MEMS and NEMS Mechanical Reliability Measurements

Our objective is to develop new mechanical tests structures and methodologies based on commercial and pre-commercial MEMS manufacturing methods, and new analytical reliability prediction tools, in order to enable device producers in the MEMS industry to assess, predict, and optimize device reliability. Test structures will focus on "theta" specimens fabricated using lithography and deep reactive ion etching, and focused ion-beam and laser machining.

We will devise specialty test structures largely based on a theta design that are amenable to mechanical testing. Finite element modeling methods will be used to design robust specimen and loading configurations. Emphasis will be placed on measuring strength, fracture resistance, and compliance. Fractographic analysis will be used to correlate mechanical testing results to fabrication conditions. We will develop methods to fabricate, in-house, prototype silicon (Si) test structures with dry reactive ion etching (DRIE) methods. Test specimens of Si and other materials such as poly-Si, SiC, poly-diamond, and alumina that are made by DRIE and Focused Ion Beam and laser micro-machining will be acquired in collaborative activities with the Center for Integrated Nanotechnology (CINT), the Pennsylvania State University, and Cornell University. We will also begin C-ring strength testing in 2008 as an alternative procedure. Both theta and C-ring specimens are ring structures that can be conveniently loaded compressively without grips.

Impact and Customers:

• The MEMS (micro-electromechanical systems) industry currently generates revenues of about $45B/year, dominated by two non-contact devices: inkjet printer heads and accelerometers. An increasingly large share is arising from micro-mirror devices, which are contact devices.

• Commercial exploitation of MEMS and ultimately NEMS (nanoelectromechanical systems) technology to a plethora of contacting sensor and actuator devices will be facilitated by the ability to make accurate reliability predictions.

• MSEL standard test specimens and methods will enable device developers to measure mechanical properties needed for devicedesign and reliability prediction, resulting in devices with longerlifetimes that can function under a broad range of operating conditions.

We completed a second round of tests on round and hexagonal theta specimens prepared by J. Beall of NIST’s EEEL laboratory in Boulder. The work focused on demonstrating the repeatability of compliance and strength measurements. Extensive three-dimensional advanced finite element modeling of these structures was done to assess the effects of potential misalignments and various modifications on the basic specimen designs. These analyses were used to design second-generation theta specimens, which are now being fabricated for testing in 2008. Extensive fractographic analysis was undertaken on the broken fragments of theta specimens to identify the fracture origins in single-crystal Si.

Fracture origin in a theta specimen

New procedures to harvest and handle the tiny fracture fragments were developed, and dozens of origin flaws were found with a field emission scanning electron microscope. The fracture origins were of the order of 100 nm to 500 nm in size and are some of the smallest fracture origin flaws ever recorded. Manuscripts on these findings are in preparation.

“A Guide to Practice for Fractography of Ceramics and Glasses”, published in 2007, has been very well-received by the nanomechanics community and will likely become a definitive work on fracture analysis of brittle materials. The Guide is now used as a teaching aid at several universities, and many forensic and failure analysis experts have requested copies of the document. Techniques detailed in the Guide were instrumental in the analyses of miniature theta specimens in our own research.

Practice guide

Finally, three new ASTM standards were recently adopted: “Size Scaling of Tensile Strengths Using Weibull Statistics for Advanced Ceramics”, C-1683; “Flexural Strength of Advanced Ceramics at Ambient Temperature - Cylindrical Rod Strength”, C-1684; and “Fractographic Analysis of Fracture Mirror Sizes in Ceramics and Glasses”, C-1678. ASTM C-1684 was supported by a comprehensive journal paper that reviewed this test and included a complete error analysis.