Electronics Configuration

Unlike the AFM deflection signal which was over 30 kHz in frequency, the response of the software and hardware was only of the order of ∼2 kHz. Thus when the tunnel current was the feedback parameter, only tunnel current variations of

≤ 2 kHz would contribute to the system feedback and therefore the tip movement.

A low-pass filter was integrated into the tunnel current channel, thus reducing any high frequency noise that may affect the tunnel current and thus the resulting topography images of the surface.

A digital signal processor (DSP) received user defined data via a PC to con-trol the microscope head, as illustrated in Figure 3.9. The DSP simultaneously

Chapter 3. Construction of a Combined AFM/STM UHV System 73

recorded the data from the microscope head. This data was processed and dis-played visually with gnome X scanning microscopy (GXSM) computer software [81]. The majority of outputs from the DSP to the microscope head were con-nected via a HV amplifier. The HV signals from the HV amplifier controlled the piezoelectric transducers of the scan tube that positioned the probe with respect to the surface. All of these channels were connected directly to the HV UHV feedthrough. The slipsticks were powered and controlled separately by a specific Omicron PSU which was connected to a switch box. The microscope user could use this switch box to switch between X and Z direction slipstick motion.

There were two output channels from the microscope head used to generate images for the STM and AFM modes of operation (as shown in Figure 3.9). These channels were the tunnel current and the TF sensor channel, connected to the tip wire and TF electrodes respectively. Both these signals were processed by the DSP so the GXSM software could generate topographic images and spectroscopy data.

Operational amplifiers (OP-AMPs) were included in the design of the system to amplify these two signals as close to the probe assembly as possible, to reduce the signal-to-noise ratio and to accommodate the input signal range of the DSP.

The tip holder ensemble had two recesses in which the OP-AMPs could be separately secured. The tip holder ensemble was grounded to provide a sealed electrical shield around the OP-AMPs. Only the contacts of the OP-AMPs pro-truding through the MACOR seal were accessible, unless the tip holder ensemble was dismantled.

Extra electrical shielding was required for the wires that connecting the probe assembly to the OP-AMPs, as these were extremely small (nanoamp) signals. The wires were very short in length to minimise signal loss, and thus it was difficult to successfully wrap a grounded thin Kapton wire around the signal wire to provide shielding. Therefore very thin coaxial wire (insulated with Kapton) was connected to the corresponding terminals instead. The coaxial shielding was then grounded, thus providing complete shielding of the sensitive wires, which received additional shielding from the grounded inner electrode of the scan tube.

The bias voltage was applied to the sample and the tunnel current measured through the STM tip, which was connected to one of the OP-AMPs situated inside the tip holder ensemble. An OPA111 OP-AMP converted the tunnel current signal from the tip wire attached to the TF into a voltage which was recorded with the DSP. The tunnel current signal was amplified by a factor of 10⁹ with the inclusion of a 1 GΩ resistor. The resistors used for both OP-AMP set-ups were physically very small, high value thick film chip resistors (RH73 series from RS Components).

The output of the tunnel current OP-AMP was input into the DSP via a low-pass filter, as explained previously. This had the added benefit of reducing the high frequency (20-32 kHz) component of the tunnel current that varied accord-ing to the oscillation frequency of the AFM/STM cantilever when operated in AFM/STM mode. Thus the time averaged tunnel current could be recorded as a separate channel alongside AFM data when the TF was excited. This also allowed for a ‘dynamic STM’ mode [82, 83, 84] in which the time averaged tunnel current flowing between the sample and oscillating tip was used as the feedback signal. In this mode the AC component of the tunnel current could also be recorded with the inclusion of a lock-in amplifier. Dynamic STM is discussed in Section 3.4.

The bias voltage applied to the sample was supplied directly from the DSP, for any operational mode of the system. Other signals not yet described but also labelled as a lower voltage signal were the power lines for the OP-AMPs previously described. The OP-AMPs were powered with ±15 V from the HV amplifier. The final lower voltage signal was the channel used to excite the tuning fork.

Tuning Fork Excitation

The qPlus-sensor (QPS), which has been described previously, required a sinusoidal voltage waveform applied to one electrode to create a varying potential difference between the TF electrodes. This caused the tines of the TF to oscillate relative to each other at the frequency of the waveform. However, this was found to be a complex design that led to difficulties with the sensor deflection. That is, it was not known to a certainty that the easyPLL plus was indeed providing the correct motion of the cantilever by both driving and monitoring the QPS.

Therefore the electrodes of the TF were not used in the excitation process, but the electrodes were still monitored as the sensor deflection system. Instead of the drive signal from the easyPLL plus driving the TF directly, it was connected to the small ‘dither’ piezoelectric transducer previously described in Section 3.1.2.

Chapter 3. Construction of a Combined AFM/STM UHV System 75

The dither piezo had a range of 0.1 µm with a 300 V potential applied. Therefore for every 1 V applied to the dither piezoelectric transducer, there was a 3.33 ˚A displacement of the tip holder from its original position. The amplitude of the TF driving signal controlled by the easyPLL plus was of the order of several volts to produce an estimated cantilever oscillation of several ˚Angstr´oms.