Patent ID: 9691773
Date: 2017-06-27
CPC Classifications: H01L

Claim:
1. An access device comprising: a substrate; a plurality of first trenches in the substrate extending along a first direction; a first oxide liner on walls of the first trench; a buried digit line (BDL) in the substrate along the first trenches, wherein the buried digit line comprises a doped region of silicon, and wherein the buried digit line does not contain metal; a plurality of second trenches formed in each first trench, wherein the second trenches present undercuts; a plurality of third trenches formed in each second trench, wherein a depth of the third trenches is deeper than that of the doped region; a second oxide liner on walls of the first, second and third trenches; a filling material conformally filling the first, second and third trenches, wherein airgaps are formed in the second trenches during the filling process; a plurality of word line (WL) trenches in the substrate extending along a second direction, wherein a depth of the word lines trenches is shallower than that of the buried digit lines; an oxide liner on walls of the word line trenches; a metal layer on parts of the walls of the word lines trenches, wherein the metal layer serves as word lines; and a filling material filled into the word line trenches.