JPH03284703A - Optical glass member for generating prescribed phase difference and production thereof - Google Patents

Abstract

PURPOSE:To obtain a phase mask having a good shape with good reproducibility by using electron beam vapor deposition which requires heating of an optical glass substrate and dissolving an Al film by utilizing the cracks of SiO2 films generated by a difference in the coeffts. of thermal expansion between Al and SiO2 at the time of the cooling thereof. CONSTITUTION:Al film patterns 7 are formed in the upper part of 1st regions 14 and 2nd regions 15 are higher than the 1st regions 14 and are formed with the 2nd SiO2 films 13 in the upper part thereof. The Al film patterns are formed on the surface of the 1st region 14 of the polished optical glass substrate 6 by a stage for lifting off the resist patterns formed by mask exposing. While the substrate 6 is kept heated by the electron beam vapor deposition, the 1st and 2nd SiO2 films 12, 13 are respectively formed in the upper parts of the patterns 7 and the substrate 6. The substrate 6 is then cooled down to room temp. to generate the cracks in the SiO2 films 12 on the patterns 7. The substrate 6 is then immersed in an acid or alkaline soln. to dissolve away the Al film patterns 7 together with the SiO2 films 12 remaining without partly being stripped. The etching stage is omitted in this way and since the mask exposing is once, the excellent pattern accuracy is obtd.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an optical glass member having steps for producing a patterned phase difference in transmitted light, and a method for manufacturing the same.

To describe a typical example of the optical member of the present invention, a λ/4 shift type diffraction grating (λ is the wavelength of light) built into a distributed feedback semiconductor laser (hereinafter referred to as a DFB laser) is manufactured. This is a phase mask for The present invention relates to an optical glass member typified by such a precise phase mask, which is a silicon dioxide (hereinafter referred to as SiO□ film) having a predetermined thickness.

) and a method for manufacturing the same.

[Conventional technology]

In recent years, active research has been conducted on DFB lasers, which always oscillate in a stable single longitudinal mode, regardless of continuous operation or high-speed modulation operation, as light sources for long-distance, large-capacity optical transmission systems. A DFB laser has a diffraction grating built into the laser element, and oscillates near the Bragg wavelength determined by the period of the diffraction grating. Bragg wavelength λ of the diffraction grating. is λ. =2nΔ/
It is given by m. Here, n is the equivalent refractive index in the laser element, Δ is the period of the diffraction grating, and m is the order of the diffraction grating.

However, a DFB laser with a built-in uniform diffraction grating has a Bragg wavelength λ. , there is no oscillation, and the Bragg wavelength λ0
It is easy to oscillate in two modes, left and right symmetrically, and it is difficult to obtain stable single mode oscillation.

Therefore, the period of the diffraction grating built into the DFB laser is set to 1.
As (m=1), λ. /(4n), the Bragg wavelength λ. A prototype is being produced that matches the vertical mode. This diffraction grating can be used as a λ/4 shift type diffraction grating, or as a DF that incorporates this λ/4 shift type diffraction grating.
The B laser is called a λ/4 shift type DFB laser.

For a first-order diffraction grating, the period is λ. Shifting by /(4n) corresponds to shifting by λ/2, and the unevenness of the diffraction grating may be reversed in the phase shift section.

The conventional method for forming such a λ/4-shifted diffraction grating utilizes the reverse photosensitivity of both positive and negative type resists during laser interference exposure (Utaka, Ak.
iba, 5akai, Matsushima
IEEEJ, (luantum Electron
ics vol, mouth E-22, pp, 1042-10
51 (1986)), and a method in which a phase mask is brought into close contact with an optical glass substrate coated with a resist, and interference fringes with a phase shift are generated due to the phase difference of light at the stepped portions of the mask (Ozaki, Funota, Yamakoshi, Ishikawa, Nakajima Institute of Electronics and Communication Engineers Technical Research Report 0QE85-60, pp. 57-64 (1
985)) are known.

Here, the principle of the method using a phase mask will be explained using a cross-sectional conceptual diagram showing the principle during interference exposure shown in FIG. In the following description, resist refers to the substance itself applied to an optical glass substrate or the like, and resist pattern refers to a pattern generated as a result of exposure. Furthermore, an aluminum film (more precisely, an aluminum film) refers to the material, and an aluminum film pattern refers to one produced by a lift-off method using a resist pattern.

The first and second laser beams 1.2 are applied at an asymmetrical angle, with a step difference d.
is incident on the phase mask 3 having the rays at a predetermined position. Here, 01 and θ2 are the incident angles of the first laser beam l, the second laser beam 20 and the phase mask 3, respectively, and α
is the inclination angle of the bisector of the angle formed by the first laser beam 1 and the second laser beam 2, and the interference fringes are incident on the phase mask 3 at an angle of α. This interference fringe is generated by the phase mask 3
The first laser beam 1 and the second laser beam 2 are refracted within the laser beam and travel at an inclination angle of α, and a phase difference occurs due to the difference in optical path at the portion where the step d is located. As a result, the resist 4 coated on the semiconductor substrate 5 is exposed to interference fringes with shifted light and dark patterns. After developing the resist 4, the semiconductor substrate 5 is etched using the formed resist pattern as a mask, thereby forming a diffraction grating with a shifted period at a predetermined position. The period Δ of the diffraction grating at this time is 8=λ, /(s
in θ1+sin θ2), and the period deviation Φ is Φ=d (tana-tancr), so by adjusting the step d, the period deviation can be reduced to 8/2.

Hereinafter, based on FIG. 3, a method for manufacturing a phase mask used for forming a λ/4-shifted diffraction grating according to the conventional technology (
Shirasaki et al., 0QE85-60: see page 4).

(a) First, a first aluminum film pattern 7 is formed on an optical glass substrate 6 polished with optical precision by a resist pattern lift-off method. The purpose of the first aluminum film pattern 7 is to periodically arrange the phase shift portions of the diffraction grating.

(b) Next, a SiO□ film 8 is formed thereon by sputtering at a low temperature over the entire region to a thickness equal to the target step d of the phase mask 3.

(C) A resist 4 is applied to the entire upper surface of the film 8, and the optical glass substrate 6 is exposed to light from the back side.

((To) At this time, the first aluminum film pattern 7 serves as a mask, and by developing, the resist pattern 9 is formed only in a portion of the first aluminum film pattern 7.

(e) Furthermore, an aluminum film 10 is vapor-deposited over the region where the resist pattern 9 is formed and over the region where the resist pattern 9 is not formed.

(g) By a lift-off method of the resist pattern 9, a second aluminum film pattern 11 is formed at alternate positions from the first aluminum film pattern 7 with the SiO film 8 in between.

(g) Next, by dry etching (for example, reactive ion etching), the SiO□ film 8 on the top of the first aluminum film pattern 7 is etched into the first aluminum film pattern using the second aluminum film pattern 11 as a mask. Etch up to part 7. At this time, the surface force of the optical glass substrate 6 (second aluminum film pattern 11
Thus, the surface of the SiO2 film 8 is protected.

(c) Finally, the first aluminum film pattern 7 and the second aluminum film pattern 11 are removed to complete the phase mask 3.

[Problem to be solved by the invention]

As a method for manufacturing a phase mask, in the conventional technique represented by the method by Ozaki et al. described on page 4, a step formed by a SiOz film on an optical glass substrate is formed using dry etching. There were also the following issues associated with this project.

1) The second aluminum film pattern 11 is formed by a lift-off method of the resist 4, and the resist pattern 9 used therein is formed by exposure and development from the back side of the optical glass substrate 6 using the first aluminum film pattern 7 as a mask. formed by.

However, since the SiO2 film 8 is formed between the first aluminum film pattern 7 and the resist pattern 9, the thickness of this SiO2 film greatly influences the pattern accuracy of the resist pattern 9.

That is, the thickness of the SiO□ film 8, that is, the step d shown in FIG. 2, needs to be 2 to 3 μm or more when the inclination angle α is 5°. The distance between the resist pattern 9 and the resist pattern 9 is 2 to 3 μm or more, and the pattern accuracy of the resist pattern 9 deteriorates.

Therefore, the precision of the second aluminum film pattern 11 formed by the lift-off method of the resist pattern 9 exposed and developed using this method also deteriorates.

2) Since the pattern of the SiO□ film 8 is formed using dry etching, side etching of the SiO□ film 8 that proceeds under the edge portion of the second aluminum film pattern 11 must be minimized. For this purpose, precise gas flow rate control and condition setting are required. Deterioration of the stepped shape of the SiO2 film 8 due to the progress of side etching expands the transition region (defectively formed portion) of the shifted portion of the phase shift type diffraction grating.

3) Since there are first and second aluminum film patterns 7.11 above and below the Si0 film 8 formed by sputtering at low temperatures, there is a possibility that aluminum may be mixed into the 5iOt film 8 during dry etching. . If aluminum is mixed in, 5
The refractive index and transmittance of the ift film 8 to the laser beam change. Therefore, when interference exposure is performed using the phase mask 3 having the pattern of the S i O film 8, not only the predetermined phase difference cannot be obtained, but also the amount of exposure to the resist 4 is insufficient. The pattern 9 becomes poorly cut, and a well-shaped diffraction grating is not formed.

4) As mentioned above, since dry etching is used,
The surface of the optical glass substrate 6 and the surface of the Sing film 8, which should not be etched, are simultaneously etched with the first aluminum film pattern 7.
Furthermore, since the second aluminum film pattern 11 is used for protection, the process becomes complicated and the yield is low.

[Means to solve the problem]

The present invention intentionally requires heating of the large optical glass substrate 6, whereas the conventional method uses a sputtering method that can form the SjO□ film 8 on the first aluminum film pattern 7 at a low temperature. Using electron beam evaporation, during cooling after completion of evaporation,
The first phenomenon occurs due to the difference in thermal expansion coefficient between aluminum and SiO□.
The SiO□ film 8 on the top of the first aluminum film pattern 7 is dissolved by dissolving the first aluminum film pattern 7 with acid or alkali using the crack in the 5in2 film 8 on the top of the aluminum film pattern 7. Remove the first aluminum film pattern 7
An inverted pattern of the SiO2 film 8 is formed in the area where there was no pattern.

Therefore, the present invention provides a method for manufacturing the phase mask 3 that is extremely simple and has a good shape with good reproducibility.

[Effect]

In the method for manufacturing a phase mask according to the present invention, the mask exposure is performed only when forming a lift-off resist pattern used in forming the first aluminum film pattern, so that pattern accuracy is excellent. Furthermore, since the pattern formation of the SiO□ film is not based on etching of the SiO2 film, there is no problem of side etching and there is no need to form an aluminum film pattern that serves as a mask during etching.
.

There is also no possibility of aluminum being mixed into the membrane.

Due to the above-described effects, the present invention allows a high-quality phase mask to be manufactured extremely easily and with good reproducibility.

〔Example〕

The present invention will be explained in detail using FIG. Proceed to steps (i), (ii), (iii), and (1v) in order from the top.

Here, the first region 14 is a region on which the aluminum film pattern 7 is formed. Further, the second region 15 is higher in height than the first region 14, and has a second sinusoid on the top thereof.
This is the area where the g film 13 and the film body 3 are formed.

(i) A first aluminum film pattern 7 is formed on the surface of the first region of the optical glass substrate 6 which has been polished with optical precision through a step of lifting off a resist pattern formed by mask exposure. The thickness of the first aluminum film pattern 7 is at least the same as that of the second SiO□ film 13 formed later.
It is formed to have a film thickness of 3.

(ii) Next, while heating the optical glass substrate 6 by electron beam evaporation, the first S i O film 12 and the second A SiO□ film 13 having a thickness of 3 is formed. First SiOz film 12, second (7) Si02 film 13
The thickness of the target phase mask is the thickness of the 30 step difference (second
Set to step d) in the figure.

(j]) This first SiO□ film 12, second SiO
After forming the Z film 13, the optical glass substrate 6 is cooled to room temperature, thereby causing cracks to occur only in the first SiO2 film 12 on the first aluminum film pattern 7.

This crack occurs when the first aluminum film pattern 7 contracts when the optical glass substrate 6 is cooled because aluminum has a much larger coefficient of thermal expansion than SiO□.
This occurs because the film 12 is strained. The first SiO□ on the first aluminum film pattern 7 where cracks have occurred
The film 12 peels off as the temperature decreases, but not all of the film peels off, and a portion remains without peeling off.

(iv) Then, the optical glass substrate 6 is immersed in acid or alkali. Examples of acids that can be used here include relatively highly concentrated hydrochloric acid, sulfuric acid, and nitric acid, and alkalis include aqueous solutions of caustic soda, caustic potassium, ammonia, and the like. For example, by immersing it in 35% hydrochloric acid, the first aluminum film pattern 7 is melted and removed together with the first S j Oz M12 that remains partially without falling off. At this time, the second Si formed directly on the optical glass substrate 6
O□ film 13 film thickness: 3 It is not peeled off because of its strong adhesion to the glass substrate 6. Therefore, the first aluminum film pattern 7
A second S i O film 13 is formed on the optical glass substrate 6 .

What is important is that the accuracy of pattern cutting between the first SiO2 film 12 to be removed on the first aluminum film pattern 7 and the second SiO2 film 13 to be left on the optical glass substrate 6 is It depends on the thickness of the aluminum film pattern 7. In the first step (i), the thickness of the first aluminum film pattern 7 is adjusted to at least the second S formed on the predetermined phase mask 3.
This is the reason why the iO□ film 13 was formed to have a thickness of 3.

Through the above steps (i) to (iv), the phase mask 3 having a predetermined step can be manufactured with extremely simple steps and with good reproducibility.

Furthermore, in the above embodiment, the first SiO2 film 12 and the second SiO film 13 are formed using electron beam evaporation, but for example, sputtering, The present invention can be applied by forming a 5iO7 film under heating using plasma CVD, ion blating, or the like.

Further, as another example, if the aluminum film is formed into a stripe pattern with a fine period, it is also possible to form a stripe pattern of the SiO□ film, which is the inverse of the stripe pattern. Here, both the surface of the optical glass substrate 6 polished with optical precision, that is, the concave portion of the pattern, and the surface of the SiOz film, that is, the convex portion of the pattern, are not damaged.

Therefore, it is also possible to manufacture a transmission diffraction grating as is.
Further, a reflective diffraction grating can be easily manufactured by coating with a metal reflective film.

Moreover, if the aluminum film is not limited to a stripe pattern but can be formed into any shape or figure, the inverted S i O,
A film pattern can be produced without damaging both the surface of the optical glass substrate and the surface of the SiOz film.

In addition, in the present invention, one combination with a difference in thermal expansion coefficient, such as aluminum and 5iOz, was explained as an example, but if we focus on the difference in thermal expansion coefficient, other combinations such as silver (Ag) and copper (
Nitride film (SisN4), silicon (Cu), gold (Au), iron (Fe), etc.
The present invention can be applied by adjusting the heating conditions by combining groups with small thermal expansion coefficients such as Si).

〔Effect of the invention〕

As explained above, according to the present invention, cracks are created in the SiO film formed on the aluminum film, making it possible to omit the etching process that was required in the conventional technology. Moreover, the number of mask exposures can be reduced to one.

As a result, since mask exposure is performed only when forming the first aluminum film pattern 7, pattern accuracy is excellent.

Also, since there is no need for dry etching,
It is no longer necessary to suppress the occurrence of side etching.

Also, S i O! during dry etching. There is no fear of aluminum being mixed into the membrane.

Furthermore, the reproducibility was definitely improved by the reduction in the number of steps.

With this manufacturing method, which has excellent pattern accuracy and no risk of side etching, it is possible to obtain a good step shape, and a phase mask with this good step shape can be manufactured by reducing the number of steps. However, the effect of reliably improving reproducibility can be obtained.

[Brief explanation of drawings]

FIG. 1 is a cross-sectional view showing the manufacturing process of a phase mask according to an embodiment of the present invention, and FIG. 2 is a cross-sectional view showing the principle of interference exposure in a method for forming a λ/4-shifted diffraction grating using a phase mask. The conceptual diagram and FIG. 3 are cross-sectional views showing the manufacturing process of a phase mask according to a conventional technique. 1... First laser beam, 2... Second laser beam, 3
... Phase mask, 4... Resist, 5... Semiconductor substrate, 6... Optical glass substrate, 7... First aluminum film pattern, 8... SiC2 film, 9... Resist Pattern, 10... Aluminum film, 11... Second aluminum film pattern, 12... First SiO□艮13... Second S
iO□ membrane.

Claims (2)

[Claims] (1) In a method for manufacturing an optical glass member having a step formed on an optical glass substrate polished with optical precision and consisting of a first region and a second region having a higher height than the first region, (i) forming an aluminum film on the first region by at least the thickness of the step between the first region and the second region; (ii) forming an aluminum film on the first region and the second region; (iii) forming an SiO_2 film by the thickness of the step between the first region and the second region under heating on each of the second regions; (iii) cooling the optical glass substrate; A step is formed on the surface of the optical glass substrate, which consists of a step of creating a crack in the SiO_2 film on top of the aluminum film on the first region, and (iv) a step of removing all the aluminum film by adding acid or alkali. A method of manufacturing an optical glass member having the above-mentioned structure and producing a predetermined retardation. (2) An optical glass member produced by the manufacturing method according to claim (1).