Centro de Microelectrónica Universidad de los Andes -

STM-Uniandes Electronic set-up

The electronic set-up of the STM-Uniandes is composed by three elements shown in the image:

1.  The pre amplification circuit

2.  The Piezo drivers and bias circuit

3.  The control and data acquisition circuitry

Following, each of these components will be thoroughly described, but firstly let us point out that the Control and acquisition part has been implemented using an electronics i/o board with an open source programming environment used for exploring the electronic arts, tangiblemedia, teaching and learning computer programming and prototyping with electronics. This was possible thanks to the help of Hernando Barragan and his fantastic project:

Control and data acquisition

As it was described above, the control and acquisition has been implemented using an electronics i/o board with an open source programming environment used for exploring the electronic arts and prototyping with electronics. This i/o board and programming environment is encouraged for people with a weak electronics background and it marry up very well with our objective of giving everyone their own experience with nanotechnology. For more information on the board and programming visit the wiring link above.

When describing the operation of an STM we mentioned that an adequate control of the displacement of the sample with respect to the probe (tip) is a must. We also mentioned that this control was done using piezoelectric elements described which present a very small mechanical deformation upon application of a potential difference. In order to control such displacement, in particular the Z axis that indicates the distance between the sample and the tip, a feedback control circuit is necessary. Let us firstly start by reviewing that a PI controller looks something like the picture below:

The image below shows an schematic of the i/o ports in the Wiring board. The analog inputs are used to read the tunneling current of the microscope, and the additional I/O pins are used to generate 4 PWM signals :

X piezo, Y piezo, Z piezo, Bias voltage

Click here to download the simplest version of an STM implemented in Wiring.

Pre amplification circuit

The scientific background section described the order of magnitude of the current measured by the STM. Since this is generally a very small quantity, in the order of nA, a high amplification stage is firstly needed to condition the current signal coming from the interaction between the sample and the probe. For this design, the preamplifier used is analog to the one described by the Interface Physics Group and their SXM Project. The figure below shows the current implementation of this circuit.

This topology is referred to as a current to voltage amplifier. A very small current signal at the input is amplified by a very sensitive operational amplifier and it delivered as a voltage to the output. Generally, this amplifications staged is ajusted so that every volt is proportional to 1nA, however, accurate calibration of this quantity requires specialized measurement equipment.

Piezo drivers and bias circuit

Since the analog signals obtained from the Wiring board are PWM signals, both the bias and piezo drive circuits start with a low pass filter so that the PWM signal is converted to a stable analog signal between 0 and 5 volts. Let us firstly explore the design of the sample bias circuit.

The PWM signal used for the sample bias has a very low frequency in the order of 1-10 hertz, hence a low-pass filter with very low block frequency is necessary. An RC filter with a 10k resistor and 220uF capacitor is then used. Since the polarization of the circuit is -10 to +10, the bias can be doubled to reach a full bias scale from 0-10volts.

Piezoelectric element deform very slightly with voltage and micrometer scale deformations are generally reached with a potential difference of over 20 or 30 volts. For this reason, additional circuitry that steps up to such potential is necessary. A piezo drive circuit is then used to increase the potential across the piezo (for more information about piezoelectrics visit http://en.wikipedia.org/wiki/Piezoelectricity ) using a capacitive charge compensation technique, pioneered by R. Bernal at Los Andes and described in [19]. The circuit starts by filtering a PWM signal of 1kHz, and then leveling it to zero and scaling so that it reaches full -10 to +10 scale. The right part is the compensation circuit that deliver a differential potential to the piezo in a -20 to +20 scale.

This same circuit is implemented three times for each of the axis of the microscope scanner. The x and y axis piezos are driven from the Wiring board using a raster fashion as shown in the figure below. Piezoelectric z is driven by a feedback control circuit implemented inside the Wiring board.