EUV ptychography with high-order harmonic generation source for actinic evaluation of EUV materials

Abstract

With technological advances over the decades extreme ultraviolet (EUV) lithography has been introduced into high-volume manufacturing (HVM) at the 7 nm and 5 nm technology nodes and is expected to be extended at 3 nm and beyond. Behind a successful introduction of EUV lithography, the need for actinic mask metrology and inspection still remains a critical challenge. Actinic (at-wavelength) solutions that satisfy not only sensitivity and resolution but also a small footprint and low cost of ownership are required. Even though non-actinic techniques using deep ultraviolet (DUV) light or electron beam can be used to identify defects on the EUV mask surface, they cannot predict the defect printability. Besides, phase defects inside Mo/Si multilayer (ML) are undetectable due to the ML structure optimized for EUV wavelength. Therefore, actinic techniques are essential for accurate defect review and printability prediction of defects. EUV pellicles are no exception to this. The pellicle’s optical properties are critical parameters for insertion in HVM and must be evaluated using actinic techniques since the poor optical properties of the pellicle leads to the throughput loss and the degradation of the imaging performance of the EUV mask. This thesis presents actinic technology that combines a coherent tabletop EUV source with a lensless imaging method for EUV mask and pellicle evaluation. An EUV ptychography microscope, based on a high-order harmonic generation (HHG) source and a coherent diffraction imaging (CDI) method, has advantages of compactness, cost-effectiveness, and monochromaticity. Ptychography, a multi-shot CDI, has been widely studied in materials science and biology research using X-rays because it is difficult to fabricate imaging lenses with a high numerical aperture. Likewise, this can supersede complex and expensive optics in the EUV system resulting in low-cost and aberration-free imaging. In ptychography, diffraction patterns recorded by the CCD detector are reconstructed into the original object image through a computational process based on Fourier and inverse Fourier transform. The phase retrieval process is necessary since the ptychographic data set has only intensity information of the object without phase information. The ptychographic iterative engine (PIE) and extended PIE (ePIE) are widely used algorithms for the phase retrieval process. The resolution of the reconstructed image is strongly dependent on the ability of the algorithm to compensate for the inevitable noise of the imaging probe and object. Therefore, in this thesis, function enhancement methods for high-fidelity mask imaging were studied. The improved ptychographic algorithm compensated for the deviation of probe information and minimized crosstalk artifacts occurring in periodic patterns by enforcing the probe information. By imaging the EUV mask with periodic line and space patterns using the EUV ptychography microscope, the feasibility of actinic mask metrology and inspection technology, as well as the ability of the amplitude and phase recovery for developing an advanced phase shift mask, were demonstrated. The optical characteristics of the EUV pellicle were also investigated using the EUV ptychography microscope. Fundamentally, the EUV pellicle placed at a distance of 2.5 mm from the mask defocuses the particles on the pellicle surface so that the particle image cannot be printed on the wafer. However, larger particles than a critical size can act as killing defects on the final wafer pattern. Thus, a defect printability study was conducted via through-pellicle imaging and showed the impact of the contaminated pellicle on the mask imaging. Furthermore, EUV reflectivity (EUVR), one of the stringent requirements of the EUV pellicle, must be maintained below 0.04% to prevent the critical dimension (CD) variation owing to overexposure of the intrafield corner and edge. However, the EUV pellicle designed with a thin membrane is prone to deformation (e.g. deflection or the formation of wrinkles) during the fabrication process or the exposure process, and such pellicle deformation could alter the EUVR of the pellicle. The effect of pellicle wrinkles on EUVR and local CD was demonstrated using the EUV ptychography microscope. It was confirmed that pellicle wrinkles could locally increase the EUVR, resulting in the degradation of the mask imaging performance. In addition to the actinic metrology and inspection of the EUV mask and pellicle, the EUV ptychography microscope can provide practical information for the development of next-generation EUV materials.