Designing for MEMS Reliability
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Designing for
MEMS Reliability Susanne Arney
Introduction Microelectromechanical systems (MEMS) devices are being manufactured in the hundreds of millions and are widely deployed for pressure sensor, accelerometer, display, and printing applications.1 This suggests customer confidence in the longterm reliability of MEMS (also known as microsystems or micromachines) under diverse stringent conditions. However, reliability-physics aspects of these early MEMS applications may have been viewed as a market differentiator, resulting in limited public dissemination of MEMSspecific physical-failure models and appropriate design solutions for long-term reliability. This article provides a review of MEMS reliability-physics issues and MEMSspecific test methodologies, failure modes, and solutions. The examples emphasize electrostatically actuated MEMS and materials choices deriving from silicon or silicon-compatible fabrication techniques leveraged from the microelectronics industry. Solutions to reliability issues can be based on design, materials, or operational choices. Reliability concepts are potentially applicable over many MEMS device types, despite differences in materials choice, fabrication technique, or microelectromechanical design.
reliability paradigm. Specifically, an interdependent relationship and tight feedback loop between all contributors to device, subsystem, and system design, fabrication, manufacturing and testing, reliability physics, and packaging can greatly accelerate time-to-market of emerging MEMS products (see Figure 1). Classic reliability-physics methodology, as applicable to MEMS, begins with an initial test plan designed to reveal failure modes or failure mechanisms through the application of a series of, for example, thermal, electrical, mechanical, and optical environmental applied conditions. A fundamental understanding of each observed failure mode or mechanism is then sought. Experiments are designed to identify and isolate each mechanism, and to determine its fundamental physical characteristics, root cause, and statistical distribution. Accelerating factors for each mechanism are then identified to permit more rapid (i.e., time-efficient) experimentation. “Overstressing” strategies for accelerating the failure of the devices relative to nominal operating conditions depend on device design, materials choices, and intended operating conditions.2 The most straightforward accelerated test design results in a
Designing for Reliability To ensure built-in reliability, MEMS reliability research has a fourfold mission: 1. To obtain a fundamental understanding of chip-level, MEMS-specific failure mechanisms; 2. To facilitate the design, packaging, manufacturability, and testing of commercially interesting MEMS research and development concepts; 3. To preview compliance and qualification testing of MEMS devices; and 4. To ensure the long-term reliability of MEMS products in the field. Commercial applications of MEMS that mandate rapid introduction into the marketplace can benefit from such a
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