Ralf K. Heilmann

Massachusetts Institute of Technology, Cambridge, Massachusetts, United States

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Publications (114)95.58 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: NASA's Chandra X-ray Observatory continues to provide an unparalleled means for exploring the high-energy universe. With its half-arcsecond angular resolution, Chandra studies have deepened our understanding of galaxy clusters, active galactic nuclei, galaxies, supernova remnants, neutron stars, black holes, and solar system objects. As we look beyond Chandra, it is clear that comparable or even better angular resolution with greatly increased photon throughput is essential to address ever more demanding science questions—such as the formation and growth of black hole seeds at very high redshifts; the emergence of the first galaxy groups; and details of feedback over a large range of scales from galaxies to galaxy clusters. Recently, we initiated a concept study for such a mission, dubbed X-ray Surveyor. The X-ray Surveyor strawman payload is comprised of a high-resolution mirror assembly and an instrument set, which may include an X-ray microcalorimeter, a high-definition imager, and a dispersive grating spectrometer and its readout. The mirror assembly will consist of highly nested, thin, grazing-incidence mirrors, for which a number of technical approaches are currently under development—including adjustable X-ray optics, differential deposition, and new polishing techniques applied to a variety of substrates. This study benefits from previous studies of large missions carried out over the past two decades and, in most areas, points to mission requirements no more stringent than those of Chandra.
    SPIE 9601, UV, X-Ray, and Gamma-Ray Space Instrumentation for Astronomy XIX, San Diego, CA; 08/2015
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    ABSTRACT: The future of x-ray astronomy depends upon development of x-ray telescopes with larger aperture areas (approximate to 3 m(2)) and fine angular resolution (approximate to 1"). Combined with the special requirements of nested grazing-incidence optics, the mass and envelope constraints of space-borne telescopes render such advances technologically and programmatically challenging. Achieving this goal will require precision fabrication, alignment, mounting, and assembly of large areas (approximate to 600 m(2)) of lightweight (approximate to 1 kg/m(2) areal density) high-quality mirrors at an acceptable cost (approximate to 1 M$/m(2) of mirror surface area). This paper reviews relevant technological and programmatic issues, as well as possible approaches for addressing these issues-including active (in-space adjustable) alignment and figure correction.
    SPIE Optical Engineering + Applications; 09/2014
  • Ralf K. Heilmann · Alex R. Bruccoleri · Dong Guan · Mark L. Schattenburg ·
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    ABSTRACT: Soft x-ray spectroscopy of celestial sources with high resolving power R = E/ΔE and large collecting area addresses important science listed in the Astro2010 Decadal Survey New Worlds New Horizons, such as the growth of the large scale structure of the universe and its interaction with active galactic nuclei, the kinematics of galactic outflows, as well as coronal emission from stars and other topics. Numerous studies have shown that a transmission grating spectrometer based on lightweight critical-angle transmission (CAT) gratings can deliver R = 3000-5000 and large collecting area with high efficiency and minimal resource requirements, providing spectroscopic figures of merit at least an order of magnitude better than grating spectrometers on Chandra and XMM-Newton, as well as future calorimeter-based missions. The recently developed CAT gratings combine the advantages of transmission gratings (low mass, relaxed figure and alignment tolerances) and blazed reflection gratings (high broad band diffraction efficiency, utilization of higher diffraction orders). Their working principle based on blazing through reflection off the smooth, ultra-high aspect ratio grating bar sidewalls has previously been demonstrated on small samples with x rays. For larger gratings (area greater than 1 inch square) we developed a fabrication process for grating membranes with a hierarchy of integrated low-obscuration supports. The fabrication involves a combination of advanced lithography and highly anisotropic dry and wet etching techniques. We report on the latest fabrication results of free-standing, large-area CAT gratings with polished sidewalls and preliminary x-ray tests.
    SPIE Astronomical Telescopes + Instrumentation; 07/2014
  • Brandon Chalifoux · Ralf K. Heilmann · Mark L. Schattenburg ·
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    ABSTRACT: Air bearing glass slumping followed by ion implantation for fine figure correction constitutes a promising process for fabricating thin glass segmented mirrors for future high-resolution x-ray telescopes. We have previously demonstrated the feasibility of both air bearing slumping and ion implantation figure correction to produce mirrors with good figure and without introducing mid spatial-frequency errors or roughness. In this work, we describe a mechanically-robust slumping tool design that can be adapted to Wolter I mirror shapes; and we describe progress on understanding ion implantation for use as a figure correction process, by using in-situ curvature measurements in a tandem ion accelerator.
    SPIE Astronomical Telescopes + Instrumentation; 07/2014
  • D. Guan · A. R. Bruccoleri · R. K. Heilmann · M. L. Schattenburg ·
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    ABSTRACT: Thin silicon dioxide films have been studied as a function of deposition parameters and annealing temperatures. Films were deposited by tetraethoxysilane (TEOS) dual-frequency plasma enhanced chemical vapor deposition with different time interval fractions of high-frequency and low-frequency plasma depositions. The samples were subsequently annealed up to 930 °C to investigate their stress behavior. Films that were deposited in high-frequency dominated plasma were found to have tensile residual stress after annealing at temperatures higher than 800 °C. The residual stress can be controlled to slightly tensile by changing the annealing temperature. High tensile stress was observed during the annealing of high-frequency plasma-deposited films, leading to film cracks that limit the film thickness, as predicted by the strain energy release rate equation. Thick films without cracks were obtained by iterating deposition and annealing to stack multiple layers. A series of wet cleaning experiments were conducted, and we discovered that water absorption in high-frequency plasma-deposited films causes the residual stress to decrease. A ∼40 nm thick low-frequency deposited oxide cap is sufficient to prevent water from diffusing through the film. Large-area free-standing tensile stressed oxide membranes without risk of buckling were successfully fabricated.
    Journal of Micromechanics and Microengineering 01/2014; 24(2). DOI:10.1088/0960-1317/24/2/027001 · 1.73 Impact Factor
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    ABSTRACT: A fabrication process has been developed to chemically polish the sidewalls of 200 nm-pitch gratings via potassium hydroxide (KOH) etching following the Bosch deep reactive-ion etching (DRIE) process. Previous KOH polishing experiments focused on micron scale features. This work is the first reported combined DRIE-KOH etching process on the nanoscale for ultrahigh aspect ratio structures with feature sizes 30 times smaller than previously published work. The primary application of the gratings is x-ray spectroscopy and requires polished sidewalls for efficient x-ray reflection. Polishing is also critical for increasing the open area by narrowing the grating bars, which increases the throughput of x-rays. The increased open area is also valuable for other applications such as ultraviolet filtration, neutron spectroscopy and biofiltration. Advanced Bosch processes leave approximately 4 nm, root mean square (RMS), of roughness on the sidewalls. This roughness needs to be reduced to below 1 nm to efficiently reflect soft x-rays with wavelengths between 1 and 5 nm. Furthermore, high aspect ratio DRIE can result in bar width variations of approximately a factor of two from the top to the middle of the channel, commonly referred to as bowing. The polishing procedure presented here removes the roughness to below the resolution of the scanning electron microscope, and was measured via an atomic force microscope to be less than 1 nm RMS. The bowing has also been reduced by at least a factor of 3. The polishing process takes advantage of the anisotropy of KOH silicon etching. Specifically, the {111} silicon planes etch approximately 100 times slower than other crystal planes. This anisotropy allows the grating bars to be etched in 50% by weight KOH at room temperature for up to 60 min. Long etches have several key requirements, including 0.2 degree alignment of the grating with respect to the {111} planes, mask roughness below 40Ȁ- ;nm and minimal defects in the silicon. If these requirements are not met, the grating will quickly be destroyed by the etch, which etches the non-{111} planes in excess of 1 μm per hour. The fabrication steps of this work are described in detail including a novel technique to align the 200 nm-pitch interference lithography image grating to the {111} planes of a 〈110〉 silicon wafer.
    Journal of vacuum science & technology. B, Microelectronics and nanometer structures: processing, measurement, and phenomena: an official journal of the American Vacuum Society 11/2013; 31(6):06FF02-06FF02-9. DOI:10.1116/1.4820901 · 1.46 Impact Factor
  • Brandon Chalifoux · Edward Sung · Ralf K. Heilmann · Mark L. Schattenburg ·
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    ABSTRACT: Achieving both high resolution and large collection area in the next generation of x-ray telescopes requires highly accurate shaping of thin mirrors, which is not achievable with current technology. Ion implantation offers a promising method of modifying the shape of mirrors by imparting internal stresses in a substrate, which are a function of the ion species and dose. This technique has the potential for highly deterministic substrate shape correction using a rapid, low cost process. Wafers of silicon and glass (D-263 and BK-7) have been implanted with Si+ ions at 150 keV, and the changes in shape have been measured using a Shack-Hartmann metrology system. We show that a uniform dose over the surface repeatably changes the spherical curvature of the substrates, and we show correction of spherical curvature in wafers. Modeling based on experiments with spherical curvature correction shows that ion implantation could be used to eliminate higher-order shape errors, such as astigmatism and coma, by using a spatially-varying implant dose. We will report on progress in modelling and experimental tests to eliminate higher-order shape errors. In addition, the results of experiments to determine the thermal and temporal stability of implanted substrates will be reported.
    Proceedings of SPIE - The International Society for Optical Engineering 09/2013; 8861. DOI:10.1117/12.2023535 · 0.20 Impact Factor
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    ABSTRACT: We report several break-through nanofabrication developments enabling high efficiency and high resolving power spectrometers in the soft x-ray band. The device is the critical-angle transmission (CAT) grating, which combines the low mass and relaxed alignment tolerances of a transmission grating with the high broad-band efficiency and high diffraction orders of a blazed reflection grating. Past work successfully demonstrated the CAT grating concept; however, the open-area fraction was often less than 20% whilst more than 50% is desired. This presents numerous nanofabrication challenges including a requirement for a freestanding silicon membrane of ultra high-aspect ratio bars at a period of 200 nanometers with minimal cross support blockage. Furthermore, the sidewalls must be smooth to a few nanometers to efficiently reflect soft x-rays. We have developed a complete nanofabrication process for creating freestanding CAT gratings via plasma-etching silicon wafers with a buried layer of SiO2. This removable buried layer enables combining a record-performance plasma etch for the CAT grating with a millimeter-scale honeycomb structural support to create a large-area freestanding membrane. We have also developed a process for polishing sidewalls of plasma-etched ultra-high aspect ratio nanoscale silicon structures via potassium hydroxide (KOH). This process utilizes the anisotropic etch nature of single crystal silicon in KOH. We developed a novel alignment technique to align the CAT grating bars to the {111} planes of silicon within 0.2 degrees, which enables KOH to etch away sidewall roughness without destroying the structure, since the {111} planes etch approximately 100 times slower than the non-{111} planes. Preliminary results of a combined freestanding grating with polishing are presented to enable efficient diffraction of soft x-rays.
    Proceedings of SPIE - The International Society for Optical Engineering 09/2013; 8861:19-. DOI:10.1117/12.2024357 · 0.20 Impact Factor
  • Ralf K. Heilmann · Alex R. Bruccoleri · Dong Guan · Mark L. Schattenburg ·
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    ABSTRACT: Large area, high resolving power spectroscopy in the soft x-ray band can only be achieved with a state-of-the-art diffraction grating spectrometer, comprised of large collecting-area focusing optics with a narrow point spread function, large-area high-resolving power diffraction gratings, and small pixel, order sorting x-ray detectors. Recently developed critical-angle transmission (CAT) gratings combine the advantages of transmission gratings (low mass, relaxed figure and alignment tolerances) and blazed reflection gratings (high broad band diffraction efficiency, utilization of higher diffraction orders). Several new mission concepts containing CAT grating based spectrometers (AEGIS, AXSIO, SMART-X) promise to deliver unprecedented order-of-magnitude improvements in soft x-ray spectroscopy figures of merit related to the detection and characterization of emission and absorption lines, thereby addressing high-priority questions identified in the Astro2010 Decadal Survey "New Worlds New Horizons". We review the current status of CAT grating fabrication, present recent fabrication results, and describe our plans and technology development roadmap for the coming year and beyond.
    Proceedings of SPIE - The International Society for Optical Engineering 09/2013; 8861:18-. DOI:10.1117/12.2024426 · 0.20 Impact Factor
  • Edward Sung · Brandon Chalifoux · Mark L. Schattenburg · Ralf K. Heilmann ·
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    ABSTRACT: Molding glass by using air bearings is a promising procedure for inexpensive and high precision glass shaping. Thin glass sheets are sandwiched between air bearings and pushed flat while being thermally cycled. In this study, a novel device for shaping glass is created and tested using 0.5 mm thick, 100 mm round, Schott D263 wafers. Numerous samples were shaped with varying values for bearing-to-glass gap and maximum temperature, and were measured with a Shack Hartmann metrology tool. Glass was shaped with bearing-to-glass gaps of >50 μm, 36+/-2.5 μm, and 30.5+/-2.5 μm. The best peak-to-valley (P-V) flatness achieved is 6.7/3.6+/-0.5 μm for front/back of the glass sheet, using a gap of 36+/-2.5 μm. The average steady-state P-V achieved is 12 μm. Using the same device parameters, the best repeatability achieved over the whole 100 mm wafer is 2.7+/-0.5 μm P-V and 9.5 arcseconds RMS slope error. When looking at 60 mm sections, the repeatability improves to <1 μm P-V and 5+/-0.5 arcsec.
    Proceedings of SPIE - The International Society for Optical Engineering 09/2013; 8861. DOI:10.1117/12.2026100 · 0.20 Impact Factor
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    ABSTRACT: Low-mass pre-main sequence (PMS) stars are strong and variable X-ray emitters, as has been well established by EINSTEIN and ROSAT observatories. It was originally believed that this emission was of thermal nature and primarily originated from coronal activity (magnetically confined loops, in analogy with Solar activity) on contracting young stars. Broadband spectral analysis showed that the emission was not isothermal and that elemental abundances were non-Solar. The resolving power of the Chandra and XMM X-ray gratings spectrometers have provided the first, tantalizing details concerning the physical conditions such as temperatures, densities, and abundances that characterize the X-ray emitting regions of young star. These existing high resolution spectrometers, however, simply do not have the effective area to measure diagnostic lines for a large number of PMS stars over required to answer global questions such as: how does magnetic activity in PMS stars differ from that of main sequence stars, how do they evolve, what determines the population structure and activity in stellar clusters, and how does the activity influence the evolution of protostellar disks. Highly resolved (R>3000) X-ray spectroscopy at orders of magnitude greater efficiency than currently available will provide major advances in answering these questions. This requires the ability to resolve the key diagnostic emission lines with a precision of better than 100 km/s.
  • [Show abstract] [Hide abstract]
    ABSTRACT: Large area, high resolving power spectroscopy in the soft x-ray band can only be achieved with a state-of-the-art diffraction grating spectrometer. Recently developed critical-angle transmission (CAT) gratings combine the advantages of transmission gratings (low mass, relaxed figure and alignment tolerances) and blazed reflection gratings (high broad band diffraction efficiency, utilization of higher diffraction orders). Since the demise of IXO there are no soft x-ray spectroscopy missions in the pipeline, but several new mission concepts containing CAT grating based spectrometers such as AXSIO promise to deliver unprecedented order-of-magnitude improvements in soft x-ray spectroscopy figures of merit. The CAT grating principle has previously been demonstrated with x rays using small wet-etched samples. We report the latest progress in the fabrication and testing of large (32x32 mm^2) CAT grating prototypes with an integrated hierarchy of low-obstruction support structures. The gratings are fabricated from silicon-on-insulator wafers using advanced lithography and highly anisotropic dry and wet etching techniques. We present our latest grating fabrication results.
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    ABSTRACT: A nanofabrication process has been developed for a novel critical-angle transmission grating for astronomical x-ray spectroscopy. The pitch of the gratings is 200 nm and the depth is 4 μm, which exceeds the state-of-the-art in aspect ratio by over a factor of 2 for ultrahigh aspect ratio freestanding nanoscale gratings with open areas on the order of 50% and spanning several square centimeters. They have a broad array of other applications, including neutral mass spectroscopy, ultraviolet filtration, and phase contrast imaging x-ray spectroscopy. The gratings are fabricated as a monolithic structure in silicon via two lithographic and pattern transfer processes, integrated together on a silicon-on-insulator wafer. The grating is patterned via interference lithography and transferred into the 4 μm device layer via a Bosch deep reactive-ion etch (DRIE). The grating channels are then filled without voids by spinning photoresist on them, which wicks into the channels. The sample is then bonded under vacuum via Crystal Bond™ to a carrier wafer, and a honeycomb pattern is etched via DRIE through the handle layer until it stops cleanly on the buried SiO2. The buried SiO2 is etched away, and the sample is separated from the carrier. The resist filling is cleaned from the channels and the grating is critical-point dried to create a freestanding structure. The freestanding gratings can now be mounted to external frames and structurally analyzed and tested for launch and deployment in space.
    Journal of Vacuum Science & Technology B Microelectronics and Nanometer Structures 11/2012; 30(6). DOI:10.1116/1.4755815 · 1.36 Impact Factor
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    ABSTRACT: Recently developed Critical-Angle Transmission (CAT) grating technology - in combination with x-ray CCD cameras and large collecting-area focusing optics - will enable a new generation of soft x-ray spectrometers with unprecedented resolving power and effective area and with at least an order of magnitude improvement in figures-of-merit for emission and absorption line detection. This technology will be essential to address a number of high-priority questions identified in the Astro2010 Decadal Survey “New Worlds New Horizons” and open the door to a new discovery space. CAT gratings combine the advantages of soft x-ray transmission gratings (low mass, relaxed figure and alignment tolerances, transparent at high energies) and blazed reflection gratings (high broad band diffraction efficiency, utilization of higher diffraction orders to increase resolving power). We report on progress in the fabrication of large-area (31× 31 mm2) free-standing gratings with two levels of low-blockage support structures using highly anisotropic deep reactive-ion etching.
    Proceedings of SPIE - The International Society for Optical Engineering 09/2012; 8443. DOI:10.1117/12.926827 · 0.20 Impact Factor
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    ABSTRACT: We discuss concepts for high-throughput (effective area 250-1400 cm2), high-resolution (spectral resolving power R > 3500) soft X-ray grating spectroscopy in missions of moderate (probe-class or smaller) scale. Such missions can achieve high-priority scientific objectives identified by the Astro2010 Decadal Survey attainable in no other way, and would provide an essential complement to any future large-area X-ray observatory equipped with non-dispersive spectrometers. We enumerate key science drivers and discuss consequences of various alternative design choices for scientific capability and overall mission size.
    Proceedings of SPIE - The International Society for Optical Engineering 09/2012; 8443:15-. DOI:10.1117/12.926810 · 0.20 Impact Factor
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    ABSTRACT: MARX is a portable ray-trace program that was originally developed to simulate event data from the trans- mission grating spectrometers on-board the Chandra X-ray Observatory (CXO). MARX has since evolved to include detailed models of all CXO science instruments and has been further modified to serve as an event simulator for future X-ray observatory design concepts. We first review a number of CXO applications of MARX to demonstrate the roles such a program could play throughout the life of a mission, including its design and calibration, the production of input data products for the development of the various software pipelines, and for observer proposal planning. We describe how MARX was utilized in the design of a proposed future X-ray spectroscopy mission called ÆGIS (Astrophysics Experiment for Grating and Imaging Spectroscopy), a mission concept optimized for the 0.2 to 1 keV soft X-ray band. ÆGIS consists of six independent Critical Angle Transmission Grating Spectrometers (CATGS) arranged to provide a resolving power of 3000 and an effective area exceeding 1000 cm2 across its passband. Such high spectral resolution and effective area will permit ÆGIS to address many astrophysics questions including those that pertain to the evolution of Large Scale Structure of the universe, and the behavior of matter at very high densities. The MARX ray-trace of the ÆGIS spectrometer yields quantitative estimates of how the spectrometer’s performance is affected by misalignments between the various system elements, and by deviations of those elements from their idealized geometry. From this information, we are able to make the appropriate design tradeoffs to maximize the performance of the system.
    Proceedings of SPIE - The International Society for Optical Engineering 09/2012; 8443. DOI:10.1117/12.926937 · 0.20 Impact Factor
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    ABSTRACT: SMART-X is a mission concept for a 2.3 m2 effective area, 0.5" angular resolution X-ray telescope, with 5' FOV, 1" pixel size microcalorimeter, 22' FOV imager, and high-throughput gratings.
    Proceedings of SPIE - The International Society for Optical Engineering 09/2012; DOI:10.1117/12.926851 · 0.20 Impact Factor
  • Herman L. Marshall · N. Schulz · R. Heilmann · K. Kochanski ·
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    ABSTRACT: We developed an instrument design capable of measuring linear X-ray polarization over a broad-band using conventional spectroscopic optics. A set of multilayer-coated flats reflects the dispersed X-rays to the instrument detectors. The intensity variation with position angle is measured to determine three Stokes parameters: I, Q, and U -- all as a function of energy. By laterally grading the multilayer optics and matching the dispersion of the gratings, one may take advantage of high multilayer reflectivities and achieve modulation factors >50% over the entire 0.2 to 0.8 keV band. This instrument could be used in a small orbiting mission or the approach could be used on a large dispersive spectrometric facility. We present progress on laboratory work to demonstrate the capabilities of key components. We thank the MIT Kavli Institute and the NASA Astrophysics Research and Analysis program for funding.
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    ABSTRACT: Future X-ray astronomical missions will need to address a number of important goals such as studying the dynamics of clusters of galaxies, determining how elements are created in the explosions of massive stars, and revealing most of the "normal" matter in the universe which is currently thought to be hidden in hot filaments of gas stretching between galaxies. In order to achieve these goals, spectrometers capable of high resolution and high throughput are necessary for the lowest X-ray energies, 0.3-1.0 keV. We present recent progress in the development of off-plane reflection grating technology for use on upcoming missions. Off-plane grating spectrometers consist of an array of gratings capable of reaching resolutions above 3000 (lambda/Deltalambda). Concept designs have been made for the International X-ray Observatory X-ray Grating Spectrometer. More recently however, we have designed an Optics Module Assembly for WHIMex, an Explorer mission concept that incorporates a Wolter telescope, steering flats, and an array of gratings. This paper will discuss these designs and the application of off-plane arrays for the future.
    Proceedings of SPIE - The International Society for Optical Engineering 09/2011; 8147. DOI:10.1117/12.895037 · 0.20 Impact Factor
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    ABSTRACT: A method of thermally shaping individual sheets of glass for the International X-Ray Observatory using porous mandrels as air bearings has been developed, which eliminates the problems of sticking and dust particleinduced distortion which plague traditional slumping methods. A detailed mathematical model of the process has been developed, allowing prediction of final glass shape based on process parameters that include air supply pressure, imperfections on the mandrel surface, glass total thickness variations and gravity vector orientation. Experiments to verify model findings are conducted under closed-loop control of pressure and apparatus tilt. Little improvement in repeatability is seen, suggesting that the error is due to unmodeled forces such as contact forces from the glass holding technique. Finally, the design process and fabrication of a third generation slumping tool is presented. In addition to scaling the design to accomodate larger flats, slumps are done horizontally to float the glass and minimize contact during the process. New capabilities of the tool include active gap measurement and control, as well as plenum air temperature monitoring.
    Proceedings of SPIE - The International Society for Optical Engineering 09/2011; DOI:10.1117/12.895413 · 0.20 Impact Factor

Publication Stats

1k Citations
95.58 Total Impact Points


  • 1996-2014
    • Massachusetts Institute of Technology
      • Kavli Institute for Astrophysics and Space Research
      Cambridge, Massachusetts, United States
  • 2004
    • Stanford University
      Palo Alto, California, United States
  • 1998-2004
    • Harvard University
      • • School of Engineering and Applied Sciences
      • • Department of Physics
      Boston, MA, United States
  • 1999
    • Carnegie Mellon University
      • Department of Physics
      Pittsburgh, PA, United States