The fracture of brittle MEMS materials is often characterized by ultimate strength measures such as the maximum stress or strain in an element at failure. It has been known for many decades that a better way to characterize the strength of a brittle material on the macro-scale is to make use of statistical measures. This is due to the nature of brittle materials in which failure occurs when a critically sized flaw exists in the region that is under tensile stress. The distribution of flaws is often random, so the strength of a brittle material can only be properly characterized by statistical measures. Working with MEMS devices, where the site scale is small, it becomes even more important to use a statistical approach. Doing so can explain two observed effects. First, there is an apparent size effect on the strength of the material. The larger the structure that is under a given stress, the larger the region where a critically sized flaw may exist, resulting a higher probability of failure. Second, two identical beams with different stress states, loaded to the same maximum stress can have dramatically different average strengths. In this paper, Weibull statistics are used to characterize the strength of one MEMS material-- polycrystalline silicon. The relevant statistical measures are obtained from the fracture of a large number of cantilever beams. It is shown that, for this material, the average failure strength of a beam loaded in uniaxial tension should be on the order of 40% lower than the average strength of identical beams loaded in cantilever bending.

Failure analysis tools have been applied to analyze failing polysilicon microengines. These devices were stressed to failure under accelerated conditions in both oxidizing and non-oxidizing environments. The dominant failure mechanism of these microengines was identified as wear of rubbing surfaces. This often results in either seized microengines or microengines with broken pin joints. Analysis of these failed polysilicon devices found that wear debris was produced in both oxidizing and non-oxidizing environments. By varying the relative percent humidity (%RH), we observed an increase in the amount of wear debris with decreasing humidity. Plan view imaging under scanning electron microscopy revealed build-up of wear debris on the surface of microengines. Focused ion beam (FIB) cross sections revealed the location and build-up of wear debris within the microengine. Seized regions were also observed in the pin joint area using FIB processing. By using transmission electron microscopy in conjunction with energy dispersive x- ray spectroscopy and electron energy loss spectroscopy, we were able to identify wear debris produced in low (1.8% RH, medium and high (39% RH) humidities.

A study to evaluate three processes used for the release of standard devices produced by MCNC using the MUMPS process was undertaken by Jet Propulsion Laboratory with the collaboration of The Aerospace Corporation, and Polytec PI. The processes used were developed at various laboratories and are commonly the final step in the production of micro- electro-mechanical systems prior to packaging. It is at this stage of the process when the devices become extremely delicate and are subject to yield losses due to handling errors or the phenomenon of stiction. The effects of post processing with HF on gain boundaries and subsequent thermal processing producing native oxide growth during packaging will require further investigation.

A critical step in surface micromachining of microelectromechanical systems (MEMS) is the process that releases, cleans, and dries the flexible structures that are crucial to MEMS functionality. Standard release methods employed today can leave residue particles and can cause sticking because of surface tension. Aggressive design requirements, liquid processing, packaging, handling, transportation, and device operation etc., can contribute to device failure due to stiction. The use of supercritical carbon dioxide has been proven in various industries to achieve ultra-clean surfaces. Recent critical research studies by academia, research laboratories and industry have shown that supercritical carbon dioxide can be successfully used to alleviate the stiction problem and provide a clean and dry surface. The absence of surface tension in the supercritical phase of a fluid provides an excellent means to overcome stiction. The advantages of supercritical carbon dioxide include its relatively low critical temperature and pressure, its high diffusivity, low surface tension, and environmentally friendly (non-ozone depleting, non- hazardous). This paper reviews the stiction problem for MEMS, and the application of critical point drying for MEMS technology.

In micro-electromechanical system (MEMS) such as sensors and actuators, thin film has been widely used as microstructural materials. MEMS materials have differences with bulk in terms of mechanical reliability. So, the electrostatically actuated test structure is presented to measure the micromechanical reliability of micromaterials as thin films forming the microactuators. The designed test structures is fabricated by using the surface micromachining processes and driven by the electrostatic force. The sharp notch in the test structure is introduced by controlling the etching condition. The displacement, deflection and curvature of free standing beam in the electrostatic test structure under the electrostatic force is analyzed on the basis of the beam bending theory. From these results, we can predict the driving characteristics and the micromechanical reliability of microactuator materials under the operating condition.

The M beam device described herein is a surface mount accelerometer. This device is built from commonly available piezo material using a novel packaging approach that resolves many of the issues that make competing accelerometers expensive. The concept includes separating the piezo material into active and inactive areas by sawing the piezo material into an M shape. The two outer legs are used for package connection (inactive area) and the inner leg is free to move as a piezoelectric beam (active area). The design equations for M beam characteristics are presented.

Many MEMs systems require controlled atmospheres or vacuum for successful operation. These atmospheres or vacuum must first be obtained and then maintained against degradation over the lifetime of the device. Vacuum and controlled atmospheres decay in quality over time due to various mechanisms including poor processing and outgassing. At the beginning of the MEMs era many researchers felt that these systems were immune to these degradation mechanisms due to their small sizes. As the technology has developed and moved in the application phase many groups have run into lifetime issues when they have tried to package their systems. Investigation demonstrated that the same problems that large-scale static vacuum systems suffer apply in the MEMs arena as well. The key to controlling this process of degradation is good processes and process control during the manufacturing phase and the introduction of gettering materials into the package to deal with long term outgassing of harmful species. The key steps of a successful generic process inclusive of getter use and activation will be discussed.

During the last decade, research and development of microelectromechanical systems (MEMS) has shown a significant promise for a variety of commercial applications including automobile and medical purposes. For example, accelerometers are widely used for air bag in automobile and pressure sensors for various industrial applications. Some of the MEMS devices have potential to become the commercial- off-the-shelf (COTS) components. While high reliability applications including aerospace require much more sophisticated technology development, they would achieve significant cost savings if they could utilize COTS components in their systems. This paper reviews the current status of MEMS packaging technology from COTS to specific application provides lessons learned, and finally, identifies a need for a systematic approach for this purpose.

Film stresses in doubly-supported bi-layer micro-ribbons have been determined by measurement of their resonant vibration frequency. The composite aluminum/silicon-nitride ribbons constitute a programmable diffraction grating in which ribbon stresses play an important role in device behavior. To determine the intrinsic film stresses in each layer, special ribbon arrays were fabricated in which the coverage of aluminum was systematically varied from end toward the center of each ribbon. With increasing aluminum coverage, the ribbons exhibit a characteristic roll-off in resonant frequency which can be used to refine longitudinal film stresses. An analytical expression describing resonant frequency as a function of partial layer coverage is derived. By least-squares fitting this function to the observed roll-off, the film stresses in each layer are determined. It is found that the silicon-nitride stresses are large and tensile (approximately 800 MPa), while those in the aluminum are small and compressive (approximately -100 MPa).; This paper reviews the relevant theory behind the approach, demonstrates its application to wafer- level data and discusses its repeatability and accuracy.

This paper describes an investigation into the development of a LIGA optical energy interrupter for use in a MEMS-based Safety and Arming (S&A) system. The energy interrupter acts as a switch to selectively couple optical power between input and output fiber optic waveguides using either a reflectance or direct coupling technique. This device is a critical component of the S&A system, which must perform its task in a zero-tolerance manner to ensure reliable and safe operation of naval ordnance. The union of fiber optic components with LIGA-based MEMS structures allows development of an energy interruption junction that will isolate the low and high voltage sections of an optical charging circuit. A movable LIGA barrier interacts with optical fibers in order to align or misalign the optical propagation path between the source and receiver in the circuit. The implementation of such a design allows for the `Safe' operational mode of an S&A system with the optic path broken, and an `Arm' mode during energy transfer. Effects such as fiber misalignment, light scattering, and multiple reflections are sources of potential failure modes for this micromachined optical system. This paper is concerned with the effects of fiber misalignment of system reliability. Three primary fiber/barrier topologies are discussed. Computer simulations used to establish the positioning and geometric tolerances in the MEMS structure to ensure maximum functional reliability are described, and experimental results are presented.

The development of a miniature underwater weapon safety and arming system requires reliable chip-to-chip bonding of die that contain microelectromechanical actuators and sensors fabricated using a LIGA MEMS fabrication process. Chip-to- chip bonding is associated for several different bond materials (indium solder, thermoplastic paste, thermoplastic film and epoxy film), and bonding configurations (with an alloy 42 spacer, silicon to ceramic, and silicon to silicon). Metrology using acoustic micro imaging has been developed to determine the fraction of delamination of samples.

As the design of Micro-Electro-Mechanical Systems (MEMS) devices matures and their application extends to critical areas, the issues of reliability and long-term survivability become increasingly important. This paper reviews some general approaches to addressing the reliability and qualification of MEMS devices for space applications. The failure modes associated with different types of MEMS devices that are likely to occur, not only under normal terrestrial operations, but also those that are encountered in the harsh environments of space, will be identified.

This paper provides an overview of high-aspect-ratio CMOS micromachining, focusing on materials characterization, reliability, and fault analysis. Composite microstrutural beam widths and gaps down to 1.2 micrometers are etched out of conventional CMOS dielectric, aluminum, and gate-polysilicon thin films using post-CMOS dry etching for both structural sidewall definition and for release from the substrate. Differences in stress between the multiple metal and dielectric layers cause vertical stress gradients and curl, while misalignment between layers causes lateral stress gradients and curl. Cracking is induced in a resonant fatigue structures at 620 MPa of repetitive stress after over 50 million cycles. Beams have withstood over 1.3 billion cycles at 124 MPa stress levels induced by electrostatic actuation. Failures due to process defects are classified according to the geometrical features of the defective structures. Relative probability of occurrence of each defect type is extracted from the process simulation results.

The reliability of a MEMS-based system depends on the producibility and environmental stability of the MEMS components as well as other components within the system. This article will focus on MEMS components in a sacrificial LIGA MEMS-based Safety & Arming (S&A) system. The process monitoring and quality assurance methods under development for LIGA MEMS S&A components are presented. These methods include techniques to study: (1) process bias, (2) material microstructure and mechanical properties, (3) mechanical response of spring-supported structures, and (4) actuator performance. Characterization of the as-produced components and materials serve as the starting point for future studies of reliability of LIGA MEMS components and systems. The utilization of several process monitoring and quality assurance methods in future reliability studies is discussed.

Compactness, complexity of the interconnections and specific packaging, which are characteristics of Microsystems (MEMS), rule out the use of statistical procedure to assess reliability in space applications. Predictable reliability is the method recommended in this paper that uses a similar approach as CALCE already did for hybrid and microelectronic circuits. This method based on a failure mechanism approach is recalled at first and an example to illustrate this procedure based on the evolution of material crystal properties under radiation is presented.

Characterization tools have been developed to study the performance characteristics and reliability of surface micromachined actuators. These tools include: (1) the ability to electrically stimulate or stress the actuator, (2) the capability to visually inspect the devices in operation, (3) a method for capturing operational information, and (4) a method to extract performance characteristics from the operational information. Additionally, a novel test structure has been developed to measure electrostatic forces developed by a comb drive actuator.

The safe, secure and reliable application of microelectromechanical systems (MEMS) devices requires knowledge about the distribution in material and mechanical properties of the small-scale structures. A new testing program at Sandia is quantifying the strength distribution using polysilicon samples that reflect the dimensions of critical MEMS components. The strength of polysilicon fabricated at Sandia's Microelectronic Development Laboratory was successfully measured using samples 2.5 microns thick, 1.7 microns wide with lengths between 15 and 25 microns. These tensile specimens have a freely moving hub on one end that anchors the sample to the silicon die and allows free rotation. Each sample is loaded in uniaxial tension by pulling laterally with a flat tipped diamond in a computer-controlled nanoindenter. The stress-strain curve is calculated using the specimen cross section and gage length dimensions verified by measuring against a standard in the SEM.

Long-term reliability of MEMS devices is increasingly important for large scale manufactured products. Traditional reliability testing used for microelectronic devices has been applied to integrated MEMS devices with the expectation that acceleration of MEMS-specific failure mechanisms would not be substantial. Rather, traditional package and circuit related failures would be accelerated. In addition, reliability tests that impart mechanical stresses on parts were expected to accelerate failures more so than traditional tests. It was found that while mechanical stresses were more effective in inducing MEMS related failures some traditional reliability tests did accelerate MEMS-related failures.