Low Electron Temperature Plasma Diagnosis: Revisiting Langmuir Electrostatic Probes

This article describes a method of measurement of the current-to-probe voltage characteristic curve of a Langmuir electrostatic probe immersed in a plasma characterized by a low electron temperature that is only one order of magnitude higher than room temperature. These plasmas are widely used in industrial processes related to surface technology, polymers, cleaning, nanostructures, etc. The measurement method complies with the strict requirements to perform representative plasma diagnosis, particularly in the ion saturation zone when the probe is polarized much more negatively that the potential of the plasma bulk surrounding the probe and allows to diagnose the plasma very quickly and locally, making it possible to better monitor and control the plasma discharge uniformity and time drift. The requirements for the Langmuir probe design, the data acquisition and data treatment are thoroughly explained and their influence on the measurement method is also described. Subsequently, the article describes different diagnostic methods of the magnitudes that characterize the plasma, based on theoretical models of that characteristic curve. Each of these methods is applied to different zones of the measured characteristic curve, the obtained results being quite similar, which guarantees the quality of the measurements. The advantages and disadvantages of each method are discussed. A series of measurements of the plasma density for different plasma conditions shows that the method is sensitive enough that the temperature of the ions needs to be taken into account in the data processing. Finally, a Virtual Instrument is included in the LabView environment that performs the diagnosis process with sufficient speed and precision, which allows the scientist to control the parameters that characterize the plasma to increase the quality and performance of the industrial processes in which the plasma diagnosis is to be used. The Virtual Instrument can be downloaded for free from a link that is included, in order to be easily adapted to the usual devices in a plasma laboratory.