Chemical Modification of Silicon Surfaces for the Application in Soft Lithography

In 1965 MOORE [58] predicted that the transistor density of integrated circuits would double every 2 years$^{1}$. 40 years later this observation called Moore’s law is still an ongoing trend. Enhancing computing performance while simultaneously lowering the fabrication costs is the driving force for the semiconductor industry to decrease the sizes of devices on a chip further and further. To escape from the inevitable size restrictions, efforts are made to find innovative architectures leading to new concepts in electronics. Molecular Electronics is a research field pointing in that direction. The idea to use single molecules as functional units was inspired by FEYNMAN in his speech „There’s Plenty of Room at the Bottom“ in 1959 $^{2}$. Unconventional techniques are required in order to address molecules in electronics. Conventional patterning techniques used in semiconductor processing like Photolithography or Electron Beam Lithography are limited with respect to resolution or economic viability. Therefore efforts are made to introduce alternative patterning methods such as Soft Lithography techniques [96]. It is characteristic for Soft Lithography methods that a soft polymer as stamp or mold is used for patterning a substrate by printing or molding. Surface and interface properties often play fundamental roles in the printing and molding processes. Tailor-made surface properties are required for many Soft Lithography applications. It is often sufficient to build up films of molecular monolayer thickness in order to modify the properties of a materials surface. Such monolayers can consist of physically adsorbed molecules or more stable of molecules that are able to chemically bind to the surface. Langmuir-Blodgett films and self-assembled monolayers are common possibilities for such surface coatings [89]. The two-dimensional surface film determines the new surface properties. Commmonly silicon is the basic semiconductor material for microelectronic devices. Its production and handling is well-established. It is therefore reasonable that also in Soft Lithography techniques silicon is used as substrate for patterning procedures or as master for stamp fabrication. Hence, the chemical modification of silicon surfaces is a matter of interest for various applications in Soft Lithography. Appropriate molecules for this intention are organosilanes, which are capable to anchor to the surface in a self-limiting self-assembly process [67] [64]. They can bear a wide range of functionalities determining the new surface properties. Furthermore, they can act as interface between inorganic and organic compounds due to their characteristic structure. Further coupling of organic molecules to the interfacial silane film is possible. [...