MOSFET with work function adjusted metal backgate
An SOI substrate, a semiconductor device, and a method of backgate work function tuning. The substrate and the device have a plurality of metal backgate regions wherein at least two regions have different work functions. The method includes forming a mask on a substrate and implanting a metal backgate interposed between a buried oxide and bulk regions of the substrate thereby producing at least two metal backgate regions having different doses of impurity and different work functions. The work function regions can be aligned such that each transistor has different threshold voltage. When a top gate electrode serves as the mask, a metal backgate with a first work function under the channel region and a second work function under the source/drain regions is formed. The implant can be tilted to shift the work function regions relative to the mask.
1. A method of backgate work function tuning comprising:
forming a mask over a portion of a semiconductor on insulator (SOI) substrate wherein the substrate has a SOI layer, a buried insulator (BOX), a bulk layer, and a metal backgate between the BOX and the bulk layer; and
implanting a species into a portion of the metal backgate to form a metal backgate structure located entirely beneath the BOX and having a first region which lacks the implanted species and a second region which contains the implanted species.
2. The method of claim 1 , further comprising: annealing the substrate after implantation.
3. The method of claim 1 , wherein the mask is a patterned gate stack.
4. The method of claim 3 , further comprising:
forming a second mask having an opening above the first region of the metal backgate structure; and
performing a second implantation to place implanted species in the first region of the metal backgate structure.
5. The method of claim 1 , wherein the implanting comprises implanting at a tilt angle.
6. The method of claim 1 , wherein the first region has a first work function and the second region has a second work function.
7. The method of claim 1 , wherein the metal backgate comprises a metal, a metal nitride, or a metal carbide, wherein the metal is selected from the group consisting of molybdenum, tungsten, titanium and tantalum.
8. The method of claim 1 , wherein the metal backgate is formed by a deposition process.
9. The method of claim 1 , wherein the mask is an insulator material or a photoresist and no device is present on the SOI layer prior to, or during, the implanting.
10. The method of claim 1 , wherein the species is selected from an element from Group 17 or 18 of the Periodic Table of Elements.
11. The method of claim 1 , wherein the species is selected from an element from Group 15 of the Periodic Table of Elements.
12. The method of claim 1 , wherein a sidewall surface of the first region directly contacts a sidewall surface of the second region, and the first and second regions have topmost surfaces and bottommost surfaces that are coplanar with each other.
13. The method of claim 1 , further comprising forming a source region and a drain region into the SOI layer after performing the implanting.
14. The method of claim 1 , wherein the mask is a dummy gate stack, and the dummy gate stack is replaced with a structure that includes a top gate electrode after the implanting.
15. The method of claim 1 , wherein an entirety of a topmost surface of the metal backgate structure directly contacts an entirety of a bottommost surface of the BOX and an entirety of a bottommost surface of the metal backgate structure directly contacts an entirety of a topmost surface of the bulk layer.
16. The method of claim 1 , wherein the implanting does not amorphize any portion of the SOI layer.
17. A method of backgate work function tuning comprising:
providing a structure comprising, from bottom to top, a bulk layer, a first buried insulator (BOX 1), a metal backgate, a second buried insulator layer (BOX 2), a semiconductor-on-insulator (SOI) layer and a mask located over a portion of the SOI layer; and
implanting a species into a portion of the metal backgate to form a metal backgate structure having a first region which lacks the implanted species and a second region which contains the implanted species, wherein the first region has a different workfunction than the second region.