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When to Raise the Red Flag Effective Dispositioning of Defective Masks Jerry Huang, Lan-Hsin Peng, and Chih-Wei Chu, ProMOS Technologies Kaustuve Bhattacharyya, Ben Eynon, Farzin Mirzaagha, Tony Dibiase, Kong Son, Jackie Cheng, Ellison Chen, and Den Wang, KLA-Tencor Corporation
Progressive mask defects are an industry-wide mask reliability problem, particularly when the defects approach the critical state where the mask either needs to be pulled out of production or sent for cleaning (or repair). This problem is especially troublesome with expensive high-end masks running deep ultraviolet (DUV) lithography. In these cases, the fab will want to sustain the problematic masks in production as long as possible, until just before the masks begin impacting the process window. This study found that while a small, growing defect may not print at the best focus exposure condition, it can still influence the process window, shrinking it significantly. Direct, high resolution reticle inspection enables early detection of these defects; however, fabs still need an effective means to disposition defective masks. A lithographic detector has been evaluated to see if it can predict the criticality of such progressive mask defects. Examining the nature of mask defect growth
In a typical fab, many masks remain problem-free (clean) even after a large number of exposures. On average, about 1% of binary masks (at 365 nm lithography) and 6% to 15% of embedded phase shift masks (EPSMs) (using DUV lithography) show a defect growth problem through the duration of their usage in the fabs1,2. A direct, high resolution mask inspection can detect these defective masks effectively. But the nature of this defect growth can be severe on some masks, which means thousands of real crystal growth-type defects on the pattern side of masks. As one can imagine, this can
Figure 1. Progressive mask defects growth in fab.
20
Spring 2006
Yield Management Solutions
render the defect review session of these problematic masks to be quite complicated. However, a mask inspection tool such as KLA-Tencor’s TeraScan STARlight has the capability of binning the defects by size as well as by type (such as “on-chrome”, “on-clear”, “on-half-tone”, etc.). This helps to disposition masks that have a reasonable number of defects. When the total defect count grows on certain masks, however, traditional review techniques may result in a very lengthy review session. Hence, it was decided that the new TeraScan STARlight-2 (SL2) should be used to evaluate a run-time mask error enhancement factor (MEEF) based detector that may isolate the defects of interest from those thousands of total defects caught by SL2. In Figure 1 below, a fab’s incoming inspection indicates that a mask arrived clean, but after only 20 days of