Research Bits: Aug. 30

Through glass vias

Researchers from the Chinese Academy of Sciences (CAS) developed a Through Glass Via (TGV) process for 3D advanced packaging, which they say enables low transmission loss and high vacuum wafer-level packaging of high-frequency chips and MEMS sensors.

TGV is a vertical interconnection technology applied in wafer-level vacuum packaging. The researchers found that it has good electrical, thermal, and mechanical properties, and achieves interconnection with the shortest distance and the minimum spacing between chips.

The researchers proposed a new TGV wafer manufacturing scheme. The newly-developed TGV wafers boasted high uniformity, high density and high aspect ratio, which enabled ultra-low leakage rate and ultra-low signal loss.

“We have overcome technical problems such as the manufacture of high-uniformity glass micropore arrays, glass dense reflow, and high-density filling of glass micropores and metals,” said Li Shan, of the Hefei Institutes of Physical Science at CAS. “They can be applied to 5G/6G high-frequency chips such as ring resonators, waveguide slot antennas, millimeter-wave antennas, as well as new 3D packaging requirements for MEMS gyroscopes and accelerometers. And we can make different sizes according to requirements.”

Laser-induced femtosecond logic gates

Researchers at the University of Rochester and the Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU) demonstrated a logic gate that operates at femtosecond timescales.

The team used laser pulses that last a few femtoseconds to generate ultrafast bursts of electrical currents. This is done, for example, by illuminating tiny graphene-based wires connecting two gold metals. The ultrashort laser pulse sets in motion, or excites, the electrons in graphene and sends them in a particular direction, generating a net electrical current.

They found that in gold-graphene-gold junctions, it is possible to generate two versions—“real” and “virtual”—of the particles carrying the charges that compose these bursts of electricity.

They explained that “real” charge carriers are electrons excited by light that remain in directional motion even after the laser pulse is turned off, while “virtual” charge carriers are electrons that are only set in net directional motion while the laser pulse is on. The virtual charge carriers only live transiently during illumination.

Because the graphene is connected to gold, both real and virtual charge carriers are absorbed by the metal to produce a net current.

The team also found that by changing the shape of the laser pulse, they could generate currents where only the real or the virtual charge carriers play a role. Thus, the two types of current could be controlled independently.

Using this augmented control landscape, the team was able to experimentally demonstrate logic gates that operate on a femtosecond timescale. In the experiment, the input signals are the shape or phase of two synchronized laser pulses, each one chosen to only generate a burst of real or virtual charge carriers. Depending on the laser phases used, these two contributions to the currents can either add up or cancel out. The net electrical signal can be assigned logical information 0 or 1, yielding an ultrafast logic gate.

“What is amazing about this logic gate,” said Ignacio Franco, an associate professor of chemistry and physics at Rochester, “is that the operations are performed not in gigahertz, like in regular computers, but in petahertz, which are one million times faster. This is because of the really short laser pulses used that occur in a millionth of a billionth of a second.”

“It will probably be a very long time before this technique can be used in a computer chip, but at least we now know that lightwave electronics is practically possible,” said Tobias Boolakee, who led the experimental efforts as a PhD student at FAU.

Franco added, “Through fundamental theory and its connection with the experiments, we clarified the role of virtual and real charge carriers in laser-induced currents, and that opened the way to the creation of ultrafast logic gates.”

Glucose fuel cell for implantable devices

Engineers from the Massachusetts Institute of Technology (MIT) and Technical University of Munich designed a new kind of glucose fuel cell that converts glucose directly into electricity. The sugar power source generates about 43 microwatts per square centimeter of electricity.

The fuel cell is able to withstand temperatures up to 600 degrees Celsius, meaning it could remain stable through the high-temperature sterilization process required for implantable devices. The researchers think the design could be made into ultrathin films or coatings and wrapped around implants to passively power electronics, using the body’s glucose supply.

“Glucose is everywhere in the body, and the idea is to harvest this readily available energy and use it to power implantable devices,” said Philipp Simons, who developed the design as part of his PhD thesis in MIT’s Department of Materials Science and Engineering. “In our work we show a new glucose fuel cell electrochemistry.”

“Instead of using a battery, which can take up 90 percent of an implant’s volume, you could make a device with a thin film, and you’d have a power source with no volumetric footprint,” said Jennifer L.M. Rupp, visiting professor at MIT’s Department of Materials Science and Engineering and associate professor of solid-state electrolyte chemistry at Technical University Munich in Germany.

MIT’s Jennifer Chu explains the concept: “A glucose fuel cell’s basic design consists of three layers: a top anode, a middle electrolyte, and a bottom cathode. The anode reacts with glucose in bodily fluids, transforming the sugar into gluconic acid. This electrochemical conversion releases a pair of protons and a pair of electrons. The middle electrolyte acts to separate the protons from the electrons, conducting the protons through the fuel cell, where they combine with air to form molecules of water — a harmless byproduct that flows away with the body’s fluid. Meanwhile, the isolated electrons flow to an external circuit, where they can be used to power an electronic device.”

Silicon chip with 30 individual glucose micro fuel cells, seen as small silver squares inside each gray rectangle. (Image credit: Kent Dayton/MIT)

The researchers designed a glucose fuel cell with an electrolyte made from ceria, a ceramic material that possesses high ion conductivity, is mechanically robust, and as such, is widely used as an electrolyte in hydrogen fuel cells. It has also been shown to be biocompatible.

“When you think of ceramics for such a glucose fuel cell, they have the advantage of long-term stability, small scalability, and silicon chip integration,” Rupp noted. “They’re hard and robust.”

Simons added, “Ceria is actively studied in the cancer research community. It’s also similar to zirconia, which is used in tooth implants, and is biocompatible and safe.”

The team sandwiched the electrolyte with an anode and cathode made of platinum, a stable material that readily reacts with glucose. They fabricated 150 individual glucose fuel cells on a chip, each about 400 nanometers thin, and about 300 micrometers wide. They patterned the cells onto silicon wafers and measured the current produced by each cell as they flowed a solution of glucose over each wafer in a custom-fabricated test station.

They found many cells produced a peak voltage of about 80 millivolts, which the researchers claim is the highest power density of any existing glucose fuel cell design.

“Excitingly, we are able to draw power and current that’s sufficient to power implantable devices,” Simons said.