II. Thin Film Deposition

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II. Thin Film Deposition Powered By Docstoc
					                   II. Thin Film Deposition
Physical Vapor Deposition (PVD)
- Film is formed by atoms directly transported from source to the substrate
through gas phase
      • Evaporation
             • Thermal evaporation
             • E-beam evaporation
      • Sputtering
             • DC sputtering
             • DC Magnetron sputtering
             • RF sputtering
      • Reactive PVD

Chemical Vapor Deposition (CVD)
- Film is formed by chemical reaction on the surface of substrate
      • Low-Pressure CVD (LPCVD)
      • Plasma-Enhanced CVD (PECVD)
      • Atmosphere-Pressure CVD (APCVD)
      • Metal-Organic CVD (MOCVD)

Oxidation
Spin Coating
Platting


  Applied Physics 298r                  1                 E. Chen (4-12-2004)
General Characteristics of Thin Film Deposition
• Deposition Rate
• Film Uniformity
     • Across wafer uniformity
     • Run-to-run uniformity
• Materials that can be deposited
     • Metal
     • Dielectric
     • Polymer
• Quality of Film – Physical and Chemical Properties
     • Stress
     • Adhesion
     • Stoichiometry
     • Film density, pinhole density
     • Grain size, boundary property, and orientation
     • Breakdown voltage
     • Impurity level
• Deposition Directionality
     • Directional: good for lift-off, trench filling
     • Non-directional: good for step coverage
• Cost of ownership and operation

    Applied Physics 298r           2             E. Chen (4-12-2004)
                       Evaporation

Load the source material-to-be-
deposited (evaporant) into the
container (crucible)                              Substrate
                                                     Film

Heat the source to high
temperature
Source material evaporates                      Evaporant
                                                  Vapor
Evaporant vapor transports to and
Impinges on the surface of the
                                    Current
substrate                                                     Crucible (energy source)
Evaporant condenses on and is
adsorbed by the surface




Applied Physics 298r         3                E. Chen (4-12-2004)
                 Langmuire-Knudsen Relation
Mass Deposition Rate per unit area of source surface:                             Substrate

                  1
        M             1
Rm = Cm   cos θ cos ϕ 2 (Pe (T ) − P )
                  2
                                                                              r
        T            r
                                                                      θ
                                                                          ϕ
 Cm = 1.85x10-2
 r:  source-substrate distance (cm)                                       P
 T: source temperature (K)
 Pe: evaporant vapor pressure (torr), function of T
 P: chamber pressure (torr)                                      Pe
 M: evaporant gram-molecular mass (g)

    Maximum deposition rate reaches at high               Source (K-Cell)
    chamber vacuum (P ~ 0)


       Applied Physics 298r             4               E. Chen (4-12-2004)
                                  Uniform Coating
Spherical surface with source on its edge:           Spherical Surface


                         r
        cos θ = cos ϕ =
                        2r0                                       ϕ
                              1                                       r
                M  Pe       2                            r0 θ
       Rm = Cm         2
                T  4r0                                          P

                                                             Pe
       Angle Independent – uniform coating!

                                                      Source (K-Cell)
Used to coat instruments with spherical surfaces



       Applied Physics 298r              5          E. Chen (4-12-2004)
                       Uniformity on a Flat Surface
Consider the deposition rate difference
between wafer center and edge:
                                                               W /2
              1                                                           Wafer
      R1 ∝      2
             r1

            1           r
                               2
                                                                    ϕ
       R2 ∝ 2 cos 2 θ = 1 4                               r1
           r2           r2                                      θ r2
Define Uniformity:
                                                                    P
                    R1 − R2
       σ (% ) =             (% )
                       R1                                      Pe
                         −2
         W 2       2
σ = 1− 1 +    ≈ W              or
                                          W
                                             = 2σ     Source (K-Cell)
         2r1  
                 2r1
                        2
                                          r1
       


        Applied Physics 298r              6         E. Chen (4-12-2004)
         Uniformity Requirement on a Flat Surface
Source-substrate distance requirement:
                                                                         160
                   W
             r>




                                            Source-Sample Distance (r)
                                                                         140
                   2σ                                                                         1%

                                                                         120                  2%
                                                                                              5%
In practice, it is typical to double this                                100                  10%
number to give some process margin:
                                                                         80

                                                                         60
                      2
             r >W
                      σ
                                                                         40

                                                                         20


 Larger r Means:                                                          0
                                                                               0         2          4     6        8   10
    bigger chamber
                                                                                               Sample Size (W)
    higher capacity vacuum pump
    lower deposition rate
                                                                                    Another Common Solution:
    higher evaporant waste                                                         off-axis rotation of the sample


         Applied Physics 298r                         7                                      E. Chen (4-12-2004)
       Thickness Deposition Rate vs. Source Vapor Pressure
                                      dh Rm                               Substrate
Thickness deposition rate                =   Ae
                                      dt   ρ                                 Film
                            1                                                                     dh
      dh Ae   M             1
         = Cm   cos θ cos ϕ 2 Pe (T )
                            2
                                                                                            r
      dt  ρ   T            r
                                                                                    θ
      T:      source temperature (K)                                                    ϕ
      Ae:     source surface area (cm2)
      ρ:      evaporant density (g/cm3)                                 Ae
                                                                                        P
       Pe is function of source Temperature!
                                                                              Pe
Example: Al                                                                        T
M ~ 27, ρ ~ 2.7, Ae ~ 10-2 cm2, T ~ 900 K
R ~ 50 cm (uniformity requirement)
                                                                      Source (K-Cell)
       dh
          = 50 Pe      (A/s)                The higher the vapor pressure, the higher the material’s
       dt                                   deposition rate


            Applied Physics 298r                  8                 E. Chen (4-12-2004)
            Deposition Rate vs. Source Temperature
Typically for different material:


    dh
       = (10 ~ 100) Pe (T )   ( A / s)
    dt

•     For deposition rate > 1 A/s:
              Pe > ~ 100 mtorr
•     Pe depends on: 1) materila
      and 2) temperature
•     Deposition rates are
      significantly different for
      different materials
•     Hard to deposit multi-
                                             Example: for Pe > 100 mtoor
      component (alloy) film
                                                 T(Al) > 1400K, T(Ta) > 2500K
      without losing stoichiometry


           Applied Physics 298r          9            E. Chen (4-12-2004)
          Heating Method – Thermal (Resist Heater)

                   Source
                   Material
                                 Resistive
Current
                                   Wire          Foil Dimple Boat




                   Crucible
                                                 Alumina Coated
                                                 Foil Dimple Boat
           Contamination Problem
          with Thermal Evaporation
  Container material also evaporates, which
       contaminates the deposited film
                                              Cr Coated Tungsten Rod


          Applied Physics 298r           10    E. Chen (4-12-2004)
      CIMS’ Sharon Thermal Evaporator




Applied Physics 298r   11    E. Chen (4-12-2004)
                  Heating Method – e-Beam Heater


                                                        e-
                           Electron Beam

Crucible                                       Magnetic Field

                                                                                  Focusing
                                                                                  Aperture
             Evaporant            Evaporant        (beam focusing
                                                   & positioning)

                                                                Cathode Filament



      Water Cooled Rotary Copper Hearth               Advantage of E-Beam Evaporation:
            (Sequential Deposition)
                                                        Very low container contamination



           Applied Physics 298r               12                E. Chen (4-12-2004)
      CIMS’ Sharon E-Beam Evaporator




Applied Physics 298r   13    E. Chen (4-12-2004)
                                       Comparison

                                 Typical                     Deposition   Temperature
Deposition    Material                            Impurity                                Cost
                                Evaporant                      Rate         Range


                            Au, Ag, Al, Cr, Sn,
                            Sb, Ge, In, Mg,
             Metal or low   Ga
 Thermal      melt-point                           High      1 ~ 20 A/s    ~ 1800 ºC      Low
              materials     CdS, PbS, CdSe,
                            NaCl, KCl, AgCl,
                            MgF2, CaF2, PbCl2

                            Everything above,
                            plus:
             Both metal
                            Ni, Pt, Ir, Rh, Ti,               10 ~ 100
 E-Beam         and                                 Low                    ~ 3000 ºC      High
                            V, Zr, W, Ta, Mo                    A/s
             dielectrics
                            Al2O3, SiO, SiO2,
                            SnO2, TiO2, ZrO2



                     Stoichiometrical Problem of Evaporation
•    Compound material breaks down at high temperature
•    Each component has different vapor pressure, therefore different deposition
     rate, resulting in a film with different stoichiometry compared to the source

        Applied Physics 298r                      14                E. Chen (4-12-2004)
           Typical Boat/Crucible Material

                        Refractory Metals
                                             Temperature for 10-mtorr
                           Melting Point
     Material                                  Vapor Pressure (Pe)
                                (ºC)
                                                      (ºC)

   Tungsten (W)                3380                    3230

   Tantalum (Ta)               3000                    3060

 Molybdenum (Mo)               2620                    2530

                       Refractory Ceramics
Graphitic Carbon (C)           3799                    2600

  Alumina (Al2O3)              2030                    1900

 Boron Nitride (BN)            2500                    1600



Applied Physics 298r            15           E. Chen (4-12-2004)
                   DC Diode Sputtering Deposition
• Target (source) and substrate are placed                       2 – 5kV
  on two parallel electrodes (diode)
• They are placed inside a chamber filled
  with inert gas (Ar)
• DC voltage (~ kV) is applied to the diode          Target (Cathode)
• Free electron in the chamber are                     e-           e-

  accelerated by the e-field
                                                            γ
• These energetic free electrons inelastically         Ar           Ar
                                                                 e- Ar+

  collide with Ar atoms
       excitation of Ar    gas glows                 Substrate (Anode)
       ionization of Ar    Ar+ + 2nd electron
• 2nd electrons repeat above process
     “gas breakdown”
     discharge glow (plasma)


           Applied Physics 298r                 16   E. Chen (4-12-2004)
                                 Self-Sustained Discharge
• Near the cathode, electrons move much faster than ions
  because of smaller mass                                                         2 – 5kV
        positive charge build up near the cathode, raising
     the potential of plasma
        less electrons collide with Ar
        few collision with these high energetic electrons
     results in mostly ionization, rather than excitation
        dark zone (Crookes Dark Space)                             Target (Cathode)
• Discharge causes voltage between the electrodes
                                                                   + + + + +             Crookes
                                                                                        Dark Space
  reduced from ~103 V to ~102V, mainly across the dark
                                                                    Ar+           Ar+
  space
                                                                          t   t
• Electrical field in other area is significantly reduced by
  screening effect of the position charge in front of
  cathode
• Positive ions entering the dark space are accelerated           Substrate (Anode)
  toward the cathode (target), bombarding (sputtering) the
  target
        atoms locked out from the target transport to the
     substrate (momentum transfer, not evaporation!)
        generate 2nd electrons that sustains the discharge
     (plasma)


              Applied Physics 298r                   17        E. Chen (4-12-2004)
Requirement for Self-Sustained Discharge
   • If the cathode-anode space (L) is less than the dark space length
             ionization, few excitation
             cannot sustain discharge
   • On the other hand, if the Ar pressure in the chamber is too low
             Large electron mean-free path
             2nd electrons reach anode before colliding with Ar atoms
             cannot sustain discharge either



Condition for Sustain Plasma:              L ⋅ P > 0.5 (cm ⋅ torr )

                                L: electrode spacing, P: chamber pressure

   For example:
   Typical target-substrate spacing: L ~ 10cm
      P > 50 mtorr


Applied Physics 298r                  18              E. Chen (4-12-2004)
                Deposition Rate vs. Chamber Pressure
                  High chamber pressure results in low deposition rate

Mean-free path of an atom in a gas ambient:
                                                     In fact, sputtering deposition rate R:
                  5 × 10 −3
               λ~           (cm)
                  P (torr )                                        1
                                                             R∝
                                                                  L⋅P
  Use previous example:
              L = 10 cm, P = 50 mtorr
                                                        Large LP to sustain plasma
     λ ~ 0.1 cm
     sputtered atoms have to go through                 small LP to maintain good
  hundreds of collisions before reaching the
                                                        deposition rate and reduce
  substrate
     significantly reduces deposition rate              random scattering
     also causes source to deposit on chamber                                        ?
  wall and redeposit back to the target



            Applied Physics 298r                19             E. Chen (4-12-2004)
                     DC Magnetron Sputtering

• Using low chamber pressure to maintain high deposition rate
• Using magnetic field to confine electrons near the target to sustain plasma




        E
            e-
            + B        +
            Cathode (Target)                           S         N           S




 Apply magnetic field parallel to the
 cathode surface
                                                               Target
    electrons will hope (cycloid) near the
                                                        S        N           S
 surface (trapped)




       Applied Physics 298r              20            E. Chen (4-12-2004)
             Impact of Magnetic Field on Ions

Hoping radius r:


         1    2m
    r~           Vd
         B     e                                   Ar+



Vd – voltage drop across dark space                            e-
     (~ 100 V)                                E
B – Magnetic field (~ 100 G)                                        r
                                                   + B

                                                         Cathode (Target)

       For electron         r ~ 0.3 cm

       For Ar+ ion:         r ~ 81 cm




     Applied Physics 298r                21       E. Chen (4-12-2004)
                                 As A Result …

  current density (proportional to ionization rate) increases by 100 times
  required discharge pressure drops 100 times
  deposition rate increases 100 times



                                         Magnetron
           Deposition Rate (R)




                                                 Non-Magnetron
                                 ~ 1mT



                                                      ~ 100mT



                                  Chamber Pressure (P)


Applied Physics 298r                        22                  E. Chen (4-12-2004)
                     RF (Radio Frequency) Sputtering
   DC sputtering cannot be used for depositing
   dielectrics because insulating cathode will cause                             13.56 MHz
   charge build up during Ar+ bombarding                                              ~
       reduce the voltage between electrodes
      discharge distinguishes

                                                                             Target
             Solution: use AC power
• at low frequency (< 100 KHz), both electrons and                                        Target Sheath
  ions can follow the switching of the voltage –             e-         e-
                                                                  Ar
      DC sputtering                                                    Ar+
                                                                                 t

• at high frequency (> 1 MHz), heave ions cannot no                                   Substrate Sheath
  long follow the switching
      ions are accelerated by dark-space (sheath)                       Substrate
  voltage
      electron neutralizes the positive charge buildup on
  both electrodes
• However, there are two dark spaces
      sputter both target and substrate at different cycle


            Applied Physics 298r                   23        E. Chen (4-12-2004)
               RF (Radio Frequency) Sputtering
                                                               13.56 MHz
                          n
         VT     AS                                                ~
              ∝
               A     
                             (n ~ 2)
         VS     T    

                                                   AT     Target
  VT   –   voltage across target sheath
                                                               VT
  Vs   –   voltage across substrate sheath
  AT   –   area of target electrode
  As   –   area of substrate electrode
                                                               VS
                                             AS
                                                        Substrate

Larger dark-space voltage develops at the
electrode with smaller area
   make target electrode small



       Applied Physics 298r             24        E. Chen (4-12-2004)
        Comparison between Evaporation and Sputtering


                 Evaporation                                  Sputtering

                                                       High energy atoms / ions
                                                              (1 – 10 eV)
              Low energy atoms
                                            • denser film
                 (~ 0.1 eV)
                                            • smaller grain size
                                            • better adhesion
                 High Vacuum                                 Low Vacuum
• directional, good for lift-off            • poor directionality, better step coverage
• lower impurity                            • gas atom implanted in the film

                 Point Source                             Parallel Plate Source
• poor uniformity                           • better uniformity


   Component Evaporate at Different Rate      All Component Sputtered with Similar Rate
• poor stoichiometry                        • maintain stoichiometry




        Applied Physics 298r               25               E. Chen (4-12-2004)
                   Chemical Vapor Deposition (CVD)
           Deposit film through chemical reaction and surface absorption


• Introduce reactive gases to the chamber
• Activate gases (decomposition)
                                                          A B              A    B
        heat
        plasma
• Gas absorption by substrate surface                 A         B      A            B


• Reaction take place on substrate surface;               W                 W


  film firmed
                                                                Substrate
• Transport of volatile byproducts away
  form substrate
• Exhaust waste




          Applied Physics 298r                26      E. Chen (4-12-2004)
                        Types of CVD Reactions
Pyrolysis (Thermal Decomposition)
   AB ( gas ) → A ( solid ) + B ( gas )

   Example
   α-Si deposited at 580 - 650 ºC:

           SiH 4 ( gas ) = Si ( solid ) + 2 H 2 ( gas )


Reduction (lower temperature than Pyrolysis)
    AB ( gas ) + H 2 ( gas, commonly used ) ↔ A ( solid ) + HB ( gas )
     Example
     W deposited at 300 ºC:

    WF6 ( gas ) + 3H 2 ( gas ) = W ( solid ) + 6 HF ( gas )
       Reversible process, can be used for chamber cleaning


      Applied Physics 298r                    27              E. Chen (4-12-2004)
                 Types of CVD Reactions (Cont.)
Oxidation

AB ( gas or solid ) + O2 ( gas, commonly used ) ↔ AO ( solid ) + [O ]B ( gas )



   Example
   Low-temperature SiO2 deposited at 450 ºC:

          SiH 4 ( gas ) + O2 ( gas ) = SiO2 ( solid ) + 2 H 2 ( gas )

     Example
     SiO2 formed through dry oxidation at 900 - 1100 ºC:

            Si ( Solid ) + O2 ( gas ) = SiO2 ( solid )




     Applied Physics 298r                    28                E. Chen (4-12-2004)
               Types of CVD Reactions (Cont.)
Compound Formation

    AB ( gas or solid ) + XY ( gas or solid ) ↔ AX ( solid ) + BY ( gas )

    Example
    SiO2 formed through wet oxidation at 900 - 1100 ºC:


     Si ( Solid ) + 2 H 2O(vapor ) = SiO2 ( solid ) + 2 H 2
    Example
    SiO2 formed through PECVD at 200 - 400 ºC:

     Si H 4 ( gas ) + 2 N 2O( gas ) = SiO2 ( solid ) + 2 N 2 + 2 H 2

    Example
    Si3N4 formed through LPCVD at 700 - 800 ºC:

     3Si H 2Cl2 ( gas ) + 4 NH 3 ( gas ) = Si3 N 4 ( solid ) + 6 H 2 + 6 HCl

     Applied Physics 298r                   29                E. Chen (4-12-2004)
                         CVD Deposition Condition
Mass-Transport Limited Deposition
- At high temperature such that the reaction rate
  exceeds the gas delivering rate
- Gas delivering controls film deposition rate




                                                      Deposition Rate (log)
- Film growth rate insensitive to temperature
- Film uniformity depends on whether reactant                                 Mass-Transport
                                                                                 Limited
  can be uniformly delivered across a wafer and                                  Regime
  wafer-to-wafer
                                                                                                  Reaction-Rate
                                                                                                     Limited
Reaction-Rate Limited Deposition                                                                     Regime

- At low temperature or high vacuum such that
  the reaction rate is below gas arriving rate                                                 1/T (K)

- Temperature controls film deposition rate
- Film uniformity depends on temperature
  uniformity across a wafer and wafer-to-wafer



         Applied Physics 298r                    30                              E. Chen (4-12-2004)
                        Low-Pressure CVD (LPCVD)

                            Heater      Heater        Heater


Reactant                                                                              Exhausted
                                                          Wafer
  Gas                                                                                    Gas
Horizontal
Quartz Tube
                              Z-1         Z-2          Z-3


  • Thermal energy for reaction activation
  • System works at vacuum (~ 0.1 – 1.0 torr), resulting in high diffusivity of reactants
           reaction-rate limited
  • Wafer can stacked closely without lose uniformity as long as they have the same
     temperature
  • Temperature is controlled around 600 - 900ºC by “flat” temperature zone through using
     multiple heaters
  • Low gas pressure reduce gas-phase reaction which causes particle cluster that
     contaminants the wafer and system


        Applied Physics 298r                     31               E. Chen (4-12-2004)
                   Plasma-Enhanced CVD (PECVD)

                                                                                            RF
• Use rf-induced plasma (as in sputtering
                                                                  Gases
                                                                                            ~
  case) to transfer energy into the reactant
  gases, forming radicals (decomposition)
• Low temperature process (< 300 ºC)
• For depositing film on metals and other
                                                                       B
                                                                                      Shaw Heads
  materials that cannot sustain high                e-       e-                 e-
                                                         A                 A+        B+
  temperature                                                     e-

• Surface reaction limited deposition;
  substrate temperature control (typically                   Substrate
  cooling) is important to ensure uniformity




          Applied Physics 298r                 32   E. Chen (4-12-2004)
                 Common CVD Reactants


      Material              LPCVD                 PECVD

                                                   SiH4
        α-Si                 SiH4
                                                  SiH2Cl2

                       Si(OC2H5)4 (TEOS)        SiH4 + N2O
        SiO2
                          SiH2Cl2 + N2O          SiH4 + O2

                          SiH4 + NH3            SiH4 + NH3
        Si3N4
                         SH2Cl2 + NH3            SiH4 + N2




Applied Physics 298r          33           E. Chen (4-12-2004)
             Comparison of Typical Thin Film Deposition Technology

                                                                   Film       Deposition      Substrate
 Process        Material     Uniformity   Impurity   Grain Size                                              Directional     Cost
                                                                  Density       Rate         Temperature


               Metal or
                 low
 Thermal                                             10 ~ 100
               melting-        Poor        High                     Poor       1 ~ 20 A/s     50 ~ 100 ºC       Yes        Very low
Evaporation                                             nm
                point
               materials
               Both metal
  E-beam          and                                10 ~ 100
               dielectrics     Poor         Low                     Poor      10 ~ 100 A/s    50 ~ 100 ºC       Yes          High
Evaporation                                             nm


               Both metal                                                     Metal:
                  and                                                           ~ 100 A/s                      Some
Sputtering     dielectrics   Very good      Low       ~ 10 nm      Good                        ~ 200 ºC                      High
                                                                              Dielectric:                      degree
                                                                                ~ 1-10 A/s


                 Mainly                              10 ~ 100                                                  Some
 PECVD                         Good       Very low                 Good       10 - 100 A/s   200 ~ 300 ºC                  Very High
               Dielectrics                              nm                                                     degree



                 Mainly
  LPCVD                      Very Good    Very low   1 ~ 10 nm    Excellent   10 - 100 A/s   600 ~ 1200 ºC    Isotropic    Very High
               Dielectrics




                  Applied Physics 298r                            34                   E. Chen (4-12-2004)

				
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