Electronics
Tunneling current amplifier
The amplifier required in a STM is a so-called transimpedance amplifier, meaning it converts the current into a voltage. As the tunneling current is very small, in the order of pico (10^(-12)) - to nanoamps (10^(-9)), the amplifier should be as close to the tip as possible and it should be well shielded. We manufactured the case from a stock box of that size, adding the connectors and soldering the cabling inside. It features a VGA plug to connect it to the control electronics. Furthermore, there is a plug for the piezos which is internally wired to the VGA connector. The case is compact enough to be mounted directly onto the microscope.
The amplifier itself is actually very simple. All you need is a low noise operational amplifier and a very big feedback resistor, 100MΩ in our case. The output voltage is the feedback resistor times the input current. Thus, 1nA are amplified to 10V. Furhtermore, capacitors at the power supply inputs are highly recommended to filter noise from the supply. Still, the work should be done carefully as any strong noise would destroy the signal. The schematic can be download from the download section.
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| Fig. 1: The amplifier in its box. |
Control electronics
To scan the surface of the sample, the STM requires electronics which would give control over the piezo actuators. There are generally two approaches to develop such electronics - one basing on the analog feedback loop and one on the digital one. The analog approach uses a logarithmic amplifier to control the distance between the probe and the sample by comparing the output signal to a set-point value and returning a signal to the high voltage amplifier driving the Z-piezo. The difference between the output and the set-point value is amplified by a differential amplifier. This value then has to be integrated using an integrator circuit and added to the voltage at the Z-piezo. All three circuits can be realised using op-amp circuits, rendering the analog feedback loop quite simple.
In the second, digital, approach, the feedback loop is realised by A/D and D/A signal converters and a microprocessor. The output signal from the current amplifier is converted into digital form by the ADC and processed in the microprocessor, which basically takes care of everything - it registers the ADC output, sends it to the host PC, compares it to a set-point value and returns a digital signal, which is then converted by the DAC transferred to the high voltage amplifiers.
In our case, we have opted for the second approach, majorly due to its versatility and the possibility to control the STM and display the scans using a PC. The self-designed printed circuit board is based on a PIC32 USB Starter Kit currently produced by Microchip. The board itself consists of four layers and requires 5 different voltages - +5V, +/- 15V and +/- 24V, provided by a linear power supply with a voltage regulator board. The whole setup is fitted into a metal box (as seen on Figure 2), which exchanges signals with the amplifier box using a VGA connector and communicates with the PC over USB.
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| Fig. 2: The control electonics box built by us. |
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