JPH05182948A - Method of forming thin film tapered structure - Google Patents

Abstract

PURPOSE:To provide a method of forming a thin film tapered structure, which is capable of obtaining an efficient waveguide optical coupling when the method is applied to the formation of the tapered structure of an optical waveguide element, is capable of obtaining also a positioning accuracy of the same degree as that in a photolithography method and is capable of forming also a tapered structure having an extremely large ratio of taper. CONSTITUTION:A method of forming a thin film tapered structure has a process, in which such three layers of first, second and third thin film layers, 2, 3 and 4 as they are etched with the same etching solution and their etching rates to the etching solution are increased as they are upper layers are laminated in order on a substrate 1 in the order of these layers 2, 3 and 4, a refractive index of the layer 3 is made larger than that of the layer 2, a mask 5 is formed on the layer 4 and the layers 4, 3 and 2 are selectively etched with an etching solution using the mask 5. In this method, the thin film tapered structure is formed into a constitution wherein the end point of the etching is provided in the middle of the layer 2.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a T in an optical integrated circuit.
The present invention relates to a method for forming a thin film taper structure applied to an E-TM mode separation element, an optical coupling element between waveguides, and a step portion such as a contact hole in a semiconductor device.

[0002]

2. Description of the Related Art Conventionally, a light receiving element, a film thickness,
As a method of guiding guided light to other optical waveguides having different refractive indexes smoothly with reduced loss, literature: IEEE, JQE, QE-2
2,845 (1986) T. Suhara, and H. Nishihara "Integrated Opt
ics Components and Devices Using Periodic Structure
s ", and literature: Proceedings of the 36th spring meeting of the 1st year of the Heisei era 1a-PB-9, Yamaguchi, Sato, Kokubun" Reduction of loss of spot size converter (SST) by overlapping overlapping tapered waveguides " There is a tapered waveguide structure shown in, and in the literature, light is guided from the waveguide to the light receiving element formed monolithically with the waveguide through the tapered structure.
Waveguides having different refractive indexes are coupled by a taper structure to couple guided light. On the other hand, in the semiconductor wiring technology, there is a method of forming the stepped portion in a tapered shape in order to prevent disconnection of the wiring material in the stepped portion such as a contact hole. Device manufacturing method ".

Here, FIG. 5 shows a guided light coupling portion to the light receiving element portion having the tapered structure shown in the above-mentioned document. In FIG. 1A, a light receiving element (PN photodiode) portion 12 is formed on the Si substrate 1, and an etching rate higher than that of the buffer layer 5 is provided on the buffer layer (thermal oxide film in this case) 5 thereon. A large high etch rate layer (not shown) is laminated, and a photoresist (not shown) is formed on the photo resist layer 12 except the upper portion of the light receiving element 12, and the buffer layer 5 and the high etch rate are formed by wet etching. The buffer layer 5 is formed into a taper shape by utilizing the difference in etch rate from the layer, the high etch rate layer is removed, and then the waveguide layer 20 is laminated on the taper-formed buffer layer 5. . Further, in FIG. 6B, the buffer layer 5 (thermal oxide film) is formed by using the LOCOS method, and the taper of the bird's beak portion formed at that time is used for coupling guided light to the light receiving element 12. Incidentally, in the above two types of taper forming methods,
The taper ratio is only up to about 1: 5.

On the other hand, as another taper forming method, there is a shadow mask method in the literature (see FIG. 6). This is because when the waveguiding layers 21 and 22 are formed by a film forming method with good step coverage such as a sputtering method or a CVD method, shadow masks 60 and 61 are installed on the substrate 1 and reactive species to the shadow mask end are provided. This is a method of forming a taper by wrapping around. In this forming method, in FIG. 6A, first, one end of the waveguide layer 22 is formed in a tapered shape, and then in FIG. 6B, the end of the shadow mask is aligned with the tapered end of the waveguide layer 22. , The next waveguide layer 21 is formed in a tapered shape. By this method, a taper angle of 1 deg or less was obtained.

[0005]

Of the above three types of taper forming methods, the taper ratio of only about 1: 5 can be obtained in FIGS. 5 (a) and 5 (b). At the taper ratio, the waveguide loss in the tapered portion is large, and in some cases, mode conversion of the guided light occurs in the tapered portion, and the converted mode has an equivalent refractive index different from the originally designed value. There is a problem that the guided optical path is not guided. Further, in the shadow mask method of FIG. 6, although a gentle taper can be obtained, there is a problem that the positioning accuracy cannot be obtained. In the optical waveguide element, in order to suppress the above-mentioned waveguide loss and unnecessary mode conversion and realize efficient guided light coupling, a taper forming method of a waveguide layer having a larger taper ratio is required, Moreover, the positioning accuracy as high as that of the photolithography method is required.

Further, as a wiring technique for a step portion such as a contact hole of a semiconductor element, there are the above-mentioned patent applications and the like. In this method, as in the case of FIG. 5A, P-Si ₃ N ₄ is formed on the insulating coating layer forming the step by plasma deposition.
Form the layers. And P-Si for the insulating coating layer
_The taper is formed on the step portion by utilizing the fact that the etching rate of the ₃ N ₄ layer with respect to BHF (etching solution of HF: NH ₄ F = 1: 10) can be changed up to about 6 times. However, even if this method is applied to the taper formation of the optical waveguide layer, the taper ratio can be made only about the etching rate difference, and the taper ratio of the optical waveguide layer is still insufficient as in the above-mentioned forming method. is there.

The present invention has been made in view of the above circumstances, and when applied to the formation of a taper structure of an optical waveguide element, efficient guided light coupling can be obtained, and the same degree as that of the photolithography method is obtained. It is an object of the present invention to provide a method for forming a thin film taper structure, which is capable of obtaining the positioning accuracy described above and is capable of forming a taper structure having an extremely large taper ratio.

[0008]

In order to achieve the above object, in the invention of claim 1, the substrate is etched with the same etching solution, and the etching rate for the etching solution becomes higher toward the upper layer. The first, second, and third thin film layers are sequentially stacked in this order, and the refractive index of the second thin film layer is larger than that of the first thin film layer,
A thin film taper structure including a step of forming a mask on a third thin film layer and selectively etching the third thin film layer, the second thin film layer and the first thin film layer with the etching solution using the mask. In the above method, the end point of etching is in the middle of the first thin film layer.

According to the second aspect of the invention, the first,
The second two thin film layers are sequentially stacked in this order, and the transparent substrate, the first thin film layer, and the second thin film layer are etched with the same etching solution to obtain a transparent substrate, a first thin film layer, and a second thin film. Layers are formed in the order of layers to increase the etching rate of the etching solution, and the refractive index of the first thin film layer is higher than that of the transparent substrate, and a mask is formed on the second thin film layer. In the method for forming a thin film taper structure including the step of selectively etching the second thin film layer, the first thin film layer and the transparent substrate by using the etching solution, the end point of etching is in the middle of the transparent substrate. Is characterized by.

According to the third aspect of the present invention, the first and second two thin film layers which are etched with the same etching solution on the substrate and have a higher etching rate for the etching solution toward the upper layer in this order. In order to form a thin film taper structure having a step of selectively laminating the second thin film layer and the first thin film layer with the etching solution using the mask, The etching is stopped in the middle of the etching with the etching solution, and then the mask is partially dissolved in a solution that dissolves the mask, and then the second thin film layer and the first thin film layer are again etched with the etching solution. It is characterized in that the second thin film layer and the first thin film layer are selectively etched by repeating the operation a plurality of times.

According to the fourth aspect of the present invention, the first and second two thin film layers which are etched with the same etching solution on the substrate and have a higher etching rate for the etching solution toward the upper layer are provided in this order. In the method for forming a thin film taper structure, which comprises a step of sequentially laminating, forming a mask on the mask, and selectively etching the second thin film layer and the first thin film layer with the etching solution using the mask, A solution that can dissolve the mask is mixed and used in an etching solution, and the mixed concentration is such that the mask solution is used by the etching solution to etch the mask when the second thin film layer and the first thin film layer are etched by the etching solution. It is characterized in that it is used as a concentration that cannot be completely dissolved.

[0012]

In the method of forming the thin film taper structure according to claim 1, when the second thin film layer is used as the optical waveguide layer and the first thin film layer is used as the buffer layer to form the tapered structure of the optical waveguide device,
At the same time that the taper is formed in the optical waveguide layer, the taper is also formed in the underlying buffer layer, so that a relatively gradual taper can be formed and the coupling loss of the guided light can be reduced. The method of forming a thin film taper structure according to claim 2, wherein when forming the taper structure of the optical waveguide element using the first thin film layer as the optical waveguide layer, the taper is formed on the optical waveguide layer and at the same time the underlying transparent substrate is also tapered. Since it is formed, a relatively gentle taper can be formed even with a simple structure without a buffer layer, and the coupling loss of guided light can be reduced.

In the method of forming a thin film taper structure according to a third aspect of the present invention, when the taper structure of the optical waveguide element is formed by using the first thin film layer as the optical waveguide layer, the etching of the taper shape and the receding of the mask layer are repeated. Therefore, a more gradual taper shape can be formed and the coupling loss of the guided light can be reduced. In the method of forming the thin film taper structure according to claim 4, when the taper structure of the optical waveguide element is formed by using the first thin film layer as the optical waveguide layer, the tapered etching and the retreat of the mask layer are simultaneously performed. A more gradual tapered shape can be formed, and the coupling loss of guided light can be reduced.

[0014]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described in detail below based on the illustrated embodiments. FIG. 1 is an explanatory view of a process of forming a thin film taper structure showing an embodiment of the invention described in claim 1. Less than,
A method of forming the thin film taper structure in the present embodiment will be sequentially described. In FIG. 1A, a SiO ₂ layer (buffer layer) 2 is formed as a first thin film layer on a semiconductor substrate as a substrate 1 by a thermal oxidation method, and Si is used as a source gas on the SiO ₂ layer (buffer layer) 2.
An optical waveguide layer 3 made of silicon oxynitride (hereinafter referred to as SiON) is formed as a second thin film layer by a plasma CVD method using H ₄ , NH ₃ , N ₂ O and an inert carrier gas, and further on it. Then, the SiON layer 4 is formed as the third thin film layer by the plasma CVD method using the same gas species as described above. The characteristics of each layer are as follows by using the methods described in: Japanese Patent Application No. 2-247688 and Japanese Patent Application No. 3-220521 previously filed by the present applicant. SiON layer 4: Etch rate about 16000Å
/ Min Optical waveguide layer (SiON layer) 3: Etch rate about 800Å / m
in, refractive index about 1.87 buffer layer (SiO ₂ layer) 2: etch rate about 500Å / m
in, Refractive index: about 1.46 Therefore, the etching rate for the same etching solution is higher in the upper layer. After forming each of the above layers, S
A photoresist layer (mask layer) 5 is formed in a desired pattern on the iON layer 4 by a photolithography technique.

Next, as an etching solution capable of etching the three layers, the photoresist layer 5 is etched with the hydrofluoric acid-based solution described in the above-mentioned prior application, and the photoresist layer 5 is not etched. Then, the SiON layer 4 and the optical waveguide layer 3 are sequentially etched. Then, the etching is terminated when the SiON layer 4 and the optical waveguide layer 3 are etched and the SiO ₂ layer 2 is partially etched. At this time, the etching rate is higher in the upper layer and the etching solution penetrates under the mask layer 5 as well, so that the etching in the lateral direction also progresses. Therefore, as shown in FIG.
A tapered shape is formed in the iO ₂ layer (buffer layer) 2.

Next, as shown in FIG. 1C, the photoresist layer 5 is removed, and the SiON layer 4 is removed with a diluted dilute hydrofluoric acid solution. At this time, the SiON layer 4 includes the optical waveguide layer (SiON layer) 3 and the buffer layer (SiO _2).
Since the etching rate is much higher than that of the layer 2), the film loss of the optical waveguide layer 3 and the buffer layer and the change of the taper shape after the removal of the SiON layer 4 are very slight, and there is no problem. Absent. Here, the taper formed on the optical waveguide layer 3 and the buffer layer (SiO ₂ layer) 2 is larger in the buffer layer (SiO ₂ layer) 2 than in the optical waveguide layer 3, as can be seen from the difference in the etching rates. Figure 1
In (c), θ ₂ > θ ₃ > θ ₁ .

Next, as shown in FIG. 1D, another optical waveguide layer 20 is formed on the optical waveguide layer (SiON layer) 3 and the buffer layer (SiO ₂ layer) 2 by the CVD method, the sputtering method, and the vapor deposition. Or the same plasma CVD method as described above. The refractive index of the optical waveguide layer 20 is, for example, about 1.55.
And Then, when the guided light 80 is guided from the left side to the optical waveguide layer 20 as shown in FIG. 1D, the SiO ₂ layer 2 as the buffer layer is partially etched in the region A, but If the layer is left sufficiently thick, the guided light 80 is guided in the optical waveguide layer 20 in the region A. Subsequently, in the region B, a taper is formed on the SiO ₂ layer 2 which is a buffer layer. However, since the taper is sufficiently loose, the guided light 80 is emitted from the optical waveguide layer 2
While being confined in 0, there is no loss due to the taper of the buffer layer 2, and the light is guided upward along the taper. Next, in the region C, since the optical waveguide layer 3 has a higher refractive index, the guided light 80 is coupled from the optical waveguide layer 20 to the optical waveguide layer 3, but in order to reduce the loss at this time, it is tapered. It needs to be joined smoothly.

In the present invention, not only the optical waveguide layer 3 but also the buffer layer 2 in the B region immediately before it is formed with a gentler taper, so that the guided light 80 is coupled from the optical waveguide layer 20 to the optical waveguide layer 3. The taper can be made relatively gentler. Therefore, as in the case of the conventional example shown in FIG. 1E, a relatively gentle taper can be formed as compared with the case where only the optical waveguide layer 3 is tapered, and therefore the optical waveguide layer 20 to the optical waveguide layer 3 are formed. Guided light 80
The coupling loss at the time of coupling can be reduced compared to the conventional example shown in FIG. In the present embodiment, the SiO ₂ layer is used as the buffer layer 2, the SiON layer formed by the plasma CVD method is used as the optical waveguide layer 3, and the SiON layer formed by the plasma CVD method is used as the layer 4 thereabove. However, it is obvious that the film forming method of each layer may be other than the film forming method shown in this embodiment.

Next, FIG. 2 is an explanatory view of a process of forming a thin film taper structure showing an embodiment of the invention described in claim 2.
Hereinafter, a method of forming the thin film taper structure in this embodiment will be sequentially described. In FIG. 2 (a), SiH ₄ , NH ₃ , N as raw material gases are provided on a substrate 1 made of transparent quartz.
_An optical waveguide layer 3 made of silicon oxynitride (hereinafter referred to as SiON) is formed as a first thin film layer by the plasma CVD method using ₂ O and an inert carrier gas, and the same gas species as described above is further formed thereon. The SiON layer 4 is formed as the second thin film layer by the plasma CVD method using. The characteristics of each layer are as follows by using the method described in the above-mentioned prior application. SiON layer 4: Etch rate about 16000Å
/ Min Optical waveguide layer (SiON layer) 3: Etch rate about 800Å / m
in, refractive index about 1.87 Substrate (transparent quartz substrate) 1: etch rate about 500Å / m
in, Refractive index: about 1.46 Therefore, the etching rate for the same etching solution is higher in the upper layer. After forming each of the above layers, S
A photoresist layer (mask layer) 5 is formed in a desired pattern on the iON layer 4 by a photolithography technique.

Next, the substrate 1 and the two thin film layers 3 and 4 are formed.
Etching is performed with the hydrofluoric acid-based solution described in the above-mentioned prior application as the etching solution capable of etching with the photoresist layer 5 as a mask, the etching proceeds in a portion not covered by the photoresist layer 5,
The ON layer 4 and the optical waveguide layer 3 are sequentially etched. Then, the etching is finished when the SiON layer 4 and the optical waveguide layer 3 are etched and the substrate 1 is further etched halfway. At this time, the upper layer has a higher etching rate, and the etching solution penetrates under the mask layer 5 as well, and the etching in the lateral direction also progresses. Therefore, as shown in FIG. A tapered shape is formed.

Next, as shown in FIG. 2C, the photoresist layer 5 is removed, and the SiON layer 4 is removed with a diluted dilute hydrofluoric acid solution. At this time, since the SiON layer 4 has a much higher etching rate than the optical waveguide layer (SiON layer) 3 and the substrate 1, the SiON layer 4
After the removal, the film loss of the optical waveguide layer 3 and the substrate 1 and the change in the taper shape are very slight and there is no problem. Here, the taper formed on the optical waveguide layer 3 and the substrate 1 is
As can be seen from the difference in the etching rates, the optical waveguide layer 3
The substrate 1 is slower than the substrate 1, and θ ₂ > in FIG.
θ ₃ > θ ₁ .

Next, as shown in FIG. 2D, another optical waveguide layer 2 is formed on the optical waveguide layer (SiON layer) 3 and the substrate 1.
0 is formed by the CVD method, the sputtering method, the vapor deposition method, or the plasma CVD method similar to the above. The refractive index of the optical waveguide layer 20 is, eg, about 1.55. Then, when guided light 80 is guided to the optical waveguide layer 20 from the left side as shown in FIG. 2D, the substrate 1 which is also a buffer layer is partially etched in the region A, but as a buffer layer. Is left sufficiently thick, the guided light 80 is guided in the optical waveguide layer 20 in the region A.
Subsequently, in the region B, a taper is formed on the substrate 1 which is also a buffer layer, but since this taper is sufficiently loose, the guided light 80 remains confined in the optical waveguide layer 20,
There is no loss due to the taper of the substrate 1, and the wave is guided upward along the taper. Next, in the C region, since the optical waveguide layer 3 has a higher refractive index, the guided light 80 is transmitted through the optical waveguide layer 20.
The optical waveguide layer 3 is coupled to the optical waveguide layer 3. However, in order to reduce the loss at this time, it is necessary to smoothly couple by taper.

In the present invention, not only the optical waveguide layer 3 but also the substrate 1 in the B region immediately before the optical waveguide layer 3 is formed with a gentler taper, so that the guided light 80 is coupled from the optical waveguide layer 20 to the optical waveguide layer 3. The taper can be made relatively gentler. Therefore, as in the case of the conventional example shown in FIG. 2E, a relatively gentle taper can be formed as compared with the case where the taper is formed only in the optical waveguide layer 3, so that the optical waveguide layer 20 to the optical waveguide layer 3 are formed. The coupling loss when the guided light 80 is coupled can be reduced as compared with the conventional example shown in FIG. In this embodiment, the substrate 1 is a transparent quartz substrate and the optical waveguide layer 3 is plasma CVD.
A SiON layer formed by the plasma CVD method is used as the SiON layer formed by the above method, and the SiON layer formed by the plasma CVD method is used so that the etching rate becomes higher as the upper layer is formed. Obviously, other than the type of substrate and the film forming method shown in the examples, it does not matter.

Next, FIG. 3 is an explanatory view of a process of forming a thin film taper structure showing an embodiment of the invention described in claim 3.
Hereinafter, a method of forming the thin film taper structure in this embodiment will be sequentially described. In FIG. 3A, first, a buffer layer 2 made of a SiO ₂ layer is formed on a semiconductor substrate 10 by a thermal oxidation method to form a substrate 1 for an optical integrated circuit. This board 1
An optical waveguide layer 3 made of silicon oxynitride (hereinafter referred to as SiON) is used as a first thin film layer by a plasma CVD method using SiH ₄ , NH ₃ , N ₂ O and an inert carrier gas as a source gas. Then, the SiON layer 4 is formed thereon as the second thin film layer by the plasma CVD method using the same gas species as described above. The characteristics of each layer are as follows by using the method described in the above-mentioned prior application. SiON layer 4: Etch rate about 16000Å
/ Min Optical waveguide layer (SiON layer) 3: Etch rate about 800Å / m
in, refractive index about 1.87 buffer layer (SiO ₂ layer) 2: etch rate about 500Å / m
in, Refractive index: about 1.46 Therefore, the etching rate for the same etching solution is higher in the upper layer. After forming each of the above layers, S
A photoresist layer (mask layer) 5 is formed in a desired pattern on the iON layer 4 by a photolithography technique.

Next, the SiON layer 4 and the optical waveguide layer 3 are formed.
Etching is performed with the hydrofluoric acid-based solution described in the above-mentioned prior application as the etching solution capable of etching with the photoresist layer 5 as a mask, the etching proceeds in a portion not covered by the photoresist layer 5,
As the ON layer 4 and the optical waveguide layer 3 are etched,
Since the etching rate of the SiON layer 4 is higher than the etching rate of the optical waveguide layer 3, the etching solution also penetrates under the photoresist layer 5 and the lateral etching also progresses, as shown in FIG. Thus, the tapered shape is formed in the optical waveguide layer 3. Then, when the etching progresses to the middle of the optical waveguide layer 3, the etching is stopped once.

Next, as a solution capable of dissolving the photoresist layer 5, for example, in a solution 70 obtained by diluting an organic solvent (eg, acetone, methyl alcohol, ethyl alcohol, isopropyl alcohol, ethyl cellosolve acetate) with water, As shown in FIG. 3C, only the eaves portion E of the photoresist layer 5 is selectively dissolved. Since the lower thin films 4 and 3 of the photoresist layer 5 are removed by the etching, the eaves portion E is dissolved by the solution 70 not only from above but also from below. On the other hand, the non-overhanging portion F of the photoresist layer 5 is dissolved in the solution 70 only from above. Therefore, at the time when the thickness d ₀ of the eaves portion E is dissolved and removed, the photoresist layer 5 remains in the non-overhanging portion F by the thickness d ₁ . Then, at this stage, the dissolution of the photoresist layer 5 is stopped. The cross section of the device at this stage is as shown in FIG. 3D, and the edge of the photoresist layer 5 which was at point e before the initial etching was made f by selective dissolution of the photoresist layer 5. Can be retracted to a point.

Next, the thin film layers 4 and 3 are again etched with the hydrofluoric acid solution. Then, since the etching proceeds with the end of the photoresist layer 5 as the point f, the position where the etching solution penetrates may recede from the point e to the point f as compared with the case where the end of the photoresist layer 5 is initially set as the point e. Thereby, as shown in FIG. 3E, a more gradual tapered shape is formed. That is, FIG.
Compared with the taper angle θ ₁ in (b), θ ₂ > θ ₁ . Then, when the etching reaches the interface between the optical waveguide layer 3 and the substrate 1, the etching is stopped. Alternatively, if the partial dissolution of the photoresist layer 5 and the etching of the thin film layer are repeated a plurality of times, a more gradual tapered shape is formed. Subsequently, the photoresist layer 5 is removed, and the SiON layer 4 is removed with a diluted dilute hydrofluoric acid solution (not shown). At this time, the SiON layer 4 becomes the optical waveguide layer 3
And the buffer layer (SiO ₂ layer) to have a much greater etch rate than the 2, the optical waveguide layer 3 and the buffer layer after removal of the SiON layer 4 (SiO ₂ layer)
The film loss of 2 and the change of the taper shape are very slight and there is no problem.

Next, as shown in FIG. 3F, another optical waveguide layer 20 is formed on the optical waveguide layer (SiON layer) 3 and the buffer layer 2 of the substrate 1 by the CVD method, the sputtering method, the vapor deposition method, Alternatively, it is formed by a plasma CVD method similar to the above.
The refractive index of the optical waveguide layer 20 is, eg, about 1.55.
Then, guided light 80 is applied to the optical waveguide layer 20 as shown in FIG.
When the light is guided from the left side as shown in FIG. 5, the refractive index of the optical waveguide layer 3 is increased in the taper portion of the optical waveguide layer 3.
The guided light 80 is coupled from the optical waveguide layer 20 to the optical waveguide layer 3 because it is larger than the refractive index. At this time, since the taper shape of the optical waveguide layer 3 is formed so as to be sufficiently gentle as compared with the conventional case (θ ₂ > θ ₁ ), the coupling loss can be reduced as compared with the conventional case. In this embodiment, the buffer layer 2 of the substrate 1 is a SiO ₂ layer, the optical waveguide layer 3 is a SiON layer formed by a plasma CVD method, and the upper layer 4 is a SiON layer formed by a plasma CVD method. Although the etching rate is increased as it goes, it is clear that the film forming method for each layer may be other than the film forming method shown in this embodiment.

Next, FIG. 4 is an explanatory view of a process of forming a thin film taper structure showing an embodiment of the present invention.
Hereinafter, a method of forming the thin film taper structure in this embodiment will be sequentially described. In FIG. 4A, first, a buffer layer 2 made of a SiO ₂ layer is formed on a semiconductor substrate 10 by a thermal oxidation method to obtain a substrate 1 for an optical integrated circuit. This board 1
An optical waveguide layer 3 made of silicon oxynitride (hereinafter referred to as SiON) is used as a first thin film layer by a plasma CVD method using SiH ₄ , NH ₃ , N ₂ O and an inert carrier gas as a source gas. Then, the SiON layer 4 is formed thereon as the second thin film layer by the plasma CVD method using the same gas species as described above. The characteristics of each layer are as follows by using the method described in the above-mentioned prior application. SiON layer 4: Etch rate about 16000Å
/ Min Optical waveguide layer (SiON layer) 3: Etch rate about 800Å / m
in, refractive index about 1.87 buffer layer (SiO ₂ layer) 2: etch rate about 500Å / m
in, Refractive index: about 1.46 Therefore, the etching rate for the same etching solution is higher in the upper layer. After forming each of the above layers, S
A photoresist layer (mask layer) 5 is formed in a desired pattern on the iON layer 4 by a photolithography technique.

Next, the SiON layer 4 and the optical waveguide layer 3 are formed.
As an etching solution capable of etching the above, as a solution y capable of dissolving the photoresist layer 5 in the hydrofluoric acid solution x described in the above-mentioned prior application, for example, an organic solvent (for example, acetone, methyl alcohol, ethyl alcohol, isopropyl alcohol, ethyl (Cersolve acetate, etc.). The mixing concentration is S by the hydrofluoric acid solution X.
During the time when the iON layer 4 and the optical waveguide layer 3 are etched, the solution y has a concentration at which the photoresist layer 5 cannot be completely dissolved. Then, when etching is carried out in a solution z (= x + y) in which this hydrofluoric acid-based solution x and a solution y capable of dissolving the photoresist layer 5 are mixed, FIG.
As shown in FIG. 3, the SiON layer 4 and the optical waveguide layer 3 are etched by the hydrofluoric acid-based solution component x in the mixed solution z, and the photoresist layer 5 has an eaves shape, but at the same time, the solution component in the mixed solution z is formed. Since the photoresist layer 5 is also partially dissolved by y, the SiON layer 4 and the optical waveguide layer 3
The edge e of the photoresist layer 5 also recedes at the same time as the etching. Therefore, as compared with the conventional case where only the SiON layer 4 and the optical waveguide layer 3 are etched, a more gradual tapered shape can be formed. Then, as shown in FIG. 4C, the etching is stopped when it reaches the interface between the optical waveguide layer 3 and the substrate 1.

A solution y that dissolves the photoresist layer 5 is used.
The surface tension of the organic solvent mixed as is generally smaller than that of water (about 73 dyn / cm) or hydrofluoric acid (about 53 dyn / cm), for example, methanol or the like is about 22 dyn / cm. Therefore, the surface tension of the mixed solution z is smaller than that of the hydrofluoric acid solution x before being mixed. For this reason, since it easily penetrates into the fine gap, the exchange of the etching solution under the photoresist layer 5 is promoted, and this also has the effect of forming a gentler tapered shape. Now,
After the etching is completed, the photoresist layer 5 is subsequently removed, and the SiON layer 4 is removed with a diluted dilute hydrofluoric acid solution (not shown). At this time, since the SiON layer 4 has a much higher etching rate than the optical waveguide layer 3 and the buffer layer (SiO ₂ layer) 2 of the substrate 1,
After removing the SiON layer 4, the optical waveguide layer 3 and the buffer layer (SiO ₂ layer) 2 of the substrate 1 are reduced in film thickness and the change in taper shape is very slight, and there is no problem.

Next, as shown in FIG. 4D, another optical waveguide layer 20 is formed on the optical waveguide layer (SiON layer) 3 and the buffer layer 2 of the substrate 1 by the CVD method, the sputtering method, the vapor deposition method, Alternatively, it is formed by a plasma CVD method similar to the above.
The refractive index of the optical waveguide layer 20 is, eg, about 1.55.
Then, guided light 80 is applied to the optical waveguide layer 20 as shown in FIG.
When the light is guided from the left side as shown in FIG. 5, the refractive index of the optical waveguide layer 3 is increased in the taper portion of the optical waveguide layer 3.
The guided light 80 is coupled from the optical waveguide layer 20 to the optical waveguide layer 3 because it is larger than the refractive index. At this time, since the tapered shape of the optical waveguide layer 3 is formed so as to be sufficiently gentle as compared with the conventional one, the coupling loss can be reduced as compared with the conventional one. In this embodiment, the buffer layer 2 of the substrate 1 is a SiO ₂ layer, the optical waveguide layer 3 is a SiON layer formed by a plasma CVD method, and the upper layer 4 is a SiON layer formed by a plasma CVD method. Although the etching rate is increased as it goes, it is clear that the film forming method for each layer may be other than the film forming method shown in this embodiment.

[0033]

As described above with reference to the embodiments, in the method of forming a thin film taper structure according to claim 1, the second thin film layer is used as the optical waveguide layer 3 and the first thin film layer is used as the buffer layer 2. When forming the taper structure of the waveguide element, the taper is formed on the optical waveguide layer 3 and at the same time, the taper is formed on the underlying buffer layer 2, so that a relatively gradual taper can be formed. The coupling loss of guided light can be reduced. Moreover, the positioning accuracy of the taper forming position is also high.

In the method of forming a thin film taper structure according to a second aspect of the present invention, when the first thin film layer is used as the optical waveguide layer 3 to form the taper structure of the optical waveguide element, the taper is formed on the optical waveguide layer 3 and at the same time the base film is formed. Since the taper is also formed on the substrate 1, a relatively gentle taper can be formed even with a simple structure having no buffer layer, and the coupling loss of the guided light can be reduced. Moreover, the positioning accuracy of the taper forming position is also high.

In the method of forming a thin film taper structure according to a third aspect of the present invention, when forming the taper structure of the optical waveguide device using the first thin film layer as the optical waveguide layer 3, taper-shaped etching and retreating of the mask layer 5 are repeated. Since this is done, a more gradual taper shape can be formed and the coupling loss of the guided light can be reduced. Further, since the retreat of the mask layer 5 can be selectively performed, it does not require the labor of forming the mask layer a plurality of times, and the positional displacement due to the alignment does not occur.

In the method of forming a thin film taper structure according to a fourth aspect of the present invention, when the taper structure of the optical waveguide element is formed by using the first thin film layer as the optical waveguide layer 3, the tapered etching and the retreat of the mask layer 5 are performed simultaneously. I'm doing so
A more gradual tapered shape can be formed, and the coupling loss of guided light can be reduced. Further, since the retreat of the mask layer 5 can be selectively performed, it does not require the labor of forming the mask layer a plurality of times, and the positional displacement due to the alignment does not occur. Also, since the surface tension of the etching solution can be lowered, the etching solution can easily enter into the fine gap, the exchange of the etching solution in the fine gap is promoted, and a more gradual taper shape can be formed. Is.

[Brief description of drawings]

FIG. 1 is an explanatory diagram of a process of forming a thin film taper structure showing an embodiment of the invention according to claim 1;

FIG. 2 is an explanatory view of a process of forming a thin film taper structure showing an embodiment of the invention described in claim 2;

FIG. 3 is an explanatory view of a process of forming a thin film taper structure showing an embodiment of the invention described in claim 3;

FIG. 4 is an explanatory view of a process of forming a thin film taper structure showing an embodiment of the invention described in claim 4;

FIG. 5 is an explanatory diagram of a method of forming a thin film tapered structure showing an example of a conventional technique.

FIG. 6 is an explanatory view of a method of forming a thin film taper structure showing another example of the prior art.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Substrate 2 ... Buffer layer (SiO ₂ layer) 3 ... Optical waveguide layer (SiON layer) 4 ... SiON layer 5 ... Photoresist layer (mask layer) 10 ... Semiconductor substrate 20 ... Optical waveguide layer

Claims (4)

[Claims] 1. A first thin film layer, a second thin film layer, a second thin film layer, and a third thin film layer, which are etched with the same etching solution on a substrate and have a higher etching rate for the etching solution toward the upper layer, in this order. Laminated, the refractive index of the second thin film layer is larger than the refractive index of the first thin film layer, a mask is formed on the third thin film layer, and the third thin film layer by the etching solution using the mask. In a method of forming a thin film taper structure having a step of selectively etching the second thin film layer and the first thin film layer, the end point of etching is in the middle of the first thin film layer, Forming method. 2. A first and a second thin film layers are sequentially laminated on a transparent substrate in this order, and the transparent substrate, the first thin film layer and the second thin film layer are etched with the same etching solution. A transparent substrate, a first thin film layer, and a second thin film layer, which are formed of a material having a higher etching rate with respect to the etching solution, and the refractive index of the first thin film layer is larger than that of the transparent substrate. In a method for forming a thin film taper structure, which comprises a step of forming a mask on a layer and selectively etching the second thin film layer, the first thin film layer and the transparent substrate by the etching solution using the mask, A method of forming a thin film taper structure, characterized in that the end point of etching is in the middle of the transparent substrate. 3. A first thin film layer and a second thin film layer, which are etched with the same etching solution and have a higher etching rate for the etching solution toward the upper layer, are sequentially laminated in this order on a substrate, Form a mask on it,
In the method for forming a thin film taper structure having a step of selectively etching the second thin film layer and the first thin film layer with the etching solution using the mask, the etching is stopped during the etching with the etching solution, Then, by partially dissolving the mask in a solution that dissolves the mask and then again etching the second thin film layer and the first thin film layer with the etching solution, the second thin film layer is repeated. And a method of forming a thin film taper structure, which comprises selectively etching the first thin film layer. 4. A first thin film layer and a second thin film layer, which are etched with the same etching solution and have a higher etching rate for the etching solution toward the upper layer, are sequentially laminated on a substrate in this order, and In a method for forming a thin film taper structure, which comprises a step of forming a mask on the mask and selectively etching the second thin film layer and the first thin film layer with the etching solution using the mask, the mask is added to the etching solution. Is used as a mixed concentration, and the mixed concentration is such that the mask solution completely dissolves the mask at the time when the second thin film layer and the first thin film layer are etched by the etching solution. A method for forming a thin film taper structure, which is used as an unobtainable concentration.