JPH08195334A - Method for bonding silicon substrate - Google Patents

Abstract

PURPOSE: To firmly bond silicon substrates at low temperature, to utilize materials which cannot stand high temperature and to prevent films from separating and voids from being produced. CONSTITUTION: The method comprises the steps of hydrophobing the surfaces of silicon substrates 1 and 2, contacting both hydrophobed silicon substrates, press-bonding them, and heating them to strengthen the bond. And the press- bonding step, the substrates 1 and 2 are brought into contact from the central portions of the substrates 1 and 2 to the peripheries.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing semiconductor substrates, and more particularly to a method for joining silicon substrates.

[0002]

2. Description of the Prior Art Bonding of silicon substrates is done by SOI
(Silicon on Insulator) This is an indispensable technique for manufacturing a substrate. Conventionally, the bonding of silicon substrates has been carried out by heating the substrates in an oxidizing atmosphere or hydrophilizing the substrates after they are contacted with each other, as disclosed in JP-A-39-17869 or JP-A-62-27040. Is used after heating, and the substrates are bonded via an oxide layer.

[0003]

However, in the conventional methods, in order to obtain a strong joint, 10
It is necessary to heat to around 00 ° C. Therefore, when an element is formed on the substrate before joining, there is a problem that the material used is extremely limited to withstand such a high temperature.

Further, in the laminated structure, there is a problem that film peeling occurs due to a difference in thermal expansion of each layer material. The conventional method also has a problem that the gas generated by the dehydration decomposition of the hydroxyl groups on the substrate surface is liable to remain as voids, which lowers the bonding strength.

(Object of the Invention) An object of the present invention is to enable strong bonding by low-temperature treatment in the bonding process of silicon substrates, and by this, materials other than materials that can withstand high temperatures can be used. Furthermore, it is to prevent film peeling and voids.

[0006]

Means for Solving the Problems In the method for joining silicon substrates according to the present invention, as means for solving the above problems, a step of hydrophobizing the surfaces of silicon substrates, a step of contacting the substrates while applying pressure, and a step of heat treatment. It is characterized by having three steps.

First, hydrophobization of the silicon surface is typically carried out by removing the natural oxide film on the surface with hydrofluoric acid. The hydrofluoric acid treatment terminates most of the silicon surface with hydrogen atoms. The concentration of hydrofluoric acid used is 10% by weight or less, and a hydrofluoric acid treatment of 1 to 5% is suitable for the hydrophobizing treatment. You may use the buffer hydrofluoric acid which adjusted pH to 3-6. Before the hydrofluoric acid treatment, impurities such as organic substances adhering to the surface of the substrate may be removed. In this case, it is necessary to avoid a treatment for forming irregularities on the substrate surface. As the hydrophobizing method, in addition to the above hydrofluoric acid treatment, the substrate surface is irradiated with accelerated inert ions in a high vacuum to form a clean silicon surface, and then hydrogen gas is introduced into the vacuum chamber to clean the substrate surface. There is a method of terminating with a hydrogen atom.

Next, the two hydrophobized substrates are brought into contact with each other while applying pressure. FIG. 1 is a schematic view of an apparatus used for contacting a substrate in the present invention. This apparatus holds the first substrate 1 flat, introduces a pressurized fluid (either liquid or gas) from a fluid inlet 4, and applies this fluid pressure to the back surface of the second substrate 2. However, the two substrates are brought close to each other. In the apparatus of FIG. 1, the fluid pressure is applied to the thin membrane 3, and the membrane bends spherically, so that the second substrate 2 in contact with it also bends spherically. As the second substrate 2 bends, the two substrates come into contact with each other from their central portions. In this contacting method, the atmospheric gas is expelled to the peripheral portion in the contacting process, so that the gas does not remain between the substrates even in the atmosphere. Of course, two substrates may be contacted in vacuum. Fluid pressure is 3kg / cm
² or more is effective enough.

Next, heating is performed in order to strengthen the bond between the two silicon substrates which are in contact with each other. In the present invention, the heating temperature is 2
If the temperature is 50 ° C or higher, practically sufficient bonding strength can be obtained.
300 ° C to 700 ° C, more preferably 500 to 700
At a temperature of ° C, a strong bond comparable to the conventional method is achieved. The bonding substrate is heated by heating means such as heater heating and lamp heating. Further, the atmosphere during heating is not limited to bonding, and may be air, inert gas, or vacuum.

The effect of heating is estimated as follows.
When the hydrogen atoms adsorbed on the surface of the silicon substrate are desorbed by heating, the silicon atoms on the outermost surface of the substrate are directly bonded to each other, or the hydrogen atoms and the oxygen atoms in the atmosphere are replaced, and The silicon atoms are bonded to each other to bond the two silicon substrates.

In the process of increasing the bonding strength during heating, when a stronger bond is formed than the peripheral portion of the substrate,
Voids often occur in the center of the substrate. It is considered that the strain cannot be relieved in the portion that has been firmly bonded, and the strain remains in the unbonded portion, resulting in a void. Such voids also occur when the strongly bonded portions are randomly formed due to the nonuniformity of the substrate temperature.

Therefore, in order to bond the silicon substrates without generating voids, it is effective to bond from one point. As a method for this, as shown in FIG. 3, the silicon substrate 5 which has been completely contacted is sandwiched between two plates 6 and 6'having relatively low thermal conductivity such as quartz or ceramics, and the substrate center behind these plates is heated. It is effective to do. By such means, the two silicon substrates are gradually heated from the center, and the strong joint portion spreads from the center to the periphery to suppress the generation of voids.

As another method, as shown in FIG. 4, the contacted silicon substrate 8 is conveyed by the conveying roller 10 in the direction of the short sides of the elongated heating means 9 and 9 ', and the joint portion is gradually strengthened from the edge of the substrate. There is a method of forming. The shielding plate 12 and the cooling roller 11 serve to control the temperature rising portion by reducing the heat conduction to the unbonded portion. In both of the methods of FIG. 3 and FIG. 4, the temperature difference between the two substrates is minimized by installing the heating means in upper and lower two sets. Further, in the method shown in FIG. 4, the same effect can be expected by moving the heating means from the edge of the substrate instead of transporting the substrate.

[0014]

According to the present invention, since the substrate surface is subjected to the hydrophobic treatment in advance, it is possible to perform strong bonding even at a low temperature.

The hydrophobizing treatment can be effectively performed by using hydrofluoric acid or buffer hydrofluoric acid.

In the hydrophobizing treatment, the surface of the substrate is irradiated with accelerated inert ions in a high vacuum to form a clean silicon surface, and then hydrogen gas is introduced to terminate the surface of the substrate with hydrogen atoms. This makes it possible to completely hydrophobize the substrate surface.

In the step of bringing the substrates into contact with each other under pressure and joining the substrates, the substrates are not included in the joining interface by sequentially bringing the substrates into contact with each other from the central portion, so that the occurrence of voids can be prevented.

The heat treatment is performed at 250 ° C. to 700 ° C.
By doing so, sufficient bonding strength can be obtained.

[0019]

【Example】

[Example 1] Two (100) silicon substrates were surface-hydrophobicized with 2% hydrofluoric acid, and immediately, while applying a gas pressure of 5 atm with the contact device of FIG. Were contacted.

Next, heating was performed in the atmosphere for 2 hours to perform bonding.
The heating temperature (heat treatment temperature) was each temperature shown in FIG.

The bending strength of the joint sample thus manufactured was measured. FIG. 2 shows the interfacial shear stress at the time of fracture calculated from the bending strength. The breaking strength of the silicon substrate measured by the same method is 55 kg / cm ² ,
It was found that heating at 0 ° C resulted in sufficiently strong bonding.

Example 2 Two (100) silicon substrates were used.
After performing an ultraviolet ray treatment at 00 ° C. for 10 minutes to remove organic impurities on the surface of the substrate, a hydrophobic treatment was performed with buffer hydrofluoric acid adjusted to PH 4 with ammonium fluoride.

After that, two substrates were brought into contact with each other in the same manner as in Example 1, and then heat treatment was performed.

When the strength was measured in the same manner as in Example 1, it was confirmed that the bonding was performed with the same strength.

COMPARATIVE EXAMPLE A (100) silicon substrate was subjected to a hydrophilic treatment, and the bonding strength of a sample subjected to contact and heat treatment was measured in the same manner as in Example 1. The bonding strength was 6 as shown in FIG.
A strong bond was obtained at a temperature of 00 ° C or higher, but sufficient strength was not obtained at a low temperature. Further, when the temperature was 400 ° C. or lower, the peeling occurred at the bonding surface when the dicing saw was cut.

[Example 3] Two (100) silicon substrates were immersed in a mixed solution of sulfuric acid and hydrogen peroxide at 85 ° C for 5 minutes and then washed with pure water for 10 minutes. Continued,
Hydrophobization treatment was performed for 20 seconds with a 2% hydrofluoric acid aqueous solution, and pure water washing was performed for 2 minutes. The two silicon substrates subjected to the above pretreatment were brought into contact with each other while applying 5 atmospheric pressure by the contact device shown in FIG. Next, as shown schematically in FIG. 3, the two contacting silicon substrates 5 are replaced with the two alumina ceramic plates 6,
It was sandwiched between 6 ', and the central portion of this alumina ceramics plate was heated by heaters 7 and 7'so that the heating temperature was 500 ° C.

When the two silicon substrates bonded as described above were irradiated with infrared rays and the transmitted light was observed with an infrared camera, no void was observed. Further, it was confirmed that the shear stress at the time of breaking was 85 kg / cm ² , and that it had sufficient strength.

Example 4 Two (100) silicon substrates were heated to 200 ° C. in an oxygen atmosphere while being exposed to ultraviolet rays.
After irradiation for 0 minutes, a hydrophobic treatment was performed with a 2% aqueous solution of hydrofluoric acid for 20 seconds, and a pure water wash was performed for 2 minutes. The two hydrophobized silicon substrates were brought into contact with each other while applying 5 atm by the contact device shown in FIG. Next, the contacted silicon substrate 8 was heated by a heating device having a sample transfer device schematically shown in FIG. In FIG. 4, 10 is a transport roller, 11 is a water-cooled roller, and 12 is a heat shield plate. The silicon substrate 8 is heated by the heaters 9 and 9 ′ installed in the central portion of the heat shield plate 12 while moving in the direction of the arrow. The temperature of the portion of the silicon substrate directly below the heater rises, but the portion separated from the heater is kept at a low temperature by the heat being blocked by the water-cooling roller, so that a strong joint does not occur. In this example, the heater power was adjusted so that the temperature of the silicon substrate immediately below the heater was 550 ° C.

The silicon substrate bonded as described above was observed with an infrared camera as in Example 3, but no void was found. The shear stress at the time of breaking is 91 kg.
Was / cm ² .

[0030]

As described above, the bonding method of the present invention has the effect of enabling strong bonding of silicon substrates at a lower temperature than conventional methods. Further, according to the method of the present invention,
Since they can be bonded at a low temperature, silicon substrates after element formation can be bonded to each other, and an effective means for forming a three-dimensional structure such as a micromachine can be provided.

Further, film peeling due to the difference in thermal expansion of the laminated structure and generation of voids can be reduced, so that
Quality and reliability can be improved.

[Brief description of drawings]

FIG. 1 is a schematic view of an apparatus used for contacting a substrate in the present invention.

FIG. 2 is a diagram of the interfacial shear stress at the time of failure of the example and the comparative example calculated from bending strength.

FIG. 3 is a schematic diagram of a heating method used in Example 3.

FIG. 4 is a schematic diagram of a heating device used in Example 4.

[Explanation of symbols]

 1 First Substrate 2 Second Substrate 3 Membrane 4 Fluid Inlet 5, 8 Contacted Silicon Substrates 6, 6 ', Ceramics Plates 7, 7', 9, 9'Heating Means 10 Conveying Rollers 11 Water Cooling Rollers 12 Heat Shield

Claims (8)

[Claims] 1. A step of hydrophobizing the surface of a silicon substrate, a step of bringing the substrates that have been hydrophobized into contact with each other under pressure to bond them, and a step of subsequently performing heat treatment to strengthen the bond. A method of bonding a silicon substrate, which comprises: 2. The method for bonding silicon substrates according to claim 1, wherein the hydrophobic treatment removes a natural oxide film on the substrate surface with hydrofluoric acid. 3. The method for joining silicon substrates according to claim 1, wherein buffer hydrofluoric acid is used for the hydrophobic treatment. 4. The hydrophobic treatment is performed by irradiating the substrate surface with accelerated inert ions in a high vacuum to form a clean silicon surface, and then introducing hydrogen gas to terminate the substrate surface with hydrogen atoms. The method for joining silicon substrates according to claim 1, wherein 5. The method of bonding silicon substrates according to claim 1, wherein in the step of bonding the substrates by bringing them into contact with each other under pressure, the substrates are sequentially contacted from a central portion of the substrates. 6. The method for bonding a silicon substrate according to claim 1, wherein the heat treatment is performed at 250 ° C. to 700 ° C. 7. The method of bonding a silicon substrate according to claim 1, wherein the heat treatment includes raising the temperature from the center of the silicon substrate. 8. The method of bonding a silicon substrate according to claim 1, wherein the heat treatment is performed by raising the temperature from an end portion of the silicon substrate.