A STUDY OF THE MECHANISM OF AC ELECTROLYTIC GRAINING OF ALUMINUM IN HYDROCHLORIC AND NITRIC-ACID

a.c. Electrolytic graining or electrograining employs an alternating v oltage or current on aluminium electrodes in a suitable electrolyte, e .g. hydrochloric or nitric acid, to produce uniformly pitted and convo luted surfaces for lithography. The resultant morphology or the electr ograined surface is dependent upon the particular electrical, solution and substrate conditions employed. Here, current understanding of ele ctrograining of aluminium in hydrochloric acid is reviewed and compare d with graining in nitric acid. Electron micrographs revealed that dur ing electrograining large hemispherical pits are formed in both electr olytes. However, close examination of the hemispherical pits formed in hydrochloric acid showed that they are composed of a large number of fine cubic pits. Conversly, these crystallographic pits are not observ ed after treatment in nitric acid; instead a hemispherical pit with a characteristic dimension is found. Such morphological appearances have been studied as a function of graining frequency and current density. In addition to the formation of pits, a non-uniform film of etch prod ucts is formed, masking completely the real surface morphology. This e tch film is deposited largely as a gel of water and amorphous AI(OH)3, aluminium particles and some chloride or nitrate ions. The formation of this etch film is explained by a cathodic filming process on the al uminium electrode. Surface alterations by treatment in both electrolyt es are explained in terms of the applied a.c. signal.