A critical component of high-performance EUV lithography source optics is the reflecting multilayer coating. The ideal multilayer will have both high reflectance and high stability to thermal load. Additionally the capping layers must provide resistance to degradations from exposure to an EUV source, and also be compatible with, or enhance, the systems used for cleaning an exposed multilayer coating. We will report on the results of development of C and B4C stabilized Mo/Si multilayers used to increase the as-deposited peak reflectivity (Rp) as well as decreasing the loss of peak reflectivity (Rp) as a function of annealing temperature. Previous results demonstrate that these layers prevent loss of Rp for temperatures up to 600º C. Results on the use of reactively-sputtered oxide capping layers such as SiO2 and ZrO2 will be presented as well, along with results of exposure testing. The deposition is performed in a dual processchamber inline magnetron system, using reactive sputtering for the production of capping layers. The reflectometer and exposure apparatus at the NIST Physics Laboratory is used for evaluation of the performance. Exposure results on the resistance to oxidation in the presence of water vapor will be presented and discussed.
The NIST Extreme Ultraviolet (EUV) Reflectometry Facility was designed in the 1990s to accommodate the largest multilayer optics envisioned at that time. However, with increasing power requirements for an EUV scanner, source collection optics have grown larger and more steeply curved than the original design would allow. To accommodate these changes, the mechanical and operational parameters of the facility have been upgraded. To access the entire surface of a larger optic, an auxiliary off-axis rotation stage has been installed allowing an increase in maximum optic size from 350 mm to 450 mm. Likewise, to deal with the deeper sags and steeper slopes of these optics, we have had to significantly expand our data analysis capabilities. In order to make these measurements, the incident radiation is reflected out of the vertical plane, allowing for measurements of effectively unpolarized radiation, an advantage for EUV lithography optics such as source collectors.
Laser-produced plasma (LPP) sources for extreme ultraviolet lithography (EUVL) systems utilize CO2 lasers operating
with wavelength 10.6μm. Since multilayer-coated optics have high reflectivity for this infrared radiation (IR), a
significant and detrimental amount of IR is passed through the EUVL system. One method to remove the IR from the
system is to utilize a binary diffraction grating. When this grating is applied directly to the surface of the primary
collector optic of the source, the majority of the IR is diverted outside the radius of the exit aperture at the intermediate
focus (IF). This paper will report details on the performance of a full size (410mm diameter) Demonstration Collector
utilizing IR rejection (IRR) technology with the capability to produce over 125X suppression of IR, equaling the
performance of a IR spectral filter. Additional details will be reported on the technology development and use of a
glassy smoothing layer to enable high EUV performance, a weighted average multilayer reflectance of 50.9% for
unpolarized EUV radiation.
The most promising wavelength for the next generation EUV lithography in terms of maximizing throughput of an
optical system was found to be 6.63nm, where highest peak reflectivity is expected at this wavelength using
La(La2O3)/B4C structures. The optical throughput at 6.63nm is expected to be ~6 times lower than at 13.5nm due to the
higher resolution of multilayers at the smaller wavelengths.
La/B4C and La2O3/B4C multilayer structures were coated at RIT by using magnetron sputtering deposition technology.
EUV reflectivity of the multilayers was tested at CXRO and NewSUBARU. The round robin measurements
demonstrated a maximum deviation of 1.9% in the peak reflectivity and 0.0063nm in the peak position. The big
difference in the peak value can be explained by presence of the higher harmonics in the probe beam at NewSUBARU
which affected the accuracy of the measurements. The maximum peak reflectivity of 48.9% was measured from La/B4C
multilayer at 6.68nm. Maximum reflectivity of La2O3/B4C structure at this wavelength was 39.2% while reflectivity at
6.63nm was measured to be 42.68%. The measured band width of the reflectivity curves was about 20% lower than for
ideal structures. La2O3/B4C structure demonstrated a larger level of the imperfections resulting in much lower
performance.
EUV reflectivity of one of the La/B4C multilayers deposited in December 2000 was measured at NewSUBARU in
January 2011 and the results were compared with the measurements performed in January 2001 at CXRO. The
maximum reflectivity dropped from 42.6% to 37.6%. Reduction of the reflectivity band width from 0.044nm to 0.04nm
was also observed.
To perform actinic inspection of patterned EUV reticles with diffraction-limited resolution at 13.5 nm wavelength aspheric optical surfaces with surface figure errors and roughnesses well below 1 nm had to be developed.
The 3D surface topologies of prototype optical components were characterized over spatial periods ranging from the clear apertures down to 25 nanometers over 6 orders of magnitude by using a portfolio of instruments.
3D topography maps were Fourier analyzed and averaged Power Spectral Densities (PSDs) computed over the entire spatial frequency range. A good fit to the PSD was achieved with a linear function on a log-log scale. RMS values were computed over several spatial period ranges.
All optical surfaces were coated with high-reflectivity coatings to maximize optical throughput at 13.5 nm for the average angle-of-incidence of each optic. The spectral reflectivity of the HR coatings, consisting of Molybdenum-Silicon bi-layers (40 periods) were measured using synchrotron instruments at the NIST/DARPA EUV Reflectometry Facility and the Center for X-Ray Optics at Lawrence Berkeley National Laboratory. Total variations (PV) of peak-position within the clear-apertures ranged from 0.005 nm to 0.020 nm, with the one exception being a highly-curved convex surface yielding a PV variation of 0.040 nm. Peak reflectivity variation was typically 0.2% to 1% PV over the clear aperture, with some of the variation being instrument precision. One optic was coated with Ruthenium only, approximately 16nm thick, with less than ±0.1 nm variation in thickness. Detailed information on the spectral reflectivity for the coatings is discussed.
In a joint-development, Rohwedder and Osmic have designed and built a low-defect dual-ion beam reactive-sputtering tool. The tool has been specifically targeted for developing low-defect lithography mask photoblank coatings intended as DUV absorbers and phase-shifting films. The Osmic/Rohwedder collaboration will continue into NGL - the present tool also serves as an R&D platform for EUVL mask blanks. The deposition tool and robotic substrate handler have been integrated and delivered to Osmic in the 2nd quarter of 2003. In this paper, we present initial capability for production of thin-film lithography coatings, including spectrophotometric performance, defect levels and film uniformity. Future reports will share results from more in-depth process development and optimization.
In a joint-development, Rohwedder and Osmic have designed and built a low-defect dual-ion beam reactive-sputtering tool. The tool has been specifically targeted for developing low-defect lithography mask photoblank coatings intended as DUV absorbers and phase-shifting films. The Osmic/Rohwedder collaboration will continue into NGL -- the present tool also serves as an R&D platform for EUVL mask blanks. The deposition tool and robotic substrate handler have been integrated and delivered to Osmic in the 2nd quarter of 2003. In this paper, we present initial capability for production of thin-film lithography coatings, including spectrophotometric performance, defect levels and film uniformity. Future reports will share results from more in-depth process development and optimization.
Molybdenum/Silicon soft x-ray optical coatings for extreme ultraviolet lithography are being developed for both projection optics and masks, and have only recently been produced on a production scale via magnetron sputtering. A number of critical factors must be met for successful development of these coatings for implementation into commercial EUV Lithography. We report on our results for several factors with a state-of-the-art in-line sputtering system. All coatings in a lithography system must match to obtain maximum energy transmission. Hence, process repeatability and characterization of the reflectivity and central wavelength is essential. Run-to-run performance and substrate uniformity is shown to have achieved less than 1% deviation in soft x-ray central wavelength between any two points on any two substrates coated in separate batches; coating uniformity on a given wafer is on the order of 0.3% total deviation. Hard x-ray measurements of d-spacing and reflectivity were correlated to soft x-ray measurements; these correlations were used to improve process control. Furthermore, the coatings must be as defect-free as possible; elimination of aerosol-based particulate generation has allowed improvements by nearly four orders of magnitude. Finally, efforts to understand and control coating stress as a function of processing parameters and post-coating annealing schedules will be reviewed. Results of the effects of deposition method, ion bombardment and interactions between sputter power, sputter pressure and deposition rate are reviewed.
W-B4C multilayers with single d-spacing period of 2.2 nm have been deposited on 330 long by 50 mm wide Si substrates to be used as monochromators for a computed tomography application. Using magnetron sputtering and a substrate masking technique, d-spacing uniformities of +/- 0.86% and +/- 1% were obtained over a 180 mm by 100 mm area for 2.2 nm and 4.2 nm d-spacings respectively. Two separate processes were used to coat the 330 mm long substrate, wherein half of the substrate was coated in each process. A similar process was used to deposit depth graded W-B4C supermirrors on Si and CVD SiC substrates for a beamline pre-mirror application. The 330 mm long by 50 mm wide Si and 300 mm long by 79 mm wide SiC substrates were coated with 20 bi-layer supermirrors with d-spacings ranging from 4.4 nm to 10.8 nm. For an angiography research application laterally graded W-B4C multilayers were deposited on 150 mm by 120 mm silicon substrates. A strong nonlinear d-spacing gradient, from 1.6 nm to 3.8 nm was achieved across the mirror's surface in an attempt to provide uniform intensity over the reflected area. The maximum and minimum d-spacing gradient was 0.06 nm/mm and 0.003 nm/mm, respectively. We measured and mapped the d-spacing gradient using a custom Cu-Ka diffraction system. The measured d-spacings were within +/- 1.5% of the intended d-spacings.
Reflectivity at (lambda) equals 0.154 nm and mechanical stress in the bulk thin films of tungsten and silicon and single d- spacing multilayers on their basis with d approximately equals 2.8 nm deposited by the magnetron sputtering technique on flat thin substrates of Si wafer (approximately 0.2 mm), glass (approximately 0.3 mm), and epoxy gold replicated aluminum foil (approximately 0.3 mm) have been studied. The interfacial roughness of the multilayers has been calculated from the x- ray reflectivity curves as the following: on Si wafer (sigma) approximately equals 0.31 nm, on glass (sigma) approximately equals 0.32 nm, and on foil (sigma) approximately equals 0.34 nm. There was not observed a significant dependence on the stress in the Si film with change in rf power, Ar gas pressure and biasing. For the W films an increase of dc power results in an increase of stress. A similar relationship is also evident for W films deposited by rf power, but this dependence is less pronounced. The influence of low temperature (up to 200 degrees Celsius) annealing on x-ray reflectivity and stress in the multilayers has been investigated. There was not found an appreciable changes in the absolute value of reflectivity or in d-spacing with annealing temperature. The stress in the coatings changes with annealing temperature from compressive to tensile. There was observed a temperature of annealing at which the stress is no longer present in the film. The absolute value of this temperature measured for W/Si multilayer is approximately 120 degrees Celsius.
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