To understand the mechanism of recently reported low-temperature Cu dry etching using an inductively coupled Cl-2/N-2 plasma with ultraviolet light irradiation, we have studied the reaction characteristics between Cu thin film and an inductively coupled Cl-2/N-2 plasma at room temperature without/with ultraviolet irradiation, Light absorption characteristics of solid etch products in ultraviolet/visible region, and mass spectrometry of gaseous desorption products during ultraviolet photon irradiation on solid etch product CuClx(s) surface. Ultraviolet irradiation increased the Cu consumption rate for the total etch reaction by approximately five times and the Cl stoichiometry x of CuClx(s) was always above 1.0 (1.2-1.3) regardless of reaction time in contrast to results without ultraviolet light irradiation. The mechanism of low-temperature Cu dry etching induced by ultraviolet light irradiation consists of selective absorption of ultraviolet radiation (300-400 nm) by the solid etch product CuClx(s) depending on the Cl stoichiometry x due to the electronic transitions of bonding electrons of CuCl (bandgap transition) or CuCl2 (charge transfer), weakening of CuClx(s) bonding, and nonthermal desorption of gaseous products. The dominant desorbed species were CuCl2 and Cu2Cl3 during the ultraviolet irradiation on CuClx(s), which is different from the Cu3Cl3 for the pure thermal desorption. Also, the distribution of gaseous products from the CuClx(s) by ultraviolet irradiation depends on the Cl stoichiometry x of CuClx(s), especially whether x < 1.0 or x > 1.0.