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]