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Experiment Research on Micro-/Nano Processing Technology of Graphite as Basic MEMS Material

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Graphite is expected to be a common choice for basic microelectromechanical-system (MEMS) material in the future. However, in order to become a basic MEMS material, it is very important for graphite to be adapted to the commonly-used micro-/nanoprocessing technology. Therefore, this paper used a directly lithography and etching process to study micro-, /nanoprocessing on graphite. The results show that the graphite surface is suitable for lithography, and that different shapes and sizes of photoresist patterns can be directly fabricated on the graphite surface. In addition, the micro-meter height of photoresist could still resist plasma etching when process nanometers height of graphite structures. Therefore, graphite with photoresist patterns were directly processed by etching, and nanometer amounts of graphite were etched. Moreover, micro-/nanoscale graphite structure with height ranges from 29.4 nm–30.9 nm were fabricated with about 23° sidewall.
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Appl.Sci.2019,9,3103;doi:10.3390/app9153103www.mdpi.com/journal/applsci
Article
ExperimentResearchonMicro/NanoProcessing
TechnologyofGraphiteasBasicMEMSMaterial
ChengZhang
1,2,3,
*,YijinLiu
4
,MinggeWu
1
andNingboLiao
1
1
CollegeofMechanicalandElectricalEngineering,WenzhouUniversity,Wenzhou325035,China
2
NanjingUniversityofAeronauticsandAstronautics,Nanjing210016,China
3
KeyLaboratoryofAirdrivenEquipmentTechnologyofZhejiangProvince,Quzhou324000,China
4
SchoolofMaterialsScienceandEngineering,UniversityofScienceandTechnologyBeijing,
Beijing100083,China
*Correspondence:zhangcheng@wzu.edu.cn;Tel.:+8615869707966
Received:10May2019;Accepted:27July2019;Published:31July2019
FeaturedApplication:ThisstudymaypromotegraphitetobecomeabasicMEMSmaterial,which
issignificantfortheexpansionofthescopeandfieldofMEMSapplications.
Abstract:Graphiteisexpectedtobeacommonchoiceforbasicmicroelectromechanicalsystem
(MEMS)materialinthefuture.However,inordertobecomeabasicMEMSmaterial,itisvery
importantforgraphitetobeadaptedtothecommonlyusedmicro/nanoprocessingtechnology.
Therefore,thispaperusedadirectlylithographyandetchingprocesstostudymicro,
/nanoprocessingongraphite.Theresultsshowthatthegraphitesurfaceissuitableforlithography,
andthatdifferentshapesandsizesofphotoresistpatternscanbedirectlyfabricatedonthegraphite
surface.Inaddition,themicrometerheightofphotoresistcouldstillresistplasmaetchingwhen
processnanometersheightofgraphitestructures.Therefore,graphitewithphotoresistpatterns
weredirectlyprocessedbyetching,andnanometeramountsofgraphitewereetched.Moreover,
micro/nanoscalegraphitestructurewithheightrangesfrom29.4nm–30.9nmwerefabricatedwith
about23°sidewall.
Keywords:graphite;MEMS;micro/nanoprocessing;lithography;etching
1.Introduction
Withthecontinuousmaturityofmicro/nanoprocessingtechnology,microelectromechanical
systems(MEMS)withsiliconasthebasicmaterialhavebeenrapidlydeveloped,andareusedin
aviation,biological,medical,andotherfields[1–3].Moreover,researchersareinvestigatingnewbasic
MEMSmaterialstomeetthespecialneedsofdifferentapplications.Forexample,flexiblematerials
suchaspolydimethylsiloxaneandpolyimidefilmhavebeeninvestigatedandusedasbasicmaterials
inpreparingflexibleandwearableMEMSdevices[4–6].Hightensilestrengthmaterialhasalsobeen
invented,suchasthenickel–molybdenum–tungstenalloy,whichisastrongmaterialinventedby
KevinJ.Hemker’steamatJohnsHopkinsUniversity.Ithasgoodtensileandhightemperature
resistancetomeetthedemandforMEMStoworkinharshenvironments[7].Withthecontinuous
expansionofthescopeandfieldofMEMSapplications,researchingfornewbasicMEMSmaterials,
suitablefordifferentapplicationfields,isreceivingincreasingattention.
Graphiteisagloballyabundantmineral.Itisacrystallinecarbonwithahexagonallayered
structurecrystallattice,eachlayerhasthesingleatomthicknessofgraphene[8].Thisstructuregives
graphiteexcellentcharacteristics,suchasconductivity,heatconduction,hightemperatureresistance,
radiationresistance,andgoodchemicalstability.Forexample,Zhengetal.foundthatsuperlubricity
characteristicsexistbetweenmicronsizedgraphitelayersintheatmosphere[9].Moreover,they
illustratedtheimportanceofsuperlubricitycharacteristicinfuturetechnologicalapplications,such
Appl.Sci.2019,9,31032of7
asdurablenano‐ andmicroelectromechanicaldevicesandmechanicalbearings[10].Theother
importantcharacteristicssuchasconductivityandgoodchemicalstability,havemadegraphitean
importantmaterialforelectrodesinthefieldoffuelcells[11].Theabovecharacteristicsmayenable
graphitebecomeanimportantchoiceforbasicMEMSmaterialsandcontributetographitebased
MEMStoworkunderharshconditions,suchasinchemicalcorrosion,friction,andwear.However,
micro/nanoprocessingtechnologyisthemainmethodtofabricateMEMSdevices.Therefore,
graphiteneedstomeettherequirementsofmicro/nanoprocessingtechnologytobecomeabasic
MEMSmaterial.Atpresent,CarbonMEMShavebeenproposedandstudied,Gongetal.[12]
proposesamethodofmixingpolyimidewithnanographiteparticleadditivetoincreasethereleasing
rateofsacrificiallayer.Wangetal.[13–15]fabricatedCMEMS/NEMSstructurebypyrolysisof
photoresistsonsiliconwafersattemperaturesrangingfrom600to1100°C.Otherresearchershave
usedplasmaenhancedchemicalvapordeposition(PECVD)todepositnanocrystallinegraphite
(NCG)andapplyNCGinMEM/NEMfield[16,17].Moreover,researchershavecarriedoutmicro
/nanoprocessingongraphiteandconstructedmicro/nanostructures.Liuetal.[18]depositedSiO2
filmsasahardmaskonthegraphitesurface,followedbylithographyandetchingprocessesonthe
graphitesurface.Evansetal.[19]usedaluminumfilmasahardmask,followedbylithographyand
etchingprocesses,andcombineditwithtransfertechnologytoproducea10nanometerthick
vermiculargraphite.Divanetal.[20]usedlithographyandliftoffprocessestofabricatemetal
microstructuresonthegraphitesurface.Junjietal.[21–23]fabricatedgraphiteMEMSwithtitanium
filmasahardmaskcoatingonthegraphite,thenprocessedphotoresistpatternsonthetitaniumfilm.
Tothebestofourknowledge,thecurrentprocessofgraphiteetchingreliesonmetalormetaloxide
asamaskmaterial,almostnoresearchonthedirectetchingofthegraphitesurfaceafterlithography
isavailable.Moreover,norelevantresearchonthestabilityofgraphicimagingduringthedirect
lithographyofthegraphitesurfaceisavailable.Besides,directlithographyiseasierforobtaining
imagesonMEMSthanprocessingmethodsusingmetalastheintermediatelayer.Inaddition,
lithographyofMEMSbasicmaterialisanimportantprocessstep,basicallybecomethefoundationof
otherMEMSprocesstechnology,besides,directlithographyonMEMSbasicmaterialcouldimprove
theprocessingefficiencyofMEMS.Therefore,stablelithographyandthedirectetchingofbasic
materialsarecrucialinthefabricationofMEMSdevices.Inthispaper,directlithographyonthe
graphitesurfacewasinvestigatedtoanalyzephotoresistimagestability,andthegraphitewith
photoresistwasdirectlyetchedsoastoanalyzetheadaptabilityofgraphiteasabasicMEMSmaterial,
whichmaysupporttheuseofgraphitetofabricateMEMS.
2.MaterialsandMethods
Opticallithography[24–27]isaparallelmicro/nanographicimagingmethodthatiscommonly
usedinMEMSprocessing.Inaddition,etchingprocess[28–31]isanimportantmicro/nanostructure
processinMEMSandthatisusuallyperformedafterlithography.Therefore,thispaperinvestigated
thedirectlithographyonthegraphitesurfacevialithographytoexaminethestabilityofdirect
photoresistpatternimagingonthegraphitesurface.Furthermore,utilizedthecommonlyused
etchingtechnique,reactiveionetching(RIE),tomicroprocessthegraphitewithphotoresistpatterns.
Firstofall,wepreparedphotoresistpatternsofdifferentshapesandsizesonthegraphitesurface,for
investigatingthestabilityofdirectphotoresistpatternimagingonthegraphitewiththeprocessshow
inFigure1a–d.Therefore,alithographicmasktemplatethatcontainsmaskpatternsofdifferent
shapesandsizeswasdesignedandfabricated.Figures2aandS1ashowthemaskunderanoptical
microscope.Themasktemplatecontainedlargequantities,neatlyarrangedshapes,anddifferent
sizes(4–20μm)ofcircularandsquaremaskpatterns.
Baseontheabovelithographicmasktemplate,weperformedopticallithographyanddirect
etchingonthehighlyorientedpyrolyticgraphite(HOPG)(Bgradegraphite,BrukerCompany,
Germany;size:12mm×12mm×2mm)surfacebyusingopticallithographyandRIE,asshownin
Figure1a,HOPGwithafreshlycleavedsurfacewasobtained.(b)Aphotoresist(AZ9912)filmlayer
wascoatedontheHOPGbyusingaspinner.(c)Thegraphitewiththephotoresistfilmwasmasked
withalithographicmasktemplate.Furthermore,itwasexposedwithanexposuretool(maskaligner
Appl.Sci.2019,9,31033of7
H9425,SichuanNanguangvacuumtechnologyco.Ltd.,China)andexposureparameter(iline;
1×reduction;theexposuredoseis40mwfor6s).(d)Thegraphitewithphotoresistwasdeveloped
andbaked.(e)ThegraphitewithphotoresistwasetchwithanO
2
basedRIE(Etchlab200reactiveion
etchingmachine;SENTECHInstrumentsGmbH;Germany).Atsametime,photoresistisalsoetched,
butitnotbeentirelyetchedwhileprocessingnanometersheightofgraphitestructures,sothe
graphitemicro/nanostructureswithphotoresistwerefabricated.(f)Thegraphitemicrostructure
withphotoresistwasplacedinacetonetoremovethephotoresist.(g)Finally,thedesignedmicro
/nanostructurepatternwasfabricatedonthegraphite.
Figure1.Processdiagramofmicro/nanotechnologyongraphitesurfacewithdirectlydirect
lithographyandetch.
3.ResultsandDiscussion
Photoresistpatternsweredirectlyprocessedonthegraphitesurfacebasedonthe
aforementionedmicro/nanotechnology,showninFigure1a–d.Inaddition,Figure2b,candS1bshow
thephotoresistpatternsobservedunderanopticalmicroscope.Figure3d–hshowthephotoresist
patternobservedunderatomicforcemicroscopy(NTEGRASolaris,TMDTCompany,Russia).As
canbeseenfromtheaboveFigures,lithographycouldbedirectlyappliedtothemicrographofthe
graphitesurface,andphotoresistpatternsofdifferentshapesandsizescanbestablyandcontrollably
processedinlargequantities.Moreover,themicrostructureofthephotoresistprocessedonthe
graphitesurfacewasneatlyarranged.Figure2b–eshowsthattheheightofthephotoresistranges
from565nm–579nm,whichmeansthephotoresisthasgoodheightuniformity.Additionally,the
photoresistpatternshowninFigure2bwasprocessedfromthemaskpatterninthemasktemplate
showninFigure2a.ThepatterncomparisoninFigure2a,bshowsthatthephotoresistpattern
obtainedbylithographyonthegraphitesurfaceissubstantiallyidenticaltothepatternonthe
designedmasktemplate.Thisfindingindicatesthatthegraphitesurfacecanbedirectlyprocessedby
lithography.Therefore,differentthephotoresistpatternscanbedesignedaccordingtorequirements,
andvariousphotoresistpatternsandphotoresistmicrostructurescanbeaccuratelyandcontrollably
processedongraphitesurfacebyopticallithography.
Appl.Sci.2019,9,31034of7
Figure2.(a)Masktemplateobservedunderopticalmicroscope;(b,c)photoresistpatternsobserved
underopticalmicroscope;(d,e)photoresistpatternobservedunderatomicforcemicroscopy;(f,g)the
resultsofthestepheightcharacterization.
ByusingtheetchingprocessshowninFigure1e–g,thedesignedmicrostructurepatternwas
machinedonthegraphite.Furthermore,thegraphitemicro/nanostructureswascharacterizedby
atomicforceandscanningelectronmicroscopy.Theresultshowsthatthephotoresistcouldstillresist
plasmaetchingbeforeitwascompletelyetched.AswecanseefromtheFigure3,partofphotoresist
isetchedbytheoxygenplasma,butmostofphotoresiststillcoveredonthegraphitesurfacewith
about45°sidewall.Althoughtheoxygenplasmaisalsoaphotoresistetching,the600nmthickness
photoresiststillcanresisttheO
2
etchingwhileprocesstensofnanometersheightofgraphite
structures.Moreover,westudiedtheamountofphotoresistetchedduringprocessgraphitemicro
/nanostructures.Theresultshowsthatabout100nmamountofphotoresistetchedduringprocess
about30nmamountofgraphite.Forexample,a1.06μmthicknessofphotoresistwouldhaveresulted
in957nmthicknessofphotoresistleftbehindafteretching,andathickerthicknessofphotoresist
wouldhaveresultedinmorephotoresistleftbehind.
Figure3.Theelectronmicrographoftheetchedphotoresistandgraphitemicrostructuresbyusing
scanningelectronmicroscope.
Appl.Sci.2019,9,31035of7
Inaddition,wedesignexperimentsvarytheoxygenflowrate(20sccm;30sccm),power(100W;
150W)andpressure(7Pa;8Pa;10Pa)tofindoptimumRIEconditionstofabricategraphitemicro
/nanostructureswithlargerdegreesidewall.Theresultshowsthattheetchrateofthegraphite
changedwiththedifferentetchconditions,e.g.,wecouldgettheetchrateof0.93nm/sandthe3.1
nm/swiththeparameterof10Pachamberpressure,100Wpower,20sccmoxygenflowrate;theetch
rateof1.2nm/swiththeparameterof8Pachamberpressure,100Wpower,20sccmoxygenflow
rate;theetchrateof1.53nm/swiththeparameterof7Pachamberpressure,100Wpower,20sccm
oxygenflowrate.Therefore,thesmallerthecavitypressure,thegreatertheetchrateofgraphite.
Besides,Figure4isthegraphitenanostructureswithabout23°sidewall(Sentech’sEtchlab200
reactiveionetchingmachine;chamberpressure,10Pa;power,100W;oxygenflowrate,20sccm;
etchingrate0.93nm/s).AswecanseefromtheFigure4,thesidewallofthemicro/nanoscalegraphite
structuresobtainedbyusingthephotoresistasamaskrelativelyclosetothatobtainedwithan
intermediatemaskfromreference[32].Moreover,theamountofgraphiteetchedrangesfrom29.4
nm–30.9nm,whichmeansamicro/nanoscalegraphitestructurewegothasuniformheight.
Therefore,althoughitisquitedifficulttoachievenanometeramountofgraphiteetchedusing
photoresistasamask,wefoundtheprocessparameter,whichis10Pachamberpressure,100W
power,20sccmoxygenflowrate.Besides,thegraphitewithphotoresistpatternscouldbefabricated
byRIEtoproduceneatlyarrangedgraphitemicro/nanostructures.Figure4a,d,3andS2showthat
circulargraphitemicrostructurecanbeprocessed.Furthermore,Figure4c–eshowsthataprocessed
graphitemicrostructurecanachievenanometeramountofgraphiteetched.Therefore,graphitecan
bedirectlysubjectedtomicro/nanoprocessingsuchasetching,andthemicro/nanoscalegraphite
structurescanbestablyandcontrollablyprocessed,therebyillustratingthattheadaptabilityof
graphiteasabasicMEMSmaterialtomicro/nanoprocessing.
Figure4.Thefabricatedgraphitenanostructurewithabout23°sidewall(a,b)atomicforce
microscopycharacterizationofthegraphitenanostructure;(ce)theresultsofthestepheight
characterizationofthegraphitenanostructure.
4.Conclusions
Inconclusion,thispaperuseddirectlylithographyandetchingtostudymicro/nanoprocessing
ongraphite.Theresultsshowthatgraphitecanbedirectlyprocessedbylithographyanddifferent
maskpatternscanbedesignedaccordingtorequirements.Moreover,variousphotoresistpatterns
canbeaccuratelyandcontrollablyprocessedongraphitewithuniformheight,andgraphitewith
photoresistpatternscanbedirectlyprocessedbyetchingnanometeramounttoformmicro
/nanoscalegraphitestructures.Inaddition,themicrometerheightphotoresistcouldresistplasma
etchingwhenprocessnanometersheightofgraphitestructures.Moreover,graphitewithphotoresist
Appl.Sci.2019,9,31036of7
patternshasbeendirectlyprocessedbyetching,andnanometeramountsofgraphitehavebeen
etched.Micro/nanoscalegraphitestructureswithheightrangefrom29.4nm–30.9nmwasfabricated.
Furthermore,agreaterthicknessofphotoresistwouldhaveresultedinmorephotoresistleftbehind.
Therefore,thisstudyprovesthatgraphitecanbecompatiblewithcommonmicro/nanoprocessing
techniques,andmaysupportstheuseofgraphitetofabricateMEMS,whichisimportantforgraphite
applications.
SupplementaryMaterials:Thefollowingareavailableonlineatwww.mdpi.com/xxx/s1,FigureS1(a)
photographofthemasktemplateobservedunderanopticalmicroscope(b)photographsofthephotoresist
patternsobservedunderanopticalmicroscope,FigureS2theelectronmicrographoftheetchedphotoresistand
graphitemicrostructure.
AuthorContributions:C.Z.conceivedanddesignedthestudy;N.L.analyzedtheexperimentaldata;C.Z.wrote
thepaper;M.W.andprovidedguidanceandmodificationofthepaper.Y.L.didsomeexperimentsand
characterizationofatomicforcemicroscopyandscanningelectronmicroscope.
Funding:ThispaperisfundedbythebasicscientificresearchprojectofWenzhou,China(No.2019G0057).
Acknowledgments:ThispaperissupportedbyKeyLaboratoryofAirdrivenEquipmentTechnologyof
ZhejiangProvinceandthebasicscientificresearchprojectofWenzhou,China.
ConflictsofInterest:Theauthorsdeclarenoconflictofinterest.
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The authors report the fabrication of polymer MEMS transducers, without using any sacrificial layers, on flexible substrates. The elimination of sacrificial layers and the process overhead associated with them is achieved through maskless grayscale UV lithography of SU-8 photoresist, and results in a simplification of the overall fabrication process. Six batches, with a total of 60 cantilevers and 60 double- clamped beams, and three batches, with a total of 1500 circular membranes, have been fabricated on commercially available, 45 um Kapton® polyimide sheets coated with a 35um copper layer. The validation of the fabrication process has been done through functional characterization of the bi-directional electro-mechanical interface and robustness to mechanical bending. The test and measurement results of the fabricated microstructures validate a good design predictability (geometry error: <5%, dynamical performance error: <4%), high reproducibility (less than 4% normalized standard deviation between individual microstructures) and an acceptable robustness against temporary bending of the substrate (less than 3% behavior change before and after bending). The approach opens the path for simple fabrication of MEMS transducers and their hybrid integration with electronics in all-flexible microsystems.
Article
For logic nodes of 7 nm and beyond, back-end-of-line (BEOL) trench patterns have a critical pitch of less than 40 nm, directly affecting the plasma etch process window of the dual damascene etch process. Feature size dependent etch depth (reactive ion etch, RIE lag), hard mask selectivity, and ultra-low-k (ULK) damage have become significant challenges that must be overcome in order to meet target device performance. Recently, atomic layer etching has been used to widen the plasma etch process window in terms of selectivity and process control [S. Sherpa, P. L. F. Ventzek, and A. Ranjan, J. Vac. Sci. Technol. A 35, 05C310 (2017); T. Tsutsumi, H. Kondo, M. Hori, M. Zaitsu, A. Kobayashi, T. Nozawa, and N. Kobayashi, J. Vac. Sci. Technol. A 35, 01A103 (2017)]. In this work, the impact of a quasiatomic layer etch (QALE) process, a conventional continuous wave plasma, and a pulsed plasma process on ULK materials were investigated to determine the benefits of an ALE process approach for BEOL etching. Both blanket ULK film and patterned ULK samples were used for this study. The ULK etch damage from each process was characterized using Fourier transform infrared spectroscopy and x-ray photoelectron spectroscopy on three different ULK films. From patterned samples, it was determined that QALE could be used to successfully suppress RIE lag in low-k materials at advanced pitches, while keeping low-k damage to a minimum. In addition, the QALE technique showed improved hard mask selectivity and resulted in lower line edge pattern roughness. Based on this study, the authors concluded that QALE is a powerful plasma etch method to overcome BEOL etch challenges at advanced pitches.
Article
Structural superlubricity, a state of ultralow friction and wear between crystalline surfaces, is a fundamental phenomenon in modern tribology that defines a new approach to lubrication. Early measurements involved nanometre-scale contacts between layered materials, but recent experimental advances have extended its applicability to the micrometre scale. This is an important step towards practical utilization of structural superlubricity in future technological applications, such as durable nano- and micro-electromechanical devices, hard drives, mobile frictionless connectors, and mechanical bearings operating under extreme conditions. Here we provide an overview of the field, including its birth and main achievements, the current state of the art and the challenges to fulfilling its potential.
Article
Ultrathin Cu(In,Ga)Se2 solar cells are a promising way to reduce costs and to increase the electrical performance of thin film solar cells. An optical lithography process that can produce sub‐micrometer contacts in a SiO2 passivation layer at the CIGS rear contact is developed in this work. Furthermore, an optimization of the patterning dimensions reveals constrains over the features sizes. High passivation areas of the rear contact are needed to passivate the CIGS interface so that high performing solar cells can be obtained. However, these dimensions should not be achieved by using long distances between the contacts as they lead to poor electrical performance due to poor carrier extraction. This study expands the choice of passivation materials already known for ultrathin solar cells and its fabrication techniques. Interface passivation of ultrathin Cu(In,Ga)Se2 solar cells is important to achieve enhanced performance of solar cells. The potential of SiO2 as a passivation layer and the implementation of nano‐patterning (production of sub‐micrometer contacts) on SiO2 by optical lithography is investigated. Co‐relation between the dimensions of sub‐micrometer contacts and its implications on performance of the ultrathin passivated solar cells is thoroughly investigated.
Article
To date structural superlubricity in microscale contacts are mostly observed in intrinsic graphite flakes that are cleaved by shearing from HOPG mesas in situ or friction pairs assembled in vacuum due to the high requirement of ultra clean interface for superlubricity, which severely limits its practical application. Here we report observation of microscale structural superlubricity in graphite flake pairs assembled under ambient condition where contaminants are inevitable present at the interfaces. For such friction pairs, we find a novel running-in phenomenon, where the friction decreases with reciprocating motions but no morphological or chemical changes can be observed. The underlying mechanism for the new running-in process is revealed to be the removal of third bodies confined between the surfaces. Our results expand the understandings of microscale superlubricity and may help to extend the application of superlubricity in practice.
Article
We develop a method for the design and fabrication of a miniaturized multichannel piezoelectric xylophone acoustic transducer with an integrated flexible ribbon cable for electrical connectivity. This transducer works in air or underwater and it is designed to be implantable for in vivo animal testing. The transducer and ribbon cable are fabricated separately using microelectromechanical systems (MEMS) techniques, and bonded with a customized wire-epoxy bonding technique. The transducer has a xylophone structure which consists of four piezoelectric bimorph cantilevers of varied lengths each tuned to a target frequency bandwidth. A parylene ribbon cable extends from the base of the xylophone to an electrode bay for external monitoring. Cantilever tip deflections are measured in response to voltage excitation to confirm the transducer functionality in air and underwater as well as to validate the finite element analysis (FEA) model, which is developed to design the transducer and study the acoustic structure interaction of the cantilever beam in a viscous fluid environment. The frequency response of the model matches closely with the measured results.