Basic steps in the CVD process Precursor Transport R R Gas Phase Reactions R Adsorption metal atom R ligand + R Adsorption Decomposition reactions R R Byproduct Transport R Desorption Difusion Nucleation + Heated substrate CVD CVD Reactor CVD CVD Reactor CVD Precursor Volatility CVD Chemical Vapor Deposition Aluminum 2.27 jiQcm, 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 H / 3 Al H CH below 330 oC H H H H Al CH 3 CH H < CH3 CH Al / H2 P-Methyl Elimination H CH 3 H H CH 3 Al above 330 oC H H > H H Al CH 3 CH 3 CH 3 H Al 4 H2 C CVD Chemical Vapor Deposition 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 CVD Chemical Vapor Deposition TMAA H CH3 H H H3C H3C Ah N H H CH3 H3C N Al CH3 ^CH3 H V H H3C N CH3 ^CH3 H H ...........N> H3C H3C CH3 (CH3)3N-AlH3 ' 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 H3C 3 ^ H H CH3 H H B....... H H (CH3)3N-BH3 + 3/2 H2 + Al H H H H | \ .......H \ H ' H H H H ...... H H DMAH ligand redistribution [(CH3)2AlH]3 * (CH3)3Al ft + AlH3 ' Al + H2 at 280 °C, low carbon incorporation CVD Chemical Vapor Deposition Tungsten 5.6 uT2cm, 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 Diketonate Ligands KETO I-M/-M ENOL 3 CH, - H+ HC 3 OO H CH OO e Name Abbreviation CM, CH, ch3 c(ch3}3 cha c(ch3)a ch, CF, C100 oC F3C CF3 OH O H 2 (g) —^ 2 H (ads) 0 1 C >250 oC C CF3 CO + CF 3 C /////////////////////////////////////////// A CVD Chemical Vapor Deposition SEM of Cu film, coarse grain, high resistivity CVD Chemical Vapor Deposition Growth rate of Cu films deposited from Cu(hfacac)2 with 10 torr of H2 1000 2 o o 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/TCK) X 1000 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 ~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 ^im/hr), small, localised areas, poor quality films. Hot filament and plasma methods: slower growth rates (0.1-10 ^im/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, SiO2, quartz, Si3N4 also form carbide layers. SiC, WC, and TiC CVD 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 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 p-doping: B2H6 incorporates B into the lattice doping with atoms larger than C very difficult, /f-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(O2CCH3)4 — SiO2 + 2 O(OCCH3)2 CVD LPCVD of ZnO from Aminoalcoholates (100) ,(101) 20 40 60 2 Theta (cleg.) go SEM of the film deposited by LPCVD at 500 °C. Bar = 1 urn. Hexagonal ZnO PDF 79-0208 CVD LPCVD of ZnO from Aminoalcoholates LPCVD,500QC, 1 h [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 by1st precursor 2/ cleaning of the reaction chamber 3/ exposition by 2nd precursor 4/ cleaning of the reaction chamber CVD ALD Atomic Layer Deposition Cycle repetitions until desired film thickness is reached 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 Figure 2. Cross-sectional SEM images for a 300-nmAl^Oj 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 Encyclopedia 0 2DD4The Computer Language Co. Inc. 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 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 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 of Si02 and A1203 Films Step A H H Jj Step B g OBu1 Buk3—Si-OBu1 )3SiOH + CH3 — i + CH4 Al Al CVD ALD of SiO2 and Al2O3 Films C, D: alkoxide - siloxide exchange SitOBu^ 3i:Oe.i':-; {Gu'OuSi Bu*G OBu* Bu*0—Si—OBu1 ■^^\^iv^^\r^\r\ ^VV^VV^VWV^. -Wv^Cvi^^'WV^ D SifOBut)^ s i OBL -;i3 A A ButO —5 i—OB u* I BJO —5 i—OB i/', O + n (Bu^OhSiOH--\ 0 +nButOH Jl Al CVD ALD of SiO2 and Al2O3 Films E: elimination of isobutene = formation of -OH f ľ I Auto— s i—OEut ButO—|i — ^n-C (C Hjfe ButQ—