two wafers to be bonded is heated from the backside, and the other is cooled from the backside, so that heat flows through the bond regions while the device regions stay relatively cool. In this work, integrated temperature sensors were used to measure the temperature at different distances from the bond region during Si to glass and Si to Si (with a -7 mum SiO2) bonds in order to verify the utility of this bonding technique. The temperature was measured to be only 25% and 37% of the bond ring temperature at 650 mum away from the bond ring for a Si to glass bond and 250 mum away from the bond ring for a Si to Si (with a ~7 mum oxide) bond respectively for bond ring temperatures up to 410degC and 200degC. Furthermore a successful Au-Si eutectic bond was demonstrated using this technique.

A Micro Oven-Control System for Inertial Sensors

This paper presents a modular and generic micromachined oven-control system for use with miniature micro-electro-mechanical system (MEMS) transducers. The micro-oven-controlled off-the-shelf commercial six-axis MEMS inertial measurement unit (IMU), Invensense MPU-6050, provides the lowest reported temperature-induced root of sum of squares bias errors of 62.71°/h and 1.920 mg from −40 °C to 85 °C for three-axis gyroscopes and three-axis accelerometers, respectively. The micro-oven control system provides thermal isolation from the surrounding environment using a micro-machined isolation platform, vacuum-sealing, and a metal package. In addition, a CMOS temperature sensor, a proportional–integral–derivative-based temperature control scheme, and least mean square and random forest compensation algorithms are utilized to reduce temperature-induced bias drifts of IMUs. The most stable axes achieve peak-to-peak bias drifts of 12.78°/h and 665.2 ug during a thermal-cycle test for gyroscopes and accelerometers, respectively. The oven’s heater power consumption is <125mW at the lowest temperature, −40 °C. This oven-control system can be applied to a wide range of MEMS sensors to reduce performance degradation due to temperature variation. [2016-0233]