1. Field of the Invention
The present invention relates to a method of manufacturing a micro-lens array, and more particularly, to a method of precisely manufacturing a micro-lens array including a micro-lens having a high numerical aperture.
2. Description of the Related Art
A micro-lens may be manufactured by a method of forming a single micro-lens using a machining process, a method of forming a micro-lens array through a photolithography process using photoresist, etc.
Among conventional methods of manufacturing a micro-lens, a machining process directly grinds or cuts glass or plastic material. Injection molding or press molding is used to mass-produce an object lens. The injection molding or press molding is a method of manufacturing a lens from a melt or semi-molten plastic or glass using a die made by a machining process, such as a cutting or abrasive process. FIG. 1 is a schematic view showing a method of manufacturing a conventional single micro-lens using such a machining process.
A conventional method of manufacturing a glass single lens using a press molding will now be described with reference to FIG. 1. Upper and lower dies 11 and 13, which are made of super-hard metal, etc., are very precisely ground in a shape corresponding to a surface of a lens. A ball or gob-shaped glass preform is inserted between the upper and lower dies 11 and 13 and is pressed and heated at a temperature of 500° C. to 600° C., thereby forming a lens.
The method using an ultraprecise die has an advantage in that it can achieve a very precise curved surface process. However, as the size of the lens is made smaller, it is difficult to manufacture a die for a lens having a high numerical aperture, that is, a die for an aspheric surface having a large radius of curvature. This is because there is a limit to the radius of curvature of a processing tool. Also, such a glass molding method has a disadvantage in that productivity is low because it takes a long time to uniformly heat a preform material through its interior.
In the case where a lens is manufactured using a plastic material, a cavity die is molded by cutting a metal very precisely, and a melt plastic is injection molded to produce a lens. In plastic injection molding, the molding temperature is 300° C. or less, which is lower than the glass molding temperature. Also, the plastic injection molding can use a die having a plurality of cavities, so that manufacturing time is shorter and productivity is higher than that of glass molding. Further, the plastic lens is lightweight and processability is excellent. The plastic lens can be manufactured at a low cost. Therefore, the plastic lens has been widely used as a pickup lens for CD and DVD applications.
However, the plastic material has a low refractive index of about 1.5, such that light refraction is insufficient. Thus, two lenses must be used to manufacture a lens for a Blue-Ray Disc having a numerical aperture of 0.85. Considerable time and effort are required to assemble and precisely adjust the position of the two lenses, thereby increasing manufacturing cost much more than in the case of a single lens. Also, the plastic is susceptible to heat, and its optical characteristics (for example, refractive index, thermal expansion, etc.) can change greatly depending on a change in temperature. Specifically, the plastic absorbs blue wavelength light, thus causing a yellowing phenomenon after use over a long period of time. As a result, the property of the material is changed. Since the object lens of the optical pickup is used to condense light, such phenomenon may cause a serious problem in such a lens.
A single lens may be manufactured using a machining process, but it is difficult to assemble the single lens with a device such as an optical module. In order to facilitate assembly, a flat type or multiple array type is desired. In order to make an array type lens, a micro process instead of a machining process is widely used. Among them, a photolithography process is most typical.
FIGS. 2A through 2E are sectional views showing sequential procedures of manufacturing a conventional micro-lens array using a photolithography process. Referring to FIG. 2A, a photoresist 23 is coated on a substrate 21. Referring to FIG. 2B, a mask M having a predetermined pattern is positioned on the photoresist 23 and an ultraviolet ray is irradiated to expose the photoresist 23. Referring to FIG. 2C, the exposed portion of the photoresist 23 is developed to form a patterned photoresist 23a. Referring to FIG. 2D, a reflow process is performed by applying heat to the patterned photoresist 23a, thus deforming the photoresist 23a into a preform 23b having a shape of a circular spherical lens. Referring to FIG. 2E, the shape of the lens preform is transferred on the substrate by anisotropic etching, such as by reactive ion etching, thereby forming a micro-lens array on the substrate.
According to the above conventional method of manufacturing a micro-lens array using a photolithography process, it is difficult to implement a high sag for obtaining a high numerical aperture. Accordingly, it is difficult to process an aspheric curved surface for aberration correction.