波长轮换与相移扫描相结合的表面形貌干涉测量方法

Surface topography measurement is very important as it can be applied in microelectronics, optical components processing and surface features characterization. The project will use wavelength switching technology with phase shift scanning method to realize measurement of surface topography. The main idea is to design three light sources with different wavelength, which two wavelengths are close, and the third one is far from them. The interference frame images are acquired by microscopic interferometry measurement method when the phase shift is driven at uniform speed and the wavelength is switched in turn. Applying smooth filtering and differential convolution operators, all the phases of every point in each interference image for each wavelength are calculated respectively. On the basis of phase calculation, the topography height of every point is decided at large scale range by comparing the phase difference of two close wavelength interferogram.Then comparing the phase differnence of two far wavelength images, the height is revised and refined at a small scale range. At the last, the height result is further revised and refined at half a wavelength scale by single wavelength phase. The wavelength switch technology uses the algorithm of large scale combining with small scale to achieve the surface morphology of high-precision absolute measurement. This technology has the advantage of long coherent length of single wavelength mesurement and needs only the displacement of half a wavelength PZT drive. In the meantime, it overcomes the disadvantage of general wavelength scanning that requires the samples to be placed at one side of the virtual reference surface. Comparing to conventional white light interference microscopy, this method has a higher stripe fineness, simplified drive structure.

表面形貌测量在微电子、光学元件加工和表面功能特性评定等方面具有重要意义。本项目应用波长轮流切换与相移扫描相结合的方式实现表面形貌的测量，主要思想是设计三个不同波长的光源，其中两波长相近，一个相差较远，应用显微干涉测量的方法，在匀速相移驱动扫描同时，通过波长轮流切换方式获取不同波长的干涉帧图像。在平滑与微分卷积算子作用下，分别对三波长干涉帧图像进行逐点逐帧相位运算；在相位运算的基础上，比较两近波长帧干涉图的相位差，在大尺度范围内确立形貌点高度；比较两远波长帧的相位差，在小尺度内用以校正或细化近波长计算结果。最后在半波长级尺度内运用单波长相位校正两远波长帧计算结果。运用这种大小尺度结合算法实现表面形貌的高精度绝对测量的波长轮换技术，既有单波长相干长度长的优点，又仅需半个波长的位移驱动，克服了单纯波长扫描要求样品处于虚参考面一侧的缺点，相比传统白光显微干涉具有更高的条纹精细度、更简洁的驱动结构。

表面形貌直接影响零部件功能特性和使用，其测量在微电子、光学元件加工和表面功能特性评定等方面具有重要意义。本项目主要研究内容如下：.（1）基于干涉相差的大尺度和小尺度结合的理论基础. 提出了基于相位差大小尺度结合思想所展开的图像数据的校正与细化算法。分别对三波长干涉帧图像进行逐点逐帧相位运算；比较两近波长帧干涉图的相位差，在大尺度范围内确立形貌点高度；比较两远波长帧的相位差，在小尺度内用以校正或细化近波长计算结果。最后在半波长级尺度内运用单波长相位校正两远波长帧计算结果。.（2）基于波长切换与相移扫描的干涉测量系统. 测量系统选择了520nm、550nm、640nm三个波长，应用Linnik干涉测量原理，在匀速相移驱动扫描同时，通过波长轮流切换方式获取不同波长的干涉帧图像。系统将测量范围扩展到4.76μm，相对于单波长的640nm，扩大了近15倍。.（3） 窄带带宽作用下的高精度相位及相位差算法. 对单波长干涉相位的识别只能测量半波长范围内的表面形貌高度。对多波长干涉相位差的识别，能够更大范围拓展表面形貌测量的高度，理论上而言，波长相差越近，尺度范围越大。. 解决的关键问题包括：.a.稳频准直同光路的多波长激光源及其稳定轮流切换. 设计了恒流源驱动电路与高精度温度控制系统以实现激光器的稳频。波长切换既要保证PZT驱动、波长切换、图像采集的顺利进行，又要保证相互时延恰当。.b.高精度的驱动系统与相位识别算法. 项目提出了一种基于椭圆拟合和Lagrange插值、满足四步法的高精度相位识别方法，把仅通过两像素点灰度值序列关联关系，把驱动步长的求解问题转化为椭圆拟合问题，并准确地获取了相移驱动相位。使重构灰度值相对误差控制在0.5%以内，驱动步长的精度为0.75nm。. 项目的研究成果突破了单波长测量精度无法超越半波长测量范围的瓶颈，既保留单波长的测量精度，也拓宽了整体的测量范围,降低了对系统的信噪比要求。