Sequential microcontroller-based control for a chemical vapor depositionprocess Technology of Applied Journal Researchand

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Availableonlineatwww.sciencedirect.com

Journal of Applied Research and Technology

www.jart.ccadet.unam.mx JournalofAppliedResearchandTechnology15(2017)593–598

Original

Sequential microcontroller-based control for a chemical vapor deposition process

Edgar Serrano Pérez

^a,∗

, Javier Serrano Pérez

^b

, Fernando Martínez Pi˜nón

^a

, José Manuel Juárez García

^c

, Omar Serrano Pérez

^d

, Fernando Juárez López

^a

aInstitutoPolitécnicoNacional,CIITEC,CerradadeCecati,Sta.Catarina,D.F.02250,Mexico

bUniversidadTecnológicadeTecámac,CarreteraFederalMéxico–Pachuca,Km.37.5,PredioSierraHermosa,C.P.55740,Tecámac,EstadodeMexico,Mexico

cCentroNacionaldeMetrología,Km4.5CarreteraaLosCués,C.P.76246MunicipioElMarqués,Querétaro,Mexico

dDepartamentodePosgrado,InstitutoPolitécnicoNacional,CIDETEC,Av.JuandeDiosBátizs/n,07700CiudaddeMéxico,Mexico Received22November2016;accepted26July2017

Availableonline11December2017

Abstract

Acost-effectivedirectliquidinjectionsystemisdevelopedforachemicalvapordepositionprocessusingamicrocontroller.Theprecursor gasphaseiscontrolledbytheprecisesequentialinjectionofaliquidprecursorsolutiontoavaporizingchamberpriordeposition.Theelectronic controlsystemallowsthehuman–machineinterfacethroughaLCDdisplayandakeypadmatrix.Thecoreoftheelectronicsystemisbasedon anelectromechanicalinjectoroperatedintimeandfrequencyasasequentialcontrolsystembyapopularPIC16F877Achip.Thesoftwarehas beendevelopedintheBASIClanguageanditcanbeeasilymodifiedthroughanICSPprogrammerfordifferentsequentialautomatizedroutines.

Theinjectioncalibrationtesthasproventhelinearityoftheinjectioncontrolsystemfordifferentoperationparameters.Theresultsreportedthe sequentialinjectionMOCVDdepositionofaluminathinfilm.

Keywords:Microcontroller;Chemicalvapordeposition;Directliquidinjection;Thinfilms

1. Introduction

Metalorganic chemical vapordeposition (MOCVD) isan importanttechniqueforthesynthesisofmaterialsintheform ofcoatingsandthinfilmsfortheelectronicsandsemiconductor industry.Thisprocess,allowsdepositingsolidmaterialsinthe formofthinfilmsandpowderfromametal-organicprecursor bymeanofachemicalreactiononathermallyactivatedsurface.

Usually,metalorganicprecursorsdonothaveenoughvapor pressureforbeingeffectivelytransportedtoathermallyactivated surface;whentheprecursorreachesthesample,itdecomposes toproduceadesiredmaterialintheformofthinfilm.Inorder toincrease the vaporpressure, precursors must be heated to increasetheirvolatilityandtransportedbymeansofacarriergas.

∗Correspondingauthor.

E-mailaddress:[email protected](E.S.Pérez).

PeerReviewundertheresponsibilityofUniversidadNacionalAutónomade México.

Theheatingandcoolingprocessoftheprecursorinsublimation andbubblertechnologieshavesomedrawbacks.Precursorscan partially decomposeafter thermalcycles, reducingthe reproducibility ofthe thinfilm depositionprocess(Sovar Samélor, Gleizes,&Vahlas,2007).Thegasphaseconcentrationisvari- able in time due to its difficult control, and this is because temperature and mass flow variables are coupled in a same device.Moreover,theevaporationratealsofluctuatesintime;it hasastrongdependenceontheremainingamountofprecursor inthecontainer.

Direct liquid injection arises as an important technique whichcanreduceintrinsicdrawbacksof conventionalprecur- sordeliverymethods.Inthissense,ifaconstantvaporprecursor concentrationcanbereached,conformalsmooththinfilmscan be producedwiththismethod;the processcanbetransferred to academic and research environments to satisfy industrial applications. Asthe precisecontrol of the mainvariablesfor the MOCVDprocess is a key factor in orderto obtain high experimental reproducibility,precise control of the precursor

https://doi.org/10.1016/j.jart.2017.07.003

1665-6423/©2017UniversidadNacionalAutónomadeMéxico,CentrodeCienciasAplicadasyDesarrolloTecnológico.Thisisanopenaccessarticleunderthe CCBY-NC-NDlicense(http://creativecommons.org/licenses/by-nc-nd/4.0/).

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andreleased.In thismanner,thereisnotenoughtimeforthe precursor to present chemicalchanges. The micro amount is controlledelectronicallybytheopeningtimeandfrequencyof theinjector,bythiswayispossibletomaintainamorestable andconstantprecursorvaporflowintothereactorchamber,with thesecharacteristics,ispossibletoincreasethereproducibilityof theexperimentsandoperatethesysteminasaferandconvenient way.

Theatomiclayerdepositionprocesseshasalsoalreadytaken anadvantageofthisdeliveryprecursormethodproducinghigh qualitythinfilms(Leskelä&Ritala,2002).Apreciseamountof precursorisinjectedsequentiallyforeachcycle;thegrowthis constantandcomplexstructurescanbeobtained(George,2010).

Inthiscontext,themicrocontrollersareusefulelectronicdevices whichcanbeadaptedforchemicallaboratoryfacilitiesformon- itoringandcontrolofdedicatedprocesses.Thelowcostandhigh availabilityof suchdevicesallowsthedevelopmentofelectro mechanicalsystemsforanalyticalinstrumentationinacademic andresearchenvironments(Kubínová& ˇSlégr,2015;Rajendran

&Neelamegam,2004;Ramana&Malakondaiah,2007;Urban, 2014,2015).

The present work is dedicated to the development of a low cost sequentialmicrocontroller-based direct liquid injection control systembased on a PIC16F877a microcontroller.

Thissystemcanbeused inadirect liquidinjectionMOCVD reactortoproducecoatings,thinfilmsandnanostructures.The microcontrollerandtheelectromechanicalinjectorarecoupled

retention around 40 years.These functionalitiesmakeof this microcontrolleranattractivedeviceforthisapplication.

BlockBcontainstheuserinterfacetothedatainputtothe controllerbymeansofakeypadwhichallowsselectingthetime andfrequencydesiredtotheoperationalparametersoftheinjec- tor.Theselectionofanoperationroutinecanbemodifiedatany timeduringtheprocessingifdesired.Ifitisdesiredtoobtain a constantgas phase feedingto the reactor,the routine must bekeptconstant,whichmakesitpossibletoobtainconformal smooththinfilms.Ifitisdesiredtomodifythereactionration ratethroughagasphasemodification,theroutinecanbemod- ified,infactautomatedroutinescanbeprogrammedtoobtain differentgrowingratesinareproducibleway.Severalinjectors canbecombinedwithawidespectrumofinjectionparameters toobtainnewcomplexmaterialswithuniqueelectrical,chemical,andmechanicalfunctionalities.Forthisreason,thetime-on andfrequencyofoperationparametersarecrucialforthispro- cess.Inthissense,withthiscontrolsystemthegasphasecan be preciselycontrolledandeachcombinationcanmodify the gas phase through the reaction chamber andmodify velocity and concentration profiles tofinally modify the reaction rate todepositfromsingletocomplexcoatingsandthinfilms.The blockDcontainsapowerelectronicstagewhichtransfersthe controlsignalprocessedfromthemicrocontrollertotheinjec- tor.Thecoreofthisstageisanelectromechanicalinjector.For thecorrectatomizationofsolutions,theinjectormustbekeptat 40psi.Thecontrolsignalistransferredtotheinjectorthrough

Pic16F877a

Microcontroller Injection arrangement LCD 2X16

Keypad Power source

Block A

Block B

Block C Block D

Figure1.Blockdiagramofmicrocontrollerbasedinjectioncontrol.

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Figure2.Circuitdiagramfortheinjectorcontrolsystem.

apowerMOSFETIRL510whichisalowcost,fastswitching devicefor inductiveloads. The systemhasbeen successfully testedfromshortsinjectionparameters,suchastime-onof5ms and0.5soffrequency.TheblockCiscomposedofaLCD12X2 alphanumericlowcostdisplay,whichallowstoseetheinjec- tion parametersoperating on real time. Because of the ICSP (in-circuitserialprogramming)option inthismicrocontroller, injectionsroutinescanbeeasilymodified.

2.1. Circuitdescription

Thecircuitdiagramoftheinjectioncontrolsystemisshown inFigure2.The keypadconsistsof amatrixoflogic buttons embeddedinthesamedevice.Usuallyoneportisneededtoread aninput,butwiththisconfiguration,areducednumberofports andcomponentsareneededtoinputdataforthemicrocontroller.

Asitis shown, 8 pinsof PortD areneeded toreadup to 16combinationsfromthe 4x4keypad,inthiscase7pinsare usedforreading12buttons(RD0-RD6).TheLCDtransference hasbeenassigned through the portAfor convenience(RA0- RA4).InPortBjusttwopinshavebeenassigned,RB3which allowsenablingthecommunicationbetweentheLCDandthe microcontroller,pinRB0isassignedtothecontrolsignaltothe gateofthepowerMOSFETstageoftheinjectionsystem.The IRL510MOSFETallowscontrollingtheinductiveloadofthe electromagneticinjector.Thisdeviceiscapableofhandlingup to5Apeakoutputcurrent,largeenoughfortheinjectionpower supply.This configuration enablesa safe operation andover temperatureprotection,makingof thiscontrol systemalong- term safe operation, although fast switching during injection operationduringlongtermtimewithshorttime-on(2ms),high currentpeaks(1.2A)fortheinductiveloadof theinjectorare wellsupportedwiththisconfiguration.A4MHzmicrocrystal isconnectedtopins13and14,respectively.Thecircuitisbuilt withjustafewmicroelectronicsanddiscretecomponentsfora

verylowprice(<100USD)incomparisonwiththeexpensive specializedequipmentsoldbylargecompanies.Thisisacost- effectiveapproachwhichcanoperatethisprocessinaneasyand efficientway.

2.2. Software

TheprogramhasbeendevelopedusingBASIC.PortsAand B havebeenselected as output totheLCD andtheinjection controlpin.BitsD3–D6havebeenselectedasinputsinorderto bereadbythemicrocontrollerandreadkeypadstate.Avariable has beeninitialized inordertostore de dataobtained by the keypadreading.Asetofconditionalcontrolsentences“if”have beennestedinordertodefinetheinjectionparameters,timeon andfrequencythattheinjectionmusttooperateforeachbutton.

Thecompiled.HEXprogramhasbeentransferredtoasection ofthePic16F877ausingtheISCPprogrammingprotocolanda lowcostprogrammer(K150).Thisprogramallowshavingthe wholesetofoperationsfortheinjector.Thediagramflowchart isshowninFigure3.

3. Materials,methodsandinjectorcalibration

Themechanicalcomponentsoftheinjectorheadhavebeen machinedinaluminum6061andnylamid^TM.Thebasewhich supportstheinjectorisconnectedtothevaporizingchamberand ithasbeenbuiltalsoinaluminum6061becauseofitsmechan- icalandthermalproperties.Aluminumhasenoughstrengthto supporttheinjectorandthethermaldissipation;also,itreduces theneedforanelectricfanwhichmustcooltheinjector.Two rodshavebeenmachinedforthispurpose.Theplatehasafast connectorforpolyurethaneflexibletransparentpipewhichtrans- portsthesolutionfromthepressurizedcontainertotheinjector feedinghead,asitcanbeseeninFigure4.

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Injection parameters selection

Display selection in LCD

Injection parameters to RB0

Stop process

End

Figure3.Flowchartoftheinjectioncontrolmicrocontroller-based.

Inordertocalibratetheinjector,5routineparametershave beenselected.Waterhasbeenselectedasthecalibrationsolu- tion.Thelinepressurehasbeensetto40psiforalltheruns.The calibrationset-upisshowninTable1.

resultsconvenientforthepulsedinjectionchemicalvapordepo- sition,itisliquidatroomtemperature.Beforedeposition,silicon sampleswerecleanedusinganisopropylalcoholultrasonicbath for 5minutesanddriedinargonflow.Thethinfilmprecursor hadbeensequentiallyinjectedtoavaporizingchamberwitha digitaltemperaturecontrollersetto140^◦C.Thethinfilmdepo- sitiontookplaceinahot-wallreactorwhichwascomposedofa thermoregulatedquartztube.Temperaturehadbeenregulated usingaresistiveloadcoupledtoadigitalcontroller.Thinfilm depositiontookplacearound350^◦C.Figure5presentsanopti- calmicrographofthesurface,ascanbeobserved;althoughthin filmsweredepositedonsiliconsubstrates,depositiononstain- less steel304L sampleholderresults interesting,any kindof specialpreparationhadbeendoneforthismaterial,thesample holder hadbeenjustcleanedusing isopropyl alcoholpriorto depositionandfurthercharacterizationforthismaterialwillbe preparedforaforthcomingmanuscript.Thereexistsanincreas- inginterestofaluminaprotectivepropertiesforstainlesssteel incorrosion applications.Thethicknessofthe siliconsample is1mm;asshowninFigure5,thisdimensionislargeenough to modify the flow precursor patterns and, consequently, the depositionpath.

Vaporizing Chamber Injector coupling base Injector supports Electro mechanical

injector

Pressurized precursor feeding line

Pressurized container

Figure4.Injectionmountingforthevaporizingchamber.

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Precurs or flo w

Silicon sample

1 mm

Figure5.Opticalmicrographofthinfilmsampledeposition.

(a) (b)

1 2 3 4

keV 0.0

0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

cps/eV

Si

O Al

C

100 µm

Figure6.Scanningelectronmicroscopy(a)andenergydispersiveX-rayspectroscopy(b)analysisofthethinfilmdepositiononsiliconsample.

The chemical vapor deposition technique takes advan- tage of this situation; using computational fluid dynamics software is possible to predict the precursor flow behav- ior in the reactor under controlled environments; deposition thickness and precursor mass fraction can be computed to predict the deposition rate for complex shapes obtaining closeresultsbetween computationalandexperimentalresults (Vergnes,Samélor,Gleizes,Vahlas&Caussat,2011).Thesur- face morphology of thin films has been investigated using the scanning electron microscope technique as shown in Figure6.

Thesameimage/positionhasbeenusedfortheenergydisper- siveX-rayspectroscopymappinganalysisinordertoinvestigate theelementalcomposition/distributionofthesample,asshown inFigure6.Itcanbeseenthatthefilmdoesnothavetrenches or blotches,as canbeobserved, a crackfree surface without poresandpinholeshasbeenobtained.Ontheotherhand,atthe samplesurfaceispossibletoobservenodules whichcouldbe formedbecauseofhomogeneousgasphasereactionsunderthe conditionsofthisprocess.

TheelementalanalysisenergydispersiveX-rayspectroscopy atthesamplesurfaceshowninFigure6revealedadistribution ofcarbon,oxygenandaluminumelementsinthesample.The siliconsignalcorrespondstothesubstrate.Withtheseresults,it ispossibletosupposeasafirstassumptionanaluminathinfilm formationinagreementwithreportsintheliteraturepresenting aluminum tri-sec-butoxide as an alumina thin film precursor (Haanappel, Van Corbach, Fransen, & Gellings, 1994). The local distributionof the elements is homogenous at the surface,asshowninFigure6,buthigheraluminumconcentrations are present atthe surfacenodules;thiscould be attributedto gasphasereactionsor,ultimately, toanincompleteprecursor decompositionpathduringthinfilmformation

5. Resultsanddiscussion

Theperformanceofamicrocontroller-basedinjectioncontrol systemwasinvestigatedusing distilledwaterinapressurized containerat40psipressure.Thesolutionconsumptionwasread beforeandafterforeachrun.Asitcanbenoted,theinjection

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Figure 7.Comparison betweendifferent injectionparameters and the flow obtained.

performanceisquitelinearinrelationtothedifferentinjection parametersadoptedfortheflowasshowninFigure7.

Thetimeparameterdetermineshowlongthevalveremains openandthefrequencydetermineshowmanytimesthevalveis openedduringonesecond.InFigure7itispossibletoseethat thethreeflowsobtainedfromcalibrationhaveaquasilinearrela- tionship.Ascanbenoted,thesequentialinjectioncontrolsystem operatesinanefficientway,asitcanachieveconstantflowduring timefortheinjectionofeachparameterset.Inordertoincrease theflowtoareactorchamberthereexisttwopossibilities:

(a) Increasingtheopeningtimefortheinjectorwithaconstant frequency

(b) Increasingthefrequencyofpulsespersecondswithacon- stantopeningtime.

Theincreaseoftheopeningtimeofthevalveseemstobea morereliablechoice,asanincreaseofthefrequencyreducesthe time-lifeoftheinjector.

6. Conclusion

Theinjectorcalibrationtesthasshownthatitispossibleto determinetheflow for eachinjection parametersforany par- ticularliquid/solution.Duringadirectliquidinjectionchemical vapordepositiontest,thesolution,whichisusuallyformedfrom asolid/liquidprecursorsdissolvedinanorganicsolvent,thecali- brationmethodmustbedoneforeachparticularsolutionbecause ofthechangesinviscosity,densityandphysicalpropertiesofthe liquidwhichcanmodifytheflowrateforeachinjectionparam- eters.Thesequentialinjectioncontrolsystemhasanacceptable precision;itaimstoobtainaconstantvaporgasphasetofeed thechamberofachemicalvapordepositionprocessforconfor- malthinfilmdeposition.Theprecursorsolutionusedwiththis

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