Advantages of Broadband Illumination for Critical Defect Capture at the 65-nm Node and Below

The physical and optical properties of materials are based on their structure. Figure 1 shows the theoretical wavelength dependence of the brightfield gray-level signal from a bridg ing defect in two thicknesses of a photoresist/BARC stack, patterned in a line and space array. In this figure, the defect gray level signal is plotted — i.e. the difference in gray level between an image with the defect and another without the defect. One stack exhibits best defect gray level in the deep ultraviolet (DUV) range, while the other shows best defect gray level in the visible range. Because the bridging defect sigAs chipmakers continue innovating with new materials, structures, and processes, they face an increase in new defect types, along with new noise sources that hamper defect detection. Tunable broadband brightfield illumination technology has several advantages over a single-wavelength approach for meeting new inspection challenges and generating higher capture rates of yield-impacting defects. Modeling studies as well as fab experience show that different defect types and device layers require different inspection wavelengths for reliable defect detection. A broadband source spanning DUV through visible wavelengths can be tuned to the optimal wavelength band to create maximum contrast, considering the specific optical properties of a given layer or defect type. Its spectrum of wavelengths also can reduce color noise-interference from underlying layers that can create nuisance defects. Minimizing nuisance defects means that detected defects correlate more strongly to yield.