JP3452733B2 - Manufacturing method of diffractive optical element - Google Patents

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a diffractive optical element.
It relates to a manufacturing method .

[0002]

2. Description of the Related Art Since glass has excellent properties such as flatness, processing accuracy, weather resistance, and heat resistance, it can be used as a microlens incorporated in a diffraction grating or a display device used for optical communication, etc. It is known to have been subjected to.

In order to perform fine processing on a glass substrate, conventionally, wet etching (chemical etching) using an etchant such as hydrofluoric acid or dry etching (physical etching) such as reactive ion etching is used.
It is generally due to.

However, in wet etching, there are problems of etchant management and processing, and in dry etching, equipment such as a vacuum container is required and the apparatus itself becomes large, and more complicated photolithography technology is used. A pattern mask or the like must be formed, which is not efficient.

In addition, commercially available wavelength separation elements such as diffraction gratings, which are relatively inexpensive, are industrially used to obtain a master by carving a metal such as aluminum with a diamond blade (ruling) to obtain a master. Originally, a method of transferring to an epoxy resin or the like is adopted.

The above-mentioned industrial method for producing a diffraction grating requires a large-scale ruling facility and the like, and it is inevitable to transfer it to an organic substance for mass production. However, although transfer to an organic material has good moldability, it has a limit in resistance to humidity and temperature.

On the other hand, laser light has strong energy, and it has been conventionally performed to raise the surface temperature of an irradiated material and to ablate (evaporate) or evaporate the irradiated portion to perform various processes. ing. In particular, since the laser light can be focused on an extremely small spot, it is suitable for fine processing.

Therefore, as a prior art in which a plurality of laser beams are made to interfere with each other to form a laser beam having a periodic light intensity distribution, and the surface of a workpiece such as a metal plate is irradiated with the laser beam to perform fine processing, Japanese Unexamined Patent Publication No. 50-42499, Japanese Unexamined Patent Publication No.
No. 253583, Japanese Patent Publication No. 7-4675, Japanese Patent Publication No. 7-47232, Japanese Patent Publication No. 7-51400, Japanese Patent Publication No. 7-102470, Japanese Patent Publication No. 8-979.
Those disclosed in Japanese Patent Publication No. 4 and Japanese Patent Publication No. 8-25045 are known.

Particularly, in Japanese Patent Publication No. 8-25045, a waveguide (thin film) having a refractive index higher than that of air and the workpiece is formed on the workpiece such as a metal plate. Laser light is radiated to the waveguide, and the interference between the light propagating in the waveguide and the radiated light forms minute irregularities on the waveguide, giving the surface of the workpiece a rainbow-colored coloring function. Is.

Masataka Murahara, et al. Applied Physics Vol. 52 No. 1 (1983) p.84, PMMA (polymethyl methacrylate), which is an organic polymer, is coated on a glass substrate, and the thin film is applied to the excimer. It has been reported that fine unevenness of an organic thin film was directly produced by ablation using laser interference light.

[0011]

In any of the above-mentioned prior arts, a thin film is formed on the surface of a base material, laser light energy is absorbed in the thin film to cause ablation, and the thin film is subjected to fine processing. However, no consideration is given to laser light energy.

That is, it is conventionally known that a laser beam having a certain intensity or more must be irradiated to cause ablation or the like. However, when a thin film is formed on the surface of a substrate, the substrate is passed through the thin film. If the energy of the laser beam reaching to the point is larger than the energy (threshold value) that causes ablation or the like in the substrate, not only fine irregularities are formed in the thin film, but also the substrate itself is processed. Thus, if a thin film and a base material having different physical properties are also microfabricated at the same time, they cannot be used as a member such as a diffraction grating that requires precision. Further, when the thin film is an organic polymer, there is a disadvantage that the weather resistance and heat resistance are poor.

[0013]

In order to solve the above-mentioned problems, the present invention provides a laser on a surface of a glass substrate rather than a glass substrate.
The thin film with excellent absorbency is formed, and
The thin film is irradiated with laser light and the thin film is irradiated with laser light.
By absorbing energy, melting, evaporation, or abrasion
The thin film according to the intensity of the laser light.
A method of manufacturing a diffractive optical element that is removed by
If the laser light uses a phase mask,
A periodic light intensity distribution can be obtained by making light interfere.
Laser light, the thin film is mainly composed of an inorganic material,
In addition, the thickness of the thin film or the absorption coefficient of the laser beam can be measured through the thin film.
The intensity of the laser light that reaches the glass substrate surface
Melt, evaporate or ablate the substrate
It is set to a value that is less than or equal to the threshold value.

The thin film includes metal oxides, metal nitrides,
A single layer of a metal carbide, a semiconductor, a glass mainly containing SiO2, a fluoride glass or a chalcogenide glass, or a multi-layered combination thereof is suitable. As a method for forming a thin film, various methods such as a sol-gel method, a sputtering method, a vacuum vapor deposition method, and a liquid phase deposition method can be applied.

The amount of energy loss when the laser beam passes through the thin film can be controlled by the thickness and absorption coefficient of the thin film, but when it is necessary to secure a predetermined thickness, , Mainly controls the absorption coefficient of laser light. Then, as a method of controlling the absorption coefficient, a method of intentionally introducing a shift in the stoichiometric ratio such as oxygen vacancies, a method of introducing defects, a method of doping ions having high absorption with respect to wavelength, a method of mixing ultrafine particles , A method of mixing a pigment or an organic dye, and the like.

Further, by using a laser beam having a periodic or regular light intensity distribution as the laser beam,
It is possible to manufacture a diffractive optical element such as a diffraction grating or a hologram incorporated in an optical coupler, a polarizer, a demultiplexer, a wavelength filter, a reflector, a mode converter, or the like. In forming unevenness with a laser beam on the thin film formed on the surface of the glass substrate, if the ablation is performed until the glass substrate is exposed on the bottom surface of the recess of the thin film, the thickness of the thin film will be diffracted as it is. It becomes the thickness of the uneven portion of the optical element of the mold.

The laser light having a periodic light intensity distribution is
It can be obtained by causing a phase mask or a plurality of laser beams to interfere with each other, and the cross-sectional shape of the periodic unevenness formed on the surface of the glass substrate can be controlled by the pulse energy of the laser beam. Further, laser light having a regular light intensity distribution can be obtained by using a mesh mask or the like.

[0018]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments and comparative examples of the present invention will be described below with reference to the accompanying drawings. The following is a table (Table) comparing the main items of the examples and comparative examples.

[0019]

【table】

Example 1 A SiO colloid containing Ag colloid dispersed on soda lime glass.
_Two thin films were formed. The forming method is sputtering, and
₂ Place the target and the silver metal chip on the target,
Sputtered at the same time. The sputtering conditions were set as follows to form a film. In addition, the target is placed underneath, 5 inches ×
Using a 20-inch quartz target, 32 silver disk-shaped chips (about 4 mm in diameter) were dispersed and placed on the target. Sputtering conditions Gas flow rate: Oxygen 3 sccm, Argon 97 sccm Sputtering pressure: 2.8 × 10 ⁻³ Torr Incident power: 3.0 Kw Substrate: Soda lime glass The obtained thin film is colored brown, but the surface is smooth. The adhesion was strong and a clear film was obtained. The film thickness was 315 nm after a film forming time of 5 minutes. Further, when the concentration of silver in the film was measured by XPS (X-ray electron spectroscopy), it was 0.9
It was 4 atom%. When the absorption spectrum of the thin film was measured, there was an absorption peak near 390 nm, which is considered to be plasmon absorption of ultrafine silver particles (colloid), and it is considered that ultrafine silver particles were formed in the glass during film formation. .

The thin film on the glass was ablated by the optical system shown in FIG. 1 and finely processed. The optical system shown in FIG. 1 uses two single mode laser beams.
Then, the light was focused again on the glass thin film, and a pattern in which the intensity changed periodically due to the interference was formed and irradiated. The laser used is Nd: YA
G laser has a pulse width of 10 ns and a repetition frequency of 10
Hz, the wavelength used is 355 nm of the third harmonic, and the irradiation energy is about 110 mJ / Pu before splitting into two beams.
It was energy of 1se. The beam was split by a 50% beam splitter, and after passing through a quartz lens, the two beams were superposed on each other to form interference fringes and a periodic light intensity distribution was formed. The angle between the laser beams was about 10 °. The beam diameter on the sample is 2 mm and the energy density is 4.14 J / c.
m ²・ Pulse. This energy is obtained by measuring the ablation energy of the thin film in advance and setting it higher than the energy.

The processed film was observed with an optical microscope and an electron microscope. 2 (a) is a 1000 × optical microscope photograph, FIG. 2 (b) is a drawing prepared based on the photograph, and FIG. 3 (a).
Is a scanning micrograph at 3500 times, and (b) is a diagram created based on the same photograph. From these figures, about 1μ
It can be confirmed that m periodical irregularities are formed.

The absorption coefficient of the thin film at 355 nm in this example was 46,212 cm ^-1 . The absorption coefficient of the soda-lime glass substrate is 0.3081 cm ^-1.
Met. The energy that reaches the substrate calculated from the absorption coefficient and thickness of the thin film is 0.95 J / cm ² · Pul.
It was se.

On the other hand, the ablation threshold of glass was experimentally obtained. The energy at the time of ablation was recorded while increasing the irradiation energy. In order to calculate the power density, the area of the ablation mark is necessary, but cracks around the irradiation mark were severe and it was not possible to accurately determine it. Approximately 8 to 10 J / cm ² · Pulse for the laser of this experiment
Met. Therefore, the energy reaching the substrate is below the ablation threshold of the substrate, and preferential ablation of the thin film occurs, which is considered to enable such fine workability.

Comparative Example 1 A SiO ₂ thin film was formed on a soda lime glass substrate by the same process as in Example 1. In this case, silver was not placed on the target so that a thin film of only SiO ₂ could be formed. The obtained thin film was laser-processed using the same optical system as in Example 1. As a result, the thin film could not be selectively processed and ablation occurred with the substrate. The energy at this time was 9 J / cm ² · Pulse. Further, when the absorption coefficient was measured, the absorption coefficient of the substrate was found to be that of Example 1.
However, the absorption coefficient of the SiO ₂ film was 0.001 cm ⁻¹ or less, which was lower than that of the substrate. Further, it is considered that the energy reaching the substrate was almost close to the ablation threshold value of the substrate, and effective processing could not be performed.

(Examples 2, 3, 4, 5) As in Example 1, a SiO ₂ thin film in which Ag colloid was dispersed was formed on soda lime glass. As the sputtering conditions, the number of silver targets and the sputtering power were adjusted to change the silver concentration. The main points of this example are summarized in the above (Table). The absorption coefficient of the film decreased as the silver concentration decreased. This glass with a thin film was ablated using interference light in the same manner as in Example 1. As a result, as in Example 1, a periodic structure was formed on the film surface, and the function as a diffraction grating was exhibited. As can be seen from the table, all of these thin films have a higher absorption coefficient than the substrate, and the laser energy reaching the substrate is lower than the ablation threshold of the substrate.

(Comparative Examples 2 and 3) Ag ₂ mixed SiO ₂ films were prepared under the same conditions as in Examples 4 and 5. This was placed in the chamber at the same time as the film formation in Examples 4 and 5, and it was considered that the characteristics of the film were completely the same. However, the substrate has a lower ablation threshold value (3.5 J / cm ² · T) than that used in Examples 1 to 5.
Pulse) material was used (see table). This board is
Ultrafine particles such as CdS and CdSe are dispersed in a glass matrix, which is widely used as a sharp cut filter. The produced glass substrate with a thin film was ablated under the same conditions as in Example 1. As a result, in Comparative Example 2, the workability of the diffraction grating was barely seen, but in Comparative Example 3, the substrate was damaged and good workability was not obtained. 355nm of this glass substrate
The processing threshold for the wavelength was 3.5 J / cm ² , and its absorption coefficient was 377 cm ⁻¹ . The absorption coefficient of the film in Comparative Example 2 is 6490 cm ^-1 , which is higher than that of the substrate. In addition, the energy passing through the thin film and reaching the substrate was 3.4 J / cm ² , which was barely below the ablation threshold of the substrate. On the other hand, the absorption coefficient of the film in Comparative Example 3 is 2235 cm ⁻¹ , and the absorption coefficient of the substrate is 377.
Although higher than cm ⁻¹ , the energy to reach the substrate is 3.
It was 9 J / cm ² · Pulse and reached the ablation threshold of the substrate. Therefore, this point is considered to be the cause of losing the workability in Comparative Example 3. Although Example 5 and Comparative Example 3 have the same thin film, it is considered that such a difference is caused by the difference in the threshold value of the substrate.

Example 6 For the same sample used in Example 2, a diffraction grating was manufactured using the apparatus shown in FIG. Specifically, a substrate provided with a phase mask in which a diffraction grating was formed via a spacer was arranged on the surface on which the Ag—SiO ₂ film was formed on the above glass substrate, and laser light was irradiated. When laser light is incident on the phase mask, as shown in FIG. 5A, a plurality of diffracted lights mainly including + first order, 0th order, and −1st order are emitted, and the phase mask emits due to interference of these diffracted lights. A periodic light intensity distribution is obtained near the side. Here, the phase mask of this embodiment has a diffraction grating period: 1055 nm,
Diffraction grating depth: about 250 nm, size: 10 mm x 5 m
m (Canada from QPS Technology Inc.) was used.
Then, as shown in FIG. 5B, a glass substrate having a thin film formed thereon was set in the region where the periodic intensity distribution was formed. As a result, as shown in FIG. 5C, the thin film is evaporated or ablated according to the periodic light intensity, and the diffraction grating having the same period as the period of the light intensity processed the thin film on the glass substrate. Formed in shape. The relationship between the glass threshold value and the thin film threshold value is the same as that described in the first and second embodiments.

The laser light used was the light of 355 nm which is the third harmonic of the Nd: YAG laser as in the first embodiment. The pulse width was about 10 nsec and the repetition frequency was 5 Hz. The energy per pulse of the laser light can be adjusted by changing the timing of the Q switch of the laser, the energy is 110 mJ / pulse, and the beam diameter is about 5 mm. In order to increase the energy density of the laser so as to be suitable for processing, the laser beam was narrowed down by a lens having a focal length of 250 nm so that the beam size on the glass substrate was about 2 mm.

At the diffraction grating formed as described above, in this embodiment, a spacer is provided so that the distance between the phase mask and the glass substrate is about 50 μm. This is to prevent evaporation from the surface of the glass substrate from adhering to the phase mask as much as possible, and this interval itself is arbitrary. For example, if the + 1st-order light and the -1st-order light are overlapped with each other, the diffraction grating can be produced even if the phase mask and the glass substrate are brought into close contact with each other, and the thickness between the phase mask and the glass substrate is about 150 μm. Even when the quartz plate was sandwiched and brought into close contact and laser irradiation was performed, a diffraction grating could be produced in the same manner as in this example. Since the phase mask is used repeatedly, it is important to prevent its contamination, and therefore interposing a spacer is an effective means.

(Examples 7 and 8) Using a borosilicate glass called BK7 as a substrate, TiO ₂ and GeO ₂ thin films were formed by the electron beam evaporation method. When these thin films are subjected to laser processing in the same manner as in Example 1,
Similarly, a periodic structure could be formed. Therefore, it can be seen that the processing threshold of these thin films is lower than 4.14 J / cm ² . The processing threshold of BK7 glass at 355 nm is 8 to 9 J / cm ² , the transmission energy calculated from the absorption coefficient (46060 cm ⁻¹ ) of TiO ₂ and the film thickness is 3.2 J / cm ² , and the absorption coefficient of GeO ₂ is 755
The transmission energy was 3.6 J / cm ² at 5 cm -1. Therefore, the processing threshold of the substrate is much higher than these values, and it is considered that the thin film laser processing could be stably performed. Although the GeO ₂ thin film is glassy,
It was revealed from the X-ray diffraction results that O ₂ was crystallized and had a crystal form of anatase structure. Therefore, it was found that the present invention can be applied not only to the glassy thin film but also to the crystalline thin film, and it is necessary to satisfy the requirements shown in the claims.

Example 9 A TiO ₂ thin film in which gold colloid was dispersed in the film was formed on a soda lime glass by the sol-gel method. The main material in thin film fabrication is titanium tetrabutoxide (TT).
B), which is a 4-fold equimolar amount of acetylacetone (A
A) was mixed. This gently promotes hydrolysis of water, which is the main reaction of the sol-gel reaction, and helps form a good quality thin film. As a hydrolysis reaction liquid, NaAuCl ₄
An aqueous solution in which was dissolved at a concentration of 0.16 mol / l was used. 12 ml of TTB and 12 of ethanol for dilution
ml, 4 ml of AA and 3 ml of NaAuCl4 aqueous solution were mixed, stirred and reacted for 30 minutes, and then coated on a glass substrate by the dip method. After the application, the film was heated in air at 400 ° C. for 15 minutes to evaporate the residual organic matter to form a strong film. At this time, ultrafine particles of gold were deposited and the thin film turned blue. This is due to the plasmon absorption of ultrafine gold particles in the TiO ₂ film. Repeat coating and heat treatment 3 times, 34
A thin film with a thickness of 0 nm was obtained. A diffraction grating was produced from this thin film using the same optical system and irradiation energy as in Example 1. As a result, a periodic structure was similarly formed on the glass substrate. The absorption coefficient of this thin film was 58000 cm ⁻¹ at 355 nm, and the energy reaching the substrate was estimated to be 0.57 J / cm ² , which was far lower than the substrate threshold. This thin film is amorphous, but the main component is the same as TiO ₂ shown in Example 7. In this example, the absorption coefficient was larger than that of the film of Example 7, and it was shown that the absorption coefficient can be adjusted by dispersing ultrafine particles such as gold ultrafine particles in the material. There is.

Example 10 As a method of obtaining a glass thin film, there is a method of precipitating SiO ₂ in a liquid phase. As such a method, A.Hishinum
a et al. Applied Surface Science 48/49 (1991) 405
The LPD method (Liquid Phase Depositio
n: liquid phase film forming method) is known. SiO using LPD method
Rhodamine 6G (R6, which is one of the organic dyes in 2
G) was mixed and a film was formed on a soda lime glass substrate. The thin film was manufactured as follows. First, SiO ₂ glass is put into a silicofluoric acid (H ₂ SiF 6) solution to obtain a saturated solution. At this time, the concentration of silicofluoric acid was set to 2 mol / l. After saturation, this solution contains about 0.2 mol / l R6G
Were mixed so that the concentration became. Then, the glass substrate was put in the liquid, and then an aluminum piece was put. Aluminum pieces are used for the equilibration of silicofluoric acid saturated with SiO _2.
It has a function of moving in the direction in which O ₂ is deposited, and deposits a SiO ₂ thin film on the glass substrate. At this time, since the dye was mixed, the dye was also introduced into the glass thin film. The obtained thin film was red, which clearly showed that the rhodamine dye was introduced into the film. A diffraction grating was produced from this thin film using the same optical system and irradiation energy as in Example 1. As a result, a periodic structure was similarly formed on the glass substrate. However, probably because the adhesion is weak, the film did not remain in part, and there was a part where the glass substrate was exposed. The absorption coefficient of this thin film is 3850 at 355 nm.
0 cm-1 and the energy to reach the substrate is 0.5
It was estimated to be 7 J / cm ² , which was much lower than the substrate threshold.

[0034]

As described above, according to the present invention,
Rather than irradiating a substrate such as a glass substrate directly with laser light for processing, a thin film with better laser absorption than the substrate is formed on the surface of the substrate, and this thin film is irradiated with laser light to form a thin film. Since it is designed to be finely processed,
Even if an excimer laser using F and F as light emission sources is not used,
d: YAG laser or other solid-state laser that is cheap and easy to use, and is thought to be unusable for direct processing on glass, 1064 nm, and its harmonics of 532 n
It is possible to use wavelengths of m, 355 nm and 266 nm.

Since the thin film is mainly made of an inorganic material,
A product with excellent environment resistance is obtained, and further, the energy reaching the substrate by adjusting the thickness of the thin film or the laser absorption coefficient is set to be lower than the processing threshold of the substrate.
Only the thin film portion can be finely processed, and a highly accurate product can be obtained.

If the thin film portion is removed by ablation or the like until the surface of the glass substrate is exposed in the concave and convex portions formed on the thin film, the thickness of the unevenness of the diffractive optical element can be determined by the thickness of the thin film. It is controllable and high precision optical elements can be obtained in a simple way.

[Brief description of drawings]

FIG. 1 is a schematic view of an apparatus for manufacturing a diffraction grating by the method of the present invention utilizing laser interference.

FIG. 2 (a) is an optical microscope photograph (× 1000) of the surface of a thin film after microfabrication, and FIG. 2 (b) is a drawing prepared based on the photograph.

FIG. 3A is a scanning micrograph (3500 times) of the surface of a thin film after microfabrication, and FIG. 3B is a drawing prepared based on the photo.

FIG. 4 is a schematic view of an apparatus for manufacturing a diffraction grating by the method of the present invention using a phase mask.

FIG. 5 (a) is a diagram for explaining the operation of the phase mask,
(B) is a diagram showing a state where a glass substrate is irradiated with laser light through the same phase mask, and (c) is a diagram showing a laser-processed glass substrate.

─────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-5-303916 (JP, A) JP-A-8-57678 (JP, A) Anna-Karin Holmer, Simultaneous machinning of parallel groove in SnO2 thin. films using a Nd: YAG laser and a kinoform, Applied Optics, May 20, 1996, Vol. 35 No. 15, p2614-2618 (58) Fields investigated (Int.Cl. ⁷ , DB name) C03C 15/00-23/00 B23K 26/00-26/18 G02B 5/18 G02B 5/30-5/32 WPI JOIS

Claims (3)

(57) [Claims] 1. A thin film, which is more excellent in laser absorption than a glass substrate, is formed on the surface of a glass substrate, and the thin film is irradiated with laser light having an intensity distribution to apply energy of the laser light to the thin film. A method of manufacturing a diffractive optical element, wherein the thin film is removed according to the intensity of laser light by causing melting / evaporation or ablation by absorption, and the laser light uses a phase mask or a laser. Laser light having a periodic light intensity distribution is obtained by causing light to interfere, and the thin film is mainly made of an inorganic material, and the thickness of the thin film or the absorption coefficient of laser light is transmitted through the thin film and the glass substrate surface Of a diffractive optical element characterized in that the intensity of the laser light reaching the laser beam is set to a value below a threshold for melting, vaporizing or ablating the glass substrate. Build method. 2. The method for manufacturing a diffraction-type optical element for a glass substrate according to claim 1, wherein the absorption coefficient of the laser light is intentionally introduced with a deviation in stoichiometric ratio such as oxygen deficiency. A method of introducing defects, a method of doping ions having high absorption with respect to wavelength, a method of mixing ultrafine particles,
A method for producing a diffractive optical element, which comprises increasing the amount by applying either a method of mixing a pigment or a method of mixing an organic dye. 3. The method for manufacturing a diffractive optical element according to claim 1 or 2, wherein the thin film is a metal oxide, a metal nitride, a metal carbide, a semiconductor, or a glass containing SiO ₂ as a main component. A method of manufacturing a diffractive optical element, characterized in that a single layer of fluoride glass or chalcogenide glass or a combination thereof is laminated in multiple layers.