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The optical coupling between gratings in diffraction-based overlay triggers a swing-curve1,6 like response of the target’s signal contrast and overlay sensitivity through measurement wavelengths and polarizations. This means there are distinct measurement recipes (wavelength and polarization combinations) for a given target where signal contrast and overlay sensitivity are located at the optimal parts of the swing-curve that can provide accurate and robust measurements. Some of these optimal recipes can be the ideal choices of settings for production. The user has to stay away from the non-optimal recipe choices (that are located on the undesirable parts of the swing-curve) to avoid possibilities to make overlay measurement error that can be sometimes (depending on the amount of asymmetry and stack) in the order of several “nm”. To accurately identify these optimum operating areas of the swing-curve during an experimental setup, one needs to have full-flexibility in wavelength and polarization choices. In this technical publication, a diffraction-based overlay (DBO) measurement tool with many choices of wavelengths and polarizations is utilized on advanced production stacks to study swing-curves. Results show that depending on the stack and the presence of asymmetry, the swing behavior can significantly vary and a solid procedure is needed to identify a recipe during setup that is robust against variations in stack and grating asymmetry. An approach is discussed on how to use this knowledge of swing-curve to identify recipe that is not only accurate at setup, but also robust over the wafer, and wafer-to-wafer. KPIs are reported in run-time to ensure the quality / accuracy of the reading (basically acting as an error bar to overlay measurement).

On-product overlay requirements are becoming more challenging with every next technology node due to the continued decrease of the device dimensions and process tolerances. Therefore, current and future technology nodes require demanding metrology capabilities such as target designs that are robust towards process variations and high overlay measurement density (e.g. for higher order process corrections) to enable advanced process control solutions. The impact of advanced control solutions based on YieldStar overlay data is being presented in this paper. Multi patterning techniques are applied for critical layers and leading to additional overlay measurement demands. The use of 1D process steps results in the need of overlay measurements relative to more than one layer. Dealing with the increased number of overlay measurements while keeping the high measurement density and metrology accuracy at the same time presents a challenge for high volume manufacturing (HVM). These challenges are addressed by the capability to measure multi-layer targets with the recently introduced YieldStar metrology tool, YS350. On-product overlay results of such multi-layers and standard targets are presented including measurement stability performance.

Immersion technology has successfully extends the application of ArF lithography in the semiconductor. However, as we further push the k 1 factor below 0.3, the patterning fidelities degrade significantly. In this paper, a novel method to quantify the mask fidelity of complex 2D patterns is proposed. With this method, the critical dimension (CD) error of both edge placement error (EPE) and corner rounding can be well described by using 2 indices "bias" and "blur" respectively. The "bias" is defined as the CD offset between the mask and the targets, and the "blur" is a derived term that can well represent the mask rounding. These 2 indices are not only able to describe the mask quality but also able to link with model parameters that are used in optical proximity correction (OPC) and some other applications. In this paper, we demonstrate the methodology and quantify the actual mask quality on the complex and critical 2D patterning in the advanced nodes.

In advanced optical lithography the requirements of focus control continues to tighten. Usable depth of focus (DoF) is already quite low due to typical sources of focus errors, such as topography, wafer warpage and the thickness of photoresist. And now the usable DoF is further decreased by hotspots (design and imaging hotspots). All these have put extra challenges to improve focus metrology, scanner focus stability calibrations and on-product correction mechanisms. Asymmetric focus targets are developed to address robustness in focus measurements using diffraction-based focus (DBF and μDBF) metrology. A new layout specific calibration methodology is introduced for baseline focus setup and control in order to improve scanner focus uniformity and stability using the measurements of the above mentioned asymmetric targets. A similar metrology is also used for on product focus measurements. Moreover, a few novel alternative methods are also investigated for on-product focus measurements. Data shows good correlation between DBF and process on record (POR) method using traditional FEM. The new focus calibration demonstrated robustness, stability and speed. This technical publication will report the data from all the above activities including results from various product layers.