A Guide to Out-of-Plane Topography of 3-D Surface Micromachined Microsensors

The topography of two microsystems that use large out-of-plane deflections as sensor elements can be characterized using Polytec’s MSA-500 Micro System Analyzer. The MSA-500 incorporates a scanning white light interferometer which was first used to measure an integrated magnetometer (magMEMS). A current half-loop is deflected with a magnetic field (Lorentz force) then measurements are taken on a flow micro-sensor that uses cantilever deflection induced by incident air flows.

Introduction

By adding several CMOS-compatible steps (such as a chemical release) and a low-temperature annealing an out-of-plane (3-D) movable part was created through surface micromachining.

When low power consumption, a high and tunable sensitivity, and a small footprint are required, such mechanical structures are interfaced with electronics on the same chip and target applications.

3-D CMOS-compatible MEMS sensors require the chemical release of the microstructures for a successful assembly. They also require the control of residual stresses built up in multilayered structures undergoing a complete thermal process. Mismatches between the layers in the thermal expansion coefficient and the plastic flow of a metallic layer cause the deflection of multilayered structures made of both elastic and plastic thin films. Monitoring the process thermal budget and stack thickness allows for the attainment of the deflection specification for multilayered micro-cantilevers.

Experimental Setup

As shown in Figure 1, Polytec’s Micro System Analyzer was mounted to a probe station sitting on a vibration isolation table to test these MEMS sensors. A pressure unit with a nozzle and goniometer also form part of the unit.

Measurement and test system including the Micro System Analyzer and pressure unit (to the right, close views of the pressure nozzle).

Figure 1: Measurement and test system including the Micro System Analyzer and pressure unit (to the right, close views of the pressure nozzle).

 

Results: 3-D Half-loop magMEMS (Integrated Magnetometer)

Figure 2 shows an SEM image of this CMOS-compatible magnetic field. An out-of-plane magnetic flux within the device is converted into a mechanical force on the M-shaped cantilever via the Lorentz force. At the anchors of the magMEMS is an integrated Wheatstone bridge composed of four piezoresistors. The off and on  state of the device and the resulting deflection is color-visualized.

SEM view of a magMEMS (on the left); resulting deflections for off and on states (on the right).

Figure 2: SEM view of a magMEMS (on the left); resulting deflections for off and on states (on the right).

Results: CMOS/MEMS Co-integrated Flow Microsensor

Figure 3 shows a magnified SEM view of the flow micro-sensor. The cantilevers bend downwards under airflow. This will lead to an increase of their capacitance and a lowering of the oscillating frequency of the integrated ring oscillator. An investigation is underway into such electrical measurements along with real-time monitoring of the cantilever topography enabled by the pressure module. Figure 3 shows a topographic scan at rest position.

MEMS flow microsensors co-integrated with SOI CMOS circuits: SEM view (on the left), 2-D and 3-D topography profiles (on the right).

Figure 3: MEMS flow microsensors co-integrated with SOI CMOS circuits: SEM view (on the left), 2-D and 3-D topography profiles (on the right).

Conclusion/Outlook

Three-dimensional surface micromachined sensors and actuators can now be built using Silicon-on-Insulator (SOI) technology, something which new CMOS-compatible microsystems are taking full advantage of. Polytec’s MSA-500 Micro System Analyzer can be used for critical topographic analysis in their development. Flow sensors can be topographically scanned and measured by using accessories that inject static pressure or constant air flow in association with a compensated objective lens. Futures studies could apply the MSA-500, which also features dynamic out-of-plane and in-plane vibration measurements.

MSA-500 - Micro System Analyzer

A premier measurement technology for the analysis and visualization of structural vibrations and surface topography in micro structures such as MEMS (Micro-Electro-Mecha- nical Systems) devices is the MSA-500. This analyzer is designed with an all-in-one combination of technologies that clarifies real microstructural response and topography. It fully integrates a microscope with Scanning Laser-Doppler Vibrometry, Strobo- scopic Video Microscopy and White Light Interferometry.

The MSA-500 can be incorporated in the MEMS design and test cycle and enables accurate 3-D dynamic and static response data that simplifies troubleshooting, improves and shortens design cycles, increases yield and performance, and reduces product cost. Direct geometry scan data acquisition is also featured with the MSA-500 for the vibration measurement.

More Info: www.polytec.com/microsystems

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

For more information on this source, please visit Polytec.

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