Abstract:
A photoresist composition including a polymer resin for forming a photoresist layer, a photosensitive chemical that changes the solubility of the photoresist layer when exposed to some form of radiation and 3-methoxybutyl acetate and 4-butyrolactone as a solvent, is provided. The composition has a good photosensitivity and remainder ratio and no unpleasant odor.

Description:
BACKGROUND OF THE INVENTION 
     (a) Field of the Invention 
     The present invention relates to a composition for a positive type photoresist for manufacturing fine circuit patterns such as liquid crystal display circuits or semiconductor integrated circuits and, more particularly, to a composition for a positive type photoresist, including polymer resin, that produces a photoresist layer, photosensitive chemical, and solvents without unpleasant odor. 
     (b) Description of the Related Art 
     For producing fine circuit patterns such as those used in liquid crystal display (LCD) circuits or semiconductor integrated circuits, a photoresist composition is uniformly coated or applied on an insulator layer or a conductive metal layer on a substrate. The coated or applied substrate is then exposed through a mask to some form of radiation, such as ultraviolet light, electrons, or X-rays. And the exposed substrate is developed to produce a desired pattern. The patterned photoresist film is used as a mask to remove the insulator layer or the conductive metal layer. The remaining photoresist layer is removed to complete the fine pattern onto the substrate surface. Photoresist compositions are classified into negative and positive types depending on whether the exposed areas of photoresist coating become insoluble or soluble. Recently, positive type photoresist compositions have been preferred because the positive type photoresists can form smaller patterns than the negative type photoresists. 
     The important properties of photoresist compositions for commercial use are photosensitivities, contrast, resolution, adhesivity to the substrate, remainder ratio and safety. 
     Photosensitivity refers to the speed how fast the photoresist responds to light. Higher photosensitivity is required, particularly in applications where a number of exposures are performed to form multiple patterns by a repeated process. The other examples are when reduced light is used, like the projection exposure techniques that use light passed through a series of lenses and mono-chromatic filters. 
     Contrast refers to a ratio between the percentage of film loss in the exposed development area and the percentage of film loss on the unexposed area. Ordinarily, development of an exposed resist coated substrate is continued until the coating on the exposed area is completely dissolved away. Thus, development contrast can be determined simply by measuring the percentage of the film coating loss in the unexposed areas when the exposed coating areas are removed entirely. 
     Resolution refers to the capability of a photoresist composition how finely to reproduce the mask image utilized during exposure on the developed exposed spaces. In many industrial applications, particularly in the manufacture of LCDs or semiconductor integrated circuits, a photoresist is required to provide a high degree of resolution for very fine line and space widths of 1 μ or less. 
     Generally, photoresist composition includes a polymer resin for producing a photoresist layer, photosensitive chemicals, and a solvent. Various attempts have been made previously to improve the photosensitivity, contrast, resolution, and the safety of positive photoresist compositions. For example, U.S. Pat. No. 3,666,473 discloses the compound of a mixture of two phenol formaldehyde novolak resins together with a typical photosensitive chemical. U.S. Pat. No. 4,115,128 discloses that an organic acid cyclic anhydride was added to the phenolic resin and naphthoquinone diazide photosensitive chemical to increase photosensitivity. U.S. Pat. No. 4,555,069 discloses that novolak resin and o-quninone diazide photosensitivity chemical and propylene glycol alkyl ether acetate solvent are used for higher photosensitivity and for increased safety. 
     Various solvents have been developed to improve physical properties of the photoresist composition and work safety. For example, ethylene glycol mono ethyl ether acetate or ethyl lactate, propylene glycol mono ethyl ether acetate may be used as solvent. However, the photoresist composition including ethyl lactate exhibits poor adhesivity to the substrate and it is difficult to be uniformly coated on a substrate. Ethylene glycol mono ethyl ether acetate or propylene glycol mono ethyl ether acetate is toxic and generates an unpleasant odor. Accordingly, there is still a need for photoresist compositions that are suitable for various industrial applications, without sacrificing any one of the properties of photosensitivity, contrast, resolution, or solubility of polymer resin. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a composition for a positive type photoresist that exhibits high photosensitivity, remainder ratio, contrast, resolution, and substrate adhesivity for varieties of industrial applications. 
     It is another object to provide a composition for a positive type photoresist that is safe, without unpleasant odor, and environmentally friendly. 
     These and other objects may be achieved by a composition for positive type photoresist of the present invention. This composition includes a polymer resin for forming a photoresist layer, a photosensitive chemical that changes solubility of the photoresist layer when exposed to some form of radiation, and a solvent of 3-methoxybutyl acetate and 4-butyrolactone. The polymer resin is preferably a novolak resin and the photosensitive chemical is preferably a diazide-type compound. The ratio of 3-methoxybutylacetate:4-butyrolactone is 60 to 80 parts by weight 3-methoxybutylacetate to 2 to 10 parts by weight 4-butyrolactone. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The useful polymer resins employed in a photoresist composition of the present invention are well known in the related arts. A novolak resin, an exemplary polymer resin, may be employed in a photoresist composition of the present invention. The novolak resin is produced by reacting aromatic alcohol such as phenol, meta and/or para cresol with formaldehyde. To improve the photoresist, the molecular weight of the resin may be controlled by adjusting the ratio among high, medium or low molecular resins depending on the purpose of the photoresist. 
     The useful diazide compound for a photosensitive chemical may be produced by reacting polyhydroxy benzophenone with a diazide-based compound such as 1,2-naphtoquinonediazide, or 2-diazide-1-naphtol-5-sulfonic acid. 
     The solvent includes 3-methoxybutyl acetate and 4-butyrolactone. The ratio of 3-methoxybutyl acetate:4-butyrolactone is preferably 60 to 80 parts by weight 3-methoxybutyl acetate to 2 to 10 parts by weight 4-butyrolactone, and more preferably 65 to 75 parts by weight 3-methoxybutyl acetate to 2 to 5 parts by weight 4-butyrolactone. 
     3-methoxybutyl acetate exceeding the above range decreases the viscosity of the composition and renders the remainder ratio poor. On the other hand, 3-methoxybutyl acetate less than the above range increases the viscosity too much such that the ability to coat becomes poor. 4-butyrolactone exceeding the above range reduces the remainder ratio. On the other hand, 4-butyrolactone less than the above range may cause problems in obtaining the homogeneous composition and particles may easily occur on the substrate. 
     In the present invention, the photoresist composition includes preferably about 10 to about 25 wt. % of the polymer resin, about 4 to 10 wt. % of the photosensitive chemical, and 65 to 85 wt. % of the solvent. 
     If the amount of the polymer resin is less than 10 wt. %, the adhesion to substrate is reduced and the produced profile becomes bad. On the other hand, if the amount thereof is more than 25 wt. %, the increased viscosity reduces the photosensitivity and the ability to coat the composition. If the amount of the photosensitive chemical is less than 4 wt. %, the resolution is reduced and the profile becomes bad. However, more than 10 wt. % of photosensitive chemical easily produces scum or particles in coating the photoresist composition on the substrate. 
     Additives such as a colorants, dyes, anti-striation agents, plasticizers, adhesion promoters, speed enhancers, and/or surfactants may be added to the photoresist composition of the present invention. Such additives help to improve the performance of the photoresist composition. 
     The photoresist composition of the present invention can be applied to a substrate by such conventional method as dipping, spraying, whirling and spin coating. When spin coating, for example, the photoresist solution can be adjusted in respect to the percentage of solid contents in order to provide a coating of the desired thickness given the type of spinning equipment utilized and the spinning process. Suitable substrates include silicon, aluminum, silicon dioxide, doped silicon dioxide, silicon nitride, tantalum, copper, polysilicon, ceramics, and aluminum/copper mixtures or polymeric resins. 
     The substrate coated with photoresist composition is heated at 20° C. to 100° C. to perform soft-baking. This step permits the evaporation of the solvent without pyrolysis of a solid component in the photoresist composition. Generally, the concentration of the solvent is preferably reduced to minimum by the soft-baking step, and thus, the soft-baking step is performed until the solvent is mostly evaporated and a thin coating layer of photoresist remains on the substrate. 
     Next, the substrate coated with the photoresist layer is selectively exposed to light, particularly, ultraviolet light using a suitable mask to obtain a desirable pattern. The exposed substrate is then dipped into an aqueous alkaline developing solution until either the exposed photoresist layer is all or almost entirely dissolved. Suitable aqueous developing solution includes an aqueous solution including alkaline hydroxides, ammonium hydroxide, or tetramethylammonium hydroxide (TMAH). 
     The substrate with the exposed photoresist removed is then taken out from the developing solution. The resulting substrate is heat-treated to improve and to increase the chemical resistance of the photoresist layer. The step is called a hard-baking step. The hard-baking is done at a temperature below the softening point of the photoresist layer, preferably at about 100° C. to 150° C. 
     The developed substrate is treated with an etchant or with vapor plasma to etch the exposed portion and the remaining photoresist protects the substrate regions which it covers. The photoresist layer is removed from the etched substrate using a stripper to complete the pattern onto the substrate surface. 
     The following examples further illustrate the present invention. 
     EXAMPLE 1 
     A photoresist composition including 6.43 wt. % of a photosensitive chemical, 19.47 wt. % of a polymer resin, 70.4 wt. % of 3-methoxybutyl acetate, and 3.70 wt. % of 4-butylolactone was drop-wise applied to 4 inch bare glass plates and the glass plates were then rotated at a constant rate. The resulting glass plates were heat-dried at 135° C. for 90 seconds to obtain a photoresist film layer with a thickness of 1.60 μ on the glass. The resulting glass plates were exposed to ultraviolet light using a mask and then dipped into a 2.38% tetramethyl ammonium hydroxide (TMAH) aqueous solution for 75 seconds to remove the exposed portions and obtain photoresist patterns. 
     It was found that the photoresist composition had a good photosensitivity. Furthermore, since the thickness is linearly increased according to the increases in the spin speed, layer thickness can be effectively predicted if the spin speed is changed. 
     The positive type photoresist composition of the present invention has increased photosensitivity and rendered a good remainder ratio and thickness of layer so that the composition can be easily applied to various industrial applications. Furthermore, the composition has no unpleasant odor so that it can contribute to a favorable work environment. 
     While the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art will appreciate that various modifications and substitutions can be made thereto without departing from the spirit and scope of the present invention as set forth in the appended claims.