JP7266275B2 - Grating structure made of quartz and method for manufacturing diffraction grating - Google Patents

Description

The present invention relates to grating structures and diffraction gratings made of quartz and methods for their manufacture.

In recent years, in the field of spectroscopy, echelle spectroscopy has come to be widely used for simultaneously measuring a wide wavelength range. In echelle spectroscopy, a high-dispersion diffraction grating that uses high-order diffracted light is combined with a vertical dispersion element such as a prism or a low-dispersion diffraction grating to fold the spectrum into a two-dimensional imaging device.

In astronomical observations, as telescopes become larger, spectroscopic observation devices also become larger. Therefore, there is a demand for the development of transmission diffraction gratings with large angular dispersion that enable miniaturization of optical systems. In particular, a diffraction grating with an incident angle (the angle between the incident light and the diffraction grating normal) and a diffraction angle (the angle between the diffracted light and the diffraction grating normal) of 45° bends the optical axis at right angles, making it This simplifies the arrangement of the optical system, contributing to the miniaturization of the device and the ease of optical adjustment. Angular dispersion: dβ/dλ, where N is the number of gratings, m is the diffraction order, β is the diffraction angle, and λ is the wavelength,
dβ/dλ = Nm/cosβ
, and the larger the product of N and m and the closer β is to 90°, the larger the value. Conventional saw-toothed surface-ruled transmission diffraction gratings are often used at a diffraction angle of 20° or less because the diffraction efficiency decreases as the diffraction angle increases.

Under these circumstances, for example, the WFOS (Wide-Field Optical Spectrometer), which is the first phase observation instrument of the 30-m telescope (TMT) currently under construction, has wavelengths from ultraviolet to near-infrared (30
0 to 1000 nm), there is a demand for the development of a transmission type diffraction grating that can accurately measure high-order diffracted light at a large diffraction angle (for example, 36° to 53°).

In the above-described conventional surface ruled transmission grating, the refractive index of the medium filling the grating must be increased as the diffraction angle increases (angle dispersion increases). For example, when the incident angle and the diffraction angle are 45°, the diffracted light does not appear outside the grating if the refractive index of the medium filling the grating is 2.3 or less. However, transparent media having a refractive index of 2.3 or higher for visible light are limited to ZnSe, ZnS, TiO ₂ , diamond, and the like. Furthermore, at a wavelength of 400 nm or less, diamond is the only transparent medium with a refractive index of 2.3 or more.

As a means for solving this problem, a thick rectangular diffraction grating (Volume Binary grating, hereinafter referred to as a VB diffraction grating) as shown in FIG. 9 has been proposed.

The VB diffraction grating can match the characteristics of S-polarized light and P-polarized light to achieve high diffraction efficiency for natural light polarized light and circularly polarized light. For example, if the refractive index of the ridge is 1.55, the refractive index of the groove is 1.0, and the incident and diffraction angles are 45°, the ratio of the groove width (S) to the ridge width (L) (duty ratio ) is 4:1, and the ratio (aspect ratio) of the depth (t) to the width (S) of the groove is about 1:20. On the other hand, for high-order diffracted light, the refractive index of the ridge is 1.55, the refractive index of the groove is 1.0, the incident and diffraction angles are 45°, the duty ratio (L:S) is 19:1, and the aspect ratio (S When :t) is about 1:40, high diffraction efficiency can be achieved in the 6th order or higher.

It is extremely difficult to fabricate grating structures with groove aspect ratios greater than 1:10, such as by anisotropic etching of quartz or UV exposure of photoresist. Non-Patent Document 1 reports a prototype of a quartz VB diffraction grating for first-order diffracted light of visible light. The VB diffraction grating described in Non-Patent Document 1 has a grating period Λ ~ 0.44 µm, L: S = 0.22 µm
m: 0.22 μm, and the groove aspect ratio is 1:10 (groove depth t=2.2 μm). However, with this shape, the diffraction efficiencies of S-polarized light and P-polarized light cannot be matched.

In Non-Patent Document 2, an attempt is made to process grooves having an aspect ratio of 1:15 (L=0.22 μm:t=3.3 μm), but an ideal shape cannot be obtained.

Thus, it would be desirable to produce a grating structure made of quartz having a high aspect ratio.

In addition to the diffraction grating, the grating structure made of quartz can also be suitably used for MEMS (Micro Electro Mechanical Systems) such as flow path modules, acceleration sensors, pressure sensors, and gyroscopes. This is because quartz transmits visible light and has heat resistance.

M. C. Gupta, S. T. Peng, "Diffraction characteristics of surface-relief gratings," Appl. Opt. 32, 2911-2917 (1993) T. Clausnitzer, E.B. Kley, H.J. Fuchs, A. Tuennermann, "Highly efficient polarization-independent transmission gratings for pulse stretching and compression." Proc. SPIE 5252, 174-182 (2004) N. Ebizuka, et. al., "Novel gratings for next-generation instruments of astronomical observations," Proc. SPIE 10233, 0M1-0M8, 2017.

In view of these problems, the object of the present invention is to provide a grating structure made of quartz.

A first aspect of the present invention is
A method for manufacturing a grating structure made of quartz, comprising:
A step of preparing an SOQ (Silicon on Quartz) substrate;
drawing a resist pattern on the silicon-side surface of the SOQ substrate;
a step of forming silicon ridges by digging grooves on the silicon side surface by etching using the resist pattern as a mask so that a silicon bonding layer in contact with the quartz layer of the SOQ substrate remains;
obtaining quartz ridges by oxidizing the entire silicon of the SOQ substrate including the silicon ridges and the bonding layer ;
A method comprising:

By forming grooves in silicon with good workability in this way by etching and then oxidizing it, a lattice structure made of quartz can be easily manufactured. With this manufacturing method, a lattice structure with a high aspect ratio can be manufactured.

Further, in the manufacturing method according to this aspect, the step of partially oxidizing the silicon and the step of removing the silicon oxide layer may be performed after the step of digging the trench. These two steps may be performed only once or may be performed multiple times.

By partially oxidizing the silicon and removing the silicon oxide layer, it is possible to reduce the width of the ridges (hereafter referred to as ridges) of the lattice structure, so that the width of the ridges can be narrower than the limit of the resist pattern. . In addition, surface roughness called scallops generated during etching can be removed to make the surface smooth.

In the manufacturing method according to this aspect, the resist pattern may be a pattern in which a plurality of fine lines with a predetermined width are arranged, and each fine line may be provided with a gap at a predetermined interval.

By using such a resist pattern, the generated ridges are also provided with gaps at predetermined intervals. Although there is a risk that the ridges may break, undulate, or collapse during oxidation, such a situation can be prevented by providing gaps in the ridges to avoid stress concentration.

In the manufacturing method according to this aspect, the resist pattern may have a connecting portion between adjacent fine lines.

By using such a resist pattern, a reinforcing portion connecting between two adjacent ridges is created. Therefore, it is possible to prevent the ridges from collapsing during oxidation.

A second aspect of the present invention is a method for manufacturing a diffraction grating,
each of the steps described above;
filling the grooves of the grating structure with a resin having a higher refractive index than quartz;
characterized by comprising

The present invention can also be regarded as a microchannel module, a pressure sensor, an acceleration sensor, a gyroscope, etc. having the lattice structure made of quartz described above. The present invention can also be viewed as an aerial imaging optical element having the high aspect ratio grating structure described above.

According to the present invention, a grating structure made of quartz can be easily produced, and in particular, a grating structure made of quartz having a high aspect ratio can also be produced.

FIG. 1 is a diagram illustrating the structure of a transmission diffraction grating according to an embodiment. FIG. 2 is a flow chart showing manufacturing steps within a transmission grating according to an embodiment. FIG. 3 is a diagram for explaining a method of manufacturing a transmission diffraction grating according to the embodiment. FIG. 4 is a diagram showing resist patterns in the first embodiment. FIG. 5 is a diagram showing the lattice structure in the first embodiment. FIG. 6 is a diagram showing numerical analysis results of the diffraction efficiency of the transmissive diffraction grating according to the first embodiment. FIG. 7 is a diagram showing resist patterns in the second embodiment. FIG. 8 is a diagram showing a lattice structure in the second embodiment. FIG. 9 is a diagram illustrating a conventional VB diffraction grating.

<First embodiment>
In recent years, in the field of spectroscopy, echelle spectroscopy has come to be widely used for simultaneously measuring a wide wavelength range. In echelle spectroscopy, a high-dispersion diffraction grating that uses high-order diffracted light is combined with a vertical dispersion element such as a prism or a low-dispersion diffraction grating to fold the spectrum into a two-dimensional imaging device.

This embodiment is a high-dispersion diffraction grating 10 (hereinafter simply referred to as diffraction grating 10) used for echelle spectroscopy. FIG. 1 shows the structure of the diffraction grating 10. As shown in FIG.

The diffraction grating 10 includes a grating structure 11 made of quartz (SiO ₂ ), a high refractive index resin 13 (filling portion) filled in grooves of the grating structure 11, and a glass substrate provided on the resin 13. 14. The grid structure 11 has a structure in which a plurality of ridges 12 are arranged in parallel on one surface of a base. Ridge 12 has an upright rectangular shape. The ridges 12 are also called fins or comb teeth.

In this embodiment, when the incident angle and the diffraction angle of the third-order diffracted light with a wavelength of 1.65 μm are 28°, the width L of the ridges 12 is 0.5 μm, and the interval S between the adjacent ridges 12 is 4 μm. .6 μm, and the repetition period Λ is 5.1 μm. Further, the height (groove depth) t of the ridge portion 12 is 16 μm, and the aspect ratio L:t is 1:32. In this embodiment, the width L and spacing S of the ridges 12 are constant, but the width L and spacing S may vary depending on the position.

Next, a method for manufacturing the diffraction grating 10 according to this embodiment will be described. FIG. 2 is a flowchart showing the manufacturing process of the diffraction grating 10 according to this embodiment. FIG. 3 is a diagram for explaining each step.

In step S10, an SOQ (Silicon on Quartz) substrate in which a quartz layer 31 and a silicon layer 32 are bonded is prepared (FIG. 3A). The quartz layer of the SOQ substrate becomes the base of the grid structure 11 and the silicon layer 32 becomes the ridges 12 of the grid structure 11 . The thickness of the silicon layer 32 may be determined according to the intended depth t of the grooves of the diffraction grating 10 (the height of the ridges 12). In this embodiment, for example, an SOQ substrate to which a silicon layer 32 having a thickness of 17 μm is bonded is used.

In step S20, a resist pattern 33 is drawn on the surface of the silicon layer 32 of the SOQ substrate by photolithography (FIG. 3B). As shown in FIG. 4, the resist pattern 33 is a stripe pattern in which a plurality of fine lines with a width of 2.0 μm are arranged at intervals of 3.1 μm.

In step S30, a groove with a height of 15.86 μm is dug by cycle etching (Bosch process) using the resist pattern as a mask (FIG. 3C). The Bosch process is mainly a deep reactive ion etching (RIE) that repeats isotropic etching using sulfur hexafluoride ( _SF6 ) gas and side wall protection using Teflon-based gas ( _{C4F8 ,} etc.). be. At this time, the silicon layer 32 is not entirely etched, leaving a bonding layer 32a that bonds to the quartz layer 31. Next, as shown in FIG. In this embodiment, the thickness of the bonding layer 32a is 1.14 μm. In the present embodiment, the sidewalls of the grating are perpendicular to the substrate and have a constant width, but a tapered grating may be formed by changing the cycle etching conditions.

The silicon is oxidized in step S40 (FIG. 3D), and the silicon oxide layer 34 is removed in step S50 (FIG. 3E). Steps S40 and S50 are repeated multiple times until the width of the ridge 12 reaches the desired value. In this embodiment, the width L of the ridge portion 12 is set from 2.0 μm to 0.22 μm by performing steps S40 and S50 twice. The interval S between the ridges 12 is 4.88 μm. Oxidation and removal of the oxide layer removes sidewall scallops caused by cyclic etching and smoothes the surface. Further, it is desired that the bonding layer 32a is also thinned but not completely removed.

In step S60, the entire silicon is heated to, for example, about 1100° C. to oxidize it (FIG. 3F). Through the steps up to this point, the grating structure 11 made of quartz is obtained. The grid structure 11 obtained by the process of step S60 has a structure in which a plurality of ridges 12 are arranged, as shown in FIG. Note that FIG. 5 is a conceptual diagram for explaining the structure, and the scale is not accurate. Since the silicon layer is thickened by oxidation, the width L of the ridges 12 of the obtained lattice structure 11 can be 0.5 μm, the interval S can be 4.6 μm, and the height t can be 16 μm.

In step S70, the grooves of grating structure 11 are filled with resin 13 having a high refractive index (FIG. 3G), and in step S80 glass substrate 14 (ZnSe, n=2.45@1.65 μm) is formed on resin 13. ) (FIG. 3H). In step S70, resin 13 is used which has a refractive index at which incident light has a critical angle on the grating side wall.

The refractive index _n2 of the resin 13 in FIG. 1 is obtained. Assuming that the angle of refraction inside the resin 13 at the interface between the glass substrate 14 and the resin 13 is _θ1 ,
sin θ ₀ =n ₂ ×sin θ ₁
is. The formula for the critical angle at the lattice sidewall is
n ₁ ×sin 90°=n ₂ ×cos θ ₁
is. From the above,
(sin θ ₀ )/n ₁ =tan θ ₁
Since θ ₀ and n ₁ are known and θ ₁ can be obtained, the refractive index n ₂ of the resin 13 can be obtained.

Since the refractive index of quartz is n ₁ =1.44@1.65 μm, for θ ₀ =28°, n ₂ ≧1.516 from the above equation. Further, when θ ₀ =45°, n ₂ ≧1.60 from the above equation.

FIG. 6 is a diagram showing the diffraction efficiency of the diffraction grating 10 according to this embodiment. The solid line in the figure indicates the diffraction efficiency for S-polarized light, and the dotted line indicates the diffraction efficiency for P-polarized light. As can be seen from the figure, the diffraction grating 10 can achieve a high diffraction efficiency of about 60% or more in the 2nd to 6th orders.

In addition, although the above description assumes use in vacuum or air (refractive index=1), it is not always necessary. It does not matter if the diffraction grating is in contact with something other than vacuum or air. Even in that case, the shape can be designed by the same technique as described above.

The transmission diffraction grating according to this embodiment can be used as a dispersive optical element other than the echelle spectroscopic measurement. Since the diffraction grating 10 has an imaging function, it can also be used as an aerial imaging optical element for projecting an image in the air.

If the grating period of the transmission type diffraction grating according to the present embodiment is set to about several tens of μm to 100 mm, it is not used as a diffraction grating, but is used as lighting by guiding external light to the ceiling or the back of the room. Applications such as windows are also possible.

(Second embodiment)
The diffraction grating of the first embodiment has a structure in which a plurality of continuous ridges 12 are arranged. In order to increase the area of the diffraction grating, it is necessary to increase the length of the ridges 12 (the length in the direction perpendicular to the plane of the paper in FIG. 1). If so, there is a risk that the ridges 12 may be broken or wavy when heated to about 1100° C. for silicon oxidation. Therefore, in this embodiment, a ridge structure having a structure different from that of the first embodiment is created.

FIG. 7 shows the resist pattern 33 in the resist pattern drawing step S20 of this embodiment. In the first embodiment, as shown in FIG. 4, a striped pattern is used, but in this embodiment, as shown in FIG. 7, a pattern including gaps 71 in the middle of the stripes is used. Although only one gap 71 is provided for one stripe in FIG. 7 to divide the stripe into two stripe segments, two or more gaps 71 may be provided for one stripe. The interval of the gap 71 may be set to a width that does not affect the performance of the manufactured diffraction grating, and in this embodiment, the interval of the gap 71 is, for example, 3 μm. Also, a connecting portion 72 is provided between two adjacent stripe segments. Although one connecting portion 72 is provided between adjacent stripe segments in FIG. 7, two or more connecting portions may be provided.

FIG. 8 shows the lattice structure 11 obtained in the process of step S60 in this embodiment. By using the resist pattern as described above, the ridges 12 of the grid structure 11 according to the present embodiment are divided by providing the gaps 81 . Further, reinforcing portions 82 corresponding to the connecting portions 72 are provided between adjacent ridge portion 12 segments. By providing the gap 81 and the reinforcing portion 82 in this manner, it is possible to prevent breakage, undulation, and collapse during oxidation heating. That is, according to this embodiment, a diffraction grating with a large area can be manufactured.

(Third embodiment)
Although the first and second embodiments use the grating structure as a diffraction grating, the grating structure described above can be used for purposes other than the diffraction grating.

The lattice structure 11 can be used, for example, to define channels and reaction vessels in a microchannel module (microfluidic device). For example, if a microchannel module having the lattice structure 11 is used when observing DNA with a fluorescence microscope, background light can be suppressed because quartz does not generate fluorescence. Alternatively, grid structure 11 can be used in other MEMS devices such as acceleration sensors, pressure sensors, gyroscopes, and the like. Alternatively, the grating structure can be used as an optical amplifier that amplifies light by covering the surface of the grating structure with a thin metal film or the like to cause surface plasmon resonance. Furthermore, by combining the lattice structure 11 with silicon, metal, nonlinear optical crystal, etc., it is expected that it can be applied to logic gates, wiring, filters, etc. of all-optical computers.

In the past, when a lattice structure transparent to light of a target wavelength was required, resin was sometimes used, but there was a problem of low heat resistance. The lattice structure made of quartz of this embodiment is transparent to light (visible light) and resistant to heat, so it can be suitably used for MEMS devices for various purposes.

10: Diffraction Grating 11: Grating Structure 12: Ridges of Grating Structure 13: Resin (Filling Portion) 14: Glass Substrate 31: Quartz Layer 32: Silicon Layer 33: Resist Pattern 34: Silicon Oxide Layer

Claims (6)

A method for manufacturing a grating structure made of quartz, comprising:
A step of preparing an SOQ (Silicon on Quartz) substrate;
drawing a resist pattern on the silicon-side surface of the SOQ substrate;
a step of forming silicon ridges by digging grooves on the silicon side surface by etching using the resist pattern as a mask so that a silicon bonding layer in contact with the quartz layer of the SOQ substrate remains;
obtaining quartz ridges by oxidizing the entire silicon of the SOQ substrate including the silicon ridges and the bonding layer ;
A method, including oxidizing a portion of the silicon and removing a silicon oxide layer are performed after creating the ridge ;
The method of claim 1. the step of partially oxidizing the silicon and the step of removing the silicon oxide layer are performed multiple times;
3. The method of claim 2. The resist pattern is a pattern in which a plurality of thin lines with a predetermined width are arranged, and each thin line is provided with a gap at a predetermined interval.
4. A method according to any one of claims 1-3. The resist pattern has a connecting portion between adjacent fine lines,
5. The method of claim 4. each step of the method for manufacturing a lattice structure made of quartz according to any one of claims 1 to 5;
filling the grooves of the grating structure with a resin having a higher refractive index than quartz;
A method of manufacturing a diffraction grating, comprising:

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