Cornell researchers who build nanoscale electronics have developed  microsensors so tiny, they can fit 30,000 on one side of a penny.

There's more to these tiny sensors than just their diminutive size:  They are equipped with an integrated circuit, solar cells and  light-emitting diodes (LEDs) that enable them to harness light for power  and communication. And because they are mass fabricated, with up to 1  million sitting on an 8-inch wafer, each device costs a fraction of that  same penny.

The team's paper, "Microscopic Sensors Using Optical Wireless Integrated Circuits," published April 17 in PNAS.

The collaboration is led by Paul McEuen, the John A. Newman Professor  of Physical Science, and Alyosha Molnar, associate professor of  electrical and computer engineering. Working with the paper's lead  author, Alejandro Cortese, Ph.D. '19, a Cornell Presidential  Postdoctoral Fellow, they devised a platform for parallel production of  their optical wireless integrated circuits (OWICs) – microsensors the size of 100 microns (a micron is one-millionth of a meter), mere specks to the human eye.

"In a certain sense, it's an old idea, building tiny sensors like this," said McEuen, who co-chairs the Nanoscale Science and  Microsystems Engineering (NEXT Nano) Task Force, part of Cornell's  Radical Collaboration initiative. "But we pushed it another order of  magnitude down in size and made it mass fabricate-able. A lot of times  when people would make these little doodads, they would wire them all  together by hand. You didn't get a million at a time. So we constrained  ourselves and said we're not going to do it unless we can make them by  the million."

The OWICS are essentially paramecium-size smartphones that can be  specialized with apps. But rather than rely on cumbersome radio  frequency technology, as cellphones do, the researchers looked to light  as a potential power source and communication medium.

Placing tiny circuits on a silicon wafer is relatively easy in the  nanotech arena, McEuen said, but adding LEDs is a special challenge  because they are made with a different material: gallium arsenide. In  order to transfer the LEDs to a wafer with the electrical components and integrate them, the researchers developed a complicated assembly  method that involved more than 15 layers of photolithography, 30  different materials and more than 100 steps.

"There are a lot of people working at larger scales where you can pick  up things and see them with your eye and touch them. This is not that,"  Cortese said. "It's at a scale that you legitimately cannot see what  you're doing unless you're under a microscope. So you really have to  gain an intuition about the nanoscale and the microscale."

Once the OWICs are freed from their substrate of silicon, they can be  used to measure inputs like voltage and temperature in hard-to-reach  environments, such as inside living tissue and microfluidic systems. For  example, an OWIC rigged with a neural sensor would be able to  noninvasively record nerve signals in the body and transmit its findings  by blinking a coded signal via the LED.

As a proof of concept, the team worked with the lab of Chris Xu,  professor of applied and engineering physics and a co-author of the  paper, and successfully embedded an OWIC with a temperature sensor in  brain tissue and wirelessly relayed the results.

McEuen, Molnar and Cortese have launched their own company, OWiC Technologies, to commercialize the microsensors. A patent application has been filed through the Center for Technology Licensing. The first  application is the creation of e-tags that can be attached to products  to help identify them.

The tiny, low-cost OWICs could potentially spawn generations of  microsensors that use less power while tracking more complicated  phenomena.

"The circuits in this paper were quite simple," Cortese said. "But  you can potentially fit thousands of transistors on one of these  devices. And that means you can increase the range of things the device  can sense, how the device communicates out, or it's ability to complete  more complex tasks. We really developed this as a platform so that a lot  of people have space to develop new devices, new applications."

Molnar and Xu are members of the Kavli Institute at Cornell for Nanoscale Science, which McEuen directs.

Contributing authors include doctoral students Conrad Smart, Michael  Reynolds, Samantha Norris, Yanxin Ji, Fei Xia, Aaron Mok and Chunyan Wu;  postdoctoral researchers Sunwoo Lee and Tianyu Wang; and Nathan Ellis,  machine shop manager of the Laboratory of Atomic and Solid State  Physics. The researchers developed their fabrication process with the  assistance of the Cornell NanoScale Science and Technology Facility.

Source: Cornell University