To improve data storage capabilities, researchers are perfecting 3D NAND flash memory, stacking memory cells to maximize space.
Researchers have discovered a faster and more efficient way to etch deep holes into 3D NAND flash memory using advanced plasma processes. By tweaking the chemistry, they doubled the etching speed and improved accuracy, paving the way for denser, higher-capacity memory storage.
Explore the future of data storage
As electronic devices become smaller and more data-intensive, the way we make digital memory needs to be improved. Researchers in a public-private partnership are exploring new ways to create atomic-scale digital memory to meet the growing need for denser data storage.
A key focus is improving the manufacturing process for 3D NAND flash memory, a technology that stores data vertically to maximize storage capacity. A recent study published in the Journal of Vacuum Science & Technology A found that using the right combination of plasma and other key materials can double the speed at which the deep, narrow holes needed for this memory can be etched. The research was conducted using simulations and experiments by scientists at Lam Research, the University of Colorado Boulder, and the U.S. Department of Energy’s Princeton Plasma Physics Laboratory ( PPPL ).
NAND flash memory is a type of non-volatile storage, meaning it retains data even if there is a power outage. “Most people are familiar with NAND flash memory because it is the type of memory found in digital camera memory cards and flash drives, and is also used in computers and mobile phones. This will make this type of memory even denser — to pack more data into the same area — increasingly important as our need for data storage grows with the use of artificial intelligence ,” said Igor Kaganovich, a principal research physicist at the Institute of Information and Communications Technology.PP.
Stack memory cells to save space
Digital memory stores information in units called cells. Data is stored as cell states, where each cell is either on or off. With traditional NAND flash memory, the cells are arranged in a single layer. In 3D NAND flash memory, multiple memory cells are stacked on top of each other to hold more data in a smaller area. It’s like swapping a wooden house for a 10-story apartment building to accommodate more people.
A key step in creating these stacks is punching holes into alternating layers of silicon oxide and silicon nitride. The holes can be etched by exposing the layered material to chemicals in the form of a plasma (partially ionized gas). Atoms in the plasma interact with atoms in the perforated layered material.
The researchers wanted to improve the way they created these holes, so that each hole was deep, narrow, and vertical, with smooth edges. Getting the exact formula right is difficult, so the researchers continued to experiment with new ingredients and temperatures.
Using plasma to create deep and narrow channels
“These processes use plasma as a source of high-energy ions,” said Yuri Barsukov, a former PPPL researcher now at Lam Research. Using the charged particles present in plasma is the easiest way to create the very small but deep circular holes needed for microelectronics, he says. However, the process, known as reactive ion etching, is not yet fully understood and could be improved. One recent development involves keeping the wafer—the sheet of semiconductor material to be processed—at low temperatures. This emerging method is called cold etching.
Traditionally, cold etching uses hydrogen gas and fluorine gas separately to create holes. The researchers compared the results of this process with a more advanced cold etching process that uses hydrogen fluoride gas to create a plasma.
“Cold etching using hydrogen fluoride plasma shows a significant increase in etch rate compared to previous cold etching processes where you use separate fluorine and hydrogen sources,” says Thorsten Lill of Lam Research, a Fremont, California-based company that provides wafer fabrication equipment and services to chip manufacturers.
Double your engraving speed with a new method
When silicon nitride and silicon oxide were tested separately, etch rates were increased for the nitride layer and oxide layer using a hydrogen fluoride plasma instead of separate hydrogen and fluorine gases. While the effect was more pronounced for silicon nitride than for silicon oxide, simultaneous etching of both materials yielded the most significant gains. In fact, the etch rate for alternating layers of silicon oxide and silicon nitride doubled, increasing from 310 nanometers per minute to 640 nanometers per minute. (A human hair is about 90,000 nanometers wide.)
“The quality of the engraving also seems to have improved, and that’s important,” says Lill.
The researchers also studied the effects of phosphorus trifluoride, a key ingredient in etching silicon dioxide to any significant degree. Although it had been used before, the researchers wanted to better understand and quantify its effects. They found that adding phosphorus trifluoride increased the etching rate for silicon dioxide by a factor of four, although it only slightly increased the etching rate for silicon nitride.
Another chemical compound the researchers studied was ammonium fluorosilicate, which is formed during the etching process when silicon nitride reacts with hydrogen fluoride. Research shows that ammonium fluorosilicate can slow down the etching process, but water can offset this effect. According to Barsukov’s simulations, water weakens the bonds of ammonium fluorosilicate. “The salt can decompose at lower temperatures than when water is present, which can speed up the etching process,” Barsukov says.
Laying the foundation for future research
Kaganovich said the research is also important because it shows how scientists from industry, academia and national laboratories can work together to answer important questions in microelectronics. It also brings together information gained from experimentalists and theorists.
“We’re building bridges to the larger community,” he said. “This is a necessary step for people to better understand the semiconductor manufacturing process.”
Lill said he appreciates collaborating with PPPL on semiconductor fabrication research because PPPL’s research provides many plasma simulation options for microelectronics.
Reference: “Low-temperature etching of silicon oxide and silicon nitride with hydrogen fluoride” by Thorsten Lill, Mingmei Wang, Dongjun Wu, Youn-Jin Oh, Tae Won Kim, Mark Wilcoxson, Harmeet Singh, Vahid Ghodsi, Steven M. George, Yuri Kagansuch and Yuri Kagansuch06 A110106 Igor 9
Funding for this research was provided by the PPPL Laboratory Directed Research and Development program.
