Measurement of Large Forces and Deflections in Microstructures

📝 Abstract

Properties of typical MEMS materials have been widely investigated. Mechanical properties of MEMS structures depend not only on the bulk material properties, but also structural factors. A measurement system has been made to measure force/deflection on microstructures to examine some of the structural properties. This is a stylus setup integrated with a load cell and a linear actuator. First, the requirements for the measurement system were established. Then the system was built up and characterized. We have successfully made measurements on a typical micromechanical structure, a cantilever accelerometer design. The stylus placement accuracy, the spring constant along the proof mass, analysis of the force/deflection curve shape and destructive tests on the cantilever have been investigated in our experiment and will be presented in this paper.

💡 Analysis

Properties of typical MEMS materials have been widely investigated. Mechanical properties of MEMS structures depend not only on the bulk material properties, but also structural factors. A measurement system has been made to measure force/deflection on microstructures to examine some of the structural properties. This is a stylus setup integrated with a load cell and a linear actuator. First, the requirements for the measurement system were established. Then the system was built up and characterized. We have successfully made measurements on a typical micromechanical structure, a cantilever accelerometer design. The stylus placement accuracy, the spring constant along the proof mass, analysis of the force/deflection curve shape and destructive tests on the cantilever have been investigated in our experiment and will be presented in this paper.

📄 Content

9-11 April 2008 ©EDA Publishing/DTIP 2008

ISBN: 978-2-35500-006-5 Measurement of Large Forces and Deflections in Microstructures

Kai Axel Hals1, Einar Halvorsen, and Xuyuan Chen Institute for Microsystem Technology, Vestfold University College, P.O. Box 2243, N-3103 Tønsberg, Norway

1 Present address: Infineon Technologies Sensonor AS, P.O.Box 196, N-3192 Horten, Norway Abstract-Properties of typical MEMS materials have been widely investigated. Mechanical properties of MEMS structures depend not only on the bulk material properties, but also structural factors. A measurement system has been made to measure force/deflection on microstructures to examine some of the structural properties. This is a stylus setup integrated with a load cell and a linear actuator. First, the requirements for the measurement system were established. Then the system was built up and characterized. We have successfully made measurements on a typical micromechanical structure, a cantilever accelerometer design. The stylus placement accuracy, the spring constant along the proof mass, analysis of the force/deflection curve shape and destructive tests on the cantilever have been investigated in our experiment and will be presented in this paper.

I.
INTRODUCTION The direct wafer level measurement of force versus deflection of MEMS structural elements is not available in conventional probe stations, but is highly desired. Verification of the elastic stiffness at specific points could be useful as design verification and possibly also in process control. Furthermore, if sufficiently large deflections could be made, it would allow for strength assessment at wafer level. For accelerometers this would be an alternative to shock testing of packaged devices or could serve as a complementary test.
Measurement of displacement versus force can be done by atomic force microscopy, but is limited with respect to displacement and scan range. Surface profilometers could in principle also be used for this purpose [1, 2], but are limited with respect to force range and are not suitable for measurements at a point. There are nano-indenters that have both the displacement and the scan range necessary to make them useful for this kind of measurements [3, 4]. These expensive instruments are optimized towards characterization of thin films and surfaces, not for test of MEMS structural elements. In particular they have much finer spatial resolution and different tip geometry than needed for measurement of large deflections of a structural element such as a beam or a suspended proof mass.
From the above, it is clear that there is a need to investigate methods for mechanical test of MEMS structural elements directly on wafer. It is therefore worthwhile to investigate if a simple and affordable approach for mechanical testing of MEMS can be found. In particular it is interesting to investigate large deflection for potential fracture test on wafer.
We have built a simple and affordable measurement setup for probing mechanical properties of MEMS structures at wafer level. In the following we give a detail description of our setup and analyze its behaviour over a wide range of loads based on measurements on micromachined silicon accelerometers.

I I.
MEASUREMENT SETUP The basic principle of the apparatus is shown in Fig. 1. A probe tip is attached to a load cell which measures the vertical force on the probe tip. The load cell is attached to a linear z- actuator which is computer controlled with known displacement. The above equipment is mounted on a fixture that is attached to a manual xy-actuator (a small xy-table). Since the z-displacement is known and the force is measured, we can obtain the force versus deflection response at any point of choice on a device. The measurement setup is depicted in Fig. 2a. The load cell was a Honeywell 25g Minigram beam load cell, the linear actuator was a Zaber CE Linear actuator. The actuator and the load cell were connected to a PC. The actuator was connected through the RS-232 interface and the load cell was read by a Texas Instruments USB 6009 ADC, both controlled by a LabView program. In addition a laboratory microscope was used for visual inspection and probe tip positioning. The whole setup was placed on an anti-vibration table.

Fig. 1. Basic principle of measurement setup. 9-11 April 2008 ©EDA Publishing/DTIP 2008

ISBN: 978-2-35500-006-5

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b) c)

Fig. 2. Measurement setup realisation: a) overview, b) probe tip and load cell, and c) probe tip.

The probe tip is a very critical element. In our initial design considerations for the setup, we analyzed the maximum stress in the tip based on the Hertz contact problem. For a tip radius of 10um, a silicon substrate and 10mN tip force, the maximum stress is several GPa for several choices of tip

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