Many sensing devices, including inertial and environmental sensors, can be miniaturized and operated with minimal power consumption thanks to micro-electromechanical systems (MEMS) technology. From the design board to the realization of the sensor, an essential step is the characterization of the behavior of the MEMS structure. Application-specific analog circuitry (ASIC) is often developed for this purpose, but this approach makes the iterative sensor development process cumbersome and time-consuming. A fast and comprehensive characterization method for MEMS devices is thus essential.
To gain a complete understanding of the behavior of a sensor, it is necessary to study the response of the MEMS structure under the influence of a drive signal or as a function of changes in the sensing environment. Several measurements – described below – are required to cover a broad range of parameters.
Frequency response analysis is essential to find the optimum drive signal to maximize the sensor's performance. This also enables the characterization of the sensor's resonance and sidebands. Backbone measurements can then be performed by varying drive parameters with a parametric sweeper; alternatively, it is possible to use parametric resonances. To monitor fast variations in the sensor's frequency response, the chirp FFT provides high spectral and temporal resolution.
This figure shows how the LabOne Sweeper tool can be used to characterize the resonance of a sensor.
Step response measurements provide the sensor's behavior as a result of a change in its drive signal or in an environmental factor. These time-resolved measurements uncover the structural properties of the sensor such as damping and quality factor.
MEMS structures with different damping can be simultaneously monitored using multiple demodulators as shown in this ring-down measurement.
Impedance analysis is used to characterize the transducer structure of the MEMS device: the impedance depends on the structure as well as on the drive signal and the environmental conditions. Accurate measurements are thus essential.
The high vertical precision and temporal resolution offered by the MFIA Impedance Analyzer or by the MF-IA option for the MFLI Lock-in Amplifier make it possible to quantify the effect of rapid changes in the environment on a sensor's capacitance (orange curve) and resistance (cyan curve).
Closed-loop sensor control forces the sensor to remain set to its optimal condition. This is achieved by locking the phase of the sensor with a phase-locked loop (PLL) or by locking other parameters such as the amplitude with a proportional-integrative-derivative (PID) controller. This type of locking scheme also increases the measurement bandwidth significantly.
The time trace shown in this figure captures how automatic gain control stabilizes the sensor by closing a PLL (blue trace) and a PID (orange trace).
The Benefits of Choosing Zurich Instruments
- All measurement strategies discussed above can be implemented and tested with Zurich Instruments lock-in amplifiers, thus eliminating the need for time-consuming ASIC development: take advantage of an all-in-one approach for your MEMS sensor applications.
- Track and control multiple resonances simultaneously using the multiple demodulators and oscillators in a single instrument.
- Zurich Instruments' analog electronics offer multiple input stages to minimize the input noise and maximize the signal-to-noise ratio for periodic signals.
- With the data acquisition module (DAQ), you can automate your measurement workflow thanks to the included LabOne software and its application programming interfaces for Python, C, MATLAB®, LabVIEW™ and .NET.
- Fast digital data transfer through USB or GbE connections allows you to record your measurement results without an additional digitizer card.
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