NANOMETER PRJXISION IN LARGE SURFACE PROFILOMETRY’
.tBlVL-66587Invi edpapwpres. at 9th lnt% Cor ONPmdm-rionEngineeriq (RIPE)Osaka,]apan - 8/30-9/l/99NANOMETERPRJXISIONIN LARGESURFACEPROFILOMETRY’Peter 2. T;rkacsBrookhavenNational LaboratoryUpton, NY 11973-5000May, 1999*This work supported98CH10886in part by the U.S. Departmentof EnergyunderContractNo.:DE-AC02--
Nanometer Precision in Large Surface ProfilometryInstrumentationPeter Z. TakacsDivision 535B, Brookhaven National LaboratoryUpton, NY 11973 USAAbstractThe Long Trace Profiler (LTP) is in use at many synchrotron radiation (SR)laboratories throughout the world and by a number of manufacturers who specializein fabricating grazing incidence mirrors for SR and x-ray telescope applications.Recent improvementsin the design and operation of the LTP system have reducedthe statistical error in slope profile measurement to the 1 standard deviation level of0.3 microradian for 0.5 meter long mirrors. This corresponds to a height error on theorder of lo-20 nanometers. This level of performance allows one to measure withconfidence the absolute shape of large cylindrical aspheres and spheres that havekilometer radii of curvature in the axial direction. The LTP is versatile enough tomake measurementsof a mirror in the face up, sideways,and face downconfigurations.We will illustrate the versatility of the current version of theinstrument, the LTP II, and present results from two new versions of the instrument:the in situ LTP (ISLTP) and the Vertical Scan LTP (VSLTP). Both of them are basedon the penta prism LTP (ppLTP) principle that utilizes a stationary optical head andmoving penta prism. The ISLTP is designed to measure the distortion of high heatload mirrors during actual operation in SR beam lines. The VSLTP is designed tomeasure the complete 3-dimensional shape of x-ray telescope cylinder mirrors andmandrels in a vertical configumtion. Scans are done both in the axial direction and inthe azimuthal direction.Keywords:Metrology;Profilometry;Figure measurement;X-ray mirrors; Aspherics1.IntroductionThe Long Trace Profiler (LTP) is a non-contact optical profiling instrument usedfor measuring the surface slope and figure errors on large cylindrical mirrors, such asthose used in x-ray beam lines at synchrotron radiation (SR) light sources [l, 21.Surface slope errors with a magnitude of a few microradians on grazing-incidencemirrors seriously compromisethe beam quality of 3rd generationSR sources.Knowledge of the precise surface curvature of meter-long cylindrical and toroidalmirror segments is essential for the proper assembly and alignment of SR beam lines.The much-improvedcommercialversion of the LTP (LTP II) is optimized formeasuring the absolute surface figure of the large, long-radius mirrors used in thesebeam lines [3].Recent improvementsand modifications to the LTP II system have significantlyimproved the repeatabilityand accuracy of the measurementand extended its1
measurement applicability to other types of optical components. These improvementsand modifications include the addition of a Dove prism to the standard LTP II opticalconfiguration [4, 51 and implementationof the penta prism LTP (ppLTP) technique[6] in two versions: the In Situ LTP (ISLTP) [7, 81 and the Vertical Scan LTP(VSLTP) [9-121. The ISLTP has been successful in measuring mirror heat loaddistortion at ELETTRA in Italy and at the Advanced Photon Source (APS) atArgonne National Laboratory. The VSLTP was developed for NASA Marshall SpaceFlight Center by Continental Optical Corporation in collaboration with BNL througha NASA SBIR grant2. LTP II Operating PrincipleBased on the principle of the pencil-beam interferometer developed by von Bieren[13, 141, the LTP is optimized for measuring large flats and spheres and especiallyaspheric surfaces with extremely long radii of curvature in the tangential direction,such as grazing incidence toroids and cylinders. The standard LTP II configurationhas a nominal scan length of 1 .meter and is designed to measure concave and convexsurfaces that have at most a total slope change of 10 mrad. The shortest radius ofcurvature, Rmin, that can be measured over a given scan length, L, must satisfy thefollowing inequality:- 10 mrud.(1)The length of the scan is limited only by the length of the air bearing translation stageused to move the optical head. The largest scan length systems currently are 2 metersat ESRF in Grenoble[ 1S]and at the APS [ 161.STATlONARYFig.1 Schematicsystem.Adiagramof the standardLTP II opticalThe LTP measures the localslope profile of a surface bymeasuring the angle at which alaser probe beam is reflected asit is translated across the surfaceby a linear translation stage [l,13, 141. A simplified schematicof the LTP optical system isshownin Fig. 1. A beamsplitting arrangement separatesa laser beam into a pair ofcollinear beams, separated by adistanceofaboutonemillimeter.The beam pair issplit again into two sets of probepairsbyapolarizingbeamsplitter (PBS), and each setis separately directed into thereference arm or the test surface2
arm of the system. Upon reflection fromthe test and reference surfaces, both setsof beams are directed back into theoptical head, where they pass through aFourier transform lens, and are broughtto a focus on a linear array detector.Each set of beam pairs produces its owninterferencefringe patternon thedetector,and the locationof theminimum in each fringe pattern is adirect measure of the local slope of thesurface.(a)Error Reduction MethodsAs the optical head of the standardLTP II is driven along the translationstage, the major source of measurementerror is caused by pitch angle error inthe direction of travel. Pitch angle errorcauses the test and reference beams tomove in onoositedirectionon thedetector (Fig. 2a).The magnitude of the Fig. 2 Phase relationship between thermal andmechanicalerrors in the test and referencecarriage pitch error can be on the orderbeams(a)without,and (b) with the Dove prismof tens of microradians.Changes ininstalled in the LTP II optical head. The pitchtemperature and temperature gradientsangle error is shown highly exaggerated.in theopticalandmechanicalcomponentsproduce angular errors that cause the fringes to move in the samedirection on the detector. The reference arm was mainly designed to correct formechanical pitch error by adding the reference signal to the test signal, since themagnitude of the pitch error is usually much greater than the thermal drift error.Unfortunately,correctingbotherrors at the same time in thestandard LTP II is not possible [46, 171. The residual error left in0the processed data is a significantGlimitationin achievinghighz -10taccuracy with the standard LTP II.g-20Various methodshave beenPn-30devised for dealing with theseerror sources[3, 17-201. We have-40chosen to implementtwo other1-5ot050100150200methods for pitch error correction:Pseudo X positionthe Dove prism method for theFig. 3 Error correction with Dove prism. External force LTP II, and the penta prism LTPapplied to the optical head causes large error signalsmethod.in test and ref beams that are completely corrected.The sense of the pitch error3.I,.,,,RMS of residual is 0.86 prad.3
l.term alone can be changed by inserting a Dove prism into the referencearm on theLTP II optical head [4, 51. This enables simultaneouscorrection of thermal andmechanical errors by a simple subtraction of the test and reference signals. By tracingthe path of the reference beam through the system in Fig. 2b, we can see that pitcherror causes the beams in both the test and reference arms to move in the samedirection. The thermal drift error phase is unchanged. Subtraction of the test andreference signals in Fig. 2b now cancels both thermal and mechanicalerrorscompletely. Fig. 3 illustrates the completeness of the correction for mechanical error.As the scan was recorded, force was applied to the optical head to produce a largepitch error. The reference subtraction completely removes both errors, leaving a 0.86urad residual, which is essentially system noise.The other pitch error reduction method is to use a scanning penta prism and astationary optical head. This scanning method is identified as the penta prism LTP(ppLTP) [6]. The penta prismis driven by a translation stage10 scans, 496 pts each @ 1 mrn/ptcontainingmechanicalpitcherrors. However, the nature ofthe penta prism is such that theprobe beam is always deviatedby a 90” angle, independent oferrors in the orientation of thepenta prism.So with thisscanningpentaprismean subtractedconfiguration,we need notmakeany correctionsfor0100200300400500mechanical pitch error. In thex(mm)ppLTP the reference signal isFig. 4 Overlay of 10 scans on a 500 mm long mirror. Curvesused to correct only for theabout zero are residuals after subtrating the average fromthermal drift errors.each. The 1 cr error bar for the average slope profile is 0.29prad.4.RepeatabilityFig. 4 shows ten scans madewith the Dove prism LTP II ona 500 mm long test mirror thatare overlaid on each other toillustrate the repeatabilityinthe measurement.Subtractingthe average from each, we canoverlay the residuals and viewthe slope error noise in eachscan, shown as the curvesabout the horizontalcenterline. The standard deviation inthe residuals is 0.93 Frad rms,while for the average of the tenthe noise is less by a factor ofAll 106000CRheightprofiles 4606 10m500015040001005030000-50010020030040050X (mm)Fig. 5 Height profiles and residuals computed from the 10 slopeprofiles. in Fig. 4.4
tfi,which gives the standard deviation in the mean as 0.29 prad rms for this dataset. The corresponding rms error bars for the height residuals in Fig. 5 are 42 rnn rmsfor a single scan and 13.3 nm rms for the average. The mean radius of the averageheightis 4606 meters with astandard deviation in the mean of27 meters. The 0.6% relative errorin the averageis an excellentrepeatabilityresult for a longradius mirror measurement.Tests to show the insensitivityof the ppLTP to errors in themechanical stage are shown in Fig.6. Curve (a) is the result of testing-15"""'c"'L'”a mirror with the penta prism020406060100mounted on a mechanical slide thatScan Position [mm]has a 25 pad pitch error, and curve(b) is the result with a penta prism Figure 6. Measurements made with the ppLTP with thepenta prism mounted on two slides of very differentmounted on a high accuracy airquality: (a) penta prism on a mechanical slide; (b)bearing slide. Though the two slidepenta prism on an air bearing slide. Large pitch angleaccuracies are very different, theerrors in the mechanicalslide do not affect themaximum difference between theaccuracy of the measurement.measured profiles is only on the order of 10 nm. By eliminating pitch error effects inthis way, the ppLTP operating routinely at ELETTM is extremely stable and capableof highly accurate measurements[21]. Based on the success of the ppLTP method,two versions of the LTP II have been developed for different applications: the ISLTPand the VSLTP.In Situ LTP (ISLTP) for High Heat Load OpticsIn order to test mirror distortion under actual high heat load operating conditionsfrom third-generationsynchrotronlight sources, the in situ LTP (ISLTP) was5.developedinitiallyat ELETTRA[7, 211 and then througha collaborationbetweenBNL, APS, and ContinentalOptical Corporation[22]. There are two basicconfigurations for in situ ppLT:P distortion tests on SR beam lines. In both cases, theLTP optical head is always operated outside the vacuum chamber, while the test andthe reference pencil beams pass from the laboratory environment into the (normallyultra-high) vacuum chamber. Depending on the specific problems and the mirrorconditions, the two configurationsfor scanning the penta prism over the mirrorsurface are illustrated in Figure 7.5.1 Configuration (a)The penta prism and translation stage are mounted outside the vacuum chamber(Fig. 7a). In this case the sampling beam must pass through a large vacuum-isolationwindow which usually limits the length of the scan to be significantly less than thelength of the mirror. Reference reflection and adjustment- units are also locatedoutside the window. In this case the entire profiler is operated under atmosphericpressure with the convenience of not having to deal with UHV components. For this5
’.Ialreason this configurationwasused for the in situ distortionmirmr under testprofile tests at ELETTRA and atthe APS [2 1,221.5.2 Configuration(b)The penta prism is scannedinside the vacuum chamber (Fig.UHV WINDOWTestRBf7b). This configuration requiresBeamBeamonly a relatively small window‘sLTPto pass both the test andreference beams through the wallof thevacuumchamber.lb1VACUUM CHAMBERHowever, it does require moreIIcomplicated ultra-high vacuum-Ilmirmr under tedX-RAY BEAMcompatiblelinear translation1stage componentsand requiresrthat the translation mechanismleaves no contaminationon themirror surface. In the case of along mirror, this solution may beFig. 7. Two alternativesfor the in situ LTP distortioneven though it mayprofile test by use of a penta prism LTP: [a] -- Penta favorableprism scans outside the vacuum chamber with the require significantdesign workbeams through a large window. [b] -- Penta prism scans to implement.inside the vacuum chamber with the beams through a6. APS Mirror Distortionsmall window. The reference beam remains stationaryRecent measurements made atin each configuration,reflecting from a fixed point ontheAdvanced Photon Source atthe surface.ArgonneNationalLaboratoryhave confirmed that distortion of mirrors subjected to the intense beams in 3rdgeneration synchrotron source beam lines is a serious problem [22]. Results from aset of measurements made on a 200 mm long Si mirror exposed to x-rays from the 2ID-A 2.4 meter, 5.5 cm period undulator are shown in Fig. 8. These measurementswere made with a beam currentof 100 ma. The estimated totalabsorbed power in the mirror is100 Watts. The scans were madeusing configuration (a) through a90 mm diameter window. Eachscan takes about 2 minutes tocomplete,and the scanningstarted immediatelyafter thefront end shutter was opened. Abaselineprofileis subtractedfrom each scan to remove the IX (mm)intrinsic surface radius of 1 kmFig. 8 - ISLTP scans of distortion on the M2C side-cooledand the distortion effects of theSi mirror in the 2-ID-A beam line at the APS. InitialUHV window.convex distortion turns into a steady-stateconcaveVACUUM CHAMBERe a- -tIsurface after about 15 minutes of exposure.6
’.The height profiles in Fig. 8 indicate that the initial transient distortion in themirror is convex, as expected as the surface begins to heat up and expand. After a fewminutes the surface returns to its original shape and then becomes more concave.After about 15 minutes a steady-state is reached with the surface in a more concaveconfiguration, corresponding to a 20% change in the initial 1 km radius of curvature.When the shutter is closed and the heat load is removed, the surface becomes evenmore concave before it returns to its original baseline shape. The change in thesteady-state curvature of the surface when it is exposed to the x-ray beam heat load issignificantly larger than predicted by finite element calculations. The cause of thisdiscrepency is currently under investigation.Vertical Scan LTP for X-Ray Telescope OpticsA critical problem in the fabrication of mirrors used in x-ray telescope systems isthe lack of adequate metrology techniques to characterize the surface figure errors inthe cylindrical aspheres used in these systems. Based on the requirements of a NASASBIR program, and utilizing the ppLTP principle described previously, a VerticalScan Long Trace Profiler (VSLTP) was developed to enable fast and accuratemeasurementof inarytests during thecommissioningof the VSLTP indicate that the instrument is capable of microradianslope measurementrepeatability and sub-micron height repeatability over its largescan range.8.SummaryThe LTP has been under development and in routine operation at BNL for the past12 years. The LTP II is considered the instrument of choice by most SR laboratoriesand mirror manufacturers for measuring x-ray mirror surface slope and figure errors.8
Owing to recent significant performance improvements, the LTP’s repeatability andaccuracy has been increased by an order of magnitude, with slope error bars fortypical measurement parameters now in the range of 0.3 urad rms and with heighterror bars in the 10 to 20 nm range. Several special versions of the LTP have beendeveloped recently, including the ISLTP and the VSLTP. The LTP has the potentialfor wider applications in the fields of high power laser optics, FEL optics, adaptiveoptics, and X-ray or large aperture telescope optics, wherever large aspheric mirrorsare used, or wherever absolute measurementof long-radius curvature or surfaceflatness is required.AcknowledgmentsDetails of the ISLTP at ELE.TTRA were kindly provided by Shinan Qian, who isnow at BNL. Haizhang Li provided information about the VSLTP from ContinentalOptical Corporation.Kevin Randall was instrumentalin facilitatingISLTPmeasurements at the APS. Lahsen Assoufid provided Dove prism measurement datafrom the LTP II at the APS. This work was supported in part by NASA MarshallSpace Flight Center under SBIR contract number NAS8-40642,by the U.S.Departmentof Energy under Contract No. DE-AC02-98CH10886,and bySINCROTRONE TRIESTE in Italy.References1. P.Z. Takacs, S.K. Feng, E/L. Church, S. Qian, and W. Liu, “Long trace profilein Advancesin Fabricationandmeasurementson cylindricalaspheres,”Metrology for Optics and Large Optics, Jones B. Arnold and Robert A. Parks,eds., Proc. SPIE 966,354-364 (1989).2. P.Z. Takacs, K. Furenlid, 1 . DeBiasse, and E.L. Church, “Surface topographymeasurementsover the 1 meter to 10 micrometer spatial period bandwidth,” inSurface Characterization and Testing II, J.E Griever& and M. Young, eds.,Proc. SPIE 1164,203-211 (11989).3. S.C. Irick, W.R. McKinney, D.L.T. Lunt, and P.Z. Takacs, “Using a straightnessreference in obtaining more accurate surface profiles”, Rev. Sci. Instrum.,63,1436-1438, (1992).4. P.Z. Takacs and C.J. Bresloff, “Significant Improvements in Long Trace Profilerin Optics for ��Radiation Beamlines II, L.EI. Berman and J. Arthur, eds., Proc. SPIE 2856, 236245 (1996).5. P.Z. Takacs, E.L. Church, C.J. Bresloff, and L. Assoufid, “Improvements in theAccuracy and Repeatablili-ty of Long Trace Profiler Measurements,”AppliedOptics (in press) , (1999).6. S. Qian, W. Jark, and P.Z. Takacs, “The Penta-Prism LTP: A Long-Trace-Profilerwith Stationary Optical Head and Moving Penta-Prism,” Rev. Sci. Instrum. 66(3), 2562-2569, (1995).7. S. Qian, W. Jark, P.Z. Takacs, K.J. Randall, and W. Yun, “In-Situ SurfaceProfiler for High Heat Load Mirror Measurement,” Optical Engineering 34 (2),396-402, (1995).9
8. S. Qian, W. Jark, and G.S. et.al, “Per&t-prism LTP Detects First In-Situ DistortionProfile,” Synchrotron Radiauon News 9 (3), (1996).9. H. Li, X. Li, M.W. Grindel, and P.Z. Takacs, “Measurementof X-ray TelescopeMirrors Using A Vertical Scanning Long Trace Profiler,” Opt. Eng. 35 (2), 330338, (1996).10. H. Li; P.Z. Takacs, and T. Oversluizen, “Vertical scanning long trace profiler: atool for metrologyof x-ray mirrors,”in Materials,Manufacturing,&Measurementfor SynchrotronRadiationMirrors, P.Z. Takacs and T.W.Tonnesson, eds., Proc. SPIE 3152, 180-l 87 (1997).11. S. Qian, H. Li, and P.Z. Takacs, “Penta-Prism Long Trace Profiler (PPLTP) forMeasurement of Grazing Incidence Space Optics,” in MuZtiZayer and GrazingIncidence X-Ray/EUV Optics III, R. Hoover and A.B.C. Walker, Jr., eds., Proc.SPIE 2805, 108-l 14 (1996).12. P.Z. Takacs, H. Li, X. Li, and M.W. Grindel, “3-D X-ray Mirror Metrology with aVertical Scanning Long Trace Profiler”, Rev. Sci. Instrum., 67, (9), (1996), CDROM.13. K. von Bieren, “Pencil Beam Interferometer for Aspherical Optical Surfaces,” inLaser Diagnostics, Proc. SPIE 343, 101-108 (1982).14. K. von Bieren, “Interferometry of Wavefronts Reflected Off Conical Surfaces,”Appl. Opt. 22,2109-2114,(1983).15. J. Susini, R. Baker, and A. Vivo, “Optical metrology facility at the ESRF,” Rev.Sci. Insts. 66 (2), 2232-2234, (1995).16. C. Bresloff and D. Mills, “The Advanced Photon Source Metrology Laboratory”,Rev. Sci. Instrum., 67, (9), (1996), CD ROM, G. K. Shenoy and J. L. Dehmer,eds.17. S.C. Irick, “Improvedmeasurementaccuracyin a long trace profiler:Compensation for laser pointing instability,” Nuclear Instruments and Methods inPhysics Research A347,226-230 (1994).18. S.C. Irick, “Determiningsurface profile from sequential interference patternsfrom a long trace profiler,” Rev. Sci. Instrum. 63 (l), 1432- 143 5 (1992).19. S.C. Irick, “Advancementsin one-dimensionalprofilingwith a long traceprofiler,” in Int’l Symp on Optical Fabrication, Testing, and Surface Evaluation,J. Tsucjiuchi, ed. 1720, 162-168 (1992).20. S. hick, “Error reduction techniques for measuring long synchrotron mirrors,” inAdvances in Mirror Technology for Synchrotron X-Ray and Laser Applications,A. Khounsary, ed. 3447,101-108 (1998).21. S. Qian, W. Jark, G. Sostero, A. Gambitta, F. Mazzolini, and A. Savoia, “Precisemeasuring method for detecting the in situ distortion profile of a high-heat-loadmirror for synchrotronradiation by use of a pentaprism long trace profiler,”Applied Optics 36 (16), 3769-3775, (1997).22. P.Z. Takacs, S.N. Qian, K.J. Randall, W.B. Yun, and H. Li, “Mirror DistortionMeasurementswith an In-Situ LTP,” in Advances in Mirror Technology forSynchrotron X-Ray and Laser Applications, A. Khounsary, ed. 3447, 117- 124(1998).10
NANOMETER PRJXISION IN LARGE SURFACE PROFILOMETRY’ Peter 2. T;rkacs Brookhaven National Laboratory Upton, NY 11973-5000 May, 1999 *This work supported in part by the U.S. Department of Energy under Contract No.: DE-AC02- 98CH10886 . Nanometer Precision in Large Surface Profilometry .
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