Simulation Of Piezoelectric Response Of Scanning Probe Microscopy/AlN Film Contact Point

AlN is widely used in MEMS devices due to its excellent piezoelectric and acoustic properties,and the piezoelectric properties of AlN are closely related to the performance of devices,so it is very important to study the piezoelectric properties of AlN.In this paper,scanning probe microscope(SPM)is used to investigate the piezoelectric properties of materials.When a bias voltage(either direct or alternating)is applied between the SPM probe and the piezoelectric sample,the surface of the sample will deform due to the inverse piezoelectric effect.The purpose of this paper is to model the complete tip model by COMSOL software.By optimizing the geometric parameters of the probe,the corresponding frequency consistent with the resonance frequency of the experimental probe NSC18(75k Hz)is obtained,and the final geometric parameters are determined.Based on the PFM method,which is widely used in the field of piezoelectric coefficient measurement,this paper discusses the influence of substrate,aspect ratio of piezoelectric film and upper electrode on the contact piezoelectric response.The simulation results show that the presence of substrate will reduce the piezoelectric response,and only when the film length/thickness is very small,the measured piezoelectric response is close to the built-in piezoelectric response of piezoelectric materials,With the increase of film length/thickness,the greater the influence of substrate clamping,the smaller the measured piezoelectric response,and finally reach a constant value.In addition,the softer the substrate is,the larger the deformation of the film/substrate interface is,and the smaller the piezoelectric response of the sample surface is.In order to increase the piezoelectric response,regardless of the substrate clamping,it is found that the effective piezoelectric constant of the structure with and without the upper electrode is almost unchanged with the magnitude and direction of the bias voltage,which is about 4pm/V,and larger than that of the probe acting as the active electrode(<3pm/V),However,the measured value of the structure is still less than the built-in piezoelectric coefficient(4.96pm/V),and the error is large.Considering the influence of pressure on the piezoelectric response,a method to measure the piezoelectric coefficient of materials in non-contact mode is urgently needed.Considering the long-range characteristics of electrostatic force,it is necessary to quantitatively analyze the electrostatic force in non-contact mode.This paper explores the electrostatic force of each part of the probe,calculates the variation of the electrostatic force contribution of each part with the tip sample gap,and comes to the conclusion that the cantilever can be ignored when the tip sample gap is ? 10 nm.Then,this paper explores the factors that affect the probe electrostatic force,including the setting of boundary conditions,grid division and geometric parameters of the probe.The setting of boundary conditions changes the actual gap between the tip and the sample,thus changing the electrostatic force on the probe.However,the error of grid division is relatively small,so it is better to divide the grid manually.In addition,by comparing the electrostatic force distance curves of the needle free cantilever and the complete needle tip model,it is found that the effect of the cantilever can be ignored in the short distance,and changing the length of the cantilever will change the electrostatic force on the cantilever,but it has little effect on the slope of the electrostatic force distance curve.The influence of tip geometry parameters(tip height,tip semi cone angle,tip radius)on the electrostatic force distance curve is found in the tip sample gap of 10nm-10 nm ? It is more obvious when the temperature is less than30 m;However,changing the tip radius can significantly change the electrostatic force at close range(<200nm);The smaller the tip half cone angle is,the faster the electrostatic force decreases with the tip sample gap,and the smaller the value when the tip half cone angle finally tends to equilibrium.Due to the complexity of the electrostatic force distance curves of different tip models,this paper starts from the analytical solution of the electrostatic force of different models,and finds that the electrostatic force on the tip is always proportional to the square of the probe bias voltage in the case of pure electrostatic,and in the case of piezoelectric,when the tip is far away from the sample surface(?1?m)The piezoelectric sample has almost no effect on the tip electrostatic force,which is equivalent to the pure electrostatic case.However,in the case of short distance,the piezoelectric sample deforms due to the inverse piezoelectric effect(much larger than the deformation caused by the electrostatic force),which changes the electrostatic force and deviates from the pure electrostatic case.Based on this situation,a method of measuring longitudinal piezoelectric coefficient based on atomic force microscope is invented.This method uses the electrostatic force bias curve of the same sample at short distance and long distance to extract the piezoelectric response.The experimental device is similar to the piezoelectric response force microscope method,but the operation is in non-contact mode,which avoids the influence of pressure on the results,In addition,there is no upper electrode,which reduces the human error in the deposition process.