Basic steps in the CVD process Precursor Transport metal atom r h> ligand Byproduct Transport Gas Phase Reactions Adsorption r[>H Decomposition A r reactions Sr -* r» Adsorption Desorption Difusion Nucleation Heated substrate CVD CVD Reactor solution injector precursor solution vapouriser To pressure control, filters, pumps and exhaust substrate silica reactor t t f ir heating CVD CVD Reactor —i iL —I V///////AW7^7Zm 0-H Gas Supply System Reactor Vacuum System CVD Precursor Volatility CVD Chemical Vapor Deposition Aluminum 2.27 jj,Qcm, 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 P-Hydride Elimination H h4 CH below 330 <>C Al CH3 H. H > H /CH3 CH, Al P-Methyl Elimination CH, above 330 »C H H H. H > < CH, H CH3 ^11 CH, Al Al 4 H2 CVD Chemical Vapor Deposition Al deposits selectively on Al surfaces, not on Si02 Laser-induced nucleation 248 nm only surface adsorbates pyrolysed 193 nm gas phase reactions, loss of spatial selectivity control TMA large carbon incorporation, A14C3, RF plasma, laser A12(CH3)6 —► 1/2 A14C3 + 9/2 CH4 under N2 A12(CH3)6 + 3 H2 —► 2 Al + 6 CH4 under H2 CVD Chemical Vapor Deposition TMAA H3C H CH3 ^CH3 ___^A1- H- H3C H3C' -Al .........N H "H ^CH3 H,CL CH3 ,^CH3 ..Al H H H3C H H' N- CH3 ^-CH3 2A1-H .........?N, H3C/ ^CH3 H3C (CH3)3N-A1H3 —►Al + (CH3)3N + 3/2 H2 below 100 °C CVD Chemical Vapor Deposition (CH3)3N-A1H3 —►Al + (CH3)3N + 3/2 H2 below 100 °C CVD Chemical Vapor Deposition Aluminoboranes H CH3 W"^ I " H H H (CH3)3N-BH3 + 3/2 H2 + Al H H A' H | N .....H I H ■H H "H "B H H DMAH ligand redistribution [(CH3)2A1H]3-► (CH3)3A1 ft + A1H3-►Al + H2 at 280 °C, low carbon incorporation CVD Chemical Vapor Deposition Tungsten 5.6 u,Qcm, a high resistance to electromigration, the highest mp of all metals 3410 °C. 2 WF6 + 3Si^2W + 3 SiF4 WF6 + 3H2 ^ W + 6 HF WF6 + 3/2SiH4 -> W + 3H2 + 3/2 SiF4 W(CO)6 -> W + 6 CO CVD Diketonate Ligands R2 Name Abbreviation CH3 CH3 Pentane-2.4-d ionate (acetylacetonate) acac CH3 CF3 1,1,1 -triHuoropentane-2,4-dionate (trHluo roacety lacetonate) ťac CF3 1,1,1,5,5,5-hexafluorope ntane-£ ,4-dionate (hexaf I uoroacety lacetonate) hfac CH3 C(CH3)3 1,1 -di methy lhexane-3,5-dionate dhd C(CHS)3 C(CH3)3 £,£,6,6-letfamethylhepiarie-3,5-dioiiate CH3 CH (CH E-m elhylhepta n e-2,4- d io na te mhd C(CH3)3 CH2CH (CH^)^ 2,2: 7-trimethy lactane-3,5-d ionate tmcd CCH5 C6H5 1,3-diphenylpropane-1,3-dionale (dibenyzoylmethariate) dor Diketonate Precursors c Polynuclear CVD 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 Chemical Vapor Deposition Scheme I CF, ■O O • xx —> F,C. -150 °C .0 CF, YY 3 FC W ox o + o o Cu ///////////////////////////////////////// H 2 (g) —► 2 H (ads) F,C CF, OH O CO + CF 3 /////////////////////////////////////////// C Cu° CVD CVD Chemical Vapor Deposition Growth rate of Cu films deposited from Cu(hfacac)2 with 10 torr of H2 370°C 350°C 330°C 310°C 1000 100 ■ r V "9 1.50 1.52 1.54 1.56 1.58 1.60 1.62 1.64 1.66 1.68 1.70 1.72 1.74 1/TfK) X1000 CVD Chemical Vapor Deposition Cu(I) precursors Disproportionation to Cu(0) and Cu(II) 2 Cu(diketonate)Ln -» Cu + Cu(diketonate)2 + n L CVD 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 Chemical Vapor Deposition Experimental conditions: temperature 1000-1400 K the precursor gas diluted in an excess of hydrogen (typical CH4 mixing ratio ~l-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 um/hr), small, localised areas, poor quality films. Hot filament and plasma methods: slower growth rates (0.1-10 um/hr), high quality films. CVD 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 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 CVD 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, Si02, quartz, Si3N4 also form carbide layers. SiC, WC, and TiC CVD Chemical Vapor Deposition Applications of diamond films: t - 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 gs -protect mechanical parts, reduce lubrication gearboxes, engines, and transmissions CVD Chemical Vapor Deposition 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 Electronic devices - doping, an insulator into a semiconductor /7-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 CVD Laser-Enhaced CVD ArF laser Substrate Heater Heated source Vacuum Pump Vacuum chamber Si(02CCH3)4 -> Si02 + 2 0(OCCH3)2 CVD LPCVD of ZnO from Aminoalcoholates Me2N weight % 100 NMe2 OH (002) (ioo)Li|in >i^iai>r in ,(101) S ™ SEM of the film deposited by LPCVD at 500 °C. Bar = 1 um. 2 Theta (deg.) Hexagonal ZnO PDF 79-0208 CVD LPCVD of ZnO from Aminoalcoholates [MeZn(tdmap)]2 CVD CVD of YF3 from hfacac Complex ALD Atomic Layer Deposition Special modification of CVD Method for the deposition of thin films Film growth by cyclic process 4 steps: 1/ exposition bylst precursor 21 cleaning of the reaction chamber 3/ exposition by 2nd precursor 4/ cleaning of the reaction chamber H HOB CVD ALD Atomic Layer Deposition Cycle repetitions until desired film thickness is reached o 1 cycle: 0.5 s - several sec. thickness 0.1- 3 A Self-Limiting Growth Mechanism High reactivity Formation of a monolayer Control of film thickness and composition Deposition on large surface area CVD 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 vs. CVD Comparison Figures Cross-sectional SEM images for a. 300-nm Al20j film (a) and a. 14-nm TiN film [b) deposited on a patterned silicon substrate. CVD 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 Precursor Properties Thermally stable Must react with surface centers (hydroxyl groups on oxide surface) Thermodynamics Kinetics Mechanisms CVD Examples of ALD High-permitivity Oxides A1(CH3) 3/H20 ZrCl4/H20 NMOS Transistor HfCl4/H20 (n-channel MOSFET) From Computer Desktop Bicyclopedia ©2uÜ4The Computer Language Co. Inc. Examples of ALD DRAM capacitors (Ba,Sr)Ti03 - 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 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 02 Ni, Cu - metal oxide reduction by hydrogen radicals formed in plasma Al - direct reduction of AlMe3 by H radicals from plasma CVD ALD of Si02 and A1203 Films Precursors: trimethylalane, tris(tert-butoxy)silanol Deposition of amorphous Si02 and nanolaminates of A1203 32 monolayers in 1 cycle Applications: microelectronics optical filters protective layers (against diffusion, oxidation, corrosion) CVD ALD of Si02 and A1203 Films Step A A CHa H H j, JLX J™ StepB o OBu1 I Buk3-Si-OBu1 (Bu"013SiDH + CH3 —p- O + CH4 Al /I CVD ALD of Si02 and A1203 Films C, D: alkoxide - siloxide exchange 3i OH J, OH -C3uL BukS—ii-"^ __-Si—OBu1 __ BulO—ii-O-li- A "A A A Al Al Al Al CVD 42 ALD of Si02 and A1203 Films Repeat Step A A CHj H H J, CVD 43 CVD