Feature Article

26 English Edition No.7

JY DivisionInformation

Grating & OEMVacuum Ultra Violet Monochromator

Erick Jourdain

Abstract

Taking the advantage of Jobin Yvon(JY) leading position in the design and realisation of diffraction grating JY hasdeveloped over the last past 30 years some of the most innovative Vacuum Ultra Violet (VUV) monochromatorsfor synchrotron centre. These monochromators, which combine the ultimate advanced diffraction gratingtechnologies and some ultra precise under vacuum mechanics, have been installed in several synchrotron centresand have demonstrated superior performances. Today the development of new compact VUV sources andapplications require compact VUV monochromators. Based on diffraction grating capabilities and our largeexperience acquired in the difficult and demanding market of under vacuum synchrotron monochromators JY iscurrently developing new compact monochromators for application such as Extreme UV lithography or X-rayPhotoemission Spectroscopy.

Introduction

In the middle of the seventies thank to the developmentof the aberration corrected holographic toroidal gratingsat Jobin Yvon (JY) we introduced on the market a newVUV monochromator concept that drastically enhancedthe spectroscopic performances in this field [1]. Duringthe eighties the Toroidal Grating Monochromator (TGM)and its associated toroidal gratings had a large and worldwide success. At that time most of the synchrotron VUVbeamline got equipped with this monochromator type andeven today some recent beamline are still been developedaround TGM like the one manufactured and installed in2001 - 2002 at Daresbury (UK). This monochromator isa perfect example of our under vacuum mechanical systemcapabilities as its scanning mechanism (goniometer)exhibits a weight of around 100 kg in order to handle upto three 300 mm × 80 mm gratings interchangeable undervacuum while the angular scanning resolution of thegoniometer is equal to 0.4″ (arcsec) (Fig. 1).

Fig. 1 Inner View of Toroidal Grating Monochromator ChamberInstalled at Daresbury (UK)

Following the development of the TGM and associatedgrating JY started some collaboration with synchrotronscientists to continue the development and manufacturingprocess of diffract ion grat ing and associatedmonochromators. Especially the last generation ofsynchrotron source has required during the last decade alarge R&D effort to adapt the instrumentation to new hugepower emitted by the source.

The synchrotron sources have been over last threedecades used for experiment that required Vacuum UltraViolet light between 1 and few hundreds nm, thanks to itscontinuous and bright emitted spectrum. Nevertheless

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synchrotron required huge and expensive instrumentationthat limits its availability. Today the development of newcompact sources, such as capillary discharge or laserinduced plasma, already enables tabletop VUV experimentswith significantly less equipments than in synchrotroncases. The JY VUV group already offers a complete rangeof compact monochromators and spectrographs especiallyadapted for this growing market of compact VUVapplication. Examples of such compact instruments arepresented in last part of this paper.

Synchrotron VUV Instrument

We at JY focus our effort on new optical components,monochromator designs and components in collaborationwith synchrotron centres to increase the resolution, theflux and minimises the harmonic rejection of themonochromator. The third generation synchrotron sourcesbrilliance has developed a need for especially dedicatedinstrumentation (cooling optics, high resolutionmonochromators, stable mechanics, etc.) adapted to thesource parameters.

In middle of the 1990 s, JY started a R&D program incollaboration with LURE (Orsay, France) optical groupto develop new beamlines and components well adaptedto third generation synchrotron sources. One of the firststeps of the program was the development of threededicated software to optimise and simulate grating andbeamline performances and therefore be able to proposecomplete project study starting from definition phase. Ashort description of the three developed software arepresented hereafter.

(1) Grating Efficiency Calculation

As the main part of a monochromator the gratingneeds to be optimised in terms of efficiency and highorder rejection. Calculation of grating efficiency andharmonics contamination is done thanks to a JYsoftware. This software based on the electromagnetictheory includes some optimisation algorithms, whichgive the possibility to define grating parameters (groovedepth, c/d) for any specific VUV configuration whereno commercial software is available.

(2) VLS Grating Holographic Parameters Calculation

In some VUV monochromator optical designsaberrations are corrected using VLS (Variable LineSpacing) gratings, which means that the groove densityis given by a polynomial law. JY already proposed asimilar solution in 1975 in the Toroidal GratingMonochromator design where aberrations is correctedwith a non uniform groove density grating obtainedby interference of two spherical wavefronts. Like inthe TGM case the VLS grating density can be obtainedby interference of two wavefronts, which give all theadvantages of holographic recording grating (no ghost,less stray light). Definition and optimisation ofholographic recording parameters that permit toproduce the VLS law on the grating is performed withsoftware developed in JY [2], which calculates theinterference produced by plane, spherical or asphericalrecording laser wavefronts.

(3) Beamline Ray Tracing and OptimisationJY can ray trace any VUV system starting from thesource (undulator, wiggler, bending magnet, etc.) upto the experimental chamber with software especiallydeveloped for grazing incidence optics. With this toolany instrument or beamline performance taking intoaccount slope errors and roughness of the opticalsurfaces can be checked and optimised by aminimisation of a merit function.

Taking benefit of these simulation softwares JY hasdeveloped a new beamline for the second-generationstorage ring, the SACO (LURE in France) *. This storagering is in operation since 1986 and presents somecharacteristics such as high emittance (source size), lowbrilliance and floor vibration not optimise for highresolution spectroscopy beamline. The challenge of thisbeamline was to realise a monochromator with the highestpossible performance in the Extreme Ultra Violet (EUV)range. The different constraints of the project weredetermined by the end users and the SACO hall experimentenvironment: The monochromator have to cover anextended wavelength range (30 - 250 nm) with a spectralresolution (λ/∆λ) better than 150,000 over the wholespectral range. This design goal was of the level of thehighest spectral resolution never achieved on a

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synchrotron beamline but with a fixed monochromator(no wavelength scanning).

* LURE is the French synchrotron radiation laboratory locatedat Orsay in Essonne. It is under joint management by theCNRS / CEA / MENRT (UMR 130). It has two storage rings(DCI and SACO) which produce photons of energies rangingfrom IR to 50 keV, a free electron laser working in the IR –from one micron to 50 microns – (CLIO), and a free electronlaser on SACO.

Fields of application using the SACO• Atomic and molecular physics• Solid state physics• Surface physics• Biology• Chemistry• Optics and detectorshttp://www.lure.u-psud.fr/

Above 30 nm normal incidence optical configurationcan still be used as some material presents reflectivityclose to 30 %, which is no more the case below 30 nm.Nevertheless this reflectivity of 30 % does not allow opticalconfiguration with numerous optics in order to maintainrelatively high monochromator throughput. Therefore asolution with a single optics monochromator based on aslightly modified Rowland circle principle (Eagle off plane)was chosen. In this design a spherical grating is the onlyoptical piece that performs the dispersion of the light andthe focalisation. The main difficulty in Rowland circledesign is the non constant position of the spherical gratingfocal point, which moves during the wavelength scanning.In order to maintain the exit slit position fixed it is thereforenecessary that the grating translates during the scanningmovement.

The focal length of the instrument has been defined at6.65 m in order to ensure the ultra high spectral resolution.This focal length value and grating groove density (4,300groove per mm) gave a theoretical spectral resolution above200,000, which was the goal value of the design. The6.65 m focal length and target spectral values have risenup some constraints on the under vacuum gratingscanning and translation mechanism. For example theheight error of the translation have to be maintain below0.5 µm over the 300 mm range while a scanning angularresolution of 0.05 arcsec have to be achieve. After a newapproach of the under vacuum translation mechanical

design and a new design of the scanning drivingmechanism (Fig. 2) we install and align the new andchallenging monochromator and its beamline in 1999.

Fig. 2 Inner View of Eagle Off Plane Monochromator ChamberInstalled at LURE (France)

Since the completion of the installation andcommissioning the monochromator has demonstratedperformances above scientist expectations with a spectralresolution of 208,000 [3] (Fig. 3) and a stray light level neverachieved (Fig. 4). This spectral resolution represents evertoday the world record of a synchrotron monochromator.

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Fig. 3 Ar Auto Ionisation Spectra Demonstrating a MonochromatorSpectrum Resolution of 208,000 (λ/∆λ)

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Fig. 4 Ne Auto Ionisation Spectra Demonstrating Low Stray LightLevel of Monochromator

Compact VUV Instruments

The EUV lithography R&D programs managed by majorsemi conductor companies have raised up new compactVUV sources developed to have strong emission line at13.5 nm. These sources like capillary discharge, laserinduced plasma or hollow cathode do not yet present thepower, repetition rate and reliability characteristics requiredfor EUV lithography project. Nevertheless they are alreadyproviding large emission wavelength range from few nmup to few hundreds with flux and repetition rate drasticallyhigher than penning discharge sources previously used inthis domain. These new sources offer therefore analternative to synchrotron for experiments that do notrequire the exceptional synchrotron characteristics. As fewexamples of experiments achievable with these sourceswe can mention EUV reflectometry, X-ray PhotoemissionSpectroscopy (XPS), EUV ellipsometry, fluorescence.

Thanks to our long experience in the Synchrotronmonochromator designs, our diffraction gratingdevelopment and production capabilities we alreadypropose the l a rges t r ange o f VUV compac tmonochromators and spectrographs adapted to these newsources. Based on toroidal diffraction grating or similardesign these instruments provide large throughput andrelative high spectral resolution compare to their fewhundred millimetres focal length. JY line-up includes theproduct models described as below for VUV application.

3.1 LHT 30The LHT 30 was the first VUV instrument in the world

designed around a toroidal aberration corrected grating.This monochromator covers the whole VUV wavelengthrange from 10 to 300 nm with three different gratings.Providing a spectral resolution of few tenths of nm it isthe instrument of choice for most of VUV experimenttypes even 30 years after is development.

3.2 VTM 300The VTM 300 (Fig. 5) designed around new generation

of aberration corrected toroidal grating allowsmonochromator and spectrograph (flat field) operationwith the same grating. A simple modification of the exitport from an exit slit to a bi-dimensional detector permitsthe use of the instrument as a monochromator to selectan excitation wavelength or as a flat field spectrograph tocharacterise the emitted light of a source or sample.Covering the EUV region from 50 to 300 nm this instrumentis mainly dedicated to lithography characterisation orfluorescence excitation around 100 nm where its singleoptics design offers an incomparable throughput.

Fig. 5 Vacuum Toroidal Monograph (VTM 300)

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3.3 TGS 300VUV source characterisation is a key challenge to improve

performances and reliabilities. The TGS 300 (Fig. 6) is theinstrument for such application. With its three interchangeableflat field toroidal gratings this spectrograph offers a uniqueopportunity to perform time resolved studies over largespectral ranges (9-32 nm, 10-110 nm or 15-170 nm) withtenths of nm spectral resolution. Equipped with two-dimensional multi channel plate (MCP) detector or VUV CCDcamera the TGS 300 is the most suitable instrument for sourcecharacterisation.

Fig. 6 Toroidal Grating Spectrograph (TGS 300)

Fig. 7 shows examples of data recordedwith the TGS300.

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Fig. 7 Examples of Data Recorded with the TGS 300(a)Spectral Image of a EUV Lithography Capillary Discharge

Source Recorded with the TGS 300 in the Range 9.5 –32 nm

(b)Spectrum of a EUV Lithography Capillary DischargeSource Deduced from the Image Presented Fig. 7(a)

3.4 PGM 200Instead of the other instrument the PGM 200 (Fig. 8)

is designed around a plane aberration corrected gratingused in conjunction with a toroidal mirror. This slightlydifferent design gives the opportunity to work with anexit beam parallel to the entrance one, which is crucialwhen space is a constraint. This is especially the casewith VUV source laser generated such as high harmonicsgeneration or plasma with gas or solid target. Similarlythan the VTM the PGM is a versatile instrument thatpermits monochromator and spectrograph operation withthe same optics.

Fig. 8 Plane Grating Monograph (PGM 200)

Conclusion

JY has acquired over the year a large experience andrenown in the designed and realisation of VUV synchrotronoptics and monochromators. This has been demonstratedby some great technical successes and a reconnaissance ofthe world wide synchrotron community. With the JY gratingteam we pursue our close collaboration with the synchrotronscientists to bring new optical solution that enhancespectroscopy capability in this field but also that will benefitin the new growing field of other VUV application.

AcknowledgmentsThe author would like to express his thanks to the whole

Jobin Yvon grating’s team for their help and advices duringthe definition of the instruments and the wonderfulperformances of the optics supplied.

The VUV instrumentation in Jobin Yvon owed a lot toD r. L e p è re , w h o d e s i g n e d a n d d e v e l o p e d t h emonochromator presented in this review.

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References

[1] D. Lepère “Monochromateur à simple rotation du réseau,à réseau holographique sur support torique pourl’ultraviolet lointain”, Nouvelle Revue d’ Optique, vol. 6(3), pp. 173 - 178 (1975).

[2] B. Deville et al. “Holographically recorded ion-etchedvariable-line-space gratings”, Proc. SPIE Vol. 3450, p. 24- 35, Theory and Practice of Surface-Relief DiffractionGratings: Synchrotron and Other Applications, WayneR. McKinney; Christopher A. Palmer; Eds.

[3] L. Nahon et al. “Very high spectral resolution obtainedwith SU5: A vacuum ultra violet undulator-basedbeamline at Super-ACO”, Review of ScientificInstruments, vol. 72, number 2, February 2001, p.1320 -1329.

Dr. Erick JourdainJobin Yvon S.A.SGratings and OEM DivisionVUV Product Manager