A seven-junction electron pump: design, fabrication, and operation

We have developed a seven-junction electron pump for use in a new standard of capacitance based on measuring the voltage produced when a known charge is placed on a capacitor. This new pump, with an error per pumped electron of 15/spl times/10/sup -9/, is about 30 times more accurate than a five-junction pump made previously at NIST. By careful design of the pump geometry, we have reduced the effect of cross capacitance and simplified device operation. Our fabrication recipe produces small, stable tunnel junctions relatively quickly and reliably. We have developed a method of tuning the pump for highly accurate electron counting. This tuning can be quickly repeated whenever fluctuations in background charges degrade accuracy.