Formation of Shallow Junctions
As modern device structures
scale down to micron and submicron dimensions, they require precise control of
dopant distributions vertically and laterally on a very fine scale. The three
main techniques used to achieve these goals are
Low energy implantations
- Arsenic is heavy enough to form shallow n+
layers with implantation energies within the reach of standard machines,
for example at 75 keV, the range of arsenic is only 500°A.
- At 75 keV Boron has a range of 2500°A but the effective boron
energy can be reduced by implanting the molecular ion BF2+.
This decreases the range.
- Another way is to implant through a surface
film such as silicon dioxide.
- The recoil effect can be used directly if we
dope by knocking dopant out from a deposited surface film using silicon
self-implantation.
Tilted ion beams
- It is the ion velocity perpendicular to the
surface that determines the projected range of an implanted ion
distribution.
- If the wafer is tilted at a larger angle to
the ion beam then the effective ion energy is greatly reduced, and this
makes it possible to achieve extremely shallow junctions. [2]
Implanted silicides and polysilicon
- Another way is to implant entirely into a
surface layer and then diffuse the dopant into the substrate.
- This is most often done when the surface film
is to be used as a conductor making contact to the substrate. Diffusion
results in steep dopant profiles without damage to the silicon lattice.
- For silicides, if the implant is beneath the
metal, it will be “snow ploughed” forward as silicide forms, resulting in
a steep dopant gradient near the interface. [2]