Damage Mechanism of Single Crystal Silicon in Laser Machining

Author:Liu Qing Yuan

Supervisor:long yu hong


Degree Year:2019





In laser machining,both brittle and ductile behaviors can be exhibited at different temperatures.The fracture behaviors of single crystal silicon vary with temperature,which cannot be reflected internally by conventional fracture mechanics method.In addition,single crystal silicon not only undergoes quasi-static mechanical effect,but also the laser-induced impact.The material undergoes not only thermal stress,but also impact effect.The crystal damage caused by laser shock is beyond the scope of fracture mechanics.The variable material properties and complex loading environment lead to the diversity of damage mechanism in laser processing of single crystal silicon.To control micro-scale damage in laser processing,it is necessary to understand the mechanism of damage formation.The main problems at present are as follows: 1)For the initiation of micro-scale fracture in laser processing,the fracture model based on continuum theory lacks physical basis,which mainly manifests that micro-scale information needs to be expressed in the fracture model.2)It is difficult to describe the fracture phenomena of single crystal silicon with only one kind of fracture theory,because of the wide range of temperatures in laser processing.In addition,the change of fracture strength under different temperatures can not be described by traditional fracture model.3)Griffith fracture model neglects dissipation in micro-scale damage.Although the non-linear fracture problem can be described by J fracture theory,there are still some problems in its application in micro-scale systems.4)The damage of materials under laser shock is beyond the scope of fracture analysis.In addition,the dynamic evolution process should be considered when the laser interacts with the damage area.Micro-scale fracture damage and impact damage widely exist in laser processing.A proper investigation of them can help to understand the mechanism of damage formation,so as to point out the direction for the realization of low damage machining.In order to solve the above problems,the damage mechanism of single crystal silicon is studied by fracture mechanics theory,molecular dynamics method,impact dynamics knowledge and experimental means.According to the damage forms of single crystal silicon in laser processing,micro-scale damage is divided into two parts: micro-scale fracture damage and impact damage.The work and achievements in this thesis are mainly as follows: 1)The fracture model is modified by introducing the crystal information,then a semi-empirical fracture model of single crystal silicon is constructed,and the molecular dynamics method is adopted to analyze the mechanism;2)The thermophysical information is used to modify the semi-empirical model.Then,the relationship between the temperature effect and the change of fracture strength in micro-scale fracture damage can be expressed.3)The limitations of equivalent J-fracture model is discussed,and the mechanism of nonlinear fracture behavior is analyzed.Then,the equivalent J-fracture model is improved with a high accuracy;4)For the damages induced by laser shock in high temperature region,the dynamic process and physical mechanism during damage evolution process are studied by combining molecular dynamics and shock dynamics principles.5)For the laser and defect interaction phenomenon,the healing process of defect region is analyzed by using molecular dynamics and shock dynamics knowledge;6)The damage characteristics of single crystal silicon in laser processing were studied by experimental method.The damage mechanism was explained by above damage theory.At the end,the processing parameters were adjusted to control the damage.Dynamic,non-linear and coupling effects can be found during damage formation process.Temperature,crystal structure and force amplitude play important roles in this process.By introducing crystal structure information into fracture model,the fracture is learned without using traditional fracture related parameters.By using thermophysical information in fracture model,the attenuation of fracture strength induced by temperature is expressed.The increase of temperature lead to the rise of enthalpy,which results in the decrease of bond energy.The non-linear fracture is analyzed by the improved micro-scale equivalent J fracture model,which overcomes the shortcomings of its predecessors and can accurately capture the information of bond breaking.It can be observed the dissipation effect in dynamic fracture process comes from atomic structure distortion,local plastic deformation and thermal dissipation.In laser induced shock process,a strong coupling effect between force and heat can be observed.In dynamic damage process,relaxation effect and the evolution of crystal structure affect energy absorption.Above the ductile-brittle transition temperature,the effect of low-amplitude stress wave can lead to the healing of defect zone.As the damages are controlled by above thermal and mechanical process,machining parameters are adjusted in experiments.The pulse frequency is increased to reduce single pulse energy.At the same time,the thermal accumulation and impact effect are eased to reduce the damage extent near the processing area.