As 3D NAND devices increase memory density by adding layers, scaling and increasing bits-per-cell, new overlay (OVL) metrology challenges arise. On product overlay (OPO) may decrease for critical thick layers such as thick deck-to-deck alignment, whereas high aspect ratio (Z-axis) structures introduce stress, tilt and deformation that require accurate and robust OVL measurements. Advanced imaging metrology (AIM®) targets, that consist of two side-byside periodic gratings in the previous and current layers, are typically used to measure OVL with Imaging Based Overlay (IBO) metrology systems. In this paper, we present a new approach that utilizes the Talbot effect in AIM to produce multiple contrast planes along the Z-axis, which enables a common focus position for both layers at a similar focus plane, resulting in improved measurement robustness. We will present Talbot effect theory, target design steps by metrology target design (MTD) simulator, actual measurement results on an advanced 3D NAND device and conclusions for such targets.
The current state of the art ADI overlay metrology relies on multi-wavelength uDBO techniques. Combining the wavelengths results in better robustness against process effects like process induced grating asymmetries. Overlay information is extracted in the image plane by determining the intensity asymmetry in the 1st order diffraction signals of two grating pairs with an intentional shift (bias). In this paper we discuss a next evolution in DBO targets where a target is created with multiple biases. These so called cDBO (continuous bias DBO) targets have a slightly different pitch between top and bottom grating, which has the effect of having a different bias values along the grating length and are complimentary to the uDBO technology. Where for the uDBO target, the diffraction results in a uniform Intensity pattern that carries the Overlay signal, for cDBO, an oscillating intensity pattern occurs, and the Overlay information is now captured in the phase of that pattern. Phase-based Overlay has an improved, intrinsic robustness over intensity-based overlay and can reduce the need for multi-wavelength techniques in several cases. Results on memory technology wafers confirm that the swing-curve (through-wavelength) behavior is indeed more stable for phase-based DBO target and that for accurate Overlay, this target can be qualified with a single wavelength recipe (compared to the uDBO dual wavelength recipe). In this paper, both initial results on a Micron feasibility wafer will be shown as well as demonstrated capability in a production environment.
CD measurements of advanced 3D-NAND Staircase process require development of new approaches in CD metrology [1]. The current CD SEM Contact Analysis used for 3D-NAND assumes that process control could be provided through a set of geometric parameters defining the contact shape (i.e. parameters of contact shape elliptic fit such as equivalent contact top diameter (Top CD), equivalent contact bottom diameter (Bottom CD), ellipticity, minor, major axis). The limitation of this approach for process control of complex structures was considered, and a new approach based on Grey Level Analysis of contact features in SEM images was proposed. However, this analysis is not enough for controlling the complicated 3D-NAND Staircase formation process steps, as contact holes with same geometric parameters but different depths cannot be separated by traditional CD SEM metrology measurement procedure (Figures 1 and 2). Thus, traditional CD SEM approach needs revisiting in order to work in situations where process control requires analysis of sophisticated Grey Level uniformity distribution. We propose a novel approach combining traditional metrology with machine learning methods. The essence of this new approach is to combine Grey Level attributes and traditional CD measured geometric parameters of the feature, obtained by traditional CD metrology flow, in a classification scheme (Figures 2 and 3). The proposed approach was qualified at Micron site demonstrating ~98% purity classification results.
The proposed approach is generic and can be extended to a large variety of process control applications. Enhancing regular metrology flow with the capability to classify Etch process quality eliminates the need for the expensive and destructive cross-sectional SEM analysis. Furthermore, this method has a clear advantage during the early R&D phase of process development as it increases the usefulness of the in-line metrology tool while the process is still immature and unstable.
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