This paper presents the results of patterned and unpatterned EUV mask inspections. We will show inspection results related to EUV patterned mask design factors that affect inspection tool sensitivity, in particular, EUV absorber material reflectivity, and EUV buffer layer thickness. We have used a DUV (257nm) inspection system to inspect patterned reticles, and have achieved defect size sensitivities on patterned reticles of approximately 80 nm. We have inspected EUV substrates and blanks with a UV (364nm) tool with a 90nm to a 120 nm PSL sensitivity, respectively, and found that defect density varies markedly, by factors of 10 and more, from sample to sample. We are using this information in an ongoing effort to reduce defect densities in substrates and blanks to the low levels that will be needed for EUV lithography. While DUV tools will likely meet the patterned inspection requirements of the 70 nm node in terms of reticle defect sensitivity, wavelengths shorter than 200 nm will be required to meet the 50 nm node requirements. This research was sponsored in part by NIST-ATP under KLA-Tencor Cooperative Agreement #70NANB8H44024.

A new algorithm for an Applied Materials CD-SEM metrology tool has been developed which gives a quantitative characterization of the quality of contact holes in photoresist. This is a non-destructive technique that allows users to assess the cross sectional profile of a contact hole from top-down measurements thus avoiding the time-consuming and expensive task of measuring cross sections. This analysis is based on the shape of the SEM (Scanning Electron Microscope) waveform that is created when a contact hole is measured. The algorithm generates a numerical grade for the contact profile, which is based on the shape of the waveform. The classification of the contact hole into different Profile Grades (either open, closed or transitional) can then be determined due to the strong correlation between the shape of the waveform and the cross-section profile of the contact hole. The Profile Grades have been found to be in excellent agreement with SEM cross-sections. When this technique is applied to contact holes across a focus-exposure matrix wafer, the algorithm gives grades which clearly delimit the domain in which the process parameters produce open, sharp-edged contacts. In many cases it was found that the CD measurement alone was insufficient to determine if a given contact hole was actually open. The combination of the CD measurements with the Profile Grades is a particularly powerful tool to determine ideal process parameters for lithography, and determination of the Profile Grade becomes essential as toolsets are pushed toward the limits of their capability. The following paper concentrates on 250 nm contact holes but presents data on contact holes ranging in size from 200 nm to 500 am.

The optical inspection of next generation lithography ͑NGL͒ patterned reticles, multilayer-coated blanks, and uncoated substrates is particularly challenging. The difficulties arise not only because of the higher sensitivity necessary at the smaller design rules, but also due to the specifics of the NGL mask materials and structures. Our research program is investigating the theoretical and practical operational limitations facing optical inspections of patterned and unpatterned NGL masks. We are constrained by the necessity to inspect only in reflected light, limitations in mask contrast, and interference effects caused by partially coherent illumination. We present inspection results and images of several types of NGL masks, blanks and substrates obtained on high resolution ultraviolet ͑UV͒ and direct ultraviolet ͑DUV͒ optical mask inspection systems. While electron projection lithography ͑EPL͒ stencil masks can be inspected in reflection, limited transmission through the mask suggests that subsurface defects will be detected with reduced sensitivity, compared to defects on the surface. For unpatterned inspections, we have demonstrated UV system sensitivity to 88 nm PSL spheres on quartz substrates and 117 nm PSLs on silicon substrates. On extreme ultraviolet masks we have achieved DUV system sensitivity down to defects as small as 80 nm in size, both in die-to-die inspections and in simulated die-to-database inspections. In addition to advancing the development of optical inspection systems for NGL reticles, these inspection results provide feedback to NGL mask developers.