基于相位展开及拼接算法的一种高精度大量程宽光谱干涉显微术

A High Precision and Large Range Measuring Method for Broadband Light Interferometric Microscopy Based on Phase Unwrapping and Stitching Algorithm

宽光谱干涉显微术广泛应用于高精密检测领域,它测量样品形貌通常采用垂直扫描干涉术对亚微米至毫米级特征进行测量,以及相移干涉术对纳米级特征进行测量。其中,相移干涉术精度可达纳米级,但量程有限,高度变化对应的相位需限制在区间内。采用包裹相位展开算法可以扩展相移干涉术的量程,也仅适用于平滑表面,当高度起伏超出焦深或者光源相干长度的限定范围时,干涉条纹模糊或对比度丧失,所解算的结果将产生较大误差甚至错误。提出一种基于相位展开及拼接算法的高精度、大量程宽光谱干涉显微测量方法,以干涉条纹调制度量化条纹质量,条纹对比度高、成像清晰的区域对应调制度较高,定义当前焦面条纹调制度高于阈值的区域为理想区域,定义焦面条纹调制度低于阈值的区域为问题区域。以相位展开算法获得理想区域中的样品相位分布,问题区域的包裹相位不进行展开。使用微位移结构纵向移动物镜焦平面,选择合理的步长,使相邻焦面位置理想区域展开后的真实相位保持部分区域重合,根据重合区域的相位值均差可以实现不同焦面位置的高精度相位拼接,最终获得扩展量程的高精度真实相位结果,进而可以恢复样品完整的表面形貌分布。该算法通过对理想区域的筛选,避免了相位在问题区域展开带来的误差,可以得到精确的测量结果。通过模拟计算和实验验证,证明了该方法不仅保持了宽光谱干涉显微术中相移干涉术的纳米级高精度,还可将其量程从数百纳米拓展到数微米。而且,该方法精度不依赖于位移部件,理论上量程可以拓展到显微物镜的全工作距离。

Broadband light interferometric microscopy is widely used for high precision profile measurement in the industry field.Vertical scanning interferometry(VSI)is usually used to measure the submicron to millimeter level features,and phase shift interferometry(PSI)to measure the nanoscale features.Among them,the precision of PSI is of nanoscale order,while its measurable range is limited because the phase changes corresponding to the height variations of the sample surface should be limited within the scope of 2π.A large amount of phase unwrapping algorithms are developed to extend the range of PSI.However,they are only suitable to smooth surfaces.When the height fluctuations exceed the limited range determined by the focal depth or the coherent length of the light source,the interference fringes will be blurred.Even the contrast will be lost.Thereafter,great measurement errors will be introduced to the calculated results.This paper proposesa high precision and large range broadband light interferometric microscopy measurement method based on the phase unwrapping and stitching algorithm.The fringe modulation value quantified the fringe quality at a given focal plane,the areas with the high modulation values generally correspond to the regions of interests(ROI)with high contrast and clear image.The ideal regions(IRs)are defined as the ROI with a modulation value greater than a given threshold within the current focal plane.Meanwhile,the problem regions(PRs)are defined as the ROI with a modulation value lower than the given threshold.Only the true phase distribution in IRs is calculated with the phase unwrapping algorithm.By vertically moving the focal plane of the objective with a translation stage at a reasonable step length,the IRs of the adjacent focal plane will be partially overlapped.According to the differences between the phase of the overlapped IRs of the adjacent focal plane,the corresponding unwrapped phase of the adjacent plane can be stitched together with high precision.Finally,the complete profile distribution of the sample is restored according to the stitched true phase with high precision.The proposed method for broadband light interferometric microscopy avoids the error caused by the phase unwrapping in the PRs.Through simulation and experiments results,we demonstrate that the proposed method maintains the nanoscale precision of PSI in broadband light interferometric microscopy and extends its range from hundreds of nanometers to several micrometers.Moreover,its accuracy does not depend on the displacement precision of the focal planes by translation stages.Theoretically,the range of our proposed method can be extended to the total working distance of the microscopic objective.