HF2PLL for KPFM Applications

Kelvin Probe Force Microscopy (KPFM) is a scanning technique that allows the detection of a electrostatic potential distribution of a surface. The HF2PLL supports several variants of KPFM, some of them are mentioned in this post.

AM-KPFM

With the AM-KPFM method, topography is recorded using PLL loop. An additional sinusoidal KPFM AC signal is applied at a frequency high enough frequency ensuring the PLL loop is not influenced. The X component of this signal is detected using an additional lock-in path. KPFM DC-bias control is done based on this X value which becomes positive or negative if the substrate is biased in a way that under-compensates or over-compensates the surface potential. If the potential is compensated correctly, no X signal will be measured with the lock-in, ensuring the surface potential can be read from the DC bias.

Hardware Requirements

hf2_application_afm-kpfm_am

FM-KPFM

With the frequency modulation KPFM method, topography is recorded using a PLL loop. An additional sinusoidal KPFM AC signal is applied to the substrate – similarly to the procedure in AM-KPFM. However, the signal is applied at low frequencies, ensuring that the PLL loop does track the change in resonance frequency which is caused by the change of the potential distribution. Lock-in detection is not done on the input signal, but on the df signal of the PLL.
Again the X component of this signal is detected using a lock-in path. KPFM DC-bias control is done based on this X value which becomes positive or negative if the substrate is biased in a way that under-compensates or over compensates-the surface potential. If the potential is compensated correctly, no X signal will be read with the lock-in, ensuring the surface potential can be read from the DC bias.

Hardware Requirements

hf2_application_afm-kpfm_fm

Fast FM-KPFM

Regarding the physical context, the direct FM-KPFM method resembles the conventional cascaded FM-KPFM method. However, the PLL is set up with a bandwidth high enough to track the substrate’s topography, but lower than the KPFM AC signal. Using the recovered center frequency of the frequency modulated signal, a direct sideband analysis can be performed using the HF2LI-MOD Dual AM/FM Modulation Option. This offers the advantage of lower requirements for the PLL tracking bandwidth. But more remarkable is the fact that no filtering has to be done on the PLL df signal and that the SNR is potentially increased by a factor of sqrt(2).

Hardware Requirements

hf2_application_afm-kpfm_fm_fast

DH-KPFM

Different to the above mentioned KPFM modes, dual-harmonic KPFM is an open loop method. The substrate potential is calculated by means of the omega and two-omega components of the detected signal. If the substrate surface potential is zero, only a two-omega component of the AC bias signal is detected. The higher the potential, the more omega component is generated whereas two-omega is always constant due to the parabolic shape of the capacitor’s potential curve.

Hardware Requirements

hf2_application_afm-kpfm_dh