Time series of the secondary electron yield were recorded using a personal computer controlling a Keithley 6517A electrometer through an IEEE 488 bus. For a complete measurement cycle the bias voltage and the voltage on one of the deflection plates needed to be changed by software. A special interface was built for this task (Figure 3.2).
To switch the beam on and off the voltage applied to one of the deflection plates was switched between two values provided by two power supplies. These deflection plates are located at the exit of the electrostatic analyzer. To avoid problems arising from the electrical connection between the deflection system and the controlling PC the switch was realized by relays providing galvanic insulation between the two systems. For the sample bias voltage it was impossible to use relays in the same way as for the deflection voltage because of the very small current to be measured. Even very small leakage currents through some mounting structure towards ground had to be avoided. This switch was realized using a battery and some electronics mechanically attached only to the vacuum interface pin. A battery powered relay controlled through a light dependent resistor (LDR) was used to switch between =0V and =18V. The only connection to ground is through the ampere meter. A light emitting diode (LED) illuminates the LDR to switch the relay. The LED has no mechanical contact at all with the switching electronics providing a maximum of insulation. Power to the switching electronics was provided by a two batteries, one for the bias voltage itself and the other supplied the driver and the relay. The LED and the relay for were controlled via the parallel port of the PC.
Figure 3.3 depicts a typical measurement cycle. A total of 16 frames of 100 measurements each are recorded with alternating settings for beam deflection and sample bias voltage. Appropriate time delays between the frames ensure that charge displacements on insulated parts of the chamber can take place without any negative influence on the current measurement. The observed time constants suggested a replacement circuit for the current measurements as shown in Figure 3.4.
resulting from has a value of a about 50 seconds and is probably due to charge displacements on the insulating parts of the probe holder occurring at the switch of the bias voltage . Using the peak charge current of when switching the value of is estimated to be of the order of 10pF yielding a value for of 10.
A value for was estimated by
where is the change in the measured current when is switched while the primary beam is not changed (see Figure 3.3). This yields a value of .
Another problem for time resolved measurement is the non shielded part of the circuit shown as shaded area A in Figure 3.4. Induction might cause additional currents in this loop introducing additional noise. By keeping the area small and by avoiding movements in proximity of the circuit during measurements the induced current can be kept low. Useful measurements may only be taken during periods of quiet laboratory environment. Some unexplained temporarily current fluctuations may be due to the nearby air conditioning system for the laboratory below or due to traffic in the nearby railway station.
March 2001 - Martin Wieser, Physikalisches Institut, University of Berne, Switzerland