Principle and Structure of the Atomic Force Microscope (AFM)
1 Force Detection Unit
In an Atomic Force Microscope (AFM) system, the force to be measured is the van der Waals force between atoms. A micro cantilever is used in this system to detect variations in interatomic force. Micro cantilevers have specified parameters such as length, width, spring constant and tip geometry. Different probes are selected according to sample properties and operating modes.
2 Position Detection Unit
In the AFM system, the cantilever deflects once interaction occurs between the tip and the sample. When a laser beam irradiates the free end of the cantilever, the position of the reflected laser changes along with cantilever deflection, generating a displacement signal. A laser spot position detector records this displacement and converts it into an electrical signal for signal processing by the SPM controller.
3 Feedback System
In the AFM system, signals captured by the laser detector are fed into the feedback loop as internal adjustment signals. The feedback system drives the scanner, which is usually made of piezoelectric ceramic tubes, to move appropriately so as to maintain a constant interaction force between the tip and the sample.
An AFM integrates the above three units to characterize sample surface properties: The micro cantilever senses the interaction force between the tip and the sample. This force deflects the cantilever. A laser beam hits the end of the cantilever; deflection shifts the reflected light and produces a displacement. The laser detector records the displacement and sends the signal to the feedback system for adjustment. Finally, the surface features of the sample are displayed as an image.
