FIELD OF THE INVENTION

The present invention relates to a surface treatment apparatus and method according to preambles of the independent claims. BACKGROUND ART

Glass is a substance that has an important role in modern day science and industry. Its chemical and physical properties make it suitable for various applications, for example flat glass, container glass, optics, optoelectronics, instrumentation, thermal insulation, and reinforcement. In general the term glass refers to any solid substance that possesses a non-crystalline (amorphous) structure and exhibits a glass transition when heated towards a liquid state. Glass may thus refer to a range of materials, for example, metallic alloys, ionic melts, aqueous solutions, molecular liquids and polymers. In the context of this specification the term glass material is used, however, to refer to a soda-lime-silica glass material, at least 70% of which is silica (S1O2).

Surface treatment refers here to a process in which a surface or a surface layer of a substrate material is exposed to an activating element such that composition of the surface or surface layer is modified. A surface treatment may thus refer to a layering process in which a substrate material is modified by exposing it to particles and allowing the particles to enter and modify a layer in the surface of the substrate material, or to deposit on the substrate material such that a surface layer is produced on the substrate material. A surface treatment may also refer to plasma processing of a surface or surface layer of a substrate material. A modified layer may be very thin compared to the general dimensions of the substrate, ranging from the order of micrometers to nanometres. However, properties of a processed surface or modified surface layer may have a significant impact to various characteristics of the treated substrate.

For this reason surface treatment of glass materials has been under keen interest for some time already. For example, one interesting layering application is deposition of functional underlayers on glass material. It is known that native silicon dioxide is a high-quality electrical insulator and can be used, for example, as a barrier material during impurity implants or diffusion, for electrical isolation of semiconductor devices, as a component in MOS transistors, or as an interlayer dielectric in multilevel metallization structures such as multichip modules. Various silicon based underlayers may be used between a transparent conducting oxide layer and a glass substrate in thin film solar cells and low-em issivity glasses. The underlayer acts as a diffusion barrier and it can eliminate interference colours that would be otherwise shown by the transparent conducting oxide. The problem with existing layering methods is that the achieved growth rates are often inadequate for industrial glass production processes.

On the other hand, surface properties of glass may have substantial effect on success and quality of post-production processing of glass substrate materials. After production, the surface of a glass substrate starts to deteriorate and/or contaminate. Before a stored or stocked glass substrate can be further processed, the surface of the glass substrate needs to be cleaned or activated to ensure good results. Conventionally, the surface of the glass substrate is cleaned by corroding or etching, which exposes a new and clean sur- face. However, these methods are not well controllable, so typically the cleaned surface structure is non-homogenous. In addition, these conventional methods are also difficult to integrate to a process line in which several process steps need to be carried out successively.

SUMMARY

An object of the present invention is thus to provide an enhanced solution for treating glass such that at least one of the above mentioned prior art problems is alleviated. The object of the invention is achieved by a surface treatment apparatus, a surface treatment system, an arrangement, and a surface treatment method, which are characterized by what is stated in the independent claims. The preferred embodiments of the invention are disclosed in the dependent claims.

The invention is based on enhancing or providing a surface treat- ment on glass material by transient electric discharges. These dielectric barrier discharges (DBD) are incurred to a space between a dielectric barrier and the glass material to be treated. In order to achieve this, the glass material must be conductive and able to receive and continuously follow charging state of a DBD electrode. During operation the glass material may then act as a DBD electrode, and the powerful transient discharges between the dielectric barrier and the glass material facilitate, expedite or accelerate the surface treatment processes on the glass material.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, embodiments will be described in greater detail with reference to accompanying drawings, in which

Figure 1 illustrates an embodiment of a surface treatment apparatus; Figure 2 is a diagram of a glass processing system;

Figure 3 shows another embodiment of the glass processing system;

Figure 4 illustrates stages of a surface treatment method. DETAILED DESCRIPTION OF SOME EMBODIMENTS

In the following description, for the purposes of clear explanation, a number of specific details are set forth in order to provide a thorough understanding of the embodiments of the invention. It is apparent to one skilled in the art that embodiments of the invention may, however, be practised without one or more of these specific details or with some equivalent arrangement. In other instances, well-known structures and units may be shown in block diagram form in order to avoid inappropriately obscuring the embodiments of the invention.

Figure 1 is a diagram of a surface treatment apparatus, capable of treating glass material according to an exemplary embodiment. The surface treatment apparatus comprises a nozzle 100 and a dielectric barrier discharge (DBD) element 102. The DBD element comprises a first electrode 104, a second electrode 106 and a dielectric element 108. Between the dielectric element 108 and the second electrode 106 is a gap 1 10 dimensioned to allow passage of a planar mass of glass material through the gap. The nozzle100 is installed operatively to the DBD element 102 such that at least during operation of the surface treatment apparatus, the nozzle may be directed to spray a flow of gas or aerosol to the gap 1 10. The surface treatment apparatus comprises also an input element 1 12 for introducing a planar mass of glass materi- al into the gap 1 10.

The input element 1 12 may be an active element that participates to transfer of the planar mass of glass material through the gap, or a passive element (e.g. an adaptor or a fit) that merely provides the planar mass of glass material with access to the gap. Figure 1 shows an example of an active input element, a conveyor apparatus in which aligned successive rollers rotate in one direction and move the planar mass of glass material to a direction 1 14 defined by a tangential line of successive roller surfaces, as shown in Figure 1 .

The term dielectric barrier discharge (DBD) refers here to a specific class of gaseous electric discharge that occurs between two electrodes. Electric discharge refers to any flow of electric charge though a gas, liquid or solid. In DBD, the electrodes are separated by at least one insulating dielectric barrier that prevents charges generated in the gas from reaching the conducting electrode surfaces. The DBD process normally operates in near-atmospheric pressure range gas and uses high-voltage (>1000 V) alternating current (AC), ranging from lower RF to microwave frequencies. With a half-cycle of the driving oscillation, the voltage applied across the gas exceeds that required for electrical breakdown, and many independent thin current filaments are formed. Herein, alternating current denotes any alternating waveform (for example, sinusoidal, sawtooth, triangular, square, and pulsed waveforms). Electrons are conducted within the filament towards the more positive electrode, and the voltage drop across the filament decreases. The voltage drop falls very quickly below the breakdown level and the discharge is extinguished. A filament typi- cally discharges in microseconds, leading to its reformation in another position over the gaseous medium. The microdischarge filaments are transient electric discharges and act as weakly ionized plasma channels that ignite when a breakdown field is reached and extinguish not far below the same field value when electron attachment and recombination reduce plasma conductivity. Mo- bility of the charges on the dielectric is low, so the lateral region allows parallel filaments to form in very close proximity to one another. Due to this, a gas-filled space between DBD electrodes may become covered by transient discharge filaments. DBD is an easy way to provide plasma for various implementations. Plasma parameters in these breakdown channels can be influenced and mod- eled and thereby optimized for many given applications.

One interesting DBD application is in the field of thin film coating of glass materials. Exposure of precursors of surface treatment particles to a plasma zone activates them such that faster deposition of coating may be achieved at lower substrate temperatures. The DBD-based procedure applies especially well to industrial glass surface treatment processes, because it ena- bles efficient deposition in relatively low substrate temperatures. In glass production it allows one to treat the surface at a later, cooled stage of the glass production process, advantageously outside (after) the molten metal bath. Also in other coating processes the low substrate temperature requirement is naturally an advantage. The DBD process applies also near-atmospheric pressure range, which means that typically no additional casing and pumping arrangements are required in the coating equipment. The required device architecture is thus simple and allows low cost production of coated glass in industrial scale.

A further advantage follows from noting that the behavior of electrical resistivity of glass material changes in varying temperature conditions. Electrical resistivity is a measure of how strongly a material opposes a flow of electric current. In normal ambient conditions a conductor, like a metal, has low electrical resistivity, and an insulator, like glass, has high electrical resistivity. However, electrical resistivity of metals increase with temperature and electrical resistivity of semiconductors, like glass, decrease with temperature. A following temperature dependency has been published for SiO2 glass material in bulk (The Physics Factbook, available online, original bibliographic entry from Gamov, George. Matter, Sky and Earth. New Jersey: Prentice Hall, 1965).:

It has been noticed that electrical resistivity of glass material, which exits from the molten metal bath in the of the order 600 degrees Celsius temperatures is such that with low current densities, glass material may act as an electrical conductor. Accordingly, the surface treatment apparatus of Figure 1 comprises a high-voltage source 1 16 configured to create between the first electrode 104 and the second electrode 106 a breakdown voltage that induces transient electric discharges 1 18 across the gap 1 10. In addition, the surface treatment apparatus comprises means 120, 122 for transferring charging state of the second DBD electrode 106 to a portion of the planar mass of glass material in the gap 1 10 such that at least part of the transient electric discharges 1 18 in the gap 1 10 may occur between the dielectric element 108 and the glass material. The charging state of an entity corresponds to the charge level of the entity and represents a part of the total electric charge of the entity that may transfer to another entity via an electric contact between them. In the embodiment of Figure 1 , the gas or aerosol sprayed from the nozzle 100 carries one or more precursors of the coating. The electric discharges and/or the plasma generated thereby accelerate the growth of the deposited layer.

For example, silane may be used as the source material for silicon, some oxidant, like O2 or CO2 as the source material for oxygen, and nitrogen or carbon added to achieve a SiO _x N _y or SiO _x C _y coating on a glass substrate. In such arrangement, DBD plasma may be used to radicalize the silane, which significantly increases the growth rate of the coating.

The charging state of the second DBD electrode 106 may be transferred to the glass material by creating an electric contact that enables between the second electrode and the glass material. The electric contact may be created with an electrically conductive element that extends from the second electrode to the gap such that at least part of the conductive element is located within the gap or in a wall of the gap. When the glass material traverses in the gap, a physical contact is created between the glass material and the region of the conductive element in the gap. An electric connection between the glass material and the electrode is created and the charging state of the second electrode transfers to the glass material. In the exemplary embodiment of Figure 1 , the electric contact is implemented by producing at least one of the rollers 120 of the input element 1 12 of highly conductive material and providing the roller 120 with a contact 122 to a grounding electrode 106 (real or virtual ground). The grounding electrode 106 provides a reference potential, and due to the electrical connection provided by the contact region 124 of the roller 120, the glass material reaches the same reference potential.

In Figure 1 , the planar mass of glass material is shown as a glass sheet. Let us assume that the glass sheet is input to the surface treatments after the molten metal bath, i.e. in a temperature between 580 C and 610 C. When the glass sheet enters the gap, at some point it gets into physical contact with the roller. Due to the lowered resistivity of the glass material in the temperature range, the charging state of the second electrode is transferred through this physical contact from the roller 120 to the glass sheet. As long as the glass sheet is in the gap, the transient electric discharges caused by the oscillating high-voltage source 1 16 occur between the dielectric element 108 and the glass sheet 124.

In many conventional DBD configurations, the first electrode and the second electrode have to be positioned directly opposite each other. In the present arrangement, the position of the second electrode may be more freely selected, as long as the charging state of the second electrode can be transferred to the planar mass of glass material before the glass material is exposed to the first electrode. The width of the gap 1 10 may be considered to correspond the distance between the first electrode and the second electrode, transverse to the direction 1 14 in which the glass material is designed to be conveyed. The dimensions for the space in which the dielectric barrier discharges during operations take place are defined by considering the top sur- face of the planar mass of glass material (the surface opposing the first electrode) as the surface of the second electrode.

In the present embodiment, the transfer of charging state to the glass material is done with a conveying element readily available or mandatorily designed into the apparatus. This provides concise and cost-effective device implementations. It is, however, noted that the contact does not need to be integrated to any other element of the apparatus, but may be a separate connection element that extends from a real or virtual ground to the level of the tangential line along which the glass material is designed to travel. The connection element may comprise a specific contact head, like a roll or a smooth drag, which allows the passing glass material to be contacted without damaging the contacted surface. A temperature-controlled flotation bed of conductive volatile material or a conveyor belt of conductive material may be applied, as well. Other methods for transferring the charging state of the second electrode are available to a person skilled in the art within the scope of protection.

In another embodiment of Figure 1 , the gas or aerosol sprayed from the nozzle 100 does not carry precursors of the coating. During operation of the apparatus, the electric discharges and/or the generated plasma operate in the space between the glass dielectric and the glass substrate and clean or activate thereby the surface or surface layer of the glass substrate.

Figure 2 is a diagram of a glass processing system incorporat- ing a surface treatment apparatus, the exemplary embodiment of which is discussed in Figure 1 . It is noted that the invention, as such, is applicable in any type of glass processing systems, as long as the dielectric barrier discharge configuration described above for the surface treatment apparatus can be established. A specifically advantageous application of the invention is in float glass production systems where required temperature ranges of glass material are inherently available and the possibility to operate in atmospheric pressure conditions is of great importance. However, the configuration may be advantageously applied also in other surface treatment processes.

Figure 2 illustrates a float glass production system 20 that com- prises a glass formation stage 21 and an annealing stage 22. In the formation stage, raw materials are fed into a furnace 23 to melt and then drawn from a delivery canal into a bath 24 of molten tin. The glass forms a floating planar ribbon, the thickness of which is very even. In the tin bath, the temperature of the glass material is reduced to the order of 600 degrees Celsius. The anneal- ing stage comprises a lehr kiln in which the formed glass is cooled gradually to normal ambient temperatures.

It has been detected that when the temperature of the glass material is between 580 to 610 degrees Celsius, the electrical resistivity of a range of commonly used glass materials enables use of a planar mass of the glass material as a dielectric barrier in DBD configuration. Accordingly, the surface treatment apparatus 25 in the float glass production system is advantageously positioned such that glass material traversing in the system is exposed to the surface treatment apparatus 25 after the tin bath 20 but before the annealing lehr 21 .

Figure 3 shows another embodiment of the glass processing system, an off-line process line for processing glass substrates 30, and especially planar glass substrates or flat glass. The term "off-line" means here that the process line is not integrated to a production line of flat glass or other glass products. After production, glass products are stored and in the storage the surface layer of the glass substrate 30 typically deteriorates or contaminates. Therefore the surface of the glass substrate 30 has to be cleaned or activated before it can be processed in the process line. Let us assume, the process line of Figure 3 is arranged to provide a coating, for example a thin film coating, on the surface of the glass substrate 30. The process line comprises transport means 31 with transport rollers 32 for transporting the glass substrates 30 in the process line. The process line also comprises two heating units 34 followed by coating units 35, respectively and an annealing or tempering unit 36 for tempering the coated glass substrate 30. In the heating units 34 the temperature of the glass substrate 30 may raised to a level required by a coating process of the coating unit 35, typically to 580 °C or higher. Therefore, at least after one of the heating units 34, there may be a glass surface activation or cleaning unit 33 that is implemented with the configuration shown in Figure 1 . The coating units 35 may be an aerosol coating unit in which small liquid droplets or particles are conducted towards the surface of the glass substrate 30 for providing a coating on the surface of the glass substrate 30. In the provided temperature range above 580 °C, the glass material may act as a DBD electrode and the plasma generated in the DBD process cleans or activates the surface of the glass substrate.

Figure 4 illustrates stages of a surface treatment method that may be implemented with a surface treatment apparatus in a glass processing system, embodiments of which have been discussed in Figures 1 and 3. It is noted that the description of the method applies many terms and concepts already defined in description of those embodiments. The method begins in a stage where an apparatus configuration comprising an alternating current (AC) high-voltage source, a first electrode, a second electrode and a dielectric ele- ment connected to the first electrode is set up such that a gap is formed between the dielectric element and the second electrode. Gas or aerosol is fed (stage 40) into the gap, and the AC high-voltage source is used to create breakdown voltages (stage 41 ) between the first electrode and the second electrode. In a DBD configuration, these breakdown voltages induce transient electric discharges across the gap. When a planar mass of glass material is introduced to the gap (stage 42) in a temperature between 580 to 610 degrees Celsius, its electrical resistivity allows it to operate as a DBD electrode. In order to achieve this, charging state of the second electrode is transferred (stage 43) to a portion of the planar mass of glass material in the gap such that at least part of the transient electric discharges in the gap begin to occur between the dielectric element and the glass material. In case the gas or aerosol com- prises precursors of a surface layering process, significant enhancement to the rate of coating is achieved with a configuration that inherently integrates to glass processing and/or production systems. If the gas or aerosol does not comprise precursors of a surface layering process, the dielectric barrier dis- charges and/or the plasma generated thereby cleans and/or activates the surface of the substrate.

It will be obvious to a person skilled in the art that, as the technology advances, the inventive concept can be implemented in various ways. The invention and its embodiments are not limited to the examples described above but may vary within the scope of the claims.