Paul B. Mirkarimi

Multilayer reflective coatings for extreme-ultraviolet lithography

Multilayer mirror coatings which reflect extreme UV (EUV) radiation are a key enabling technology for EUV lithography. So/Si multilayers with reflectances of 67.5 percent at 13.4 nm are now routinely achieved and reflectances of 70.2 percent at 11.4 nm were obtained with Mo/Be multilayers. High reflectance is achieved with careful control of substrate quality, layer thicknesses, multilayer materials, interface quality, and surface termination. Reflectance and film stress were found to be stable relative to the requirements for application to EUV lithography. The run-to- run reproducibility of the reflectance peak position was characterized to be better than 0.2 percent, providing the required wavelength matching among the seven multilayer- coated mirrors used in the present lithography system design. Uniformity of coating was improved to better than 0.5 percent across 150 mm diameter substrates. These improvements in EUV multilayer mirror technology will enable us to meet the stringent specifications for coating the large optical substrates for our next-generation EUV lithography system.

Practical approach for modeling extreme ultraviolet lithography mask defects

An approximate method is proposed to calculate the extreme ultraviolet (EUV) scattering from a defect within a multilayer coating. In this single surface approximation (SSA) the defective multilayer structure is replaced by a single reflecting surface with the shape of the top surface of the multilayer. The range of validity of this approximation has been investigated for Gaussian line defects using two-dimensional finite-difference-time-domain simulations. The SSA is found to be valid for sufficiently low aspect ratio defects such as those expected for the critical defects nucleated by particles on the mask substrate. The critical EUVL defect size is calculated by combining the SSA with a multilayer growth model and aerial image simulations. Another approximate method for calculating the aerial image of an unresolved defect is also discussed. Although the critical substrate defects may be larger than the resolution of higher numerical aperture cameras, the point defect approximation provides a useful fra...

Localized defects in multilayer coatings

We present a non-linear continuum model of the growth of localized defects in multilayer coatings nucleated by particles on the substrate. The model is valid when the deposition and etch fluxes are near normal incidence so that shadowing effects are negligible. Three-dimensional simulations of defects in Mo/Si multilayer films nucleated by arrays of lithographically patterned particles are shown to be in good agreement with experimental measurements. Our results confirm that incorporating ion beam etching in the multilayer deposition process significantly suppresses the defect growth. This has a potentially important application in the fabrication of defect-free masks for extreme ultraviolet lithography.

High-performance Mo-Si multilayer coatings for extreme-ultraviolet lithography by ion-beam deposition

An ion-beam deposition system has been used to fabricate Mo-Si multilayer coatings for masks and imaging optics to be used for extreme-ultraviolet lithography. In addition to high reflectivity and excellent profile control, ion-beam deposition has the capability to smooth rough substrates. For example, we achieved reflectivity of 66.8% on a substrate with 0.39-nm roughness. Smoothing can be further enhanced with a second ion source directed at the multilayer coating. The smoothing capabilities relax the requirement on the finish of the mirror and the mask substrates and could dramatically reduce the cost of these components. Thickness profile control is in the +/-0.01% range, and the figure error added to the mirror substrate by errors in the multilayer thickness is less than 0.1 nm. Peak reflectivities obtained on smooth substrates are 67.5-68.6%.

Advances in multilayer reflective coatings for extreme-ultraviolet lithography

Multilayer mirror coatings which reflect extreme UV (EUV) radiation are a key enabling technology for EUV lithography but must meet stringent requirements in terms of film quality, stability, and thickness control across multi optical elements up to 300 nm in diameter. Deposition technology has been dramatically improved to meet those specifications for thickness control and repeatability over large curved optical substrates. Coating uniformity was improved to +/- 0.055 percent peak-to-valley (P-V) on 140- mm flats and +/- 0.1 percent P-V across 160 mm curved substrates. the run-to-run reproducibility of the reflectance peak wavelength was improved to 0.13 percent on flats to enable fabrication of wavelength-matched sets of optics. Multilayers with reflectances of 67.5 percent at 13.42 nm and 70.2 percent at 11.34 nm are typically achieved for Mo/Si and Mo/Be multilayers, respectively. Also, we have recently achieved a reflectance of 70.1 percent at 13.5 nm for a Mo/Si multilayer deposited with a modified process. The reflectance and stress of these multilayers appear to be stable relative to the requirements for application to EUV lithography. These improvements in EUV multilayer mirror technology enable us to meet the stringent specifications for coating the large optical substrates for our next- generation EUV lithography system. The primary remaining issues are improving the run-to-run wavelength repeatability on curved optics to realize the maximum optical throughput, and verifying long-term stability of the multilayers within the environment of a production EUV lithographic system.

Substrate-Integrated Hollow Waveguides: A New Level of Integration in Mid-Infrared Gas Sensing

A new generation of hollow waveguide (HWG) gas cells of unprecedented compact dimensions facilitating low sample volumes suitable for broad- and narrow-band mid-infrared (MIR; 2.5-20 μm) sensing applications is reported: the substrate-integrated hollow waveguide (iHWG). iHWGs are layered structures providing light guiding channels integrated into a solid-state substrate material, which are competitive if not superior in performance to conventional leaky-mode fiber optic silica HWGs having similar optical pathlengths. In particular, the provided flexibility in device and optical design and the wide variety of manufacturing strategies, substrate materials, access to the optical channel, and optical coating options highlight the advantages of iHWGs in terms of robustness, compactness, and cost-effectiveness. Finally, the unmatched modularity of this novel waveguide approach facilitates tailoring iHWGs to almost any kind of gas sensor technology providing adaptability to the specific demands of a wide range of sensing scenarios. Device fabrication is demonstrated for the example of a yin-yang-shaped gold-coated iHWG fabricated within an aluminum substrate with a footprint of only 75 mm × 50 mm × 12 mm (L × W × H), yet providing a nominal optical absorption path length of more than 22 cm. The analytical utility of this device for advanced MIR gas sensing applications is demonstrated for the gaseous constituents butane, carbon dioxide, cyclopropane, isobutylene, and methane.

Mo/Si and Mo/Be multilayer thin films on Zerodur substrates for extreme-ultraviolet lithography

Multilayer-coated Zerodur optics are expected to play a pivotal role in an extreme-ultraviolet (EUV) lithography tool. Zerodur is a multiphase, multicomponent material that is a much more complicated substrate than commonly used single-crystal Si or fused-silica substrates. We investigate the effect of Zerodur substrates on the performance of high-EUV reflectance Mo/Si and Mo/Be multilayer thin films. For Mo/Si the EUV reflectance had a nearly linear dependence on substrate roughness for roughness values of 0.06-0.36 nm rms, and the FWHM of the reflectance curves (spectral bandwidth) was essentially constant over this range. For Mo/Be the EUV reflectance was observed to decrease more steeply than Mo/Si for roughness values greater than approximately 0.2-0.3 nm. Little difference was observed in the EUV reflectivity of multilayer thin films deposited on different substrates as long as the substrate roughness values were similar.

Sub-70 nm extreme ultraviolet lithography at the Advanced Light Source static microfield exposure station using the engineering test stand set-2 optic

Static microfield printing capabilities have recently been integrated into the extreme ultraviolet interferometer operating at the Advanced Light Source synchrotron radiation facility at Lawrence Berkeley National Laboratory. The static printing capabilities include a fully programmable scanning illumination system enabling the synthesis of arbitrary illumination coherence (pupil fill). This new exposure station has been used to lithographically characterize the static imaging performance of the Engineering Test Stand Set-2 optic. Excellent performance has been demonstrated down to the 70 nm equal line/space level with focus latitude exceeding 1 μm and dose latitude of approximately 10%. Moreover, equal line/space printing down to a resolution of 50 nm has been demonstrated using resolution-enhancing pupil fills.

Advances in the reduction and compensation of film stress in high-reflectance multilayer coatings for extreme-ultraviolet lithography

Due to the stringent surface figure requirements for the multilayer-coated optics in an extreme UV (EUV) projection lithography system, it is desirable to minimize deformation due to the multilayer film stress. However, the stress must be reduced or compensated without reducing EUV reflectivity, since the reflectivity has a strong impact on the throughput of a EUV lithography tool. In this work we identify and evaluate several leading techniques for stress reduction and compensation as applied to Mo/Si and Mo/Be multilayer films. The measured film stress for Mo/Si films with EUV reflectances near 67.4 percent nm is approximately -420 MPa, while it is approximately +330 MPa for Mo/Be films with EUV reflectances near 69.4 percent at 11.4 nm. Varying the Mo-to-Si ratio can be used to reduce the stress to near zero levels, but at a large loss in EUV reflectance. The technique of varying the base pressure yielded a 10 percent decrease in stress with a 2 percent decrease in reflectance for our multilayers. Post-deposition annealing was performed and it was observed that while the cost in reflectance is relatively high to bring the stress to near zero levels, the stress can be reduced by 75 percent with only a 1.3 percent drop in reflectivity at annealing temperatures near 200 degrees C. A study of annealing during Mo/Si deposition was also performed; however, no practical advantage was observed by heating during deposition. A new non-thermal buffer-layer technique was developed to compensate for the effects of stress. Using this technique with amorphous silicon and Mo/Be buffer-layers it was possible to obtain Mo/Be and Mo/Si multilayer films with near zero net film stress and less than a 1 percent loss in reflectivity. For example a Mo/Be film with 68.7 percent reflectivity at 11.4 nm and a Mo/Si film with 66.5 percent reflectivity at 13.3 nm were produced with net stress values less than 30 MPa.

An ion-assisted Mo-Si deposition process for planarizing reticle substrates for extreme ultraviolet lithography

Substrate particles are a serious concern in the fabrication of reticles for extreme ultraviolet lithography (EUVL) because they nucleate defects in the reflective multilayer films that can print in the lithographic image. We have developed a strategy for planarizing reticle substrates with smoothing-layers and, in this letter, we investigate the smoothing properties of an ion-assisted Mo-Si deposition process. We have observed that ion-assistance can significantly improve the particle-smoothing properties of Mo-Si multilayer films and can do so without a significant increase in the high-spatial frequency roughness of the multilayer film. An ion-assisted Mo-Si smoothing-layer approach to reticle substrate planarization, therefore, shows significant promise for defect mitigation in EUVL reticles.