JPH08199118A - Joining of silicon to silicon - Google Patents

Abstract

PURPOSE: To provide a method for joining silicon to silicon by which the joining can be carried out even at a relatively low temperature without using glass and the joining strength is high. CONSTITUTION: One surface to be joined of one silicon single crystal substance 2 is brought into close contact with one surface to be joined of another silicon single crystal substance 4 through an insulating film 3 formed on either or both of their surfaces and a high voltage is then applied across the silicon single crystal substances to produce the dielectric breakdown in the insulating film. Thereby, both the surfaces of the silicon single crystal substances are mutually joined. The insulating film is preferably a wet oxide film on the silicon surface.

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 single crystal bodies such as silicon wafers, which are useful when manufacturing, for example, a micromachine or a sensor.

[0002]

2. Description of the Related Art Conventionally, micromachines and SOI (Si (Si
Licon On Insulator) etc. are useful for bonding silicon wafers to each other, and direct bonding method or anodic bonding method is adopted as the bonding method for such silicon wafers. .

The direct bonding method is a method in which silicon wafers having extremely smooth surfaces are superposed on each other and a bond is generated by an interatomic force at the interface, and the silicon wafers can be bonded directly or via SiO _2. . The principle is that when smooth silicon wafers with hydroxyl groups on the surface are piled up, hydrogen bonds are generated, and when they are heated to a high temperature, they are dehydrated and condensed at the interface, and the remaining oxygen diffuses and bonds to silicon. It is believed that.

In the anodic bonding method, smooth surfaces of a silicon wafer and glass are brought together, heated to 300 to 400 ° C., and a negative charge of about 500 V is applied to the glass side. An electric attractive force is generated, which leads to a chemical bond at the interface. When bonding silicon wafers to each other, a silicon wafer on which a silicon oxide film having a thickness of about 1 μm is formed by a wet thermal oxidation method is used, and the silicon oxide films of the silicon wafers are combined to about 850 ° C. There is a method of heating and applying a voltage of about 30 V to electrostatically bond it. In addition, there is a method of bonding via a Pyrex glass, and a method of sandwiching a Pyrex glass thin film between silicon wafers, or a low melting point glass is formed on the silicon wafer by RF sputtering to a thickness of 1 to 5 μm. There is a method of applying a voltage of about several tens of volts which does not cause dielectric breakdown and joining at a temperature of about 50 ° C. or higher.

[0005]

However, each of the above-mentioned methods has a serious problem. First of all, the direct bonding method is 400 to 400% to obtain sufficient bonding strength.
Since heat treatment at a high temperature of about 1000 ° C. is required, element deterioration may occur in the manufacturing process of a micromachine or a sensor that requires bonding in the middle of the process or in a process close to the final stage.

In addition, the method of bonding through the silicon oxide film in the anodic bonding method requires heating at about 850 ° C., and therefore has the same problem as the direct bonding method. On the other hand, R
In the anodic bonding method through a low melting point glass by F sputtering, bonding can be performed at a temperature of about 50 ° C., but the bonding strength is about 30 kgf / cm ² and there is a problem that the bonding strength is low, and Pyrex glass is used. There is a problem that the joining through causes contamination such as sodium. It is reported that this method cannot bond at room temperature.

In the joining of silicon wafers, it is required that the joining can be performed at a temperature as low as possible from the viewpoint of preventing the occurrence of residual stress and preventing the deterioration of elements.
If there is a polishing step after joining, etc., it is necessary to have strength that can withstand this, so it is desirable that the joining strength be as high as possible, and it is desirable not to use glass that causes contamination, but conventionally, such requirements were satisfied. There was no possible joining method.

The present invention has been made in view of the above demands, and an object of the present invention is to provide a silicon-silicon bonding method capable of bonding at a relatively low temperature without using glass and having high bonding strength. .

[0009]

In order to achieve the above-mentioned object, the present invention provides the following silicon-silicon bonding method. (1) One surface of one silicon single crystal body to be bonded and one surface of another silicon single crystal body to be bonded are brought into close contact with each other through an insulating film formed on one or both of the both surfaces, and these silicon single crystals are bonded to each other. A silicon-silicon bonding method, characterized in that a high voltage is applied between crystal bodies to cause dielectric breakdown in the insulating film to bond the both surfaces of both silicon single crystal bodies. (2) The silicon-silicon bonding method according to (1), wherein the insulating film is a silicon oxide film obtained by thermally oxidizing the surface of a silicon single crystal. (3) The silicon according to (1) or (2) above, wherein a small current is passed between the silicon single crystals for a predetermined time after dielectric breakdown.
Silicon bonding method. (4) The silicon-silicon bonding method according to the above (1) to (3), which is performed in an atmosphere at a pressure lower than atmospheric pressure.

[0010]

According to the silicon-silicon bonding method of the present invention, the surfaces of silicon single crystals such as silicon wafers are brought into close contact with each other via an insulating film such as a silicon oxide film, and a high voltage is applied between these silicon single crystals. Then, the silicon oxide film is bonded by causing dielectric breakdown, and the principle is different from the conventional bonding method, and it is characterized by the bonding by dielectric breakdown.

That is, in the conventional anodic bonding method, silicon wafers are electrically insulated from each other by an insulating film such as glass, and a voltage which does not cause dielectric breakdown is applied to bond the silicon wafers. In essence, this is different in that both silicon single crystals are electrically connected to each other by dielectric breakdown.
Although the reason why the silicon wafers can be bonded to each other by such dielectric breakdown is not clear, surprisingly, the silicon wafers can be firmly bonded over the entire surface. Further, the temperature at the time of joining can be sufficiently performed even at 200 ° C. or lower, and since glass does not have to be used, contamination with sodium can be prevented.

[0012]

EXAMPLES The silicon-silicon bonding method of the present invention will be specifically described below. According to the bonding method of the present invention, the surfaces of the silicon single crystals are bonded to each other. In this case, it is necessary to form an insulating film in advance on the surfaces to be bonded of both silicon single crystals. The surface to be joined is preferably a smooth surface. The insulating film may be formed on both one smooth surface of one silicon single crystal body and one smooth surface of the other silicon single crystal body, or as shown in FIG. 1A, a silicon wafer or the like. No. 1 silicon single crystal body 2
The insulating film 3 may be formed on one of the smooth surfaces and the insulating film may not be formed on the smooth surface 4a of the silicon single crystal body 4 such as the other silicon wafer.

The insulating film is preferably a silicon oxide film obtained by wet-oxidizing the surface of the silicon single crystal. Also, C
It may be a silicon oxide film deposited by VD or the like. A glass film may be used, but it is not preferable because it may be contaminated with sodium. The thickness of the insulating film sandwiched between the silicon single crystal bodies (the total of these when formed on both surfaces to be bonded) is not particularly limited, but with a natural oxide film it is impossible to bond because a current flows through it in about several volts. The lower limit can be about 10 nm and the upper limit can be up to several μm, but a preferable range is about 100 to 1000 nm.

Such a silicon single crystal is shown in FIG.
As shown in (B), the flat surfaces of both silicon single crystal bodies 2 and 4 are brought into close contact with each other with the insulating film 3 interposed therebetween. It should be noted that the bonding can be performed with sufficient strength without applying any particular pressure when closely contacting. It is considered that this is because a large electrostatic attractive force is generated on the contact surfaces of the silicon single crystal bodies due to the application of the voltage, resulting in strong adhesion, and it is considered that this is the reason why the entire contact surfaces can be bonded.

It is desirable that the atmosphere at the time of joining is a vacuum state. If a gas such as air is present, bubbles may be generated on the contact surface, resulting in non-uniform bonding. The temperature of the contact surfaces to be joined does not need to be heated to a particularly high temperature and may be a relatively low temperature. Specifically, it is preferably about 150 to 500 ° C., but sufficient bonding can be achieved at a temperature below this range, and a temperature exceeding this range is also possible.

A high voltage is applied as shown in FIG. 1B in a state where the surfaces to be joined of both silicon single crystal bodies are in close contact with each other. In this case, unlike anodic bonding, a positive or negative voltage may be applied to any of the silicon single crystal bodies to apply a high voltage to the insulating film. As shown in FIG. 2, as the applied voltage is increased, dielectric breakdown occurs at a voltage according to the type and thickness of the insulating film. At this time, the current value suddenly rises. Therefore, in order to protect the device, the applied voltage is lowered as shown in FIG. 2, and the current value is kept at 0.1 mA or more at a low voltage and kept for several minutes to several tens of minutes. Preferably. The voltage that causes dielectric breakdown is generally about several tens to several hundreds of volts. Conversely speaking, the thickness of the insulating film is preferably such that the breakdown voltage is within this range. The applied voltage after the dielectric breakdown is about several to several tens of volts, and the current value is 0.1.
It is preferably about 10 mA.

By such an operation, as shown in FIG. 1C, the contact surfaces of the two silicon single crystal bodies 2 and 4 are firmly bonded to each other to form the bonding surface 6, and the bonded silicon single crystal material 7 is formed.
Can be manufactured. Although the insulating film 3 disappears in FIG. 1C, the state of the insulating film is actually unknown and will be clarified by future research.

Although the bonding method of the present invention described above is completely different in principle from the anodic bonding method, the apparatus used can be a general anodic bonding apparatus as it is. FIG. 3 shows the outline of the anodic bonding apparatus. This apparatus includes a vacuum chamber 11 on a vibration isolation table 10 and an XY located below the vacuum chamber 11.
A Zθ adjustment stage 12 is installed. In the vacuum chamber 11, an upper electrostatic chuck (cathode) 13 holding one silicon wafer 2 to be bonded and a lower electrostatic chuck 14 holding the other silicon wafer 4 face each other with a predetermined distance therebetween. The upper electrostatic chuck 13 is fixed to the vacuum chamber. The lower electrostatic chuck 14 is
The lower electrostatic chuck 14 is supported by an XYZθ adjusting stage 12 via a hot plate 15 which is in close contact with and heats the lower electrostatic chuck 14, and the XYZθ adjusting stage 12 adjusts the vertical movement and inclination of the lower electrostatic chuck 14. be able to. Further, in the vacuum chamber 11, a turbo vacuum pump 20 and a rotary pump 2 for evacuating the inside are provided.
1 is connected via a pipe, and a pipe for introducing a pressure adjusting gas into the inside is a mass flow controller 22.
Connected through. An alignment microscope 30 is installed on the vacuum chamber 11, and the infrared camera 3
The position of the upper and lower wafers can be aligned with the monitor 1 and the monitor 32. Although not shown, a high voltage DC power supply applied between silicon wafers is provided.

According to the bonding method of the present invention, flat wafers are bonded to each other, or, as shown in FIG. 4A, a flat wafer and a wafer having a recess are bonded to each other, and as shown in FIG. 4B. , Bonding of the convex surfaces of the wafers in a state where the concave portions of the wafers provided with the concave portions face each other, bonding in which a bonding surface and a non-bonding surface are alternately formed as shown in FIG. Various applications are possible in the fields of sensors and micromachines. [Examples and Comparative Examples] <Example 1> A silicon wafer having a diameter of 4 inches and a silicon oxide film having a thickness of 7800Å formed on the entire surface by a wet oxidation method, and a silicon wafer having a diameter of 4 inches of a bare wafer having no silicon oxide film formed thereon. Was installed in the anodic bonding apparatus shown in FIG. These wafers are superposed, dielectric breakdown is caused by an applied voltage of 800 V (one wafer is applied via a silicon oxide film) under the condition of a temperature of 400 ° C. in vacuum, and after the dielectric breakdown, 10 It was kept for 40 minutes while applying a current of about 20 V and about 1 mA.

The obtained bonded wafer was cut into a square having a side of 15 mm as shown in FIG. Then, as shown in FIG. 6, a rod R is bonded to both surfaces of the bonded wafer 7a cut by an adhesive, and a force for pulling the rod up and down is applied to the rod R,
The force when the joining surface 6 was peeled off was defined as the joining strength. In addition,
From the strength of the adhesive, about 200 kgf / cm ² is the upper limit of the bond strength that can be measured. If this value is exceeded, the rod and the bonded wafer will separate.

The values of the bonding strength of each part of the bonded wafer are also shown in FIG. In the figure,> means peeling off at the adhesive part,
This indicates that the bonding strength beyond that could not be measured. Further, a photograph of the X-ray topography of the bonded wafer is shown in FIG.
The average bonding strength of the bonded wafers is 185 kgf / cm ^2.
Met. In addition, when showing the bonding strength by other bonding methods as a reference, the bonding strength by the direct bonding method is 1000 ° C.
About 120 kgf / cm ² at the heat treatment, the heat treatment at 400 ° C. to about 50 kgf / cm ² (Applied Physics Vol. 56 No. 3 (1987)), anodic bonding via the low-melting glass by RF sputtering (IEICE Magazine C-
II Vol. J72-C-II No. 2 pp. 1
81-183 (February 1989), the maximum is about 30k.
It is gf / cm ² . <Comparative Example 1> A voltage was applied to the contact wafer in the same manner as in Example 1. However, the increase in the applied voltage was stopped immediately before the dielectric breakdown occurred, the voltage application was stopped, and the silicon wafer was taken out. It wasn't joined at all. <Comparative Example 2> An attempt was made to join wafers having a natural oxide film, but a current flowed at several volts, and joining was not possible. <Example 2> A bonded wafer was obtained by the same operation as in Example 1 except that the bonding temperature was 170 ° C. The bonding strength distribution of this bonded wafer is shown in FIG. The average bonding strength was 124 kgf / cm ² . <Example 3> An oxide film having a thickness of 200 nm was formed on only one side of a silicon wafer, and a bonded wafer was obtained in the same manner as in Example 1. The average bonding strength was 150 kgf / cm ² .

[0022]

According to the silicon-silicon bonding method of the present invention, it is possible to firmly bond silicon single crystal bodies to each other at a relatively low temperature.

[Brief description of drawings]

FIG. 1 is a cross-sectional view illustrating a process of a silicon-silicon bonding method of the present invention.

FIG. 2 is a graph showing a voltage and a current applied between bonded silicon single crystal bodies.

FIG. 3 is a schematic view showing an apparatus used in the joining method of the present invention.

FIG. 4 shows an embodiment of the joining method of the present invention.
(C) is a sectional view, respectively.

5 is a plan view showing the bonding strength of each part of the bonded wafer obtained in Example 1. FIG.

FIG. 6 is a cross-sectional view showing a method for measuring the bonding strength in an example.

7 is a photograph of an X-ray topograph of the silicon wafer bonded in Example 1. FIG.

8 is a plan view showing the bonding strength of each part of the bonded wafer obtained in Example 2. FIG.

[Explanation of symbols]

 2 One single crystal 3 Insulating film 4 Other single crystal 6 Bonding surface 7 Bonding

Claims (4)

[Claims] 1. A single surface of one silicon single crystal body to be bonded and another surface of another silicon single crystal body to be bonded are brought into close contact with each other through an insulating film formed on one or both of the both surfaces. A silicon-silicon bonding method characterized in that a high voltage is applied between silicon single crystal bodies to cause a dielectric breakdown in the insulating film to bond the both surfaces of both silicon single crystal bodies. 2. The silicon-silicon bonding method according to claim 1, wherein the insulating film is a silicon oxide film obtained by thermally oxidizing the surface of a silicon single crystal. 3. The silicon according to claim 1, wherein a small current is passed between the silicon single crystal bodies for a predetermined time after the dielectric breakdown.
Silicon bonding method. 4. The silicon-silicon bonding method according to claim 1, wherein the silicon-silicon bonding method is performed in an atmosphere at a pressure lower than atmospheric pressure.