JPH0582404A - Method for joining silicon substrate - Google Patents

Abstract

PURPOSE:To provide a method for joining silicon substrates by which heat treatment can be performed at a relatively low temperature and a firm joining strength is obtained and, at the same time, a uniform joining strength and joining property can be obtained over the entire surface of a substrate. CONSTITUTION:At the time of performing hydrophilic treatment on the polished surfaces of two silicon substrates to be joined together after polishing the joining surfaces to mirror surfaces, the hydrophilic treatment on at least one of the silicon substrates performed in an atmosphere containing oxygen produced by a plasma generating device 1 using a high-frequency power source 5, while the self-bias voltage across the plasma and electrode plate 4 (cathode) is adjusted to 125-225V, so that a plasma oxide layer containing such a sub-oxide as Si<3+>, Si<2+>, Si<+>, etc., can be formed. Thereafter, the two silicon substrates are joined together by directly sticking the joining surfaces which have been subjected to the hydrophilic treatment to each other and heat-treating them at 200-450 deg.C for 5-120 minutes.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for joining silicon substrates.

[0002]

2. Description of the Related Art Conventionally, as a method for joining two silicon substrates, there is a wafer direct joining (W, D, B) method (for example, T. Abe et al: JPn, J. App.
l. Phys. 29 (1990) L2315, Shinpo Yu;
Applied Physics 56 (1987) 373. ). This method does not have an intermediate layer of an adhesive or the like at the bonding interface and can eliminate the influence of thermal strain. However, heat treatment at 800 to 1100 ° C. or higher is necessary to obtain sufficient bonding strength.
Therefore, in Japanese Patent Laid-Open No. 3-91227, as a hydrophilic treatment for a polished surface of a silicon substrate, it is reacted at a relatively low temperature by reacting with oxygen ions or oxygen radicals to form an oxide layer on the bonding surface of the silicon substrate. ing.

[0003]

However, in the method according to this publication, the bonding strength largely varies from 0.7 to 2.9 kgf / mm ^2, and the bonding area ratio (actual bonding area / total bonding area) is 20. There was a problem that it changed to 90%.

An object of the present invention is to provide a method for bonding silicon substrates, which can be heat treated at a relatively low temperature, can obtain strong bonding strength, and can obtain uniform bonding strength and bondability over the entire surface of the substrates. is there.

[0005]

According to the present invention, the bonding surfaces of two silicon substrates are mirror-polished, and the polishing surfaces are hydrophilized, and the hydrophilized bonding surfaces are directly adhered to each other. A method of joining silicon substrates, wherein the two silicon substrates are bonded to each other by heat treatment, wherein in the hydrophilic treatment of the polishing surface, at least one of the two silicon substrates has a polishing surface. The hydrophilization treatment of the sub-oxide, that is, the sub-oxide, that is, the self-bias voltage between the plasma and the cathode electrode is set to 100 to 250 V in an atmosphere containing oxygen using a plasma generator with a high frequency power source, S shown in FIG. 11 (b)
i ^3+, Si ²⁺ shown in FIG. 11 (c), a method of bonding a silicon substrate to form an oxide layer having a Si ⁺ shown in FIG. 11 (d) and its gist. Incidentally, SiO FIG 11 (a) ₂
Indicates Si ⁴⁺ .

[0006]

Function: The respective bonding surfaces of the two silicon substrates are mirror-polished. Then, in the hydrophilic treatment of the polished surface of at least one of the two silicon substrates, a self-bias between the plasma and the cathode electrode is performed in an atmosphere containing oxygen by using a plasma generator with a high frequency power source. An oxide layer containing suboxide is formed under the condition that the voltage is 100 to 250 volts. Due to the oxide layer containing the sub-oxide, many dangling bonds of silicon that effectively work at the bonding interface are formed.

After that, the hydrophilic bonding surfaces are brought into direct contact with each other and subjected to heat treatment to bond the two silicon substrates to each other. At this time, since there are many silanol groups (Si—OH) at the bonding interface due to the oxide layer containing suboxide, the density of hydrogen bonds becomes high, and the hydrogen bonds change to Si—O—Si covalent bonds at a relatively low temperature. It is possible to obtain high bond strength even at low temperature.

[0008]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT An embodiment of the present invention will be described below with reference to the drawings. FIG. 1 shows a plasma generator 1.

In the chamber 2, a pair of electrode plates 3 and 4 are vertically opposed to each other. The upper electrode plate 3 is grounded to serve as an anode electrode, and the lower electrode plate 4 has a high frequency power source 5
Are connected to form a cathode electrode. Also, chamber 2
Is provided with a gas inlet 6 and a gas outlet 7. Oxygen gas is introduced from the gas inlet 6, and the gas in the chamber 2 is evacuated from the gas outlet 7 by the exhaust pump 8 to achieve the highest vacuum degree. Can be 1 Pa or less. Then, oxygen gas is introduced into the chamber 2 through the gas introduction port 6, and plasma of oxygen gas is generated between the two electrode plates 3 and 4. A silicon substrate 9 is arranged on the lower electrode plate 4, and the silicon substrate 9 serves as the electrode plate 3.
It is installed in a so-called cathode-coupled state facing the. In this state, the chamber 2 is evacuated to 1P
Then, oxygen gas is introduced from the gas introduction port 6, and electric power of the high frequency power source 5 causes a discharge between the electrode plates 3 and 4 to generate oxygen plasma.

Next, as shown in FIG. 2, a bonding process for bonding the first silicon substrate 10 and the second silicon substrate 11 will be described. First, the first silicon substrate 10 and the second silicon substrate 11 whose at least one surface is mirror-polished are subjected to, for example, 1.1.1 trichloroethane boiling, acetone ultrasonic cleaning, and pure water cleaning in order to sufficiently perform the cleaning. Wash. After that, the natural oxide film is removed with a mixed solution of HF: H ₂ O = 1: 20. Then, the mirror-polished surface of the first silicon substrate 10 is placed in the chamber 2 of the plasma generator 1 shown in FIG.

Then, the chamber 2 is evacuated to 1P.
Then, oxygen gas is introduced from the gas introduction port 6, and the electric power of the high frequency power source 5 causes a discharge between the electrode plates 3 and 4 to generate oxygen plasma. The gas pressure at this time is 1
The discharge power is set to 25 Pa and the discharge power is set to about 50 to 300 W / m ² . In addition, the self-bias potential generated between the plasma generation region 13 and the lower electrode plate (cathode) 4 is 125 to 225.
Make it a bolt. As a result, the surface of the first silicon substrate 10 is exposed to the oxygen plasma and the plasma oxide layer 12 is formed on the surface.
Will be formed.

At this time, oxygen radicals (oxygen with suffix * in FIG. 1) or oxygen ions (indicated by O ⁺ in FIG. 1) exist in the oxygen plasma, and these are in a chemically active state. Therefore, the oxidation reaction can easily proceed even at room temperature. Furthermore, since the first silicon substrate 10 is arranged in the cathode-coupled state as described above, oxygen ions easily reach the first silicon substrate 10, which promotes the oxidation reaction. Further, the first silicon substrate 10 facing the plasma generation region 13 has only the upper surface, and there is no plasma damage to the lower surface. Further, since the treatment is performed at room temperature, even if the element is formed on the lower surface which is not oxidized, the element characteristic is not deteriorated.

On the other hand, on the mirror-polished surface of the second silicon substrate 11, known thermal oxidation, chemical vapor deposition, sputtering,
The oxide film 14 is formed by a method such as vapor deposition. Furthermore, H
Chemical surface treatment is performed by immersing in a ₂ SO ₄ —H ₂ O ₂ mixed solution at a liquid temperature of 80 ° C. or higher.

Then, the first silicon substrate 10 having the plasma oxide layer 12 formed thereon and the second silicon substrate 11 having the oxide film 14 formed by a predetermined method are washed in pure water and dried with nitrogen. Silicon substrate 1
Controls the molecular weight of water adsorbed on the surface of 0,11. After that, as shown in FIG. 2, the surface of the first silicon substrate 10 on which the plasma oxide layer 12 is formed and the second silicon substrate 11 are formed.
The oxide film 14 surfaces are adhered to each other. As a result, the two silicon substrates 10 and 11 are bonded by the hydrogen bond of the silanol groups formed on the surface and the water molecules adsorbed on the surface. Further, the bonded silicon substrates 10 and 11 are dried in a vacuum of 10 Pa or less. At this time, in order to compensate the warp of the silicon substrates 10 and 11, 30 gf
A load of / cm ² or more may be applied.

Further, as shown in FIG. 3, the silicon substrates 10 and 11 in the bonded state are arranged in the chamber 15. The chamber 15 can be made to have an ultimate vacuum of 10 Pa or less by an exhaust pump 16. Then, while removing excess water molecules existing at the bonding interface between the silicon substrates 10 and 11, the heater 17 in the chamber 15 performs heat treatment at 200 to 450 ° C. for 5 to 120 minutes to bond the silicon substrates 10 and 11. ..

Here, when the plasma oxide layer 12 is formed by the oxygen plasma treatment in the apparatus shown in FIG. 1 as in the present embodiment, the plasma generation region 13 and the lower side of the plasma generation region 13 are particularly changed by changing the discharge power and the gas pressure. It is possible to change the self-bias potential generated between the electrode plates (cathodes) 4. Therefore, the plasma oxidation processing is performed by changing the discharge power, the gas pressure, and the processing time variously, and the first silicon substrate 10
A plasma oxide layer 12 was formed on top. Furthermore, FIGS.
The through holes 18 having a hole diameter of 1 mm are formed at a pitch of 3 mm in the second silicon substrate 11 as shown in FIG. 2, and the second silicon substrate 11 and the first silicon substrate 10 are formed with the plasma oxide layer 12 interposed therebetween. And joined. After the bonding, the first silicon substrate 10 and the second silicon substrate 11 were cut by dicing at the positions indicated by broken lines in FIGS. Then, the bondability of both substrates 10 and 11 to each of the chips and the breakdown voltage by applying pressure through the through hole 18 were measured.

The results are shown in Table 1. In Table 1, the judgment was made by taking the joining rate and the breakdown voltage into consideration, and was marked with "○" for practically preferable and "X" for practically unfavorable.

As can be seen from this table, the bias potential is 1
It can be seen that when the voltage is 25 V to 225 V, good bondability is exhibited and the bond strength is high. However, even if the bias potential is 100 V or less or 500 V or more, the discharge is not stable, so that the process cannot be established.

[0019]

[Table 1]

Next, the plasma oxidation layer 12 formed by performing the plasma oxidation process with various bias potentials is subjected to XPS (X
Line photoelectron spectroscopy). The result is shown in FIG. As can be seen from this figure, the Si _2p photoelectron spectrum on the outermost surface of the plasma oxide layer 12 is changed by changing the bias potential. In particular, those having a high bias potential (400 V, 500 V) have high Si ⁴⁺ (corresponding to SiO ₂ ), and it can be seen that oxidation is advanced. In addition, high bias potential (400V, 50
It can also be seen from the ratio of Si ⁴⁺ and Si ⁰ (metal) that the relative oxide film thickness becomes thicker at ⁰ V). On the other hand, when the bias potential is as low as 125 V and 225 V, the 2p peak of silicon is shifted to the side where the energy is lower than 103.4 eV, and sub oxides such as Si ³⁺ and Si ²⁺ are contained in the plasma oxide layer 12. It turns out that there are many on the outermost surface.

Further, the thermally oxidized Si (Si ⁴⁺ ) is 2.2.
Expands to double the volume. For this reason, as shown in FIG. 7, some Si atoms diffuse in the oxide film while remaining unoxidized, or as shown in FIG. 8, unoxidized Si atoms cause substrate diffusion as interstitial Si. To do Further, as shown in FIG. 9, it is considered that vacancies in the silicon substrate are consumed at the SiO ₂ interface, and point defects due to oxygen defects or non-crosslinking oxidation in the plasma oxide layer 12 are likely to exist. Therefore, dangling bonds, dimer bonds, etc. are easily generated. Similarly, it is presumed that plasma oxidation causes the above-mentioned mechanism, and a large number of oxidizing species that can be active species are generated even at low temperatures. Therefore, the first silicon substrate 10 formed with the plasma oxide layer 12 at a low bias potential and the second silicon substrate 11 are formed.
Is effectively joined even at a relatively low temperature of about 450 ° C. × 2 hr. Also, in the pressure resistance test, it is presumed that the strength is strong because of strong bonding.

Next, a semiconductor type pressure sensor to which the present invention is applied will be described with reference to FIG. FIG. 10 is a structural diagram showing an example of a semiconductor pressure sensor manufactured by applying the bonding method of the embodiment. The present pressure sensor is composed of a first silicon substrate 19 as a detector and a second silicon substrate 20 as a pedestal. A p-type diffusion layer 21 is formed on the upper surface of the n-type first silicon substrate 19 to form a diffusion strain gauge. Further, a protective film 22 is formed on the upper surface of the first silicon substrate 19, and the contact hole 2 for electrically connecting to the p-type diffusion layer 21 is formed in the protective film 22.
3, 24 are formed, and the electrodes 25, 26 are wired in the contact holes 23, 24. Further, a recess 27 is formed on the lower surface of the first silicon substrate 19 by a known method such as anisotropic etching with an alkaline solution or isotropic etching with boiling nitric acid, and a diaphragm 28 is processed. On the other hand, a pressure introduction hole 29 is provided in the second silicon substrate 20 as a pedestal.

Then, the above-mentioned predetermined element formation and processing
And the lower surface of the first silicon substrate 19 that has been subjected to
The upper surface of the second silicon substrate 20 is mirror-polished. This
On the underside of the first mirror-polished first silicon substrate 19.
A zuma oxide layer 30 is formed. In addition, the second silicon substrate 2
Oxide film 31 is formed on the mirror-polished surface of 0. ₂
SO_Four-H₂O₂Immerse in a mixed solution at a liquid temperature of 80 ° C or higher
Chemical surface treatment is applied. And the plasma oxide layer
First silicon substrate 19 having 30 formed thereon and a predetermined method
Second silicon substrate 20 having oxide film 31 formed in
And are washed in pure water and dried with dry nitrogen etc.
Controlling the amount of water molecules adsorbed on the substrate surface (surfaces to be joined)
To do. After that, through the plasma oxide layer 30 and the oxide film 31,
The diaphragm 28 of the first silicon substrate 19 and the second
By aligning the pressure introducing hole 29 of the silicon substrate 20 of
To join.

The pressure sensor manufactured in this way is
The first silicon substrate 19 and the second silicon substrate 20 are bonded after the elements such as the diffusion layer 21 and the electrodes 25 and 26 are formed. Therefore, as described above, in order to obtain high bonding strength as a high-voltage sensor, it is not necessary to perform heat treatment at a high temperature as in the past during this bonding process, and therefore the element characteristics of the element portion are changed or deteriorated due to the high temperature of the heat treatment. There is nothing to do. Further, since it is not soaked in an acidic solution as in the conventional case in the hydrophilic treatment of the bonding surface, there is no need to specially coat a protective film for an acid resistant solution on the element forming surface,
Further, since the element formation can be performed by the normal process, it is not necessary to increase the number of processes or change the process.

Further, a plasma oxide layer similar to the plasma oxide layer 30 of the first silicon substrate 19 may be formed on the bonding surface of the second silicon substrate 20 without forming the oxide film 31. However, it has been found that when the surfaces of the first silicon substrate 19 and the second silicon substrate 20 on the bonding side are plasma-oxidized, voids or the like are likely to occur at the interface. However, since a strong bond is obtained, the first silicon substrate 19 and the second silicon substrate 20 can be integrated using the same material and without an intermediate layer such as an adhesive. Therefore, it is chemically stable, and there is no problem of temperature drift due to the difference in thermal expansion coefficient.

As described above, in this embodiment, two pieces of silicon are used.
Each bonded surface of the substrates 10 and 11 is mirror-polished, and each polished surface is polished.
When performing the hydrophilic treatment, the two silicon substrates 10,
Parent of the polished surface of at least one of the 11 silicon substrates
For the hydration treatment, the plasma generator 1 with the high frequency power source 5 is used.
Using an atmosphere containing oxygen, plasma and electrode plate 4
The self-bias voltage between (cathode electrode) is 100 to
Si at 250 volts³⁺, Si²⁺, Si⁺etc
Forming a plasma oxide layer 12 having a suboxide of
I was allowed to do it. After that, bond the hydrophilic surfaces together.
Two silicon substrates 1 that come into close contact with each other and are heat treated
0 and 11 were joined to each other. In other words, the bonding surface
(H₂SO_Four/ H₂O₂) To make it hydrophilic
Instead, the joint surface is treated with oxygen ions or oxygen radicals.
React to form plasma oxide layer 12 to render it hydrophilic
And S as the plasma oxide layer 12
i³⁺, Si ²⁺Which produces a large number of sub-oxides such as
Is. That is, the sub-oxide in the plasma oxide layer 12 is
Silano per unit area at the bonding interface due to the id
The number of radicals (Si-OH) increases and the hydrogen bond strength improves.
Or, there are many unbonded hands that work effectively for joining.
Be done. As a result, the density of hydrogen bonds is also high and relatively low.
Can change from hydrogen bond to Si-O-Si covalent bond at high temperature
Therefore, high bonding strength can be obtained even at low temperature. Moreover, all
Since it can be effectively joined on the surface, variations in strength can be reduced,
In addition, when measured by XPS analysis,
In Si²⁺, Si³⁺Bonding is especially good even though there are many
Met.

Thus, at a relatively low temperature (450 ° C. ×
Within 2 hours), heat treatment can be performed, and strong bonding strength can be obtained, and uniform bonding strength and bonding property can be obtained over the entire surface of the substrate. Also, even if the element is made of wiring such as aluminum,
It is possible to directly bond to another silicon substrate without damaging the element portion, and high reliability can be secured.

The present invention is not limited to the above embodiment, and for example, the self-bias voltage between the plasma and the cathode electrode in the plasma generator using the high frequency power source may be 100 to 250 volts.

[0029]

As described in detail above, according to the present invention,
It exhibits an excellent effect that heat treatment can be performed at a relatively low temperature, a strong bonding strength is obtained, and uniform bonding strength and bonding property are obtained over the entire surface of the wafer.

[Brief description of drawings]

FIG. 1 is a diagram showing a plasma generator.

FIG. 2 is a diagram showing first and second silicon substrates.

FIG. 3 is a diagram showing a vacuum bonding apparatus.

FIG. 4 is a diagram showing a silicon substrate for testing.

5 is a cross-sectional view taken along the line AA of FIG.

FIG. 6 is a diagram showing measurement results by XPS.

FIG. 7 is a diagram for explaining the structure of silicon.

FIG. 8 is a diagram for explaining the structure of silicon.

FIG. 9 is a diagram for explaining the structure of silicon.

FIG. 10 is a cross-sectional view when the present invention is embodied in a pressure sensor.

FIG. 11 is a diagram for explaining a sub oxide.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Plasma generator 4 Electrode plate (cathode electrode) 5 High frequency power supply 10 First silicon substrate 11 Second silicon substrate 12 Plasma oxide layer

Claims (1)

[Claims] 1. Bonding surfaces of two silicon substrates are mirror-polished, each polishing surface is hydrophilized, and the hydrophilized bonding surfaces are directly adhered to each other.
A method of joining silicon substrates for joining two silicon substrates to each other, wherein in the hydrophilic treatment of the polishing surface, the hydrophilic treatment of the polishing surface of at least one of the two silicon substrates is performed by high frequency An oxide layer having a sub-oxide is formed by using a plasma generator with a power supply in an atmosphere containing oxygen and a self-bias voltage between the plasma and the cathode electrode of 100 to 250 V. And a method for joining silicon substrates.