Publications

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Manufacturing of integrated circuits (ICs) using a split foundry process expands design space in IC fabrication by employing unique capabilities of multiple foundries and provides added security for IC designers [1] Defect localization and root cause analysis is critical to failure identification and implementation of corrective actions. In addition to split-foundry fabrication, the device addressed in this publication is .comprised of 8 metal layers, aluminum test pads, and tungsten thru-silicon vias (TSVs) making the circuit area > 68% metal. This manuscript addresses the failure analysis efforts involved in root cause analysis, failure analysis findings, and the corrective actions implemented to eliminate these failure mechanisms from occurring in future product.

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This report summarizes design and modeling activities for the MEMS passive shock sensor. It provides a description of past design revisions, including the purposes and major differences between design revisions but with a focus on Revisions 4 through 7 and the work performed in fiscal year 2008 (FY08). This report is a reference for comparing different designs; it summarizes design parameters and analysis results, and identifies test structures. It also highlights some of the changes and or additions to models previously documented [Mitchell et al. 2006, Mitchell et al. 2008] such as the way uncertainty thresholds are analyzed and reported. It also includes dynamic simulation results used to investigate how positioning of hard stops may reduce vibration sensitivity.

This report summarizes the functional, model validation, and technology readiness testing of the Sandia MEMS Passive Shock Sensor in FY08. Functional testing of a large number of revision 4 parts showed robust and consistent performance. Model validation testing helped tune the models to match data well and identified several areas for future investigation related to high frequency sensitivity and thermal effects. Finally, technology readiness testing demonstrated the integrated elements of the sensor under realistic environments.

This report describes activities conducted in FY07 to mature the MEMS passive shock sensor. The first chapter of the report provides motivation and background on activities that are described in detail in later chapters. The second chapter discusses concepts that are important for integrating the MEMS passive shock sensor into a system. Following these two introductory chapters, the report details modeling and design efforts, packaging, failure analysis and testing and validation. At the end of FY07, the MEMS passive shock sensor was at TRL 4.

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Qualification of microsystems for weapon applications is critically dependent on our ability to build confidence in their performance, by predicting the evolution of their behavior over time in the stockpile. The objective of this work was to accelerate aging mechanisms operative in surface micromachined silicon microelectromechanical systems (MEMS) with contacting surfaces that are stored for many years prior to use, to determine the effects of aging on reliability, and relate those effects to changes in the behavior of interfaces. Hence the main focus was on 'dormant' storage effects on the reliability of devices having mechanical contacts, the first time they must move. A large number ({approx}1000) of modules containing prototype devices and diagnostic structures were packaged using the best available processes for simple electromechanical devices. The packaging processes evolved during the project to better protect surfaces from exposure to contaminants and water vapor. Packages were subjected to accelerated aging and stress tests to explore dormancy and operational environment effects on reliability and performance. Functional tests and quantitative measurements of adhesion and friction demonstrated that the main failure mechanism during dormant storage is change in adhesion and friction, precipitated by loss of the fluorinated monolayer applied after fabrication. The data indicate that damage to the monolayer can occur at water vapor concentrations as low as 500 ppm inside the package. The most common type of failure was attributed to surfaces that were in direct contact during aging. The application of quantitative methods for monolayer lubricant analysis showed that even though the coverage of vapor-deposited monolayers is generally very uniform, even on hidden surfaces, locations of intimate contact can be significantly depleted in initial concentration of lubricating molecules. These areas represent defects in the film prone to adsorption of water or contaminants that can cause movable structures to adhere. These analysis methods also indicated significant variability in the coverage of lubricating molecules from one coating process to another, even for identical processing conditions. The variability was due to residual molecules left in the deposition chamber after incomplete cleaning. The coating process was modified to result in improved uniformity and total coverage. Still, a direct correlation was found between the resulting static friction behavior of MEMS interfaces, and the absolute monolayer coverage. While experimental results indicated that many devices would fail to start after aging, the modeling approach used here predicted that all the devices should start. Adhesion modeling based upon values of adhesion energy from cantilever beams is therefore inadequate. Material deposition that bridged gaps was observed in some devices, and potentially inhibits start-up more than the adhesion model indicates. Advances were made in our ability to model MEMS devices, but additional combined experimental-modeling studies will be needed to advance the work to a point of providing predictive capability. The methodology developed here should prove useful in future assessments of device aging, however. Namely, it consisted of measuring interface properties, determining how they change with time, developing a model of device behavior incorporating interface behavior, and then using the age-aware interface behavior model to predict device function.

Optical actuation of microelectromechanical systems (MEMS) is advantageous for applications for which electrical isolation is desired. Thirty-two polycrystalline silicon opto-thermal actuators, optically-powered MEMS thermal actuators, were designed, fabricated, and tested. The design of the opto-thermal actuators consists of a target for laser illumination suspended between angled legs that expand when heated, providing the displacement and force output. While the amount of displacement observed for the opto-thermal actuators was fairly uniform for the actuators, the amount of damage resulting from the laser heating ranged from essentially no damage to significant amounts of damage on the target. The likelihood of damage depended on the target design with two of the four target designs being more susceptible to damage. Failure analysis of damaged targets revealed the extent and depth of the damage.

Failure analysis and device characterization of MEMS components are critical steps in understanding the root causes of failure and improving device performance. At the wafer and die level these tasks can be performed with little or no sample preparation. Larger challenges occur after fabrication when the device is packaged, capped, sealed, or otherwise obstructed from view. The challenges and issues of MEMS failure analysis lie in identifying the root cause of failure for these packaged, capped, and sealed devices without perturbing the device or its immediate environment. Novel methods of gaining access to the device or preparing the device for analysis are crucial to accurately determining the root cause of failure. This paper will discuss issues identified in performing root cause failure analysis of packaged MEMS devices, as well as the methods employed to analyze them.

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Optical actuation of microelectromechanical systems (MEMS) is advantageous for applications for which electrical isolation is desired. Thirty-two polycrystalline silicon opto-thermal actuators, optically-powered MEMS thermal actuators, were designed, fabricated, and tested. The design of the opto-thermal actuators consists of a target for laser illumination suspended between angled legs that expand when heated, providing the displacement and force output. While the amount of displacement observed for the opto-thermal actuators was fairly uniform for the actuators, the amount of damage resulting from the laser heating ranged from essentially no damage to significant amounts of damage on the target. The likelihood of damage depended on the target design with two of the four target designs being more susceptible to damage. Failure analysis of damaged targets revealed the extent and depth of the damage.

Thermal actuators have proven to be a robust actuation method in surface-micromachined MEMS processes. Their higher output force and lower input voltage make them an attractive alternative to more traditional electrostatic actuation methods. A predictive model of thermal actuator behavior has been developed and validated that can be used as a design tool to customize the performance of an actuator to a specific application. This tool has also been used to better understand thermal actuator reliability by comparing the maximum actuator temperature to the measured lifetime. Modeling thermal actuator behavior requires the use of two sequentially coupled models, the first to predict the temperature increase of the actuator due to the applied current and the second to model the mechanical response of the structure due to the increase in temperature. These two models have been developed using Matlab for the thermal response and ANSYS for the structural response. Both models have been shown to agree well with experimental data. In a parallel effort, the reliability and failure mechanisms of thermal actuators have been studied. Their response to electrical overstress and electrostatic discharge has been measured and a study has been performed to determine actuator lifetime at various temperatures and operating conditions. The results from this study have been used to determine a maximum reliable operating temperature that, when used in conjunction with the predictive model, enables us to design in reliability and customize the performance of an actuator at the design stage.

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