Deep physics, materials science enhance dielectrics, varactors

When doing analog design, especially at higher frequencies ranging into the microwave region, it’s normal to focus on devices and the performance they enable in the specific topology. But there’s another aspect of microwave design that’s important to keep in mind: the role of advanced materials and the atomic-scale physics that allows conception, construction, and test of the advanced devices need to reach toward the multi-gigahertz part of the spectrum.

This is demonstrated by a recent Cornell University-led development related to voltage-tunable capacitors, or varactors, that combine high performance with low loss—and the road to get there. Traditional varactor technologies, while effective, often hit a performance ceiling due to intrinsic material limitations, particularly when it comes to dielectric losses that degrade signal quality.

A federal research program was initiated in 1999 to find materials for varactors that would offer lower dielectric losses at higher frequencies. The “back story” of success here is yet another example of how progress is often not linear, predictable, or obvious, despite the way it’s often portrayed.

The research team’s success here is due to persistence and following a very different path, as the project has been a long journey. While nearly every scientific team in the program focused on using barium strontium titanate, the Cornell team looked at layered crystalline materials, a type of perovskite structure known as Ruddlesden-Popper thin films, characterized by their exceptionally low energy loss at microwave frequencies.

Unfortunately, these films also had a major drawback: according to the accepted understanding of their crystal symmetry, they shouldn’t have been able to provide the tunability needed for practical devices.

A member of the research team was developing a new technique for measuring the dielectric properties of thin films across a wide range of frequencies. One of his measurements of strontium titanium oxide with composition Sr₄Ti₃O₁₀, a layered Ruddlesden-Popper thin film, suggested something remarkable: the supposedly untunable material might, in fact, be tunable after all.

But there was a problem: the effect only appeared in an in-plane geometry, in which the electric field moved sideways through the material. Real-world devices such as voltage-tunable capacitors used in microwave circuits generally require an out-of-plane design, in which the electric field moves vertically through the film, enabling smaller, more efficient components.

Researchers spent a decade trying to find a way to preserve their low microwave loss while making them more tunable and more practical. They then asked a more radical question: what if they could change the symmetry of the material itself? If so, it might be possible to change the symmetry in a specific family of Ruddlesden-Popper compounds made from barium, strontium, titanium, and oxygen.

In a true multi-institution effort with collaborators at Cornell, the University of Connecticut, Rice University, the University of Maryland, Boise State University and the National Institute of Standards and Technology (NIST), they engineered a new version of the material by inserting carefully spaced rock-salt layers. The strategy effectively rewrote the material’s internal rules, allowing it to exhibit the out-of-plane behavior needed for practical devices while preserving the low-loss characteristics that had made the Ruddlesden-Popper thin films attractive in the first place.

By engineering a film structure that introduces a unique rock-salt atomic layer interleaved with every “n” perovskite unit cell, the researchers created a new class of thin films whose symmetry properties could be precisely controlled (Figure 1).

Figure 1 Researchers used advanced microscopy to confirm the atomic structure of an engineered Ruddlesden-Popper material. The diagrams show how alternating layers in the crystal helped produce the material’s unusual combination of tunability and low energy loss. Source: Cornell University

From possible breakthrough to despair, then to a solution

But this success led to another dead-end, as the new out-of-plane devices posed an entirely different metrology problem. The frequencies most relevant for modern communications systems are among the most difficult to measure accurately because at those high frequencies, the signal from the material can be distorted by the test structure itself—the metal electrodes, wiring, and geometries surrounding the dielectric. So, when the researchers first tested the new Ruddlesden-Popper devices at microwave frequencies, the results were confusing.

Addressing this issue, a NIST-based group began to develop a new metrology approach capable of characterizing the material in an out-of-plane, metal-insulator-metal capacitor geometry at frequencies beyond the reach of conventional techniques. They added a “control structure” using a sheet of metal that had the same topology as the device. Measuring that control structure let the team perform an additional round of calibration, subtracting away distortions caused by the test structure itself, and isolating the dielectric’s true microwave response (Figure 2).

Figure 2 The microwave measurement setup used by the NIST team in Boulder, Colorado. Source: NIST via Cornell University

Their custom-tailored composition exhibits a remarkable relative tunability of 51% under an applied electric field of 250 kV/cm, which is almost double the performance of many conventional tunable dielectrics. At the same time, it maintains an impressively low dielectric loss that translates to a material quality factor of about 200. For the best version, the measured dielectric tuning figure of merit (FOM) showed tenfold improvement for out-of-plane tunable dielectrics at 10 GHz.

Figure 3 Various perspectives on microwave characterization are displayed at ambient temperature. Source: Cornell University

Will this lead to new varactors that you can buy? Obviously, it’s too early to say; there are still many potential obstacles on the path to commercialization, if it even happens.

But I do think the right screenwriter could make an exciting story out of this long quest with its advances, insight, contrary thinking, roadblocks, and eventual success. It would be nice to see a true story of science discovery and innovation captured and brought to a more general audience (can you think of any recent ones other than the 2023 blockbuster movie Oppenheimer?).

The work is detailed in their intense paper with a deceptively simple title “Breaking symmetry yields a low-loss out-of-plane tunable microwave dielectric” published in Nature Electronics; while that paper is behind a paywall, a “student” preprint copy is posted at ResearchGate here. In addition, there’s a fairly technical yet very readable description of the work posted at Bioengineer.org (why there—I can’t say).

Bill Schweber is a degreed senior EE who has written three textbooks, hundreds of technical articles, opinion columns, and product features. Prior to becoming an author and editor, he spent his entire hands-on career on the analog side by working on power supplies, sensors, signal conditioning, and wired and wireless communication links. His work experience includes many years at Analog Devices in applications and marketing.

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