Method for forming a memory device with C-shaped deep trench capacitors

1. A method for forming a memory device, comprising:

providing a substrate;

forming a capacitor in the substrate or overlying the substrate, wherein the capacitor has a C-shaped pattern at a cross section parallel to a surface of the substrate; and

forming a word line on the substrate;

wherein the capacitor is a C-shaped deep trench capacitor, and the step of forming the C-shaped deep trench capacitor comprises:

patterning the substrate to form a C-shaped deep trench; and

fabricating the C-shaped deep trench capacitor according to the C-shaped deep trench;

wherein the step of fabricating the C-shaped deep trench capacitor according to the C-shaped deep trench comprises:

forming a capacitor at a lower portion of the C-shaped deep trench and a first polysilicon layer is formed on the capacitor in the C-shaped deep trench;

forming a collar dielectric layer covering an upper sidewall of the C-shaped deep trench;

forming a second polysilicon layer on the capacitor in the C-shaped deep trench; and

selectively opening a portion of the collar dielectric layer neighboring a central portion of an inner edge of the C-shaped deep trench;

wherein the method for forming a memory device further comprises:

performing at least one implanting process to dope a portion of the substrate neighboring the central portion of the inner edge of the C-shaped deep trench for forming a buried strap;

forming a third polysilicon layer in the C-shaped deep trench and on the second polysilicon layer; and

forming an insulating layer on the third polysilicon layer in the C-shaped deep trench.

2. The method for forming a method for forming a memory device as claimed in claim 1 , wherein the step of patterning the substrate to form the C-shaped deep trench comprises:

forming a hard mask layer on the substrate;

forming a pillar structure on the hard mask layer;

forming a ring-shaped spacer surrounding the pillar structure;

selectively etching a portion of the ring-shaped spacer to form a C-shaped spacer;

depositing a sacrificial layer to cover the C-shaped spacer, the pillar structure and the hard mask layer;

polishing the sacrificial layer till the C-shaped spacer is exposed;

removing the C-shaped spacer to form a C-shaped opening in the sacrificial layer;

etching the hard mask layer using the sacrificial layer as a mask to transfer the pattern of the C-shaped opening to the hard mask layer; and

etching the substrate using the hard mask layer as a mask to form the C-shaped deep trench.

3. The method for forming a memory device as claimed in claim 2 , wherein the ring-shaped spacer is formed of polysilicon and the step of selectively etching a portion of the ring-shaped spacer to form the C-shaped spacer comprises:

performing an implanting process with a tilt-angle to implant dopants into a portion of the ring-shaped spacer; and

performing an etching process using NH 4 OH as a primary etchant to remove an undoped portion of the ring-shaped spacer, wherein the remaining doped portion of the ring-shaped spacer constitute the C-shaped spacer.

4. The method for forming a memory device as claimed in claim 1 , wherein the step of opening the portion of the collar dielectric layer neighboring the central portion of the inner edge of the C-shaped deep trench comprises:

conformally depositing a sacrificial polysilicon layer in the C-shaped deep trench;

performing an implanting process with a tilt-angle to substantially implant dopants into a portion of the sacrificial polysilicon layer, wherein the portion neighboring the central portion of the inner edge of the C-shaped deep trench is not implanted;

performing a selective etching process having a greater etching rate at the undoped portion of the sacrificial polysilicon layer than that of the doped portion of the sacrificial polysilicon layer; and

etching the collar dielectric layer using the selectively etched sacrificial polysilicon layer as a mask,

wherein dopants of the implanting process with the tilt-angle comprise BF 2 and the selective etching process uses NH 4 OH as a primary etchant.

5. The method for forming a memory device as claimed in claim 1 , further comprising forming a word line on the insulating layer, the substrate and the buried strap, wherein the entire buried strap is covered by the word line.