on 29.11.2017 by Romain Stomp
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.
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.
|Base technique||Amplitude modulation (AM)||Frequency modulation (FM)|
|Sensitive to||Force (through amplitude)||Force gradient (through phase)|
|KPFM mode||AM-KPFM||1ωD-KPFM||Sideband FM-KPFM||2ωD-KPFM||Heterodyne FM-KPFM|
|Electrical drive||f1 or off-resonance||f0, 90° phase-shifted with respect to mechanical drive||fm||2f0, 90° phase shifted with respect to mechanical drive||f1-f0|
|Detection||X-component at f1||Dissipation channel||X-component at f0 ± fm||Dissipation channel||X-component at f1|
|Setpoint||Nullify X1||Dissipation value equal to value without bias modulation||Nullify X3-X2||Dissipation value equal to value without bias modulation||Nullify X1|
|Comments||Can operate < 1 V drive amplitude||Requires PLL to lock mechanical phase and AGC to measure dissipation||Typical drive amplitude VAC ~ 2 V||Requires PLL to lock mechanical phase and AGC to measure dissipation||Can demodulate at higher bandwidth at f1|
|Recommended instrument configuration||
For HF2LI: HF2LI-PID
Most commonly used closed-loop single-pass KPFM techniques. The topography is always recorded in standard tapping mode (in air) on in non-contact AFM mode (in vacuum) at the mechanical drive f0.
f0 = cantilever resonance frequency
f1 = second eigenmode of the cantilever
fm = electrical modulation frequency
on 29.11.2017 by Romain Stomp