Characterization of ALD copper thin films on palladium seed layers for molecular electronics

Atomic layer deposition (ALD) is a powerful tool for nanoelectronics. In addition to its role for growing ultrathin films in microelectronics, ALD can be used for controlling critical dimensions on the nanometer scale. In molecular tunnel junctions, the spacing between the metal electrodes must be small enough to adsorb individual or small groups of molecules, on the order of 1-2 nm. This condition makes ALD an ideal process to fabricate nanoelectronic devices because it provides subnanometer thickness control.;A method for fabricating monolithic nanoscopic tunnel junctions (MNTJs) for tunneling spectroscopy measurements using atomic layer deposition (ALD) of copper on palladium seed layers has recently been introduced (Gupta et al., Appl. Phys. Lett. v. 207). A critical need for tunneling spectroscopy and other molecular electronics measurements is to characterize the composition and structure of the electrodes. The ALD grown layers are characterized here using planar thin films as models for the electrode composition and structure. ALD grown copper films using a varying number of cycles on thick palladium seed layers were investigated using transmission electron microscopy (TEM), Auger electron spectroscopy (AES), glancing incidence x-ray diffraction (GIXRD), and x-ray photoelectron spectroscopy (XPS) to investigate the chemical composition and structure of the electrodes. Electron diffraction and GIXRD show that as copper is deposited, the deposited layer progresses from palladium-rich to becoming predominately copper. In contrast, AES data show that significant palladium consistently remains on the surface of the growing film. The divergence in surface and bulk behaviors is attributed to palladium surface segregation that is driven by hydrogen adsorption during the ALD process. A diffusion model was created to explain the behavior of palladium movement through the film. Platinum was demonstrated to be a viable alternative seed layer for copper ALD growth, leading to pure copper films, in contrast with results for palladium. This is because hydrogen adsorption does not induce platinum surface segregation. A smaller diffusion rate for platinum in copper may also contribute to the purity of the ALD copper layers on platinum. However, whereas palladium leads to relatively smooth surfaces, platinum seed layers produce copper films that have larger grains and rougher surfaces. Overall, we demonstrate how composition and structure of the electrodes can be measured and controlled using ALD deposited layers. Extension of the methods to real devices with three dimensional structures is discussed. This approach is promising for future nanoelectronic devices based on active molecular devices.