Via hole structure and process

Abstract: When an insulating film of a multilayer interconnected board or the like is made of two or more coatings so as to build up the required thickness, it can be difficult to provide a via hole with a suitably tapered shape such that when a metallic film for wiring is formed on the composite insulating film, the metal film can completely fill up the via holes. The via holes 7 of the invention are given a gradually increasing diameter from the bottom toward the top of the insulating layer 8 comprising a plurality of preferably photosensitive insulating resin films 2,5, by forming an aperture in each film 2,5 as it is applied, these apertures having successively smaller diameters.

Description: The present invention relates to a via hole structure for interlayer connection formed in an insulating film and a process for the formation thereof. More particularly, in the present invention, fine via holes for interlayer connection are formed in a protective film, an interlayer insulating film, or other film of a multilayer interconnected board, a printed circuit for high-density mounting, a printed circuit board, a wiring board, an electronic component, etc. so as to have a shape such that in the subsequent step of depositing a metallic film for wiring, the metallic film can successfully fill up the via holes. In recent years, polyimide resin has attracted attention as an organic insulating film. Although polyimide has heat resistance, an insulating property, and toughness, it does not inherently have a photosensitive property. For this reason, in order to form fine via holes in a polyimide film, it is necessary to use a process wherein etching is conducted through the use of a resist mask. This involves the following troublesome steps. At the outset, a polyimide precursor to which no photosensitive property has been imparted is coated on a substrate such as a printed circuit board or a silicon wafer by spin coating, spray coating, roll coating, or the like. A photoresist is then coated thereon by the same coating method as that described above. The coatings are prebaked to evaporate the solvent contained in the polyimide precursor or photoresist to form a photosensitive plate. The photosensitive plate is then covered with a photomask having a via hole pattern formed thereon, and light such as an ultraviolet ray is applied from the top of the photomask to conduct exposure. The resist is then developed, thereby forming a resist pattern corresponding to the via hole pattern. In the development of the photoresist, since the photoresist is usually treated with a solvent, it is necessary to conduct post-baking after the treatment. Then the resist pattern is used as a mask and subjected to plasmas etching and wet etching to etch the polyimide film, thereby forming via holes in the polyimide film. Finally, the photoresist film is peeled off and imidation heat-treatment is performed, thereby forming a polyimide film having via holes formed therein. At the time of the wet etching, since the polyimide film is subjected to isotropic etching, when via holes having a large aspect ratio (thickness of film/diameter of via hole) are formed, the upper side wall is unfavourably shaved and tapered, which hinders an increase in the density. This is because the taper angle of the wet etching is generally fixed and is as small as 40°. A photosensitive polyimide which enables via holes to be formed without the aid of any photoresist has been developed for the purpose of solving the above problem and reducing the troublesome treatment step. The photosensitive polyimide has a photosensitive functional group attached to a molecule of the polyimide per se, which causes an exposed region alone to be photoreacted and thus make the solubility of the exposed region lower than that of the unexposed region. This in turn causes only the exposed region, corresponding to the mask pattern, to remain following development with a solvent. Further, rinsing is conducted through the use of ethyl alcohol or isopropyl alcohol, incapable of dissolving the exposed portion, to complete the removal of dissolved dust. Finally, heat treatment is carried out to remove the photosensitive functional group having a poor heat resistance through depolymerization, thus causing only the polyimide portion having a good heat resistance to be left for imidization. The method of introducing the photosensitive functional group varies depending upon the manufacturers of the material. Examples include a covalent bond type and an ionic bond type. Figure 6 is a schematic cross-sectional view of a multilayer interconnected board. In the drawing, 1 designates a substrate, 8 an insulating layer, 13 a wiring layer, and 7 a via hole for connection between wiring layers. The thickness of a single coating of polyimide is only about 10 µm, so that in order to impart a satisfactory insulating property to the insulating layer 8, it is necessary to carry out two or more coatings. When a via hole 7 is formed in an insulating layer 8 comprising a plurality of polyimide film layers, it is useful for the via hole to be normally tapered as shown in Fig. 5 (a) from the viewpoint of the subsequent formation of a metallic film. The metallic film for wiring is usually formed by a dry process such as vacuum deposition or sputtering. In this case, when the shape of the via hole is such that the side is vertical as shown in Fig. 5 (b) or reversely tapered as shown in Fig. 5 (d), it becomes difficult for the metallic film for the wiring to be deposited within the via hole, or there occurs a region where the metallic film for wiring cannot be completely deposited. In order to solve this problem, proposal has been made of the following process for the formation of a via hole in a multi-coating insulating layer. As shown in Fig. 4 (a), a first polyimide film 2 is coated on a substrate 1 and subjected to selective exposure and development to form first via holes 4. Then, as shown in Fig. 4 (b), a second polyimide film layer 5 is coated on the first. Thereafter, as shown in Fig. 4 (c), a negative working glass mask 6 on which a pattern or via holes having a larger diameter than that of the first via holes 4 has been formed is placed so that the centres of the holes coincide with those of the first via holes 4, and the assembly is then exposed. Then, after a development treatment is conducted as shown in Fig. 4 (d), a curing treatment is conducted to form via holes 14 wherein the aperture of the via holes in the upper layer is larger than that of the aperture of the via holes in the lower layer as shown in Fig. 4 (e). Although the above-described via holes are widened upward, they are not of a tapered form. This often hinders a metallic film from completely filling up the via holes at the time of the deposition of a metallic film for wiring. Accordingly, an object of the present invention is to eliminate the above-described drawback and to provide a via hole structure wherein via holes are formed in an insulating film of a multilayer interconnected board or the like so as to have a shape such that a metallic film can completely fill up the via holes when a metallic film for wiring is formed on the insulating film. According to a first aspect of the present invention a via hole structure is provided for a multilayer interconnected board including an insulating layer itself comprising a plurality of insulating resin film or photosensitive insulating resin film layers and a wiring layer comprised of an electric conductor, in which the aperture of the via hole formed in the insulating layer gradually increases from the bottom toward the top of the insulating layer. The process for the formation of via holes in accordance with the invention includes the steps of forming a first preferably photosensitive insulating resin film, forming a first via hole in the first film, forming a second such film on the first, and forming a second via hole having a smaller diameter than that of the first via hole in the second insulating resin film, the centre of the second via hole 7 coinciding with that of the first. The via holes 7 of the invention are thus given a gradually increasing diameter, or aperture area, from the bottom toward the top of the composite insulating layer 8; this is achieved by forming an aperture in each component film 2, 5 as it is applied, these apertures having successively smaller diameters. In the via hole structure, it is preferred that the diameter of the upper part of the via holes be 300 µm or less and the preferably photosensitive insulating resin material comprise a polyimide or a polyimide precursor. A metal wiring film is generally formed in the via holes by the dry process. The total thickness of the insulating layer is advantageously in the range of 2 to 100 µm. The via hole structure can be formed in a protective film, an interlayer insulating film, or other film of a multilayer interconnected board, a printed circuit for high-density mounting, a printed circuit board, a wiring board, an electronic component, etc. In the process for the formation of via holes, the above-described insulating resin film is preferably formed by dissolving a resin material in at least one solvent and applying the resultant coating solution. The present inventors have found that in forming via holes 7 in an insulating layer 8 comprising a plurality of preferably photosensitive insulating resin film layers 2,5, normally tapered via holes having an aperture increasing generally continuously from the bottom toward the top of the insulating layer can be formed by forming a first insulating resin film layer 2, forming first via holes 4 in the first layer, forming a second photosensitive insulating resin film layer 5, and forming second via holes 7 having a smaller size than that of the first via holes 4 in such a way that their centres coincide with those of the first via holes 4. When the photosensitive resin insulating film comprises three layers or more, the above-described coating and via-formation steps may be successively conducted for upper photosensitive insulating resin film layers. Further, it was experimentally confirmed that the taper angle of the via holes can be freely varied by adjusting the size of the via holes formed in the lower photosensitive insulating resin film layers. It is not always necessary to use photosensitive insulating resin films, although it is highly desirable for the uppermost layer; use may be made of non-photosensitive insulating resin films for the lower layers. This, however, involves a drawback that the process of forming via holes becomes troublesome. It is also possible to use a non-photosensitive insulating resin film for all the layers including the uppermost layer. In this case as well, the process of forming via holes becomes still more troublesome and further the shape of the via holes becomes somewhat inferior to the case where use is made of a photosensitive insulating resin film. Examples of the invention will now be described with reference to the accompanying drawings, in which Figs. 1 (a)-(f) schematically show a process for forming via holes according to the first and second examples; Figs. 2 (a)-(c) are cross-sectional views of via holes observed under SEM; Figs. 3 (a)-(e) show a process for forming via holes according to the third example; Figs. 4 (a)-(e) show a process for forming via holes according to the prior art; Figs. 5 (a)-(c) show via holes having various shapes; and Fig. 6 is a cross-sectional view of a multilayer interconnected board. First Example As shown in Fig. 1 (a), a first photosensitive polyimide precursor varnish layer 2 (non-volatile matter: 17.5% by weight) was spin-coated on a 3-in. silicon wafer 1, subjected to a pretreatment and prebaked at a temperature of 110°C for one hour. The film thickness after prebaking was 14 µm. Then, a negative glass mask 3 having a pattern of via holes having a diameter of 50 µm formed thereon was placed on the silicon wafer 1, which was then irradiated with ultraviolet light having a wavelength of 365 nm at an exposure of 250 mJ/cm². As shown in Fig. 1 (b), the exposed film was subjected to ultrasonic development using a N-methyl-2-pyrrolidine solution and rinsed with an ethyl alcohol solution. This caused the exposed pattern regions to be left, while the unexposed portions, representing the via hole portions, were eluted. The film was then semi-cured at a temperature of 200°C for 30 minutes to form a 10µm-thick first polyimide film layer 2 having first via holes 4 formed therein. As shown in Fig. 1 (c), a second photosensitive polyimide precursor varnish layer 5 was spin-coated thereon and prebaked at a temperature of 110°C for one hour. As shown in Fig. 1 (d), a negative glass mask 6 having a pattern of via holes having a diameter of 20 µm formed thereon was placed so that the centers coincided with those of the first via holes 4, followed by irradiation with ultraviolet light having a wavelength of 365 nm at an exposure of 350 mJ/cm². As shown in Fig. 1 (e), the exposed film was subjected to ultrasonic development using a N-methyl-2-pyrrolidone solution and rinsed with an ethyl alcohol solution. This caused the exposed pattern regions to be left,with the via hole portions as the unexposed portions being eluted, thereby forming second via holes 7. As shown in Fig. 1 (f), the film was semi-cured at a temperature of 200°C for 30 minutes and then at a temperature of 350°C for 30 minutes to form an insulating layer 8 having a total film thickness of 15 µm, which comprised two polyimide film layers 2,5 having normally tapered via holes 7 formed therein. The section of a via hole 7 formed in an insulating layer 8 comprising two polyimide film layers 2,5 was observed under a scanning electron microscope (SEM). As a result, as shown in Fig. 2 (a), it was observed that via holes comprising a normally tapered structure having a lower aperture of 18 µm and an upper aperture of 45 µm were formed. A 2 µm-thick copper film and a 2 µm-thick aluminum film were formed by sputtering and vacuum deposition, respectively, and the section of the film was observed under a scanning electron microscope (SEM). As a result, no breaking of wire was observed. Comparative Example For comparison, via holes were formed in the same manner as that of Example 1, except that the aperture of the via holes formed in the first polyimide film layer 2 and the aperture of the via holes formed in the second polyimide film layer were 20 µm and 50 µm, respectively. The section of a via hole formed in the insulating layer was observed under a scanning electron microscope (SEM). As a result, there was no evidence of the formation of a normally tapered structure. Further, it was observed that the shape of the via holes in the upper layer had a somewhat reversely tapered structure. A 2 µm-thick copper film and a 2 µm-thick aluminum film were formed by sputtering and vacuum deposition, respectively, and a section of the film was observed under a scanning electron microscope (SEM). As a result, as shown in Fig. 2 (b), in both cases, breaking of wire was observed on the side portion of the via holes. In the drawing, the copper film or aluminum film is designated by a reference numeral 13. Second Example As shown in Fig. 1 (a), a first photosensitive polyimide precursor varnish layer 2 (nonvolatile matter: 14.5 % by weight) was spin-coated on a bell-shaped ceramic substrate 1 and prebaked at a temperature of 110°C for one hour. The film thickness after prebaking was 9 µm. Then, a negative glass mask 3 having a pattern of via holes having a diameter of 60 µm formed thereon was placed on the ceramic substrate, which was then irradiated with ultraviolet light having a wavelength of 365 nm at an exposure of 250 mJ/cm². As shown in Fig. 1 (b), the exposed film was subjected to ultrasonic development using a mixed solution comprising N-methyl-2-pyrrolidone and methyl alcohol and rinsed with an ethyl alcohol solution This caused the exposed pattern regions to be left, with the via hole portions as the unexposed portions being eluted. Then, the film was semi-cured at a temperature of 200°C for 30 minutes to form a 5 µm-thick polyimide film 2 as a first layer having first via holes 4 formed therein. As shown in Fig. 1 (c), a second photosensitive polyimide precursor varnish layer 5 was spin-coated thereon and prebaked at a temperature of 110°C for one hour. As shown in Fig. 1 (d), a negative glass mask 6 having a pattern of via holes having a diameter of 30 µm formed thereon was placed so that the centers coincided with those of the first via holes 4 having a diameter of 60 µm formed in the first layer, followed by irradiation with ultraviolet light having a wavelength of 365 nm at an exposure of 400 mJ/cm². As shown in Fig. 1 (e), the exposed film was subjected to ultrasonic development using a mixed solution comprising N-methyl-2-pyrrolidone and methyl alcohol and rinsed with an ethyl alcohol solution. This caused the exposed pattern regions to be left with the via hole portions as the unexposed portions being eluted, thereby forming second via holes 7. As shown in Fig. 1 (f), the film was semi-cured at a temperature of 200°C for 30 minutes and then at a temperature of 300°C for 30 minutes to form an insulating layer 8 having a total film thickness of 10 µm which comprised two polyimide film layers 2,5 having normally tapered via holes 7 formed thereon. The section of a via hole 7 formed in an insulating layer 8 comprising two polyimide film layers 2,5 was observed under a scanning electron microscope (SEM). As a result, as shown in Fig. 2 (a), it was observed that via holes comprising a normally tapered structure having a lower aperture of 30 µm and an upper aperture of 50 µm were formed. A 2 µm-thick copper film and a 2 µm-thick aluminum film were formed by sputtering and vacuum deposition, respectively, and the section of the film was observed under a scanning electron microscope (SEM). As a result, no breaking of wire was observed. Third Example As shown in Fig. 3 (b), a third photosensitive polyimide precursor varnish layer 9 was spin-coated on the insulating layer comprising two polyimide film layers 2,5 shown in Fig. 3 (a) prepared according to the second example, and prebaked at a temperature of 110°C for one hour. As shown in Fig. 3 (c), a negative glass mask 10 having a pattern of via holes having a diameter of 20 µm formed thereon was placed so that the centers coincided with those of the via holes 7 having a diameter of 30 µm formed in the second polyimide film layer 5, followed by irradiation with ultraviolet light having a wavelength of 365 nm at an exposure of 400 mJ/cm². As shown in Fig. 3 (d) , the exposed film was subjected to ultrasonic development using a mixed solution comprising N-methyl-2-pyrrolidone and methyl alcohol and rinsed with an ethyl alcohol solution. This caused the exposed pattern regions to be left with the via hole portions as the unexposed portions being eluted, thereby forming third via holes 11. As shown in Fig.3 (c), the film was semi-cured at a temperature of 200°C for 30 minutes and then at a temperature of 300°C for 30 minutes to form an insulating layer 12 having a total film thickness of 15 µm which comprised three polyimide film layers 2,5,9 having via holes 11 formed therein. The section of a via hole formed in the insulating layer 12 comprising three polyimide film layers 2,5,9 was observed under a scanning electron microscope (SEM). As a result, as shown in Fig. 2 (c), it was observed that via holes comprising a normally tapered structure having a lower aperture of 18 µm and an upper aperture of 50 µm were formed. From the above results, it can be easily estimated that even when the insulating layer comprises at least three layers, via holes having a normally tapered structure can be formed by making the apertures of via holes formed in the uppermost photosensitive insulating film layer smaller than that of via holes formed in the lower photosensitive insulating film. As described above, in the via hole structure and the process for the formation thereof according to the present invention, since via holes having a normally tapered structure can be formed by successively forming via holes in a plurality of insulating resin film or photosensitive insulating resin film layers constituting the insulating layer so as to have apertures gradually reduced from the lower layer toward the upper layer, a metallic film can successfully fill up the via holes at the time of the formation of a metallic film for wiring conducted in a subsequent step, which greatly contributes to an improvement in the reliability of electronic components.

Claims: A via hole structure of a multilayer interconnected board including an insulating layer (8), itself comprising a plurality of insulating resin films (2,5), and an electrically conducting wiring layer (13) lying on the insulating layer (8) and filling the via hole (7), characterised in that the aperture of the via hole (7) formed in the insulating layer (8) increases gradually from the bottom toward the top of the insulating layer. A via hole structure as set forth in claim 1, wherein the insulating resin films (2,5) are made of a photosensitive resin. A via hole structure as set forth in claim 1 or 2, wherein the diameter of the top of the aperture of the via hole (7) is 300 µm or less. A via hole structure as set forth in any preceding claim, wherein the insulating resin comprises polyimide. A via hole structure as set forth in any preceding claim, wherein the thickness of the insulating layer (8) is in the range of 2 to 100 µm. A via hole structure as set forth in any preceding claim and further including a metal film formed in the via hole (7). A protective film or interlayer insulating film of a printed circuit for high-density mounting, a printed circuit board, a wiring board, or an electronic component, including one or more via hole structures as set forth in any preceding claim. A process for the formation of a via hole structure in an insulating layer (8) comprising a plurality of insulating resin films (2,5), including the steps of: forming a first insulating resin film (2) and forming a first aperture (4) in the first insulating resin film; and forming a second insulating resin film (5) on the first and forming a second aperture (7) in the second insulating resin film (5) whose centre coincides with that of the first aperture (4); characterised in that the second aperture (7) is smaller than the first aperture (4). A process according to claim 8, wherein the insulating resin films (2,5) are made of a photosensitive resin. A process according to claim 8 or 9, wherein the insulating resin films (2,5) are each formed by applying a coating solution of an insulating resin in at least one solvent. A process according to any of claims 8 to 10, wherein the insulating resin comprises polyimide or a polyimide precursor. A process according to any of claims 8 to 11, and including the further step of forming a metal film in the via hole (7) by a dry process. A process according to any of claims 8 to 12, wherein the thickness of the insulating layer (8) is in the range of 2 to 100 µm. A process according to any of claims 8 to 13, in which the insulating layer (8) is a protective film or an interlayer insulating film of a printed circuit for high-density mounting, a printed circuit board, a wiring board, or an electronic component.

Inventors: HORIKOSHI EIJI FUJITSU LTD KAM, KAWAMURA ISAO FUJITSU LTD KAMI, MIYAHARA SHOICHI FUJITSU LTD K, SASAKI MAKOTO FUJITSU LTD KAMI, TANI MOTOAKI FUJITSU LTD KAMIK, HORIKOSHI, EIJI, FUJITSU LTD. 1015, KAMIKODANAKA, KAWAMURA, ISAO, FUJITSU LTD. 1015, KAMIKODANAKA, MIYAHARA, SHOICHI, FUJITSU LTD. 1015, KAMIKODANAKA, SASAKI, MAKOTO, FUJITSU LTD. 1015, KAMIKODANAKA, TANI, MOTOAKI, FUJITSU LTD. 1015, KAMIKODANAKA
Applicants: FUJITSU LTD, FUJITSU LIMITED
Application year: 1992
Main classification: H05K3
IPC codes: H05K3, H01L21, H01L23