Silicon buried digit line access device and method of forming the same
An access device includes a plurality of first digit lines (DL) trenches extending along a first direction, buried digit lines between each DL trench, second and third trenches separating the digit lines, a filling material filling the digit line trenches comprising airgaps in each second trench, a plurality of word line (WL) trenches extending along a second direction, metal word lines deposited on the walls of the word line trenches, a filling material filling the word line trenches.
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.
2. The access device according to claim 1 , wherein the first direction and the second direction are perpendicular to each other.
3. The access device according to claim 1 , wherein the filling material is deposited through a High Density Plasma process (HDP) or a Plasma-enhanced chemical vapor deposition (PECVD) process.
4. The access device according to claim 1 , wherein a top surface of the metal layer on parts of the walls of the word line trenches is lower than a top surface of the trench walls.
5. The access device according to claim 1 , wherein the metal layer comprises titanium nitride (TiN).
6. The access device according to claim 1 , wherein a top surface of the filling material in the word line trenches is lower than a top surface of the trench walls, but higher than a top surface of the metal layer.
7. The access device according to claim 1 , wherein the filling material of in word line trenches comprises air gaps.