JPH1126878A - Apparatus and method for measuring phase shift amount of diffraction grating - Google Patents

Abstract

(57) [Summary] [PROBLEMS] To provide an apparatus and method for measuring a phase shift amount of a diffraction grating, which can accurately measure a phase shift amount. SOLUTION: A light source 14, a semi-transparent mirror 16 for splitting light emitted from the light source, a reflecting mirror 18 for reflecting a part of the light split by the semi-transparent mirror, and split by the semi-transparent mirror. A supporting means 12 for supporting the diffraction grating so that another part of the light is incident on the diffraction grating to be measured and returns from the diffraction grating to the translucent mirror; A detecting means for simultaneously detecting the light passing through the mirror and the light returned from the diffraction grating and passing through the translucent mirror;

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an apparatus and a method for measuring a phase shift amount of a phase shift diffraction grating used for a distributed feedback semiconductor laser having a phase shift type diffraction grating, for example.

[0002]

2. Description of the Related Art A distributed feedback semiconductor laser having a phase shift type diffraction grating is shown, for example, in FIG. Such a semiconductor laser includes an active layer 1, a light guide layer 2, and a diffraction grating 3. The diffraction grating 3 is formed on the surface of the substrate 3a. The light seeps out of the active layer 1 and propagates, and a part of the light is diffracted by a change in the refractive index in a direction parallel to the active layer 1. Only light having a wavelength matching the period of the diffraction grating 3 is returned to the active layer 1, amplified, and oscillates.

The diffraction grating 3 has a phase shift 4, and the peaks and valleys of the periods of the left and right portions of the phase shift 4 are shifted by π phase. By providing the phase shift 4, light of a specific wavelength is strongly output from the laser. When the phase shift amount deviates from π, the output of light of a specific wavelength decreases. As described above, in the distributed feedback semiconductor laser having the phase shift type diffraction grating, the oscillation wavelength and the single longitudinal mode oscillation probability depend on the amount of phase shift.

[0004] For example, FIGS. 3 and 4 show the relationship between the amount of phase shift, the oscillation wavelength, and the normalized threshold gain difference. The diffraction grating 3 is, for example, as shown in FIG.
The length of the resonator is 300 μm, the length from one end of the resonator to the phase shift 4 is L ₁ , and the length from the phase shift 4 to the other end of the resonator is L ₂ . Normalized coupling coefficient κL = 1.
5

FIG. 3 shows that the phase shift 4 corresponds to the center of the resonator (L ₁
= L ₂ ), and FIG. 4 shows that the phase shift 4 is different from the center of the resonator (L ₁ = 100 μm, L ₂ = 200 μ).
m). As shown in FIG.
Normalized threshold gain difference 0.3 required for single longitudinal mode operation
The allowable range of the phase shift amount to achieve the above is 0.35 to
Although relatively wide at 1.65π, the shift of the oscillation wavelength is large, and the phase shift amount must be suppressed to 0.8 to 1.2π in order to keep the oscillation wavelength deviation within ± 0.2 nm.
Also, taking into account the oscillation wavelength deviation due to the variation of the laser structure, the allowable range of the phase shift amount is 0.9 to 0.9.
1.1π.

On the other hand, as shown in FIG. 4, in order to increase the light extraction efficiency from one side of the laser end face, in the case of a structure in which the position of the phase shift 4 is shifted by 50 μm from the center, it is necessary for the single longitudinal mode operation. The allowable range of the amount of phase shift for achieving a normalized threshold gain difference of 0.3 or more is also 0.
It needs to be suppressed to 9 to 1.1π. By the way, as a method of manufacturing a phase shift type diffraction grating, there are a method of combining a phase shift mask method with a two-beam interference exposure method, and an electron beam direct writing method. In the electron beam method, the amount of phase shift is strictly determined by the setting at the time of writing, but the exposure time is enormously long,
It has the disadvantage of poor productivity. On the other hand, the method of combining the two-beam interference exposure method and the phase shift mask method has much better productivity, but the phase shift amount is not strictly determined due to the setting of the optical system and the variation of the phase shift mask. There is a disadvantage that.

Therefore, inspecting the amount of phase shift of a diffraction grating manufactured by a method combining a two-beam interference exposure method with a phase shift mask method can improve the yield of a distributed feedback semiconductor laser having the phase shift diffraction grating. Required for

[0008]

Conventionally, as shown in FIG. 5, the amount of phase shift is determined by shifting the position of the phase shift 4 of the two phase shift type diffraction gratings 3 and disposing them so as to face each other. Two electron micrographs were taken at a position straddling the shift 4 and measured from the mutual phase relationship between the peak and valley edges. FIG. 6A shows a photograph of the part A in FIG.
FIG. 6B shows a photograph of a portion B in FIG. FIG. 6 (A)
Reads [Phi ₁ from reading the [Phi ₂ from (B) of FIG. 6, it is possible to obtain the phase shift amount as Φ = Φ ₂ -Φ _1.

However, the electron microscope photograph has a high magnification of about 100,000 times, and if the measurement surfaces of the two diffraction gratings 3 are not strictly flush with each other, both diffraction gratings 3 are taken at the time of electron microscope photographing. There was a problem that it was difficult to focus simultaneously. For this reason, even if one diffraction grating 3 is in focus, the other diffraction grating 3 is not in focus, and the position of the edge of one diffraction grating 3 cannot be accurately detected.

As another method of measuring the amount of phase shift, as shown in FIG. 7, a resist diffraction grating 5 having the same period and having no phase shift is formed on a phase shift type diffraction grating 3, and an electron microscope photograph is taken. There is also a method of taking two sheets over the phase shift 4 and measuring from the mutual phase relationship. Also in this case, similarly to the previous method, FIG. 8A shows a photograph of the portion C in FIG. 7, and FIG. 8B shows a photograph of the portion D in FIG.
Φ ₁ is read from FIG. 8A, and Φ ₂ is read from FIG.
And the phase shift amount can be obtained as Φ = Φ ₂ −Φ ₁ . However, in this method, measured values become uncertain due to warpage of the substrate and the like. For example, FIG. 8C shows a photograph of the portion E in FIG. 7, from which Φ ₃ is read. Originally, it should be Φ ₁ = Φ ₃ , but this may not be the case. Then, Φ = Φ ₂ −Φ ₁ will also not be accurate.

An object of the present invention is to provide an apparatus and a method for measuring the amount of phase shift of a diffraction grating, which can accurately measure the amount of phase shift.

[0012]

According to the present invention, there is provided an apparatus for measuring the amount of phase shift of a diffraction grating, comprising a light source, a semi-transparent mirror for splitting light emitted from the light source, and a part split by the semi-transparent mirror. And a supporting mirror for returning a part of the light branched by the translucent mirror to the diffraction grating to be measured and returning from the diffraction grating to the translucent mirror. Supporting means, and detecting means for simultaneously detecting light reflected by the reflecting mirror and passing through the translucent mirror, and light returned from the diffraction grating and passing through the translucent mirror. Is what you do.

In the above configuration, as a result of the reflected light from the reflecting mirror and the diffracted light from the diffraction grating being combined by the translucent mirror, moire fringes are generated due to interference. Focusing on the phase relationship between the diffracted lights on both sides of the phase shift, the diffracted light on the right side of the phase shift is shifted in wavefront phase by the phase shift sandwiched between the diffracted lights on the left side.
When there is no phase shift, the interference condition between the incident light and the reflected light is the same, so that there is no difference in the light and dark of the moiré fringes caused by the interference. Therefore, the phase difference appears as a light-dark phase difference of the moire fringes as it is. Therefore, the phase shift amount can be measured by measuring the light / dark phase difference of the moiré fringes sandwiching the phase shift.

In the method for measuring the amount of phase shift of a diffraction grating according to the present invention, a photoresist is applied on a substrate having a diffraction grating having a phase shift to be measured formed on the surface thereof, and the photoresist is exposed by a predetermined exposure method. Then, the photoresist is developed, and a pattern formed on the photoresist is detected. Also in this case, the pattern of the photoresist becomes moire fringes, and by detecting the moire fringes, the amount of phase shift can be measured.

In the method for measuring the amount of phase shift of a diffraction grating according to the present invention, coherent light is added to the diffracted light of the diffraction grating having the phase shift to be measured, and the moiré is generated from the diffracted light and the interference light. A fringe is generated, and a phase shift amount is detected from the moiré fringe.

[0016]

FIG. 1 is a diagram showing an embodiment of a diffraction grating phase shift amount measuring apparatus 10 according to the present invention. FIG.
, The substrate 3a having the diffraction grating 3 is
It is supported by. The diffraction grating 3 has a phase shift 4. This diffraction grating 3 can be used in a distributed feedback semiconductor laser having the phase shift type diffraction grating shown in FIG.

The measuring device 10 comprises a light source 14 and a light source 1
A semi-transparent mirror 16 for splitting the light emitted from the light source 4 and a reflecting mirror 1 for reflecting a part of the light split by the semi-transparent mirror
8, a support 12 for supporting the diffraction grating 3 so that another part of the light split by the semi-transparent mirror 16 enters the diffraction grating 3 and returns from the diffraction grating 3 to the semi-transparent mirror 16; And a detecting means 20 for simultaneously detecting the light reflected by 18 and passing through the translucent mirror 16 and the light returned from the diffraction grating 3 and passing through the translucent mirror 16.

The light source 14 emits parallel light over a predetermined width. Part of the light that has entered the translucent mirror 16 is reflected by the translucent mirror 16 and proceeds toward the reflecting mirror 18. The light reflected by the reflecting mirror 18 enters the translucent mirror 16 along the same optical path as when entering the reflecting mirror 18, and at least a part of the light passes through the translucent mirror 16. Another part of the light that has entered the translucent mirror 16 passes through the translucent mirror 16 and enters the diffraction grating 3. This light is reflected by the diffraction grating 3 as diffracted light,
The light enters the translucent mirror 16 along the same optical path as when entering the diffraction grating 3, and at least a portion is reflected by the translucent mirror 16. Therefore, the diffraction grating 3 is arranged obliquely with respect to the optical axis connecting the light source and the translucent mirror 16 according to the diffraction angle.

The detecting means 20 comprises an imaging means such as an electron microscope or a photographic device, and detects the reflected light from the reflecting mirror 18 and the diffracted light from the diffraction grating 3 simultaneously. As a result of combining the reflected light from the reflecting mirror 18 and the diffracted light from the diffraction grating 3 with the translucent mirror 16, Moire fringes as shown in FIG. 9 are generated due to interference. As shown in FIG.
Focusing on the phase relationship between the diffracted lights on both sides of the phase shift 4, the diffracted light on the right side of the phase shift 4 has a wavefront phase shifted by the phase shift 4 sandwiched between the diffracted lights on the left side. . When there is no phase shift 4, the interference condition between the incident light and the reflected light is the same, so that there is no difference in the light and dark of the moiré fringes caused by the interference. Because of the difference,
The phase difference appears as a light-dark phase difference of the moire fringes as it is.

FIGS. 11 and 12 are diagrams showing moire fringes generated by interference between the diffracted light on both sides of the phase shift 4 and the reflected light from the reflecting mirror 18. FIG. Thus, by measuring the light / dark phase difference of the moire fringes generated on both sides of the phase shift 4, the phase shift amount can be measured. The period of the pattern of the peaks and valleys of the diffraction grating 3 is 200 to 240 nm, while the moiré fringes can be observed at a period of 1 to 3 mm. Can be observed.

FIG. 13 is a view showing a modification of the phase shift amount measuring apparatus 10 for a diffraction grating. The measuring apparatus 10 also includes a light source 14, a semi-transparent mirror 16 for splitting light emitted from the light source 14, a reflecting mirror 18 for reflecting a part of the light split by the semi-transparent mirror 16, and a semi-transparent mirror 16. A support 12 for supporting the diffraction grating 3 so that another part of the split light is incident on the diffraction grating 3 and returns from the diffraction grating 3 to the translucent mirror 16, and a reflecting mirror 18 and a translucent mirror And a detecting means 20 for simultaneously detecting the light passing through the diffraction grating 16 and the light returned from the diffraction grating 3 and passing through the translucent mirror 16.

In the measuring apparatus 10, the light source 14 is composed of, for example, a He-Cd laser which emits light having a long coherence length at a wavelength at which diffraction light is generated by the diffraction grating 3 to be measured. Further, between the light source 14 and the translucent mirror 16, a convex lens 22 for expanding the light emitted from the light source 14 as necessary for measurement and a convex lens 24 for converting the expanded light into a parallel plane wave are arranged. Is done. Light source 14 and convex lens 22
Suitable mirrors 26 and 28 are arranged between the two.

Further, the support 12 is configured so that the angle of incidence on the diffraction grating 3 can be changed in order to return the diffracted light from the diffraction grating 3 to the translucent mirror 16 in the same optical path as the incident optical path. . In the embodiment, the support member 12 is formed so as to be rotatable in a direction indicated by an arrow, and can correspond to a diffraction angle of the diffraction grating 3. The detecting means 20 is connected to an image processing device 30 including a display. The operation of this example is the same as the operation of the previous example.

FIG. 14 is a diagram showing an example in which the diffraction grating 3 is manufactured by a method of combining the two-beam interference exposure method with the phase shift mask method. As shown in FIG. 1A, an InP substrate 3a is prepared, and a photoresist 32 is applied on the substrate 3a. A phase shift mask 34 is placed on the photoresist 32, and exposure is performed from two opposite directions (the respective incident angles θ10 and θ20) indicated by arrows in the figure. The phase shift mask 34 has a step around its middle.

As shown in FIG. 3B, when the photoresist 32 is developed, a diffraction pattern 32a is formed on the photoresist 32 by interference due to exposure from two directions.
By using the phase shift mask 34 having a step, a phase shift 32b can be formed in the intermediate portion. Next, (C)
As shown in (2), reactive ion etching with ethane gas is performed to transfer the diffraction pattern 32a of the photoresist 32 to the substrate 3a. Next, as shown in (D), when the photoresist 32 is removed, the diffraction grating 3 having the phase shift 4 is formed on the substrate 3a. However, in this method, since the phase shift 4 cannot be formed with strict control as described above, it is desired to measure the amount of phase shift simply and reliably. Further, according to the present invention, the amount of phase shift can be measured simply and reliably.

In one example of manufacturing the diffraction grating 3 having the phase shift 4, the light source 14 is a 325 nm wavelength He-C
d laser, incident angle θ10 = 59 ° 00′54 ″, incident angle θ20 = 48 ° 43′47 ″, phase shift mask step is 2.14 μm, phase shift mask refractive index is 1.475,
The recorded diffraction grating period is 202 nm, and the phase shift amount is π.

FIGS. 15 to 17 are views showing an embodiment of the method of measuring the amount of phase shift of the diffraction grating according to the present invention. In this example, the diffraction grating 3 is manufactured by the manufacturing method described with reference to FIG. The diffraction grating 3 is formed on the surface of the substrate 3a and has a phase shift 4. This diffraction grating 3 can be used in a distributed feedback semiconductor laser having the phase shift type diffraction grating shown in FIG.

In this embodiment, as shown in FIG. 15, after manufacturing the diffraction grating 3 on the substrate 3a, a photoresist 40 is applied to the surface of the diffraction grating 3. then,
The photoresist 40 is exposed by exposure from two directions (without the phase shift mask 34) under the same exposure conditions as described with reference to FIG. Then, as shown in FIG. 16, the photoresist 40 is developed. Then, a peak and valley pattern having the same period as the peak and valley diffraction pattern of the diffraction grating 3 is formed on the surface of the photoresist 40. in this case,
Since the phase shift mask 34 is not used, the pattern of the photoresist 40 has no phase shift. But,
Portion 4 where the exposure amount at the time of exposure is located on both sides of phase shift 4
0a, 40b, the thickness of the portions 40a, 40b located on both sides of the phase shift 4 changes. In FIG. 16, the right portion 40b is thicker than the left portion 40a.

FIG. 17 shows the photoresist 40 formed on the phase shift 4 in this way, and after exposing and developing, the pattern on the surface of the photoresist 40 is detected by an imaging and detecting means 20 such as an electron microscope or a camera. Is shown. The pattern on the surface of the photoresist 40 becomes moire fringes as shown in FIG. 18 due to interference between the diffracted light and the reflected light at the time of exposure. Since the phase is shifted on both sides of the phase shift 4 of the diffraction grating 3, FIG.
As shown in the figure, on both sides of the phase shift 4 of the diffraction grating 3,
There is a difference between the light and dark of the moire fringes caused by the interference, and the phase shift amount of the phase shift 4 can be measured from the phase difference between the light and dark of the moiré fringes.

FIG. 20 is a view for explaining the exposure in FIG. The following relational expression is established based on the relation shown in FIG. Note that the refractive index of air is 1, and the refractive index of the photoresist 40 is n. The wavelength of the light source is λ, and the period of the diffraction grating 3 is Λ. θ10 and θ20 are incident angles in two-way exposure, θ11 and θ21 are refraction angles at the photoresist 40, and θ12 and θ22 are diffraction angles at the diffraction grating 3.

[0031]

(Equation 1)

[0032]

(Equation 2)

[0033]

(Equation 3)

[0034]

(Equation 4)

[0035]

(Equation 5)

[0036]

(Equation 6)

[0037]

(Equation 7)

As described above, when the photoresist 40 is exposed from two directions at incident angles θ10 and θ20, respectively,
The photoresist 40 is exposed by the respective incident light and the reflected light transmitted through the photoresist 40 and reflected by the diffraction grating 3. The reflected light transmitted through the photoresist 40 and reflected by the diffraction grating 3 is a so-called reflected light R1.
0, 210 and reflected diffracted lights D10, D20. The refraction angles θ11 and θ21 are equal to the reflection angles of the reflected light R10 and 210 on the surface of the diffraction grating 3. Therefore, Expressions (6) and (7) show that the diffraction angle of the diffracted light of one incident light is
This indicates that the angle is equal to the reflection angle of the reflected light of the other incident light.

Therefore, the diffracted light of one incident light interferes with the reflected light of the other incident light, and the pattern on the surface of the photoresist 40 becomes moire fringes. As shown in FIG. 19, the moiré fringes are shifted in the portions 40a and 40b on both sides of the phase shift 4 of the diffraction grating 3 corresponding to the phase shifts on both sides of the phase shift 4 of the diffraction grating 3. Therefore, portions 40a, 4a on both sides of the phase shift 4 of the diffraction grating 3
By comparing the 0b moiré fringes, the shift amount of the phase shift 4 can be measured.

In the above embodiment, coherent light is added to the diffracted light of the diffraction grating having the phase shift to be measured, and moire fringes are generated from the diffracted light and the coherent light. The phase shift amount is detected from the moire fringes. In this way, the diffraction grating can be inspected and used for a distributed feedback semiconductor laser having a phase shift diffraction grating.

[0041]

As described above, according to the present invention,
It is possible to provide an apparatus and a method for measuring a phase shift amount of a diffraction grating that can accurately measure a phase shift amount.

[Brief description of the drawings]

FIG. 1 is a diagram showing an embodiment of a diffraction grating phase shift amount measuring apparatus according to the present invention.

FIG. 2 is a diagram illustrating an example of a distributed feedback semiconductor laser having a phase shift diffraction grating.

FIG. 3 is a diagram illustrating a relationship between a phase shift amount, an oscillation wavelength, and a normalized threshold gain difference in the laser of FIG. 2;

FIG. 4 is a diagram illustrating a relationship between a phase shift amount, an oscillation wavelength, and a normalized threshold gain difference when a position of a phase shift is changed.

FIG. 5 is a diagram showing an example of a conventional measurement method.

6 is a partially enlarged view of FIG. 5, (A) is an enlarged view of a part A of FIG. 5, and (B) is an enlarged view of a part B of FIG. 5;

FIG. 7 is a diagram showing another example of a conventional measurement method.

8 is an enlarged view of a part of FIG. 7, (A) is an enlarged view of a part C of FIG. 7, (B) is an enlarged view of a part D of FIG. 7,
FIG. 8B is an enlarged view of a portion E in FIG.

FIG. 9 is a diagram showing moire fringes generated using the apparatus of FIG. 1;

FIG. 10 is an enlarged view of a diffraction grating.

11 is a diagram showing an example in which the moiré fringes of FIG. 9 are formed on both sides of the phase shift.

FIG. 12 is an enlarged view of FIG. 11;

FIG. 13 is a diagram showing a modification of the apparatus of FIG. 1;

FIG. 14 is a diagram illustrating an example of a method for manufacturing a diffraction grating.

FIG. 15 is a diagram showing an embodiment of a method for measuring a phase shift amount of a diffraction grating according to the present invention.

FIG. 16 is a view showing a step of developing the photoresist of FIG. 15;

FIG. 17 is a view showing a step of detecting moiré fringes from above the photoresist of FIG. 15;

FIG. 18 is a diagram showing moire fringes detected in FIG. 17;

FIG. 19 is a diagram showing an example in which the moiré fringes of FIG. 18 are formed on both sides of the phase shift.

FIG. 20 is a view showing an exposure method in the photoresist exposure step of FIG. 14;

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Active layer 2 ... Light guide layer 3 ... Diffraction grating 3a ... Substrate 4 ... Phase shift 12 ... Supporter 14 ... Light source 16 ... Translucent mirror 18 ... Reflecting mirror 20 ... Detecting means 22 ... Convex lens 24 ... Convex lens 30 ... Image processing Apparatus 32 Photoresist 34 Phase shift mask 40 Photoresist

Claims (7)

[Claims] 1. A light source, a semi-transparent mirror for splitting light emitted from the light source, a reflecting mirror for reflecting a part of light split by the semi-transparent mirror, and split by the semi-transparent mirror A supporting means for supporting the diffraction grating so that another part of the light is incident on the diffraction grating to be measured and returns from the diffraction grating to the translucent mirror, and is reflected by the reflecting mirror and passes through the translucent mirror. And a detecting means for simultaneously detecting the reflected light and the light returned from the diffraction grating and passing through the translucent mirror. 2. The light source emits light having a long coherence length at a wavelength at which diffracted light is generated by a diffraction grating to be measured. Between the light source and the translucent mirror, the light emitted from the light source is measured. The apparatus for measuring the amount of phase shift of a diffraction grating according to claim 1, further comprising an optical system that expands as necessary and converts the plane wave into a plane wave. 3. A diffraction grating for returning the diffracted light from the diffraction grating to the semi-transparent mirror in the same optical path as the incident optical path of the other part of the light split by the semi-transparent mirror. The phase shift amount measuring apparatus for a diffraction grating according to claim 1, wherein the apparatus is configured to be able to change an incident angle with respect to. 4. A photoresist is applied on a substrate having a diffraction grating having a phase shift to be measured formed on a surface thereof, the photoresist is exposed by a predetermined exposure method, and the photoresist is developed. A method for measuring a phase shift amount of a diffraction grating, comprising detecting a pattern formed on a resist. 5. The diffraction grating is formed by irradiating light from two directions, and the predetermined exposure method of the photoresist is to expose from the same two directions as when exposing the diffraction grating. 5. The method for measuring the amount of phase shift of a diffraction grating according to claim 4, wherein: 6. A coherent light is added to diffracted light of a diffraction grating having a phase shift to be measured, and moire fringes are generated from the diffracted light and the coherent light. A method for measuring the amount of phase shift of a diffraction grating, comprising detecting an amount. 7. The method according to claim 6, wherein a phase shift amount of the phase shift diffraction grating used in the distributed feedback semiconductor laser having the phase shift type diffraction grating is measured. .