JPH02132873A - Formation of diffraction grating and distributed feedback type semiconductor laser - Google Patents

Abstract

PURPOSE:To manufacture a phase shifting diffraction grating which is uniform throughout the whole face and excellent in reproducibility by a method wherein a surface flattened layer is formed as a first layer on a semiconductor substrate or an epitaxial growth layer on a semiconductor substrate. CONSTITUTION:Polyimide 2 is applied onto an InP substrate 1, Si 3 is evaporated thereon, positive type photoresist 4 is applied to form a resist pattern 4A, and the Si 3 is etched using the resist pattern 4A as a mask to form an Si pattern 3A. Then, the polyimide 2 is etched using a part of the Si pattern 3A as a mask to obtain a polyimide pattern 2A. In succession, the etched part of the polyimide 2 is covered with an SiO2 protective film 6, then the rest part if obliquely etched, the protective film 6 is removed, and then the InP substrate 1 is etched using the polyimide pattern 2A as a mask to form a phase shifting diffraction grating 100.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to a method for producing a diffraction grating that is essential for producing a distributed feedback semiconductor laser used as a light source for optical communications.

Conventional technology Distributed feedback semiconductor laser (hereinafter referred to as DFB laser)
oscillates in a single longitudinal mode, that is, at a single wavelength, near the plug wavelength determined by the period of the diffraction grating built into the element. For this reason, it is being put into practical use as a light source capable of long-distance, large-capacity transmission even in optical communication systems using optical fibers having wavelength dispersion. However, in a DFB laser that has a uniform diffraction grating within the element, the wavelength that provides the minimum threshold gain does not coincide with the Puwogg wavelength in principle, and there are two longitudinal modes that can oscillate close to each other on both sides. Because of this, these two
There was a problem in that stable single longitudinal mode oscillation could not be obtained with a good yield due to instability caused by competition between the two longitudinal modes.

Therefore, in order to make all the elements oscillate in a single longitudinal mode at the Bragg wavelength by matching the Bragg wavelength with the wavelength that gives the minimum threshold gain, the phase of the left and right diffraction gratings is changed to λn/4 near the center of the element. A phase-shift type DFBL has been proposed that has a structure shifted by (λn: wavelength of light propagating within the element). However, in the case of a first-order diffraction grating, λ/
4. To create a shifted diffraction grating, it is necessary to reverse the unevenness of the left and right diffraction gratings, which generally requires a more complicated creation process than the process of creating a uniform diffraction grating over the entire surface. As a method for creating a phase shift diffraction grating,
For example, Electronics Letters Vol. 20 No. 4,
Pages 1008-101o (published November 22, 1984)
An interference exposure method that utilizes the reverse photosensitivity of the positive type and negative type 7 photoresists described above is known.

Below, a brief explanation will be given with reference to Fig. 3. InP
Negative photoresist 19 with a film thickness of 700 on the substrate 18
(a), and then apply a positive photoresist 20 on the negative photoresist 19, and form a resist pattern 20A covering only half of the cavity length using normal gluing lithography technique. (C). Then, using the resist pattern 2OA as a mask, the negative photoresist 19 is etched with sulfuric acid to form the pattern 1.
9A, a positive photoresist pattern 20 is formed.
A is completely removed (d) and the positive photoresist 21 is applied again.
is coated over the entire surface with a film thickness of 700A (lil).

After exposure by a two-beam interference exposure method using a He--Cd laser beam with a wavelength of 3260, a resist pattern 30 shown in (f) is obtained by development. Furthermore, the resist pattern 3
After etching the InP substrate 18 using 0 as a mask,
The negative-type photoresist 18 and the positive-type photoresist are completely removed to create a phase shift grating 1oO shown in (cr).

Problems to be Solved by the Invention However, the above-mentioned conventional technique requires a step of applying two types of 7-year-old resists adjacently, and furthermore, the shape of the underlying semiconductor substrate and the surface of the epitaxial growth layer on the substrate is There was a problem in that it was not possible to create a phase shift diffraction grating that was uniform throughout the entire area and had good reproducibility.

A first object of the present invention is to create a phase-shifted diffraction grating that is uniform and has good reproducibility over the entire surface of a semiconductor substrate and an epitaxially grown layer on the substrate. A second object is to enable manufacturing of a phase-shifted DFB laser even in a structure in which an active layer is buried in a groove in a current blocking layer.

Means for Solving the Problems In order to achieve the above object, the technical solution of the present invention is as follows:
First, a first layer that is a surface flattening layer is formed on a semiconductor substrate or an epitaxially grown layer on the substrate, a second layer that is an intermediate layer is formed on the first layer, and a second layer that is an intermediate layer is formed on the first layer. forming a third layer of photoresist on the third layer; forming a periodic resist pattern on the third layer; and etching the second layer using the resist pattern as a mask; etching the third layer while masking a portion on the layer; covering the etched portion of the third layer with a protective film, and then performing inclined dry etching on the remaining portion; After removing the protective film,
The method includes at least a step of etching the semiconductor substrate or an epitaxial growth layer on the substrate using the third layer as a mask. In a second method, the phase shift grating is formed directly on an InP grading layer or an InGaAsP waveguide layer in a groove in a current blocking layer on a semiconductor substrate.

Function: Firstly, by forming a surface flattening layer as the first layer, the fine irregularities and intricately processed shapes of the base layer are absorbed, and the second layer and the third layer The layer is flattened over the entire surface. Therefore, a periodic resist pattern with good uniformity over the entire surface is formed on the third layer, and a periodic pattern with good uniformity over the entire surface is also transferred to the second layer.

After masking a portion of the second layer and etching the first layer, covering the etched portion with a protective film,
The remaining portion of the first layer is etched at an angle using dry etching to form a phase-shifted mask pattern in the first layer. Even if the thickness of the first layer changes, the shift amount can be set to an arbitrary value K by changing the inclination angle of the substrate with respect to the beam during the dry etching. As described above, it is possible to create a phase shift diffraction grating that is uniform over the entire surface and has good reproducibility, regardless of the shape of the base.

Second, by forming the surface planarization layer on the InP cladding layer or InGaAsP waveguide layer in the trench in the current blocking layer on the semiconductor substrate, it is possible to form the IuP cladding layer or the InGaAsP waveguide layer, which was previously difficult. A phase-shifted DFB laser in which a phase-shifted diffraction grating is directly formed on an IuGaAsF waveguide layer is manufactured.

EXAMPLE Hereinafter, a first example of the present invention will be described with reference to FIGS. 1a to 1h. As shown in Figure 1a, In
Coat polyimide 2 with a film thickness of 1000 on P substrate 1,
Next, Si 3 with a thickness of 250 nm is deposited by electron beam, and a positive type photoresist 4 with a thickness of 700 nm is further applied.

Next, we used a He-Cd laser beam with a wavelength of 3260 nm to
After performing beam interference exposure, development is performed to obtain a resist pattern 4A shown in Φ). And the resist pattern 4A
With St as a mask, reactive ion beam etching (hereinafter referred to as RIBE) was performed using CF4 gas.
Co forming pattern 3A). Then, a positive type photoresist is formed and using normal gluing lithography technique (d
A positive resist pattern 6A shown in ) is obtained. Next, 0
RIBE (Reactive Io) using 2 gas 6o
(n Beam Etching) to obtain a polyimide pattern 2A shown in (e). And plasma CVD
When the S102 film 6 is debossed, an S102 film θ is formed on the exposed part of the surface of the InP substrate 1 and on the StS.
Deposition occurs, but positive photoresist pad 5A
is etched by oxygen plasma (f). Next, oblique etching is performed using O2 gas 60 to tilt the substrate surface at an angle of 49° with respect to the RIBE beam to selectively remove a part of the two polyimide patterns.
A mask pattern similar to S102 6+ polyimide pattern 2A shown in ) is obtained. Furthermore, after removing Si○2 with hydrofluoric acid and etching the surface of the substrate 1 using the remaining mask pattern with an enoting solution of saturated bromine water: phosphoric acid: water = 2:1:15, CF4
RIBE with gas is performed to obtain a phase-shifted diffraction grating 1oO shown in 》.

As described above, in this example, since a polyimide flattening layer is used, a phase shift diffraction grating that is uniform and has good reproducibility can be formed over the entire surface regardless of the shape of the base. In this example, a λ/4 shifted diffraction grating was obtained, but the amount of shift can be set arbitrarily by changing the tilt angle in the step <q). Moreover, even if the thickness of the polyimide film changes, the amount of shift can be easily controlled by changing the inclination angle.

A second embodiment of the invention will now be described with reference to FIGS. 2a to 2e. As shown in Figure 2a, n-I
n-InP 8 , p-InP 9 ,
n-I! A current blocking layer is created by liquid phase epitaxial growth in the order of IP 1 0. Next, a V-groove 70 is formed in the current prosodic layer by ordinary photolithography technology and wet etching (b), and n-InP 11, y-doped InGaAsP is formed in this groove 7o.
The active layer 1 2 and the p-InGaAsP waveguide layer 13 are grown by liquid phase epitaxial growth in this order (C). At this time n-
These layers are also formed on InP10. Then, by the method shown in the first embodiment of the present invention, p-InGaA
A phase shift diffraction grating is formed on the sP waveguide layer 12 (d
). (d) is a diagram including a partially cutaway cross section. moreover,
After performing liquid phase epitaxial growth in the order of p-InP 14 and p-InGaAsP16, p-electrodes 16 and n
An electrode 17 is deposited to obtain a phase-shifted DFB laser shown in (e).

As described above, according to this embodiment, even in a structure in which the active layer is buried in the groove in the current blocking layer, in which it has been difficult to form a diffraction grating, it is possible to
A phase-shifted DFB laser in which a phase-shifted diffraction grating is formed directly on a GaAsP waveguide layer can be manufactured.

Effects of the Invention As described above, the effects of the present invention include a phase shift that is uniform over the entire surface and has excellent reproducibility and controllability of the amount of phase shift, regardless of the shape of the semiconductor substrate surface or the surface of the epitaxial growth layer on the substrate. Diffraction gratings can be created. In addition, even in a structure in which the active layer is buried in a groove in the current block layer, where it has been difficult to form a diffraction grating, it is also possible to form a phase shift grating directly on the InP cladding layer or InGaAsP waveguide layer. D
An FB laser can be produced.

[Brief explanation of the drawing]

Figures 1a to 1h are cross-sectional views of the manufacturing process of a phase-shifted diffraction grating according to the first embodiment of the present invention, and Figures 2a to 2e are sectional views of the manufacturing process of a phase-shifted DFB laser according to the second embodiment of the present invention. The perspective views and FIGS. 3A to 3Q are cross-sectional views of the manufacturing process of a conventional phase shift diffraction grating. 1.^...InP substrate, 2...polyimide,
3. Old...St, 4...Positive photoresinut, 60...02 gas. Name of agent: Patent attorney Shigetaka Awano and 1 other person1
Figure 4A Figure IJI-1 River,r Figure Figure Kasumi

Claims (2)

[Claims] (1) A first layer that is a surface flattening layer on a semiconductor substrate or an epitaxial growth layer on the substrate, a second layer that is an intermediate layer on the first layer, and a second layer that is an intermediate layer on the first layer; forming a third layer of photoresist on the third layer; forming a periodic resist pattern on the third layer; and etching the second layer using the resist pattern as a mask; etching the third layer while masking a portion on the layer; covering the etched portion of the third layer with a protective film, and then subjecting the remaining portion to inclined dry etching; A method for producing a diffraction grating, comprising at least the step of etching the semiconductor substrate or an epitaxial growth layer on the substrate using the third layer as a mask after removing the protective film. (2) A distributed feedback semiconductor having a structure in which a diffraction grating formed by the method described in claim 1 is built in and an active layer is buried in a groove in a current blocking layer epitaxially grown on a semiconductor substrate. laser.