Henry G. Hughes

Review of viscous damping in micromachined structures

Damping in a vibratory mechanical device plays an important role in modulating the response of the system. It is of critical importance to understand the nature of damping and to be able to effectively control it for micro-machined devices such as sensors or actuators. For example, if damping is too low in a micro-machined lateral accelerometer, the severe degree of resonance of the accelerometer upon an impact of external force may produce such a large signal that cripples its control circuitry resulting in total system failure. High damping (near critical) is generally desired for accelerometers. As for yaw rate gyroscopic sensors, on the other hand, low damping is required in order to achieve sufficient sensitivity of the system under a given driving force and for certain types of applications. Therefore, in designing a MEMS device, the consideration of damping must be taken into account at the earliest stage. Micro-Electro-Mechanical-System (MEMS) devices are often operated in an isolated environment filled with nitrogen or other types of gas such that the gas functions as a working fluid and dissipates energy. A gas film between two closely spaced parallel plates oscillating in normal relative motion generates a force, due to compression and internal friction, which opposes the motion of the plates. The damping, related to energy loss of the system, due to such a force is referred to as squeeze film damping. In other cases, two closely spaced parallel plates oscillate in a direction parallel to each other, and the damping generated by a gas film in this situation is referred to as shear damping. Under the small motion assumption, both flow induced force is linearly proportional to the displacement and velocity of the moving plates. The coefficient of the velocity is the damping coefficient. The review will start with a discussion on effective viscosity coefficients that were used in some of the early work by T. Veijola et al., M. Andrews et al. and others. In general, the capping pressure for a micro-machined system is below or much below the atmospheric pressure. As pressure decreases, the mean free path of the gas molecules ( nitrogen for example) increases. When the mean free path is comparable to the air gap between two plates, one may no longer be able to treat the gas as continuum. Therefore, an effective viscosity coefficient is introduced such that governing equations of motion for fluid at relatively high pressures can still be used to treat fluid motion at low pressures where the mean free path is comparable or even larger than the air gap of the plates

The package integration of RF-MEMS switch and control IC for wireless applications

The integration of microelectromechanical systems (MEMS) switch and control integrated circuit (IC) in a single package was developed for use in next-generation portable wireless systems. This packaged radio-frequency (RF) MEMS switch exhibits an insertion loss under -0.4 dB, and isolation greater than -45 dB. This MEMS switch technology has significantly better RF characteristics than conventional PIN diodes or field effect transistor (FET) switches and consumes less power. The RF MEMS switch chip has been integrated with a high voltage charge pump plus control logic chips into a single package to accommodate the low voltage requirements in portable wireless applications. This paper discusses the package assembly process and critical parameters for integration of MEMS devices and bi-complementary metal oxide semiconductor (CMOS) control integrated circuit (IC) into a single package.

Characterization of glass on electronics in MEMS

Microelectromechanical systems (MEMS) have been around for many years. However, reliability issues, increasing costs, and die sizes are pushing the technology beyond its current capabilities. Integrating a micromachined sensor with its control circuitry on a single piece of silicon offers a cost and a performance advantage over the conventional two chip sensor. The enhancements offered by an integrated MEMS device are leading to many new challenges. The ability to encapsulate the MEMS device without affecting the integrated circuit is a key concern. One method of hermetically sealing the MEMS uses a frit glass which can cause potential damage to the integrated circuit because of the sealing parameters used. In standard CMOS processing, the integrated circuit is not subjected to high temperatures once the devices are built, whereas in wafer level packaging, high temperatures are involved. The high temperatures and the glass composition associated with the sensor capping process could be detrimental to these devices. A test vehicle was developed, therefore, to evaluate the compatibility of the CMOS and the sensor capping processes. The electrical results suggest that the glass and bonding process do not degrade the transistor performance.

Chemical gas sensors on silicon

Gas sensors have been used and available for many years. While there are many different types of gas sensors and detecting materials, metal oxides are commonly used for the detection of various gases, such as carbon monoxide (CO), methane (CH4), and other hydrocarbons. Such sensors are used in environmental, commercial HVAC, home, and industrial applications. One of these metal oxides, tin oxide, is very stable in air at temperatures > 500 degrees C, and is readily processed using ceramic or thin-film techniques. The thin films on silicon can be doped with inpurities using standard diffusion or spin-on methods, which influence the sensitivity, selectivity, and stability of the sensors. Thin- film tin oxide on silicon CO gas sensors have been characterized. The structure, chemistry, and response results are reported.

Integration of SAW RF Rx filter stacked on a transceiver chip in a QFN package

Enhanced integration of mobile phone components is driven by demands for reduced form factor and cost. Because SAW filters must be fabricated on piezoelectric substrates, they are difficult to monolithically integrate on semiconductor chips. Here we report on the integration of a compact wafer-scale packaged SAW filter stacked over a transceiver chip in a quad flat-pack no-lead (QFN) package. An integrated passive device interposer provided redistribution and matching. We demonstrated the successful integration of both EGSM and DCS filters in such modules. SAW compact models based on the coupling of modes model were developed to facilitate system design.

Some Mesomorphic Properties of N-(p-Azidobenzylidene) — Anilines

Synthesis of several p-substituted alkyl and alkoxy azidobenzylidene anilines was carried out to study their mesomorphic transition temperatures and the affect made upon these transitions by the linear azide moiety.