At wavelength coherent scatterometry microscope using high-order harmonics for EUV mask inspection

Abstract

In this review, we describe our research on the development of the 13.5 nm coherent microscope using high-order harmonics for the mask inspection of extreme ultraviolet (EUV) lithography. EUV lithography is a game-changing piece of technology for high-volume manufacturing of commercial semiconductors. Many top manufacturers apply EUV technology for fabricating the most critical layers of 7 nm chips. Fabrication and inspection of defect-free masks, however, still remain critical issues in EUV technology. Thus, in our pursuit for a resolution, we have developed the coherent EUV scatterometry microscope (CSM) system with a synchrotron radiation (SR) source to establish the actinic metrology, along with inspection algorithms. The intensity and phase images of patterned EUV masks were reconstructed from diffraction patterns using ptychography algorithms. To expedite the practical application of the CSM, we have also developed a standalone CSM, based on high-order harmonic generation, as an alternative to the SR-CSM. Since the application of a coherent 13.5 nm harmonic enabled the production of a high contrast diffraction pattern, diffraction patterns of sub-100 ns size defects in a 2D periodic pattern mask could be observed. Reconstruction of intensity and phase images from diffraction patterns were also performed for a periodic line-and-space structure, an aperiodic angle edge structure, as well as a cross pattern in an EUV mask.

Robust and reliable actinic ptychographic imaging of highly periodic structures in EUV photomasks

“…Robust and reliable imaging of highly periodic structures remains one of the most challenging problems for ptychography, as seen both in previous publications [5,9,13] and in the results shown above. Special treatments must be applied during the reconstruction process to improve the image quality, for example, modulus enforced probe was used by Gardner et al in [9] and by Kim et al in [13], and TV regularization was necessary to realize the results shown above.…”

“…Secondly, we used a diode laser at slightly shorter wavelength, 405nm, to image a semiconductor sample from imec [12]. This sample is more challenging for two reasons: (1) The sample has low contrast, which leads to low diffraction efficiency into high orders and low SNR; (2) The sample is covered with 2D periodic arrays (a 3.6um pitch) of square pillars (1.8um lateral size), which is challenging for ptychography due to the lack of diversity in diffraction patterns [5,9,13]. The amplitude image of a ptychographic reconstruction is shown in Fig.…”

“…As extreme ultraviolet (EUV) lithography is moving to high volume manufacturing, the fabrication and inspection of defect-free EUV photomasks remains one of the most critical and challenging issues in EUV metrology [1][2][3][4][5][6]. Although some inspection can be done using DUV or electron beams, non-actinic inspection techniques struggle with low spatial resolution and/or exaggeration or distortion of the actual impact of photomask defects on patterned wafers.…”

“…Although researchers have demonstrated ptychographic reconstruction of micron-and submicron-scale features, the quality and reliability of these images still needs validation, and the challenge of ptychographic imaging of highly periodic structures, for example, 1D line-space and 2D contact array structures, remains to be definitively addressed. This challenge is due to the lack of diversity in the diffraction data [5,9,13] used in ptychographic reconstruction. Consequently, a versatile tabletop-scale tool for robust and reliable actinic EUV photomask microscopy has not previously been implemented.…”

High-repetition-rate seeded free-electron laser with direct-amplification of an external coherent laser

“…Highly coherent EUV and soft x-ray laser sources facilitate scientific and industrial researches such as spectroscopy and advanced nano-technologies. As one of these laser sources, HHG has been explored for applications in the nanostructure characterization and transient nanoscale dynamic of materials [26,27]. The limited power of EUV radiation generated by HHG, however, hinders its application.…”