A detailed characterization of across chip line width variation (ACLV) has been carried out on the latest Nikon scanners with a combination of advanced metrology techniques in Texas Instruments, including scatterometer-based image field and CD fingerprinting, lens aberrations measurement using a Litel in-situ interferometer, and illumination source imaging with a pin-hole camera. This paper describes the application of the above techniques in our investigation of the root causes for pattern CD bias between vertical and horizontal features. Illumination source radiance distribution is found sometimes to have a significant impact on V-H bias and the final overall ACLV on production wafers. Examples are given to demonstrate a comprehensive methodology that is used to quantitatively break down the overall CD errors and correlate them back to the basic optical and imaging components. It is shown through pupil-gram analysis that the ellipticity in partial coherence is typically within 1+/-1% for conventional illuminations settings on the advanced Nikon scanners while the uneven radiance distribution across the source plays a major role in V-H pattern CD bias. For scanners with low and uniform lens coma aberrations, the V-H bias after removing the contribution from image field errors is found to follow a linear relationship with the source radiance non-uniformity described also in terms of ellipticity. It is shown that radiance ellipticity is a bigger concern for off-axis illuminators. Tighter design rules patterned with off-axis illumination are more vulnerable to source radiance non-uniformity as well as lens aberrations. Illuminator induced V-H bias across the slit is compared to the signature caused by lens aberrations specifically uneven x,y-coma. Implications to exposure tool specification, control, and matching are further explored through experiments and lithography simulation for the current 130nm production and the future technology nodes in development.
Lens spherical error is an important lens aberration used to characterize lens quality and also has a significant contribution to across chip line width variation (ACLV). It also impacts tool-to-tool matching efforts especially when the optical lithography approaches sub-half wavelength geometry. Traditionally, spherical error is measured by using CD SEM with known drawbacks of poor accuracy and long cycle time. At Texas Instruments, an in-house scatterometer-based lens fingerprinting technique (ScatterLith) performs this tedious job accurately and quickly. This paper presents across slit spherical aberration signatures for ArF scanners collected using this method. The technique can successfully correlate these signatures with Litel lens aberration data and Nikon OCD data for spherical aberration errors as small as 10mλ. ACLV contributions from such small spherical errors can be quantified using this method. This provides the lithographer with an important tool to evaluate, qualify and match advanced scanners to improve across chip line width variation control.