The Large Synoptic Survey Telescope (LSST) Project is a public-private partnership that is well into the
design and development of the complete observatory system to conduct a wide fast deep survey and to
process and serve the data. The telescope has a 3-mirror wide field optical system with an 8.4 meter
primary, 3.4 meter secondary, and 5 meter tertiary mirror. The reflective optics feed three refractive
elements and a 64 cm 3.2 gigapixel camera. The LSST data management system will reduce, transport,
alert and archive the roughly 15 terabytes of data produced nightly, and will serve the raw and catalog data
accumulating at an average of 7 petabytes per year to the community without any proprietary period. The
project has completed several data challenges designed to prototype and test the data management system
to significant pre-construction levels. The project continues to attract institutional partners and has acquired
non-federal funding sufficient to construct the primary mirror, already in progress at the University of
Arizona, build the secondary mirror substrate, completed by Corning, and fund detector prototype efforts,
several that have been tested on the sky. A focus of the project is systems engineering, risk reduction
through prototyping and major efforts in image simulation and operation simulations. The project has
submitted a proposal for construction to the National Science Foundation Major Research Equipment and
Facilities Construction (MREFC) program and has prepared project advocacy papers for the National
Research Council's Astronomy 2010 Decadal Survey. The project is preparing for a 2012 construction
The Large Synoptic Survey Telescope (LSST) project has evolved from just a few staff members in 2003 to about 100 in
2010; the affiliation of four founding institutions has grown to 32 universities, government laboratories, and industry.
The public private collaboration aims to complete the estimated $450 M observatory in the 2017 timeframe. During the
design phase of the project from 2003 to the present the management structure has been remarkably stable. At the same
time, the funding levels, staffing levels and scientific community participation have grown dramatically. The LSSTC
has introduced project controls and tools required to manage the LSST's complex funding model, technical structure and
distributed work force. Project controls have been configured to comply with the requirements of federal funding
agencies. Some of these tools for risk management, configuration control and resource-loaded schedule have been
effective and others have not. Technical tasks associated with building the LSST are distributed into three subsystems:
Telescope & Site, Camera, and Data Management. Each sub-system has its own experienced Project Manager and
System Scientist. Delegation of authority is enabling and effective; it encourages a strong sense of ownership within the
project. At the project level, subsystem management follows the principle that there is one Board of Directors, Director,
and Project Manager who have overall authority.
LSST will be a large, wide-field groundbased telescope designed to obtain sequential images of the entire visible sky every few nights. The optical design involves a 3-mirror system with an 8.4 m primary, which feeds three refractive correcting elements inside a camera, providing a 10 square degree field of view sampled by a 3 Gpixel focal plane array. The total effective system throughput, AΩ = 319 m2 deg2, is nearly two orders of magnitude larger than that of any existing facility. The survey will yield contiguous overlapping imaging of 20,000 square degrees of sky in 6 optical bands covering the wavelength regime 320-1060 nm.
The 8.4m Large Synoptic Survey Telescope (LSST) is a wide-field telescope facility that will add a qualitatively new capability in astronomy. For the first time, the LSST will provide time-lapse digital imaging of faint astronomical objects across the entire sky. The LSST has been identified as a national scientific priority by diverse national panels, including multiple National Academy of Sciences committees. This judgment is based upon the LSST's ability to address some of the most pressing open questions in astronomy and fundamental physics, while driving advances in data-intensive science and computing. The LSST will provide unprecedented 3-dimensional maps of the mass distribution in the Universe, in addition to the traditional images of luminous stars and galaxies. These mass maps can be used to better understand the nature of the newly discovered and utterly mysterious Dark Energy that is driving the accelerating expansion of the Universe. The LSST will also provide a comprehensive census of our solar system, including potentially hazardous asteroids as small as 100 meters in size. The LSST facility consists of three major subsystems: 1) the telescope, 2) the camera and 3) the data processing system. The baseline design for the LSST telescope is a 8.4m 3-mirror design with a 3.5 degree field of view resulting in an A-Omega product (etendue) of 302deg2m2. The camera consists of 3-element transmisive corrector producing a 64cm diameter flat focal plane. This focal plane will be populated with roughly 3 billion 10μm pixels. The data processing system will include pipelines to monitor and assess the data quality, detect and classify transient events, and establish a large searchable object database. We report on the status of the designs for these three major LSST subsystems along with the overall project structure and management.
The development of defect-free reticle blanks is an important challenge facing the commercialization of extreme ultraviolet lithography (EUVL). The basis of an EUVL reticle are mask blanks consisting of a substrate and a reflective Mo/Si multilayer. Defects on the substrate or defects introduced during multilayer deposition can result in critical phase and amplitude defects. Amplitude- or phase-defect repair techniques are being developed with the goal to repair many of these defects. In this paper we discuss the selection of a capping layer for amplitude-defect repair, and report on experimental results of the reflectance variation over the amplitude-defect repair zone for different capping layers. Our results suggest that carbon and silicon carbide are the leading candidates for capping layer materials. We further performed a quantitative assessment of the yield improvement due to defect repair. We found that amplitude- and phase-defect repair have the potential to significantly improve mask blank yield, and that yield can be maximized by increasing the number of Mo/Si bilayers.
Low-defect mask blanks remain a key technical challenge to Extreme Ultraviolet Lithography (EUVL). The mask blank is ion-beam sputter-coated with an 81-layer Mo/Si multilayer stack for high reflectance at l = 13.4nm. The current mask coating process can achieve a median added defect level of 0.05 defects/cm2 (12 added defects 90nm or larger on a 200mm Si-wafer test substrate), but this must be reduced by about a factor of 10 to meet mask cost requirements for EUVL. To further reduce the particle defect level, we have studied pathways for particle transport, using test particles and particles native to the coating process, and combined the results into a computational model of particle transport in an ion-beam sputter system. At process pressure, gas drag is negligible for particles above 100nm, so particles travel ballistically until they hit a surface. Bounce from chamber walls allows particles to reach all surfaces in the chamber if they have initial velocities above ~100m/s. The ion beam has sufficient momentum to entrain slower particles and accelerate them toward the sputter target, where some can bounce to the substrate. The model shows preliminary agreement with experimental defect distributions on witness wafers at various positions within the coating chamber.
The high volume inspection equipment currently available to support development of EUV blanks is non-actinic. The same is anticipated for patterned EUV mask inspection. Once potential defects are identified and located by such non-actinic inspection techniques, it is essential to have instrumentation to perform detailed characterization, and if repairs are performed, re-evaluation. The ultimate metric for the acceptance or rejection of a mask due to a defect, is the wafer level impact. Thus, measuring the aerial image for the site under question is required. An EUV Aerial Image Microscope (“AIM”) similar to the current AIM tools for 248nm and 193nm exposure wavelength is the natural solution for this task. Due to the complicated manufacturing process of EUV blanks, AIM measurements might also be beneficial to accurately assessing the severity of a blank defect. This is an additional application for an EUV AIM as compared to today’s use.
In recognition of the critical role of an EUV AIM for the successful implementation of EUV blank and mask supply, International SEMATECH initiated this design study with the purpose to define the technical requirements for accurately simulating EUV scanner performance, demonstrating the feasibility to meet these requirements and to explore various technical approaches to building an EUV AIM tool.
Static and scanned images of 100 nm dense features were successfully obtained with a developmental set of projection optics and a 500W drive laser laser-produced-plasma (LPP) source in the Engineering Test Stand (ETS). The ETS, configured with POB1, has been used to understand system performance and acquire lithographic learning which will be used in the development of EUV high volume manufacturing tools. The printed static images for dense features below 100 nm with the improved LPP source are comparable to those obtained with the low power LPP source, while the exposure time was decreased by more than 30x. Image quality comparisons between the static and scanned images with the improved LPP source are also presented. Lithographic evaluation of the ETS includes flare and contrast measurements. By using a resist clearing method, the flare and aerial image contrast of POB1 have been measured, and the results have been compared to analytical calculations and computer simulations.
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.
The EUV Engineering Test Stand (ETS) has demonstrated the printing of 100-nm-resolution scanned images. This milestone was first achieved while the ETS operated in an initial configuration using a low power laser and a developmental projection system, PO Box 1. The drive laser has ben upgraded to a single chain of the three-chain Nd:YAG laser developed by TRW. The result in exposure time is approximately 4 seconds for static exposures. One hundred nanometer dense features have been printed in step-and-scan operation with the same image quality obtained in static printing. These experiments are the first steps toward achieving operation using all three laser chains for a total drive laser power of 1500 watts. In a second major upgrade the developmental wafer stage platen, used to demonstrate initial full-field imaging, has been replaced with the final low-expansion platen made of Zerodur. Additional improvements in the hardware and control software have demonstrated combined x and jitter from 2 to 4 nm RMS Over most of the wafer stage travel range, while scanning at the design scan speed of 10 mm/s at the wafer. This value, less than half of the originally specified jitter, provides sufficient stability to support printing of 70 nm features as planned, when the upgraded projection system is installed. The third major upgrade will replace PO Box 1 with an improved projection system, PO Box 2, having lower figure error and lower flare. In addition to these upgrades, dose sensors at the reticle and wafer planes and an EUV- sensitive aerial image monitor have been integrated into the ETS. This paper reports on ETS system upgrades and the impact on system performance.
EUV mask blanks are fabricated by depositing a reflective Mo/Si multilayer film onto super-polished substrates. Small defects in this thin film coating can significantly alter the reflected field and introduce defects in the printed image. Ideally one would want to produce defect-free mask blanks; however, this may be very difficult to achieve in practice. One practical way to increase the yield of mask blanks is to effectively repair multilayer defects, and to this effect we present two complementary defect repair strategies for use on multilayer-coated EUVL mask blanks. A defect is any area on the mask which causes unwanted variations in EUV dose in the aerial image obtained in a printing tool, and defect repair is correspondingly defined as any strategy that renders a defect unprintable during exposure. The term defect mitigation can be adopted to describe any strategy which renders a critical defect non-critical when printed, and in this regard a non-critical defect is one that does not adversely affect device function. Defects in the patterned absorber layer consist of regions where metal, typically chrome, is unintentionally added or removed from the pattern leading to errors in the reflected field. There currently exists a mature technology based on ion beam milling and ion beam assisted deposition for repairing defects in the absorber layer of transmission lithography masks, and it is reasonable to expect that these this technology will be extended to the repair of absorber defects in EUVL masks . However, techniques designed for the repair of absorber layers can not be directly applied to the repair of defects in the mask blank, and in particular the multilayer film. In this paper we present for the first time a new technique for the repair of amplitude defects as well as recent results on the repair of phase defects.
The Engineering Test Stand (ETS) is an EUV lithography tool designed to demonstrate full-field EUV imaging and provide data required to accelerate production-tool development. Early lithographic results and progress on continuing functional upgrades are presented and discussed. In the ETS a source of 13.4 nm radiation is provided by a laser plasma source in which a Nd:YAG laser beam is focused onto a xenon- cluster target. A condenser system, comprised of multilayer-coated and grazing incidence mirrors, collects the EUV radiation and directs it onto a reflecting reticle. The resulting EUV illumination at the reticle and pupil has been measured and meets requirements for acquisition of first images. Tool setup experiments have been completed using a developmental projection system with (lambda) /14 wavefront error (WFE), while the assembly and alignment of the final projection system with (lambda) /24 WFE progresses in parallel. These experiments included identification of best focus at the central field point and characterization of imaging performance in static imaging mode. A small amount of astigmatism was observed and corrected in situ, as is routinely done in advanced optical lithographic tools. Pitch and roll corrections were made to achieve focus throughout the arc-shaped field of view. Scan parameters were identified by printing dense features with varying amounts of magnification and skew correction. Through-focus scanned imaging results, showing 100 nm isolated and dense features, will be presented. Phase 2 implementation goals for the ETS will also be discussed.
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.
We propose the use of optical proximity correction on absorber features to compensate for the effect of sub-resolution multilayer defects that would otherwise induce a critical error in linewidth. A series of defect printability and compensation experiments utilizing programmed multilayer defects are presented which demonstrate this approach. The amount of absorber removal for defect compensation depends on system imaging performance and the quality of the absorber removal process. A process flow for the mask fabrication, defect characterization and compensation is presented.
A clean, high-power Extreme Ultraviolet (EUV) light source is being developed for Extreme Ultraviolet Lithography (EUVL). The source is based on a continuous jet of condensable gas irradiated with a diode-pumped solid state laser producing a time-averaged output power of 1700 W at 5000 - 6000 Hz. An illumination system is being assembled to collect and deliver the EUV output from the source and deliver it to a reticle and projection optics box to achieve an EUV exposure rate equivalent to ten 300-mm wafers per hour.
Extreme UV Lithography (EUVL) is one of the leading candidates for the next generation lithography, which will decrease critical feature size to below 100 nm within 5 years. EUVL uses 10-14 nm light as envisioned by the EUV Limited Liability Company, a consortium formed by Intel and supported by Motorola and AMD to perform R and D work at three national laboratories. Much work has already taken place, with the first prototypical cameras operational at 13.4 nm using low energy laser plasma EUV light sources to investigate issues including the source, camera, electro- mechanical and system issues, photoresists, and of course the masks. EUV lithograph masks are fundamentally different than conventional photolithographic masks as they are reflective instead of transmissive. EUV light at 13.4 nm is rapidly absorbed by most materials, thus all light transmission within the EUVL system from source to silicon wafer, including EUV reflected from the mask, is performed by multilayer mirrors in vacuum.
The imaging specifications for extreme ultraviolet lithography (EUVL) projection optics parallel those of other optical lithographies. Specifications are scaled to reflect the 100 nm critical dimension for the first generation EUVL systems. The design being fabricated for the Engineering Test Stand, an EUVL alpha tool, consists of a condenser with six channels to provide an effective partial coherence factor of 0.7. The camera contains four mirrors; three of the mirrors are aspheres and the fourth is spherical. The design of the optical package has been constrained so that the angles of incidence and the variations in the angle of incidence of all rays allow for uniform multilayer coatings. The multilayers introduce a slight shift in image position and magnification. We have shown that a system aligned with visible light is also aligned at 13.4 nm. Each mirror must be fabricated with an RMS figure error of less than 0.25 nm and better than 0.2 nm RMS roughness. Optical surfaces that exceed each of these specifications individually have been fabricated. The success of EUVL requires that these specifications be met simultaneously. Keywords: EUV projection lithography, optical design, multilayer coatings, aspheric optics
EUV lithography (EUVL) is a leading candidate as a stepper technology for fabricating the '0.1 micrometers generation' of microelectronic circuits. EUVL is an optical printing technique qualitatively similar to DUV lithography (DUVL), except that 11-13 nm wavelength light is used instead of 193-248nm. The feasibility of creating 0.1 micrometers features has been well-established using small-field EUVL printing tools, and development efforts are currently underway to demonstrate that cost-effective production equipment can be engineered to perform full-width ring-field imaging consistent with high wafer throughput rates. Ensuring that an industrial supplier base will be available for key components and subsystems is crucial to the success of EUVL. In particular, the projection optics are the heart of the EUVL imaging system, yet they have figure and finish specifications that are beyond the state-of-the-art in optics manufacturing. Thus it is important to demonstrate that industry will be able to fabricate and certify these optics commensurate with EUVL requirements. Indeed, the goal of this paper is to demonstrate that procuring EUVL projection optical substrates is feasible. This conclusion is based on measurements of both commercially-available and developmental substrates. The paper discusses EUVL figure and finish specifications, followed by examples of ultrasmooth and accurate surfaces, and concludes with a discussion of how substrates are measured and evaluated.
The assembly of an optical system requires the correction of aberrations in the entire imaging field by making selected rigid-body motions of the optical elements. We present a rigorous method for determining which adjustment motions, called compensators, to use for alignment. These compensators are found by employing techniques from linear algebra that choose the most independent vectors from a set which are interdependent. The method finds the applied to a four-mirror scanning ring-field EUV lithography system. It is shown that out of 32 degrees of freedom in the configuration of the optical elements, only eight compensators are required on the optics. By adjusting these compensators a misaligned configuration giving 30(lambda) wavefront error can be assembled to (lambda) /50 in the absence of measurement noise.
In some white-light imaging applications it is desirable to make the optical elements extremely thin. Numerous practical advantages are derived if the lenses are one to two orders of magnitude thinner than conventional refractive lenses. For example, these lens systems provide minimal weight and compactness, and are potentially low cost. Although diffractive lenses are only a few microns thick, they have severe chromatic aberrations. Approaches are compared for making the lenses thinner and techniques are examined for correcting chromatic aberrations. There are basic limits to the polychromatic MTF and chromatic correction that can be achieved.
A radial, modulo-m2(pi) diffractive lens and a modulo-2(pi) diffractive lens are superposed in a single integrated optical element. The resulting compound lens both introduces optical power and cancels material dispersion. The lens is a microthin, planar achromat suitable for broadband imaging applications. An f/5, 100 mm focal length lens is fabricated by precision diamond-turning. A master mold is generated from the diamond turned copper; the master is then used to form a second-generation replica in an optical quality, uv-cured photopolymer. The measured effective V-number of the replicated lens is 213. Measured narrowband (2 nm) resolution is 120 lp/mm with a contrast of 10%.
SC123: Introduction to Extreme Ultraviolet Lithography
Extreme Ultraviolet Lithography (EUVL) is the next step for optical lithography moving toward shorter wavelengths. EUVL is a leading technology for device fabrication with feature sizes of 70 nm and below, and is being developed in the U.S., Japan and Europe. The implementation of EUV requires new technologies in sources, optics and resists materials, different from those of the more traditional optical techniques. The course introduces EUVL and the status of developments in the US and worldwide. It focuses on practical issues associated with the design and use of an EUVL stepper. The instructors address the details of the materials and optics, discuss the image formation process, the resist properties, mask design and fabrication, and the current design of lithographic tools.