Lecture 11 PVD - – “pressure” at sample surface is...

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Unformatted text preview: – “pressure” at sample surface is much lower • few monolayers per sec « Pequiv ~ 10-6 Torr high vacuum to avoid contamination – “line-of-sight” deposition, poor step coverage heating of source material – potential problem: thermal decomposition rates ~ 0.1- few nm/sec – typically Pvapor ~ 10-4 Torr immediately above source Dean P. Neikirk © 2001, last update February 2, 2001 • • • Microsystems Principles 21 10-6 10-5 10-4 10-3 10-2 10-1 100 0 500 Na Al 1500 Ti 2000 Pt Cr Ni Ti 2500 W Mo Dept. of ECE, Univ. of Texas at Austin adapted from Campbell, p. 284. Temperature (°C) 1000 In Ag Ga Au : melting point Physical vapor deposition: thermal evaporation September 21, 2001 • Ion beam evaporation • Inductive heating evaporation • At >10kv incident electrons can produce x-rays • Redeposition of metal droplets blown of source by vapor – Uses a stream of high energy electrons (5-30 keV) to evaporate source material – Can evaporate any material – Electron-beam guns with power up to 1200 kw – Drawbacks • Electron-beam evaporation • Limited to low melting point metals • Small filament size limits deposit thickness – Uses resistive heating to evaporate a metallic filament – Drawbacks • Resistive evaporation Evaporation vapor pressure (Torr) Evaporation Microsystems Principles September 21, 2001 Microsystems Principles – Accurately controlled alloy compounds are difficult to achieve – No in situ substrate cleaning – Poor step coverage – Variation of deposit thickness for large/multiple substrates – X-ray damage • Limitations September 21, 2001 – Films can be deposited at high rates (e.g., 0.5 µm/min) – Low energy atoms (~0.1 ev) leave little surface damage – Little residual gas and impurity incorporation due to highvacuum conditions – No substrate heating • Material is heated to attain gaseous state • Carried out under high-vacuum conditions (~5x10-7 torr) • Advantages Evaporation • • • • • • • Resistance Heated Evaporation Thermal conductivity Thermal expansion Electrical conductivity Wettability and reactivity R. B. Darling / EE-527 R. B. Darling / EE-527 – Au, Ag, Al, Sn, Cr, Sb, Ge, In, Mg, Ga – CdS, PbS, CdSe, NaCl, KCl, AgCl, MgF2, CaF2, PbCl2 Simple, robust, and in widespread use. Can only achieve temperatures of about 1800°C. Use W, Ta, or Mo filaments to heat evaporation source. Typical filament currents are 200-300 Amperes. Exposes substrates to visible and IR radiation. Typical deposition rates are 1-20 Angstroms/second. Common evaporant materials: – – – – • Engineering considerations: – Graphitic Carbon (C); MP = 3700°C, P* = 10-2 torr at 2600°C – Alumina (Al2O3); MP = 2030°C, P* = 10-2 torr at 1900°C – Boron nitride (BN); MP = 2500°C, P* = 10-2 torr at 1600°C • Refractory ceramics: – Tungsten (W); MP = 3380°C, P* = 10-2 torr at 3230°C – Tantalum (Ta); MP = 3000°C, P* = 10 -2 torr at 3060°C – Molybdenum (Mo); MP = 2620°C, P* = 10-2 torr at 2530°C • Refractory metals: Evaporation Support Materials hearth 22 Iheater hot filament – Either high current or high voltage, typically 1-10 kW.B. Darling / EE-527 R. • Electrical power: – Evaporation rate is set by temperature of source, but this cannot be turned on and off rapidly. A mechanical shutter allowsevaporant flux to be rapidly modulated. • Mechanical shutter: – Hearth – Thickness monitor – Bell jar • Cooling water: – Need 10-6 torr for medium quality films. – Can be accomplished in UHV down to 10-9 torr. • Vacuum: Evaporation System Requirements Dean P. Neikirk © 2001, last update February 2, 2001 • essentially eddy current losses Vacc B field Dept. of ECE, Univ. of Texas at Austin source material – inductively heat material (direct for metals) • source material “directly” heated by electron bombardment – can generate x-rays, can damage substrate/devices • Ibeam ~ 100 mA, Vacc ~ kV « P ~ kWatts – electron beam evaporator • tungsten (mp 3410Û&  WDQWDOXP PS Û&  PRO\EGHQXP PS 2617Û& YHU\ FRPPRQ ³KHDWHU´ PDWHULDOV • reaction with boat potential problem electron beam – resistively heat “boat” containing material • main heating mechanisms Thermal evaporation recirculating cooling water evaporation cones of material cathode filament (-10,000 V) R. B. Darling / EE-527 – Allows sequential deposition of layers with a single pump-down. – Allows larger evaporation sources to be used. • Multiple pocket rotary hearth is also preferred: – Allows a larger evaporant surface area for higher deposition rates. – Allows initial charge to be “soaked” or preheated. – Allows evaporant source to be more fully utilized. • Sweeping or rastering of the evaporant source is useful for: Filament is out of direct exposure from evaporant flux. Magnetic field can be used for beam focusing. Magnetic field can be used for beam positioning. Additional lateral magnetic field can be used produce X-Y sweep. • 270° bent beam electron gun is most preferred: – – – – R. B. Darling / EE-527 beam forming aperture magnetic field Electron Beam Heated Evaporation - 2 4-pocket rotary copper hearth (0 V) pyrolytic graphite hearth liner 270 degree bent electron beam Electron Beam Heated Evaporation Source R. B. Darling / EE-527 – X-rays can also be generated by high voltage electron beam. More complex, but extremely versatile. Can achieve temperatures in excess of 3000°C. Use evaporation cones or crucibles in a copper hearth. Typical emission voltage is 8-10 kV. Exposes substrates to secondary electron radiation. R. B. Darling / EE-527 – Everything a resistance heated evaporator will accommodate, plus: – Ni, Pt, Ir, Rh, Ti, V, Zr, W, Ta, Mo – Al2O3, SiO, SiO2, SnO2, TiO2, ZrO2 • Typical deposition rates are 10-100 Angstroms/second. • Common evaporant materials: • • • • • alumina crucible in tantalum box chromium coated tungsten rod foil trough alumina coated foil dimple boat Electron Beam Heated Evaporation - 1 alumina crucible with wire basket wire basket wire helix wire hairpin foil dimple boat Resistance Heated Evaporation Sources • Incident vapor molecules normally have a kinetic energy much higher than kBT of the substrate surface. • Whether an atom or molecule will stick depends upon how well it can equilibrate with the substrate surface, decreasing its energy to the point where it will not subsequently desorb. R. B. Darling / EE-527 – Adsorb and then desorb after some residence time τa. – Immediately reflect off of the surface. • This can lead to physisorption or chemisorption. – Adsorb and permanently stick where they land (rare!). – Adsorb and permanently stick after diffusing around on the surface to find an appropriate site. • Molecules impinging upon a surface may: Condensation of Evaporant - 2 R. B. Darling / EE-527 • Box coaters are used for evaporating large substrate materials, often up to several meters in size. • Large amounts of source material are required, but cannot be all heated at once because of realistic power limitations. • Two popular techniques: • Condensation of a vapor to a solid or liquid occurs when the partial pressure of the vapor exceeds the equilibrium vapor pressure of the condensed phase at this temperature. • The vapor is “supersaturated” under these conditions. • This is only true if condensation takes place onto material which is of the same composition as the vapor. • When a material is first deposited onto a substrate of a different composition, a third adsorbed phase must be included to describe the process. R. B. Darling / EE-527 – The impinging molecule loses its kinetic energy to a chemical reaction which forms a chemical bond between it and other substrate atoms. • Chemisorption: – The impinging molecule loses kinetic (thermal) energy within some residence time, and the lower energy of the molecule does not allow it to overcome the threshold that is needed to escape. • Adsorption is the sticking of a particle to a surface. • Physisorption: Adsorption R. B. Darling / EE-527 • Both can be adapted for either resistance heated or electron beam heated evaporation systems. – Powder trickler source – Wire feed source High Throughput Evaporation Techniques Condensation of Evaporant - 1 • • • • • • • • • R. B. Darling / EE-527 Adsorbed monomers Subcritical embryos of various sizes Formation of critically sized nuclei Growth of nuclei to supercritical size and depletion of monomers within their capture zones Nucleation of critical clusters within non-depleted areas Clusters touch and coalesce into new islands, exposing fresh substrate areas Adsorption of monomers onto fresh areas Larger islands grow together leaving holes and channels Channels and holes fill to form a continuous film Observed Growth of a Deposited Film R. B. Darling / EE-527 • If the impingement rate stops, then the adsorbed molecules will all eventuallydesorb. • Condensation of a permanent deposit will not occur, even for low substrate temperatures, unless the molecules interact. • Within the mean residence time, surface migration occurs and clusters form. • Clusters have smaller surface-to-volume ratios, and therefore desorb at a reduced rate. • Nucleation of a permanent deposit is therefore dependent upon clustering of the adsorbed molecules. Condensation of Evaporant - 5 ∆Gdes 1 exp kT υ0 B – R = deposition rate in molecules/cm2-sec. -2 – ns = surface density of deposited molecules in cm . R. B. Darling / EE-527 • Under a constant impinging vapor flux ofR, the surface density of the deposit is then: ∆Gdes R ns = Rτ a = exp kT υ0 B – ∆Gdes = free activation energy for desorption. • This is the frequency at which the molecule “attempts” to desorb. – υ0 = kBT/h = vibrational frequency of adsorbed molecule (~1014 Hz) τa = • Mean residence time for an adsorbed molecule: Condensation of Evaporant - 4 R. B. Darling / EE-527 • If αT < 1, ( Er > Es), then some fraction of the impinging molecules will desorb from the surface. – Es, Ts = energy, temperature of substrate surface. • (Those which have adsorbed, but have not permanently found a site.) – Ev, Tv = energy, temperature of impinging vapor molecules. – Er, Tr = energy, temperature of resident vapor molecules; • Thermal accomodation coefficient: E − Er Tv − Tr αT = v = Ev − Es Tv − Ts Condensation of Evaporant - 3 September 21, 2001 Microsystems Principles – Deposition rate of some materials quite low – Some materials (e.g., organics) degrade due to ionic bombardment – Incorporation of impurities due to low-medium vacuum • Limitations Sputtering Sputtering September 21, 2001 Microsystems Principles – Can use large area targets for uniform thickness over large substrates – Easy film thickness control via time – Ease of alloy deposition – Substrate surface can be sputter cleaned (etched) – Step coverage – Sufficient material for many depositions – No x-ray damage • Uses high energy particles (plasma) to dislodge atoms from source surface • Carried out under low-medium vacuum (~10 mtorr) • Advantages Sputtering • Too much heat will desorb the deposited film, evaporating it away! (But this can be used for cleaning… )R. B. Darling / EE-527 – Quartz IR lamps from frontside – Ta, W, or Mo foil heaters from backside – Graphite impregnated cloth heaters from backside • Substrate heaters: – Increase surface diffusivity of adsorbed molecules. – Performs annealing of deposited film. • (Shortens the residence time.) – Increase thermal energy of adsorbed molecules. • Control of condensation of the evaporant is achieved through the control of substrate temperature Ts. • Higher substrate temperatures: Condensation Control 400 600 800 1000 Ion energy (eV) Ti Si W Cr Al Pt Au Dept. of ECE, Univ. of Texas at Austin adapted from: Campbell, p. 295 200 target E field 24 target E field plasma Dept. of ECE, Univ. of Texas at Austin S-gun conical magnetron adapted from: Campbell, p. 298 planar magnetron B field plasma B field – ion energies ~ few hundred eV ; ejected atoms ~ tens eV – ~10-2 Torr, λ ~ 5 mm – better step coverage than evaporation typically inert (noble) gas used to form incident ions – rf plasma: dielectrics • rates up to ~1 µm / minute – magnetic field used to confine plasma, electric field (“bias”) to accelerate – dc plasma: metals plasma generates high density, energetic incident particles Dean P. Neikirk © 2001, last update February 2, 2001 • • 23 0 1 2 3 Sputtering Dean P. Neikirk © 2001, last update February 2, 2001 – can deposit almost anything • use moderate energy ion bombardment to eject atoms from target • “purely” physical process Sputtering sputtering yield (ejected atoms / incident ion) ...
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