Precise generation of ultra-low currents

Measuring lowest currents is becoming increasingly important in many fields such as, for example, semiconductor or medical instrumentation industries. In medical radiation diagnostics and therapy, for example, dosimeters based on the ionization of an irradiated gas are typically used to determine radiation levels. These devices measure very low currents in the pA region. The accuracy of the measurement is decisive for a precise dose determination. Testing and calibration of such dosimeters and other picoamperemeters require electrical current source standards that can produce constant low currents with high accuracy over long periods of time. An important method to generate extremely low currents is based on charging and discharging a capacitor. Applying a voltage that increases or decreases linearly in time (dU/dt = constant) to a capacitor of capacitance C, generates a constant current I = C · dU/dt. By measuring the capacitance and the time derivative of the voltage the generated current to be traced back to the units of capacitance, voltage and time.

A new current source of this type for highest specificities has been developed at PTB. In order to calibrate measuring instruments that average over long periods of time as well, a voltage ramp generator was developed based on an electronic integrator. Continuous and variable measuring times of up to 35 minutes can be realized. The relative fluctuation of the voltage ramp remains below 1 · 10^-5.

The new device produces user-defined currents of 10 pA to 0,1 fA. At 1 pA, the uncertainty amounts to only 0,01 percent. At 10 fA it is still 0,05 percent.

This calibration technique is currently being transferred to a renowned German manufacturer of dosimeters. The method can gain even greater economic importance for the semiconductor industry. Here, the capability to measure lowest currents in the fA region is one of the typical requirements for modern picoamperemeters, as spurious leakage currents in the cells of modern memory devices are of the same order of magnitude. The new procedure has made the calibration of such devices more reliable and accurate.