A long-recognized important objective in the constant advancement of monolithic IC (Integrated Circuit) technology is the scaling-down of IC dimensions.  Such scaling-down of IC dimensions reduces area capacitance and is critical to obtaining higher speed performance of integrated circuits.  Moreover, reducing the area of an IC die leads to higher yield in IC fabrication.  Such advantages are a driving force to constantly scale down IC dimensions. 
Referring to FIG.  1, a common component of a monolithic IC is a MOSFET (Metal Oxide Semiconductor Field Effect Transistor) 100 which is fabricated within a semiconductor substrate 102.  The scaled down MOSFET 100 having submicron or nanometer dimensions includes a drain extension junction 104 and a source extension junction 106 formed within an active device area 126 of the semiconductor substrate 102.  The drain extension junction 104 and the source extension junction 106 are shallow junctions to minimize short-channel effects in the MOSFET 100 having submicron or nanometer dimensions, as known to one of ordinary skill in the art of integrated circuit fabrication. 
The MOSFET 100 further includes a drain contact junction 108 with a drain silicide 110 for providing contact to the drain of the MOSFET 100 and includes a source contact junction 112 with a source silicide 114 for providing contact to the source of the MOSFET 100.  The drain contact junction 108 and the source contact junction 112 are fabricated as deeper junctions such that a relatively large size of the drain silicide 110 and the source silicide 114 respectively may be fabricated therein to provide low resistance contact to the drain and the source respectively of the MOSFET 100. 
The MOSFET 100 further includes a gate dielectric 116 and a gate structure 118 which may be comprised of polysilicon.  A gate silicide 120 is formed on the polysilicon gate structure 118 for providing contact to the gate of the MOSFET 100.  The MOSFET 100 is electrically isolated from other integrated circuit devices within the semiconductor substrate 102 by shallow trench isolation structures 121.  The shallow trench isolation structures 121 define the active device area 126, within the semiconductor substrate 102, where the MOSFET 100 is fabricated therein. 
The MOSFET 100 also includes a spacer 122 disposed on the sidewalls of the gate structure 118 and the gate dielectric 116.  When the spacer 122 is comprised of silicon nitride (Si.sub.3 N.sub.4), a spacer liner oxide 124 is deposited as a buffer layer between the spacer 122 and the sidewalls of the gate structure 118 and the gate dielectric 116 and between the spacer 122 and the semiconductor substrate 102. 
As the dimensions of the MOSFET 100 are further scaled down to tens of nanometers, short channel effects are more likely to disadvantageously affect the operation of the MOSFET 100, as known to one of ordinary skill in the art of integrated circuit fabrication.  Referring to FIG.  2, to prevent short channel effects as the dimensions of the MOSFET 100 are further scaled down, halo regions 132 and 134 are formed to be beneath the drain extension junction 104 and the source extension junction 106 in the semiconductor substrate 102.  A drain halo region 132 is formed to be beneath the drain extension junction 104, and a source halo region 134 is formed to be beneath the source extension junction 106. 
The halo regions 132 and 134 are implanted with a halo dopant that is opposite in type to the dopant within the drain and source extension junctions 104 and 106.  For example, when the drain and source extension junctions 104 and 106 are implanted with an N-type dopant for an NMOSFET (N-channel Metal Oxide Semiconductor Field Effect Transistor), the halo regions 132 and 134 are implanted with a P-type dopant for preventing short channel effects of the NMOSFET.  On the other hand, when the drain and source extension junctions 104 and 106 are implanted with a P-type dopant for a PMOSFET (P-channel Metal Oxide Semiconductor Field Effect Transistor), the halo regions 132 and 134 are implanted with an N-type dopant for preventing short channel effects of the PMOSFET.  Such halo regions 132 and 134 are known to one of ordinary skill in the art of integrated circuit fabrication. 
Referring to FIG.  2, the halo dopant within the halo regions 132 and 134 diffuse due to transient enhanced diffusion during any integrated circuit fabrication process that heats up the semiconductor substrate 102.  In that case, the drain halo region 132 extends into the drain contact junction 108, and the source halo region 134 extends into the source contact junction 112, to undesirably increase the series resistance of the drain and source contact junctions 108 and 112.  In addition, the drain and source halo regions 132 and 134 extend toward the channel region 136 of the MOSFET 100 between the drain and source extension junctions 104 and 106, to undesirably increase the short channel effects of the MOSFET 100.  Furthermore, the drain and source halo regions 132 and 134 may also extend vertically along the semiconductor substrate 102 from transient enhanced diffusion, undesirably occupying greater volume of the semiconductor substrate 102. 
Nevertheless, halo regions are desired for reducing short channel effects of a MOSFET as the dimensions of the MOSFET are further scaled down.  Thus, a mechanism is desired for forming halo regions to be confined to be substantially only beneath the drain and source extension junctions with minimization of transient enhanced diffusion of the halo dopant.