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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.
Generally, we can use the sampling scheme optimization for a set of different features and their measured parameters in parallel. Especially in logic, but also for memory, the focus and dose dependencies of several features may be different. Hence, we optimized the distribution of the measured sites to create a perfect representation of the systematic fingerprint for all important anchor features within one single sampling scheme.
For the verification of the approach we investigated two cases. The first case are dense CD measurements, which are usually needed to create and update intra-field dose corrections. We minimize the number of measured sites significantly and distribute the remaining sites over different fields to ensure a good coverage of the systematic effects. Finally, that allows us a much higher update frequency of the dose corrections and yields in smaller CDU values.
The second case optimized the throughput of an OCD metrology system. The applied high-density sampling scheme for the focus monitoring done on reference wafers takes a lot of time during measuring. That specific type of measurement is done for monitoring and updating the focus reference corrections. With our proposed solution, we can achieve the same quality with respect to the reference measurement with more 50% less measured sites.
Using In-Device Metrology (IDM), we show results of non-destructive overlay measurements on 3D-NAND memory holes. Once the overlay signal has been determined, the remaining asymmetry information in the measurement can be used to characterize tilt phenomena densely through the memory array.
Using hyper-dense in-device measurements show the overlay effects of intra-die stress. A new lithography scanner model is used to correct specifically for such intra-die overlay fingerprints.
This new integrated flow consists of applying ILT to the difficult core region and traditional rule-based assist features (RBAFs) with OPC to the peripheral region of a DRAM contact layer. Comparisons of wafer results between the ILT process and the non-ILT process showed the lithographic benefits of ILT and its ability to enable a robust single patterning process for this low-k1 device layer. Advanced modeling with a negative tone develop (NTD) process achieved the accuracy levels needed for ILT to control feature shapes through dose and focus. Details of these afore mentioned results will be described in the paper.
When setting up the rules for RBAF, not all patterns are considered. Thus, applying RBAF for contact layers may result in decreased process margin for certain patterns since the same rule is applied globally. MBAF, on the other hand, can maximize the process margin for various patterns as it generates AF (Assist Feature) to locations that maximize the margin for the patterns considered. However, MBAF method is very sensitive to even a slight change of a target, which influences the locations of the AF. This leads to generating different OPCed CD of the main features, even for those that should not be affected by the changed target. Once the OPCed CD is changed, it is impossible to obtain the same mask CD even when the mask is manufactured with the same method. If this case occurs during mass production, the entire layer needs to be confirmed after each revision which leads to unnecessary time loss.
In this paper, we suggest a new OPC method to prevent this issue. With this flow, OPCed shapes of unchanged patterns remain the same while only the changed targets are OPCed and replaced into the corresponding location, while the boundaries between those regions are corrected using a model based boundary healing. This method can reduce the overall OPCTAT as well as the time spent in verifying the entire layout after each revision. Details of these results will be described in this paper. After further studies, this flow can also be applied to ILT.
In this paper, we are going to certify that the overlay values extracted from optical measurement cannot represent the circuit level overlay values. We will also demonstrate the possibility to correct misregistration between two layers using the overlay data obtained from the DBM system.
In this paper, we have compared various mark designs with real cell in terms of aberration sensitivity under the specific illumination condition. The specific illumination model was used for aberration sensitivity simulation while varying mask tones and target designs. Then, diffraction based simulation was conducted to analyze the effect of aberration sensitivity on the actual overlay values. The simulation results were confirmed by comparing the OL results obtained by diffraction based metrology with the cell level OL values obtained using Critical Dimension Scanning Electron Microscope.
One of the technologies is NTD(Negative Tone Development) which uses inverse development compared to PTD(Positive Tone Development). The exposed area is eliminated by positive developer in PTD, whereas the exposed area is remained in NTD. It is well known that NTD has better characteristics compared to PTD in terms of DOF(Depth of Focus) margin, MEEF(Mask Error Enhancement Factor), and LER(Line End Roughness) for both small contact holes and isolated spaces [1]. Contact hole patterning is especially more difficult than space patterning because of the lower image contrast and smaller process window [2]. Thus, we have focused on the trend of both NTD and PTD contact hole patterns in various environments. We have analyzed optical performance of both NTD and PTD according to size and pitch by SMO(Source Mask Optimization) software. Moreover, the simulation result of NTD process was compared with the NTD wafer level performance and the process window variation of NTD was characterized through both results. This result will be a good guideline to avoid DoF loss when using NTD process for contact layers with various contact types.
In this paper, we studied the impact of different sources on various combinations of pattern sizes and pitches while estimating DOF trends aside from source and pattern types.
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