The Langmuir probe (LP) is an electrostatic measurement method in which a conductor is introduced into the plasma. A voltage ramp typically in the range of 0 V to a few 10 V is applied to this wire and the resulting current is measured. The typical current-voltage characteristic then provides plasma parameters. Typically, a conductor is used which length is significantly greater than its diameter. Tungsten, for example, is used as the material. Three regions are distinguished in the current-voltage characteristic: In the ion saturation region, the measured current is negative. Here, due to the negative potential, ions are attracted and electrons are repelled so that, depending on the applied voltage, only very energetic electrons reach the measuring wire. The smaller the difference between the applied voltage and the so-called floating potential, the more electrons can overcome the potential until ion and electron current equalize at the floating potential and the net current flow is zero. By increasing the voltage, the electron current becomes larger than the ion current, because more and more electrons overcome the opposing field depending on their kinetic energy. The inflection point describes the plasma potential. From this voltage on, all electrons in the immediate vicinity of the wire are attracted. The further increase of the current follows from the extension of the influence volume of the probe by the higher voltage. Thus, in the electron saturation current, electrons are withdrawn from the probe from an increasingly larger area.

Using the second derivative of the current with respect to the applied voltage, the electron energy distribution function can be determined. The electron temperature, for example, can be determined from the slope. The electron density can be calculated either from the integral of the distribution function or, in the case of a Maxwell distribution, from the plasma potential. The special feature of the in-house developed APS4 is on the one hand the spatial positioning of the probe inside the vacuum along one axis. On the other hand, the probe wire can be retracted so that it is protected from the gas, plasma or deposition. In addition, it can be annealed free in case of oxidation or coating. For high-frequency plasmas, the Langmuir probe has effective RF compensation directly on the probe wire and a bandpass filter to protect the electronics.

Das Besondere der In-House-Entwicklung APS4 ist zum einen die Verfahrbarkeit der Sonde im Vakuum entlang einer Achse. Zum anderen kann der Sondendraht zurückgefahren werden, sodass er vor dem Gas bzw. dem Plasma geschützt ist. Zudem kann er bei Oxidation oder Beschichtung freigeglüht werden. Für Hochfrequenzplasmen verfügt die Langmuirsonde über eine effektive HF-Kompensation direkt am Sondendraht und einen Bandpassfilter zum Schutz der Elektronik.