Characterization and mitigation of 3D mask effects in extreme ultraviolet lithography

Abstract

Abstract:The reflection and diffraction of extreme ultraviolet (EUV) light from lithographic masks and the projection imaging of these masks by all-reflective systems introduce several significant imaging artifacts. The off-axis illumination of the mask causes asymmetric shadowing, a size bias between features with different orientations and telecentricity errors. The image contrast varies with the feature orientation and can easily drop far below intuitively expected values. The deformation of the wavefront or phase of the incident light by thick absorbers generates aberration-like effects, especially variations of the best-focus (BF) position vs. the pitch and size of the imaged patterns. Partial reflection of light from the top of the absorber generates a weak secondary image, which superposes with the main image. Based on a discussion of the root causes of these phenomena, we employ mask diffraction and imaging analysis for a quantitative analysis of these effects for standard EUV masks. Simulations for various non-standard types of mask stacks (e.g. etched multilayers, buried shifters, etc.) and for various non-standard absorber materials are used to explore the imaging capabilities of alternative masks for EUV lithography. Finally, an outlook at anamorphic systems for larger numerical apertures is given.

Ni-Al Alloys as Alternative EUV Mask Absorber

“…The imaginary part κ or extinction coefficient determines the attenuation, while the real part n or refraction coefficient determines the phase velocity. To reduce mask absorber height-dependent M3D effects, a material that absorbs more EUV and has higher κ than Ta is necessary [2]. Best focus shifts through pitch are caused by phase distortion due to a mismatch in refraction coefficient n at the vacuum and absorber interface, hence a material with n close to unity is preferred [13].…”

“…The combination of EUV light at oblique incidence, absorber thickness, and non-uniform mirror reflectance through incidence angle, creates photomask-induced imaging aberrations, known as mask 3D (M3D) effects [2]. These effects are experimentally observable, as feature orientation-dependent shadowing effects [3], best focus variation through pitch [4], feature size-dependent pattern shift through focus [5,6], and pattern asymmetry and contrast loss [7].…”

“…These effects are experimentally observable, as feature orientation-dependent shadowing effects [3], best focus variation through pitch [4], feature size-dependent pattern shift through focus [5,6], and pattern asymmetry and contrast loss [7]. Selecting the correct mask absorber material and thickness helps in reducing M3D effects [2,8,9]. This approach is complementary to other M3D mitigation strategies, such as source optimization [10], sub-resolution assist feature placement [11], and transition to anamorphic high-NA EUV lithography [12].…”

“…More Al leads to smaller phase deformation as the refractive coefficient n is closer to unity. The thicker Ni x Al y absorbers have smaller best focus range, as the impact of phase deformation on best focus shift is reduced for darker absorbers with lower reflectance [2]. Figure 14d illustrates the range of pattern shifts through focus for pitches from 32 nm to 100 nm, expressed in milliradians (mrad).…”

Attenuated phase shift mask for extreme ultraviolet: can they mitigate three-dimensional mask effects?

“…The understanding, characterization, and mitigation of three-dimensional (3-D) mask effects including telecentricity errors (TCEs), contrast fading, and best focus (BF) shifts become increasingly important for the performance optimization of future extreme ultraviolet (EUV) projection systems and mask designs. [1][2][3][4] One approach to mitigate the 3-D mask effects is to employ alternative absorber materials. Figure 1(a) shows the possible material options versus the range of refractive indices n and extinction coefficients for a wavelength of 13.5 nm.…”