  In this paper evolution of silicon surface topography, under low energy ion
bombardment, is investigated at higher oblique incident angles in the range of
63\degree-83\degree. Si(100) substrates were exposed to 500 eV argon ions.
Different surface morphology evolves with increasing angle of incidence.
Parallel-mode ripples are observed up to 67\degree which undergo a transition
to perpendicular-mode ripples at 80\degree. However, this transition is not a
sharp one but undergoes a series of unusual pattern formation at intermediate
angles. Complete smoothening of silicon surface is observed at incident angles
beyond 80\degree. The observed patterns are attributed to surface confined
viscous flow and sputter erosion under ion bombardment.
