Design method for an off-axis reflective anamorphic optical system with aberration balance and constraint control.

This paper proposes a design method for an off-axis reflective anamorphic optical system (ORAOS). This method first applies vector aberration theory to establish a mathematical model to balance the aberration of an ORAOS. It then builds the error function of structural parameters and constraints through spatial ray tracing and grouping design. Next, it introduces automatic adjustment of weight factors for dynamic balance of aberrations and constraints. A particle swarm simulated annealing algorithm is used to iteratively calculate the initial structure of the ORAOS. Finally, we use an extreme ultraviolet (EUV) lithographic projection objective with off-axis six-reflective anamorphic mirrors (${\beta _x} = 1/4,{\beta _y} = 1/8$) as an example to verify the effectiveness of this method. We obtain an EUV lithographic anamorphic objective with a numerical aperture of 0.55 and a root mean square wavefront error better than $1/30\lambda$ (${\hat{\text I}}\gg\, = 13.5\;{\rm nm}$).

Design method for off-axis aspheric reflective optical system with extremely low aberration and large field of view.

This paper proposes a grouping design method based on a combination of spatial ray tracing and aberration correction to construct the initial structure for an off-axis multi-reflective aspheric optical system. This method establishes a mathematical parameter model of the optical system to satisfy the aberration balance and multi-constraint control requirements. The simulated annealing particle swarm algorithm is applied to calculate the initial optical system structure. Finally, an extreme ultraviolet (EUV) lithography projection objective with six-reflective aspheric mirrors is used as an example to verify the reliability and effectiveness of this method. A 0.33 numerical aperture EUV lithographic objective with wavefront error better than 1/70λ(λ=13.5nm) root mean square (RMS) is obtained.

Mo/Si multilayers sputtered onto inclined substrates: experiments and simulations.

We performed experiments involving the fabrication of Mo/Si multilayer coatings and established a model of the deposition process. The surface and interface roughness, surface power spectral density, layer structures, and coating reflectivity were characterized for different substrate inclination angles. The surface and interface roughness increase and the coating reflectivity decreases with an increase in the substrate inclination angle, especially for large angles (50-70°). The model was applied to explain this phenomenon, and a proposal to reduce the interfacial roughness caused by substrate inclination angles is presented.

Extended shift-rotation method for absolute interferometric testing of a spherical surface with pixel-level spatial resolution.

An improved shift-rotation method for the absolute testing of spherical surfaces is developed to obtain pixel-level spatial resolution and a low noise propagation ratio. The absolute testing process includes multiple rotational tests and two lateral shifting tests with large shifts. A wavefront reconstruction algorithm based on subaperture division and least squares fitting is proposed to reconstruct the surface figure of the test optics. Numerical simulation results show that the method reveals high-frequency figures missed in the traditional Zernike-based shift-rotation method. The algorithm error is lower than 0.4%, and the noise propagation ratio can be reduced by 70% using large shifts. The absolute testing of spherical optics is carried out to verify this method. One spherical surface was tested with the presented absolute testing method and the method using a point diffraction interferometer. The difference of the measurement results based on the two methods showed that the testing uncertainty reached 0.19 nm root mean square (RMS), which indicated that the presented method has potential subnanometer testing uncertainty.

Low-stress and high-reflectance Mo/Si multilayers for extreme ultraviolet lithography by magnetron sputtering deposition with bias assistance.

To explore the potential of achieving low-stress and high-reflectance Mo/Si multilayers deposited by conventional magnetron sputtering with bias assistance, we investigated the effects of varying Ar gas pressure, substrate bias voltage, and a bias-assisted Si ratio on the stress and extreme ultraviolet (EUV) reflectance of Mo/Si multilayers. To reduce the damage of ion bombardments on an Si-on-Mo interface, only the final part of the Si layer was deposited with bias assistance. Bias voltage has strong influence on the stress. The compressive stress of Mo/Si multilayers can be reduced remarkably by increasing bias voltage due to the increase of Mo-on-Si interdiffusion and postponement of Mo crystallization transition. Properly choosing gas pressure and a bias-assisted Si ratio is critical to obtain high EUV reflectance. Appropriately decreasing gas pressure can reduce the interface roughness without increasing interdiffusion. Too much bias assistance can seriously reduce the optical contrast between Mo and Si layers and lead to a remarkable decrease of EUV reflectance. Thus, by appropriately choosing gas pressure, bias voltage, and a bias-assisted Si ratio, the stress values of Mo/Si multilayers can be reduced to the order of -100 MPa with an EUV reflectance loss of about 1%.

Control of lateral thickness gradients of Mo-Si multilayer on curved substrates using genetic algorithm.

An inversion method based on a genetic algorithm has been developed to control the lateral thickness gradients of a Mo-Si multilayer deposited on curved substrates by planar magnetron sputtering. At first, the sputtering distribution of the target is inversed from coating thickness profiles of flat substrates at different heights. Then, the speed profiles of substrates sweeping across the target are optimized according to the desired coating thickness profiles of the primary and secondary mirrors in a two-bounce projection system. The measured coating thickness profiles show that the non-compensable added figure error is below 0.1 nm rms, and the wavelength uniformity across each mirror surface is within ±0.2% P-V. The inversion method introduced here exhibits its convenience in obtaining the sputtering distribution of the target and efficiency in coating iterations during process development.

Influence of substrate temperatures on the properties of GdF(3) thin films with quarter-wave thickness in the ultraviolet region.

High-quality coatings of fluoride materials are in extraordinary demand for use in deep ultraviolet (DUV) lithography. Gadolinium fluoride (GdF3) thin films were prepared by a thermal boat evaporation process at different substrate temperatures. GdF3 thin film was set at quarter-wave thickness (∼27 nm) with regard to their common use in DUV/vacuum ultraviolet optical stacks; these thin films may significantly differ in nanostructural properties at corresponding depositing temperatures, which would crucially influence the performance of the multilayers. The measurement and analysis of optical, structural, and mechanical properties of GdF3 thin films have been performed in a comprehensive characterization cycle. It was found that depositing GdF3 thin films at relative higher temperature would form a rather dense, smooth, homogeneous structure within this film thickness scale.

Optimal design and fabrication method for antireflection coatings for P-polarized 193 nm laser beam at large angles of incidence (68°-74°).

Most of the optical axes in modern systems are bent for optomechanical considerations. Antireflection (AR) coatings for polarized light at oblique incidence are widely used in optical surfaces like prisms or multiform lenses to suppress undesirable reflections. The optimal design and fabrication method for AR coatings with large-angle range (68°-74°) for a P-polarized 193 nm laser beam is discussed in detail. Experimental results showed that after coating, the reflection loss of a P-polarized laser beam at large angles of incidence on the optical surfaces is reduced dramatically, which could greatly improve the output efficiency of the optical components in the deep ultraviolet vacuum range.

Broadband multilayer-coated normal incidence blazed grating with approximately 10% diffraction efficiency through the 13-16 nm wavelength region.

Diffraction gratings used in extreme UV are typically coated with periodic multilayer thin films. These coatings have a small bandwidth, thus leading to a narrow usable spectral region of multilayer gratings. Well-designed aperiodic multilayer coatings could provide high reflectivity over a much broader wavelength region, so they could broaden the usable spectral region of multilayer gratings. We designed and deposited an aperiodic Mo/Si multilayer coating onto a blazed grating substrate. At an incidence angle of 10 degrees, the -2nd-order diffraction efficiency of the multilayer grating is approximately 10% through the wavelength range of 13-16 nm.

High-efficiency multilayer-coated ion-beam-etched blazed grating in the extreme-ultraviolet wavelength region.

We describe a simple method to fabricate blazed gratings used in the extreme ultraviolet wavelength region. The method uses an argon and oxygen mixed-gas ion beam to directly etch the grating substrate through a rectangular profile photoresist grating mask. With this method the etched grating groove profile can be well controlled. An Mo/Si multilayer-coated specimen with a blaze angle of 1.9 degrees was fabricated and measured. At an incident angle of 10 degrees and a wavelength of 13.62 nm, the diffraction efficiency of the negative second order reaches 36.2%.