Smartphones have a scaling problem. Specifically, the radiofrequency
  (RF) filters that every phone—and every wireless device in general—uses
  to extract information from isolated wireless signals are too big, too
  flat, and too numerous. And without these filters, wireless
  communications simply wouldn’t work at all.

  “They are literally the entire backbone of wireless systems,” says
  [1]Roozbeh Tabrizian, a researcher at the University of Florida in
  Gainesville.

  So Tabrizian and other researchers at the University of Florida have
  now developed an alternative three-dimensional RF filter that can save
  space in smartphones and [2]IoT devices. If these 3D filters one day
  replace bulky stacks of 2D filters, it would leave more room for other
  components, such as batteries. They could also make it easier to push
  wireless communications into [3]terahertz frequencies, an important
  spectrum range [4]being researched for 6G cellular technologies.

  “Very soon, we’ll have trillions of devices connected to wireless
  networks, and you need new bands, you just need a whole range of
  frequencies and a whole range of filters.” —Roozbeh Tabrizian,
  University of Florida

  The filters currently used by wireless devices are called [5]planar
  piezoelectric resonators. Each resonator is a different thickness—a
  resonator’s specific thickness is directly tied to the band of wireless
  frequencies that resonator responds to. Any wireless device that relies
  on multiple bands of spectrum—[6]increasingly commonplace
  today—requires more and more of these flat resonators.

  But planar resonator technology has revealed a number of weaknesses as
  wireless signals proliferate and as the spectrum those signals relies
  on broadens. One is that it’s getting more difficult to make the
  filters thin enough for the new swathes of spectrum that wireless
  researchers are interested in harnessing for next-gen communications.
  Another involves space. It’s proving increasingly challenging to cram
  all of the signal filters needed into devices.

  A top-down image of five silver-colored vertical fins of different
  lengths rising up from a gray surface. The vertical fins for
  ferroelectric-gate fin resonators can be constructed in the same manner
  as FinFET [7]semiconductors. Faysal Hakim/Roozbeh Tabrizian/University
  of Florida

  “Very soon, we’ll have trillions of devices connected to wireless
  networks, and you need new bands, you just need a whole range of
  frequencies and a whole range of filters,” says Tabrizian. “If you open
  up a cell phone, there are five or six specific frequencies, and that’s
  it. Five or six frequencies cannot handle that. It’s as if you have
  five or six streets, and now you want to accommodate the traffic of a
  city of 10 million people.”

  To make the switch to a 3D filter, Tabrizian and his fellow researchers
  took a page from another industry that made the jump to the third
  dimension: semiconductors. When, in the continuous quest to shrink down
  chip sizes, it seemed like the industry might finally be hitting end of
  the road, a new approach [8]that raised electron channels above the
  semiconductor substrate breathed new life into Moore’s Law. The chip
  design is called FinFET (for “fin field-effect transistor,” where “fin”
  refers to the shark-fin-like vertical electron channel).

  “The fact that we can change the width of the fin plays a huge role in
  making the technology much more capable.” —Roozbeh Tabrizian,
  University of Florida

  “We definitely got inspired [by FinFETS],” says Tabrizian. “The fact
  that planar transistors were converted to fins was just to make sure
  the effective size of the [9]transistor was smaller while having the
  same active area.”

  Despite taking inspiration from FinFETs, Tabrizian says there are some
  fundamental differences in the way the vertical fins need to be
  implemented for RF filters, compared to chips. “If you think of
  FinFETs, all the fins are nearly the same width. People are not
  changing the dimension of the of the fin.”

  Not so for filters, which must have fins of different widths. That way,
  each fin on the filter can be tuned to different frequencies, allowing
  one 3D filter to process multiple spectrum bands. “The fact that we can
  change the width of the fin plays a huge role in making the technology
  much more capable,” says Tabrizian.

  Tabrizian’s group have already manufactured multiple three-dimensional
  filters, called ferroelectric-gate fin (FGF) resonators, that spanned
  frequencies between 3 and 28 GHz. They also constructed a spectral
  processor comprised of six integrated FGF resonators that covered
  frequencies between 9 and 12 GHz (By way of comparision, [10]5G’s
  coveted midband spectrum falls between 1 and 6 GHz). The researchers
  [11]published their work in January in Nature Electronics.

  It’s still early days for 3D filter development, and Tabrizian
  acknowledges that the road ahead is long. But again taking inspiration
  from FinFETs, he sees a clear path of development for FGF resonators.
  “The good news is we can already guess what a lot of these challenges
  are by looking at FinFET technology,” he says.

  Incorporating FGF resonators into commercial devices someday will
  require solving several manufacturing problems, such as figuring out
  how to increase the density of fins on the filter and improving the
  electrical contacts. “Fortunately, since we already have FinFETs going
  through a lot of these answers, the manufacturing part is already being
  addressed,” Tabrizian says.

  One thing the research group is already working on is the [12]process
  design kit, or PDK, for FGF resonators. PDKs are commonplace in the
  semiconductor industry, and they function as a kind of guidebook for
  designers to fabricate chips based on components detailed by a chip
  foundry.

  Tabrizian also sees a lot of potential for future manufacturing to
  integrate FGF resonators and semiconductors into one component, given
  their similarities in design and fabrication. “It’s human innovation
  and creativity to come up with new types of architectures, which may
  revolutionize the way that we think about having resonators and filters
  and transistors.”

References

  1. https://www.ece.ufl.edu/people/faculty/roozbeh-tabrizian/
  2. https://spectrum.ieee.org/iot-mechanical-hijack
  3. https://spectrum.ieee.org/tag/terahertz
  4. https://spectrum.ieee.org/at-the-6th-annual-brooklyn-5g-summit-some-eyes-are-on-6g
  5. https://en.wikipedia.org/wiki/Crystal_oscillator
  6. https://spectrum.ieee.org/wi-fi-7
  7. https://spectrum.ieee.org/topic/semiconductors/
  8. https://spectrum.ieee.org/how-the-father-of-finfets-helped-save-moores-law
  9. https://spectrum.ieee.org/tag/transistor
 10. https://spectrum.ieee.org/5g-faa
 11. https://www.nature.com/articles/s41928-023-01109-5
 12. https://en.wikipedia.org/wiki/Process_design_kit

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