Experimental studies of the deposition of films formed on the sidewalls of a photoresist pattern in the etching of platinum films with Ar and halogen mixed gas plasmas were performed. In platinum etching with Ar/halogen gas plasmas there is no enhancement of the etch rate by adding halogen gases, and deposition films are formed on the sidewall of a photoresist pattern. The thickness of the sidewall deposition film is minimized in etching with pure Ar plasma. The addition of halogens increases the thickness of the sidewall deposition film. These films were analyzed by x-ray photoelectron spectroscopy (XPS) to investigate their formation mechanism. Carbon was detected even in the deposition films formed during etching with gases that did not include carbon. From the results of XPS analysis it was found that the etching reaction products from the photoresist relate to the formation of these films, and that the reason for the increase in the film thickness is the increase in the etch rate of the photoresist by adding halogens. Therefore in the etching of platinum films it is important to suppress the etching of the photoresist to decrease the thickness of the sidewall deposition film.