JPH0786106A - Method for manufacturing composite substrate material - Google Patents

Abstract

(57) [Summary] [Object] To provide a method of manufacturing a composite substrate material which is composed of wafers having different coefficients of thermal expansion, has no warp and is practical. [Structure] A quartz plate 11 having both sides mirror-polished has a coefficient of thermal expansion different from that of the quartz plate 11, and is sandwiched between glass plates 12 having at least one surface mirror-polished. After the mirror surfaces are in close contact with each other so that they face each other, they are heated and directly bonded to each other firmly without warping.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of manufacturing a composite substrate material, and more particularly to a method of manufacturing a warp-free composite substrate material.

[0002]

2. Description of the Related Art In general, substrates having a laminated structure of materials having different properties are currently used in various fields,
When obtaining such a substrate, various thin film forming methods utilizing chemical or physical methods are used. However, a technique called wafer bonding has been attracting attention in recent years as an alternative method. This technique involves mirror-finishing the surface of a single-crystal or poly-crystal wafer, cleaning the mirror surfaces so that dust and organic substances do not exist at their interfaces, and then heating them while they are in contact with each other in a clean atmosphere. By doing so, it is possible to obtain a bonded wafer having a bonding strength equivalent to that of the thin film forming method described above.
Adhesion can be seen even at room temperature if the surfaces are clean,
By undergoing heat treatment, stronger bonding that can withstand various post-treatments can be obtained. (Hereinafter, this method is referred to as a direct bonding method.)

[0003]

However, in the above technique, when the substrates having different properties, particularly the substrates having different coefficients of thermal expansion are directly joined, the above-mentioned heating process is performed, so that the substrates between the joined substrates are subjected to each other. There is a problem that warpage occurs due to the difference in thermal expansion coefficient. Further, because of this warp, when forming an electrode pattern directly on a bonded wafer or performing a fine processing, it is not possible to perform a precise processing over a wide range, and thus a fine pattern formation represented by photolithography can be performed. It was an obstacle to using the technology.

Further, when one substrate or both substrates are mechanically polished to obtain a thin film layer of a single crystal, it is difficult to form a uniform thin plate due to the warp, and the film quality is inferior but uniform film. It has not been possible to make full use of its superiority over the conventional thin film forming technology capable of obtaining a large thickness.

The present invention solves the above-mentioned conventional problems, and provides a manufacturing method for obtaining a bonded wafer having no warp even when substrates having different thermal expansion coefficients are directly bonded. It is intended.

[0006]

In order to achieve the above object, the present invention has one surface of a first substrate, both surfaces of which are mirror-polished, having a coefficient of thermal expansion different from that of the first substrate. A material having a second substrate, at least one surface of which is mirror-polished, adhered so that the mirror surfaces face each other, and the other surface of the first substrate has substantially the same coefficient of thermal expansion as the second substrate. The third substrate, of which at least one surface is mirror-polished, is likewise adhered so that the mirror surfaces face each other, and then heated to firmly and directly bond each other.

[0007]

As described above, when the substrates having different thermal expansion coefficients are joined to each other, the warpage of the substrates caused by the difference in the thermal expansion coefficient is caused by the two substrates having substantially the same thermal expansion coefficient. It is sandwiched and brought into close contact with each other, and heat treatment cancels them out, so that a bonded substrate having no warp can be obtained even if the substrate sandwiched in the middle has any coefficient of thermal expansion.

[0008]

【Example】

(Embodiment 1) FIGS. 1A and 1B are side views showing the steps of this embodiment. In this embodiment, as a substrate to be bonded, as shown in FIG.
ATcut crystal plate 11 for MHz oscillator and two 10m
M-square, 0.25 mm thick soda-lime glass plate 12 for substrates
Was used. In addition, these substrates, that is, the crystal plate 11,
The surface of the glass plate 12 to be adhered is mirror-polished.

The mirror-polished bonding surface of these substrates is cleaned by a precision cleaning technique used in the semiconductor field so that particles and organic substances do not exist, and in a clean atmosphere in a clean room, As shown in Fig. 1 (b), the contact surfaces are closely contacted with each other so that dust or dirt does not enter between them.
Integrate as shown in.

Even with this as it is, the bonding strength between these substrates shows an adhesive strength of about ²⁰ to 30 kg / cm ^{2 in} terms of tensile strength, but the adhesive strength at this stage is the hydrogen between OH groups existing at the bonding interface. It is due to binding and easily peels off when water penetrates into the interface. However, when heat treatment is applied to this substrate, a dehydration reaction occurs at the adhesive interface, hydrogen bonds are replaced by covalent bonds via oxygen, and chemically and physically stable and strong direct bonding uses an adhesive. Achieved without. The adhesive strength at this stage is
The tensile strength shows a value of 100 kg / cm ² or more, and it is a stable joint that does not peel off even when immersed in water or acid.

Therefore, in order to obtain the above-mentioned stable bonding, the substrate as shown in FIG. 1 (b) is subjected to heat treatment at 400 ° C. for 1 hour in an electric furnace. c)
A quartz substrate 11 having a sandwich structure in which a glass plate 12 is sandwiched between the glass plates 12 as described in (1), and a warp-free composite substrate material having a strong and stable joint can be obtained. The bonding strength of both bonding interfaces is 100 kg / cm ² or more, and there is no problem in strength. Also, as a result of measuring the warp of the substrate with a surface roughness meter, the amount of warp depends on the processing accuracy of the substrate Within the range of warpage, no new warpage due to heat treatment was observed.

By the way, quartz is a biaxial crystal, and its coefficient of thermal expansion differs depending on its crystal axis. For example, at a typical heat treatment temperature of 400 ° C. in this combination, 185 × in a direction parallel to the axis called the a axis.
10 ^-7 / ° C, 10 in the direction parallel to the axis called the c-axis
It has a value of 7 × 10 ⁻⁷ / ° C. The a-axis and the c-axis are orthogonal to each other. ATcut is a quartz plate that is cut at an angle with respect to the c-axis of the quartz single crystal, and the thermal expansion coefficient of the quartz plate 11 of 10 mm square used in this example is 400 ° C. 186 × 10 ^-7 / in the horizontal direction on 11 parallel sides
C., having a value of ^{132.times.10.sup.-7} / .degree.

On the other hand, the soda-lime glass plate 12 used in this embodiment has an isotropic coefficient of thermal expansion, and its value is 87 ×.
It is 10 ^-7 / ° C. Due to the difference in the coefficient of thermal expansion, when the crystal plate 11 and the glass plate 12 are directly bonded without using the sandwich structure, different warps are generated in the vertical directions on the bonded substrate planes after the heat treatment. This is because the glass and quartz have different thermal expansion coefficients, especially in the case of a biaxial crystal such as quartz, it is difficult to match the thermal expansion coefficient of both substrates, unless this method is used,
It is difficult to obtain a warp-free substrate.

An embodiment of manufacturing a crystal resonator using a direct bonding substrate having a structure in which the crystal plate is sandwiched by glass plates will be described with reference to the drawings. 2A to 2E show cross-sectional views of the manufacturing process of the crystal unit.

As shown in FIG. 2 (a), a warp-free direct bonding substrate having a structure in which a crystal plate 21 is sandwiched by glass plates 22 is provided with openings as shown in FIG. 2 (b). The mask 23 is covered except for the portion. Then, etch from both sides with an etching solution containing hydrofluoric acid,
As shown in FIG. 2C, the openings 2 are formed on both sides of the crystal plate 21.
4 is formed. The glass part remaining in this process is the crystal plate 21.
Play a role of holding. Since the etching rate of the crystal with respect to the etching liquid is slower than that of glass, the crystal portion functions as an etch stop, and the mask 23 is removed by using the organic solvent to obtain the structure shown in FIG. 2D. Finally, by forming a pair of excitation electrodes 25 sandwiching the crystal plate 21, a basic structure of the crystal resonator as shown in FIG. 2E is obtained.

Further, since this substrate has no warp, the mask 23 can be formed by using a fine pattern forming technique as used in the field of semiconductors, so that a micro crystal oscillator can be precisely manufactured.

In this embodiment, a glass plate having a different coefficient of thermal expansion from that of quartz is used in order to make the effect easy to understand. It is natural that a quartz-glass bonded substrate having very little distortion and no warp can be obtained.

(Embodiment 2) In this embodiment, as in Embodiment 1, an ATcut quartz plate for a 30 MHz oscillator having a 10 mm square and a soda lime glass plate for a substrate having a 10 mm square and a 0.25 mm thickness are used. 3A to 3D are side views showing the steps of this embodiment. The cleaning method and the joining conditions are the same as in the first embodiment. The difference is that, as shown in FIG. 1A, the glass plate 31 has a structure in which a glass plate 31 is sandwiched between a crystal plate 32 and a crystal plate 33 having a silicon film 34 formed on at least the surface on the side bonded to the glass plate 31. is there. The bonding surface of these substrates is mirror-polished.

After precisely cleaning the bonding surfaces of these three substrates, the mirror surfaces are brought into close contact with each other so that the mirror surfaces face each other, and further heat treatment is carried out at 400 ° C. for 1 hour in an electric furnace in order to strengthen the bonding. . As a result, as shown in FIG.
A warp-free direct-bond composite substrate material was obtained. In this case, the silicon film 34 is as thin as several thousand A and is not as dense as that of a single crystal. Therefore, even if the coefficient of thermal expansion is different, it plays a role of a buffer layer and does not warp the substrate. Absent.

This substrate is double-sided polished as shown in FIG. 3C, and the crystal plate 32 is polished to a desired thickness. At this time, there is no separation from the joint surface during polishing, and the adhesive strength is sufficient to withstand the polishing process. Further, since this substrate has no warp, the crystal part is uniformly polished and can be thinned to 5 μm or less.
After that, as shown in FIG. 3D, one crystal plate 33 is lifted off by using an etchant that selectively etches the silicon film 34, for example, hydrazine.

In this way, a composite substrate material in which the crystal plate 32 of 5 μm or less is strongly bonded directly to the glass plate 31 is obtained. Further, this substrate has no warp and can be precisely processed by using a fine pattern forming technique used in semiconductor technology.

An example of manufacturing a crystal resonator using the direct bonding substrate having the above structure will be described with reference to the drawings. FIGS. 4A to 4E are cross-sectional views of the manufacturing process of the crystal unit.

A glass plate as shown in FIG. 4B is formed on the surface of the composite substrate material in which a crystal plate 41 having a very thin thickness of 5 μm or less as shown in FIG. A mask 43 for forming an opening is formed in a part of 42. This substrate is etched from the glass side with an etching solution containing hydrofluoric acid to form openings 44 as shown in FIG. 4 (c). The glass portion remaining in this step plays a role of holding the crystal plate 41. In addition, since the etching rate of quartz for etching liquid is slower than that of glass,
The crystal portion functions as an etch stop, and the structure shown in FIG. 4D is easily obtained by removing the mask 43 with an organic solvent. Finally, by forming a pair of excitation electrodes 45 sandwiching the crystal plate 41, the basic configuration of the crystal resonator as shown in FIG. 4E is completed.

In such a crystal unit, the thickness of the crystal plate 41 can be made extremely thin by the process of the present embodiment, so that the crystal unit can be operated at a super high frequency. Further, the holding of the crystal plate 41 is realized by direct bonding, and since it is not necessary to use an adhesive, the characteristics are stable.

In the present embodiment, as a method for obtaining such a substrate, the silicon film 3 is formed on the bonding surface of the crystal plate 33.
Although No. 4 was formed, it goes without saying that the same effect can be obtained by forming a silicon film on the bonding surface on the glass plate side or both. (Embodiment 3) As described in Embodiment 2, by forming the thin film layer on the joint surface, the substrate material can be lifted off, and further, those which cannot be directly joined can be joined.

In the direct bonding, the flatness of the surfaces to be bonded is very important, and the thin film layer can be used to obtain this flatness. Next, the example is shown. 5A to 5C are side views showing the steps of this embodiment.

As shown in FIG. 5A, the silicon substrate 5
A structure in which 1 is sandwiched between two gallium arsenide substrates 52 is formed. Although both sides of the silicon substrate 51 to be sandwiched are mirror-polished, both sides of the gallium arsenide substrate 52 do not necessarily have to be polished.

Next, as shown in FIG. 5B, a silicon oxide film 53 is formed on the bonding surface side of the gallium arsenide substrate 52. After that, the surface of the silicon oxide film 53 is mirror-polished to have a surface roughness of 5
It is flattened so as to be not more than 00 nm.

Further, the silicon oxide film 53 polished to a mirror surface
Then, the surface of the silicon substrate 51 is cleaned and adhered by the same method as described above. And finally 30 in the electric furnace
By applying heat treatment at 0 ° C. for 1 hour, FIG.
A silicon gallium arsenide composite substrate material having no warp as shown in (1) can be obtained. Since the silicon oxide film 53 is as thin as several thousand A or less and is not as dense as that of a single crystal, it functions as a buffer layer, and the gallium arsenide substrate 52 and the silicon substrate 51.
Since there is no strain in the single crystal, it rarely affects the characteristics of the single crystal. Further, since the silicon oxide film 53 is transparent in a wide wavelength range, it does not prevent the passage of light even when this substrate is used as a substrate for an optical element. for that reason,
This substrate can be put to practical use as a composite substrate for optical elements.

Further, after forming the silicon oxide film 53,
By polishing only the silicon oxide film 53, the mechanical strain generated during polishing does not affect the characteristics of the substrate, so that a composite substrate material having excellent characteristics can be obtained.

(Embodiment 4) Embodiment 4 is almost the same as Embodiment 3, except that a silicon nitride film is used as one intermediate layer at the time of bonding instead of a silicon oxide film. next,
The example is shown. FIG. 6 is a side view showing the steps of this embodiment.

As shown in FIG. 6A, the silicon substrate 6
1 is sandwiched between gallium arsenide substrates 62 and 63 to form a structure. Although both sides of the silicon substrate 61 to be sandwiched are mirror-polished, both sides of the gallium arsenide substrates 62 and 63 do not necessarily have to be polished.
Next, as shown in FIG. 6B, a gallium arsenide substrate 62
A silicon oxide film 64 is formed on the bonding surface side, and a silicon nitride film 65 is formed on the bonding surface side of the other gallium arsenide substrate 63.

Then, the surfaces of the silicon oxide film 64 and the silicon nitride film 65 are mirror-polished to clean the bonding surface, and the mirror surfaces are brought into contact with each other to be in close contact with each other. Finally, by applying a heat treatment at 300 ° C. for 1 hour in an electric furnace, as shown in FIG.
A silicon gallium arsenide composite substrate material having no warp as shown in (c) can be obtained. Since the silicon oxide film 64 and the silicon nitride film 65 are as thin as several thousand A or less and are not as dense as a single crystal, they function as a buffer layer, and the gallium arsenide substrate 6
2, 63 and the silicon substrate 61 are not distorted,
Less likely to affect the characteristics of the single crystal. Further, since the silicon oxide film 64 is transparent in a wide wavelength range,
Even when this substrate is used as a substrate for an optical element, it does not hinder the passage of light. Therefore, this substrate can be put to practical use as a composite substrate for optical elements.

Further, by polishing only the thin film layer after forming the silicon oxide film 64 and the silicon nitride film 65, the mechanical strain generated during polishing does not affect the characteristics of the substrate, so that the characteristics are good. A composite substrate material is obtained. Further, since the buffer layers for bonding use materials of different properties such as silicon oxide and silicon nitride, it is possible to selectively lift off one of the substrates by appropriately selecting an etching solution. A composite substrate material that does not warp due to the material can be easily obtained.

The materials that can be bonded are not limited to those described above, and any material can be used as long as the bonding surface is a mirror surface. Further, even materials that cannot be joined can be joined by interposing a thin film buffer layer. Therefore, any combination of composite substrate materials can be obtained using this method.

(Fifth Embodiment) In the fourth embodiment, it is described that a two-layer composite substrate material composed of different materials can be easily obtained from a composite substrate material having a sandwich structure by bonding via a buffer layer. . In this example, a method for obtaining the same two-layer composite substrate material as in Example 4 without using a buffer layer at the time of bonding will be described with reference to the drawings. FIG. 7 is a side view showing the steps of this embodiment.

As shown in FIG. 7A, a structure in which the indium phosphide substrate 71 is sandwiched between the glass substrate 73 and the silicon substrate 72 is formed. For the glass substrate 73, borosilicate glass having a coefficient of thermal expansion of 32 × 10 ⁻⁷ / ° C. was used. The coefficient of thermal expansion of silicon is 35 × 10 ⁻⁷ / ° C., and the coefficient of thermal expansion of indium phosphide is 45 × 10 ⁻⁷ / ° C. The difference in coefficient of thermal expansion between silicon and borosilicate glass is very small compared to the difference in coefficient of thermal expansion between indium phosphide and indium phosphide. The bonding surface of these substrates is mirror-polished.

Next, as shown in FIG. 7B, an indium phosphide substrate 71, a glass substrate 73 and a silicon substrate 7 are provided.
The bonded surface of No. 2 is cleaned, a sandwich structure is formed and integrated, and then heat treatment is performed at 300 ° C. for 1 hour in an electric furnace to obtain a composite substrate material having no warpage as shown in FIG. 7C. Can be obtained. At this time, if there is a large difference in the coefficient of thermal expansion between the two substrates on the sandwiching side, the bonded substrate will warp as a whole. However, in the present embodiment, since the silicon substrate 72 and the borosilicate glass substrate 73 have a similar coefficient of thermal expansion, the substrate did not warp.

Further, as shown in FIG. 7C, by removing the glass portion by etching, a warp-free composite substrate material composed of silicon and indium phosphide can be easily obtained.

Further, in the present embodiment, the silicon substrate 72 and the glass substrate 73 having a thermal expansion coefficient close to that of silicon are used for sandwiching the indium phosphide substrate 71, but the present invention is not limited to this, and other materials can be used. It is easily conceivable that various composite substrate materials can be obtained by sandwiching a specific material between materials having similar thermal expansion coefficients. However, since the coefficient of thermal expansion of glass can be freely changed depending on its composition, the coefficient of thermal expansion can be easily adjusted, and processing is also easy, which is convenient for obtaining such a structure.

Further, the temperature of the heat treatment in each embodiment is not limited to the one given in this embodiment, and needless to say, it may be a temperature at which the respective material characteristics are not changed.

[0042]

As described above, according to the present invention, it is possible to prevent the warp of the substrate when the wafers having different thermal expansion coefficients are brought into close contact with each other and heated to directly bond the wafers, and the processing accuracy of the bonded wafers can be greatly improved. Can be improved. Moreover, since no adhesive is used for adhesion, a substrate that can withstand various post-processes can be obtained. Furthermore, since the material to be joined is a single crystal material, the composite substrate material has good crystallinity of each material.

Therefore, by applying the method of the present invention to a product, downsizing can be achieved, the characteristics can be improved, the cost can be significantly reduced, and the method can be practically used.

[Brief description of drawings]

FIG. 1 is a side view showing a process according to a first embodiment of the present invention.

FIG. 2 is a cross-sectional view showing a manufacturing process for manufacturing a crystal unit from the bonded substrate obtained in the first embodiment.

FIG. 3 is a side view showing a process according to a second embodiment of the present invention.

FIG. 4 is a cross-sectional view showing a manufacturing process for manufacturing a crystal unit from the bonded substrate obtained in Example 2;

FIG. 5 is a side view showing the steps of Example 3 of the present invention.

FIG. 6 is a side view showing the steps of Example 4 of the present invention.

FIG. 7 is a side view showing the steps of Example 5 of the present invention.

[Explanation of symbols]

 11 Crystal Plate 12 Glass Plate 21 Crystal Substrate 22 Glass Substrate 23 Mask 24 Opening 25 Excitation Electrode 31 Glass Substrate 32 Crystal Substrate 33 Crystal Substrate with Silicon Film Formed 34 Silicon Film 41 Crystal Substrate 43 Mask 44 Opening 45 Excitation Electrode 51 Silicon Substrate 52 Gallium Arsenide Substrate 53 Silicon Oxide Film 61 Silicon Substrate 62 Gallium Arsenide Substrate 1 63 Gallium Arsenide Substrate 2 64 Silicon Oxide Film 65 Silicon Nitride Film 71 Indium Phosphorus Substrate

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Yutaka Taguchi, 1006 Kadoma, Kadoma City, Osaka Prefecture Matsushita Electric Industrial Co., Ltd. (72) Inventor, Kazuo Eda, 1006 Kadoma, Kadoma City, Osaka

Claims (6)

[Claims] 1. A mirror-polished second substrate having a coefficient of thermal expansion different from that of the first substrate on one surface of the mirror-polished first substrate, at least one surface of which is mirror-polished. A third member in which the first substrate and the second substrate are in close contact with each other and are made of a material having a thermal expansion coefficient substantially the same as that of the second substrate, and at least one surface of which is mirror-polished.
Similarly, the substrates are adhered so that their mirror surfaces face each other, and then heated to firmly and directly bond the substrates to each other. 2. The method for manufacturing a composite substrate material according to claim 1, wherein the third substrate is a substrate made of the same material as the second substrate. 3. The method for producing a composite substrate material according to claim 1, wherein the third substrate is a glass substrate. 4. The method according to claim 1, wherein at least one of the second substrate and the third substrate is directly bonded to the first substrate material via silicon or a silicon compound. Manufacturing method of composite substrate material. 5. The method for producing a composite substrate material according to claim 4, wherein the silicon compound is silicon oxide. 6. The method for producing a composite substrate material according to claim 4, wherein the silicon compound is silicon nitride.