Tips for MEMS mechanical testing
In a recent presentation, the company is considering optical and electrical testing scenarios versus the mechanical testing of MEMS for which it has developed a MEMS Probe station, the FT-MPS02.
The wafer-level MEMS testing instrument offers direct mechanical and electrical testing of the MEMS’ characteristics, a more complete solution than today’s most common alternatives such as electrical response measurement or optical vibration analysis, according to Femtotools.
In his brochure, the company’s CEO Dr. Felix Beyeler highlights that most standard instrumentation used for testing MEMS has been designed for semiconductors and is optimized for electrical testing only, which means the mechanical properties of the MEMS are typically only measured at a late stage in the R&D process in final tests.
Yet, he explains, only true mechanical testing with a direct probe contact and concurrent electrical testing or stimulation allows researchers to precisely characterize the relationship between a MEMS’ electrical signals and mechanical properties, with force-deflection-time data and without having to make any mathematical assumptions.
It also allows researchers to validate or improve their models, optimize their processes and detect problems early, at wafer level to select good dies only for further packaging. In some cases, the tool could also be used to refine material model libraries in CAD tools relying on Finite Element Analysis.
With a force resolution down to 5nN at the tip of its 50um by 50um silicon probe (optionally, the tip can be furnished by a sharp tungsten probe with a tip radius smaller than 2um), the MEMS probe station can characterize most MEMS sensors and actuators (which can create forces in the Nanonewton to Millinewton range).
On unpackaged wafers and chips, the unit can measure sensor output signal versus applied deflection/load, actuation force versus driving signal, actuation deflection versus driving signal. It can directly measure many other parameters that optical or electrical testing would miss or would have to derive from models, such as stiffness, linearity, elastic/plastic deformation, hysteresis, stiction, over- and underetch, adhesion force, breaking force.
The CEO also pitches the MEMS probe stations against atomic force microscopes (AFM) which are really optimized for surface metrology, hence suffering geometrical limitations due to cantilever geometry but also lacking the combined mechanical and electrical testing capability of the FT-MPS02.
The presentation then reviews several testing examples, among them, for a cantilever-type piezoresistive sensor, characterized with a vertical bending test.
One interesting aspect of the new instrument is that it can perform direct cantilever calibration on the wafer without the need for device packaging, highlighting property distribution of stiffness and output voltage at wafer-level, ultimately creating a wafer map and providing a yield rate evaluation.
This, Beyeler concludes, improves the efficiency of the MEMS R&D process in small labs, from design houses to academia.
Visit FemtoTools AG at www.femtotools.com
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