Mask Blanks And Their (Sometimes Invisible) Defects

The ITRS roadmap for lithography aggressively shrinks many features on the mask from the critical dimension to the size of defect that must be eliminated. When a defect is larger than 30% of the minimum image size, it is considered detrimental to the mask performance. Unfortunately, the sensitivity of current inspection systems does not keep the pace dictated by ITRS for development work. The 65 nm node mask development teams are largely unaware of defects smaller than 90 run. This gap only widens for NGL masks. The scarcity of high-sensitivity inspection systems is leaving many mask makers and their customers unsure of the impact, or even presence of, defects on the reticle. This paper will provide an:analysis of mask blank defects using,several different inspection systems with both low and high sensitivity levels. Defect sources from carriers and shipping will be explored. Finally, the likelihood of removing these particles once they have been lodged on the mask surface is discussed.