A Study Of The Polarized Laser Scattering Detection Of Subsurface Damage In Ground Silicon Wafers

Monocrystalline silicon is a dominant substrate material in semiconductor industry.It is widely used in integrated circuits and photovoltaic cells.Currently,it is the primary material supporting the development of modern advanced technologies.Since monocrystalline silicon is hard and brittle,grinding is a mostly applied process to machine it.However,it is inevitable to induce subsurface damage into a silicon wafer during grinding.Subsurface damage is detrimental to the surface integrity of a machined surface and causes degradation in the performance and reduction in the life of the final part made of the wafer.Detecting subsurface damage could optimize the subsequent damage-removal processes,which is important to improving the overall manufacturing efficiency and quality of silicon wafer.However,it currently lacks an efficient method for detecting subsurface damage,which reduces the overall manufacturing efficiency of silicon wafer and increases the production cost.To increase the detection efficiency of subsurface damage,this paper conducts an in-depth study on the method for detecting subsurface damage using polarized laser scattering.Firstly,this paper studies the subsurface damage distribution in monocrystalline silicon under different grinding conditions.Then,based on the theory of electromagnetic field scattering an electromagnetic scattering model of subsurface damage is established and the principle of polarized laser scattering detection is elucidated.According to the distribution of subsurface damage,a polarized laser scattering detection system is designed and established in the paper.Finally,this paper studies the subsurface damage distribution in the monocrystalline silicon ground by different wheels with the polarized laser scattering detection system and reveals the relationship between the polarized laser scattering detection signal and the depth of subsurface damage.Therefore,this study is of significance to the development and application of the polarized laser scattering detection.The main research contents and conclusions are as follows.(1)Subsurface damage distributions in monocrystalline silicon under different parameters are revealed.The configuration distribution of the subsurface damage in ground silicon wafers is studied by the statistics method.It is found that grinding with coarse abrasive grains leads to subsurface cracks preferentially distributed along the {111} cleavage plane,while grinding with fine abrasive grains leads to subsurface cracks distributed along other crystalline planes instead of the {111} planes.Based on the formation mechanism of subsurface damage,a physically predictive model of the subsurface damage depth is built by combining the dislocation kinetics for crack initiation and fracture mechanics for crack propagation.The applicability and accuracy of the model are verified by experiments.The model reveals the distribution of subsurface damage depth,indicating that from the center to the edge of a ground wafer,the depth of cut of an abrasive grain increases,which leads to the decrease in strain rate and dynamic brittleness,correspondingly,resulting in the increase in the depth of subsurface damage.(2)Based on the theory of electromagnetic scattering,this paper establishes an electromagnetic scattering model of the subsurface damage in the monocrystalline silicon.The principle of the polarized laser scattering detection is elucidated and the polarized laser scattering electromagnetic field distributions of the grinding surface and subsurface damage are studied.It is found that when the linearly polarized laser scattering is used to detect subsurface damage,single scattering from a ground surface reserves polarization,while multi-scattering from subsurface damage shows depolarization.Detecting the depolarization signal can reveal the information on subsurface damage.In terms of grinding surface scattering,the study finds that surface roughness and the incident angle of the laser beam are two parameters that influence measurement results.However,the influence of grinding surface is negligible at the incident angle of zero degree.In terms of subsurface damage scattering,it is found that s-polarization and p-polarization of the incident laser beam leads to two different distributions of scattering field at the interfaces of silicon-to-crack and crack-to-silicon and the scattering of s-polarization leads to more depolarization,which is more beneficial to the accurate detection of subsurface damage.According to the distribution of the grinding-induced subsurface damage,this paper therefore proposes a solution to detect subsurface damage in a ground silicon wafer and to minimize the influence of surface roughness by setting the incident laser beam s-polarized and perpendicular to the grinding surface.(3)According to the proposed solution,a polarized laser light scattering detection system is designed and built and the influences of the grinding surface roughness and subsurface damage distribution to the polarized laser scattering detection are studied by both theoretical calculation and experiments with the system.Based on the calculation,the depolarization signals caused by the grinding surfaces scattering are far less than those caused by subsurface damage in different wafers.Based on the experiments,it is found that the influence of surface roughness to polarization laser light scattering detection is negligible,subsurface damage is the main influence factor in the detection using the polarized laser scattering detection system.(4)This paper studies the influences of residual stress to the polarized laser scattering detection.It is found that residual stress results in the photoelastic effect which influences the polarized laser scattering detection.When the polarization direction of an incident laser is parallel or perpendicular to the principal stresses of the residual stress,the influence of residual stress to the polarized laser scattering detection vanishes.Based on the distribution of residual stress which indicates that the principal stresses of the residual stress are found aligned with the grinding marks,this paper proposes a new method for detecting subsurface damage by using polarized laser scattering detection.The proposed method suggests that in the polarized laser scattering detection,the polarization direction of the incident laser is aligned with the grinding marks to minimize the influence of residual stress and increase the detection accuracy of subsurface damage.(5)Damage distributions in the silicon wafers ground under different conditions are studied and a relationship is built between the polarized laser scattering detection signal and the distribution of subsurface damage.It turns out that the polarized laser scattering detection signal is determined by both the depth and density of subsurface damage between which the former is a more dominant factor.The polarized laser scattering system can be used to detect subsurface damage with a depth as small as 0.1 ?m and the detection signal is proportional to the power of the depth of subsurface damage.