Kelvin Probe Force Microscopy (KPFM)

Electrostatic interaction is ubiquitous in the field of SPM and often needs to be discriminated against other interactions sensed by the AFM probe. By applying a known bias voltage between tip and sample, the surface potential can be measured by means of Kelvin Probe Force Microscopy. Over the last decade, many different KPFM techniques, with amplitude or frequency modulation, in a single or multi-pass line scan, have emerged. Hence there is a need to appropriately chose the right method depending on the environmental conditions and sensors. Thanks to its dual signal generator with AC+DC adder, the HF2PLL is suited to support all modes, as both mechanical and electrostatic excitation can be generated and used as reference for multi-frequency detection schemes.

Objectives: enable HF2PLL users to configure any type of KPFM measurements, options can be upgraded as the need for new measurements technique grow.
 
Benefits: all in one box measurement solution for both mechanical and electrostatic excitation and multi-harmonic or multi-frequency detection.
 
Relevant products: HF2LI, HF2LI-MF, HF2LI-PLL, HF2LI-PID, HF2LI-MOD
MFLI, MF-PID, MF-MD, MF-MOD

The following table provides an overview of current KPFM modes.

KPFM Mode

DH-KPFM
(Dual Harmonic)

Dual Lock-in KPFM

AM-KPFM

FM-KPFM
(tandem)

Fast-FM-KPFM
(sideband)

Application conditions

Ambient Ambient Vacuum or UHV Vacuum, UHV or low temperature Vacuum, UHV or low temperature

Number of passes

Single or double pass Single pass Single pass Single pass Single pass

1st frequency (f0)

Intermittent contact mode, amplitude modulation at or near resonance (f0) Intermittent contact mode, amplitude modulation at or near resonance (f0) Amplitude or frequency tracking (PLL) on first resonance (f0) Frequency tracking (PLL) on first resonance (f0) Frequency tracking (PLL) on first resonance (f0)

2nd frequency (nf0, f1)

Second (2f0) & third (3f0) harmonic detection Mechanical or electrostatic (phase shifted) excitation of 2nd eigenmode (f1)
 
Electrostatic (phase shifted) excitation of 2nd eigenmode (f1) AC modulation of frequency shift near resonance (fm) AC modulation of frequency shift near or away from resonance (fm)

CPD* feedback on

None (open loop operation) Phase of second demodulated signal (f1) X component of second demodulated signal (f1)
 
X component of demodulated frequency shift (from PLL output) X2-X1 of sideband modulation signal (direct demodulation)

Sensors

Standard cantilever Standard cantilever, requires PD* with high BW* >1 MHz Standard cantilever, requires PD* with high BW* >1 MHz
 
Stiff cantilever or quartz sensors (tuning fork, LER*) or MEMS Stiff cantilever or quartz sensors (tuning fork, LER*) or MEMS

Comments

Post-data processing by user, no feedback required on bias Single pass technique in air, phase sensitive Not sensitive to force gradient but to force (effect of static capacitance) Sensitive to force gradient. AM mode not possible with quartz sensors.
 
Leaves topography feedback intact. Increases speed of KPFM feedback

Required product

HF2LI HF2LI
HF2LI-PID
HF2PLL HF2PLL HF2PLL
HF2LI-MF
HF2LI-MOD

References

Publication B. J. Rodriguez, University College Dublin, Ireland Publication Dominik Ziegler, ETH Zurich, Switzerland Publication Thilo Glatzel, University of Basel, Switzerland Publication Christian Loppacher, Universite Aix-Marseille, France  

Related blogs

HF2PLL for KPFM Application       How to set-up Kelvin Probe Feedback (FM-KPFM) with Direct Sideband Detection

(*) Abbreviations

BW Bandwidth
LER Length Extension Resonator
CPD Contact Potential Difference
PD Phote Diode

 

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