Kelvin probe force microscope
The Kelvin probe (english Kelvin probe, KP ) is in the nondestructive measurement of the work function and in the study of Delaminierungsprozessen of polymer, oxide, and metal interfaces use. The combination of a Kelvin probe with an atomic force microscope is called a scanning Kelvin microscope or a Kelvin probe force microscope ( engl. scanning Kelvin probe microscope, SKPM, or Kelvin probe force microscopy, KPFM ).
The name of the Kelvin Probe dates back to Lord Kelvin.
Principle
When two metals are brought into contact, so flow -energy electrons from the metal Fermi level higher to that of lower Fermi level until they are at the same height (see contact potential ). For this flow of electrons, an electric field and a contact voltage. The contact stress resulting from the difference in work function:
The two metals have played a capacitance. For the charge that has flowed applies:
Measurement
When measured with a Kelvin probe to the conductive probe, which is set on the sample with piezoelectric actuators of the vibration and the sample as two capacitor plates behavior. By the oscillation, a current is influenziert which is linearly dependent on the difference in work function and non-linearly on the distance between the probe and sample. The current to zero can be controlled by an external voltage. Thus, the difference in work function is determined, because it is.
The construction of a similar KPFM an atomic force microscope, and can provide a better spatial resolution than the Kelvin probe. The probe used in this case is a very fine, conductive tip. Due to the small geometry of the tip, the capacitance between the tip and sample, and thus the influenzierte current is very small. Using an atomic force microscope, however, the measurement of small forces is possible. Therefore, in this structure, instead of the current, the electrostatic force between the tip and the sample is measured. By applying a voltage between the tip and sample, the force can also be eliminated in the ideal case, that is true.