Innovative applications: extending photomask registration tool for critical dimension measurement to achieve high efficiency

In the semiconductor industry, photomask quality depends on various aspects including critical dimension (CD), overlay to other layers, defects, etc. Conventionally, photomask metrology tasks are performed by separate tools. For example, KLA LMS IPRO is used for registration, while CD-SEM or optical microscope is used for CD measurements. However, current CD tools have time-consuming measurements and difficulties to providing spatial CD variation across the photomask. To address these challenges, LMS IPRO tool, originally designed for registration measurement, provides a portable and fast CD measurement solution. The feasibility of CD measurement on LMS IPRO comes from its image processing mechanism. Algorithms analyze the intensity of captured images to figure out the pattern edges. The calculated CD commonly has deviation to the actual CD due to the unknown edge intensity threshold. Thus, we fed LMS IPRO the actual CD to do calibration before using this function in production scenarios. The calibrated CD ranges from 0.5μm to 13μm, which covers the mature technology node product sizes at Quanyi Mask Optoelectronic Technology Co. Ltd (QYMask). Verification results proved that LMS IPRO meets QY Mask's mature-node mask CD measurement specifications. Therefore, it could (a) be temporary substitution in case optical CD tool is down, (b) concurrently measure CD and registration, (c) provide fast pre-check of CD uniformity.