WO2013160652A1

WO2013160652A1 - A method for photoimaging a substrate

Info

Publication number: WO2013160652A1
Application number: PCT/GB2013/050650
Authority: WO
Inventors: Chris WALL; Clive Landells
Original assignee: Electra Polymers Limited
Priority date: 2012-04-27
Filing date: 2013-03-15
Publication date: 2013-10-31
Also published as: TW201401965A; GB201207417D0

Abstract

The invention provides a method of photoimaging a substrate. The method comprises the steps of: forming a photoresist layer on at least a part of the surface of the substrate;coating the photoresist layer with a non-photopolymerisable, liquid barrier layer;positioning a coversheet on the substrate in contact with the liquid barrier layer; applying radiation through the coversheet and liquid barrier layer to regions of the photoresist layer to cure the material of the photoresist layer in the regions; removing the phototool from the substrate; and washing the surface of the substrate with one or more developer medium. Uncured photoresist and the material of the liquid barrier layer are removed from the substrate in the wash step. The method of the invention may be used, for example, in the production of printed circuit boards.

Description

A METHOD FOR PHOTOIMAGING A SUBSTRATE

FIELD OF THE INVENTION

The present invention relates to a photoimaging method. In particular, the present invention relates to a photoimaging method in which a water-soluble liquid barrier layer is provided between the photoresist and a coversheet which may be clear, or patterned such as a phototool. The photoresist is exposed to UV light through the coversheet and the liquid barrier layer and cured in exposed regions thereof. The barrier layer and uncured photoresist may then be removed in a suitable developer such as a standard aqueous wash. Advantages in terms of the exposure energy required to accomplish curing of the photoresist and also improved resolution may be obtained using the invention.

BACKGROUND OF THE INVENTION

During the manufacture of printed circuit boards, a number of different types of polymerisable photoresist are used, either to define the circuit pattern to be etched or plated, or to form a protective soldermask coating on the board before the components are soldered in place. The resist coatings are usually either applied as liquids and dried to give a tack free surface or laminated as films. The resist pattern is created by exposing the resist, imagewise, to UV light, normally through a photographic master or phototool, consisting of transparent areas and areas which are opaque to the UV light, which is laid on the surface of the resist and held in place by a vacuum.

The photoresist material under the transparent areas of the phototool receives UV light and is polymerised. Photoresist material under the opaque areas of the phototool receives no light, remains unpolymerised and is removed by washing with a suitable developer which can be an organic solvent or, more commonly these days, a mild alkaline solution.

Alternatively, the photoresist can be polymerized by direct exposure, using a laser or LED system, to create the image pattern by "writing" onto the resist surface without the use of a phototool. The same or similar processes are also used to produce plates for lithographic printing, for example in flat panel production, or to prepare sheet metal for patterning or "chemical milling" by etching. Numerous variations on the method described above have been proposed over the years. Thus, US-A-4698294 discloses a method in which a substrate is pre-coated with a UV curable monomer before laminating a photosensitive film onto the substrate. US-A-4954421 discloses a method in which a phototool is coated with a a photoresist which is then exposed through the phototool, before laminating onto the substrate which may have also been coated with photoresist prior to lamination and re-exposure through the phototool. In another method, as for example disclosed in US-A-4548884, US-A-4424089 and EP-A-0141868, the substrate is coated with photoresist and the phototool laminated onto the wet resist by the use of rollers or squeegees. Exposure through the phototool takes place as the phototool is laminated in place.

WO-A-2010/007405 discloses a method and apparatus to apply a liquid resist and laminate the phototool to the wet resist. Application of radiation to the resist is carried out whilst the resist is in its liquid form, which is stated to lead to improvements.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention, there is provided a method of photoimaging a substrate, comprising:

forming a photoresist layer on at least a part of the surface of the substrate; coating the photoresist layer with a non-photopolymerisable, liquid barrier layer;

positioning a coversheet on the substrate in contact with the liquid barrier layer;

applying radiation through the coversheet and liquid barrier layer to regions of the photoresist layer to cure the material of the photoresist layer in the regions; removing the phototool from the substrate; and

washing the surface of the substrate with one or more developer medium; wherein uncured photoresist and the material of the liquid barrier layer are removed from the substrate in the wash step;

and wherein the cured regions have dimensions greater than 1 μηι.

The method of the present invention leaves cured resist material behind on the surface of the substrate in the form of a resist pattern which corresponds to the pattern of radiation applied through the coversheet. In an aspect, the present invention relates to the photoimaged substrate obtained or obtainable by the method in accordance with the first aspect of the present invention.

The method of the present invention may be used in a variety of photoimaging applications. Broadly speaking, these fall into the following categories.

(i) Methods in which the surface of the substrate exposed by the resist pattern is subjected to an etching process. In such methods, the photoresist is referred to as an etch resist. An example of such a method is the production of a printed circuit board (PCB) in which the substrate comprises a surface layer or cladding of a conductor material, such as copper, which is removed in an etching procedure to leave a pattern of conductor material. Another example is the production of flat panel displays in which the substrate is a plate for a flat panel display. Such plates are provided on their surface with thin films of metal or semi- conductor material which may be removed in an etching process in regions not protected by the etch resist. A still further example is photochemical machining, also known as chemical etching or photo etching, in which an etchant is used to corrosively machine away selected areas of metal sheets which are exposed in a pattern corresponding to the pattern of the etch resist.

(ii) Methods in which a substance is applied to the surface of the substrate exposed by the resist pattern. An example of such a method is the production of PCBs in which the resist pattern is a soldermask. Exposed metal surfaces of the substrate are contacted with solder or a solder powder-containing paste or plated with metal, usually via an electroless or immersion process, which remains on the exposed metal, to protect the exposed areas and preserve the solderability of the underlying copper contact areas, . Another example is in the electroplating of structures, such as PCBs; in such methods, the resist pattern is referred to as a plating resist. A conductor material is then deposited by plating on exposed regions of the substrate.

(iii) Photolithographic printing methods, such as offset lithography, in which a resist pattern is applied to a printing plate; the resist pattern is a photographic negative of the desired image to be printed.

In typical embodiments of the present invention, such as those involved in the production of PCBs, the surface of the substrate is a planar surface.

In embodiments of the method of the invention, a single surface of the substrate may be treated in accordance with the invention. In other embodiments, more than one surface of the substrate, for example the two opposing surfaces of a planar substrate, may be treated, either in parallel or sequentially. For example, in methods where the substrate is a substrate of a PCB, one or both of the two major planar surfaces of the substrate may be treated in accordance with the invention.

The surface of the substrate may be a metal or non-metal. In an embodiment, the surface material is the material of the bulk of the substrate. In other embodiments, the substrate may comprise a base material and a surface layer. The surface layer may extend partially or fully around the base material of the substrate. In embodiments, the substrate may comprise a base material which has first and second planar surfaces, with the surface layer provided over one or both sides thereof, for example by a lamination method, or another method for attaching the layer.

In embodiments, the surface layer may be provided at discrete regions on the surface of the base material, as for example in embodiments in which the substrate is a PCB which comprises a non-conductive base material and conductive tracks or pathways on the surface of the base material.

The photoresist layer formed on the surface of the substrate is preferably a continuous layer which is substantially uniform in thickness and has a solid, as opposed to liquid, form. It may be formed on the surface of the substrate by applying a liquid photoresist material to the said surface, which is then dried to give a tack-free photoresist layer. Alternatively, a pre-formed film of photoresist material may be laminated to the substrate surface.

The photoresist layer for PCB manufacture, will typically have a thickness of 3 - 25 μηι, preferably 5 - 15μηι after drying in the case of a liquid etch resist, or 15 - 100, preferably 25 - 75 μηι after drying in the case of a liquid plating resist. In the case of a liquid photoimageable soldermask the thickness of the photoresist, as measured on the top of circuit traces may be in the range of 1 - 100μηι preferably 10 - 45μηι after drying. The thickness as measured between circuit traces will be affected by the height and separation of the traces but may be in the range of 10 - 120μηι, preferably 20 - 60μηι after drying. Photoresist materials are well known in the art and are curable on exposure to actinic radiation, such as ultra violet light. One example of a suitable photoresist material which may be used for forming solder masks on PCB is a liquid photoimageable solder mask (LPISM). Other types of resists which are used for PCB manufacture are etch resists, plating resists, dielectric insulating layers, flexible coverlays (for flexible circuits). Photoresists are also used in the manufacture of lithographic plates, for etching metal items (chemical milling), for etching or sandcarving glass. Resist materials can also be used in applications such as the manufacture of solar panels and flat panel displays.

The photoresist material may, for example, be one in which curing of the polymerisable material is accomplished by photoinduced free radical polymerization, where the resist material includes a radical photoinitiator. Alternatively, the photoresist material may be one in which curing is accomplished by photoinduced ionic (either cationic or anionic) reactions, in which the resist material includes either a cationic or anionic photoinitiator.

In accordance with the invention, the photoresist layer is coated with a layer of a liquid barrier material. The material of the liquid barrier layer should be selected to be substantially inert with respect to the material of the photoresist, by which is meant that the material of the liquid barrier material should not significantly interact, either physically or chemically, with the material of the photoresist such that the formation of the required resist pattern is impeded or hindered. Undesirable interactions may also include attack of the surface of the photoresist, resulting in changes in the surface finish of the exposed areas after removal of the barrier layer and/or development of the photoresist, such as loss of gloss or "matting down" of the surface.

According to the invention, after exposure to radiation and removal of the coversheet, the uncured photoresist and the material of the liquid barrier layer are removed from the substrate in the wash step. The cured photoresist regions have dimensions greater than 1 μηι. The dimensions referred to are dimensions in a plane parallel to the substrate. By way of example, in the case of photoresist which is used as an etch resist, the regions may be narrowly spaced lines or tracks, for example interconnected lines or tracks, or zones of cured photoresist separated by clear spaces of similar dimensions as the tracks, where the photoresist has been removed in the wash step. In other embodiments, for example, plating resists or solder masks, the cured photoresist regions may have dimensions greater than 10 μηι or greater than 20 μηι or greater than 50 μηι or may cover practically all of the substrate. Within these larger regions there may be smaller regions with dimensions greater than 1 μηι or greater than 10 μηι or greater than 20 μηι or greater than 50 μηι, separated from the main region by spaces of similar dimension, where the photoresist has been removed in the wash step.

In one presently preferred embodiment, a single developer medium is used in which both the material of the liquid barrier layer and the uncured photoresist are soluble. Both the liquid barrier layer and the uncured photoresist are removed in the wash step using the single developer medium.

In another embodiment, a first developer medium is used in which the material of the liquid barrier layer is soluble but the photoresist is not, and a second developer medium, different from the first developer medium is used in which the uncured photoresist is soluble. The two developer mediums are used in sequence with the first developer medium being used to remove the liquid barrier layer before the second developer medium is used to remove uncured resist material.

The or each developer medium may be selected from an aqueous wash medium comprising water, or a mixture of water and other water soluble materials provided to improve the effectiveness of the wash medium, or an organic wash medium comprising an organic solvent.

In the case of an aqueous wash medium this may be an alkaline medium, for example an aqueous solution of sodium or potassium carbonate. The pH of the alkaline wash medium may typically be 10.5 - 1 1.5. The aqueous wash medium may contain, for example, one or more defoamer, surfactant and/or base (such as tri methyl ammonium hydroxide which is used to make the solution alkaline).

In the case of an organic solvent medium, this may for example be selected from a polar and a non-polar organic medium or mixtures thereof. Non-limiting examples of polar organic media include species containing a hydroxy or carboxyl functionality such as ethanol, isopropyl alcohol, methoxy propanol, dipropylene glycol monomethyl ether, butyl carbitol, ethyl diglycol acetate, Texanol^® (2,2,4- trimethyl-1 ,3-pentanediol monoisobutyrate and nonanol. Non-limiting examples of non-polar organic media include hydrocarbon solvents such as Ruetasolve Dl (Diisopropylnaphthalene), Solban™ 60 (Isoparaffinic solvent), Petsol D100 (C9-C16 kerosine, f.p. 100-103 deg C), BAS220 (High f.p., 70°C, aromatic naphtha), Comsol 101-X (Nitroethane) and Banner Oil 380 (Mineral oil).

The developer medium or mediums are matched to the liquid barrier material and the photoresist. Thus, where a single developer medium is used, this is a developer medium in which both the material of the liquid barrier layer and the uncured photoresist are soluble or otherwise removable. This may be an aqueous based developer medium or an organic developer medium.

Where two different developer mediums are used, the first developer medium selected is one which is capable of removing the liquid barrier material but not the uncured resist material, and the second developer medium selected is one which is capable of removing the uncured photoresist material. For example, the first developer medium may be an aqueous wash medium in which the liquid barrier material is soluble and the second developer medium may be an organic solvent medium in which the uncured photoresist is soluble.

In the washing step, the substrate is washed with the developer medium or mediums to remove the uncured photoresist and the material of the liquid barrier layer. The washing step is carried out with an amount of the developer medium or mediums and for a time sufficient to remove the liquid barrier layer and/or the uncured photoresist material.

Examples of suitable liquid barrier materials which are soluble in an aqueous developer medium are water, a C2 - C6 mono alcohol or a C2 - C6 polyol, or mixtures thereof. One presently preferred liquid barrier material which is soluble in an aqueous wash medium is glycerine, or a mixture of glycerine and water in which the weight ratio of glycerine to water is in the range of 10: 1 to 1 :10, or from 5:1 to 1 :5, or from 3:1 to 1 :1 , or about 2:1. Another suitable polyol is ethylene glycol, optionally in mixture with water. Mixtures involving two or more C2 - C6 polyols are also operable, for example mixtures comprising glycerine and ethylene glycol.

Examples of suitable liquid barrier materials which are soluble in organic solvent developer mediums are Ruetasolve Dl (Diisopropylnaphthalene), Solban™ 60 (Isoparaffinic solvent), Petsol D100 (C9-C16 kerosine), which may be used with dipropylene glycol monomethyl ether and butyl carbitol as organic developer medium.

As regards the resist material, non limiting examples where the uncured resist material is soluble in an aqueous wash medium are resists based on acid- functionalised (meth)acrylate esters of resins such as acrylic copolymers, Bisphenol- A epoxies, Phenolic or cresylic novolac epoxies, polyesters, polyethers, urethanes, styrene-maleic anhydride co-polymers or mixtures thereof.. Examples of uncured resist materials which would be soluble in organic solvent developer solutions are resists based on non acid-functionalised (and acid-functionalised) (meth)acrylate esters of resins such as acrylic copolymers, Bisphenol-A epoxies, Phenolic or cresylic novolac epoxies, polyesters, polyethers, urethanes or mixtures thereof

The liquid barrier material may be deposited on the photoresist layer by any suitable technique, and may for example be deposited as a substantially uniform and continuous layer. For example the layer be applied using a spray, a brush, a roller or by dip coating. Desirably, the liquid barrier material is applied in a manner to avoid trapping air between the photoresist and the barrier layer.

The liquid barrier layer may further comprise additional components such as surfactants, debubbling agents, leveling agents, plasticisers and viscosity modifiers. Any such additional components should be of a nature and proportion such that the barrier layer is removable in the washing step.

For example, the liquid barrier layer may further comprise one or more photoinitiators. In an embodiment, the photoinitiator, or mixture of photoinitiators, is soluble or dispersible in the liquid barrier material. The amount of photoinitiator used may be 1- 20%.

An example of a photoinitiator which may be used in the invention is 2- hydroxy-2-methyl-1-phenyl-1-propanone.

The use of a photoinitiator in the liquid barrier material has been found to improve exposure speed.

The thickness of the layer of barrier material is preferably as low as possible. It may be controlled by adjusting the viscosity of the liquid to achieve thicknesses in the range 0.1 - 10μηι, preferably 0.1 - 5μηι, more preferably 0.1 - 3μηι. The thickness of a barrier layer, applied over a photoresist such as a soldermask, on a preformed circuit pattern, will vary according to the location on the board and the topography of the surface of the circuit board and photoresist thereon. Typical thickness using a glycerine/glycol based barrier liquid such as that described in the examples, may be 1.5 - 1.7μηι.

By having a relatively narrow thickness between the phototool and the substrate, the invention may be used to obtain very fine features without undercut, that is to say exposure of photoresist which is close to the edge of a portion of the phototool which is intended to exclude radiation and prevent it from reaching the photoresist.

Following application of the liquid barrier layer, the coversheet is positioned on the substrate in contact with the liquid barrier layer. A suitable pressure may be exerted in this step in order to spread the liquid material as a uniform layer between the coversheet and the resist layer, and to exclude air. The pressure applied during the lamination of the coversheet also causes the barrier liquid to flow and even out any surface imperfections and unevenness in the substrate which also benefits the resolution. A roller based system may, for example, be used to apply a suitable force pressing the coversheet into contact with the layer of barrier material.

As a result of positioning the coversheet in this manner, the coversheet is spaced from the photoresist layer by the liquid barrier layer without direct contact between the coversheet and the photoresist layer.

Where a phototool is used in the invention, this may be a positive or negative image, depending on the application. Thus, for example where the method is to be used to apply a solder mask, the image is a positive image of the exposed areas of a PCB to be soldered.

The coversheet, or in embodiments the phototool, may be made of any suitable material. In preferred embodiments the coversheet may be made of a suitable flexible material which enables the coversheet to flex in operation of the method. By way of example, the flexible material may be a flexible plastics material. Examples of plastics materials include any plastic which allows transmission of the curing radiation, such as uv radiation, to the photoresist. For example, the plastics material may be a polyester (polyethylene terephthalate or "PET"), available as Mylar™, or Melinex™, polyethylene, polypropylene, PVC, polystyrene, acrylic, polycarbonate, cellulose acetate.

The coversheet may have a thickness of at least 25 μηι, up to 500 μηι, for example from 25 to 175μηι, or from 125 to 175 μηι

Plastics materials suitable for use as the coversheet are available from

Grafix Plastics (http://www.grafixplastics.com).

The coversheet may be supported and positioned by suitable methods. Methods for making phototools and for using them in photoimaging methods are well known in the art.

The radiation used in the method may be any suitable radiation which is capable of curing the material of photoresist. In typical embodiments, the radiation is actinic radiation, having a wavelength of, for example, 200-850nm. The radiation is used at an intensity and for a time sufficient to cure the photoresist in the exposed areas.

The source of radiation may be any one which is suitable for use. Examples of suitable sources are known in the art and include ultra violet lamps, ultra violet LEDs or lasers.

The method of the invention may be used to create a photoresist pattern having features which have dimensions greater than 1 μηι. That is to say, all dimensions of the features are greater than 1 μηι in size. In an embodiment, all dimensions of the features (and thus the exposed regions of the photoresist in the method) are greater than or equal to 10 μηι.

By way of example, the method of the invention may be used to create fine lines of cured photoresist or to create fine spaces between areas of cured photoresist, such fine lines or spaces corresponding to features of electrical circuitry. The fine lines may have widths less than 200 μηι, or less than 150 μηι, or less that 100 μηι. The fine lines may have widths greater than 10 μηι, or greater than 20 μηι or greater than 50 μηι.

In one embodiment of the invention, the substrate is a blank printed circuit board (PCB) comprising a dielectric base having a conducting metal (e.g. copper) layer bonded to one or both of its sides. The method of the invention is used to apply a temporary resist mask to the substrate. After the coversheet or phototool, and the liquid barrier and uncured photoresist have been removed in the washing step, the substrate is etched to remove metal (e.g. copper) and leave behind a pattern of metal corresponding to the resist pattern. The temporary resist is then removed leaving the desired metal pattern.

In another embodiment of the invention, the substrate is a pre-formed PCB. The method of the invention is used to apply a solder mask to areas of the substrate which are not to be soldered. After removal of the phototool and washing, solder or another protective coating is applied to the exposed areas of the substrate and components can then be soldered in place. The invention may also be used in other applications, such as in the production of flat panel displays or solar panels.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1a and Figure 1 b illustrate a simple prior art photoimaging method. Figure 2a and Figure 2b illustrate a photoimaging method of the invention.

DESCRIPTION OF THE DRAWINGS

Figure 1a and Figure 1 b illustrate a prior art photoimaging method.

A substrate 100 is provided. The substrate 100 has a photoresist layer 1 10 deposited thereon. The photoresist layer 110 is typically deposited as a liquid by, for example, spraying. The liquid layer is dried to form photoresist payer 110.

A phototool 120 is applied on top of the photoresist layer 1 10. The phototool has a positive or negative image of a desired pattern which allows light to pass through some parts of the phototool but not others.

Light radiation 130, typically uv radiation is directed at the layered structure 100, 110, 120, and passes through the phototool 120 to fall on exposed areas 1 10a of the photoresist layer 110. The radiation cures the photoresist material in regions 1 10a.

The phototool 120 is then removed (Figure 1 b) and the uncured portions of the photoresist layer 110 are removed by washing with a suitable developer, such as an aqueous sodium or potassium carbonate solution or an organic solvent. The photoresist material is selected to be removable when in an uncured state using the aqueous wash medium. Remaining in situ are the cured regions of photoresist layer 1 10a.

In subsequent steps (not illustrated), the structure in Figure 1 b may be further treated. For example, the surface of the substrate 100 may be etched to remove material at the surface, such as a metal conducting material. Alternatively, a material may be applied to the exposed surface of the substrate 100, for example a solder material.

Figure 2a and Figure 2b illustrate a photoimaging method of the present invention, where like numerals represent the same feature as in Figures 1 a and 1 b. The method of the present invention differs from that of the prior art as illustrated in Figures 1 a and 1 b in that a liquid barrier layer 140 is provided between the photoresist layer 1 10 and the phototool 120. In the method, the photoresist payer 1 10 is imaged in the same manner as Figure 1 a, with light radiation passing through the phototool 120 and the liquid barrier layer 140 before reaching the photoresist 1 10 where it causes exposed photoresist material to be cured.

The phototool 120 is removed (Figure 2b) and the liquid barrier layer 140 and uncured portions of the photoresist layer 110 are removed by washing with a suitable developer such as an aqueous sodium or potassium carbonate solution or an organic solvent . The liquid barrier layer 140 is selected to be removable in the same step as the photoresist removal, using the same developer as that chosen for the photoresist, or in a separate step, using a different developer medium.

In embodiment, the liquid barrier layer 140 comprises glycerine, and optionally water. In addition, the liquid barrier material may comprise a photoinitiator which is miscible in the liquid barrier layer 140.

The present invention will now be illustrated by reference to the following non-limiting examples.

Example 1

The candidate liquid barrier materials set forth in Table 1 were tested for ease of removal in aqueous 1 % sodium carbonate solution and to assess their compatability with the surface of the photoresist. They were applied drop wise to the surface of a dried, photoimageable soldermask (Carapace™ EMP1 10 from Electra Polymers Ltd), left for a few seconds and then removed by wiping. The effect on the surface finish of the soldermask was noted. To assess their ease of removal in the aqueous developer, the process was repeated and drops were washed with the aqueous solution. The ease of removal was noted. In the case of the PVA film, the film was moistened with water and laminated to the surface of the soldermask before attempting to remove it by washing in the aqueous solution.

Table 1

Substance Surface finish Developability of soldermask

Glycerine OK OK

Water OK/patchy OK

2:1 Glycerine/water OK OK

These results illustrate that glycerine would be a suitable barrier layer to use where the resist is an aqueous developing soldermask such as EMP1 10 and the developer is aqueous 1 % sodium carbonate solution. Other candidate barrier layers, whilst not being suitable for use with the soldermask and aqueous 1 % sodium carbonate solution as the developer will nevertheless find utility with other resist materials and developers.

Example 2

A piece of printed circuit board was coated by screenprinting with a layer of a liquid photoimageable solder mask (LPISM), Carapace® EMP110 from Electra Polymers Ltd, and dried. A liquid barrier layer was applied to the surface of the resist by pouring a quantity of material in a line at one end of the circuit board. A suitable phototool was then laminated to the surface of the substrate in contact with the liquid barrier layer by using a roller to spread the barrier liquid over the surface of the resist between the phototool and the resist in such as way as to exclude air from between the phototool and the photoresist.

After exposure of the substrate to UV radiation through the phototool, the phototool was removed and the surface of the substrate washed with an aqueous 1 % sodium carbonate solution.

The effect on exposure speed, for two different formulations, of using a liquid barrier layer comprising glycerine compared to the absence of a liquid barrier layer, was measured using a Stouffer© 21 step exposure guide. Different exposure energy levels were used with a standard, mercury vapour lamp, unfiltered and filtered to allow only light of wavelength 355nm +/- 5nm. A difference of 2 steps is equivalent to a double (or halving) of exposure speed. Exposure speed was measured on resist coated over FR4 laminate. The results obtained are set forth in Table 2.

Table 2

The effect on exposure speed, for Formulation 1 , of incorporating a miscible photoinitiator into the glycerine was compared with the use of no glycerine and glycerine only as barrier liquid. The test was carried out using a Riston© 17 step exposure guide where 5 steps indicates a doubling (or halving) of the exposure speed. Exposure speed was measured on resist applied over copper. The results obtained are set forth in Table 3.

Table 3

The effect on exposure speed, for Formulation 1 , of incorporating a miscible photoinitiator into the glycerine was compared with no glycerine as barrier liquid. The test was carried out using a Stouffer© 21 Step exposure guide where 2 steps indicates a doubling (or halving) of the exposure speed. Exposure speed was measured on resist applied over FR4 epoxy/fiberglass laminate. The results obtained are set forth in Table 4.

Table 4

The composition of the liquid barrier layer for the formulation containing photoinitiator was

Glycerine - 72.7%

Ethylene glycol - 18.2%

2-Hydroxy-2-methyl-1-phenyl-1-propanone - 9.1 %

Example 4

An experimental set up similar to that used in Example 1 was employed.

The effect of using glycerine + initiator, as a barrier layer was compared with the effect of exposure in air, using no vacuum or phototool (to simulate the effect of using a laser or LED lamp for direct imaging). Exposure was carried out at 375 and 415nm, using an LED lamp array, and using a conventional Hg metal halide lamp. Exposure energy was measured using an International Light IL390C light bug. The results obtained are set forth in Table 5.

Table 5

The results in Table 5 show the effect of carrying out exposure even without a phototool to protect the resist surface. This replicates the effect of using a laser or LED system to create the image by "drawing" the pattern required directly onto the resist. This is traditionally quite a slow process and requires special resists with a greater photosensitivity/speed. Liquid resists, in particular soldermasks, (unlike dry film resists, which have a coversheet) suffer badly from oxygen inhibition which results in matting down of the surface after developing. This experimental data shows the improvement in photospeed and gloss retention obtained by using the barrier liquid.

A number of organic solvents were evaluated for their suitability for removing the glycerine based barrier liquid and as developers for the photoresist (EMP110). The results are given in Table 6 below.

Table 6

From this series of experiments it was possible to identify materials which would have suitability as a barrier liquid (i.e. have no effect on the surface of the photoresist) but which are capable of being developed (removed) using an organic developer medium, as set forth in Table 7 below.

Table 7

Developer

Barrier liquid Dipropylene Butyl

glycol, carbitol

monomethyl

ether

Ruetasolve Dl Yes Yes

(Diisopropylnaphthalene)

anner nera o o o

Thus, diisopropylnaphthalenejor example, is a suitable liquid barrier layer which can be developed in dipropylene glycol monomethyl ether or butyl carbitol as organic developer medium.

Additional materials (resin solutions in organic solvent and plasticiser) were evaluated for their compatibility with the photoresist and their ability to dissolve the photoinitiator. Results are given in Table 8 below.

Table 8

Barrier liquid Attack on photoresist Dissolves10% photoinitiator (2-Hydroxy- Removable in

(EMP110) surface 2-methyl-1 -phenyl-1 -propanone) Butyl carbitol

Ruetasolve Dl

No Yes Yes (Diisopropylnaphthalene)

Ruetasolve Dl + hydrocarbon resin

(Picco A140 from Eastman Chemical) No Yes Yes in ratio 65:35 parts by weight.

Ruetasolve Dl + liquid epoxy resin

(Beckopox 1 16 from Cytec) in ratio No Yes Yes 50:50 parts by weight.

Rheofos 95 plasticiser (mixture of

No Yes No triaryl phosphates)

From these results it can be seen that all of the above mixtures are compatible with the photoresist, will dissolve the photoinitiator and can be removed in an organic solvent (Butyl Carbitol) and are therefore suitable for use as barrier liquids, with (or without) photoiniator, in accordance with one or more of the embodiments of the invention.

Claims

1. A method of photoimaging a substrate, comprising:

and wherein the cured regions have dimensions greater than 1 μηι.

2. A method according to claim 1 , wherein the liquid barrier layer has a thickness in the range of from 0.1 to 10 μηι.

3. A method according to claim 1 or 2 wherein the washing is conducted using a single developer medium and wherein the uncured photoresist and liquid barrier layer are soluble in the single developer medium.

4. A method according to claim 3, wherein the single developer medium is an aqueous developer medium.

5. A method according to claim 3, wherein the single developer medium is an organic developer medium.

6. A method according to claim 1 or 2, wherein the washing is conducted using first and second developer mediums in sequence, and wherein the liquid barrier layer is soluble in the first developer medium and the uncured photoresist is soluble in the second developer medium.

7. A method according to claim 6, wherein the first developer medium is an aqueous developer medium and the second developer medium is an organic developer medium.

8. A method according to claim 6, wherein the first developer medium is an organic developer medium and the second developer medium is an aqueous developer medium.

9. A method according to claim 4 or 7, wherein the liquid barrier layer comprises water, a C2 - C6 mono alcohol or a C2 - C6 polyol, or mixtures thereof.

10. A method according to claim 8, wherein the liquid barrier layer is soluble in an organic solvent but not soluble in an aqueous medium.

1 1. A method according to any preceding claim, wherein the liquid barrier layer includes a photoinitiator.

12. A method according to any preceding claim, wherein the coversheet is made of a flexible material.

13. A method according to claim 12 wherein the coversheet is made of a flexible plastics material.

14. A method according to any preceding claim wherein the liquid material forms a uniform layer between the coversheet and the resist layer excluding air.

15. A method according to any preceding claim, wherein the substrate comprises a surface layer or cladding of a conductor material and the photoresist is an etch resist.

16. A method according to any one of claims 1 to 11 , wherein the photoresist is a photoimageable soldermask.

17. A method according to any one of claims 1 to 11 , wherein the photoresist is a plating resist.

18. A method according to claim 12, 13 or 14, which is part of a printed circuit board production process.

19. A photoimaged substrate produced by the method of any preceding claim.