JP3213724B2 - Bonding method and method of manufacturing optical integrated circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a bonding method and a method for manufacturing an optical integrated circuit, and more particularly to a bonding method for directly bonding a compound semiconductor crystal and a magneto-optical crystal and a method for manufacturing an optical integrated circuit.

[0002]

2. Description of the Related Art In an optical integrated circuit in which a semiconductor optical device and a magneto-optical device are integrated, the semiconductor optical device is formed on a wafer made of a compound semiconductor crystal, and the magneto-optical device is a wafer made of a magneto-optical crystal. Formed on top. In such an optical integrated circuit, it is required to reduce the optical coupling loss between the semiconductor optical element and the magneto-optical element, and for that purpose, the optical waveguide of these elements is formed with high dimensional accuracy. And it is necessary to match their optical axes with high precision.

In order to form such an optical waveguide with high dimensional accuracy, it is effective to use an epitaxial growth method. According to the epitaxial growth method, a thin film having a desired thickness can be formed with high accuracy and high reproducibility.
Therefore, an optical waveguide can be formed with sufficient dimensional accuracy by forming a thin film made of a light transmitting material on these wafers by using the epitaxial growth method and patterning the thin film. Since the compound semiconductor crystal and the magneto-optical crystal have completely different crystallographic properties, the thin films are separately formed on these wafers in the manufacture of an optical integrated circuit.

[0004] As a method of bonding the wafers, a method using an adhesive or solder is known. However, in order to reduce the optical coupling loss between the semiconductor optical device and the magneto-optical device to a sufficiently low level when using such a method, the thickness of the adhesive layer or the solder layer should be about 10 nm. It must be less than and must be realized with high reproducibility. Such control is extremely difficult in practice. Therefore, when an adhesive or solder is used for bonding the wafers, a large optical coupling loss occurs between the semiconductor optical device and the magneto-optical device, making it impossible to realize an optical integrated circuit.

A direct bonding method is known as a method for bonding the wafers without causing a large optical coupling loss. Bonding of wafers using this direct bonding method is performed by immersing these wafers in a predetermined acidic solution to modify the surfaces with hydroxyl groups, bringing the surfaces modified with hydroxyl groups into contact with each other, and further heating. . According to this method, these wafers can be joined without any interposition of an adhesive or the like, and thus the above-described problems regarding the adhesive and the solder do not occur.

However, when a compound semiconductor crystal and a magneto-optical crystal such as rare-earth iron garnet are joined using a conventional direct joining method, a direct junction may not be formed. Further, even if a direct junction can be formed, the following problem occurs. That is, processes such as optical waveguide formation and electrode formation by patterning the thin film are performed on the bonded wafer, but when the conventional direct bonding method is used, the bonding strength is insufficient. Therefore, these wafers are peeled off when the above process is performed.

As described above, a sufficient bonding strength cannot be obtained by the conventional direct bonding method, so that it is difficult not only to carry out various processes after bonding but also to manufacture an optical integrated circuit itself.

[0008]

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned problems, and makes it possible to join a compound semiconductor crystal and a magneto-optical crystal with a sufficiently high strength by a direct joining method. It is intended to provide a joining method.

Another object of the present invention is to provide a method for manufacturing an optical integrated circuit using such a bonding method.

[0010]

In order to solve the above-mentioned problems, the present invention relates to a joining method for joining a compound semiconductor crystal and a magneto-optical crystal.
Modifying the surface of the magneto-optical crystal with a hydroxyl group, activating the second surface of the magneto-optical crystal, modifying the activated second surface with a hydroxyl group, and combining the first surface modified with a hydroxyl group with a hydroxyl group A heating step in a state of contacting the second surface modified with the step (c).

The present invention also relates to a method of manufacturing an optical integrated circuit in which a semiconductor optical device and a magneto-optical device are integrated, wherein the semiconductor optical device is substantially made of a compound semiconductor crystal and the semiconductor optical device is formed. A step of joining the first substrate and a second substrate substantially made of a magneto-optical crystal and on which the magneto-optical element is formed, wherein the joining is performed by a first surface of the first substrate. Modifying with a hydroxyl group, activating the second surface of the second substrate, modifying the activated second surface with a hydroxyl group, and modifying the first surface modified with a hydroxyl group.
Providing a method for manufacturing an optical integrated circuit, which comprises heating the surface of the optical integrated circuit and the second surface modified with a hydroxyl group in contact with each other.

In the direct joining method, the hydroxyl group density on the joining surface has a very large effect on the joining strength. That is,
For example, a sufficiently high bonding strength can be obtained by modifying the second surface with a hydroxyl group at a high density.

According to the bonding method of the present invention, the second surface of the magneto-optical crystal is activated, for example, by exposing the second surface to a plasma such as an oxygen plasma. The second surface activated by such a method has high reactivity to water. By bringing pure water or the like into contact with the activated second surface, the second surface can be modified (or terminated) with a hydroxyl group at high density, and water molecules can be adsorbed thereon. According to the bonding method of the present invention, for such a reason, the compound semiconductor crystal and the magneto-optical crystal can be bonded with a sufficiently high strength.

In the method of manufacturing an optical integrated circuit according to the present invention, the first base and the second base are joined by the above-described joining method. Therefore, a sufficiently high direct bond can be formed between these substrates.

According to the method of manufacturing an optical integrated circuit of the present invention,
Usually, prior to bonding these substrates, a thin film made of a light-transmitting material is formed on at least one of the first and second substrates, and the thin film is patterned.
Further, the first and second thin films are usually formed by using an epitaxial growth method. By using the epitaxial growth method, the first and second thin films can be formed to a desired thickness and with high accuracy, so that optical coupling loss can be more effectively suppressed.

In the method of the present invention, the first surface and the second surface need to be sufficiently smooth. When the smoothness of the first surface and the second surface is low, if necessary, smoothing by polishing is performed before joining.

In the method of the present invention, the compound semiconductor crystal
For example, a crystal such as GaAs or InP may be used.
And the magneto-optical crystal is Y_ThreeFe_FiveO₁₂And (LuNd
Bi)_Three(FeAl)_FiveO₁₂Multi-element oxide crystals such as
Can be mentioned. Note that Y_ThreeFe_FiveO₁₂Ya (LuN
dBi)_Three(FeAl)_FiveO₁₂Magneto-optical crystals such as
Gd having the same crystal structure_ThreeGa_FiveO₁₂And C
a, Mg, Zn-added Gd _ThreeGa_FiveO₁₂On a substrate made of crystals
Is epitaxially grown. Also, as a light transmissive material
The Ga_1-xAl_xAs (0 ≦ x ≦ 1, GaAlAs and
Abbreviations), Ga_xIn_1-xAs_yP_1-y(Abbreviated as GaInAsP)
Notation), (BiY)_ThreeFe_FiveO₁₂, (CeY)_ThreeFe_FiveO₁₂,
And (BiGd)_ThreeFe_FiveO₁₂And the like.

In the method of the present invention, the bonding between the compound semiconductor crystal and the magneto-optical crystal is performed, for example, by the following procedure. A method for manufacturing an optical integrated circuit will be described as an example.

First, a wafer made of a compound semiconductor crystal and a wafer made of a magneto-optical crystal are prepared. These wafers are mirror-polished as required to smooth their surfaces. Next, a thin film made of a light-transmitting material is formed on at least one of these using a normal epitaxial growth method. Here, a case is considered in which the thin film is formed on both a wafer made of a compound semiconductor crystal and a wafer made of a magneto-optical crystal. Further, the thin film formed on the wafer made of the compound semiconductor crystal and the thin film formed on the wafer made of the magneto-optical crystal are patterned by using a lithography technique.

The optical integrated circuit formed by this method has a shape in which the main surfaces of these wafers are partially bonded to each other, and the optical waveguide is formed on the bonding surface side in each wafer. . That is, at the junction, the optical waveguide is located between these wafers. Normally, it is impossible to pattern a thin film located between these wafers after bonding, so that in order to form an optical integrated circuit, at least an optical waveguide at a bonding portion is formed before bonding the wafers.

After patterning the thin film, the organic substances adhering to the surface of the wafer are removed by washing with ethanol, acetone or the like. Thereafter, the wafer made of the magneto-optical crystal is quickly carried into the vacuum vessel. After arranging the wafer on one of a pair of electrodes opposed to each other in the vacuum vessel so that the joint surface is exposed to plasma, the gas in the vacuum vessel is exhausted. At this time, the ultimate degree of vacuum in the vacuum vessel affects the degree of cleaning of the wafer surface, and thus is desirably 10 ⁻³ Pa or less.

After evacuation, a trace of high-purity oxygen is introduced into the vacuum vessel. The purity of the introduced oxygen is usually 99.99.
9% or more. The introduction of high-purity oxygen is usually controlled so that the pressure in the vacuum vessel is about 0.1 to 100 Pa.

Thereafter, a high frequency (1) is applied between the pair of electrodes.
A microwave of 3.56 MHz is applied to generate plasma discharge in the vacuum chamber. At this time, an appropriate amount of high-purity oxygen is continuously introduced into the vacuum vessel.

After the plasma treatment is performed, for example, for about 1 second to 3 minutes, the application of the microwave is stopped to stop the plasma discharge in the vacuum vessel. Further, the introduction of high-purity oxygen into the vacuum vessel is stopped, and the gas in the vacuum vessel is exhausted. At this time, the ultimate degree of vacuum in the vacuum vessel is, for example, 10 ⁻³ Pa or less.

Thereafter, the wafer subjected to the plasma treatment is carried out of the vacuum vessel and immediately immersed in pure water. Usually, the immersion time in pure water is 1 second or more. As described above, the surface of the wafer made of the magneto-optical crystal is modified with the hydroxyl group.

Next, the wafer made of the magneto-optical crystal and the wafer made of the compound semiconductor crystal are brought into contact with each other in the form described for the optical integrated circuit. Further, by heating these wafers, a direct bond is formed. The bonding surface of the wafer made of the compound semiconductor crystal is modified in advance with a hydroxyl group before bonding these wafers.

The surface treatment of the compound semiconductor wafer may be performed by any method as long as the bonding surface can be modified with a hydroxyl group. For example, if the bonding surface is phosphoric acid (H ₃ P
Conventional methods of treating with an acidic solution such as an aqueous solution of O ₄ ) can be used. Further, the same method as described for the wafer made of the magneto-optical crystal may be used. In this case, a stronger direct bond can be formed.

The heating temperature of these wafers is, for example, 100 to 700 ° C., and the heating time is, for example, 1 second to 12 seconds.
0 minutes. After bonding the wafers as described above, if necessary, an optical waveguide is formed by patterning the thin film formed on the wafer by using a lithography technique, and further, electrodes and the like are formed on the wafer. Thus, an optical integrated circuit is obtained.

In these post-bonding steps, the bonded wafer is subjected to, for example, wet etching using a reactive solution, dry etching in plasma, crystal regrowth, resist coating and baking thereof. Such various processes are performed as a device process required for manufacturing an optical integrated circuit.

As described above, according to the conventional direct bonding method, it was not possible to form a bond having a strength enough to withstand such a device forming process. For it,
According to the method of the present invention, a direct bond having a sufficiently high strength can be formed, so that the wafer does not peel even when the above-described device forming process is performed. Therefore, according to the method of the present invention, the device-forming process can be performed after bonding these wafers.

As described above, according to the present invention, since various device processes can be performed after wafer bonding, optical axes can be aligned with high accuracy in the in-plane direction, and optical coupling loss can be reduced. can do. Furthermore, since the optical integrated circuit can be cut into chips after completion, more chips can be obtained from a pair of wafers.

That is, according to the present invention, an optical integrated circuit can be manufactured with a high degree of freedom, and cost reduction, improvement in mass productivity, and high performance can be realized.

[0033]

Embodiments of the present invention will be described below with reference to the drawings. [Modification of Wafer Surface with Hydroxyl Group] First, it was confirmed by the following method that the bonding surface was modified with a hydroxyl group at a high density according to the method of the present invention.

That is, a wafer made of Gd ₃ Ga ₅ O ₁₂ and a magneto-optical crystal (LuNdBi) ₃ (FeA
l) Each of the ₅ O ₁₂ wafers was subjected to an oxygen plasma treatment for 30 seconds under the conditions described above.
Next, the wafer after the oxygen plasma treatment was immersed in pure water for 10 minutes.

Further, the hydrophilicity of the surface of the immersed wafer was evaluated by a contact angle method. The contact angle method is
Predetermined volume (1 microliter in this embodiment)
Is dropped on the surface of the object to be measured, as shown in FIG.
This is a method for evaluating the hydrophilicity of the surface of the measurement target by measuring the angle (contact angle) θ between the surface of the measurement target 1 and the surface of the water droplet 2. When a small contact angle is obtained by this contact angle method, it is determined that the hydrophilicity of the wafer surface is high and the density of hydroxyl groups on the wafer surface is high.

For comparison, the hydrophilicity of the surface of an untreated wafer, a wafer treated with an acidic aqueous solution, and a wafer subjected to only oxygen plasma treatment were evaluated by a contact angle method. As the acidic aqueous solution, an H ₃ PO ₄ aqueous solution was used.

In a conventional treatment method in which the surface of a wafer is modified with a hydroxyl group by immersing the wafer in an acidic solution, Gd ₃ Ga ₅ O ₁₂ or (LuNdBi) ₃ (FeA
l) For a wafer made of another element oxide crystal such as ₅ O ₁₂ , an H ₃ PO ₄ aqueous solution can be used.

Table 1 below shows the treatment methods and the measurement results.

[0039]

[Table 1]

As shown in Table 1, both the Gd ₃ Ga ₅ O ₁₂ wafer and the (LuNdBi) ₃ (FeAl) ₅ O ₁₂ wafer were subjected to oxygen plasma treatment and pure water treatment according to the method of this embodiment. When immersion was performed, a smaller contact angle could be realized as compared with the case where the treatment was performed by another method. That is, it was confirmed that according to the method of the present invention, the surface of the magneto-optical crystal can be modified with a hydroxyl group at a high density.

[Direct Bonding] Next, it was confirmed that a direct bond having a sufficient strength was formed according to the method of the present invention. Instead of directly measuring the bonding strength, the following method is used to determine whether a direct bond is formed and whether the formed direct bond has sufficient resistance to the device fabrication process. Examined.

That is, first, a compound semiconductor crystal is used.
The bonding surface of the InP wafer is H _TwoSO_FourAqueous solution and H_Two
O_TwoLiquid mixture with aqueous solution (H_TwoSO_Four: H_TwoO_Two: H_TwoO = 5:
1: 1) for 3 minutes. Next, this wafer and
Perform oxygen plasma treatment and immersion in pure water under the above conditions
Gd_ThreeGa_FiveO₁₂Contact the wafers and remove these wafers
Heated at 220 ° C. for 90 minutes. Note that the contact of these wafers
The joint surface is smoothed in advance. The above-mentioned processing
Performed on sample, this process forms a direct bond
I checked whether all samples were smelled
Thus, a direct bond could be formed.

For comparison, it was examined whether or not a direct bond was formed when the conventional method was used. That is, Gd subjected to oxygen plasma treatment and immersion in pure water
Instead of ₃ Ga ₅ O ₁₂ wafers, bonding surface H ₃ PO ₄ is performed for a plurality of samples processed as described above in the same manner except for using Gd ₃ Ga ₅ O ₁₂ wafers were immersed for 10 minutes in an aqueous solution Then, it was examined whether or not a direct bond was formed by this treatment. As a result, in the conventional method, a direct bond was not formed in two-thirds or more of the samples.

Next, the resistance of the direct junction formed by the method according to the present embodiment to the device-forming process was examined.
Table 2 below shows the conditions and results of the device fabrication process.

[0045]

[Table 2]

As shown in Table 2, the direct junction formed by the method according to the present example had sufficient resistance to any of the processes. From the above results, it was confirmed that according to the method of the present invention, a direct junction having practically sufficient strength can be reliably formed between the compound semiconductor crystal and the magneto-optical crystal.

[Optical Integrated Circuit] It has been confirmed by the following method that an optical integrated circuit having a small optical coupling loss can be realized according to the method of the present invention.

First, a thin film made of GaInAsP, which is a light transmitting material, was formed on one main surface of a wafer A made of InP, which is a compound semiconductor crystal, by an epitaxial growth method. The source gas is (C ₂ H ₅ ) ₃ G
a, (CH ₃ ) ₃ In, AsH ₃ , and PH ₃ were used, and the thickness of the thin film was 0.4 μm.

Also, on one main surface of the wafer B made of Gd ₃ Ga ₅ O ₁₂ , a magneto-optical crystal (LuNdBi) ₃ (FeA
It was deposited a thin film made of l) ₅ O _12. As raw materials, Lu ₂ O ₃ , Nd ₃ O ₃ , Bi ₃ O ₃ , Fe ₂ O ₃ , Al ₂ O ₃
And the thickness of the thin film was 0.5 μm.

Next, the thin film formed on the wafer B was patterned using lithography and etching to form optical waveguides having widths of 2 μm, 3 μm, 5 μm, and 10 μm, respectively.

Further, the above-described oxygen plasma treatment and immersion in pure water were performed on the bonding surfaces of the wafers A and B.

Thereafter, the surfaces of the wafers A and B on which the thin films were formed were partially brought into contact with each other. Furthermore, these wafers A and B were heated at 220 ° C. for 90 minutes to form a direct bond.

Next, the thin film formed on the wafer A was collectively patterned by using a lithography technique and an etching technique to form an optical waveguide. In addition, the peeling of the wafer due to the formation of the optical waveguide did not occur.

The optical waveguide formed as described above has extremely high dimensional accuracy not only in the in-plane direction of the wafer but also in the thickness direction. Further, the optical axes of the optical waveguides could be matched with extremely high precision between these wafers. That is, it was confirmed that the method of the present invention can realize an optical integrated circuit with small optical coupling loss.

[0055]

As described above, in the method of the present invention, since the surface of the magneto-optical crystal is modified with hydroxyl groups after being activated, a high hydroxyl group density can be realized. Therefore, a strong direct junction can be formed between the compound semiconductor crystal and the magneto-optical crystal.

That is, according to the present invention, a bonding method capable of bonding a compound semiconductor crystal and a magneto-optical crystal with sufficiently high strength by a direct bonding method, and an optical integrated circuit using such a bonding method Is provided.

[Brief description of the drawings]

FIG. 1 is a side view schematically showing a contact angle method.

[Explanation of symbols]

 1: Measurement object 2: Water drop

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int. Cl. ⁷ , DB name) G02B 6/12-6/14 H01S 5/00-5/50 H01L 41/22 JICST file (JOIS)

Claims (6)

(57) [Claims] 1. A bonding method for bonding a compound semiconductor crystal and a magneto-optical crystal, wherein the first surface of the compound semiconductor crystal is modified with a hydroxyl group, and the second surface of the magneto-optical crystal is oxygen plasma. Exposing the activated second surface to a hydroxyl group,
And a step of heating in a state where the first surface modified with the hydroxyl group and the second surface modified with the hydroxyl group are in contact with each other. 2. A step of modifying the second surface with a hydroxyl group.
Contacting water with said activated second surface
The bonding method according to claim 1, comprising: 3. A step of modifying the first surface with a hydroxyl group.
Prior to the step of activating the first surface further comprises
The method according to claim 1 or 2, wherein
Joining method. 4. The semiconductor optical device and the magneto-optical device are integrated.
A method of manufacturing an integrated optical circuit, wherein the semiconductor optical device is substantially made of a compound semiconductor crystal.
A first substrate formed and substantially free of magneto-optic crystal.
Bonding the second base on which the magneto-optical element is formed
Comprising the step of step of the bonding includes modifying the first surface of the first substrate by a hydroxyl group, a second surface of the magneto-optical crystal is exposed to an oxygen plasma
Activating , modifying the activated second surface with hydroxyl groups,
And the first surface modified with the hydroxyl group and the hydroxyl group
Heating in contact with the modified second surface.
And a method for manufacturing an optical integrated circuit. 5. The method according to claim 1 , further comprising:
A light transmissive material on at least one of the second surfaces;
This thin film is formed by an epitaxial growth method.
Characterized by further comprising a step of patterning
The method for manufacturing an optical integrated circuit according to claim 4. 6. The method according to claim 1, wherein the first surface is not modified with a hydroxyl group.
And activating the first surface.
6. The method for manufacturing an optical integrated circuit according to claim 4 or claim 5.