| Popis: |
As K1 factor for mass-production of memory devices has been decreased to almost its theoretical limit, the process window of lithography is getting much smaller and the production yield has become more sensitive to even small variations of the process in lithography. So it is necessary to control the process variations more tightly than ever. In mass-production, it is very hard to extend the production capacity if the tool-to-tool variation of scanners and/or scanner stability through time is not minimized. One of the most critical sources of variation is the illumination pupil. So it is critical to qualify the shape of pupils in scanners to control tool-to-tool variations. Traditionally, the pupil shape has been analyzed by using classical pupil parameters to define pupil shape, but these basic parameters, sometimes, cannot distinguish the tool-to-tool variations. It has been found that the pupil shape can be changed by illumination misalignment or damages in optics and theses changes can have a great effect on critical dimension (CD), pattern profile or OPC accuracy. These imaging effects are not captured by the basic pupil parameters. The correlation between CD and pupil parameters will become even more difficult with the introduction of more complex (freeform) illumination pupils. In this paper, illumination pupils were analyzed using a more sophisticated parametric pupil description (Pupil Fit Model, PFM). And the impact of pupil shape variations on CD for critical features is investigated. The tool-to-tool mismatching in gate layer of 4X memory device was demonstrated for an example. Also, we interpreted which parameter is most sensitive to CD for different applications. It was found that the more sophisticated parametric pupil description is much better compared to the traditional way of pupil control. However, our examples also show that the tool-to-tool pupil variation and pupil variation through time of a scanner can not be adequately monitored by pupil parameters only, The best pupil control strategy is a combination of pupil parameters and simulated CD using measured illumination pupils or modeled pupils. |
