Journal of Aerospace Engineering

Despite great advancements in thin-film growth and deposition techniques, determination of the residual stress and Young’s modulus for thin films has continued to be a challenge. The bulge test is a potentially powerful tool for characterizing these mechanical properties but is underutilized because of its sensitivity to experimental error. The bulge test is highly sensitive to sample and test preparation, accuracy of the deflection measurement, accuracy in the measurement of film dimensions, and selection of the correct equation to be used for extraction of the Young’s modulus and residual stress. In this study, effort has been made to consolidate the findings from various reports in the literature, and a thorough error analysis has been conducted. A discussion on the experimental technique used to extract Poisson’s ratio from the bulge test is also presented. Finally, the technique is used to determine the residual stress and Young’s modulus of 3C-SiC thin films grown on silicon.

Hydrogels have a large number of potential applications in microelectromechanical technology as sensors and actuators. In this paper we try to understand the physics of dissolvable hydrogels and investigate the various parameters controlling the dissolution process. Hydrogels, crosslinked via disulfide bonds, when immersed in a solution containing the disulfide cleaving agent can break covalent cross links causing the hydrogel to dissolve and thereby indicating the presence of the cleaving agent. Such a concept can be used to develop biochemical sensors, sacrificial structures in microfluidic systems, and other applications. The mechanism of the hydrogel dissolution process has been studied in detail and a mathematical model has been developed. From the vanishing time of the dissolvable hydrogel, a significant amount of qualitative and quantitative information about the solution can be obtained. A large number of factors governing the hydrogel dissolution process were investigated by simulations and experiments.

Microelectromechanical (MEMS) gyroscopes have wide-ranging applications including automotive and consumer electronics markets. Among them, complementary metal-oxide semiconductor-compatible MEMS gyroscopes enable integration of signal conditioning circuitry, multiaxis integration, small size, and low cost. This paper reviews various designs and fabrication processes for integrated MEMS gyroscopes and compares their performance. Operational principles and design issues of MEMS vibratory gyroscopes are also addressed.

Capacitive micromachined ultrasonic transducers (CMUTs), introduced about a decade ago, have been shown to be a good alternative to conventional piezoelectric transducers in various aspects, such as sensitivity, transduction efficiency, and bandwidth. In this paper, we discuss the principles of capacitive transducer operation that underlie these aspects. Many of the key features of capacitive ultrasonic transducers are enabled with micromachining technology. Micromachining allows us to miniaturize device dimensions and produce capacitive transducers that perform comparably to their piezoelectric counterparts. The fabrication process is described briefly, and the performance of the CMUT transducers is evaluated by demonstrating characterization results. It is shown that the transduction efficiency as defined by the electromechanical coupling coefficient can be close to unity with proper device design and operating voltage. It is also shown that CMUTs provide large bandwidth (123% fractional bandwidth) in immersion applications which translate into high temporal and axial resolution. Finally, the feasibility of using CMUTs is demonstrated by showing imaging examples in air and in immersion.

We discuss two types of micromachined flow sensors realized by using novel microfabrication processes—a hot-wire anemometer (based on thermal transfer) and a biologically inspired flow sensor (based on momentum transfer). Both sensors are enabled by a new, efficient three-dimensional assembly technique called the plastic deformation magnetic assembly method. The sensors can be packaged in high-density, two-dimensional arrays efficiently, with each sensor node capable of performing two-component or three-component flow sensing. We first discuss the development of new hot-wire anemometers (HWA). The HWA uses a thermal element (hot wire) that is made of Pt/Ni/Pt film with a measured temperature coefficient of resistance of 2,700 ppm/°C. The thermal element is elevated out of plane by using support beams made of polyimide, a polymer material. Both steady-state and transient characteristics of the sensor have been experimentally obtained. The second type of flow sensor is based on momentum transfer principles and inspired by fish lateral line sensors. Each sensor consists of a vertical cilium attached to a horizontal cantilever. Fluid flow imparts moment on the vertical cilium, and causes the horizontal cantilever to bend. The fabrication process and preliminary measurement data are presented.