Photoreactive surface processing

A method of cleaning a substrate surface, the cleaning being done photoreactively without damaging the surface, comprising

• delivering a laser beam of UV radiation at an acute angle to the surface, the beam striking the surface at a long and narrow reaction region, said beam delivering 500 to 2,000 mJ/cm.sup.2 energy density measured normal to said surface when said beam is incident on said surface at an angle of 80.degree. with respect to normal;
• moving the beam and the substrate relative to one another to cause the beam to sweep the surface;
• and
• while the beam is sweeping the surface, providing a directed flow of a reactant gas or gas mixture at the reaction region so that the gas is excited by the UV laser beam and chemically interacts with foreign material on said substrate surface, the acute angle of the beam being of a value such that foreign material is removed form the substrate surface.