Study On The Mechanism And Application Of Electric Field Assisted Friction-induced Selective Etching

Since the 20th century,the continuous progress of micro-nano processing technology has promoted the rapid development of the semiconductor industry.Micro-nano functional structures are the research hotspot and focus in micro-nano processing technology,which is widely used in micro-electromechanical systems,spectral detection,photoelectric detection,environmental monitoring,energy storage and microelectronic sensing.However,the miniaturization trend of integrated devices poses a significant challenge to traditional micro-nano fabrication techniques.Friction-induced selective etching method is a new micro-nano processing method based on the innovation of scanning probe lithography.This method has attracted more and more attention due to its advantages of wide application of materials,simple process,and flexible processing.Nowadays,the laws and mechanisms of this method for processing Si and Ga As have matured,and its application in nanoimprinted templates and nanofluidic chips has been explored.However,the processing method still has shortcomings such as limited processing depth and low processing efficiency,which limit its practical application in industry to a certain extent.Therefore,it is necessary to explore new friction-induced selective etching processing methods by means of external field assistance,which can not only improve its processing capabilities,but also deepen the understanding of the mechanism of friction-induced selective etching and promote its practical process.In this thesis,we have systematically carried out research on electric field-assisted friction-induced selective etching processing methods,mechanisms and related applications through in-situ nanomechanical testing system,probe scribing processing equipment,atomic force microscope,scanning electron microscope and other analysis and characterization equipment.The rhodamine 6G solution was used as the detection molecule to test the Raman spectrum enhancement performance of the prepared micro-nano structures.The research work of this thesis is as follows:（1）In order to solve the problem of limited depth of friction-induced selective etching processing,this thesis introduces external voltage into friction-induced selective etching processing.The processing law of friction-induced selective etching under applied voltage was studied.The rapid removal of silicon damage areas and the processing of deeper silicon micro-nano structures were achieved.The constant voltage-assisted friction-induced selective etching processing method was developed and its processing mechanism was clarified.Under the action of voltage,the holes（h⁺）in the silicon substrate aggregates to the probe scanning area,which promotes further etching reaction between HF and silicon atoms,and increases the processable depth of maskless friction-induced selective etching from 300 nm to 600 nm.Under the combined action of the positive voltage and the trench structure,the electric field at the tip of the trench is enhanced,and the h⁺in the reaction system moves toward the probe scanning area,so that a large amount of h⁺accumulates in this area,which accelerates the etching reaction on the sidewalls and bottom of the scratched area.Under the combined action of the negative voltage and the trench structure,the concentration of h⁺forms a gradient difference between the silicon surface and the backside,so that the etching reaction mainly occurs at the bottom of the trench,and the etching reaction on sidewall of the trench is slow.（2）Since the depth of structures processed by maskless friction-induced selective etching is limited,this thesis pioneers the use of ketone-aldehyde resin as an acid etchant mask for isotropic etching,and introduce metallic silver into friction-induced selective etch processing to further increase its processable depth to 100-micron level.The silver-assisted friction-induced selective etching processing method was developed and its processing mechanism was explained.The ketone-aldehyde resin is used to replace the traditional etching mask,and the mask pattern is formed on the surface of the silicon substrate by the probe scanning process,which simplifies the process of preparing the mask.The ketone-aldehyde resin mask is very stable in both silver deposition and chemical etching stages,which blocks the chemical reaction between silicon and HF,and effectively protects the silicon substrate.Silver is deposited directionally in the probe scanning area and catalyzes the reduction of H2O2 to release h⁺.The continuous input of h⁺greatly improves the processing capability and etching rate of friction-induced selective etching.（3）During the processing of silver-assisted friction-induced selective etching,the motion of h⁺after input is arbitrary and difficult to control.In this thesis,the external voltage and silver were introduced into friction-induced selective etching processing,and the constant voltage and silver synergistically enhanced friction-induced selective etching processing method was established.A mask pattern was formed on the surface of the silicon substrate by probe scanning processing,and the silver catalyst was deposited and silicon micro-nano structure was prepared by electrochemically etching.The moving direction of h⁺was regulated by the action of voltage,so that the etching reaction occurs in the direction of the depth of the microstructure to control the characteristic profile of the prepared structure.Under the same etching time,the depth of the structure etched with voltage and silver is about 12 times that of without voltage and silver.（4）The proposed silver-assisted and constant voltage and silver synergistic-enhanced friction-induced selective etching methods were used for the preparation of SERS-active substrates,respectively,which directly fabricated microchannels with nanowire nested structures by one-step chemical etching.This method greatly simplifies the fabrication process of the existing microfluidic SERS active substrate,and realizes the application in surface-enhanced Raman spectroscopy.The detection limit,uniformity and reproducibility of the microfluidic SERS substrates were tested by rhodamine 6G molecules.The detection limits of the SERS active substrates prepared by the two processing methods were 10^-9 M and 10^-11 M,respectively,and their Raman spectral enhancement factors（EF）were 2.725×10⁵ and6.832×10⁷,respectively.In addition,a facile processing method to prepare large-area SERS active substrates is proposed and its Raman-enhanced properties are characterized.The Raman enhancement effect of the SERS substrate with both nanowire and pyramid structures is much higher than that of the single-structure SERS substrate,which has a detection limit of 10^-12 M for rhodamine 6G molecule.