Application Description

Kelvin probe force microscopy (abbreviated as KPFM, KFM or SKFM) is a technique based on atomic force microscopy (AFM) that is used to study the electronic properties of nanoscale materials and devices. KPFM quantifies the local contact potential difference (CPD) between an AFM probe and the sample surface by detecting a capacitive electrostatic force. In the case of metal surfaces, the KPFM signal is directly related to the work function of the material, while for semiconductor CPD will be related to the doping profiles of semiconductors or surface photo-voltage (SPV) of photo-sensitive thin films. Most KPFM methods described on this page are referred to as closed-loop single-pass techniques, where the local CPD is actively tracked and imaged at the same time as the surface topography or other force contributions.

Open-loop variants of KPFM can be seen as an extension to electrostatic force microscopy (EFM), where a sinusoidal electric modulation gives rise to three spectral components: a static DC term, and two AC components at the fundamental and second harmonic of the bias modulation frequency. When the CPD is not actively tracked by a feedback loop, its value can be computed from the 2 AC components in the so-called dual-harmonic (DH-KPFM) mode; this is particularly relevant for measurements in liquids.

Measurement Strategies

In a typical KPFM setup, applying a probing AC bias voltage superimposed onto a DC voltage generates an electrostatic force between tip and sample that can be measured with a standard lock-in detection technique (see figure). Depending on the measurement scheme (see table), the relevant demodulated component of the force or the force gradient is fed into a PID loop, which in turn adjusts the DC bias voltage to minimize the electrostatic force. The CPD value of interest is reached when the electrostatic contribution is canceled out by the applied DC source. Many existing KPFM modes fall into one of two categories, namely amplitude-modulated KPFM (AM-KPFM) and frequency-modulated KPFM (FM-KPFM). AM-KPFM modes are robust and easy to implement, but their resolution is limited by the large stray capacitance from the cone and cantilever geometry. AM-KPFM can be useful for large and fast surface inspection, and it can usually operate with smaller AC drive voltage. FM-KPFM modes unlock ultimate surface potential resolution due to its force gradient sensitivity, but they are more complicated to optimize and operate in stable conditions over rough surfaces. Recent technical advances in heterodyne FM-KPFM in air, and in 2ω dissipation KPFM (2ωD-KPFM) in vacuum, represent the state-of-the-art methods in terms of quantitative measurements as these modes are the least prone to artefacts.

Compared to more traditional dual-pass techniques – one pass for the topography, one pass for the electrostatic contribution – single-pass measurements reduce bias artefacts in the topography, improve the surface potential resolution and reduce measurement times. As single-pass KPFM techniques require fine adjustment of many parameters, the Zurich Instruments LabOne^® control software ensures that the overall optimization process is more consistent and systematic thanks to multiple demodulators at various harmonics or frequencies, multiple feedback loops as well as phase shifters and the parametric sweeper functionality.