Improved ptychographic inspection of EUV reticles via inclusion of prior information

Abstract

The development of actinic mask metrology tools represents one of the major challenges to be addressed on the roadmap of extreme ultra violet (EUV) lithography. Technological advancements in EUV lithography result in the possibility to print increasingly fine and highly resolved structures on a silicon wafer, however the presence of fine-scale defects, interspersed in the printable mask layout, may lead to defective wafer prints. Hence the development of actinic methods for review of potential defect sites becomes paramount. Here, we report on a ptychographic algorithm that makes use of prior information about the object to be retrieved, generated by means of rigorous computations, to improve the detectability of defects whose dimensions are of the order of the wavelength. The comprehensive study demonstrates that the inclusion of prior information as a regularizer in the ptychographic optimization problem results in a higher reconstruction quality and an improved robustness to noise with respect to the standard ptychographic iterative engine (PIE). We show that the proposed method decreases the number of scan positions necessary to retrieve an high quality image and relaxes requirements in terms of signal to noise ratio (SNR). The results are further compared with the state-of-art total variation based ptychographic imaging.

A priori information in ptychographic image reconstruction for EUV mask metrology

“…The additional TV constraint might help to recover information where DM fails. Furthermore, we will investigate the possibility to use the mask design file as prior knowledge, similarly to Ansuinelli et al 16 The main challenge for a layout based prior is the alignment of the reconstructed image to the mask reference during the iteration procedure.…”

“…As the problem is non-smooth, one cannot directly apply a gradient based optimization technique. 16 ADMM finds the solution by using partial derivatives of the augmented Lagrangian. 17 Minimizing the derivatives for all variables directly gives update functions, that can be summarized to the algorithm presented in Tab.…”

Towards fast ptychography image reconstruction of EUV masks by deep neural networks

“…[3][4][5][6] Ptychography, a computational imaging method that images a sample by processing a series of diffraction patterns obtained by illuminating the object at overlapping probe positions, is a promising alternative to conventional actinic mask inspection methods, owing to its robustness, flexibility and relatively simple and lens-free optical design. 7,8 On the other hand, the need for a scanning illumination, the requirement for a large overlap among adjacent probe positions, the need to handle a significant volume of data, and the slow iterative phase retrieval process pose throughput challenges that need to be properly evaluated and addressed.…”

Ultrafast high-harmonic nanoscopy of magnetization dynamics

“…2c) in the holes compared to the film within the circular FOV. Only weak variations in the FOV are observed for single-helicity reconstructions, including fringes near the perimeter from wave-guiding and edge-diffraction effects [46,47]. Some contrast arises from thickness variations and contaminations of the specimen and the substrate.…”