Multiple harmonic compensation of Langmuir probes in rf discharges

Abstract

We report on the use of an active Langmuir probe with three-harmonic compensation to diagnose rf discharge plasmas driven at 13.56 MHz. The plasma generates many harmonics on the fundamental, the first few being strongest. This gives a multi-harmonic rf voltage across the probe sheath that is removed by applying a rf signal to the probe that is matched both in amplitude and in phase for each harmonic. The probe I-V characteristic can then be analysed using dc theory. We show here that only when the rf harmonic amplitude approaches about 2T e is it necessary to compensate for that harmonic. For the commercial processing rig used this only occurs for the second harmonic at low pressures, <5 × 10 −3 mbar. Here the addition of second-harmonic compensation shifts the probe I-V curve, making it markedly more positive than for fundamental-only compensation. Despite this the values obtained for the electron density n e and temperature T e changed by less than 10%. For most plasmas in which the harmonics have amplitudes below T e the use of fundamental-only compensation is adequate for all but the most precise measurements.

The Effect of Positive Ion Energy on Plasma Polymerization: A Comparison between Acrylic and Propionic Acids

“…8,10 We control the ion energy at the substrate by applying an RF potential to the substrate which nulls the RF sheath potential. To reach the minimum ion energy (floating potential), the RF voltage applied must be matched in both phase and amplitude to the RF potentials already present in the sheath 11 that no current, and hence no power, flows into the plasma. Once this condition is satisfied, the bulk plasma is unperturbed and independent control of the ion energy is achieved.…”

“…The RF bias potential is provided by a separate power supply, which amplifies a signal taken from an electronic module. The module uses a phase reference signal, taken from a pickup electrode, to provide an input into the amplifier which can be varied in both phase and amplitude with respect to the pickup . In this way, it is possible to provide a voltage to the substrate which is matched to the RF potential in the plasma, for the 13.56 MHz fundamental and the first two harmonics at 27.1 and 40.7 MHz.…”

Application of a Langmuir probe AC technique for reliable access to the low energy range of electron energy distribution functions in low pressure plasmas

“…Thus, various RF compensation techniques with active or passive filter designs have been developed over the last decades. In practice, however, it is often extremely difficult to achieve sufficient RF compensation for various plasma conditions 10 and it has been shown that already small RF distortions can lead to highly erroneous EEDFs especially near zero electron energy [11][12][13] .…”

Langmuir probe measurements at incomplete rf-compensation

“…The evaluation described above of such disturbed characteristics gives incorrect plasma parameters. To avoid this problem the probe tip is forced to follow the varying plasma potential either by driving the probe with an rf-voltage [4,5], or by increasing the impedance between probe and ground [6].…”