Inspired by the need to avoid damage and reaction associated with the physical deposition processes for organic interfaces and the desire to weaken the bonds at metal-semiconductor and other solid interfaces, we notice that an emerging technology for contact formation, namely, the nanoTranfer Printing (nTP) technique28, 29, 71 can satisfy both needs, with some modifications. Therefore, we plan to development an instrument with essentially the same advantages as the nTP technique, but with added option to choose a wide range of metals as the stamping material and added capabilities to monitor the junction electrical characteristics as the interface is formed. This instrument is a stainless steel ultrahigh vacuum (UHV) system that will enable novel experiments on fundamental mechanism of interfaces and practical contact technology development. A schematic of a representative experiment that can be conveniently conducted in this instrument is shown in Fig. 3. Well-controlled surfaces are first prepared in UHV and then press together to study the evolution of electrical properties as a function of the applied pressure. The use of a soft metal (Au) layer on a soft substrate (elastomeric poly-dimethylsiloxane, PDMS) facilitates the formation of a conformal, gap-less contact to a hard material, as depicted in Fig. 4. The use of UHV allows the surface of the soft metal (Au, Al, etc.) to be coated with another ultrathin layer of desired conducting material, which is then forced into an intimate contact with the hard surface by pressure, as Fig. 4 illustrates. Such a transfer step allows a wide range metals to be used as “ink” in stamping experiments in UHV.

     The instrument under development is capable of forming interfaces between a variety of materials, under atomically clean conditions. A necessary condition is that one of the materials is soft enough to conform to the other surface under pressure. So, for example, soft metals can be pressed against hard semiconductors or insulators; soft organic layers can also be squeezed against hard metal or semiconductor surfaces. Therefore, this instrument can be used to study many materials interfaces of current interest. It is certainly well suited for a detailed study of the Schottky barrier height formation at interfaces stamped in vacuum, which may provide an ultimate test of the latest concept of interface dipole formation. With the expected “unpinning” of the Fermi level, one is also in position to investigate and develop novel contact strategies for semiconductors. Furthermore, the proposed instrument allows the introduction of dipolar layers, such as SAM, at metal-semiconductor junctions for additional flexibility in barrier height control. The proposed instrument is poised to widen the scope of science and technologies of ohmic contact. One notes that stamping/printing is similar in spirit to vacuum wafer-bonding, which has been widely studied in recent years.72 However, there is little reported work of bonding metals and semiconductors in vacuum, likely because the advantages in this approach have not been obvious. Rather, the role played by metal in direct vacuum-bonding has often been as an interface “glue” layer to join together different semiconductors.73, 74