入射光照对典型光刻胶纳米结构的光学散射测量影响分析

作为一种快速、低成本和非接触的测量手段,光学散射测量在半导体制造业中的纳米结构三维形貌表征方面获得了广泛关注与运用.光学散射测量是一种基于模型的测量方法,在纳米结构待测参数的逆向提取过程中,为降低参数之间的耦合性,通常需要将结构的光学常数作为固定的已知量,即假设结构的材料光学常数不受光学散射仪入射光照的影响.事实上,这一假设对于半导体制造业中的绝大多数材料是成立的,但某些感光材料的光学常数有可能随着入射光的照射时间增加而发生改变,而由此产生的误差会在一定程度上传递给待测形貌参数的逆向提取值.本文针对聚甲基丙烯酸甲酯光刻胶薄膜培片和光栅结构分别开展了光学散射测量实验与仿真研究,结果表明该光刻胶材料的光学常数随着入射光照时间增加而变化,进而导致光栅结构形貌参数的提取结果较大地偏离于真实值,不容被忽视.这一研究发现将为更进一步提高光刻胶纳米结构三维形貌参数的测量精确度提供理论依据.

Influence of incident illumination on optical scattering measurement of typical photoresist nanostructure

Optical scatterometry,as a fast,low-cost,and non-contact measurement instrument,is widely used in the profile characterization of nanostructure in the semiconductor manufacturing industry.In general,it involves two procedures,i.e.the forward optical modeling of sub-wavelength nanostructures and the reconstruction of structural profiles from the measured signatures.Here,the general term signature means the scattered light information from the diffractive grating structure,which can be in the form of reflectance,ellipsometric angles,Stokes vector elements,or Mueller matrix elements.The profile reconstruction process is an inverse problem with the objective of optimizing a set of floating profile parameters(e.g.,critical dimension,sidewall angle,and height)whose theoretical signatures can best match the measured ones through regression analysis or library search.During solving the inverse problem,the refractive index and distinction coefficient of the material of nanostructure are assumed to be constants and they are generally fixed.This assumption is valid for most of the materials in semiconductor industry,but not for certain materials that are very photosensitive.That is,the optical constants of photosensitive materials may vary with the illumination time of the incident light beam in spectroscopic ellipsometer,and the error caused by the variation of optical constants propagates to the final extracted results of structural profiles,which should not be neglected,especially for high precision and accuracy metrology.Experiments performed on SiO2 and polymethyl methacrylate(PMMA)thin films are conducted and demonstrate that the extracted geometric parameters and optical constants of SiO2 film do not change with illumination time increasing,while the twenty groups of values of extracted refractive index n and distinction coefficient k of PMMA resist film vary obviously,and the difference between the extracted maximum and minimum film thickness has reached 40.5 nm,which to some extent illustrates that the above assumption is not valid for PMMA resist,so that the incident light beam of spectroscopic ellipsometer has a great influence on the extracted film thickness.Further,simulations based on a three-dimensional PMMA grating also indicate that the error of optical constant has considerably transferred to the extracted profile parameters.This finding is of significance for improving the accuracy of nanostructure characterization in optical scatterometry.