A Novel Objective for EUV Microscopy and EUV Lithography Allowing High-Throughput Imaging

Background

Extreme ultraviolet (EUV) and x-ray imaging technologies are foundational to advanced fields such as semiconductor lithography, microscopy, and spectroscopy. In EUV lithography, for example, the ability to print ever-smaller features on integrated circuits is directly tied to the performance of optical systems operating at wavelengths near 13.5 nm. However, EUV and x-ray photons are strongly absorbed by most materials, necessitating the use of reflective rather than refractive optics, typically in vacuum environments. Achieving high-resolution, high-throughput imaging in these regimes requires optical elements that can efficiently reflect and focus light while minimizing image distortions. The demand for improved imaging systems is driven by the need for greater precision, higher yield, and faster processing in semiconductor manufacturing, as well as by the pursuit of more detailed and accurate scientific measurements in microscopy and diagnostics.

Current approaches, such as the widely used Schwarzschild objective, employ pairs of concentric spherical mirrors to form images. While these systems can correct certain aberrations for rays close to the optical axis (paraxial rays), they are not well-suited for the non-paraxial, wide-angle rays typical in EUV lithography and x-ray imaging. The Bragg condition, which governs efficient multilayer or crystal reflection at these wavelengths, can only be satisfied locally on conventional mirror surfaces, severely limiting the usable reflective area and thus photon throughput. Attempts to expand the Bragg-reflecting area—such as lateral grading of multilayer spacing—introduce new aberrations by breaking the system’s symmetry. Adding more optical elements to correct these distortions reduces throughput and increases complexity, exposure times, and risk of damage to sensitive sources. Additionally, some prior solutions are impractical due to their size or fixed configurations, restricting their utility in real-world lithography or imaging setups. As a result, existing technologies face a trade-off between image quality, efficiency, and practicality, highlighting the need for a fundamentally improved approach.

Technology

The described technology is an advanced imaging apparatus designed for extreme ultraviolet (EUV) and x-ray applications, such as EUV lithography, microscopy, and spectroscopy. It features at least one pair of concentric spherical reflectors—a convex and a concave mirror—sharing a common center. These reflectors are engineered to simultaneously satisfy the Bragg condition across their entire surfaces, using multilayer coatings for EUV or spherically bent crystals for x-rays. A unique aspect is the ability to rotate the entire ray pattern about the axis of symmetry that passes through the object, image, and common center, enabling continuous adjustment of Bragg angles and variable de-magnifications. This rotational degree of freedom allows optimization for photon throughput and minimization of aberrations, while maintaining stigmatic (distortion-free) imaging. The apparatus adheres to the mathematical condition ( R_1 \cos(\theta_1) = R_2 \cos(\theta_2) = R_T ), ensuring precise alignment and imaging performance. The design supports full-sphere or annular reflectors, accommodates apertures for ray selection, and is adaptable for single-exposure imaging of entire masks, crucial for high-throughput semiconductor manufacturing.

What differentiates this technology is its ability to overcome the fundamental limitations of conventional Schwarzschild objectives and prior EUV/x-ray imaging systems. Traditional designs suffer from restricted usable reflective area, image distortions (such as astigmatism and coma), and require scanning exposures due to only local satisfaction of the Bragg condition. By enabling the simultaneous, global fulfillment of the Bragg condition and introducing a rotational degree of freedom, this apparatus allows for a continuum of Bragg angle pairs and magnifications, greatly enhancing design flexibility. This leads to significantly larger effective reflective areas, higher photon throughput, and the elimination of image aberrations even for non-paraxial rays. The apparatus supports isotropic magnification, enabling true-to-scale imaging essential for advanced lithography, and can be integrated into existing systems with minimal modifications. Analytical methods and computational tools further facilitate precise configuration for specific applications, making this solution a major advancement in EUV and x-ray optics for both scientific and industrial use.

Benefits

• Simultaneous fulfillment of the Bragg condition across entire spherical reflector surfaces enables larger usable reflective areas and higher photon throughput.
• Rotational degree of freedom of the ray pattern allows continuous adjustment of Bragg angles and magnifications, optimizing performance for different applications.
• Elimination of image distortions such as astigmatism, coma, and spherical aberrations even for non-normal incidence rays ensures high-quality stigmatic imaging.
• Supports single-exposure imaging of entire masks without scanning, increasing throughput and reducing complexity in EUV lithography.
• Isotropic magnification provides true-to-scale images critical for semiconductor manufacturing.
• Compatible with existing EUV lithography systems, requiring only replacement of objectives without fundamental technology changes.
• Applicable to a wide range of wavelengths, including EUV (10–15 nm) and x-rays, enabling use in microscopy, lithography, spectroscopy, and plasma diagnostics.
• Design flexibility through adjustable parameters (rotation angle, radii ratio, Bragg angles) allows optimization to maximize reflectivity and minimize aberrations.

Applications

• EUV lithography for semiconductor manufacturing
• EUV microscopy for biological imaging
• X-ray imaging for plasma diagnostics
• EUV spectroscopy instrumentation

Additional info

This optical system employs concentric spherical reflectors (convex and concave) sharing a common center, configured to simultaneously satisfy the Bragg condition for EUV/x-rays. A unique rotational freedom of the ray pattern about the symmetry axis permits continuous adjustment of Bragg angles and de-magnifications. This optimizes photon throughput, minimizes aberrations, and ensures stigmatic, distortion-free imaging with larger reflective areas for single-exposure applications.