Abstract
Rules are derived to obtain specifications on radiance, power, lifetime, and cleanliness of the source for an actinic patterned mask inspection system. We focus on the physical processes and technological aspects governing the requirements of radiation sources for reticle inspection. We discuss differences and similarities to scanner with respect to magnification, system etendue, and image recording. Source radiance requirements are estimated from a perspective of targeted throughput and defect detection sensitivity. The derivations consider the influence of photon shot noise on signal detection and conservation laws of light etendue and radiant flux. We describe the scaling laws for required radiance with targeted sensitivity index, optical contrast, field size, and system throughput. In addition, we address the limits on the required brightness and minimum repetition rate set by mask damage threshold. Finally, system and source cleanliness requirements and criticality of the source availability and lifetime are discussed. The analysis can be applied to other microscopy-based metrology and inspection applications.
Highlights
At extreme ultraviolet (EUV) wavelengths, diffraction limited optical imaging with a lateral half-pitch resolution in the single-digit nanometer range is achievable with state-of-the-art reflective optics.[1,2] The inspection of mask blanks and patterned masks for EUV lithography at 13.5 nm requires the ability to scan large surfaces for the presence of small but printable defects as rapidly as possible.[3]
CCD-type detector is used in time delay integration (TDI) mode, which is well suited for lowlight applications and imaging of moving objects.[8]
The challenging requirement for mask inspection is that reflectivity changes stay below 0.1% for 25 inspections, which means that changes in lattice constant should be smaller than 5 pm
Summary
At extreme ultraviolet (EUV) wavelengths, diffraction limited optical imaging (printing) with a lateral half-pitch resolution in the single-digit nanometer range is achievable with state-of-the-art reflective optics.[1,2] The inspection of mask blanks and patterned masks (reticles) for EUV lithography at 13.5 nm requires the ability to scan large surfaces for the presence of small but printable defects as rapidly as possible.[3]. In the mask shop, patterned masks are inspected multiple times during fabrication, and during photolithography exposures in the wafer fab, masks are inspected periodically to ensure that the mask remains defect free This is critical for masks used in EUV lithography, for which these inspections are currently being performed at 193 nm wavelength in production. Two different illumination/imaging modes combined with advanced database modeling are used to enhance contrast and defect sensitivity.[4] These tools support mask development and production down to 7 nm node. The requirements on source radiance are derived based on the desired throughput, optical resolution, and defect sensitivity, for the specific case of an actinic EUV mask inspector using basic illumination and imaging modes, and a single, simple defect type. The final part underlines the requirements on source cleanliness and availability
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