An artist’s representation of a hole etched into alternating layers of silicon oxide and silicon nitride using plasma, to make 3D NAND flash memory. Researchers want to refine how they make these holes so each one is deep, narrow, and vertical, with smooth sides. Credit: Kyle Palmer/ PPPL Communications Department. CC BY.

The hardware for one type of flash memory can be made twice as fast with a recipe, which includes a plasma made from hydrogen fluoride.

To store ever more data in electronic devices of the same size, the manufacturing processes for these devices need to be studied in greater detail.

By investigating new approaches to making digital memory at the atomic scale, researchers engaged in a public-private partnership are aiming to address the endless demand for denser data storage.

One such effort has focused on developing the ideal manufacturing process for a type of digital memory known as 3D NAND flash memory, which stacks data vertically to increase storage density.

The narrow, deep holes required for this type of memory can be etched twice as fast with the right plasma and other key ingredients.

NAND flash memory is a type of nonvolatile data storage, meaning that it retains data even if the power to the device is off.

Igor Kaganovich, a principal research physicist at US Department of Energy’s Princeton Plasma Physics Laboratory (PPPL), said: “Most people are familiar with NAND flash memory because it’s the kind that is in the memory cards for digital cameras and thumb drives. It is also used in computers and mobile phones.

“Making this type of memory denser still — so that more data can be packed into the same footprint — will be increasingly important as our data storage needs grow due to the use of artificial intelligence.”

Stacking memory cells to save space

Digital memory saves information in units called cells. Data is saved as the cell’s state, 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, many memory cells are stacked on top of each other to fit more data in a smaller footprint. It’s akin to replacing a bungalow with a ten-storey apartment to house more people.

A critical step in creating these stacks involves carving 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 plasma (partly ionised gases). Atoms in the plasma interact with atoms in the layered material, carving out the holes.

Researchers want to refine how they make these holes so each one is deep, narrow, and vertical, with smooth sides.

Getting the recipe just right has been tough, so scientists have continued to test new ingredients and temperatures.

Using plasma to create deep, narrow channels

Yuri Barsukov, a former PPPL researcher now working at Lam Research, said: “These processes use plasma as a source of high-energy ions.”

“Using the charged particles found in plasma is the easiest way to create the very small but deep, circular holes needed for microelectronics.”

However, the process, known as reactive ion etching, isn’t fully understood and could be improved.

One recent development involves keeping the wafer – the sheet of semiconductor material to be processed – at a low temperature. This emerging approach is called cryo etching.

Traditionally, cryo etching uses separate hydrogen and fluorine gases to make the holes.

The researchers compared results from this process to a more advanced cryo-etching process that uses hydrogen fluoride gas to create the plasma.

Lam Research’s Thorsten Lill, said: “Cryo etch with the hydrogen fluoride plasma showed a significant increase in the etching rate compared to previous cryo-etch processes, where you are using separate fluorine and hydrogen sources.”

Doubling etch rates

When silicon nitride and silicon oxide were tested separately, the etch rate increased for the nitride and the oxide layer using the hydrogen fluoride plasma instead of the separate hydrogen and fluorine gases.

While the effect for silicon nitride was more pronounced than for silicon oxide, etching both materials simultaneously yielded the most significant increase.

In fact, the etching rate for the alternating silicon oxide and silicon nitride layers more than doubled, increasing from 310 nanometres per minute to 640 nanometres per minute. For comparison, a human hair is roughly 90,000 nanometres in width.

Lill said: “The quality of the etch seems to have improved as well, and that’s significant.”

The study is published in the Journal of Vacuum Science & Technology A.