JP3134391B2 - Silicon substrate bonding method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

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

[0002]

2. Description of the Related Art Conventionally, as a method of bonding two silicon substrates, there is a wafer direct bonding (W, D, B) method (for example, T. Abe et al: JPn, J. App).
l. Phys. 29 (1990) L2315, Yu Shinbo;
Applied Physics 56 (1987) 373. ). According to this method, there is no intermediate layer made of an adhesive or the like at the bonding interface, and the influence of thermal distortion can be eliminated. However, heat treatment at 800 to 1100 ° C. or higher is required to obtain sufficient bonding strength.
Therefore, Japanese Patent Application Laid-Open No. 3-91227 discloses a method for hydrophilizing a polished surface of a silicon substrate by reacting it with oxygen ions or oxygen radicals to form an oxide layer on the bonding surface of the silicon substrate. ing.

[0003]

However, according to the method disclosed in this publication, the bonding strength varies greatly from 0.7 to 2.9 kgf / mm ^2, and the bonding area ratio (actual bonding area / total bonding area) is 20%. Or about 90%.

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

[0005]

According to the present invention, each bonding surface of two silicon substrates is mirror-polished, each polished surface is subjected to a hydrophilic treatment, and the hydrophilic bonding surfaces are directly adhered to each other. A bonding method for bonding the two silicon substrates to each other by heat-treating the two silicon substrates, wherein the polishing surface of at least one of the two silicon substrates in the hydrophilizing treatment of the polishing surface is performed. The hydrophilization treatment is performed using a plasma generator with a high-frequency power supply, under an atmosphere containing oxygen, and in a state where the self-bias voltage between the plasma and the cathode electrode is set to 100 to 250 volts, a suboxide, that is, S shown in FIG.
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
^Is shown as Si ⁴⁺ .

[0006]

The joint surfaces of the two silicon substrates are mirror-polished. Then, in the hydrophilizing 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 using a plasma generator using a high-frequency power supply. At a voltage of 100 to 250 volts, an oxide layer having a suboxide is formed. Due to the oxide layer having this suboxide, many dangling bonds of silicon that effectively work at the junction interface are formed.

Thereafter, the bonding surfaces that have been made hydrophilic are directly adhered to each other, and are subjected to a heat treatment to bond the two silicon substrates to each other. At this time, the density of hydrogen bonds increases due to the large number of silanol groups (Si-OH) at the bonding interface formed by the oxide layer having a suboxide, and changes from hydrogen bonds to Si-O-Si covalent bonds at a relatively low temperature. And high bonding strength can be obtained even at low temperatures.

[0008]

DESCRIPTION OF THE PREFERRED EMBODIMENTS One 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 arranged to face each other. The upper electrode plate 3 is grounded to serve as an anode electrode, and the lower electrode plate 4
Are connected to form a cathode electrode. 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 so that the ultimate vacuum Can be set to 1 Pa or less. Then, oxygen gas is introduced into the chamber 2 from the gas inlet 6, and oxygen gas plasma 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 is
, So-called cathode couple. In this state, the inside of the chamber 2 is evacuated for 1P.
After that, oxygen gas is introduced from the gas inlet 6, discharge is caused between the electrode plates 3 and 4 by the power of the high frequency power supply 5, and oxygen plasma is generated.

Next, as shown in FIG. 2, a description will be given of a bonding step in a case where the first silicon substrate 10 and the second silicon substrate 11 are bonded. First, the first silicon substrate 10 and the second silicon substrate 11 each having at least one surface mirror-polished are sequentially subjected to, for example, 1.1.1 trichloroethane boiling, acetone ultrasonic cleaning, and pure water cleaning, and then sufficiently. Wash. After that, the natural oxide film is removed with a mixed solution of HF: H ₂ O = 1: 20. Thereafter, the first silicon substrate 10 is placed in the chamber 2 of the plasma generator 1 shown in FIG. 1 with the mirror-polished surface facing upward.

Then, the chamber 2 is evacuated to 1P
a, oxygen gas is introduced from the gas inlet 6, and discharge is caused between the electrode plates 3 and 4 by the power of the high frequency power supply 5 to generate oxygen plasma. The gas pressure at this time is 1 to
The discharge power is set to about 50 to 300 W / m ^{2 at} 25 Pa. Further, the self-bias potential generated between the plasma generation region 13 and the lower electrode plate (cathode) 4 is set to 125 to 225.
Bolt. As a result, the surface of the first silicon substrate 10 is exposed to oxygen plasma, and
Is formed.

At this time, oxygen radicals (oxygen with a suffix * in FIG. 1) or oxygen ions (indicated by O ⁺ in FIG. 1) are present 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 state of the cathode couple as described above, oxygen ions can easily reach the first silicon substrate 10, and therefore the oxidation reaction is promoted. 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 where oxidation is not performed, deterioration of element characteristics does not occur.

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 evaporation. Furthermore, H
A chemical surface treatment of immersing in a ₂ SO ₄ —H ₂ O ₂ mixture at a liquid temperature of 80 ° C. or higher has been performed.

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 dry nitrogen. And then dry the silicon substrate 1
The molecular weight of water adsorbed on the surface of 0,11 is controlled. Thereafter, 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
The oxide films 14 are brought into close contact with each other. As a result, the two silicon substrates 10 and 11 adhere to each other by a hydrogen bond between silanol groups formed on the surface and 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, to compensate for the warpage of the silicon substrates 10 and 11, 30 gf
/ Cm ² or more.

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

Here, when the plasma oxidized layer 12 is formed by the oxygen plasma treatment in the apparatus shown in FIG. 1 as in this embodiment, the discharge power and the gas pressure are changed, so that the plasma generation region 13 and the lower side are formed. The self-bias potential generated between the electrode plates (cathode) 4 can be changed. Therefore, the plasma oxidation treatment is performed by changing the discharge power, the gas pressure, and the treatment time variously, and the first silicon substrate 10
A plasma oxide layer 12 was formed thereon. 4 and 5
A through hole 18 having a hole diameter of 1 mm is formed at a pitch of 3 mm in a second silicon substrate 11 as shown in FIG. 1 and such a second silicon substrate 11 and the first silicon substrate 10 are interposed via a plasma oxide layer 12. 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 the two substrates 10 and 11 to each chip and the withstand 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 in consideration of the joining ratio and the withstand voltage, and ○ was given for those which were practically preferable, and × was given for those which were not practical.

As can be seen from this table, when the bias potential is 1
It can be seen that good bondability is exhibited when the voltage is between 25 V and 225 V, and that a large bond strength is obtained. However, when the bias potential is less than 100 V or more than 500 V, the discharge is not stable, and the process cannot be realized.

[0019]

[Table 1]

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

The thermally oxidized Si (Si ⁴⁺ ) is 2.2.
Expand to twice the volume. Therefore, as shown in FIG. 7, some Si atoms diffuse in the oxide film without being oxidized, or as shown in FIG. 8, unoxidized Si atoms diffuse into the substrate as interstitial Si. Or 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, non-crosslinking oxidation, etc. in the plasma oxide layer 12 are likely to be present. For this reason, a dangling bond, a dimer bond, and the like are easily generated. Similarly, it is presumed that the above-described mechanism also occurs by plasma oxidation, and that many oxidizing species that can become active species even at low temperatures are generated. Therefore, the first silicon substrate 10 having the plasma oxide layer 12 formed at a low bias potential and the second silicon substrate 11
Is effective even at a relatively low temperature of about 450 ° C. × 2 hours. Also in the pressure resistance test, it is presumed that the strength is strong due to the strong bonding.

Next, a semiconductor 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. This pressure sensor includes a first silicon substrate 19 as a detection unit 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. A protective film 22 is formed on the upper surface of the first silicon substrate 19, and the protective film 22 has a contact hole 2 for making an electrical connection with the p-type diffusion layer 21.
3 and 24 are formed, and electrodes 25 and 26 are wired in contact holes 23 and 24. Further, on the lower surface of the first silicon substrate 19, a concave portion 27 is formed 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 introducing hole 29 is provided in the second silicon substrate 20 as a pedestal.

Then, the predetermined element formation and processing described above are performed.
And a pedestal of the lower surface of the first silicon substrate 19 subjected to
The upper surface of the second silicon substrate 20 is mirror-polished. This
The lower surface of the mirror-polished first silicon substrate 19 is
A zuma oxide layer 30 is formed. Also, the second silicon substrate 2
The oxide film 31 is formed on the mirror-polished surface of _Two
SO_Four-H_TwoO_TwoImmerse in a mixture at a temperature of 80 ° C or higher
A chemical surface treatment has been applied. And the plasma oxide layer
First silicon substrate 19 with 30 formed thereon and predetermined method
Silicon substrate 20 having oxide film 31 formed by
And washed in pure water, dried with dry nitrogen, etc.
Controls the amount of water molecules adsorbed on the substrate surface (joining surfaces)
I do. After that, through the plasma oxide layer 30 and the oxide film 31
And the diaphragm 28 of the first silicon substrate 19 and the second
Of the pressure introducing hole 29 of the silicon substrate 20 of FIG.
Join.

The pressure sensor thus manufactured is
After forming elements such as the diffusion layer 21 and the electrodes 25 and 26, the first silicon substrate 19 and the second silicon substrate 20 are joined. Therefore, as described above, in order to obtain a high bonding strength as a high-pressure sensor, a heat treatment at a high temperature as in the related art is not required in the bonding process, so that the element characteristics of the element portion change or deteriorate due to the high temperature of the heat treatment. I will not do it. In addition, since the bonding surface is not immersed in an acidic solution as in the prior art in the hydrophilization treatment, there is no need to specifically apply a protective film for an acid-resistant solution to the element forming surface,
In addition, since the element can be formed by a normal process, there is no need to increase the number of processes or change the processes.

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 is known that when the surfaces of the first silicon substrate 19 and the second silicon substrate 20 to be bonded are plasma-oxidized, voids and the like are easily generated at the interface. However, since strong bonding is obtained, the first silicon substrate 19 and the second silicon substrate 20 can be integrated using the same material without using an intermediate layer such as an adhesive. Therefore, it is chemically stable, and there is no problem of temperature drift due to a difference in thermal expansion coefficient.

As described above, in this embodiment, two silicon
Each joint surface of the substrates 10 and 11 is mirror-polished, and each polished surface is
When performing the hydrophilic treatment, respectively, two silicon substrates 10,
11 is a parent of the polished surface of at least one of the silicon substrates.
In the water treatment, the plasma generator 1 using the high-frequency power source 5 is turned on.
Plasma and electrode plate 4 in an atmosphere containing oxygen.
(Cathode electrode) between 100 and
Si at 250 volts³⁺, Si²⁺, Si⁺etc
Forming a plasma oxide layer 12 having a sub-oxide of
I tried to do it. After that, the hydrophilic bonding surfaces are directly
The two silicon substrates 1 are brought into close contact with each other and heat-treated.
0,11 were joined together. In other words, the bonding surface is
(H_TwoSO_Four/ H_TwoO_Two) To make it hydrophilic
Instead, bond the interface with oxygen ions or oxygen radicals.
React to form the plasma oxidized layer 12 and make it hydrophilic.
And the plasma oxidation layer 12
i³⁺, Si ²⁺That produce a large number of suboxides such as
It is. That is, the sub-oxide in the plasma oxide layer 12
Silano per unit area at the bonding interface
And the hydrogen bonding force is improved
Or many unjoined hands that work effectively for joining are formed.
It is. As a result, the density of hydrogen bonds also increases,
It can change from hydrogen bond to covalent Si-O-Si bond at temperature
Therefore, high bonding strength can be obtained even at a low temperature. In addition,
Surface can be joined effectively, reducing the variation in strength,
When measured by XPS analysis, the suboxide
Si inside²⁺, Si³⁺Is particularly good, though there are many on the surface
Met.

In this manner, a relatively low temperature (450 ° C. ×
(Within 2 hours), a strong bonding strength can be obtained, and a uniform bonding strength and bonding property can be obtained over the entire surface of the substrate. In addition, even if the element is provided with 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. For example, the self-bias voltage between the plasma and the cathode electrode in the plasma generator using the high-frequency power supply may be 100 to 250 volts.

[0029]

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

[Brief description of the 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 view showing a vacuum bonding apparatus.

FIG. 4 is a view showing a test silicon substrate.

FIG. 5 is a sectional view taken along line AA of FIG. 4;

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

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

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

FIG. 9 is a diagram illustrating a structure of silicon.

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

FIG. 11 is a diagram illustrating a suboxide.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Plasma generator 4 Electrode plate (cathode electrode) 5 High frequency power supply 10 1st silicon substrate 11 2nd silicon substrate 12 Plasma oxide layer

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-3-91227 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) H01L 21/02 H01L 27/12

Claims (1)

(57) [Claims] 1. A bonded surface of two silicon substrates is mirror-polished, each of the polished surfaces is subjected to a hydrophilic treatment, and the hydrophilic bonded surfaces are directly adhered to each other, and are heat-treated.
A method of bonding silicon substrates, wherein two silicon substrates are bonded to each other, wherein the hydrophilic treatment of the polished surface of at least one of the two silicon substrates is performed by a high-frequency process. An oxide layer having a sub-oxide is formed by using a plasma generator with a power supply and in an atmosphere containing oxygen in a state where a self-bias voltage between the plasma and a cathode electrode is 100 to 250 volts. Method for bonding a silicon substrate.