  Copper foil impurities are hampering scalable production of high-quality
graphene by chemical vapor deposition (CVD). Here, we conduct a thorough study
on the origin of these unavoidable contaminations at the surface of copper
after the CVD process. We identify two distinct origins for the impurities. The
first type is intrinsic impurities, originating from the manufacturing process
of the copper foils, already present at the surface before any high-temperature
treatment, or buried into the bulk of copper foils. The buried impurities
diffuse towards the copper surface during high-temperature treatment and
precipitate. The second source is external: silica contamination arising from
the quartz tube that also precipitate on copper. The problem of the extrinsic
silica contamination is readily solved upon using an adequate confinement the
copper foil samples. The intrinsic impurities are much more difficult to remove
since they appear spread in the whole foil. Nevertheless, electropolishing
proves particularly efficient in drastically reducing the issue.
