Research On The Measurement Of Oxide Surface Potential By Kelvin Probe Force Microscope

Kelvin probe force microscopy?KPFM?is currently the best technique for characterizing the electrical properties of nanostructures.It can simultaneously measure the topography and surfaceelectrical properties without any damages to the sample.KPFM has been widely applied to the measurement of electrical properties of metal and semiconductor materials.In recent years,KPFM has been applied to measure electrical properties of organic materials,biological materials,and ferroelectric materials.However,the traditional mode KPFM relies on an external DC feedback loop to measure the surface potential.Therefore,it is also calledthe closed-loop mode KPFM.Applying a DC voltage will cause surface charge accumulation in insulator samples and energy band bending in semiconductor samples,and it will make the measurement results inaccurate.Therefore,it is of great significance to study the KPFM without the DC feedback loop.In this paper,a KPFM that does not need a DC feedback loop is studied,which is called thedual-harmonic mode KPFM.Experiments is made to study and optimize the dual-harmonic mode KPFM.Andthis method is used to measure the surface charge density of the insulator.Through systematic research,the following results were obtained:Firstly,this paper studies the measurement conditions of the dual-harmonic mode KPFM.The curves of A_?-V_acand A_2?-V²_acat different tip-sample distances are measured.The results show that the measurement result is the best,when the tip-sample distance is 100 nm.At the same time,the experiment uses the reference sample method to calibrate the potential ratio coefficient,and the result shows that the value of the potential proportional coefficient under this experimental condition is0.45.Secondly,this paper optimizes the system parameters of thedual-harmonic mode KPFMThe main parameters include frequency,AC voltage and time constant.The KPFM signal diagrams of the dual-harmonic mode KPFM under different system parameters are measured,and through analysis and comparison,the dual-harmonic mode KPFM has the best measurement effect when the frequency is 50 k Hz,the AC voltage is 2⁴ V,and the time constant is 10 mV.Thirdly,the measurement effectof the dual-harmonic mode KPFMis verified.On the one hand,the paper studies the resolution of the dual-harmonic mode KPFM and the potential resolution is at least 50 mV.On the other hand,the experiment compares the closed-loop KPFM with the dual-harmonic mode KPFM by changing the current and gate voltage applied to the sample.According to the data,the ground electrode potential is about 0 V when measured by the dual-harmonic mode KPFM,while theground electrode potential shift significantlywhen measured by the closed-loop KPFM.The results show that the potential measured by dual-harmonic mode KPFM is more stable and more accurate.Finally,the dual-harmonic mode KPFMis applied to the measurement of surface charge density of insulators.In the experiment,we study the phenomenon of surface charge accumulation and its variation of the mirror film in the plasma environment.The results show that the plasma treatment will make the mirror filmnegatively charged,and the quantitywill eventually reach saturation.At the same time,the higher the plasma intensity,the shorter the time for the mirror film to indicate the saturation of the charge,and the greater the absolute value of the saturated charge density.In this paper,through studying the dual-harmonic mode KPFM,we get a KPFM method without the DC feedback loop.Compared with the closed-loop mode KPFM,the dual-harmonic mode KPFM method is more accurate and stable.At the same time,the use range of the dual-harmonic mode KPFM is more extensive,and it can be used to measure the surface potential of metal materials,semiconductor materials,insulator materials,pressure sensitive materials,etc.This method can even be measured in a liquid environment.In summary,the dual-harmonic mode KPFM is of great significance for the study of the electrical properties of dielectric materials,the modification of dielectrics and the miniaturization of electronic devices.