Semiconductor on insulator vertical transistor fabrication and doping process
A process for conformally doping through the vertical and horizontal surfaces of a 3-dimensional vertical transistor in a semiconductor-on-insulator structure employs an RF oscillating torroidal plasma current to perform either conformal ion implantation, or conformal deposition of a dopant-containing film which can then be heated to drive the dopants into the transistor. Some embodiments employ both conformal ion implantation and conformal deposition of dopant containing films, and in those embodiments in which the dopant containing film is a pure dopant, the ion implantation and film deposition can be performed simultaneously.
1. A process for fabricating a vertical transistor on a semiconductor-on-insulator wafer consisting of an underlying substrate, an intermediate insulator layer and a top crystalline semiconductor active layer, said process comprising:
sculpting from said active layer a 3-dimensional source-channel-drain structure having horizontal and vertical surfaces and comprising a channel with a source and drain at either end of the channel;
forming a thin gate oxide layer over a section of the channel;
forming a 3-dimensional gate structure over said thin gate oxide layer;
while supporting the wafer in a plasma immersion ion implantation reactor chamber, introducing into the chamber a dopant-containing process gas and applying RF plasma source power into the chamber to generate from said process gas a plasma; and
applying RF plasma bias power or voltage to the wafer to draw ions from the plasma toward the wafer across a plasma sheath at a sufficient energy to implant the ions in the source-drain-channel structure.
2. The process of claim 1 further comprising:
setting said bias power or voltage at a sufficiently high level to attain a threshold conformality of ion implant dose on the horizontal and vertical surfaces of the source-channel-drain structure.
3. The process of claim 2 wherein:
the step of forming a 3-dimensional gate structure comprises forming a polycrystalline semiconductor gate; and
a threshold conformality of ion implant dose is attained on the horizontal and vertical surfaces of the 3-dimensional gate structure.
4. The process of claim 1 wherein the level of said RF plasma bias power or voltage is sufficient to promote scattering of the ions in the plasma sheath of the plasma by enhancing the thickness of the plasma sheath.
5. The process of claim 1 wherein the level of said RF plasma bias power is sufficient to establish a wafer bias voltage on the order of a sub-kilovolt to several kiloVolts.
6. The process of claim 1 further comprising maintaining the RF plasma source power at a sufficiently low level to enhance the conformality of the ion implant dose.
7. The process of claim 6 wherein the level of the RF plasma source power is on the order of hundreds of Watts.
8. The process of claim 1 further comprising maintaining a sufficiently high pressure over the wafer to enhance the conformality of the ion implant dose.
9. The process of claim 8 wherein the pressure over the wafer is on the order of about 80 milliTorr.
10. The process of claim 1 further comprising enhancing the conformality of the ion implant dose by supplementing the dopant containing process gas with a second gas species having a collision cross-section that is greater than the collision cross section of the dopant containing process gas.
11. The process of claim 10 wherein the supplementing step comprises supplying into the chamber said second gas species at a gas flow rate that is between about 5% and 95% of the gas flow rate of the dopant containing gas.
12. The process of claim 10 wherein said second gas species comprises Xenon, Kr, Ar, Ne, He or H2.
13. The process of claim 1 further comprising:
maintaining the RF plasma source power at a sufficiently low level to enhance the conformality of the ion implant dose; and
maintaining a sufficiently high pressure over the wafer to enhance the conformality of the ion implant dose.
14. The process of claim 13 wherein:
the level of said RF plasma bias power is sufficient to establish a wafer bias voltage on the order of sub-kV to kiloVolts;
the level of the RF plasma source power is on the order of hundreds of Watts; and
the pressure over the wafer is on the order of about 80 milliTorr.
15. The process of claim 14 further comprising:
supplementing the dopant containing process gas with a second gas species having a collision cross-section that is greater than the collision cross section of the dopant containing process gas.
16. The process of claim 15 wherein:
said second gas species comprises xenon, Kr, Ar, Ne, He or H2.
17. The process of claim 1 wherein said dopant containing process gas comprises one of B2H6, BF3, PH3, PF3, AsH3, AsF5, PF5, AsF3.
18. The process of claim 1 further comprising performing a post-implant anneal process on the wafer.
19. The process of claim 1 wherein the threshold conformality is on the order of magnitude of about 10%.
20. The process of claim 1 wherein:
said reactor comprises an external reentrant conduit having a pair of openings into said chamber defining a toroidal path extending across a processing zone overlying said workpiece; and
the step of generating a plasma comprises generating an RF oscillating plasma current along said toroidal path.
21. The process of claim 1 further comprising maintaining a bias voltage on the wafer corresponding to an ion implantation depth profile that peaks at or near the surfaces of the vertical transistor.
22. The process of claim 21 wherein the ion implantation depth profile peak is within a fraction of a micron of each surface of the vertical transistor.
23. The process of claim 1 further comprising maintaining said wafer at a sufficiently high temperature to prevent accumulation of ion bombardment damage or amorphization of the source-channel-drain structure.
24. The process of claim 23 wherein said temperature is on the order of 500 to 600 degrees C. or more.