Method for forming an insulating layer

Abstract: Methods are disclosed for forming a wrinkle-free insulating layer from a resin having both photo-curing and thermosetting properties. The concentration gradient of the residual solvent on a resin layer is reduced by coating a liquid photosensitive epoxy resin solution as an insulating layer to the substrate 10, drying it, passing the substrate 10 by a roll coater 1 charged with a solvent for the resin, and allowing the solvent to be absorbed by the resin layer. In this case, a gap is maintained so that the surface of the resin layer does not come into contact with the solvent application roll 8. After pattern-exposure, the insulating resin layer is developed and cured by heating. Instead of allowing the resin layer to absorb the solvent, it is also possible to affix a Mylar film to the resin layer so as to reduce the concentration gradient of the residual solvent in the resin layer.

Description: Field of the invention The present invention relates to a method for forming an insulating layer and more particularly, concerns a method for forming a wrinkle-free insulating layer from a resin having photo-curing and thermosetting properties. Background Art Conventionally, as methods for forming a pattern of an insulating resin layer such as a solder mask on a substrate, two methods have been generally employed; one is to thermoset the insulating resin after forming a pattern of the resin layer by screen pattern printing during resin coating, and the other is to apply a resin having both photo-curing and thermosetting properties to the substrate and cure the resin by heating after forming a pattern (photo-via holes 11 as shown in Figure 3) through photo-curing and developing. For example, as illustrated in Figure 4, a lower-layer circuit pattern is first formed 100 on the substrate, and secondly, an insulating resin solution having both photo-curing and thermosetting properties is applied 102 onto the substrate and this lower-layer circuit pattern. After being dried 104, this resin layer is pattern-exposed 108 (photo-cured). The pattern thus formed is developed 110 and photo-cured 112 as a final cure. When fine pattern formation is required, the latter method is chosen from a point of view of resolution. This method involves a two-stage setting of resins, that is, photo-curing and thermosetting. Among resins, during the stage of photo-curing, some are photo-cured to the inside because the absorption coefficient of the resin is low and light passes to the inside fully, and some are photo-cured only on the surface layer because of high absorption coefficient of the resin. When a resin is photo-cured to the inside, there is no likelihood of wrinkles occurring on the surface of the resin layer formed after photo-curing followed by thermosetting. On the other hand, when a resin is photo-cured only on the surface layer, wrinkles are formed on the surface of the resin layer after photo-curing followed by thermosetting. The formation of these wrinkles spoils the smoothness of the surface and raises problems associated with inspection, testing, and surface treatment. In particular, when the resin is employed as an insulating inner layer for use in multi-layer boards, it is essentially required to have even surface of the insulating resin layer by grinding, etc., in order to form an upper-layer circuit pattern. It is also necessary to grind the roughness resulting from said wrinkles in addition to the roughness derived from a lower-layer circuit pattern. Accordingly, an amount of a resin greater than the ground quantity, should be applied to grind the uneven area. When a smoothness needed for the formation of a circuit pattern by conventional etching is spoiled in the inner layer, such defects as short circuiting and broken wires result. It is an objective of the present invention to provide a method for forming insulating layers which prevent the occurrence of the above-mentioned wrinkles on the surface of an insulating resin layer without spoiling resolution and electric and physical characteristics of that resin when forming on a substrate a pattern of the insulating resin layer having both photo-curing and thermosetting properties. Disclosure of the Invention As a result of the inventors' intensive studies, it was concluded that the cause of the occurrence of wrinkles on the surface of the insulating resin layer is the concentration gradient of a residual solvent in the cross-sectioning direction of the resin layer occurring during the drying process 104 after coating of an insulating resin solution in the processes shown in Figure 4. For example, it has been confirmed (see for example US-A-3 529 960) that allowing the solvent remaining in the resin layer to be nil or nearly so by natural drying prevents the occurrence of wrinkles on the surface of the insulating resin layer, but that it requires many hours and is not practical to completely remove the residual solvent without thermosetting. As a further result of their studies, they found out that the occurrence of wrinkles on the surface of the insulating resin layer after final curing 112 can be prevented by reducing the concentration gradient of the residual solvent in the cross-section of the resin layer after the drying 104 of the solvent, thus perfecting the present invention. The invention discloses a method of forming a smooth surfaced insulating layer on a substrate according to claims 1 or 3. In the method for forming a smooth-surfaced insulating layer according to one aspect of the present invention, the concentration gradient of the residual solvent is reduced by absorbing an additional solvent through the surface of the insulating resin layer after the coating and drying of the insulating resin solution. In the method for forming a smooth-surfaced insulating layer according to another aspect of the present invention, the concentration gradient of the residual solvent is reduced by providing on a surface of an insulating resin layer preventing the permeation of the solvent after the coating and drying of the insulating resin solution and heating the insulating resin layer. The present invention is advantageous in that the occurrence of wrinkles is effectively prevented by treating the insulating resin layer in such a way that the residual solvent in it will be reduced because the solvent distributes evenly throughout the insulating resin. Brief Description of the Drawings The invention will now be described by way of example only, with reference to the accompanying drawings in which: Figure 1 is a process flow diagram showing a method for forming the insulating layer according to the present invention; Figure 2 is a schematic cross-sectional diagram showing the roll coater for use in the process of solvent absorption by the method of Figure 1; Figure 3 is a schematic top view of the state of the insulating layer; (A) is a comparison case, and (B) is the method of Figure 1; and Figure 4 is a process flow diagram of a conventional process for forming an insulating layer. Detailed Description of the Invention Figure 1 is a process flow diagram explaining the method for forming a smooth-surfaced insulating layer according to the present invention. As shown in Figure 1, the insulating layer is formed as follows: A A lower-layer circuit pattern is formed 100 on the substrate (the process of forming a lower-layer circuit pattern). B Subsequently, an insulating resin solution having both photo-curing and thermosetting properties is applied 102 onto the substrate and onto the lower-layer circuit pattern formed thereon and forms an insulating resin layer (process of coating an insulating resin solution). Epoxy resin having photosensitivity can, for example, be used as an insulating resin having both photo-curing and thermosetting properties. Typically, a liquid photosensitive epoxy resin of Probimer 52 (Ciba Geigy Japan Ltd.) may be used. C This insulating resin layer is dried 104 (pre-cured) (the process of drying). That is to say, the solvent in the insulating resin layer is evaporated through pre-curing, and the tackiness on the surface disappears. Consequentially, the solvent transpires since the surface of the insulating resin layer comes into contact with the air having 0% solvent concentration. Thus, a concentration gradient of the residual solvent then occurs in the inner portion (in the cross-sectional direction). D The concentration gradient of the dried insulating resin layer is reduced 106 (the process of reducing the concentration gradient of the residual solvent). E The insulating resin layer treated to reduce the concentration gradient of the residual solvent is pattern-exposed 108 (photo-cured) to form a pattern (the process of pattern-exposure [the process of photo-curing]). F The pattern thus formed is developed 110 (the process of developing). G The developed pattern is thermoset 112 as a final cure (the process of thermosetting). The aforementioned processes A - C and E - G are virtually the same as in the past; therefore, the means and materials employed in the prior art may be used. The existence of process D, that is, the process of reducing 106 the concentration gradient of the residual solvent, is the particular strength of the method of forming a smooth-surfaced insulating layer according to the present invention. The occurrence of wrinkles on the surface of the insulating resin layer can be prevented by reducing the residual solvent in the resin to be nil or nearly so. Nevertheless, it takes many hours and is not practical to completely remove the residual solvent without thermosetting. On the other hand, unless the solvent is used, there should be no occurrence of wrinkles due to the occurrence of the concentration gradient of the solvent. However, at present, this case is not practical in view of the fact that as to photosensitive epoxy resin, types containing solvent are conventionally used. As mentioned above, it is of course also impractical to pre-cure the resin layer until the solvent is gone because this process takes far too much time. Thus, the means that may be practically employed is to reduce or get rid of the concentration gradient of the residual solvent occurring during the pre-cure process. In the present invention, the reduction of the concentration gradient, i.e. homogenization, of the residual solvent may be accomplished through a variety of methods. For example, it is possible to provide on the surface of the insulating resin layer a barrier layer impermeable to the solvent used, heat the insulating resin layer, and evenly distribute the solvent within the portion enclosed by this barrier layer. An impermeable film (such as polyethylene terephthalate film) may be used as the barrier layer. They may also be pressed in a press and then heated, or held between heated belts. Furthermore, the evenness of the concentration gradient of the residual solvent may also be accomplished by heating in an autoclave under pressure. Or, the concentration gradient of the residual solvent may be reduced by applying solvent absorption process to the pre-cured insulating resin layer and absorbing the solvent throughout the surface. When the concentration gradient of the residual solvent is reduced by the solvent absorption process, the solvent (thinner) for the insulating resin comprising the insulating layer can be used as a solvent for this process. This solvent may be different from, or the same as, the solvent of the insulating resin solution used in process B. Vaporizing solvent and bringing the solvent vapor into direct contact with the surface of the resin layer, or directly applying the solvent to the surface of the resin layer are proposed as methods for absorbing the solvent into the surface of the resin layer. Whenever the solvent in a gas or liquid phase is brought into contact with the surface of the resin layer, it is absorbed into the resin layer, and the bias of the concentration gradient of the solvent absorbed into the resin layer is reduced, and the advantage of wrinkles being eliminated is obtained by either of the methods. Figure 2 is a schematic diagram of a roll-coater used for the process of solvent absorption by the method for forming a smooth-surfaced insulating layer according to the present invention. Numeral 1 is the roll-coater, 2 a belt conveyor, 3 a cover, 4 a belt, 5 an inlet, 6 an outlet, 7 a conveyance roll, 7A a backup roll, 8 a solvent application roll, 9 a doctor bar, 9A a solvent supplier, and 10 the substrate. The process of solvent absorption as the process of reducing the concentration gradient of the residual solvent D is explained in detail with reference to Figure 2. The roll coater 1 possesses the belt conveyor 2 and the cover 3 covering thereover. The inlet 5 and the outlet 6 for the substrate 10 are furnished between the belt 4 of the belt conveyor 2 and the cover 3. In this illustration, the belt conveyor 2 possesses one each of the conveyance rolls 7 at positions corresponding to the inlet 5 and the outlet 6, respectively, and there is a backup roll 7A between them. The solvent application roll 8 is set over belt conveyor 2 corresponding to the backup roll 7A. This solvent application roll 8 is set at a gap d as close as possible to the upper surface of the substrate 10 on the belt 4 without making contact. The doctor bar 9 is disposed opposite to the solvent application roll 8 and feeds a solvent to the solvent application roll 8 from the solvent supplier 9A. The substrate 10, with a dry insulating resin layer on its surface after passing through the aforementioned process C, is transported through inlet 5 and transported on the belt 4 of belt conveyor 2 driven by rolls 7 and 7A and arrives beneath the solvent application roll 8. During the passing of the substrate, the solvent vapors evaporated from the surface of the solvent application roll 8 are absorbed by the resin layer. In this case, since the solvent application roll 8 does not come into contact with the substrate, the lower-layer circuit pattern provided on the substrate will not be deformed. The substrate 10, having completed the process of solvent absorption, is transported through outlet 6 to the next process of pattern exposure (photo-curing). Thereafter, the substrate is processed in the same manner as in the prior art, and an insulating layer is formed. Example 1 Onto the copper-clad substrate 10 for printed circuit boards on which circuits have been formed by the conventional technique, a liquid photosensitive epoxy resin of Probimer 52 (Ciba Geigy Japan Ltd.) was applied to a dried thickness of 60 µm, and dried by heating at 90°C for 2 hours. Subsequently, the roll coater 1, shown in Figure 2, was used and the Probimer 52 thinner fed by the solvent supplier 9A was evaporated from the surface of the solvent application roll 8. The resin coated substrate was passed by the belt conveyor 2 by roll coater 1 charged with solvent vapors at a speed of 3J/cm 2 at the gap d of 1.5 mm between the upper surface of the substrate and the lower end of the solvent application roll 8 so that there is no contact between the coated and dried resin surface and the solvent application roll 8. Then the substrate was developed by exposing to light of 3J/cm 2 through the mask film by a high-pressure mercury vapor lamp and thermoset by heating at 135°C for 2 hours. The surface of the resin coated substrate was free from wrinkles and was satisfactory. Example 2 Onto the surface of the pre-cured, liquid photosensitive epoxy resin Probimer 52, a Mylar (registered trade name of polyethylene terephthalate manufactured by Du Pont Ltd.) film was affixed using a film applicator. During this film application, the temperature of a rubber roll was 80°C. In one of the samples, the Mylar film was peeled off and exposed to light (comparison case), while, in another sample, after being heat-treated for 30 min in an oven maintained at 80°C, the Mylar film was peeled off and the sample was exposed to light (the present invention). These two samples were simultaneously thermoset after being developed under the same conditions. The results are shown in Figure 3. Figure 3 is a schematic top view of the state of the insulating layer; (A) is a control case, and (B) is the present invention. As shown in Figure 3(A), in the control case sample, the occurrence of wrinkles 12 was clearly observed adjacent to the photo-via holes 11 and between the photo-via holes 11, whereas in the sample of the present invention, no wrinkles were observed (Figure 3(B)). In the present invention, when a pattern of an insulating resin layer having photo-curing and thermosetting properties is formed on a substrate, the occurrence of wrinkles in the photo-cured, developed and thermoset insulating layer is effectively prevented by following the coating and drying of the insulating resin solution, the insulating resin layer being treated in such a way that the concentration gradient of the residual solvent is reduced, and the concentration gradient of the residual solvent in the cross-sectional direction of the insulating resin layer thereby being reduced. In particular, the occurrence of wrinkles in the insulating layer is effectively prevented because, after the coating and drying of the insulating resin solution, the solvent is to be absorbed through the surface of the insulating resin layer. At the same time, the occurrence of wrinkles in the insulating layer is effectively prevented because, after the coating and drying of the insulating resin solution, the insulating resin layer is heated after providing a barrier layer impermeable to the solvent on the surface of the insulating resin layer. Moreover, the occurrence of wrinkles in the insulating layer is effectively prevented because, after the coating and drying of the insulating resin solution, the insulating resin layer is allowed to pass near a roller impregnated with the solvent so that the solvent is absorbed through the surface of the insulating resin layer. Furthermore, the occurrence of wrinkles in the insulating layer is also effectively prevented, because a film is affixed onto the surface of the insulating resin layer and the insulating resin layer is heated, so that the barrier layer impermeable to the solvent can be provided on the surface of the insulating resin layer and the insulating resin layer can be heated.

Claims: A method of forming a smooth-surfaced insulating layer on a substrate (10) from resin having photo-curing and thermosetting properties, the method comprising the steps of: forming (100) a circuit pattern on the substrate; applying (102) an insulating resin solution having photo-curing and thermosetting properties onto the substrate and circuit pattern to form an insulating resin layer; drying (104) the insulating resin layer; photo-curing (108) the insulating resin layer to form a pattern; developing (110) the pattern formed by photo-curing; and thermosetting (112) the developed pattern; characterized in that the method further comprises the step of: after the drying (104) step, reducing (106) the concentration gradient of residual solvent in the cross sectional direction of the insulating resin layer by absorbing the solvent through the surface of said insulating resin layer. A method, as claimed in claim 1, wherein the absorption of the solvent is carried out through the surface of said insulating resin layer in a way that said insulating resin layer can pass close to a roller (8) impregnated with the solvent. A method of forming a smooth-surfaced insulating layer on a substrate (10) from resin having photo-curing and thermosetting properties, the method comprising the steps of: forming (100) a circuit pattern on the substrate; applying (102) an insulating resin solution having photo-curing and thermosetting properties onto the substrate and circuit pattern to form an insulating resin layer; drying (104) the insulating resin layer; photo-curing (108) the insulating resin layer to form a pattern; developing (110) the pattern formed by photo-curing; and thermosetting (112) the developed pattern; characterized in that the method further comprises the step of: after the drying (104) step, reducing (106) the concentration gradient of residual solvent in the cross sectional direction of the insulating resin layer by heating said insulating resin layer after providing a barrier layer on its surface which prevents the permeation of said solvent through it. A method, as claimed in claim 3, wherein the heating of said insulating resin layer is carried out after providing a barrier layer on its surface which prevents the permeation of said solvent through it by affixing a film on the surface of said insulating resin layer.

Inventors: SHIRAI MASAHARU, YAMADA SHINJI, SHIRAI, MASAHARU, YAMADA, SHINJI, YAMADA, SHINJI 27-4 MIDORI-CHO
Applicants: IBM, INTERNATIONAL BUSINESS MACHINES CORPORATION
Application year: 1993
Main classification: G03F7
IPC codes: H01L21, H05K3, G03F7