Micro-machines are fit for space

Electronics Times, Oct 11, 1999

At the University of California's Berkeley Sensor & Actuator Center (BSAC), researchers have demonstrated functional logic circuits made with micro-electromechanical system (MEMS) technology.

Although the initial devices used were relatively large and slow, scientists at the Defence Evaluation & Research Agency (Dera) in Malvern, UK, have been developing quicker, smaller structures that could eventually make the technology as compact and fast as their electronic counterparts.

Although unlikely to compete with conventional electronics for mainstream applications, MEMS-based logic chips should have advantages in systems exposed to unwanted heat or other energy.

According to Ezekiel Kruglick, formerly of UC Berkeley, now at Optical Micro Machines in La Jolla, California,: "Mechanical computation can function in much higher temperatures and in radiation-filled environments.

"The [ability to withstand] higher temperatures means that the logic can run in harsh environments, such as jet engines, which means that MEMS sensors can be integrated with logic in such environments.

"The radiation hardness is a potential boon to satellite builders."

Although conventional electronics have been wildly successful, they are based on a very obvious weakness - they require knowledge about operating conditions in order to work. To design the smallest and most efficient transistors, engineers have to make assumptions about the energy of charge carriers within the semiconductor and the location of the Fermi level relative to valence and conduction bands.

When chips get too hot or are subjected to ionising radiation, these assumptions no longer hold and currents can flow when they are not supposed to.

At BSAC and Dera, researchers have attempted to sidestep this problem using mechanical, rather than electronic, switching. Both their systems are based on simply completing (or breaking) a circuit using a micro- mechanical structure.

When electrical contact is made, current flows. When the circuit is broken, the electrons can only leap the divide if the voltage is extremely high, enough to allow a spark or field emission of electrons. Thus, the chances of unwanted current flowing are minimised, even if there is a great deal of extra energy in the system.

The BSAC device, developed by Kruglick while doing his PhD research under Kristofer Pister, is a relay based on thermal actuation.

Six pairs of actuators pull a gold-coated crossbar into two electrodes, shorting them. Though micro-mechanical logic is not an entirely new idea, the devices that had previously been suggested for the job were impractical.

According to Kruglick, for a relay to do the job, it has to be able to carry enough current to switch itself and several other relays down the line. Where previous devices could barely switch themselves, the Berkeley relay was able to run with a fanout of about 10.

To determine the correct fanout value, Kruglick and his colleagues had to take account of reliability issues. These included increasing the power supply voltage to compensate for the way resistive relays can reduce the voltage available for switching - a problem particular to this type of implementation - while at the same time limiting the voltage to twice that required for switching in order to prevent the propagation of intermediate logic states.

Berkeley researchers also examined the way the device 'wore in', becoming more reliable after running for more than 1000 cycles.

Dera researchers Mark McNie and Keven Brunson, on the other hand, chose to use a cantilever as their switching structure. The arm is electrostatically pulled down towards a pad in the centre, but makes contact with the substrate only at the end. Although they have not yet reported operational logic, they have been able to show how they would build simple Boolean cells.

What could make the Dera technology (or future iterations of it) attractive compared with that demonstrated by UC Berkeley is their projected size: Dera expect to be able to drive their systems at 1MHz with low (1V) switching voltage, whereas the Berkeley relays would operate at more like 10kHz.

Just as compelling, these switches are not just potentially fast, but tiny. The footprint of the Dera devices is just 100 micro m sq compared with 10000 micro m sq for one device from BSAC.

Though much work is required to come up with a micro-mechanical structure that is sufficiently fast, small, reliable and manufacturable to be practical, there does seem to be a market waiting for the new technology when it is ready.

The satellite application is one that Kruglick suggests could be particularly important.

"Right now, each chip they put up in a satellite can cost tremendous amounts of money - of the order of $10000 for each little chip - and most of that money goes toward certification and testing," he said. "Once any technology is established as being effectively immune to radiation, the prices would drop 100-fold and satellites [would] get much cheaper."