JP2007187732A - Diffractive optical element and its manufacturing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a diffractive optical element capable of improving characteristics such as diffraction efficiency, and to provide the convenient manufacturing method of the diffractive optical element. <P>SOLUTION: The diffractive optical element contains a light-transmissive layer comprising a DLC film (1) and the light-transmissive layer contains a plurality of high refractive index regions 1b and low refractive index regions 1a which are periodically and alternately arranged in directions parallel to the layer surface so as to operate as a refractive index modulation type diffraction grating, wherein the surface of each of the high refractive index regions 1b has a projecting shape 1d operating as a relief type diffraction grating. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an improvement in characteristics of a diffractive optical element and a method for easily manufacturing the improved diffractive optical element.

  As is well known, a diffractive optical element is an optical element that can generate various functions by utilizing the diffraction phenomenon of light. More specifically, diffractive optical elements having functions such as wavelength coupling, power coupling, polarization coupling, wavelength plate, optical isolator, or lens are known.

  In general, a diffractive optical element is manufactured by forming a diffraction grating layer on a translucent substrate. Based on the structural difference of the diffraction grating layer, the diffractive optical element is roughly classified into a relief type and a refractive index modulation type. Diffraction occurs in the relief type diffractive optical element depending on whether a phase difference is caused by a phase of light passing through a thick portion in the relief of the diffraction grating layer being delayed compared to a phase of light passing through a thin portion. On the other hand, diffraction occurs in the refractive index modulation type diffractive optical element because the phase difference of the light passing through the high refractive index region of the diffraction grating layer is delayed compared to the phase of the light passing through the low refractive index region. Because it occurs.

  FIG. 6 is a schematic sectional view illustrating an example of a conventional method for producing a relief type diffractive optical element (Non-Patent Document 1: “Ultra-precision processing of microlens (array) and mass production technology”). Association Press, April 28, 2003, pp. 20-21 and pp. 71-81).

  In FIG. 6A, a positive photoresist layer 12 is formed on the Si substrate 11 and irradiated with ultraviolet light 14 a through a first photomask 13.

  In FIG. 6B, the exposed resist layer 12 is developed to form a first resist pattern 12a. Then, using the first resist pattern 12a as a mask, a groove having a predetermined depth is formed by reactive ion etching (RIE) represented by an arrow 14b.

  In FIG. 6C, by removing the first resist pattern 12a, a relief type diffractive optical element 11a having a binary level (the phase of light is modulated in two stages) can be obtained. Note that the groove width, depth, spacing, etc. in the relief are set so as to obtain the best diffraction efficiency in accordance with the purpose of each of the two-level or multi-level relief type diffractive optical elements.

  FIGS. 6D to 6F illustrate a process of manufacturing a four-level relief type diffractive optical element following the same process as FIGS. 6A to 6C.

  In FIG. 6D, a second resist layer 15 is further formed on the upper surface of the Si substrate 11a formed in the same process as in FIG. 6C, and ultraviolet light 14c is applied through the second mask 16. Irradiate.

  6E, the exposed second resist layer 15 is developed to form a second resist pattern 15a as shown in FIG. Using the second resist pattern 15a as a mask, etching to a predetermined depth is further performed by RIE represented by an arrow 14d.

In FIG. 6 (f), the second resist pattern 15a is removed to obtain a relief type diffractive optical element 11b that includes a four-stage thickness change and can cause a four-level phase change. Note that a higher diffraction efficiency can be obtained with a multi-level diffractive optical element than with a two-level diffractive optical element. Further, by repeating the above-described photolithography and RIE processes N times, a 2 ^N level relief type diffractive optical element can be manufactured. From this, it will be understood that if the number of steps N is very large, the step-like relief approaches a continuous slope.

  On the other hand, although a refractive index modulation type diffractive optical element can be manufactured in principle, it has been difficult to manufacture a practical refractive index modulation type diffractive optical element. This is because, for example, it is known that the refractive index can be increased by irradiating quartz glass with an energy beam such as ultraviolet light or X-ray. In this case, the refractive index change Δn is about 0.01 or less. Because it is small.

  Therefore, in recent years, attempts have been made to produce a refractive index modulation type or relief type diffractive optical element using a photopolymer layer (see, for example, JP-A-7-306529 of Patent Document 2). According to Patent Document 2, the refractive index can be increased by about Δn = 0.1 by irradiating the photopolymer layer with an energy beam.

  However, the method of producing a diffractive optical element using a photopolymer layer requires a process that is not easy to control, such as polymerization using energy beam irradiation or generation of fine bubbles. Not only energy beam irradiation but also complicated processes such as heat treatment and development are required. Further, when a refractive index modulation type diffractive optical element is manufactured using a photopolymer, an additional oxygen shielding film is required, and when a relief type diffractive optical element is manufactured, an additional negative type photo element is required. The production process is very complicated because a polymer layer is required.

  Furthermore, diffractive optical elements manufactured using a photopolymer layer tend to deteriorate in characteristics over time, and are particularly low in heat resistance and cannot be used stably with high intensity light. There is a problem that it can not be used in the environment of.

  Recently, the present inventors have disclosed that a refractive index modulation type diffractive optical element can be produced using a DLC (diamond-like carbon) film (for example, Patent Document 1: JP 2004-163892 A). That is, the present inventors have confirmed that the refractive index can be easily and significantly increased by irradiating the DLC film with an energy beam.

  Such DLC films can be formed by plasma CVD (chemical vapor deposition) on silicon substrates, glass substrates, polymer substrates, and various other substrates. The translucent DLC film obtained by such plasma CVD usually has a refractive index of about 1.55.

  As the energy beam for increasing the refractive index of the DLC film, ultraviolet (UV), X-ray, synchrotron radiation (SR) light, ion beam, electron beam, or the like can be used. SR light generally includes electromagnetic waves in a wide wavelength range from ultraviolet light to X-rays.

For example, the amount of change in refractive index can be increased to about Δn = 0.65 by implanting He ions at a dose of 5 × 10 ¹⁷ / cm ² under an acceleration voltage of 800 keV. Note that the refractive index can be similarly increased by implanting ions such as H, Li, B, and C. Further, the amount of change in the refractive index can be increased up to about Δn = 0.65 by irradiating with SR light having a spectrum of 0.1 to 130 nm. Furthermore, in UV light irradiation, for example, if KrF excimer laser light with a wavelength of 248 nm is pulsed at a frequency of 100 mW / mm ^{2 at} an irradiation density of 160 mW / mm ² , the refractive index change amount can be increased to about Δn = 0.22. Can do. Note that the refractive index can be similarly increased by irradiation with excimer laser light such as ArF (193 nm), XeCl (308 nm), XeF (351 nm), or Ar laser light (488 nm). The refractive index change amount of these DLC films by energy beam irradiation is the refractive index change amount by conventional UV irradiation of quartz glass (Δn = about 0.01 or less) or the refractive index change amount of photopolymer (Δn = 0). It can be seen that it is significantly larger than about .1 or less.

In FIG. 7, a method of manufacturing a refractive index modulation type diffractive optical element using such a DLC film is illustrated in a schematic cross-sectional view. In this method of manufacturing a refractive index modulation type diffractive optical element, for example, a DLC film 22 is formed on a silica (SiO ₂ ) glass substrate 21 by plasma CVD. Then, a mask 24 a formed on another silica glass substrate 23 a is overlaid on the DLC film 22.

As shown in FIG. 7, UV (ultraviolet) light 25 a is irradiated onto the DLC film 22 from above in a state where the mask 24 a is overlaid on the DLC film 22. As a result, in the DLC film 22, the region covered with the mask 24a and not irradiated with the UV light 25a does not change the refractive index, and maintains the refractive index n ₁ as deposited by plasma CVD. ing. On the other hand, a region in the DLC film 22 that is not covered with the mask 24a and irradiated with the UV light 25a undergoes a refractive index change, and the refractive index is increased to n ₂ . After the UV light irradiation, the silica glass substrate 23 a and the mask 24 a are removed from the DLC film 22. The refractive index modulation type diffractive optical film 22 thus obtained includes binary refractive indexes of n ₁ and n ₂ and acts as a two-level refractive index modulation type diffraction grating.

In FIG. 8, a method for producing a three-level refractive index modulation type diffractive optical element is illustrated by a schematic cross-sectional view. In FIG. 8, a second mask 24b on the silica glass substrate 23a is further formed on the DLC film 22 including two-level refractive index modulation of n ₁ and n ₂ formed by the same method as in FIG. Overlaid. In that state, the UV light irradiation 25b is performed again.

At this time, the second mask 24b has an opening for irradiating only the selected region in the region of the high refractive index n ₂ in the DLC film formed in the process of FIG. . Accordingly, after the irradiation of UV light 25b, a relatively high refractive index n selected regions refractive index of the _second region is increased to a higher n _3. That is, the refractive index modulation type diffractive optical film 22 produced in FIG. 8 functions as a diffraction grating including three levels of refractive index modulation of n ₁ , n ₂ , and n ₃ .

In this way, a refractive index modulation type diffractive optical film including a desired multi-level refractive index modulation is obtained by repeatedly performing UV light irradiation on the DLC film while sequentially using a mask having a partially corrected pattern. be able to. As described above, since the multi-level refractive index modulation type diffraction grating can generate higher diffraction efficiency than the two level refractive index modulation type diffraction grating, the light utilization efficiency can be further improved.
JP 2004-163892 A JP-A-7-306529 "Ultra-precision processing and mass production technology of microlens (array)", Technical Information Association, April 28, 2003, pp. 20-21 and pp. 71-81

  As described above, in order to manufacture a conventional general relief type diffractive optical element, a groove needs to be carved by etching in a translucent substrate, so that the substrate needs a certain thickness. Moreover, the etching process requires labor and time, and it is not easy to accurately adjust the depth of the groove to be carved by etching. In particular, in order to produce a multi-level relief type diffractive optical element with high diffraction efficiency, such an etching process must be repeated.

  Further, as described above, the production of a diffractive optical element using a photopolymer layer is not easy to control and requires complicated steps. Furthermore, a diffractive optical element using a photopolymer layer is likely to deteriorate over time, and is not particularly resistant to heat and strong light.

  On the other hand, compared to the conventional case of irradiating an energy beam to silica glass or a photopolymer layer, if the energy beam is radiated to the DLC film, the refractive index can be increased by an order of magnitude. However, a stable and practical refractive index modulation type diffractive optical element can be manufactured. However, even in a refractive index modulation type diffractive optical element using a DLC film, it may be desired to further improve the diffraction efficiency. In order to produce a multi-level refractive index modulation type diffractive optical element, generally, many times of masking and energy beam irradiation must be repeated even in a DLC film.

  In view of the prior art state, an object of the present invention is to simply provide a diffractive optical element having further improved characteristics such as diffraction efficiency.

  The diffractive optical element according to the present invention includes a translucent layer made of a DLC film, and the translucent layer is a plurality of layers arranged alternately and alternately in a direction parallel to the layer surface so as to act as a refractive index modulation type diffraction grating. The high-refractive index region and the low-refractive index region are characterized in that the surface of the high-refractive index region has a convex shape that acts as a relief type diffraction grating. Such a diffractive optical element can have, for example, an excellent wavelength branching function that splits one light beam including a plurality of wavelengths into a plurality of light beams depending on the wavelengths.

  In the boundary region between the high refractive index region and the low refractive index region in the DLC film, the refractive index is preferably changed continuously. Further, the boundary region between the high refractive index region and the low refractive index region may be parallel to the direction orthogonal to the layer surface of the translucent layer, or inclined with respect to the layer surface of the translucent layer. It may be parallel to the direction. As the translucent layer, a translucent DLC film can be preferably used.

  In the method for manufacturing the diffractive optical element as described above, the diffractive optical element includes a light-transmitting DLC film, and the DLC film is irradiated with ultraviolet rays through a phase grating mask, and by the ultraviolet irradiation, In the DLC film, a plurality of high-refractive index regions and low-refractive index regions that are periodically and alternately arranged in a direction parallel to the film surface so as to act as a refractive index modulation type diffraction grating are formed. A convex shape acting as a relief type diffraction grating is formed on the surface of the high refractive index region.

  Further, in the method for manufacturing a diffractive optical element, the diffractive optical element includes a light-transmitting DLC film, and the DLC film has interference between the + 1st order diffracted light and the −1st order diffracted light that have passed through the phase grating mask. A high refractive index region is formed in the region irradiated with light and the interference light is irradiated, and a convex surface can be formed.

  Furthermore, in the method for manufacturing a diffractive optical element, the DLC film is irradiated with interference light of 0th-order diffracted light and + 1st-order diffracted light or −1st-order diffracted light that has passed through the phase grating mask, and the interference light is irradiated. A high refractive index region may be formed in the formed region, and a convex surface may be formed.

  As the ultraviolet ray, for example, a KrF laser including a wavelength of 248 nm can be preferably used.

  According to the present invention as described above, characteristics such as diffraction efficiency can be improved in the diffractive optical element, and the diffractive optical element thus improved can be easily manufactured.

In the cross-sectional view of FIG. 1, a manufacturing process of a diffractive optical element according to an embodiment of the present invention is schematically illustrated. First, for example, when a DLC film 1 containing carbon as a main component is deposited by a well-known plasma CVD on a silica glass substrate (not shown), various saturated or unsaturated hydrocarbon gases or vapors are used as source gases. Can be used as More specifically, methane (CH ₄ , molecular weight 16, saturated), acetylene (CH≡CH, molecular weight 26, unsaturated), cyclopropane (C ₃ H ₆ , molecular weight 42, saturated), propane (C ₃ H ₈ , Molecular weight 44, saturated), butyne (CH ₃ C≡CCH ₃ , molecular weight 54, unsaturated), butylene (CH ₂ ═CHCH ₂ CH ₃ , molecular weight 56, unsaturated), benzene (C ₆ H ₆ , molecular weight 78, Saturation), hexane (C ₆ H ₁₄ , molecular weight 86, saturation), octane (C ₈ H ₁₈ , molecular weight 114, saturation) and other hydrocarbon gases or vapors to form a DLC film containing carbon as a main component It can be deposited by plasma CVD.

In the manufacturing method of the diffractive optical element shown in FIG. 1, for example, a relief-type phase grating mask made of glass (diffraction grating) is applied to a DLC film 1 having a thickness of about 1 μm and a refractive index of n = 1.55. 2) are arranged close to each other with a distance of about 100 μm. In this state, for example, a KrF laser beam (wavelength 248 nm) 3 is irradiated for 1 hour at an energy density of 16 mw / mm ² , whereby a diffractive optical element can be manufactured. At this time, the refractive index of the region exposed to the interference light between the + 1st order diffracted light and the −1st order diffracted light from the phase grating mask 2 is increased. On the other hand, the refractive index of the region not exposed by the interference light is maintained as it is.

  In this case, the interference light between the + 1st order diffracted light and the −1st order diffracted light appears at a period that is ½ of the uneven period of the relief type phase grating mask 2. Therefore, the relief type phase grating mask 2 formed with a concavo-convex period twice as long as the period of the desired high refractive index region in the DLC film can be used. In addition, the intensity of the interference light increases as the center of the width of the high refractive index region is increased. Therefore, in the DLC film 1 in FIG. 1, the refractive index continuously changes in the vicinity of the interface between the low refractive index region and the high refractive index region (that is, acts as an infinite level diffraction grating), and high diffraction efficiency. Can be obtained. If desired, an amplitude type phase grating mask obtained by patterning a chromium film, a chromium oxide film, an aluminum film, or the like can be used instead of the relief type phase grating mask 2.

  FIG. 2 is a schematic cross-sectional view showing an example of the diffractive optical element obtained by the manufacturing method of FIG. In this figure, in the DLC film 1, a region 1a that is not exposed by the interference light of the + 1st order diffracted light and the −1st order diffracted light remains as it is, and the refractive index n = 1.55 is relatively low. Maintained as an area. On the other hand, the region 1b exposed by the interference light has a significantly increased refractive index, and can be, for example, a region having a relatively high refractive index n = 1.70. These high refractive index regions 1b can be formed with a period of 0.5 μm, for example.

  By the way, the surface 1d corresponding to the high refractive index region 1b manufactured by the method as shown in FIG. 1 is convex as compared with the surface 1c corresponding to the low refractive index region 1a as shown in FIG. . Such a height difference in the uneven surface shape can be, for example, 50 nm to 100 nm. The reason why such an uneven surface shape is formed on the DLC film by UV irradiation is not necessarily clear at present, but such an uneven surface shape can act as a relief type diffraction grating. Moreover, the uneven surface shape is formed as a continuous curved surface (acts as an infinite level diffraction grating), and can act as a relief type diffraction grating having high diffraction efficiency.

  That is, the diffractive optical element obtained in FIG. 2 includes a relief type diffraction grating having a high diffraction efficiency superimposed on a refractive index modulation type diffraction grating having a high diffraction efficiency. Can produce high diffraction efficiency.

FIG. 3 is a schematic cross-sectional view illustrating an example of the operation of the diffractive optical element as shown in FIG. As shown in this figure, if light 4 including wavelengths λ ₁ and λ ₂ is incident on the diffractive optical element 1, the incident light 4 is diffracted at different diffraction angles depending on the wavelength. The That is, the incident light 4 is branched (spectral) into light having the wavelength λ ₁ and light having the wavelength λ ₂ . In FIG. 3, for the sake of simplicity of explanation, the case of incident light including two wavelengths λ ₁ and λ ₂ is described. However, a color filter for light including wavelengths of, for example, blue, green, and red is used. Needless to say, a diffractive optical element that can act as the above can also be manufactured.

  In the diffractive optical element shown in FIG. 3, the boundary region between the high refractive index region 1b and the low refractive index region 1a is illustrated as being parallel to the film thickness direction. Needless to say, the boundary region may be inclined with respect to the film thickness direction, as shown. For this purpose, for example, in the manufacturing method of FIG. 1, the ultraviolet light 3 is incident obliquely with respect to the film surface of the DLC film 1, and exposure is performed by interference light of 0th-order diffracted light and + 1st-order diffracted light or -1st-order diffracted light. Can be used. However, the interference light between the 0th-order diffracted light and the + 1st-order diffracted light or the −1st-order diffracted light appears in the same period as the uneven period of the phase grating mask 2. Therefore, the phase grating mask 2 formed with irregularities having the same period as compared with the period of the desired high refractive index region 1b in the DLC film 1 must be used.

  The schematic cross-sectional view of FIG. 5 shows another preferable application example of the diffractive optical element in which the boundary region between the high refractive index region and the low refractive index region is inclined with respect to the film thickness direction. In FIG. 5, the irregular shape on the surface is not shown.

  That is, in the DLC diffraction grating film 1 on the glass substrate S, the boundary region 1e between the high refractive index region and the low refractive index region is inclined with respect to the film thickness direction. In this case, for example, the incident light L1 is refracted into the light L2 when entering the DLC film 1, and is high at a predetermined Bragg reflection angle θ in the boundary region 1e between the high refractive index region and the low refractive index region. The light L3 is diffracted with efficiency. Since the boundary region 1e is inclined with respect to the thickness direction of the DLC film, the diffracted light L3 can be emitted in a direction orthogonal to the DLC film surface. That is, the diffracted light L3 can be efficiently incident so as to be orthogonal to the surface of the liquid crystal panel, for example.

  As described above, according to the present invention, characteristics such as diffraction efficiency can be improved in a diffractive optical element, and such an improved diffractive optical element can be easily manufactured and provided.

FIG. 6 is a schematic cross-sectional view for explaining a manufacturing process of a diffractive optical element according to an embodiment of the present invention. It is typical sectional drawing which shows an example of the diffractive optical element obtained by this invention. It is a typical sectional view which illustrates the spectrum function of the diffractive optical element obtained by the present invention. FIG. 5 is a schematic cross-sectional view illustrating the spectral function of a diffractive optical element in which a boundary region between a high refractive index region and a low refractive index region is inclined with respect to the film thickness direction in the present invention. FIG. 10 is a schematic cross-sectional view showing another preferred application example of the diffractive optical element in which the boundary region between the high refractive index region and the low refractive index region is inclined with respect to the film thickness direction in the present invention. It is typical sectional drawing which shows an example of the preparation process of the relief type | mold diffractive optical element by a prior art. It is typical sectional drawing which shows an example of the preparation processes of the refractive index modulation type | mold diffractive optical element by a prior art. FIG. 8 is a schematic cross-sectional view illustrating an example of a process of manufacturing a three-level refractive index modulation type diffractive optical element following the manufacturing process of FIG. 7.

Explanation of symbols

  1 DLC film, 1a low refractive index region, 1b high refractive index region, 1c concave surface, 1d convex surface, 1e boundary region between low refractive index region and high refractive index region, 2 relief type phase grating mask (diffraction grating) 3 energy beam (ultraviolet laser beam), 4 incident light, 11 silicon substrate, 11a 2 level relief microlens, 11b 4 level relief microlens, 12 first resist layer, 13 first mask, 14a exposure, 14b RIE, 14c exposure, 14d RIE, 15 second resist layer, 16 second mask, 21 silica glass substrate, 22 DLC film, 23a, 23b silica glass substrate, 24a, 24b mask, 25a, 25b UV light.

Claims (9)

  A translucent layer comprising a DLC film, and the translucent layer includes a plurality of high-refractive index regions and a plurality of low-refractive index regions periodically arranged alternately in a direction parallel to the layer surface so as to act as a refractive index modulation type diffraction grating. A diffractive optical element comprising a refractive index region, and a surface of the high refractive index region also has a convex shape acting as a relief type diffraction grating.   2. The diffractive optical element according to claim 1, wherein the diffractive optical element has a wavelength branching function capable of splitting one light beam including a plurality of wavelengths into a plurality of light beams depending on the wavelength.   The diffractive optical element according to claim 1, wherein a refractive index continuously changes in a boundary region between the high refractive index region and the low refractive index region.   4. The diffractive optical element according to claim 1, wherein a boundary region between the high refractive index region and the low refractive index region is parallel to a direction orthogonal to a layer surface of the translucent layer. .   4. The boundary region between the high refractive index region and the low refractive index region is parallel to a direction inclined with respect to a layer surface of the translucent layer. Diffractive optical element. A method for producing the diffractive optical element according to claim 1, comprising:
The diffractive optical element includes a translucent DLC film,
Irradiating the DLC film with ultraviolet rays through a phase grating mask,
By the ultraviolet irradiation, in the DLC film, a plurality of high refractive index regions and low refractive index regions that are periodically and alternately arranged in a direction parallel to the film surface so as to act as a refractive index modulation type diffraction grating are formed. And a convex shape acting as a relief type diffraction grating is also formed on the surface of the high refractive index region. A method for manufacturing the diffractive optical element of claim 4, comprising:
The diffractive optical element includes a translucent DLC film,
The DLC film is irradiated with interference light of + 1st order diffracted light and −1st order diffracted light that has passed through a phase grating mask, and the high refractive index region is formed in the region irradiated with the interference light. A method of manufacturing a diffractive optical element, wherein a convex surface is formed. A method for manufacturing the diffractive optical element of claim 5, comprising:
The diffractive optical element includes a translucent DLC film,
The DLC film is irradiated with interference light of 0th order diffracted light and + 1st order diffracted light or −1st order diffracted light that has passed through the phase grating mask, and the high refractive index region is formed in the region irradiated with the interference light. And a method of manufacturing a diffractive optical element, wherein the convex surface is formed.   The method for manufacturing a diffractive optical element according to claim 6, wherein the ultraviolet ray is a KrF laser including a wavelength of 248 nm.

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