High Speed - Atomic Force Microscope

Students learn the principle of the laser deflection system and how to adjust the system.They can take an image in contact mode and check the difference between slow feedback speed (constant height mode) and fast feedback speed (constant force mode). With distance curves, they can calculate the sensitivity of the system and calculate the contact force used in the experiments.

For the daily use of AFM, the imaging in dynamic mode is more common than the contact mode, because tip and sample are spared.The students learn the relation between force constant and resonance frequency, repeat some knowledge about harmonic oscillating systems, resonances and Q-factors and get introduced to the basics of lockin amplification technique, which is used to evaluate the amplitudes and phase shifts.In the acquired images, material contrast can be obtained in the phase image in addition to the topography.

Current-AFM measures electrical current flowing between tip and sample during scanning. This mode is used to study conductive properties of materials at the nanoscale.

In contact mode, lateral forces are detectable and can be used to find material contrast in inhomogeneous samples. The experiment explains how LFM is working and the students learn to understand, how to distinguish between topography-related signals and material-related signals.

Kelvin Probe Force Microscopy (KPFM) is a technique that allows to detect work function differences on surfaces as well as local charges. It has undergone a long development and there are many different kind of KPFM operation modes known. All KPFM mode have in common, that an alternating plus a dc bias is applied between tip and sample. The forces resulting from the alternating voltage are used to generate a feedback signal that controls the value of the dc bias. Usually, a signal of a lockin amplifier is compensated to zero with the KPFM feedback. Two of the available Anfatec SPM controllers offer KPFM capability: the AMU2.6 controller type offers it for lockin frequencies up to 1 MHz and the AMU2.9-HighSpeed controller version offers it for frequencies up to 10 MHz. In both cases, the lockin card includes four independent 2-phase lockin amplifiers. They can be configured in a way, that AM-KPFM, Side band KPFM on the 2nd cantilever resonance or FM-KPFM experiments can be performed.

AM-KPFM image of an Al / silicon surface taken with a DEP01 cantilever with Pt coating. Topography (left) and Surface Potentoal (right). The Al wire is buried into the silicon surface and shows the more positive surface potential. The silicon is covered with a thin oxide laser and the surface potential slightly decays in dependence on the distance from the Al edge.

Resonance enhanced FM-KPFM image of an Al / silicon surface taken with a DEP01 cantilever with Pt coating. Topography (left) and Surface Potentoal (right). In comparison to the AM-KPFM image, the potential contrast is better localized due to the side band detection. There are also more potential features visible on the Al surface. The potential difference between Al and Si is measured with 190 mV.

In Magnetic Force Microscopy (MFM), each line of the images is taken twice: the first trace is used to get the topography information. The 2nd trace is following the topography in a certain height of several nm and mainly detects long range interactions, such as interactions caused by magnetic forces. MFM employs cantilevers with a magnetic coating, usually CrCo coated silicon tips.

Topography (left) and MFM Amplitude (right) images of a Bruker reference sample for MFM. Image Size: 1.5 µm x 1.5 µm.

Topography (left) and MFM Phase (right) images of a Bruker reference sample for MFM. Image size: 40 µm x 40 µm

EFM separates electrical forces from topography using modulated voltage applied between tip and sample. It enables the study of capacitance variations and electrical properties.