Introduction:
Langmuir probes play a pivotal role in plasma diagnostics, unravelling plasma characteristics for effective process control. By delivering accurate measurements of key plasma parameters, Langmuir probes enable meticulous optimisation, leading to increased efficiency and improved performance across applications. As a frontrunner in plasma diagnostic solutions, Impedans Ltd. offers cutting-edge Langmuir probe technologies, empowering industries to leverage precise insights and elevate their plasma-based processes to new levels of excellence.
Section 1: Understanding Langmuir Probes
Langmuir probes consist of a thin conducting electrode, which is placed inside the bulk plasma. The prob is biased to an electric potential V, with respect to a reference electrode (which can be the grounded metallic wall of the plasma chamber), to collect electron and/or ion currents from the plasma. This current is used to build up a current voltage (I-V) characteristic of the plasma, from which key plasma parameters are extracted. These parameters shed light on plasma’s behaviour, aiding in diagnosing instabilities, monitoring changes, and ensuring efficient process control.
The design of Langmuir probes is carefully crafted to facilitate accurate measurements with minimum perturbation to the plasma. The probe tip, the area exposed to plasma, must be small to ensure local measurements and can be spherical (a), cylindrical (b) or planar (c, in the figure 1) which have uses depending on the plasma environment. A spherical and planar tip will have a greater collection area and be better suited to a lower density plasma. It is the conducting tip which is exposed to plasma and the rest of the probe must be sufficiently insulated in order to protect it from the plasma environment.
If the Langmuir probe is being used in an RF plasma, RF chokes (inductors) with high impedance at plasma excitation frequency must be placed close to the probe tips. These chokes must have a resonance frequency band covering the RF frequency used to drive the plasma and also its harmonics, so that the impedance of the inductor at the driving frequency is around 100kOhms or more. This allows the measurement tips to ‘float’ with the RF, giving accurate data.
The circuitry needed for a Langmuir probe is quite basic, only requiring circuitry to apply variable voltages to the probe and measure the current the probe draws from the plasma. For this reason. Langmuir probes exhibit remarkable versatility, adapting to a range of plasma environments and process conditions. From laboratory experiments to industrial applications, these probes offer real-time insights into plasma properties. Whether within fusion devices, space exploration, semiconductor manufacturing, or plasma-based material processing, Langmuir probes empower researchers and engineers to fine-tune processes and achieve optimal outcomes.
Section 2: The I-V Characteristics and Key Parameters Measured
The current measured from the probe is a function of both ions and electrons travelling across the plasma sheath that is formed around the probe tip. In order to build up a characteristic I-V curve, the current entering the sheath must be calculated from the current measured at the probe tip. Langmuir probes typically utilise various theoretical models to perform this calculation including Lafromboise theory at low pressures where the sheath around the probe can be thought of as collisionless, and Allen, Boyd, Reynolds theory for higher pressures where ions have typically much shorter mean free paths. Once the current travelling across the sheath edge is calculated, an I-V characteristic curve such as the one shown in figure 2 can be produced.
Some of the important coordinates are highlighted above, where Vf is the floating potential, Vp is the plasma potential, Vsat is a probe potential in the electron saturation region of the characteristic, I(Vp) is the probe current at the plasma potential and I(Vsat) is the probe current at the chosen Vsat. The I-V characteristic curve obtained from a Langmuir probe measurement consists of distinct regions that convey important plasma characteristics. The regions include:
- Ion Saturation Region: This is region to the left of Vf where the probe bias is increasingly more negative, with respect to the plasma potential. The electrons are repelled and the probe current is dominated by the positive ions. The drained ion current from the plasma is limited by the electric shielding of the probe and the current decreases slowly for very negative V≪Vp. Sheath expansion theory, governing the ion collection, is applied to this region to calculate the ion number density ni.
- Electron Retardation Region: Moving from Vf to Vp the probe collects increasingly more electron current as the potential barrier formed between the probe and plasma decreases (becoming 0 at Vp). The number of electrons that reaches the probe in this region is dependent on the energy distribution of the electrons, hence this region can be used to build up the electron energy distribution function (EEDF) and measure the electron temperature. In this region, electron current increases exponentially when the electrons are in thermal equilibrium.
- Electron Saturation Region: The exponential growth of electron current with Vp should continue until Vp = Vs, when none of the electrons is repelled by a negative potential. To the right of Vp, the probe potential repels ions and attracts electrons; electron saturation occurs. For Vp > Vs, electron current increases
slowly as the collection area grows due to an increase in sheath thickness, the shape of the curve depending on the shape of the probe tip.
Section 3: Impedans Langmuir Probe Solutions
Impedans Ltd., a leading provider of advanced plasma diagnostic solutions, offers cutting-edge Langmuir probe technologies to address the needs of the plasma research and industrial sectors. Their Langmuir probes offer real time measurement of key plasma parameters, with complex Langmuir analysis of raw data being done by their state of the art free software to make using an Impedans Langmuir probe accessible to users of all experience levels. Below figure 3 and 4 are an example of a screenshot of the Langmuir software being used in an Argon CCP plasma, giving all the parameters measured and easily exported for the user.
Impedans’ Langmuir probes are compatible with various plasma sources, being used in over 100 publications. Whether in a laboratory setting or an industrial facility, Impedans’ probe systems deliver reliable results and contribute to improved plasma diagnostics.
Section 4: Applications and Benefits of Langmuir Probes
Langmuir probes find extensive applications across numerous industries where plasma diagnostics and process control are critical. Below is a schematic listing of some examples of the applications that a Langmuir probe can be implemented in.
In thin film deposition applications, film uniformity is highly dependent on plasma density uniformity. A Langmuir probe provides highly localised measurements of plasma parameters, and as such can be used to build up a special distribution of the plasma density. Zoubian et al (2021) [1] demonstrated below plasma density measurements in figure 6 across space of ECR microwave plasma sources supplied by solid state generators. Impedans also offer linear drives for this spatial mapping of plasma parameters.
In thruster applications, a key parameter for performance is the divergence of the plasma plume. In general, the more concentrated the plume is, the thruster will generate more acceleration. K Dannenmayer et al (2011) [2] used an Impedans Langmuir probe with its linear drive system to investigate this and map out thruster plasma plumes EEDFs, electron temperatures, plasma potential and electron density as shown by the figure 7 below.
Impedans Langmuir probes have been demonstrated to have sufficient time resolution in order to build a picture of how plasma parameters evolve in pulsed ICP [3], pulsed CCP [4] and HiPIMS [5] set ups. Langmuir probes have also been used in the device optimisation of sterilisation sources [6]. In this study the plasma with inductive coupling was found to be more efficient at destroying biological matter than the plasma operated in a capacitive mode at the same applied power.
Conclusion:
Langmuir probes are indispensable tools for understanding plasma characteristics and achieving precise process control. By leveraging the working principle of Langmuir probes and extracting essential plasma parameters, scientists and engineers gain valuable insights into plasma behaviours. Impedans Ltd. stands as a trusted provider of advanced Langmuir probe solutions, offering cutting-edge technologies that empower plasma diagnostics and process optimization. To explore Impedans’ range of Langmuir probes and learn more about their plasma diagnostic solutions, visit the Impedans website and unlock the potential for enhanced plasma processes.
References:
1 Zoubian, Fadi & Renaut, Nicolas & Latrasse, Louis. (2021). Distributed elementary ECR microwave plasma sources supplied by solid state generators for production of large area plasmas without scale limitation: plasma density measurements and comparison with simulation. Plasma Research Express. 3. 10.1088/2516-1067/ac0499.
2 Dannenmayer, K., Kudrna, P., Tichý, M., & Mazouffre, S. (2011). Measurement of plasma parameters in the far-field plume of a Hall effect thruster. Plasma Sources Science and Technology, 20(6). https://doi.org/10.1088/0963-0252/20/6/065012
3 Liu, W., Xue, C., Gao, F., Liu, Y. X., Wang, Y. N., & Zhao, Y. T. (2021). Time-resolved radial uniformity of pulse-modulated inductively coupled O2/Ar plasmas. Chinese Physics B, 30(6). https://doi.org/10.1088/1674-1056/abe3f5
4 Anjum, Z., & Rehman, N. U. (2020). Temporal evolution of plasma parameters in a pulse-modulated capacitively coupled Ar/O2mixture discharge. AIP Advances, 10(11). https://doi.org/10.1063/5.0019527
5 Stranak, V., Herrendorf, A. P., Drache, S., Cada, M., Hubicka, Z., Bogdanowicz, R., Tichy, M., & Hippler, R. (2012). Plasma diagnostics of low pressure high power impulse magnetron sputtering assisted by electron cyclotron wave resonance plasma. Journal of Applied Physics, 112(9). https://doi.org/10.1063/1.4764102
6 Bol’shakov, A. A., Cruden, B. A., Mogul, R., Rao, M. V. V. S., Sharma, S. P., Khare, B. N., & Meyyappan, M. (2004). Radio-Frequency Oxygen Plasma as a Sterilization Source. AIAA Journal, 42(4), 823–832. https://doi.org/10.2514/1.9562
