Practical aspects of Kelvin probe force microscopy

Abstract

We discuss practical aspects of Kelvin probe force microscopy ͑KFM͒ which are important to obtain stable images of the electric surface potential distribution at high spatial resolution ͑Ͻ100 nm͒ and high potential sensitivity ͑Ͻ1 mV͒ on conducting and nonconducting samples. We compare metal-coated and semiconducting tips with respect to their suitability for KFM. Components of the metal coating can become detached during scanning, introducing sudden offset jumps in the potential maps ͑typically up to 350 mV between adjacent scan lines͒. However, n-doped silicon tips show no substantial tip alterations and, therefore, provide a stable reference during the experiment ͑offset jumps typically up to 40 mV between adjacent scan lines͒. These semiconducting tips must be electrically connected via contact pads. We use InGa and colloidal silver pads which are easily applied to the substrate supporting the cantilever and have a low enough differential contact resistance ͑350 ⍀ and 2.2 k⍀, respectively͒. Furthermore, we introduce a simple procedure to fine tune the feedback which detects the electric surface potential and show how the basic KFM setup has to be modified to gain access to the necessary control signals.

Open loop Kelvin probe force microscopy with single and multi-frequency excitation

“…The result of which is that absolute (i.e., the real values for the system being measured) SP measurements in CL-KPFM are subject to the feedback effect and can vary typically within an instrument-dependent ~ 1 V range [23]. During CL operation, the measured SP can be further influenced by non-zero tracking errors and the choice of feedback parameters (excitation frequency, phase projection, and feedback gains) [22,23]. Some forms of parasitic signals can be avoided.…”

“…Under the condition of F ω = 0, the measured SP can then be determined independently from measurement parameters (equation 1b). In practice, however, the interpretation of the recorded SP requires careful consideration of the influence of the instrumentation, and in particular the bias feedback loop [22,40].…”

“…Conventional KPFM is characterized by CL bias feedback and, similar to other SPM techniques which rely on a feedback approach, is susceptible to a wide range of imaging and feedback artifacts [22][23][24]. An ideal feedback system minimizes the difference between an input signal (in this case the cantilever oscillation amplitude) and a setpoint (in this case 0 V as CL-KPFM is a nulling technique) by varying an output parameter (the dc bias applied to the tip or sample).…”

“…We thus define any condition where the feedback loop is not optimized and/or where the input signal contains non-electrostatic and/or parasitic contributions as the feedback effect (i.e., non-ideal feedback). It is widely reported that feedback effects result in SP values determined by CL-KPFM that can depend on the feedback parameters [22], the amplitude and phase reference of the excitation voltage [23,24], the tip-sample distance [25], as well as topographical crosstalk [26,27].…”

Eletrização de dielétricos: novas propostas para resolver velhos problemas

“…Os materiais são analisados por microscopias de varredura por sonda (SPM) 36 nos seguintes modos de operação: microscopia de força Kelvin (KFM) ou microscopia de varredura de potencial elétrico (SEPM), 37,38 que são baseadas no método de Kelvin 15 e microscopia de força elétrica (EFM). 39 Além das microscopias de sonda foram utilizadas microscopias analíticas, principalmente a microscopia eletrônica de transmissão acoplada à espectroscopia de perda de energia de elétrons (EELS-TEM), gerando imagens de espectroscopia de elétrons (ESI-TEM).…”