Discharge power supply and modes of operation
Discharge Power
In the GD source, the anode is normally held at ground potential
and a potential is applied to the sample. This power supplied to the sample may be
RF or DC.
RF (radio frequency)
For RF, the inner surface of
the sample reaches a negative dc bias potential. This dc potential
arises from the combination of two factors:
- the area of the sample surface
inside the plasma is much smaller than the area of the inside
of the anode, making it easier, because of charge density (i.e.
electrons repel each other), for electrons to flow to the anode
rather than to the sample; and
- the large mass of the argon
atom, means the positive argon ions cannot follow the rapid changes
in rf potential and so travel in the opposite direction to the
average electron flow.
The
DC bias potential in RF sources is generally a little lower than
the equivalent DC voltage in DC sources. The difficulty of measuring
the RF currents means that most RF sources are controlled by the
applied RF power, which typically varies from 5 W to 80 W.
To
see how to measure RF currents, click here,
then click on Search and enter: Development
of special hardware for RF-GD-OES.
DC (direct current)
If DC, the applied voltage
is typically varied between 400-1200 V and the DC current produced
typically varies between 20-120 mA.
First published on the web: 15 May 2000.
Author: Richard Payling
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Capacitively coupled discharges
In a glow discharge used for analytical purposes, the sample to be analysed forms one of the two electrodes. Usually the sample is held at negative potential in order to cause ablation of material by cathodic sputtering. As the electrodes are an integral part of the electrical circuit maintaining the discharge, insulating materials can not be analysed in this configuration using a continuous direct power supply. If the sample-electrode is non-conductive, applying a DC voltage across the two electrodes will only lead to a short breakdown followed by the creation of a surface charge; no current can flow through the non-conductive sample. A way around this is to apply an alternating voltage, for example RF, between the two electrodes which then alternate between cathodic and anodic behaviour in each half-cycle; the most commonly used frequency is 13.56 MHz (a band allowed by the International Telecommunications Union). This results in a capacitively coupled discharge, so-called because the electrodes and their sheaths form a capacitor (Fig.). High plasma densities can be achieved with sufficient RF input power. The electric field in the chamber transfers energy to both electrons and ions. At the usual RF frequencies and pressures used, because of collisional heating in main plasma body and collisionless heating across the sheath, ions and electrons have different behaviour according to their masses and are therefore not at thermal equilibrium. The resulting high electron temperature sustains the discharge via electron impact ionisation of the fill gas. Capacitively coupled RF plasmas are more efficient in converting the power from the supply into the plasma. RF discharges, compared to equivalent DC discharges also produce much lower energy ions at the cathode, thus minimising sputtering damage at the surface. This is important for example in some microelectronic applications. When the surface area of the two electrodes of an RF discharge is very different a significant negative DC bias voltage may develop at the smaller electrode. Due to the high mobility of electrons the time average of the current, measured over an rf-cycle, remains zero. This asymmetric configuration allows a stable plasma to be sustained and causes continuous ion bombardment of only one electrode, the sample. It thus allows the efficient sputtering of both conductive and insulating material.
First published on the web: 15 March 2008
Author: Hakan Candan . The text is based on a lecture given by Philippe Belenguer at the first GLADNET training course in Antwerp Sept. 2007.
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DC (direct current)
In
DC, the three main source parameters for monitoring and controlling
the source are current, voltage and pressure. It 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 current |
principally by ions bombarding the sample,
as in DC |
| Potential distribution |
similar in RF to DC |
| Electric fields |
similar in RF to DC |
| Density of ground state |
similar in RF to DC |
| Ion densities |
similar in RF to DC |
| Ar intensities |
similar in RF to DC |
| Net sputtering rate |
similar in RF to DC |
| Sputtered species |
similar in RF to DC |
Differences
Characteristic |
Result |
| Ionization |
a + g in RF, g only in DC |
| Electron impact ionization |
more efficient in RF than DC |
| Plasma potential |
lower in RF than DC |
| Ion density |
drops more slowly away from the sample in RF
than DC |
| Plasma cell |
more filled with argon ions than DC |
| Excitation |
more efficient in RF than DC |
| Population of excited levels |
higher in RF than DC |
| Cu intensites |
higher 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)
History of RF vs DC
Werner Grimm invented the first
modern DC GD-OES source in 1967 and the first commercial DC instrument,
RSV Analymat, was released about 1979.
RF was introduced into GD-OES at Renault by Richard Passetemps in 1988 and the first commercial instrument,
JY 50S GDS, a combined DC and RF instrument, became available in
1992. At the same time Ken Marcus in USA was developing his
RF source.
In the 1990s, independent studies, particularly
by Delwyn Jones and Dick Payling at BHP in Australia using
one of the first JY 50S GDS,(4) compared the performance
of RF and DC and demonstrated the ability of RF GD-OES to analyse
both conductive and non-conductive materials. BHP then proposed
a quantitative model for RF and the first wholly RF instrument,
JY 5000 RF, was released in 1995, followed by the JY 10000 RF in
1996.
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
Anal. 20, 787 (1993).
First published on the web: 1 June 2000.
Author: Richard Payling
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![[RF Voltage]](images/rf_cha6.gif)
DC Bias
If a pure sinusoidal RF voltage is applied to the sample, it has an
amplitude Vrf (typically 700 V) and produces a DC bias
potential on the inside surface of the sample Vdc. The
potential on the sample surface therefore varies during each RF cycle
between a small positive value, Vp, typically
30-50 V, and a large negative value, 2 x Vrf + Vp,
typically -1360 V. The average is Vrf + Vp,
ie 660 V, which equals Vdc.
The threshold voltage for sputtering is about 300 V, so a peak value in
excess 300 V will cause sputtering. Since 300 V peak corresponds to a DC
bias of about 140 V, sputtering will occur in RF at much lower voltages
than in DC. An argon atom can typically travel about 1 mm during half
an RF cycle. This means sputtering will only occur during that part of the
cycle where Vrf > 300 V, ie during approximately 60%
of the cycle. Hence, as reported, sputtering rates in RF are
approximately 60% of those for DC.(1)
Impedance
To first order, the impedance of the GD plasma is independent of the
applied RF power. This crucial result was first reported by Fabienne
Canpont in 1993.(2) It means, to first order, we can vary
the RF power without changing the impedance. Since the emission yield
depends on impedance, we can therefore vary the power without also changing the emission yield.
This has consequences I will talk about later.
In an RF circuit using a matching box, the parallel capacitor Cmod varies mostly with the real part of the load impedance (resistance) and
the series capacitor Cpha varies mostly with the
imaginary part of the load impedance (phase). If we increase the applied
RF power, both Cmod and Cpha remain
nearly constant. If we increase the plasma pressure, the plasma resistance
decreases and Cmod decreases while Cpha stays about the same. If we increase the gap between the sample and anode,
the plasma impedance decreases, the plasma being less obstructed, and Cmod decreases.
If Rg is the source resistance, and if varying RF power
does not change Rg, then
where Wi is the incident (or applied) RF
power, and the factor 2 comes Vrf being defined
here as peak voltage rather than rms voltage. For a fixed sample-to-anode
gap, the source resistance varies with the sample matrix (ie, with
the secondary electron emission yield of the sample) and the pressure. Rg is typically 20 KW.
Effective Power
In RF systems some power is lost in cables and through radiation.
Manufacturers try to keep such losses to a minimum. The lost power can be
thought of as a further resistance Rloss, in parallel
with the source resistance. The power in the plasma, ie the 'effective'
power Weff, is then
where Vg is the RF peak voltage on the
inside surface of the sample and Vrf is the applied RF
peak voltage. For metal samples Vg = Vrf,
for non-conducting samples, Vg < Vrf.
Canpont has suggested we can estimate Wloss by measuring the power without argon in the source. This is equivalent to
making Rg infinite. The effective power is then the difference in power with and without argon.(2)
References:
(1) R Payling, D G Jones and S Gower, Surf.
Interface Anal. 20 (1993) 959.
(2) F Canpont, These (Doctorat), Université Claude Bernard
Lyon I, France, 1993, pp 40, 53, 70.
First published on the web: 12 November 2002.
Author: Richard Payling
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