How to minimize CD variation and overlay degradation induced by film stress

It is getting harder to minimize feature size to satisfy bit growth requirement. 3D NAND flash memory has been developed to meet bit growth requirement without shrinking feature size. To increase the number of memory cells per unit area without shrinking feature size, we should increase the number of stacked film layers which finally become memory cells. Wafer warpage is induced by the stress between film and wafer. Both of film stress and wafer warpage increase in proportion to stacked film layers, and the increase of wafer warpage makes CD uniformity worse. Overlay degradation has no relation with wafer warpage, but has indirect relation with film stress. Wafer deformation in film deposition chamber is the source of overlay degradation. In this paper, we study the reasons why CD uniformity and overlay accuracy are affected by film stress, and suggest the methods which keep CD uniformity and overlay accuracy safe without additional processes.