High-G manoeuvres

6 December 2012

Researchers from Seoul National University and Chonbuk National University have developed a MEMS acceleration switch with a triggering threshold that can be raised electronically on demand.

From the top

MEMS technologies allow the creation of compact, high-sensitivity acceleration switches with low fabrication costs and power consumption; suitable for monitoring applications requiring long lifetimes and low cost.

MEMS acceleration switches were first suggested in 1972 and since then much work has gone into their development. Only a small part of these efforts have been focused on making such switches tunable i.e. switches whose threshold acceleration – the acceleration value at which the switch triggers – can be adjusted in use.

Previous tunable acceleration switches have included cantilever-type and vertically actuating switches. In these approaches an additional electrode is patterned under the cantilever to induce an electrostatic force that can be used to reduce the threshold acceleration of the switch. This does lead to an electrical pull-in phenomenon between the tuning electrodes that latches the switch in the on position, which is not desirable for some applications.

However, the main limitation of these approaches is that their structure means that the force can only act to decrease the base threshold acceleration, it cannot be used to increase it. This has limited the range of applications for which MEMS-based acceleration switches can be used. The Chonbuk and Seoul team’s design addresses this problem, and their prototype switch can be tuned, electrically, from a base threshold acceleration of 11 g up to around 30 g.  

Looking up

The design the team report in this issue uses comb drive actuators (a stator and a rotor) to apply a force that resists the closing of the contacts in the switch, this force is controlled by adjusting the potential difference applied between the comb drives and thereby the threshold acceleration can be raised. The ability to increase the threshold could lead to a greater range of applications for MEMS acceleration switches. For use in monitoring earthquakes or impacts to items in shipment, a threshold level of only a few ‘g’ is sufficient but in airbag systems a threshold level of 50-200 g is required to detect a car crash. Military applications can require significantly higher levels again – 100-100,000g for fuze or safety and arming systems.

In addition to the challenge of designing a structure that would allow the raising of the threshold there were significant challenges in realising a prototype, as team member Professor Jung-Mu Kim explained, “Throughout the whole process, fabrication errors were a challenging issue. During the DRIE releasing process, footing and thermal isolation effects severely affected the performance of the switch. To overcome the problem, we adopted a recently-reported technique of patterning a conductive metal layer under the silicon structures. We patterned a chrome layer under the cavity of silicon wafer, which acted as a path for accumulated charges and thermal energy, successfully preventing fabrication failure during DRIE process”.

Looking both ways

The design has potential beyond allowing the threshold acceleration to be tuned upwards. With only a slight change to the design, in the positioning of the comb actuators, this method can also be used to reduce the threshold acceleration. The Seoul team are now working on joining the two ranges of tuning. By adjusting the comb actuator designs the team are attempting to fabricate a switch that will allow the threshold acceleration to be adjusted in both directions, allowing a very wide tuning range.

The team’s wider MEMS work includes RF, optical, bio and inertial measurement unit (IMU) MEMS. The acceleration switch is part of their IMU research and their long-term goal is to implement a tunable acceleration switch in safety and arming units of military equipment, for which they are also developing a mechanically-latchable acceleration switch design.

In the nearer future they expect the first implementation of this kind of switch is more likely to be in shipment monitoring or alarm systems, which could benefit from threshold tuning but have lower ‘g’ and reliability requirements. However, before such commercial implementations can be realised the team will study the properties of these devices in terms of life-cycle tests, shock resistance and off-axis interference measurement. The devices will also need to be hermetically packaged as their performance can be altered by external pressure, humidity, or particles.

Further reading

This article is based on the Letter: MEMS acceleration switch capable of increasing threshold acceleration (new window).

A PDF version (new window) of this feature article is also available.

Journal content

Cover of Electronics Letters, Volume 49, Issue 25

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