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Using Pattern-Quality Confirmation to Control a Metal-level DUV Process with a Top-down CD SEM Chien-Sung Liang, Haiqing Zhou, Mark Boehm, Ricky Jackson, KFAB Photolithography Module, Texas Instruments Chih-Yu Wang, Mike Slessor, KLA-Tencor
As critical-feature patterning processes increase in complexity and sensitivity, conventional critical-dimension (CD) mea surements may not afford the level of process control required for effective device production. By comparing recorded top-down scanning-electron-microscope (SEM) images to a predefined reference image, Pattern Quality Confirmation (pQC) enables a more detailed analysis of measurements captured by KLA-Tencor 8xxx CD-SEMs. An example of the utility of this additional information is discussed for a metal interconnect level patterned with a deep-ultraviolet (DUV) photolithography process. In particular, we demonstrate that, for certain ranges of focus-exposure conditions, conventional post-develop CD measurements remain well within specification; however, when etched, the resulting metal-line CDs are significantly below the lower specification limit. The pQC image analysis results predict the observed post-etch CD variations and, consequently, offer sensitivity to yield-limiting focus drifts and excursions, enabling effective product-dispositioning (rework) decisions.
Introduction
At virtually every processing step, modern semiconductor manufacturing relies on highly automated metrology for process control. For lithography and etch processes, automated critical-dimension scanningelectron microscopes (CD SEMs) fill this role; however, there exists a set of yieldrelevant excursions, e.g., profile changes, scumming, etc., which can elude detection by standard CD measurements. The contraction of multi-dimensional image information into a single CD value can obscure such subtle effects, which are evident in the images themselves. Pattern Quality Confirmation (pQC) is a method that utilizes some of this image information to enable detection of subtle changes as part of a fully-automated CD SEM metrology step. In effect, a comparison is 56
Fall 2001
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made between a known “good� template, and the image obtained during measurement of the feature of interest. A correlation score provides the metric of comparison; a perfect match returns a score of 100, no match returns a score of zero. On KLA-Tencor 8xxx CD SEMs, this measurement option is available in two flavors: a onedimensional or linescan pQC correlation score, and a two-dimensional or image-based pQC correlation score. Both one- and two-dimensional pQC measurements have been demonstrated to provide additional value to top-down CD SEM measurements; 1,2,3 in this work, we explore the use of two-dimensional pQC in monitoring an aluminum-interconnect patterning process. The process of interest employs deep-ultraviolet (DUV) photoresist application and exposure techniques along with a conventional metal-etch process. For this process and layer, a thick layer of photoresist is required to pattern the desired interconnect structure, and the lithography process is found to be relatively sensitive to variations in exposure-tool focus and energy. In turn,