CD Variations Correction by Local Transmission Control of Photomasks Done with a Novel Laser based Process

As IC feature sizes become smaller and smaller, requirements for Critical Dimension (CD) variations control have become a critical issue. A new process for the control and correction of intra-field CD variations (Critical Dimension Control or CDC) was applied and it's influence on defects detection and photo-masks inspection capabilities at different modes of inspection was investigated. CD Control (CDC) of the photomask is a process in which Deep UV transmittance is selectively altered by patterns of small partially scattering shading elements (Shade in Element ) inside the quartz. The shading elements are formed by a process of shooting an ultrafast laser beam focused inside the mask substrate, resulting in localized intra-volume breakdown inside the quartz which creates local pixels of modified index of refraction (delta n). An array of such pixels with constant density constitutes one shading element. Process patterns are predetermined according to a CD variations map which may be supplied from wafer CD SEM, Optical CD or mask aerial imaging simulation tool (AIMS). Thus by changing local photomask transmission levels, it is possible to correct for the CD variations inside the field. Attenuation level, or optical density of the shading elements depends on the laser pulse energy, distance between pixels, number of layers and the size of the shading element itself. Since photomask transmittance is being changed, qualification of the impact of the transmittance changes on the defect detection and analysis capabilities are required. In this study, the principles of patterning of scattering elements inside transparent media by focusing of ultra-short laser pulses were introduced and explained. Analysis of the effects to both mask and wafer due to the CDC process was verified by full printing process applied to wafers, and by aerial imaging simulation tool. More tests for CDC required also tests by automatic reticle inspection tool to be production-worthy for the 65nm node and beyond.