The plasma sheath around planar probes: effects of ion collisions

In this work, experimental and theoretical study of pulsed discharge in argon has been carried out. The experimental data on the dynamics of electron density, electron temperature, and plasma potential have been obtained by probe measurements (using hairpin and Langmuir probes) in the afterglow between two RF pulses. This dynamics is thoroughly analyzed by comparison with the results of the kinetic PIC MC simulations. The main processes that cause electron cooling have been revealed for two different time stages. The first afterglow stage is characterized by the rapid decrease of the electron temperature. During this stage, inelastic electron collisions with Ar atoms in the ground state play an important role in the electron cooling. During the second longscale stage, only diffusion along the electron energy scale via Coulomb or elastic collisions can provide the observed effect of the gradual electron cooling. The effects of elastic and Coulomb collisions on the electron cooling have been separated and analyzed by means of PIC MC simulations. The temporal evolution of the electron concentration spatial distribution and the plasma boundary sheaths size has been also analyzed on the base of 1D PIC MC simulations. A good agreement between the probe measurements and the PIC simulations data on plasma density, ambipolar potential and electron temperature is observed. To perform a more detailed study of EEDF and electron temperature dynamics, the relative intensity of Ar( 2 p 1 )→Ar( 1 s 2 ) emission at 750 nm has been measured. A very good agreement between the measured emission intensity and the calculated excitation rate of Ar( 2 p 1 ) at different time scales validates the electron temperature temporal dynamics obtained by the used PIC model.

Section: Resultsmentioning

confidence: 91%

Experimental and PIC MCC study of electron cooling—re-heating and plasma density decay in low pressure rf ccp argon afterglow

Proshina

Рахимова

Kovalev

et al. 2020

“…In the previous simulation of the sheath near a planar probe [34], it was discovered that both the effective ion collection area A s and k-value decreased with the pressure increase. It was shown that for each of the given plasma density n 0 and probe potential V pr values the A s value decreased in a similar way as its value in the collisionless case A s 0 [33].…”

Section: Numerical Model: Pic MCC Simulationmentioning

confidence: 97%

“…The 2D numerical model of ion current collection by a planar probe (electrode) [34] is used. A detailed description of the model can be found in [34]. The main model assumptions are as follows.…”

Section: Numerical Model: Pic MCC Simulationmentioning

confidence: 99%

“…A higher accuracy can be provided by the numerical model of the ion motion in plasma based on the PIC MCC method. In the recent paper [34], the 2D structure of the sheath around a disk-shaped planar probe was calculated with the kinetic PIC model involving ion collisions. The decrease of the effective ion collection area A s and, especially, of the plasma density at the sheath boundary n s (reduction of the k n n s 0 = value) with the pressure increase was shown there.…”

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

Relation between the ion flux and plasma density in an rf CCP discharge

Bogdanova

Lopaev

Zyryanov

et al. 2018

Self Cite

Ion flux and plasma density are some of the most essential parameters for plasma-assisted process control. There is also a simple and well-known relation between them based on the Bohm criterion. This relation allows the avoidance of directly measuring both the ion flux and the plasma density simultaneously, but the question is, how accurate this estimation would be. This work represents the study of the sensitivity of the relation to plasma conditions, such as gas pressure, type of gas and plasma density. The experiments are carried out in an asymmetric rf dual-frequency CCP discharge in noble gases-Ar and Xe-and in molecular gas-N 2 . The gas pressure is varied from 20 mTorr up to 200 mTorr and the plasma density range is 10 9 to 5.6 ×10 10 cm −3 . Analysis of the experimental data is made by using the 2D PIC MCC model of the ion current collection by a planar electrode. The results show that there is some sensitivity of the relation to the type of gas and pressure, but the dependence of the relation on plasma density turns out to be slight. Therefore, the pressure parameterization of the relation for a certain gas can be used to establish plasma density by measuring ion current to an electrode and vice versa.

“…This, in turn, requires "real-time" feedback through sensors that provide information about the process and thus make the process control truly precise. the cylindrical probe [12] and the flat probe [13], and with simplified numerical approaches for these probes [14]. The relation between ion flux and plasma density in a df rf CCP discharge in various gases has been investigated both experimentally and using the 2D PIC MCC simulation in [15] to obtain the pressure dependence of this relation.…”

Section: Introductionmentioning

confidence: 99%

‘Virtual IED sensor’ for df rf CCP discharges

Bogdanova¹,

Lopaev²,

Рахимова³

et al. 2021

Ion-assisted surface processes are the basis of modern plasma processing. Ion energy distribution (IED) control is critical for precise material modification, especially in atomic-level technologies such as atomic layer etching. Since this control should be done in "real time", it requires "real-time" feedback using fast process sensors. In the general case of an industrial plasma reactor, when direct IED measurement is not possible, the IED can be estimated using the concept of a "virtual IED sensor". In this paper, a similar "virtual IED sensor" is considered using an asymmetric dual-frequency (df) rf CCP discharge as an example. It is based on a fast calculation method of the IED at an rf-biased electrode. This approach uses the experimentally measured sheath voltage waveform and plasma density (or ion flux) as input data, and also includes Monte-Carlo simulation of ion motion in the sheath to take into account the effect of ion-neutral collisions. To validate this approach, experiments were carried out using various plasma diagnostics in several gases: argon and xenon as examples of plasma with atomic ions and nitrogen as an example of plasma with molecular ions. It is shown that in all cases it is possible to obtain an adequate IED estimation, close to the experimental one, in a reasonably short time ( tens of seconds when using a modern PC). The results obtained demonstrate the possibility of using a virtual IED sensor in real plasma processing.So, on the one hand, modern plasma processing requires the development of new plasma approaches and systems to ensure an appropriate level of control. On the other hand, this control requires the development of fast sensors.The dual-frequency (df) rf ICP and CCP discharges are widely used today in plasma surface treatment. The simultaneous use of high-frequency and low-frequency rf voltages is aimed at realizing separate control of ion flux and energy. This idea was presented in the works of Goto in the early 1990s [1,2]. Since then, these discharges have been extensively studied both experimentally and theoretically [3,4]. However, one of the main results of these studies is that the two frequencies are generally coupled and the desired functional separate control can only be achieved over a limited range of discharge parameters (frequencies, pressures, and input powers).The multi-frequency discharge excitation was also investigated [5] to reveal the role of the third frequency in controlling the IED shape [6]. In order to better control the ion energy, some new methods of rf discharge excitation have recently been developed. One of them is the excitation of a symmetric rf CCP discharge with two frequencies (the fundamental frequency and its second harmonic) with a phase shift between them. In this configuration, the dc self-bias voltage appears due to the electrical asymmetry effect (EAE) [7]. This voltage is adjusted by the phase shift value between the two frequencies and is used for additional control of ion energy. This idea was further developed in the so-call...