a substrate support assembly having a horizontally rotatable chuck base, chuck pins disposed on top of the chuck base to support a substrate, a purge gas inlet hole extending from an underside center of the chuck base to an interior of the chuck base in an upward and downward direction thereof, and a purge gas outlet hole extending radially from the purge gas inlet hole and then extending upwardly to penetrate top of the chuck base;

a spin motor having a hollow tube-shaped driving shaft adapted to rotate the substrate support assembly; and

a purge gas supply assembly connected to the driving shaft through a magnetic bearing in a state of not rotating, extending vertically in a state of top of the purge gas supply assembly being spaced apart from an underside of the chuck base, and having a hollow hole penetratingly extending therealong in an upward and downward direction thereof and formed facing the purge gas inlet hole to supply purge gas to the purge gas inlet hole,

wherein the purge gas discharged upward from the purge gas outlet hole through the purge gas inlet hole is supplied between the substrate and the top of the chuck base and flows outside the substrate edge, purge gas introduced between the top of the purge gas supply assembly and an underside of the chuck base is exhausted to the outside in a direction away from the substrate via the magnetic bearing,

wherein, in a longitudinal sectional view, the chuck base comprises an inside part and an outside part with respect to the purge gas outlet hole, the inside part being lower in height than the outside part.