RF Inductive Discharges

1
RF Inductive Discharges

• Comparison between capactive and inductive
  discharges
• Principles of magnetic induction
• History and configurations TCP, ICP
• RF sheath
• Advantages and disadvantages

2
Comparison between CCP and ICP

• Electric field energy vs. magnetic field energy
• Voltage coupling vs. current coupling
• High capacitance vs high inductance between the
  electrode and plasmas
• High vs. low ion bombardment energy
• RIE/RF sputtering vs. HDP etching/CVD

3
Principles of Magnetic Induction

• Magnetic induction Faradays law

Ip
IRF
4
History and Configurations

• Electrodeless ring discharge by Hittorf (1884)
• Controversy of capacitive inductive
  characteristics over 50 years (capacitive at low
  density and transition to inductive at high
  density)
• 1970s high pressure (gt20 mTorr) in
  cylindrical geometry, open air induction torch
• late 1980s planar coil configuration, low
  pressure (lt50 mTorr), low aspect ratio geometry
  (l/R ? 1)

RF Source
Gas Feed
TCP
ICP
Dielectric
RF Source
Multipoles
Dielectric
RF Bias
RF Bias
Vacuum Pump
Substrate
Vacuum Pump
Substrate
5
RF Sheath with Dielectric Window
Why is the inductive discharge at high voltage
considerably more efficient? The answer is that
only a small fraction of the RF voltage appears
across the sheath, such that the ion energy loss
is considerably lower.

• Using the modified Child law, the sheath
  thickness for smltltb-R is

• This sheath potential is much smaller than that
  of a capacitive discharge

• The smaller number of turns is favorable!

6
Advantages and Disadvantages
Operating regime of inductive plasma torches and
TCPs
Parameter Low value Typical value High value
Frequency 10 kHz 13.56 MHz 100 MHz Gas
pressure 10 Torr 760 Torr 10 atm Power
level 1 kW 30 kW 1 MW Efficiency 20 35
50 Gas temperature 1000 K 10000 K 20000 K
Frequency 1 MHz 13.56 MHz 100 MHz Electron
density, ne 1017/m3 1018 /m3 Gas pressure 1
mTorr 20 mTorr Power level 2 kW

• Advantages
• coupled to the plasma across a dielectric window
  or wall rather than direct contact
• low voltage across all plasma sheaths - ion
  energy control possible
• independent control of the ion/radical fluxes
  and the ion-bombarding energy (through substrate
  electrode power)
• Disadvantages
• difficult to achieve large-area plasma sources