INSTITUTE OF PHYSICS PUBLISHING JOURNAL OF MICROMECHANICS AND MICROENGINEERING J. Micromech. Microeng. 15 (2005) 792–801 doi:10.1088/0960-1317/15/4/017 Micro-structure mechanical failure characterization using rotating Couette flow in a small gap Danny Blanchard, Phil Ligrani, Bruce Gale and Ian Harvey Department of Mechanical Engineering, University of Utah, 50 South Central Campus Drive Rm. 2110, Salt Lake City, UT 84112, USA Received 27 September 2004, in final form 19 January 2005 Published 25 February 2005 Online at stacks.iop.org/JMM/15/792 Abstract A new method for testing the failure rates of micro-mechanical structures is presented. The technique uses Couette flow in a narrow-gap channel to induce different forces and loading on an array of structures. The Couette flow is induced by a rotating disc, which allows multiple devices to be tested at different designed stress levels depending on rotation speed and radial position. As an example, SU-8 micro-structures are used to present a general testing procedure that can be applied to other components. The forces acting on the micro-structures are due to fluid shear stress, centrifugal forces from rotation and form drag, all of which are characterized as they vary with radial position for one rotation rate. One series of tests with a single circumferential row of identical micro-structures is performed to determine the relative importance of these forces on revolutions to failure and failure rate of the structures in the row. Two additional series of tests are conducted to determine the effects of also adding unsteady loading from wakes to the structures. This unsteady loading from wakes is induced by time-varying velocity and pressure variations, which are imposed when additional rows of micro-structures are placed at smaller radial positions compared to the row being tested. Weibull failure rate approaches are used to provide information on failure rate as dependent upon cumulative revolutions. As such, the testing approach is useful for failure characterization of thin-film adhesion, self assembled nano-layers and micro-mechanical structures. Nomenclature C d normalized drag coefficient H gap height between spinning disc surface and flat plate F D-C total drag force for a cylinder in cross-flow F D-N total drag force for tested micro-structures obtained from numerical flow predictions r radial position μ dynamic viscosity ω angular velocity 1. Introduction The development of micro-fabrication technology has opened the door to the creation of micro-electro-mechanical systems (MEMS). These devices include a variety of sensors, actuators and other devices. Reliability, failure analysis and strength characterization of device components are important for performance, useful to optimize fabrication processes and materials systems, and needed to predict failure modes and reliability. Many different devices and techniques are employed to test the strength, fatigue and reliability characteristics of materials. Tensile testing is performed on bulk coupons using an Instron machine, an MTS machine or a micro-actuator [1–7], or by using a rotating substrate to apply centrifugal forces to test for tensile strength [8]. Shear testing can be performed by loading the end of a post with a probe [9]. Bending tests are performed by deflecting cantilever beams or membranes [10–12], bending the test material [13] or bending the substrate on which the specimen is bonded [14]. Torsional testing is performed on submicron single crystal silicon by 0960-1317/05/040792+10$30.00 © 2005 IOP Publishing Ltd Printed in the UK 792