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Comparison DC / RF

DC (direct current)

In DC, the three main source parameters for monitoring and controlling the source are current, voltage and pressure. It is not possible to keep all three source parameters constant for all samples during calibration, analysis, and depth profiling. Hence the source can only be operated in one of three different modes: with current, voltage or pressure variable.

It is now common practice to operate in DC with constant current and voltage and variable pressure.

RF (radio frequency)

In RF, the main parameters for monitoring and controlling the source are applied power and pressure, DC bias voltage, applied voltage, and blank power. Few instruments have all these options.

It has been common practice to keep applied power and pressure constant for all samples during calibration, analysis, and depth profiling.

New modes of operation are being developed, eg constant (applied-blank) power and constant applied voltage with variable pressure. Blank power is measured with no argon in the source.

Hence the modes are separable into those that use constant pressure and those that use variable pressure.

There is no great mystery about RF (radio frequency) glow discharges compared to DC (direct current) glow discharges. RF and DC glow discharge plasmas are similar in many ways. So similar, in fact, that the RF glow discharge has been described as a DC discharge with a superimposed high-frequency field.(1)

The basic plasma processes producing the GD-OES signals are the same: ion and atom bombardment of the sample by the plasma gas, excitation of the sputtered atoms through inelastic collisions with energetic electrons and metastable atoms, followed by de-excitation and photon emission. There are subtle differences, e.g. in electron densities and energy distributions—RF glow discharges tend to have fewer but more energetic electrons—but the main difference is that RF can analyse both conductors and non-conductors and DC cannot.

But as RF develops, its superiority to DC, even in the analysis of conducting materials, is slowly emerging:

  • wider range of operating parameters
  • more stable plasma
  • less affected by surface oxides
  • greater sputtering depth.

Theoretical Comparison

In a recent theoretical study of the similarities and differences between RF and DC, Bogaerts and Gijbels found the following, for copper, using constant power and pressure in RF and DC:

Similarities

Characteristic

Result

Electrical currentprincipally by ions bombarding the sample, as in DC
Potential distributionsimilar in RF to DC
Electric fieldssimilar in RF to DC
Density of ground statesimilar in RF to DC
Ion densitiessimilar in RF to DC
Ar intensitiessimilar in RF to DC
Net sputtering ratesimilar in RF to DC
Sputtered speciessimilar in RF to DC

Differences

Characteristic

Result

Ionizationa + g in RF, g only in DC
Electron impact ionizationmore efficient in RF than DC
Plasma potentiallower in RF than DC
Ion densitydrops more slowly away from the sample in RF than DC
Plasma cellmore filled with argon ions than DC
Excitationmore efficient in RF than DC
Population of excited levelshigher in RF than DC
Cu intensiteshigher in RF than DC, x10 for atomic lines

To see their whole paper, click here. The a-ionization that occurs only in RF is from 'wave-riding' electrons, ie electrons with low energy which gain energy as the field in front of the sample expands during the positive-going cycle of the RF.(3)

References:

  1. M R Winchester, C Lazik and R K Marcus, Spectrochim. Acta 46B, 483 (1991).
  2. A Bogaerts and R Gijbels, J. Anal. Atom. Spectrom.; 15; 2000; 1191-1201 DOI: 10.1039/b000519n
  3. P Belenguer, PhD Thesis, Université Nancy I, France (1990).
  4. R Payling and D G Jones, Surf. Interface Anal20, 787 (1993).

First published on the web: 1 June 2000.

Author: Richard Payling