针对超精密加工后单晶金刚石衬底表面损伤层具有超薄(试验仅几个纳米)且透明的特点,提出一种基于光谱椭偏的测量和表征方法,实现衬底损伤层厚度和折射率的无损测量和表征.首先,建立"粗糙层+纯基底"两层光学模型,利用离散型穆勒矩阵椭偏测量模式测量加工前籽晶衬底,分析测量数据获得其光学常数,作为后续加工损伤层椭偏数值反演的基础,以避免损伤层与衬底间椭偏参数耦合;然后,根据衬底加工后的特征,建立"粗糙层+损伤层+纯基底"三层光学模型,采用多点拟合分析策略,在此基础上,实现粗磨和精磨两个典型加工阶段金刚石衬底损伤层的无损表征,并进一步探究单面磨削和双面磨削损伤层差异.结果表明,籽晶折射率与金刚石折射率理论值接近,且随波长的变化趋势一致,说明测量模式和拟合策略可行;粗磨后衬底损伤层的厚度和折射率均高于精磨后衬底损伤层的厚度和折射率;双面磨削与单面磨削损伤层的折射率在红外波段基本一致,在紫外-可见波段具有差异.损伤层厚度椭偏测量结果与透射电子显微镜(Transmission electron microscope,TEM)测量结果进行比对分析,验证椭偏测量方法的准确性.所提方法可无损测量单晶金刚石衬底超薄损伤层厚度和折射率,表征超精密加工后衬底表面质量,有助于金刚石衬底超精密加工过程的工艺优化.
Non-destructive Measurement and Characterization of Damage Layer of Single-crystal Diamond Substrate after Ultra-precision Machining
Aiming at the characteristics of ultra-thin(the experiment involves only a few nanometers)and transparent damage layer on the surface of single-crystal diamond substrate after ultra-precision machining,a method based on spectral ellipsometry is proposed to achievenon-destructive measurement and characterization of the thickness and refractive index of the damage layer.Firstly,a two-layer model of"roughness layer+pure substrate"is established,and the seed crystal substrate before processing is measured by discrete Mueller matrix ellipsometry mode,and its optical constants are obtained by analyzing the measurement data,which are used for the ellipsometric numerical inversion of the subsequent processing damage layer.This can prevent the coupling of ellipsometric parameters between the damage layer and the substrate.Secondly,according to the characteristics of the substrate after processing,a three-layer optical model of"roughness layer+damage layer+pure substrate"is established,and a multi-point fitting analysis strategy is used to achieve non-destructive characterization of the damage layer of diamond substrate in two typical machining stages of rough grinding and fine grinding is realized,and the difference in the damage layer between single-sided grinding and double-sided grinding is further explored.The results show that the refractive index of the seed sample is very close tothe theoretical value of the diamond,and their variation trends with wavelength are consistent with each other,which indicates the measurement mode and the substrate refractive index fitting strategy are feasible.The thickness and refractive index of the damage layer after rough grinding are higher than those after fine grinding.The refractive index of the damage layer after double-sided grinding and single-sided grinding is almost the same in the infrared band,but different in the ultraviolet-visible band.The ellipsometry results of damage layer thickness are compared with those observed by transmission electron microscope(TEM),which has verified the accuracy of ellipsometry characterization.The proposed method can non-destructively measure the thickness and refractive index of the ultra-thin damage layer of the single-crystal diamond substrate,and characterize the surface quality of the substrate after ultra-precision machining,which will contribute to the process optimization of ultra-precision machining of diamond substrates.
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