Characterisation of the mechanical properties of MEMS devices using nanoscale techniques
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J2.3.1
Characterisation of the mechanical properties of MEMS devices using nanoscale techniques Nicholas X. Randall and Richard A. J. Soden CSM Instruments Inc., 197 First Avenue, Needham MA 02494 USA
ABSTRACT This paper focuses on recent developments in the localised characterisation of the mechanical properties of Microsystems and MEMS devices and structures. Conventional indentation techniques provide a highly powerful method for measuring the load and depth response of bulk and coated materials, but can also be used to measure the mechanical properties of very small micro-machined silicon structures. Beam structures, such as are used for accelerometers, need to be characterised in terms of the number of cycles to failure, the spring constant or the energy required to bend the beam by a required amount. Such localised testing needs to be adapted to work at various distances from the origin of the beam with a positioning accuracy of less than a micron. Initial studies have proved to be highly repeatable. A range of examples is presented which covers a range of application areas, including accelerometer beam structures, microswitches and printer head structures. The basic instrumental concepts are explained together with the modifications required for testing small structures in a localised way. Keywords: MEMS, mechanical properties, indentation, scratch, accelerometer, microswitch, printer head
INTRODUCTION The development of Micro Electro Mechanical Systems (MEMS) has, in recent years, been more focussed on the fabrication process than on issues concerning reliability and the link to mechanical properties. Many issues related to tribology, mechanics, surface chemistry and materials science should be considered in the operation and manufacture of many MEMS devices. At present, very few analytical instruments are available for accurate characterisation of localised properties and specific zones of interest. The indentation method for assessing mechanical properties at low loads and shallow depths is already well established for the characterisation of thin films as well as bulk materials. The depth-sensing indentation method produces a load-displacement curve from which quantitative property values can be calculated using a variety of approaches1-3. Additionally, the indenter (of defined geometry) can be accurately controlled in terms of load, depth and speed of application. Provided that the indenter can be accurately positioned on a site of interest, then the technique can be used to probe the local mechanical properties of different structures, either with single or more complicated loading cycles. Previous work has already shown the application of indentation at the nanometer scale for characterising the properties of extremely small structures such as integrated circuit bonding pads4. In addition to the quasi-static indentation method, where an indenter is moving solely in the vertical direction, the simple addition of an accurate displacement stage enables the sample to be moved horizontally, thus enabling
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