Effects of surface roughness on palladium-silicon Schottky diode hydrogen sensors using laser ablation

This dissertation focuses on fabricating Pd-Si Schottky barrier diodes (SBD) to create Hydrogen sensors and using laser ablation techniques to roughen the metal's and semiconductor's surfaces. Then, the sensors were tested in a H2 environment to study their altered electrical characteristics.;Specifically, Pd-Si Schottky diodes were fabricated by sputtering Pd on a processed Si wafer using a plasma-enhanced physical vapor deposition process (i.e., a Technics Hummer V system). The diodes were analyzed using an XE-100 Scanning Probe Microscope (SPM) to determine the Pd's film thickness. Electrical contacts were added to create the hydrogen sensors and the sensors were analyzed before and after being exposed to H2 using the National Instruments (NI) Elvis Curve Tracer to generate I-V curves. Multimeters were used to measure resistance during exposure to H2.;Laser ablation was used to vary the roughness of the Pd surface, the n-well surface, and the Pd and n-well surfaces of the fabricated devices. Untreated and laser-ablated Schottky diode sensors and resistive sensors were then exposed to H2 and were analyzed to examine their electrical characteristics. The effects of ablation were studied to compare sensor performance. This ablation work was sponsored by Dr. Mary Helen McCay through the National Center for Hydrogen Research at the Florida Institute of Technology under NASA Glenn Research Center, Grant ;Type 1 and Type 3 sensors were successfully created in the MicroElectronics Laboratory at F.I.T. The Type 1 Sensors exhibited Schottky diode behavior characteristics, with the resistance decreasing during exposure to H 2. The current and voltage increased monotonically during this exposure, and the diodes did not pass current in the reverse bias direction. The sensors showed a decreasing Is as I increased, which is consistent with the diode equation, since the saturation current is inversely proportional to the forward current. Also, Is increased in the control sensor after it was exposed to H2, further supporting the fact that all thin film sensors exhibited Schottky diode behavior. High resistances and subsequently low currents meant that there were only a few conductive channels in the Pd hydride, and also meant that the Type 1 sensors had very thin Pd films, on the order of 10-20 nm. While industry has created films of 100-200 nm, this work contributes to sensor fabrication because of the very thin films we were able to create in the lab.;The Type 3 sensors acted as resistor-based devices and there was a significant increase in resistance during H2 exposure.;It was theorized by (Kaltenpoth 2003) that creating micro-channels in the Pd surface or the n-well would create better conduction through the device, thus increasing the sensitivity of the sensor to H 2. Our investigation confirmed this experimental hypothesis and built on this database. For the Type 3 thick film sensors, the laser power settings created micro-channels. For the Type 1 thin film sensors, ablating the n-well created micro-channels and was the most promising technique that resulted in the most sensitive hydrogen sensor. Using laser ablation in this investigation as a micro- and nano-roughening technique created a baseline on which future studies can further research the technique by using other low powered lasers to create the "microchannels" in the n-wells or the n Si substrate.;Specifically, the research showed that for the Type 1 thin film SBD sensors, for time critical applications such as a hydrogen gas leak on the Shuttle, the control and n-well ablated sensors are better suited than the Pd-ablated sensor. Also, while the control sensor had the fastest relative response to H2, the overall drop in resistance may only be detected by sensitive, expensive equipment. Thus, if cost is an important consideration, the n-well ablated sensor is most suitable, since it has the greater drop in resistance during this time interval and this drop can be easily detected by most equipment. Finally, if there is an application where H2 does not have to be detected immediately, the Pd-ablated sensor would work nicely, since after an initial delay in response, there is a nice fast, linear drop in resistance.;This research is invaluable in that we were able to thoroughly investigate factors affecting H2 sensor performance by successfully designing, creating and testing both thick and thin film hydrogen sensors, both types of which were built from raw, factory wafers. The expertise gained during this research effort and the wealth of data that has been analyzed can be accessed by the academic community and industry for further research in creating effective hydrogen sensors for the aerospace industry, for government agencies such as NASA, and for other commercial industries. (Abstract shortened by UMI.)...