Nanowire mesh FET with multiple threshold voltages
Nanowire-based field-effect transistors (FETs) and techniques for the fabrication thereof are provided. In one aspect, a FET is provided having a plurality of device layers oriented vertically in a stack, each device layer having a source region, a drain region and a plurality of nanowire channels connecting the source region and the drain region, wherein one or more of the device layers are configured to have a different threshold voltage from one or more other of the device layers; and a gate common to each of the device layers surrounding the nanowire channels.
1. A method of fabricating a FET, comprising the steps of:
forming a plurality of device layers oriented vertically in a stack, each device layer having a source region, a drain region and a plurality of nanowire channels connecting the source region and the drain region;
configuring one or more of the device layers to have a different threshold voltage from one or more other of the device layers; and
forming a gate common to each of the device layers surrounding the nanowire channels.
2. The method of claim 1 , further comprising the steps of:
providing a silicon-on-insulator (SOI) wafer;
forming an alternating series of silicon and sacrificial layers on the wafer;
etching a stack of nanowire bars into the silicon and sacrificial layers; and
removing the sacrificial layers from the stack.
3. The method of claim 2 , wherein a top layer of the stack is a silicon layer, the method further comprising the steps of:
etching the top silicon layer to form a plurality of nanowire bars therein; and
laterally thinning the nanowire bars formed in the top silicon layer.
4. The method of claim 3 , wherein the top silicon layer of the stack is thinner than one or more other of the silicon layers in the stack.
5. The method of claim 2 , wherein each of the sacrificial layers comprises silicon germanium, the method further comprising the step of:
doping each of the sacrificial layers with an n-type or a p-type dopant; and
diffusing the dopant from the sacrificial layers to the silicon layers in the source and drain regions of the device layers.
6. The method of claim 3 , wherein the step of laterally thinning the nanowire bars formed in the top silicon layer further comprises the step of:
oxidizing the nanowire bars formed in the top silicon layer.
7. The method of claim 2 , further comprising the step of:
forming a plurality of nanowire hardmasks over the stack; and
using the nanowire hardmasks as masks during the etching step to form the nanowire bars.
8. The method of claim 1 , further comprising the step of:
forming a dielectric on the nanowire channels prior to forming the gate.
9. The method of claim 1 , wherein one or more of the device layers are configured to have a threshold voltage V t1 and one or more other of the device layers are configured to have a threshold voltage V t2 , the method further comprising the step of:
configuring the nanowire channels of the one or more device layers with the threshold voltage V t1 to have at least one of a width and a thickness that is less than a width and thickness of the nanowire channels in the device layers with the threshold voltage V t2 .