PECVD - plasma enhanced CVD

• Characteristics:
  • It was created in order to deposit at low temperature
  • Thermal energy required for reactions, diffusion, etc is replaced with plasma energy
  • The gases are dissociated in the gas phase by collisions with high-energy electrons (accelerated by the applied bias)
  • The rest of the processes are similar to CVD, only the dissociation processes are different; also the ionisation/excitation processes occur and  ions bombard the film surface
• Advantages:
  • Low temperature applications
    • Energy for reactions is provided by plasma particles
    • Better step coverage due to energetic bombardment
• Disadvantages:
  • Uniformity - plasma nonuniformities are reflected in film nonuniformities (the layer is usually thicker in the centre of the wafer)
  • Throughput - usually in the PECVD reactors the wafers are horizontally placed, thus the throughput is much lower than in the case of LPCVD
  • Damage - the ions can sputter the deposited film, a remote plasma is used for a lower damage (substrate is not placed inside the plasma, but at a distance from the plasma and the radicals diffuse toward the substrate) 
• Applications: passivation, ILD - dielectric between metal lines, gate dielectric in TFTs, when deposition is done on substrates that cannot withstand high temperature such as glass, plastic, metal 
• HDP - high density plasma (ECR or ICP)
• high density of electrons
• created usually at low pressure
• is used to obtain Si₃N₄ with better film properties  from N₂
• disadvantage = lower deposition rate than PECVD
• Ion bombardment at low pressure etches the excess material found at the corners of the structure(overhang), improving the step coverage 
  • Too much bombardment however can produce structures with triangular shape, like in the picture below