JP2876546B2 - Method of forming diffraction grating and semiconductor device using the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for forming a diffraction grating, and more particularly to a method for forming a phase shift type diffraction grating.

[0002]

2. Description of the Related Art As a light source for optical communication, a distributed feedback (DFB) laser or a distributed Bragg reflection (DBR) laser using a diffraction grating as a reflector of a laser resonator has a stable dynamic single mode. Due to its characteristics, research and development are being actively conducted today.

[0003] A few years ago, a λ / 4 shift DFB laser was developed to improve these high dynamic unity. Since the λ / 4 shift type DFB laser oscillates in one axis mode completely matching the Bragg wavelength, the number of elements oscillating in the two-axis mode, which has been a problem in the conventional structure, is reduced.

The structure for realizing the λ / 4 shift effect can be roughly classified into the following two methods. (1) The phase is shifted by λ / 4 during the period of the diffraction grating. (2) Partially adjust the propagation constant of the waveguide to shift the phase of light equivalently.

[0005] Of these, relatively simple realization can be realized by applying a general diffraction grating preparation technique (1).
This is the method. Conventionally known methods for producing a phase shift type diffraction grating are roughly divided into the following five methods. The outlines of these five methods are shown in FIGS.

The first method is the simplest direct writing method using an electron beam (FIG. 7). The second method is a method of performing interference exposure using the reverse photosensitivity of a positive resist and a negative resist (FIG. 8). First, ONNR which is a negative resist, OBC which is a buffer (anti-interference agent), and AZ which is a positive resist are sequentially laminated on an InP substrate.
After exposure and development by photolithography, the exposed portion of the AZ layer is selectively removed, and using this as an etching mask, patterning is performed with an etchant (H ₂ SO ₄ ). After removing the remaining AZ layer, AZ and PVA (sensitivity prevention) are removed. Are again laminated and holographically exposed to produce a phase-shifted diffraction grating. The third method is a method in which a step corresponding to a phase shift is provided on a quartz plate, and the substrate is brought into close contact with the substrate to perform interference exposure (FIG. 9). The fourth method is a method in which a quartz plate having a step corresponding to a phase shift is incorporated into an optical system as a phase mask to perform interference exposure (FIG. 10). The fifth method is a method in which a part is inverted and transferred to a substrate using a SiN _x film by ECR-CVD using ordinary interference exposure (FIG. 11).

[0007]

However, the above-mentioned conventional methods of manufacturing a phase shift type diffraction grating include:
Each has the following problems.

That is, in the first method, a fine pattern can be directly drawn with high accuracy, but the proximity effect (grating interval (for example, 0.25 μm)) generated between adjacent gratings can be obtained.
A diffraction grating having a period of 0.3 μm or less is drawn on a thick resist or a thick substrate due to back scattering due to electron beam diameter (for example, 0.1 μm) and electron beam scattering on the substrate surface coated with the resist. It is difficult to do
There are problems such as low throughput and low yield.

The second method requires a complicated preparation process, and it is difficult to control the conditions due to differences in exposure conditions or development conditions between positive resists and negative resists having different sensitivities, poor reproducibility, and low yield. There is a problem that is bad.

In the third method, if the contact mask (phase mask) is not sufficiently adhered to the photoresist film coated on the substrate surface, the phase transition region is formed by Fresnel diffraction of a light beam irradiated for interference exposure. There is a problem of spreading.

In the fourth method, a phase plate, which is a quartz plate having a step corresponding to a phase shift, is incorporated in an optical system to project this phase shifter obliquely (a sample surface on which two light beams are obliquely projected). (Because it is necessary to form an image on the sample, the projection becomes oblique.) There is a problem that an aberration is generated on the sample surface, and the formation area of the diffraction grating is limited.

In the fifth method, it is difficult to control the etching conditions of the SiN _x film on the resist and the substrate, and it is necessary to perform the etching of the substrate twice. However, there is a problem that the dimensional accuracy of the depth varies.

Accordingly, an object of the present invention is to solve these problems in the prior art method of manufacturing a phase shift type diffraction grating, and to form a phase shift type diffraction grating on a substrate by a simple manufacturing method. An excellent diffraction grating that performs actual monitor observation by diffraction efficiency, achieves pattern dimensional accuracy and pattern shape, depth stability, and controllability by reliable formation, and can obtain a diffraction grating with deep grooves. And a semiconductor device using the same.

[0014]

According to the present invention, a diffraction grating having a reference period is formed, and the diffraction grating is coated with a different material different from the grating material. It is characterized in that a diffraction grating having a period that is positively inverted from the initial period is formed by etching including an etching step.

Further, a step is provided between a region where a diffraction grating having a reference period is formed and a region where the phase is reversed, and a diffraction grating is formed on the entire substrate including the step region by two-beam interference exposure. Is characterized in that, after coating with a different material different from the diffraction grating material so as to have different film thicknesses on both sides, the inverted material is formed by simultaneously etching the different material and the grating forming material. Further, the present invention provides a semiconductor device or device including a diffraction grating formed by the above-described method for forming a diffraction grating.

[0016]

Embodiment 1 Hereinafter, an embodiment of the present invention will be described with reference to the drawings. Figure 1,
2 and 3 are process schematic diagrams showing a method for producing a diffraction grating of the present invention.

[0017] First of all, for example, using the semiconductor as a substrate 1, which was washed with detergent, followed after repeated ultrasonic cleaning with an organic solvent 2-3 times, and dried with N ₂ blowing, 200 Heat treatment at 30 ° C. for 30 minutes.

Next, a positive photoresist for ordinary photolithography (trade name: AZ-1370: manufactured by Hoechst) is applied to the entire substrate, and a positive photoresist layer 2 is formed on the substrate 1. Drying is performed at 90 ° C. for 25 minutes (FIG. 1A).

Next, in order to form a phase region and a phase shift region, mask exposure is performed by a photomask using a normal photolithography technique. Subsequently, development and rinsing are sequentially performed with a dedicated developer for the positive photoresist layer 2 and ion-exchanged water to form a resist pattern 3 (FIG. 1B).

Subsequently, using the resist pattern 3 formed by the above method as an etching mask, wet or dry etching is performed on the semiconductor substrate 1 to transfer the pattern. (FIG. 1 (c)) At this time, the etching depth is preferably the same as the desired diffraction grating depth or more preferably. Further, the etching mask is removed to obtain a pattern for forming a phase region and a phase shift region (FIG. 1D).

Then, a positive photoresist (trade name: Microposit-1400-17: manufactured by Shipley Co.) for interference exposure is desirably applied to the entire surface of the substrate 1 to form a positive photoresist layer 6. Is formed on the substrate 1 and then dried at 80 ° C. for 20 minutes. At this time, the thickness of the formed positive photoresist layer 6 is 600 to 800.
Å position is appropriate.

Subsequently, a He-Cd laser (λ = 325 n
m) as sufficiently parallel rays, the two laser beams 4,
The positive photoresist layer 6 is periodically exposed by irradiating the positive photoresist layer 6 in two directions (in the illustrated example, two directions forming an angle of θ with respect to the perpendicular) in the illustrated example. (FIG. 2 (e)) Thereafter, a dedicated developer for the positive photoresist layer 6 (trade name: Microposit 450 Developer: Shipley Co., Ltd.)
And develop. As a result, a diffraction grating pattern 7 of a positive type photoresist is formed on the substrate 1 in accordance with the degree of sensitivity based on the intensity distribution of the interference fringes generated by the two-beam interference exposure (FIG. 2F).

Subsequently, using the diffraction grating pattern 7 of the positive type photoresist as an etching mask, the semiconductor substrate 1 is wet-etched, for example, H ₃ PO ₄ -based or N-type.
The semiconductor diffraction grating 8 is obtained on the substrate 1 by performing etching using an H ₄ OH-based etchant and, in the case of dry etching, using a Cl ₂ -based gas, and removing the etching mask layer (FIG. 2 (g)).

Next, there will be described a step of forming a phase shift diffraction grating having a phase opposite to the phase of the semiconductor diffraction grating 8. First, the substrate 1 having the semiconductor diffraction grating 8 is repeatedly subjected to ultrasonic cleaning with an organic solvent two to three times, dried by N ₂ blow, and subjected to a heat treatment at 90 ° C. for 30 minutes. Subsequently, a photoresist (trade name: AZ-1350J: manufactured by Hoechst) diluted to a desired value is applied to the entire surface of the semiconductor substrate 1 as the etching mask layer 9, and the etching mask layer 9 is formed at 80 ° C. for 20 minutes. Perform drying. At this time, the thickness of the photoresist to be formed is controlled by controlling the steps for forming the phase region and the phase shift region, the viscosity of the photoresist, and the number of rotations of the coating by the spinner performed in FIGS. The thickness of the region where the phase shift diffraction grating is formed is as small as possible, and the thickness of the region where the phase shift diffraction grating is formed is as large as possible (FIG. 2 (h)).

Next, etching is performed for an appropriate time by dry etching using reactive ion beam etching (RIBE) using Cl ₂ gas. The conditions at this time were as follows: ultimate vacuum 2.0 × 10 ⁻⁶ Torr, Cl ₂ flow rate 1
5 SCCM, etching pressure 7.5 × 10 ^-4 Torr ~
The etching rate of the semiconductor substrate 1 under this etching condition is 1.5 × 10 ⁻³ Torr, the ion extraction voltage is 400 V, and the selectivity with the resist mask (etching mask) 9 is approximately 4: 4. It was one.

In the above-mentioned etching, the etching mask layer 9 applied and formed on the entire surface is first etched and reduced during the etching process (FIG. 2H).
(See (i)), when the lower semiconductor substrate 1 is exposed,
Depending on the selectivity (ratio of the etching rate) between the etching mask layer 9 and the semiconductor substrate 1, the etching rate is determined at the boundary between the etching mask layer 9 and the semiconductor substrate 1,
Unique etching proceeds (FIG. 2 (i), FIG. 3)
(See (j), (k) and (l)).

The detection of the end point of the etching will be described with reference to FIGS. 1 to 3 and FIG.

That is, a photoresist is applied and formed as an etching mask layer 9 on the entire surface of the substrate 1 of the semiconductor diffraction grating 8, and the substrate 1 is irradiated with a predetermined laser beam.
The irradiated laser light generates diffracted light by the diffraction grating of the substrate 1. It can be detected by performing etching while monitoring the intensity of the diffracted light. In the process of etching, that is, in the process of forming an inverted phase from the original phase, the intensity of the diffracted light decreases as shown in FIG. This is a process of forming a lattice, and can be controlled by detecting the maximum time as the end point.

FIG. 4 shows FIGS. 2 (h), (i) and 3 (j).
(L) shows the correspondence of the diffracted light intensity in the process of forming the inverted phase diffraction grating. Thus, a phase shift diffraction grating 10 having a phase inverted from the phase of the semiconductor diffraction grating 8 created first is formed.

Finally, the remaining etching mask layer 9 in the phase region is removed to obtain a desired phase shift diffraction grating 10 on the semiconductor substrate 1 (FIG. 3 (m)).

As described above, in the present embodiment, a simple method is used in which a photoresist is applied to the entire surface of the diffraction grating 8 obtained by the conventional holographic interference method, and dry etching is performed on the entire surface without exposure and development. According to the method, a phase shift type diffraction grating (a phase inversion of the initial diffraction grating) can be obtained accurately and stably.

Thus, the accuracy and stability are good, for example, DF
A phase shift type diffraction grating can be manufactured for a B-laser or the like, and it has been possible to overcome a decrease in the yield of an element that oscillates in two modes, which was observed in a conventional DFB laser having a uniform diffraction grating structure.

Embodiment 2 Hereinafter, a second embodiment of the present invention will be described. FIG.
It is explanatory drawing which shows the manufacturing method when this invention is applied to a ridge type DFB semiconductor laser.

For example, an n-type GaAs buffer layer 12 (thickness 0.5 μm) and an n-type AlGaAs cladding layer 13 (thickness 1.5 μm) are formed on an n-type GaAs substrate 11 by a molecular beam epitaxy (MBE) apparatus or the like. , Active layer 14 (AlG
aAs MQW, AlGaAs bulk, GaAs bulk, etc .: thickness 0.1 μm), p-type AlGaAs barrier layer 15
(Thickness: 0.1 μm), p-type AlGaAs light guide layer 16
(Thickness: 0.2 μm) is epitaxially grown in this order.

Next, the p-type AlGaAs light guide layer 1
1 (a) to (d), FIGS. 2 (e) to (i), and FIGS. 3 (j) to (m), a phase shift type diffraction grating 17 having a desired pitch is formed on the substrate 6. .

Subsequently, a p-type AlGaAs cladding layer 18 (1.5 μm thick) and a p-type GaAs cap are formed on the p-type AlGaAs optical guide layer 16 on which the phase shift type diffraction grating 17 is formed by, for example, a liquid phase growth method. Layer 19 (thickness 0.
3 μm) is again epitaxially grown in this order.

Next, after patterning the ridge by photolithography, etching is performed, a silicon nitride film 20 is formed by plasma CVD, and only the ridge of the ridge is etched to form an implantation region. The width of the ridge, that is, the implantation area is 3.0 μm. Subsequently, an n-type electrode 21 and a p-type electrode 22 are formed by vapor deposition, and a cavity surface is formed by cleaving. Further, antireflection coating is applied to both resonator surfaces to obtain a ridge type DFB laser having a phase shift type diffraction grating structure.

The ridge type DFB laser having the phase shift type diffraction grating structure according to the second embodiment can reduce the number of elements that oscillate in two modes compared to the ridge type DFB laser having a uniform diffraction grating structure without phase shift. It can improve single mode characteristics. Further, the threshold current of the element can be reduced.

Embodiment 3 Hereinafter, a third embodiment of the present invention will be described. FIG.
It is explanatory drawing which shows the manufacturing method when this invention is applied to a BH type DFB semiconductor laser.

For example, n-type GaAs is used in an MBE device or the like.
An n-type AlGaAs cladding layer 24 (thickness 1.5 μm) and an active layer 25 (AlGaAs MQW, Al
GaAs bulk, GaAs bulk, etc .: thickness 0.1μ
m), a p-type AlGaAs carrier block layer 26 (thickness 0.05 μm), and a p-type AlGaAs light guide layer 27 (thickness 0.2 μm) are epitaxially grown in this order.

Next, the p-type AlGaAs light guide layer 2
7, and FIGS. 1 (a) to 1 (d) and FIGS.
(I) A phase-shifting diffraction grating 28 having a desired pitch is formed in accordance with the steps shown in FIGS.

Subsequently, a p-type AlGaAs cladding layer 29 (1.5 μm thick) and a p-type AlGaAs cap layer are formed on the p-type AlGaAs optical guide layer 27 on which the phase shift type diffraction grating 28 has been formed by, for example, liquid phase growth method. Epitaxial growth is performed in the order of 30 (thickness: 0.3 μm).

Next, a stripe having a width of about 2.0 μm is patterned at the center by photolithography.
Etching is performed up to the n-GaAs substrate 23, leaving a stripe-shaped epitaxial growth layer, and completely removing other portions.

Subsequently, the p-type AlG was added again to the portion where the epitaxial growth layer was removed by the selective epitaxial growth method.
aAs buried layer 31 (2.0 μm thick), n-type AlG
It grows in the order of the aAs buried layer 32 (2.0 μm in thickness). Subsequently, an n-type electrode 33 and a p-type electrode 34 are formed by vapor deposition, and a cavity surface is formed by cleaving. Further, antireflection coating is applied to both resonator surfaces to obtain an embedded DFB laser having a phase shift type diffraction grating structure.

The embedded DFB laser having the phase shift type diffraction grating structure according to the third embodiment can reduce the number of elements that oscillate in two modes compared with the embedded DFB laser having the uniform diffraction grating structure without phase shift. Can,
Improve unimodality. Further, the threshold current of the element can be reduced.

Other Embodiments Although the present invention has been described based on the embodiments, the present invention is not limited to the first embodiment.
The present invention is not limited to the third embodiment. For example, the semiconductor diffraction grating 8 used in the first embodiment has a triangular shape, but may have a rectangular shape or a trapezoidal shape. And the forming method is essentially the same. In the above embodiment, a photoresist is used as the etching mask layer of the phase shift diffraction grating 10. However, for example, an oxide film or a nitride film of a SiO ₂ or Si ₃ N ₄ film, or a metal material may be used. In short, the material may be determined according to the etching conditions at the time of dry etching. The substrate is not limited to the semiconductor substrate 1, but may be glass, optical glass, or the like.

Furthermore, reactive ion beam etching (RIBE) is used in the etching method of the phase shift diffraction grating 10, but sputter etching, ion milling, reactive ion etching (RIE), etc. may be used, and the etching gas is Cl _2. The selection is not limited to this, and may be selected according to the material and various etching conditions.

In addition, in the second and third embodiments, the present invention is applied to the ridge type DFB laser and the buried type DFBB laser. However, the present invention is not limited to this.
It can be widely applied to optical devices such as lasers and filter couplers.

[0049]

As described above, according to the present invention,
After forming the semiconductor diffraction grating formed by the conventional method,
A different material such as photoresist is coated on the whole surface, and this is etched as it is by dry etching to form a phase shift type diffraction grating with inverted phase.
(1) It can be created by a simple creation method, (2) The dimensional accuracy and shape stability of the pattern are improved, (3) The transition region can be shortened, and (4) The controllability of the depth of the diffraction grating is improved. .

Therefore, it is possible to improve the characteristics of an optical element such as a laser such as a DFB or a filter.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view showing a process of an embodiment of the present invention.

FIG. 2 is a cross-sectional view showing the steps of the embodiment of the present invention.

FIG. 3 is a cross-sectional view showing the steps of the embodiment of the present invention.

FIG. 4 is a diagram showing an end point detection method according to the embodiment of the present invention.

FIG. 5 is an explanatory view showing a method of manufacturing a DFB laser in which the present invention is applied to a second embodiment.

FIG. 6 is an explanatory view showing a method of manufacturing a DFB laser in which the present invention is applied to a third embodiment.

FIG. 7 is a configuration diagram showing a conventional method for manufacturing a diffraction grating.

FIG. 8 is a process cross-sectional view showing a conventional method for manufacturing a diffraction grating.

FIG. 9 is a configuration diagram showing a conventional method for manufacturing a diffraction grating.

FIG. 10 is a configuration diagram showing a conventional method for manufacturing a diffraction grating.

FIG. 11 is a process sectional view illustrating a conventional method for manufacturing a diffraction grating.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Substrate 2, 6 ... Photoresist layer 3 ... Resist pattern 4, 5 ... Laser beam 7 ... Diffraction grating pattern 8 ... Semiconductor diffraction grating 9 ... Etching mask layer 10, 17, 28 ... Phase shift diffraction grating 11, 23 n-type GaAs substrate 12 n-type GaAs buffer layer 13, 24 n-type AlGaAs cladding layer 14, 25 active layer 15 p-type AlGaAs barrier layer 16, 27 ... P-type AlGaAs optical guide layer 18, 29 p-type AlGaAs cladding layer 19 p-type GaAs cap layer 20 silicon nitride film 21, 33 n-type electrode 22, 34 p-type electrode 26 p-type AlGaAs carrier block layer 30 p-type AlGaAs cap layer 31 p-type AlGaAs buried layer 32 n-type AlGaAs buried Embedding layer

Claims (3)

(57) [Claims] 1. An etching method comprising: forming a diffraction grating having a reference period on a substrate; coating the diffraction grating with a different material different from the grating material; and simultaneously etching the different material and the grating forming material. A method of forming a diffraction grating having a period that is periodically inverted from the initial period. 2. A step is provided between a region where a diffraction grating having a reference period is formed and a region where the phase is reversed, and a diffraction grating is formed on the entire substrate including the step region by two-beam interference exposure. After coating the diffraction grating with a different material different from the diffraction grating material so as to have different film thicknesses on both sides, forming the inverted periodic diffraction grating by simultaneously etching the different material and the grating forming material. 2. A method for forming the diffraction grating according to 1. 3. A semiconductor device comprising a diffraction grating formed by the method of forming a diffraction grating according to claim 1.