Lecture 9 - GUC - Faculty of Information Engineering & Technology

ELCT 705 :Semiconductor TechnologyLecture 09: Thermal OxidationAssoc. Prof. Dr. Mohamed Ragaa BalboulDepartment of Electronic and Electrical Engineering

Silicon Oxidation SystemOxidation system is an oven capable of temperature from 600-1200 o C and asimple gas distribution system capable of introducing O 2 or H 2 O.Most systems in use today horizontally oriented, the wafers are loaded into thefurnace on boats which hold 10-50 wafers.At 1000 o C, oxidation rates in H 2 O are on the order of 0.1 nm sec -1 andapproximately double for a 100 o C temperature rise.Furnace is divided in three to five zones for temperature control, the outer zonesare designed to help compensate heat losses out the ends. Thermocouples monitorthe temperature in each sections and controller delivers power to maintain a flattemperature profile in the center zone.

Rapid Thermal Oxidation (RTO)Decreasing the size of VLSI devices hasin recent years demanded very shorthigh-temperature steps, such gateoxides are < 10 nm thick, requiring short,well-controlled cycles.RTO systems use a lamp heatedchamber which can heat a wafer up tooxidation temperature at a rate of 100oC sec -1 .Hold the wafer there while the oxide isgrown, and then cool it back down toroom temperature, again in a fewseconds.

Mechanism of Thermal Oxide GrowthWhen silicon is oxidized, the oxidation process occurs at the Si/SiO 2 interface.Because of this, a new interface is constantly forming and moving downward intothe silicon substrate.The oxide would like to expand by 30 % in all three dimensions to accommodatethe oxygen atoms. The silicon substrate prevents expansion in the two lateraldirections, the only option is for the oxide to expand upward by 2.2 times.

Basics of Thermal Oxidation ProcessO 2 /H 2 OHigh TSiO 2X SiO0.44 X SiOSiliconSilicon1 mm Si Oxidized 2.25 mm SiO 2material density Molecular weightSi 2.33 g/cm 3 28.086 g/moleSiO 2 2.2 g/cm 3 60.085 g/moleddSiSiO2(28.086 / 2.33)(60.085/ 2.2)0.44

Mechanism of Thermal Oxide GrowthThe grown oxide on the left side occupies approximately twice the volumeof the silicon consumed in the oxidation.A bird’s beak shape is observed because of lateral oxidation under thenitride mask.LOCOS Structure

Silicon-Oxide InterfaceThe perfect interface between Si and SiO 2 is one major reason why Si is used forsemiconductor devices (instead of Ge…)SiO 2 is amorphous even though it grows on a crystalline substrate.SiO 2SiO 2• Stable and reproducibleinterface with Si• Excellent electricalinsulator: energy gapE g = 8-9 eV.SiWafer• High breakdown electricfield: >10 7 V/cm.• Easily selectively etchedusing lithography masks

Structure of SiO 2Basic structure of silica: asilicon atom tetrahedrally bondsto four oxygen atoms.SiO 4 tetrahedra are the basic unitsfrom which SiO 2 forms.These terahedra bond togetherby sharing oxygen atoms.Such shared atoms are calledbridging oxygen atoms.These phases are oftenreferred to as fused silica.The various crystalline andBridging oxygenamorphous forms of SiO 2 arisebecause of the ability of the bridging oxygen bonds to rotate, allowingthe position of one tetrahedron to move with respect to its neighbours.OSi

Structure of SiO 2Single crystal (quartz) 2.65 g/cm 3Amorphous (thermal oxide). 2.21 g/cm 3

SiO 2 Growth Kinetics (Deal-Grove Model)Gas diffusion Solid state diffusion SiO 2 formationConcentrationof main gasflowConcentration at(inside) the oxidesurface. C o C s .Concentration ofoxide at theinterfaceGas fluxDiffusion fluxthrough SiO 2Reaction fluxat interfaceAccording to Henry’s law, the concentration of a gas species dissolved in a solid isproportional to the partial pressure of that species at the solid surface.

SiO 2 Growth Kinetics (Deal-Grove Model)The basic model for oxidation was developed by Deal and Grove (D-G model).F 1 : flux of oxidizing species transported from the gas phase to the gas-oxide interface.Fh( CCh(C*1 g G Sowhere, h g is mass transfer coefficient in cm/sec, ish=h g /HkT, where H is Henry‟s constant.)F 2 : flux across the existing oxide towardthe silicon substrate. Using Fick’s law ofdiffusion, D (cm 2 /sec) is oxidantdiffusivity in the oxide, x is oxidethickness, thenF2CDxDCOCxF 3 : flux reacting at the Si/SiO 2 interfaceOIC)*solubility of oxidant in SiO 2 andF k C 3 s I where, k s is the interface rate reaction constant (cm/sec).C

SiO 2 Growth Kinetics (Deal-Grove Model)In steady state condition, the three fluxes representing the oxidation processmust be equal since they occur in series with each other.F1 F2 F3Now we have two equations to solve two unknown C I and C oCO ksx1Dks1hCksxDOO*C*CI1*CkshksxDO*Cksx1DOPhysically, we can think of the overall process as involving two interface reactions(h and k s ) and a diffusion process.The process at the oxide surface (gas absorption) occurs very rapidly comparedto the chemistry occurring at Si/SiO 2 and hence h can be neglected compared tok s .

SiO 2 Growth Kinetics (Deal-Grove Model)There are two limiting cases that are of interest1. When k s x o /D 1, in this case, the oxidant is reacting at theinterface as fast as it arrives and the overall growth rate is limited by the diffusionprocess.k s x/D > 1, Diffusion limited

Oxide Growth Rate (R)Oxide growth rate, R (cm/s), given by ratio of oxygen flux, F (cm -2 .s -1 ),and number, N 1 (cm -3 ), of oxidant molecules to form unit volume of SiO 2 .RFN1dxdtoN1*kSC kS1 hkSxDofor O 2 as oxidant (dry oxidation)3Ncm221 2.310/Si OSi 2H2 SiOfor H 2 O as oxidant (wet oxidation)22 3N 4.610/ cm12O SiO22 2H2Integrating this equation from an initial oxide thickness x i to finalthickness x o leads us to our final result describing the oxide growthkinetics:Nxxo11kShkSxDodxokSCi 0*tdt

SiO 2 Growth Kinetics (Deal-Grove Model)Rate equation (previous page) can be solved according to the boundarycondition asLettingand substituting gives2xo*2DCBx2o2xi/ NII2DCN xB2i*xo xi*k C / NsBAxo xB / AiII*C kNwhere B and B/A are often termed the parabolic and linear rate constantsrespectively. Physically, they represent the contributions of F 2 (oxidantdiffusion) and F 3 (interface reaction), respectively.tst

SiO 2 Growth Kinetics (Deal-Grove Model)It is sometimes convenient to rewrite the linear parabolic growth law inthe following formwhere2xoBt xB /xo2iA t t AxBIn these expressions, x i or t account for any oxide present at the start of theoxidation. Solving the parabolic equation leads to oxide thickness in terms ofgrowth timeix oA t t 112 2 A / 4B

SiO 2 Growth Kinetics (Deal-Grove Model)There are two limiting forms of the linear parabolic growth law, theseoccur when one of the two terms dominates leading to2xoBB( t t )Ax o)xB /oA t tThin Oxides: One can neglect parabolic term and oxide thickness becomesThick Oxides: One can neglect linear term and oxide thickness becomesx 0tx 0t2x oB(ttttOxide growth rate slows down with increasing oxidation time: Fromlinear (reaction limited) to parabolic (diffusion limited) rate constants

Determine B and B/A from ExperimentB and B/A are normally determined experimentally by extracting themfrom growth data. Since, k s is particular contains a lot of “hidden” physics.2xoBxB /oA t txoB t txoA

Rate Constants B and B/AWhen oxidations are performed on flat unpatterned surface, on lightly dopedsubstrate, in O 2 or H 2 O ambients and when oxide thickness is larger than about20 nm, the growth kinetics are usually well described by the linear parabolic law.Under such conditions, values for B and B/A can be readily extracted.BB C1 exp( E1/ kT) C2exp( E2/ kT)AIn these expressions, E 1 and E 2 are the activation energy, C 1 and C 2 are thepreexponential constants.The rate constants B and B/A have physical meaning of oxidant diffusion andinterface reaction rate respectively

Rate Constants B and B/A

Oxidation Rates for (100) Si in Dry O 2Dry oxidation: shows higher quality than wet oxidation and generallyuseful for producing oxide films up to 100-200 nm. Films thicker thanthis would normally be grown using H 2 O ambient.Both rate constant Band B/A are muchlarger for H 2 O thanfor O 2 .

Oxidation Rates for (100) Si in H 2 OWet oxidation: grows much faster than it dose in dry O 2 . The principal reason for thisis the oxidant solubility in SiO 2 (C * ), is much higher for H 2 O than for O 2 .Disadvantage: oxides grown wet are less dense, with a more open structure, becauseout-diffusion of H 2 creates „voids‟ along its path.For dry O 2C * 5×10 16 cm -3For H 2 OC * 3×10 19 cm -3

Dependence of Growth Kinetics on PressureThe linear parabolic model predicts that the oxide growth rate should be directlyproportional to oxidant pressure (bulk gas pressure P G ).*C HP GBoth B/A and B are proportional to P G , the gas pressure.BAHP kGN 1SB2DHPN1GDeal-Grove model can be used with the following corrections:BA BAiPB ( B)iPFor H 2 O oxidation, the growth rate is proportional to P,where i refer to the values at 1 Atm.BA BAiPnB ( B)iPFor O 2 oxidation, the relationship is not linear,where n 0.7 – 0.8.

MicronsDependence of Growth Kinetics onCrystal OrientationCrystal planes that have higher densities of atoms will oxidize faster.Oxidation on the crystal plane occurs at a higher rate because there are ahigher number of surface atoms/chemical bonds than the plane.BA111 B 1.68 A 1001.51.71.9BA110 B 1.45 A 1002.12.3-0.4 -0.2 0 0.2 0.4Microns

2D SiO 2 Growth KineticsShaped silicone structures oxidize differently than simple flat surfaces.Oxidation is slower for convex or concave corners, concave corner is even slowerthan convex corner.Several physical mechanisms are950 °C oxidation 1100 °C oxidationImportant in understanding theseResults such as1. Crystal orientation: Shapedsurfaces involve manysurface orientation.2. 2D Oxidant Diffusion: Incorner regions, oxidanttransport to Si/SiO 2 is a2D/3D transport problem.3. Stress due to volumeexpansion: Oxide layersformed on silicon are undersignificant compressivestress.