Thank you for the reply. Yes, that is a problem when using XPS to determine the carbon. Besides, I wish to quantify the carbon in terms of mass, like ug/cm2. XPS usually give the ratios of the surface elements, could these ratios be converted into mass in XPS analysis?
There is a chance to calculate the mass coverage, if you assume that you have an adsorbate. Then each atom counts, and using the weight of a carbon you may estimate the total mass coverage (see enclosed paper). But as soon as you have a film, this will not work properly, because in that case density comes into play, and you do not know the density in general.
Another way to quantify carbon may be Low Energy Ion Spectroscopy (ISS) - but still the same problems compared to XPS with contaminations! In the enclosed paper, we were unable to detect carbon.
XPS will be suitable, but you carbon film thickness needs to be less than 10 -15 nm to look at the relative amount of carbon to copper (assuming you have a cohesive film instead of islands)
Depending on your requirements you can potentially do a measurement of carbon through the formation and measurement of a gaseous product. For example methanation of surface carbon is possible via reaction with H2. For an example see Journal of Catalysis, 43, 363-366 (1976) and similar articles.