Methods for fabricating integrated circuits with improved active regions

1. A method for fabricating an integrated circuit, the method comprising:

providing a semiconductor substrate having an upper surface and including active regions and isolation regions formed in a low voltage device area and in a high voltage device area, wherein the isolation regions are formed with an upper surface at a step height over the active regions, and wherein the isolation regions have sidewalls in contact with the active regions;

selectively etching the upper surface of the isolation regions to reduce the step height and selectively etching the sidewalls of the isolation regions to form voids between the isolation regions and the active regions in the high voltage device area to expose active side surfaces; and

oxidizing the upper surface and the active side surfaces to form a gate oxide layer over the low voltage device area and the high voltage device area.

2. The method of claim 1 wherein selectively etching the upper surface of the isolation regions to reduce the step height and selectively etching the sidewalls of the isolation regions to form voids between the isolation regions and the active regions in the high voltage device area comprises masking the low voltage device area while etching the high voltage device area.

3. The method of claim 1 wherein selectively etching the sidewalls of the isolation regions to form the voids comprises forming the voids with a void depth from about 150 Å to about 200 Å below the upper surface of the isolation region.

4. The method of claim 1 further comprising forming doped regions in the high voltage device area and forming doped regions in the low voltage device area, wherein:

forming the doped regions in the high voltage device area comprises:

forming a sacrificial layer over the high voltage device area and over the low voltage device area;

forming a first mask over the sacrificial layer covering the low voltage device area and partially covering the high voltage device area to define first exposed portions of the high voltage device area;

forming the doped regions in the first exposed portions;

etching the first exposed portions of the high voltage device area, wherein etching the first exposed portions of the high voltage device area comprises selectively etching the upper surface of the isolation regions to reduce the step height and selectively etching the sidewalls of the isolation regions to form voids between the isolation regions and the active regions in the first exposed portions of the high voltage device area to expose active side surfaces;

removing the first mask;

forming a second mask covering the low voltage device area and partially covering the high voltage device area to define second exposed portions of the high voltage device area;

forming the doped regions in the second exposed portions;

etching the second exposed portions of the high voltage device area, wherein etching the second exposed portions of the high voltage device area comprises selectively etching the upper surface of the isolation regions to reduce the step height and selectively etching the sidewalls of the isolation regions to form voids between the isolation regions and the active regions in the second exposed portions of the high voltage device area to expose active side surfaces; and

removing the second mask; and

forming the doped regions in the low voltage device area comprises:

forming a third mask partially covering the low voltage device area and covering the high voltage device area to define third exposed portions of the low voltage device area;

forming the doped regions in the third exposed portions;

removing the third mask;

forming a fourth mask partially covering the low voltage device area and covering the high voltage device area to define fourth exposed portions of the low voltage device area;

forming the doped regions in the fourth exposed portions; and

removing the fourth mask.

5. The method of claim 1 wherein selectively etching the upper surface of the isolation regions to reduce the step height comprises eliminating the step height between the upper surface of the semiconductor substrate and the upper surface of the isolation regions.

6. The method of claim 1 wherein selectively etching the upper surface of the isolation regions to reduce the step height and selectively etching the sidewalls of the isolation regions to form voids between the isolation regions and the active regions in the high voltage device area to expose the active side surfaces comprises:

forming a first mask covering the low voltage device area and partially covering the high voltage device area to define first exposed portions of the high voltage device area;

etching first voids into the isolation regions in the first exposed portions;

forming the doped regions in the first exposed portions;

removing the first mask;

forming a second mask covering the low voltage device area and partially covering the high voltage device area to define second exposed portions of the high voltage device area;

forming the doped regions in the second exposed portions;

etching second voids into the isolation regions in the second exposed portions; and

removing the second mask.

7. The method of claim 1 wherein the upper surface of the semiconductor substrate in the active regions is not etched while selectively etching the upper surface of the isolation regions to reduce the step height and selectively etching the sidewalls of the isolation regions to form voids between the isolation regions and the active regions in the high voltage device area to expose active side surfaces.

8. The method of claim 1 wherein selectively etching the upper surface of the isolation regions to reduce the step height and selectively etching the sidewalls of the isolation regions to form voids between the isolation regions and the active regions in the high voltage device area to expose the active side surfaces comprises:

forming a first mask covering the low voltage device area and partially covering the high voltage device area to define first exposed portions of the high voltage device area;

etching first voids into the isolation regions in the first exposed portions;

removing the first mask;

forming a first cap screen oxide layer on the semiconductor substrate after removing the first mask; and

removing residue from the semiconductor substrate after forming the first cap screen oxide layer.

9. The method of claim 1 further comprising:

masking the high voltage device area; and

forming doped regions in the low voltage device area before oxidizing the upper surface and the active side surfaces to form the gate oxide layer over the low voltage device area and the high voltage device area.

10. The method of claim 1 further comprising forming a sacrificial layer overlying the semiconductor substrate, wherein selectively etching the upper surface of the isolation regions to reduce the step height and selectively etching the sidewalls of the isolation regions to form the voids between the isolation regions and the active regions in the high voltage device area to expose the active side surfaces comprises:

forming a first mask covering the low voltage device area and partially covering the high voltage device area to define first exposed portions of the high voltage device area;

etching into the sacrificial layer overlying the first exposed portions to form first recessed sacrificial layer portions;

removing the first mask;

forming a second mask covering the low voltage device area and partially covering the high voltage device area to define second exposed portions of the high voltage device area;

etching into the sacrificial layer overlying the second exposed portions to form second recessed sacrificial layer portions;

removing the second mask; and

performing an etch process to remove the sacrificial layer, wherein the etch process etches the voids into the isolations regions underlying the first recessed sacrificial layer portions and the second recessed sacrificial layer portions.

11. The method of claim 10 further comprising:

forming doped regions in the first exposed portions after forming the first mask;

forming doped regions in the second exposed portions after forming the second mask; and

selectively forming doped regions in the low voltage device area after removing the second mask.

12. The method of claim 1 further comprising forming a sacrificial layer overlying the semiconductor substrate, wherein selectively etching the upper surface of the isolation regions to reduce the step height and selectively etching the sidewalls of the isolation regions to form the voids between the isolation regions and the active regions in the high voltage device area to expose the active side surfaces comprises

forming a sacrificial layer mask over the sacrificial layer overlying the low voltage device area;

recessing the sacrificial layer to form a recessed sacrificial layer overlying the high voltage device area;

removing the sacrificial layer mask; and

performing an etch process to remove the sacrificial layer, wherein the etch process etches the voids into the isolations regions underlying the recessed sacrificial layer.

13. The method of claim 12 further comprising, after performing the etch process to remove the sacrificial layer and before forming the gate oxide layer over the low voltage device area and the high voltage device area:

forming a first mask covering the low voltage device area and partially covering the high voltage device area to define first exposed portions of the high voltage device area;

forming doped regions in the first exposed portions;

removing the first mask;

forming a second mask covering the low voltage device area and partially covering the high voltage device area to define second exposed portions of the high voltage device area;

forming doped regions in the second exposed portions;

removing the second mask; and

selectively forming doped regions in the low voltage device area.

14. A method for fabricating an integrated circuit, the method comprising:

providing a semiconductor substrate with a high voltage device area including an active region having an upper surface and an isolation region having an upper surface, wherein the isolation region has a sidewall in contact with a side surface of the active region;

simultaneously recessing the upper surface of the isolation region and etching the sidewall of the isolation region to form a void between the sidewall of the isolation region and the side surface of the active region; and

oxidizing the upper surface of the active region and the side surface of the active region in the high voltage device area to form oxidized material and to define a convex interface between the oxidized material and underlying active material in the active region in the high voltage device area.

15. The method of claim 14 wherein an etch process utilizing dilute hydrofluoric acid (DHF) is performed to simultaneously recess the upper surface of the isolation region and etch the sidewall of the isolation region to form the void.

16. The method of claim 14 wherein oxidizing the upper surface and the side surface of the active region in the high voltage device area comprises selectively oxidizing the upper surface of the active region to form an oxide layer, wherein the oxide layer is not formed on the isolation region.

17. The method of claim 14 wherein simultaneously recessing the upper surface of the isolation region and etching the sidewall of the isolation region to form the void between the sidewall of the isolation region and the side surface of the active region comprises forming the void with a bottom surface lower than the upper surface of the isolation region by a void depth of from about 150 Å to about 200 Å.