JP4873015B2 - Manufacturing method of blazed diffraction grating - Google Patents

Description

  The present invention relates to a method of manufacturing a diffraction grating which is a wavelength separation / selection element used in a spectroscope, an optical spectrum analyzer, a WDM monitor, a demultiplexer, synchrotron radiation spectroscopy, extreme ultraviolet (VUV) spectroscopy, and the like. The present invention relates to a method for manufacturing a blazed diffraction grating (blazed holographic grating) manufactured using a holographic exposure method.

  The diffraction grating is a wavelength separation / selection element used in a spectroscope, a demultiplexer, or the like. Various types of diffraction gratings are known depending on the cross-sectional shape of the grating groove, and one of them is a blazed diffraction grating (also called a blazed holographic grating) in which the cross-sectional shape of the groove is serrated. A basic manufacturing method of a conventional general plane blazed diffraction grating will be described with reference to FIG. 3 (see, for example, Patent Document 1).

  First, as shown in FIG. 3A, a photoresist layer 2 is formed by applying a photoresist to the surface of a flat substrate 1 such as quartz or glass. The photoresist layer 2 is exposed and developed with interference fringes due to two-beam interference to form a resist pattern 3 having a sine half wave shape (or a sine full wave shape) as shown in FIG. 3B (holographic). Exposure method). Then, using this resist pattern 3 as a mask, etching with an ion beam is performed from an obliquely upward direction so that a desired blaze angle is formed on the substrate 1 until the resist pattern 3 disappears. The groove 4 is formed (FIGS. 3C to 3E). Thereafter, in order to increase the reflectivity of the surface, as shown in FIG. 3 (f), a reflective film 5 is formed by coating the surface of the grating groove 4 with a reflective material such as aluminum or gold, and a certain wavelength. A blazed diffraction grating in which the energy of the diffracted light is concentrated is completed.

  Normally, when the substrate 1 is etched using the photoresist as a mask as described above, the substrate 1 is scraped at a speed higher than that of the photoresist. In particular, in order to cut the substrate 1 in a sawtooth shape (that is, to cut the substrate 1 immediately below the resist pattern 3 in a wedge shape), the etching rate with respect to the substrate 1 is faster than the etching rate with respect to the resist pattern 3, that is, the selectivity. Ion beam etching is performed using an etching gas having an etching rate greater than 1 (= etching rate for substrate material (eg, glass) / etching rate for photoresist). Further, under such an etching gas condition, as shown in FIG. 3C, it is oblique to the substrate 1 in the same direction as the blaze direction (basically from an oblique direction inclined by the same angle as the blaze angle θ0). An ion beam is incident.

  The relationship between the blaze direction and the cross-sectional shape of the grating grooves 4 is as shown in FIG. 3 (f). The same direction as the blaze direction means that the substrate 1 is obliquely upward within the angle range α in the figure. It means to go against.

  According to the above conventional method, there is an advantage that the substrate 1 can be efficiently etched to form a sawtooth-shaped lattice groove in a relatively short time. However, when an etching gas having a high selectivity is used, a lattice groove having a relatively large blaze angle can be easily formed, but it is difficult to produce a lattice groove having a small blaze angle of several degrees or less. Therefore, in order to reduce the blaze angle, it is necessary to use an etching gas having a small selection ratio. Then, the angle of the edge opposite to the blaze angle across the blaze surface 4a (angle β in FIG. 3E). Tends to increase, and the diffraction efficiency tends to decrease. In particular, in recent years, it has been necessary to shorten the blaze wavelength in order to handle short-wavelength light with a spectroscope or the like, and a diffraction grating having an extremely small blaze angle of about 2 ° or less has been demanded. However, with the conventional manufacturing method, it has been difficult to obtain a blazed diffraction grating having a small blaze angle and high diffraction efficiency.

  Further, since the spectroscopic optical system can be configured without using an imaging element such as a concave mirror with a concave diffraction grating, not only a flat type but also a concave type is widely used as a blazed diffraction grating. In recent years, polychromator spectrometers that can measure multiple wavelengths simultaneously using a concave blazed diffraction grating have become more compact due to improvements in photodiode array detectors. In the grating, it is necessary to reduce the radius of curvature of the concave surface, but the smaller the radius of curvature, the smaller the incident angle of the ion beam for forming the grating groove (the angle formed with the normal to the substrate). Absent. Therefore, it is difficult to form a grating groove with a small blaze angle, as compared with a plane blazed diffraction grating.

JP-A-2005-157118

  The present invention has been made to solve the above-mentioned problems, and a first object of the invention is to provide a method for manufacturing a plane blazed diffraction grating capable of achieving a high diffraction efficiency with a small blaze angle. It is in.

  The second object of the present invention is to provide a manufacturing method for easily manufacturing a concave blazed diffraction grating having a small blaze angle, which has been particularly difficult to manufacture.

The method for manufacturing a blazed diffraction grating according to the first aspect of the present invention made to achieve the first object,
a) a resist pattern production step of forming a sine full wave or sine half wave resist pattern by a holographic exposure method on a photoresist layer provided on a substrate;
b) Using an etching gas having a selection ratio of less than 1 with respect to the substrate on which the resist pattern is formed and irradiating an ion beam obliquely in a direction opposite to the blaze direction, the resist pattern and the substrate surface are An ion beam etching process for etching;
By executing the above, a lattice groove pattern having a sawtooth cross section is engraved on the substrate.

  The method for manufacturing a blazed diffraction grating according to the first invention is particularly useful for forming a grating groove pattern having a small blazed angle on a flat blazed diffraction grating having a flat substrate.

  In the first invention (the same applies to the second invention below), the etching gas selection ratio is defined by the etching rate for the substrate / the etching rate for the photoresist layer (resist pattern). For example, when a mixed gas in which oxygen is mixed with a fluorine-based gas is used as an etching gas, the selection ratio can be controlled by appropriately selecting the mixing ratio of oxygen.

  In addition, the state where the ion beam is irradiated “from an oblique direction opposite to the blaze direction” refers to a line parallel to the arrow indicating the direction of the ion beam and a line parallel to the arrow indicating the blaze direction on the substrate. And the angle on the side in the direction of the blaze direction is an acute angle. Therefore, the state of irradiating the ion beam “from an oblique direction opposite to the blaze direction” and the state of irradiating the ion beam “from an oblique direction in the same direction as the blaze direction” are mirror-symmetric with respect to the normal to the substrate. The ion beam is irradiated onto the substrate from a certain angle range.

  When ion beam etching is performed on a substrate such as glass on which a resist pattern is formed using an etching gas having a selection ratio of less than 1, the etching rate of the substrate is reduced with respect to the resist pattern. That is, the resist pattern disappears in a relatively short time, but the substrate is less likely to be scraped. Therefore, the groove formed by transferring the sine full wave or sine half wave of the resist pattern becomes shallow, and it becomes easy to form a small blaze angle of, for example, several degrees or less, and minute protrusions (roughness) are formed on the resist surface before transfer. Even if there is, stray light is reduced because it becomes smaller after transfer to the substrate. In addition, since the ion beam is irradiated obliquely in the direction opposite to the blaze direction, the incident angle of the ion beam is adjusted to an angle at which the ion beam does not hit one surface of the sine full wave or sine half wave of the resist pattern. The angle of the edge opposite to the blaze angle across the blaze surface can be made smaller than before. Therefore, the diffraction efficiency can be increased.

The method for manufacturing a blazed diffraction grating according to the second invention made to achieve the second object,
a) a resist pattern preparation step of forming a sine full wave or sine half wave resist pattern by a holographic exposure method on a photoresist layer provided on a concave substrate;
b) An etching gas having a selectivity of 1 or more is used for the substrate on which the resist pattern is formed, and an ion beam is irradiated from an oblique direction in the same direction as the blaze direction on the rear half surface in the direction of the blaze direction. A first ion beam etching step for etching the resist pattern and the substrate surface,
c) Using an etching gas having a selection ratio of less than 1 on the substrate on which the resist pattern is formed, irradiate the ion beam from an oblique direction opposite to the blaze direction on the front half surface in the direction of the blaze direction. A second ion beam etching step for etching the resist pattern and the substrate surface,
By executing the above, a lattice groove pattern having a sawtooth cross section is engraved on the concave substrate.

  In the method for manufacturing a blazed diffraction grating according to the second aspect of the present invention, “first” and “second” in the first and second ion beam etching steps do not define a temporal order. The process may be executed first.

  In the blazed diffraction grating manufacturing method according to the second aspect of the invention, in the first and second ion beam etching steps, the ion beam is irradiated in a state where the other half surface of the substrate to be etched is masked on the other half surface. Good.

  In the blazed diffraction grating manufacturing method according to the second aspect of the invention, the concave substrate is divided into two regions, one front surface and one rear surface in the blaze direction, and ions are formed from different directions according to the curved shape of each region. The photoresist and the substrate are etched by irradiating the beam. That is, when the ion beam is irradiated from an oblique direction in the same direction as the blaze direction on the rear half surface in the blaze direction, the incident angle of the ion beam with respect to the substrate increases toward the outer periphery. Even if it is used, the blaze angle can be reduced. On the other hand, when the ion beam is irradiated on the front half surface in the direction of the blaze direction obliquely in the same direction as the blaze direction, it is difficult to reduce the blaze angle because the incident angle of the ion beam with respect to the substrate is small. The blaze angle is reduced by using the same method as the manufacturing method.

  According to the method for manufacturing a blazed diffraction grating according to the first aspect of the present invention, a planar blazed diffraction grating having a small blaze angle and good diffraction efficiency can be easily obtained. Also, since ion beam etching is performed with a selection ratio of less than 1, that is, with a slow etching rate of the substrate, the roughness of the grating surface due to the transfer of the resist pattern can be reduced, and the diffraction grating having a smoother blazed surface Can be obtained. As a result, it is possible to further reduce stray light and configure a spectroscope with a small loss of light.

  Further, according to the blazed diffraction grating manufacturing method of the second invention, a grating groove having a small blaze angle can be easily formed over the entire concave surface. Thereby, it is possible to shorten the blaze wavelength even for a diffraction grating having a small curvature radius used in a short focus spectroscope, and it is possible to realize a short wavelength region spectrum with a compact spectroscope. Further, by using an etching gas having an optimum ion incidence direction and an optimum selection ratio in each region by increasing the number of divisions, a lattice groove having a smaller blaze angle can be formed over the entire area.

The schematic sectional drawing for demonstrating the procedure of the manufacturing method of the plane blazed diffraction grating which is one Embodiment of 1st invention. The schematic sectional drawing for demonstrating the procedure of the manufacturing method of the concave blazed diffraction grating which is one Embodiment of 2nd invention. The schematic sectional drawing for demonstrating the procedure of the manufacturing method of the conventional general plane blaze | braze type | mold diffraction grating.

Explanation of symbols

1 ... Substrate (planar substrate)
11 ... Substrate (concave substrate)
2, 12 ... Photoresist layer 3, 13 ... Resist pattern 4, 14 ... Lattice groove 4a ... Blaze surface 5, 15 ... Reflective coating 16, 17 ... Masking material

  A procedure for manufacturing a plane blazed diffraction grating according to an embodiment of the first invention will be described with reference to FIG.

(1) Photoresist layer forming step The surface of the flat substrate 1 made of optical glass or the like is optically polished, and the surface is cleaned by ultrasonic cleaning. Thereafter, a photoresist is applied to a substantially uniform thickness by, for example, spin coating, and baked in a convection oven. Thereby, a photoresist layer 2 is formed on the surface of the substrate 1 (FIG. 1A).

(2) Step of producing resist pattern A workpiece on which the photoresist layer 2 is formed on the surface of the substrate 1 is set in a holographic exposure apparatus, and a latent image of interference fringes having a predetermined density due to two-beam interference is exposed on the photoresist layer 2. Then, the photoresist layer 2 of the exposed workpiece is developed with a dedicated developer and rinsed with pure water to produce a resist pattern 3 on which a diffraction grating pattern is formed. In general, since the intensity distribution of the interference fringes of two-beam interference is sinusoidal, the cross-sectional shape of the manufactured resist pattern 3 is also sinusoidal full wave or sinusoidal half wave (FIG. 1B). The groove depth of the resist pattern 3 can be determined by controlling the exposure time and the development time.

(3) Ion Beam Etching Step With respect to the substrate 1 on which the resist pattern 3 is formed, from obliquely above in the direction opposite to the blaze direction (the direction of the arrow from right to left in FIG. 1) (the normal line P of the substrate 1) Reactive ion beam etching is performed on the resist pattern 3 and the substrate 1 by irradiating with an ion beam (with an incident angle of θ1 from At this time, an etching gas having a selection ratio of less than 1 is used. Therefore, the resist pattern 3 is shaved relatively quickly, and the substrate 1 is shaved relatively slowly.

  At this time, as shown in FIG. 1C, the resist pattern 3 having a sine full wave cross section is scraped from the slope on which the ion beam hits. The angle γ of this slope is substantially equal to the incident angle θ1 of the ion beam. Since the photoresist thickness is thin at the bottom portion of the resist pattern 3, the photoresist disappears from this portion first, and the surface of the substrate 1 is exposed. Then, the ion beam starts to directly hit the substrate 1, but since the selection ratio is small, the cutting speed of the substrate 1 is slow. On the other hand, since the resist pattern 2 can be scraped faster, the portions where the photoresist disappears spread one after another in the direction opposite to the blaze direction, and the surface of the substrate 1 hits the ion beam (FIG. 1D).

  Since the cutting speed of the substrate 1 is slow, the inclination of the exposed surface (that is, the blazed surface 4a) is reduced, and the inclination is further reduced as the etching proceeds. On the other hand, the surface 4b opposite to the blaze surface 4a is shaved with substantially the same inclination as that of the ion beam. Thus, when the photoresist completely disappears, the etching end point is reached. At this time, the resist pattern 3 having a sine full wave shape is transferred to the lattice groove 4 pattern having a sawtooth cross section having a small blaze angle θ0. (FIG. 1 (e)).

(4) Reflective film forming step After the etching is completed, the substrate 1 on which the lattice grooves 4 are engraved is washed, and a reflective film 5 is formed on the surface of the lattice grooves 4 by vacuum deposition (FIG. 1F). ). An appropriate material may be used for the reflective coating 5 according to the wavelength range used, and for example, aluminum, gold, platinum, or an X-ray multilayer film can be used. It is also possible to use the reflective coating 5 without coating.

  As described above, it is possible to obtain a planar blazed diffraction grating having a small blaze angle and a small edge angle opposite to the blaze angle across the blaze surface, that is, a high diffraction efficiency.

  Next, a procedure for manufacturing a concave blazed diffraction grating according to an embodiment of the second invention will be described with reference to FIG.

(1) Photoresist layer forming step The surface of the concave substrate 11 made of optical glass or the like is optically polished, and the surface is cleaned by ultrasonic cleaning. Thereafter, a photoresist is applied to a substantially uniform thickness and baked in a convection oven. Thereby, a photoresist layer is formed on the surface of the substrate 11.

(2) Resist Pattern Preparation Step Similar to the above-described plane blazed diffraction grating, a resist pattern 13 having a sine full wave or sine half wave cross section is formed on the surface of the substrate 11 by a holographic exposure method (FIG. 2 ( a)).

(3) First ion beam etching step First, masking is performed using a masking material 16 on the substrate 11 on which the resist pattern 3 is formed so that the ion beam does not strike the front half surface in the blaze direction (the left half surface in FIG. 2). , And reactive ion beam etching of the resist pattern 13 and the substrate 11 by irradiating an ion beam obliquely from above in the same direction as the blaze direction (at an incident angle of an angle θ2 from the normal line P of the substrate 11). (FIGS. 2B and 2C). At this time, an etching gas having a selectivity of 1 or more is used, and preferably an etching gas having 2 or more is used.

  Since the surface of the substrate 11 is an arc-shaped concave surface, the substantial incident angle increases as it goes to the outer periphery (as it goes to the right in FIG. 2), and the blaze angle can be reduced. In addition, since the selection ratio is large, the substrate 11 is likely to be deeply cut like a wedge in the portion where the photoresist disappears and the substrate 11 is exposed. Then, when the photoresist is completely disappeared and the lattice grooves 14 are formed on the half surface, this etching is finished (FIG. 2D).

(4) Second Ion Beam Etching Step This time, the substrate 11 is masked so that the ion beam does not strike the rear half surface in the blaze direction (the right half surface in FIG. 2) in which the lattice grooves 14 are first formed. After the masking is performed using the material 17, the etching method in the first invention, that is, the resist remaining on the half surface in front of the blaze direction by irradiating the ion beam from the oblique direction opposite to the blaze direction. Reactive ion beam etching is performed on the pattern 13 and the substrate 11 (FIGS. 2E and 2F). At this time, an etching gas having a selectivity of less than 1 is used. In this case, since the selection ratio is small, the substrate 11 is relatively hard to be shaved, and the blaze angle can be reduced as described above. Then, when the photoresist completely disappears and the lattice grooves 14 are formed on the remaining half surface, this etching is finished (FIG. 2 (g)).

  By forming the grating grooves 14 on the surface of the concave substrate 11 by half by ion beam etching having different two-stage conditions as described above, the grating grooves 4 having a sawtooth cross section having a small blaze angle as a whole are formed. A pattern can be produced.

(5) Step of forming a reflective film A reflective film 15 is formed on the surface of the substrate 11 on which the grating grooves 14 are formed by a vacuum vapor deposition method in the same manner as the above-mentioned plane blazed diffraction grating (FIG. 2 (h)).

  As described above, a concave blazed diffraction grating having a small blaze angle in the entire substrate 11 can be obtained. In this case, unlike the flat blazed diffraction grating, the blaze angle is not constant and differs depending on its position.

  A specific example of manufacturing a planar blazed diffraction grating according to an embodiment of the first invention will be described.

  In this example, a flat substrate made of BK7 optical glass having a size of about 60 mm × 60 mm × 10 mm was used as the substrate 1. The substrate 1 may be other than BK7. For example, BSC2, Pyrex (PYREX: registered trademark of Corning), soda glass, quartz glass, Zerodur (manufactured by SCHOTT, ZERODU: registered trademark of Carl Zeiss), Christon Low thermal expansion crystal glass such as (registered trademark of HOYA Corporation) is useful. The surface of the substrate 1 was optically polished, and a photoresist layer 2 was formed on the surface using a spinner. As a photoresist, MP1805 manufactured by Shipley Co., Ltd. was used, spin coating was performed at a speed of 3000 rpm for 40 seconds, followed by baking in a convection oven at 90 ° C. for 30 seconds, and a photoresist layer 2 having a thickness of about 0.3 μm. Formed. The photoresist is not limited as long as it can be holographically exposed. In addition to the MP1800 series manufactured by Shipley, OFPR5000 manufactured by Tokyo Ohka Co., Ltd. may be used.

  Thereafter, development was performed after two-beam interference exposure of laser light, whereby a sinusoidal diffraction grating pattern was formed on the substrate 1. The laser used for the exposure is a He—Cd laser (wavelength: 441.6 nm), but an Ar + ion laser or the like can be used as long as the wavelength is sensitive to photoresist (about 350 to 450 nm). Further, a developing solution of MP303A manufactured by Shipley Co., Ltd. was used for development. As a result, the lattice grooves of the resist pattern 3 formed on the substrate 1 are sinusoidal with 600 / mm and a groove depth of 0.2 μm. Since the intensity distribution of the interference fringes of the two-beam interference is a sine wave, the grating groove pattern of the photoresist is also a sine wave.

The workpiece produced as described above was mounted in a vacuum chamber of an ion beam etching apparatus, and reactive beam etching was performed by reducing the pressure inside the vacuum chamber. Specifically, the resist pattern 3 was irradiated with an ion beam obliquely from above in the direction opposite to the blaze direction at an incident angle θ1 = 76 ° from the normal to the substrate 1. At this time, a gas obtained by mixing CF ₄ and O ₂ gas at a mixing ratio O ₂ / (CF ₄ + O ₂ ) = 30 [%] was used as an etching gas. The selection ratio at this time is 0.7, which is 1 or less. The gas pressure during etching was 2 × 10 ⁻² Pa.

  Ion beam etching under the above conditions was performed until the resist pattern 3 was completely transferred and engraved into the pattern of the lattice grooves 4 of the substrate 1. The etching time required for this was about 10 minutes. As a result of such treatment, a plane blazed diffraction grating having a small blaze angle with a blaze angle θ0 of 2 ° could be obtained.

  A specific example of manufacturing a concave blazed diffraction grating according to an embodiment of the second invention will be described.

  In this example, a concave substrate made of BK7 optical glass having a radius of about 30 mm × 30 mm × 10 mm and a curvature radius of 80 mm was used as the substrate 11. In addition, the same thing as Example 1 can use another thing as the board | substrate 11. FIG. After optically polishing the surface of the substrate 11, a photoresist layer was formed on the surface using a spinner. The material and formation conditions of the photoresist layer are the same as in Example 1. Thereafter, a sinusoidal resist pattern 13 was formed on the substrate 11 by holographic exposure and development, and the conditions were the same as in Example 1. Thereby, the lattice grooves of the resist pattern 13 formed on the substrate 11 are sinusoidal with 600 / mm and a groove depth of 0.2 μm.

The workpiece produced as described above was mounted in a vacuum chamber of an ion beam etching apparatus, and reactive beam etching was performed by reducing the pressure inside the vacuum chamber. Specifically, first, on the front half surface in the direction of the blaze direction, a convex semicircular glass having the same curvature is used as a masking material 16 so that the resist pattern 13 is not scratched. As a mask, the other half surface, that is, the rear half surface in the direction of the blaze direction is etched. At this time, the lattice groove pattern was irradiated with an ion beam from obliquely above in the same direction as the blaze direction at an incident angle θ2 = 78 ° from the normal to the substrate 11. At this time, a gas in which CF ₄ and Ar gas were mixed at a mixing ratio Ar / (CF ₄ + Ar) = 40 [%] was used so that the selection ratio was 2 or more as an etching gas. The selection ratio at this time is 2.1. The gas pressure during etching was 2 × 10 ⁻² Pa.

  Ion beam etching under the above conditions was performed until the resist pattern 13 was completely transferred and engraved into the pattern of the lattice grooves 14 of the substrate 11. The etching time required for this was about 13 minutes.

Next, the previously etched half surface is covered with a heat-resistant tape as the masking material 17, and the remaining half surface (the front half surface in the direction of the blaze direction) is exposed, and the incident angle θ3 = 71 ° is the same as the blaze direction. The ion beam was irradiated obliquely from above. At this time, a gas in which CF ₄ and O ₂ gas were mixed at a mixing ratio O ₂ / (CF ₄ + O ₂ ) = 30 [%] was used so that the selection ratio was 1 or less as an etching gas. The selection ratio at this time is 0.7. The gas pressure during etching was 2 × 10 ⁻² Pa.

  Ion beam etching of the remaining half surface under the above conditions was performed until the resist pattern 13 was completely transferred and engraved into the pattern of the lattice grooves 14 of the substrate 11. The etching time was about 7 minutes. The blaze angle θ0 of the concave blazed diffraction grating thus completed was not uniform, but averaged 3 °. In the conventional method, it was difficult to form a lattice groove having a blaze angle as small as 3 ° on the substrate 11 having such a small radius of curvature. However, according to the manufacturing method according to the second invention, it is relatively easy to manufacture. can do.

Claims (2)

A method of manufacturing a blazed diffraction grating by engraving a concave groove-shaped substrate with a sawtooth cross-sectional grating groove pattern,
a) a resist pattern preparation step of forming a sine full wave or sine half wave resist pattern by a holographic exposure method on a photoresist layer provided on a concave substrate;
b) For the substrate on which the resist pattern is formed, an etching gas having an etching rate for the substrate / etching rate for the photoresist layer of 1 or more is used, and in a direction orthogonal to the extending direction of the lattice grooves to be formed. there, the slope of the shorter of the saw teeth on the front side, the half front side from the blaze direction is the direction toward the rear direction of the rear side when facing the longer slope of the rear side, and the blaze direction A first ion beam etching step of etching the resist pattern and the substrate surface by irradiating an ion beam obliquely in the same direction;
c) with respect to the substrate on which the resist pattern has been formed, the etching rate to the etching rate / the photoresist layer using an etching gas is less than 1 for the substrate, the front side of half of the blaze orientation, the blaze A second ion beam etching step of etching the resist pattern and the substrate surface by irradiating an ion beam from an oblique direction opposite to the direction;
Method for producing a blazed diffraction grating, characterized in that it comprises a. In the first and second ion beam etching step, the blazed diffraction grating according to claim 1, characterized in that an ion beam is irradiated while masking the other half is not a half of the substrate is etched object Production method.

Images