Printed wiring boards are conventionally produced by photolithography. Photolithography refers to a method in which a photosensitive resin composition is coated onto a substrate, an exposed portion of the photosensitive resin composition is polymerized and cured by exposing to a light pattern, a resist pattern is formed on the substrate by removing the unexposed portion with a developing solution, and after having formed a semiconductor pattern by etching or plating treatment, the resist pattern is separated and removed from the substrate to form a semiconductor pattern on the substrate.
In the photolithography method as described above, in the lamination of a layer composed of a photosensitive resin composition (to be referred to as a “photosensitive resin layer”) on a substrate, either a method in which a photoresist solution is coated onto the substrate and dried, or a method in which a photosensitive resin laminate (referred to as a “dry film resist”), comprised by sequentially laminating a support, a photosensitive resin layer and as necessary, a protective layer, is laminated onto a substrate, is used. The latter dry film resist method is frequently used when producing printed wiring boards.
The following provides a brief description of a method for producing a printed wiring board using the dry film resist method as described above.
First, in the case of having a protective layer such as a polyethylene film, this is separated from the photosensitive resin layer. Next, the photosensitive resin layer and a support are laminated on a substrate such as a copper clad laminate using a laminator so that the substrate, photosensitive resin layer and support are laminated in that order. Next, the exposed portion is polymerized and cured by exposing the photosensitive resin layer to activating light, such as i-rays (365 nm) emitted by a super high pressure mercury lamp, through a photomask having a wiring pattern. Next, the support composed of polyethylene terephthalate and the like is separated. Next, the unexposed portion of the photosensitive resin layer is removed by dissolving or dispersing with a developing solution such as a weakly alkaline aqueous solution to form a resist pattern on the substrate. Next, known etching treatment or pattern plating treatment is carried out by using the formed resist pattern as a protective mask. Finally, the resist pattern is separated from the substrate to produce a substrate having a conductor pattern, namely a printed wiring board.
Accompanying the decreasing size of wire spacing in printed wiring boards in recent years, demands on the dry film resist method to provide higher resolution are increasing. In addition, higher sensitivity is also being required from the viewpoint of improving productivity. On the other hand, exposure methods have become increasingly diversified according to the application, and mask-less exposure methods such as laser direct writing, which eliminate the need for a photomask, are demonstrating rapid proliferation. The light sources used in mask-less exposure can be broadly divided into those using i-rays (365 nm) and those using h-rays (405 nm), and these light sources are used according to the application by taking advantage of their respective merits. In the case of dry film resist methods, emphasis is placed on being able to use both types of exposure devices under the same conditions, or in other words, demonstrating equal sensitivity for both types of exposure devices and being able to form highly sensitive, high-resolution and highly adhesive resist patterns.
In the case of photosensitive resin compositions for dry film resists, benzophenone, Michler's ketone and derivatives thereof conventionally used as photopolymerization initiators demonstrated localized absorbance in the vicinity of a wavelength of 360 nm. Thus, the sensitivity of a dry film resist using these photopolymerization initiators decreases as the wavelength of the exposure light source approaches the visible region, and it is difficult to obtain adequate resolution and adhesion for light sources having a wavelength of 400 nm or more.
In addition, other photopolymerization initiators in the form of thioxanthone and derivatives thereof can be combined to demonstrate high sensitivity to exposure light sources having a wavelength in the vicinity of 380 nm by selecting a suitable intensifier. However, even if such combinations are used, there are many cases in which adequate sensitivity is unable to be obtained in the formed resist pattern, and sensitivity again decreases with respect to exposure light sources having a wavelength of 400 nm or more.
Patent Document 1 discloses photoreaction initiators having high photosensitivity and satisfactory image reproducibility in the form of hexaaryl biimidazole and 1,3-diaryl-pyrazoline or 1-aryl-3-aralkenyl-pyrazoline, and describes examples of fabricating a dry film resist. However, when the inventors of the invention fabricated dry film photoresists having photosensitive resin layers containing compounds specifically described in Patent Document 1 in the form of 1,5-diphenyl-3-styryl-pyrazoline and 1-phenyl-3-(4-methyl-styryl)-5-(4-methyl-phenyl)-pyrazoline, the compounds remained in the photosensitive resin layer in the form of undissolved substances, thereby preventing use as dry film resists.
Although Patent Documents 2 and 3 describe examples using 1-phenyl-3-(4-tert-butyl-styryl)-5-(p-tert-butyl-phenyl)-pyrazoline, although this compound demonstrates high sensitivity to h-rays having a wavelength of 405 nm, sensitivity equal to that of exposure by h-rays was unable to be obtained with i-rays having a wavelength of 365 nm. For reasons such as this, there is a need for a photosensitive resin composition that demonstrates favorable compatibility as a photosensitive resin composition for dry film resist methods, has equal sensitivity to both i-ray and h-ray light sources, and demonstrates satisfactory sensitivity along with high resolution and adhesion.    Patent Document 1: Japanese Unexamined Patent Publication No. H04-223470    Patent Document 2: Japanese Unexamined Patent Publication No. 2005-215142    Patent Document 3: Japanese Unexamined Patent Publication No. 2007-004138