CHEMICAL VAPOR DEPOSITION OF THIN FILM MATERIALS FOR ...

pinch-off to indicate the lack of a channel region near the drain. The drain current is now weakly
dependent upon the drain voltage and controlled primarily by the gate source voltage (VGS).
In the past few decades, the continuous scaling down of the MOSFET in size has
revolutionized the development of the microelectronic industry. The channel length of the
MOSFET structure has been shrinked from microns to tens of nanometers. At this scale, a lot
more functionalities can be realized in a same size wafer and meanwhile the cost of fabrication
can be decreased. However, with this scaling down, few operational problems have been found.
For example, the voltage applied to the gate has to be reduced to maintain reliability. To
maintain performance, the threshold voltage needs to be reduced too. With the reduced Vth, the
transistor cannot be switched from completely off to completely on with the limited voltage
swing available. Subthreshold voltage can thus consume as much as half of the total power
consumption. Another problem is with the gate oxide layer: as the thickness of this layer going
down to few atomic layers, the electron tunneling effect occurs and makes the power
consumption even higher. High-k materials (such as hafnium silicate or oxide) have been
developed to replace the traditional oxide material since the layer can be made thicker while
maintaining the same capacitance.[34-36] To work with these new high-k materials, the
properties of the gate electrode materials have to be considered as well. The traditional gate
electrode material polysilicon, for example, when coupled with a very thin (~1 nm) dielectric
layer, a phenomenon observed is poly depletion, where a depletion layer is formed in the
polysilicon layer next to the dielectric layer when the transistor is in the inversion. To avoid this
problem, metal gate materials such as tantalum carbide, tantalum nitride and titanium nitride can
be used with high-k dielectrics.[37-39]
32