JPH0474875A - Production of diffraction grating - Google Patents

Abstract

PURPOSE:To simultaneously and easily form two kinds of diffraction gratings of different periods with one time of exposing by applying a resist on a substrate and partially covering the resist with a translucent mask, then subjecting the resist to two-beam interference exposing followed by developing and etching the substrate with the resulted patterns as a mask. CONSTITUTION:The resist 12 which has the min. value of the developing rate with respect to an exposing intensity when baked at 110 to 120 deg.C prior to exposing is applied on the substrate 1 and is baked at 110 to 120 deg.C. The translucent mask 13 is then disposed on a part of the resist 12 and the resist 12 is exposed by the two-beam interference exposing method at the exposing intensity at which the max. value and min. value of the exposing intensity at the points not coated with the translucent mask 13 are on both sides of the intensity to minimize the developing rate and the max. value of the exposing intensity is below the intensity to minimuze the developing rate at the points covered with the translucent mask 13. The resist 12 is thereafter developed and is patterned to the diffraction grating 23a. The substrate is etched with this pattern as a mask, by which the diffraction grating 23b is formed.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a method for manufacturing a diffraction grating, and particularly to a method for manufacturing a diffraction grating in which diffraction gratings with two types of periods are simultaneously formed.

[Conventional technology]

FIG. 5 is a diagram showing a conventional interference exposure apparatus used for manufacturing a diffraction grating. In the figure, a laser light source 4 emits laser light 17a. Laser beam 17a is a half mirror
5, the laser beam 17b is emitted.

These laser beams 17b and 17C are reflected by mirrors 6a and 6b, respectively, and are irradiated onto the resist 2 coated on the substrate 1.

Further, FIG. 6 is a cross-sectional process diagram and an exposure intensity distribution diagram showing a method of forming a diffraction grating by a conventional interference exposure method.

Next, the manufacturing process of a conventional diffraction grating will be explained with reference to FIG.

First, a resist 2 is applied onto a substrate 1 as shown in FIG. 6(a), and then the resist 2 is periodically exposed using a three-beam interference exposure method as shown in FIG. 6(b).

At this time, the exposure intensity of the laser light irradiated onto the resist 2 changes periodically as shown in FIG. 6(C), so when the exposed resist 2 is developed, the resist 2 changes as shown in FIG.
The diffraction grating 3a shown in d) is patterned. After that, using the patterned resist 2 as a mask, the substrate 1 is
By etching, a diffraction grating 3b is formed as shown in FIG. 6(e).

Next, the principle of the interference exposure method used in the conventional method of manufacturing a diffraction grating will be explained with reference to FIG.

In the apparatus shown in FIG. 5, laser light 17a emitted from a laser light source 4 is split into two directions by a half mirror 5, reflected by a mirror 6, and then recombined on the substrate 1. At this time, due to the interference of the laser beam, the intensity of the light on the substrate has a periodic distribution represented by . Here, λ represents the wavelength of the laser beam, and θ represents the angle of incidence of the laser beam onto the substrate.

Conventional diffraction gratings are fabricated using the above principle, and are performed by exposing a resist coated on a substrate to light at a periodic pitch, followed by development and etching.

FIG. 2 is a cross-sectional structural diagram in the resonator length direction showing a distributed feedback (DFB) semiconductor laser having both primary and secondary diffraction gratings. In the figure, p-type InGaAsP guide layer 7. InGaAsP active layer 8;
are sequentially stacked on a P-type 1nP substrate la on which is formed.

Next, the operation of this DFB semiconductor laser will be explained.

In a DFB laser, light generated in the active layer is reflected by the diffraction grating and confined inside the element, so if the diffraction grating is formed at a uniform pitch throughout the element, the optical density at the center of the element will be high. , hole burning, etc., which adversely affect device characteristics. The semiconductor laser shown in FIG. 2 solves these problems.

Light is reflected by a diffraction grating more efficiently with a lower-order diffraction grating. Therefore, as shown in FIG. 2, by making the diffraction grating 10 at the center of the element higher-order than the diffraction grating 11 near the end face of the resonator substrate, the intensity of light reflected at the center of the element can be reduced near the end face of the resonator. can be made smaller than the reflected light intensity at . As a result, the light generated in the active layer 8 is not confined only to the center of the element, but most of the light travels to the vicinity of the cavity end face and is reflected by the diffraction grating 11. Therefore, the density of light does not become high only in the central part of the element, and the density of light becomes uniform over the entire element, making hole burning less likely to occur and improving laser characteristics. Further, in this laser, the laser beam can be extracted perpendicularly to the substrate by the second-order diffraction grating 10.

[Problem to be solved by the invention]

Since the conventional diffraction grating is manufactured as described above, a diffraction grating with the same period is formed over the entire surface of the substrate. Therefore, there is a problem that a semiconductor laser having both a first-order and second-order diffraction grating as shown in FIG. 2 cannot be manufactured.

The second invention was made to solve the above-mentioned problems, and aims to provide a method for manufacturing a diffraction grating that can simultaneously form diffraction gratings with two types of periods on the same plane by one interference exposure. purpose.

[Means to solve the problem]

The method for manufacturing a diffraction grating according to the present invention includes coating a substrate on which a diffraction grating is to be formed with a resist whose development speed has an extreme value with respect to the exposure intensity, and further covering the resist partially with a semi-transparent mask. Then, the light intensity is set such that the maximum and minimum values of the exposure intensity in either the area covered by the semi-transparent mask or other areas are on both sides of the intensity at which the development speed reaches the extreme value. A diffraction grating is manufactured by performing interference exposure using a photoresist, and then etching the substrate using the pattern obtained by developing the resist as a mask.

[Effect]

The method for manufacturing a diffraction grating in this invention is to apply a resist whose development speed has an extreme value with respect to the exposure intensity onto a substrate forming a diffraction grating, and then partially cover the resist with a semi-transparent mask. , the light intensity is such that the maximum and minimum values of the exposure intensity in either the area covered by the semi-transparent mask or other areas are on both sides of the intensity at which the development speed reaches an extreme value. Since the diffraction grating is manufactured by carrying out interference exposure and etching the substrate using the pattern obtained by developing this post-processing resist as a mask, two types of diffraction gratings with different periods can be produced by one interference exposure. Can be formed simultaneously by exposure.

〔Example〕

An embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 is a cross-sectional process diagram and a light intensity distribution diagram showing a method for manufacturing a diffraction grating according to an embodiment of the present invention.

In the figure, on the substrate 1 is a resist (image reversal resist) 1 whose development speed has a minimum value with respect to the exposure intensity.
2 is applied, and a translucent mask 13 is applied on the resist 12.
is partially placed.

Next, the manufacturing process of the diffraction grating according to this example will be explained.

First, as shown in FIG. 1(a), the substrate 1 on which the diffraction grating is formed is baked at a temperature of 110 to 120°C before exposure so that the development speed has a minimum value with respect to the exposure intensity. Naru resist (image reversal resist) 1
Apply 200°C then about 50°C at a temperature of 110-120°C.
After baking for a minute, a semi-transparent mask 13 is placed on a part of the resist 12 as shown in FIG. The maximum and minimum values are on both sides of the intensity where the phenomenon speed is minimum, and in the area covered with the semi-transparent mask 13, the resist is exposed at an exposure intensity such that the maximum value of the exposure intensity is less than the intensity where the phenomenon speed is minimum. 12 is exposed.

FIG. 1(C) is a diagram showing the light intensity distribution at this time. Thereafter, the resist 12 is developed so that the portions of the resist 12 exposed to the light intensity at which the development speed becomes minimum remain, and the resist 12 is patterned into a diffraction grating 23a shown in FIG. 1(d).

Thereafter, the patterned resist 12 is used as a mask and the substrate 1 is etched with a suitable etching solution, such as HB r /
HN O: I/Hz O mixed solution, HBr/HN
O,/CH30H mixed solution or 13 r2 /
By etching with a CH, OH mixed solution or the like, a diffraction grating 23b is formed as shown in FIG. 1(e).

Next, the effect of a resist (image rehearsal resist) in which the development speed has a minimum value with respect to the exposure intensity will be explained.

Image reversal resist is 110-120 before exposure
It is known that by baking at a temperature of °C, the phenomenon speed becomes a minimum value with respect to the exposure intensity α, as shown in Fig. 3 (see the June 1986 issue of Submaterials).
.

Further, the exposure intensity in exposure by the interference exposure method has a periodic distribution as shown in FIG. Here, the period of the distribution is (1)
This is the number eight in the formula.

Therefore, by exposing so that the maximum value of the exposure intensity due to interference exposure is above the point where the phenomenon becomes the minimum value (point α in Figure 3) and the minimum value of the exposure intensity is below the α point, the development speed can be reduced to the minimum value. It is possible to develop the resist so that only the parts exposed to light at an intensity of Become.

On the other hand, if the maximum value of the exposure intensity due to interference exposure is below the α point or the minimum value of the exposure intensity is above the α point, a resist will remain with the period of the interference fringes obtained by interference exposure, and a diffraction grating with the period of the interference fringes will remain. is formed.

Therefore, by covering a part of the substrate with a semi-transparent mask, the maximum value of the exposure intensity in the area not covered by the mask is above the α point, and the minimum value is below the α point,
By setting the maximum value of the exposure intensity to be below the α point in the area covered by the mask, a periodic diffraction grating and a period A/2 diffraction grating can be simultaneously formed by one interference exposure.

Next, a process for manufacturing the semiconductor laser shown in FIG. 2 using the method for manufacturing a diffraction grating according to this embodiment will be described.

A P-type 1nP substrate 1a is used as the substrate 1 shown in FIG.
A diffraction grating is manufactured on a substrate la according to the flow shown in FIG. That is, first, 110 to 12
A resist (image reversal resist) whose development rate has a minimum value with respect to exposure intensity by betaening at a temperature of 0°C is applied as shown in Figure 1(a) at 110 to 120°C. After baking at a temperature of
As shown in Fig. 3), by partially covering the substrate with a semi-transparent mask 13, and using the interference exposure method, the maximum and minimum values of the exposure intensity are minimized in the phenomenon speed in the areas not covered by the semi-transparent mask 13. In the area covered by the semi-transparent mask 13, exposure is performed so that the maximum value of the exposure intensity is equal to or less than the intensity at which the phenomenon speed becomes minimum. Here, the radius and interval of the semi-transparent mask 13 are approximately 300 μm. Thereafter, a diffraction grating is formed by etching with a suitable etching solution. After that, the bandgap equivalent wavelength is 1.15
An InGaAsP541 layer 7 with a composition of μm, an InC with a composition of a band gap equivalent wavelength of 1.3 μm; an aAsP active layer 8 and an n-type 1nP cladding layer 9 are sequentially formed, and separated by cleavage at the center of the -order diffraction grating. As a result, the DFB laser shown in FIG. 2 is completed.

In the above example, a diffraction grating with a period of A/2 is fabricated in an area covered with a semi-transparent mask, and a diffraction grating with a period of A/2 is fabricated in an area not covered with a semi-transparent mask. By exposing the entire resist surface, the maximum value of the exposure intensity is above the α point and the minimum value is below the α point in the area covered by the semi-transparent mask, and the minimum value of the exposure intensity is in the area not covered by the mask. Even when the value is set to be equal to or higher than the α point, the same effects as in the above embodiment can be obtained.

〔Effect of the invention〕

As described above, according to the present invention, a resist whose development speed has an extreme value with respect to the exposure intensity is coated on a substrate forming a diffraction grating, and the resist is further partially covered with a semi-transparent mask. Then, the light intensity is set such that the maximum and minimum values of the exposure intensity in either the area covered by the semi-transparent mask or other areas are on both sides of the intensity at which the development speed reaches the extreme value. Since the diffraction grating is manufactured by carrying out interference exposure using a photoresist and etching the substrate using the pattern obtained by developing the resist as a mask, two types of diffraction gratings with different periods can be exposed in one interference exposure. This has the advantage that a semiconductor laser having both -order and second-order diffraction gratings can be easily formed.

[Brief explanation of drawings]

FIG. 1 is a cross-sectional process diagram and an exposure intensity distribution diagram showing a method of manufacturing a diffraction grating according to an embodiment of the present invention. FIG. A cross-sectional side view of a DFB laser with a diffraction grating, Figure 3 is 1
A diagram showing the relationship between the exposure intensity and the phenomenon speed of the image reversal resist after being betaed at 10 to 120°C, Figure 4 is a diagram showing the exposure intensity distribution by interference exposure method, and Figure 5 is an interference exposure apparatus. 6 are a cross-sectional process diagram and an exposure intensity distribution diagram showing a method of forming a diffraction grating by a conventional interference exposure method. 1 is a substrate, 1a is a p-type 1nP substrate, 7 is a p-type nGaA
sP guide layer, 8 is an InGaAsP active layer, 9 is an n-type 1nP cladding layer, 10 is a second-order diffraction grating in the center of the resonator, 11 is a first-order diffraction grating near the end face of the resonator, 12
is an image reversal resist, 13 is a semi-permeable mask,
23a is a diffraction grating made of a resist pattern, and 23b is a diffraction grating transferred onto the substrate. Note that the same reference numerals in the figures indicate the same or equivalent parts.

Claims (1)

[Claims] (1) A process of applying a resist whose development speed has an extreme value with respect to the exposure intensity on the substrate forming the diffraction grating, and partially covering the resist with a semi-transparent mask, and then applying the resist with the semi-transparent mask. performing interference exposure at a light intensity such that the maximum and minimum values of the exposure intensity are on both sides of the intensity at which the development speed reaches an extreme value only in either the covered area or the other area; A method for manufacturing a diffraction grating, comprising the steps of: developing and patterning the resist; and etching the substrate using the patterned resist as a mask.