As the semiconductor industry rapidly approaches the 3nm lithography node, on-product overlay (OPO) requirements have become tighter, which drives metrology performance enhancements to meet the reduction in overlay (OVL) residuals. The utilization of multiple measurement wavelengths in Imaging- Based Overlay (IBO) has increased in the past few years to meet these needs. Specifically, the color per layer (CPL) method allows for optimizing the OVL measurement conditions per layer, including focus, light, wavelength (WL), and polarization customization which enhance the metrology results. CPL is applicable for multiple technology segments (logic, foundry, DRAM, 3D NAND), relevant for different devices (DRAM high stack layers, NAND channel holes, etc.), and can work well for both thin and thick layers for standard and EUV lithography processes. In this paper, we will review the benefits of CPL for multiple DRAM and NAND critical layers. We will describe how CPL can contribute to measurement accuracy by quantifying the OVL residual reduction in comparison to single-wavelength (SWL) measurement conditions.
KEYWORDS: Overlay metrology, Semiconducting wafers, Advanced process control, Scanners, Scatterometry, Process control, Signal processing, Metrology, Control systems, Optical parametric oscillators
As the cell size of memory devices continues to shrink, tighter on-product overlay (OPO) specs require more accurate and robust overlay control. The overlay error budget mainly consists of the reticle, scanner, process, and metrology errors. The metrology budget is generally required to be <10% of the OPO control budget so that the accuracy and robustness of overlay metrology become more crucial as pattern size gets smaller on current 1x nm DRAM nodes. For overlay control in high-volume manufacturing (HVM), the primary optical overlay metrology typically used is Image-Based Overlay (IBO). In many cases, scatterometry overlay (SCOL), using a direct grating-scanning method, was shown to achieve more accurate After Development Inspection (ADI) overlay measurements. Using a tunable source and customized illumination pupil to directly scan within the grating cell, this technology improves accuracy by reducing the contribution of pattern surroundings in the scribe line, resulting in improved OPO control stability. Since the purpose of overlay control is to minimize actual device pattern misregistration, as measured after the etching process (AEI), achieving accurate and stable characterization of the systematic deviation between ADI and AEI overlay known as Non-Zero-Offset (NZO) is critically important. Accurate NZO applied to the scanner via the Advanced-Process-Control (APC) loop enables effective scanner overlay control at the post-lithography ADI step. This paper demonstrates a new scatterometry overlay technology adopted in DRAM use cases that resulted in OPO and NZO stability improvement. In addition, we demonstrate an efficient method to monitor HVM run-to-run overlay performance and NZO stability by comprehensive dataset modeling combining ADI and AEI.
As DRAM technology continues to evolve, advanced nodes shrink the device dimensions and raise the requirements for on-product overlay control to reduce residual error. Increased process complexity also demands tighter accuracy and robustness in metrology control, which necessitates new and innovative metrology enhancements and methods. Scatterometry-based overlay (SCOL®) metrology is a unique overlay metrology architecture that uses angle-resolved pupil imaging for overlay analysis and calculation. KLA’s SCOL metrology system offers wide-spectrum tunable laser and multi-wavelength (MWL) illumination patterns along with custom-designed advanced algorithms that provide multiple measurement conditions to meet unique layer and target requirements. This paper demonstrates improved overlay metrology accuracy and residual error on DRAM FEOL critical layer with SCOL technology. Multiwavelength and rotated quadrupole (RQ) illumination in the metrology tool are utilized to provide significantly improved residuals compared with the traditional single-wavelength (SWL) and on-axis illumination.
In this paper, the rAIMTM (robust AIM) overlay target was investigated in terms of the stability versus the POR AIM® (Advanced Imaging Metrology) target used for imaging-based overlay (IBO) measurement at after development inspection (ADI). The targets were designed using KLA’s MTD AcuRate™, metrology target design software that performs simulations based on the optical properties related to relative permittivity and permeability about the material of each of the layers. Using advanced device layers, we studied the performance of the POR AIM target versus the newly designed rAIM target for imaging-based overlay measurements. For each target, we quantified the optical contrast, kernel signal, correctable modeled terms, total measurement uncertainty (TMU), and overlay (OVL) residuals from the modeled data through various wavelengths inside the Moiré effect regime in the case of rAIM. We demonstrate that there is an OVL measurement performance improvement using the rAIM target versus the POR AIM target. The measured optical properties of the rAIM target and comparison to the POR AIM target will be presented.
In the latest 3D NAND devices there is a larger focus on measurement accuracy control, coupled with more traditional minimization of Total Measurement Uncertainty (TMU). Measurement inaccuracy consumes an increasingly significant part of the overlay (OVL) budget, requiring control and optimization.
In this paper we will show the improvement in imaging OVL measurement accuracy using wave tuning (WT) capability combined with advanced algorithms to address 3D NAND process challenges. In addition to new OVL target designs that take advantage of WT capability, we also demonstrate improvement in OVL model residuals through optimization of measurement bandwidth, focus position and number of grab frames. Improvements in precision and tool-to-tool matching are also realized through both optimization of the region of interest (ROI) and splitting measurement areas using a dual-recipe technique.
Overlay process control is a critical aspect of integrated circuit manufacturing. Advanced DRAM manufacturing overlay error budget approaches the sub-2nm threshold, including all sources of overlay error: litho processing, non-litho processing, metrology error, etc. Overlay measurement quality, both for accuracy and robustness, depends on the metrology system and its recipe setup. The optimal configuration depends on the layer and materials involved. Increased flexibility of metrology setup is of paramount importance, paired with improved methods of recipe optimization.
Both optical image-based overlay (IBO) and scatterometry diffraction overlay (SCOL®) are necessary tools for overlay control. For some devices and layers IBO provides the best accuracy and robustness, while on others SCOL provides optimum metrology. Historically, wavelength selection was limited to discrete wavelengths and at only a single wavelength. At advanced nodes IBO and SCOL require wavelength tunability and multiple wavelengths to optimize accuracy and robustness, as well as options for polarization and numerical aperture (NA). In previous studies1,2,3 we investigated wavelength tunability analysis with landscape analysis, using analytic techniques to determine the optimal setup. In this report we show advancements in the landscape analysis technique for IBO through both focus and wavelength, and comparisons to SCOL. A key advantage of imaging is the ability to optimize wavelength on a per-layer basis. This can be a benefit for EUV layers in combination with those of 193i, for example, as well as other applications such as thick 3D NAND layers. The goal is to make accurate and robust overlay metrology that is immune from process stack variations, and to provide metrics that indicate the quality of metrology performance. Through both simulation and on-wafer advanced DRAM measurements, we show quantitative benefits of accuracy and robustness to process stack variability for IBO and SCOL applications.
Methodologies described in this work can be achieved using Archer™ overlay metrology systems, ATL™ overlay metrology systems, and 5D Analyzer® advanced data analysis and patterning control solution.
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