The measurement and control of the ion energy distribution function at a surface in an RF plasma

Using an RF-driven collecting surface, mounted on the front end of a high-resolution energy-resolved mass spectrometer, the ion energy distribution functions (IEDFs) within a 13.56 MHz argon discharge have been measured and controlled. A technique of RF signal feedback has been developed, in which the RF amplitude and phase (fundamental and first two harmonics) in the sheath are varied, so manipulating the mean bombarding ion energies and widths of the IEDFs. For high RF sheath potentials, the IEDFs are broad and bi-modal. However as the sheath potential drop is reduced (with imposed matched RF signal) the IEDFs narrow, eventually becoming single peaked when no RF potential drop is present in the sheath. The observed widths in the IEDFs broadly agree with simple modelling predictions for the `high frequency' RF sheath regime; however we calculate, from the ion spectral data, marginally thinner sheaths than given by the Child-Langmuir law (derived using the mean DC sheath potential).