Research Bits: Dec. 13

Electronic-photonic interface for data centers

Engineers at Caltech and the University of Southampton integrated an electronic and photonic chip for high-speed communication in data centers.

“There are more than 2,700 data centers in the U.S. and more than 8,000 worldwide, with towers of servers stacked on top of each other to manage the load of thousands of terabytes of data going in and out every second,” said Arian Hashemi Talkhooncheh, a Caltech graduate student.

“There is a continuous demand for increasing the speed of data communication between different chips not only in data centers but also in high-performance computers. As the computing power of the chips scale, the communication speed can become the bottleneck, especially under stringent energy constraints,” said Azita Emami, professor of electrical engineering and medical engineering at Caltech.

Key to the design was intensive co-optimization, said Hashemi. “We had to optimize the entire system all at the same time, which enabled achieving a superior power efficiency. These two chips are literally made for each other, integrated into one another in three dimensions.”

The resulting optimized interface between the two chips allows them to transmit 100 gigabits of data per second while producing just 2.4 pico-Joules per transmitted bit. The researchers said that this improves the electro-optical power efficiency of the transmission by a factor of 3.6 compared to the current state-of-the-art.

“As the world becomes more and more connected, and every device generates more data, it is exciting to show that we can achieve such high data rates while burning a fraction of power compared to the traditional techniques,” said Emami.

Improved AFM for circuit defects

Researchers at the National Institute of Standards and Technology (NIST) developed a way to simultaneously locate individual electrical flaws in multiple microcircuits on the same chip using an atomic force microscope (AFM).

AFM uses a sharp tip with a cantilever that can scan individual wires under the surface of a chip. However, static electric fields from neighboring wires can impact results and interfere with the ability to clearly image defects in the wire undergoing scanning.

To address this, the researchers applied specific AC voltages, supplied by an external generator, to individual neighboring wires instead of to the tip. The same AC voltage was supplied to neighboring wires as to the wire undergoing scanning, except that the voltages to the neighbors were exactly out of phase. Whenever the voltage to the wire of interest reached its highest value, the voltages to the neighboring wires were at their lowest.

“The out-of-phase voltages exerted electrostatic forces on the AFM tip that opposed the force exerted by the scanned wire. Those oppositely directed forces translated into regions of high contrast on an AFM image, making it easier to distinguish the signal from the wire of interest,” said the researchers.

The team demonstrated the technique using a test chip featuring four pairs of wires buried 4 micrometers beneath the surface, finding it produced clear and accurate images of defects. By tailoring the AC voltages applied to each wire so that they have different frequencies, the researchers showed that they could image defects in several adjacent wires at the same time.

Because the technique depends on an AC voltage applied remotely, to the wires rather than the AFM, the researchers dubbed the technique remote bias-induced electrostatic force microscopy.

“Applying a voltage to the wires instead of the AFM tip may seem like a small innovation, but it makes a big difference,” said Joseph Kopanski, a NIST scientist. “The method does not require a new instrument and could be easily implemented by the semiconductor industry,” he added.

CDs to sensors

Researchers from Binghamton University found a way to recycle CDs and turn them into flexible biosensors.

They developed a process that separates a gold CD’s thin metallic layer from the rigid plastic, allowing it to be fashioned into sensors to monitor electrical activity in human hearts and muscles as well as lactose, glucose, pH, and oxygen levels. The sensors can communicate with a smartphone via Bluetooth.

The fabrication is completed in 20 to 30 minutes without releasing toxic chemicals or needing expensive equipment, and it costs about $1.50 per device. According to the paper, “this sustainable approach for upcycling electronic waste provides an advantageous research-based waste stream that does not require cutting-edge microfabrication facilities, expensive materials or high-caliber engineering skills.”

By removing the gold layer of a CD from the plastic underneath, Binghamton University researchers have created flexible biosensors for numerous applications. (Credit: Matthew Brown / Binghamton University)

Ahyeon Koh, assistant professor in the Department of Biomedical Engineering at Binghamton, explained the method, which is similar to the creation of graphene. “We loosen the layer of metals from the CD and then pick up that metal layer with tape, so we just peel it off. That thin layer is then processed and flexible.”

To create the sensors, the researchers used a Cricut cutter, which is commonly used by crafters. The flexible circuits could be removed and stuck onto a person. A smartphone app could be used by medical professionals or patients to track readings.

Matthew Brown, at the time a PhD student at Binghamton, said, “We used gold CDs, and we want to explore silver-based CDs, which I believe are more common. How can we upcycle those types of CDs with the same kind of process? We also want to look at if we can utilize laser engraving rather than using the fabric-based cutter to improve the upcycling speed even further.”

Koh thinks that the sensors could be created by anyone with step-by-step instructions. “Everybody can create those kinds of sensors for their users. We want these to become more accessible and affordable, and more easily distributed to the public.”