Thin Film Deposition techniques. SCPY663 Sem2/2009 (Physics-MUSC)

Transcription

1 Thin Film Deposition techniques Thermal Evaporation 1

2 An evaporator Evaporator SCPY663 Sem2/2009 (Physics 2

3 Evaporation rate all materials have an equilibrium vapor pressure, p e (T) (T: source temperature) at sufficiently i high h temperature, t the gas impingement i rate Φ Φ (p e ) can be high enough to cause deposition of material (thin-film growth) on a cold substrate (T S <<T, T S : substrate temperature) cold substrate, T s vapor T>> T s hot source, T 3

4 Evaporation rate Gas impingement rate for thermal evaporation (Knudsen equation, Ch.2 p.8) Φ e N A ( p e p 2πMRT h ) p e : equlibrium vapor pressure of the source p h : hydeostatic pressure acting on the evaporant Usually: p h 0 (vacuum) Φ pe gk torr MT molecules 2 cm s pe gk torr MT ML s 1 ML 3 Å Γ e M T K g p torr g 2 cm s Γ e : mass evaporation rate typically 10-4 g/(cm 2 s)at 10-2 torr!! p e p e (T)!! 4

5 Clausius Clapeyron equation (solid liquid or liquid - vapor equlibrium) dp e dt ΔH T ΔV ΔH: molar enthalpy difference ΔV: molar volume difference ΔV Vgas RT / p dpe dt ΔH ( T ) 2 RT p e ΔH(T) He molar heat of evaporation p H e T ) p exp RT e( 0 Good approximation, but not exact (non perfect gas behavior, ΔH(T) const.) Water: H e 40.6 kj/mol p(373k) 10 5 Pa - p(273k) 10 3 Pa - p(77k) Pa 5

6 Clausius-Clapeyron equation only applicable if system is in equilibrium (molar evaporation and condensation rates, Q v and Q c c, are balanced) orifice diameter << λ p e evaporation rate can be calculated Q c Q v Q v evaporation rate must be measured q q Knudsen cell vacuum evaporator Rule of thumb: if p e (T M ) 10-3 torr then T >T M is required 6

7 Vapor pressure of selected elements Al Cu 1350K melting temperature T m 7

8 Angular distribution of evaporants: cosine law r ө da s K-cell Ф J o J ocos ө ө da r Emitting flux from any point on the surface; J v Total evaporation rate from a source of area A; Q J v A For sphere of radius r; cosine flux distribution J 0 J θ KQ 2 r J cosθ J 0 cosθ cosφ Receiving Flux on da ; J 2 r K geometry factor 1 4 π 1 π ; point source 0 r da r ; disc shape source e.g. boat Deposition rate is determined in J J cos 2 Ф In the typical case of disc-shaped source Фө ; J J o cos 4 ө /r 2 8

9 Evaporation of Compounds and Alloys only very few compounds evaporate as molecules vapor composition and film stoichiometry do not differ from that of the source e.g. SiO, B 2 O 3, GeO, SnO, AlN, CaF 2, MgF 2 ) most compounds decompose, e.g. (1) Ag 2 Se(s) 2Ag(g)+½Se 2 (g) (2) SiO 2 2( (s) SiO(g)+½O 2 2(g) evaporate from separate sources (1) or introduce O 2 partial pressure (2- reactive evaporation) evaporated metal alloy films are widely used: Al-Cu metallization in integrated circuits Fe-Ni magnetic data storage etc. 9

10 metal atoms in an alloy are less tightly bound than atoms in an inorganic compound constituents nearly evaporate independently of each other, enter vapor phase as single atoms metallic melts are solutions and follow thermodynamics binary alloy AB: interaction energy A-B usually differs from energies A-A, A, B-B B (real solution) partial pressure of A in AB at T, p A partial pressure of pure A at T, p A (0) p A γ A.X A.p A (0) γ A : activity coefficient, X A: mole fraction Knudsen equation flux ratio Φ Φ A B γ AXA pa (0). γ (1- X ) p (0) B M M γ i, M i, and T determine the vapor and film composition A B B A 10

11 Is it feasible to evaporate an Al-2wt%Cu alloy at T1350K? Φ Φ p p Al Cu 98/ MAl 2 / M Cu X 3 (0) 10 X 4 (0) 2 10 Al γ CuΦ γ Φ Al p p Cu (0) (0) M M Al( Cu Al Cu Al Al Cu Cu 15 ( γ Al γ Cu ) the melt should have the composition 13wt-% Cu in order to compensate for the preferential evaporation of Al melt volume should be large to avoid composition changes due to preferential evaporation better: evaporation from dual sources maintained at different temperatures If keeping Al at 1350K, what should be Cu temperature to have Al-2wt%Cu? 11

12 Evaporation Source resistance-heated evaporation sources (few V, 10-50A) tungsten wire sources evaporant wets W & is retained by surface tension refractory metal sheet sources (Ta, W, Mo) for poor wetting evaporants or powders crucible cible sources (Al 2 O 3, BN, graphite, WC, indirectly heated by W wires or sheets) for evaporants that alloy with Ta, W, Mo) 12

13 Knudsen Cell Heater Crucible 13

14 disadvantages: contamination, alloy formation, chemical reaction of source material and evaporant possible outgassing of hot source material small evaporation rates, low input power small evaporant volume 14

15 Electron Beam Evaporator suitable for almost all evaporants heated filament: thermionic electron emission no direct lines of sight filament-evaporant evaporant and filament-substrate: no contamination of the film/coating of the filament electron acceleration ( kv) electron deflection by magnetic field (Lorentz force) high power (kw): can evaporate high melting point materials water cooled crucible: material only melts at the surface, no alloying with crucible etc. 1 W/cm 2 for evaporation 0.1 W/cm 2 for kinetic energy of vapor atoms 10 W/cm 2 for radiation i heat loss kw/cm 2 for heat conduction into the crucible power consumption 15

16 Electron Beam Evaporator Crucible rotation water cooling flanges 16

17 Electron Beam Evaporator The electron current density j e leaving the hot filament is due to thermionic emission, as expressed by Richardson's equation: j e 2 qφ Φ AT exp kt where A is Richardson s constant ( A/m 2 ), q is elementary electronic charge, and Φ is the work function. The e-beam evaporation source is a relatively high intensity source, as it is not necessary to raise a furnace or other enclosure to the temperature of the evaporant. The evaporant flux can be so dense near the evaporant surface that it is in laminar flow. The beam intensity of e-beam evaporators has been described with a cos n (θ) law. J Ω n ( ϑ ) + C J cos ( ϑ ) n ~ (1 C)cos with the exponent n varying from 2 to at least 6, as J increases. Crucibles are typically made of materials with a high melt point, like Al 2 O 3, graphite, TiN, BN, etc., and often cooled by water. 17

18 Laser-beam evaporation High energy laser beam is used to evaporate the target material. It heats the source surface only and can be operated at a high pressure. A laser beam evaporation system: 1 CO 2 laser, 2 ZnSe window, 3 crucible, 4 target material, 5 pump, 6 gauge, 7 mass flow controller, 8 mirror, 9 substrat, 10 substrate heater 18

19 Pulsed Laser deposition pulsed laser deposition i (PLD) "flash evaporation" method. Special deposition technique for specialized thin films of unusual stoichiometries or super-lattices. pulsed excimer laser for evaporation directed through a viewport at a target mounted in a vacuum chamber. condensible vapor is produced after target absorbing a powerful laser beam strikes a target and vaporizes a thin surface region. vaporized region of the target ~ several hundred to 1000 angstroms thick. conical plume of evaporant caused by ablation of the material is created. The axis of the vapor plume normal to the target's surface; follows a cosine distribution rule. Such a visible plume appears when the emitted vapor is ionized by the laser, forming a plasma. characteristic speed of the evaporant particles (which can be both neutrals and ions) cm/sec ~ kinetic energy of - 3 ev. The film growth rates can approach 0.5 μm/min. KrF excimer laser most frequently used lasers operating at wavelength248 nm. Thus the energy carried by a photon is hνhc/λ, which should be over the energy required for an atom escaping from the solid target, hν >E condense. 19

20 Excimer laser used for PLD: Laser medium ArF KrF XeCl XeF Wavelength (nm) Pulse energy (mj) Average power (W) Gas lifetime (106 pulse) Some representative laser parameters are as follows: i) The wave length is in the UV range ii) A pulse on the order of 25 ns in duration (the pulse duration, δt) ) iii) At a power density j of W/cm 2 at the target The illuminating an area of the target (δa) of typically 0.1 cm 2 At a repetition rate (f) of 50 Hz. The fluence of this typical pulse (j δt) is thus 6 J/cm 2. The incident energy per pulse is mj. The instantaneous SCPY663 power Sem2/2009 is (Physics- 7 W, and the average power is 30W. 20

21 The fluence of this typical pulse (jδt) is thus 6 J/cm 2. The incident energy per pulse is mj. The instantaneous power is W, and the average power is 30 W. One of the most successful PLD applications has been the preparation of high temperature superconducting thin films. PLD seems unusually effective in recreating in the thin film the stoichiometric composition of the complex, multi-component target materials; the vaporization is so fast that segregation is nearly impossible. Sometimes preserving stoichiometry is assisted by performing PLD with a high partial pressure (in the mtorr range) of reactive gas, such as oxygen, due to the absence of hot filaments or other hot components. The drawbacks of PLD are the relatively small deposition area, poor thickness uniformity, and the surface outgrowths that lead to a rough film surface. 21

22 Example I Determine the rate of loss of MgO by evaporation when exposed to a temperature of 2000K. MgO(s)Mg(g)+O(g) g(g) (g) Given the Gibbs Free Energy ΔG T J 22

23 Sources for Evaporating multi-component film Figure Methods for evaporating multicompoment films include (a) single source evaporation, (b) multisource simultaneous evaporation, and (c) multisource sequential evaporation 23

24 Example II Consider a drop of water inside the room temperature vacuum chamber. If the drop forms a hemisphere of radius r 0, and if the drop remains at room temperature, calculate the time it will take to evaporate the drop. Assume r 0 1 mm. 24

25 Evaporation rate Gas impingement rate for thermal evaporation (Knudsen equation, Ch.2 p.8) Φ e N A ( p e p 2πMRT h ) p e : equlibrium vapor pressure of the source p h : hydeostatic pressure acting on the evaporant Usually: p h 0 (vacuum) Φ pe gk torr MT molecules 2 cm s pe gk torr MT ML s 1 ML 3 Å Γ e M T K g p torr g 2 cm s Γ e : mass evaporation rate typically 10-4 g/(cm 2 s)at 10-2 torr!! p e p(t) e!! 25

26 Example III An evaporator is used to deposit aluminum. The aluminum boat is maintained at a uniform temperature of 1100 C. If the evaporator planetary has a radius of 40 cm and the diameter of the crucible is 5 cm, what is the deposition rate of aluminum? If the chamber also has a background pressure of 10-6 torr of water vapor and the water vapor is assumed to be at room temperature, determine the ratio of the arrival rated of the aluminum atoms and the water molecules, the mass density of solid aluminum is 2.7 g/cm 3. 26

27 Planetary SCPY663 Sem2/2009 (Physicshttp:// 27

28 Vapor pressure of selected elements Al 1373K1100C melting temperature T m 28

29 29

30 Step Coverage Aspect Ratio (AR) step height/step diameter Standard evaporation : Discontinuous film for 1< AR Marginal for 0.5< AR<1 Continuous for AR <0.5 Improvements : i) Substrate Heating high h deposition rate ii) Substrate Rotating 30

31 Angular distribution of evaporants: cosine law r ө da s K-cell Ф J o J o cos ө ө da r Emitting flux from any point on the surface; J v Total evaporation rate from a source of area A; Q J v A For sphere of radius r; cosine flux distribution J 0 J θ KQ 2 r J cosθ J 0 cosθ cosφ Receiving Flux on da ; J 2 r K geometry factor 1 4 π 1 π ; point source 0 r da r ; disc shape source e.g. boat Deposition rate is determined in J J cos 2 Ф In the typical case of disc-shaped source Фө ; J J o cos 4 ө /r 2 31

32 Deposition Geometry Figure 12.3 The geometry of deposition for a wafer (A) in an arbitrary yposition and (B) on the surface of a sphere. Figure 3-4 Evaporation from (a) point source, (b) surface source. 32

33 Cos n θ distribution Figure 3-5 Calculated lobe-shaped vapor clouds with various cosine exponents. (From Ref.9.) 33

34 Electron Beam Evaporator The electron current density j e leaving the hot filament is due to thermionic emission, as expressed by Richardson's equation: j e 2 qφ Φ AT exp kt where A is Richardson s constant ( A/m 2 ), q is elementary electronic charge, and Φ is the work function. The e-beam evaporation source is a relatively high intensity source, as it is not necessary to raise a furnace or other enclosure to the temperature of the evaporant. The evaporant flux can be so dense near the evaporant surface that it is in laminar flow. The beam intensity of e-beam evaporators has been described with a cos n (θ) law. J Ω n ( ϑ ) + C J cos ( ϑ ) n ~ (1 C)cos with the exponent n varying from 2 to at least 6, as J increases. Crucibles are typically made of materials with a high melt point, like Al 2 O 3, graphite, TiN, BN, etc., and often cooled by water. 34

35 Thickness uniformity for point source d Φ 0 ; ρ material bulk density 2 4πρh d d {1 + ( l / h) } for surface source 1 {1 + ( l / h) 3/ 2 Φ d 0 ; ρ material bulk density 2 πρh d d } ; 35