Title: Improvements relating to chemical reactions

Field of the Invention

This invention concerns a method of and apparatus for performing a chemical reaction on a substrate, and is concerned particularly, but not exclusively, with a continuous process for electroless deposition of metal.

Background to the Invention

Electroless deposition is a well known process that has been used for many years to apply a metal coating to a substrate. The process involves immersing a substrate in a succession of baths of reagents, with a typical process involving 6 different baths. The substrate may be in the form of a web of material, passed through successive baths.

Summary of the Invention

According to one aspect of the present invention there is provided a method of performing a chemical reaction on a substrate, comprising applying a reaction component in liquid form to a surface of the substrate, to form a liquid film thereon, while the substrate is conveyed along a path.

The substrate is preferably generally planar, in the form of a flat sheet or flat elongate strip of material. The substrate is preferably of flexible material. The substrate is preferably in the form of a web of flexible material, for conveying around rollers. A non-exhaustive list of suitable substrates includes, for example, plastics materials, paper, metal foils and fabrics.

The reaction component may be applied to the substrate surface by a range of different deposition techniques, including spraying, blade coating, continuous weir coating, printing, e.g. inkjet, flexographic, litho, gravure and screen printing, etc. The application

process is to be contrasted with and distinguished from immersion techniques, such as dipping in a bath of liquid. Application is typically in a generally downward direction, onto an upper, generally horizontal surface of the substrate, but this is not essential. The reaction component may be applied to form a continuous coating on the web, or may be applied in a selective manner according to a pattern, for example by printing techniques such as inkjet printing.

The applied reaction component may react with the substrate itself, or with one or more other reaction components previously applied to the substrate surface, e.g. by a deposition process such as those referred to above or by an immersion process, either as a continuous coating or as a pattern on selected areas only.

The method of the invention is applicable to a wide range of chemical reactions, including dissolution, with reaction component(s) being reactive or catalytic. Chemical reactions of interest include etching, stripping, photographic development and metallisation, with the substrate and reaction component or components being selected appropriately. For example, the method may be used for electroless deposition of materials such as electroless plating of a metal layer on a substrate. A suitable activator, e.g. a palladium catalyst, may be applied to a surface of the substrate in a preliminary step, e.g. by inkjet printing. In accordance with the invention, an electroless plating solution may then be applied to the substrate surface while the substrate is conveyed along a path, to form a liquid film on the moving substrate and catalyst and to react therewith to form a metal layer. The path length and speed of movement of the substrate are selected to allow sufficient time for the reaction to take place. Any remaining reactants, e.g. surplus or spent plating solution may then be removed from the substrate.

More than one reaction component in liquid form may be applied to the surface of the substrate, in accordance with the invention, simultaneously or sequentially, for reaction with the substrate or previously applied reaction component(s).

The substrate is conveniently conveyed along a path by being passed from one reel to another, optionally being passed over one or more intervening rollers, with the substrate describing a path that can be linear, serpentine, circular etc.

The invention is typically used as one stage of a multi-stage continuous process, e.g. for continuous production or conversion of items or for chemical reaction involved in the performance of tests or analyses in continuous fashion. The substrate is preferably conveyed along the path continuously, but could instead be stopped intermittently.

The thickness of the film of liquid will depend on a number of factors including the viscosity of the reaction component, the speed of movement of the substrate, rate of reaction etc, and may vary between wide limits e.g. 1 ran to 10 cm, typically being in the range 10 microns to 5 mm, more typically 100 microns to 2 mm.

In contrast to known methods that involve passing a substrate web through a bath containing a reaction component, the current invention displays distinct advantages, especially in situations where the reaction component has a finite lifetime and/or is unstable and a compromise must be reached between reactivity and bath stability.

1. The amount of reagent used may be reduced to the minimum required for the reaction to complete. This obviously reduces waste.

2. The reagent may be made-up as it is applied to the substrate. This removes the problem of large volumes of reagent ageing and thus requiring replacement of a whole bath of liquid.

3. If the reaction rate is increased by heating then this may be done continuously as the reaction component is applied. This would remove long warm-up times and heat loss associated with heating large baths of liquid.

One of the problems associated with the application of thin liquid films to a substrate is that of the stability of the liquid film. In many situations, the surface energies of the substrate and liquid are such that substrate wetting is not favoured and the film can become unstable, resulting in beading of the liquid rather than a continuous coverage. This

problem may be overcome by the addition of surfactant materials to the liquid in order to better match the surface energy to that of the substrate. Alternatively, a surface treatment (such as a discharge treatment or a self assembled monolayer (SAM)) may be applied to the substrate material in order to improve wetting. Although these methods may improve wetting, in many cases they may also inhibit the reaction with the first reaction component.

In a preferred aspect of the current invention the problem of substrate wetting is addressed by applying a covering means or covering layer to the liquid film in order to stabilise the film. The covering means may be a further portion of substrate material or it may be any other solid material which is able to maintain contact with the liquid film on the substrate.

In most cases the surface tension of the liquid will seek to hold close contact with the two bounding sheets of material (substrate and covering means) and will hold all three layers together in a stable configuration.

Placing a solid covering means in contact with the liquid film also has the advantage of preventing evaporation from the surface of the film. Similarly, the solid covering means will also reduce degradation in liquids which are sensitive to exposure to the atmosphere.

There may or may not be relative movement between the covering means and the substrate. In cases where the covering means is moving with respect to the substrate, the relative motion may enhance the rate of reaction by the action of the agitation caused by the shearing of the liquid film.

The substrate may thus be conveyed along a path provided with a cover that contacts the upper surface of the liquid film. The cover may be stationary, e.g. being constituted by a fixed layer of material such as a layer of polytetrafluoroethylene (PTFE), so that there is relative movement between the cover and the substrate and film. Alternatively the cover may move, e.g. being in the form of a moving layer of material such as a web or loop of material, possibly in the form of a further portion of substrate material (either a further portion of the substrate being treated or a separate portion of substrate). The cover may

move at the same speed and in the same direction as the substrate and film, in which case there will be no relative movement. Otherwise there will be relative movement.

Under certain circumstances the speed of movement of the substrate is dictated by considerations such as the need to print on the material at a given print speed or the need to produce a certain elongation rate or strain in the material. If these speeds are relatively high then providing sufficient linear length in order to achieve a particular reaction time may require apparatus having an excessively large footprint. For example, if a system needs to print features on a substrate at 0.5 m/s and these features then need to be exposed to a developer solution for a period of 2 minutes, then the developer solution would need to be carried on the substrate for a distance of 60 m. This could be undesirably long for many production situations. Many systems in the prior art have implemented a so-called serpentine route to reduce the space taken up by a particular reaction. This involves wrapping the substrate backwards and forwards around a series of rollers in order to take longer to traverse a particular length of space.

Such a strategy may also be applied to the current invention in order to reduce the length of substrate carrying system required to provide a particular reaction time at a particular speed of substrate.

Advantageously, the serpentine arrangement may be combined with the idea of covering the thin liquid film by winding a web in such a way that alternate coils of the web make contact with each other such that a separate material is not needed in order to provide a stabilising cover to the substrate.

In a further aspect the invention provides apparatus for performing a chemical reaction on a substrate, comprising means for conveying the substrate along a path; means for applying a reaction component in liquid form to a surface of the substrate to form a liquid film thereon; and covering means for covering the liquid film.

The conveying means conveniently comprise one or more rollers around which the substrate is conveyed, with the substrate describing a path that can be linear, serpentine, circular, etc.

The liquid application means conveniently comprise liquid deposition means such as spraying means, coating means, printing means, etc.

The covering means is desirably arranged to contact the surface of the liquid film on the substrate.

The covering means may be a further portion of substrate material, or it may be any other solid material.

The covering means may be stationary, i.e. fixed with respect to the substrate path, e.g. being constituted by a fixed layer of material such as a layer of PTFE. Alternatively, the covering means may move, e.g. being in the form of a moving layer of material such as a web or loop of material, possibly in the form of a further portion of substrate material (either a further portion of the substrate being treated or a separate portion of substrate). The covering means may be arranged to move at the same speed and in the same direction as the substrate and film, in which case there will be no relative movement. Alternatively, the covering means may be arranged to move in the same direction, but at a different speed as the substrate and film, in which case there will be relative movement. The covering means may alternatively be arranged to move in the opposite direction to the substrate and film, again producing relative movement.

The covering means may comprise a web of material arranged to be conveyed along a path following that of the substrate and slightly spaced therefrom. Such covering means may be in the form of a continuous loop of material passed around rollers. Alternatively, the covering means may be in the form of an elongate strip of material passed from a supply reel to a take up reel, possibly passing over one or more intervening rollers.

In a preferred embodiment, the apparatus comprises an arrangement of a plurality of superimposed webs of flexible substrate material each adapted to be passed around associated rollers in parallel, aligned paths, with one web of substrate acting as the covering means for the web of substrate immediately therebelow. Two, three or more webs of substrate may be superimposed in this way.

In another preferred embodiment, a web of flexible substrate material is arranged to be passed around at least two rollers, with the substrate passing around each roller more than once so that one face of the substrate constitutes the surface on which the liquid film forms and the other face of the substrate constitutes the covering means. Such an arrangement conveniently uses, e.g. three rollers arranged in the form of a triangle, four rollers arranged in the form of a rectangle such as a square, five rollers arranged in the form of a pentagon, etc. The substrate is passed from a supply reel to a take up reel. The supply reel may be located externally of the rollers, with the take up reel within the rollers, or vice versa. In each case the reels may either be in the same plane as the rollers or displaced from this plane. A further possibility is for both the supply and take up reels to be displaced from the plane of the rollers.

The invention will be described, by way of illustration, with reference to the accompanying drawings, in which Figures 1 to 6 are each schematic illustrations of a different arrangement for performing the method of the invention.

Detailed Description of the Invention

Figure 1 illustrates an arrangement in which a web 3 of Melinex polyester material (Melinex is a Trade Mark of Dupont Teijin Films) is unwound from a reel 1 and rewound back on to reel 2. The Melinex web 3 constitutes the substrate on which a chemical reaction is to be performed, namely electroless deposition of a copper layer.

As a preliminary step, one surface of the Melinex web 3 (the upper surface as shown in the Figure) has had a palladium acetate activator solution applied thereto in a pattern on only

selected areas of the surface by inkjet printing, generally as described in WO 2004/068389. In particular, the following activator solution was prepared:

% (by weight)

Palladium acetate 2.0

Irgacure 1700 3.25

Irgacure 819 1.25

DPGDA 30.5

DPHA 3.0

Actilane 505 10.0

Diacetone alcohol 47.5

PVP K30 2.5

Viscosity, cPs (25 ⁰ C) 17.6

Palladium acetate is present as an activator. Igracure 1700 and Igracure 819 are UV photo-initiators supplied by Ciba Speciality Chemicals, Macclesfield, UK - Irgacure is a Trade Mark. DPGDA is dipropylene glycol diacrylate, a UV-curable reactive diluent monomer supplied by UCB, Dragenbos, Belgium. DPHA is dipentaerythritol hexacrylate, a UV-curable hexafunctional monomer, supplied by UCB, Dragenbos, Belgium. Actilane 505 is a UV-curable reactive tetrafunctional polyester acrylate oligomer supplied by Akzo Nobel UV Resins, Manchester, UK. The monomers and oligomers are in liquid form. Diacetone alcohol is a solvent for the palladium acetate. PVP K30 is a grade of polyvinyl pyrrolidinone supplied by ISP, Tadworth, UK.

PVP constitutes a water soluble chemical functionality. The monomers and oligomers, Actilane 505, DPHA and DPGDA, react to form a polymer that constitutes a water insoluble chemical functionality.

This fluid was printed with a XJ500/180 print head (available from Xaar of Cambridge, England) at 180 x 250 dpi. The samples were then cured under a Fusion 500 Watt H-bulb, in 4 passes of 20 metres/min each, resulting in formation of an activator layer.

In accordance with the invention, a liquid reaction component in the form of an electroless copper plating solution was applied at a controlled rate to the upper surface of web 3 from a liquid dispenser 5 comprising a syringe and plunger, while the web 3 is moving below the dispenser, to form a stable film of liquid 4 on the moving web. In this embodiment, the electroless copper plating solution comprises Enplate 872 A, B and C reagents (Enplate is a Trade Mark) which are available from Enthone-OMI and are in common use as component solutions for electroless copper plating. In particular, the electroless copper plating solution has the following composition:

% (by weight)

Enplate 872 A 10.713

Enplate 872 B 10.713

Enplate 872 C 3.571 water balance to 100%

Enplate 872 A contains copper sulphate. Enplate 872B contains a cyanide complexing agent and formaldehyde. Enplate 872 C contains sodium hydroxide.

The electroless copper plating solution forms a stable film 4 about 2 mm thick on the moving substrate web 3. The solution is allowed to remain on the web for a transit time of about 2 minutes. After this time a continuous film of copper metal has built up on the previously deposited catalytic material on the web. The remaining spent plating solution is removed from the web using squeegee 6.

Figure 2 shows an arrangement which is similar to that of Figure 1 , but with the addition of a solid PTFE block 7 constituting a cover for film 4. Block 7 is positioned around 200

microns above the moving Melinex web 3 in such a way that the surface tension of the liquid 4 holds the web in close contact with the solid block.

In Figure 3 the same basic web configuration is again used but in this case a second web 8 is uncoiled and placed above the first web 3. These webs are brought sufficiently close together that the surface tension of the liquid 4 holds the two materials in intimate contact. The second web may run at the same or different rate as the first web. Where the webs are run at the same rate, then there will be no relative motion between the two systems and therefore no drag placed on the reaction system by the second surface. In this case the second web helps to maintain a stable film over the first web and also prevents evaporation of the liquid. This results in quicker and more reproducible plating.

Figure 4 shows a similar configuration to that of Figure 3 , but in this case the uncoiling and recoiling web 8 is replaced by a continuous loop 9 which is constantly rotated above the moving web 3. This may be at the same linear speed or different linear speed to the surface of the moving web 3. In the case that it moves at the same linear speed, the system will not experience any extra drag associated with the shearing of the liquid 4. This embodiment has the advantage that it may run continuously without needing to replace the second web of material. This also means that much less material is needed for the second web.

Figure 5 shows a serpentine arrangement combined with the idea of covering the thin liquid film by winding the web in such a way that alternate coils of the web make contact with each other. In this arrangement, a separate material is not needed in order to provide a stabilising cover to the web.

In the configuration shown in Figure 5 the associated sides of the web are shown moving in different directions and therefore create a high degree of shear across the thin liquid film (typically 1-2 mm thick). This shear is beneficial and serves to agitate the liquid and maintain the reaction. However, since the liquid has a finite viscosity it may produce a significant resistance to the shearing motion of the opposing sides of the web.

Figure 6 shows a modified configuration in which the web is repeatedly wrapped around itself. This has the benefit of having neighbouring layers of the web moving in the same direction and thus reducing the drag associated with high shear rates on the liquid film. However this configuration still maintains the benefits of reducing the footprint required to keep a given length of web live and also allows the concentric layers of the web to cover each other and maintain the stabilising effect on the thin liquid film.

After being wound off roller 1 the moving web 3 has a reactive liquid applied and is then wrapped repeatedly around rollers 10, 11 and 12 and is then rolled back onto roller 2 in the centre of the configuration. Alternatively, roller 2 may be angled out of the plane of the diagram such that the web exits in a plane substantially perpendicular to the plane of the figure. The web may then be wound onto another roller in a location away from the system. Alternatively, the web may enter another such system of rollers where a similar or different reactive liquid is applied and allowed to react.