Flexible MEMs

 

The printed MEMS are also flexible, so they could be used to make sensors with irregular shapes. And since the stamping process dispenses with the harsh chemicals and high temperatures ordinarily required for the fabrication of MEMS, it could allow MEMS to incorporate a wider range of materials.

Instead of using a wafer (which is usually the case in conventional MEMs production), the MIT researchers begin with a grooved sheet of a rubbery plastic, which is coated with the electrically conductive material indium tin oxide. The researchers use what they call a “transfer pad” to press a thin film of metal against the grooved plastic. Between the metal film and the pad is a layer of organic molecules that weaken the metal's adhesion to the pad. If the researchers pull the pad away fast enough, the metal remains stuck to the plastic.

“It's kind of similar to if you have Scotch tape on a piece of paper,” says Corinne Packard, a postdoc in the Research Lab of Electronics at MIT who led the work, along with professors of electrical engineering Vladimir Bulovic and Martin Schmidt. “If you peel it off slowly, you can delaminate the tape very easily. But if you peel fast, you'll rip the paper.”

Once the transfer pad has been ripped away, the metal film is left spanning the grooves in the plastic like a bridge across a series of ravines. Applying a voltage between the indium-tin-oxide coating and the film can cause it to bend downward, into the groove in the plastic: the film becomes an “actuator” — the moving part in a MEMS. Varying the voltage would cause the film to vibrate, like the diaphragm of a loudspeaker; selectively bending different parts of the film would cause them to reflect light in different ways; and dramatically bending the film could turn a smooth surface into a rough one. Similarly, if pressure is applied to the metal film, it will generate an electric signal that the researchers can detect. The film is so thin that it should be able to register the pressure of sound waves.

 

The discovery of the manufacturing technique, was a happy accident. The researchers were actually trying to use a printing technique to build an electrical circuit. They had created a plastic stamp with a pattern molded into it and were trying to transfer that pattern to a thin sliver film. They had expected that the plastic would pull away the silver it made contact with, leaving behind an electrode that could control an organic light-emitting diode.