JP3960643B2 - Optical element manufacturing method - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
INDUSTRIAL APPLICABILITY The present invention is used in the fields of optical communication and optical measurement, as a diffraction grating used as a polarization beam splitter, a coupling grating, etc., a diffractive optical element used as a hologram, a birefringent element, a light scatterer, etc. The present invention relates to a method for manufacturing an optical element such as a photonic crystal.
[0002]
[Prior art]
As shown in FIG. 8A, it is known that a diffraction grating in which a dielectric multilayer film has periodic unevenness in one direction has excellent characteristics as a polarizing beam splitter. (Rong-Chung et al., OPTICS LETTERS Vol.21, No.10, p761, 1996)
Further, as shown in FIG. 8B, a diffraction grating in which periodic irregularities are formed in two directions on a dielectric multilayer film has been proposed as a three-dimensional photonic crystal. (E. Yablonovitch, Journal of Optical Society of America B Vol.10, No.2, p283, 1993)
[0003]
The dielectric multilayer film itself is currently used in various fields as a mirror, and as a manufacturing method, techniques such as an electron beam evaporation method, a heating evaporation method, or a sputtering method have already been established.
[0004]
In addition, since the technology for forming periodic irregularities in a dielectric multilayer is also a technology similar to patterning technology such as VLSI, applying this patterning technology to a dielectric multilayer film results in periodicity on the dielectric multilayer film. Diffraction gratings with typical irregularities can be manufactured.
As a specific patterning technique, wet etching (chemical etching) using an etchant such as hydrofluoric acid or dry etching (physical etching) such as reactive ion etching can be considered.
[0005]
[Problems to be solved by the invention]
Diffraction gratings and the like can be manufactured by applying the film formation method and the etching method described above, but there are problems in the management and processing of etchants in wet etching, and equipment such as a vacuum vessel in dry etching. And the apparatus itself becomes large, and the pattern mask must be formed by more complicated photolithography techniques, specifically resist coating, drying, exposure, baking, development, etc., which is not efficient.
[0006]
Furthermore, when etching a dielectric multilayer film in which two or more layers are laminated, there is a difference in the etching rate of each layer, and it is difficult to obtain a clean cross-sectional shape.
[0007]
[Means for Solving the Problems]
In order to solve the above-described problems, the optical element manufacturing method according to the present invention forms a dielectric multilayer film composed of two or more layers having different dielectric constants on the surface of a substrate such as a glass substrate, and the dielectric multilayer film. On the other hand, the laser beam having an intensity distribution is irradiated and the energy of the laser beam is absorbed into the dielectric multilayer film to cause melting / evaporation or ablation, and a part of the dielectric multilayer film is removed according to the laser beam intensity. As a result, the dielectric convex portions having a lattice constant of the order of the wavelength of the light are periodically arranged and left on the substrate surface.
[0008]
Here, the material constituting the dielectric multilayer film is preferably a material having a lower threshold value for causing melting / evaporation or ablation with respect to laser light than the base material and having a large film adhesion force. Silicon oxide, titanium oxide, cerium oxide, germanium oxide, magnesium fluoride, calcium fluoride, tantalum oxide and the like are suitable.
The form of each film constituting the dielectric multilayer film may be glass (amorphous), single crystal, or polycrystalline.
[0009]
The laser beam has a periodic intensity distribution in one direction, for example. This laser light having a periodic intensity distribution in one direction can be obtained by interfering with a phase mask or two laser lights.
The laser beam has a periodic intensity distribution in two directions, for example. Laser light having a periodic intensity distribution in these two directions can be obtained by interfering with three or more laser lights.
[0010]
As the laser light, an excimer laser such as KrF, an Nd-YAG laser, a Ti: Al ₂ O ₃ laser and its harmonics, a dye laser, or the like is used. Preferably, the dielectric multilayer film to be processed has a reflectivity. Use low-area laser light.
[0011]
DETAILED DESCRIPTION OF THE INVENTION
Example 1
First, a procedure for forming a dielectric multilayer film on the surface of the substrate will be described.
As the base material, a borosilicate glass substrate (BK7 glass substrate) is used, the substrate is cleaned with alcohol before film formation, and this base material is heated to 300 ° C. in a vapor deposition apparatus, as shown in FIG. A dielectric multilayer film in which nine layers of SiO ₂ and TiO ₂ were alternately laminated was formed.
[0012]
As a raw material of SiO ₂ , a SiO ₂ disk having a diameter of 2 inches was used, and the thickness per layer of SiO ₂ was about 104 nm.
Further, Ti ₂ O ₃ granules were used as a raw material for TiO ₂ , and this was evaporated in an oxygen atmosphere to form a film. The thickness per layer of TiO ₂ was about 50 nm.
[0013]
The dielectric multilayer film produced as described above has the same configuration as a so-called dielectric multilayer film mirror, and becomes a mirror having a reflectance peak at about 600 nm.
As a result of measuring the spectrum, it was found that the reflectance is not so large for light with a wavelength of 300 to 400 nm, and the laser light can sufficiently pass through the film. It was decided to use.
[0014]
Specifically, the third harmonic (355 nm) of an Nd: YAG laser was used. The energy of the laser beam was 350 mJ / pulse, the pulse width was 5 nsec, and the repetition frequency was 5 Hz when it was emitted from the laser light source. The beam diameter was about 7 mm.
[0015]
As shown in FIG. 2, the laser beam is divided into two beams by a beam splitter and adjusted so as to overlap again on the substrate through different optical paths. In order to form clear interference fringes on the substrate, it is necessary that the optical path lengths of the two beams are almost equal, and the energy of each beam is substantially equal.
[0016]
In this embodiment, the difference between the optical path lengths of the two beams is 2 cm or less, which is sufficiently smaller than the spatial length of the laser light pulse, 150 cm, and forms clear interference fringes. We were able to. In the present embodiment, the energy of the two beams has a ratio of about 1: 2 due to the difference in mirror loss between the respective optical paths. The clarity of is not lost.
In order to increase the energy density, the laser beam was narrowed down with a lens having a focal length of 200 mm so that the beam size on the substrate was about 2 mm.
[0017]
In this embodiment, since laser irradiation is performed in the atmosphere, air discharge occurs at the lens focal position. In order to remove the influence of this discharge, the position of the glass substrate was adjusted to be closer to the lens than the focal position of the lens. Therefore, in FIG. 2, the two beams before the lens are drawn in parallel, but in actuality, they are incident on the lens with a slight angle.
[0018]
In this way, after adjusting the optical system, the dielectric multilayer film produced previously is set at the position where the interference fringes of the laser beam are formed and irradiated with several pulses of laser light. As shown in FIG. 4 and FIG. 4, the dielectric multilayer film is removed in accordance with the intensity of the laser beam at a location where the energy of the laser beam is absorbed by the dielectric multilayer film and exceeds a threshold value causing melting, evaporation or ablation. As a result, a diffraction grating was formed in which a large number of dielectric protrusions were arranged periodically along one direction.
Here, FIG. 3A is an optical micrograph (1000 times) of a dielectric convex portion formed by laser light irradiation, FIG. 4B is a diagram created based on the photo, and FIG. 4 is the dielectric multilayer film. It is an expansion perspective view of the surface.
[0019]
Here, it should be noted that in the case of the present embodiment, among the layers constituting the dielectric multilayer film, the SiO ₂ layer has a lower absorption coefficient with respect to the laser beam of 355 nm, compared with TiO ₂ . It is difficult to evaporate. Nevertheless, in this embodiment, SiO ₂ is evaporated together with TiO _{2 because} SiO ₂ is also heated and evaporated together when the upper and lower TiO ₂ layers are evaporated.
[0020]
As described above, the method for manufacturing a diffraction grating according to the present invention can be carried out even when the threshold value causing melting, evaporation or ablation of each layer of the dielectric multilayer film is different, but under such conditions, the film is damaged. In order to reduce this, it is necessary that the adhesion of the film is strong. In addition to SiO ₂ and TiO ₂ , cerium oxide, germanium oxide, magnesium fluoride, calcium fluoride, and tantalum oxide are conceivable as film configurations that satisfy this condition. Moreover, even if it is the same film | membrane structure, the adhesive force of a film | membrane can also be improved with a manufacturing method like ion assist electron beam vapor deposition, for example.
[0021]
In the above embodiment, interference fringes by two laser beams are used. However, the present invention is not limited to this, and interference fringes of three or more laser beams can also be used in the method for manufacturing a diffraction grating of the present invention. In this case, the shape of the produced diffraction grating is a diffraction grating having periodicity along two orthogonal directions as shown in FIG. That is, it becomes a kind of three-dimensional photonic crystal.
[0022]
A similar photonic crystal can be realized by rotating the substrate by 90 ° using the optical system of FIG. 2 and processing it twice from different directions.
[0023]
(Example 2)
A diffraction grating was manufactured using the apparatus shown in FIG. 6 on the borosilicate glass substrate on which the dielectric multilayer film was formed in the same manner as in Example 1. Here, FIG. 6 is a schematic view of an apparatus for manufacturing a diffraction grating by the method of the present invention using a phase mask, FIG. 7 (a) is a diagram for explaining the action of the phase mask, and FIG. The figure which shows the state which has irradiated the laser beam to the board | substrate, (c) is a figure which shows the glass substrate by which laser processing was carried out.
[0024]
Specifically, a substrate provided with a phase mask in which a diffraction grating was formed via a spacer was placed on the surface on which the dielectric multilayer film was formed on the glass substrate, and laser light was irradiated.
[0025]
When laser light is incident on the phase mask, as shown in FIG. 7A, a plurality of diffracted lights mainly including + 1st order, 0th order, and −1st order are emitted, and the phase mask emits light due to interference of these diffracted lights. A periodic light intensity distribution is obtained in the vicinity of the side pole.
Here, as the phase mask of this example, a diffraction grating period: 1055 nm, a diffraction grating depth: about 250 nm, and a size: 10 mm × 5 mm (Canada made by QPS Technology Inc.) were used.
Then, as shown in FIG. 7B, a glass substrate on which a thin film was formed was set in a region where this periodic intensity distribution was formed. As a result, as shown in FIG. 7 (c), the thin film evaporated or ablated in accordance with the periodic light intensity, and a diffraction grating having the same period as the light intensity period processed the thin film on the glass substrate. Formed in shape.
[0026]
The laser beam used was 355 nm light, which is the third harmonic of the Nd: YAG laser, as in Example 1. 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, and the beam diameter was about 5 mm at an energy of 110 mJ / pulse. In order to increase the energy density of the laser so as to be suitable for processing, the laser beam was narrowed down with a lens having a focal length of 250 nm so that the beam size on the glass substrate was about 2 mm.
[0027]
In this embodiment, the distance between the phase mask and the glass substrate is set to about 50 μm by the spacer. 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 overlap, 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. In the case where laser irradiation was performed with a quartz plate sandwiched between and closely attached, a diffraction grating could be produced in the same manner as in this example. The phase mask is used repeatedly, and it is important to prevent the contamination of the phase mask. Therefore, interposing a spacer is an effective means.
[0028]
【The invention's effect】
As described above, according to the present invention, after forming the dielectric multilayer film on the surface of the substrate, the dielectric multilayer film is irradiated with laser light having an intensity distribution, and a part of the dielectric multilayer film is irradiated. Is produced in accordance with the intensity of the laser beam, and other portions are left as dielectric convex portions periodically arranged having a lattice constant of the order of the wavelength of the light, thereby producing a diffraction grating or a photonic crystal. Therefore, a diffraction grating or the like can be easily manufactured without going through complicated steps such as photolithography and etching.
[0029]
In the evaporation process of the dielectric multilayer film by laser, even if the thresholds causing melting / evaporation or ablation of each layer are different, evaporation of the intermediate layer occurs when the upper and lower layers evaporate. In comparison, the difference between the layers is less likely to appear, and a step is less likely to occur on the side surface of the dielectric convex portion.
[Brief description of the drawings]
FIG. 1 is an enlarged perspective view showing a state in which a dielectric multilayer film is formed on the surface of a substrate. FIG. 2 is a schematic view of an apparatus for manufacturing a diffraction grating by the method of the present invention using laser interference. Fig. 4 is an optical micrograph (1000x) of a dielectric convex portion formed by laser light irradiation, and Fig. 4 (b) is a diagram created based on the photo. Fig. 4 is periodic in one direction produced by the method of the present invention. FIG. 5 is a perspective view of a diffraction grating having periodic irregularities formed in two directions, produced by the method of the present invention. FIG. 6 is a diagram of a diffraction grating formed by a method of the present invention using a phase mask. FIG. 7A is a diagram for explaining the action of a phase mask, FIG. 7B is a diagram showing a state in which a glass substrate is irradiated with laser light through the phase mask, and FIG. ) Is a diagram showing a laser processed glass substrate. [Fig. 8] (a) Prior art The perspective view of the diffraction grating which formed the periodic unevenness | corrugation in 1 direction shown in literature, (b) is the perspective view of the diffraction grating which formed the periodic unevenness | corrugation in 2 directions shown in prior art literature. Figure

Claims (4)

In a method of manufacturing an optical element in which dielectric convex portions in which two or more kinds of layers having different dielectric constants are laminated are periodically arranged so as to have a lattice constant of about the wavelength of light, the method includes: After the dielectric multilayer film is formed on the dielectric multilayer film, the dielectric multilayer film is irradiated with laser light having an intensity distribution, and the dielectric multilayer film absorbs the energy of the laser light, thereby causing melting, evaporation or ablation. Part of the dielectric multilayer film is removed according to the intensity of the laser beam, and the other part is left as a periodically arranged dielectric convex part having a lattice constant of about the wavelength of the light. A method for manufacturing an optical element. 2. The method of manufacturing an optical element according to claim 1, wherein the dielectric multilayer film is at least 2 selected from silicon oxide, titanium oxide, cerium oxide, germanium oxide, magnesium fluoride, calcium fluoride, and tantalum oxide. A method for producing an optical element, comprising laminating more than one kind of film. 2. The method of manufacturing an optical element according to claim 1, wherein the laser light has a periodic intensity distribution in one direction, and the laser light having a periodic intensity distribution in one direction is a phase mask or two pieces of light. A method for producing an optical element, which is obtained by causing laser light to interfere. 2. The method of manufacturing an optical element according to claim 1, wherein the laser light has a periodic intensity distribution in two directions, and the laser light having a periodic intensity distribution in the two directions includes three or more laser lights. A method for producing an optical element obtained by interfering with the optical element.

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