Trace-metal contamination in proton exchange membrane fuel cells caused by laser-cutting stains on carbon-coated metallic bipolar plates

Trace-metal contamination poses a threat to performance and stability of proton exchange membrane fuel cells (PEMFCs). In this study the source of origin and degree of metal dissolution from carbon-coated 316L bipolar plates (BPPs) are evaluated after a long-term PEMFC test run under conditions resembling a real-life automotive application. Despite intact carbon-coating, metal dissolution occurs from uncoated oxycarbide stains on the plates’ surface. Which correlates with post-mortem detection of chromium, iron and nickel in the membrane electrode assembly. The rate of cell voltage decrease throughout the high current operations is found to be twice as high in the presence of metal ions. Metal dissolution can be correlated with transients in cell voltage during dynamic current load cycling as a result of temporary global fuel starvation. The observed difference in metal dissolution on the anode and cathode BPP indicates weak galvanic coupling between the bipolar plate(s) and the electrode layer(s).     

Synthesis of C-axis-oriented AlN thin films on high-conducting layers: Al, Mo, Ti, TiN, and Ni

Thin piezoelectric polycrystalline films such as AlN, ZnO, etc., are of great interest for the fabrication of thin film bulk/surface acoustic resonators (TFBARs or TFSARs). It is well-known that the degree of c-axis orientation of the thin films correlates directly with the electromechanical coupling. However, the degree of c-axis orientation of the piezoelectric film is, in turn, influenced by other parameters such as the structure of the substrate material, the matter of whether the c-axis is up or down (polarity), and the growth parameters used. The correlation of these three aspects with the electromechanical coupling of the AlN-thin films, is studied here. Thin AlN films, prepared in a magnetron sputtering system, have been deposited onto thin Al, Mo, Ni, Ti, and TiN films. Such thin high-conducting layers are used to form the bottom electrode of TFBAR devices as well as to define a short-circuiting plane in TFSAR devices. In both cases, they serve as a substrate for the growth of the piezoelectric film. It has been found that the degree of orientation and the surface roughness of the bottom metal layer significantly affects the texture of the AlN films, and hence its electroacoustic properties. For this reason, the surface morphology and texture of the metal layers and their influence on the growth of AlN on them has been systematically studied. Finally, FBARs with both Al and Ti electrodes have been fabricated and evaluated electroacoustically.     

On the thermal stability of atomic layer deposited TiN as gate electrode in MOS devices

The work function of ALD TiN was found to be above 5 eV after RTP annealing below 800/spl deg/C in a nitrogen atmosphere, while higher annealing temperatures cause a drop in work function by about 0.3-0.5 eV. The effect was found for TiN metal gates on both SiO/sub 2/ and Al/sub 2/O/sub 3/ gate dielectrics in MOS-capacitors and was seen in C-V as well as in I-V measurements. On the contrary, annealing of SiO/sub 2/ capacitors in oxygen-enriched N/sub 2/ atmosphere increased the work function. A variation in EOT of less than 2 A was demonstrated for the various annealing temperatures, concluding that the ALD TiN is stable in contact with the different dielectric materials. However, the decrease in work function that is found in this investigation may implicate that ALD TiN is less suitable as a metal gate for pMOSFETs.     

Electrical characterization of AlN MIS and MIM structures

Aluminum nitride (AlN) thin films have been deposited on p-Si[100] and Mo-Si[100] substrates. The sputter deposited Mo was polycrystalline, predominantly showing a [110] orientation. Thin AlN films were grown under different process conditions in a physical vapor deposition (PVD) system to attain highly textured polycrystalline films as well as close to amorphous films. MIS and MIM structures were fabricated and electrical properties such as the dielectric constant, leakage current, and high-frequency behavior were investigated. It is found that the dielectric constant is 10 and does not change with the crystallinity of the films. High-frequency measurements up to 10 GHz show no frequency dispersion of the capacitance. The leakage current stays relatively constant between films and is believed to be Poole-Frenkel controlled. Capacitance-voltage (C-V) measurements for MIS structures revealed the presence of charges in the interface layer between the substrate and the dielectric film. The temperature dependence of the capacitance has also been studied.