Preventing 3D NAND Layer Bending: Understanding the Cause

Key Takeaways

3D NAND flash memory faces significant manufacturing challenges, including tier bending and collapse, particularly as layer counts increase.
Research using SEMulator3D identified the importance of matching compressive stress and axial strain in SiN and oxide layers to improve process yield.
Adjustments in material thickness and cantilever length can mitigate tier collapse risks and enhance overall device performance.

Understanding 3D NAND Flash Memory Challenges

3D NAND flash memory technology is based on stacking multiple layers of silicon nitride (SiN) and oxide (TEOS) materials to facilitate higher memory densities. Current manufacturing processes achieve stacks exceeding 200 layers, with expectations for over 300 layers soon. However, this multi-layering presents challenges such as tier bending and tier collapse, which can hinder production efficiency.

Research conducted with SEMulator3D, a process modeling platform, explored the factors contributing to tier collapse. Key culprits include intrinsic stress and strain within the SiN and TEOS materials during the manufacturing process, which can be exacerbated by the cantilever’s length and the techniques employed for material removal. As layer counts increase, the cantilever—formed during manufacturing—grows longer, heightening its vulnerability to collapse. Measurements indicate that the cantilever length can extend from 550 nm in a 200-layer structure to 700 nm in a 300-layer structure.

To effectively address these issues, initial design experiments employed virtual Design of Experiments (DOE) methodology. This approach assessed the interplay between material stress, layer thickness, and cantilever length. Results unveiled that reducing and balancing compressive stress and axial strain of both SiN and oxide layers is crucial for enhancing yield and process efficiency.

In testing different scenarios, the study found that significant displacement and even tier collapse were noted when the Young’s Modulus (Ey) of SiN reached high levels, particularly when both SiN and oxide layer thicknesses increased. Conversely, at lower Ey values, displacement was minimal, suggesting that material properties play a critical role in device stability.

A follow-up DOE further refined these findings, comparing titanium silicon nitride (SiN) intrinsic stress and Ey values. Insights from this modeling activity reveal that compressive SiN allows broader displacement ranges compared to tensile SiN, underscoring the requirement for precise material selections during the manufacturing process.

The SEMulator3D Stress Analysis package is highlighted as an effective tool for engineers to simulate and analyze these stresses without the necessity of costly and lengthy physical testing. This capability facilitates faster process development, enhancing the decision-making process for in-fab experimental work.

In summary, as the complexity of 3D NAND structures increases, the necessity to manage stress within the material layers becomes paramount. Adjustments in material characteristics and manufacturing techniques will be vital in overcoming the challenges posed by tier bending and collapse, driving efficiencies in NAND flash memory production.

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