JPH09167847A - Manufacture of semiconductor pressure sensor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enable a diaphragm to be restrained from being deformed and set uniform in thickness when it is formed through such a manner that two silicon substrates are pasted together, one of the silicon substrates is ground and polished for the formation of a diaphragm and to lessen stress applied to the diaphragm in a thermal treatment process where a pressure detector and a peripheral circuit are formed on a silicon substrate after the diaphragm is formed. SOLUTION: A silicon oxide film 4 is formed on the surface of a first silicon substrate 1 where a recess 11 is previously formed, and then the silicon oxide film 4 is removed from the surface of the substrate 1 other than the recess 11 by etching. By this setup, when the substrates 1 and 2 are pasted together, a cavity 12 is filled up with the silicon oxide film 4, so that a diaphragm 3 is less deformed when the silicon substrate 2 is ground and polished, and a pressure change is lessened in a thermal treatment process. Lastly, the silicon oxide film 4 is removed when a hole is bored in the first silicon substrate 1.

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 a semiconductor pressure sensor which detects pressure and acceleration by detecting strain of a diaphragm, for example.

[0002]

2. Description of the Related Art As a semiconductor pressure sensor, a semiconductor substrate is locally thinned to form a diaphragm, and a pressure detecting element such as a piezo element is formed on the surface side of the diaphragm to prevent the distortion of the diaphragm due to the pressure application. It is known that pressure is converted into an electric signal by detecting with a pressure detecting element. As one of the methods for manufacturing this kind of semiconductor pressure sensor, a method of bonding two silicon wafers is known, and is described in, for example, Japanese Patent Laid-Open No. 3-68175.

Here, an example of a method for manufacturing a semiconductor pressure sensor using a wafer bonding technique is shown in FIG. First, the recess 11 is formed on the first silicon wafer 1 as shown in FIG. 8A, then the second silicon wafer 2 is bonded as shown in FIG. 8B, and then the second silicon wafer 2 is attached. After shaving the substrate 2 with a glider, the diaphragm 3 is polished to a thickness of about 10 μm as shown in FIG. 8C.
To form After that, as shown in FIG. 8D, for example, the pressure detection element 31 is formed at a position near the end of the diaphragm 3, and then a hole 12 is opened in the first silicon substrate 1 as shown in FIG. 8E. By opening the recess 11 by
A pressure sensor is formed which measures the differential pressure across the diaphragm 3.

Further, as a method of manufacturing a semiconductor pressure sensor using the bonding of two silicon substrates, FIG.
As shown in FIG. 9A, the second silicon substrate 22 having the epitaxial layer 21 formed on the surface thereof is bonded to the surface of the first silicon substrate 1 having the recess 11 formed therein, and FIG.
As shown in FIG. 9, for example, the polysilicon layer 23 of the second silicon substrate 22 is ground and thinned by a grinder, and then the second silicon substrate 22 is electrochemically etched to leave a thin epitaxial layer 21 (see FIG. )), A technique for forming the diaphragm 3 in this manner is also known.

[0005]

However, in the method shown in FIG. 8, when the second silicon substrate 2 becomes thin in the grinding / polishing process, the portion corresponding to the diaphragm 3 is changed to the one shown in FIG.
As shown in 0, it bites into the concave portion 11 to be curved and deformed, so that the diaphragm 3 finally becomes uneven,
Therefore, it is difficult to make the thickness uniform, and it is difficult to perform highly accurate pressure detection.

Further, in the manufacturing process for forming the pressure detecting element and the peripheral circuit on the second silicon substrate 2, for example, in the diffusion step, the silicon substrate is heated to a high temperature, and the pressure inside the cavity is largely changed, as shown in FIG. As described above, the flow stress of silicon (the stress at the limit leading to plastic deformation) sharply decreases as the temperature rises, so the stress applied to the diaphragm may enter the region of plastic deformation, and the diaphragm may cause plastic deformation. In particular, since high precision is required for pressure detection in the future, a CMOS circuit may be formed in the same chip as a pressure detection element in order to compensate for fluctuations in output signal due to variations in diaphragm thickness and temperature changes. In this case, since the substrate is heated to a high temperature of 1200 ° C. during the well forming process, the diaphragm is likely to be plastically deformed. It has been clarified by experiments by the present inventors that this plastic deformation causes crystal defects in silicon on the diaphragm. Such crystal defects weaken the fracture strength of the diaphragm (lower the yield point), The reliability of mechanical strength in use is reduced.

On the other hand, according to the method of electrochemically etching as shown in FIG. 7, a large pressure such as mechanical polishing is not applied during the formation of the diaphragm, so that the diaphragm can be formed to have a uniform thickness. There are advantages. However, in this method, since the epitaxial layer must be formed on the second silicon substrate, the manufacturing cost of the substrate becomes higher and higher, and the cost of the semiconductor pressure sensor eventually increases. Furthermore, the above-mentioned problems in the high temperature process in forming the pressure sensing element and the peripheral circuit on the diaphragm still remain.

The present invention has been made under such circumstances, and an object thereof is to provide a method of manufacturing a semiconductor pressure sensor which can improve the mechanical reliability of the diaphragm. Another object of the present invention is to provide a method of manufacturing a semiconductor pressure sensor in which the diaphragm is less deformed even if the diaphragm is formed by grinding and polishing.

[0009]

According to the first aspect of the present invention, there is provided the following:
Forming a concave portion for a diaphragm in the semiconductor substrate, filling the concave portion with a filling material, bonding a second semiconductor substrate to the concave surface of the first semiconductor substrate, Thinning one of the first semiconductor substrate and the second semiconductor substrate to form a diaphragm; forming a pressure detection element on the diaphragm; and the first semiconductor substrate or the second semiconductor substrate.
A hole is opened from the other side of the semiconductor substrate to open the recess, and the filling material in the recess is removed through the hole.

According to a second aspect of the invention, in the invention of the first aspect, the step of filling the recess corresponds to the step of forming a thin film on the surface of the first semiconductor substrate on which the recess is formed, and the recess. Forming a mask on the thin film to remove the thin film in other regions, thereby leaving the thin film in the recess.

[0011]

1 and 2 are process drawings showing a manufacturing process according to an embodiment of the present invention. The same parts as those in FIG. 6 are designated by the same reference numerals. In this embodiment, first, as shown in FIG. 1A, for example, a depth of 1 to 1 is formed on the surface of a first silicon substrate 1 which is a first semiconductor substrate.
A concave portion (trench) 11 having a cavity size of 2 μm and a length and width of 500 μm × 500 μm is formed, and then FIG.
As shown in FIG. 5, a silicon oxide film (SiO ₂ film) 4 slightly thinner than the depth of the recess 11 is formed on the surface of the first silicon substrate 1 by, for example, CVD (Chemical Vapor Deposition). After that, as shown in FIG. 1C, a resist mask 5 is formed on the silicon oxide film 4 in the recess 11, and the silicon oxide film 4 in the recess 11 is left by leaving the silicon oxide film 4 in the recess 11 by wet etching using, for example, hydrofluoric acid. The oxide film 4 is removed, and then the resist mask 5 is removed.

As a result, the inside of the recess 11 is filled with the silicon oxide film 4 as shown in FIG. 1 (d). The silicon oxide film 4 corresponds to the embedding material in the present invention, but other material such as silicon nitride may be used as the embedding material as long as it does not diffuse into silicon. The thickness of the silicon oxide film 4 is formed, for example, by about 1000 angstroms if the depth of the recess 11 is, for example, 1 μm. The reason for slightly reducing the thickness is that the silicon oxide film 4 is recessed by 11 μm. This is because if they protrude from the substrate, it becomes impossible to bond the substrates described below.

Although the recess 11 is schematically illustrated in the drawing, since the size of the recess 11 is, for example, about several hundreds of μm as described above, an alignment error when the resist mask 5 is formed. Is not a problem, so SiO
The embedding of ₂ can be easily implemented. A slight gap is formed between the concave portion 11 and the silicon oxide film 4. However, since the concave portion 11 is sufficiently larger than this gap, the silicon oxide film 4 causes at least about 4/5 of the volume of the concave portion 11. The above is satisfied.

After that, as shown in FIG. 2A, the second silicon substrate 2 is attached to the surface of the first silicon substrate 1, and the second silicon substrate 2 is ground by a grinder to be thinned to some extent (see FIG. 2 (b)), and a cavity 1 for further polishing to form a recess 11 or a diaphragm.
A diaphragm 3 having a thickness of 10 to 20 μm is formed on the substrate 3. The first and second silicon substrates 1 and 2 each have a thickness of, for example, 1000 on the surface of the single crystal silicon substrate.
A silicon oxide film having a thickness of about angstrom is used. Since the silicon oxide film (not shown) and the silicon oxide film 4 for filling have different properties, only the silicon oxide film 4 can be removed by wet etching.

Subsequently, as shown in FIG. 2D, a pressure detecting element 31, for example, a P-type dopant 2 to 2 is formed on the diaphragm 3.
Form a piezo element that is diffused by about 5 μm, and further
On the silicon substrate 2 of the peripheral element, for example, the pressure detecting element 3
A CMOS circuit 32 for performing temperature compensation or the like on the electric signal corresponding to the pressure detected in 1 is formed. After that, as shown in FIG. 2E, a hole 12 is opened from the back surface side of the first silicon substrate 1 by wet etching with hydrofluoric acid to open the recess 11, and the silicon oxide film 4 in the recess 11 is melted. The semiconductor pressure sensor for measuring the differential pressure is obtained by removing it through the hole 12.

According to the above-described embodiment, when the second silicon substrate 2 is ground and polished to form the diaphragm 3, the silicon oxide film 4 in the cavity 13 causes the silicon substrate 2 to enter the cavity 13. Curvature is suppressed,
The uniformity of the thickness of the diaphragm 3 becomes high, and accurate pressure detection can be performed.

During the thermal process for forming the pressure detecting element 31 and the peripheral circuit 32, for example, the diffusion process, the stress of the diaphragm 3 is relaxed by the following mechanism. That is, as shown in FIG.
If the inside of the cavity 13 is not filled with the silicon oxide film 4, and the volume inside the cavity 13 is set to V1, the gas inside the cavity 13 expands and the volume increases by ΔV. On the other hand, in the case where the cavity 13 is filled with the silicon oxide film 4 as shown in FIG. 3B, if the volume inside the cavity 13 is V2, the increase in volume when the temperature becomes high is similar to that of the diaphragm. Since the degree of curvature of 3 is almost the same, it can be expressed as ΔV. Although the pressure inside the cavity 13 rises due to the temperature rise, V2 is 1 of V1 as described above.
Since it is smaller than about / 5, the volume increase rate is larger when the silicon oxide film 4 is present (ΔV / V2) than when it is not present (ΔV / V1). This means that filling the cavity 13 with the silicon oxide film 4 reduces the pressure change with respect to the temperature change. This can be expressed by a formula: Boyle-Charle's formula PV = n
Differentiating RT and rearranging it gives the following equation. ΔP · V + P · ΔV = nR · ΔT (1) ΔP = nR · ΔT / VP−ΔV / V (2) That is, it can be seen from Equation (2) that the smaller V is, the smaller the pressure change is. In this way, if the pressure change in the cavity 13 is small, the stress applied to the diaphragm 3 can be relieved.

FIG. 4 shows the temperature dependence of the flow stress of silicon (critical stress that causes plastic deformation) and the stress on the diaphragm for a 500 μm square rectangular diaphragm. A region NG surrounded by a line connecting □ (flow stress) and a region NG surrounded by a line connecting ■ are plastic deformation regions when the inside of the cavity is under positive pressure and under negative pressure, respectively.
In the heat treatment process, the stress of the diaphragm is
It is necessary not to get inside.

In the conventional method, when the pressure in the cavity is adjusted so as not to reach the negative pressure side region NG as indicated by ◯ (the pressure Po in the cavity when the silicon substrates are bonded together is 0.212 atm). , Temperature about 11
When the temperature exceeds 70 ° C., the region NG on the positive pressure side is affected, and in this example, the theoretical upper limit of the heat treatment temperature is about 1170 ° C.
Becomes Further, when Po is 0.23 atm, as shown by ●, when it exceeds about 1080 ° C., it is applied to the region NG on the positive pressure side, and the upper limit of the thermal process temperature is changed from 1170 ° C. only by slightly shifting Po by about 0.02 atm. 1080 ° C
The temperature becomes too high and the thermal process is considerably limited.

On the other hand, according to the present invention, Po is 0.2a
When set to tm, a thermal process can be performed up to around 1300 ° C. as indicated by ▲, and any semiconductor process whose maximum temperature is usually 1200 ° C. can be supported. For example, a well of a CMOS circuit can be formed. Become.
The characteristic indicated by x is when Po is set to 0.24 atm, and it can be seen that the thermal process can be performed to a considerably high temperature even if the setting of Po is slightly deviated.

Further, since the silicon oxide film exists in the cavity, even if the pressure in the cavity is negative, the height of the silicon oxide film can be adjusted to prevent the diaphragm from being curved inward. Therefore, it is more advantageous than the conventional method. As described above, in the above-described embodiment, the temperature range of the thermal process is widened, which also means that the stress applied to the diaphragm is relaxed, thus improving the mechanical reliability of the diaphragm and further improving the diaphragm. The pressure sensor can be made thin and can be made with high sensitivity.

In the above, according to the present invention, instead of grinding / polishing the second silicon substrate 2, the first silicon substrate 1 is ground / polished to form a diaphragm, and a hole is opened from the second silicon substrate 2 side. You may make it open a cavity. In the present invention, instead of mechanically thinning the silicon substrate, an electrochemical polishing method may be used as described in the section of the prior art.

Furthermore, the present invention is applicable not only to the piezoresistive pressure sensor but also to the production of a capacitive pressure sensor.
FIG. 5 is a diagram showing a capacitive pressure sensor. As for the diaphragm 61 of the silicon substrate 6, a silicon oxide film is formed in the recess 62 and the pressure detection element 63 is formed in the diaphragm 61 as in the previous embodiment. The latter silicon oxide film is removed. This capacitive pressure sensor is configured by adhering a glass substrate 7 to a silicon substrate 6, and detects a capacitance change of a cavity 71 forming a reduced pressure area by a metal layer 72 and a pressure detection element 63.

The semiconductor pressure sensor of the present invention also includes an acceleration sensor for obtaining acceleration based on an electric signal from the pressure detecting element. FIG. 6 shows an example of a process for manufacturing the acceleration sensor. In this example, after forming the pressure detection element 31 made of, for example, a piezoresistive element on the diaphragm 3, a portion other than the cut region S is masked on the diaphragm 3. 8 (FIG. 6A), and then the cut region S is etched (FIG. 6B) to remove the silicon oxide film 4 in the recess 11 from the cut region. In this way, as shown in FIG. 7, an acceleration sensor in which the pressure detecting element 31 is provided on the supporting portion of the cantilevered diaphragm 3 is obtained. As for the acceleration, as the diaphragm 3 moves up and down due to the vertical acceleration, stress is generated in the supporting portion,
This is converted into an electric signal by the pressure detection element 31 and detected.

[0025]

According to the present invention, since the stress of the diaphragm is small during the thermal process for producing the pressure detecting element, the peripheral circuit, etc., the mechanical reliability of the diaphragm can be improved. Also, even if the diaphragm is formed by the grinding / polishing process, the deformation of the diaphragm is suppressed,
Since the thickness can be made uniform, the pressure can be accurately detected.

[Brief description of the drawings]

FIG. 1 is a process drawing showing an embodiment of the present invention.

FIG. 2 is a process drawing showing an embodiment of the present invention.

FIG. 3 is an explanatory diagram for explaining a pressure difference in a concave portion between a conventional method and a method of the present invention in a heat treatment step.

FIG. 4 is a characteristic diagram showing the relationship between the stress of a diaphragm and temperature between the conventional method and the method of the present invention.

FIG. 5 is an explanatory view showing a capacitive semiconductor pressure sensor manufactured by the method of the present invention.

FIG. 6 is a process drawing showing a process for manufacturing an acceleration sensor according to the present invention.

FIG. 7 is a schematic perspective view showing an acceleration sensor.

FIG. 8 is a process chart showing an example of a conventional method for manufacturing a semiconductor pressure sensor.

FIG. 9 is a process drawing showing another example of the conventional method for manufacturing a semiconductor pressure sensor.

FIG. 10 is an explanatory diagram showing a state in which unevenness is formed on the diaphragm in the diaphragm manufacturing process.

FIG. 11 is a schematic characteristic diagram schematically showing the relationship between flow stress of silicon and temperature.

[Explanation of Codes] 1 First Silicon Substrate 11 Recess 12 Hole 2 Second Silicon Substrate 3 Diaphragm 31 Pressure Sensing Element 4 Silicon Oxide Film 5 Resist Mask 6 Silicon Substrate 61 Diaphragm 63 Pressure Sensing Element 7 Glass Substrate 72 Metal Layer

Claims (2)

[Claims] 1. A step of forming a concave portion for a diaphragm in a first semiconductor substrate, a step of filling the concave portion with a filling material, and a second semiconductor substrate on a surface of the first semiconductor substrate on which the concave portion is formed. A step of forming a diaphragm by thinning one of the first semiconductor substrate and the second semiconductor substrate, a step of forming a pressure detection element on the diaphragm, A method of manufacturing a semiconductor pressure sensor, which comprises: forming a hole from the other side of the semiconductor substrate or the second semiconductor substrate to open the recess, and removing a filling material in the recess through the hole. 2. The step of filling the recess includes the step of forming a thin film on the surface of the first semiconductor substrate on which the recess is formed,
2. A semiconductor pressure sensor according to claim 1, further comprising the step of forming a mask on the thin film corresponding to the recess to remove the thin film in other regions, thereby leaving the thin film in the recess. Production method.