  We describe a technique for making one-dimensional ohmic contacts to
individual graphene layers encapsulated in hexagonal boron nitride (h-BN) using
CF4 and O2 plasmas. The high etch selectivity of h-BN against graphene (>1000)
is achieved by increasing the plasma pressure, which enables etching of h-BN,
while graphene acts as an etch stop to protect underlying h-BN. A low-pressure
O2 plasma anisotropically etches graphene in the vertical direction, which
exposes graphene edges at h-BN sidewalls. Despite the O2 plasma bombardment,
the lower h-BN layer functions as an insulating layer. Thus, this method allows
us to pattern metal electrodes on h-BN over a second graphene layer. Subsequent
electron-beam lithography and evaporation fabricate metal contacts at the
graphene edges that are active down to cryogenic temperatures. This fabrication
method is demonstrated by the preparation of a graphene Hall bar with a
graphite back-gate and double bilayer-graphene Hall bar devices. The high
flexibility of the device geometries enabled by this method creates access to a
variety of experiments on electrostatically coupled graphene layers.
