from nanometers to gigaparsecs: the role of nanostructures in...

From nanometers to gigaparsecs: The role of nanostructures in unravelingthe mysteries of the cosmos

Mark L.Schattenburga)

Space Nanotechnology Laboratory, Center for Space Research, Massachusetts Institute of Technology,77 Massachusetts Avenue, Cambridge, Massachusetts 02139

~Received 1 June 2001; accepted 24 September 2001!

High launch costs and the extreme distance to astrophysical objects place a premium onastrophysical instrumentation with the highest attainable sensitivity and resolution at the lowestpossible weight and cost. Many interesting and useful optical phenomena occur when the size andplacement accuracy of features are comparable to, or smaller than, the wavelength of light. Theseconsiderations have compelled us to develop a variety of nanotechnoligies that have now beenutilized in space physics instrumentation on nine missions. These include 200- and 400-nm-periodmembrane-supported transmission gratings for high-resolution spectroscopy of astrophysical x-raysources, mesh-supported transmission gratings for solar extreme ultraviolet~EUV! monitoring, andUV nanofilters with 45 nm slots that are key components of atom cameras observing Earth’smagnetosphere. This article will describe instruments on space missions where we have appliednanotechnology. One application is the NASA Chandra Observatory x-ray telescope, for which wemanufactured a large quantity of transmission gratings for high-resolution spectroscopy. Chandra isnow returning a torrent of high-quality x-ray images and spectra from such interesting objects assupernova remnants, the accretion disks around black holes and neutron stars, stellar coronae,galaxy cluster cooling flows, and other x-ray-emitting objects up to gigaparsecs distant.@A shortastronomy lesson: As Earth orbits Sol, nearby stars in the sky appear to wobble due to parallax. Ata distance of one parsec~a ‘‘parallax-second’’!, the diameter of Earth’s orbit (;1.531011 m!subtends one arcsecond, so a parsec is around 3.3 light years, or 3.131016 m. For reference, thenearest star is around a parsec away, our Milky Way galaxy is a few kiloparsecs across, nearbygalaxies are megaparsecs away, and the known universe is measured in gigaparsecs (1025 m!.#Another application is the atom ‘‘camera’’ on the NASA Imager for Magnetopause-to-AuroraGlobal Exploration~IMAGE! spacecraft that studies Earth’s magnetosphere, the belt of plasmaaround the Earth formed by swept-up ions from the Solar wind trapped in the bottle of Earth’smagnetic field. The camera images the magnetosphere in the ‘‘light’’ of neutral atoms, rather thanphotons, emitted from the plasma due to charge exchange processes. We developed nanofilters,consisting of 500-nm-thick gold foils with 45-nm-wide slots, that are designed to block unwanteddeep-UV and EUV photons which would otherwise overwhelm the detector with a million-to-onenoise-to-signal ratio, thus allowing the camera to detect the weak atom fluxes. IMAGE is nowsending back spectacular atom movies of the magnetosphere revealing a wealth of new informationabout this complex and dynamic environment. Finally, I describe work in our laboratory aimed atdeveloping microtechnology for the shaping and assembly of glass microsheet optics tofew-nanometer accuracy. We believe these new x-ray optics will spawn a new generation ofdiffraction-limited x-ray telescopes with massive collecting areas and resolution approaching 0.1microarcsecond~;1 picoradian!. These new telescopes may enable the direct imaging of themassive black holes believed to lurk at the center of most galaxies. ©2001 American VacuumSociety. @DOI: 10.1116/1.1418410#

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I. INTRODUCTION

Our universe is a bizarre place, filled with many mysteous and wonderful objects. There are supernova explosthat during their fury emit more energy than all the billioof stars in our galaxy combined. There are black holesneutron stars, literally eating hapless normal stars likesun, the superheated gas glowing brightly in x-rays as isucked down the deep gravitational wells. There are su

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2319 J. Vac. Sci. Technol. B 19 „6…, Nov ÕDec 2001 1071-1023Õ200

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massive black holes like the one believed to lurk at the ceof our galaxy, many with more mass than all the stars ingalaxy combined and spraying huge jets of relativistic pticles into space many times longer than our galaxy. Thare just a sampling of the many strange objects under insive study by astronomers around the globe.

These studies are shedding light on some of the cenmysteries of the Universe: Why does only;10% of the massof the Universe appear to be visible? What is the naturethe mysterious ‘‘missing matter?’’ What powers the enomous relativistic jets shooting from quasars?~Massive black

23191Õ19„6…Õ2319Õ10Õ$18.00 ©2001 American Vacuum Society

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2320 Mark L. Schattenburg: From nanometers to gigaparsecs: Role of nanostructures 2320

holes are suspected.! How did the universe get here and whis its ultimate fate?

Increasingly, astronomers are turning to space instrumtation to avoid the perturbing effects of the atmospherestudy wavelength bands otherwise inaccessible. Therebeen a great deal of recent interest in the application of mand nanotechnology to space hardware. This interest is nral given the extreme need for spacecraft systems miniaization due to astronomically high launch costs and thegent need for ‘‘smart’’ materials and systems that cperform in hostile environments far from human supportrepair. Much of this work has focused on such micrelectromechanical systems~MEMS! devices as microthrusters, sensors, and actuators. Nanotechnology for advamaterials and biological applications are also of great inteto the space community.

Our laboratory has concentrated on developing micronanotechnology for space physics instrumentation. This tenology has been applied to a variety of missions rangfrom x-ray astronomy telescopes, which study objects asas the edge of the known universe, to instruments studynearby plasmas around the Sun and Earth. In general,efforts have focused on solving particularly challengispace optics problems in the UV and x-ray bands, wherspecific optical task needs to be performed with maximoptical performance and minimum weight.

Nanofabricated optical components developed inlaboratory can generally be characterized as passive stures marked by a high degree of accuracy, organization,dimensional control. For example, of importance are gratiwith precise control of period and line geometry, and velow levels of defects ~pinholes!. We have developedadopted, or adapted a variety of nanofabrication tools to fricate these structures, including methods of nanolithogphy, high-fidelity pattern transfer, nanometrology, and micelectromechanical systems~MEMS! processing.

Typical nanofabricated structures for space instrumetion utilize extremely thin and flimsy components that mube accurately registered, and yet at the same time neesurvive the extreme stresses of rocket vibration and the lathermal excursions typical of spacecraft environments, wout risk of failure or compromise to accuracy. The interfaof flimsy and precisely placed nanostructures with massspacecraft structural members, in an environment of lamechanical and thermal disturbances, is what we callscaling problem. This problem is generally solved by utiling a scaled hierarchy of micromechanical supports thateventually mated to robust spacecraft components usingoxy bonding and single-point and/or kinematic mountischemes. The use of low-outgassing epoxies andmatched, low-thermal-expansion materials are also veryportant. Extensive finite-element modeling and environmtal testing are essential to confirm component survivabilFortunately, nanostructures are typically fabricated inform of thin membranes and/or have such small membthat their mechanical resonances are safely above the petypical rocket acoustic spectra.

J. Vac. Sci. Technol. B, Vol. 19, No. 6, Nov ÕDec 2001

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II. NANOTECHNOLOGY FOR SPACE MISSIONS

This section describes details of the instrumentationnine space missions for which our laboratory has develonanotechnology. Nanotechnology for future missions isscribed in Sec. III. In this article I use the encompassing tenanotechnology to describe structures with features insub-100 nanometer range, and also larger features in thecrometer range where placement accuracy in the few naneter range is required.

A. The Chandra Observatory x-ray telescope:Gratings for high-resolution x-ray spectroscopy

The Chandra Observatory is a large NASA x-ray tescope that was launched into Earth orbit by the Space Shin July of 1999. Chandra is providing astrophysicists whigh-resolution x-ray imaging and spectroscopy of astphysical sources in the 0.1–10 keV energy band~wave-lengths between 0.1 and 14 nm!.1,2 Chandra utilizes grazingincidence reflective optics of the Wolter I design to imagerays~see Fig. 1!. X rays experience two reflections to focubouncing first from paraboloids of revolution and then frohyperboloids. Four nested shell pairs are used in ordeincrease collecting area. At the focal plane are two selectaimagers: a microchannel plate detector and a charge coudevice~CCD! camera.

In astronomy, imaging and spectroscopy are complemtary and indispensable tools. High-resolution spectroscopa particularly powerful tool in the x-ray band for the studyhot plasmas. The universe is filled with a plethora of bizaobjects harboring plasmas in the one- to ten-million-degtemperature range that emit bright and richly detailed x-spectra. Examples of these objects are the shock wavesupernova explosions and the accretion disks surrounblack holes and neutron stars that orbit and consume nostars. Initial results from Chandra confirm the value of higresolution x-ray spectra and reveal a wealth of informatabout such plasmas, including temperatures, element adances, velocities, and ionization lifetimes.3–5

FIG. 1. Chandra x-ray telescope utilizes grazing-incidence optics ofWolter I design. X rays bounce first from paraboloids of revolution and thfrom hyperboloids. The telescope collecting area is increased by the ufour nested shell pairs.

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On Chandra, two instruments provide high-resolutispectroscopy. The low energy transmission grating spectreter ~LETGS!, built by the Space Research OrganizatiNetherlands~SRON!, uses 1.0mm-period gold transmissiongratings for spectroscopy in the 0.15–14 nm wavelenband,6,7 while the high energy transmission grating spetrometer ~HETGS!, built by MIT, uses 200 and 400nanometer-period gold transmission gratings, fabricatedour laboratory, for spectroscopy in the 0.15–3 nm walength band.8

The optical principal of the HETGS is depicted in Fig.X rays reflecting from the telescope mirrors are focusthrough the grating array and dispersed along an imagersisting of a row of six CCD chips, also made by MIT.9 Onthe detector, short wavelengths land near the zero-orderage while long wavelengths land farther away. In orderprovide coverage of the extremely large telescope bawidth, the outer-two optic shells are mounted with gratinthat provide double the dispersion of those used withinner two. Spectral overlapping is avoided by mountingtwo grating sets offset by a small angle, such that the speforms a shallow ‘‘X’’ on the detector.

A photograph of the HETGS and an individual gratinelement are shown in Fig. 3. The spectrometer consists1.1-m-diameter toroidal aluminum disk in a Rowland circgeometry, to which are mounted 196 gratings with 400period ~MEGs! and 144 gratings with 200 nm perio~HEGs!. Gold grating lines are supported by submicrothickness polyimide membranes bonded to Invar frames~seeFig. 4!. The grating bars are designed to provide;p phaseshift of the transmitted x rays, maximizing first-order efciency @Fig. 4~b!#. The spectral resolution goal ofl/Dl.1000 at the 1 nm wavelength placed stringent limits onallowable magnitude of grating period and rotation errors.achieve this goal, all gratings were required to have the speriod to within 250 parts per million and to be rotationaaligned to within 1 arc min. The use of frames madelow-thermal-expansion Invar metal, in conjunction withsingle-point cantilevered mounting scheme@mounting tabvisible in Fig. 3~b!# were essential to control mounting ditortions.

The high resolution and efficiency of the HETGS enascientists to obtain x-ray spectra of unprecedented qua

FIG. 2. Optical design of the high energy transmission grating spectrom~HETGS!. X rays are focused through the grating disk and dispersed intX-shaped spectrum across a row of x-ray CCD detectors.

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Figure 5 depicts the spectra of the x-ray binary star Cape3

Narrow fluorescent emission lines from highly ionized sicon, magnesium, neon, iron, and oxygen are visible inspectra, the narrowness due to the tight control of the graperiod during manufacturing.

The gratings were patterned by interference lithograp~IL ! as depicted in Fig. 6. Our IL system represents overyears of continuous development and has been highly omized for high volume, high yield grating production.beam from al5351.1 nm argon ion laser is split into twbeams, expanded by spatial filters, and then recombinedintersection angle 2U. The resulting standing-wave gratinimage, with periodp5l/(2 sinU), is captured in a photo-sensitive resist. Active fringe-locking optoelectronics estalish a high-contrast grating image.

A macroview of the fabrication process is depicted in F7. Details of our fabrication process are availabelsewhere.10 In the first step@Fig. 7~a!#, 100-mm-diametersilicon wafers are coated with 0.5–1.0mm of polyimide,which will later form the support membrane, a plating baconsisting of 5 nm of chromium and 20 nm of gold, andtrilevel resist consisting of 0.5–1.0mm of antireflection coat-

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ing ~ARC!, 15 nm of tantalum pentoxide, and 200 nmresist. Our group was the first to apply ARC and trilevresist processing to IL.11 In the second step@Fig. 7~b!#, thetrilevel resist is patterned by IL and converted to gold us

FIG. 4. ~a! Design of HETGS x-ray transmission grating element~HEGtype!. The polyimide support membrane is substantially transparent tox-ray band of interest.~b! Depiction ofp-phase shifting principle of x-raytransmission gratings, which suppresses zero order and maximizesorder efficiency.

FIG. 5. Chandra telescope HETGS spectra of the x-ray binary star Ca~Ref. 3!. Astrophysical plasmas in the (1 – 10)3106 degree range emitbright fluorescent spectra in the 0.1–10 nm wavelength band, revealiwealth of detailed information.


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a process described in more detail below and depicted in8. In the third step the substrate is spin etched frombackside using an HF/HNO3 mixture, stopping on the poly-imide layer @Fig. 7~c!#.12 Finally, an Invar frame is alignedand bonded to the resulting membrane@Fig. 7~d!# and theframe is cut out@Fig. 7~e!#.

A microview of the fabrication process is depicted in Fi8. First the substrates are coated with the six layers descrpreviously @Fig. 8~a!# and then patterned by IL@Fig. 8~b!#.Next the resist is used as a mask to etch into the interlayeCF4 RIE @Fig. 8~c!#, and subsequently the interlayer is usas a mask to etch into the ARC by oxygen RIE@Fig. 8~d!#.The resulting polymer form is used as a mold for gold eletroplating @Fig. 8~e!#, followed by stripping of the interlayerand ARC@Fig. 8~f!#. Finally the substrate is spin etched frothe backside and the resulting membrane is bondedmetal frame as described previously@Fig. 8~g!#.

Electron micrographs of intermediate steps in the fabrition process are shown in Fig. 9. The resist after interfere

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FIG. 7. Membrane-supported transmission grating fabrication process~mac-roview!.

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lithography and development is shown in Fig. 9~a!, the ARCafter oxygen RIE down to the plating base is shown in F9~b!, and the gold grating after ARC stripping is shownFig. 9~c!.

FIG. 8. Membrane-supported transmission grating fabrication process~mi-croview!.

FIG. 9. Electron micrographs of intermediate transmission grating procing steps.~a! Resist after interference lithography.~b! ARC after oxygenRIE down to plating base.~c! Gold grating lines after electroplating anresist stripping.


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B. The SOHO and GOES missions: Gratings for solarUVÕEUV monitoring

The NASA/ESA Solar and Heliospheric Observato~SOHO! was launched in December 1995. Our laboratofabricated 200-nm-period gold transmission gratings forSolar EUV monitor~SEM!, built by the University of South-ern California and International Radiation Detectors Corwhich monitors the Solar extreme ultraviolet~EUV! l530.4nm He II line.13,14 In the EUV and soft x-ray bands the polyimide support membranes used for the Chandra gratingsopaque and need to be avoided. Additional fabrication stwere utilized for the SOHO gratings to pattern, using contlithography, an integral gold support mesh over the 200-nperiod grating, consisting of a 4.0-mm-period grating crossedwith a 150-mm-period grating~see Fig. 10!.

More recently we developed a much stronger integnickel honeycomb support mesh using contact and interence lithographies to produce gold transmission gratingsUV/EUV spectrometers on the NASA/NOAA GeostationaOperational Environmental Satellites~GOES! N, O, P, and Qmissions, planned for launch in 2002, 2005, 2007, and 20The spectrometers, built by Panametrics and InternatioRadiation Detectors Corps., will monitor four solar EUbands betweenl58–100 nm and the hydrogen Lyman alphwavelength at 121.6 nm. We fabricated gratings with 2400, and 600 nm periods using a process described inII C. The Solar instruments will help understand, and phaps give advanced warning of, solar storms that can damsatellites and endanger astronaut health.

C. The IMAGE and TWINS missions: Nanofiltergratings for neutral atom imaging of Earth’smagnetosphere

The NASA Imager for Magnetopause-to-Aurora GlobExploration~IMAGE! satellite, launched by NASA in March2000, uses an arsenal of instruments to study Earmagnetosphere.15,16The magnetosphere is the belt of plasmaround the Earth formed by swept-up ions from the So

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wind trapped in the bottle of Earth’s magnetic field. Thcomplex magnetohydrodynamic environment can chadramatically as the Earth is buffeted by powerful sostorms, giving rise to a variety of phenomena, dubb‘‘space weather.’’ The aurora borealis or ‘‘northern lights’’one manifestation of these geomagnetic storms. Spweather is of great interest to the space community sithese storms are a danger to satellites and astronauts.

Traditional methods of magnetospheric study have blimited to sending space probes through this region andtempting to deduce its complex onionskin structure fropoint measurements of plasma density. New instrumentsIMAGE have enabled, for the first time, the direct higresolution imaging of the magnetosphere in the ‘‘light’’neutral atoms, rather than photons, emitted from the pladue to charge exchange processes.17 Energetic neutral atom~predominantly oxygen! are created when ions, trapped aspiraling in Earth’s magnetic field, collide with neutral hdrogen atoms in Earth’s tenuous geocorona. The ion captan electron from the hydrogen atom, forming an energneutral atom~ENA! that subsequently leaves the magnesphere, while the hydrogen atom becomes ionized and tits place in the trap. These neutral atoms contain directformation about the composition, energy, and density oftarget plasma population. IMAGE is now sending back sptacular atom images of the magnetosphere that revewealth of new information about this dynamic environme

The medium energy neutral atom~MENA! instrument onIMAGE, built by the Southwest Research Institute~SWRI!

FIG. 11. IMAGE mission medium energy neutral atom~MENA! camerainstrument concept. At the bottom of the detector a microchannel p~MCP! stack, backed by a position-sensitive anode~PSA!, amplifies andsenses charged particles. Energetic neutral atoms~ENAS! are first colli-mated and then ionized by a 2.6-nm-thick carbon foil. An acceleratingproduces a pulse of electrons on the PSA and starts a clock. The positiotime of the ion hitting the PSA allows measurement of the directionenergy, respectively, of the primary ENA. UV nanofilters prevent deep-photons from flooding the detector.


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and Los Alamos National Laboratory~LANL !, is sensitive toENAs in the 1–30 keV range.18 The MENA imager is atime-of-flight slit camera that utilizes a collimator array anspacecraft spin to produce images with 8°34° angular reso-lution over a 140°3~360° field of view~see Fig. 11!. ENAsentering the detector are collimated by a set of platesthen ionized by traversing a 2.5-nm-thick carbon foil. Aaccelerating grid produces a pulse of electrons that starclock. A segmented position-sensitive anode then determthe polar angle of the ENA from the position of the ion aits energy from the time of flight.

Unfortunately, this class of detectors suffers from a debtating noise problem due to the intense and ubiquitodeep-UV radiation, predominantly hydrogen Lyman alpha~l5121.6 nm!, originating from the Sun and scattered bEarth’s atmosphere, which overwhelms the sensitive detewith a million-to-one noise-to-signal ratio. We developenanofilter gratings, consisting of 500-nm-thick gold foiwith 45-nm-wide slots, that are designed to block unwandeep-UV and EUV photons, thus allowing the camera totect the weak atom fluxes@see Fig. 12~a!#. The position ofthe filters in the instrument is visible in Fig. 11, between tcollimator and carbon foil. The narrow slots in the nanofilteallow atoms to stream through while blocking deep-UV wa discrimination exceeding 106, which has been confirmedby extensive modeling and measurement@Fig. 12~b!#.19,20

These studies show that control over grating slot geometrparticularly critical: every 10 nm growth of slot width degrades the ability of the filter to block deep-UV by a factor10.

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Nanofilters are much more difficult to fabricate than tChandra gratings. The sub-45 nm slot width, the requiremthat gratings are free standing to allow atom transmissthe need of a robust nickel support mesh which requirenitride etch barrier, and the need for extremely low deflevels to prevent UV leakage, almost double the numbeprocess steps, many of which are extremely unforgivingrequired extensive development.

Our nanofilter fabrication process is depicted in Fig. 1Details of our fabrication process are available elsewher21

First the substrates are coated with six layers similar toChandra gratings, except that low-stress silicon-rich nitris substituted for polyimide@Fig. 13~a!#. The sacrificial ni-tride will later serve as a durable etch-stop barrier durHF/HNO3 spin etching. The resist is then patterned by intference lithography~IL !, the trilevel resist is wet and plasmdeveloped, the lines are electroplated with gold, and thesist stripped, using processes similar to the Chandra grat@Figs. 13~b1!–13~b3!#. The target ARC linewidth of 4563nm was achieved through precise control of resist dose

FIG. 13. Mesh-supported transmission grating fabrication process.


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ing lithography and substrate temperature during RIE. Thadditional resist is spun over the gold grating and a comnation of IL and contact lithography is used to patternintegral nickel support mesh consisting of a 4.0-mm-periodgrating and a 346.4-mm-period triangle grid@Fig. 13~c!#. De-tails of the support mesh design are depicted in Fig. 14.wafer is then spin-etched from the back, stopping onnitride barrier layer@Fig. 13~d!#.

At this point an optional pinhole-plugging step is peformed @Figs. 13~e1! and 13~e2!#. While pinholes are not atremendous problem for gratings intended for spectroscofor the nanofilter gratings even a single 20mm pinhole persquare centimeter will leak well over one part per millionUV. We developed a simple self-plugging procedure thatvolves spinning resist over the grating and backlighting wactinic UV radiation@Fig. 13~e1!#, followed by resist devel-opment and nickel electroplating@Fig. 13~e2!#.

The grating membrane is then bonded to stainless sframes and cut out, similar to the Chandra [email protected]~f!#. Finally the nitride barrier and plating base are rmoved by plasma etching@Fig. 13~g!#.

Electron micrographs of intermediate steps in the fabrition process are shown in Fig. 15. The ARC lines afterthography and oxygen RIE are shown in Fig. 15~a!, and a topview of the gold grating after plating and ARC strippingshown in Fig. 15~b!.

We also fabricated similar gratings for two additionneutral-atom cameras: the NASA Two Wide-Angle NeutrAtom Spectrometers~TWINS!, scheduled for launch in 2002and 2003, which will provide stereoscopic images of tmagnetosphere.

The MENA camera on IMAGE is now sending back spetacular atom ‘‘movies’’ of geomagnetic storms. Figure 1shows two frames from a movie of the magnetic stormAugust 12, 2000. A huge blob of plasma has been deposon the dark side of the earth, which over 12 h has spraround the equator in a so-called ring current.22 Duringstorms as much as a million amperes can flow in these

FIG. 14. Support mesh design for freestanding nanofilter transmission ging. ~left! Nanofilter grating and 4.0-mm-period nickel supporting grating~Middle! Expanded view showing 4.0-mm-period nickel support grating and346.4-mm-period nickel triangle-grid support mesh.~Right! Expanded viewshowing 346.4-mm-period nickel triangle mesh and stainless steel fligframe. Note the single-side cantilevered mounting scheme.

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rents, inducing similar currents in long transmission linand pipelines on the Earth. Intense magnetic storms hbeen known to destroy satellites and have even knockedthe North American power grid.23

FIG. 16. Frames taken 12.5 h apart from an atom camera ‘‘movie’’ ofmagnetospheric storm of August 12, 2000~Ref. 22!. The small white circledepicts Earth while the white loops are magnetic field lines. The view inframes is from a pole with the Sun side of the Earth in the upper right hcorner and the night side in the lower left.

FIG. 17. Silicon microcomb design.~a! Reference combs are designed to make highly accurate single-point contact with x-ray mirror foils.~b! Spring combshave flexible springs that impart minute forces to properly shape and position foils.

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III. NANOTECHNOLOGY FOR FUTURE SPACEMISSIONS

A number of new nanotechnologies are under develment in our laboratory for possible use in future space msions, including the NASA Constellation X and MAXIMmissions. These include 100-nm-period gold transmissgratings fabricated by achromatic interference lithograph24

super-smooth x-ray reflection gratings,25 and scanning-beaminterference lithography that will lead to new high-resolutigratings with unprecedented control of period and phase.26,27

This last section will describe silicon microcomb technolothat we have pioneered for the nano-accurate assembfoil x-ray optics.

Traditional x-ray optics, such as depicted in Fig. 1, agenerally fabricated by the extremely expensive and labous method of cutting out the optic shells from huge blocof low-expansion glass, followed by painstaking polishinFor example, the superb optics on Chandra achieve subsecond resolution, but at a cost well in excess of 100 milldollars and yet yielding little more effective collecting arthan an amateur telescope. A number of groups worldware developing alternative foil optic designs based on gmicrosheets only a few hundred microns thick, which anested by the hundreds to provide large effective areasvery little mass.28 Unfortunately, these groups have not beable to simultaneously achieve both large collecting areahigh resolution, due to the difficulty of shaping and assebling thin foils to the required submicron tolerances. Weaddressing these issues with micromachined structures.

Our microcomb design is depicted in Fig. 17. They cosist of two types: reference combs@Fig. 17~a!# and springcombs@Fig. 17~b!#. The combs are CAD designed and thfabricated by lithographic patterning and etching all the wthrough silicon wafers, in cookie cutter fashion, using tBosch plasma etch process.29 Electron micrographs of completed combs are shown in Fig. 18.

The combs are used to establish a metrology refereframe in a piece of tooling called an assembly truss.30,31Theassembly truss utilizes a high-accuracy reference block, sas a flat or cylinder, to which the ends of all reference comare registered. Because the combs are lithographicallyfined, all their surfaces are accurate to a small fraction omicron. The reference block and the comb design accura

FIG. 18. Electron micrographs of microcombs.~a! Spring comb.~b! Refer-ence comb.


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establish the contact surfaces of all the reference comb tto precisely position the foils to their correct locations ashapes in three-space. The spring combs are designesliding actuation in the assembly truss, so that the sprteeth apply controlled amounts of force to nudge the foagainst the reference teeth~see Fig. 19!. We have built andtested an assembly truss that has demonstrated submassembly accuracy, corresponding to subarcsecond opquality.30 This represents a;1003 improvement over previ-ous foil optic assembly technology.32

The short wavelength of x-rays potentially enables1000-fold improvement in resolution over visible light telscopes, but diffraction-limited performance has been difficto achieve. As lithographic accuracy improves over the ndecade, one could imagine fabricating microcombs withmensional tolerances approaching 10 nm. It turns outthis is the required figure and assembly tolerance for facating diffraction-limited optics forl51 nm x rays, due tothe tremendous sin~U! relaxation of optic figure tolerance agrazing angles.33 A proposed NASA mission, the Micro Arcsecond X-ray Imaging Mission~MAXIM !,34 may perhapsutilize x-ray interferometer optics based on silicon micrcomb assembly. If MAXIM can be realized, telescope relutions of 1 picoradian~0.1 microarcsecond! will becomepossible, some 106 times better than conventional telescopeSuch a resolution would enable the direct imaging ofsupermassive black holes believed to lurk at the centegalaxies.

ACKNOWLEDGMENTS

Words cannot express the author’s profound sensegratitude to Professor Claude R. Canizares and ProfeHenry I. Smith for their unwavering support and encourag

FIG. 19. Depiction of silicon microcomb assembly principle. Combs adesigned to assemble;200-mm-thickness glass microsheets in a specpiece of alignment tooling called an assembly truss. Spring combs aresigned to slide in the assembly truss, forcing the microsheets againsfixed reference combs.

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