JPH0690323B2 - Method of manufacturing diffraction grating - Google Patents

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 for manufacturing a diffraction grating on a semiconductor substrate.

(Prior Art) In recent years, research and development of single-axis mode semiconductor lasers have been actively conducted as a light source of a long-distance, large-capacity optical transmission system. Among them, the distributed feedback (DFB) semiconductor laser is easy to manufacture compared to other single-axis mode semiconductor lasers because of its stability of single axis mode controllability and wide temperature range. It is getting attention.

The DFB laser has a structure with a uniform diffraction grating inside the device, and oscillates at a single wavelength near the Bragg wavelength determined by the period of this diffraction grating. However, in the conventional DFB laser having a uniform diffraction grating, there are two closely oscillating modes that are symmetric with respect to the Bragg wavelength.
This means a biaxial mode operation, which is not preferable for a single axis mode laser. Usually, a DFB laser has a structure in which one end has a concave surface and the other end has a non-reflective coating that suppresses edge reflection in order to suppress Fabry-Perot mode. As described above, the reflection mirror loss characteristic of the DFB laser having the asymmetric end face reflectance is asymmetric with respect to the Bragg wavelength, and one longitudinal mode tends to be selected. However, since the loss characteristic of the reflector significantly changes depending on the position of the end reflector on the diffraction grating,
In the conventional DFB laser, many elements oscillate in two modes. Therefore, in order to eliminate such instability, a prototype DFB laser having a structure in which the period of the diffraction grating is shifted by λg / 4 (where λg is the wavelength of light propagating in the device) inside the DFB laser has been manufactured. The DFB laser having such a structure is called a λ / 4 shift type DFB laser.

The λ / 4-shift type DFB laser is characterized in that it oscillates in a single axial mode that completely matches the Bragg wavelength.
For this reason, the number of elements that oscillate in the biaxial mode found in the conventional elements is extremely small, which is very promising in terms of yield improvement. By the way, the period Λ of the DFB laser diffraction grating
Has the following relationship with the oscillation wavelength λ.

Here, n _e is an equivalent refractive index in the device, m is the order of diffraction gratings, lambda] g is the wavelength of light propagating in the element. Therefore,
The period of the first-order diffraction grating (m = 1) is Given in. From this, it is understood that the λ / 4 shift type diffraction grating using the first-order diffraction grating may be obtained by inverting the concavities and convexities of the left and right diffraction gratings with the shift region as a boundary. Conventionally,
The following four methods have been proposed as a method of manufacturing a diffraction grating for obtaining such a diffraction grating.

As a first manufacturing method, a novolac positive photoresist and a cyclized rubber negative photoresist are overcoated, and a λ / 4 shift diffraction grating is formed by utilizing the reverse photosensitivity of both photoresists. The method that has been realized is given. The literature on this method is Utaka et al., Electronics Letters, Vol. 2, November 22, 1984.
The papers listed in Vol. 0, No. 4, pages 1008 to 1010 can be cited.

As a second method, an intermediate layer is sandwiched between a novolac-based positive photoresist and a negative photoresist having a photosensitivity opposite to that of the positive photoresist, so that the positive photoresist and the negative photoresist are mixed. A method of realizing a λ / 4 shift type diffraction grating by performing interference exposure so as not to match is given. As a literature of this method, the paper described in Utaka et al., Proc. 2a-N-9, p. 202, Proceedings of the 46th Joint Symposium on Applied Physics in Autumn 1985 can be cited.

As a third method, there is a method in which a quartz contact mask having a gap corresponding to a phase shift is brought into close contact with a photoresist-coated semiconductor substrate to perform interference exposure. As a literature of this method, there can be mentioned the papers described by Shirasaki et al., Technical Report OQE 85-60, 57-64, IEICE.

A fourth method is a method in which a quartz contact mask having a step corresponding to the phase shift is incorporated in the optical system. The literature of this method is as follows: Proceedings of Tsuji et al.
2a-N-10, page 22 can be cited.

(Problems to be Solved by the Invention) The above-described conventional method for manufacturing a diffraction grating has the following drawbacks. That is, in the first method, since a cyclized rubber type photoresist is used as the negative type photoresist, there is a drawback that the resolution is poor and a diffraction grating having a good shape with good reproducibility cannot be manufactured. .

In the second method, since a novolac-based photoresist is used as the negative photoresist, the resolution is good, but the number of man-hours is large, such as two development processes and two etching processes.
Moreover, there are drawbacks that the production conditions are very strict and the yield is low. In the third method, there is a problem in adhesion between the contact mask and the semiconductor substrate, and it is difficult to form the diffraction grating on the entire surface of the semiconductor substrate. In the fourth method, the diffraction grating formed area was extremely small, about 7 mm ² (circle with a radius of 1.5 mm) due to aberration, and there was a drawback that mass productivity was lacking.

An object of the present invention is to provide a diffraction grating having a good shape as a diffraction grating for use in a λ / 4 shift type DFB laser and having a deep groove, that is, a large coupling constant, with good yield over the entire surface of a semiconductor substrate. It is to provide a manufacturing method of.

(Means for Solving the Problems) In the method for manufacturing a diffraction grating according to the present invention, after the negative photoresist thin film is selectively formed on the photoresist applied to the entire surface of the substrate, the normal line of the substrate is applied. In a method of manufacturing a diffraction grating in which two laser beams are irradiated from a symmetrical angle with respect to a tilted direction as a central axis to perform interference exposure, mixing or reaction of the photoresist and the negative photoresist thin film is prevented. The method is characterized by including a step of forming an intermediate layer having a role between the photoresist and the negative photoresist thin film.

(Operation) The principle of the present invention will be described. An object of the present invention is to form a diffraction grating having a good shape and a deep groove on the entire surface of a semiconductor substrate with a high yield as described above. Then, how can we form a diffraction grating with a good shape and a deep groove? For that purpose, a photoresist having a good resolution may be used. The method of the present invention succeeded in eliminating the drawbacks of the first method by using only one type of photoresist having good resolution.

Next, how can we make a diffraction grating with high yield? For that purpose, it is desirable that the development and etching steps be reduced to once, and that the development and etching conditions have a margin. In the method of the present invention, since one type of photoresist is used, the development and etching steps are each performed once, and compared with the second method using two types of photoresist, the development and etching conditions are It has the advantage of high tolerance and solves the problem of the second method.

Finally, in terms of forming a diffraction grating on the entire surface of the substrate, since the method of the present invention forms a film on a photoresist, the adhesion problematic in the third method is very good. Thus, the third drawback is overcome. Further, in the method of the present invention, the optical system is formed by only a mirror, a lens, a beam splitter, and a beam expander,
The formation area of the diffraction grating is not limited by the influence of aberration.

That is, the defect existing in the fourth method does not exist in the method of the present invention.

Now, the principle of the present invention will be described below by using mathematical expressions. 1
In order to fabricate a λ / 4 shift type diffraction grating using different types of photoresist, it is necessary to obtain a state in which the contrast of the interference fringes caused by the interference of two laser beams used for exposure is reversed between the two regions. I have to. Such a state is 1
The difference Δl between the optical path difference Δl ₁ of the two laser beams entering one area and the optical path difference Δl ₂ of the _two laser beams entering the other area is Will be realized when the relationship is satisfied. However, here, λe is the wavelength of the laser light. This concrete method is the second
Shown in the figure. After applying the photoresist 2 on the semiconductor substrate 1, the refractive index n ₂ and the thickness are set so as to cover the photoresist 2.
A thin film 3 (intermediate layer) of d ₂ is formed. A negative photoresist thin film 4 (hereinafter referred to as a dielectric film) having a refractive index n ₁ and a thickness d ₁ is partially formed on the film 3. By doing this, unlike the case where a contact mask is used, the adhesion is sufficiently good,
The diffraction grating can be formed on the entire surface of the semiconductor substrate. The semiconductor substrate 1 is tilted by an angle ψ, and two laser beams with an incident angle θ are irradiated onto the horizontal plane from above to perform interference exposure. In FIG. 2, the broken line indicates 2 when the dielectric film 4 does not exist.
The optical paths of the two laser lights are shown, and the solid line shows the optical paths of the two laser lights when the dielectric film 4 is present. Now, assuming that the dielectric film 4 exists, assuming that the left and right optical path lengths are equal at the point C where the two laser beams intersect and the lights are mutually strengthened (Δl ₁ = 0), + n ₁ + n ₂ = n ₂ + n ₁ (1) However, here, in order to simplify the calculation, the point F is determined so that the light phase at the point F becomes equal to the light phase at the point H. That is, from equation _{(1) = n 1 (-} ) + n 2 (-) ... and to determine the point F so (2). Here, as can be seen from FIG. Is. In Expression (3), θ ₁ and θ ₂ are incident angles of the laser light from the left with respect to the film and the horizontal plane when entering the photoresist 2, and θ ₄ and θ ₃ are laser light from the right to the film 3 and the photo, respectively. It is an incident angle with respect to a horizontal plane when entering the resist 2. Substituting equation (3) into equation (2), Becomes Next, when the dielectric film 4 is not present, it is assumed that the difference in optical path length between the two laser beams is λe / 2 (Δ1 ₂ = λe /
2). By doing so, Δ1 = | Δ1 ₁ −Δ1 ₂ | = λe / 2 is satisfied, and the brightness and darkness of the interference fringes are inverted in the region where the dielectric film 4 is present and the region where the dielectric film 4 is not present. You can get the status. Expressing the above assumptions as an equation, Becomes However, here, it is = + cos ((theta)-(theta) ₁ ) (6-a) = cod ((theta)-(theta) ₄ ) (6-b). Substituting equations (3), (4), and (6) into equation (5) and rearranging Therefore, the film thickness d ₁ of the dielectric film 4 is determined by the equation (7). Also, from Snell's law, sin (θ−Ψ) = n ₁ sin (θ ₁ −Ψ) = n ₂ sin (θ ₂ −
Ψ) (8) sin (θ + Ψ) = n ₁ sin (θ ₄ + Ψ) = n ₂ sin (θ ₃ +
Ψ) (9) and from equations (8) and (9) Becomes From equations (7), (10), and (11), the refractive index n ₁ , the incident angle θ of the laser beam with respect to the horizontal plane, the tilt angle Ψ of the substrate 1,
When the wavelength λe of the laser light is determined, the film thickness d _{1 of the} dielectric film 4
Can be determined. In this case, the period Λ of the diffraction grating is Becomes Figures 3 and 4 show Λ = 2000Å and 2400Å respectively.
Angle Ψ of the substrate 1 and the dielectric film 4 for manufacturing the λ / 4-shifted diffraction grating of 3D using the He-Cd laser light of wavelength 3250Å
FIG. 3 is a diagram showing the relationship between the film thickness d ₁ and the incident angles θ of the left and right laser beams. However, the refractive index n ₁ = 1.55 is set here. When the thickness of the dielectric film 4 is 2 μm, from the figure, when Λ = 2000Å, θ = 54.6 °, Ψ = 5.0 °, and when Λ = 2400Å, θ = 4.
It can be seen that 3.0 ° and Ψ = 7.0 are sufficient.

(Example) Hereinafter, the Example of this invention is described in detail using drawing.
FIGS. 1 (a) to 1 (f) are views for explaining a method of manufacturing a diffraction grating according to an embodiment of the present invention in the order of steps thereof. In the step shown in FIG. 1A, an ODUR-120 film 2 (manufactured by Tokyo Ohka Co., Ltd.), which is a novolac-based negative photoresist, is applied on an InP semiconductor substrate 1 so as to have a film thickness of 500 Å. In the step shown in FIG. 1B, a silicon nitride film is formed as an intermediate layer 3 so as to cover the ODUR-120 film 2 by using an ECR deposition device (manufactured by Nichiden Anelva). In the step shown in FIG. 1C, an NNR film 4 (manufactured by Nagase), which is a cyclized rubber-based negative photoresist, is formed on the silicon nitride film 3 to a thickness of 2 μm.
After being coated so as to have a thickness of m, it is covered with a 300 μm pattern Cr mask and exposed to mercury. After exposure to mercury, the NNR film is developed with an NNR-dedicated developer (made by Nagase Co., Ltd.), and then interference exposure is performed using two laser beams having a wavelength of 3250Å at an incident angle of 54.6 ° with respect to a horizontal plane and a substrate inclination angle of 5.0 °. This is λ = 2000Åλ / 4
It corresponds to the shift diffraction grating. When making a λ / 4 shift diffraction grating with Λ = 2400Å, two laser beams with a wavelength of 3250Å are used to perform interference exposure at an incident angle of 43.0 ° to the horizontal plane and Ψ = 7.0 °. In the step shown in FIG. 1D, the NNR film 4
(Manufactured by Nagase) is removed using a mixed solution of H ₂ O: H ₂ SO ₄ : H ₂ O ₂ = 1: 5: 1 (30 ° C.), and then the silicon nitride film 3 is subjected to HF 7% buffered fluoride. Remove with acid. In the step shown in FIG. 1 (e), ODUR-120 film 2 (manufactured by Tokyo Ohka Co., Ltd.) is developed using a developer for ODUR (manufactured by Tokyo Ohka), and then rinse solution for ODUR (manufactured by Tokyo Ohka). Rinse with. In the step shown in FIG. 1 (f), the InP semiconductor substrate 1 is subjected to HBr: H ₂ O using the periodically formed ODUR-120 film 2 as an etching mask.
_The diffraction grating 5 could be obtained by etching with a mixed solution of ₂ : H ₂ O = 10: 0.1: 100.

Next, an example in which a film other than the silicon nitride film is used as the intermediate layer 3 will be described. In the step shown in FIG. 1 (a),
As in the above-mentioned embodiment, the novolac photoresist film 2 is applied on the InP semiconductor substrate 1 so as to have a film thickness of 1500 Å. In the step shown in FIG. 1B, an OBC film (manufactured by Tokyo Ohka Co., Ltd.) is applied as the intermediate layer 3 so as to cover the novolac photoresist film 2 so as to have a film thickness of 1 μm. In the step shown in FIG. 1 (c), a NONCRON film 4 (manufactured by Tokyo Ohka Co., Ltd.), which is a water-soluble photoresist, is formed on the OBC film 3 with a film thickness of 0.8 μm.
After being coated so as to have a thickness of m, it is covered with a 300 μm pattern Cr mask and exposed to mercury. After exposure to mercury, the NONCRON film with water 4
Is developed, and interference exposure is performed using two laser beams with a wavelength of 3250Å. In the step shown in FIG. 1D, the OBC film 3 and N
The ONCRON film 4 is removed at the same time. In the step shown in FIG. 1E, the novolac-based photoresist film 2 is developed and then rinsed. In the step shown in FIG. 1F, the InP semiconductor substrate 1 was etched using the novolac photoresist film 2 formed in a periodic pattern as an etching mask, and the diffraction grating 5 could be obtained. Thus, this time, it was found that the method of the present invention is effective by the two examples.

Here, a silicon nitride film or an OBC film is used as the film 3 capable of suppressing the mixing or reaction of the photoresist film 2 and the dielectric film 4, but other than this, for example, a photoresist film, a SiO ₂ film, It may be an amorphous silicon film or the like. Although the NNR film, which is a cyclized rubber type negative photoresist, and the water-soluble photoresist NONCRON are used as the dielectric film 4 here, other negative photoresist films may be used.

(Effects of the Invention) In the method of manufacturing a diffraction grating of the present invention, since one type of photoresist 2 is used by being exposed in stripes, the diffraction grating 5 having the same shape on both sides of the transition region and having a good shape is used. Can be obtained over the entire surface of the semiconductor substrate. In addition, the dielectric film 4
By inserting the thin film 3 between the photoresist 2 and the photoresist 2, the dielectric film 4 can be formed or removed without adversely affecting the photoresist 2.

In the method of manufacturing the diffraction grating using the contact mask,
Although the adhesion between the photoresist and the contact mask becomes a problem, it is not necessary to care about such adhesion in the method of manufacturing the diffraction grating of the present invention. As described above, the present invention has three effects.

According to the present invention, it was possible to obtain a saw-tooth diffraction grating with a period of 2000Å and a depth of 1000Å and a saw-like diffraction grating with a period of 2400Å and a depth of 1200Å without irregularities or bridges. When a semiconductor laser was manufactured using this diffraction grating, the threshold current was reduced by several mA compared with the diffraction grating formed by the conventional method, and the yield was improved to 98%. The method proved to be effective.

[Brief description of drawings]

1A to 1F are process drawings of an embodiment of the present invention. In FIG. 1, 1 is a semiconductor substrate, 2 is a photoresist, 3 is an intermediate layer, 4 is a dielectric film, and 5 is a diffraction grating. FIG. 2 is a diagram for explaining the principle of the method of manufacturing a diffraction grating according to the present invention. The numbers in the figure correspond to the numbers in FIG. Figures 3 and 4 show a cycle of 2000Å and cycle, respectively.
FIG. 6 is a diagram showing a relationship between a substrate inclination angle Ψ for obtaining a 2400 Å λ / 4 shift diffraction grating, a thickness d _{1 of} a dielectric film, and an incident angle θ of laser light with respect to a horizontal plane.

Claims (1)

[Claims] 1. A negative photoresist thin film is selectively formed on the photoresist coated on the entire surface of the substrate.
A method of manufacturing a diffraction grating, which comprises performing interference exposure by irradiating two laser beams from a symmetrical angle with a central axis being a direction inclined with respect to a normal line of the substrate, wherein the photoresist and the negative photoresist thin film are provided. A method of manufacturing a diffraction grating, comprising the step of forming an intermediate layer having a function of preventing mixing or reaction with the photoresist between the photoresist and the negative photoresist thin film.