3 Thin Film DepositionQuality – composition, defect density, mechanical and electrical propertiesUniformity – affect performance (mechanical , electrical)Thinning leads to RVoids: Trap chemicals lead to cracks (dielectric) large contact resistance and sheet resistance (metallization)AR (aspect ratio) = h/w with feature size in ICs.
4 Examples Poor step coverage with increasing AR Thinning causes metal resistance to increase, generates heat and lead to failure
5 Chemical Vapor Deposition Methods of Deposition:Chemical Vapor Deposition (CVD):APCD, LPCVD, HDPCVD Physical Vapor Deposition (PVD: evaporation, sputtering)Cold wall reactorAtmospheric Pressure : APCVDCold wall reactors (walls not heated - only the susceptor)Flat on the susceptorLow pressure: LPCVD – batch processing.Hot wall reactor
6 Atmospheric Pressure Chemical Vapor Deposition Steps in depositionTransport by forced convectionTransport of byproducts by forced convection@ the surface (4): decomposition, reaction, surface migration attachment etc.By diffusion through boundary layerDiffusion through the B. LDesorption of by productsadsorption(3) May be desorption which depends on a sticking coefficient (4)Growth rate for Si deposition N=5•1022cm-3transportMole fraction of the incorporating species in the gas phase.reactionPartial pressureSteady stateAs in Deal-Grove model for oxidationV = 0.14 m/min1 torrPtotal = 1 atm = 760 torrTotal concentration in the gas phaseCT = 1 * 1019 cm-35 * 1022
7 Determined by the Smaller of ks or hG Growth KineticsDetermined by the Smaller of ks or hGTwo limiting cases1) Surface reaction ks << hGControl: fast transportslow reactionkS limited deposition is VERY temp sensitive.hG limited deposition is VERY geometry(boundary layer) sensitiveLimited by transport2) Mass transfer or gas phase diffusion hG << ksTemperature uniformity more important than the gas flow wafers vertically poly-SiPut wafers flat to ensure flow the Si surface.EpitaxyFast reaction and slow transport.Both are linear with time (t)Light massAPCVDSiO2heavySiH4 the fastest growth(111) Si shows slower v – fewer attachment sites than in (100) SiEa 1.6 eV for all Si sources H desorption from the Si surface.With H2 as a gas carrier
8 Boundary Layer – Diffusion to the Surface Gas moves with the constant velocity U.Boundary layer (caused by friction ) increases along the susceptor, mass transfer coefficient hG decreases, gas depletion caused by consumption of the reacting species (concentrations decrease)Growth rate decreases along the chamberUse tilted susceptorUse T gradient 5-25°CGas injectors along the tubeUse moving beltviscosityB.L.Deposition of alloys DIFFICULT – various reactions, kinetics (species, precursors)Use PVD rather than CVDgas density
9 Doping in CVD for EPITAXY Intentional and Unintentional The dopant sources at the surface go through:dissociation of hydride gaslattice site incorporationburying of dopants by other atoms in the filmAutodoping: 1 - 4Si source + Dopant (AsH3, PH3, or B2H6)Simulation very inaccurate : chamber design etc.In deposition , the doping,°CoutdiffusionautodopingoutdiffusionT&time of CVDautodopingThe growth is faster than the diffusionCalculate all distributions (=contributions) to get C(x,t)
10 Low Pressure Chemical Vapor Deposition Operate at the surface reaction limited regime lower deposition temperaturesLarger lower PDG1/Ptotallow pressurehG increases ~ 100 X for 760 torr 1 torrtransport less importantSurface reaction controls the growthLPCVD reactors useVertical wafer stackingP = 0.25 – 2.0 torrT = 300 – 900 °C ( + 1 °C) temperature gradient increase 5 – 25% to compensate reactants depletion distributed feeding.Less autodoping (at lower P)Fewer particulates.Possible disadvantages:Deposition rates may be too low, Film quality decreasesShadowing (less gas-phase collisions) due to directional diffusion to the surface deterioration of the step coverage and filling.
11 Plasma Enhanced Chemical Vapor Deposition(PECVD) Used when :Low T required (dielectrics on Al, metals) but CVD at decreased T gives increased porosity, poor step coverage.Good quality films – energy supplied by plasma increases film density, composition, step coverage for metal decreases but WATCH for damage and by product incorporation.13.56 MHzP 50 mtorr - 5 torrPlasma: ionized excited molecules, neutrals, fragments, ex. free radicals very reactive the Si surface enhanced increase deposition ratesIons, electrons, neutrals = bombardmentOutgassing , peeling , cracking stress.°CHigh Density Plasma CVD dense layers ( SiO2) at low T (150 °C) and low P ( m torr); T increases to 400°C by bombardmentSeparate RF (gives substrate biasing bombardment) from plasma generation (Electron Cynclotron resonance ECR and Inductively coupled Plasma ICP)Controlled bombardment (angular -> sputtering) preferential sputtering of sloped surface improved planarization and filling
13 Very low pressure (P < 10 –5 torr) - long mean free path. Physical Vapor Deposition (PVD) – no chemical reactions (except for reactive sputtering)EvaporationAdvantages:Little damagePure layers (high vacuum)Disadvantages:Not for low vapor pressure metalsNo in-situ cleaningPoor step coverageVery low pressure (P < 10 –5 torr) - long mean free path.purer – no filaments, only surface of the source meltedX-rays generated trapped charges in the gate oxides anneal it !
14 Evaporation Deposition rates: Ideal cosine emission Wafer holders to increase uniformity of depositionUse spherical holders & rotate them in a planetary configurationProjected areaThe largest flux for the perpendicular directionPractical casesKnudsen –cell like behaviornonuniform depositionAffected by a crucible (melt,) l r and cos k v Emitted fluxes from cruciblessource here is || to the waferlower because of cosi emission
15 Evaporation Partial Pressure (Pe) of the source (target) No alloys – partial pressure differences1-10 mtorrNeeded for reasonable v m/minUse separate sources and e-beamDepositionStep Coverage Poor :Long mean free path (arrival angle not wide = small scattering) and low T (low energy of ad-atoms)Sticking coefficient high T) no desorption and readsorption poor step coverageHeating can increase Sc but may change film properties (composition, structure)Sticking coefficientRarely used in IC fabrication
16 Use ultra clean gasses and ultra clean targets Sputter DepositionHigher pressures 1 –100 mtorr ( < 10-5 torr in evaporation) -> contaminations!Use ultra clean gasses and ultra clean targetsMajor Technique in Microelectronics for:DC Sputtering (for metal)Alloys (TiW, TiN etc)good step coveragecontrolled propertiesConductiveAl, W, TiAr inert gas at low pressure.No free radicals formed by Ar (ex. O, H ,F as was for PECVD)
17 DC SputteringLow concentrations of electrons -> few collisions -> large voltage here0.1 –10 mme-The source of material to be depositedI+low e--conc.e- collide with Ar atomsexcitation=glowConductive !Ar ions (+) strike the target and sputter ions electrons do not have high energy yet dark glow anode sheath.(Crookes dark space)ions+ potentialpositive potential (form next to each surface = anode)I+ and e- strike the surface:µe larger (smaller mass than for ions)more electrons than ionsE field due to charge imbalance Vp (10 V develops) to decrease e- accumulationIons get neutralized by e- and diffuse to the wafer surfacesecondary electrons!They sustain the plasmaVery small sputtering at the anode can be used for bias sputtering and ionized sputtering
18 Sputtering Cathode DC Sputtering CONDUCTOR secondary electrons 10-20 eVelectrons sustain the plasma - ionization of the gasCan be incorporated in the filmdesorption (small)Bombardment by I+ , e- charges and neutralsadsorptionWAFERSmigrationheatingcan be used in hot sputterreflowReactive SputterDeposition = add gas:. N Ti N. O Ti O2
19 SPUTTERING YIELDNeutralmuch smaller differences in sputtering yields then in partial pressures of components (target) in evaporation+TargetgasTarget atomEffect of mass (gas) & EnergyTiWsteady state sputteringEffect of mass (target)implantation
20 - V1/V2=(A2/A1)m + RF Sputter Deposition Dielectric Instead of DC: 13.6 MHz RF coupled capacitively to plasmaDC sputter cannot be used for dielectricssecondary e-plasma extinguished (VZ ) in 1-10µse- charge on electrode (e- are fast so they keep up with RF)Moree-on the walls-charge built-uppotential VPpotential@ the target ( area)faster, smallertenths of voltsA1A2several 100VwafersFor A1=A2 Ions would bombard the target and the deposited layerV1/V2=(A2/A1)mm=1-2= NON-CONDUCTINGA2Oscillating (with RF) e-ionization yieldpressurecan be usedlarge A1 area+magnet e- trajectory Magnetron Sputter Deposition have better ionization yieldsdeposition rates (10-100X)better film quality (Ar needed)use in DC & RF ( heating of the target since I )
21 BIAS SPUTTERING - Small (-) Bias @ the Wafer Chuck 50 – 300V on wafers – 2000V on targetFor :precleaning = sputter etchbetter planarity step coverageproperties (stress, compos.)Not used much particulates from flakingUse High Density Plasma CVD instead orCollimated Sputtering and Ionized SputteringWider arrivalsmallinextended sources = not point sources evap;in point sourcesCollimate the beam by using holes to direct the ions to the wafers : that u
23 VARIOUS DEPOSITION TECHNIQUES mass transfer regimeT oxide evaporationCl- cleaning (HCL etc)0.5 – 2 torr – 200 torr!VLSILow T: -Si instead of poly_Sigive better step coverage than sputtering& better doping controlSelectivity with Cl, poor for SiHCLConditions: density of SiO2, step coverage, contaminations (C, H, N) all that determines T, pressure and gas used.TEOSporous BPSG 4-8% B, P)reflow for planarization, steam, N2LPCVD (conformal, °C)Barrier! Al – TIBA or DMAH Al-HC - better rCu or Al-Cu sputtered on CVD Al 400 0Cplugs= salicide400 – 7000C
24 Polysilicon @ low T amorphous Si As, P the grain boundariesr ( B does not ! )columnar structure625 °CAs & P deposition rate of poly – Si use doping after poly depositionB Vpoly - Si