Method of low-temperature plasma generation, method of an electrically conductive or ferromagnetic tube coating using pulsed plasma and corresponding devices

Abstract

The present invention resides in the unifying idea of synchronizing a positive voltage pulse supplied to an electrically conductive or ferromagnetic tube and a exciting negative voltage pulse on a hollow cathode induced on the background of a high-frequency capacitive discharge. In one embodiment, the invention relates to a method of generating low-temperature plasma in a vacuum chamber comprising a hollow cathode and an electrode, the method comprising the step of igniting the pulsed DC discharge in the hollow cathode wherein the positive voltage pulse at least partially overlaps with the negative voltage pulse, and the positive voltage pulse at least partially overlaps with the negative voltage pulse on the hollow cathode. In another embodiment, the present invention relates to a method of coating the inner walls of hollow tubes which utilizes the above-mentioned low-temperature plasma generation process. In another embodiment, the invention relates to a low-temperature plasma generating device comprising a hollow cathode located in the vacuum chamber, a RF plasma source, a pulse DC burst source, and a bipolar pulse source. In another embodiment, an object of the invention is an apparatus adapted to coat the inner sides of hollow tubes comprising a low-temperature plasma generating device.

Claims

1. A device for generating a pulsed discharge low-temperature plasma comprising a DC pulse source connected in parallel to a RF source; an electrode and a hollow cathode placed in a vacuum chamber, a bipolar source synchronized with the DC pulse source via a control pulses generator, wherein the electrode is connected to the bipolar source, wherein the bipolar source is configured to apply a positive voltage pulse on the electrode; and the hollow cathode is connected to the RF source and the DC pulse source, wherein DC pulse source is configured to initiate a pulse discharge on a background of RF capacity discharge; and wherein the device is configured to set the positive voltage pulse on the electrode in at least in part overlap with a negative voltage pulse applied on the hollow cathode.

2. A device suitable for coating an inner surface of an electrically conductive or ferromagnetic hollow tube by a thin film via low-temperature plasma comprising the device according to claim 1, wherein, the hollow cathode is a coated tube situated in a vacuum chamber, the electrode is equipped with a hollow cathode at its end; and the hollow cathode and the coated tube are electrically insulated.

3. The device according to claim 2, wherein the coated tube is attached to a cooling.

4. The device according to claim 3, wherein the cooling is a water cooling.

5. The device according to claim 4, wherein the water cooling is attached to a bellows, wherein the bellows are electrically insulated from sides of the vacuum chamber.

6. The device according to claim 4, wherein the device further comprises at least one oscillography probe connected to a digital oscilloscope and the electrode and/or the water cooling.

7. The device according to claim 4, wherein the water cooling is connected to a low-frequency filter via measuring resistance, wherein the low-frequency filter is consisting of inductor and capacitor with the bipolar pulse source.

8. The device according to claim 2, wherein the vacuum chamber further comprises at least one opening for entrance gas flow.

9. The device according to claim 2, wherein the electrode is equipped with a water cooling.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) Specific embodiments of the methods and devices of the invention are schematically shown in the accompanying drawings, wherein:

(2) FIG. 1 represents an example of the time overlap of pulsed voltages and currents and their mutual phases of a plasma source with a hollow cathode and an electrode.

(3) FIG. 2 represents an example of a device for generating pulsed low-temperature plasma.

(4) FIG. 3 represents the preferred embodiment of the example of the time behaviour of the pulse voltages and currents and their mutual phase applied on a plasma source having a hollow cathode and an electrically conductive substrate in the shape of a long tube.

(5) FIG. 4 represents a schematic diagram of the preferred embodiment of a device suitable for pulsed plasma deposition of an adhesive film on the inner surface of an electrically conductive or ferromagnetic tube of any length.

(6) Figures illustrating the schematics of the device according to the invention and demonstrating the effects of its use and the following examples of particular embodiments of the device do not in any way limit the scope of protection set forth in the claims but merely illustrate the essence of the invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

(7) Generating Low-Temperature Plasma: A Method and a Device

(8) In the example carried out according to the first embodiment of the invention, low-temperature plasma 11 is generated in the vacuum chamber 1. The vacuum chamber 1 comprises a hollow cathode 14 and an electrode 21E. The time sequence of the applied voltage U.sub.E+ and U.sub.C− and the passing currents through the electrode 21E and the hollow cathode 14 is shown in FIG. 1.

(9) In the first step of the low-temperature plasma generation process 11, the RF frequency capacitive RF voltage generated by the RF source 16 is supplied to the hollow cathode 14. In the second step, a positive voltage pulse U.sub.E+, from a bipolar voltage source 20, is supplied to the electrode 21E. After a certain time .sub.T, but at the same time as the positive voltage pulse U.sub.E+ is still present on the electrode 21E, a negative voltage pulse U.sub.C is supplied to the hollow cathode 14 by a DC source 18 which generates the low-temperature plasma 11.

(10) During pulse time-overlap, the potential difference between the hollow cathode 14 and the electrode 21E is generated. Within the experiment, a 1 kV potential difference was achieved, with 400V being the difference between the positive voltage pulse and the ground and 600V between the negative voltage pulse and the ground. The potential difference allowed a fast (in units of μs) and reliable ignition of a high ionization discharge between the hollow cathode 14 and the coated tube 21 for this short time (10-50 μs) to form transient high ion- and electron-10.sup.12 cm.sup.−3) and high ionization (30-60%) of deposition particles without arcing.

(11) Preferably, a negative voltage pulse U.sub.E− is added to the electrode 21E after the positive voltage pulse U.sub.E+ is terminated, which contributes to the bombarding of the surface of the electrode by accelerated positive ions. (The preferred embodiment is to some extent shown in FIG. 3, where U.sub.E+=U.sub.+, followed by U−)

(12) An example of a device according to the third embodiment of the invention is shown in FIG. 2. The low-temperature plasma generating device comprises a metal vacuum chamber 1 which is preferably separated by a plate valve 2 and pumped by a vacuum pump 3. In the interior of the vacuum chamber 101, the electrode 13 is placed through a dielectric bushing 5 whereby this electrode 13 is preferably provided with water cooling 6. The working gas 4 is supplied to the electrode 13. The electrode 13 is electrically connected to the RFc power source 15, preferably via the separating capacitor C.sub.m, and connected in parallel to the DC pulse source 18, preferably through the stabilizing and measuring resistor R.sub.c on which the voltage U.sub.c is measured by a pair of voltage oscilloscopic probes 16 and a LC cell formed by the inductor L.sub.c and the capacitor C.sub.c to the DC pulse source 18 which is controlled by the pulse from the pulse generator 17. In a further preferred embodiment, a pair of oscilloscopic probes 16 is connected to a digital oscilloscope 19. On the digital oscilloscope 19, it is possible to display or store data in digital form, wherein the data may be time course of the U.sub.c supplied on the hollow cathode 14 relative to the ground walls of the vacuum chamber 1 and further the time course of the voltage U.sub.zs, by which, it is possible to calculate the flow of electric current I.sub.c on the hollow cathode 14 according to the relationship

(13) $\begin{array}{cc}{I}_c=\frac{U_{\mathrm{zs}}-U_c}{R_c}.& \left(I\right)\end{array}$

(14) A hollow cathode 14 is connected at the end of the electrode 13, wherein the inner wall of the hollow cathode 14 is sputtered by a DC pulse discharge 11. The discharge in the hollow cathode 14 closes in the initial phase of the working pulse across the electrode 21E.

(15) The initial phase is the time when a positive voltage pulse from the bipolar pulse source 20 is applied to the coated electrically conductive tube 21. In the next working phase of the working pulse, the coating is secured via the RF plasma 7 present in the electrode 21E.

(16) Coating of Electrically Conductive and/or Ferromagnetic Low-Temperature Plasma Tubes: Method and Equipment

(17) FIG. 3 shows the time courses of the currents and voltages in the periphery of the hollow cathode 14 and the conductive coating tube 21. The embodiment of the device according to the fourth embodiment of the invention is shown on FIG. 4.

(18) In the example carried out according to the second embodiment of the invention, stable plasma formed according to the method described in the example above is utilized. In this example, the electrode 21E is an electrically conductive tube 21 having an internal diameter of 9 mm and a length of 200 mm. In another embodiment, the method can also be used for ferromagnetic tubes.

(19) In the first step of the method of generating low-temperature plasma 7, 11, an alternating voltage RF is generated on the hollow cathode which is generated by the RF source 16. In the second step, a positive voltage pulse U+ is supplied to the coated tube 21 from a bipolar voltage source 20. After a certain time .sub.T but simultaneously at the time the positive voltage pulse U+ is still present on the coated tube 21, a negative voltage pulse U.sub.C is supplied to the hollow cathode 14 with a DC source 18 which generates the low-temperature plasma 11.

(20) Preferably, a negative voltage pulse U− is applied to the coating tube 21 after the positive voltage pulse U+ is terminated.

(21) In another preferred embodiment, the coating tube 21 is cooled by means of water cooling 6.

(22) More preferably, the plasma 7, resp. 11, is stabilized by means of the stabilizing measuring resistor R.sub.c and further through the LC a cell formed by the inductor L.sub.c and the capacitor C.sub.c.

(23) More preferably, another working gas is supplied into the inner part 101 of the chamber 1 which is not led through the hollow cathode 14 into the chamber 1. The advantage is that the additional working gas does not contaminate the inner surface of the hollow cathode 14 due to the flowing gas through the internal volume of the hollow cathode 14.

(24) More preferably, the hollow cathode 14 is cooled.

(25) An example of a device according to a fourth embodiment of the invention is shown in FIG. 4. The device suitable for coating the inner surface of hollow electrically conductive or ferromagnetic tubes 21 by means of a low-temperature plasma 11 is a metal vacuum chamber 1 which is preferably separated by a plate valve 2 and pumped by a vacuum pump 3. In the interior space 101 of the vacuum chamber 1, an electrode 13 is located, for example through a dielectric bushing 5, whereby this electrode 13 is preferably equipped with water cooling 6. Through the electrode 13, a working gas 4 is fed into the interior space 101 of the vacuum chamber. The electrode 13 is electrically connected to the RFc power source 15 via the decoupling capacitor C.sub.m and connected in parallel to the DC pulse source 18, preferably via the stabilizing and measuring resistor R.sub.c, on which voltage U.sub.c is measured by two voltage oscilloscope probes 16, with an LC cell formed by the inductor L.sub.c and a capacitor C.sub.c, the DC pulse source 18 being controlled by the control pulse generator 17. At the end of the electrode 13 the hollow cathode 14 is connected, the inner wall of the hollow cathode 14 is sputtered by a DC pulse discharge 11. The discharge in the hollow cathode 14 closes in the initial phase of the working pulse over the coated electrically conductive or ferromagnetic tube 21 such that the sputtered particles in the plasma 11 reach the surface of the inner wall of the coated tube 21. The initial phase is the time when a positive voltage pulse from the bipolar pulse source 20 is applied to the coated electrically conductive tube 21. In the next working phase of the working pulse, the coating is secured via the RF plasma 7 which is present in the coated tube 21. The hollow cathode 14, together with the carrier electrode 13, is covered by an electrically conductive tube 12 due to the electrical insulation of the hollow cathode 14 and the coated conductive tube 21.

(26) Preferably, the coated tube 21 is in both thermal and electrical contact with the metallic tube cooling 9, preferably water, acting as an electrode, which is attached to the movable bellows 10 provided with a linear movement through the dielectric bushings 5. The bellows 10 is from the walls of the grounded vacuum chamber 1 electrically insulated.

(27) More preferably, the oscilloscope probes 16 are coupled to the digital oscilloscope 19. On the digital oscilloscope 19 it is then possible to display or store in digital form the time course of the U.sub.c voltage on the hollow cathode 14 relative to the ground walls of the vacuum chamber 1 and, it is possible to calculate the course of the electric current Ic by the hollow cathode 14 according to the relationship

(28) $\begin{array}{cc}{I}_c=\frac{U_{zc}-U_c}{R_c}.& \left(I\right)\end{array}$

(29) More preferably, the bipolar pulse source 20 is connected to the condenser 9 via an LC filter formed by the C.sub.sp capacitor and the inductor Ls and further to the measuring resistor R.sub.s to which oscilloscopic voltage probes 16 are connected. The flow of the electric current I.sub.s on the coated tube 21 and can be obtained from the measured during the stresses U.sub.s and U.sub.zs according to:

(30) $\begin{array}{cc}{I}_s=\frac{U_{\mathrm{zs}}-U_s}{R_s}.& \left(I\right)\end{array}$

(31) More preferably, the opening 4′ of the vacuum chamber 1 works as the inlet of further working gas.

INDUSTRIAL APPLICABILITY

(32) The methods and devices according to the invention can be used for the industrial coating of 3D objects, in particular the inner surfaces of electrically conductive tubes with hardly accessible inner coated surface. In a new method and apparatus according to the invention it is possible to coat the internal surfaces of the ferromagnetic tubes.