Mechanical Testing of MEMS-based Micromirror using FT-RS1000 Microrobotic System

By AZoNano Staff Writers

Table of Content

Introduction
MEMS-based Micromirror
FT-RS1000 Microrobotic System
Measurement of Deflection Range and Torsional Stiffness
Conclusion
About FemtoTools AG

Introduction

Microelectromechanical systems (MEMS) feature microscopic mechanical structures, which significantly impact the device’s performance. MEMS’ relative geometrical tolerances also tend to be poor.

Moreover, differences in stress and properties of materials may influence the device’s specifications. Hence, the measurement of the mechanical characteristics of the microfabricated structures is critical for MEMS development or quality control.

MEMS-based Micromirror

A MEMS-based micromirror has been developed by the institute of Micro and Nanosystems at ETH Zurich. Designed for large mirror deflections, the micromirror features a mirror plate that is suspended by SU8 polymer flexures. The mirror is electrostatically actuated by interdigitated, vertical comb drives.

The micromechanical testing of this micromirror provided a better understanding about its functionality, mechanical properties, and characteristics parameters such as the maximum deflection, the rotational stiffness of the torsion spring, and the hysteresis of the SU8 torsional springs.

Figure 1. MEMS micromirror chip

Figure 2. Measurement setup

FT-RS1000 Microrobotic System

FemtoTools has designed a compact micromechanical instrument, the FT-RS1000 Microrobotic System, which can be used for mechanical characterization of micromirrors. In order to measure displacement and forces perpendicular to the MEMS chip plane, the FT-S100 Microforce Sensing Probe is placed vertically on the system.

Users can employ this system configuration to perform the micromechanical testing on both chip and wafer levels. Figure 2 shows a micromirror placed in a chip carrier for testing. To observe the micromirror during sample alignment and compression test, a side view microscope camera is utilized.

Measurement of Deflection Range and Torsional Stiffness

In order to carry out precise micromechanical characterization of the micromirror, the following compression- testing procedure needs to be followed:

  • The joysticks of the FT-RS1000 Microrobotic System are used to place the FT-S100 Microforce Sensing Probe above the non-movable part of the chip and then a compression test is carried out. This ensures that the actual micromirror properties are determined and the sample holder and the flexibility of the measurement system do not impact the consecutive measurements.

  • With the help of joysticks, the FT-S100 Microforce Sensing Probe is aligned relative to the micromirror. Then, the probe tip is positioned over one side of the suspended mirror plate. The graphical user interface with the camera image is shown in figure 3.

  • The “Find Contact” functionality is utilized to locate the contact point between the micromirror plate and the FT-S100 sensor probe. The sensor probe will continue to move downwards until a small threshold force is calculated. Following this, the sensor probe is automatically retracted to a position of 30 µm above the mirror surface.

  • Finally, the automated compression test is started. The FT-RS1000 Microrobotic System samples the position data of the integrated optical encoders and also the force signal of the FT-S100 Force Sensing Probe. In order to prevent damage to the Microforce Sensing Probe or the micromirror, the FT-RS1000 system automatically halts the compression test at the specified force threshold of 100 µN.

  • This compression test is again carried out at different locations on the mirror plate. The deflected mirror plate during the compression is shown in figure 4.

Figure 3. The Microforce Sensing Probe located above the micromirror plate

Figure 4. Deflected mirror plate during compression testing

Figure 5. Measurement locations

Figure 6. Force-Position data

The selected measurement locations a, b and c are located at different distances to the rotation axis of the micromirror plate, as illustrated in figure 5. The measurement results of force-position exported during the three compression tests are shown in figure 6.

The slope ‘a’ shows a linear relationship between the force and the displacement. At a deflection of 145 µm, the highest deflection of the mirror plate is reached and the mirror plate contacts the underlying chip substrate. This can be viewed in the graph as a sudden increase of the device stiffness.

The other measurements ‘b’ and ‘c’ near to the rotation axis are steeper, given that a larger force is needed to deflect the mirror at these locations. The FT-RS1000 Microrobotic System records the force data and x,y,z position which allow precise computation of the rotational stiffness.

Figure 7. Rotational stiffness

The above figure shows the rotation angle against torsional moment. It can be observed that at a moment of 27 nNm, the highest deflection is reached, related to a rotation angle of 10.6° for all three measurements.

Conclusion

The FT-RS1000 Microrobotic System supplied by FemtoTools has been successfully utilized for the mechanical testing of the electrostatic micromirror. The maximum deflection angle as well as the rotational stiffness has been acquired from the measurement data. Likewise, the micromechanical tests can also be carried out on various MEMS devices on a wafer.

About FemtoTools AG

FemtoTools is a Swiss high-tech company that offers award-winning, ultra high-precision instruments for mechanical testing and robotic handling in the micro- and nanodomains. This new generation of instruments meets the challenging requirements of semiconductor technology microsystem development, materials science, micromedicine and biotechnology.

FemtoTools’ microrobotic handling and measurement instruments feature highly sensitive microforce sensing probes and force sensing microgrippers that are the result of a specially developed microelectromechanical system (MEMS)-based manufacturing process. The unmatched sensitivity and accuracy of our innovative systems redefines the standards for true quantitative investigations in the micro- and nanodomains.

FemtoTools’ easy-to-use microrobotic handling and measurement instruments have exceeded customer’s expectations and create exciting new possibilities, as demonstrated by numerous recent scientific advancements that have used our instruments.

This information has been sourced, reviewed and adapted from materials provided by FemtoTools.

For more information on this source, please visit FemtoTools.

Date Added: Jun 24, 2013 | Updated: Aug 16, 2013
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