Interaction of Vapor-Deposited Metals with SAMs
We have demonstrated that the interaction of PVD-deposited metals with SAMs is governed by a single, general scheme. In the first stages of deposition, at low coverages, metal atoms such as Al, Ca and Cu are adsorbed because they form weak complexes at the SAM/vacuum interface. Metals that cannot form such complexes are scattered from the surface. The adsorbed metal atoms then either react with the SAM terminal group or penetrate through the monolayer. In some cases, such as vapor deposition of Cu on -COOH terminated SAMs, most adsorbed metal atoms form a weak complex with the terminal group and eventually penetrate through the SAM, while a small proportion have sufficient kinetic energy to react with the terminal group. For metals that do not form weak complexes, only atoms with energy sufficient to react with the terminal group or methylene chain can adsorb. In all cases, adsorbed metal atoms provide sites for the nucleation of metallic islands and/or overlayers.
Many metal-molecule interactions are complex processes that depend not only on the chemical species involved but also on the experimental conditions. In collaboration with D.B. Janes (Purdue University) we have recently shown that the conductivity of a metal/molecule/semiconductor device, Au/CH3(CH2)17S/GaAs, is correlated with the contact structure, which is in turn related to the metal deposition conditions used. In this case, a four-fold difference in the amount of Au penetration leads to an approximately hundred-fold difference in the conductivity, demonstrating that deposition conditions must be carefully controlled.
New Techniques for Metal Deposition on Organic Surfaces
Chemical Vapor Deposition
There are many technologies besides PVD used to deposit metals and other materials in semiconductor-based microelectronics, including atomic layer deposition (ALD) and chemical vapor deposition (CVD). CVD is employed in a broad range of commercial applications to grow oxide, metal, semiconductor, glass and compound thin films. There have been few studies of CVD on organic thin films because high thermal activation temperatures (T ≥ 200 °C) are needed which are incompatible with most organic thin films, including SAMs. However, we have recently shown that the ALD precursor trimethylaluminum (TMA) can be used to deposit thin films of both aluminum and aluminum oxides on SAMs at room temperature. TMA reacts with -OH and -COOH terminated SAMs to form a surface-bound dimethyl aluminum complex. Trimethylaluminum does not react with -CH3 terminal groups. If deposition is carried out using a nitrogen-purged glove box, an alumina film is grown on -CH3, -OH and -COOH terminated SAMs. Alumina films are strongly bound to -COOH and -OH terminated SAMs but only weakly adsorbed on -CH3 terminated SAMs. These films can be removed from -CH3 terminated SAMs by rinsing in organic solvents. If the base pressure of the deposition chamber is lowered to below 10-8 torr, a metallic Al overlayer is selectively deposited on -OH and -COOH terminated SAMs, with no reaction observed on -CH3 terminated SAMs.
Electroless Deposition
Copper is widely used in microchips but it interacts only weakly with polymers, leading to poor interfacial adhesion and diffusion of copper into organic films, leading to shorts. Studies of PVD of Cu on -COOH, -CO2CH3, -OH, -OCH3 and -CH3 terminated SAMs have also demonstrated that Cu penetrates through SAMs. An alternative method by which to deposit a metal overlayer is electroless deposition, which is commonly employed to deposit metals on polymer films. In this method deposition occurs via the chemically promoted reduction of metal ions without an externally applied potential. We have studied the electroless deposition of copper on SAMs, using a plating solution containing copper sulphate, formaldehyde (reducing agent) and EDTA (complexing agent) . At 22 °C copper deposits on both -CH3 and -COOH terminated SAMs, but not on -OH terminated SAMs. This is because the hydroxyl terminal groups react with formaldehyde in the plating solution to form acetals, which prevent Cu deposition. At higher deposition temperatures (45 °C), no Cu deposits on -CH3 terminated SAMs, while more Cu deposits on -COOH terminated SAMs. Copper also penetrates through -CH3 and -COOH terminated SAMs to the Au/S interface. This suggests that electroless deposition may not be suitable for creation of metal/SAM/metal or metal/SAM/semiconductor device architectures.