Structural and Electronic Characterization of Metal-Molecule-Silicon Junctions Produced by Flip Chip Lamination
Michael A. Walsh, Mariona Coll, Curt A. Richter, Christina A. Hacker
Organic molecules are of great interest for nanoelectronic applications because they are a versatile material which can be used to make devices, such as photovoltaics, chemical and biological sensors, or transistors. The integration of organic molecules with silicon is particularly interesting due to the many advantages these hybrid devices can offer for post-CMOS technology. Despite the potential of silicon-based molecular electronics, fabrication of the devices still presents difficult challenges. Issues involving electrode surface roughness, molecule-electrode connection, and physical structure of the molecular packing all are important factors when creating devices. A new fabrication technique, flip chip lamination (FCL), has been developed which overcomes many of these challenges by enabling uniform, dense molecular films to be covalently bound to both silicon and gold electrodes at either end. The process begins with a thin gold film deposited on a silicon surface functionalized with a fluorinated release layer. The gold film is then transferred onto a plastic substrate (PET) using nanoTransfer Printing (nTP). This process creates an ultrasmooth gold (uSAu) film on the PET. Thiol-based bifunctional molecules are then used to form a self-assembled monolayer (SAM) on the uSAu on PET. The substrate is then “flipped” and the nTP step is used once again to promote covalent attachment of the SAM to a hydrogen-passivated silicon substrate. This transfer process enables highly dense, uniform molecular SAMs to be covalently attached between both a metal and semiconductor electrode with relatively low defect and pinhole concentrations. These structures can then be tested for both their physical and electronic properties. Several bifunctional molecules with varying end groups were used to explore the role of chemical functionality on the physical and electrical properties of the metal-molecule-semiconductor devices. Several techniques including infrared spectroscopy, spectroscopic ellipsometry, contact angle measurements, and scanning probe microscopy were used to characterize the molecules at the interface prior to FCL. The effects of the FCL process on the chemical and physical properties of the imbedded molecular layer were interrogated with polarized-backside incident infrared spectroscopy. Electrical measurements of aliphatic and aromatic monolayers highlight the transport mechanisms of hybrid organic-inorganic junctions.