Shallow trench isolation (STI) with trench liner of increased thickness

1. An integrated circuit comprising:

a substrate having first and second high voltage transistor regions and first and second low voltage transistor regions, the substrate including a first trench between and adjacent to the first and second high voltage transistor regions, a second trench between and adjacent to the first and second low voltage transistor regions, and a third trench between the first and second trenches and between and adjacent to the second high voltage transistor region and the second low voltage transistor region, wherein the trenches have openings at the top surface of the substrate;

a thicker silicon dioxide layer lining the first trench and a first portion of the third trench adjacent to the second high voltage transistor region, the thicker silicon dioxide layer having rounded corners at the openings of the first trench and the first portion of the third trench;

a thinner silicon dioxide layer lining the second trench and a second portion of the third trench adjacent to the second low voltage transistor region;

a silicon nitride layer present on the thinner silicon dioxide layer and lining the second trench and the second portion of the third trench but not present on the thicker silicon dioxide layer and not lining the first trench and the first portion of the third trench; and

dielectric material filling the first, second, and third trenches.

2. The integrated circuit of claim 1 , wherein the thicker silicon dioxide layer is at least three times as thick as the thinner silicon dioxide layer.

3. The integrated circuit of claim 1 , wherein the high voltage transistor regions are suitable for flash memory cells.

4. The integrated circuit of claim 1 , wherein the high voltage transistor regions are suitable for transistors having an operating voltage in the range of approximately 3.3 volts and higher.

5. The integrated circuit of claim 1 , wherein the low voltage transistor regions are suitable for transistors having an operating voltage in the range of approximately 1.2 volts to 2.5 volts.

6. The integrated circuit of claim 1 , wherein the integrated circuit is a programmable logic device.

7. An integrated circuit comprising:

a substrate having first and second high voltage transistor regions and a low voltage transistor region, the substrate including a first trench between and adjacent to the first and second high voltage transistor regions and a second trench between and adjacent to the second high voltage transistor region and the low voltage transistor region, wherein the trenches have openings at the top surface of the substrate;

a thicker silicon dioxide layer lining the first trench and a first portion of the second trench adjacent to the second high voltage transistor region, the thicker silicon dioxide layer having rounded corners at the openings of the first trench and the first portion of the second trench;

a thinner silicon dioxide layer lining a second portion of the second trench adjacent to the low voltage transistor region;

a silicon nitride layer present on the thinner silicon dioxide layer and lining the second portion of the second trench but not present on the thicker silicon dioxide layer and not lining the first trench and the first portion of the second trench; and

dielectric material filling the first and second trenches.

8. The integrated circuit of claim 7 , wherein the thicker silicon dioxide layer is at least three times as thick as the thinner silicon dioxide layer.

9. The integrated circuit of claim 7 , wherein the high voltage transistor regions are suitable for flash memory cells.

10. The integrated circuit of claim 7 , wherein the high voltage transistor regions are suitable for transistors having an operating voltage in the range of approximately 3.3 volts and higher.

11. The integrated circuit of claim 7 , wherein the low voltage transistor region is suitable for transistors having an operating voltage in the range of approximately 1.2 volts to 2.5 volts.

12. The integrated circuit of claim 7 , wherein the integrated circuit is a programmable logic device.

13. An integrated circuit comprising:

a substrate having first and second low voltage transistor regions and a high voltage transistor region, the substrate including a first trench between and adjacent to the first and second low voltage transistor regions and a second trench between and adjacent to the second low voltage transistor region and the high voltage transistor region, wherein the trenches have openings at the top surface of the substrate;

a thinner silicon dioxide layer lining the first trench and a first portion of the second trench adjacent to the second low voltage transistor region;

a thicker silicon dioxide layer lining a second portion of the second trench adjacent to the high voltage transistor region, the thicker silicon dioxide layer having a rounded corner at the opening of the second portion of the second trench;

a silicon nitride layer present on the thinner silicon dioxide layer and lining the first trench and the first portion of the second trench but not present on the thicker silicon dioxide layer and not lining the second portion of the second trench; and

dielectric material filling the first and second trenches.

14. The integrated circuit of claim 13 , wherein the thicker silicon dioxide layer is at least three times as thick as the thinner silicon dioxide layer.

15. The integrated circuit of claim 13 , wherein the high voltage transistor region is suitable for flash memory cells.

16. The integrated circuit of claim 13 , wherein the high voltage transistor region is suitable for transistors having an operating voltage in the range of approximately 3.3 volts and higher.

17. The integrated circuit of claim 13 , wherein the low voltage transistor regions are suitable for transistors having an operating voltage in the range of approximately 1.2 volts to 2.5 volts.

18. The integrated circuit of claim 13 , wherein the integrated circuit is a programmable logic device.