Nanusens creates nanosensors in CMOS
“Our first silicon nano-sensor samples from 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, Josep Montanyà i Silvestre. “And the sensitivity is an order of magnitude above what is needed for a motion sensor in most applications.”
Stiction, the friction that can prevent stationary surfaces from moving, is a major source of failure for this type of sensor as it is based on Van der Waals forces and is area dependent rather than mass dependent. In inertial sensors stiction can be caused by a shock that takes the moving mass beyond its normal range of travel and “sticking” where the mass meets an end-stop. The solution is to provide stiffer suspension points but this reduces the sensitivity of the sensor.
Nanusens claim to have been able to provide both high sensitivity measurement and avoiding stiction while manufacturing in CMOS.
Nanusens has gone to smaller dimensions with linear feature sizes of 0.3-micron compared with 1 to 2 micron conventionally used for MEMS. This reduces the Van der Waals attractive forces as the surface area reduction is in two dimensions. A reduced size of proof mass would also decrease sensitivity but this is offset by reducing the gap between it and the fixed electrode.
“Therefore, by reducing all the dimensions of the device, we keep the sensitivity and we increase the reliability,” added Montanyà. “In fact, we have such a gain in reliability, that we can increase sensitivity and still have a very reliable device.”
Next: Smaller dimensions with broad applications
The new nano-sensors are made using standard CMOS processes and mask techniques. The inter-metal dielectric is etched away through the pad openings in the passivation layer using vapour hydrogen fluoride to create the nanosensor structures. The holes are then sealed and the chip packaged as necessary. As only standard CMOS processes are used, and the sensors can be directly integrated with active circuitry as required, the sensors can potentially have high yields similar to CMOS devices.
The company, formed by former executives of the now defunct Baolab Microsystems, has plans to develop CMOS-based MEMS for three products: a low-cost accelerometer for wearables and toys; a high performance accelerometer for augmented reality applications and an RF tuner circuit for use in smartphones.
Rajesh Nair, vice president of product management at Globalfoundries, commented: “We have built a strong relationship with Nanusens and we look forward to advance this new generation of sensor designs using our standard 0.18-micron processes.”
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