Electron cyclotron resonance harmonics and wall material effects in a microwave discharge

In this article we investigate a 2.45 GHz microwave discharge operated at the electron cyclotron resonance (ECR) and the second harmonic, by applying a divergent magnetic field where the maximum is changed from 1400 to 500 G. The magnetic field is continuously decreased so that the ECR zone is moved axially along the chamber from about 15 cm away to very close to the microwave window. When the field is reduced to below 875 G the ECR disappears and only the second harmonic is present in the source. We observe that the second harmonic is not efficient enough for breakdown at microwave power of less than 500 W and gas pressure of 0.48 Pa, and it is shown that an overdense plasma can not be generated by use of the second harmonic (independent of gas type). It is also shown that the wall material close to the ECR zone is extremely important, probably due to an effect similar to that of the antenna material in capacitively and inductively coupled plasma sources. The possibility of measuring the thickness and location of the effective ionizing resonance zone is discussed. A calculation of the plasma density as a function of the magnetic field is performed, which takes into account the conservation of magnetic flux and the secondary emission from the wall, and excellent agreement with the measured results is obtained