RF Power Analysis


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Why do I need anything more than my RF powermeter?

The determination of the real power consumed in RF gas discharges and of the discharge impedance has been a subject of intense investigations for the last 15 years. The reason for spending this effort is that the electrical properties of such discharges can give an insight in both understanding and controlling the processes.

The use of an RF (SWR) powermeter in the RF transmittance line for measuring the power dissipation in the discharge is nowadays not acceptable as it is very well known that a stray impedance exists between the point where the SWR is placed and the discharge itself.

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The power consumption on this parasitic impedance can vary from 20% to 80% of the total RF generator power, depending on:

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 the frequency of plasma excitation and

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 the elements that are used in the electrical circuit:

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matching network

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shunt circuit

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RF electrode shield

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RF electrode length

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.....

Thus, the power dissipation in this parasitic impedance leads to a very significant error in the calculation of the power actually consumed in the discharge. Moreover, this error cannot be considered as systematic because the power consumption on the stray impedance is not constant but it is affected by the discharge impedance.

bulletOn the other hand, the discharge impedance depends on process parameters like:
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frequency

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pressure

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gas composition

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power

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flow

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Therefore, power consumption in the external circuit is affected in such a manner that cannot be predicted without the application of a more sophisticated method of power measurement.  It goes without saying that any attempt to evaluate the influence of any of the external discharge parameters listed above on the process will introduce also an influence due to the modification of the discharge impedance (leading to a modification of the discharge power). Nevertheless, current literature is full with examples of experiments examining the influence of many macroscopic parameters (even the power as measured by the SWR bridge) without ever actually measuring the power itself! 

PTLUP has been involved since 1994 in the development and continuous improvement of a method for the measurement of the power actually consumed in low pressure RF discharges.  You can find more information by following the links:

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