Making MEMS at nano-scale avoids stiction issues, says startup

August 04, 2017 // By Graham Prophet
Nanusens solves the problem of stiction in MEMS inertial sensors by going smaller and creating nano-sensors in standard CMOS. The Barcelona, Spain-based 2014-startup has announced that its technology has been successfully used to solve the problem of stiction in MEMS inertial sensors, which is a major source of failure for this type of sensor.

The company’s proposition is to build sensors for volume applications such as smartphones that are small, low cost, high performance and that offer very high reliability.

 

"Our first silicon nano-sensor samples from [silicon foundry] GlobalFoundries exceeded our expectations showing outstanding resilience to stiction, with the devices going through more than 10,000 switching cycles, each equivalent to more than 1000G shocks," explained Nanusens' CEO, Dr Josep Montanyà i Silvestre. "And the sensitivity is an order of magnitude above what is needed for a motion sensor in most applications."

 

The problem of stiction in MEMS is caused by attractive forces - such as Van der Waals and Casimir effect - that occur on microscopic levels. These are surface-area dependent and not mass dependent. In an inertial sensor design, there is a proof mass connected to a spring. This mass moves when there is an acceleration and the movement is detected by the mass acting as one plate of a capacitor and the change in capacitance is measured relative to a second fixed electrode. However, if there is a large movement such as from a shock or collision, the mass goes beyond the normal range of travelling and touches a surface enclosing the sensor where it 'sticks' due to the attractive forces and stops working. This can be countered by having stronger springs but this reduces the sensitivity of the sensor. A solution to increase the sensitivity could be to increase the mass but this results in a greater surface area for the mass and so, unfortunately, more attractive forces.

 

The approach used by Nanusens is to reduce the sensor design by an order of magnitude from MEMS with linear feature sizes of 1-2 microns to Nano-MEMS (NEMS) where the feature sizes are typically 0.3 µm. This reduces the attractive forces significantly as the surface area reduction is in two dimensions, i.e. almost two orders of magnitude reduction. Reducing


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