The characterisation of e-beam evaporated and magnetron sputtered carbon films fabricated for atomic oxygen sensors

Carbon coatings, for atomic oxygen sensor applications, have been deposited onto alumina substrates and subjected to atomic oxygen and various heat treatments. The effect of these treatments has been studied. Carbon was fabricated using two methods: electron-beam (e-beam) evaporation and dc-magnetron sputtering. The carbon films were exposed to 5 eV atomic oxygen (AO) fluences ranging from 0.5×1019 to 7.9×1020 atoms cm−2 and their resultant characteristics investigated. E-beam evaporated carbon films were found to be graphitic in content and conducting. The sputtered carbon films were insulating and their resistivity remained high even after high-temperature annealing. From Raman spectroscopy it was possible to determine the ID/IG ratio in the carbon from the intensity of the characteristic D and G peaks. After AO exposure, Raman spectroscopy shows the appearance of a characteristic D peak in the e-beam evaporated carbon films, implying the formation of sp3 clusters. For the magnetron sputtered films, the D peak broadened, the broadening being proportional to ordering in the films. A plot of the G peak position with temperature shows the trend in graphitisation as temperature is increased. From XPS analysis, there was an increase in the surface concentration of the O1s peak at the expense of the C1s peak, signifying that the carbon reacts with oxygen atoms. The reaction of carbon to oxygen is not a physical process, but involves the removal of carbon by a chemical process of CO or CO2, which is lost to the surrounding environment.