Microfluidic Chip Design & Fab
Microfluidic Chip Design & Fab ME EN 5960
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This 12 page Class Notes was uploaded by Miss Leatha Gottlieb on Monday October 26, 2015. The Class Notes belongs to ME EN 5960 at University of Utah taught by B. Gale in Fall. Since its upload, it has received 53 views. For similar materials see /class/230016/me-en-5960-university-of-utah in Mechanical Engineering at University of Utah.
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Date Created: 10/26/15
Thin film processes grown films typically converted from ori 39nal substrate material example Si02 formed by oxidation of Si substrate C fDH deposited films j crystalline polycrystalline amorphous electrodeposition not standard It process liquid phase deposition 39 quot0 Standard 39 Pmcess Fundamentals of Micromachining vapor phase deposition PVD physical vapor deposition Dr Bruce Gale CVD chemical vapor deposition BIOEN 6421 EL EN 5221 and 6221 ME EN 5960 and 6960 Physical Vapor Deposition ThlnFlhn DepOSItlon Spinon Films 4 Polyimide PI photoresist PR Gas Phase Gas Phase 4 spin0n glass Tra nsport Physical Vapor Deposition PVD 4 Evaporation Evaporatlon Condensatlon 4 Sputtering Chemical Vapor Deposition CVD Condensed Phase Condensed Phase 4 Oxidation solld or Ilquld usually solld 4 LPCVD 4 PECVD R B Darling EESZ7 Silicon Oxides SiO2 Stages of Thin Film Growth Uses Island Stage diffusion masks surface passivation gate insulator MOSFET Channel Stage isolation insulation Formation Coalescence Stage Continuous Film Stage bridging oxygen nonbridging oxygen silicon mp 173239 C glass is unstabl below quotmark modifier 171039 C M r BUT devitri cation rate ie quot9 quot 939 crystallization below 1000 C negligible hydroxyl group R B DarlmgEggm Thermal Ox1dation of S1licon Modesofmn Film Growth Formation of the oxide of silicon on the silicon 1 VolmerWeber island growth surface is known as oxidation i a Thermal Oxidation is characterized by M Volmerand A WeberZ Phys Chem 119 p 2771926 temperatures 900 1200 C Two main processes 2 FrankVan der Merwe layer growth ideal epitaxy 4 D Oxidation Ty F c Frank and J H Van der Merwe Pm R Soc London Ser A 193 p 205 1949 0 815 02 gt 8102 I aim 1000 C 4 Wet Oxidation 3 StranskiKrastanovz layers islands s1 5 2H20 gtSi02 2H2 4 Dry oxidation produces a better more dense oxide as u J N Stranski and L Krastanov Ber Almd Wiss Win 146 p 797 1933 compared to wet ox1dat10n R B Dzrlmg EEASZ7 Oxide growth kinetics basic model is the Grove and Deal Model supply of oxidizer is mited by diffusion through oxide to growth interface JN FIck s FIrst Law ux ADM ax uoueuuaouoo Growth of SiO2 from Si in dry ltlt 20 ppm H20 oxygen Si 02 gt Sio2 once an oxide is formed how does this chemical reaction continue does the oxygen go in or the 5 con go out density I formula differences pSioz 225 gmlcm3 GMW 60 pSi 23 gmlcm3 GMW 28 oxide d thick consumes a layer 044d thick of Si bare silicon in air is always covered with about 1520 A of oxide upper limit of 40 A it is possible to prepare a hydrogen terminated Si surface to retard this native oxide formation uouenuaauoa N0 i ited by the solid solub y limit ofthe oxidizer in the oxide N002 5 x1016 cm 100039 c N0H20 3 x1019 cm 100039 c flux of oxidizerj at SiO2 I Si interface consumed to form new oxide j39 1quot N1 k is the chemical reaction rate constant in steady state flux in must equal flux consumed steadistate N0 4 N1 X j kN14De solve for N1 sub back into flux eq Wet oxidation of Si overall reaction is Si2H20 gt Sio2 H2 proposed process H20 SiOSi gt SiOH SiOH diffusion of hydroxyl complex to SiO2 Si interface SiOH SiOSi SiSi gt H2 SiOH SiOSi this results in a more open oxide with lower density weaker structure than dry oxide pwet 215gmlcm3 Limiting behavior of Grove amp Deal oxidation model Ar 17 1 x1 1A24B 1J short times 17 1 Al 1 17 1 1 41 t z 1 41 2 H A24B J Q m 2 l 2 A24B J thickness is linearly increasing with time characteristic of a reaction rate BIA is the linear rate constant 3 A 21 J l A n k linear rate constant depends on reaction rate between oxidizer and s con k AND solid solub ty of oxidizer in oxide NO temperature dependence mainly from reaction rate ted process Relation between flux and interface position flux oxidizer molecules crossing interface per unit area per unit time cm392 sec391 rate of change of interface position dx I dt interface velocity cm sec1 n of oxidizer molecules per unit volume of oxide N ZforHO ZforHO FMr 2 225gtlt10 crrr3r 2 GMWSiOZ L 1 for o l 1 for o cm393 then relation is just Q j DNon d1 n x Dk now integrate with appropriate initial condition Limiting behavior of Grove amp Deal oxidation model long times l 2sz All a z dig3 dependence39 parabo39 t ickness2 octime characteristic of a diffusion limited process B is the parabolic rate constant B 2DN0 parabolic rate constant depends on diffusivity of oxidizer in oxide D AND solid solubI ty of oxidizer in oxide NO temperature dependence mainly from diffusivity Grove and Deal relation setting Zle A function ofwhat s diffusing what it39s diffusing in and what it reacts with ZDNoln B function ofwhat s diffusing and what it39s diffusing in initial cond39 39on x t 0 xi integration gives A r 17 1 1 1 1 x0 2 L A24B J where 1 represents an offset tIme to account for any oxide present att 0 2Axa 7 Effect of Si doping on oxidation kinetics Pressure Effects on Oxidation grow thick oxides at reduced boron k gt 1 slow SiO2 diffuser timetemperature product k Co I CSi 3 Pressures spa53 s ea39 dopants accumulate in oxide concentration of oxidizer in oxide little effect on linear rate constant BA Nok n can increase parabolic rate constant B 2JNo n really only signi cant for men gt 102 cmquot5 0 phosphorus k lt 1 slow SiO2 diffuser kC XICSi 01 dopants pileupquot at silicon surface oxide silicon uoueuuaouoo increase of 1 atm pressure temperature is reduced 30 C little effect on parabolic rate constant B pressures up to 25 atm adapted from Sxe2nd p 122 increases inear rate constant BIA have beenuse j 39 39 39 quot39 39 39 39 39 39 39 quot39 again really only signi cant for commercial systems 1 10 N gt 10 cm HiPOx FOX Time noun Ph svh ms suoniui ssauxoiu apixo oxide silicon uoueuuaouoo Types OfCVD Oxidation thicknesses Atmospheric Pressure CVD APCVD wet oxidation 640 Torr partial pressure is typical LOW Pressure dry OXidation vapor pressure over liquid water Plasma Enhanced CVD PECVD o o o a z e E E g z x E a w E gtlt O suwsiiuJ muxaiuz spixo 100 Time hours Time hours Chemical Vapor Deposition CVD CVD formation of nonvolatile solid lm on substrate by reaction of vapor phase chemicals 0 Steps in CVD Gases are introduced into a reaction chamber Gas species move to the substrate Reactants are adsorbed on the substrate Filmforming chemical reactions Desorption and removal of gaseous byproducts Low Temperature Oxidation of Silicon LTO SiO2 is formed using three types of CVD Processes APCVD Most commonly used method LPCVD and PECVD SiPL Oz gtSiOz 2H2 240 550 C 200 500 nmmin optimal and 1400 nmmin possible Deposition rate increases slowly with increased T 310 450 C Deposition rate can also be increased by increasing the O2 SiH4 ratio APCVD 325 C ratio 31 475 C ratio 231 550 C ratio 601 LPCVD 360 C ratio 11 450 C ratio 145 1 Deposition can occur in the APCVD as low as 130 C For LPCVD Window 100 330 C 212 torr and 14 nmmin at 300 C CVD reactions 0 Heterogeneous occur at wafer surface Desirable Produce good quality films 0 Homogeneous occur in gas phase Undesirable Form gas phase clusters of material Consume reactants Reduce deposition rate Chemical vapor deposition general characteristic of gas phase chemical reactions pressures typically atmospheric to 50 mTorr 9v ranges from ltlt 1 pm to 1 mm reactions driven by thermal temperatures 100 1000 C higher temperature processes increase surface migrationmobility plasma optical example materials polycrystalline silicon poly silicon dioxide I I 39 39 39 39 39 39 glasses P56 856 BPSG silicon nitride Low Temperature oxide formation by APCVD LPCVD PECVD vs Thermal Oxidation of Silicon ADVANTAGES Low temperatures Fast Deposition rates especially APCVD Good Step Coverage especially PECVD DISADVANTAGES Contamination especially PECVD Inferior electrical properties of PECVD films as compared with thermally grown ones Less dense films are obtained CVD Reaction Rate R R R0 CXpEakT a where Ea activation energy eV 4 k Boltzmann constant a T temperature K Surface reaction rate increases with increasing temperature at very high temperature 4 Reaction rate gt reactant arrival rate a Masstransport limited At low temperatures 4 Reaction rate lt reactant arrival rate 4 Reaction rate limited CVD system design hot wall reactors chamber wall heat entlre system tube thermally driven reactions furnace heater requlresleaktlght loadlock chemical scrubbers lters vacuum pumps atmospheric high deposition rates low pressure LPCVD lower rates good uniformity plasma assisted CVD PECVD gas supply graphite rf electrode PECVD SiH4 2N20 gtSi02 2N2 2H2 200 400 C RF 01 5 torr Low ratio of N20 SiL will increase n leading to formation of silicon rich films Lower deposition temperatures and higher ratios of NZOSiO2 will lead to less dense hns and faster etch rates HF etch rate is a measure of the hn s density Densi cation of films Basic configurations Cold wall reactors horizontal tube reactor parallel plate plasma reactor P39asma rfellc odes heat substrate only using resistive heating pass current through susceptor inductive heating external rf elds create eddy currents in conductive susceptor optical heatingamps generate IR absorbed by susceptor advantages reduces contamination from hot furnace walls reduces depo on on chamber walls disadvantages more complex to achieve temperature uniformity hard to measure temperature inherently a nonisothermal system graphite susceptor wafers gasinlet wafers gasinlet pump ancake con guration is barrel reactor r single wafer systems from m VWWV apphedmaterials cornprod uctspdd mm mmquot CV D Chemistries Gas flow n CVD systems SlllCOl l Odee purely turbulent ow reactants are well mixed no geometric 9 OdeatIOIlI 1 02 9 102 reactants to wafer surface typical of LPCVD tube furnace design 4 Wet OX1dat10n 1 2H20 9 102 2H2 interaction of gas flowvvith surfaces away from surfaces flow is primar 9 SlH4 02 9 102 2H2 fric 39on forces velocity to zero at surfaces 4 N20 9 layproducts causes formation ofstagnant houndarylayer 4 SiCle2 N20 9 SiO2 byproducts 4 SiOC2H54 9 SiO2 byproducts tions on supply of reactant supply limited by diffusion across boundary layer geometry of wafers relative to gas flow critl al for lm thickness uniformity to improve boundary layer uniformity can tilt wafer wrt gas flow Material examples polysilicon USES gates high value resistors local interconnects deposition silane pyrolysis 6003970039 C SiH4 gt Si 2H2 atmospheric cold wall 5 silane in hydrogen 1l2 pmlmin LPCVD 1 Ton39 hot wall 20100 silane hundreds nmlmin grain size dependent on growth temperature subsequent processing 39 95039 C phosphorus diffusion 20 min 1 um grain size 1050 C oxidation 13 pm grain size insitu doping ptype diborane BZHG p 0005 Qcm BISi 25x103 can cause substantial increase in deposition rate ntype arsine AsHa phosphine PH3 p 002 Qcm can cause substantial decrease in deposition rate dope after deposition implant diffusion CVD Chemistries 0 Silicon Nitride 3s1H4 4NH3 9 Si3N4 12H2 SiClZH2 NH3 9 Si3N4 byproducts SiH4 4N20 9 Si3N4 by products SiH4 N2 9 Si3N4 by products Metal CVD tungsten WF63H2 r W6HF cold wall systems 300 C can be selective adherence to SiOz problematic TiN often used to improve adhesion causes long initiation time before w deposition begins frequently used to ll deep high aspect ratioquot contact vias aluminum triisobutylaluminum TIBA LPCVD 200 300 C tens nmlmin deposition rate copper Cu Bdiketones 100 200 C CVD Chemistries Polysilicon SiH4 9 Si 2H2 Silicon Carbide Polycrystalline Diamond Parylene polymerized pXylylene Refractory Metals ZWF6 3SiH4 9 2W 3SiF4 6H2 IIVI compounds e g CdSe Safety issues in CVD most gases used are toxic pyrophoric flammable explosive or some combination of these silane SiH4 toxic burns on contact with air phosphine very toxic ammable ammonia toxic corrosive how to deal with this monitor limit maximum flow rate from gas sources helps with dispersal problem associated with gases double walled tubing all welded distribution networks CVD silicon dioxide thermally driven reaction midtemperature 500 0 LTO lowtemp oxide T lt 50039C 5m 02 Sio2 H2 coldwall atmospheric 01 pmmin hotwal LPCVD 001 pmmin plasmaenhanced reaction PECVD low temperature 250 C high temperature 70039C tetraethyl orthosl cate TEOS SiOczH5 gt 2 byproducts new materials sul 39 n with lower dielectric constants k lt 3 fluorinated oxides spinon glasses organics high k dielectrics k gt 25100 s gate insulators decou 9 caps Deposited thin films need to be able to add materials on top of silicon both conductors and insulators deposition methods physical vapor deposition PVD thermal evaporation sputtering chemical vapor deposition CVD general requirements good electrical characteristics free from pinholes cracks low stress good adhesion chemical compatl y with both layer below and quotabovequot at room temperature and under depos Silicon nitride Si3N4 uses diffusivity of 02 H20 is very low in nitride mask against oxidat39 n protect against watercorrosion diffusivity of Na also very low protect against mo Ion contamination deposition stoichiometric formulation is SiN4 in practice SilN ratio varies from 07 N rich to 11 Si rich LPCVD 700 C 900 0 asiH 4NH SiaN 12H2 3Si2c 39 SiIN ratio 075 48quotn H p 39lcm3n20k67 stress 10 Gdynelcmz tensile PECVD 250 C 350 0 aSiH bNH SixNyHl cH2 a 39H bN2 S39xNyH 4NH3 Sm chI 6H2 39 p 2428 glcm3 n 1825 k 69 stress 26 5T Gdynelcm2 Impact of pressure on deposition conditions pressure influences e meanfreepathAolt1P e conmmlnatlon ratequot re up very high vaerrum e N pressure 71 mrnnmlemp k lum Kinetic theory of gases for a gas at STP e N 27 x10quot3 moleculescm3 rme mmnsphere e 1Paseal1132anr1n5mms fraction of molecules k Alsthemeanfree path A x n1 cmP lin nascais 53 gtlt1 3 cmP lin anr at mnmlemp and one almnsphere 1 132 xquot pascal 76 anr1mmHg traveling distance d without co mg is Kc as 7 39 mm Impact of pressure on deposition conditions quotmaul Imlul mmur nimimlinn zvn39 momenoe A 3 srmree 1 mn39 momenoe 4 we39ll use mm m Velocity distribution for ideal gas velocity dist A 500 mileshour at rm temp rate of surface bombardment flux nor 4 Jux1022wcm2sec PNMT P in Torr M is grzmamnleculz mass 4 r4 molecules perunit area bombard rate ribution is Maxw ri H m quot23quotquot wily inw 2 2 xl m m Torr summary of SiO2 characteristics low pressure x ltlt system K gt step plasma SiHA 02 TEOS thermal temperature 200 C 45U C 700 C 1 0001 composl on Si013 H Si02H SiOz Si 02 step coverage non conformal non conformal conformal quotconformalquot thermal sta ty dens stable stable density glcma 22 22 stress Mdynelcmz 1C dielectric strength MVlcm index of refraction 6328 nm 9 low freq 49 adapted 39om Sze 2nd p 259 10391 Torr It 7 05 mm small compared to system large compared to wafer features isotropic arrival at at surface BUT no scattering 39 side hole top at surface location Jf 180 incidence shadowing by comers of M random zing ns t k 180 incidence assumes mate 39 I does NOT migrate after arrival vacuum conditions x gt system k gtgt step case 10395 Torr It 7 5 meters long compared to almost everything anisotropic arrival at all surfaces geometric shadowingquot dominates anisotropic deposition neof gghtquot no randomizing very dependent on source con guration relative to sample surface assumes material does NOT migrate after arrival
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