AND APPARATUS

Field of the invention

The invention relates to a plasma technology for deposition of coatings onto metal and dielectric substrates, in particular, a plasma polymerization technology and equipment utilized for the given type of technologies.

Background of the invention

Various plasma polymerization methods and apparatuses are currently known and used for performing polymerization processes.

The European application EP0152256A2 (IPC G02 B 1/10, published on 21.08.1985) describes a method for plasma deposition of polymer coatings onto optical products and an apparatus for performing the same. The problem to be solved involves formation of a polymer coating on a surface of an optical product (lenses). A protective coating to be deposited should prevent product surfaces from being stained with oils or fats. The known method comprises the steps of igniting and sustaining a high-frequency discharge in a mixture of oxygen and hydrocarbons, with aliphatic hydrocarbons being preferably used as hydrocarbons. A substrate is placed into discharge plasma, and a polymer coating of predetermined properties is deposited from a discharge volume onto said substrate. The apparatus for implementing this method comprises a vacuum chamber with a gas evacuation system, a system for introducing a reaction gas into the vacuum chamber, a gas-discharge device with two electrodes, each connected to a high-voltage source, and a holder with a sample to be processed, said holder being located between the discharge electrodes.

The patent US4693799 (IPC C07C 3/24, published on 15.09.1987) discloses a method of depositing a polymer coating onto a film movable between electrodes through a discharge volume, wherein a plasma is generated using a high-frequency discharge device. The discharge volume is filled with a mixture of hydrocarbons or a mixture of organometallic compounds. Optimum time characteristics of a discharge are chosen depending on a reaction gas to be utilized. The resulted coating formed on the film has a low friction coefficient, a high durability and strength, and the film proper does not necessitate additional lubrication during usage thereof. An apparatus for performing the method comprises a vacuum chamber with evacuation means and means for filling the chamber with a reaction gas. A film to be processed is moved by a film-traction mechanism between the electrodes connected to a pulsed high-frequency voltage source. Another method for plasma deposition of coatings (the published international application WO91/12092 (IPC B05D 3/06, 3/14, 3/00, 3/02, published on 22.08.1991), in particular, for depositing a protective corrosion-resisting coating onto a steel substrate uses a low-temperature plasma of gaseous hydrocarbons. The plasma is generated by igniting a direct current discharge between the discharge electrodes. The steel substrate under process functions as a cathode in the gas-discharge device. Anodes are located around the cathode and fitted with a magnetic system which generates a shielding magnetic field above the surfaces thereof. Magnetrons are utilized as magnetomotive force sources. A plasma-polimerizable working gas is introduced along with an inert gas into the vacuum chamber at a substrate pretreatment stage and in the process of depositing an organosilane film onto the substrate under process.

Also known is a method for plasma deposition of polymer coatings, including the steps of filling a discharge chamber with a reaction gas containing at least one plasma- polymerizable monomer gas, igniting and sustaining a pulsed-periodic gas discharge with a repetitive pulse-train (Patent RU 2190484, IPC B05D 3/06, published on 10.10.202). In the process of plasma polymerization, a polymer coating is deposited onto a surface of a manufacturable sample which is moved through a discharge volume. In particular, an aluminum foil was employed as a manufacturable sample and it was connected to a pulsed power supply unit to function as an anode. An array consisting of hollow cathodes was positioned over the aluminum foil. In order to hold the electrons in the discharge volume, an external magnetic system was used for creating in the vacuum chamber cavity of a non- homogeneous stationary magnetic field declining towards the center of the discharge chamber.

An apparatus for performing the polymer coating deposition method comprises a vacuum chamber with a gas evacuation system, a system for introducing a reaction gas containing acetylene and nitrogen, and a gas-discharge device enabling ignition and sustaining of a pulsed-periodic gas discharge. Discharge electrodes of the apparatus are connected via a matching system to a pulsed-periodic high-frequency signal source.

In a method of forming a polymer coating, described in European application EP0002889A2 (IPC C08F 2/52, published on 1 1.07.1979), a plasma is generated by igniting a glow discharge. A metal substrate onto which a polymer coating is deposited functions as a passive electrode of a capacitive high-frequency discharge. An oppositely arranged discharge electrode is connected to a high-frequency voltage source. The known method utilizes a fluorocarbon gas as a reaction monomer gas. The closest analog to the present invention is a method for plasma deposition of polymer coatings and an apparatus for performing the same, which are disclosed in an international application W099/27156 (IPC C23C 16/44, published on 03.06.1999). The prototype method involves plasma polymerization of a coating deposited onto a metal surface of a sample under process. The deposition process is provided by generating a plasma using a gas-discharge device enabling the ignition and sustaining of a discharge under a direct current mode and a pulsed mode.

A metal sample to be processed is located on an anode, a discharge chamber is evacuated to a predetermined vacuum extent and a mixed reaction gas is introduced into the chamber until a predetermined pressure is established in the chamber. The said reaction mixed gas contains a non-saturated aliphatic hydrocarbon monomer-gas (acetylene) or a fluorine- containing monomer gas and a non-polymerizable gas (nitrogen). A partial pressure of the non-polymerizable gas is chosen within a range of from 50% to 90% of the total pressure of the mixed gas. A suitable voltage is applied to the electrodes in order to enable the ignition of an electric discharge to thereby generate plasma with positive and negative ions and radicals of non-saturated aliphatic hydrocarbons and non-polymerizable gas. In the process of plasma polymerization, a polymer coating is formed on the surface of the anode and the manufacturable sample, said polymer coating having hydrophilic or hydrophobic properties depending on the selected mixed reaction gas and parameters of the discharge. In case a high- frequency discharge is used, a polymer coating can be deposited onto ceramic or polymer samples.

The apparatus for performing the method for plasma deposition of polymer coatings comprises a vacuum chamber with a gas evacuation system including a rotor pump and a diffusion pump. Acetylene and nitrogen introducing pipelines are connected to the vacuum chamber. A gas-discharge device comprises a central electrode for locating a manufacturable sample thereon, and peripheral grounded electrodes. The central electrode functioning as an anode is connected to a positive pole of a power supply unit. The peripheral electrodes function as cathodes and define a discharge volume.

The known prototype method allows coatings to be produced which provide an increased adhesion to a paint to be subsequently applied and a high corrosion resistance. However, on realizing the method, the polymer coatings are deposited at an insufficient deposition rate. Furthermore, in the process of depositing a coating from a mixed reaction gas containing acetylene and nitrogen, considerably large-sized dust macro particles are produced because of polymer agglomeration. When deposited on the sample, these macro particles deteriorate the properties of the coating deposited.

In the process of implementing the above-described prototype methods, fast-moving electrons are generated in cathode regions of a direct current discharge or in electrode regions of a high-frequency discharge, said fast-moving electrons effectively dissociating the basic gases and enabling formation of monomer radicals, positive and negative ions. In the electrode regions a process is initiated for aggregation of monomers into chains. Polymeric chains, monomers, atoms and ions of non-polymerizable gas formed in the electrode regions diffuse into the volume of the plasma filling a discharge space, including the region where manufacturable samples are located.

It was ascertained by experimental investigations that in the process of plasma deposition of polymer coatings onto the sample functioning as an anode, a coating deposition rate was directly proportional to the current supplied to the sample. Thus, the coating deposition rate directly depended on the rate at which charged particles were transferred to the manufacturable sample. The structure of the coating to be formed and the properties of the polymer coating depended on the concentration of monomers in the discharge volume and the polymerization rate thereof.

At the same time it is common knowledge that on burning of the glow discharge, an increase in a discharge current, that is, an increase in the current supplied to the manufacturable sample is accompanied by an increase in a cathode potential drop value. In this case the concentration and energy of electrons in the cathode region of the discharge is increased thereby stipulating a growth of monomer concentration in the discharge volume and an increase in the polymerization rate. As a consequence, a plasma polymerization process performed using known methods does not allow the monomer concentration and the polymerization rate to be independently adjusted in the discharge volume for the purpose of controlling the structure and properties of the deposited coating, and the polymer coating deposition rate to be simultaneously controlled.

Disclosure of the invention

The present invention is based on the solution of technical problems connected with implementing a continuous process for a high-speed plasma deposition of polymer coatings and enabling an independent controlling of a polymer coating deposition rate as well as coating structure and properties. The solution of the indicated technical problems allows the efficiency of the process to be increased due to an independent controlling of a coating deposition rate and properties of a polymer coating to be deposited in accordance with predetermined requirements.

The technical results are achieved by implementing a method for plasma deposition of polymer coatings, involving the following steps: locating a manufacturable sample in a vacuum chamber, filling the vacuum chamber with a reaction gas containing at least one plasma-polymerizable monomer gas, generating a plasma and depositing a polymer coating onto the surface of the manufacturable sample during plasma polymerization.

According to the present invention, a plasma is generated by igniting and sustaining a two-stage glow discharge in two spatially separated discharge volumes. The first discharge volume is separated from the second discharge volume by a perforated electrode having aperture sizes (cross-sections) greater than 0.1 mm. The indicated minimum size of the perforated electrode apertures corresponds to a Debye radius (Debye shielding radius) rated for parameters of the glow discharge plasma generated in the first discharge volume. The manufacturable sample is located on the electrode which is then placed in the second discharge volume in a position opposite the perforated electrode. Thereafter, a positive or negative shift potential relative to the perforated electrode is supplied to the electrode functioning as a sample holder.

The technical results are reached through the occurrence of the following physicochemical processes and phenomena.

A method enabling plasma deposition of polymer coatings having properties adjustable during a treatment process is based on the usage of a two-stage glow discharge for plasma generation. A first discharge stage provides dissociation of primary gases contained in the mixed reaction gas, formation of monomers and polymerization thereof. A second discharge stage provides the creation of a directed flow of reactive particles and polymer chains forming a polymer coating on the surface of a manufacturable sample.

The glow discharge is ignited and sustained in the first discharge volume by means of a gas-discharge device. Changing of the gas-discharge device parameters, including an electric power applied to the electrodes, a partial pressure of the reaction gas components, geometrical sizes of the electrodes, and interelectrode space sizes, causes changes in a reaction gas dissociation rate, monomer concentration and a length of polymer chains formed in the first discharge volume. The adjustment of polymer coating properties in a specific process can be provided by varying the voltage applied to the discharge electrodes, and/or by varying the distance between the perforated electrode and the electrode functioning as the holder for the manufacturable sample.

The employment of the perforated electrode for separating the first discharge volume from the second discharge volume enables an independent controlling of the deposition rate and the coating properties. For this purpose, the size of the apertures in the perforated electrode should be greater than a Debye radius which is rated according to the parameters of the plasma generated in the first discharge volume, and should be of the order of 0.1 mm for the average parameters of the glow discharge. At a selected distance, the electric field of the charged particles generated in the interelectrode space is shielded in the glow discharge plasma to allow the reactive particles (monomer radicals, electrons and ions) and the primary polymer chains to diffuse freely from the first discharge volume into the second discharge volume wherein the manufacturable sample is located. Polymer coatings are produced on the surface of the manufacturable sample due to the interaction between the reactive particles formed in the first discharge volume with the surface of the sample located in the second discharge volume.

With changes in the potential supplied to the sample located on the electrode there occur changes in the particle flux intensity and, accordingly, in the polymer coating deposition rate. With that, the properties of a polymer coating to be deposited and its structure will be delimited by specific features of the polymer chains formed in the first discharge volume, i.e., it will be dependent on the processes occurring at the first stage of the gas-discharge device.

An inductive high-frequency discharge can be used in the first discharge volume. In this case the concentration of monomers and primary polymer chains in the first discharge volume is controlled by varying the power supplied to the plasma through an inductor. The polymer coating deposition rate is independently controlled by varying the potential of the electrode carrying the manufacturable sample relative to the perforated electrode and accordingly relative to the glow discharge plasma.

A capacitive high-frequency discharge can be ignited and sustained in the first discharge volume between the discharge electrodes, one of said electrodes being the perforated electrode. In the given variant of embodiment of the method, the adjustment of polymer coating properties and the coating deposition rate is carried-out in the similar manner.

It is also possible to ignite and sustain a direct current glow discharge in the first discharge volume between the discharge electrodes, the perforated electrode being one of the said discharge electrodes. The high-frequency discharge or direct-current discharge formed as a pulsed discharge can be used in the first discharge volume. It has been known that on transition from a discharge space breakdown stage to a stationary discharge formation stage, the electron concentration increases during a short time period by a several orders of magnitude. Average electron energy is considerably higher at a discharge transient stage than at a stationary discharge mode. It is also known that molecule excitation, ionization and dissociation processes are essentially dependent on an average electron energy, so the short-pulse processes at the discharge transient stages are more efficient as compared to the stationary discharge mode. An increase in the dissociation rate results on its turn in an increased concentration of monomer radicals in the discharge volume. As a consequence, the polymerization rate is increased.

A selected discharge voltage pulse time should be not less than the time needed for achieving a maximum radical monomer formation rate. Optimum discharge pulse time values range between 10 μ≤ and 100 ms.

Upon stopping electrical power the lifetime of radicals in the discharge volume is substantially greater than the lifetime of electrons, so the discharge pulse interval should be greater than the time during which the electrons drift onto the chamber wall, but smaller than the lifetime of the radicals in the discharge volume. The discharge pulse interval is preferably chosen between 10 μ≤ and 100 ms.

Depending on the reaction gas composition and predetermined properties of the polymer coating, the shift potential supplied to the electrode functioning as the holder for the manufacturable sample may be of positive or negative polarity. The shift potential is generally adjusted within a range of from 50V to 500V. The rate of plasma deposition onto the manufacturable sample for forming the polymer coating can be also adjusted by changing the position of the electrode located in the second discharge volume relative to the perforated electrode. In such a case, the deposition rate of reactive particles onto the surface of the s manufacturable ample depends on the distance between the electrodes, said distance being adjusted during the plasma polymerization process.

The technical results are also reached through the usage of an apparatus for plasma deposition of polymer coatings, which comprises a vacuum chamber, a gas-discharge device, a system for supplying electric power to the gas-discharge device, a holder for a sample to be processed, a system for evacuating a gas from the vacuum chamber cavity, and a system for introducing a reaction gas into the vacuum chamber. According to the present invention, the gas-discharge device is made two-stage and consists of two spatially separated discharge volumes with a perforated electrode between them. The said perforated electrode have aperture sizes not less than 0.1 mm. The indicated minimum size of the apertures in the perforated electrode is determined according to a Debye radius value rated according to the characteristics of plasma of the glow discharge which is ignited and sustained in the first discharge volume. There is an electrode in the second discharge volume, which is positioned opposite the perforated electrode and adapted to function as the holder for the manufacturable sample. The system for supplying electric power to the gas-discharge device involves a voltage source to which are connected the perforated electrode and the electrode functioning as the holder for the manufacturable sample.

The glow discharge can be generated in the first discharge volume with the use of an inductive type high-frequency discharge generator. In another variant of embodiment, a capacitive type high-frequency discharge generator functions as the first stage of the gas- discharge device, said generator including two discharge electrodes, the perforated electrode being one of said discharge electrodes. For supplying electric power to the gas-discharge device, a voltage source can be used which is capable of generating voltage pulses for the purpose of igniting and sustaining a pulsed high-frequency discharge in the first discharge volume.

The discharge can be generated in the first discharge volume by means of a direct- current glow discharge generator comprising two discharge electrodes, the perforated electrode being used as one of said electrodes. The discharge generator can be of the type to operate in a pulsed mode of operation.

Brief description of drawings

The invention is further exemplified by a description of a specific example of embodiment with reference to an accompanying drawing (see Fig. 1) illustrating a diagram of an apparatus for plasma deposition of polymer coatings.

Preferable example of embodiment of the invention

As an example of embodiment of the invention there is a description of a method for plasma deposition of polymer coatings and an apparatus for implementing the method, using a capacitive type high-frequency discharge generator operating in a pulsed mode in the first discharge volume. The apparatus utilized in the process for plasma deposition of polymer coatings is a part of a plasma chemical reactor and comprises a vacuum chamber 1 , a gas evacuation system 2, a branch pipe 3 of a reaction gas introducing system, a gas-discharge device, and a system for supplying electric power to the gas-discharge device. The gas-discharge device is made two- stage and consists of two spatially separated discharge volumes. In the first discharge volume there is a discharge electrode 4 and a perforated electrode 5 formed as a steel wire screen with a screen transmittance of 90%. In the example of embodiment under investigation, the perforated electrode 5 is under the ground potential. A minimum cross-sectional size of apertures 6 provided through the perforated electrode 5 is 2 mm, i.e., it is much greater than a Debye radius rated for a glow discharge plasma and making 0.1 mm. The perforated electrode 5 separates the first discharge volume from the second discharge volume, the latter being disposed between the rear side of the perforated electrode 5 and an electrode 7 with a manufacturable sample 8 located thereon.

An interelectrode space in the first discharge volume between the discharge electrode 4 and the perforated electrode 5 is 12cm and it can be adjusted (before starting a process) within a range of from 10cm to 15cm. The interelectrode space in the second discharge volume between the perforated electrode 5 and the electrode 7 functioning as the holder for the manufacturable sample 8 is 3 cm and it can be adjusted during the treatment process within a range of from 1cm to 5cm by using a displacement mechanism for displacing the electrode 7 (not shown in the drawing).

The system for supplying electrical power to the gas-discharge device includes a high- frequency pulse source (HPS) 9 to which source the discharge electrode 4 is connected, and a shift voltage source (SVS) 10. The perforated electrode 5 is grounded and the electrode 7 functioning as the holder for the manufacturable sample 8 is connected to a positive pole of the shift voltage source 10.

The method for plasma deposition of polymer coatings is performed with the employment of the above-described apparatus in the following manner.

Before the plasma deposition process is started, the cavity of the vacuum chamber 1 is evacuated using the gas evacuation system 2 for evacuating the gases to the residual pressure level of about ~10 ^"3 Pa. The first discharge volume is thereafter filled through the branch pipe 3 of the reaction gas introducing system with a reaction gas containing a plasma-polymerizable monomer gas. In the involved example of embodiment of the invention, a reaction gas was a mixture of acetylene and nitrogen used in equal volume ratios at the general gaseous mixture pressure of 5Pa. On applying of high-frequency voltage pulses from the high-frequency voltage source 9 to the discharge electrode 4, a pulsed capacitive high-frequency discharge was ignited between the electrode 4 and the grounded perforated electrode 5. The electric power supplied from the high-frequency voltage source to the discharge space was in the range of 1000V.

The high-frequency voltage pulse time varied within a range of from 10μ$ to 100ms.

With the indicated range of supply pulse time values, the glow discharge was kept at a pulsed mode. Under such conditions a maximum reactive particles formation rate was reached due to an abrupt increase in an average electron energy at a transient stage of the glow discharge: from a breakdown discharge stage to a stationary discharge stage. In the electrode regions 1 1 of the high-frequency discharge, fast-moving electrons were generated which effectively dissociated the reaction gas in a region 12 of the glow discharge plasma. The fast-moving electrons caused the formation of monomer radicals, positive and negative ions which diffused into the region 12, wherein the resulted monomers are aggregated into polymer chains.

On applying to the electrode 7 with the manufacturable sample 8 located thereon of a positive shift potential of 250V relative to the grounded perforated electrode 5, a current of about 200mA was generated. The shift potential was applied to the electrode 7 from the shift voltage source 10 switched between the electrodes 5 and 7. In the process of controlling the deposition rate, the shift potential value varied within a range of from 50V to 500V using the controllable shift voltage source 10.

The current between the electrodes 5 and 7 gives an indication of transition of negatively charged particles from the first discharge volume onto the sample 8 through the second discharge volume. The deposition rate of the polymer coating onto the sample 8 is in direct proportion to the electric current supplied to the electrode 7.

The reactive particles and the polymer chains transferred from the first discharge volume into the second discharge volume via apertures 6 formed in the perforated electrode 5. With the aperture sizes greater than a Debye radius, i.e., greater than 0.1mm for the parameters of the glow discharge in the first discharge volume, negatively charged particles along with neutral reactive particles freely moved from the first discharge volume into the second discharge volume. Owing to selecting a certain size of the apertures 6, there occurred shielding of an electric field in the region of the apertures. The said electric field being generated as a result of separation of unlike charges.

The polymer coating was formed on the surface of the manufacturable sample 8 as a result of interaction of the reactive particles and primary polymer chains with the surface under the treatment process. The polymer coating deposition rate was controlled by changing the positive shift potential value within a range of from 50V to 500V or by moving the electrode 7 in conjunction with the manufacturable sample 8 relative to the perforated electrode 5. The position of the electrode 7 was adjusted with the use of a controllable moving mechanism (not shown in the drawing) for moving the electrode 7 within a range of interelectrode spaces of from 1 cm to 5 cm. With changing of the shift potential value from 50V to 500V, the electric current supplied to the electrode 7 and the sample 8 correspondingly increased from 20mA to 300mA. The deposition rate of the polymer coating onto the sample 8 was correspondingly increased.

In the process of depositing the polymer coatings onto the sample 8, the structure and properties of the deposited coating were controlled independent of processes occurred in the second discharge volume, i.e., regardless of an actual coating deposition rate. It was determined by the results of investigations that the structure and physicochemical properties of the coating formed on the surface of the manufacturable sample 8 were dependent on the concentration of reactive monomer particles in the first discharge volume and the polymerization rate thereof.

Therefore, by changing the power applied to the discharge space of the first discharge volume the properties of the deposited polymer coating can be adjusted. The increase in the glow discharge current results in the increase of near-electrode potential drop value. This leads to an increased electron concentration and enhanced electron energy distribution in the electrode regions 1 1 of the glow discharge. An increased concentration and energy of electrons in the first discharge volume, in return, directly influences the rate of formation of monomer radicals and ions.

In the example of embodiment of the invention under investigation the hydrophilic properties of the polymer coating deposited onto the sample 8 and the coating deposition rate were independently adjusted. By controlling the discharge current in the first discharge volume, with the fixed coating deposition rate in the second discharge volume, a highly hydrophilic polymer coating was deposited onto the surface of the sample 8. A wetting angle of the polymer coating was not greater than 20°. At the same time, a wetting angle of the polymer coating deposited during plasma polymerizing process, when the sample 8 was located on one of the discharge electrodes (4 or 5) in the first discharge volume, that is, in case a single-stage gas-discharge device was used, was 83°.

The obtained experimental data suggest that polymer coatings with a desired structure and certain properties can be produced at a high capacity of the coating deposition process by independent controlling of a polymer formation rate and the coating deposition rate. Industrial applicability

In the above-described example of embodiment of the invention a certain type of a glow discharge (a capacitive high-frequency glow discharge) was used and the electric power supply system was connected in a certain manner to the electrodes of the gas-discharge device, however this does not deny the possibility of utilizing other discharge types. As an example, an inductive high-frequency discharge or direct-current discharge may be used for generating a gas discharge in the first discharge volume. Along with that, other variants of embodiment of the invention are possible, in particular, those enabling application of a negative shift potential to a perforated electrode for the purpose of providing a positively charged particles flux onto the surface of a manufacturable sample. The application of such unessential variances is defined by particular requirements specified for the coating chemical composition and properties. Such particular requirements primarily involve hydrophilic or hydrophobic properties, adhesion and sorption properties.

The invention can be utilized in a wide range of processes for deposition of polymer coatings onto the surfaces of products designed for different applications. The method of plasma deposition and the apparatus for performing the same can be used particularly in the processes for depositing hydrophilic coatings, protective corrosion-resistant coatings and adsorptive coatings.