C. Montcalm, A. Badeen, D. Burbidge, R. Bruce, G. Carlow, J. Dane, N. Firdawsi, G. E. Laframboise, A. Miles, J.-P. Noel, R. Rinfret, B. Sullivan, R. Bardazzi, S. Lorenzini, L. Giunti
This paper presents the design, manufacture and characterization results of two optical interference filters to be used in the Lightning Imager (LI) optical head on the Meteosat Third Generation (MTG) mission. The first optical filter is a Solar Rejection Window (SRW) to limit the solar thermal radiation absorbed by the optical head while the second optical filter is a Narrow Band Filter (NBF) intended to only pass the lightning discharge emission wavelengths. Each filter has its own distinctive design considerations and manufacturing challenges. The SRW must pass wavelengths from the 760 to 780 nm spectral range and reject (block) wavelengths from the ultraviolet (UV) to the mid-wave infrared (MWIR) and up to 16.3° angle of incidence (AOI) and over a large temperature range (as when exposed to direct sunlight). The NBF is designed to pass only the oxygen emission triplet, centered around the 777.6 nm (vacuum) wavelength and rejecting other wavelengths. Considering the AOI of the light and the temperature excursion, the center wavelength (CWL) uniformity has to be better than 0.04% peak-to-valley (PV) over the 114 mm diameter clear aperture, which is a formidable challenge. We achieved a coating thickness uniformity less than ±0.01% PV, exceeding the prescribed specification. Post-deposition annealing was carried out to tune the bandpass to within pico-meters (pm) of the target CWL value while maintaining the desired CWL uniformity. To ensure that both the SRW and NBF filter meet the desired optical and physical specifications, a comprehensive series of optical and physical characterization tests, along with durability tests, were carried out on each deposition batch.
Over the past decade, tremendous strides have been made in the design, manufacture and measurement of optical thin
film filters. Driven in part from the challenging demands of fiber optic communication (telecom) filters, the
manufacture of optical coatings has advanced significantly through improved optical monitoring technologies and
algorithms; improved deposition technologies; and, very importantly, the ability to fully automate all aspects of the
coating process. This improvement in optical coating technology has since been applied to filters used in other diverse
fields ranging from bio-medical instrumentation to sensors to astronomy. In this paper, advanced optical thin film filters
will be described along with their applications, both in telecom and spectroscopic fields.
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.
The Engineering Test Stand (ETS) is a developmental lithography tool designed to demonstrate full-field EUV imaging and provide data for commercial-tool development. In the first phase of integration, currently in progress, the ETS is configured using a developmental projection system, while fabrication of an improved projection system proceeds in parallel. The optics in the second projection system have been fabricated to tighter specifications for improved resolution and reduced flare. The projection system is a 4-mirror, 4x-reduction, ring-field design having a numeral aperture of 0.1, which supports 70 nm resolution at a k1 of 0.52. The illuminator produces 13.4 nm radiation from a laser-produced plasma, directs the radiation onto an arc-shaped field of view, and provides an effective fill factor at the pupil plane of 0.7. The ETS is designed for full-field images in step-and-scan mode using vacuum-compatible, magnetically levitated, scanning stages. This paper describes system performance observed during the first phase of integration, including static resist images of 100 nm isolated and dense features.
A model has been developed to predict the cost of extreme ultraviolet lithography (EUVL) masks. The mask blank for EUVL consists of a low thermal expansion material substrate having a square photomask form factor and is coated with reflective Mo/Si multilayers. Absorber layers are deposited on the multilayer and patterned. EUVL mask patterning will use evolutionary improvements in mask patterning and repair equipment. One of the challenges in implementing EUVL is to economically fabricate multilayer-coated mask blanks with no printable defects. The model of mask cost assigns yield and time required for each of the steps in fabricating EUVL masks from purchase of a polished substrate to shipment of a patterned mask. Data from present multilayer coating processes and present mask patterning processes are used to estimate the future cost of EUVL masks. Several of the parameters that significantly influence predicted mask cost are discussed in detail. Future cost reduction of mask blanks is expected from learning on substrate fabrication, improvements in low defect multilayer coating to consistently obtain <0.005 defects cm-2, and demonstration of multilayer smoothing which reduces the printability of substrate defects. The model predicts that the price range for EUVL masks in production will be S30-40K, which is comparable to the price of complex phase shift masks needed to use optical lithography for 70 nm critical dimension patterning.
The Engineering Test Stand (ETS) is an EUV laboratory lithography tool. The purpose of the ETS is to demonstrate EUV full-field imaging and provide data required to support production-tool development. The ETS is configured to separate the imaging system and stages from the illumination system. Environmental conditions can be controlled independently in the two modules to maximize EUV throughput and environmental control. A source of 13.4 nm radiation is provided by a laser plasma source in which a YAG laser beam is focused onto a xenon-cluster target. A condenser system, comprised of multilayer-coated mirrors and grazing-incidence mirrors, collects the EUV radiation and directs it onto a reflecting reticle. A four-mirror, ring-field optical system, having a numerical aperture of 0.1, projects a 4x-reduction image onto the wafer plane. This design corresponds to a resolution of 70 nm at a k1 of 0.52. The ETS is designed to produce full- field images in step-and-scan mode using vacuum-compatible, one-dimension-long-travel magnetically levitated stages for both reticle and wafer. Reticle protection is incorporated into the ETS design. This paper provides a system overview of the ETS design and specifications.
Synchrotron-based reflectometry is an important technique for the precise determination of optical properties of reflective multilayer coatings for Extreme Ultraviolet Lithography (EUVL). Multilayer coatings enable normal incidence reflectances of more than 65% in the wavelength range between 11 and 15 nm. In order to achieve high resolution and throughput of EUVL systems, stringent requirements not only apply to their mechanical and optical layout, but also apply to the optical properties of the multilayer coatings. Therefore, multilayer deposition on near-normal incidence optical surfaces of projection optics, condenser optics and reflective masks requires suitable high-precision metrology. Most important, due to their small bandpass on the order of only 0.5 nm, all reflective multilayer coatings in EUVL systems must be wavelength-matched to within +/- 0.05 nm. In some cases, a gradient of the coating thickness is necessary for wavelength matching at variable average angle of incidence in different locations on the optical surfaces. Furthermore, in order to preserve the geometrical figure of the optical substrates, reflective multilayer coatings need to be uniform to within 0.01 nm in their center wavelength. This requirement can only be fulfilled with suitable metrology, which provides a precision of a fraction of this value. In addition, for the detailed understanding and the further development of reflective multilayer coatings a precision in the determination of peak reflectances is desirable on the order of 0.1%. Substrates up to 200 mm in diameter and 15 kg in mass need to be accommodated. Above requirements are fulfilled at beamline 6.3.2 of the Advanced Light Source (ALS) in Berkeley. This beamline proved to be precise within 0.2% (rms) for reflectance and 0.002 nm (rms) for wavelength.
We present the results of coating the first set of optical elements for an alpha-class extreme-ultraviolet (EUV) lithography system, the Engineering Test Stand (ETS). The optics were coated with Mo/Si multilayer mirrors using an upgraded DC-magnetron sputtering system. Characterization of the near-normal incidence EUV reflectance was performed using synchrotron radiation from the Advanced Light Source at the Lawrence Berkeley National Laboratory. Stringent requirements were met for these multilayer coatings in terms of reflectance, wavelength matching among the different optics, and thickness control across the diameter of each individual optic. Reflectances above 65% were achieved at 13.35 nm at near-normal angles of incidence. The run-to-run reproducibility of the reflectance peak wavelength was maintained to within 0.4%, providing the required wavelength matching among the seven multilayer-coated optics. The thickness uniformity (or gradient) was controlled to within plus or minus 0.25% peak-to-valley (P-V) for the condenser optics and plus or minus 0.1% P-V for the four projection optics, exceeding the prescribed specification for the optics of the ETS.
The performance of beryllium-based multilayer coatings designed to reflect light of wavelengths near 11 nm, at normal incidence, is presented. These multilayer coatings are of special interest for extreme ultraviolet lithography (EUVL). The beryllium-based multilayers investigated were Mo/Be, Ru/Be and a new material combination Mo2C/Be. The highest reflectivity achieved so far is 70% at 11.3 nm with 70 bilayers of Mo/Be. However, even though high reflectivity is very important, there are other parameters to satisfy the requirements for an EUVL production tool. Multilayer stress, thermal stability, radiation stability and long term reflectance stability are of equal or greater importance. An experimental characterization of several coatings was carried out to determine the reflectivity, stress, microstructure, and long term stability of these coatings. Theoretically calculated reflectivities are compared with experimental results for different material pairs; differences between experimental and theoretical reflectivities and bandwidths are addressed.
Two new sets of projection optics for our prototype 10X reduction EUV lithography system were coated with Mo/Si multilayers. The coating thickness was graded across the optics by using shadow masks to ensure maximum throughput at all incidence angles in the camera. The overall deviation of the wavelength response across the clear aperture of each mirror is below 0.01 percent RMS. However, the wavelength mismatch between two optics coated in different runs is up to 0.07 nm. Nevertheless, this is till within the allowed tolerances, and the predicted optical throughput loss in the camera due to such wavelength mismatch is about 4 percent. EUV reflectances of 63-65 percent were measured around 13.40 nm for the secondary optics, which is in good agreement with the expected reflectance based on the substrate finish as measured with AFM.
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.
The capabilities of the EUV 10x microstepper have been substantially improved over the past year. The key enhancement was the development of a new projection optics system with reduced wavefront error, reduced flare, and increased numerical aperture. These optics and concomitant developments in EUV reticles and photoresists have enabled dramatic improvements in EUV imaging, illustrated by resolution of 70 nm dense lines and spaces (L/S). CD linearity has been demonstrated for dense L/S over the range 100 nm to 80 nm, both for the imaging layer and for subsequent pattern transfer. For a +/- 10 percent CD specification, we have demonstrated a process latitude of +/- micrometers depth of focus and 10 percent dose range for dense 100 nm L/S.
Two new Schwarzschild cameras have been fabricated for the EUV 10x microstepper. The surface topography of the mirrors was characterized over the full range of spatial frequencies both before and after multilayer coating. EUV scattering from the individual mirrors was measured and compared with the surface profilometry. A knife-edge test was used to directly measure the flare of the assembled cameras. The flare measured in this way is in excellent agreement with the contrast of isolated printed lines and with the point spread function of the camera as determined by EUV interferometry. The measured flare of the camera is also in good agreement with the flare calculated from the combined surface profile measurements of the individual mirrors. Consistent with the improvements made in the surface finish of the mirror substrates, a significant reduction in the flare is observed as compared with previously existing cameras.
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.
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.
A new set of mirrors for the SANDIA 10X microstepper has been fabricated. The optics have been tested by optical profilometry, atomic force microscopy, EUV reflectometry and EUV scattering. THese measurements allow one to predict the performance of the camera. Mo/Si multilayer coatings with the required thickness profile were produced by DC magnetron sputtering using shadow masks in front of the rotating substrates. The figure errors of the new mirrors are considerably smaller than those obtained previously, while mid-spatial frequency roughness still needs improvement. This roughness reduces mostly the throughput of the system; i.e. most of the scattered light occurs outside the field of the camera and there is only a small reduction of contrast or resolution.
Mo/Si multilayer mirrors with a high reflectance at normal incidence in the 232 - 236 angstrom spectral region have been deposited by rf magnetron sputtering for use in a XUV Ge-laser. The mirrors had a peak reflectance of 26% in this wavelength region. Characterization by TEM and XRD indicates good thickness control in the deposition process and low interface roughness, although interdiffusion is present at the interfaces. Preliminary experiments indicate that the XUV laser output intensity was increased when a multilayer mirror was added to allow a double pass through the gain medium.
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