Metallization Challenges in 3D NAND (invited)
As 2D planar NAND flash memory scaling became increasingly difficult and cost-prohibitive, 3D NAND architecture emerged as a viable alternative. Thin film deposition in complex geometries and Reactive Ion Etching of multilayer stacks has been pivotal in enabling high yield, low cost 3D NAND production. Continued development for future nodes will require revolutionary approaches due to increased loading densities and cost/bit density requirements. This talk discusses the challenges, production-worthy solutions and future directions in a gate-last approach for 3D NAND metallization.
Due to its thermal stability and excellent electric conductivity at small dimensions, tungsten (W) is widely used for both front-end and back-end metallization in the semiconductor industry. In Charge Trap 3D NAND, W control gates or Word Lines (WL) are formed by a replacement process. However, uniform deposition of barrier layers and nucleation films and void-free gap-fill is not a trivial problem in complex geometries and aspect ratios that routinely exceed 50:1. Meanwhile, 3D NAND extendibility requires W-WL thickness reduction so that total stacked pairs or memory layers can keep increasing over future technology nodes, while meeting acceptable levels of resistance. This makes it even more difficult for gap-filling in sideways recessed 3D fins with shrinking dimensions. In addition, severe wafer warp associated with the increased volume of W, imposes limitations on the number of 3D layers that can be fabricated. Moreover, residual fluorine in the WLs from the WF6 precursor diffuses into inner dielectrics and creates reliability problems -making WL metallization a very key technology that determines 3D NAND scaling.
In this talk, we will review the metallization challenges faced in recent 3D NAND nodes, lessons learnt and the upcoming challenges as we continue 3D NAND scaling for the many years to come. This talk will discuss some of the promising technologies for lowering WL resistance, controlling fluorine content and stress management that include: ALD deposition, fluorine-free barriers/films, and touch upon alternative metals for future generations.