Inspecting mask defects with through-focus scanning optical microscopy

An advanced technology that uses 13.5nm light, extreme UV (EUV) lithography uses mirrors and masks to pattern features 32nm wide or even smaller onto microchips, allowing the creation of denser and thus faster, more powerful computers. Before this technology can be widely applied, however, the industry must find a way to reduce the defect density of the masks.1 The sources of defects must be mitigated or eliminated at each step of the mask manufacturing process, which requires methods of identifying defects. Our research, with experimental results at visible wavelengths and simulations in the deep UV, suggests that 3D data sets from through-focus scanning optical microscopy (TSOM) could inexpensively provide information about buried defects that are either difficult or impossible to detect using current methods. Mask defects can be categorized by whether they change the phase or amplitude of reflected EUV light. Amplitude defects are located on top or near the top of the multilayer mask structure. Phase defects, however, are generally formed either by the deposition of multiple layers over substrate defects (particles or pits) or by particles added during the multilayer deposition. Buried defects are harder to detect than surface defects. Repair and mitigation techniques differ by defect type. Most commercial inspection tools, however, provide no information about amplitude or phase change. Characterization techniques such as electron microscopy, ion microscopy, or atomic force microscopy can provide information on surface topography but are unable to report the size and depth location of a defect underneath multiple layers. Also, electron and ion microscopes provide limited or no information about the core of the buried defects. Methods with unique capabilities, such as actinic Figure 1. Construction of a through-focus scanning optical microscopy (TSOM) image using a conventional microscope. (a) Optical images are acquired at multiple heights, above and below the best focus position. (b) Images are stacked at their respective heights to form the 3D optical data set. (c) Optical intensity data extracted along a vertical cross section passing through the center of the target (a nanoparticle) is plotted as a 2D TSOM image.