Lateral shearing interferometry for high-NA EUV wavefront metrology

We present a lateral shearing interferometer suitable for high-NA EUV wavefront metrology. In this interferometer, a geometric model is used to accurately characterize and predict systematic errors that come from performing interferometry at high NA. This interferometer is compatible with various optical geometries, including systems where the image plane is tilted with respect to the optical axis, as in the Berkeley MET5. Simulation results show that the systematic errors in tilted geometries can be reduced by aligning the shearing interferometer grating and detector parallel to the image plane. Subsequent residual errors can be removed by linear fitting

Studying resist stochastics with the multivariate poisson propagation model

Progress in the ultimate performance of extreme ultraviolet resist has arguably decelerated in recent years suggesting an approach to stochastic limits both in photon counts and material parameters. Here we report on the performance of a variety of leading extreme ultraviolet resist both with and without chemical amplification. The measured performance is compared to stochastic modeling results using the Multivariate Poisson Propagation Model. The results show that the best materials are indeed nearing modeled performance limits

Upgrade to the SHARP EUV mask microscope

The Sharp High-NA Actinic Reticle review Project (SHARP) is a synchrotron-based, extreme ultraviolet (EUV) microscope dedicated to photomask research. A potential upgrade to the SHARP microscope is presented. The upgrade includes changing the light path in the instrument from its current off-Axis configuration to an on-Axis configuration. This change allows for an increased working distance of 2.5 mm or more. A central obscuration, added to the zoneplate aperture, blocks stray light from reaching the central part of the image, thus improving the image contrast. The imaging performance of the two configurations is evaluated by means of ray tracing

Implications of image plane line-edge roughness requirements on extreme ultraviolet mask specifications

Line-edge roughness (LER) and the related effect of contact size variation remain as significant challenges facing the commercialization of extreme ultraviolet (EUV) lithography. LER is typically viewed as a resist problem; however, recent simulation results have shown that the mask can indeed be an important contributor. Problems arise from both mask absorber LER as well as mask multilayer roughness leading to random phase variations in the reflected beam (see Fig. 1). The latter effect is especially important as higher coherence off-axis illumination conditions are used and defocus is considered. Here we describe these effect in detail and explore how they will impact EUV mask requirements for the 22-nm half-pitch node and beyond. Figure 2 shows modeling results for 22-nm lines printed in a 0.32-numerical aperture system with 100-nm defocus assuming a mask with 0.24-nm rms multilayer roughness and no absorber edge roughness (unlike the example in Fig. 1). The impact of the phase roughness on the printed line-edge roughness is clearly evident and demonstrates the basic problem with mask roughness. The more detailed modeling-based analysis to be presented will account for performance throughout the process window as well as non-stochastic resist effects. We note that the mean-field resist effect is important to consider because, in practice, the resist is the limiting resolution element in the system and therefore dominates the mask-error enhancement factor (MEEF). As is typically the case with projection-optic-induced MEEF, the resist-induced MEEF will lead to even tighter mask requirements. Note that we do not consider resist stochastic effects since the purpose of this study is isolate mask-induced sources of image-plane roughness

Multilayer Phase-Only Diffraction Gratings: Fabrication andApplication to EUV Optics

The use of phase-only diffractive devices has long played an important role in advanced optical systems in varying fields. Such devices include gratings, diffractive and holographic optical elements, diffractive lenses, and phase-shift masks for advanced lithography. Extending such devices to the increasingly important regime of extreme ultraviolet (EUV) wavelengths, however, is not trivial. Here, we present an effective fabrication and etch process enabling high-resolution patterning of Mo/Si multilayers for use in EUV phase devices, providing another method for fabrication of high numerical aperture diffractive devices or high-resolution EUV phase shift masks

Spatial scaling metrics of mask-induced induced line-edge roughness

Mask contributors to line-edge roughness (LER) have recently been shown to be an issue of concern for both the accuracy of current resist evaluation tests as well the ultimate LER requirements for the 22-nm production node. Problems arise from mask absorber LER as well as mask reflector or surface roughness leading to random phase variations in the reflected beam. Not only do these mask contributors effect to total measured LER in resist, but they also effect LER spatial characteristic such as the LER power spectral density and related descriptors of correlation length and roughness exponent. Noting that these characteristics are important in the understanding of LER, it is crucial to understand how mask effects impact these parameters. Moreover, understanding how these metrics respond to mask effects may lead to an experimental mechanism for experimentally evaluating the importance of mask contributors to LER. Here we use computer modeling to study the LER spatial metrics arising from mask effects. We further describe the effects of illumination conditions and defocus on the metrics and compare the results to those expected from intrinsic resist LER

Achieving diffraction-limited performance on the Berkeley MET5

The Berkeley MET5, funded by EUREKA, is a 0.5-NA EUV projection lithography tool located at the Advanced Light Source at Berkeley National Lab. Wavefront measurements of the MET5 optic have been performed using a custom in-situ lateral shearing interferometer suitable for high-NA interferometry. In this paper, we report on the most recent characterization of the MET5 optic demonstrating an RMS wavefront 0.31 nm, and discuss the specialized mask patterns, gratings, and illumination geometries that were employed to accommodate the many challenges associated with high-NA EUV interferometry

Mask roughness induced LER control and mitigation: aberrations sensitivity study and alternate illumination scheme

Here we conduct a mask-roughness-induced line-edge-roughness (LER) aberrations sensitivity study both as a random distribution amongst the first 16 Fringe Zernikes (for overall aberration levels of 0.25, 0.50, and 0.75nm rms) as well as an individual aberrations sensitivity matrix over the first 37 Fringe Zernikes. Full 2D aerial image modeling for an imaging system with NA = 0.32 was done for both the 22-nm and 16-nm half-pitch nodes on a rough mask with a replicated surface roughness (RSR) of 100 pm and a correlation length of 32 nm at the nominal extreme-ultraviolet lithography (EUVL) wavelength of 13.5nm. As the ideal RSR value for commercialization of EUVL is 50 pm and under, and furthermore as has been shown elsewhere, a correlation length of 32 nm of roughness on the mask sits on the peak LER value for an NA = 0.32 imaging optic, these mask roughness values and consequently the aberration sensitivity study presented here, represent a worst-case scenario. The illumination conditions were chosen based on the possible candidates for the 22-nm and 16-nm half-pitch nodes, respectively. In the 22-nm case, a disk illumination setting of {sigma} = 0.50 was used, and for the 16-nm case, crosspole illumination with {sigma} = 0.10 at an optimum offset of dx = 0 and dy = .67 in sigma space. In examining how to mitigate mask roughness induced LER, we considered an alternate illumination scheme whereby a traditional dipole's angular spectrum is extended in the direction parallel to the line-and-space mask absorber pattern to represent a 'strip'. While this illumination surprisingly provides minimal improvement to the LER as compared to several alternate illumination schemes, the overall imaging quality in terms of image-log-slope (ILS) and contrast is improved