Attota Ravikiran, Silver Richard, Dixson Ronald
Precision Engineering Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, USA.
Appl Opt. 2008 Feb 1;47(4):495-503. doi: 10.1364/ao.47.000495.
We present a detailed experimental study of a new through-focus technique to measure critical dimension linewidth with nanometer sensitivity using a bright field optical microscope. This method relies on analyzing intensity gradients in optical images at different focus positions, here defined as the focus metric (FM) signature. The contrast of an optical image of a structured target, where a particular structure is repeated several times, varies greatly as it is moved through-focus if the spacing between the structures is such that the scattered field from the features interferes. Complex, distinguishable through-focus optical response occurs under this condition giving rise to the formation of several cyclic high and low contrast images. As a result it exhibits several FM signature peaks as opposed to a single FM peak for structures nearly isolated. This complex optical behavior is very sensitive to the dimensions of the target geometry. By appropriately analyzing the through-focus optical image, information can be obtained regarding the target. An array of lines is used as a structured target. Linewidth measurements were made by using experimental through-focus optical data obtained using a bright field microscope and simulated optical data. The optical results are compared with reference metrology tools such as a critical dimension atomic force microscope and critical dimension scanning electron microscope.
我们展示了一项详细的实验研究，该研究采用一种新的全聚焦技术，通过明场光学显微镜以纳米级灵敏度测量关键尺寸线宽。此方法依赖于分析不同聚焦位置光学图像中的强度梯度，在此将其定义为聚焦度量（FM）特征。对于具有特定结构且重复多次的结构化目标，若结构之间的间距使得特征散射场产生干涉，那么当该目标在聚焦过程中移动时，其光学图像的对比度会有很大变化。在这种条件下会出现复杂且可区分的全聚焦光学响应，从而形成多个高低对比度循环图像。结果是，与几乎孤立的结构只有一个FM峰值不同，它会呈现出多个FM特征峰。这种复杂的光学行为对目标几何尺寸非常敏感。通过适当地分析全聚焦光学图像，可以获取有关目标的信息。使用一系列线条作为结构化目标。通过使用明场显微镜获得的实验全聚焦光学数据以及模拟光学数据进行线宽测量。将光学测量结果与诸如关键尺寸原子力显微镜和关键尺寸扫描电子显微镜等参考计量工具的测量结果进行比较。
