Research Bits: July 11

Modeling ALE

Scientists at U.S. Department of Energy’s (DOE) Princeton Plasma Physics Laboratory (PPPL), in coordination with Lam Research, modeled atomic layer etching (ALE) for semiconductor fabrication.

“This would be one little piece in the whole process,” said David Graves, associate laboratory director for low-temperature plasma surface interactions at PPPL and a professor in the Princeton Department of Chemical and Biological Engineering. Insights gained through modeling, he said, “can lead to all sorts of good things, and that’s why this effort at the Lab has got some promise.”

“The simulations basically agreed with experiments as a first step and could lead to improved understanding of the use of ALE for atomic-scale etching,” said Joseph Vella, a post-doctoral fellow at PPPL. He added that an improved understanding will enable PPPL to investigate such things as the extent of surface damage and the degree of roughness developed during ALE.

The model simulated the sequential use of chlorine gas and argon plasma ions to control the silicon etch process on an atomic scale. Plasma, or ionized gas, is a mixture consisting of free electrons, positively charged ions and neutral molecules. The plasma used in semiconductor device processing is near room temperature, in contrast to the ultra-hot plasma used in fusion experiments.

“A surprise empirical finding from Lam Research was that the ALE process became particularly effective when the ion energies were quite a bit higher than the ones we started with,” Graves said. “So that will be our next step in the simulations — to see if we can understand what’s happening when the ion energy is much higher and why it’s so good.”

Switch array for lidar chip

Engineers at the University of California Berkeley built a high-resolution lidar (light detection and ranging) chip using based on a focal plane switch array (FPSA), a semiconductor-based matrix of antennas that gathers light like the sensors found in digital cameras.

The resolution of 16,384 pixels on a 1-centimeter-square chip is a big increase over the 512 pixels or less found on FPSAs until now. It is also scalable to megapixel sizes using CMOS technology.

Lidar works by capturing reflections of light emitted by its laser. Getting the power and resolution available in large, mechanical spinning lidar into a chip format has proved difficult.

“We want to illuminate a very large area,” said Ming Wu, professor of electrical engineering and computer sciences and co-director of the Berkeley Sensor and Actuator Center at the University of California, Berkeley. “But if we try to do that, the light becomes too weak to reach a sufficient distance. So, as a design trade-off to maintain light intensity, we reduce the areas that we illuminate with our laser light.”

The FPSA helps with this. It consists of a matrix of tiny optical transmitters, or antennas, and switches that rapidly turn them on and off. This way, it can channel all available laser power through a single antenna at a time.

Switching, however, poses a problem. Thermo-optic switches are commonly used in lidar chips, but large numbers together can generate too much heat to operate properly. Instead, the team adopted MEMS switches that physically move the waveguides from one position to another.

“The construction is very similar to a freeway exchange,” Wu said. “So, imagine you are a beam of light going from east to west. We can mechanically lower a ramp that will suddenly turn you 90 degrees, so that you are going from north to south.”

When the switch turns on a pixel, it emits a laser beam and captures the reflected light. Each pixel is equivalent to 0.6 degrees of the array’s 70-degree field of view. By cycling rapidly through the array, the FPSA builds up a 3D picture of the world around it. Mounting several of them in a circular configuration would produce a 360-degree view around a vehicle.

Further increasing the FPSA resolution is needed for commercialization, Wu said. “While the optical antennas are hard to make smaller, the switches are still the largest components, and we think we can make them a lot smaller.”

Increasing range from the current iteration’s 10 meters would also be needed. “We are certain we can get to 100 meters and believe we could get to 300 meters with continual improvement,” he said.

Wu believes that lidar won’t be limited to automotive applications. “Just look at how we use cameras. They’re embedded in vehicles, robots, vacuum cleaners, surveillance equipment, biometrics and doors. There will be so many more potential applications once we shrink lidar to the size of a smartphone camera.”

Stretching without distortion

Researchers from The Korea Institute of Machinery and Materials (KIMM) developed a stretchable micro-LED meta-display that can be stretched up to 25% without image distortion.

Typically, stretchable materials like rubber distort, shrinking in one direction when pulled in the other. The new 3-inch display uses mechanical metamaterials with a negative Poisson’s ratio, which were applied to a circuit board. The Poisson’s ratio refers to the ratio at which the width of material shrinks when it is stretched lengthwise. When a mechanical metamaterial with a Poisson’s ratio of -1 is stretched lengthwise, it demonstrates the effect of stretching at the same ratio widthwise.

It can be manufactured by transferring a micro-LED device to a large-area meta-circuit board using roll-based large-area transfer technology. After micro-LED transfer, the meta-display is manufactured through reflow and the board cut into a kirigami pattern using a laser, which enables the stretching.

In demonstrations, the display was attached to a hemispherical surface with a radius of 80 cm without wrinkles or creases, making it possible to build skin-attachable displays that conform to the human body.

The team plans to continue follow-up research and investigate applications in mini-LEDs and graphene, as well as pursue commercialization.