We have studied the characteristics of ruthenium etching reactions and the etch selectivity of Ru to photoresist in ozone gas. The Ru was etched at a maximum etch rate of 950 nm/min at between 100 and 150 degreesC by the formation of a volatile product, RuO4. However, the etch rate decreased at temperatures above 150 degreesC, since the formation of RuO2 on the Ru surface hindered the etching reaction. A comparison between the etching reaction of Ru and that of photoresist revealed that Ru was etched predominantly by ozone, whereas the photoresist was etched predominantly by atomic oxygen. This difference in etching behavior enables us to control the etch selectivity between Ru and photoresist. Below 150 degreesC, the Ru/photoresist selectivity was higher than 40 due to a small amount of atomic oxygen. Conversely, above 300 degreesC, the selectivity of photoresist/Ru was higher than 40 due to RuO2 formation. Therefore, a high degree of selectivity was achieved by manipulating the temperature.