Computing consumes energy at an exponentially increasing pace. Business intelligence and consulting firm Enerdata says that information, communication, and technology use between 5% and 9% of all the electricity in the world.
If growth is unchecked, by 2030 computers might consume up to 20% of the world’s energy production. Engineers need to flatten the energy demand curve for computing as soon as possible because the industry is switching from fossil fuels to renewable energy sources and power networks are already being stressed by weather-related events.
The multifunctional thin film group led by Jon Ihlefeld is contributing. They are looking into a way to use materials that would let the semiconductor industry put both computing and memory on the same chip.
According to Ihlefeld, an associate professor of materials science and engineering and electrical and computer engineering at the University of Virginia School of Engineering and Applied Science, “Right now we have a computer chip that does its computing activities with a little bit of memory on it.”
The computer chip sends a signal down the line every time it needs to communicate with the larger memory bank, which uses energy. Energy requirements increase with distance. In the modern period, the separation can be quite wide, up to several centimeters.
In an ideal world, we would put them in direct contact with one another.
For it, memory components that work with the rest of the integrated circuit are necessary. Ferroelectrics, or materials with the ability to store and release a charge on demand, are one type of material ideal for memory devices. Most ferroelectrics, though, can’t be used with silicon and don’t work well when shrunk down to the size needed for tiny devices now and in the future.
In Ihlefeld’s lab, researchers are serving as matchmakers. Their work increases the electrical and optical properties of materials—a department of materials science and engineering research strength—that enable modern computation and communication. The Charles L. Brown Department of Electrical and Computer Engineering’s research strength in fabrication and characterization of a variety of materials is another area in which they are experts.
Hafnium oxide, which is currently employed in the production of computers and cell phones, is their subject of interest. The drawback is that hafnium oxide is not ferroelectric in its native condition.
An Honorable Mention for Shelby Fields
It has been known for the past 11 years that it is possible to manage the atoms of hafnium oxide in order to create and maintain a ferroelectric phase or structure. When a thin film of hafnium oxide is heated, a process called “annealing,” the atoms can move into the crystallographic pattern of the ferroelectric material. When the thin film cools, its crystallographic structure becomes solid.
Many theories have been put forth as to why the ferroelectric phase forms. Shelby Fields, who graduated from UVA this year with a Ph.D. in materials science engineering, submitted significant work to clarify how and why hafnium oxide transforms into its useful ferroelectric phase.
A hafnium oxide-based thin film can be stabilized by being sandwiched between a metal substrate and an electrode, as shown in Fields’ paper, Origin of Ferroelectric Phase Stabilization via the Clamping Effect in Ferroelectric Hafnium Zirconium Oxide Thin Films, which was published in August in Advanced Electronic Materials. Studies have shown that more of the film stabilizes in the ferroelectric crystalline phase when the top electrode is in place during thermal annealing and cooling.
Fields stated, “The community had all kinds of theories as to why this happened, and it turns out we were mistaken.” The hafnium oxide was believed to be stopped from stretching out and returning to its original, non-ferroelectric form by the top electrode, which we believed to impose some sort of mechanical force spreading laterally across the plane of the electrode. According to my research, the electrode has a clamping effect while the mechanical tension is moving out of plane.
Because the whole sandwich, which is made up of the substrate, the thin film, and the electrode, works as a capacitor, this discovery could change the materials that semiconductor makers use for the electrodes.
“We now see why the top layer is such a crucial factor. “In the future, people who want to combine compute and memory on a single chip will have to carefully consider every processing step,” Fields remarked.
Fields’ paper is a summary of the latter section of his dissertation study. In earlier research that was published, Fields showed how to quantify mechanical stresses and very thin films; the tiny materials made stress measurements experimentally challenging.
Samantha Jaszewski, Ale Salanova, Takanori Mimura, and Brian Sheldon from Brown University; David Henry from Sandia National Labs; Kyle Kelley from Oak Ridge National Lab; and Helge Heinrich from UVA’s Nanoscale Materials Characterization Facility are all contributors to this collaborative study. Grants from the Semiconductor Research Corporation and the US Department of Energy’s 3D Ferroelectric Microelectronics Energy Frontier Research Center paid for the study.
According to Fields, we sought to support our characterization of the behavior of the substance with statistics rather than just anecdotal descriptions. “I’m delighted we were able to give the neighborhood more information about this clamping effect. We can engineer the top layer to improve the clamping effect because we know how important it is, and we may also engineer the bottom layer to aid in this effect. The semiconductor industry would greatly benefit from having the ability to manipulate the crystalline phase with just one experimental variable. I wish someone would ask and respond to that query.
It denotes the location.
Samantha Jaszewski, a materials science and engineering doctoral student who works with Ihlefeld’s Multifunctional Thin Film research team, may be that person. Jaszewski is also trying to figure out how chip designers can change the way the material acts and what makes the ferroelectric phase of hafnium oxide stable.
The subject of Jaszewski’s research is the atomic structure of hafnium oxide in both its natural and ferroelectric phases, with particular emphasis on the function of oxygen atoms. In the October 2022 issue of Acta Materialia, her seminal paper, Impact of Oxygen Content on Phase Constitution and Ferroelectric Behavior of Hafnium Oxide Thin Films Deposited by Reactive High-Power Impulse Magnetron Sputtering, is published.
As the name suggests, hafnium oxide is made up of oxygen and hafnium atoms. According to Jaszewski, “sometimes we are lacking those oxygen atoms in specific locations, and that helps stabilize the ferroelectric phase.”
Some of these oxygen vacancies can be tolerated in their natural, non-ferroelectric state, but not enough of them to maintain the ferroelectric phase. Since there aren’t many ways to make a precise measurement, it’s hard to figure out the exact number and location of oxygen vacancies that make hafnium oxide ferroelectric.
To get around this issue, Jaszewski measured oxygen vacancies in the team’s thin films using a variety of approaches and connected those results with ferroelectric characteristics. She found that the amount of oxygen vacancy required for the ferroelectric phase is substantially larger than previously believed.
The method of choice for determining oxygen vacancy concentrations was X-ray photoelectron spectroscopy. Jaszewski says that contributing factors that go beyond what most people who use this spectroscopic method measure cause a significant undercount of the oxygen vacancies.
The results of Jaszewski’s research further suggest that oxygen vacancies might be one of, if not the most crucial, factors in maintaining the material’s ferroelectric phase. More study is required to comprehend why there are vacancies. She would also like to see other research teams test the oxygen vacancies using the same method she did. This would help her prove what she has found.
Conventional wisdom holds that the hafnium oxide is stabilized by the size of the crystal, or “grain,” but Jaszewski’s research disproves this notion. Three samples with comparable grain diameters and various oxygen vacancy concentrations were created by Jaszewski. Because the phases of these samples were different, her research led her to believe that oxygen vacancy concentration is more important than grain size.
The work was co-authored by group members Fields and Salanova as well as others from numerous research groups both inside and outside of UVA, with Jaszewski serving as the paper’s lead author. The Semiconductor Research Corporation and Jaszewski’s graduate research fellowship from the National Science Foundation both contribute to the cost of her studies.
Jaszewski is focusing her research on hafnium oxides to better understand how the substance reacts to an electric field. This phenomenon is referred to as wake-up and fatigue in the semiconductor industry.
The ferroelectric characteristics of this material enhance or “wake up” when an electric field is applied. “The ferroelectric characteristics deteriorate as the electric field is applied, a phenomenon known as fatigue,” explained Jaszewski.
She has found that putting an electric field on a ferroelectric structure for the first time makes it better, but the benefits go away over time.
According to Jaszewski, the ferroelectric characteristics deteriorate when the field is applied more and more.
The next step is to look at how the arrangement of oxygen atoms in the material affects waking up and feeling tired. This means looking at where vacancies are dynamically located.
These ground-breaking investigations “explain the existence and stabilization of ferroelectric hafnium oxide,” according to Ihlefeld. “We can build hafnium oxide thin films to be even more stable and perform even better in a real application based on these new discoveries. We can assist semiconductor companies in understanding the root causes of issues and how to avoid them in their next manufacturing lines by conducting this fundamental research.
