11 hours ago by Denis Paiste MIT electrical engineering graduate student Farnaz Niroui works in a glovebox, where she prepares a sample for deposition of gold. The glovebox is attached through a transfer line to a thermal evaporator that deposits the gold coating onto squeezable switches, or squitches, which Niroui designs, fabricates, and tests in the Organic and Nanostructured Electronics Lab at MIT. Credit: Denis Paiste/Materials Processing Center

A longstanding problem in designing nanoscale electromechanical switches is the tendency for metal-to-metal contacts to stick together, locking the switch in an "on" position. MIT electrical engineering graduate student Farnaz Niroui has found a way to exploit that tendency to create electrodes with nanometer-thin separations. By designing a cantilever that can collapse and permanently adhere onto a support structure during the fabrication process, Niroui's process leaves a controllable nanoscale gap between the cantilever and electrodes neighboring the point of adhesion.

Niroui, who works in Professor Vladimir Bulovi's Organic and Nanostructured Electronics Laboratory (ONE Lab), presented her most recent findings Jan. 20 at the IEEE Micro Electro Mechanical Systems (MEMS) Conference in Portugal. MIT collaborators include professors Jeffrey Lang in electrical engineering and Timothy M. Swager in chemistry. Their paper is titled "Controlled Fabrication of Nanoscale Gaps Using Stiction."

Stiction, as permanent adhesion is called, is a very important challenge in electromechanical systems and often results in device failure. Niroui turned stiction to her advantage by using a support structure to make nanoscale gaps. "Initially the cantilever is fabricated with a relatively larger gap which is easier to fabricate, but then we modulate the surface adhesion forces to be able to cause a collapse between the cantilever and the support. As the cantilever collapses, this gap reduces to width much smaller than patterned," she explains.

"We can get sub-10-nanometer gaps," she says. "It's controllable because by choosing the design of the cantilever, controlling its mechanical properties and the placement of the other electrodes, we can get gaps that are different in size. This is useful not only for our application, which is in tunneling electromechanical switches, but as well for molecular electronics and contact-based electromechanical switches. It's a general approach to develop nanoscale gaps."

Niroui's latest work builds on her earlier work showing a design for a squeezable switchor "squitch"which fills the narrow gap between contacts with an organic molecular layer that can be compressed tightly enough to allow current to tunnel, or flow, from one electrode to another without direct contactthe "on" positionbut that will spring back to open a gap wide enough that current cannot flow between electrodesthe "off" position. The softer the filler material is, the less voltage is needed to compress it. The goal is a low-power switch with repeatable abrupt switching behavior that can complement or replace conventional transistors.

Niroui designed, fabricated, tested, and characterized the cantilevered switch in which one electrode is fixed and the other moveable with the switching gap filled with a molecular layer. She presented her initial findings at the IEEE MEMS Conference in San Francisco last year in a paper titled, "Nanoelectromechanical Tunneling Switches Based on Self-Assembled Molecular Layers." "We're working right now on alternative designs to achieve an optimized switching performance," Niroui says.

"For me, one of the interesting aspects of the project is the fact that devices are designed in very small dimensions," Niroui adds, noting that the tunneling gap between the electrodes is only a few nanometers. She uses scanning electron microscopy at the MIT Center for Materials Science and Engineering to image the gold-coated electrode structures and the nanogaps, while using electrical measurements to verify the effect of the presence of the molecules in the switching gap.

Building her switch on a silicon/silcon-oxide base, Niroui added a top layer of PMMA, a polymer that is sensitive to electron beams. She then used electron beam lithography to pattern the device structure and wash away the excess PMMA. She used a thermal evaporator to coat the switch structure with gold. Gold was the material of choice because it enables the thiolated molecules to self-assemble in the gap, the final assembly step.

For the initial tunneling current demonstration, Niroui used an off-the-shelf molecule in the gap between electrodes. Work is continuing with collaborators in Swager's chemistry lab to synthesize new molecules with optimal mechanical properties to optimize the switching performance.

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