Previous Lecture - - - - Next Lecture

Physics of Thin Films

PES 449 / PHYS 549


Chemical Vapor Deposition (CVD) Modifications

 Ohring Chapter 4 Section 6


Low Pressure CVD (LPCVD)

gas pressures around 1 mtorr - 1 torr (rather than 1 atm)
higher initial gas concentrations

lower P => higher D of gas to substrate

surface reaction often becomes rate limiting

advantages

  • better film uniformity
  • better film coverage over steps
  • fewer defects


Plasma Enhanced CVD (PECVD)

plasma in vicinity of substrate
Plasma breaks up gas molecules
  • higher reactivity
  • can use lower temperatures
  • can use lower pressures

electrons in plasma:

  • ionize gas to keep plasma going
  • "activate" gas by dissociation to enhance CVD
    • typically about 1% of gas is activated

pressures are higher than in sputter deposition

ions suffer more collisions in gas phase => less energy when reach cathode

=> minimal sputtering effects

ion energy depends on gas pressure and cathode voltage

can use RF plasma discharges (especially for insulating films)

charge builds up on insulating surfaces - reverse polarity before charge saturates

low frequencies (< 1 MHz) allows ions to reach cathode and bombard it

high frequencies (> 1 MHz) reverses direction before ions reach substrate

anode and cathode may be symmetric (identical processes at each electrode) or may be asymmetric (allowing greater use of ion bombardment at one electrode)

process parameters

  • substrate temperature
    • control by external heater
    • very little heating from PECVD process
  • gas flow
    • higher flow rates can increase deposition rate and uniformity
      • but wastes gas
  • pressure
    • changes the energy of ions reaching electrodes
    • can change deposition rate
    • increases pressure may lead to chemical reactions in the gas
    • effects also depend on gas concentration
  • power
    • effects the number of electrons available for activation and the energy of those electrons
    • increased power may lead to chemical reactions in gas
    • increased power increases deposition rate
  • frequency
    • changes plasma characteristics
    • changes ion bombardment characteristics
    • sometimes use a dual frequency system to control these two processes independently


Laser Enhanced CVD (LECVD)

also called Photoassisted CVD (PCVD)

use laser to enhance surface reactions

2 processes

  • pyrolytic
    • heats substrate to enhance reactions
  • photolytic
    • gas phase dissociation of molecules to enhance reactivity
    • typically use UV radiation


Metalorganic CVD (MOCVD)

also called Organometallic vapor phase epitaxy (OMVPE)

use organometallic source gasses

ex: (CH3)3Ga . . . . tri-methyl Gallium

advantage: volatile at relatively low temperatures


Previous Lecture Previous Lecture - - - INDEX OF LECTURES - - - Next Lecture Next lecture