Sources of variability in long-term testing of silicon tip field emission

This paper will examine the variability in field emission during life testing of silicon tip devices fabricated at the Rutherford Appleton Laboratory. Life tests at both constant voltage and software-controlled constant current have been carried out. Variability in emitted currents was expected from several non-linear effects. Firstly, the well-known Fowler-Nordheim response of field emission current to voltage has some interesting consequences. If the emitted current i is related to gate voltage V by a simplified form of the Fowler-Nordheim relation. The accuracy of the high-specification Keithley 487 voltage source used in the laboratory testing is specified as 0.15% + 40 mV. At typical operating voltages this corresponds to a fluctuation in emission current of 1-2%. This implies that even in idealised conditions, with identical field emitters running at constant voltage, variation in the emission current can be expected. A finite element model incorporating the Fowler-Nordheim equation has been used to predict the response of emission current to tip diameter, using the measured range of ∼28-40 nm as an input parameter. The basic noise level of 1-2% will be increased as a consequence of the distribution of tip diameters.