Ultraviolet photoetching of copper

Abstract

Excimer laser etching of copper using CCl4 and CF2Br2 vapors is described. These gases, unlike pure halogen gas, are inert toward copper in the absence of light. For pressures of 2–10 Torr, 248-nm dissociation of CCl4 or CF2Br2 results in etching that is linear in pressure, and is approximately the same for both gases. At 351 nm, only CCl4 causes etching. This wavelength variability, examined together with the gas-phase absorption spectra, suggests that gas-phase photolysis occurs prior to etching for CF2Br2 at 248 nm, but that a surface mediated dissociation likely occurs for CCl4 at 248 and 351 nm. A numerical solution of the one-dimensional heat flow equation was used to obtain the laser-induced surface temperature-time profile for the case of a bare copper surface, and copper covered with differing thicknesses of CuCl. CuCl is the reaction layer assumed to form when CCl4 dissociates on copper. Surface melting is not indicated, implying that etching occurs either by transient evaporation (solid→vapor) or by a quantum (nonthermal) process. Numerical solution of an etching model based upon transient evaporation of the CuCl reaction layer gives calculated etch rates that are many orders of magnitude smaller than the experimental rates. It is thus argued that nonthermal photophysical processes are responsible for material removal from the surface. The implications of these results for excimer laser etching of copper in Cl2 or Br2 gases are discussed.