  We investigate the etching of a pure hydrogen plasma on graphite samples and
graphene flakes on SiO$_2$ and hexagonal Boron-Nitride (hBN) substrates. The
pressure and distance dependence of the graphite exposure experiments reveals
the existence of two distinct plasma regimes: the direct and the remote plasma
regime. Graphite surfaces exposed directly to the hydrogen plasma exhibit
numerous etch pits of various size and depth, indicating continuous defect
creation throughout the etching process. In contrast, anisotropic etching
forming regular and symmetric hexagons starting only from preexisting defects
and edges is seen in the remote plasma regime, where the sample is located
downstream, outside of the glowing plasma. This regime is possible in a narrow
window of parameters where essentially all ions have already recombined, yet a
flux of H-radicals performing anisotropic etching is still present. At the
required process pressures, the radicals can recombine only on surfaces, not in
the gas itself. Thus, the tube material needs to exhibit a sufficiently low H
radical recombination coefficient, such a found for quartz or pyrex. In the
remote regime, we investigate the etching of single layer and bilayer graphene
on SiO$_2$ and hBN substrates. We find isotropic etching for single layer
graphene on SiO$_2$, whereas we observe highly anisotropic etching for graphene
on a hBN substrate. For bilayer graphene, anisotropic etching is observed on
both substrates. Finally, we demonstrate the use of artificial defects to
create well defined graphene nanostructures with clean crystallographic edges.
