STM-Uniandes Mechanical set-up
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A precise control of the position of the tip/probe with respect to the sample is necessary if we want to achieve detectable tunneling currents. The positioning process is done in two stages. Firstly, a coarse positioning is achieved by manually placing the tip over the sample at a tunneling distance (~nA). When this is achieved, the tip is scanned around the sample using a pair of piezoelectric elements in a scanner configuration, and a third piezo controls the tunneling distance using the controller described in the electronics section.
Coarse positioning
At Uniandes, we implemented a similar system to the one used in the SXM Project, however the high standard pieces needed to reproduce such system at a low cost are generally difficult to obtain in the local market. This mechanical system is based in the principle of a seesaw in which linear movement in one end can be scaled down to the movement of the other end as explained below.
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Seesaw scheme |
This principle can be easily implemented using a pair of metallic plates in which one of the metallic plates sits on top of the second one and can be displaced using 3 screws in a tripod fashion. Two screws in the front can perform as the pivot of the seesaw system and the third screw would set the distance d1 above shown. Such a system would schematically look as the one below.
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| Largo positioning mechanism |
Even though this systems would theoretically deliver any amount of distance scaling, in reality L1 is a finite distance hence the scaling values would not reduce more than 30 to 50 times. A standard screw would displace about 1mm per turn then if we manually rotated the screw with a precision of about 2 degrees, the finest approaching precision would be in the order of 120 nanometers.
Obtaining screws that would displace less than the example above would mean elevating the cost of fabrication and maintenance of this system, hence at Uniandes we decided to use a standard 0.5mm per turn screws and a 1/150 reducing gear easily obtained in the local market. This set-up would deliver a fine precision of about 1nm, as it is needed.
Fine positioning
Sample holder
Vibration isolation
A scanning probe microscopes set-up has to be such that the probe can be steadily placed at the order of tenths of nanometers from the sample. When trying to achieve this, mechanical vibrations are a main issue since modern buildings transmit structural vibrations nearly everywhere. To avoid this, a SPM has to include an appropriate mechanical isolation system that can reduce vibrations to about one pico meter (since we are trying to control nanometer positions) [15]. This would mean an attenuation of −120 dB when the input vibrations are in the order of a couple micrometers.
The mechanical isolation set-up not only includes the elements that reduce vibrations transfer from the outside world toward the tip-sample, but also all the elements that can hold wires and connections in place and enhance the robustness of the instrument.
It has been reported that mechanical vibrations range from 10 to 100 Hz [16]. There are many elements, such as motors and fans, that contribute considerably to this mechanical inputs. Their typical frequency is around or a factor of the electrical network frequency (60Hz in Colombia). Other vibration sources include people walking and the movements of buildings themselves. Those range from 1 Hz to 15 and 25 Hz.