Vacuum nanogap formation in multilayer structures by an adhesion-controlled process

Abstract

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. The 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 5nm and an area larger than 7mm2. Such electrodes can be used for efficient energy conversion and cooling in the mixed thermionic and thermotunneling regime.