Unraveling the Future: Exploring the Advancements in 3D NAND Memory Technology
The world of data storage is constantly evolving, and 3D NAND flash technology is at the forefront of this revolution. However, scaling the z-pitch (vertical pitch) in 3D NAND presents a set of unique challenges that must be addressed to ensure reliable and high-performance memory storage.
The Challenges of Z-Pitch Scaling
The key challenges in z-pitch scaling involve achieving finer vertical stacking with high accuracy and reliability due to the increasing density of layers. These challenges include physical and process limitations, patterning constraints, and mechanical and material stresses.
Physical and process limitations arise as vertical layers increase, making it difficult to control layer thickness and uniformity. Scaling bump pitch (interconnect pitch) beyond 10 µm towards 2 µm faces significant challenges in manufacturing precision and defect control.
Patterning constraints are another issue, with lithography methods such as self-aligned double patterning (SADP) and quadruple patterning used to reduce lateral pitch, but vertical pitch scaling in 3D structures being more complex and influenced by these lateral capabilities. Traditional lithography tools pose challenges for patterning very fine features reliably.
Mechanical and material stresses also play a significant role. High layer counts produce mechanical strain and potential defects in stacking and bonding layers, especially as nanometer-scale variations can cause bond failure.
Solutions to Z-Pitch Scaling Challenges
Solutions to these challenges largely focus on advanced lithography and process integration techniques. Multiple patterning techniques, such as advanced double and quadruple patterning methods, are employed to pattern critical layers and achieve precise feature sizes needed for smaller pitch.
Improved metrology and defect detection are essential as pitches shrink. Detecting nanometer-scale height, tilt, and contamination variations becomes crucial to prevent bond failures and ensure yield.
Innovative material and process engineering is another approach. Using CMOS-compatible, low-temperature fabrication processes enables better layer control and integration of advanced actuators or strain relief mechanisms to mitigate mechanical stresses.
Advanced lithography technologies, including inverse lithography and mask optimization, are explored to overcome patterning limits at small scales.
The Future of 3D NAND
Imec, a world-leading research and innovation hub, is developing key technologies to support aggressive z-pitch scaling while preserving memory operation and reliability. One such technology is the integration of airgaps between adjacent word-lines in 3D NAND, which reduces electrostatic coupling between memory cells.
Another innovation is the charge trap cut, which would drill even deeper into the memory cell, potentially increasing the memory window of the cell and enabling more bits per cell.
To increase the storage density of 3D NAND, companies are investing in several complementary tools, including tier stacking, increasing the number of bits per cell, and reducing the x-y pitch of the GAA cell (lateral scaling).
The airgap integration scheme is considered a crucial step in enabling future z-pitch scaling. These developments will allow the memory industry to gradually move to 100 Gb/mm of data storage, a demand driven primarily by cloud computing and AI applications.
Looking Ahead
As the industry continues to push the boundaries of 3D NAND, researchers are taking a hard look at more radical cell architectures to carry the memory roadmap beyond 2030. These include arranging the conduction channels of the memory cells horizontally instead of vertically, or connecting the charge trap memory cells through a trench-based architecture.
The manufacturing process for the GAA channel in 3D NAND involves stacking alternating layers of conductor and insulator, forming cylindrical holes, and depositing alternating layers of silicon oxide and SiN on the sidewalls of the hole. The charge trap cell, which stores charges in insulators, has replaced the floating-gate transistor in 3D NAND, improving read and write performance and allowing for higher densities.
In conclusion, the challenges in z-pitch scaling for 3D NAND flash are significant, but with advancements in lithography, process integration, and material engineering, the industry is well-positioned to overcome these hurdles and continue delivering high-density, reliable memory storage solutions.
Science and technology play crucial roles in addressing the challenges of z-pitch scaling in 3D NAND flash storage. While physical and process limitations, patterning constraints, and mechanical and material stresses pose formidable issues, solutions lie in the use of advanced lithography and process integration techniques, improved metrology and defect detection, innovative material and process engineering, and the exploration of advanced lithography technologies. Future research efforts focus on more radical cell architectures to extend the memory roadmap beyond 2030.
