Christopher C. Walton

EUV multilayers for solar physics

We present an overview of currently available EUV multilayer coatings that can be used for the construction of solar physics instrumentation utilizing normal-incidence optics. We describe the performance of a variety of Si-based multilayers, including Si/B4C and new Si/SiC films that provide improved performance in the wavelength range from 25 n 35 nm, as well as traditional Si/Mo multilayers, including broad-band coatings recently developed for the Solar-B/EIS instrument. We also outline prospects for operation at both longer and shorter EUV wavelengths, and also the potential of ultra-short-period multilayers that work near normal incidence in the soft X-ray region.

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.

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.

Sub-diffraction-limited multilayer coatings for the 0.3 numerical aperture micro-exposure tool for extreme ultraviolet lithography

Multilayer coating results are discussed for the primary and secondary mirrors of the micro-exposure tool (MET): a 0.30 NA lithographic imaging system with a 200 microm x 600 microm field of view at the wafer plane, operating in the extreme ultraviolet (EUV) region at an illumination wavelength around 13.4 nm. Mo/Si multilayers were deposited by DC-magnetron sputtering on large-area, curved MET camera substrates. A velocity modulation technique was implemented to consistently achieve multilayer thickness profiles with added figure errors below 0.1 nm rms demonstrating sub-diffraction-limited performance, as defined by the classical diffraction limit of Rayleigh (0.25 waves peak to valley) or Marechal (0.07 waves rms). This work is an experimental demonstration of sub-diffraction- limited multilayer coatings for high-NA EUV imaging systems, which resulted in the highest resolution microfield EUV images to date.

Multilayer optics for an extreme-ultraviolet lithography tool with 70-nm resolution

One of the most critical tasks in the development of extreme ultraviolet lithography (EUVL) is the accurate deposition of reflective multilayer coatings for the mirrors comprising the EUVL tool. The second set (Set 2) of four imaging optics for an alpha-class EUVL system has been coated successfully. All four mirrors (M1, M2, M3, M4) were Mo/Si- coated during a single-deposition run with a production- scale DC-magnetron sputtering system. Ideally, the multilayer coatings should not degrade the residual wavefront error of the imaging system design. For the present EUVL camera, this requirement is equivalent to depositing multilayer coatings that would add a figure error of less than 0.11 nm rms. In addition, all mirrors should be matched in centroid wavelength, in order to insure maximum throughput of the EUVL tool. In order to meet these constraints, the multilayer deposition process needs to be controlled to atomic precision. EUV measurements of the coated mirrors determined that the added figure errors due to the multilayer coatings are 0.032 nm rms (M1), 0.037 nm rms (M2), 0.040 nm rms (M3) and 0.015 nm rms (M4), well within the aforementioned requirement of 0.11 nm rms. The average wavelength among the four projection mirrors is 13.352 nm, with an optic-to-optic matching of 1(sigma) =0.010 nm. This outstanding level of wavelength matching produces 99.3% of the throughput of an ideally matched four-mirror system. Peak reflectances are 63.8% (M1), 65.2% (M2), 63.8% (M3) and 66.7% (M4). The variation in reflectance values between the four optics is consistent with their high frequency substrate roughness. It is predicted that the multilayer coatings will not introduce any aberrations in the lithographic system performance, for both static and scanned images of 70 nm - dense features.

Normal-incidence reflectance of optimized W/B 4 C x-ray multilayers in the range 1.4 nm < λ < 2.4 nm

We have fabricated W/B(4)C multilayers having periods in the range d = 0.8-1.2 nm and measured their soft-x-ray performance near normal incidence in the wavelength range 1.4<l<2.4 nm . By adjusting the fractional layer thickness of W we have produced structures having interface widths ó ~ 0.29 nm (i.e., as determined from normal-incidence reflectometry), thus having optimal soft-x-ray performance. We describe our results and discuss their implications, particularly with regard to the development of short-wavelength normal-incidence x-ray optics.

Static microfield printing at the advanced light source with the ETS Set-2 optic

While interferometry is routinely used for the characterization and alignment of lithographic optics, the ultimate performance metric for these optics is printing in photoresist. The comparison of lithographic imaging with that predicted from wavefront performance is also useful for verifying and improving the predictive power of wavefront metrology. To address these issues, static, small-field printing capabilities have been added to the EUV phase- shifting point diffraction interferometry implemented at the Advanced Light Source at Lawrence Berkeley National Laboratory. The combined system remains extremely flexible in that switching between interferometry and imaging modes can be accomplished in approximately two weeks.

Actinic detection of sub-100 nm defects on extreme ultraviolet lithography mask blanks

We present recent experimental results from a prototype actinic (operates at the 13 nm extreme ultraviolet wavelength) defect inspection system for extreme ultraviolet lithography mask blanks. The defect sensitivity of the current actinic inspection system is shown to reach 100 nm in experiments with programmed defects. A method to cross register and cross correlate between the actinic inspection system and a commercial visible-light scattering defect inspection system is also demonstrated. Thus, random, native defects identified using the visible-light tool can reliably be found and scanned by our actinic tool. We found that native defects as small as 86 nm (as classified by the visible-light tool) were detectable by the actinic tool. These results demonstrate the capability of this tool for independent defect counting experiments.

A Kirkpatrick-Baez microscope for the National Ignition Facility

Current pinhole x ray imaging at the National Ignition Facility (NIF) is limited in resolution and signal throughput to the detector for Inertial Confinement Fusion applications, due to the viable range of pinhole sizes (10-25 μm) that can be deployed. A higher resolution and throughput diagnostic is in development using a Kirkpatrick-Baez microscope system (KBM). The system will achieve <9 μm resolution over a 300 μm field of view with a multilayer coating operating at 10.2 keV. Presented here are the first images from the uncoated NIF KBM configuration demonstrating high resolution has been achieved across the full 300 μm field of view.

Atomic-precision multilayer coating of the first set of optics for an extreme-ultraviolet lithography prototype system

We present our results of coating a first set of optical elements for an extreme-ultraviolet (EUV) lithography system. The optics were coated with Mo-Si multilayer mirrors by dc magnetron sputtering and characterized by synchrotron radiation. Near-normal incidence reflectances above 65% were achieved at 13.35 nm. The run-to-run reproducibility of the reflectance peak wavelength was maintained to within 0.4%, and the thickness uniformity (or gradient) was controlled to within +/-0.05% peak to valley, exceeding the prescribed specification. The deposition technique used for this study is an enabling technology for EUV lithography, making it possible to fabricate multilayer-coated optics to accuracies commensurate with atomic dimensions.