JP2527833B2 - Method of manufacturing diffraction grating - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for manufacturing a diffraction grating, and more particularly to a method for manufacturing a diffraction grating in which diffraction gratings of two types of periods are simultaneously formed.

[Conventional technology]

FIG. 5 is a diagram showing an interference exposure apparatus used for manufacturing a conventional diffraction grating. In the figure, laser light 17a is emitted from a laser light source 4. The laser light 17a is separated by the half mirror 5 into laser lights 17b and 17c.
7c is reflected by mirrors 6a and 6b, respectively, and is applied to the resist 2 applied on the substrate 1.

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

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

First, the substrate 2 is coated with the resist 2 as shown in FIG. 6 (a), and then the resist 2 is periodically exposed by the two-beam interference exposure method as shown in FIG. 6 (b). At this time, the exposure intensity of the laser beam irradiated on the resist 2 changes periodically as shown in FIG. 6 (c), so that when the exposed resist 2 is developed, the resist 2 is changed to that shown in FIG. 6 (d). The diffraction grating 3a shown is patterned. After that, the substrate 1 is etched by using the patterned resist 2 as a mask to form the diffraction grating 3b as shown in FIG. 6 (e).
Is formed.

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

In the apparatus shown in FIG. 5, the laser light 17a emitted from the laser light source 4 is split into two directions by the half mirror 5 and reflected by the mirror 6 to be multiplexed again on the substrate 1. At this time, the intensity of the light on the substrate is changed by the interference of the laser light. It has a distribution of period Λ represented by. Here, λ represents the wavelength of the laser light, and θ represents the incident angle of the laser light on the substrate.

The conventional diffraction grating is produced by using the above-mentioned principle, and is performed by exposing a resist coated on a substrate at a pitch of a period Λ, and then developing and etching.

FIG. 2 is a sectional structure view in the cavity length direction showing a distributed feedback (DFB) semiconductor laser having both first-order and second-order diffraction gratings. In the figure, p-type InGaAsP guide layer 7, InG
The aAsP active layer 8 and the n-type InP cladding layer 9 are the p-type InP substrate 1 on which the first-order diffraction grating 11 and the second-order diffraction grating 10 are formed.
stacked on top of a.

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

In the DFB laser, the light generated in the active layer is reflected by the diffraction grating and is confined inside the element.Therefore, if the diffraction grating is formed with a uniform pitch throughout the element, the light density in the central part of the element increases. The element characteristics are adversely affected by hole burning and the like. The semiconductor laser of FIG. 2 solves such a problem. Reflection of light by the diffraction grating is more efficiently performed by the low-order diffraction grating. Therefore, as shown in FIG. 2, the diffraction grating 10 in the central portion of the element is replaced by the diffraction grating 11 near the end face of the resonant substrate.
By making the order higher, the reflection intensity of light in the central portion of the device can be made smaller than the reflection intensity of light in the vicinity of the end face of the resonator. As a result, the light generated in the active layer 8 is not confined only in the central portion of the element, and most of the light travels to the vicinity of the cavity end face and is reflected by the diffraction grating 11. Therefore, the light density does not increase only in the central portion of the element, the light density becomes uniform over the entire element, hole burning hardly occurs, and the laser characteristics can be improved. Further, in the present laser, the laser beam can be extracted perpendicularly to the substrate by the secondary diffraction grating 10.

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

The present invention has been made to solve the above problems, and an object thereof is to obtain a method of manufacturing a diffraction grating capable of simultaneously forming diffraction gratings of two types of periods on the same plane by one time of interference exposure. And

[Means for solving the problem]

In the method for manufacturing a diffraction grating according to the present invention, a resist having a development speed extreme with respect to the exposure intensity is applied on a substrate on which the diffraction grating is formed, and the resist is partially covered with a semi-transparent mask. After that, the light intensity is such that the maximum and minimum values of the exposure intensity in only one of the part covered with the semi-transparent mask and the other part become the intensity on both sides of the intensity at which the developing speed becomes the extreme value. Interference exposure is carried out, and then the above substrate is etched using the pattern obtained by developing the resist as a mask to manufacture a diffraction grating.

[Action]

The method of manufacturing a diffraction grating according to the present invention comprises applying a resist having a development rate extreme with respect to the exposure intensity onto a substrate on which the diffraction grating is formed, and further partially covering the resist with a semi-transparent mask. , The light intensity such that the maximum and minimum values of the exposure intensity at both the portion covered by the semi-transparent mask or only that portion becomes the intensity on both sides of the intensity at which the developing speed reaches the extreme value. Since the diffraction grating is manufactured by performing exposure and then etching the substrate using the pattern obtained by developing the resist as a mask, two types of diffraction gratings having different synchronisms are subjected to a single interference exposure. Can be formed at the same time.

〔Example〕

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

FIG. 1 is a sectional process diagram and a light intensity distribution diagram showing a method of manufacturing a diffraction grating according to an embodiment of the present invention. In the figure, a resist (image reversal resist) 12 having a minimum developing speed with respect to the exposure intensity is applied on the substrate 1, and a semitransparent mask 13 is partially arranged on the resist 12.

Next, the manufacturing process of the diffraction grating according to the present embodiment will be described.

First, as shown in FIG. 1 (a), by baking on a substrate 1 on which a diffraction grating is formed at a temperature of 110 to 120 ° C. before exposure, the developing speed has a minimum value with respect to the exposure intensity. Apply a resist (image reversal resist) 12. Next, after baking at a temperature of 110 to 120 ° C. for about 5 minutes, a semitransparent mask 13 is placed on a part of the resist 12 as shown in FIG. Where the mask 13 is not covered, the maximum and minimum values of the exposure intensity are on both sides of the intensity at which the phenomenon speed becomes minimal,
The resist 12 is exposed with an exposure intensity such that the maximum value of the exposure intensity is less than or equal to the intensity at which the phenomenon speed becomes a minimum at the area covered with the semi-transparent mask 13. FIG. 1 (c) is a diagram showing the light intensity distribution at this time. After that, when the resist 12 is developed so that the resist 12 in the portion exposed by the light intensity at which the developing speed is minimized remains, the resist 12 is patterned into the diffraction grating 23a shown in FIG. 1 (d). After that, the substrate 1 is masked with the patterned resist 12 and an appropriate etching solution such as HBr / HNO ₃ / H ₂ O mixed solution, HBr / HNO ₃ / CH ₃ O.
Etching with H mixed solution or Br ₂ / CH ₃ OH mixed solution etc. produces a diffraction grating as shown in Fig. 1 (e).
23b is formed.

Next, the operation of the resist (image reversal resist) which makes the developing speed have a minimum value with respect to the exposure intensity will be described.

It is known that when an image reversal resist is baked at a temperature of 110 to 120 ° C. before exposure, the phenomenon speed has a minimum value with respect to the exposure intensity α as shown in FIG. See the June 1986 issue).

The exposure intensity of the exposure by the interference exposure method has a periodic distribution as shown in FIG. Here, the distribution period is Λ in the equation (1).

Therefore, exposure is performed so that the maximum value of the exposure intensity due to the interference exposure is below the point where the phenomenon becomes the minimum value (point α in FIG. 3) and the minimum value of the exposure intensity becomes below the point α, so that the phenomenon speed becomes the minimum value. It is possible to leave only the resist of the exposed portion with the intensity that makes it possible to form a diffraction grating with half the period of the interference pattern obtained by the interference exposure method by etching after that. Become.

On the other hand, the maximum value of the exposure intensity by interference exposure is less than α point,
Alternatively, when the minimum value of the exposure intensity is α point or more, the resist remains at the period of the interference pattern obtained by the interference exposure, and the diffraction grating having the period of the interference pattern is formed.

Therefore, by covering a part of the substrate with a semi-transparent mask, the maximum value of the exposure intensity where the mask is not covered is α point or more and the minimum value is the α point or less. By setting the maximum value of the exposure intensity to be equal to or less than the point α, the diffraction grating with the period Λ and the diffraction grating with the period Λ / 2 can be simultaneously formed by one interference exposure.

Next, using the method for manufacturing the diffraction grating according to the present embodiment, the second
A process of manufacturing the semiconductor laser shown in the figure will be described.

A p-type Inp substrate 1a is used as the substrate 1 shown in FIG.
A diffraction grating is manufactured on the substrate 1a according to the flow shown in the figure. That is, first, FIG. 1 (a) shows a resist (image reversal resist) which has a minimum development rate with respect to the exposure intensity by baking at a temperature of 110 to 120 ° C. on the substrate. To apply. Then 110-1
After baking at a temperature of 20 ° C., the substrate is partially covered with a semi-transparent mask 13 as shown in FIG. 1 (b), and exposed by the interference exposure method where it is not covered with the semi-transparent mask 13. The maximum value and the minimum value of the intensity are on both sides of the intensity at which the phenomenon speed becomes the minimum, and the exposure is performed so that the maximum value of the exposure intensity becomes equal to or less than the intensity at which the phenomenon speed becomes the minimum, when the semi-permeable mask 13 is covered. . Here, the width and interval of the semi-permeable mask 13 are 300 μm.
The degree. Then, a diffraction grating is formed by etching with an appropriate etching solution. After that, an InGaAsP guide layer 7 having a composition having a bandgap-equivalent wavelength of 1.15 μm, an InGaAsP active layer 8 having a composition having a bandgap-equivalent wavelength of 1.3 μm, and an n-type InP clad layer 9 are sequentially formed, and cleaved at the center of the primary diffraction grating. By separating, the DFB laser shown in FIG. 2 is completed.

In the above-mentioned embodiment, the case where the diffraction grating with the period Λ is formed where it is covered with the semi-transparent mask and the diffraction grating with the period Λ / 2 is formed where it is not covered is shown. By exposing over the entire surface of the resist, the maximum value of the exposure intensity is α point or more and the minimum value is α point or less when covered with the semi-transparent mask, and the minimum exposure intensity is not applied when not covered with the mask. Even when the value is set to be the α point or more, the same effect as that of the above-described embodiment is obtained.

〔The invention's effect〕

As described above, according to the present invention, a resist having a development speed extreme with respect to the exposure intensity is applied on the substrate on which the diffraction grating is formed, and the resist is partially covered with a semi-transparent mask. After that, the light intensity is such that the maximum and minimum values of the exposure intensity in only one of the part covered with the semi-transparent mask and the other part become the intensity on both sides of the intensity at which the developing speed becomes the extreme value. Since the diffraction grating is manufactured by etching the substrate using the pattern obtained by developing the resist as a mask, two types of diffraction gratings having different periods are subjected to the interference exposure once. The semiconductor laser having both the first-order diffraction grating and the second-order diffraction grating can be easily formed.

[Brief description of drawings]

FIG. 1 is a sectional process diagram and exposure intensity distribution diagram showing a method of manufacturing a diffraction grating according to an embodiment of the present invention, and FIG. 2 is a primary diffraction grating near the end face of the resonator and a secondary diffraction grating at the center of the resonator. Sectional side view of DFB laser with diffraction grating, Fig. 3 shows 110-
FIG. 4 is a diagram showing the relationship of the phenomenon speed to the exposure intensity of the image reversal resist after baking at 120 ° C., FIG. 4 is a diagram showing the exposure intensity distribution by the interference exposure method, FIG. 5 is a diagram showing the interference exposure apparatus, FIG. FIG. 6 is a 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 InP substrate, 7 is a p-type InGaAsP guide layer, 8 is an InGaAsP active layer, 9 is an n-type InP clad layer, 10 is a secondary diffraction grating at the center of the resonator, and 11 is a resonator. First-order diffraction grating near the end face, 12 is an image reversal resist, 13
Is a semi-transparent mask, 23a is a diffraction grating by a resist pattern, and 23b is a diffraction grating transferred onto a substrate. The same reference numerals in the drawings indicate the same or corresponding parts.

Claims (1)

(57) [Claims] 1. A step of coating a resist having a development rate extreme with respect to an exposure intensity on a substrate on which a diffraction grating is formed, and partially covering the resist with a semi-transparent mask and then the semi-transparent mask. A step of performing interference exposure with light intensity such that the maximum and minimum values of the exposure intensity in only one of the portion covered with the mask and the other portion become the intensity on both sides of the intensity at which the developing speed becomes the extreme value. And a step of developing and patterning the resist, and a step of etching the substrate using the patterned resist as a mask.