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We propose here an alternative path to investigate and discriminate the root causes of LWR using only wafer data. It is based on Local Critical Dimension Uniformity (LCDU) decomposition [2], a methodology used to identify and quantify the individual LCDU contributors. The decomposition approach requires a smart sampling of the wafer print, in which an array of contact hole is measured in different dies multiple times. For such an approach to be successful, it is critical to ensure that the measurement locations are individually identified. Hence, it is necessary to anchor the metrology to a reference feature. A linear nested model [3] is then used to quantify the three main variability components (mask, shot noise, and metrology). This approach allows to sample thousands of features at mask, a task that would not be practically achievable through direct mask measurements.
In this work, LWR decomposition is implemented for the first time. To this aim, 18nm lines at 36nm pitch, printed by EUV lithography, were used. We specifically worked with a pattern including programmed defects, used as anchoring features for the metrology. In order to limit the impact of the metrology noise, expected to be higher for lines as compared to CH, we sampled over 8000 anchored measurements per image (in the CH case, only 81 measurements per image were needed). The LWR decomposition results indicated the dominance of the metrology noise, as expected. In addition, the mask contribution was observed to be less relevant that the shot noise.
To verify the accuracy of the LWR decomposition results, Power Spectral Density (PSD) analysis on wafer and mask SEM images was used. The metrology noise contribution was removed at both mask and wafer level using an un-biasing normalization of the PSD curves [4]. The comparison with the PSD analysis confirmed the feasibility of LWR decomposition, opening the way to a more effective diagnostic technique for roughness and stochastics.
This paper provides a thorough experimental assessment of the implementation of vote-taking, and discusses its pro’s and con’s. Based on N=4 vote-taking, we demonstrate the capability to mitigate different types of mask defects. Additionally, we found that blending different mask images brings clear benefit to the imaging, and provide experimental confirmation of improved local CDU and intra-field CDU, reduction of stochastic failures, improved overlay, ... Finally, we perform dedicated throughput calculations based on the qualification performance of ASML’s NXE:3400B scanner.
This work must be seen in the light of an open-minded search for options to optimally enable and implement EUV lithography. While defect-free masks and EUV pellicles are without argument essential for most of the applications, we investigate whether some applications could benefit from vote-taking.
This is important for imaging at the edges of an image field when fields are printed very close to each other on the wafer (so-called butted fields, with zero field to field spacing). DUV light is reflected from the reticle black border (BB) into a neighboring exposure field on the wafer. This results in a CD change at the edges and in the corners of the fields and therefore has an impact on CD uniformity. Experimental CDU results are shown for 16 nm dense lines (DL) and 20 nm isolated spaces (IS) (N7 logic design features) in the fields exposed at 0 mm and 0.5mm distance on the wafer. Areas close to the edge of the image field are important for customer applications as they often contain qualification and monitoring structures; in addition, limited imaging capabilities in this area may result in loss of usable wafer space.
In order to understand and control OOB DUV light, it must be measured in the scanner. DUV measurements are performed in resist using a special OOB reticle coated with Aluminum (Al) having low EUV reflectance and high DUV reflectance. A model for DUV light impact on the imaging is proposed and verified. For this, DUV reflectance data is collected in the wavelengths range 100-400 nm for Al and BB and the ratio of reflectances of these materials is determined for assumed scanner and resist OOB spectra. Also direct BB OOB test is performed on the wafer and compared to Al OOB results. The sensitivity of 16 nm DL and 20 nm IS to OOB light is experimentally determined by means of double exposure test: a wafer with exposed imaging structures undergoes a second flood exposure from a DUV reflective material (Al or BB).
Finally, several OOB mitigation strategies are discussed, in particular, suppression of DUV light in the scanner (~3x improvement), recent successes of DUV suppression for 16 nm imaging resist (~1.8x improvement) and DUV reflectance mitigation in the reticle black border (~3.8x). An overview of OOB test results for multiple NXE systems will be shown including systems with new NXE:3350 optics with improved OOB suppression.
Black border, mask 3D effects: covering challenges of EUV mask architecture for 22nm node and beyond
Source power is the major challenge to overcome in order to achieve cost-effectiveness in EUV and enable introduction into High Volume Manufacturing. With the development of the MOPA+prepulse operation of the source, steps in power have been made, and with automated control the sources have been prepared to be used in a preproduction fab environment.
Flexible pupil formation is under development for the NXE:3300B which will extend the usage of the system in HVM, and the resolution for the full system performance can be extended to 16nm. Further improvements in defectivity performance have been made, while in parallel full-scale pellicles are being developed.
In this paper we will discuss the current NXE:3300B performance, its future enhancements and the recent progress in EUV source performance.
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