Double trench well formation in SRAM cells

1. A method of comprising:

forming a plurality of deep trenches having a first depth in a silicon substrate;

forming a deep n-well or deep p-well around a bottom of each of the plurality of deep trenches;

filling the plurality of deep trenches with a first oxide;

forming a first or second shallow trench between each pair of adjacent deep trenches, each first and second shallow trench having a second depth less than the first depth;

forming a first p-well or first n-well, respectively, above each deep n-well or p-well;

forming a second n-well at a bottom of each first shallow trench;

filling the first shallow trenches with a second oxide;

forming a second p-well at a bottom of each second shallow trench; filling the second shallow trenches with the second oxide;

forming a p+ region above each second n-well on each side of each first shallow trench; and

forming an n+ region above each second p-well on each side of each second shallow trench.

2. The method according to claim 1 , comprising forming the plurality of deep trenches to a depth of 1500 angstroms (Å) to 5000 Å.

3. The method according to claim 1 , comprising forming the plurality of deep trenches by etching with sulfur hexafluoride (SF 6 ) or nitrogen trifluoride (NF 3 ).

4. The method according to claim 1 comprising forming the first p-well or first n-well, respectively, above each deep n-well or p-well by implanting a p-type or n-type dopant with an energy of 5 kiloelectron volts (keV) to 80 keV.

5. The method according to claim 1 , comprising forming the first and second shallow trenches between the deep trenches by:

forming a self-aligned hardmask over the silicon substrate; and

etching the first and second shallow trenches through openings in the self-aligned hardmask to a depth of 700 Å to 3000 Å.

6. The method according to claim 5 , comprising forming the hardmask in a self aligned way by depositing high aspect ratio process (HARP) material, TEOS, polysilicon and/or nitride as layers and forming spacers.

7. The method according to claim 5 , comprising etching the first and second shallow trenches to form a silicon fin between each deep trench and each adjacent first and second shallow trench, the silicon fins having a pitch of 40 (nanometers) nm to 100 nm.

8. The method according to claim 7 , comprising etching the first and second shallow trenches to form silicon fins each having a width of 8 nm to 50 nm.

9. The method according to claim 7 , comprising forming each second n-well or second p-well by:

covering each second or first shallow trench, respectively, and adjacent silicon fins with a photomask; and

implanting n-type or p-type dopants through the bottom of each first or second shallow trench, respectively, at an energy level of 10 keV to 30 keV.

10. The method according to claim 9 , comprising doping silicon fins adjacent opposite sides of each first or second shallow trench through a mask with p+ or n+ dopants, respectively, to a depth of 50 Å to 1000 Å, after filling the second shallow trenches.

11. The method according to claim 9 , comprising forming the photomask with openings having a width of 80 nm.

12. The method according to claim 11 , comprising forming the photomask by allowing for misalignment of ½ trench plus fin width.

13. The method according to claim 1 , comprising heating the first and second oxides in a high temperature environment for a duration that is sufficient to cause densification of the first and second oxides.

14. The method according to claim 1 , comprising forming four first shallow trenches and two second shallow trenches to form a static random-access memory (SRAM) cell comprising at least four n-type metal-oxide-semiconductor (NMOS) field effect transistors and at least two p-type metal-oxide-semiconductor (PMOS) field effect transistors.

15. A method of comprising:

forming a plurality of deep trenches having a depth of 1500 Å to 5000 Å in a silicon (Si) substrate;

forming a deep n-well or a deep p-well across the Si substrate through the plurality of deep trenches;

filling each of the plurality of deep trenches with the first oxide;

forming a plurality of first and second shallow trenches having a depth of 700 Å to 3000 Å in the Si substrate, each pair of adjacent deep trenches having one first or second shallow trench therebetween;

forming a first p-well or first n-well, respectively, above the deep n-well or deep p-well;

forming a first photoresist over the second shallow trenches and adjacent Si substrate;

forming a second n-well through a bottom of each first shallow trench;

filling each first shallow trench with a second oxide;

removing the first photoresist;

forming a second photoresist over the first shallow trenches and adjacent Si substrate;

forming a second p-well through a bottom of each second shallow trench;

filling each second shallow trench with the second oxide;

removing the second photoresist;

forming a p+ region above each second n-well on each side of each first shallow trench through a first mask; and

forming an n+ region above each second p-well on each side of each second shallow trench through a second mask.

16. The method according to claim 15 , comprising heating the first and second oxides at a temperature and for a duration sufficient to cause densification of the first and second oxides prior to forming the p+ regions and n+ regions.