In our attempt to build an AFM there are a number of technical challenges that we will need to overcome. Any nanoscale measurements have to be really sensitive as we are trying to measure really tiny things. This sensitivity means that measurements are prone to be disrupted by small variations in the environment. A vibration that is fractions of a millimetre is likely too small for you to feel but it is about 1000 times the size of features we are trying to measure. This can cause some pretty serious problems and obscure any features you are trying to measure.
TECHNICAL CHALLENGE 1: Vibration control.
We intend to use diode laser manufactured for reading DVDs. We can buy them as ready-made components pretty cheaply (about $5 each) however we will need to design circuitry to control the lasers in a number of ways. Firstly we will have to power the laser diode. These diodes are quite fragile and any surges in current or high voltages are liable to fry them.
TECHNICAL CHALLENGE 2: Stable power supplies for laser diodes.
We will also have to be able to move the laser diode around with high precision. We need to move the diode up and down in order to focus the laser beam on the cantilever or sample. Since we are effectively measuring the distance from the laser diode to our cantilever the laser diode needs to stay still as otherwise all we will measure is the motion of the laser diode. We also want to move the laser left and right so that we can find samples and for scanning in the optical profilometer.
TECHNICAL CHALLENGE 3: Precision motion of laser – vertical and horizontal.
The optics within the AFM will translate any change in height of our sample to an electronic signal. This signal will likely be very noisey and also small. We will need to be able to filter noise from our output signal, amplify any output signal and calibrate the electronic signal with a change in height.
TECHNICAL CHALLENGE 4: Getting information out of the system.
In our AFM there will be a sharp tip and a sample. We will need to be able to move these parts relative to one another as otherwise the tip will just sit on top of our sample. Any relative motion must be smooth and controllable. This is because the information we will get out from the DVD head will be a signal in time. This means that we will have information about the profile of the substrate beneath the tip in the time domain. In order to relate that to a spatial profile we will need to calibrate this signal with the relative motion of tip and substrate. If the motion is noisy and random we will not have a good calibration. We generate the motion by placing the sample on a stage and moving the stage around.
TECHNICAL CHALLENGE 5: Precision motion of the stage.
We need to be able to use our microscope. This seems blindingly obvious but we will need to write some software which will allow the microscope to interface with a computer. We want to be able to control the microscope in a straightforward user friendly manner. We also want this software to be free to download and use which means that we are restricted to open source programming languages.
TECHNICAL CHALLENGE 6: Write software and firmware allowing the microscope to interface with a computer and for control of the microscope with the computer.