|
In recent years, optical metrology methods, based on spectroscopic ellipsometry (SE), have gained a strong foothold for use in measuring the 2-dimensional profiles of integrated device features. While optical methods generally provide superior metrology performance compared to other metrology methods, there still remain some challenges to meet precision and accuracy requirements as integrated device geometries continue to shrink at an aggressive rate. Process engineers are employing several new techniques in order to meet device patterning requirements while maintaining manufacturing-worthy process windows. These techniques include thinning the photoresist layers and adding additional underlayer films to act as hard masks for subsequent pattern transfer steps. Thinning the photoresist layer reduces the aspect ratio of patterned grating targets, which, in turn, reduces the signal-to-noise (S/N) ratio of the optical profile measurement. The additional films in the process stack below the gratings increase the number of optical interfaces that must be taken into account when building the optical model for the measurement. The increased complexity of the optical model increases the likelihood that there will be cross-correlation between the underlying films and the grating profile parameters (Critical Dimension or CD, Height, and Sidewall Angle or SWA). The reduction in S/N and increase in cross-correlation often have negative impacts on the precision and overall accuracy of the reported values for the grating profile parameters. In this paper, we will discuss a methodology to overcome the issues described above. The methodology involves performing a standard SE film thickness measurement on an open pad area in close proximity to the grating target of interest. The thickness values are then fed forward to a subsequent SE measurement of the grating target. With the underlayer thickness values fixed based on the film thickness measurement, only the grating profile parameters are solved for during the grating measurement. Decoupling the underlayer film measurement from the grating measurement, greatly reduces, and even eliminates cross-correlation between parameters. Both SE measurements are completed with a total move-acquire-measurement (MAM) time that is <10 seconds per pair, and the resulting values reported for CD, Height, and SWA are more accurate when compared to reference metrology such as Atomic Force Microscopy (AFM). Supporting data will be presented from measurements taken on a 65nm technology node gate lithography process. Using the feed-forward process, the correlation and slope of profile parameters measured via SE compared to AFM measurements is greatly improved. Furthermore, systematic anti-correlation between resist height and SWA that was observed during simultaneous measurement of the film stack and grating is eliminated when the film measurement is decoupled and fed-forward into the grating measurement.
|
