Vertical Scaling

The second approach to 3D NAND scaling is in the vertical direction. This is accomplished by adding more pairs, but this becomes less cost effective as the height and the aspect ratio increase. Shrinking the thickness of each pair allows more layers at the same stack height. But in shrinking the stack height, it gets progressively difficult to remove the silicon nitrate and to fill the space with metal in the replacement gate process.

With added pairs, thicker hard mask deposition and etch are needed for post-processing. Ironically, a thicker hard mask increases the overall stack height, resulting in a higher aspect ratio. In order to resolve this issue, Applied Materials has more selective and low-stress hard mask films compared to conventional hard mask, which enables the hard mask to be thinner.

After the staircase formation, the etched-out area must be filled with dielectric films. This is done with Applied Materials’ Plasma Enhanced High Aspect Ratio Process or PE-HARP dielectric gap fill process. This film has a very high deposition rate with tunable stress and also exhibits the lowest shrinkage postprocess compared to the alternatives.

The next key process for vertical scaling is metal gap fill for high aspect ratio contact. In 3D NAND, CVD tungsten (W) is widely used to make many connections in the vertical direction. With an increase in the stack height, this metal fill process faces two challenges. First, the deep contacts have a barrel-shaped profile. When the contacts are filled with W, the contact can be pinched off at the top, trapping corrosive gas inside, which can damage large parts of the die afterwards. Second, these metals typically grow with a high tensile stress, which can deform the wafer and crack adjacent delicate features. To address these challenges, Applied Materials pioneered Seam Suppressed Tungsten technology. This process includes a nucleation and treatment step that suppresses the film growth on the top and allows more uniform, seamless bottom-up filling.