Advances to smaller memory chip nodes has brought about challenges of accurately fabricating and monitoring increasingly narrow and high aspect ratio (HAR) features. Among these, the HAR etching process is considered the most critical and intricate step in the entire manufacturing flow. The production of high-quality HAR features require methodologies to tackle a range of process errors, including incomplete etching, bowing, twisting, tilting and CD variation. We present both theoretical insights and experimental evidence to highlight CD small-angle x-ray scattering (CDSAXS) as a powerful method for monitoring process variations such as tilt and bending signatures. This nondestructive approach holds immense potential as a rapid screening tool to detect etch process excursions. This method is model-free and eliminates the need for lengthy geometric-model-building processes that may take days to weeks. Moreover, it mitigates the risk of model failure and reduces engineering hours, making it a cost-effective and efficient solution.
3D NAND flash memory stacks cells vertically in multiple layers. One of the critical processes in chip-making is slit trench landing control since the word-line gate replacement of silicon nitride films and the isolation of plural memory blocks are through the means of slit structure. However, during the gate of silicon nitride removal in slit trenches, the architecture of memory blocks may collapse due to the lack of sufficient supporting patterns surrounding the slit trenches. For better silicon nitride removal control, there are various irregular patterns of dummy pillars (PIL) designed in as supporting structures to provide better mechanical property, which can prevent memory blocks from collapsing while removing silicon nitride in gate replacement processing. Hence, an effective measurement is needed to indicate the bottom shifting of PIL during the etching recipe optimization. In this work, we demonstrate the deployment of small angle X-ray scattering (SAXS) technology with global tilt extraction (GTE) that successfully predicts PIL landing location displacement, which matches destructive gauging data. GTE is a modeless method to determine global tilt, i.e., center line shifting (CLS) of the bottom relative to the topmost on the target pattern. GTE is proven to be a reliable method for in-line process monitoring as well as for use in PIL etching recipe optimization for 3D NAND development.
KEYWORDS: X-rays, Semiconducting wafers, Etching, Metrology, 3D metrology, Back end of line, Scattering, 3D modeling, Transmission electron microscopy, Optical alignment
3D NAND has become mainstream storage devices in a past decade and the stacking cell layers now reaches to more than 300 layers. As stack gets higher, more and more etching process challenges are brought into 3D NAND high aspect ratio (HAR) structure patterning. Among the HAR etching processes, the deep contact etches in the back-end-of-line (BEOL) are patterned after other HAR structures such as channel holes, deep trenches or other dummy patterns. Any unexpected overlapping of deep contacts with other patterns would lead to fatal product yield loss due to such wrong circuit connections and leakage currents. Hence, a reliable and fast monitoring methodology for profile tilting is extremely important. In this article, it is first time to demonstrate the employment of the small angle x-ray scattering (SAXS) technology and global tilt extraction (GTE) by using Axion @TM from KLA-Tencor to measure bottom tilting behavior for deep contact structure in 200+ layers of 3D NAND product. As regarding for the necessaries of gathering layout pitch, detail film stacks and hole dimensions by regular model base approach, the GTE is a pure modeless method by using x-ray scattering images to determine the central point shifting of the bottom relative to the topmost on the target pattern. With GTE help, the development cycle time for etch recipe tuning and BEOL process optimization is effectively improved, as well as demonstrating better process control to sustain product yield.
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