The Atomic ForceMicroscope:The Atomic Force Microscope (AFM) is a form of scanning probemicroscopy (SPM) involving the detection of interatomic forces that happenbetween a probe tip and a sample. The AFM consists of a cantilever with a sharptip (probe) at its end that is used to scan the specimen surface. Thecantilever is typically silicon or silicon nitride and is used to scan across asample in an effort to obtain information about its surface (topography). Thetip is integrated into a cantilever (figure 2) which moves up and down tracingthe interaction of the surface of a sample. When the tip is brought into proximityof a sample surface, forces between the tip and the sample lead to a deflectionof the cantilever according to Hooke’s law. The probe is scanned in a similarpattern to the SEM where it moves in a raster pattern across the sample inorder to generate an image in an x, y and z pattern. The AFM can either use theprobe in a contact or non-contact mode.
A great advantage of the AFM is thatthe specimen being examined can be non-conducting, unlike for the SEM. In thismanner, the AFM can be used to study almost any sample.The AFM generates resolution by calculating the vertical andlateral deflections of the cantilever. This is completed by reflecting a laserbeam off the cantilever which is reflected to a position sensitive photodiode (PSPD)that consists of two sections. Consequently, any small change in deflection ofthe cantilever produces a magnified reflection of displacement on thephotodiode relative to the cantilever (American society for Microbiology,2002).AFM has considerable advantages over the SEM. The ATMprovides more information with regard to the 3D surface of the specimen.
Anothermajor advantage of the AFM is that it does not require any pre-treatment thatmay cause damage or disfigurement to the sample. AFM also does not require avacuum and can be performed in air and liquid environments. Disadvantages ofthe AFM include the single scan image size produced. The AFM has a lower depthof field compared to the SEM because it only scans an area of micrometres, notmillimetres like the SEM. During analysis, the AFM has a slower scanning speedto the SEM.
Due to the nature of AFM probes, they cannot normally measure steepwalls or overhangs. Specially made cantilevers and AFMs can be used to modulatethe probe sideways as well as up and down to measure sidewalls. Thesecantilevers however are considerably more expensive and have a lower lateralresolution. What is AFM used for?· Inorganics, polymers, coatings and bio-samples· Personal care products, the measuring of the change innanoscale mechanical properties (modulus and friction) of hair, teeth and skin.
· Investigation of the force in which is required to removenanoparticles from a surface.· The topography and nano mechanical properties of coatings.The challenges facedwith measurement· Calibration, quantification and understanding of AFM modes(including that of force spectroscopy,multi-frequency modes, frequency modulation mode, lateralforce and amplitude modulation mode.· Obtaining important and additional information from AFM (mechanical,chemical, electrical).· Imaging soft samples at a high resolution whilst workingon minimising damage.
Procedure:As per manualResults anddiscussion:For this particular experiment, a calibration grid withdifferent areas was examined first.Scan 1a was performed and the results were read as follows:· 0.5 second per line· Had 64 points per line· Was tilted· Had poor resolution· Also viewed in 3-D· White line obscured visualisation of scan indicatingdirt/dust.
Dirt can be viewed as heights on the screenScan 1b un-tilted was performed and the results were read asfollows:· 1 second per line· Had 64 points per line· Un-tilted. Measurements can’t be taken unlesstilt plane is removed· Measurement of pillar at 12.5µm (length)compared to standard length of 10µm and 119.2nm (depth) compared tostandard distance of 100 nm.· Measurement of square holes 12.89µm (length)and 119.6nm (depth)Scan 1b 128 points per line was performed and the resultswere read as follows:· 1 second per line· Had 128 points per line· Un-tilted· Measurement of pillars at 13.28µm(length) and 116.9nm (depth)· Measurement of square holes at 12.1µm(length) and 111.7nm (depth Scan 2 was performed and the results were read as follows:· 1 second per line· Had 128 points per line· Measurement of circular holes at 7.813µm(length) and 125.4nm (depth)· Circular holes should have 5µm between them.· Depressions were shown with dark colours.· Elevations were shown with light colours.Scan 2 3d was performed and the results were read asfollows: 1 second per line Had 128 points per line Measurement of circular holes at 7.0um (length) and 108.0nm (depth)A microchip was examinedThe depth of the valley of the chip was measured.· Depths in the trench – 0.223µm· Length of the hole – 1.362µmLastly, Staphylococcus aureus was examined.The characteristic look resembled tiny bunches of grapes,some of which were stacked on top of one another, and others higher up on thescan.· Width of the bacteria = 0.976µm,0.585µm (very small clusters)· 0.2µm flattened · Force 29N