Study On The Influence Of Water Layer Characteristics On Quality Of Underwater Laser Cutting Monocrystalline Silicon

An aqueous solution assisted laser machining technology,which is a composite laser machining technology possessing the scribing and cool the work-piece simultaneously,is proposed in order to reduce the thermal damage of laser machining hard and brittle materials.Among them,laser processing underwater is a simple way with good universality,but the processing quality depends heavily on the characteristics of water layer.At present,most researches focus on the influence of water layer thickness on laser cutting quality.However,the mechanism of the effect of water layer thickness and flow characteristics on machining quality is not clear.Based on this,this thesis focuses on the mechanism of laser irradiation removal of mono-crystalline silicon under the influence of water layer thickness and flow characteristics and optimize the water layer,in order to finally achieve low damage laser processing of mono-crystalline silicon.The above work provides a reference for the optimization of water layer characteristics in the near non-damage laser micromachining of hard brittle materials.This thesis mainly carries out the following work:1.The basic theories related to underwater laser processing process used in this thesis are summarized and analyzed,including the laser wavelength selection for underwater laser processing of mono-crystalline silicon,the effect of laser on spot size when passing through glass and water layers,the absorption and reflection of laser by water layer,and the effect of laser-induced cavitation bubbles and plasma on material removal.2.The optimization design of the closed container for underwater laser processing is completed,and realize the control of the thickness and velocity of the water layer.Fluent software is used to simulate and analyze the stability of water layer flow in different water layer thickness and different water layer velocity of the optimized closed container in processing region.Under the premise of controllable water layer characteristics,COMSOL Multiphysics software is used to realize the direct coupling analysis of arbitrary multiple physical fields,where a heat transfer and convection model is established to analyze the thermal procedure of underwater laser scribing mono-crystalline silicon wafer,and the control mechanism of water layer characteristics on processing quality and thermal damage is studied.3.According to the smooth particle hydrodynamics(SPH)method,the dynamic behavior of the interaction between the flowing water layer and the molten materials in the laser cutting process under the water layer is simulated,and the influence of the flowing water layer on the laser processing is analyzed.The complex cavitation bubble evolution process in laser-water-material interaction under different characteristics of water layer is studied by experimental observation method,whereby the experiment,and the influence of different water layer characteristics on cavitation bubbles and the effect of cavitation bubbles on silicon material removal are analyzed.4.The laser cutting experiment of single crystal silicon wafer is carried out in a container with controllable water layer characteristics.The influence laws of laser power,water layer thickness,inlet flow rate,laser frequency and laser scanning speed on scribing groove depth and groove width and heat affected zone(HAZ)are analyzed by orthogonal experiment.The process parameters which have significant effects on the width,depth and HAZ of the tank are determined through variance analysis of the experimental results.The cutting quality and removal mechanisms of mono-crystalline silicon in aqueous solution with different water layer characteristics are studied.5.In laser processing under flowing water layer,there are many,highly complex and nonlinear process parameters in the process of laser-water layer-material interaction,which makes it difficult to establish a direct relationship between process parameters and processing quality.However,the main process parameters and value range that affect the processing quality are identified through the above mechanisms research.On this basis,a large number of processing experimental data are collected,and a prediction model of the relationship between processing parameters and processing quality is established by using BP neural network,where the optimal combination of process parameters is found.Finally,laser cutting experiment is used to verify the model.In this thesis,the water layer of laser cutting mono-crystalline silicon under water is stable and controllable by optimizing the control container of water layer characteristics.On this basis,the thermal-damage,material removal mechanisms of laser cutting mono-crystalline silicon and the interaction mechanism of laser-water layer-material under different water layer characteristics are studied.The main influencing factors of processing quality are clarified by analyzing the material removal mechanisms.Then,the relationship between the complex parameters and the machining quality is established through the neural network,and the water layer thickness and flow characteristics is optimized by adopting this model,so as to realize the low damage removal of mono-crystalline silicon.The research of this project aims to reduce thermal damage and obtain silicon wafers with high surface quality on the premise of ensuring material processing efficiency,in order to provide certain reference basis for semiconductor device manufacturing.