Abstract: A method of cleaning a substrate surface, the cleaning being done photoreactively without damaging the surface. A laser beam of UV radiation is delivered at an acute angle to the surface of the substrate, the beam striking the surface at a long and narrow reaction region. The beam and the substrate are moved relative to one another to cause the beam to sweep the surface. While the beam is sweeping the surface, a flow of a reactant gas is provided at the reaction region so that the gas is excited by the UV laser beam. The acute angle of the beam is of a value such that foreign material is removed without essentially damaging the surface of the substrate or leaving a residue that would inhibit further processing of the substrate surface.