The process for creating the narrow, deep holes needed for a specific type of flash memory has become significantly quicker, thanks to a specialized formula that includes hydrogen fluoride plasma.
To accommodate the increasing amount of data needed in compact electronic devices, a closer examination of their manufacturing processes is essential. Researchers involved in a collaboration between public and private sectors are exploring innovative methods for constructing digital memory at the atomic level, aiming to meet the ever-growing demand for more compact data storage.
One initiative is dedicated to refining the production method for 3D NAND flash memory, a form of digital memory that stacks data vertically to boost storage capacity. A recent study published in the Journal of Vacuum Science & Technology A reports that using the appropriate plasma and other vital components can double the speed at which these narrow, deep holes are etched. This research involved simulations and experiments by experts from Lam Research, the University of Colorado Boulder, and the Princeton Plasma Physics Laboratory (PPPL), which is part of the U.S. Department of Energy.
NAND flash memory is a type of nonvolatile storage; it retains data even when the device is powered off. “Most people are familiar with NAND flash memory because it’s commonly found in memory cards for digital cameras and USB drives. It’s also utilized in computers and smartphones. Enhancing the density of this memory is crucial as our data storage requirements increase, particularly with the rise of artificial intelligence,” explained Igor Kaganovich, a leading research physicist at PPPL.
Maximizing Space with Stacked Memory Cells
Digital memory holds information in units called cells, where each cell represents a binary state of either on or off. Traditional NAND flash memory arranges cells in a single layer, while 3D NAND flash memory stacks many cells atop one another to optimize storage in a smaller area. It’s similar to replacing a single-story house with a high-rise building to accommodate more residents.
A key part of creating these stacked configurations involves drilling holes through alternating layers of silicon oxide and silicon nitride. This drilling process entails exposing the layered structure to chemicals in the form of plasma (partially ionized gases), where the atoms in the plasma interact with those in the materials being etched to form the holes.
Researchers aim to enhance the hole-making process so each hole is precisely deep, narrow, and vertical, with smooth walls. Perfecting this technique has proven challenging, prompting continuous trials with new materials and temperature settings.
Utilizing Plasma for Precise Channel Creation
“These etching processes employ plasma as a high-energy ion source,” said Yuri Barsukov, a former researcher at PPPL who now works at Lam Research. He noted that using charged particles from plasma presents the most straightforward method for creating the very small but deep holes necessary for microelectronics. However, the reactive ion etching methodology is not fully understood and has potential for improvement. A significant recent advancement is maintaining a low temperature for the wafer during processing, referred to as cryo etching.
Standard cryo etching utilizes separate hydrogen and fluorine gases to form the holes. The research team assessed the outcomes of this traditional method against a more sophisticated cryo-etching process using hydrogen fluoride to generate the plasma.
“Using cryo etching with hydrogen fluoride plasma demonstrated a considerable boost in the etching rate compared to earlier cryo-etching methods employing separate hydrogen and fluorine gases,” said Thorsten Lill from Lam Research, a company based in Fremont, California, that provides fabrication equipment and services to chip manufacturers.
Doubling the Etching Speeds
When evaluating silicon nitride and silicon oxide separately, a notable increase in etching rates was observed for both when hydrogen fluoride plasma was used rather than the separate hydrogen and fluorine gases. The improvement was particularly significant for silicon nitride, though maximum elevation occurred when etching both materials at once. In fact, the etching rate for the alternating layers of silicon oxide and silicon nitride more than doubled, rising from 310 nanometers per minute to 640 nanometers per minute. (For context, a human hair is approximately 90,000 nanometers thick.)
“The quality of the etch has also noticeably improved, which is crucial,” Lill added.
The researchers further examined the role of phosphorus trifluoride, a critical component when achieving significant etching of silicon dioxide. Though previously used, the researchers sought to clarify its specific effects. They discovered that incorporating phosphorus trifluoride quadrupled the etching rate for silicon dioxide, although it had only a minimal impact on silicon nitride.
Another substance investigated was ammonium fluorosilicate, which forms during the etching reaction between silicon nitride and hydrogen fluoride. Their findings showed that this compound could hinder the etching process, but the presence of water could mitigate this issue. According to Barsukov’s simulations, water weakened the bonds of ammonium fluorosilicate. “The salt can break down at a lower temperature in the presence of water, accelerating the etching process,” Barsukov noted.
Establishing a Basis for Future Research
Kaganovich emphasized that this research is significant as it highlights the fruitful collaboration between industry, academia, and national laboratories to tackle crucial inquiries within microelectronics. It also fosters a collective approach combining insights from experimentalists and theorists. “We’re forging connections within the broader scientific community,” he stated. “This is a vital step towards a deeper understanding of semiconductor manufacturing processes for everyone.”
Lill expressed his appreciation for collaborating with PPPL on semiconductor research, as PPPL’s expertise in plasma simulation greatly contributes to advancements in microelectronics.
This research was supported by the Laboratory Directed Research and Development program from PPPL.
