ASU Research E-Magazine: The Skinny on Thin Films

New types of ceramic thin films may prove key elements in the development of smaller, faster, more powerful electronic devices.

Need a quick primer on the rapid advance of technology? All you need to do is take a close look at the props used in feature films that are more than five years old. The actors are using portable telephones and laptop computers that seem hopelessly big and clunky.

As technology marches on, electronic devices continue to shrink in size. Even as they get smaller in physical dimensions, they increase in speed and are packed with more memory than ever before. In 2004, the new generation of svelte cell phones can send text messages, check e-mail, and even take photos. Phones today are much more than just phones.

The electronics industry continues to improve its products at an astounding pace simply because of the ability to exponentially decrease the size of circuit building blocks. New designs and materials allow researchers to tightly pack more and more tiny circuits onto microchips. Unfortunately, the materials now in use can’t get much smaller and still provide the same level of performance.

The electronics industry is looking toward materials scientists such as Sandwip Dey to process new materials. Dey is a professor of materials engineering and an adjunct professor of electrical engineering at Arizona State University.

Dey works with ceramics. To most people, the term ceramics conjures images of dinnerware, pots, cups, and sinks. To a materials scientist, ceramics are usually insulators; they do not conduct electricity.

Because of this property, ceramic materials have always been used in electronic devices. Ceramics also have a variety of other properties that can be useful in electronics and other applications.

Dey and his colleagues work at the interface of chemistry, physics, materials science, and electrical engineering to create new ceramics. He explains that ceramics are used to make gate oxides. Gate oxides pose one of the major hurdles engineers must overcome to shrink the size of electronic devices.

The basis of most modern electronics is complementary metal oxide semiconductors (CMOS). The CMOS consists of gate metals, gate oxides, and semiconductors with different impurities.

Semiconductors are materials that conduct electricity when they contain impurities or are heated to a high temperature. The best known semiconductor material is silicon. Gates are structures that control the flow of electricity.

In CMOS devices, the gate is made of the gate oxide, an insulating ceramic material, with a gate metal on top. As electronic devices continue to get smaller, the insulator must be thinner. But it also needs to be more insulating to account for everything on the device being more tightly packed.

To create reliable devices, scientists must meet extremely stringent processing and property requirements. These restrictions require new gate oxide and gate metal pairs that Dey and his ASU team are evaluating right now.

“The central theme behind our research is to determine the relationships between processing, structure, and properties,” Dey says. “Once we understand this, we can make materials with very high performance because we can control them.”

Dey’s focus on the processing-structure-properties relationship began years ago during his postdoctoral fellowship at the University of Illinois at Urbana-Champaign. Each afternoon, members of the electrical engineering, materials science, and physics departments would get together for coffee. During stimulating—and sometimes heated—conversations with colleagues, Dey got the message. If he were to ever exploit the multi-functional materials he works on, he needed to make reliable and reproducible thin films. To do that, he needed to understand and eventually control the relationships between structure, processing, and properties.

Scientists know that a good-sized chunk of any material exhibits certain properties. However, a tiny amount of the same material made into a thin film will often exhibit different properties. Why? Because so much of the material is exposed to and interacting with every bit of the surrounding environment. Understanding the physics of the interfaces and how they influence the overall properties is critical. Also, different processing schemes induce different structures in thin films. These different structures also change the film’s properties.

Dealing with the gate insulator problem is a major effort in Dey’s laboratory. As insulators, ceramics can be used to filter electronic noise in wireless communication systems or in high density random access memory (RAM). But ceramic materials have other properties that are also useful in electronics.

For example, ceramics are ferroelectric. This means that they can be permanently polarized. When a voltage is applied across a ceramic material, positive and negative charges will line up in an orderly fashion. Dey says that this makes them suitable for non-volatile memory, which is what a computer uses to remember settings when the power is turned off.

“For the next generation of memory, this is one of the technologies people are studying,” Dey continues. “The military is very interested because these materials are radiation hard—the system will retain its memory even if there’s a nuclear explosion.”

Ceramics also respond to pressure and heat, which makes them useful in sensors and actuators. Ceramics have been used in infrared detectors. They have the advantage of working at room temperature. Most infrared detectors must be cooled.

Dey’s group also is looking at how ceramics can be incorporated into molecular electronics and biosensors. He has placed a layer of organic molecules between metal and ceramic thin films. This “sandwich” can be used to tune electrical characteristics of bio-nanodevices or even be the basis of a novel biosensor.

The ASU researcher also is developing ceramic nanostructures for gene and drug delivery. His current study aims to develop biodegradable ceramic nanostructures that will release a drug slowly over time. Once the structures are created, they will be tested for safety and efficacy.

“With some of the future technologies we’re looking at, I think we have the potential to make a significant impact,” Dey adds. “This impact is not just going to be in the form of writing a research paper. We will impact the commercial market in a big way.”—Linley Erin Hall