  In this study, we electroplate Cu electrode on Si substrate to realize a
large-area vacuum nanogap for electron tunneling. We used cathode coating,
cathode rotation, asymmetric current regime, and electrolyte temperature
stabilization to obtain the regular geometry of the Cu electrode and reduce its
internal tension. Subsequently, internal tension was altered to achieve the
predefined surface curvature (concave or convex). For 12-mm diameter Ag/Cu
electrode, we achieve the curvature of 40 nm/mm from the Ag side. Reduction of
the electrode diameter to 3 mm allowed curvature as low as of 2.5 nm/mm. It
also allowed fabrication of two conformal electrodes having a nanogap of less
than 5 nm wide, over the area of 7 mm2. Such electrodes can be used for
efficient energy conversion and cooling in the mixed thermionic and
thermotunneling regime.
