Thin-film deposition

							Thin-film deposition
•Thin-film deposition is any technique for
 depositing a thin film (about 1 mm) of material
 onto a substrate or onto previously deposited
 layers.
•"Thin" is a relative term, but most deposition
 techniques allow layer thickness to be
 controlled within a few tens of nanometers,
 and some (molecular beam epitaxy) allow one
 layer of atoms to be deposited at a time.
Critical radius
•Critical radius is the minimum size that must be
 formed by atoms clustering together in the liquid
 before the solid particle is stable and begins to
 grow
•The total free energy is the sum of surface energy
 and bulk energy.
•The critical energy can be obtained by taking the
 derivation to the total free energy.
                 4 3
 Δ G  4πr  γ   πr Gv
          2

                 3   
     d ΔG
 Let       0
       dr
                 2γ
 We have r*  -
                 Gv
  PVD (Physical vapor deposition)
• Physical vapor deposition (PVD) is a group of
  vacuum coating techniques used to deposit thin films
  of various materials onto various surfaces.
• Two technologies are often used:
  (1) Evaporator:
       - Thermal evaporator: Material is heated to attain
  gaseous state.
      - Electron beam evaporator
  (2) Sputtering:
      - Carried out under high-vacuum conditions
        (~5x10-7 torr)
      - Plasma as the particle source to strike the
        target
   Physical Vapor Deposition (continued):
• Advantages:
• 1. films can be deposited at high rates (e.g., 0.5 mm/min)
• 2. low energy atoms (~0.1 eV) leave little surface damage
• 3. little residual gas and impurity incorporation due to
  high-vacuum conditions
• 4. no substrate heating

Limitations:
• 1. accurately controlled alloy compounds are difficult to
  achieve
• 2. no in situ substrate cleaning
• 3. poor step coverage
• 4. variation of deposit thickness for large/multiple
  substrates
• 5. x-ray damage
•Thermal evaporator uses an electric
 resistance heater to melt the material
 and raise its vapor pressure to a useful
 range. This is done in a high vacuum,
 both to allow the vapor to reach the
 substrate
•An electron beam evaporator fires
 a high-energy beam from an
 electron gun to boil a small spot
 of material.
 Sputtering (濺 鍍)
•Sputtering is a physical process whereby atoms in
 a solid target material are ejected into the gas
 phase due to bombardment of the material by
 energetic ions.
•The ions for the sputtering process are supplied
 by a plasma that is induced in the sputtering
 equipment.
•Sputtering relies on a plasma (usually a noble gas,
 such as Argon) to knock material from a "target" a
 few atoms at a time.
Sputtering Yield
           sputtered atoms       Mm Em
       S                   
            bombing ions       M  m 2 U M
       M : mass of target atom
       m : mass of bombing ion
       Em : kinetic energy of bombing ion
       U M : Bonding energy of target metal
        : striking angle
A magnetron sputter gun
                  CVD
•Chemical vapor deposition (CVD) is a
 chemical process often used in the
 semiconductor industry for the deposition
 of thin films of various materials.
             Process of CVD

• Vaporization and Transport of Precursor
 Molecules into Reactor
• Diffusion of Precursor Molecules to Surface
• Adsorption of Precursor Molecules to Surface
• Decomposition of Precursor Molecules on
 Surface and Incorporation into Solid Films
• Recombination of Molecular Byproducts and
 Desorption into Gas Phase
Types of Chemical Vapor Deposition

•Atmospheric pressure chemical vapor
 deposition (APCVD)
•Low pressure chemical vapor deposition
 (LPCVD)
•Plasma assisted (enhanced) chemical
 vapor deposition (PACVD, PECVD)
APCVD
•Atmospheric pressure chemical vapor
 deposition
•Operate at 1 atm.
•Fast deposition rate
•Continuous
•May subject to
 particle
 contamination.
 LPCVD
•Low Pressure Chemical Vapor Deposition (LPCVD)
 is a technique in which one or more gaseous reactors
 are used to form a solid insulating or conducting
 layer on the surface of a wafer under low pressure
 and high temperature conditions.
PECVD

•Plasma Enhanced Chemical Vapor Deposition
 (PECVD) is a technique in which one or more
 gaseous reactors are used to form a solid
 insulating or conducting layer on the surface
 of a wafer enhanced by the use of a vapor
 containing electrically charged particles or
 plasma, at lower temperatures.