Radio-frequency discharge power measurement with emphasis on collisionless electron heating in capacitive sheaths
Gahan, David (2006) Radio-frequency discharge power measurement with emphasis on collisionless electron heating in capacitive sheaths. PhD thesis, Dublin City University.
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Many of today’s processing plasma tools are operated at low pressures to achieve high etch directivity and reduce side erosion on the wafer. At these pressures electron-neutral collisions are rare and the electrons cannot gain energy through the Ohmic heating process. Instead the heating mechanism is attributed to a stochastic process between the electrons and the sheath electric field. Theoretical models of this stochastic process include the hard wall approximation and the pressure heating effect. The former is inconsistent with electron current conservation at the sheath whilst the later shows a difference in power absorption when electron loss to the electrodes is considered.
This thesis examines the effects of electron current on power coupling in a capacitive sheath by controlling this current with an additional DC bias applied to an rf biased electrode. Experimental and particle-in-cell model results for a low pressure argon plasma are compared and presented. Results show th a t the electron power absorption is more effective when the electron conduction current is removed, as predicted by the earlier theoretical work.
The model also shows a high harmonic content on the sheath voltage which is attenuated by removing the electron current. These high frequency harmonics are measured in the experiment, with an unbiased probe connected to a spectrum analyzer, and their correlation with the electron current is in agreement with the model results. It is found th a t the high frequency oscillations do not contribute to the power absorption in an average sense.
Finally a novel rf power sensor is presented and compared with an industrial standard power meter. The design incorporates directional coupler and I-V probe techniques to determine the power. This sensor is found to out perform the standard meter over a wide range of conditions. Its ability to measure the power at the fundamental as well as harmonic frequencies makes it a particularly useful plasma diagnostic.
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