Scaling New Heights: How Molybdenum is Revolutionizing 3D NAND Architectures

In our interconnected world, we depend on computing power to reliably store ever-growing quantities of data. However, to meet this demand, the semiconductor industry must pack more performance and greater storage capability into a smaller footprint. And so, the 3D architecture emerged.

NAND, a non-volatile memory, is a critical component across major electronic end-use markets—including smartphones, servers, PCs, tablets, and USB drives. With AI and cloud infrastructure driving the demand for greater computing power and data storage, chipmakers have migrated from 2D/planar (reaching its scaling limit at the 15 nm node) to 3D NAND memory with 3D stacking becoming the primary way to achieve more storage in a smaller footprint. Performance has been improved with each generation of technology by the scaling of the dimension, the addition of more and higher performing transistors, and the improvement of electrical transmission.

These three-dimensional vertically stacked memory architectures offer exponential gains in flash memory storage, but they also introduce fundamental new challenges at the device fabrication and integration level. These challenges become more acute as architecture is pushed to 300 layers and beyond. As the number of device layers keeps increasing, the demands on the materials in the layers and aspect ratios become more extreme.

Goodbye Tungsten, Hello Molybdenum: Redefining Performance in Word Lines and Bit Lines

Acting as conductive material, tungsten has been the key metal for word lines and bit lines in 3D NAND architecture for years. Deposited using Chemical Vapor Deposition (CVD) or Atomic Layer Deposition (ALD), tungsten successfully fills the narrow, high-aspect ratio channels within 3D structures. However, as layer counts increase, filling these channels becomes increasingly difficult.

In this new era of flash memory, a new contender has emerged, Molybdenum (Mo). As a promising alternative to tungsten (W), Mo has several favorable properties for word lines and bit lines in 3D NAND architecture due to several key advantages that address the challenges of advanced memory scaling:

• Lower Resistivity at Small Dimensions: As 3D NAND scales down, the cross-sectional area of word lines shrinks, increasing resistance. Molybdenum has a lower resistivity than tungsten at these small dimensions, which helps reduce RC delay and improve program/erase speeds.

• Better Scalability and Embeddability: Tungsten becomes harder to embed cleanly as layer counts increase and dimensions shrink. This leads to voids that trap unwanted fluorine atoms from its precursor (WF₆), which can degrade dielectric materials and cause leakage failures. Molybdenum, when deposited using fluorine-free precursors like Molybdenum Dichloride Dioxide (MoO₂Cl₂)_, can avoid this issue and enable better embeddability with fewer defects. Molybdenum word lines with improved embeddability have also shown significant lower leakage failure rates compared to tungsten.
• Enhanced Reliability for High-Layer Stacks: As NAND stacks grow taller (300+ layers), maintaining structural and electrical integrity becomes more difficult. Molybdenum supports both vertical and horizontal scaling more effectively, making it a better fit for next-generation architecture.

Rethinking Your Etchant

As 3D NAND structures grow taller, they introduce significant challenges for dry etching high aspect ratio channels and interconnects. However, conventional etching processes designed for tungsten are not effective for molybdenum. In a Mo-filled structure, the recess interacts differently with etchants and cleans. Top-to-bottom uniformity often suffers when using typical etchants like phosphoric acid/acetic acid/nitric acid (PAN). Conventional acidic or alkaline cleans can leave Mo residue on the side walls and result in local inconsistent Mo etching from layer to layer.

Since these conventional etching processes are not effective for molybdenum, the development of high-selectivity Mo etchants is essential.

Entegris offers selective high-performance etchants customized for Mo to help address high aspect ratios and vertical densification. These formulations combine etching and cleaning in a single step resulting in no residues and enabling longer bath life and shelf life.

Molybdenum: The Precursor Changing Advanced Chip Design

Molybdenum is a promising new alternative for 3D NAND applications and is enabling the next generation architectures for advanced devices. As a leader in precursors and delivery systems, Entegris is positioned to help our customers with a smooth transition to molybdenum.

For more information about the transition to Mo, see our white paper, “Migrating to Molybdenum: Comprehensive IC Solutions to Streamline the Transition.”