Method of changing the surface of a glass substrate containing silver, by using a laser beam

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

TECHNICAL FIELD
 The present invention relates to a laser processing method for glass
 substrates, and a diffraction grating and a microlens array which can be
 obtained therefrom.
 BACKGROUND ART
 Silicate glass composed primary of SiO.sub.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 Unto 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 glass substrate already heated to 300 through 700.degree. 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 of 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.sub.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.sub.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 having a curved line cross section on the
 glass substrate surface.
 Further 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 diffraction grating and 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 on the glass substrate, absorbing energy of the
 laser beam into the glass substrate, and removing the glass by melting,
 evaporation or ablation due to the energy, wherein microscopic concavities
 and convexities are formed on a surface of the glass substrate, by
 partially varying the spacial distribution of the intensity of the laser
 beam applied upon the surface of the glass substrate, thereby removing a
 greater amount of glass where the intensity is stronger, and less where
 the intensity is weaker.
 A diffraction grating or a microlens array which can be incorporated into
 an optical coupler, a polariscope, a spectroscope, a reflector or a mode
 transducer, etc., can be manufactured by using a laser beam having
 periodical or regular distribution in intensity.
 The laser beam having the regular intensity distribution can be obtained by
 a phase mask or interference between two laser beam, and the periodical
 cross-sectional configuration of the concavities and convexities formed on
 the surface of the glass substrate can be controlled by the pulse energy
 of the laser beam. And, the laser beam having the regular intensity
 distribution can be obtained by using a mesh-like mask, etc.
 For achieving the another object mentioned above, according to the present
 invention, a laser processing method for a glass substrate comprises:
 disposing a mask at the focus position on the incident side of a lens,
 disposing the glass substrate at the focus position on the exit side of
 said lens, radiating the laser beam on the mask thereby forming a Fourier
 transform image on a surface of said glass substrate at the focus position
 of the exit side of said lens, absorbing energy of the Fourier transform
 image into the glass substrate, and removing the glass by melting,
 evaporation or ablation due to the energy, thereby forming a plurality of
 concavities periodically distributed on said glass substrate.
 Here, the laser beam penetrating the openings of the mask shows a
 rectanglar intensity distribution in which the intensity is nearly equal
 at the central and the peripheral portions. However, the Fourier image of
 the laser beam penetrating said mask shows a sinusoidal intensity
 distribution which has greater value at the central portion and a lesser
 value on the peripheral portion thereof. As the result of this, it is
 possible to form a number of concavities spreading on the surface of the
 glass substrate in two dimensions, with smoothly curved lines including
 arc lines in the cross sectional view. For example, applying it to a
 planar microlens array, it is possible to form a convex lens with high
 accuracy.
 Similarly for achieving the another object mentioned above, according to
 the present invention, a laser processing method for a glass substrate
 comprises: coinciding the focal point on the exit side of a first lens
 with the focal point on the incident side of a second lens, disposing a
 first mask at the focal point on the incident side of said first lens,
 disposing a second mask at focal point on the exit side of said first
 lens, disposing a glass substrate at the focal point on the exit side of
 said second lens, radiating the laser beam on the first mask thereby
 forming a Fourier transform image at the focal point on the exit side of
 said first lens as well as forming a part of a Fourier transform image on
 a surface of said glass substrate disposed at the focal point on the exit
 side of said second lens, absorbing energy of the formed image into the
 glass substrate, and removing the glass by melting, evaporation or
 ablation due to the energy, thereby forming a plurality of concavities
 periodically distributed on said glass substrate.
 The pattern of the concavities formed on the glass substrate surface by
 such a method is coincident with that of the first mask, but the
 cross-sectional configuration thereof is curved smoothly. And, the power
 of the image can be adjusted by changing the focal length of the two
 lenses.
 Here, the Fourier transform image is formed on the glass substrate by
 disposing the glass substrate at the focal point on the exit side of the
 lens, however, according to the present invention, it is also possible to
 dispose the glass substrate away from the focal point. In this case, not
 the Fourier transform image, but a periodical structure differing from
 that of the mask is transferred.
 As the masks, not only are a mask having openings, such as a copper sheet
 (a copper mesh) on which are arranged rectangular or circular holes in two
 dimensions, and a mask obtained from patterning of layers by metal
 evaporation on a fused quartz substrate applicable, but also a mask of so
 called phase type, which gives phase shift to the beam is applicable.
 Further for achieving the another object mentioned above, according to the
 present invention, a laser processing method for a glass substrate
 comprises: radiating the laser beam 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 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.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
 Hereinafter, the embodiments of the present invention will be explained in
 detail.
 EXAMPLE 1
 An ion exchange was conducted using a device as shown in FIG. 2. A glass
 substrate to be processed was in the form of a sheet of silicate glass
 having a thickness of 2 mm and composed mainly of SiO.sub.2 and containing
 Al.sub.2 O.sub.3, B.sub.2 O.sub.3, Na.sub.2 O, F, etc. A molten salt
 placed in a quartz container was a mixture of 50 mol % of silver nitrate
 and 50 mol % of sodium nitrate.
 Specimens of the glass substrate were immersed in the molten salt in the
 quartz container for 12 minutes. The molten salt was kept at 285.degree.
 C. in an electric furnace, and the reactive atmosphere was air.
 Na ion (one-valence positive ions) in the surface of the glass substrate
 are eluted, 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 microanalyzer, were about 5 .mu.m.
 Then, a diffraction grating was manufactured by using the device shown in
 FIG. 3. Concretely describing, on the surface of the glass substrate,
 which surface is treated by the ion exchange, is positioned a substrate
 including a phase mask, on which mask the diffraction grating is formed,
 via a spacer, and then the laser beam is radiated thereupon.
 Upon incidence of the laser beam onto the phase mask, as shown in FIG.
 4(a), primarily the diffraction beams of +1 and -1 of order exit, and the
 periodical distribution in intensity of the light beam can be obtained by
 the interference between those diffraction beams in the vicinity of the
 pole on the exit side of the phase mask. And, the frequency of the
 interference, if the laser beams are parallel on the incident side,
 coincides with the spacing of the diffraction grating of the phase mask.
 Here, the phase mask, having a spacing of the diffraction grating: 1055
 nm, depth of the diffraction grating: about 250 nm, size: 10 mm.times.5 mm
 made by QPS Technology Inc., Canada) is used, and therefore, intensity
 distribution of the beam having frequency of around 1055 nm can be
 obtained.
 On the region where the periodical intensity distribution is formed, as
 shown in FIG. 4(b), the glass substrate is set. As the result thereof, as
 shown in FIG. 4(c), glass is evaporated or ablated depending on said
 periodical light intensity, and a diffraction grating having a frequency
 equal to that of the light intensity is formed on the glass substrate.
 Here, the laser beam which is used is light beam of third high harmonic
 wave of the Nd:YAG laser of 355 nm. The pulse width is about 10 nsec, the
 repetition frequency is 5 Hz. And, energy per one pulse of the laser beam
 is able to be adjusted by changing the timing of the Q switch of the
 laser. In this embodiment, the laser which is used radiates a laser beam
 of about 90 mJ maximum pulse energy and about 5 mm beam diameter.
 The evaporation or ablation by the laser beam is, generally, non-linear,
 thus, the evaporation does not occur until the laser beam exceeds a
 certain intensity. In the case of the glass substrate used in this
 embodiment, with a laser of wavelength of 355 nm, the ablation does not
 occur until the intensity increases to more than 3 to 4 J/cm.sub.2 /pulse
 in energy density. As mentioned above, because the energy density of the
 laser beam applied is about 0.46 J/cm.sub.2, the ablation will not occur
 as it is. Therefore, in order to increase the energy density, the laser
 beam is focussed by a lens having focus distance of 250 mm, thereby
 obtaining beam size of about 2 mm on the glass substrate.
 In the concrete radiation method of the embodiment, first, the laser beam
 is decreased in the intensity, and the optical path is adjusted so that it
 enters nearly perpendicular from the substrate side of the phase mask.
 After that, the energy of the laser beam is gradually increased by
 changing the timing of the Q switch of the laser beam source. The ablation
 of the glass first occurs when the optical energy comes to about 80
 mJ/pulse, and keeping this condition, 5 pulses of the laser beam are
 radiated, then the radiation of the laser beam is stopped.
 The configuration of the diffraction grating formed as mentioned above is
 shown in FIG. 5 and FIG. 6. Here, FIG. 5(a) is a picture of the surface of
 a diffraction grating, observed by a scanning type electron microscope
 (10,000 power), FIG. 5(b) is a drawing which is made on the basis of the
 above picture, FIG. 6(a) is a picture of the cross section of the
 diffraction grating observed by a scanning type electron microscope
 (10,000 power), and FIG. 6(b) is a drawing which is made on the basis of
 the above picture. As is apparent from those drawings, the frequency of
 the diffraction grating is nearly equal to that of the phase mask which
 was used, and the configuration of the diffraction grating comes to be
 curved surface following the constructive frequency of the beam intensity
 distribution. Moreover, the surface of the diffraction grating is very
 smooth.
 In the above, measurements are made near the center of the diffraction
 grating, however, since the laser beam intensity is lower than that of the
 central portion on the peripheral portions thereof, a diffraction grating
 having a configuration different from that of the central portion is
 formed. Namely, because the ablation occurs even in the recess portion of
 the intensity distribution at the central portion of the radiation area of
 the laser beam (the portion where the beam intensity is highest), the
 convexities and concavities of the manufactured diffraction grating come
 to be smoothly curved. On the contrary to this, in the peripheral portion
 of the laser radiation area (the portion where the beam intensity is low),
 the ablation occurs only in the projecting portion of the beam intensity
 distribution, and as the result of this, the configuration becomes
 trapezium in the cross section. In this instance, since the top surface of
 the diffraction grating was originally the surface of the glass substrate,
 it cannot be so smoothed.
 As mentioned above, it is ascertained that the diffraction grating changes
 in the formed configuration, due to the difference in the beam intensity
 between the central portion and the peripheral portions of the laser beam.
 Moreover in this embodiment, it is acknowledged that the cross-sectional
 configuration of the diffraction grating is also changed in the same
 manner by changing the laser beam intensity itself.
 By manufacturing the diffraction grating in this manner, the diffraction
 grating can be very easily manufactured on the glass with low price,
 without the necessity of a special vacuum container.
 By the way, the distance between the phase mask and the glass substrate is
 maintained at about 50 mm by a spacer in the present embodiment. This is
 for the purpose of inhibiting the evaporated materials of the glass
 substrate surface from adhering on the phase mask as much as possible, and
 the distance itself is at random. For example, within the area where the
 light beams of +1 and -1 of the order overlap, the diffraction grating can
 be manufactured even if the phase mask is closely attached to the glass
 substrate. In case of radiating the laser on the glass substrate and the
 phase mask which are put on one another via a quartz plate having
 thickness of about 150 mm therebetween, the diffraction grating can be
 manufactured in the same manner of the present embodiment. Since the phase
 mask can be used repeatedly, it is important to protect it from dirt, and,
 interposition of the spacer is effective means to do this.
 EXAMPLE 2
 In this embodiment, in place of using the phase mask which was used in the
 embodiment mentioned above, the periodical distribution of the intensity
 is formed by utilizing an interference between two laser beams.
 Namely, as shown in FIG. 7, the laser beam is split into two through a beam
 splitter and put one onto the other again with a certain angle. Then the
 periodical distribution of light intensity is formed in the portion where
 the two light beams overlap each other. The frequency is determined by the
 angle being defined by the two overlapping laser beams.
 In this embodiment, the optical system is so constructed that the incident
 angle of the two laser beams is about 20.degree.. In this case, the
 frequency of the distribution in the beam intensity is about 1020 nm.
 Then, the processable glass of the same kind as that used in the above
 embodiment 1 is positioned at the portion where the two laser beams
 overlap, and is radiated with the laser beam. As the result of this, the
 ablation occurs. The lens in the drawing is used for increasing the energy
 density on the glass surface, and the energy density when the ablation
 occurs comes to be a similar value to that of the above-mentioned
 embodiment.
 The frequency of the formed diffracting grating is measured, and the
 measured value comes to be nearly equal to the predicted value. The cross
 section is measured or observed by the scanning type electron microscope,
 and it is ascertained that the diffraction grating having smooth curved
 surface is formed, the same as the embodiment 1.
 Here, there is a difference between the embodiment 1 and the embodiment 2
 in the means for forming the periodical intensity distribution, and each
 of them has its own advantages and disadvantages, respectively.
 Namely, the method of using the chase mask, since it is simple in the
 construction of the optical system and provides good reproducibility, is
 advantageous when the diffraction grating having the same frequency is
 produced. On the other hand, when the frequency must be frequently
 changed, the method using the interference between the two laser beam is
 advantageous.
 EXAMPLE 3
 As shown in FIG. 8, a mask made of copper and having a mesh pattern is
 closely attached to the glass substrate, and the laser beam is applied on
 it. The laser beam is so focused by the lens having the focus distance of
 250 mm that it has a beam size of about 2 mm on the glass substrate.
 The glass which is used here is the same kind that is processable by the
 laser and is also used in the above-mentioned embodiment 1. The condition
 for the diffusing Ag ion into the glass is the same as that of the
 embodiment 1, except that the temperature of the molten salt is
 300.degree. C., and the laser beam which is used here is the second high
 harmonic of the Nd:YAG laser having wavelength of 532 nm. The pulse width
 is about 10 nsec, and the repetition frequency is 5 Hz. And, the energy of
 the laser beam per one pulse can be adjusted by changing the timing of the
 Q switch of the laser. In the case of the laser which is used in this
 embodiment, the maximum pulse energy is about 90 mJ and the beam diameter
 is 5 mm.
 In processing the glass substrate by the laser processing, first, the laser
 beam is decreased in intensity, and the optical path is adjusted so that
 it enters nearly perpendicular from the substrate side of the phase mask.
 After that, the energy of the laser beam is gradually increased by
 changing the timing of the Q switch of the laser beam source. The ablation
 of the glass is acknowledged when the optical energy comes up to about 4
 J/cm.sup.2 /pulse, and while keeping this condition, 5 pulses of the laser
 beam are radiated, and then the radiation of the laser beam is stopped.
 The configuration of the concavities and convexities formed on the glass
 substrate is shown in FIG. 9. Here, FIG. 9(a) is a picture of the plan
 view of the glass substrate processed, and FIG. 9(b) is a drawing which is
 made on the basis of the above picture. As is apparent from those
 drawings, the mask pattern having mesh distance of 50 .mu.m is accurately
 transferred on the glass substrate. No cracking can be observed in the
 peripheral portion of any concavity. Further, the pattern of interference
 light of the diffraction beams can be observed at the period of more or
 less 1 .mu.m. This means that the transferring of the pattern in the
 microscopic order of around 1 .mu.m is possible. In this embodiment, a
 laser beam having a wavelength of 532 nm is applied, but a similar result
 can be obtained with a laser beam having a wavelength of 355 nm.
 Furthermore, the materials of the mask are not limited to copper, but
 other materials which have superior heat conductivity, such as aluminum,
 gold etc., and materials which have a high melting point, such as
 tungsten, stainless, steel, tantalum, etc. can be used.
 Here, the glass substrate, on which surface is transferred the mask pattern
 mentioned above, if treated by the process of filling the concavities with
 plastics of high refractive index, can be applicable as a microlens array
 which may be incorporated into a liquid crystal display device and/or a
 plasma display device.
 EXAMPLE 4
 In this embodiment, as shown in FIG. 10, without attaching the copper mask
 on the surface of the glass substrate, the laser beam is radiated on the
 glass substrate which is positioned on the optical path of the lens.
 The kind of glass which is used, the condition for diffusing the Ag ions
 into the glass, and the used laser beam are same as for the
 above-mentioned example 3.
 In FIG. 10, the glass is set at the position where a real image is formed
 by the lens (focus distance of 100 mm). In the same manner as in the
 embodiment 3, the ablation of the glass is acknowledged when the optical
 energy comes up to about 4 J/cm.sup.2 /pulse, and while keeping this
 condition, 10 pulses of the laser beam are radiated, and then the
 radiation of the laser beam is stopped.
 The configuration of the concavities and convexities formed on the glass
 substrate is shown in FIG. 11. Here, FIG. 11(a) is a picture of the plan
 view of the glass substrate processed, and FIG. 9(b) is a drawing which is
 made on the basis of the above picture. As is apparent from those
 drawings, the mesh is transferred on to the glass substrate in reduced
 size compared to that of the embodiment 3, in spite of using the same mask
 as the embodiment 3. In this way, reduction and enlargement of the
 transfer is possible by using the lens.
 In the above, is described an example of manufacturing a glass substrate
 for use as a planar microlens array using a mask. A glass substrate for
 use as a microlens array can be formed by interfering three or more laser
 beams, as shown in FIGS. 12(a) and (b).
 Here, in the embodiment, as the glass substrate, the silicate glass system
 containing Al.sub.2 O.sub.3, B.sub.2 O.sub.3, Na.sub.2 O, F and treated
 with the Ag ion exchange is disclosed, however, other glasses with the Ag
 ion exchange treatment, and even other glasses without the Ag ion exchange
 treatment, if they have processability by the laser, can be used as the
 object to be processed in accordance with the method of the present
 invention.
 Moreover, the shape of the glass material as the object to be processed in
 accordance with the method of the present invention is not be limited to
 being sheet-like, but also the present invention is applicable to other
 shapes including cylinder-like and so on.
 EXAMPLE 5
 FIG. 13 shows an optical system for exercising a laser processing relating
 to example 5, wherein the mask is positioned at the focus on the incident
 side of the projection lens and the glass substrate is positioned at the
 focus on the exit side of the projection lens.
 As the glass substrate 3, the glass mainly containing Al.sub.2 O.sub.3,
 B.sub.2 O.sub.3, Na.sub.2 O, F, having a thickness of 2 mm, and being
 treated with the Ag ion exchange is used. The Ag ion exchange treatment is
 conducted in the following steps.
 A molten salt comprising a mixture of 50 mol % of silver nitrate and 50 mol
 % of sodium nitrate was placed in a quartz container. Specimens of the
 glass substrate were immersed in the molten salt in the quartz container
 for 15 minutes. The molten salt was kept at 300.degree. C. in an electric
 furnace, and the reactive atmosphere was air.
 By this treatment, Na ions in the surface of the glass substrate are
 eluted, diffusing Ag ions in the molten salt into the glass (ion
 exchange). The thicknesses of the layers into which the Ag ions were
 diffused, as measured by a microanalyzer, were about 5 .mu.m.
 As the laser beam source, the light beam of third highest harmonic wave of
 the Nd:YAG laser (wavelength: 355 nm, pulse width: 10 nsec, repetition
 frequency: 5 Hz) is used. The energy per one pulse of the laser beam is
 able to be adjusted by changing the timing of the Q switch of the laser.
 in the case of the laser beam which is used in this embodiment, the
 maximum pulse energy is about 90 mJ and the beam diameter thereof is about
 5 mm.
 A lens having a focus distance of 250 mm is used and as the mask is used a
 copper mesh constructed with a number of holes having a diameter of about
 100 .mu.m arranged in two dimensions.
 In the above, upon the radiating of the laser beam, a Fourier transform
 image of the mask is formed on the surface of the glass substrate.
 Here, the spacial distribution in the intensity of the laser beam when it
 penetrates the mask shows a rectangle, as shown in FIG. 14(a), where the
 intensity is nearly equal at the central and the peripheral portions. On
 the other hand, the spacial distribution in the intensity of the laser
 beam of the Fourier transform image shows a sinusoidal wave, as shown in
 FIG. 14(b).
 And, recess portions having a curved (arc-like) cross-section are formed on
 the surface of the glass substrate corresponding to the spacial
 distribution in the intensity of the laser beam. The evaporation or
 ablation by the laser beam is, generally, non-linear, thus, the
 evaporation does not occur until the laser beam exceeds a certain
 intensity. In the case of this embodiment, no evaporation nor ablation
 occurs regarding the components greater than 3 in the order of the Fourier
 transform image because the intensity is small.
 However, even if using masks of the opening-type, the spots of higher than
 that in the order can be recorded or transferred, depending on the design
 of the masks.
 EXAMPLE 6
 FIG. 15 shows an optical system for exercising a laser processing relating
 to example 6, wherein the focus position of the exit side of the first
 projection lens is made coincident with the focus position of the incident
 side of the second projection lens, a first mask is positioned at the
 focus of the incident side of the first projection lens, a second mask is
 positioned at the focus of the exit side of the first projection lens, and
 the glass substrate is positioned at the focus of the exit side of the
 second projection lens.
 The laser beam source, the first mask, the first and second projection
 lenses and the glass substrate, used are the same as those of the above
 embodiment 5.
 In the above, upon the radiating of the laser beam, the spots of the orders
 except for 0 and 1 of the Fourier transform image are shut by the second
 mask, and only the image of the orders of 0 and 1 of the Fourier transform
 is made incident upon the second projection lens, whereby an image is
 formed on the surface of the glass substrate. This image is such that the
 components in the orders or more than 2 are eliminated, and the
 cross-sectional configuration of the recess portion formed on the glass
 base is smoothly curved. The power of the image can be adjusted by
 changing the focus distances of the first and the second lenses.
 FIG. 16 is a cross-sectional view of a planar microlens array, and the
 planar microlens array is constructed by filling plastic of high
 refractive index into the concavities of the glass substrate obtained by
 the above embodiments 5 and 6.
 EXAMPLE 7
 FIG. 17 shows outlining construction of a processing apparatus which is
 applied for exercising the laser processing method of example 7. The
 processing device is constructed with a radiation source of a laser beam,
 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. 18.
 Concretely, as the above glass substrate, the glass mainly contains
 Sio.sub.2 addition thereto, Al.sub.2 O.sub.3, B.sub.2 O.sub.3, Na.sub.2 O,
 F, and has a thickness of 2 mm. The glass substrate was immersed in the
 molten salt (kept at 300.degree. 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 .mu.m.
 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 .mu.m. 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.sup.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. 19.
 FIG. 20 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. 20, 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 in the above, according to the present invention, under
 processing the glass by the laser beam, patterns can be very finely
 formed, compared to the conventional method, with very high accuracy and
 in less time, because the microscopic concavities and convexities can be
 formed on the surface of the glass substrate by partially varying the
 spacial distribution of the intensity of the laser beam, i.e., removing a
 greater amount of glass where the intensity is stronger, and less where
 the intensity is weaker.
 Therefore, it is very effective when applying it to the process of
 manufacturing the diffraction grating, the planar microlens array, etc.
 And according to the present invention, by disposing the glass substrate at
 the position of forming the Fourier transform image, it is possible to
 form a number of concavities spreading on the surface of the glass
 substrate in two dimensions, with smoothly curved lines including
 arc-lines in the cross-sectional view. Consequently, applying it to a
 planar microlens array, for example, it is possible to form a convex lens
 with high accuracy.
 And according to the present invention, the Fourier transform image is
 formed on the glass substrate surface again after removing the high
 frequency components of the Fourier transform image once formed, whereby
 concavities having smoothly curved cross-sections can be obtained, though
 being the same to the first mask in the plane view. And, the power of the
 image can also be adjusted by changing the focus distances of the two
 lenses.
 Once the pattern of concavities and convexities to be formed on the glass
 substrate has been determined, it is enough to produce the mask on the
 basis of said pattern, therefore any type of pattern can be formed easily.
 And 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, 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 interior
 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.