JP2801672B2 - Method for manufacturing semiconductor wafer - Google Patents

Description

The present invention relates to a method of manufacturing a single semiconductor wafer by directly bonding two substrates without using an adhesive.

(Prior art) Activate cleaning of a substrate such as silicon polished to a mirror surface,
When the polished surfaces are brought into contact at room temperature, they come into close contact with each other by their own force. Further, by heat-treating the adhered substrate, the adhesive strength is increased, and a strong bonded wafer is obtained. The adhesion at room temperature is due to the interaction of OH groups formed on the substrate surface, and the subsequent increase in strength due to heat treatment is due to the dehydration condensation reaction, etc., where the bonds between the OH groups are changed from Si-O-Si bonds to Si-Si bonds. It is thought to change to. An increase in strength due to heat treatment is observed at temperatures above 200 ° C.

With this direct bonding technology, the resulting wafer is thermally and chemically stable because there is no foreign matter such as adhesive at the interface, and pn junctions and dielectric embedding can be easily performed. Has been applied to In addition, semiconductor substrates such as silicon, dielectric substrates such as quartz and glass, and silicon substrates
By bonding substrates made of different materials such as a GaAs substrate, an SOI substrate or a GaAs on Si wafer can be easily obtained.

However, the conventional direct bonding technique has the following problems.

First, when substrates made of different materials are directly bonded to each other by a bonding technique, the substrates may be peeled or broken during the heat treatment. This is a result of the occurrence of stress during the heat treatment due to the different thermal expansion coefficients of the two substrates.
If the heat treatment temperature is lowered in order to reduce the generated stress, sufficient bonding strength cannot be obtained.

Second, voids (unbonded portions) may occur at the interface of the bonded substrate depending on the preprocessing conditions when the substrates are bonded to each other after performing preprocessing such as cleaning processing on the two substrates. is there. Also, even if there is no void when bonding,
Thereafter, when heat treatment is performed, voids may be generated depending on the heat treatment conditions. The generation of voids is observed both in bonding substrates of different materials and bonding substrates of the same material. These voids not only have a significant effect on the electrical characteristics and yield of the semiconductor device formed using the bonded wafer, but also adversely affect the device manufacturing process. Particularly, in the case of bonding between substrates made of different materials, these voids tend to cause peeling or cracking of the substrate.

(Problems to be Solved by the Invention) As described above, in the conventional direct bonding technology, it is difficult to obtain a wafer having a sufficient bonding strength when two substrates are made of different materials, and the interface is determined by pretreatment and heat treatment conditions. There is a problem that voids are generated in the glass.

An object of the present invention is to provide a method for manufacturing a semiconductor wafer in which substrates having different coefficients of thermal expansion are reliably bonded with sufficient bonding strength.

Another object of the present invention is to provide an excellent method for manufacturing a semiconductor wafer having no voids at the bonding interface.

[Structure of the Invention] (Means for Solving the Problems) In the first method of the present invention, when two substrates having different coefficients of thermal expansion are directly bonded to each other, the two substrates are bonded together at room temperature and then adhered to each other. The first heat treatment is performed at an intermediate temperature, the thickness of one substrate is reduced, and then the second heat treatment is performed again at a high temperature.

A second method of the present invention is characterized in that at least one of the substrates is immersed in pure water at 80 ° C. or higher at the end of a pretreatment step such as cleaning and drying of the substrates prior to the substrate bonding. .

(Action) In the first method, the intermediate temperature in the first heat treatment step is equal to or lower than a temperature at which the substrate is cracked or peeled,
Any temperature may be used as long as an adhesive strength that can withstand the next step of reducing the thickness is obtained. This first treatment can be omitted if the adhesive strength at room temperature satisfies the above conditions. The step of reducing the thickness can be performed by polishing, etching, or the like. Since a certain degree of adhesive strength is required for mechanical processing, the first heat treatment temperature is set to be relatively high. In the case of etching, the adhesive strength may be low,
The first heat treatment temperature can be lowered. The temperature of the second heat treatment is higher than that of the first heat treatment in order to firmly bond the two substrates. The appropriate temperature varies depending on the substrate material, but is set within a range in which heat does not cause melting, deformation, crystal defects, or the like of the substrate.

The reason why a strong bonded wafer can be obtained because the substrate does not crack or the like by the first method will be described in detail below with reference to FIG. FIG. 1A shows a state in which a first substrate 11 and a second substrate 12 having different coefficients of thermal expansion are bonded at room temperature. Here, the first substrate 11 is the second substrate.
It is assumed that the thermal expansion coefficient is larger than that of the substrate 12. When the temperature of the bonded substrates is raised for heat treatment, the first substrate 11 having a large thermal expansion coefficient tends to extend from the second substrate 12.
However, since both substrates are tightly integrated, the first substrate 11
Is prevented from being stretched by the second substrate 12 and the first substrate 11
Generates a compressive stress. Conversely, tensile stress is generated in the second substrate 12. These stresses increase in proportion to the heat treatment temperature. This is the cause of peeling or cracking when heat treatment is performed on the close contact substrates having different coefficients of thermal expansion at a high temperature. In general, a solid material used as a semiconductor wafer is strong against a compressive stress but weak against a tensile stress. Therefore, in this case, the tensile stress generated in the second substrate 12 becomes a problem.

On the other hand, these stresses are related to the thickness of the substrate other than the temperature. For example, when the first substrate 11 is relatively thinner than the second substrate 12 as shown in FIG. 1B, the stress generated in the first substrate 11 increases. Further, when the first substrate 11 is thin, the force applied to the second substrate 12 becomes smaller, so that the stress generated in the second substrate 12 becomes smaller than that in the case of FIG. That is, as shown in FIG.
When the substrate 11 is made thinner, the compressive stress generated on the first substrate 11 increases, and the tensile stress generated on the second substrate 12 decreases. As described above, since the tensile stress generated in the second substrate 12 is a cause of peeling or the like, if the first substrate 11 is made thinner than the second substrate 12, the peeling does not occur at a high temperature. Heat treatment becomes possible.

How thin the first substrate 11 should be depends on the materials of the first substrate 11 and the second substrate 12, the original thickness, the heat treatment temperature for obtaining the required strength, and the like.
I can't say it. According to the experiments performed by the present inventors, the first substrate 11 needs to be heat-treated at a temperature of several hundred degrees Celsius or more, which is necessary for firmly bonding commonly used substrates such as GaAs, silicon, quartz, and glass. It is sufficient if the thickness of the second substrate 12 is 1/5 or less.

In addition, even if there is a film such as an oxide film on the surface to which one or both substrates are bonded, there is no problem if it is about 10 μm or less and sufficiently thinner than the second substrate.

If the thickness of the first substrate 11 is set to about 1/5 of the thickness of the second substrate 12 from the beginning, there is no problem even if the heat treatment is performed at a high temperature. However, the thickness of the substrate is usually several hundred μm, and a thickness of 1/5 is too thin to handle the substrate. Conversely, if the thinner is several hundred μm and the other is five times as thick, the wafer obtained by direct bonding is too thick,
It interferes with the subsequent steps. Also, the cost is increased.

Therefore, as in the present invention, it is useful to take a step of performing a heat treatment at an intermediate temperature at which peeling or the like does not occur on the contact substrate, then reducing the thickness of one of the substrates, and then performing a heat treatment again at a high temperature. .

Next, according to the second method of the present invention in which the substrate is immersed in high-temperature pure water at the end of the pretreatment, generation of voids at the interface of the obtained bonded wafer is effectively suppressed. This has been confirmed experimentally. Observation of voids at the interface of the bonded wafer is performed using a scanning radiation thermometer (for example, Thermovision 68 manufactured by AGA).
0).

The reason why the voidless bonded wafer is obtained by the second method is that excessive water molecules on the substrate surface are evaporated or vaporized and removed by high-temperature pure water immersion treatment, and an appropriate amount of water molecules necessary for the voidless bonding are removed. It is considered that an adsorbed state was obtained. Therefore, from this, according to the second method, deterioration of the bonding interface due to excessive moisture is avoided, the electrical characteristics of the interface are improved, and the characteristics of the semiconductor element are improved.

(Example) Example 1 A mirror-polished silicon substrate and a quartz substrate were prepared.
Both silicon and quartz substrates are 100mm in diameter and 500μ in thickness
m. First, both substrates were cleaned. Cleaning is performed on both substrates.
Processing of a mixture of sulfuric acid and hydrogen peroxide → processing of a mixture of hydrochloric acid and hydrogen peroxide → washing with water. Finally, the silicon substrate was treated with dilute hydrofluoric acid and washed with water. After washing, both substrates were immersed in pure water at about 90 ° C., dried with a spinner, and brought into close contact by bringing mirror surfaces into contact with each other in a clean atmosphere. Next, a first heat treatment was performed.
The temperature is 300 ° C., the atmosphere is nitrogen, and the time is 1 hour. Next, the thickness of the silicon substrate was reduced by polishing. Next, a second heat treatment was performed at 1100 ° C. for 1 hour in a nitrogen atmosphere.

In the sample wafer in which the silicon substrate was thinned to 100 μm or less, no peeling or the like was observed at all in the subsequent second heat treatment step. In addition, it withstood the subsequent device processes. The bonded state of the two substrates can be visually observed through the quartz substrate, but no void was observed.

In the case of a sample whose silicon substrate was less polished and whose thickness was 120 μm or more, it was destroyed in the second heat treatment step at 1100 ° C.

2 (a) to 2 (f) show the steps of manufacturing a high breakdown voltage diode wafer to which the method of this embodiment is specifically applied. The silicon substrate 21 and the quartz substrate 22 are subjected to the cleaning treatment as described above, and the silicon substrate 21 has n ⁻
A mold layer 24 and a 0.5 μm thermal oxide film 23 were formed ((a)).
After bonding them at room temperature, a first heat treatment was performed ((b)). Thereafter, the silicon substrate 21 was polished to a thickness of 30 μm ((c)).

Next, a groove 25 was dug by selectively etching the silicon substrate 21, and ap ⁺ type layer 26 was formed by diffusion on the side surface of the exposed silicon layer ((d)). This diffusion was performed at 1100 ° C. This is the second
Heat treatment step. Then, after forming a thermal oxide film 28 on the side surface, a polycrystalline silicon film 26 was deposited on the entire surface ((e)). Finally, use a silicon layer to planarize the surface.
Polishing was performed so that the thickness of 21 became 21 μm, and a derivative-separated wafer was obtained ((f)).

FIG. 3 shows a high voltage diode formed on the wafer thus obtained by a normal process.

FIG. 4 shows a similar application example, in which a Si substrate 41 and a quartz substrate 42 are bonded to form a separation groove 43 having vertical side walls by a reactive ion etching method.

Example 2 A mirror-polished silicon substrate and GaAs substrate were prepared.
The silicon substrate has a diameter of 76 mm and a thickness of 400 μm, and the GaAs substrate has a diameter of 76 mm and a thickness of 450 μm.

First, both substrates were cleaned. The cleaning of the GaAs substrate is boiled with methacrene → substitution with acetone → substitution with ethanol → rinsing with water → boiling with hydrochloric acid → rinsing with water. The cleaning of the silicon substrate is the same as in the first embodiment.

After the cleaning, both substrates were dried with a spinner, and the mirror surfaces were brought into close contact with each other in a clean atmosphere. Next, a first heat treatment was performed. The temperature was 400 ° C., the atmosphere was argon, and the time was 1 hour. GaAs substrate thickness 20μ by etching
m and then a second heat treatment was performed at 600 ° C. for 1 hour in an argon atmosphere.

As a result, there was no peeling or destruction of the substrate, the bonding strength was sufficient, and the subsequent device process was stopped.

Example 3 A number of silicon substrates having one surface mirror-polished were prepared.
All have plane orientation (100), p-type with specific resistance of 10Ωcm, diameter 7
6 mm, thickness 450 μm. These substrates were first cleaned. The cleaning treatment is a mixture treatment of a hydrogen peroxide solution and sulfuric acid and an aqua regia treatment.

30 of the cleaned substrates were immersed in boiling pure water and taken out to a clean atmosphere, and mirror surfaces were brought into contact to obtain an adhesive wafer (A type). The other 30 substrates were immersed in pure water at 80 ° C and taken out in a clean atmosphere.
Similarly, the mirror surfaces were brought into contact with each other to obtain a bonded wafer (B type).

A total of 15 A type wafers and 15 B type wafers
For 30 sheets, the presence or absence of voids at the interface was examined with a scanning radiation thermometer using an InSb detector. No voids were observed in any of them.

In addition, 1000 wafers in a nitrogen atmosphere
Heat treatment was performed at ℃ for 1 hour. When void inspection was performed again by the scanning radiation thermometer, no void was detected in any of the wafers.

For comparison, a bonded silicon wafer was prepared under the same conditions except that the temperature of the pure water treatment was lowered. One of them is
This is a type subjected to pure water treatment at 23 ° C. (C type), and the other set is a type subjected to pure treatment at 70 ° C. (D type). No voids were detected in these C-type and D-type bonded wafers before the heat treatment. However, in a nitrogen atmosphere, 1
After heat treatment at 000 ° C for 1 hour, C type
Voids occurred in 10 out of 50 sheets (50%), and voids occurred in 7 out of 15 sheets (47%) in the D type.

Example 4 The same 40 silicon substrates as in Example 3 were prepared.
A silicon oxide film (0.7 μm) is formed on the surface of ten substrates, and a silicon nitride film (0.4 μm) is formed on the other ten substrates.
μm). Then, all the substrates were subjected to a cleaning treatment in the same manner as in Example 3, further immersed in boiling pure water, taken out in a clean atmosphere, and the substrate on which the oxide film was formed and the substrate on which no film was formed were removed. Mirror surfaces were bonded together to make ten bonded wafers (E type), and similarly, a substrate on which a nitride film was formed and a substrate on which no film was formed were brought into mirror contact with each other to make ten bonded wafers (F type). ).

These wafers were inspected for voids in the same manner as in Example 3, but no voids were found on any of the wafers. Further, even after performing the same high-temperature heat treatment as in Example 3 on these wafers, generation of voids was not recognized.

Example 5 A GaAs substrate having one surface mirror-polished was prepared, and this was subjected to a cleaning treatment. The cleaning treatment is a boiling treatment with tricrene and acetone and an acid treatment. Thereafter, the substrate was immersed in boiling pure water and taken out in a clean atmosphere, and two mirror surfaces were bonded together to obtain an adhesive wafer.

The resulting bonded wafer was inspected for voids as in Example 3, but no voids were detected.

Further, the obtained wafer was subjected to a heat treatment at 500 ° C. for 1 hour in a nitrogen atmosphere. No voids were detected in the heat-treated wafer.

Example 6 An InP substrate having one surface mirror-polished was prepared, and was subjected to a cleaning treatment under the same conditions as in Example 5. Thereafter, the substrate was immersed in pure water at 90 ° C., taken out in a clean atmosphere, and two mirror surfaces were stuck together to obtain an adhesive wafer.

The resulting bonded wafer was inspected for voids as in Example 3, but no voids were detected.

Further, the obtained wafer was heat-treated under the same conditions as in Example 5. No voids were detected in the heat-treated wafer.

Example 7 A GaP substrate having one surface mirror-polished was prepared, and was subjected to a cleaning treatment under the same conditions as in Example 5. Thereafter, the substrate was immersed in boiling pure water and taken out in a clean atmosphere, and two mirror surfaces were bonded together to obtain an adhesive wafer.

The resulting bonded wafer was inspected for voids as in Example 3, but no voids were detected.

Further, the obtained wafer was heat-treated under the same conditions as in Example 5. No voids were detected in the heat-treated wafer.

[Effects of the Invention] As described above, according to the present invention, it is possible to obtain a semiconductor wafer in which substrates having different coefficients of thermal expansion are firmly and directly bonded. Further, according to the present invention, it is possible to obtain an excellent semiconductor wafer in which no void is generated at the bonding interface.

[Brief description of the drawings]

FIGS. 1 (a) and 1 (b) are diagrams for explaining the operation of the present invention, and FIGS. 2 (a) to 2 (f) are diagrams showing a manufacturing process of a dielectric isolation wafer of a specific embodiment. FIG. 3 is a view showing the structure of a high voltage diode using the obtained wafer, and FIG. 4 is a sectional view of another dielectric isolation wafer. 11 first substrate, 12 second substrate, 21, 41 silicon substrate, 22, 42 quartz substrate, 23 oxide film, 24 n
^- -type layer, 25, 43 ...... groove, 26 ...... p ⁺ -type layer, 27 ...... polycrystalline silicon film, 28 ...... oxide film.

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁶ , DB name) H01L 21/02 H01L 27/12

Claims (2)

(57) [Claims] A semiconductor wafer is manufactured by directly bonding polished surfaces of two substrates having different coefficients of thermal expansion, at least one of which is a semiconductor, and at least one surface of which is mirror-polished, when manufacturing a semiconductor wafer. A method for manufacturing a semiconductor wafer, comprising: bonding a substrate at room temperature, performing a first heat treatment at an intermediate temperature, reducing the thickness of one substrate, and then performing a second heat treatment at a high temperature. 2. A method for producing a semiconductor wafer, wherein at least one of the substrates is a semiconductor, and at least one side of each of the substrates is mirror-polished, and the polished surfaces of the two substrates are directly bonded to each other to produce a semiconductor wafer
A method for manufacturing a semiconductor wafer, comprising cleaning two substrates, immersing the substrates in pure water at a temperature of 80 ° C. or higher, taking out the substrates, and bonding and heat-treating the two substrates.