Determination of DICD Best Focus by Top-down CD-SEM

摘要

As critical-dimension shrink below 0.18 μm, the SPC (Statistical Process Control) based CD (Critical Dimension) control in lithography process becomes more difficult. Increasing requirements of a shrinking process window have called on the need for more accurate decision of process window center. However in practical fabrication, we found that top-down CD-SEM showed its limitations in process window center determination, especially for the best focus. For instance, in some extreme focus situation, resist pattern will show a severe undercutting profile which will affect the DICD reading by top-down CD-SEM with fixed measurement algorithm. This kind of DICD measurement error will finally affect the process window center determination (especially best focus) and in-line DICD monitoring, which will lead to the cost of scrap and loss of time for trouble-shooting. In this paper, we will present a detailed study of DICD best focus determination in case of top-down DICD by experiment and simulation. Further a possible solution to this problem will be described in the latter part of this paper.