The instability of solidification process occurring under the cooling of a metal melt is shown to be able to initiate short‐wave periodical structures on the surface provided the impurity concentration in the melt is sufficiently high. The experiments were carried out using titanium (plus yttrium) and copper (plus tin) targets melted by means of an ion beam. The mechanism of this phenomenon is associated with the solidification front instability caused by the constitutional supercooling of the liquid phase due to the impurities forced out into the latter under front motion. A dispersion equation for the solidification waves on the interphase boundary is derived for the thin melt case, its solutions are investigated as functions of the impurity concentration and heat transfer regime. The calculated values of the structure periods and excitation criteria are in good accordance with the experimental data. In our opinion, the short‐wave structures occurring on the solidified surface of liquid metal ion source [A. Wagner, T. Venkatesan, P. M. Petroff, and D. Barr, J. Vac. Sci. Technol.19, 1186 (1981)] may be caused by the above mentioned instability of the melt solidification process. In course of ion source operation, the impurities are produced by the cathode sputtering of the extractor [G. S. Galovich and A. Wagner, J. Vac. Sci. Technol. B6, 2108 (1988)].