Scanning Kelvin Probe Microscopy (SKPM)
Block diagram of the circuit used for the SKPM mode. On the first pass the photodetector ouput is used to control the Z piezolectric scanner. On the second pass the photodetector ouput is used to control the potential between the probe and surface.
The force between tow conducting materials is given by the following equation:
In this equation:
- F = force
- C = Capacitance
- z = Probe to sample distance
- V = Probe-sample potential difference
Assuming the change in capacitance with distance is negligible for small changes in distance, the force between the cantilever and surface is proportional to the voltage difference squared.
If a cantilever is held a fixed distance from a surface, and a potential is place between the probe and sample, electrostatic forces between the cantilever and surface will bend the cantilever at all voltages except when the probe and sample voltages are the same.
In the SKPM an oscillating voltage between a probe that is near a surface causes the cantilever to oscillate when the probe and surface are at different potentials. A feedback circuit varies the potential so that the cantilever oscillation is minimized. As a scan is made, the output of the feedback electronics is a potential map of a surface.
In the SKPM a feedback loop controls the potential between the probe and surface. The feedback loop controls the voltage so that the oscillations of the cantilever from electrostatic forces are minimized.
This graph shows the amplitude of vibration of a cantilever as a function of voltage applied to the cantilever. The sample is held at 0 Volts. As expected the vibration amplitude is parabolic.
Two Pass Imaging
In the SKPM two pass imaging is used for measuring a topography image surface potential map.
First Pass: The first pass of the probe over the surface is used to measure surface topography. Typically, this topography measurement is made in vibrating mode(tapping) by mechanically oscillating the probe as it is scanned across the surface. In this first pass a feedback loop is utilized to assure that the vibration amplitude(or phase) is held constant as the probe is scanned across the surface. The motion of the probe in the Z axis is stored in a computers memory.
Second Pass: In the second pass, the probe is raised a fixed distance above the surface, and the topography stored in the first scan is utilized to scan the probe at a fixed distance above the surface. During this scan, the mechanical oscillation is disabled. In the second scan, the potential is measured with a feedback loop that adjusts the voltage between the probe and surface to minimize the vibrations of the cantilever at a fixed frequency.
In two pass imaging the probe is scanned across the same region of a sample two times.
In the first scan the topography is measured, in the second pass the potential map is created.
Two pass imaging generates two images, a topography image, and a potential image.
At the right is an example of SKPM on a test sample. In the test sample metal traces are held at a a different potential than the substrate.
Topography image of a test pattern used for calibrating SKPM images. At the right of the image are three traces that are at a different potential than the substrate.
The SKPM image is a potential map of the surface and shows the three traces at the right of the image are at a different potential than the substrate.
The SKPM software is integrated with the AFMWorkshop AFMControl software. Once installed on the host computer, the KPFM software is accessed with a modes tab at the top of the window. AFMControl software is written in the industry standard National Instrument Labview instrument control environment.
The probe holder for SKPM includes a special probe clip designed specifically for making electrical measurements with AFMWorkshop microscopes. To assure the greatest sensitivity, the probe clip has is comprised of a conducting spring that is electrically shielded.
Our universal probe holders accommodate probes from all major probe manufacturers, giving maximum flexibility. The probe holder is easily removed from the AFM light lever and Changing probes takes only a few minutes.
The SKPM mode option is compatible with all of the AFMWorkshop atomic force microscopes. The option includes all of the parts and software necessary for making SKPM images:
- SKPM feedback control Electronic Module
- Probe Holder
- Insulating Sample Stage
- 60 Pin interface Cable
- 2 Pin interface cable
- Reference Sample
- Power cord
- SKPM Mode Software