CVD_ALD_MLD Basic steps in the CVD process R ligand metal atom Precursor Transport Gas Phase Reactions R RR Adsorption R R R + Adsorption Desorption R R R Decomposition reactions Difusion Nucleation R R R R + Heated substrate Diffusion of precursor to a surface Diffusion of products from surface Silicon CVD CVD_ALD_MLD CVD_ALD_MLD CVD Reactor Cold-wall reactor CVD_ALD_MLD CVD Reactor Hot-wall reactor CVD Kinetics CVD_ALD_MLD Deposition depends on the sequence of events: (1) Diffusion of precursor to surface (2) Adsorption of precursor at surface (3) Chemical reaction at surface (4) Desorption of products from surface (5) Diffusion of products from surface • The slowest event will be the rate-determining step CVD Kinetics CVD_ALD_MLD Growth Rate Model F1 = precursor flux from bulk of gas to substrate surface F1 = hG ⋅ (CG - CS) hG = mass-transfer coefficient hG = D /  D = gas diffusion constant D = Do T 3/2 / P  = boundary layer thickness (related to gas velocity) CG, CS = precursor conc. at bulk of gas and at substrate surface (conc. gradient – driving force for diffusion) F2 = flux consumed in film-growth reaction (rate of chemical reaction) F2 = kS ⋅ CS kS = surface-reaction rate constant: kS = A exp (Ea/kT) Steady state F1 = F2 = F CVD Kinetics CVD_ALD_MLD Growth Rate Model F1 = F2 (rate of transport = rate of reaction) hG ⋅ (CG - CS) = kS ⋅ CS CS = CG / (1 + kS/hG) F = kS hG CG / (kS + hG) Growth rate (thickness growth rate) dy / dt = F /  y = film thickness  = atomic density of film Steady state F1 = F2 = F GS G hk C dt dy 11 11    Growth Rate CVD_ALD_MLD GS G hk C dt dy 11 11    Growth rate is determined by: a) Concentration of a precursor in bulk of gas mixture b) By the smaller of hG and kS kS << hG = Surface reaction limited dy/dt  exp(Ea/kT) hG << kS = Mass transport limited dy/dt  T3/2 Deposition rate vs. Temperature CVD_ALD_MLD dy / dt Deposition rate vs. Temperature CVD_ALD_MLD Growth Rate Dependence on Flow Velocity CVD_ALD_MLD F1 = hG ⋅ (CG - CS) hG = mass-transfer coefficient hG = D /   = boundary layer thickness At constant T Low flow rate U  large boundary layer thickness   slow mass-transfer CVD_ALD_MLD Precursor Volatility          12 0 1 2 11 ln TTR H p p subl CVD_ALD_MLD Chemical Vapor Deposition Aluminum 2.27 cm, easily etched, Al dissolves in Si, GaAs + Al AlAs + Ga Gas diffusion barriers, Al on polypropylene, food packaging = chip bags, party balloons, high optical reflectivity TIBA Al CH3 H CH3 H H below 330 oC -Hydride Elimination CH3 CH3 H H Al H Al CH3 CH3 H H H2 Al CH3 H CH3 H H above 330 oC -Methyl Elimination CH3 HH H Al CH3 Al CH3 HH H H2 C CVD_ALD_MLD Al deposits selectively on Al surfaces, not on SiO2 Laser-induced nucleation 248 nm only surface adsorbates pyrolysed 193 nm gas phase reactions, loss of spatial selectivity control TMA large carbon incorporation, Al4C3, RF plasma, laser Al2(CH3)6 1/2 Al4C3 + 9/2 CH4 under N2 Al2(CH3)6 + 3 H2 2 Al + 6 CH4 under H2 Chemical Vapor Deposition CVD_ALD_MLD Chemical Vapor Deposition TMAA Al H H Al H H N N H H CH3 H3C CH3 H3C CH3 H3C Al H H N H CH3 CH3 H3C Al H H N H CH3 CH3 H3C CH3 H3C H3C N (CH3)3N-AlH3 Al + (CH3)3N + 3/2 H2 below 100 C CVD_ALD_MLD Chemical Vapor Deposition (CH3)3N-AlH3 Al + (CH3)3N + 3/2 H2 below 100 C Decomposition mechanism of TMAA on Al CVD_ALD_MLD Chemical Vapor Deposition Aluminoboranes Al H H B H H N H H CH3 CH3 H3C (CH3)3N-BH3 + 3/2 H2 + Al Al H H B H H H H H H B B H H H H DMAH ligand redistribution [(CH3)2AlH]3 (CH3)3Al  + AlH3 Al + H2 at 280 C, low carbon incorporation CVD_ALD_MLD Chemical Vapor Deposition Tungsten 5.6 cm, a high resistance to electromigration, the highest mp of all metals 3410 C. 2 WF6 + 3 Si  2 W + 3 SiF4 WF6 + 3 H2  W + 6 HF WF6 + 3/2 SiH4  W + 3 H2 + 3/2 SiF4 W(CO)6  W + 6 CO CVD_ALD_MLD O O H3 C CH3 HH KETO ENOL O O H3 C CH3 H H O O H3 C CH3 H O O H3 C CH3 - H+ Diketonate Ligands CVD_ALD_MLD Diketonate Precursors Mononuclear Polynuclear CVD_ALD_MLD Chemical Vapor Deposition Copper(II) hexafluoroacetylacetonate excellent volatility (a vapor pressure of 0.06 Torr at r. t.), low decomposition temperature, stability in air, low toxicity, commercial availability deposition on metal surfaces (Cu, Ag, Ta) the first step, which can already occur at -150 C, a dissociation of the precursor molecules on the surface (Scheme I). An electron transfer from a metal substrate to the single occupied HOMO which has an anti-bonding character with respect to copper dxy and oxygen p orbitals weakens the Cu-O bonds and facilitates their fission. CVD_ALD_MLD Chemical Vapor Deposition Scheme I -150 o C CF3 CF3F3 C F3 C O O O Cu O e O Cu O F3 C CF3 + F3 C CF3 O O F3 C CF3 OOH C CO + CF 3 2 H (ads)H 2 (g) Cu o >250 o C CF3C C O >100 o C CVD_ALD_MLD Chemical Vapor Deposition SEM of Cu film, coarse grain, high resistivity CVD_ALD_MLD Chemical Vapor Deposition Growth rate of Cu films deposited from Cu(hfacac)2 with 10 torr of H2 CVD_ALD_MLD Chemical Vapor Deposition Cu(I) precursors Disproportionation to Cu(0) and Cu(II) 2 Cu(diketonate)Ln  Cu + Cu(diketonate)2 + n L O Cu O R R L O Cu O R R LL L: PMe3, PEt3, CO, CNt Bu, SiMe3 CVD_ALD_MLD Chemical Vapor Deposition Diamond films activating gas-phase carbon-containing precursor molecules: •thermal (e.g. hot filament) •plasma (D.C., R.F., or microwave) •combustion flame (oxyacetylene or plasma torches) CVD_ALD_MLD Chemical Vapor Deposition Experimental conditions: temperature 1000-1400 K the precursor gas diluted in an excess of hydrogen (typical CH4 mixing ratio ~1-2vol%) Deposited films are polycrystalline Film quality: •the ratio of sp3 (diamond) to sp2-bonded (graphite) carbon •the composition (e.g. C-C versus C-H bond content) •the crystallinity Combustion methods: high rates (100-1000 µm/hr), small, localised areas, poor quality films. Hot filament and plasma methods: slower growth rates (0.1-10 µm/hr), high quality films. CVD_ALD_MLD Chemical Vapor Deposition Hydrogen atoms generated by activation (thermally or via electron bombardment) H-atoms play a number of crucial roles in the CVD process: H abstraction reactions with hydrocarbons, highly reactive radicals: CH3 (stable hydrocarbon molecules do not react to cause diamond growth) radicals diffuse to the substrate surface and form C-C bonds to propagate the diamond lattice. H-atoms terminate the 'dangling' carbon bonds on the growing diamond surface, prevent cross-linking and reconstructing to a graphite-like surface. Atomic hydrogen etches both diamond and graphite but, under typical CVD conditions, the rate of diamond growth exceeds its etch rate whilst for graphite the converse is true. This is the basis for the preferential deposition of diamond rather than graphite. CVD_ALD_MLD Chemical Vapor Deposition Diamond initially nucleates as individual microcrystals, which then grow larger until they coalesce into a continuous film Enhanced nucleation by ion bombardment: damage the surface - more nucleation sites implant ions into the lattice form a carbide interlayer - glue, promotes diamond growth, aids adhesion Diamond laser window CVD_ALD_MLD Chemical Vapor Deposition Substrates: metals, alloys, and pure elements: Little or no C Solubility or Reaction: Cu, Sn, Pb, Ag, and Au, Ge, sapphire, diamond, graphite C Diffusion: Pt, Pd, Rh, Fe, Ni, and Ti the substrate acts as a carbon sink, deposited carbon dissolves into the metal surface, large amounts of C transported into the bulk, a temporary decrease in the surface C concentration, delaying the onset of nucleation Carbide Formation: Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Fe, Co, Ni, Y, Al B, Si, SiO2, quartz, Si3N4 also form carbide layers. SiC, WC, and TiC CVD_ALD_MLD Chemical Vapor Deposition Applications of diamond films: Thermal management - a heat sink for laser diodes, microwave integrated circuits active devices mounted on diamond can be packed more tightly without overheating Cutting tools - an abrasive, a coating on cutting tool inserts CVD diamond-coated tools have a longer life, cut faster and provide a better finish than conventional WC tool bits Wear Resistant Coatings -protect mechanical parts, reduce lubrication gearboxes, engines, and transmissions CVD_ALD_MLD Chemical Vapor Deposition Electronic devices - doping, an insulator into a semiconductor p-doping: B2H6 incorporates B into the lattice doping with atoms larger than C very difficult, n-dopants such as P or As, cannot be used for diamond, alternative dopants, such as Li Optics - protective coatings for infrared optics in harsh environments, ZnS, ZnSe, Ge: excellent IR transmission but brittle the flatness of the surface, roughness causes attenuation and scattering of the IR signal CVD_ALD_MLD Laser-Enhaced CVD Si(O2CCH3)4  SiO2 + 2 O(OCCH3)2 ArF laser Substrate Heated source Heater Vacuum chamber Vacuum Pump CVD_ALD_MLD LPCVD of ZnO from Aminoalcoholates SEM of the film deposited by LPCVD at 500 °C. Bar = 1 μm. Hexagonal ZnO PDF 79-0208 CVD_ALD_MLD LPCVD of ZnO from Aminoalcoholates CVD_ALD_MLD CVD of YF3 from hfacac Complex CVD_ALD_MLD ALD Atomic Layer Deposition Special modification of CVD Method for the deposition of thin films Film growth by cyclic process 4 steps: 1/ exposition by1st precursor 2/ cleaning of the reaction chamber 3/ exposition by 2nd precursor 4/ cleaning of the reaction chamber CVD_ALD_MLD ALD Atomic Layer Deposition Cycle repetitions until desired film thickness is reached 1 cycle: 0.5 s – several sec. thickness 0.1- 3 Å Self-Limiting Growth Mechanism High reactivity Formation of a monolayer Control of film thickness and composition Deposition on large surface area CVD_ALD_MLD ALD vs. CVD Comparison ALD Carried out at room temperature Control over number of deposited layers = film thickness Reactor walls inactive – no reactive layer Separate loading of reactive precursors Self-limiting growth Precursor transport to the reaction zone does not have to be highly uniform (as in CVD) Solid precursors CVD_ALD_MLD ALD vs. CVD Comparison CVD_ALD_MLD Precursor Properties Selection of suitable combination of precursors Molecular size influences film thickness Gases, volatile liquids, solids with high vapor pressure Typical precursors: Metallic - halogenides (chlorides), alkyls, alkoxides, organometallics (cyclopentadienyl complexes), alkyl amides Nonmetallic - water, hydrogen peroxide, ozone, hydrides, ammonia, hydrazine, amines CVD_ALD_MLD Precursor Properties Thermally stable Must react with surface centers (hydroxyl groups on oxide surface) Thermodynamics Kinetics Mechanisms CVD_ALD_MLD Examples of ALD High-permitivity Oxides Al(CH3) 3/H2O ZrCl4/H2O HfCl4/H2O CVD_ALD_MLD Examples of ALD DRAM capacitors (Ba,Sr)TiO3 – Sr and Ba cyclopentadienyl compounds and water as precursors Nitrides of transition metals TiN - TiCl4 and NH3 TaN - TaCl5/Zn/NH3 WN - WF6 and NH3 WCxNy CVD_ALD_MLD Examples of ALD Metallic films Difficult by ALD: metal surface has no reaction sites, low reactivity with reducing agents W - WF6 and Si2H6 Ru, Pt - organometallic precursors and oxygen applies to all precious metals capable of catalytic dissociation of O2 Ni, Cu – metal oxide reduction by hydrogen radicals formed in plasma Al – direct reduction of AlMe3 by H radicals from plasma CVD_ALD_MLD ALD of SiO2 and Al2O3 Films Precursors: trimethylalane, tris(tert-butoxy)silanol Deposition of amorphous SiO2 and nanolaminates of Al2O3 32 monolayers in 1 cycle Applications: microelectronics optical filters protective layers (against diffusion, oxidation, corrosion) CVD_ALD_MLD ALD of SiO2 and Al2O3 Films Step A Step B CVD_ALD_MLD ALD of SiO2 and Al2O3 Films C, D: alkoxide - siloxide exchange CVD_ALD_MLD ALD of SiO2 and Al2O3 Films E: elimination of isobutene = formation of -OH CVD_ALD_MLD 50 ALD of SiO2 and Al2O3 Films F: elimination of butanol = condensation G: elimination of water = condensation OH OH OH OH CVD_ALD_MLD ALD of SiO2 and Al2O3 Films 51 Repeat Step A CVD_ALD_MLD CVD_ALD_MLD MLD - Molecular Layer Deposition Sequential, self-limiting reactions A and B for MLD growth using two homobifunctional reactants CVD_ALD_MLD AB MLD CVD_ALD_MLD ABC MLD CVD_ALD_MLD Diols vs. Polyols homobifunctional precursors can react twice with the AlCH3* surface species, double reactions lead to a loss of reactive surface sites and decreasing growth rate CVD_ALD_MLD ABCD MLD growth of an alumina-siloxane CVD_ALD_MLD AB Lewis Acid-Lewis Base Reactions CVD_ALD_MLD Alucone