Comparison of optical and electrical measurement techniques for CD metrology on alternating aperture phase-shifting masks

This paper presents a comparison of optical and electrical techniques for critical dimension (CD) metrology on binary and alternating aperture phase shifting masks. For the first time, measurements obtained from on-mask electrical CD structures are compared with optical measurements made using a deep ultra-violet (DUV) mask metrology system. Initial results show that the presence of alternating phase shifting trenches between the chrome blocking features has a detrimental effect on the optical measurements. In addition the optical metrology system appears to have problems with the measurement of the narrowest isolated features due to calibration related issues. Electrical CD measurements are seen as a way of probing the limits of optical tool calibration, and for highlighting and managing the need to extend the complexity of the calibration schedule.