Trench power MOSFET

1. A method comprising:

forming a trench in a semiconductor region, wherein the semiconductor region is of a first conductivity type;

forming a conductive layer extending into the trench and on an edge of the trench; etching the semiconductor region to extend the trench deeper into the semiconductor region, with the conductive layer acting as an etching mask;

performing a tilt implantation using the conductive layer as a part of an implantation mask to form a Doped Drain (DD) region in the semiconductor region, wherein the DD region is of the first conductivity type; forming a first dielectric layer on a bottom and sidewalls of the trench;

forming a field plate in the trench and over a bottom portion of the first dielectric layer;

forming a second dielectric layer over the field plate; and

forming a main gate in the trench and over the second dielectric layer.

2. The method of claim 1 , wherein the forming the conductive layer comprises:

depositing a blanket conductive layer comprising horizontal portions and vertical portions, wherein the trench has a remaining portion unfilled by the blanket conductive layer; and

etching the blanket conductive layer to remove the horizontal portions, wherein parts of the vertical portions in the trench remain as the conductive layer.

3. The method of claim 2 , wherein a top end of the conductive layer is lower than a top surface of the semiconductor region.

4. The method of claim 1 , wherein the DD region is overlapped by the conductive layer.

5. The method of claim 1 , wherein the main gate is joined to the conductive layer to form a continuous conductive region.

6. The method of claim 1 further comprising forming a mask layer at the bottom of the trench, wherein a topmost end of the mask layer is lower than a top of the trench, with the tilt implantation performed using the mask layer as an additional implantation mask, and the DD region has a bottom surface higher than the bottom of the trench.

7. The method of claim 1 further comprising:

depositing an inter-layer dielectric over the main gate; and

forming a source region and a drain region on opposite sides of the semiconductor region.

8. The method of claim 1 further comprising forming a body region of a second conductivity type opposite to the first conductivity type, wherein the body region contacts a sidewall of the DD region.

9. A method comprising:

etching a semiconductor region to form a trench;

forming a mask layer at a bottom of the trench;

performing a tilt implantation to form a Doped Drain (DD) region in the semiconductor region, with the tilt implantation performed using the mask layer as an additional implantation mask, wherein a topmost surface of the mask layer defines a bottom surface of the DD region, wherein the DD region has portions on opposite sides of the trench, and a bottom surface of the DD region is higher than a bottom of the trench;

forming a first dielectric layer lining the bottom and sidewalls of the trench, with the first dielectric layer having an edge contacting an edge of the DD region;

forming a field plate in the trench and over a bottom portion of the first dielectric layer;

forming a second dielectric layer over the field plate; and

forming a main gate in the trench and over the second dielectric layer.

10. The method of claim 9 , wherein the trench comprises a portion that is overlapped by the DD region.

11. The method of claim 9 , wherein the tilt implantation is performed with a conductive layer in the trench as an implantation mask.

12. The method of claim 11 further comprising:

depositing a blanket conductive layer comprising horizontal portions and vertical portions; and

etching the blanket conductive layer to remove the horizontal portions, wherein parts of the vertical portions in the trench remain as the conductive layer.

13. The method of claim 9 , wherein a bottom surface of the first dielectric layer is in physical contact with a top surface of the semiconductor region.

14. The method of claim 9 further comprising removing the mask layer after the tilt implantation.

15. The method of claim 9 , wherein the top surface of the mask layer is between a top of the trench and the bottom of the trench.

16. A method comprising:

etching a semiconductor region to form a trench;

oxidizing a surface layer of the semiconductor region to form an oxide layer, wherein the surface layer is exposed to the trench;

forming a conductive layer in the trench and on sidewalls of the trench, wherein the conductive layer has a bottom contacting a top surface of the oxide layer;

etching the oxide layer and the semiconductor region to extend the trench deeper into the semiconductor region, with the conductive layer acting as an etching mask;

forming a mask layer extending to a bottom of the trench, wherein the mask layer has a topmost surface at an intermediate level between a top surface of the semiconductor region and the bottom of the trench;

performing a tilt implantation to form a Doped Drain (DD) region in the semiconductor region, with the tilt implantation performed using the mask layer as an implantation mask, wherein the topmost surface defines a position of a bottom surface of the DD region, wherein the DD region is overlapped by the conductive layer;

implanting the semiconductor region to form a body region, with the body region and the DD region having opposite conductivity types, wherein the body region has a bottom end at an intermediate level between a top surface and a bottom surface of the DD region, and the body region contacts a sidewall of the DD region; and

forming a drain region overlapped by the DD region, wherein the drain region is electrically connected to the DD region.

17. The method of claim 16 , wherein when the tilt implantation is performed, the conductive layer has a top surface lower than a top surface of the semiconductor region.

18. The method of claim 16 , wherein when the tilt implantation is performed, a bottom surface of the conductive layer is higher than a bottom of the trench.

19. The method of claim 16 further comprising:

forming a first dielectric layer on a bottom and sidewalls of the trench, with the first dielectric layer having an edge contacting an edge of the DD region;

forming a field plate in the trench and over a bottom portion of the first dielectric layer;

forming a second dielectric layer over the field plate; and

forming a main gate in the trench and over the second dielectric layer.

20. The method of claim 16 , wherein the bottom surface of the DD region is substantially coplanar with the topmost surface of the mask layer.