JPH11311709A - Production of polarizing diffraction grating - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high performance polarizing diffraction grating in a fine pitch by forming a diffraction grating and an ion exchanged-region on an optical crystal plate. SOLUTION: In this producing method, a mask resist 5 is formed on an optical crystal plate 1 and an ion exchanged region 6 is formed by ion implantation method. The energy of ions to be implanted is changed in several stages so as to implant ions of different energy in different stages. After the mask resist 5 is removed, the crystal plate is processed in such a manner that the phase difference produced by the difference between the optical pass length of the light transmitting through the ion exchange region 6 and the optical pass length of the light transmitting through the non-ion exchange region is as (2n+1)π for the phase difference ϕo of the ordinary ray component and 2nπ for the phase difference ϕe of the extraordinary ray component, wherein n is an integer.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a polarizing diffraction grating which is used in various optical devices utilizing a laser or the like and has an ion exchange region having a different diffraction efficiency depending on the polarization direction formed on an optical crystal. is there.

[0002]

2. Description of the Related Art FIGS. 5A and 5B show a typical optical crystal using lithium niobate.
3 shows a cross section of a polarizing diffraction grating in which a phase change of π occurs.

FIG. 5 (a) shows a process of the creation.
A mask resist 5 having a period is formed on a lithium niobate crystal plate 1. Then, this lithium niobate crystal plate 1 is immersed in a benzoic acid solution 9 to form an ion exchange region 6 (a region in which lithium ions and hydrogen ions are replaced) of a predetermined concentration up to a predetermined depth L1 shown in FIG. I do.

Thus, the refractive index No of the lithium niobate crystal plate 1 with respect to the ordinary light ray component is ΔNo in the ion exchange portion, and the refractive index Ne of the lithium niobate crystal plate 1 with respect to the extraordinary light ray component is also the ion exchange portion. ΔNe changes.

Next, etching is performed to form an ion exchange region 6.
When a groove having a predetermined depth L2 is formed and the mask resist 5 is removed, a polarizing diffraction grating shown in FIG. 5B is formed. At this time, the optical path length difference between the optical path 14 transmitted through the ion exchange region 6 and the optical path 15 transmitted through the other is π as the phase difference φo of the ordinary light ray component and φ as the phase difference φ of the extraordinary light ray component.
The depth L1 and the etching depth L2 of the ion exchange region are set so as to satisfy the conditions of (Equation 1) and (Equation 2) so that e becomes 0.

[0006]

(Equation 1)

[0007]

(Equation 2)

Here, 10 is incident light, 11 is 0th-order diffracted light, 12 is 1st-order diffracted light, and 13 is -1st-order diffracted light. When the incident light 10 is incident on the diffraction grating having the structure shown in FIG. 5B, no phase change occurs in the optical path 14 and the optical path 15 with respect to the extraordinary light ray component, and the optical path does not function as a diffraction grating. Therefore, the incident light travels straight through the lithium niobate crystal plate 1 as the zero-order light 11. On the other hand, the ordinary light ray component of the incident light causes a phase change of π in the optical path 14 and the optical path 15, so that the ordinary light ray component is incident on the optical diffraction grating, and the first-order diffracted light 12 and -1st order diffracted light 1
3 and exit from the lithium niobate crystal plate 1.

[0009]

In various optical devices using such a polarizing diffraction grating, a polarizing diffraction grating having a larger diffraction angle is realized in order to reduce the size of the device itself and its components. Is required. For example, when using a polarizing diffraction grating to divide and guide light on each of the divided light receiving elements, the distance between the polarizing diffraction grating and the light receiving element is reduced by realizing a larger diffraction angle. I can do it. As a result, there are merits such as a reduction in the size of the optical device as a whole, or the possibility of integrally forming the polarizing diffraction grating and the light receiving element.

Increasing the diffraction angle can be achieved by reducing the pitch of the polarizing diffraction grating. However, in the conventional method for producing a polarizing diffraction grating, after a mask resist is formed to obtain a desired shape, ion exchange is performed by immersion using benzoic acid or the like, and the immersion is isotropic. As shown in FIG. 5 (a), ion exchange occurs by penetrating into the back side of the mask resist 5.

As a result, the boundary between the ion exchange region 6 and the non-ion exchange region becomes unclear. This is not a problem if the desired shape and size of the polarizing diffraction grating is sufficiently large compared to the boundary region that becomes unclear. However, in the case where the desired shape and dimensions of the polarizing diffraction grating are unclear, and in the case where the pitch is small, which is about the same as the boundary region width, the optical performance is greatly reduced, and a polarizing grating having a small practical pitch is required. It was difficult to create.

[0012]

In order to solve the above-mentioned problems, in the method for producing a polarizing diffraction grating of the present invention, a method of forming an ion exchange region has a period of fine pitch in a lithium niobate crystal plate. An ion implantation method is used to form a mask resist and perform ion exchange with a uniform depth and concentration.

However, the concentration distribution in the depth direction of ion implantation has a maximum value as shown in FIG. 2, and the depth at which the concentration becomes maximum depends on the ion energy at the time of ion implantation. Therefore, in order to form a uniform ion concentration distribution to a desired depth, the energy of ions to be implanted is changed stepwise to form an ion exchange region so that the integrated concentration becomes uniform.

In the subsequent step, the ion exchange region is etched to a predetermined depth or a film for adjusting the optical path length is formed on the ion exchange region. On this occasion,
The difference between the optical path length passing through the ion-exchange region and the optical path length passing through the non-ion-exchange region is an odd multiple of half the wavelength of the monochromatic light used in the ordinary light beam component, and used in the extraordinary light beam component. It is an integral multiple of the wavelength of the monochromatic light, or, in the ordinary light ray component, an integral multiple of the wavelength of the monochromatic light used,
In the extraordinary light beam component, the depth of the ion exchange layer and the etching, or the material and thickness of the film are determined so as to be an odd multiple of a half wavelength of the monochromatic light used.

Thus, the uniformity of the ion-exchange layer can be improved, the boundary between the ion-exchange region and the non-ion-exchange region can be clarified, and a high-performance polarizing diffraction element having a small pitch can be formed. It is.

Furthermore, a polarizing diffraction grating can be realized by forming a protective film or an antireflection film on the diffraction grating. Also in this case, the difference between the optical path length passing through the ion-exchange region and the optical path length passing through the non-ion-exchange region is an odd multiple of the half wavelength of the monochromatic light used in the ordinary light component, and an extraordinary light component. In the above, it becomes an integral multiple of the wavelength of the monochromatic light used, or, in the ordinary light ray component, an integer multiple of the wavelength of the monochromatic light used, and, in the extraordinary light ray component, an odd multiple of a half wavelength of the monochromatic light used. It is necessary to determine the ion exchange region depth, etching depth, and film thickness.

In the ion implantation, etching, and film forming steps, the ion exchange region is set to a temperature at which diffusion is difficult.
By controlling the optical crystal plate, it is possible to further clarify the boundary between the ion exchange region and the non-ion exchange region,
It is possible to form a high-performance polarizing diffraction element with a small pitch.

[0018]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The invention described in claim 1 of the present invention is as follows.
One of the extraordinary ray component and the ordinary ray component causes a phase change between the light path passing through the ion exchange region formed in the optical crystal plate and the light path passing through the non-ion exchange region of the optical crystal plate. In the method for producing a polarizing diffraction grating to be generated, the surface of the optical crystal plate is ion-implanted from a mask resist applied to obtain a desired diffraction grating pattern by changing the ion energy stepwise, and After etching the surface so that the exchange region has a predetermined depth, by removing the mask resist, the optical path length transmitting through the ion exchange region and the optical path length transmitting through the non-ion exchange region The difference between the ordinary light ray component is an odd multiple of half the wavelength of the monochromatic light used, and the extraordinary light ray component is an integral multiple of the wavelength of the monochromatic light used.
Alternatively, the wavelength of the ordinary light ray component is an integral multiple of the wavelength of the monochromatic light used, and the extraordinary light ray component is an odd multiple of the half wavelength of the monochromatic light used.

Thus, the uniformity of the ion-exchange layer is improved, and the boundary between the ion-exchange region and the non-ion-exchange region is clarified, so that a fine-pitch, high-performance polarizing diffraction grating can be formed. It becomes possible.

According to a second aspect of the present invention, a mask for adjusting an optical path length is formed on the ion exchange region after the ion implantation instead of etching the ion exchange region. It is characterized in that the resist is removed. As in the first aspect, the uniformity of the ion-exchange layer is improved, and the boundary between the ion-exchange region and the non-ion-exchange region is clarified to form a fine-pitch, high-performance polarizing diffraction grating. Becomes possible.

According to a third aspect of the present invention, in the first and second aspects, the temperature of the optical crystal plate is controlled in the ion implantation and the subsequent steps, and the diffusion of the ion exchange region is performed. By pressing, the boundary between the ion exchange region and the non-ion exchange region is clarified from claims 1 and 2,
This is a method for producing a polarizing diffraction grating capable of forming a highly functional polarizing diffraction grating with a finer pitch.

According to a fourth aspect of the present invention, in the method of the first to third aspects, a protective film or an anti-reflection film is formed on the surface of the polarizing diffraction grating. Thereby, the polarizing diffraction grating can be protected and the optical characteristics can be improved.

The invention according to claim 5 is characterized in that a lithium niobate crystal plate is used as the optical crystal plate. Lithium niobate crystal plate has characteristics such as stability of optical crystal, optical characteristics, especially comparatively large nonlinear optical characteristics, etc., and it is a general-purpose material, so it is possible to obtain desired optical characteristics at low price Becomes

(Embodiment 1) An embodiment of the present invention will be described in detail with reference to the drawings. However, in this embodiment, as in the conventional example, an example in which lithium niobate is used as the optical crystal substrate and a phase difference of an odd multiple of π is generated in the ordinary light ray component will be described.

FIG. 1 shows a step-by-step method of forming a polarizing diffraction grating in one embodiment of the present invention, wherein 1 is a lithium niobate crystal plate, 2 is a photoresist film, and 3 is a mask. Reference numeral 4 denotes light for exposing the photoresist, 5 denotes a mask resist, 6 denotes an ion exchange region, and 7 denotes a film for adjusting an optical path length and protecting a diffraction grating.

First, in FIG. 1A, a lithium niobate crystal plate 1 on which a polarizing diffractive optical element is to be formed is provided with:
A photoresist film 2 is applied. Next, the photoresist film 2 is exposed in a desired periodic pattern using the light 4 and the mask 3 for exposing the photoresist film 2 in FIG.

Then, in FIG. 1C, the photoresist film 2 is developed to form a mask resist 5. FIG.
In (d), ion implantation is performed to implant hydrogen ions into the surface of the lithium niobate crystal plate 1. The implanted hydrogen ions are exchanged for existing lithium ions, and lithium diffuses from the surface of the lithium niobate crystal plate 1. Further, the mask resist 5 has a mask effect for ion implantation, and an ion exchange region 6 is formed. The ion implantation will be described later in detail.

In FIG. 1E, the surface of the ion exchange region 6 of the lithium niobate crystal plate 1 is etched to form a groove. Since the mask resist 5 also has a masking effect on etching, a concave lattice can be formed on the surface of the lithium niobate crystal plate 1 by etching.

Next, by removing the mask resist 5, the polarizing diffraction grating shown in FIG. 1F is formed. Here, the depth L3 of the ion exchange region and the etching depth L4 are
The phase difference resulting from the difference between the optical path length passing through the ion exchange region 3 and the optical path length passing through the non-ion exchange region is an odd multiple of π in the phase difference φo of the ordinary ray component, and
The phase difference φe of the extraordinary light beam component is determined to be an integral multiple of 2π, and each phase difference is represented by the following (Equation 3) and (Equation 4).

[0030]

(Equation 3)

[0031]

(Equation 4)

Here, the etching is performed after the ion implantation, but the polarization diffraction grating can be formed by the reverse. In addition, at the time of ion implantation and the subsequent steps, the temperature of the lithium niobate crystal plate 1 is set to a predetermined temperature 15 ° C.
By controlling the temperature to 0 ° C. or lower, diffusion of hydrogen ions can be suppressed, and the boundary between the ion exchange region and the non-ion exchange region can be made clearer.

Further, as shown in FIG. 1 (g), the uneven lattice on the surface of the lithium niobate crystal plate 1 is formed by an anti-reflection film, for example, an MgF2 film, a protection film, for example, a film such as SiO, or a multilayer using them in combination. It is also possible to form a stacked film and flatten it.

At this time, in consideration of the refractive index of the material used for flattening, it is necessary to adjust the etching depth and the depth of the ion exchange region in advance so that the optical characteristics after flattening become desired. is there.

The conditions for the case where the film 7 is formed are shown below as calculation examples. At this time, assuming that the refractive index of the film 7 for the ordinary light component and the extraordinary light component is Noc1 and Nec1, respectively, the depth L3 of the ion exchange region and the etching depth L4 are the optical path length passing through the ion exchange region 3. The phase difference resulting from the difference from the optical path length transmitted through the non-ion exchange region is an odd multiple of π in the phase difference φo of the ordinary light beam component, and an integer multiple of 2π in the phase difference φe of the extraordinary light beam component. And each phase difference is represented by the following (Equation 5) and (Equation 6).

[0036]

(Equation 5)

[0037]

(Equation 6)

As a final surface flattening, a step of polishing after film formation may be added. Also in this film forming step, similarly to the above description, by controlling the temperature of the lithium niobate crystal plate 1 to a predetermined temperature of 150 ° C. or lower, diffusion of hydrogen ions is suppressed, and the The boundary of the non-ion exchange region can be made clearer.

The ion implantation direction, implantation depth, and implantation amount can be accurately and arbitrarily controlled using the acceleration direction, the acceleration amount of the implanted ions, and the ion current value at the time of implantation. 5, the ion exchange region 6 can be formed with high accuracy.

However, as shown in the schematic diagram of FIG. 2, the concentration of ions implanted with a single ion energy is
The ion-exchange region 6 has an implantation depth that is not uniform in the depth direction but has a maximum ion concentration. The implantation depth depends on the ion implantation energy.
Therefore, in order to form a uniform ion concentration in the depth direction, ion energy is implanted in multiple stages.

In the present embodiment, an example in which ion implantation is performed in three stages with three types of ion energies will be described. FIG.
4 schematically shows the concentration distribution in the depth direction of ion implantation. First,
It is implanted step by step from high ion energy (a) to low ion energy (c). The ion concentration distribution (ac) at each ion energy is represented by a dotted line,
The entire ion concentration distribution is shown by a solid line. Since the ion implantation depth depends on the ion energy, it is possible to make the ion concentration distribution uniform by changing the ion energy stepwise and controlling the implantation time at each ion energy.

(Embodiment 2) Another embodiment of the present invention will be described in detail with reference to the drawings. However, in this embodiment, as in the conventional example, an example in which lithium niobate is used as the optical crystal substrate and a phase difference of an odd multiple of π is generated in the ordinary light ray component will be described.

FIG. 4 shows another embodiment of the present invention.
The method for forming a polarizing diffraction grating is shown step by step, 1 is a lithium niobate crystal plate, 2 is a photoresist film, 3 is a mask, 4 is light for exposing the photoresist, 5 is a mask resist. , 6 are ion exchange regions, 7, 8
Is a membrane. In this embodiment, an example is shown in which lithium niobate is used as an optical crystal and a phase difference of an odd multiple of π is generated in the ordinary light ray component as in the first embodiment.

First, in FIG. 4A, a lithium niobate crystal plate 1 on which a polarizing diffractive optical element is to be formed is provided.
A photoresist film 2 is applied. Next, the photoresist film 2 is exposed in a desired periodic pattern using the light 4 and the mask 3 for exposing the photoresist film 2 shown in FIG.

Then, in FIG. 4C, the photoresist film 2 is developed to form a mask resist 5. FIG.
In (d), ion implantation is performed to implant hydrogen ions into the surface of the lithium niobate crystal plate 1. The implanted hydrogen ions are exchanged for existing lithium ions, and lithium is converted to the lithium niobate crystal plate 1. Diffuses from the surface. Further, the mask resist 5 has a mask effect for ion implantation, and an ion exchange region 6 is formed. The ion implantation is performed in the same manner as in the first embodiment.

Next, in FIG. 4E, a film 8 is formed on the upper surface of the ion exchange region 6 of the lithium niobate crystal plate 1. In the first embodiment, the ion exchange region is etched by a predetermined depth. Instead, a film 8 is formed here to adjust the optical path length. This film 8
Is not particularly limited as long as it has a higher refractive index than air.

Next, by removing the mask resist 5, the polarizing diffraction grating shown in FIG. 4F is formed. Here, the depth L5 of the ion exchange region and the thickness L6 of the membrane 8 are determined by the phase difference resulting from the difference between the optical path length passing through the ion exchange region 6 and the optical path length passing through the non-ion exchange region. Is determined to be an odd multiple of π in the phase difference φo of the ordinary light ray component and an integer multiple of 2π in the phase difference φe of the extraordinary light ray component, and the respective phase differences are expressed by the following (Equation 7) and (Equation 8). ).

[0048]

(Equation 7)

[0049]

(Equation 8)

Further, as shown in FIG. 4 (g), the surface of the lithium niobate crystal plate 1 is coated with an antireflection film, for example, MgF.
It is also possible to form a two-layer film or a protective film, for example, a film of SiO or the like, or a multilayer laminated film using both of them, and flatten it. At this time, it is necessary to adjust the film thickness and the depth of the ion-exchange region in advance so that the optical characteristics after the flattening become desired in consideration of the refractive index of the material used for the flattening.

The conditions when the film 7 is attached are shown below as a calculation example. Here, the depth L5 of the ion exchange region and the film thickness L6
Is the difference between the optical path length passing through the ion-exchange region 6 and the optical path length passing through the non-ion-exchange region, where the refractive indices of the ordinary light component and the extraordinary light component of the film 7 are Noc2 and Nec2, respectively. The resulting phase difference is determined so as to be an odd multiple of π in the phase difference φo of the ordinary light ray component and to be an integer multiple of 2π in the phase difference φe of the extraordinary light ray component. It is expressed by (Equation 10).

[0052]

(Equation 9)

[0053]

(Equation 10)

As a final surface flattening, a polishing step after film formation may be added. Also in this film forming step, similarly to the above, by controlling the temperature of the lithium niobate crystal plate 1 to a predetermined temperature of 150 ° C. or less, diffusion of hydrogen ions is suppressed, and the ion exchange region and the non-ion The boundaries of the exchange area can be made clearer.

In the above embodiments of the present invention, lithium niobate is used as the crystal plate. However, it goes without saying that the above-described production method can be applied to other suitable optical crystals. Further, in the first and second embodiments, the example is described in which the phase difference of the ordinary light ray component is an odd multiple of π.
The phase difference between either the ordinary ray component or the extraordinary ray component is π
The same polarization diffraction grating can be obtained even when the condition that the phase difference of other light components is an odd multiple of 2π and an integer multiple of 2π is satisfied.

[0056]

As described above, according to the present invention, the boundary region between the ion-exchange region and the non-ion-exchange region can be clarified, and the ion-exchange region can be accurately formed into a predetermined shape. Therefore, a high-performance polarizing diffraction grating with a fine pitch can be formed.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view illustrating a method for producing a polarizing diffraction grating according to an embodiment of the present invention.

FIG. 2 is a schematic diagram of ion implantation concentration in the depth direction when conventional ion implantation is performed.

FIG. 3 is a schematic diagram of ion implantation concentration in the depth direction when ion implantation is performed in the embodiment of the present invention.

FIG. 4 is a cross-sectional view showing a method for producing a polarizing diffraction grating according to another embodiment of the present invention.

FIG. 5 is a cross-sectional view of a polarizing diffraction grating in a conventional mode.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Lithium niobate crystal plate 2 Photoresist film 3 Mask 4 Light for exposing photoresist 5 Mask resist 6 Ion exchange region 7,8 film 9 Benzoic acid solution 10 Light incidence 11 0th-order diffracted light 12 1st-order diffracted light 13 − First-order diffracted light 14, 15 Optical path

Claims (5)

[Claims] 1. One of an extraordinary ray component and an ordinary ray component between a light path passing through an ion exchange region formed in an optical crystal plate and a light path passing through a non-ion exchange region of the optical crystal plate. However, in the method for producing a polarizing diffraction grating that generates a phase change, ion implantation is performed by gradually changing ion energy from a mask resist applied to obtain a desired diffraction grating pattern on the surface of the optical crystal plate. Then, after etching the surface so that the ion-exchange region has a predetermined depth, the mask resist is removed, so that the optical path length through the ion-exchange region and the non-ion-exchange region are transmitted. The difference from the optical path length is an odd multiple of half the wavelength of the monochromatic light used for the ordinary light ray component, and an integer multiple of the wavelength of the monochromatic light used for the extraordinary light ray component. In other words, the ordinary light beam component has an integer multiple of the wavelength of the monochromatic light used, and the extraordinary light beam component has an odd multiple of a half wavelength of the monochromatic light used. Method. 2. After the ion implantation, instead of etching the ion exchange region, a film for adjusting the optical path length is formed on the ion exchange region, and then the mask resist is removed. The method for producing a polarizing diffraction grating according to claim 1, wherein: 3. The method according to claim 1, wherein the temperature of the optical crystal plate is controlled in the ion implantation and subsequent steps.
3. The method for producing a polarizing diffraction grating according to 2. 4. The method according to claim 1, wherein a protective film or an antireflection film is formed on the surface of the optical crystal plate after removing the mask resist. 5. The method according to claim 1, wherein a lithium niobate crystal plate is used as the optical crystal plate.