RU2449513C1 - Vacuum-arc device - Google Patents

Abstract

FIELD: electricity.

SUBSTANCE: in vacuum-arc device, analog control unit, switching device and additional ballast resistances the value of which determines cathode clearance current and heating current respectively are included.

EFFECT: providing stable fault-free operation of extended structure plasma source, thickness uniformity of coating formed on treated products, accelerated cathode entering into operational mode.

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Description

The invention relates to the field of vacuum-plasma technology and can be used for coating in vacuum.

Among the most promising methods of surface treatment of materials in vacuum is a method that uses streams of metal plasma generated by a vacuum-arc discharge with an integrated cold cathode. This type of technological equipment provides high efficiency in obtaining ionized and high-speed flows of matter, opens up wide possibilities for controlling the technological process of plasma coating of various pure metals and for carrying out plasma-chemical synthesis of simple and complex compounds, reproducing alloys and producing complex combined coatings, as well as conducting ionic spraying the treated surface, and alloying the surface volume. The flow of the process in vacuum ensures both the purity of the resulting coatings and high adhesive properties on substrates with different physicochemical parameters.

Vacuum arc discharge is an independent discharge developing in the vapor of the cathode material. The emission center of the discharge is a cathode spot, characterized by small dimensions, in which the temperature reaches a boiling point in a very short time, which determines the intensive sputtering (destruction) of the cathode material, which ensures high efficiency of the generation processes in the discharge.

The technological vacuum-arc device fulfills its functional purpose only if the zone of the likely existence of the spot is the working surface of the cathode.

The control of cathode spots and increasing the reliability of their retention in a given erosion zone is one of the most pressing problems in the development of plasma sources with an integrated cold cathode.

In the known device [RF Patent No. 2072642, MKI H05H 1/50, C23C 14/35. Application No. 94021421/25 from 06/07/94. Vacuum-arc plasma source. Abramov I.S., Bystrov Yu.A., Lisenkov A.A. and other BI No. 3. 1997] presents the design of an extended plasma source intended for processing long or large products, operating in a pulsed mode and forming a directed tape stream.

The source consists of an extended cylindrical cathode, an arcing screen, arbitrarily located on the side of the current input, an anode and an extended magnetic system oriented along the cathode and mounted on the opposite side with respect to the generated plasma stream.

When working on the working surface of the cathode at the ignition electrode, cathode spots form, which in an external magnetic field move to the current input. When cathode spots fall into the gap between the cathode and the arcing screen, the vacuum-arc discharge is extinguished. The interval between igniting pulses is greater than or equal to the average lifetime of the cathode spots on the cathode surface.

However, at the initial moment of time, uncontrollable high-speed cathode spots appear on the cathode surface, which, despite the superimposed external magnetic field, carry out random movements along the cathode surface to clean it, including on its non-working part.

This mode of operation is emergency and leads to disruption of the installation and the contamination of the workpiece.

Closest to the claimed vacuum-arc device operating in a pulsed mode and forming a directed tape flow is the device presented in [RF Patent No. 2180472, MKI H05H 1/50, C23C 14/35. Application No.20001103758 / 25 of 02/07/2000. Vacuum-arc plasma source. Vetrov N.Z., Kuznetsov V.G., Lisenkov A.A. and other BI No. 7. 03/10/2002].

The design of the vacuum-arc device consists of an extended massive cylindrical cathode located inside a thin-walled cylindrical screen having a longitudinal window forming a working surface on the cathode from the ignition electrode to the arcing screen located at the current input. The anode function is performed by the wall of the vacuum chamber, on the external side of which there is an extended magnetic system oriented along the cathode. The arc discharge is supplied from a stabilized power source.

The value of the operating current I _slave is selected from the condition of stable burning of the discharge in the coating spraying mode.

In the process of work, it was revealed that at the initial moment of the evaporator operation, an independent arc discharge arises, which exists exclusively on contaminated sections of the cathode working surface. In this case, favorable conditions are created for the transition of cathode spots to an insulated screen, which leads to an emergency. The probability of this process is the higher, the higher the value of the working current of the discharge I _{slave is} selected.

After removing contaminants from the working surface of the cathode, an arc discharge occurs from the cathode material. In this case, in each subsequent pulse, at a constant power supplied to the cathode, the lifetime of the discharge on the cathode material from the moment of its excitation to extinction is determined by the type of cathode material and its initial temperature.

In the operating mode of the evaporator (coating spraying mode), a prolonged transitional mode is observed associated with the heating of the massive cathode body to the operating temperature. At this stage, the use of a fixed value of the discharge current I _slave leads to the fact that not all current pulses reach the arcing screen, which leads to the formation of a coating that is uneven in thickness.

The objective of the invention is the creation of a trouble-free vacuum-arc device of an extended design due to the reliable fixation of the cathode spots on the working surface of the cathode and the accelerated exit of the cathode into operation.

The solution of the problem will achieve the following technical results:

- increase the reliability of the plasma source of an extended design;

- increase the uniformity of thickness of the formed coating on the processed products.

The problem is solved due to the fact that in the vacuum arc device containing an extended cathode, an arc suppression screen, an ignition electrode, an anode, an extended magnetic system and a power source with ballast resistance that determines the discharge working current, an analog control unit is included in the electric arc supply circuit , a switching device and additional ballast resistances, the value of which respectively determines the cleaning current and the heating current of the cathode, while alternately connecting ballast resistances tions to arc power supply circuit is carried out analog control unit through the switching device when the arc discharge current pulses of predetermined duration in each mode of operation of the evaporator.

In the inventive device, the use at each operating mode of the evaporator of its own value of the discharge current allows you to:

- with a minimum value of the discharge current, ensure both the discharge retention on the working surface of the cathode and the effective cleaning of the working surface of the cathode from surface contaminants;

- at the maximum value of the discharge current, reduce the time to reach the working temperature of the cathode to ensure the operating mode of evaporation.

The essence of the invention is illustrated by drawings.

Figure 1. Design of a vacuum arc plasma source.

Figure 2. Operating modes of a vacuum-arc plasma source.

Figure 3. The movement of the cathode spot on the working surface.

The design of the vacuum-arc device (Fig. 1) consists of a water-cooled extended cylindrical cathode 1 located inside a thin-walled cylindrical screen 2 having a longitudinal window forming a working surface on the cathode 1 from the ignition electrode 3 to the arcing screen 4 located at the current input 5. The vacuum chamber 6 performs the functions of the anode, on the external side of which there is an extended magnetic system 7, oriented along the cathode 1. The workpiece 8 is located in the working volume, ennoe on an electrically insulating disc 9, which provides rotation motor 10. The source 11 provides a sputtering process on a workpiece 8 regulated negative voltage. The arc is powered from a direct current source 12, into the electric circuit of which an analog control unit 13 is included, which controls the switch 14, which ensures its alternate connection to the ballasts of R ₁ , R ₂ and R ₃ .

The resistance R ₁ (position I) ensures the flow in the electric circuit of the cleaning current I _Pts . The resistance R ₂ (position II) provides the current flow of the cathode heating I _times . The resistance R ₃ (position III) provides the flow of the working current I _slave , selected from the conditions of stability of the arc discharge and determined by the design of the cathode assembly, cathode material, the distance between the electrodes of the cathode and anode. The resistance values in this case are correlated as: R ₁ > R ₃ > R ₂ .

The analog control unit 13 consists of a recording device and a device for determining the duration of current pulses associated with the flow of I _times and I _Pts .

The principle of operation of the proposed pulsed plasma source of an extended design is presented in Figure 2. On igniting electrode 3 with a predetermined frequency and amplitude in a vaporizer U pulses supplied _by providing a stable arc initiation.

At the initial time, the switch 14 is in position I and the ballast resistance R _{1 is} included in the discharge electric circuit. When the ignition electrode 3 is triggered at time t ₀ on the working surface of the cathode 1 due to the presence of surface contaminants generated due to the adsorption of the main atoms of the contaminants - oxygen and carbon, a solenoid 7 uncontrolled by an external magnetic field randomly moves and very quickly covers the whole surface discharge.

Each material is characterized by thermophysical properties and its value of the minimum critical current I _crit , capable of supporting discharge. The area of the cathode spot, being the concentration of heat, in which the temperature reaches a boiling point in a very short time, determines the intense evaporation of the cathode material.

The magnitude of the arc discharge current determines the number of spots simultaneously existing on the working surface of the cathode. On thin-film coatings of contaminants, the current that closes at each cathode spot is only a few amperes.

Surface impurities and defects, even at low concentrations, have a significant effect on the thermionic properties of metals and lead to a noticeable spread in the work function (more than 0.1 eV). These conditions greatly facilitate the maintenance of a developing discharge that can exist at a lower value of the discharge current than the main arc discharge from the cathode material. Therefore, when conducting the cleaning mode of the working surface of the cathode at the initial moment of time, the discharge current I _Pc , determined by the resistance value R ₁ , is selected significantly less than the current value that is minimally critical for the cathode material I _{crit crit} .

The removal of surface contaminants, at which there is an arc discharge, occurs in the time interval t ₀ -t ₁ at a current I _Pts = (0.2 ... 0.4) I _slave . In subsequent attempts to excite the discharge, at a selected current value of I _Pts , the discharge does not occur on the working surface of the cathode. The analog device 13, without fixing the flow of the discharge current in the circuit, registers short pulses generated by the ignition electrode 3, and, controlling the switch 14, provides switching the power circuit to position II (Figure 1), connecting the ballast resistance R ₂ .

Simultaneously with the switching of the power circuit, a single pulse is generated of the transition of the analog device to the second element of the switch control unit, which is responsible for the timely completion of the cathode cleaning process.

Subsequently, an arc discharge exists only in the vapors of the cathode material, and to maintain it, it is necessary to increase the discharge current to a value of at least I _crit .

и магнитном

полях смещается в сторону максимума индукции магнитного поля

, учитывающего влияние магнитного поля

, создаваемого током, протекающим в плазме

, и магнитным полем

, создаваемым током, протекающим по катоду

, и представляет собой точечный источник тепла, нагревающий катод вдоль пути своего перемещения. Тепло распространяется, в основном, в направлении, перпендикулярном оси перемещения.3 is a moving character of the cathode spot on the working surface of the water-cooled cathode 1 _Cat diameter 2R and the inner diameter of the cooling system 2R _OHL. Cathode spot subject to movement in crossed electric

and magnetic

fields is shifted towards the maximum magnetic field induction

taking into account the influence of the magnetic field

created by the current flowing in the plasma

, and magnetic field

generated by current flowing along the cathode

, and represents a point source of heat, heating the cathode along its path of movement. Heat is distributed mainly in the direction perpendicular to the axis of movement.

The cathode spot, as a heat source of influence on the cathode surface, is modeled in the form of a circle of radius R, within which the heat flux q supplied from the discharge and interacting with the surface is constant for any moment of time, and outside it is equal to zero.

The cathode with respect to the effective size of the heat source is a semi-infinite body, therefore, at an infinite distance from the surface, the heat flux is zero, and the temperature is constant. In the initial state, the cathode surface temperature is the same at all points T _{t = 0} = T ₀ .

To maintain the stability of the discharge burning, the power supplied directly to the evaporating surface is expended on heating the cathode in the cathode spot to the temperature necessary to reproduce the required amount of vaporized material per unit time. The time taken to warm up the cathode to the required temperature is determined by the initial temperature of the cathode and the power supplied from the discharge. Thus, the speed of movement of the cathode spots will be determined by the temperature of the cathode and the speed of movement of the heat flux in front of the spot.

The continuous movement of the cathode spots from the ignition electrode 3 towards the current input 5 along the limited working surface of the cathode 7, regardless of the initial distribution, leads to the establishment of a certain average equilibrium temperature determined by the geometrical dimensions of the cathode and its cooling conditions (Figure 3).

When moving, the cathode spot at all subsequent moments of time moves to a less heated portion of the cathode, therefore its speed is determined by the level of input power and the time it takes to reach the operating temperature in the new cathode cell. The time taken to warm up the surface depends on the initial temperature at the location.

An increase in the cathode temperature during operation leads to a decrease in the formation time of the main arc discharge in subsequent pulses.

Figure 2 shows that the first current pulses in the mode of heating the cathode have a long duration τ, which is explained by the movement of the cathode spots on an unheated working surface.

As the cathode is heated, the duration of the current pulses decreases τ ₁ > τ ₂ > τ _n = τ _them . The analog device 13 compares the duration of the current pulses τ _them with a reference pulse duration τ _gene generated by the clock generator.

Upon reaching the current pulse of a given duration τ _them = τ _gene , determined by the travel time from the ignition electrode 3 to the extinguishing screen 4, the analog device 13, controlling the switch 14, switches the power circuit to position III (Figure 1), connecting the ballast resistance R ₃ .

To reduce the time interval t ₁ -t ₂ , during which the cathode body is heated up and its operating mode is reached, the discharge current value is selected from the condition I _times = (1.5 ... 2.0) I _slave .

After establishing the working temperature on the cathode in the period of time t ₂ -t ₃ the coating is sprayed (Figure 2) at a current I _slave , determined by the design of the arc discharge, the distance of the cathode-substrate and the number of processed products.

An example of the implementation of the proposed vacuum-arc plasma source with an extended cylindrical cathode is a technological device with a cathode length of 450 mm, operating in a pulsed mode and used for coating zirconium carbide on grid electrodes of powerful generator lamps up to 500 mm high. The magnitude of the discharge current I _slave = 200 A. Selected operating modes: cleaning mode (position I, Figure 1) cleaning current I _och = 50 A; cathode heating mode (position II, FIG. 1) I _times = 300 A; spraying mode (position III, Figure 1) I _slave = 200 A.

The developed vacuum-arc device with the specified operating modes ensures its stable trouble-free operation.

Claims (1)

A vacuum-arc device containing an extended cathode, an extinguishing screen, an ignition electrode, an anode, an extended magnetic system, a power supply with ballast resistance, which determines the discharge working current, characterized in that an analog control unit, a switching device, and additional ballast resistances, the value of which respectively determines the current for cleaning and heating the cathode, while alternately connecting the ballast resistances to the arc supply circuit It is activated by an analog control unit through a switching device when the current pulses reach the arc discharge of a given duration in each mode of operation of the evaporator.

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