JP2006039392A - Thin film and diffraction optical element and their manufacturing methods - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a practical refractive index modulating type diffraction optical element inexpensively in a simple and easy manner. <P>SOLUTION: In the manufacturing method for an diffraction optical element, a thin film (1) which includes carbon as a main component and has Knoop hardness that is greater than 30 and less than 200 and a refractive index ≤1.54, is prepared and a plurality of regions (2a) of the thin film is irradiated with energy beams (4) in order to form a refractive index modulation structure for generating diffraction operation for the thin film and refractive index of these areas is increased. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a diffractive optical element manufactured using a thin film, and more particularly to an improvement in a refractive index modulation type diffractive optical element and a manufacturing method thereof.

  As is well known, a diffractive optical element that causes diffraction of light can be used in various applications. For example, wavelength multiplexers / demultiplexers, optical couplers, optical isolators and the like used in the optical communication field can be manufactured using diffractive optical elements. In a liquid crystal projector or the like, for example, a diffractive optical element can be used as a polarization separator.

  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 refractive index modulation type and a relief type.

FIG. 17 is a schematic cross-sectional view showing an example of a refractive index modulation type diffractive optical element. This refractive index modulation type diffractive optical element includes a diffraction grating layer 12a formed on a translucent substrate 11, and a refractive index modulation structure is formed on the diffraction grating layer 12a. That is, in the diffraction grating layer 12a, local regions having a relatively small refractive index n ₁ and local regions having a relatively large refractive index n ₂ are periodically and alternately formed. A diffraction phenomenon may occur due to a phase difference generated between the light that has passed through the low refractive index n ₁ region and the light that has passed through the high refractive index n ₂ region.

The diffraction grating layer 12a having a refractive index modulation structure can be formed using a material whose refractive index increases by receiving energy beam irradiation, for example. For example, it is known that the refractive index of quartz glass doped with Ge is increased by ultraviolet irradiation. It is also known that the refractive index increases by irradiating quartz glass with X-rays. That is, a quartz glass layer having a refractive index n ₁ is deposited on the translucent substrate 11, and the glass layer is irradiated with an energy beam in a periodic pattern to locally increase the refractive index to n ₂ . A diffraction grating layer 12a as shown in FIG. 17 can be formed.

  FIG. 18 is a schematic cross-sectional view showing an example of a relief type diffractive optical element. This relief type diffractive optical element includes a diffraction grating layer 12b formed on a translucent substrate 11, and a relief structure is formed on the diffraction grating layer 12b. That is, in the diffraction grating layer 12b, local regions having a relatively large thickness and local regions having a relatively small thickness are periodically and alternately formed. A diffraction phenomenon may occur due to a phase difference that occurs between the light that has passed through the large thickness region and the light that has passed through the small thickness region.

  The diffraction grating layer 12b having a relief structure can be formed, for example, by depositing a silica-based glass layer on the translucent substrate 11 and processing the glass layer using photolithography and etching.

FIG. 19 is a schematic sectional view showing another example of the refractive index modulation type diffractive optical element. The refractive index modulation type diffractive optical element of FIG. 19 is similar to that of FIG. 17, but the diffraction grating layer 12c in FIG. 19 has three different refractive indexes n ₁ , n ₂ and n ₃ . Local areas are arranged periodically. As described above, the local region having the three levels of refractive indexes n ₁ , n ₂ , and n ₃ in the diffraction grating layer 12c is formed by, for example, depositing a silica-based glass layer having a refractive index n ₁ on the substrate 11; It can be formed by irradiating the glass layer with energy beams of two different energy levels.

  According to the diffraction grating including a local region having a multi-level refractive index, the diffraction efficiency can be improved as compared with a diffraction grating including a simple region having a refractive index of 2 (binary) level. Also, instead of gradual refractive index changes, it can be inferred that a diffraction grating containing multi-level refractive index changes can have higher diffraction efficiency than a diffraction grating containing binary level refractive index changes. A diffraction grating that includes a continuous refractive index change may also have a higher diffraction efficiency than a diffraction grating that includes a binary level refractive index change. Here, the diffraction efficiency means the ratio of the sum of diffracted light energy to incident light energy. That is, from the viewpoint of using diffracted light, a higher diffraction efficiency is preferable.

  FIG. 20 is a schematic cross-sectional view showing another example of a relief type diffractive optical element. The relief type diffractive optical element of FIG. 20 is similar to that of FIG. 18, but local regions having three different thicknesses are periodically arranged in the diffraction grating layer 12d in FIG. Yes. As described above, the local region having a three-level thickness in the diffraction grating layer 12d is formed by, for example, depositing a silica-based glass layer on the substrate 11 and processing the glass layer by photolithography and etching. It can be formed by repeating twice. Thus, even with a diffraction grating including a local region having a multi-level thickness, the diffraction efficiency can be improved as compared with a diffraction grating including a simple two-level thickness.

  The refractive index modulation type diffractive optical element described above can be manufactured in principle, but it is difficult to obtain a practical refractive index modulation type diffractive optical element. This is because, for example, the refractive index change Δn obtained by irradiating an energy beam to quartz glass is about 0.01 at most, and it is difficult to form an effective diffraction grating layer. In other words, in the refractive index modulation type diffractive optical element, the diffraction efficiency also depends on the magnitude of the refractive index change amount Δn in the refractive index modulation structure, and when Δn is small, a practical diffraction efficiency is obtained. It is because it cannot be obtained.

Therefore, at present, a relief type is generally used as a diffractive optical element. However, photolithography and etching necessary for manufacturing a relief type diffractive optical element are quite complicated processing steps, and require considerable time and labor. Moreover, it is not easy to control the etching depth with high accuracy. Furthermore, in the relief type diffractive optical element, since fine irregularities are formed on the surface, there is a problem that dust and dirt are likely to adhere.
JP 2003-248193 A

  On the other hand, Japanese Patent Application Laid-Open No. 2003-248193 of Patent Document 1 reports that the refractive index can be increased by irradiating a DLC (diamond-like carbon) film with a high energy beam. According to the teaching of this report, the amount of change Δn in the refractive index of the DLC film due to the high energy beam irradiation can be increased as compared with the case of the conventional silica glass.

  However, DLC films are generally made by vapor deposition, but must be made under rigorous and well-examined deposition conditions. The DLC film has properties similar to diamond, and has a high hardness and a high refractive index. That is, since the DLC film has a high refractive index from the beginning, in order to obtain a large refractive index change Δn, it is necessary to irradiate a very high energy beam to further increase the refractive index.

  In view of the situation in the prior art as described above, a main object of the present invention is to provide a practical refractive index modulation type diffractive optical element more simply and at low cost.

  According to the present invention, a method for producing a diffractive optical element comprises preparing a thin film containing carbon as a main component and having a Knoop hardness of greater than 30 and less than 200 and a refractive index of less than 1.54. In order to form a refractive index modulation structure for generating the above, a plurality of regions of the thin film are irradiated with an energy beam to increase the refractive index of those regions.

  The thin film preferably further contains at least one of hydrogen, nitrogen, and fluorine. The thin film may have a thickness in the range of 0.3 μm to 20 μm.

  As the energy beam, an SR (synchrotron radiation) light beam having 95% or more of photons having an energy of less than 280 eV may be used. The energy beam may be a laser beam having a wavelength of less than 400 nm. More preferably, the laser beam is an ultraviolet light beam having a wavelength of less than 300 nm. The laser beam can be a pulsed laser beam. Furthermore, ultraviolet lamp light including light having a wavelength of less than 400 nm can be used as the energy beam.

  The temperature of the thin film during irradiation with the energy beam is preferably 100 ° C. or lower. The refractive index modulation structure may be formed to produce a diffractive effect for light having a wavelength selected from 450 nm to 650 nm, 1.2 μm to 1.7 μm, 3 μm to 5 μm, and 8 μm to 12 μm.

According to the present invention, the diffractive optical element includes carbon as a main component, and has a refractive index modulation structure including a low refractive index region and a high refractive index region for generating a diffractive action. The structure is characterized by an average Knoop hardness of greater than 40 and less than 700. The Knoop hardness in the low refractive index region is preferably larger than 30 and smaller than 200, and the Knoop hardness in the high refractive index region is preferably larger than 40 and smaller than 700. The extinction coefficient in the low refractive index region is preferably less than 3 × 10 ⁻⁴ , and the extinction coefficient in the high refractive index region is preferably less than 5 × 10 ⁻³ . The Knoop hardness and extinction coefficient in the high refractive index region are larger than the Knoop hardness and extinction coefficient in the low refractive index region. In an actual diffractive optical element, it is often difficult to measure a Knoop hardness and an extinction coefficient by distinguishing a fine energy beam irradiation region and a non-irradiation region. In that case, the average value is that the Knoop hardness is greater than 30 and less than 700, or the extinction coefficient is less than 5 × 10 ⁻³ . If the area ratio between the energy beam irradiation region and the non-irradiation region is known, the Knoop hardness and the extinction coefficient can be obtained by distinguishing the irradiation region and the non-irradiation region by simulation.

According to the present invention, in a method of manufacturing a thin film capable of increasing the refractive index by irradiation with an energy beam using plasma CVD (chemical vapor deposition), the substrate is disposed on the substrate electrode in the CVD reaction chamber. The raw material gas introduced into the reaction chamber contains a saturated hydrocarbon gas having a molecular weight larger than 60 and smaller than 150 and not containing benzene nuclei at a flow rate ratio of 25% or more and 100% or less. gas pressure is set lower than 700Pa higher than 5 Pa, by applying a 2W / cm ² less than the high-frequency power larger than 0.1 W / cm ² to the substrate electrode, an amorphous thin film containing carbon as a main component Is deposited on a substrate.

  In addition, it is preferable that source gas also contains the gas containing nitrogen or a fluorine. Moreover, it is preferable that neither hydrogen gas nor a noble gas is contained in source gas. During CVD, the substrate is preferably maintained at a temperature in the range of 5 ° C to 100 ° C. The amorphous thin film is preferably heat-treated at a temperature in the range of 100 ° C. to 380 ° C. in vacuum or air.

According to the present invention, the thin film is mainly composed of carbon and hydrogen, contains hydrogen in a mass ratio in the range of more than 1/12 and less than 1/6 with respect to carbon, and is larger than 0.3 μm and smaller than 20 μm. It has a thickness within the range and a Knoop hardness greater than 30 and less than 200. Such a thin film may have a refractive index in the range of 1.45 or more and less than 1.54 for light in at least one wavelength range of 450 nm to 650 nm and 1200 nm to 1700 nm. In addition, it is preferable that at least one of nitrogen and fluorine is additionally contained at a mass ratio in the range of less than 19/60 with respect to carbon. Such a thin film may have a refractive index in the range of 1.35 or more and less than 1.54 for light in at least one wavelength range of 450 nm to 650 nm and 1200 nm to 1700 nm. Furthermore, the thin film according to the present invention may have an extinction coefficient of less than 3 × 10 ⁻⁴ . The Knoop hardness disclosed in the present invention can be measured with a Knoop hardness meter by applying an appropriate load in the range of, for example, 5 gf to 50 gf. The refractive index and extinction coefficient can be obtained by spectroscopic ellipsometry, optical simulation using transmission / reflection spectra, and the like.

  According to the present invention, it is possible to prepare a thin film having a low hardness and a low initial refractive index as compared to the DLC film, and by simply irradiating the thin film with a relatively low energy beam, a large refractive index can be obtained. A refractive index modulation type diffractive optical element including the change amount Δn can be provided at low cost.

  1 to 3 are schematic cross-sectional views illustrating a manufacturing process of a refractive index modulation type diffractive optical element according to an embodiment of the present invention. In the drawings of the present application, dimensional relationships such as length and thickness are appropriately changed for the sake of clarity and simplification of the drawings, and do not reflect actual dimensional relationships.

First, as shown in FIG. 1, an amorphous thin film 2 deposited on a translucent substrate 1 by plasma CVD is prepared. The thin film 2 in the present invention contains carbon as a main component, and has a Knoop hardness greater than 30 and less than 200, a refractive index less than 1.54, and an extinction coefficient less than 3 × 10 ⁻⁴ . . The thin film 2 may additionally contain at least one of hydrogen, nitrogen, and fluorine in order to reduce the refractive index. Such a thin film 2 can be produced on a silicon substrate, a glass substrate, and other various translucent substrates 1 by plasma CVD as described later.

  In addition, there is no special restriction | limiting in the thickness of the thin film 2, It can set to arbitrary thickness. However, if the thin film is too thick, it is not preferable because the light absorption effect by the film becomes too large. On the other hand, if the thin film is too thin, it tends to be difficult to obtain a sufficient diffraction effect. In the present invention, a thin film having a thickness in the range of 0.3 to 20 μm is preferably used for the refractive index modulation type diffractive optical element. However, if a thin film having a smaller extinction coefficient is obtained, a thicker thin film can be used, and if the refractive index change rate can be increased, a thinner thin film can be used.

  In FIG. 2, a gold mask 3 is formed on the thin film 2 by, for example, a lift-off method. In the gold mask 3, for example, gold stripes having a width of 0.5 μm can be repeatedly arranged with an interval of 0.5 μm. That is, the gold mask 3 can be formed in a line and space pattern, for example. Thereafter, the thin film 2 is irradiated with an energy beam 4 such as SR light or laser light through the opening of the gold mask 3.

  As is well known, in order to produce an efficient diffraction effect on light in a specific wavelength band, the line and space width in such a line-and-space is suitable for the specific wavelength band. Can be adjusted. As such a specific wavelength band, in addition to a visible light band of 450 nm or more and 650 nm or less, a communication wavelength band such as 1.2 μm or more and 1.7 μm or less, 3 μm or more and 5 μm or less, or 8 μm or more and 12 μm or less is also used. can do. Of course, in general, the longer the wavelength of light used, the larger the line and space widths in the diffraction grating.

  Of the thin film 2 partially covered with the gold mask 3, the refractive index of the region not irradiated with the energy beam 4 does not change, but the refractive index of the region 2a irradiated with the energy beam 4 is increased. The amount of change in refractive index in the thin film 2 according to the present invention is much larger than the amount of change in refractive index obtained in quartz-based glass, and a diffraction grating layer having sufficiently high diffraction efficiency can be formed.

  In FIG. 3, the gold mask 3 is removed by etching, and a refractive index modulation type diffractive optical element is obtained. Note that the diffraction grating layer 2 in this diffractive optical element is a so-called binary level diffraction grating layer including two types of regions having a relatively low refractive index and a relatively high refractive index.

  As specific examples of the thin film 2 in the present invention, a hydrogenated carbon film, a nitrogen-added hydrogenated carbon film, and a fluorine-added hydrogenated carbon film were prepared. In order to increase the refractive index of these thin films, two types of SR light having different energy distributions were used. The first type of SR light has a wide energy distribution of 50 eV to 2000 eV, and has an energy peak at about 800 eV. On the other hand, the second type of SR light has a relatively low energy distribution in the range of 50 eV to 200 eV, and has an energy peak at about 100 eV. Of course, the SR light having a relatively low energy distribution can be obtained from a small synchrotron, and if desired, can be obtained by attenuating a high energy component from the radiation of a large synchrotron with a Pt mirror. it can.

Table 1 shows the extinction coefficient of the thin film 2 after the SR light irradiation as described above. With any type of SR light irradiation, the refractive index of the thin film 2 is increased to a range of 1.65 to 1.70. In Table 1, with respect to any of the hydrogenated carbon film, the nitrogen-added hydrogenated carbon film, and the fluorine-added hydrogenated carbon film, the extinction when light having a wavelength of 1550 nm is transmitted through 3 to 10 samples. The range from the minimum value to the maximum value of the coefficient is shown. In the display of the extinction coefficients, for example, “5.5E-4” means “5.5 × 10 ⁻⁴ ”, and so on.

  As shown in Table 1, regardless of whether the thin film 2 is a hydrogenated carbon film, a nitrogen-added hydrogenated carbon film, or a fluorine-added hydrogenated carbon film, the first type having relatively high energy It can be seen that the increase in the extinction coefficient after increasing the refractive index is small when the second type of SR light having relatively low energy is irradiated as compared with the case where the SR light is irradiated. This is considered to be because when a beam with too high energy is irradiated, even the inner core electrons of carbon are excited to form defects, and such defects in the thin film increase the extinction coefficient. In the above example, the second type SR light has a low energy distribution in the range of 50 eV to 200 eV. However, the refractive index increases when about 95% or more of the photons have a low energy of less than 280 eV. A subsequent increase in extinction coefficient can be suppressed.

  Of course, a small extinction coefficient means a high light transmittance, and it goes without saying that it is preferable as a diffractive optical element. That is, if the thin film 2 of the present invention is used, the refractive index can be sufficiently increased even when SR light having a relatively low energy is irradiated, and an increase in the extinction coefficient can be suppressed. In other words, according to the present invention, a diffractive optical element having excellent transparency and high diffraction efficiency can be obtained simply and at low cost.

  As another specific example, the hydrogenated carbon film, the nitrogen-added hydrogenated carbon film, and the fluorine-added hydrogenated carbon film as the thin film 2 are irradiated with an ultraviolet laser having a wavelength of 248 nm using a KrF excimer laser device. Their refractive index was increased to about 1.65. The results are shown in Tables 2 and 3. In Table 2, the initial refractive index of the hydrogenated carbon film, the nitrogen-added hydrogenated carbon film, and the fluorine-added hydrogenated carbon film and the refractive index after the ultraviolet laser irradiation have wavelengths of 520 nm, 1550 nm, 4 μm, and 9 μm. Shown with respect to light. Correspondingly, in Table 3, the initial extinction coefficient of the hydrogenated carbon film, the nitrogen-added hydrogenated carbon film, and the fluorine-added hydrogenated carbon film and the extinction coefficient after the ultraviolet laser irradiation are 520 nm and 1550 nm. Shown for light at wavelengths of 4 μm and 9 μm.

  As can be seen from Tables 2 and 3, the hydrogenated carbon film, the nitrogen-added hydrogenated carbon film, and the fluorine-added hydrogenated carbon film can sufficiently increase the refractive index to about 1.65 even by ultraviolet laser irradiation. . In addition, an increase in the extinction coefficient is further suppressed in the case of ultraviolet laser irradiation compared to the case of SR light irradiation, which is more preferable from the viewpoint of translucency of the diffractive optical element. Furthermore, the laser device is simpler and less expensive than the synchrotron device, and is preferable from the viewpoint of providing the diffractive optical element more easily and at lower cost.

  As another specific example, lines of a high refractive index region having a width of 3 μm are formed by direct writing at a space interval of 5 μm using a pulse laser device without using the gold mask 3 as shown in FIG. It was done. In this case, an amorphous hydrogenated carbon film was used as the thin film, which had a refractive index of less than 1.54 for light having a wavelength of 630 nm. The used pulse laser device generates laser light including a fundamental wavelength of 1064 nm, a second harmonic wavelength of 532 nm, a third harmonic wavelength of 355 nm, and a fourth harmonic wavelength of 266 nm, and an output of 0.2 μJ / pulse , 30 ps pulse width, 10 Hz pulse period, and 3 μm beam diameter. As a result of direct drawing with a laser beam having such a fine diameter, a diffractive optical element capable of producing a diffractive action on infrared light was obtained.

  As a further specific example, a hydrogenated carbon film having an area of 50 mm × 50 mm used as a thin film in the present invention is maintained at various temperatures, and the thin film is scanned for 4 hours while scanning an ultraviolet laser beam having a width of 10 mm and a wavelength of 248 nm. Irradiated. The extinction coefficient of the hydrogenated carbon film after the laser light irradiation is shown in the graph of FIG.

  That is, in the graph of FIG. 4, the horizontal axis represents the temperature of the thin film during irradiation with ultraviolet laser light, and the vertical axis represents the extinction coefficient of the thin film after irradiation. From this graph, it can be seen that when the refractive index is increased by irradiating the thin film with an energy beam, the extinction coefficient tends to increase as the set temperature of the thin film exceeds 100 ° C. In other words, when the refractive index is increased by irradiating the thin film with an energy beam, the temperature of the thin film is preferably set to 100 ° C. or lower in order to suppress an increase in the extinction coefficient. The temperature of the thin film can be measured by thermography (a type of radiation thermometer). It is also possible to maintain the thin film on the substrate at a predetermined temperature by circulating a temperature-controlled refrigerant in the substrate holder.

  As a further specific example, a hydrogenated carbon film having various hardnesses was used as a thin film in the present invention, and the influence of the hardness on the amount of change in the refractive index by energy beam irradiation was examined. First, the initial Knoop hardness of the hydrogenated carbon film before being irradiated with the energy beam was measured under a load of 15 g. And the initial refractive index of the hydrogenated carbon film | membrane which has various initial hardness was measured. As a result, a hydrogenated carbon film having a high initial hardness tended to have a high initial refractive index.

  Thereafter, a hydrogenated carbon film having various initial hardnesses was irradiated with an ultraviolet laser having a wavelength of 248 nm, and the refractive index change amount Δn after irradiation in the thin film was measured. As a result, Δn = 0.28 for the initial hardness 55 film, Δn = 0.18 for the initial hardness 180 film, Δn = 0.08 for the initial hardness 480 film, and Δn = 0.08 for the initial hardness 1200 film. 05. That is, it can be seen that a thin film having a lower initial hardness has a lower initial refractive index, and a large amount of refractive index change can be easily generated by energy beam irradiation. However, the hydrogenated carbon film having an initial hardness of 20 was damaged by ultraviolet laser irradiation. From this, it is considered that a hydrogenated carbon film having an initial hardness that is too low has a low density and is mechanically unstable.

  From the investigation of the relationship between the hardness and the refractive index in the thin film as described above, the hydrogenated carbon film having an initial Knoop hardness of 30 or more and less than 200 has an initial refractive index of about 1.45 or more and less than 1.54. It turned out that it is preferable at the point which can obtain the big refractive index change amount comparatively easily by irradiation. In the nitrogen-added hydrogenated carbon film and the fluorine-added hydrogenated carbon film, the minimum refractive index can be further reduced, and an initial refractive index of about 1.35 or more and less than 1.54 can be obtained.

  As described above, when the thin film of the present invention is irradiated with an energy beam, the refractive index of the irradiated region increases, but the hardness tends to increase accordingly. The thin film in which the initial low refractive index region and the high refractive index region after the energy beam irradiation are mixed can be measured as having a Knoop hardness in the range of 40 to 700.

The thin film as described above can be produced by various methods, and can be produced by using, for example, plasma CVD. First, when a thin film containing carbon as a main component is deposited by well-known plasma CVD, various saturated or unsaturated hydrocarbon gases or vapors can be used as a raw material gas. 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, saturated) ), Hexane (C ₆ H ₁₄ , molecular weight 86, saturation), octane (C ₈ H ₁₈ , molecular weight 114, saturation) and other hydrocarbon gases or vapors are used to perform plasma CVD on thin films containing carbon as the main component Can be deposited.

  The inventors introduced these hydrocarbon gases or vapors into the plasma CVD reaction chamber to actually produce a thin film. More specifically, a 4-inch (about 10 cm diameter) wafer-shaped quartz glass substrate was placed on a disk-shaped substrate electrode having a diameter of 15 mm in the reaction chamber. Then, under the conditions of a reaction chamber pressure of 50 Pa and a substrate temperature of room temperature, high frequency power of 100 W having a frequency of 13.56 MHz was applied to the substrate electrode, and a thin film containing carbon as a main component was produced on the quartz glass substrate. The various thin films thus obtained were measured for refractive index and extinction coefficient with respect to light having a wavelength of 1550 nm.

  5 and 6 show the refractive index and extinction coefficient measured for these thin films. That is, the horizontal axes of the graphs of FIGS. 5 and 6 both represent the molecular weight of the raw material hydrocarbon, and the vertical axes of the graphs of FIGS. 5 and 6 represent the refractive index and extinction coefficient of the obtained thin film, respectively. . In these graphs, a black diamond mark represents a saturated hydrocarbon not containing a benzene nucleus, and a cross mark represents an unsaturated hydrocarbon or a saturated hydrocarbon containing a benzene nucleus.

From the viewpoint of the obtained thin film having a relatively small initial refractive index of less than 1.54, as can be seen from FIG. 5, the raw material hydrocarbon preferably has a molecular weight greater than 60 and less than 150. Furthermore, from the point of view that the resulting thin film has a relatively small initial extinction coefficient of less than 3 × 10 ^{−4, as} can be seen from FIG. 6, the feed hydrocarbon has a molecular weight greater than 60 and less than 150 and benzene Saturated hydrocarbons that do not contain nuclei are preferred.

  In general, a hydrocarbon gas or vapor having a high molecular weight tends to proceed with a CVD reaction even when the applied high-frequency power density is small, and a hydrogenated carbon film having a small extinction coefficient is easily obtained. On the other hand, in hydrogenated carbon films made from unsaturated hydrocarbons or hydrocarbons containing benzene nuclei, double bonds or triple bond unsaturated bonds or benzene nuclei are likely to remain in the film, resulting in a large extinction coefficient. Prone.

Next, the present inventors investigated the influence of the carrier gas on the optical characteristics of the thin film when introducing the raw material hydrocarbon including the carrier gas into the CVD reaction chamber. The results when Ar is used as the carrier gas are shown in FIGS. That is, the horizontal axis of the graphs of FIGS. 7 and 8 represents the gas flow rate ratio between C ₆ H ₁₂ and (Ar + C ₆ H ₁₂ ), and the vertical axes of the graphs of FIGS. 7 and 8 represent the thin film obtained. Refractive index and extinction coefficient are shown respectively.

As can be seen from these graphs, a thin film having a small initial extinction coefficient of less than 3 × 10 ⁻⁴ can be easily obtained unless the gas flow ratio of Ar as a carrier gas is large. However, in order to obtain a thin film having a small initial refractive index of less than 1.54, it is preferable not to include a carrier gas. Such a tendency was the same even when hydrogen gas or other rare gas was used as the carrier gas. For example, when hydrogen gas is used as the carrier gas, the hydrogen gas tends to act to increase the diamond property of the thin film, and tends to increase the initial refractive index of the thin film, which is not preferable. In general, noble gases are not preferred because they tend to increase the initial extinction coefficient by increasing the sp ² bonding property of the carbon film.

Furthermore, the present inventors also examined the influence of the pressure in the CVD reaction chamber on the properties of the thin film. In this case, hexane was used as the source gas, and the pressure in the reaction chamber was set to various different values. The results are shown in FIGS. 9 and 10. That is, the horizontal axes of the graphs of FIGS. 9 and 10 both represent the pressure in the reaction chamber (Pa), and the vertical axes of the graphs of FIGS. 9 and 10 represent the refractive index and the extinction coefficient of the obtained thin film, respectively. Yes. As can be seen from these graphs, in order for the thin film to have a small initial refractive index of less than 1.54 and a small initial extinction coefficient of less than 3 × 10 ⁻⁴ , the pressure in the reaction chamber is within a range higher than 5 Pa and lower than 700 Pa. It is preferable to set to.

Furthermore, the present inventors also examined the influence of the high frequency power density applied to the substrate electrode on the properties of the thin film. In this case, hexane was used as the raw material gas, and the high frequency power density was set to various values. The results are shown in FIGS. 11 and 12. That is, the horizontal axes of the graphs of FIGS. 11 and 12 both represent power density (W / cm ² ), and the vertical axes of the graphs of FIGS. 11 and 12 represent the refractive index and extinction coefficient of the obtained thin film, respectively. Represents. As can be seen from these graphs, in order for the thin film to have a small initial refractive index of less than 1.54 and a small initial extinction coefficient of less than 3 × 10 ⁻⁴ , the high frequency power density applied to the substrate electrode is 0. it is preferably set within a large 2W / cm ² less than the range from 1W / cm ^2.

Furthermore, the present inventors also investigated the influence of nitrogen or fluorine addition on the properties of the thin film. In this case, only hexane, hexane and nitrogen gas, or hexane and carbon tetrafluoride were used as the source gas. Nitrogen gas or carbon tetrafluoride was mixed in the raw material gas at a flow rate ratio of 10%. The pressure in the CVD reaction chamber was set to 70 Pa. In the properties of the thin film thus produced, the thin film produced only from hexane had a refractive index of 1.515 and an extinction coefficient of 3.4 × 10 ⁻⁵ . The thin film to which nitrogen was added had a refractive index of 1.482 and an extinction coefficient of 5.5 × 10 ⁻⁵ . The thin film doped with fluorine had a refractive index of 1.421 and an extinction coefficient of 8.2 × 10 ⁻⁵ .

As can be seen from this, the initial refractive index can be lowered by adding nitrogen or fluorine to the thin film. The amount of change in refractive index Δn due to energy beam irradiation can be increased. On the other hand, the addition of nitrogen or fluorine to the thin film tends to slightly increase its initial extinction coefficient, but it is still possible to maintain a relatively low extinction coefficient of less than 3 × 10 ⁻⁴ .

Furthermore, the present inventors also examined the influence of the substrate temperature during film formation on the properties of the thin film. In this case, cyclohexane was used as the source gas, and the substrate temperature was set to various different values. The results are shown in FIGS. 13 and 14. That is, the horizontal axes of the graphs of FIGS. 13 and 14 both represent the film formation temperature (° C.), and the vertical axes of the graphs of FIGS. 13 and 14 represent the refractive index and extinction coefficient of the obtained thin film, respectively. Yes. As can be seen from these graphs, in order for the thin film to have a small initial refractive index of less than 1.54 and a small initial extinction coefficient of less than 3 × 10 ⁻⁴ , the substrate temperature is within the range of 5 ° C. to 100 ° C. It is preferable to set to.

  Furthermore, the present inventors also examined the influence of heat treatment on the thin film on its mechanical and optical properties. In this case, octane was used as the source gas, and high frequency power of 80 W was applied to the substrate electrode. The produced thin film was heat-treated at various temperatures in vacuum or in the atmosphere. In the heat treatment, the thin film was heated to a predetermined heat treatment temperature within 30 minutes, held at the heat treatment temperature for 2 hours, and then lowered to room temperature in 1 hour. The mechanical and optical properties of the thin film so heat treated are shown in FIGS.

  The horizontal axes of the graphs of FIGS. 15 and 16 both represent the heat treatment temperature (° C.), and the vertical axes of the graphs of FIGS. 15 and 16 represent the mechanical durability and extinction coefficient of the heat treated thin film, respectively. . In these graphs, a black diamond mark represents heat treatment in a vacuum, and a black square mark represents heat treatment in the atmosphere. In the measurement of durability in FIG. 15, the time when the thin film after heat treatment was immersed in hot water at 95 ° C. and the film was peeled or cracked was defined as the durability time, and this durability time was heat-treated. It is calculated as a multiple in comparison with the thin film that is not. As can be seen from the graph of FIG. 15, from the viewpoint of improving the durability of the thin film, it is preferable to heat-treat the thin film at a temperature in the range of 100 ° C. to 380 ° C. in vacuum or in the air. On the other hand, as can be seen from the graph of FIG. 16, the heat treatment temperature is preferably in the range of 100 ° C. to 380 ° C. in order to prevent deterioration of the initial extinction coefficient of the thin film.

  Furthermore, the present inventors investigated the interrelationship between the hydrogen content, Knoop hardness, and optical properties of the hydrogenated carbon film as a thin film. In this case, cyclohexane was used as the source gas, and a hydrogenated carbon film having a thickness in the range of 3 μm to 5 μm was produced. The change in the hydrogen content of the thin film can be caused by changing the plasma CVD conditions such as the CVD reaction chamber pressure, the substrate temperature, and the applied high frequency power. Table 4 shows the Knoop hardness, and the refractive index and extinction coefficient for light having wavelengths of 1550 nm and 520 nm for hydrogenated carbon films having different hydrogen contents.

  As can be seen from Table 4, when the hydrogen content in the hydrogenated carbon film is as low as 4% by mass or less, the Knoop hardness in the film is as high as 1000 or more, and the initial extinction coefficient and initial refractive index are high. growing. On the other hand, as the hydrogen content in the hydrogenated carbon film increases, the Knoop hardness decreases in the film, and the initial refractive index and initial extinction coefficient also decrease. However, as described above, if the hydrogen content in the hydrogenated carbon film is too high, the mechanical properties of the film deteriorate and the film is easily damaged when irradiated with an energy beam.

  Furthermore, the present inventors investigated the correlation between Knoop hardness and optical properties when nitrogen or fluorine was added to a hydrogenated carbon film as a thin film. In this case, cyclohexane and nitrogen gas or cyclohexane and carbon tetrafluoride were used as source gases, and a thin film having a thickness in the range of 3 μm to 5 μm was produced. Table 5 shows the Knoop hardness, the refractive index and the extinction coefficient with respect to light having wavelengths of 1550 nm and 520 nm for hydrogenated carbon films having different amounts of nitrogen or fluorine added.

  As can be seen from the comparison between Table 4 and Table 5, by including nitrogen or fluorine in the hydrogenated carbon film, the Knoop hardness tends to be low in the film, and the initial extinction coefficient is suppressed while suppressing an increase. The refractive index can be reduced.

  In the above-described embodiment, an example in which laser light is used for ultraviolet irradiation has been described. Needless to say, ultraviolet lamp light including light having a wavelength of less than 400 nm can also be used.

  According to the present invention, a thin film having a low hardness and a low initial refractive index compared to a DLC film can be prepared, and a large change in refractive index can be achieved by simply irradiating the thin film with a relatively low energy beam. A refractive index modulation type diffractive optical element including the amount Δn can be provided at low cost.

It is typical sectional drawing illustrating the process of producing the diffractive optical element by this invention. It is typical sectional drawing illustrating the process of producing the diffractive optical element by this invention. It is typical sectional drawing illustrating the process of producing the diffractive optical element by this invention. It is a graph which shows the influence which the substrate temperature during ultraviolet laser irradiation has on the extinction coefficient of a hydrogenated carbon film. 2 is a graph showing the refractive index measured for thin films deposited from hydrogenated carbon of various molecular weights. 2 is a graph showing extinction coefficients measured for thin films deposited from hydrogenated carbon of various molecular weights. Gas flow rate ratio of C ₆ H ₁₂ and (Ar + C ₆ H ₁₂₎ is a graph showing the effect on the refractive index of the thin film. Gas flow rate ratio of C ₆ H ₁₂ and (Ar + C ₆ H ₁₂₎ is a graph showing the effect on the extinction coefficient of the thin film. It is a graph which shows the influence which the pressure in a CVD reaction chamber has on the refractive index of a thin film. It is a graph which shows the influence which the pressure in a CVD reaction chamber has on the extinction coefficient of a thin film. It is a graph which shows the influence which the high frequency power density applied to the substrate electrode for plasma CVD has on the refractive index of a thin film. It is a graph which shows the influence which the high frequency power density applied to a board | substrate electrode has on the extinction coefficient of a thin film. It is a graph which shows the influence which the substrate temperature in film-forming has on the refractive index of a thin film. It is a graph which shows the influence which the substrate temperature in film-forming has on the extinction coefficient of a thin film. It is a graph which shows the influence which the heat processing with respect to a thin film has on the mechanical durability. It is a graph which shows the influence which the heat processing with respect to a thin film has on the extinction coefficient. It is a typical sectional view showing an example of a refractive index modulation type diffractive optical element. It is typical sectional drawing which shows an example of a relief type | mold diffractive optical element. It is a typical sectional view showing another example of a refractive index modulation type diffractive optical element. It is typical sectional drawing which shows another example of a relief type | mold diffractive optical element.

Explanation of symbols

  1 translucent substrate, 2 thin film containing carbon as a main component, 2a high refractive index region, 3 gold mask, 4 energy beam.

Claims (33)

Providing a thin film comprising carbon as a major component and having a Knoop hardness of greater than 30 and less than 200 and a refractive index of less than 1.54;
Manufacturing a diffractive optical element, wherein a plurality of regions of the thin film are irradiated with an energy beam so as to form a refractive index modulation structure for generating a diffractive action on the thin film, thereby increasing the refractive index of those regions. Method.   The method for manufacturing a diffractive optical element according to claim 1, wherein the thin film additionally contains at least one of hydrogen, nitrogen, and fluorine.   The method for manufacturing a diffractive optical element according to claim 1, wherein the thin film has a thickness in a range of 0.3 μm to 20 μm.   4. The method of manufacturing a diffractive optical element according to claim 1, wherein the energy beam is an SR light beam in which 95% or more of photons have energy of less than 280 eV. 5.   4. The method of manufacturing a diffractive optical element according to claim 1, wherein the energy beam is a laser beam having a wavelength of less than 400 nm.   6. The method of manufacturing a diffractive optical element according to claim 5, wherein the laser beam is an ultraviolet light beam having a wavelength of less than 300 nm.   The method of manufacturing a diffractive optical element according to claim 5, wherein the laser beam is a pulsed laser beam.   4. The method of manufacturing a diffractive optical element according to claim 1, wherein the energy beam is ultraviolet lamp light including light having a wavelength of less than 400 nm.   The method of manufacturing a diffractive optical element according to claim 1, wherein the temperature of the thin film during irradiation with the energy beam is 100 ° C. or less.   It has carbon as a main component, and has a refractive index modulation structure including a low refractive index region and a high refractive index region for generating a diffractive action, and the average Knoop hardness of the refractive index modulation structure is larger than 30 and is 700 A diffractive optical element characterized by being smaller.   The diffractive optical element according to claim 10, wherein the Knoop hardness of the low refractive index region is greater than 30 and less than 200, and the Knoop hardness of the high refractive index region is greater than 40 and less than 700. 12. The diffractive optical element according to claim 10, wherein an average extinction coefficient in the refractive index modulation structure is less than 5 × 10 ⁻³ . 13. The extinction coefficient of the low refractive index region is less than 3 × 10 ⁻⁴ , and the extinction coefficient of the high refractive index region is less than 5 × 10 ⁻³ . The diffractive optical element according to item 4.   The diffractive optical element according to any one of claims 10 to 13, wherein the diffractive optical element is manufactured by the method for manufacturing a diffractive optical element according to any one of claims 1 to 9. A method of manufacturing a thin film capable of increasing a refractive index by irradiation with an energy beam using plasma CVD,
A substrate is disposed on the substrate electrode in the plasma CVD reaction chamber,
The raw material gas introduced into the reaction chamber contains a saturated hydrocarbon gas having a molecular weight larger than 60 and smaller than 150 and not containing benzene nuclei at a flow rate ratio of 25% or more and 100% or less.
The gas pressure in the reaction chamber during CVD is set higher than 5 Pa and lower than 700 Pa,
Wherein by applying a 2W / cm ² less than the high-frequency power larger than 0.1 W / cm ² to the substrate electrode, wherein the depositing an amorphous thin film containing carbon on the substrate as a major component Thin film manufacturing method.   The method of manufacturing a thin film according to claim 15, wherein the source gas also includes a gas containing nitrogen or fluorine.   The method for producing a thin film according to claim 15 or 16, wherein the source gas contains neither hydrogen gas nor rare gas.   The method for producing a thin film according to any one of claims 15 to 17, wherein the substrate is maintained at a temperature within a range of 5 ° C to 100 ° C during the CVD.   The method for producing a thin film according to claim 15, wherein the amorphous thin film is heat-treated at a temperature in a range of 100 ° C. to 380 ° C. in a vacuum or in the air.   It contains carbon and hydrogen as main components, contains hydrogen in a mass ratio within a range greater than 1/12 and less than 1/6 with respect to carbon, has a thickness within a range greater than 0.3 μm and less than 20 μm, and greater than 30 and 200 A thin film characterized by having a smaller Knoop hardness.   21. The thin film according to claim 20, further comprising at least one of nitrogen and fluorine in a mass ratio within a range of 19/60 with respect to carbon.   The thin film according to claim 20, wherein the thin film has a refractive index within a range of 1.45 or more and less than 1.54 with respect to light in at least one wavelength range of 450 nm to 650 nm and 1200 nm to 1700 nm.   The thin film according to claim 21, wherein the thin film has a refractive index within a range of 1.35 or more and less than 1.54 with respect to light in at least one wavelength range of 450 nm to 650 nm and 1200 nm to 1700 nm. The thin film according to any one of claims 20 to 23, wherein the thin film has an extinction coefficient of less than 3 x ^10-4 .   The thin film according to any one of claims 20 to 24, wherein the thin film is produced by the manufacturing method according to any one of claims 15 to 19. A method for producing a diffractive optical element using the thin film according to any one of claims 20 to 25, comprising:
Manufacturing a diffractive optical element, wherein a plurality of regions of the thin film are irradiated with an energy beam so as to form a refractive index modulation structure for generating a diffractive action on the thin film, thereby increasing the refractive index of those regions. Method. A diffractive optical element produced using the thin film according to any one of claims 20 to 25,
A diffractive optical system characterized in that an energy beam is irradiated to a plurality of regions of the thin film so as to form a refractive index modulation structure for generating a diffractive action on the thin film, thereby increasing the refractive index of those regions. element.   27. The method for manufacturing a diffractive optical element according to claim 26, further comprising characteristics of the manufacturing method according to any one of claims 1 to 9.   28. The diffractive optical element according to claim 27, wherein the diffractive optical element is manufactured using the characteristics of the manufacturing method according to any one of claims 1 to 9.     The thin film according to any one of claims 20 to 25, wherein the thin film is used in a wavelength band of wavelengths of 450 to 650 nm, 1.2 to 1.7 µm, 3 to 5 µm, and 8 to 12 µm.     It is used in any wavelength band of wavelengths 450 to 650 nm, 1.2 to 1.7 µm, 3 to 5 µm, and 8 to 12 µm, according to any one of claims 10 to 14, 27, and 29 The diffractive optical element described.   26. A method for modifying a thin film, comprising: irradiating the thin film according to any one of claims 20 to 25 with an energy beam so that the Knoop hardness is larger than 40 and smaller than 700.   26. A modified thin film characterized by being modified so that the Knoop hardness is larger than 40 and smaller than 700 by irradiating the thin film according to claim 20 with an energy beam.

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