Markus Lutz

Long-Term and Accelerated Life Testing of a Novel Single-Wafer Vacuum Encapsulation for MEMS Resonators

We have developed a single-wafer vacuum encapsulation for microelectromechanical systems (MEMS), using a thick (20-mum) polysilicon encapsulation to package micromechanical resonators in a pressure <1 Pa. The encapsulation is robust enough to withstand standard back-end processing steps, such as wafer dicing, die handling, and injection molding of plastic. We have continuously monitored the pressure of encapsulated resonators at ambient temperature for more than 10 000 h and have seen no measurable change of pressure inside the encapsulation. We have subjected packaged resonators to >600 cycles of -50 to 80degC, and no measurable change in cavity pressure was seen. We have also performed accelerated leakage tests by driving hydrogen gas in and out of the encapsulation at elevated temperature. Two results have come from these hydrogen diffusion tests. First, hydrogen diffusion rates through the encapsulation at temperatures 300-400degC have been determined. Second, the package was shown to withstand multiple temperature cycles between room and 300-400degC without showing any adverse affects. The high robustness and stability of the encapsulation can be attributed to the clean, high-temperature environment during the sealing process

Single wafer encapsulation of MEMS devices

Packaging of micro-electro-mechanical systems (MEMS) devices has proven to be costly and complex, and it has been a significant barrier to the commercialization of MEMS. We present a packaging solution applicable to several common MEMS devices, such as inertial sensors and micromechanical resonators. It involves deposition of a 20 /spl mu/m layer of epi-polysilicon over unreleased devices to act as a sealing cap, release of the encapsulated parts via an HF vapor release process, and a final seal of the parts in 7 mbar (700 Pa) vacuum. Two types of accelerometers, piezoresistive and capacitive sensing, were fabricated. Piezoresistive accelerometers with a footprint smaller than 3 mm/sup 2/ had a resolution of 10 /spl mu/g//spl radic/Hz at 250 Hz. Capacitive accelerometers with a 1 mm/sup 2/ footprint had a resolution of 1 mg/spl radic/Hz over its 5 kHz bandwidth. Resonators with a quality factor as high as 14,000 and resonant frequency from 50 kHz to 10 MHz have also been built. More than 100 capacitive accelerometers and 100 resonators were tested, and greater than 90% of the resonators and accelerometers were functional.

A precision yaw rate sensor in silicon micromachining

A new generation of production-ready yaw rate sensor, based on silicon micromachining, is presented. The sensor is designed for mass production and high performance applications. A combination of surface and bulk micromachining leads to an advantage in design, signal evaluation and packaging. This paper discusses the design of the sensing element: two bulk micromachined oscillating masses each of which supports two surface micromachined accelerometers for detection of the Coriolis force. Mechanical balancing of the sensor is avoided by implementation of a new dry etching process and precise photolithography. The electrodynamic actuation and high Q-value of the oscillator allow packaging at atmospheric pressure. Characterization results of the device are presented.

Impact of geometry on thermoelastic dissipation in micromechanical resonant beams

Thermoelastic dissipation (TED) is analyzed for complex geometries of micromechanical resonators, demonstrating the impact of resonator design (i.e., slots machined into flexural beams) on TED-limited quality factor. Zener first described TED for simple beams in 1937. This work extends beyond simple beams into arbitrary geometries, verifying simulations that completely capture the coupled physics that occur. Novel geometries of slots engineered at specific locations within the flexural resonator beams are utilized. These slots drastically affect the thermal-mechanical coupling and have an impact on the quality factor, providing resonators with quality factors higher than those predicted by simple Zener theory. The ideal location for maximum impact of slots is determined to be in regions of high strain. We have demonstrated the ability to predict and control the quality factor of micromechanical resonators limited by thermoelastic dissipation. This enables tuning of the quality factor by structure design without the need to scale its size, thus allowing for enhanced design optimization

Investigation of energy loss mechanisms in micromechanical resonators

Micromechanical resonators with resonant frequencies from 500 kHz to 10 MHz were built and examined for several energy loss mechanisms. Thermoelastic damping, clamping loss and air damping were considered. The devices were shown to be limited by thermoelastic damping, providing experimental verification of this phenomenon at the microscale. Resonators with scaled dimensions also matched well with scaling theory of damping at a given pressure. An energy loss mechanism other than thermoelastic dissipation, most likely clamping loss, was shown to be dominant for resonators whose ratio of length to width was less than 10:1. The devices were fabricated using a single-wafer encapsulation process.

Encapsulated submillimeter piezoresistive accelerometers

While micromachined accelerometers are widely available and used in various applications, some biomedical applications require extremely small dimensions (<mm) or mass (<mg) that cannot be fulfilled with commercially available accelerometers. In this work, we present a fully packaged piezoresistive accelerometer that has the smallest dimension (0.034mm/sup 3/) ever published. We achieve miniaturization by using a film encapsulation technique with a thick epitaxial polysilicon layer. This packaging technique enables the dimensions of the die to be only tens of microns larger than the micromechanical structure. We have fabricated accelerometers as small as 0.034mm/sup 3/ (387/spl mu/m/spl times/387 /spl mu/m/spl times/230/spl mu/m) with noise floor of 0.25mg//spl radic/Hz. These ultra-miniature motion sensors have potential opening up new frontiers in biomedical science and engineering.

Using MEMS to Build the Device and the Package

MEMS devices must be packaged to be used. Unfortunately, MEMS packages are challenging to develop, and the packaging of MEMS devices often dominates the cost of the product. In recent years, our group has worked with a team from Bosch to develop and demonstrate a novel wafer-scale encapsulation approach for MEMS. This process uses MEMS fabrication steps to build the device and the package at the same time. The main advantage of this approach is that the wafers emerge from the fabrication facility with all the fragile MEMS structures completely buried within the wafer, allowing all existing standard handling and packaging approaches, such as wafer-dicing, pick/place, and injection mold packaging to be used. This encapsulation process enables CMOS integration, embedding, and extreme miniaturization of complete systems. In this paper, we describe some advantages for performance, size and cost that can come from this approach.

Wafer Scale Encapsulation of MEMS Devices

MEMS packaging has always been a field of great importance since it can dominate the cost and size of a final working device. Considering this, we have concentrated on developing a wafer-scale encapsulation scheme which uses a thick epi-poly (epitaxially deposited poly silicon) layer as the sealing layer. This approach allows the use of conventional post processing, such as dicing, wire bonding, and other standard handling and mounting techniques. We also can minimize the chip area used for packaging, in some cases reducing the chip size by ×5 from what was required for silicon fusion bonded covers. This packaging scheme can be used for various MEMS devices and can be integrated with other electronics. This paper will discuss the packaging process and show some preliminary results.Copyright

The long path from MEMS resonators to timing products

Research on MEMS Resonators began over 50 years ago. In just the last 10 years, there has been a series of important technological developments, and (finally!) success at commercialization. The presentation will highlight some key milestones along this path, describe some of the critical technology steps, and outline some of the important non-technological events within SiTime - all of these factors contributed to the successful outcome.

Fully encapsulated sub-millimeter accelerometers

In this paper we present design, fabrication, and characterization results for the smallest published fully-packaged accelerometers. The miniaturization is realized by utilizing an advanced packaging scheme using a thick film epitaxial grown polysilicon encapsulation technique. Using this approach, released, encapsulated MEMS devices can be fabricated with exterior dimensions only 10s of microns larger than the micromechanical element. This advantage enables us to make accelerometers almost 2 orders of magnitude smaller than others. We have fabricated accelerometers as small as 0.034mm/sup 3/ (387/spl times/387/spl times/230/spl mu/m) with noise floor of 0.25mg/sqrt(Hz).