A laser processing method for a glass substrate, and a microlens array obtained thereby

Abstract: A laser processing method for removing glass by melting, evaporation or ablation from sheet-like glass substrate for forming microscopic concavities. Diffraction grating and planar microlens array obtained thereby.

Description: TECHNICAL FIELD The present invention relates to a laser processing method for glass substrates, and a microlens array which can be obtained thereby. BACKGROUND ART Silicate glass composed primary of SiO 2 is highly transparent and can easily be molded (deformed) at high temperatures. Sheets of silicate glass, which have been formed with holes or concavities and convexities by microscopic topographic processing, are widely used as glass substrates for optical components used for optical communications and display devices. In order to make a hole in a sheet of silicate glass according to microscopic topographic processing, it has been the general practice to process the sheet of silicate glass with wet etching (chemical etching) using an etchant of hydrofluoric acid or the like, or dry etching (physical etching) such as reactive ion etching. However, wet etching suffers problems with respect to management and processing of the etchant. Dry etching requires pieces of equipment such as a vacuum container, needs a large-scale apparatus, and is not efficient because a pattern mask has to be produced by complex photolithography. Laser beams have an intensive energy, and have heretofore been used to increase the temperature of a surface of a material to which the laser beam is applied thereby to ablate or evaporate a portion of the material to which the laser beam is applied, for processing the material in various ways. Since the laser beam can be focused into a very small spot, it is suitable for microscopic topographic processing of a material. Then, in Japanese Patent Laid-Open No. 54-28590 (1979), there is disclosed processing of a glass substrate surface by radiating it with a laser beam while moving a table in X-Y directions, on which table is fixedly mounted the class substrate already heated to 300 through 700 °C. Although, as mentioned above, the concavities and convexities of desired shape can be formed on the glass surface by moving the table in X-Y directions, the concavities and convexities cannot be created if it is for instance a microscopic pattern. such as of a diffraction grating. Moreover, the movement of the table generates dust, which results in defects in the products and decreases productivity thereof. As another method of manufacturing a planar microlens array etc., a stamper method has been already known, in which lens material is injected into a mold frame and the molded patterns are transplanted on the glass substrate and baked, however, it requires accurate positioning during the pattern transplanting process and the baking process, and it takes time. As another method of manufacturing a planar microlens array etc., it has been proposed to obtain a convex lens by forming concavities arc shaped in cross-section on the glass substrate surface with a wet etching and injecting plastic material of high refractive index into the formed concavities, thereby forming the convex lens with the concavities, however, the wet etching has the problems as mentioned above. Then, it is conceivable to form the concavities into which the plastic of high refractive index is injected by radiating a laser beam through a mask, however, since the laser beam has tendency of going straightforward and it has almost same intensity within area of one spot after passing through the openings of the mask, then the wall or the concavity formed on the glass substrate comes to be about perpendicular to the glass substrate, whereby it is impossible to obtain the cross-section of perfectly continuous arc shape. Therefore, it cannot be mounted onto apparatus requiring extremely high accuracy, such as a liquid crystal display, as it is, and it needs more or less treatment by wet etching and takes time. Laser beams are generated by an infrared laser such as a CO 2 laser, a Nd:YAG laser, a laser comprising a Nd:YAG laser combined with a wavelength conversion capability for producing a laser beam whose wavelength ranges from a near-infrared region through a visible region to an ultraviolet region, and an ultraviolet laser such as an excimer laser such as an ArF or Krf laser. If the CO 2 laser of long wavelength is used, cracking due to thermal strain occurs violently. If the ultraviolet KrF laser (wavelength of 248 nm) is used, cracking occurs around the area where the laser beam is applied, therefore it is not suitable for the microscopic topographic processing. Thus, the use of the ArF excimer laser of wavelength of 193 nm is optimum as the laser beam for glass processing, however, even when such an ArF excimer laser is used, because of absorption by air, it is needed to replace the air with absorption-free gas such as Ar, etc. or to keep a vacuum in order to allow the laser beam to reach as far away as possible. DISCLOSURE OF THE INVENTION The present invention has been made to resolve the conventional problems mentioned above, and an object thereof is to provide a laser processing method able to form microscopic concave patterns on a glass substrate surface with accuracy and within a short time period. Another object thereof is to provide a laser processing method to form a large number of the concavities on the glass substrate surface without movement of the glass substrate and by changing the light path. Furthermore another object thereof is to obtain a microlens array in accordance with the above method. For achieving the object mentioned above, according to the present invention, a laser processing method for a glass substrate comprises: radiating the laser beam having a regular intensity distribution on the glass substrate, absorbing energy of the laser beam into the glass substrate, and removing a part of the glass by melting, evaporation or ablation due to the energy, wherein microscopic concavities are formed on a surface of said glass substrate by changing the optical path of the laser beam with optical path changing means, thereby moving a spot position of the laser beam radiated on the surface of said glass substrate. Here, the optical path changing means can be constructed with a first mirror for moving the spot position of the laser beam in a X-direction on the surface of said glass substrate, and a second mirror for moving the spot position of the laser beam in a Y-direction on the surface of said glass substrate. For the mirrors, it is preferable to use a galvano mirror which turns through a small amount of angle depending on the current conducting through it. Furthermore, in the conventional art, the laser beam which is applicable to glass processing is limited to an ArF excimer laser of wavelength of 193 nm, and the device is big and complicated because of the necessity of replacement with non-absorbing gas, such as Ar or vacuum. However, according to a preferred embodiment of the present invention, it is experimentally ascertained that a laser beam having a wavelength longer than the above-mentioned is applicable to glass processing, by introducing silver into the glass in the form of Ag atoms, Ag colloid or Ag ions, without cracking or breakage, and the trace of the laser radiation is very smooth. However, in case that the glass contains silver in uniform concentration, such as the conventional light sensitive glass and/or antibacterial glass, no increase in processability can be found, therefore, it is necessary that it has a concentration slope of the silver showing the highest concentration at a side surface to be processed and gradually decreasing to the other side surface thereof. This is according to the mechanism shown in Fig. 1 and will be explained below. As shown in Fig. 1 (a), the laser beam is applied onto the surface having the highest Ag ion concentration. Then, as shown in Fig. 1 (b), the Ag ion is resolved to be a colloid (very fine particles of Ag) on the surface having the highest Ag ion concentration of the glass substrate. The Ag colloid particles absorb energy of the laser beam, as shown in Fig. 1 (c), and melting, evaporation or ablation occurs, whereby the glass of the surface layer is removed. After removing the glass of the surface layer, the same phenomenon occurs in subsequent glass layer, and concavities or penetrating holes are formed at the last as shown in Fig. 1 (d). In this way, since the glass is gradually removed from the top surface of the glass substrate, therefore cracking or breakage is hard to occur. On the contrary to this, in the glass substrate containing silver in uniform concentration or no silver, ablation occurs inside of the glass substrate, and therefore cracking or breakage occur easily. The invention also provides a planar microlens array as defined in claim 4. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 (a) to (d) are drawings for explaining the basic idea of a laser processing method relating the present invention; Fig. 2 is a block diagram of a device for use in ion exchange; Fig. 3 is a drawing outlining construction of a processing device which is applied for exercising a laser processing method of the present invention; Fig. 4 is a perspective view of the glass substrate during the processing; Fig. 5 is a cross-sectional view of the planar microlens array; and Fig. 6 is a cross-sectional view of a two dimensional optical fiber array. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Hereinafter, embodiments of the present invention will be explained in detail. Example Fig. 3 shows outlining construction of a processing apparatus which is applied for exercising the laser processing method of the present example. The processing device is constructed with a radiation source of a laserbeam, a first mirror, a second mirror, a lens and a table on which a glass substrate of sheet shaped is mounted. The first and the second mirrors are constructed with a galvano mirror which turns through a small amount of angle depending on the current conducting through it, and the first and the second mirrors are positioned so that the turning axes thereof cross or intersect at right angle. The lens is so positioned that the laser beam reflected by the second mirror is focused on the same plane, in this embodiment on the surface of the glass substrate. In the above, the spot position of the laser beam from the radiation source shifts in a X-direction on the surface of the glass substrate by rotating the first mirror, and the spot position of the laser beam shifts in a Y-direction on the surface of the glass substrate by rotating the second mirror. Consequently, by combining those operations, it is possible to shift the spot position of the laser beam to any position on the surface of the glass substrate, whereby the concavities can be sequentially formed on the glass substrate at a constant distance as shown in Fig. 4. Concretely, as the above glass substrate, the glass mainly contains SiO 2 in addition thereto, Al 2 O 3 , B 2 O 3 , Na 2 O, F, and has a thickness of 2 mm. An ion exchange was conducted using a device as shown in Fig. 2. The glass substrate was immersed in the molten salt (kept at 300°C) including a mixture of 50 mol% of silver nitrate and 50 mol % of sodium nitrate in the quartz container for 86 hours, thereby eluting Na ions in the surface of the glass substrate, and diffusing Ag ions in the molten salt into the glass. The thicknesses of the layers into which the Ag ions were diffused, as measured by a X-ray microanalyzer (XMA), were about 160 µm. By mounting the glass substrate mentioned above and moving the spot position of the laser beam of wavelength of 355 nm, the third highest harmonic wave of the Nd:YAG laser, a plurality of concavities are formed on the glass substrate at a distance of 125 µm. The pulse width of the laser beam which is used is about 10 nsec, the repetition frequency thereof is 5 Hz, and the radiation energy per one pulse is 30 J/cm 2 /pulse. One hundred pulses are shot to each one of the spots. A planar microlens array can be produced by filling plastic of high refractive index into the concavities of the glass substrate obtained by the above embodiment, as shown in Fig. 5. Fig. 6 shows a two dimension optical fiber array constructed as one element combined with the above sheet-type microlens array. This two dimension optical fiber array also can be obtained by forming holes piercing the glass substrate by the laser beam, inserting the one ends of the optical fibers into the holes, then fixing them with plastic which is curable by ultraviolet radiation, in the same manner as the planer microlens array. With forming the piercing holes at the positions corresponding to the positions of the lens portions of the planar microlens array mentioned above, as shown in Fig.6 the light beams incident upon the lens portions of the sheet-type microlens array are converged and enter into the optical fibers at the one end thereof. As explained above, according to the present invention, in processing the concavities and convexities of predetermined pattern on the glass substrate by the laser beam, a spot position of the radiated laser beam is moved by changing the optical path of the laser beam by the function of an optical path changing means, preferably including mirrors, while fixedly mounting the glass substrate, whereby the microscopic pattern of the concavities and convexities can be formed on a surface of said glass substrate, with accuracy and in short time. Since the table on which the glass substrate is mounted does not move, therefore, it is possible to suppress the generation of dust in the process and to increase productivity of the products. Further, the optical path changing means is constructed with a first mirror for moving the spot position of the laser beam vertically on the surface of the glass substrate, and a second mirror for moving the spot position of the laser beam horizontally on the surface of the glass substrate, whereby any kind of concavities and convexities can be formed on the glass substrate. Moreover, by constructing the mirrors with galvano mirrors, it is possible to perform very fine processing. In particular in the processing methods mentioned above, the glass substrate to be treated contains silver in the form of atoms, colloid or ions in it, and has a silver concentration showing the highest value on the top surface and gradually decreasing from the surface to the inferior thereof, and therefore no cracking nor breakage occurs even if using a laser beam having a relatively long wavelength. And, with using the laser beam having a relatively long wavelength, no consideration is needed about the absorption of the laser beam in air, and the device itself becomes simple. INDUSTRIAL APPLICABILITY. The laser processing method for the glass substrate according to the present invention is contributable to the fabrication of optical products, including a diffraction grating, a microlens array, etc. The diffraction grating according to the present invention can be incorporated in an optical coupler, a polariscope, a spectroscope, a reflector and a mode transducer, etc., and the microlens array according to the present invention can be incorporated in a liquid crystal display device, etc.

Claims: A laser processing method for a glass substrate, comprising radiating a laser beam having a regular intensity distribution on the glass substrate, absorbing energy of the laser beam into the glass substrate, and removing a part of the glass by melting, evaporation or ablation due to the energy, wherein microscopic concavities are formed on a surface of said glass substrate by changing the optical path of the laser beam with optical path changing means, thereby moving a spot position of the laser beam radiated on the surface of said glass subscrate. A laser processing method for a glass substrate as defined in claim 1, wherein said optical path changing means comprises a first mirror for moving the spot position of the laser beam in a X-direction on the surface of said glass substrate, and a second mirror for moving the spot position of the laser beam in a Y-direction on the surface of said glass substrate. A laser processing method for a glass substrate as defined in claim 1 or claim 2, wherein said glass substrate contains silver in the form of Ag atoms, Ag colloid or Ag ions to a predetermined depth or as a whole from the surface, and has a concentration slope of the silver showing the highest concentration at a side surface to be processed and gradually decreasing to the other side surface thereof. A planar microlens array which can be incorporated in a liquid crystal display element, wherein said planar microlens array is provided with regularly arranged concavities formed by radiation of a laser beam having a regular intensity distribution on a surface of a sheet-like glass substrate. A planar microlens array as defined in claim 4, wherein said sheet-like glass substrate constructing said diffraction grating contains silver in the form of Ag atoms, Ag colloid or Ag ions to a predetermined depth or as a whole from a surface, and has a concentration slope of the silver showing the highest concentration at a side surface to be processed and gradually decreasing to the other side surface thereof.

Inventors: HAMANAKA KENJIRO, KOYAMA TADASHI, OIKAWA MASAHIRO, TSUNETOMO KEIJI, HAMANAKA, KENJIRO, KOYAMA, TADASHI, OIKAWA, MASAHIRO, TSUNETOMO, KEIJI, Hamanaka, Kenjiro, Nippon Sheet Glass Co., Ltd., KOYAMA, TADASHI, C/O NIPPON SHEET GLASS CO., LTD., Oikawa, Masahiro, c/o Nippon Sheet Glass Co., Ltd., TSUNETOMO, KEIJI, C/O NIPPON SHEET GLASS CO., LTD.
Applicants: NIPPON SHEET GLASS CO LTD, NIPPON SHEET GLASS CO., LTD.
Application year: 1997
Main classification: C03B23
IPC codes: G02B6, C03B29, C03B23, G02B3, B23K26, C03C23