Lithography scaling issues associated with III–V MOSFETs

In this work we investigate fabrication issues associated with scaling down the gate length and source drain contact separation of a III–V MOSFET. We used high resolution electron-beam lithography and lift-off for gate and ohmic contact patterning to fabricate gate-last lithographically-aligned MOSFETs. This work considers the effect of variations in resist thickness on gate lengths and also the fabrication of long narrow gaps using electron-beam lithography. The study showed that the effect of resist thickness variation on metal linewidth is insignificant. A difference of around 2–3 nm was found between PtAu linewidths fabricated using 150 and 280 nm thick resist. A VB6 lithography tool was found to be useful for linewidth measurements. We showed that the choice of resist is critical to gap formation, and that PMMA is not well suited to this task.     

Effects of daily precipitation and evapotranspiration patterns on flow and VOC transport to groundwater along a watershed flow path

MTBE and other volatile organic compounds (VOCs) are widely observed in shallow groundwater in the United States, especially in urban areas. Previous studies suggest that the atmosphere and/or nonpoint surficial sources could be responsible for some of those VOCs, especially in areas where there is net recharge to groundwater. However, in semiarid locations where annual potential evapotranspiration can exceed annual precipitation, VOC detections in groundwater can be frequent. VOC transport to groundwater under net discharge conditions has not previously been examined. A numerical model is used here to demonstrate that daily precipitation and evapotranspiration (ET) patterns can have a significant effect on recharge to groundwater, watertable elevations, and VOC transport. Ten-year precipitation/ET scenarios from six sites in the United States are examined using both actual daily observed values and "average" pulsed precipitation. MTBE and tetrachloroethylene transport, including gas-phase diffusion, are considered. The effects of the precipitation/ET scenarios on net recharge and groundwater flow are significant and complicated, especially under low-precipitation conditions when pulsed precipitation can significantly underestimate transport to groundwater. In addition to precipitation and evapotranspiration effects, location of VOC entry into the subsurface within the watershed is important for transport in groundwater. This is caused by groundwater hydraulics at the watershed scale as well as variations in ET within the watershed. The model results indicate that it is important to consider both daily precipitation/ET patterns and location within the watershed in order to interpret VOC occurrence in groundwater, especially in low-precipitation settings.