Partially recessed DRAM cell structure
View Patent ↗A dynamic random access memory (DRAM) cell structure (and method for making a DRAM cell structure) that is more suitable than current DRAM structures for implementation in ever decreasing semiconductor fabrication geometries. The DRAM cell structure comprises a deep trench (DT) capacitor formed in a substrate. A recess is formed in the substrate proximate the deep trench capacitor. A gate is formed that extends into the recess but does not completely occupy the recess. A source is formed in the substrate in a region beneath the recess. A drain is formed in the substrate in a region laterally and vertically offset from the source. A channel between the source and drain is created beneath the gate along a side wall of the recess. Thus, the depth of the recess determines the length of the channel region. With this DRAM cell structure, it is easier to avoid the high doping concentration issue and the short channel effect. Consequently, this DRAM cell structure can be employed with smaller fabrication technologies.
1. A dynamic random access memory (DRAM) cell structure, comprising:
a. a deep trench capacitor formed in a substrate;
b. a recess formed in the substrate proximate the deep trench capacitor;
c. a gate extending into but not completely occupying the recess;
d. a source in the substrate beneath the recess;
e. a drain in the substrate laterally and vertically offset from the source whereby a channel between the source and drain is created beneath the gate along a side wall of the recess; and
f. a plug of polysilicon material in the recess adjacent the gate that reduces resistance between the source and the deep trench capacitor.
2. The DRAM cell structure of claim 1 , wherein the recess is formed to a depth in the substrate greater than a depth of the drain.
3. The DRAM cell structure of claim 1 , wherein a depth of the recess determines the length of the channel region and reduces dopant diffusion between the source and the drain.
4. The DRAM cell structure of claim 1 , wherein the recess is formed to a depth so that the gate in a cell array area and the gate in a peripheral circuit area are at substantially the same level.
5. The DRAM cell structure of claim 1 , wherein a junction of the source with the channel region and a junction of the drain with the channel region are not in the same plane.
6. The DRAM cell structure of claim 5 , wherein the channel comprises silicon oxide along the side wall of the recess beneath the gate, on a portion of a horizontal surface above the recess connected to the side wall, and on a portion of a floor of the recess connected to the side wall.
7. The DRAM cell structure of claim 6 , wherein the junction of the source with the channel region is at the portion of the floor of the recess and the junction of the drain with the channel region is at the portion of the horizontal surface above the recess.
8. The DRAM cell structure of claim 5 , wherein the channel comprises silicon oxide along the side wall of the recess beneath the gate and on a portion of a floor of the recess connected to the side wall.
9. A dynamic random access memory (DRAM) cell structure, comprising:
a. a deep trench capacitor formed in a substrate;
b. a polysilicon layer overlying the deep trench capacitor
c. a recess formed in the substrate proximate the deep trench capacitor;
d. a gate formed at the recess but not completely occupying the recess to create a channel comprising a side wall of the recess, a portion of a horizontal surface above the recess connected to the side wall and a portion of a horizontal surface of a floor of the recess connected to the side wall;
e. a source formed by doping the substrate in a region beneath the recess proximate the polysilicon layer;
f. a drain in the substrate formed by doping the substrate in a region laterally and vertically offset from the source by the channel; and
g. a plug of polysilicon material in the recess adjacent the gate that reduces resistance between the source and the deep trench capacitor.
10. The DRAM cell structure of claim 9 , wherein the recess is formed to a depth in the substrate greater than a depth of the region in which the drain is formed.
11. The DRAM cell structure of claim 9 , wherein a depth of the recess determines the length of the channel and reduces dopant diffusion between the source and the drain.
12. The DRAM cell structure of claim 9 , wherein the recess is formed to a depth so as to make the channel sufficient long to facilitate scaling-down with semiconductor fabrication technologies without the need for relatively high channel doping concentration.
13. The DRAM cell structure of claim 9 , and further comprising a bit line contact in the substrate that makes electrical contact with the drain, and wherein the gate makes electrical contact with a word line.
14. The DRAM cell structure of claim 9 , wherein a junction of the source with the channel is at the portion of the floor of the recess and a junction of the drain with the channel is at the portion of the horizontal surface above the recess.
15. A dynamic random access memory (DRAM) cell structure, comprising:
a. deep trench capacitance means formed in a substrate for storing a charge;
b. recess means in an active area of the substrate proximate the deep trench capacitance means;
c. source region means formed in the substrate beneath the recess means for transporting charge to and from the deep trench capacitance means;
d. drain region means formed in the substrate laterally and vertically offset from the source region means;
e. gate region means having silicon oxide on a side wall and a portion of a floor of the recess means for controlling charge transport across the channel between the source region means and drain region means; and
f. means for plugging a space in the recess means not occupied by the gate region means for reducing resistance between the source region means and the deep trench capacitance means.
16. The DRAM cell structure of claim 15 , wherein the recess means is formed to a depth in the substrate greater than a depth of the drain region means.
17. The DRAM cell structure of claim 15 , and further comprising a channel between the source and the drain comprised of silicon oxide grown on a portion of a horizontal surface above the recess means connected to the side wall of the recess means, the side wall of the recess means, and a portion of the floor of the recess means connected to the side wall.
18. The DRAM cell structure of claim 17 , wherein a junction of the source region means with the channel is at the portion of the floor of the recess means and a junction of the drain region means with the channel is at the portion of the horizontal surface above the recess means.
19. A dynamic random access memory (DRAM) cell structure, comprising:
a. first and second deep trench capacitors formed in a substrate, each deep trench capacitor filled with polysilicon and having silicon oxide grown on its walls;
b. a polysilicon layer overlying the first and second deep trench capacitors;
c. an isolation layer of silicon oxide material formed in the substrate between a top portion of the first and second deep trench capacitors and extending over the polysilicon layer;
d. a first recess formed in the substrate proximate the first deep trench capacitor on a first side of the isolation layer;
e. a second recess formed in the substrate proximate the second deep trench capacitor on a second side of the isolation layer;
f. a first gate formed at the first recess but not completely occupying the first recess having silicon oxide on a side wall of the first recess;
g. a second gate formed at the second recess but not completely occupying the second recess having silicon oxide on a side wall of the second recess;
h. a first source formed by doping the substrate in a region beneath the first recess;
i. a first drain in the substrate formed by doping the substrate in a region laterally and vertically offset from the first source;
j. a second source formed by doping the substrate in a region beneath the second recess;
k. a second drain in the substrate formed by doping the substrate in a region laterally and vertically offset from the second source; and
l. a plug of polysilicon material in each of the first and second recesses adjacent the first and second gates, respectively, wherein the plug reduces resistance between the corresponding source and deep trench capacitor.
20. The DRAM cell structure of claim 19 , wherein the first and second recesses are formed to a depth in the substrate greater than a depth of the region in which the first and second drains, respectively, are formed.
21. The DRAM cell structure of claim 19 , wherein a first channel between the first source and the first drain comprises silicon oxide on a portion of a horizontal surface above the first recess connected to the side wall of the first recess, and on a portion of a floor of the first recess connected to the side wall of the first recess, and wherein a second channel between the second source and second drain comprises silicon oxide on a portion of a horizontal surface above the second recess connected to the side wall of the second recess and on a portion of a floor of the second recess connected to the side wall of the second recess.
22. The DRAM cell structure of claim 19 , and further comprising a first bit line contact that makes electrical contact with the first drain, a second bit line contact that makes electrical contact with the second drain, and wherein the first and second gates make electrical contact with a word line.