JPS6314322B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method of manufacturing a silicon diffraction grating for an optical demultiplexer.

Optical demultiplexers are important as wavelength multiplexing devices or wavelength demultiplexing devices for optical fiber communications, and the performance of these devices is that they have low optical loss, excellent resolution, and low price. This is especially important. Diffraction gratings for optical demultiplexers manufactured from silicon single crystals are attracting attention because they can be manufactured in large quantities at low cost.
Conventional silicon diffraction gratings have difficulty in increasing the light dispersion angle, making it difficult to apply them to optical communication systems with high multiplicity. It has been desired to realize a manufacturing method.

Generally, the dispersion angle of a diffraction grating is determined by the change in the incident wavelength.
It can be expressed as a ratio of the change in diffraction angle dθ to dλ, and the following relational expression holds true using the grating spacing d of the diffraction grating.

dθ/dλ∝1/d (1) Therefore, in order to increase the dispersion angle of the optical demultiplexer, it is necessary to develop a technique for narrowing the grating spacing d of the diffraction grating.

However, the minimum limit of the lattice spacing d of a diffraction grating made of silicon single crystal is limited mainly by the following two points.

(i) Limits of creating fine patterns on etching masks using photolithography (ii) Limits of reproducibility and controllability of anisotropic etching Next, we will manufacture a diffraction grating for an optical demultiplexer from a conventional silicon single crystal substrate. The manufacturing method and its problems will be explained.

When chemically etching a silicon single crystal with a chemical having a certain composition, the etching rate exhibits a significant difference depending on the crystal plane orientation of the silicon single crystal. For example, when a KOH aqueous solution is used as an etchant, the plane orientation of the silicon single crystal is 100.
The etching speed of the surface changes more than 10 times faster than that of the 111 surface.

As shown in FIG. 1a, a lattice-shaped etching mask 2 is formed on a silicon single crystal wafer 1, which is a substrate whose crystal plane is tilted by θ° from the <100> direction, and then the anisotropic etchant is etched. A diffraction grating having a slanted V-shaped cross section as shown in FIG. 1b is formed by etching. The single crystal plane exposed here has the slowest etching rate111
side (hereinafter this side will be referred to as A side). moreover,
As the etching continues, the etching progresses gradually on the A side as shown in FIG. As the etching continues,
As shown in FIG. 1d, the final stage is reached, and side etching progresses from both sides of the lower part of the etching mask 2, and finally the entire surface of the silicon single crystal under the etching mask 2 is etched away, and the etching mask 2 itself is etched away. Peel it off and finish etching.

However, the conventional method shown in FIGS. 1a to 1d has two major problems as described below.

The problem in FIG. 1 is that it is necessary to use an etching mask that is extremely resistant to the chemicals used as the etchant, making it difficult to miniaturize the diffraction grating pattern.

In other words, etching from stage a to stage b in Figure 1 is completed in a short time, but etching from stage b to stage c and d
At this stage, the etching time becomes very long, so an etching mask with extremely strong chemical resistance is required, and in such cases, the thickness of the etching mask is usually increased. Therefore, it becomes difficult to miniaturize the diffraction grating pattern.

The problem with the conventional method shown in FIG. 2 is that the etching conditions using the anisotropic etchant must be extremely strict.

That is, although a long etching time is required from step b to step c and d in FIG. 1, the etching rate must be extremely strictly controlled to prevent overetching or underetching. FIG. 2 shows an example of a defect caused by a lack of etch control. In FIG. 2, 4 indicates underetching, and side etching is still insufficient. 5 indicates a normal etching state. Further, 6 indicates overetching, and if etching is continued from a normal etching state, a second diffraction grating 6a is generated. When a defect like the second diffraction grating 6a occurs, the resolution of the diffraction grating decreases and the diffraction loss increases, which significantly impairs the performance of the diffraction grating.

This invention is an attempt to solve the problems of the conventional method described above, and after forming a rectangular groove with a predetermined depth and a predetermined width in a silicon single crystal substrate in the first step, the surface orientation is adjusted in the second step. To provide a method for manufacturing a silicon diffraction grating, in which the etching time is shortened by performing dependent etching from the rectangular groove and increasing the side etching speed, and the amount of etching can be controlled with high precision. It is an object.

Hereinafter, FIGS. 3 a to d will be described for one embodiment of the present invention.
Please refer to the following for details.

First, as shown in FIG. 3a, a lattice-shaped etching mask 2 is formed on a silicon single crystal wafer 1, which is a substrate, in the same manner as in the conventional method described above.
Next, as the first step, as shown in Figure 3b,
A rectangular groove 7 having a depth h and a width l is formed in a portion of the silicon single crystal wafer 1 between the etching masks 2.
The side surfaces of this rectangular groove 7 are preferably flat and perpendicular to the etching mask 2 side, and such rectangular groove 7 can be formed by a method such as ion milling. Note that, hereinafter, the vertical side surface of the rectangular groove as described above will be referred to as the S-plane. Next, as a second step, etching is performed from the rectangular groove 7 portion using the crystal plane orientation dependence of the etching rate of the silicon single crystal. That is, etching is carried out using the same anisotropic etchant as in the conventional method described above, and after passing through the etching state with the cross-sectional shape shown in FIG. 3c, a diffraction grating with the cross-sectional shape shown in FIG. 3d is completed. The amount of side etching in this second step is defined as x ₀ . Furthermore, when the diffraction grating is completed, all the surfaces of the silicon single crystal wafer that are in direct contact with the etching solution are the A planes, and for this reason, the etching speed of the lower part of the etching mask 2 after the diffraction grating is completed is rapidly slowed down. .

Next, the operation of the method of manufacturing a silicon diffraction grating for an optical demultiplexer according to the above-described embodiment of the present invention will be described in comparison with a conventional method. FIG. 4 shows the relationship between the amount of side etching and etching time using an anisotropic etchant. In Figure 4, line 8
is the etching characteristic according to the method of the present invention, and line 9 is the etching characteristic according to the conventional method. Let the side etching speed of the A side (line 9) be R, and the side etching speed of the S side (line 8) be mR.
m indicates the dependence of the etching rate on crystal plane orientation, and m is usually considerably larger than 1. In line 9 according to the conventional method, etching of the S side is completed at time t' after etching is started, and etching of the A side is started, so that the etching rate also becomes R. Let the side etching amount at this time be x ₀ . This x ₀ has a size defined by the depth h and width of the rectangular groove 7 shown in FIG. 3b.

Considering the reproducibility and yield in the manufacture of silicon diffraction gratings, as shown in Figure 2 and described above, the reproducibility and controllability of anisotropic etching have a large effect on the reproducibility and yield performance of the manufacture of diffraction gratings. Affect. Etching speed is not limited to anisotropic etchants, but generally varies depending on the composition, temperature, flow condition of etching solution, etc.
This tendency also occurs when a KOH aqueous solution is used as an anisotropic etchant for silicon single crystals. Lines 10 and 11 in FIG. 4 show changes over time in the amount of side etching when the overall etching rate increases by α for some reason. Next, consider the amount of side etching. The amount of side etching changes as shown below as the etching conditions change.

The side etching speed of side A is (1+α)R The side etching speed of the S plane is (1+α)mR Here, it is assumed that α<<1.

If etching is carried out for a predetermined period of time t' to t'' in this state, over-etching will occur.As shown in FIG.
If Δx″ is used, then Δx″=αx ₀ ……(2) is obtained by simple calculation using the conventional method. Then, in the method according to the invention of line 10,
When the side etching amount reaches x ₀ , the etching speed changes. If the time at that time is t'-Δt', then x ₀ = (1+α)mR(t'-Δt') From now on, Δt'≒α/mRx ₀ Therefore, the overetching amount is Δx' = (1+α)RΔt ′=(1+α)Rαx ₀ /mR ≒αx ₀ /m ……(3), and from the above equations (2) and (3), Δx′≒1/m・Δx″.In other words, in the method of this invention, , it can be seen that the amount of overetching is reduced to approximately 1/m of that of the conventional method.

As explained above, in the method for manufacturing a silicon diffraction grating for an optical demultiplexer according to the present invention, after forming rectangular grooves in a silicon single crystal substrate in the first step,
By performing surface orientation-dependent etching from the rectangular groove in the process and increasing the side etching speed of the lower part of the etching mask, the etching time can be shortened, making it possible to use an etching mask with a fine pattern, and reducing the amount of etching. Since it has excellent controllability, it has the advantage of being suitable for manufacturing silicon diffraction gratings with large dispersion angles.

[Brief explanation of the drawing]

1A to 1D are partial perspective sectional views showing a conventional method for manufacturing a silicon diffraction grating in the order of steps; FIG. 2 is a partial perspective sectional view showing an example of a defect in a silicon diffraction grating;
3A to 3D are partial perspective cross-sectional views showing the manufacturing method of a silicon diffraction grating according to an embodiment of the present invention in the order of steps, and FIG. It is a figure showing comparison with a method. 1...Silicon single crystal wafer (substrate), 2...
Etching mask, 7... rectangular groove.

Claims (1)

[Claims] 1. In a method for manufacturing a silicon diffraction grating for an optical demultiplexer that utilizes the crystal plane orientation dependence of the etching rate of a silicon single crystal, a grating-shaped etching mask is formed on a silicon single crystal substrate in the first step, and then the etching mask is A rectangular groove with a predetermined depth and a predetermined width is formed in the silicon single crystal substrate between the masks, and then, in a second step, surface orientation-dependent etching is performed from the rectangular groove using a chemical etching solution, and the side of the lower part of the etching mask is etched. A method for manufacturing a silicon diffraction grating characterized by increasing the etching speed.