If the pattern is misaligned by θ= 1, the actual size of the KOH pit will be (cos θ+ sin θ ) and hence the edge length variation will be:

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1 ME141BIntroductiontoMEMS November1 st,2010 ExampleHomework#2 Problem1: WeintendtouseKOHetchingtoformadiaphragmona(100)siliconwafer.Itwilbe asquaremembranewithathicknesswell definedbyanetchstop(e.g., electrochemicaletchstopofansoiwafer),suchthattherearenorealthickness variationsinthediaphragmitself.whatfeaturesizeisrequiredtoproduceasquare diaphragmwitha400mmsidelengthby20mmthicknessonasiliconwaferthatis 500mmthick(thiswaferthicknessincludesthemembranethickness)?Whatisthe edgelengthvariationofthediaphragmiftheetchmaskismisaligned1degreetothe <110>directin?Assumingthesensitivityofapressuresensorvariesastheinverse fourthpowerofthediaphragmedgelength,whatpercentageofvariationcanbe attributedtomaskmisalignment? Problem1Solution: Thefigureshowsthedesireddiaphragmfeaturesona(100)siliconwafer.The maskfeaturesizeacanbeexpressedasafunctionofwaferthicknesstw,diaphragm thicknesstd,diaphragmdimensiondandtheintersectionangleθbetween{111}and {100}planes: Substitutingthenumbers,wegeta=1079.7um. Ifthepatternismisalignedbyθ=1,theactualsizeoftheKOHpitwillbe(cosθ+sin θ)andhencetheedgelengthvariationwillbe: Thisinturntranslatestotheedge lengthvariationforthediaphragmδd.

2 IfthesensitivitySofapressuresensorvariesastheinversefourthpowerofthe diaphragmedgelengthd,i.e. thenthepercentagevariationattributedtovariationsinwaferthicknessis: Problem2:Debugandrecreateaprocessandmasksetforapressure sensing silicondiaphragm You are a young junior faculty member who has just hired your first graduate student, Wayford Roppar. You have developed an idea for using a sealed cavity pressure sensingsilicondiaphragm(1mmacrossand15μmthick)thatyou resure willmakeyoufamousandassureyourtenure.youaskwayfordtodesignaprocess flowforcreatingthissimplestructure,andwayfordreturnswiththeprocessflow detailedinfigure1. Being a seasoned MEMS designer, you immediately notice several critical errors with Wayford s process (things that won t work or won t produce the result that Wayfordshowsinthecrosssections).Pleasefindthecriticalerrorsinthisprocess flow and, where possible, suggest alternate approaches. Do not worry about the accumulationoferrors,butrathertreateachstepassumingthatthestructureupto thatstepcouldbecreated. Then recreate a correct process flow along with the device cross sections at each stepandtheassociatedmaskset(withdimensions).

3 Processsteps: 1.Startwithadouble side polishedn typesiliconwafer. 2.Performphotolithographyusing1 μm thickpositivephotoresisttodefine thediaphragmarea. 3.Deep reactive ionetchthesilicontoformthediaphragm;ashresist. 4.Anodicallybondthesiliconwafertoapyrexwafer.

4 Problem2Solution: ProposedProcessStep 1. Start with a double side polished n typesiliconwafer 2. Perform photolithography using 1 μm thick positive photoresist to define thediaphragmarea 3. Deep reactive ion etch the silicon to formthediaphragm;ashresist. Error Noneyet! Must precede with a clean (RCA or piranha) 1μmofPRisverythinwhenusedasa mask in DRIE. In other words, since the selectivityofdrietosiliconoverpris~ 50:1, etching though ~ 500μm of Si would require more than 10μm of PR. Also, DRIE would lead to a non uniform membrane thickness with variations on the order of the required thickness (15 μm). This will make the device function improperlyiffabricatedatall. 4.Anodicallybondthesiliconwafertoa Mustprecedewithcleaningwafer pyrexwafer Correctedprocess: 1.Startwithdouble sidepolishedsoiwafer,devicelayer15μmthick,oxide layer1μmthick,substrate500μmthick.rcaclean. 2.DepositLPCVDnitride,0.5μmthick(willcoatbothsides). 3. Spin 1 μm thick positive photoresist on bottom side and perform photolithographyusingmask1tothebottomside. 4.DryetchthenitrideonthebottomsideusingCF4/H2plasmaforexample. Ashresist. 5.KOHetchthesiliconfromthebottomsideusingtheembeddedoxidelayer as an etch stop. If the dimensions of Mask 1 were calculated correctly, the resultantprofileonthetopsidemustbe1mmacross. 6.Piranhacleantoremoveallresistresidue. 7.Etchtheremainingnitridein85%phosphoricacid. 8.EtchtheexposedoxideusingBOEfor~10minutes.RCAclean. 9.AnodicallybondthepatternedSOIwafertoaPyrexwafer.

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6 Problem3: Thefigurebelowshowsanelectricaltrapthatusesdielectrophoresis(DEP)totrap cells.youwilldesignaprocessandmasksetthatwillproducethisstructure(notto scale). Both metal layers must be 0.5 μm thick gold (though other metals are acceptable beneath the gold). The substrate B and layer A both must be electrical insulators. The gold linewidth is 10 μm, and all other critical parameters are specifiedinthefigure. You are asked to create a table of process steps, along with process flow crosssectional diagrams and masks. Specify materials and the proposed etch methods, and be sure to include as steps in your process the required wafer cleans, applicationofphotoresist,andstrippingofphotoresist.youdonotneedtoinclude dimensions in your mask set in this problem (but do draw the geometries correspondingly).

7 Problem3solution: Step Description StartingMaterial:GlassWafer 4 or6 willbeusedasinsulatinglayerb 1 Clean PiranhaorRCA1,solventcleanwouldbeacceptable 2 Photolithography UsingnegativeresistandMask1.Thicknessofresistatleast 1.5μm(3timesthatofthelayertobeliftedoff).Negative resistnecessaryforliftoffprocesslateron 3 DepositAu Tibilayer E beamevaporation(goodforlift offtobeperformedinnext step).thicknessofgold~0.5μm,thicknessoftitanium~100 Å.Tiusedasadhesionlayer. 4 Lift offau Tibilayer Acetone.Followedbywaterrinse. 5 Clean Solventcleangoodhere PirahnamayeatupsomeoftheTi layerandmayleadtodelaminationofgoldlayer 6 DepositSiliconOxide PECVD,about10μmthickness.Willbeusedasinsulating layera. 7 Photolithography UsenegativeresistandMask2.Thicknessofresistmustbe greaterthan1.5um. 8 DepositAu Tibilayer E beamevaporation.thicknessofgold~0.5μm,thicknessof titanium~100å.tiusedasadhesionlayer. 9 Lift offau Tibilayer Acetone.Followedbysolventcleanandwaterrinse. 10 Photolithography Spincastpositivethickphotoresist,prebake;exposeMASK3, develop,postbake 11 EtchOxide DryetchusingCF4/H2plasma.Anisotropicandselectiveover Si. 12 StripResist AshwithanAsher 13 Clean Solventcleanok. Ï

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