ASU Research E-Magazine: LEEM Reveals Mysteries at the Surface

Scientists like answers, not mysteries. In the continuing quest for information from the world of the ultra-microscopic, ASU scientists are steps ahead of their colleagues at other institutions because they have one of the few low energy electron microscopes (LEEM) available in the world today. ASU researchers use the LEEM to watch and understand the formation of thin films on the surface of semiconductor devices.

“As we build more sophisticated devices, we have to grow more things or put more things on the surface of these devices,” says Ignatius Tsong, an ASU professor of physics. “When you jam more onto the surface, you really need to know what you are doing. Using the LEEM allows us to observe the process as it is happening. In fact, we can introduce modifications right in the microscope and observe the results as they occur.”

The LEEM at ASU is the work of Ernst Bauer. Bauer currently divides his time between ASU and the Technical University of Clausthal in Germany, where he is a professor and director of the Physics Institute. By 1996, Bauer plans to devote full time to his work at ASU as a distinguished research professor.

Bauer pioneered thin film growth and invented and developed techniques to study the thin films. LEEM is one of those techniques. Tsong believes the combination of the LEEM and Bauer’s presence strengthens ASU’s national role in semiconductor research.

“Ernst Bauer developed the theoretical principles of thin film growth and coined the terminology that all of us in this field use today,” Tsong says. “In our work, he is something of a legend.”

Bauer felt that ASU was the best place to continue his research after retiring from his position in Germany. “There is very good potential for research in combination with the local industry here in Arizona,” he says.

Bauer continues to fine-tune the LEEM he built for use by ASU scientists. “I want it to work as perfectly as possible,” he explains. “I feel I must demonstrate its full capabilities before we attempt collaborative work with industry.”

Scientists use different methods to grow thin films of semiconductor materials such as silicon and gallium arsenide. One process involves using chemical vapors to deposit thin films of metal or polysilicon on a defined area. In other processes, heating is required.

six video images taken from the LEEM
LEEM video images showing real-time growth of silicon nitride islands on a silicon substrate. The silicon surface is heated to 900 degrees Celsius and exposed to ammonia gas. The images are in sequence, left to right and top to bottom, beginning from the introduction of the ammonia at 0:36 seconds through 12:30. The bright features are the islands.

At the College of Engineering and Applied Sciences, ASU researchers are studying manufacturing techniques to develop a better control system for limited production of semiconductor devices. “As a process, the formation of semiconductor materials at the molecular level is still magic, to an extent,” says Michael Kozicki, an ASU professor of electrical engineering. “We don’t know nearly as much as we ought to. We know enough to get by.”

For most of the past 40 years, Bauer’s work has been an attempt to shed more light on this “magic” process. LEEM is one result of that work. Development of the instrument spans nearly three decades and two continents.

The LEEM at ASU is a Y-shaped device. An electron gun at the tip of one of the branches emits a stream of slow, low-energy electrons. The electron stream bombards the surface of the semiconductor device under study, which is located at the base of the Y.

“These slow electrons are reflected, because the surface acts like a mirror,” Bauer explains.

At the other branch of the Y, the stream of reflected electrons is collected to produce an image of the device’s surface. Scientists can photograph the surface or view in real time on a computer screen.

“The process also allows us to get a three-dimensional view of the surface,” Bauer says.

Despite the small number of LEEM instruments in operation worldwide, Bauer says that a large amount of valuable information on surface structure and processes already has been collected using the technique. Still, he says, there still is plenty of room for improvement.

“Future efforts will focus on image processing and analysis,” he says. “The amount of data produced by a well functioning microscope is immense. But without efficient image-analysis techniques, much information can be lost.

“There is a need for simpler analytical methods that can be incorporated into the variety of surface science systems being used today,” Bauer adds. But rather than building a LEEM for every laboratory, Bauer envisions “a few sophisticated instruments with very experienced users.”

Such a situation is strikingly similar to that in the United States today, where only two functional LEEMs exist: one at IBM laboratories and one at ASU.—Grant E. Smith