Quantitative Study On The Signals Of Electrostatic Force In Scanning Probe Microscopic Images

Scanning probe microscopy techniques,including atomic force microscopy（AFM）,magnetic force microscopy,and electrostatic force microscopy,can probe the surface morphology,electrical,magnetic,and electrochemical properties of samples at microscale and have attracted increasing attention in recent years.For electronic materials such as semiconductors,the images obtained by scanning probe microscopy can be significantly distorted or deformed due to electrostatic forces.However,current research on the electrostatic force signals in scanning probe microscopy images is mainly qualitative,lacking quantitative analysis.This thesis focuses on the quantitative analysis of the electrostatic forces impact on scanning probe microscopy imaging,starting from both AFM and EFM aspects.Firstly,a new method using AFM tapping mode to detect the charge density of the sample is proposed.Secondly,the electrostatic force expression of multilayer semiconductor structures is derived by Green’s function theory,achieving quantitative research on the electrostatic force image signal of multilayer film structures by EFM.The innovation and research content of this thesis include:1.A quantitative analysis framework for electrostatic force signals in AFM morphology images is constructed.First,an analytical model of probe vibration in AFM tapping mode is established using Fourier transformation.Qualitative analysis based on this model shows that when there is electrostatic attraction between the probe and the sample,the morphology image will produce an upward offset.Then,using the DMT（Derjaguin,Muller,and Toporov）atomic interaction force model and focusing on point-charge-type electrostatic forces,a time-domain differential equation of probe motion is established.The results of numerical calculations using the Runge-Kutta method show that when using an“SCM-PIT”model probe,with a free amplitude of 40 nm and a detection amplitude setting value of 25 nm,and a sample charge density of 6.875×10¹⁰ cm^-2,the upward displacement of the sample surface morphology is approximately 1.5 nm.In addition,AFM morphology testing was performed on a single crystal silicon sample,which was found to be in good agreement with the above theoretical analysis.2.A model for the electrostatic forces between a metal probe and a semiconductor thin film sample is constructed for EFM measurements,and the effects of semiconductor thin film structures and carrier concentration on the EFM image signals are quantitatively studied.Firstly,the Green’s function expression for a single-layer semiconductor thin film and a semiconductor/insulator bilayer is derived using the mirror charge method and its derivative Green’s function theory,based on Debye-Huckel semiconductor linear theory,and then the Green’s function recursive formula for semiconductor/insulator multilayer structures is obtained.On this basis,an analytical model of the electrostatic forces between the conductive probe and different semiconductor thin film structures is derived,and numerical calculations are used to obtain the EFM image signals under different conditions.The study found that when scanning multilayer film structures with EFM,the electrostatic force corresponding to the first layer is significantly greater when the first layer is a semiconductor material than when it is a dielectric material,and that the magnitude of the electrostatic force in multilayer films can be quantitatively reflected by the EFM signal.Numerical simulations show that when using an“SCM-PIT”model probe to test a multilayer structure composed of P-type single crystal silicon and silicon dioxide,with a resistivity of approximately 1Ω-cm,and a probe-to-sample distance of 50 nm,applying a+4 V DC bias to the conductive probe,an EFM image signal can be computed,with a frequency shift of approximately-46.8 Hz.Experimental verification has shown that the test results are in good agreement with the above theoretical analysis.