  In this study, we regulate adhesion between thin metal films to produce a
large-area vacuum nanogap for electron tunneling. Multilayer structures
comprising thin metal films with adjustable adhesion were fabricated. The
Cu/Ag/Ti/Si structures were grown on Si substrates and include thin Ti and Ag
films and a thick Cu layer. The Ag and Ti films were deposited on the Si
substrate under vacuum, and a thick Cu layer was subsequently electroplated
onto the Ag surface. Later, the sandwich was separated and a vacuum nanogap was
opened to produce two Ag/Cu and Ti/Si conformal electrodes. The adhesion
strength between the Ti and Ag films was precisely adjusted by exposing the
structures to dry O2 after Ti growth but before Ag growth. The resulting
adhesion needed to be sufficiently high to allow electroplating of Cu and
sufficiently low to allow subsequent separation. Either heating or cooling was
used to separate the sandwiches. T he structures separated as a result of the
different thermal expansion parameters of the Si and Cu electrodes and the low
adhesion between the Ti and Ag layers. After separation, the Ag and Ti surfaces
were analyzed optically using a Michelson interferometer. Adhesion regulation
and optimization of the electroplating regime allowed fabrication of two
conformal electrodes with a nanogap smaller than 5 nm and an area larger than 7
mm2. Such electrodes can be used for efficient energy conversion and cooling in
the mixed thermionic and thermotunneling regime.
