Method for fabricating a semiconductor structure

1. A method for fabricating a semiconductor structure, said method comprising:

providing a semiconductor layer and a gate stack on the semiconductor layer, wherein the semiconductor layer comprises (i) a channel region directly beneath the gate stack, and (ii) first and second semiconductor regions essentially not covered by the gate stack, and wherein the channel region is disposed between the first and second semiconductor regions;

removing the first and second semiconductor regions; and

doping regions directly beneath the removed first and second semiconductor regions so as to form first and second source/drain regions, respectively, such that top surfaces of the first and second source/drain regions are below a top surface of the channel region;

wherein said removing the first and second semiconductor regions comprises:

doping the first and second semiconductor regions of the semiconductor layer with first dopants; and

etching away the first and second semiconductor regions but leaving essentially intact the gate stack and regions of the semiconductor layer which are not doped with the first dopants,

wherein said doping the first and second semiconductor regions with the first dopants is performed such that each region of the first and second semiconductor regions of the semiconductor layer has a thickness in a first direction perpendicular to a top surface of the channel region that increases discontinuously when moving in a second direction away from the gate stack, and

wherein the second direction is parallel to the top surface of the channel region.

2. The method of claim 1 , wherein the gate stack comprises: (i) a gate dielectric region on the semiconductor layer and (ii) a gate region on the gate dielectric region, and wherein the gate region is electrically isolated from the channel region by the gate dielectric region.

3. The method of claim 1 , wherein said doping the first and second semiconductor regions with the first dopants further comprises forming spacers on sidewalls of the gate stack.

4. The method of claim 1 , wherein the semiconductor layer comprises silicon, and wherein the first dopants in the first and second semiconductor regions comprise germanium.

5. The method of claim 1 , wherein the thickness is a step function of spatial position along the second direction directed away from the gate stack, wherein the step function comprises step shaving a magnitude equal to the thickness, and wherein the magnitude of the steps increase when moving in the second direction away from the gate stack.

6. The method of claim 1 , wherein the step function consists of three steps.

7. A method for fabricating a semiconductor structure, said method comprising:

providing a semiconductor layer and a gate stack on the semiconductor layer, wherein the semiconductor layer comprises (i) a channel region directly beneath the gate stack, and (ii) first and second semiconductor regions essentially not covered by the gate stack, and wherein the channel region is disposed between the first and second semiconductor regions;

removing the first and second semiconductor regions;

doping regions directly beneath the removed first and second semiconductor regions so as to form first and second source/drain regions, respectively, such that top surfaces of the first and second source/drain regions are below a top surface of the channel region;

providing an underlying dielectric layer wherein the semiconductor layer is on the underlying dielectric layer;

implanting second dopants in a top layer of the underlying dielectric layer except in a separating dielectric region of the top layer directly beneath the gate stack;

etching away the top layer of the underlying dielectric layer except the separating dielectric region; and

epitaxially growing a semiconductor material to fill the removed top layer of the underlying dielectric layer before said removing the first and second semiconductor regions is performed.

8. The method of claim 7 ,

wherein said implanting second dopants comprises using the gate stack as a mask to implant the second dopants in the top layer of the underlying dielectric layer except in the separating dielectric region, and

wherein said etching away the top layer of the underlying dielectric layer is performed such that regions of the underlying dielectric layer which are not doped with the second dopants are essentially intact.

9. The method of claim 8 , wherein the second dopants comprise nitrogen, and wherein the underlying dielectric layer comprises silicon dioxide.

10. The method of claim 7 , wherein said epitaxially growing the semiconductor material is performed until a top surface of epitaxially grown semiconductor regions is at a level lower than a bottom surface of the first and second semiconductor regions.

11. A method for fabricating a semiconductor structure, said method comprising:

providing (i) an underlying dielectric layer, (ii) a semiconductor layer on the underlying dielectric layer, and (iii) a gate stack on the semiconductor layer;

implanting first dopants in a top layer of the underlying dielectric layer except in a separating dielectric region of the top layer directly beneath the gate stack;

removing the top layer of the underlying dielectric layer except the separating dielectric region;

epitaxially growing semiconductor regions to fill the removed top layer of the underlying dielectric layer; and

implanting second dopants in semiconductor regions of the semiconductor layer and the epitaxially grown semiconductor regions on opposing sides of the gate stack so as to form first and second source/drain regions such that the separating dielectric region is disposed between the first and second source/drain regions.

12. The method of claim 11 , wherein said implanting first dopants comprises using the gate stack as a mask to implant the first dopants in the top layer of the underlying dielectric layer except in the separating dielectric region.

13. The method of claim 11 , wherein said removing the top layer of the underlying dielectric layer comprises etching away regions of the underlying dielectric layer doped with the first dopants but leaving essentially intact regions of the underlying dielectric layer not doped with the first dopants;

a wet-etching process that etches away regions of the underlying dielectric layer doped with the first dopants but leaves essentially intact regions of the underlying dielectric layer not doped with the first dopants.

14. The method of claim 13 , wherein the method further comprises:

before said etching away is performed, removing regions of the semiconductor layer so as to expose regions of the underlying dielectric layer that are doped with the first dopants.

15. The method of claim 14 , wherein said removing the regions of the semiconductor layer comprises:

forming gate spacers on side walls of the gate stack; and

using the gate stack and the gate spacers as a mask to RIE (reactive ion etching) away the regions of the semiconductor layer so as to expose the regions of the underlying dielectric layer that are doped with the first dopants.

16. The method of claim 11 , wherein the first dopants comprise nitrogen, and wherein the underlying dielectric layer comprises silicon dioxide.