Stock material or miscellaneous articles – Structurally defined web or sheet – Including aperture
Reexamination Certificate
1999-07-27
2003-04-15
Watkins, III, William P. (Department: 1772)
Stock material or miscellaneous articles
Structurally defined web or sheet
Including aperture
C428S131000, C428S156000, C359S619000, C359S621000, C205S118000, C205S122000, C205S220000, C205S188000, C205S317000
Reexamination Certificate
active
06548144
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a semispherical microstructure and an array of semispherical microstructures, a microlens (a semispherical microstructure usable as a lens), and a fabrication method of the semispherical microstructure. In this specification, the term “semispherical” is used in a broad sense including “semicylidrical” or the like, as well as “semispherical” in an ordinary sense or the like.
2. Description of the Related Background Art
A microlens array typically has a structure of arrayed minute lenses each having a diameter from several microns to several hundreds of microns and an approximately semispherical profile. The microlens array is usable in a variety of applications, such as liquid-crystal display devices, light receivers and inter-fiber connections in optical communcation systems.
Meanwhile, earnest developments have been made with respect to a surface emitting laser and the like which can be readily arranged in the form of an array at narrow pitches between the devices. Accordingly, there exists a great need for a microlens array with narrow lens intervals and a large numerical aperture (NA).
Likewise, a light receiving device, such as a charge coupled device (CCD), has been more and more downsized as semiconductor processing techniques develop and advance. Therefore, also in this field, the need for a microlens array with narrow lens intervals and a large NA is increasing.
In the microlens array field, a desirable structure is a microlens with a large light-condensing efficiency which can highly efficiently utilize light incident on its lens surface.
Further, similar desires exist in prospective fields of optical information processing, such as optical parallel processing-operations and optical interconnections. Furthermore, display devices of active or self-radiating types, such as electroluminescent (EL) panels, have been extensively studied and developed, and a highly-precise and highly-luminous display has been thus proposed. In such a display, a desire increases for a microlens array which can be produced at a relatively low cost and with a large area as well as with a small lens size and a large NA.
A highly-precise and highly-luminous liquid-crystal (LD) display has also been developed. Also in this field, there is an increased desire increases for a microlens array which can be produced at a relatively low cost and with a large area, and which can concentrate light into an LD portion to increase its luminance.
In those situations, there are presently various prior art methods of fabricating microlenses.
In a prior art microlens-array fabrication method using an ion exchange method (see M. Oikawa, et al., Jpn. J. Appl. Phys. 20(1) L51-54, 1981), a refractive index is raised in a distributed condition at plural places on a substrate of multi-component glass by using an ion exchange method. A plurality of lenses are thus formed at high-refractive index portions.
A method, in which photosensitive glass is thermally treated and its non-sensitized portion is crystallized to expand its surface, is also known.
In these methods, however, a lens diameter cannot be made large, compared with intervals between lenses, and it is hence difficult to design a lens with a large NA.
Further, the fabrication of a large-area microlens array is not easy since a large scale manufacturing apparatus, such as an ion diffusion apparatus, is required to produce such a microlens array.
Moreover, an ion exchange process is needed for each glass, in contrast with a molding method using a mold. Therefore, variations of lens qualities, such as a focal length, are likely to increase between lots unless the management of fabrication conditions in the manufacturing apparatus is carefully conducted. In addition to the above, the cost of this method is relatively high, as compared with the method using a mold.
Further, substrate material is limited to glass in those methods. Particularly, in the ion exchange method, alkaline ions for ion-exchange are indispensable in a glass substrate, and, therefore, the material of the substrate is limited to alkaline glass. The alkaline glass is, however, unfit for a semiconductor-based device which needs to be free of alkaline ions.
Furthermore, since a thermal expansion coefficient of the glass substrate greatly differs from that of a substrate of a light radiating or receiving device, misalignment between the microlens array and the devices is likely to occur due to a misfit between their thermal expansion coefficients as an integration density of the devices increases.
Moreover, a compressive strain inherently remains on the glass surface which is processed by the ion exchange method. Accordingly, the glass tends to warp, and hence, a difficulty of junction or bonding between the glass and the light radiating or receiving device increases as the size of the microlens array increases.
In another prior art microlens-array fabrication method using a resist reflow method (see D. Daly, et al., Proc. Microlens Arrays Teddington., p23-34, 1981), resin formed on a substrate is cylindrically patterned using a photolithography process and a microlens array is fabricated by heating and reflowing the resin. Lenses having various shapes can be fabricated at a low cost by this resist reflow method. Further, this method has no problems of thermal expansion coefficient, warp and so forth, in contrast with the ion exchange method.
Further, in this method, the resist is directly patterned on a device, such as the surface emitting laser, and reflowed. Hence, the microlens can be formed directly on the device and an alignment process of bonding the microlens and the device can be omitted.
In the resist reflow method, however, the profile of the microlens is strongly dependent on the thickness of resin, wetting condition between the substrate and resin, and heating temperature. Therefore, variations between lots are likely to occur while a fabrication reproducibility per a single substrate surface is high.
Further, when adjacent lenses are brought into contact with each other due to the reflow, a desired lens profile cannot be secured due to the surface tension. Accordingly, it is difficult to achieve a high light-condensing efficiency by bringing the adjacent lenses into contact and reducing an unused area between the lenses. Furthermore, when a lens diameter from about 20 or 30 microns to about 200 or 300 microns is desired, the thickness of deposited resin must be large enough to obtain a spherical surface by the reflow. It is, however, difficult to uniformly and thickly deposit the resin material having desired optical characteristics (such as refractive index and optical transmissivity). Thus, it is difficult to produce a microlens with a large curvature and a relatively large diameter.
In another prior art method, an original plate of a microlens is fabricated, lens material is deposited on the original plate and the deposited lens material is then separated. The original plate or mold is fabricated by an electron-beam lithography method (see Japanese Patent Application Laid-Open No. 1 (1989)-261601), or a metal-plate etching method (see Japanese Patent Application Laid-Open No. 5 (1993)-303009). In these methods, the microlens can be reproduced by molding, variations between lots are unlikely to occur, and the microlens can be fabricated at a low cost. Further, the problems of alignment error and warp due to the difference in the thermal expansion coefficient can be solved, in contrast with the ion exchange method.
In the electron-beam lithography method, however, an electron-beam lithographic apparatus is expensive and a large amount of investment in equipment is hence needed. Further, it is difficult to fabricate a mold having a large area more than 100 cm
2
(10 cm-square) because the electron beam impact area is limited.
Further, in the metal-plate etching method, since an isotropic etching using a chemical action is principally employed, an etching of the metal plate into a desired profile c
Teshima Takayuki
Yagi Takayuki
Canon Kabushiki Kaisha
Fitzpatrick ,Cella, Harper & Scinto
Watkins III William P.
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