Postek Michael T, Lowney Jeremiah R, Vladar Andras E, Keery William J, Marx Egon, Larrabee Robert D
National Institute of Standards and Technology, Gaithersburg, MD 20899-0001.
J Res Natl Inst Stand Technol. 1993 Jul-Aug;98(4):415-445. doi: 10.6028/jres.098.032.
X-ray masks present a measurement object that is different from most other objects used in semiconductor processing because the support membrane is, by design, x-ray transparent. This characteristic can be used as an advantage in electron beam-based x-ray mask metrology since, depending upon the incident electron beam energies, substrate composition and substrate thickness, the membrane can also be essentially electron transparent. The areas of the mask where the absorber structures are located are essentially x-ray opaque, as well as electron opaque. This paper shows that excellent contrast and signal-to-noise levels can be obtained using the transmitted-electron signal for mask metrology rather than the more commonly collected secondary electron signal. Monte Carlo modeling of the transmitted electron signal was used to support this work in order to determine the optimum detector position and characteristics, as well as in determining the location of the edge in the image profile. The comparison between the data from the theoretically-modeled electron beam interaction and actual experimental data were shown to agree extremely well, particularly with regard to the wall slope characteristics of the structure. Therefore, the theory can be used to identify the location of the edge of the absorber line for linewidth measurement. This work provides one approach to improved x-ray mask linewidth metrology and a more precise edge location algorithm for measurement of feature sizes on x-ray masks in commercial instrumentation. This work also represents an initial step toward the first SEM-based accurate linewidth measurement standard from NIST, as well as providing a viable metrology for linewidth measurement instruments of x-ray masks for the lithography community.
X射线掩膜呈现出一种与半导体加工中使用的大多数其他物体不同的测量对象，因为支撑膜在设计上是X射线透明的。在基于电子束的X射线掩膜计量中，这一特性可被用作优势，因为根据入射电子束能量、衬底成分和衬底厚度，该膜也可以基本上是电子透明的。掩膜上吸收体结构所在的区域基本上是X射线不透明的，也是电子不透明的。本文表明，使用透射电子信号进行掩膜计量，而非更常用的二次电子信号，可以获得出色的对比度和信噪比水平。透射电子信号的蒙特卡罗建模被用于支持这项工作，以确定最佳探测器位置和特性，以及确定图像轮廓中边缘的位置。理论建模的电子束相互作用数据与实际实验数据之间的比较显示出非常好的一致性，特别是在结构的壁斜率特性方面。因此，该理论可用于识别吸收线条边缘的位置以进行线宽测量。这项工作提供了一种改进X射线掩膜线宽计量的方法，以及一种用于商业仪器中测量X射线掩膜上特征尺寸的更精确边缘定位算法。这项工作还代表了朝着美国国家标准与技术研究院（NIST）首个基于扫描电子显微镜（SEM）的精确线宽测量标准迈出的第一步，同时为光刻领域的X射线掩膜线宽测量仪器提供了可行的计量方法。
