JP4650390B2 - Diffraction grating manufacturing method - Google Patents

Description

  The present invention relates to a method for manufacturing a high-precision diffraction grating by eliminating the influence of stray light when forming a groove pattern by a holographic exposure method.

  One method of manufacturing a diffraction grating is a holographic exposure method in which a groove pattern is formed on the surface of a substrate by generating and exposing interference fringes. The principle of the holographic exposure method will be described with reference to FIG. FIG. 1A is an overall view showing this exposure method, and FIG. 1B is an enlarged view of the vicinity of the substrate.

First, the photosensitive layer 12 is formed by applying a photosensitive agent to the upper surface of the substrate 11. Next, the coherent light 13 is divided into two light beams 131 and 132 having the same intensity by the beam splitter 14, and the two light beams 131 and 132 are irradiated on the surface of the photosensitive layer 12 so as to overlap each other. . As a result, the light 131 and the light 132 interfere to form an interference fringe 15 on the surface of the photosensitive layer 12, and the photosensitive layer 12 is exposed according to the intensity of light in the interference fringe 15.
When the photosensitive layer 12 is developed after being exposed in this manner, a portion where the substrate 11 is exposed and a portion where the photosensitive agent remains are formed in a stripe shape in the photosensitive layer 12 according to the pattern of the interference fringes 15. Then, an ion beam method or the like is used to etch the upper surface of the substrate 11 using the remaining photosensitive agent as a mask, whereby a striped uneven pattern, that is, a diffraction grating groove is formed on the upper surface of the substrate 11.

  In this method, a part of the light 131 and 132 reaching the photosensitive layer 12 passes through the photosensitive layer 12 and enters the substrate 11 (passing light 161 and 162). A part of the transmitted light 161 and 162 is reflected by the lower surface 111 of the substrate 11 (reflected light 171 and 172) and is incident on the photosensitive layer 12 again. When the reflected light 171 and the reflected light 172 interfere with each other in the photosensitive layer 12, interference fringes that become noise are superimposed on the regular interference fringes 15. When a diffraction grating having such noise interference fringes is used for spectroscopic measurement, stray light is generated due to the disturbance of the diffraction grating pattern, and the measurement accuracy is lowered.

  Patent Document 1 describes that, as shown in FIG. 2A, a black paint 18 is applied to the lower surface of the substrate when performing holographic exposure. Thereby, the passing lights 161 and 162 can be absorbed by the black paint 18, and the reflected lights 171 and 172 incident on the photosensitive layer 12 can be suppressed. Further, this document also describes that the lower surface of the substrate is ground glass as shown in FIG. Thereby, it is possible to scatter the passing lights 161 and 162 on the ground glass-like lower surface 19, and to prevent the interference between the reflected lights and the formation of noise interference fringes.

  However, when black paint is used, the black paint cannot completely absorb the passing light, so that noise interference fringes are formed (although the amplitude is smaller than the case without black paint). It cannot be completely prevented. Further, when the lower surface of the substrate is made of frosted glass, noise interference fringes are not formed, but the scattered light is incident on the entire photosensitive layer with a substantially uniform intensity, so that the contrast of the original interference fringes is lowered. Arise.

Japanese Unexamined Patent Publication No. 07-098404 ([0007] to [0015], FIG. 1)

  The problem to be solved by the present invention is to provide a method for manufacturing a diffraction grating capable of preventing the generation of noise interference fringes and a reduction in contrast of the original interference fringes.

In order to solve the above problems, a method for manufacturing a diffraction grating according to the present invention includes two coherent light beams having the same intensity on the surface of a photosensitive layer formed on the upper surface of a substrate capable of transmitting predetermined coherent light. A diffraction grating manufacturing method including a step of forming a groove pattern in the photosensitive layer by forming an interference fringe by entering
The lower surface of the substrate is a glossy surface,
In the groove pattern manufacturing process,
Each of the two coherent lights is divided into front incident light for incidence on the surface of the photosensitive layer and lower incidence light for incidence on the lower surface of the substrate before the photosensitive layer,
Within the substrate, the intensity of each of the two lower surface incident light and the intensity of the inner surface lower surface reflected light that passes through the photosensitive layer and the substrate and reflects on the lower surface of the surface incident light are the same, And the intensity and phase of the lower surface incident light or / and the front surface incident light are adjusted so that the lower surface incident light and the lower surface reflected light in the substrate interfere and disappear by reversing the phases of both.
It is characterized by that.

The exposure apparatus according to the present invention causes the two coherent lights having the same intensity to enter the surface of the photosensitive layer formed on the upper surface of the substrate through which predetermined coherent light can pass and the lower surface is a glossy surface. In an exposure apparatus for producing a groove pattern in the photosensitive layer by forming interference fringes,
a) Provided on the optical path in front of the photosensitive layer for each of the two coherent lights, the surface incident light for making the coherent light incident on the surface of the photosensitive layer and the lower surface for making it incident on the lower surface of the substrate A beam splitter that splits the incident light;
b) a bottom-surface incident optical system that causes each of the two bottom-surface incident light to enter the bottom surface of the substrate;
c) Intensity of each of the two lower surface incident lights passing through the lower surface of the substrate in the substrate and the inner lower surface of the surface incident light that passes through the photosensitive layer and the substrate and is reflected by the lower surface of the substrate. Intensity adjusting means for adjusting the intensity of the lower surface incident light or / and the surface incident light so that the intensity of the reflected light is the same;
d) The lower surface incident light is such that the lower surface incident light and the lower surface reflected light in the substrate interfere with each other and disappear by reversing the phases of the two lower surface incident light and the inner lower surface reflected light in the substrate. Phase adjusting means for adjusting the phase of the light or / and the surface incident light; and
It is characterized by providing.

  In the present application, for the sake of convenience, in order to show the positional relationship of each component, the photosensitive layer side is expressed as the “upper” side with reference to the substrate, but this is merely a convenience notation for showing one directionality, The direction at the time of implementation of the diffraction grating manufacturing method of the present invention is not specified at all.

  According to the present invention, each of the two reflected light beams in the lower surface of the substrate generated from the two front surface incident lights has the same intensity and inverted phase as the lower surface incident light in the substrate. . As a result, it is possible to prevent noise interference fringes from occurring due to interference between the reflected light from the bottom surface of the two substrates. On the other hand, the original interference fringes can be formed in the same manner as in the past by using two front surface incident lights.

  The intensity of the lower surface incident light and / or the front surface incident light can be adjusted by using an ND (neutral density) filter or the like that is usually used for light intensity control. In addition, the phase of the lower surface incident light and / or the front surface incident light is obtained by using a phase variable element such as an element having a variable total thickness made by opposing two wedge-shaped glass substrates, or changing the optical path length. Can be adjusted.

  When the reflected light on the bottom surface of the substrate is scattered on the bottom surface of the substrate, the scattered light enters the entire photosensitive layer as described above, and the contrast of the original interference fringes is lowered. Cannot be lost by interfering. Therefore, in the present invention, the lower surface of the substrate is a glossy surface.

  According to the present invention, since the lower surface reflected light and the lower surface incident light in the substrate can be canceled by interfering with each other in the substrate, it is possible to prevent generation of noise interference fringes due to two conventionally generated reflected light from the lower surface in the substrate. it can. Further, in the present invention, since the lower surface of the substrate is a glossy surface, the reflected light on the lower surface in the substrate and the incident light on the lower surface can be more reliably lost to prevent the generation of noise interference fringes, and the scattered light is spread over the entire photosensitive layer. It is possible to prevent the contrast of the original interference fringes from being lowered due to incidence.

  By using the diffraction grating produced by the method of the present invention, the above-described effects can be obtained, and therefore, more precise measurement is possible particularly in the measurement field that handles weak light such as Raman spectroscopy. .

  An embodiment of a diffraction grating manufacturing method according to the present invention will be described with reference to FIG. FIG. 3 is a schematic block diagram of the exposure apparatus 20 for carrying out this method. Note that the thick solid line and the thick broken line in FIG. 3 indicate the optical path of the laser beam.

  The laser beam output from the light source 21 is first divided into two equal parts by the zeroth beam splitter 22. The two laser beams split by the zeroth beam splitter 22 are further divided into two equal parts by the first beam splitter 231 and the second beam splitter 232. Here, one of the laser beams divided by the first beam splitter 231 is referred to as first surface incident light 281 and the other is referred to as first lower surface incident light 291. Similarly, one of the laser beams divided by the second beam splitter 232 is referred to as second surface incident light 282 and the other is referred to as second lower surface incident light 292.

  The first beam splitter 231 and the second beam splitter 232 adjust so that the first surface incident light 281 and the second surface incident light 282 overlap in the photosensitive layer 12 on the surface of the substrate 11. The substrate 11 is held by the substrate holder 27. As the substrate holder 27, for example, a clamp that supports the side surface of the substrate 11 from the left and right and that does not block the incidence of light on the upper and lower surfaces of the substrate 11 is used.

  The first lower surface incident light 291 uses the reflecting mirrors 261 and 262, and the second lower surface incident light 292 uses the reflecting mirrors 263 and 264 to adjust the optical path so as to enter the lower surface of the substrate 11, respectively. A first ND filter 241 is provided on the optical path of the first lower surface incident light 291 so that the first lower surface incident light 291 in the substrate 11 has the same intensity as the first substrate lower surface reflected light 311. The intensity of the light 291 is adjusted. Further, the first phase variable element 251 is provided on the optical path of the first lower surface incident light 291, and the first lower surface incident light 291 and the first lower surface reflected light 311 in the substrate 11 are adjusted so as to be inverted. Similarly, on the optical path of the second lower surface incident light 292, the second ND filter 242 for making the second lower surface incident light 292 and the second inner surface lower surface reflected light 312 in the substrate 11 have the same intensity and the phase of both are set. A second phase variable element 252 for inversion is provided.

  A method of manufacturing a diffraction grating using the exposure apparatus 20 will be described with reference to FIG. First, a photosensitive layer 12 made of a material sensitive to laser light emitted from the light source 21 is formed on the upper surface of the substrate 11 (FIG. 4A). Next, the substrate 11 is fixed to the substrate holder 27 and laser light is output from the light source 21. As a result, the first surface incident light 281 and the second surface incident light 282 are incident on the surface of the photosensitive layer 12, and both interfere to form an interference fringe 15 (FIG. 4B). A corresponding pattern 34 is formed on the photosensitive layer 12.

  At this time, a part of the first front surface incident light 281 passes through the upper surface of the photosensitive layer 12 and the substrate 11 and enters the substrate 11, and a part of the transmitted light is reflected by the lower surface of the substrate 11 (first 1 substrate inner bottom surface reflected light 311). On the other hand, the first lower surface incident light 291 enters the lower surface of the substrate 11, and part of the light passes through the substrate 11. As described above, the intensity and phase of the first lower surface incident light 291 are adjusted by the first ND filter 241 and the first phase variable element 251, so that the first lower surface incident light 291 </ b> A in the substrate 11 and the first substrate inside The lower surface reflected light 311 disappears due to interference. Similarly, the second lower surface incident light 312 that enters the substrate 11 and is reflected by the lower surface of the substrate 11 out of the second surface incident light 282 and the second lower surface incident light 292A in the substrate 11 disappear due to interference. . Thereby, it is possible to prevent noise interference fringes from occurring due to interference between the first substrate inner lower surface reflected light 311 and the second substrate inner lower surface reflected light 312.

  After the pattern 34 is formed on the photosensitive layer 12 in this way, the surface of the substrate 11 is etched from above the pattern 34 using a method such as reactive ion beam etching as in the conventional diffraction grating manufacturing method (FIG. 4). (c)) After that, by removing the pattern 34, the diffraction grating 30 in which the grating grooves are formed on the surface of the substrate 11 can be obtained (FIG. 4 (d)). Further, using the diffraction grating 30 thus obtained as a master, the master is first pressed against the resin 351 applied to the surface of the glass substrate 361 and directly cured by heating to transfer the negative type of the master (negative master 37, FIG. 4 (e )) Next, after the master 30 and the negative master 37 are separated, the mold of the negative master 37 is transferred to the resin 352 similarly applied to another glass substrate 362 (FIG. 4 (f)), whereby a replica diffraction grating 38 is obtained. Can be obtained (FIG. 4 (g)).

Next, a method for adjusting the intensity of the first lower surface incident light 291 and the second lower surface incident light 292 in the diffraction grating manufacturing method of the present embodiment will be described. Here, the first lower surface incident light 291 will be described as an example, but the second lower surface incident light 292 can be similarly adjusted.
First, a case where the first front surface incident light 281 and the first lower surface incident light 291 are s-polarized light will be described.
When the refractive index of the substrate 11 is n, the incident angle on the upper surface of the substrate 11 is θ, and the refractive angle on the upper surface of the substrate 11 is χ (FIG. 5), the first surface incident light incident on the upper surface of the substrate 11 from a vacuum. The transmission coefficient ts1n and the reflection coefficient rs1n of 281 are obtained using the refraction angle χ and the incident angle θ.
ts1n = 2 cosθ / (cosθ + n cosχ) (1)
rs1n = (cosθ-n cosχ) / (cosθ + n cosχ) (2)
It is expressed.
The light refracted at the refraction angle χ and entering the substrate 11 is incident on the lower surface of the substrate 11 at the incident angle χ. A part of the light is transmitted from the lower surface of the substrate 11 to the vacuum at the refraction angle θ, and the rest Reflects at a reflection angle χ. In this case, the transmission coefficient tsn1 and reflection coefficient rsn1 of s-polarized light on the lower surface of the substrate 11 are
tsn1 = 2n cosχ / (n cosχ + cosθ)… (3)
rsn1 = (n cosχ-cosθ) / (n cosχ + cosθ) (4)
It is expressed.
The intensity of the first substrate inner lower surface reflected light 311 on the lower surface of the substrate 11 is (ts1n × rsn1) times the first front surface incident light 281.
On the other hand, the intensity of the first lower surface incident light 291A entering the substrate 11 from the lower surface of the substrate 11 is ts1n times the intensity before entering the substrate 11.
Therefore, the intensity of the s-polarized light of the first lower surface incident light 291 before entering the substrate 11 is changed to that of the s-polarized light of the first surface incident light 281.
(ts1n × rsn1) / ts1n times
= rsn1 times
= (n cosχ-cosθ) / (n cosχ + cosθ) times (5)
By adjusting the first ND filter 241 so as to satisfy the above, the intensities of the first lower surface incident light 291A and the first lower surface reflected light 311 in the substrate 11 can be matched.

The intensity can be similarly calculated when the first front surface incident light 281 and the first lower surface incident light 291 are p-polarized light. That is, the transmission coefficient tp1n and the reflection coefficient rp1n of p-polarized light incident on the upper surface of the substrate 11 from the vacuum are:
tp1n = 2 cosθ / (n cosθ + cosχ) (6)
rp1n = (n cosθ-cosχ) / (n cosθ + cosχ) (7)
The transmission coefficient tpn1 and reflection coefficient rpn1 on the lower surface of the substrate 11 of light transmitted from the upper surface of the substrate 11 into the substrate 11 are
tpn1 = 2n cosχ / (cosχ + n cosθ)… (8)
rpn1 = (cosχ-n cosθ) / (cosχ + n cosθ)… (9)
It is expressed. On the other hand, the intensity of the first lower surface incident light 291A in the substrate 11 is tp1n times before entering the substrate 11. Therefore, in order to match the intensities of the first lower surface incident light 291A and the first lower surface reflected light 311 in the substrate 11, the intensity of the p-polarized light of the first lower surface incident light 291 before being incident on the substrate 11 is set. P-polarized light of the first surface incident light 281
(tp1n × rpn1) / tp1n times
= rpn1 times
= (cosχ-n cosθ) / (cosχ + n cosθ) times (10)
The first ND filter 241 may be adjusted so that

  In the calculation shown here, light reflection and absorption in the photosensitive layer 12 were ignored. Therefore, actually, the first lower surface incident light 291A and the first lower surface reflected light 311 in the substrate 11 and the second lower surface incident light 292A and the second lower surface reflected light 312 in the substrate 11 are completely transmitted. In order to cancel, the preliminary experiments are performed to finely correct the intensities of the first lower surface incident light 291 and the second lower surface incident light 292 from the values obtained by the above calculation.

The diffraction grating manufacturing method and the exposure apparatus according to the present invention are not limited to the above examples.
In the exposure apparatus 20 described above, an example is shown in which the first ND filter 241 is disposed between the first beam splitter 231 and the reflecting mirror 261, and the first phase variable element 251 is disposed between the reflecting mirror 262 and the lower surface of the substrate 11. However, the first beam splitter 231 and the first phase variable element 251 may be disposed at any position between the first beam splitter 231 and the lower surface of the substrate 11. The same applies to the second beam splitter 232 and the second phase variable element 252.

  Moreover, the phase of the 1st lower surface incident light 291 and the 2nd lower surface incident light 292 can be adjusted with the following method instead of using a phase variable element. The exposure apparatus 40 shown in FIG. 6 is similar to the exposure apparatus 20 described above, and includes reflecting mirrors 261 and 262 on the optical path of the first lower surface incident light 291 in order to make the lower surface incident light incident on the lower surface of the substrate 11. Reflective mirrors 263 and 264 are provided on the optical path of the second lower surface incident light 292, respectively. Then, by moving these reflecting mirrors and changing the optical path lengths of the first lower surface incident light 291 and the second lower surface incident light 292, the first lower surface incident light 291 and the second lower surface incident light that enter the substrate 11. The phase of 292 can be adjusted. As described above, in the exposure apparatus 40, the first surface incident light 281 and the second surface incident light 282 are reflected by the lower surface of the substrate 11 and the first lower surface incident light 291 and the second lower surface without providing a phase variable element. The incident light 292 can be adjusted to have an opposite phase. Except for this phase varying means, the exposure apparatus 40 has the same configuration as the exposure apparatus 20. The operation of the exposure apparatus 40 is the same as that of the exposure apparatus 20 except for the phase adjustment method.

  In the above exposure apparatuses 20 and 40, the first (second) beam splitter 231 (232) converts the laser light into first (second) surface incident light 281 (282) and first (second) lower surface incident light 291 (292). However, the intensity of the first (second) lower surface incident light 291 (292) may be adjusted by changing the intensity ratio at the time of division.

  Up to this point, an example of adjusting the intensity and phase of the first (second) lower surface incident light 291 (292) has been shown, but instead of the first (second) lower surface incident light 291 (292) or the first (second) The intensity and phase of the first (second) front surface incident light 281 (282) may be adjusted together with the second lower surface incident light 291 (292). For example, an ND filter or a phase variable element may be provided on the optical path of the first (second) bottom surface incident light 291 (292) between the first (second) beam splitter 231 (232) and the photosensitive layer 12, By changing the distance between the 1 (second) beam splitter 231 (232) and the photosensitive layer 12, the intensity and phase of the first (second) surface incident light 281 (282) can be adjusted.

The figure for demonstrating the principle of a holographic exposure method. The longitudinal cross-sectional view which shows the conventional method for preventing formation of a noise interference fringe. The schematic block diagram which shows one form of the apparatus for enforcing the diffraction grating manufacturing method which concerns on this invention. The longitudinal cross-sectional view which shows the process of the diffraction grating manufacturing method which concerns on this invention. FIG. 4 is a longitudinal sectional view for explaining an incident angle and a refraction angle on an upper surface and a lower surface of a substrate 11. The schematic block diagram which shows another form of the apparatus for enforcing the diffraction grating manufacturing method which concerns on this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 11 ... Substrate 111 ... Substrate lower surface 12 ... Photosensitive layer 13 ... Light 131, 132 ... Divided light 14 ... Beam splitter 15 ... Interference fringe 161 ... Passing light 171 ... Reflected light 18 ... Black paint 19 ... Ground glass-like lower surface 20, 40 ... exposure device 21 ... light source 22 ... 0th beam splitter 231 ... first beam splitter 232 ... second beam splitter 241 ... first ND filter 242 ... second ND filter 251 ... first phase variable element 252 ... second phase variable element 261, 262, 263, 264 ... Reflector 27 ... Substrate holder 281 ... First surface incident light 282 ... Second surface incident light 291 ... First lower surface incident light 292 ... Second lower surface incident light 291A ... First incident on substrate 11 Lower surface incident light 292A ... Second lower surface incident light 30 incident on the substrate 11 ... Diffraction grating 311 ... Lower surface reflected light 312 in the first substrate ... Lower in the second substrate Surface reflected light 34 ... patterns 351 and 352 formed on the photosensitive layer 12 ... resin 361 and 362 ... glass substrate 37 ... negative master 38 ... replica diffraction grating

Claims (2)

A step of forming a groove pattern in the photosensitive layer by making two coherent lights having the same intensity incident on the surface of the photosensitive layer formed on the upper surface of the substrate capable of transmitting predetermined coherent light to form interference fringes. A diffraction grating manufacturing method comprising:
The lower surface of the substrate is a glossy surface,
In the groove pattern manufacturing process,
Each of the two coherent lights is divided into front incident light for incidence on the surface of the photosensitive layer and lower incidence light for incidence on the lower surface of the substrate before the photosensitive layer,
Within the substrate, the intensity of each of the two lower surface incident light and the intensity of the inner surface lower surface reflected light that passes through the photosensitive layer and the substrate and reflects on the lower surface of the surface incident light are the same, And the intensity and phase of the lower surface incident light or / and the front surface incident light are adjusted so that the lower surface incident light and the lower surface reflected light in the substrate interfere and disappear by reversing the phases of both.
A diffraction grating manufacturing method characterized by the above. The interference fringes are formed by causing the two coherent lights having the same intensity to enter the surface of the photosensitive layer formed on the upper surface of the substrate through which predetermined coherent light can pass and the lower surface is a glossy surface. In an exposure apparatus for producing a groove pattern in a photosensitive layer,
a) Provided on the optical path in front of the photosensitive layer for each of the two coherent lights, the surface incident light for making the coherent light incident on the surface of the photosensitive layer and the lower surface for making it incident on the lower surface of the substrate A beam splitter that splits the incident light;
b) a bottom-surface incident optical system that causes each of the two bottom-surface incident light to enter the bottom surface of the substrate;
c) Intensity of each of the two lower surface incident lights passing through the lower surface of the substrate in the substrate and the inner lower surface of the surface incident light that passes through the photosensitive layer and the substrate and is reflected by the lower surface of the substrate. Intensity adjusting means for adjusting the intensity of the lower surface incident light or / and the surface incident light so that the intensity of the reflected light is the same;
d) The lower surface incident light is such that the lower surface incident light and the lower surface reflected light in the substrate interfere with each other and disappear by reversing the phases of the two lower surface incident light and the inner lower surface reflected light in the substrate. Phase adjusting means for adjusting the phase of the light or / and the surface incident light; and
An exposure apparatus comprising:

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