JP6839573B2 - Manufacturing method of substrate for electrostatic induction device and substrate for electrostatic induction device - Google Patents

Description

The present invention relates to a method for manufacturing a substrate used for an electrostatic induction device that performs electromechanical conversion using an electrostatic induction action, and a substrate for an electrostatic induction device .

An electromechanical converter that converts between electric power and electric power by electrostatic interaction generated by using an electret having a property of holding electric charge semipermanently is known. (See, for example, Patent Document 1)

Patent Document 1 discloses a substrate having an electret film formed on its surface as a substrate used for an electromechanical converter. In electromechanical converters that utilize electrostatic induction, the surface potential of the electret film is required to be high, but the electret film made of a silicon oxide film formed by thermally oxidizing a conventional flat surface is sufficiently high. The surface potential could not be obtained. The cause is that the volume of the silicon oxide film per unit area is insufficient. To increase the volume of the silicon oxide film per unit area, it is effective to increase the thickness of the silicon oxide film, but for that purpose, a long-time silicon oxidation treatment is required, and the thickness of the silicon oxide film should be increased. Was difficult.

In the substrate for an electrostatic induction device according to Patent Document 1, an electret film is formed by forming grooves and protrusions on the surface of the silicon substrate and injecting an electric charge into the silicon oxide film formed by thermally oxidizing the surface. ing. The purpose of forming grooves and protrusions on the surface of a silicon substrate in Patent Document 1 is to improve the surface potential, which is a drawback of an electret film formed by thermally oxidizing a flat surface of a conventional silicon substrate. In Patent Document 1, a groove and a convex portion are formed on a flat surface of a silicon substrate, and a concave-convex shape is provided on the surface of the silicon substrate to increase the surface area. That is, the surface area was increased by newly adding the side surface of the convex portion as a surface. Then, the surface potential was improved by oxidizing the entire surface of the groove and the convex portion including the surface newly added as the surface of the silicon substrate.

Japanese Patent No. 4514782 (page 18, FIG. 1, FIG. 4)

However, in Patent Document 1, the core portion inside the convex portion remains in a silicon state without being oxidized, and therefore, this core portion does not contribute as an electret film. Therefore, the effect of the improvement by Patent Document 1 is not sufficient, and further improvement is desired.

Therefore, an object of the present invention is to provide a method for manufacturing a substrate for an electrostatic induction device, which increases the thickness of a silicon oxide film and improves the surface potential of an electret film by a simple method.

In order to solve the above problems, the manufacturing process of the substrate for the electrostatic induction device of the present invention
A step of forming grooves and protrusions on a silicon substrate, a heat oxidation step of heat-treating all the protrusions into a silicon oxide film, and a charging step of charging the silicon oxide film.
A step of removing the silicon oxide film in the thickness direction to partially expose the silicon substrate.
The groove includes a plurality of first grooves having a first width and a plurality of second grooves having a second width wider than the first width, and is formed in the second groove. The silicon oxide film was completely removed in the thickness direction, the silicon substrate was exposed in the second groove, and the silicon oxide film in the first groove was formed into polar shapes independent of each other. It is characterized by.

It is preferable that the depth of the first groove and the depth of the second groove are the same.
Further, it is preferable that the substrate for an electrostatic induction device of the present invention is charged with silicon oxide films having polar shapes independent of each other.

According to the above-mentioned method for manufacturing a substrate for an electrostatic induction device, a thick silicon oxide film can be formed in a short time by forming all the convex portions into a silicon oxide film. Therefore, an electret film having a high surface potential can be obtained, and a substrate for an electrostatic induction device provided with a high-performance electret film can be easily obtained.

It is sectional drawing explaining the manufacturing process of the electret substrate in 1st Embodiment of this invention. It is sectional drawing explaining the manufacturing process of the electret substrate in 1st Embodiment of this invention. It is a schematic block diagram of the oxidizing apparatus used in the heating oxidation step in the manufacturing process of the electret substrate shown in FIG. It is a schematic block diagram of the charging apparatus used in the charging process in the manufacturing process of the electret substrate shown in FIG. It is sectional drawing explaining the manufacturing process of the electret substrate in 2nd Embodiment of this invention. It is sectional drawing explaining the manufacturing process of the electret substrate in 2nd Embodiment of this invention. It is sectional drawing of the electret substrate explaining the condition of silicon oxide film formation. It is sectional drawing of the electret substrate explaining the condition of silicon oxide film formation. It is sectional drawing of the electret substrate explaining the condition of silicon oxide film formation. It is a graph which shows the relationship between the film thickness of a silicon oxide film, and the oxidation time. It is a perspective view and cross-sectional view of the electret substrate which shows the arrangement of the pattern of a groove and a convex part in 3rd Embodiment of this invention. It is a perspective view and cross-sectional view of the electret substrate which shows the arrangement of the pattern of the groove and the convex part in 4th Embodiment of this invention.

In the description of the embodiment of the present invention, an electret substrate most widely used as a substrate for an electrostatic induction device will be described as an example.
Hereinafter, a specific embodiment of a manufacturing process of a substrate for an electrostatic induction device (hereinafter, abbreviated as “electret substrate”) will be described in detail with reference to the drawings.
In the explanation, the drawings used in the embodiments are schematic drawings, and the dimensions, shapes, and arrangement positions do not accurately reflect the actual shapes and situations, and some exaggerations are made to make the drawings easier to see and understand. ing.

[Explanation of Manufacturing Process of Electret Substrate in First Embodiment: FIGS. 1 and 2]
The manufacturing process of the electret substrate in the first embodiment of the present invention has three steps. The three steps are a concave-convex forming step of forming grooves and convex portions on a silicon substrate, a heating oxidation step of forming all of the convex portions into a silicon oxide film, and a charging step of charging the silicon oxide film.

Hereinafter, the manufacturing process of the electret substrate in the first embodiment will be described with reference to FIGS. 1 to 4.
1 and 2 are cross-sectional views showing a manufacturing process of an electret substrate in the first embodiment.
The first step of the manufacturing process of the electret substrate is the unevenness forming step of forming the groove 3 and the convex portion 2 on the surface 1a of the silicon substrate 1.

The unevenness forming step of forming the groove 3 and the convex portion 2 on the surface 1a of the silicon substrate 1, which is the first step, will be described with reference to FIG. 1A.
In the unevenness forming step of forming the groove 3 and the convex portion 2, first, a patterned resist layer (not shown) is arranged on the surface 1a of the silicon substrate 1. In the resist layer pattern, a resist layer having a width of P1 was arranged with an interval G1 between adjacent resist layers. In the present embodiment, the pattern width P1 of the resist layer is set to 1 μm, and the distance G1 between adjacent resist layers is set to 1 μm. Here, the portion that becomes the convex portion 2 is a portion where the resist layer is arranged, and the portion that becomes the groove 3 is a portion without the resist layer.

Next, etching is performed using a D-RIE (Deep Reactive Ion Etching) apparatus. Then, the portion of the surface 1a of the silicon substrate that is not covered with the resist layer is removed in the depth direction to form the groove 3. Then, the portion covered with the resist layer is not etched and becomes the remaining convex portion 2. The depth d of the groove 3 is determined by the etching time by the D-RIE apparatus, and in the present embodiment, the etching time is set so that the depth d of the groove 3 is 5 μm, that is, the height d of the convex portion 2 is 5 μm. Controlled. By removing the patterned resist layer after the etching is completed, a concave-convex shape is formed on the surface 1a of the silicon substrate 1.

In FIG. 1A, the flat surface 1a before etching is changed to an uneven shape. The four surfaces that form the concave-convex shape are the upper surface 2U of the convex portion 2, the left side surface 2L, the right side surface 2R, and the bottom surface 3B of the groove 3. However, since the upper surface 2U of the convex portion 2 and the lower surface 3B of the groove 3 correspond to the surface 1a before etching, the newly increased surfaces are the two surfaces of the left side surface 2L and the right side surface 2R of the convex portion 2. is there. As a result, the surface area of the silicon substrate 1 is increased by the amount of these two surfaces.

Next, the second step is a heating oxidation step of forming all of the convex portions 2 into a silicon oxide film. The apparatus used in the thermal oxidation step is the thermal oxidation apparatus 30 shown in FIG. 3, and first, the configuration of the thermal oxidation apparatus 30 will be described with reference to FIG.
The components of the heating oxidation apparatus 30 are a nitrogen gas cylinder 31, a closed container 33, a heating oxidation furnace 36, and pipes 32, 35.
The nitrogen cylinder 31 is _{filled with nitrogen gas N 2} at high pressure. The closed container 33 contains the potassium hydroxide aqueous solution 34. The input pipe 32 connects the nitrogen cylinder 31 and the closed container 33, and the end thereof is arranged in the potassium hydroxide aqueous solution. The output pipe 35 is a pipe connecting the closed container 33 and the heating oxidation furnace 36. Then, one end of the output pipe 35 is in the closed container 33 and is arranged above the liquid level of the potassium hydroxide aqueous solution 34, and the other end is arranged in the heating oxidation furnace 36.

Next, the thermal oxidation step will be described using the thermal oxidation apparatus 30 shown in FIG.
First, the silicon substrate 1 is placed in the heating oxidation furnace 36 shown in FIG. In order to prevent dust from adhering, it is recommended to place it upright as shown in the figure. The heating oxidation furnace 35 is heated to 1000 ° C. using a heating heater (not shown).
After the temperature of the silicon substrate 1 has been sufficiently raised, the valve (not shown) of the nitrogen cylinder 31 is opened to allow the nitrogen gas N ₂ to flow through the input pipe 32. Then, the nitrogen gas N ₂ is bubbled into the potassium hydroxide aqueous solution 34 from the other end _{of the input pipe 32.} The nitrogen gas N _{2 functions as a} carrier gas and introduces steam containing potassium hydroxide into the heating oxidation furnace 36 via the output pipe 35. Then, the surface of the heated silicon substrate 1 is thermally oxidized in an atmosphere containing alkali ions of potassium hydroxide to form a silicon oxide film containing potassium ions.

Next, the growth of the silicon oxide film containing potassium ions will be described with reference to the cross-sectional views of the silicon substrate 1 shown in FIGS. 1 (b) and 1 (c).
FIG. 1B shows a state in which the heat oxidation step has progressed to the middle, and FIG. 1C shows a state in which the heat oxidation step has been completed.

Prior to the explanation of the heat oxidation step, the growth mechanism of the silicon oxide film will be described. Oxidation of silicon is carried out by the insertion of oxygen atoms and potassium atoms between silicon atoms, so that the volume increases when silicon changes to a silicon oxide film. As a result, the silicon oxide film grows while increasing its volume toward both the groove side on the outer side of the convex portion of the silicon substrate and the core side of the convex portion.

The shape of the silicon oxide film when the heat oxidation step has progressed halfway is shown in FIG. 1 (b). Here, the uneven shape of the silicon substrate 1 before oxidation is shown by a dotted line.
The silicon oxide film D containing potassium grows on both the outer and inner sides of the concave-convex shape of the silicon substrate 1 before oxidation.

The silicon oxide film Din is an oxide film that grows toward the inside of the uneven shape before oxidation, and starts from three surfaces, the upper surface 2U, the right side surface 2R, and the left side surface 2L of the convex portion 2, which is the starting surface of the oxidation. It grows toward the inside of the convex portion 2. As a result, the silicon inside the convex portion 2 is oxidized to form a silicon oxide film, so that the silicon portion inside the convex portion 2 is reduced and the silicon core 2a remains.
On the other hand, the silicon oxide film Dout is an oxide film that grows toward the outside of the uneven shape before oxidation, and is formed on the bottom surface 3B of the groove 3, the right side surface 2R and the left side surface 2L of the convex portion 2, which are the starting surfaces of the oxidation. It grows in the direction of filling the groove 3 with the third surface as the starting surface. As a result, the shape of the groove is narrower and shallower than the shape of the groove 3 before oxidation.

The heat oxidation step is further advanced, and the heat oxidation step is completed 12 hours after the start of the heat oxidation. A cross-sectional view of the silicon substrate 1 at the stage when the heat oxidation step is completed is shown in FIG. 1 (c).
As a result of the oxide film Din growing further inside the convex portion 2, the portion of the silicon core 2a remaining inside the convex portion 2 also becomes an oxide film, and the entire inside of the convex portion 2 is a silicon oxide film containing potassium ions. It will be filled with 2D.
Further, as a result of the oxide film Dout grown on the groove 3 side on the outside of the convex portion 2 further growing and filling the groove 3a, the silicon oxide film 3D containing potassium ions is formed between the adjacent convex portions 2. Become.
As a result, the silicon portion of the convex portion 2 and the void portion of the groove 3 before oxidation both become a silicon oxide film D and become one.

The potassium ion is positively charged, but since the potassium ion forms a pair with the electron of the silicon atom, the positive charge of the potassium ion and the negative charge of the electron cancel each other out, and the potassium ion is electrically neutral. Is.
Therefore, since the silicon oxide film D is electrically neutral and has no surface potential, it does not exhibit the performance as an electret film.

As a result of the heat oxidation step, the thickness t1 of the silicon oxide film D shown in FIG. 1C was 6 μm. The breakdown is 5 μm of the height d of the convex portion 2 and 1 μm of the thickness t0 of the thermal oxide film on the surface of the silicon substrate 1. 1 μm of the thickness t0 of the thermal oxide film is the thickness produced by the thermal oxidation treatment for 12 hours in the thermal oxidation step.
On the other hand, since it takes several hundred hours or more to obtain a silicon oxide film having a thickness of 6 μm by the method of thermally oxidizing the flat surface of the silicon substrate, it is practically difficult to manufacture the silicon oxide film according to the first embodiment. The method of forming is extremely efficient.

The next, third step is a step of charging the silicon oxide film, which is a step of charging the electrically neutral silicon oxide film D to develop the properties of an electret film having a surface potential. ..
The device used in the charging step is the charging device 40 shown in FIG. 4, and its components are a charging power supply 43, two electrodes, a negative electrode terminal 41 and a positive electrode plate 42, and a switch 44.
Here, the positive electrode plate 42 is a plate on which the silicon substrate 1 on which the silicon oxide film D is formed in the previous heating oxidation step is placed. The positive electrode of the charging power supply 43 is connected to the positive electrode plate 42, and the negative electrode of the charging power supply 43 is connected to the negative electrode terminal 41, which is the other electrode, via the switch 44.

The charging process will be described with reference to the charging device 40 of FIG. 4 and FIG.
FIG. 2 is a cross-sectional view of the silicon substrate 1, which schematically shows the presence of potassium ions and the presence of electrons in the silicon oxide film D.
First, the silicon substrate 1 shown in FIG. 2A is placed on the positive electrode plate 42 shown in FIG. At this point, the potassium ion K shown in FIG. 2A is present inside the silicon oxide film D.

Next, the negative electrode terminal 41 shown in FIG. 4 is brought into contact with the surface of the silicon oxide film D, and the switch 44 is closed to energize. The voltage of the charging power supply 43 was 1 kv.
At this time, since the potassium ion K, which is an alkaline ion, is positively charged, as shown in FIG. 4, it is attracted to the negative electrode terminal 41, which is the negative electrode, inside the silicon oxide film D, and is bonded to an electron. Then, it loses its positive charge and becomes neutral.

Then, as shown in FIG. 2B, as a result of the potassium ion K leaving inside the silicon oxide film D, only the electron E paired with the potassium ion K remains.
Since the electron E has a negative charge, the silicon oxide film D is negatively charged to become an electret film 21 having a surface potential.
In this way, the silicon substrate 1 becomes an electret substrate 20 having an electret film 21.

The charging step may be performed by corona charging. Corona charging is a method of irradiating a silicon oxide film with electrons generated by corona discharge generated at a high voltage and injecting electrons into the silicon oxide film to charge the silicon oxide film.

[Explanation of Manufacturing Process of Electret Substrate in Second Embodiment: FIGS. 5 and 6]
The manufacturing process of the electret substrate in the second embodiment of the present invention has four steps.
These four steps are a concave-convex forming step of forming grooves and convex portions on a silicon substrate, a heating oxidation step of forming all the convex portions into a silicon oxide film, and removing the silicon oxide film in the thickness direction to partially remove the silicon oxide film. This is a polar separation step for exposing the silicon oxide film and a charging step for charging the silicon oxide film. Here, the polar separation step of removing the silicon oxide film in the thickness direction and partially exposing the silicon substrate is not in the first embodiment, but is a step characteristic of the present embodiment.

Next, the manufacturing process of the electret substrate in the second embodiment will be described with reference to FIGS. 5 and 6. 5 and 6 are cross-sectional views of the silicon substrate 1 showing the manufacturing process of the electret substrate in the second embodiment.
The manufacturing process of the electret substrate according to the second embodiment is almost the same as the manufacturing process according to the first embodiment, and the difference is that the pole separation steps shown in FIGS. 5 (c) and 5 (d) are added. .. That is, it is provided with a step of removing the silicon oxide film in the thickness direction to partially expose the silicon substrate 1.

The first step in the second embodiment is a concave-convex forming step of forming the groove 3, the groove 4, and the convex portion 2 on the surface 1a of the silicon substrate 1, and will be described with reference to FIG. 5A.
FIG. 5A is a cross-sectional view of the silicon substrate 1 at the stage when the unevenness forming step is completed, and the unevenness forming step for forming the groove 3, the groove 4, and the convex portion 2 is basically the same as that of the first embodiment. Since the same is true, the same elements are designated by the same reference numerals, and the description of overlapping parts will be omitted.

First, a resist layer having a striped pattern corresponding to the width of the convex portion 2 is arranged on the surface 1a of the silicon substrate 1. Next, the silicon substrate 1 is etched using a D-RIE apparatus. Then, the portion not covered with the resist layer is etched in the thickness direction of the silicon substrate 1 to form the groove 3 and the groove 4, and the portion covered with the resist layer and remaining without being etched becomes the convex portion 2.
The arrangement of the groove 3, the groove 4, and the convex portion 2 is shown in FIG. 5 (a). A group T in which three convex portions 2 are arranged so as to be separated from each other by a groove 3 is formed, and the group T is arranged so as to be separated by a groove 4.

Here, the shape dimensions of the groove 3, the groove 4, and the convex portion 2 will be described. The width P1 of the convex portion 2 was set to 1 μm, the width G1 of the groove 3 was set to 1 μm, and the width G2 of the groove 4 was set to 4 μm. Since the depths of the grooves 3 and 4 are set to 5 μm, the height d of the convex portion 2 is also 5 μm.

The next second step is a heating oxidation step of forming the surface of the silicon substrate 1 into a silicon oxide film, which will be described with reference to FIG. 5 (b).
FIG. 5 (b) is a cross-sectional view of the silicon substrate 1 showing a state in which the heat oxidation step is completed, and the heat oxidation step according to the second embodiment shown in FIG. 5 (b) is basically the first described with reference to FIG. Since it is the same as the heat oxidation step of 1 embodiment, the description of the overlapping portion will be omitted.

FIG. 5B shows the state of the silicon substrate 1 when the heat oxidation step is completed 12 hours after the start of heat oxidation.
Similar to the first embodiment, the entire inside of the convex portion 2 is a silicon oxide film 2D.
Further, the groove 3 is filled with the oxide film grown from the convex portions sandwiching the groove 3 from both sides to form the silicon oxide film 3D, as in the first embodiment.
As a result, in the group T in which the three convex portions 2 are arranged so as to be separated from each other by the groove 3 before oxidation, all the gaps between the silicon of the convex portion 2 and the groove 3 become a silicon oxide film. Together, it becomes a silicon oxide film D.
On the other hand, since the groove 4a has a wide groove, it is not filled with the silicon oxide film and maintains the shape of the groove.

The third step below is a polar separation step of removing the silicon oxide film in the thickness direction to partially expose the silicon substrate, which will be described with reference to FIGS. 5 (c) and 5 (d).

In the electrode separation step, etching is performed on the silicon substrate 1 without providing a resist layer, and the silicon oxide films of E1 and E2 shown by hatching in FIG. 5C are removed. E1 is a silicon oxide film in the portion of group T, and E2 is a silicon oxide film in the bottom portion of the groove 4a.
Here, the end of etching is determined by monitoring the substance to be etched. That is, only silicon oxide is detected at the beginning of etching, but as the etching time advances, silicon is detected in addition to silicon oxide, so that the etching is terminated at this point. This monitored silicon means that the silicon substrate 1 is exposed on the bottom surface 5 of the groove 4b shown in FIG. 5 (d).
In this way, the silicon oxide film D is completely cut off from each other by the groove 4b, and becomes a polar shape independent of each other.

The next fourth step is a charging step of charging the silicon oxide film D, which will be described with reference to FIG. 6 which is a cross-sectional view of the silicon substrate 1, but basically the first embodiment described with reference to FIG. Since it is the same process as the above, the description of the overlapping part is omitted.

In the charging step shown in FIG. 6, before the charging voltage is applied, the silicon oxide film D contains potassium ion K, and as shown in FIG. 6A, this potassium ion K is a silicon atom. It is paired with an electron. After the potassium ion K is discharged by the application of the charging voltage, only the electron E shown in FIG. 6B remains. Since the electron E has a negative charge, the silicon oxide film D is negatively charged and becomes an electret film 61 that expresses a surface potential.

In this way, the silicon substrate 1 becomes an electret substrate 60 having an electret film 61 separated and arranged in a polar shape.
In an electret motor or an electret generator, which is an electromechanical converter utilizing electrostatic induction, the substrate to be used must have an electret film separated and arranged, and therefore has an electret film 61 separated and arranged. The electret substrate 60 is the optimum substrate.

[Explanation of Dimensional Conditions for Forming Oxide Films: FIGS. 7A, 7B, 7C]
Next, the dimensional conditions for forming the groove 3, the groove 4, and the convex portion 2 in the first embodiment and the second embodiment will be described with reference to FIGS. 7A, 7B, and 7C.
7A, 7B and 7C are cross-sectional views of the silicon substrate 1 showing the dimensional relationship between the groove 3 and the convex portion 2, and FIG. 7A shows a concave-convex forming step for forming the groove 3 and the convex portion 2. b) shows a heat oxidation step of heat-treating the silicon substrate 1 to form a silicon oxide film on the surface thereof.

Prior to the explanation of oxide film formation using FIGS. 7A, 7B, and 7C, the relationship between the film thickness of the silicon oxide film and the oxidation time will be described with reference to FIG.
The thickness of the silicon oxide film depends on the oxidation time. FIG. 8 is a graph showing the relationship between the oxidation time and the thickness of the silicon oxide film, in which the horizontal axis represents the oxidation time and the vertical axis represents the film thickness of the silicon oxide film.
When the heating oxidation time is 12 hours, the film thickness T1 of the silicon oxide film is 1 μm. When the heating oxidation time is further lengthened, for example, the film thickness T2 of the silicon oxide film becomes 2 μm at 50 hours, and the film thickness T3 of the silicon oxide film becomes 3 μm at 200 hours. In this way, as the oxidation time increases, the thickness of the silicon oxide film increases, but the increase decreases. Therefore, it takes a long time to obtain a thick silicon oxide film by thermally oxidizing the flat surface of the silicon substrate, which is very difficult.

In each of FIGS. 7A, 7B, and 7C, the oxidation time is 12 hours, so that the film thickness of the silicon oxide film formed on the surface of the silicon substrate 1 is 1 μm.
The depth of the groove 3, that is, the height of the convex portion 2 is set to 5 μm, but since an oxide film of 1 μm grows on all surfaces of the silicon substrate 1, this value forms the oxide film described below. Does not affect the conditions to be used. Therefore, a thick oxide film can be obtained by increasing the height of the convex portion 2.

First, the conditions for forming an oxide film will be described with reference to FIG. 7A. This condition is a condition applied to the first embodiment shown in FIG. 1 and the second embodiment shown in FIG.
In FIG. 7A (a), the width of the convex portion 2 is 1 μm, and the width of the groove 3 is also 1 μm.

As shown in FIG. 7A (b), since the heating oxidation time is 12 hours, the thickness of the oxide film on the surface of the silicon substrate 1 is 1 μm, and the width of the convex portion 2 is 1 μm. , All of which are silicon oxide films 2D.
On the other hand, since the volume of the groove 3 increases when silicon changes to a silicon oxide film, the silicon oxide film grows from the convex portions 2 on both sides toward the gap of the groove 3. Here, it is considered that the surface of the oxide film grows 0.5 μm in the direction of filling the groove, which is half the thickness of the surface oxide film of 1 μm. Then, the groove 3 having a groove width of 1 μm is filled with a silicon oxide film grown by 0.5 μm from each of the convex portions 2 on both sides of the groove 3, and becomes a silicon oxide film 3D.

As a result, the silicon oxide film 3D that fills the groove 3 and the silicon oxide film 2D inside the convex portion 2 are integrated, and the entire upper surface of the silicon substrate 1 becomes the silicon oxide film D.
Under the conditions of FIG. 7A, the thickness of the silicon oxide film formed on the surface of the silicon substrate 1 is 6 μm. This is the sum of the height of the convex portion 2 of 5 μm and the thickness of the oxide film formed on the surface of the silicon substrate of 1 μm.

Next, the conditions for forming the oxide film will be described with reference to FIG. 7B. This condition is a condition applied to the second embodiment shown in FIG. 5 (b).
In FIG. 7B (a), the width of the convex portion 2 is 1 μm, and the width of the groove 3 is 2 μm.
In the heat oxidation step shown in FIG. 7B (b), since the width of the convex portion 2 is 1 μm, all the silicon of the convex portion 2 is oxidized to form a silicon oxide film 2D.
On the other hand, since the width of the groove 3 is 2 μm, even if the silicon oxide film from the convex portions 2 on both sides grows in the gap of the groove 3, the gap cannot be filled and the gap remains as the groove 3a.

As described above, when the oxide film is formed under the conditions of FIG. 7B, a groove can be formed between the oxide films D. Therefore, by adding the step of removing the oxide film in the groove between the oxide films D described with reference to FIG. 6 of the second embodiment, the silicon oxide film formed on the surface of the silicon substrate is completely formed into a polar shape. Can be placed.
An electret substrate having an electret film obtained by charging a silicon oxide film arranged in a polar shape is useful as an electret substrate for forming a pole for an electromechanical converter utilizing an electrostatic induction action.

Next, the conditions for forming the oxide film will be described with reference to FIG. 7C. This condition indicates the condition of the reference example for the present application.
In FIG. 7C (a), the width of the convex portion 2 is 2 μm, and the width of the groove 3 is 1 μm.
After the heat oxidation treatment shown in FIG. 7C (b), the width of the convex portion 2 is 2 μm, so that the core portion 2a of the convex portion 2 is not oxidized and remains as silicon. On the other hand, since the width of the groove 3 is 1 μm, the silicon oxide film from the convex portions 2 on both sides grows and the groove 3 void is filled to form the silicon oxide film 3D.
Then, since the silicon oxide film D becomes an oxide film having a silicon portion inside thereof, when the oxide film is charged to form an electret film, the electret film becomes non-uniform, and the uniformity of the surface potential cannot be obtained.
In order to form a thick and uniform oxide film on the surface of the silicon substrate 1 as described above, the formation conditions as shown in FIGS. 7A and 7B are important.

[Explanation of pattern arrangement of grooves and convex portions in the third embodiment and the fourth embodiment: FIGS. 9A and 9B]
In the present invention, since a large number of grooves and protrusions are formed on the surface of the silicon substrate, there is a concern that the silicon substrate may warp in the manufacturing process and the uniformity of the silicon oxide film may decrease. In particular, when a large-sized silicon substrate is used, this warpage becomes large, and countermeasures are required.
The third embodiment and the fourth embodiment are intended to prevent warpage when a large-format silicon substrate is used.

The arrangement of the groove and convex pattern according to the third embodiment of the present invention will be described with reference to FIG. 9A.
9A (a) is a perspective view showing the arrangement of patterns of grooves and protrusions formed on the large-sized silicon substrate 90, and FIG. 9A (b) is A-A of FIG. 9A (a) after the heat oxidation step. A'cross-sectional view, FIG. 9A (c) is a cross-sectional view taken along the line BB'of FIG. 9A (a) after the heat oxidation step.

The pattern arrangement of the groove and the convex portion formed on the silicon substrate 90 will be described with reference to FIG. 9A (a).
A groove and a convex pattern Pu whose longitudinal direction is parallel to the Y axis are formed on the upper surface 90u of the silicon substrate 90, and a groove and a convex portion whose longitudinal direction is parallel to the X axis are formed on the lower surface 90d. The pattern Pd of the part is provided. Therefore, the longitudinal direction of the groove and convex pattern is orthogonal to the upper surface 90u and the lower surface 90d of the silicon substrate 90.
The shape and dimensions of the groove and the convex portion are the conditions described in FIG. 7A, and the width of the groove and the width of the convex portion are 1 μm, and the depth of the groove and the height of the convex portion are 5 μm.

Next, the shape of the silicon oxide film in the heat oxidation step will be described with reference to FIGS. 9A (b) and 9A (c).
In FIG. 9A (b), as described under the conditions for forming the oxide film according to FIG. 7A, the groove of the pattern Pu is filled with the silicon oxide film, the convex portion becomes the silicon oxide film up to the core, and the silicon oxide film in which both are integrated. It becomes PuS. The grooves and protrusions of the pattern Pd are also integrated in the same manner to form a silicon oxide film PdS.
Also in FIG. 9A (c), both the groove and the convex portion form a silicon oxide film and are integrated to form a silicon oxide film PuS and PdS.

By forming grooves and convex portions whose longitudinal directions are orthogonal to each other on the upper and lower surfaces by this pattern arrangement, the stress generated by the unevenness forming process is alleviated, and the warpage of the silicon substrate generated during heating in the heating oxidation step is alleviated. Will be done. As a result, the surface of the silicon substrate is flat and uniformly oxidized, and a homogeneous silicon oxide film can be obtained.
Further, according to this pattern arrangement, electret films can be formed on both sides of one substrate. Therefore, in particular, electricity utilizing the electrostatic induction action required for a substrate having electret films on both sides of the electret substrate. It is useful as a substrate for mechanical converters.

Next, the arrangement of the groove and convex pattern according to the fourth embodiment of the present invention will be described with reference to FIG. 9B.
FIG. 9B (a) is a perspective view showing the arrangement of patterns of grooves and protrusions formed on the large-sized silicon substrate 91, and FIG. 9B (b) is a CC in FIG. 9B (a) after the heat oxidation step. 'It is a cross-sectional view.
As shown in FIG. 9B (a), the pattern arrangement of the groove and the convex portion in the fourth embodiment includes the pattern Px of the groove and the convex portion whose longitudinal direction is parallel to the X axis and the groove and the convex portion parallel to the Y axis. The pattern Py of the part is staggered. Therefore, the longitudinal directions of the groove and convex patterns are orthogonal to each other in the adjacent pattern.

The shape of the silicon oxide film in the heat oxidation step will be described with reference to FIG. 9B (b).
As described under the conditions for forming the oxide film according to FIG. 7A, the grooves of the pattern Py are filled with the silicon oxide film, the convex portions are the silicon oxide film up to the core, and the two are integrated into the silicon oxide film PyS. The grooves and protrusions of the pattern Px are also integrated in the same manner to form a silicon oxide film PxS.

With this pattern arrangement, the longitudinal directions of the grooves and convex patterns are orthogonal to each other in the adjacent patterns, so that the stress generated by the unevenness forming process is alleviated, and the warpage of the silicon substrate that occurs during heating in the heating oxidation step. Is relaxed. As a result, the surface of the silicon substrate is flat and uniformly oxidized, and a homogeneous silicon oxide film can be obtained.
In particular, it is useful as a substrate for an electromechanical converter utilizing an electrostatic induction action, which requires an electret substrate having an electret film on one side thereof.

As described above, according to the method for manufacturing a substrate for an electrostatic induction device according to the present invention, a thick silicon oxide film can be formed in a short time by forming all the grooves and protrusions into a silicon oxide film. Therefore, an electret film having a high surface potential and made of a silicon oxide film can be obtained, and as a result, a substrate for an electrostatic induction device provided with a high-performance electret film can be easily obtained.

1, 90, 91 Silicon substrate 1a Surface of silicon substrate 2 Convex parts 2D, 3D, D, Din, Dout, PdS, PuS, PxS, PyS Silicon oxide film 2L, 2R, 2U Convex parts 2 surfaces 3, 3a, 4 4, 4a, 4b Groove 3B Bottom of groove 3 5 Bottom of groove 4b 20, 60 Electric substrate 21, 61 Electric substrate 30 Thermal oxidation device 36 Heating oxidation furnace 40 Charging device E Electronic G1 Width of groove 3 G2 Width of groove 4 K Potassium Ion P1 Convex 2 width Pd, Pu, Px, Py Groove and convex pattern

Claims (3)

The process of forming grooves and protrusions on a silicon substrate,
A heat oxidation process that heat-treats all the protrusions into a silicon oxide film,
The charging process that charges the silicon oxide film and
A step of removing the silicon oxide film in the thickness direction to partially expose the silicon substrate is provided.
The groove includes a plurality of first grooves having a first width, and a plurality of first grooves.
A plurality of second grooves having a second width wider than the first width are provided.
The silicon oxide film formed in the second groove is completely removed in the thickness direction, and the film is completely removed.
The silicon substrate is exposed in the second groove, and the silicon substrate is exposed.
A method for manufacturing a substrate for an electrostatic induction device , which comprises forming the silicon oxide film in the first groove into polar shapes independent of each other. The method for manufacturing a substrate for an electrostatic induction device according to claim 1 , wherein the depth of the first groove and the depth of the second groove are the same. A substrate for an electrostatic induction device manufactured by the manufacturing method according to claim 1 or 2 , wherein the silicon oxide films having polar shapes independent of each other are charged.

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