A surface treatment device that ejects a combination of precursor substances as a directed flow of surface treatment particles. Planar objects are conveyed along a defined plane through the particle flow, a region on the surface of the planar object that the particle flow hits forming a region of direct impact. The device comprises directing means for directing the particle flow to travel along the surface of the planar object in an extended impact region outside the region of direct impact; and flow control means for controlling the extent of the extended impact region which may include a vortex flow. The exposure of the treated surface with the particle flow increases and the probability of the desired surface treatment processes to take place increases.