RU199908U1 - LENGMUIR PROBE FOR DIAGNOSTICS OF GAS-METAL PLASMA - Google Patents

Abstract

The claimed technical solution relates to the field of probe diagnostics of plasma parameters and can be used in installations for studying parameters (concentration of charged particles, electron temperature, potential) of a plasma containing in the gas phase a significant proportion of ions or neutral metal atoms or other substances capable of forming conductive coatings The essence of the utility model lies in the fact that in the well-known Langmuir probe containing the tip, the tip-holder rod is placed in a cylindrical protective shell, and the holder rod is passed through a dielectric sleeve placed inside the protective shell, tightly onto the tip-holder rod a dielectric washer with an outer diameter d determined by the ratio 0.8D≤d≤0.9D is put on, located at a distance l from the plane of the dielectric sleeve, determined by the ratio 0.05L≤l≤0.1L, where D is the inner diameter of the protective shell, L - distance from tip to dielectric The implementation of the combination of these features makes it possible to achieve the goal of the utility model - to increase the MTBF in diagnostics of gas-metal plasma. This goal is achieved by shielding from the flow of vapors and metal ions with a dielectric washer most of the surface of the dielectric sleeve between the tip-holder rod and the sheath, with other parameters of the probe being unchanged. The utility model was created within the framework of the Grant of the President of the Russian Federation (No. MK-154.2020.8 ).

Description

The claimed technical solution relates to the field of probe diagnostics of plasma parameters, and can be used in installations for studying parameters (concentration of charged particles, electron temperature, potential) of a plasma containing in the gas phase a significant proportion of ions or neutral atoms of metals or other substances capable of forming conductive condensation coating.

Known probe Langmuir [US Patent US 7,015,703 B2] for measuring the parameters of HF plasma, including a cylindrical tip placed in the plasma, placed inside a dielectric thin-walled cylindrical shell and fixed inside it by a vacuum-tight dielectric sleeve. At the end of the shell closest to the tip, the plates of the capacitor are fixed, which is an element of the electrical circuit designed to filter the signal. Inside the same tube, from the side open to atmospheric pressure, the electrical circuit elements are mounted, which are necessary for reliable measurement of the parameters of the plasma generated by the high-frequency generator.

The disadvantage of this design of the probe is its inability to diagnose plasma containing vapors of metals or other substances capable of forming conductive films during condensation, since the deposition of a conductive film on the bushing and the inner surface of the dielectric shell will inevitably lead either to a short circuit between the tip and capacitor plates, or to a multiple increase in the surface area of the tip, and, as a result, to erroneous measurements of plasma parameters.

The Langmuir probe is known [US Patent US 6,553,853 B2], in which a thin long cylindrical tip intended for direct contact with the plasma is inserted into a small hole in the convex end of the dielectric shell, inside which a vacuum-tight dielectric sleeve protected from contact with the plasma is installed, and also the conductors and elements of the electrical circuit necessary to receive the probe signal from the tip. The annular reference electrode, necessary for filtering plasma noise, is located outside the dielectric shell at some distance from the tip, and is covered with a dielectric coating to isolate it from the plasma. Thus, this design of the probe allows for some time to diagnose plasma containing vapors of metals or other conductive substances, since the probability of a short circuit between the tip and the annular reference electrode due to the deposition of a conductive film from the plasma is reduced, due to the presence of a dielectric coating on the annular electrode, and the developed surface of the end face of the dielectric shell due to its rounding.

The disadvantage of the device is the tight abutment of the tip to the surrounding end face of the dielectric shell, which, in the case of diagnostics of plasma containing metal vapors, inevitably leads to the deposition of a conductive film on the end face, uncontrollably increasing the receiving area of the tip surface, and hence leading to incorrect measurements of plasma parameters.

The closest in technical essence to the proposed utility model is the Langmuir probe described in [D.B.Zolotukhin, V.A. Burdovitsin, E. Oks, A.V. Tyunkov, Yu. G. Yushkov, “On the influence of electron-beam metal evaporation on parameters of beam plasma in medium vacuum” // Physics of Plasmas. - 2019. - Vol. 26, No. 5. - P. 053512, https://doi.org/10.1063/1.5095165]. The mentioned probe contains a tip and a rod-holder of the tip, placed in a cylindrical protective shell, and the rod-holder is passed through a dielectric sleeve placed inside the protective shell. Placing a dielectric sleeve that insulates the tip and the rod-holder of the tip from the protective shell, deep in the cylindrical channel of the protective shell, limits the possibility of contact of the surface of the dielectric sleeve with the plasma, which means it minimizes plasma deposition on the sleeve of the conductive coating, thereby making it possible to use this probe for diagnostics plasma containing in the gas phase a significant proportion of ions or neutral atoms of metals or other substances capable of forming conductive coatings during condensation.

The disadvantage of the device is the short MTBF in the diagnosis of gas-metal plasma due to the occurrence of a short circuit between the rod-holder of the tip and the shell due to the plasma spraying on the dielectric sleeve of the conductive film.

The purpose of the proposed technical solution is to increase the time between failure of the probe in the diagnosis of gas-metal plasma.

This goal is achieved by the fact that in the known Langmuir probe containing the tip, the tip-holder rod is placed in a cylindrical protective shell, and the holder rod is passed through a dielectric sleeve placed inside the protective shell, a dielectric washer with an outer diameter is tightly put on the tip-holder rod d, determined by the ratio 0.8D≤d≤0.9D, located at a distance l from the plane of the dielectric sleeve, determined by the ratio 0.05L≤l≤0.1L, where D is the inner diameter of the protective shell, L is the distance from the tip to the dielectric bushings.

The technical result provided by the above set of features is an increase in the operating time of the probe to failure when diagnosing a gas-metal plasma with this probe. The technical result is achieved due to the screening from the flow of vapors and metal ions by a dielectric washer of the greater part of the surface of the dielectric sleeve between the rod-holder of the tip and the shell, with other parameters of the probe unchanged.

The essence of the proposed technical solution is illustrated by the drawing presented in figure 1, and is as follows.

Fig. 1: 1 - tip, 2 - rod-holder of the tip, 3 - cylindrical protective shell, 4 - dielectric sleeve, 5 - dielectric washer.

Like the prototype, the probe includes a tip 1, a rod-holder 2 of the tip, placed in a cylindrical protective shell 3, and the rod-holder 2 is passed through a dielectric sleeve 4 placed inside the protective shell 3. The tip of the probe 1 is in direct contact with the plasma, and the holder rod 2 is used to transfer voltage to the tip 1 to take the probe characteristics, as well as to rigidly fix the tip 1 relative to the rest of the probe structure. Dielectric sleeve 4 is necessary for electrical isolation of the holder rod 2 and tip 1 from the sheath 3. The sheath 3 is designed to limit the area of contact with the plasma of the receiving surface of the tip 1, as well as to prevent contact with the plasma of other structural elements of the probe. The difference from the prototype consists in the presence of a dielectric washer 5, tightly put on the holder rod 2. The dielectric washer 5 serves to shield from the flow of vapors and metal ions most of the surface of the dielectric sleeve 4 between the holder rod 2 and the shell 1. The dielectric washer 5 has an external diameter d, defined by the ratio 0.8D≤d≤0.9D, and is located at a distance l from the plane of the dielectric sleeve 4, and the distance l is determined by the ratio 0.05L≤l≤0.1L, where D is the inner diameter of the protective shell 3, L is the distance from tip 1 to dielectric sleeve 4.

The device works as follows.

The probe is placed in the region where the gas-metal plasma is generated, while when voltage is applied to the holder rod 2 and, accordingly, the tip 1, the charged plasma particles enter the receiving surface of the tip 1. Some of the plasma particles penetrate into the gap between the tip 1 and the shell 3, and vapors and metal ions from among the particles that have penetrated inside are deposited on the holder rod 2, the inner surface of the shell 3, as well as on the dielectric washer 5 and on the narrow ring-shaped surface of the dielectric sleeve 4, not protected by the dielectric washer 5. It is due to the shielding with the dielectric the washer 5 of a part of the surface of the dielectric sleeve 4, which prevents the deposition of a conductive film between the shell 3 and the holding rod 2, a positive effect is achieved - an increase in the probe's MTBF.

The diameter d of the dielectric washer 5 is selected from the following considerations. At d <0.8D, the protection of the bushing surface by the washer becomes ineffective due to the small cross-section of the washer, which leads to the deposition of a conductive film on the bushing by metal vapors scattered on the gas. When d> 0.9D, the probability of shorting the gap between the washer and the shell increases as a result of the deposition of a conductive film on them, which again leads to an uncontrolled increase in the area of the receiving surface of the tip, and, hence, to probe failure.

The distance l, at which the washer is located from the plane of the dielectric sleeve, was chosen from the following considerations. At l <0.05L, the risk of the formation of an electrically conductive area between the adjoining surfaces of the washer and the sleeve increases, due to the condensation of metal vapors from the plasma on them. At l> 0.1L, the shielding efficiency decreases due to the freer penetration of metal vapors scattered by the gas into the space between the sleeve and the washer.

The results presented in the Table illustrate the positive effect inherent in the claimed technical solution, namely, an increase in the operation time of the probe to failure when diagnosing the gas-metal beam plasma generated by the electron-beam evaporation of a copper target in helium at a pressure of 10 Pa, at the use of the proposed technical solution in comparison with the prototype.

Table. Increase in the operating time of the probe when using the proposed technical solution in comparison with the prototype, with other parameters being the same for the experiment.

Helium pressure, Pa Evaporable target MTBF (i.e. until a short circuit occurs between the tip and the sheath), min Prototype ten Copper 28 Declared solution ten Copper 63

Claims (1)

A Langmuir probe containing a tip, a tip-holder rod, placed in a cylindrical protective shell, and the holder-rod is passed through a dielectric bushing placed inside the protective shell, characterized in that a dielectric washer with an outer diameter d determined by the ratio 0.8D≤d≤0.9D, located at a distance l from the plane of the dielectric sleeve, determined by the ratio 0.05L≤l≤0.1L, where D is the inner diameter of the protective shell, L is the distance from the tip.

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