A methodology of obtaining the local critical dimension uniformity of contact hole arrays by using optical scatterometry in conjunction with machine learning algorithms is presented and discussed. Staggered contact hole arrays at 44 nm pitch were created by EUV lithography using three different positive-tone chemically amplified resists. To introduce local critical dimension uniformity variations different exposure conditions for dose and focus were used. Optical scatterometry spectra were acquired post development as well as post etch into a SiN layer. Reference data for the machine learning algorithm were collected by critical dimension scanning electron microscopy (CDSEM). The machine learning algorithm was then trained using the optical spectra and the corresponding calculated LCDU values from CDSEM image analyses. It was found that LCDU and CD can be accurately measured with the proposed methodology both post lithography and post etch. Additionally, since the collection of optical spectra post development is non-destructive, same area measurements are possible to single out etch improvements. This optical metrology technique can be readily implemented inline and significantly improves the throughput compared to currently used electron beam measurements.
As development of stacked Nanosheet Gate All-Around (GAA) transistor continues as the candidate technology for future nodes, several key process points remain difficult to characterize effectively. With the GAA device strategy, it is critical to have an inline solution that can provide a readout of physical dimensions that have an impact on the threshold voltage (VT) and yield. Metrology challenges for obtaining these metrics arise from increasingly dense arrays coupled with both high aspect ratios, high numbers of correlated parameters, and increasingly complex 3D geometries. Large area metrology structures can be used for 3D parameters’ process monitoring through techniques such as scatterometry and xray diffraction (XRD) which deliver averaged results over that area, but variation impacting specific devices cannot currently be understood without destructive cross-section. Prior work to characterize the dimensions of these GAA devices has primarily featured optical metrology, X-ray metrology, and critical-dimension scanning electron microscopy (CDSEM), but these techniques have their own challenges at the critical process points. Atomic force microscopy (AFM) had not been utilized due to the aspect ratios and small trench widths which were inaccessible to conventional techniques. However, due to recent advances in scanning and novel probe technologies, AFM is well-suited now to solve these local, three-dimensional challenges. Through this study, we demonstrate AFM characterization of a key process point in the GAA process flow for multiple structures with varying channel lengths, after epitaxial (epi) growth along the Si sidewall. The AFM scan results are compared to CDSEM images for top-down corroboration of topography and to other reference metrology for height correlation. The impact of measured variations in epi height to device performance is also reviewed.
Emerging memory technologies such as Resistive Memory (RRAM) have gained a lot of attention to meet the requirements of a potential analog computing element, due to its non-volatile characteristics, scalability and energy efficiency. An RRAM device typically consists of a resistive switching layer (e.g. HfO2) sandwiched between two metal electrodes. Since oxygen vacancies are critical to the functioning of the device, it is desirable to achieve residue free etching using oxygen-less plasmas, and preferably minimize exposure to ambient environment. In this work, we discuss the RRAM patterning challenges and their impact on the device characteristics including the switching/forming voltage.
Voids in copper lines are a common failure mechanism in the back end of line (BEOL) of integrated circuits manufacturing, affecting chip yield and reliability. As subsequent process nodes continue to shrink metal line dimensions, monitoring and control of these voids gain more and more importance [1]. Currently, there is no quantitative in-line metrology technique that allows voids to be identified and measured. This work aims to develop a new method to do so, by combining scatterometry (also referred to as Optical Critical Dimension or Optical CD) and low-energy x-ray fluorescence (LE-XRF), as well as machine learning techniques. By combining the inputs from these tools in the form of hybrid metrology, as well as with the incorporation of machine learning methods, we create a new metric, referred to as Vxo, to characterize the quantity of void. Additionally, the results are compared with inline electrical test data, as higher amounts of voids were expected to increase the measured resistivity. This was not found to be the case, as the impact of the voids was much less of a factor than variation in the cross-sectional area of the lines.
As device scaling continues, controlling defect densities on the wafer becomes essential for high volume manufacturing (HVM). One type of defect, the non-selective SiGe nodule, becomes more difficult to control during SiGe epitaxy (EPI) growth for p-type field effect transistor (pFET) source and drain. The process window for SiGe EPI growth with low nodule density becomes extremely tight due to the shrinking of contact poly pitch (CPP). Any tiny process shift or incoming structure shift could introduce a high density of nodules, which could affect device performance and yield. The current defect inspection method has a low throughput, so a fast and quantitative characterization technique is preferred for measuring and monitoring this type of defect.
Scatterometry is a fast and non-destructive in-line metrology technique. In this work, novel methods were developed to accurately and comprehensively measure the SiGe nodules with scatterometry information. Top-down critical dimension scanning electron microscopy (CD-SEM) images were collected and analyzed on the same location as scatterometry measurement for calibration. Machine learning (ML) algorithms are used to analyze the correlation between the raw spectra and defect density and area fraction. The analysis showed that the defect density and area fractions can be measured separately by correlating intensity variations. In addition to the defect density and area fraction, we also investigate a novel method – model-based scatterometry hybridized with machine learning capabilities – to quantify the average height of the defects along the sidewall of the gate. Hybridizing the machine learning method with the model-based one could also eliminate the possibility of misinterpreting the defect as some structural parameters. Furthermore, cross-sectional TEM and SEM measurement are used to calibrate the model-based scatterometry results. In this work, the correlation between the SiGe nodule defects and the structural parameters of the device is also studied. The preliminary result shows that there is strong correlation between the defect density and spacer thickness. Correlations between the defect density and the structural parameters provides useful information for process engineers to optimize the EPI growth process. With the advances in the scatterometry-based defect measurement metrology, we demonstrate such fast, quantitative, and comprehensive measurement of SiGe nodule defects can be used to improve the throughput and yield.
Multi-channel gate all around (GAA) semiconductor devices require measurements of more target parameters than FinFET devices, due in part to the increased complexity of the different structures needed to fabricate nanosheet devices. In some cases, multiple measurement techniques are required to be used in a hybrid-metrology technique in order to properly extract the necessary information. Optical scatterometry (optical critical dimension, or OCD) is an inline metrology technique which is used to measure the geometrical profile of the structure, but it may not ordinarily be sensitive to very small residues. X-ray based metrologies, such as x-ray fluorescence (XRF) can be used to identify which materials are present in the structure, but are not able to measure profile information for complex 3D structures.
This paper reviews a critical etch process step, where neither OCD nor XRF can extract all of the necessary information about the structure on their own, but, when hybridized, are able to provide enough information to solve the application. In GAA structures, the nanosheets are formed from alternating layers of thin SiGe and Si layers which are deposited on a bulk Si substrate. To form the nFET channel, the SiGe must be removed. However, in some cases, there is still remaining SiGe residue on the surface of the Si nanosheets, present in small amounts that are difficult to measure with conventional OCD. Additionally, it is desirable to know at which level of the stacked nanosheets the residue is present. In order to properly characterize the amount of SiGe remaining, data from both OCD and XRF are used. By measuring before and after the etch, the XRF can calculate the percentage of SiGe that is remaining after the etch. This percentage can be used as a constraint in the OCD model to allow the OCD to accurately measure the amount of SiGe, and to enable the OCD model to identify the location of the residue.
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