The University of Illinois Develops A 3D Microchip Inductor to Fully Utilize 3D Structure Space
Engineers from the University of Illinois have developed a microchip inductor that could greatly expand the capabilities of future chips.
The research, which was published in the journal, Science Advances, showed that by using fully-integrated, self-rolling magnetic nanoparticle-filled tubes, the new inductor can ensure a condensed magnetic field distribution as well as energy storage in 3D space—while at the same time keeping the tiny footprint required to fit on a chip.
The team behind the new study was led by Xiuling Li, an electrical and computer engineering professor at the University of Illinois and interim director of the Holonyak Micro & Nanotechnology Laboratory.
Expanding Microchip Capabilities
Engineers have been working on making microchips smaller for decades.
Many of the technological advancements in smartphone technology—and more generally, the IoT—would not have been possible without the miniaturization of several electronics components. When looking at microchips inductors, in particular, it is noticeable that these components are usually made of 2D spirals of wire. Each turn of the wire creates stronger inductance.
This is a complex technology that has improved electronics consistently in the last few years. Nonetheless, a 2D structure also means that there is a space limit on the two-dimensional surface of chips.
Researchers have tried to experiment with 3D structures to circumvent these obstacles, but their successes are currently constrained by existing capabilities in three-dimensional structure construction, current handling, and magnetic material integration. Building on a previous study, Xiuling Li’s team created 3D inductors using 2D processing by switching to self-rolled-up membrane nanotechnology, which allows for wire spiralling out of the plane and is divided by an insulating thin film from turn to turn.
When fully unrolled, the wire membranes were 1 millimetre long (about 100 times smaller than traditional 2D inductors). “A longer membrane means more unruly rolling if not controlled,” Li explained.
“Previously, the self-rolling process was triggered and took place in a liquid solution,” she added. “However, we found that while working with longer membranes, allowing the process to occur in a vapor phase gave us much better control to form tighter, more even rolls.” In other words, by using these 3D components on standardized 2D microchips, developers should be able to use up to 100 times less chip space.
A scanning electron microscope micrograph of a rolled micro inductor architecture. Image used courtesy of Xiuling Li et al., Science Advances (2020)
Induction in Microchips
At a basic level, an inductor is a passive two-terminal electrical component storing energy in a magnetic field when electric current flows through it.
As this happens, a relationship is created between the direction of the magnetic flux, which circulates around the conductor, and the direction of the current flowing through the same conductor. This phenomenon is called “Fleming’s Right Hand Rule”. A secondary voltage is also induced into the same coil by the movement of the magnetic flux as it resists or opposes any changes in the electrical current that facilitates its flow.
Inductors are usually formed with wire strongly wrapped around a central core, which is often shaped as a straight cylindrical rod or a continuous ring or loop to concentrate their magnetic flux. In the case of microchip inductors, they are typically made of iron or ferrite and are placed on the top of a printed circuit board (PCB) with solder paste, and then soldered.
"The most efficient inductors are typically an iron core wrapped with metal wire, which works well in electronic circuits where size is not as important a consideration," Li said, commenting on the new findings. "But that does not work at the microchip level, nor is it conducive to the self-rolling process, so we needed to find a different way", she added.
To solve this issue, the researchers filled the already-rolled membranes with an iron oxide nanoparticle solution using a tiny dropper. "We take advantage of capillary pressure, which sucks droplets of the solution into the cores," Li explained. “The solution dries, leaving iron deposited inside the tube. This adds properties that are favorable compared to industry-standard solid cores, allowing these devices to operate at higher frequency with less performance loss.”
Xiuling Li, electrical and computer engineering professor that led the study focused on developing the microchip inductor. Image used courtesy of Xiuling Li.
Chip inductors are primarily used in electrical power and electronic devices designed to transmit and receive radio frequencies signals to and from other devices. of these capabilities and compact size, they are often used in power lines, RF transceivers, computers, and even in microchips implanted in animals.
While the new findings present an interesting potential for future microchips performances, Li said that the new microchip inductors still have a variety of issues that need addressing. "As with any miniaturized electronic device, the grand challenge is heat dissipation," she said.
Li explained how the team is currently working with collaborators from Stanford University, Hefei University of Technology, China, and the University of Twente, The Netherlands to find materials that are better at dissipating the heat generated during induction.
“If properly addressed, the magnetic induction of these devices could be as large as hundreds to thousands of millitesla,” Li estimated, “Making them useful in a wide range of applications including power electronics, magnetic resonance imaging and communications."