I think it means the implanting of the S/D is happening at an angle.

Yawei,

   First, implantation is used to precisely control the doping of Silicon.
The knobs for controlling implanation are dose, concentration, and the
energy, depth. The challenge is that Silicon is a crystal and the implanted
dopant tends to find the lattice and travel much father than intended. The
dopant stops when it "hits" something. To prevent this phenomena, process
engineers use several approaches, including off-axis implantation. By
implanting "across" the lattice, the likelyhood of hitting the lattice increases
and the depth of the implant can be more precisely controlled.  To be honest,
I have never heard of anybody actually intentionally implanting on-axis. BTW,
other mtehods include scattering oxide and amorphization. Scattering oxides
are usually thin oxide layers that tend to randomly scatter the dopant.
Amorphization, sorry the spelling may be incorrect, is done by hurling
something big and neutral, usually Aragon,  at the Silicon prior to the
implant. The Aragon, turns the Silicon matrix into amorphous mush so
Silicon scatters the dopant.

                                                               Best Regards,

                                                                  Sheldon