JP2001277194A - Micropump manufacturing method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a micropump manufacturing method capable of forming an outlet, inlet, and chamber of micropump without resorting to a joining process for constituent members. SOLUTION: A substitute 50 is formed from a first layer 1 consisting of monocrystal silicon, a second layer 2 consisting of SiO2, and a third layer 3 consisting of monocrystal silicon, which are laminated one over another. An anisotropic wet etching is applied from the counter second layer side of the first layer 1 so that an inlet 21 and an outlet 22 are formed. The third layer is provided with pores 39, through which the second layer 2 is subjected to wet etching. Thereby the inlet 21 and outlet 22 are put in communication with each other, and a chamber 23 capable of making vermicular motion with a piezoelectric element 24 is formed. The pores 39 are blocked by film formation.

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 micropump used for conveying a minute amount of fluid.

[0002]

2. Description of the Related Art FIG. 6 is a sectional view schematically showing the principle of a micropump as described above. This micropump has a structure without a check valve, and an outlet portion 91 and an inlet portion 92 are formed in a lower layer 95 made of single crystal silicon,
On this lower layer 95, an upper layer 96 made of single crystal silicon is provided. The outlet portion 91 is formed so that the cross-sectional area becomes gradually larger toward the upper layer 96 side.
On the other hand, is formed such that the cross-sectional area gradually decreases toward the upper layer 96 side. The upper layer 96 is provided with a variable-capacity chamber 93 that allows the outlet 91 and the inlet 92 to communicate with each other. A vibrating film 97 is formed on a portion of the upper layer 96 located above the chamber 93. The piezoelectric element 94 is mounted on the vibration film 97.

[0003] In a micropump without a check valve formed as described above, a liquid is conveyed by utilizing a difference in fluid resistance. That is, when a predetermined AC voltage is applied to the piezoelectric element 94, the vibration film 97 starts to vibrate. When the vibration film 97 becomes convex toward the piezoelectric element 94 due to this vibration, the volume of the chamber 93 increases, and the liquid is sucked into the chamber 93. At this time, the fluid resistance given to the liquid sucked into the chamber 93 is larger at the outlet portion 91 than at the inlet portion 92 due to the shapes of the outlet portion 91 and the inlet portion 92. Therefore, the liquid cannot flow from the outlet 91, but flows into the chamber 93 from the inlet 92. On the other hand, when the vibrating film 97 is convex toward the lower layer 95, the volume in the chamber 93 is reduced, and the liquid is discharged from the chamber 93. At this time, the fluid resistance given to the liquid discharged from the chamber 93 is larger at the inlet 92 than at the outlet 91 due to the shapes of the outlet 91 and the inlet 92. Therefore, the liquid cannot flow out of the inlet 92, but flows out of the chamber 93 from the outlet 91. In this way, in the above-described micropump, the inlet 92 is provided without the check valve.
The liquid can be conveyed from the outlet section 91 to the outlet section 91.

The outlet section 91 and the inlet section 92, which enable a micropump without check valves, are each tapered in vertical section as described above. Such an outlet part 91 and an inlet part 92 are formed by anisotropic wet etching. That is, (10) of single crystal silicon
The (0) plane is defined as both planes of the lower layer 95, and wet etching is performed from this (100) plane with a KOH aqueous solution. The (111) plane formed by this wet etching is used as the side wall slope of the outlet 91 and the inlet 92. Accordingly, in the conventional micropump manufacturing method, first, a lower layer 95 made of single-crystal silicon having both surfaces (100) is prepared. The outlet portion 91 and the inlet portion 92 are formed by performing wet etching on both surfaces of the lower layer 95. Similarly, an upper layer 96 made of single crystal silicon having both surfaces (100) is prepared. Then, wet etching is performed on both surfaces of the upper layer 96 to form a portion serving as the chamber 93 and the vibration film 97. Then, the lower layer 95 and the upper layer 96 are joined, and finally, the piezoelectric element 94 is attached to the vibration film 97 of the upper layer 96. As a result, an outlet portion 91 and an inlet portion 92 whose cross-sectional areas change in opposite directions are formed in the lower layer 95,
92 is communicated with a chamber 93.

[0005]

In the above-described method for manufacturing a micropump, the lower layer 95 and the upper layer 96 are joined. Such bonding is performed by heating a silicon wafer using a natural oxide film of single crystal silicon. However, in order to perform such bonding firmly, the bonding surface needs to be flat and clean at the atomic level. If the bonding surface has irregularities or becomes dirty, voids are generated and the bonding strength is reduced, resulting in a problem that the reliability of the micropump is reduced. In the above manufacturing method, the lower layer 95 is formed.
And the upper layer 96, there may be a case where the positional relationship between the outlet portion 91 and the inlet portion 92 and the chamber 93 is slightly shifted during the bonding. Since the fluid resistance given to the carrier fluid depends on the positional relationship between the outlet portion 91 and the inlet portion 92 and the chamber 93, there is a problem that the displacement at the time of joining lowers the pump performance.

As a conventional example of a method for manufacturing a micropump provided with a check valve, Japanese Patent Application Laid-Open Nos. 11-257232, 4-66785, and 3-2331 have been disclosed.
No. 77 publication. However, in a micropump with a check valve, not only the outlet, the inlet, and the chamber, which form a hollow space in the member, must be formed, but of course, a check valve must be provided. Therefore, in each of the above publications, a glass substrate is anodically bonded to a silicon substrate to form a hollow space, which is used as an outlet portion, an inlet portion, and a chamber. Is configured. Therefore, also in this case, there is a problem that reliability is reduced at the time of joining, and a problem is also caused that pump performance is deteriorated due to a positional relationship between the outlet, the inlet, and the chamber.

The present invention has been made to solve the above-mentioned conventional problems, and an object of the present invention is to form an outlet portion, an inlet portion, and a chamber of a micropump without joining members. Another object of the present invention is to provide a micro pump manufacturing method.

[0008]

Accordingly, a method of manufacturing a micropump according to the present invention comprises a first layer, a second layer made of a material having an etching characteristic different from that of the first layer, and laminated on the first layer. A first step of preparing a raw material comprising a material having an etching characteristic different from that of the second layer and having a third layer laminated on the second layer;
A second step of performing etching from the layer side to form an inlet portion and an outlet portion reaching the surface on the second layer side in the first layer; and a second layer existing at least between the inlet portion and the outlet portion. And a third step of forming a chamber that allows the inlet and the outlet to communicate with each other and allows the chamber to perist from the inlet to the outlet.

In this micropump manufacturing method, a second layer having an etching characteristic different from those of the first and third layers is provided between the first and third layers. With such a structure, the first layer is etched from the side opposite to the second layer to form an inlet and an outlet, and by etching the second layer, the inlet and the outlet communicate with each other and peristalsis is possible. Forming a chamber. Fluid can be transferred from the inlet to the outlet by peristalizing the chamber with a predetermined actuator or the like, so that the inlet, outlet, and chamber constituting the micropump are formed without joining members. Becomes possible.

[0010] The method of manufacturing a micropump according to the present invention comprises:
Prior to the third step, a fourth step of forming pores in the third layer and allowing the second layer to communicate with the outside of the raw material through the pores; In the above, the etching of the second layer for forming the chamber is performed by wet etching through the pores formed in the fourth step, and further, after the third step, the communication through the pores is cut off. It is also characterized by having a fifth step.

If the second layer is wet-etched through the pores, the communication between the chamber and the outside can be easily performed by appropriately selecting the cross-sectional area of the pores regardless of the area or shape of the chamber. It becomes possible to cut off.
Further, for example, by appropriately arranging a plurality of pores, it is possible to easily form a chamber having a predetermined shape while easily blocking the communication. When the distance between the opposing side walls of the pores is about 1.2 μm or less, the communication can be easily cut off by, for example, film formation by a CVD method or the like.

[0012]

Next, a specific embodiment of the micropump manufacturing method of the present invention will be described in detail with reference to the drawings.

FIG. 1 and FIG. 2 are schematic sectional process views sequentially showing the above-mentioned manufacturing method. First, a first layer 1 having a thickness of about 500 μm made of single crystal silicon, a second layer 2 having a thickness of about 30 μm made of SiO 2 laminated on the first layer,
A substrate 50 having a third layer 3 made of single-crystal silicon and having a thickness of about 3 μm, which is stacked in layers, is prepared as a raw material (FIG. 1A). Here, the first layer 1 and the third layer 3 constitute the surfaces 31, 30 of the substrate 50, respectively, and the surfaces 31, 30 correspond to the (100) plane of single-crystal silicon, respectively. Such a substrate 50 can be obtained, for example, by bonding a quartz plate to a single-crystal silicon wafer and further bonding a single-crystal silicon substrate thereto.

Next, the surface 31, 30 of the substrate 50 formed by the first layer 1 and the third layer 3 has a thickness of about 0.2 μm.
i3N4 (hereinafter, simply referred to as “SiN”) layers 4, 5
Is formed (FIG. 2B). And the SiN layer 4
Then, a resist pattern having two openings is formed by photolithography, and the SiN layer 4 is etched using the resist pattern as a mask. The size of the etched portions 32 and 33 formed by this etching is about 810 μm × 810 μm, respectively. After the etching, unnecessary resist is removed with a sulfuric acid boiler or the like (FIG. 3C). Then, in this state, anisotropic wet etching of single-crystal silicon using a KOH aqueous solution is performed via the etching units 32 and 33. By this etching, the entrance portion 21 and the exit portion 22 having the (111) plane of the single crystal silicon as side wall surfaces 34 and 35, respectively.
Is formed on the first layer 1 until the second layer 2 is reached. The formed inlet portion 21 and outlet portion 22 are provided with side wall surfaces 34, 3
5 and the laminating surface 47 are tapered in the vertical (lamination direction) cross-section at an angle α of 55 °, and the cross-sectional area becomes gradually smaller toward the second layer 2. Second layer 2 of entrance 21 and exit 22
The dimensions on the side are about 100 μm × 100 μm (FIG. 4D). Subsequently, about 0.1 mm from the first layer 1 side of the substrate 50.
An SiN film 36 having a thickness of 3 μm is formed, and the inlet 21 and the outlet 22 are covered with the SiN film 36 (FIG. 4E).

On the other hand, a resist pattern is formed on the surface of the SiN layer 5 via a mask 38 as shown in FIG. Although the drawing is partially shown in FIG.
Pores 37 each having a square having a side L of about 1.2 μm.
Are arranged at an arrangement pitch P of about 3.0 μm. The range in which the pores 37 are arranged is about 11 mm × 1
Over 1 mm. Such a mask 38 is made of SiN
When provided on the layer 5, the inlet 21 and the outlet 22 are located in a region below (on the side of the first layer 1). Then, through the mask 38, the resist pattern is formed by a photolithography technique. Therefore, also in this resist pattern, pores having a cross section of a square having a side of about 1.2 μm are formed at the entrance 21 and the exit 2.
Approximately 11 mm x 11 m that covers 2 and 3 from above (third layer 3 side)
In the region of m, a pitch of about 3.0 μm is formed.

Next, SiN is applied through this resist pattern.
The layer 5 is etched, and subsequently the third layer 3 is dry-etched. According to the dry etching, the etching can be performed perpendicularly to the resist pattern. Therefore, the third layer 3 is provided with pores 39 having a square shape with a side of about 1.2 μm according to the resist pattern (see FIG. 2 (a)). The pores 39 are also juxtaposed at a pitch of about 3.0 μm over a range of about 11 mm × 11 mm covering the inlet 21 and the outlet 22 according to the resist pattern. Since one side of the pores 39 is about 1.2 μm, the adjacent distance between the pores 39 is about 1.8 μm.

Next, the pores 39 thus formed are formed.
An etching solution (for example, BHF) is supplied through
The layer 2 is subjected to wet etching. Since this etching is an isotropic wet etching, the etching region extends not only immediately below the pore 39 but also to the periphery thereof. Then, the second layer 2 is removed over substantially the entire area of about 11 mm × 11 mm in which the pores 39 are arranged, and the chamber 23 is formed (FIG. 2B). Then, next, the SiN layers 4, 5
And the SiN film 36 is removed by wet etching. Then, the inlet 21 and the outlet 22 open in the surface 31 of the first layer 1 (FIG. 3C).

Further, on the third layer 3, plasma CVD is performed.
The SiN layer 6 is formed by a method. Since the side of the cross section of the pore 39 provided in the third layer 3 is about 1.2 μm,
The SiN supplied by the plasma CVD method does not reach the inner part of the pore 39, and almost only the opening of the pore 39 is filled with SiN. Approximately 0.1 mm formed in this way.
On the SiN layer 6 of 5 μm, a polysilicon layer 7 of about 0.5 μm is further formed. Then, the vibration film 25 is formed by the SiN layer 6, the polysilicon layer 7, and the third layer 3.
Is constructed (FIG. 2D). Finally, a plurality of piezoelectric elements 24 are bonded on the polysilicon layer 7 (FIG. 3E).

FIG. 4 is a transparent plan view showing the micropump manufactured as described above. An inlet 21 and an outlet 22 are provided at both ends of the chamber 23, and a plurality of piezoelectric elements 24 are provided in a row between the inlet 21 and the outlet 22. The planar shape of the chamber 23 is also substantially square, and the length N of one side is about 11
mm. The planar shape of the vibration film 25 is substantially the same as that of the chamber 23. Since the vibration film 25 is reinforced by the polysilicon layer 7, the vibration film 25 can flexibly vibrate with the vibration of the piezoelectric element 24. So entrance 2
An AC voltage is applied to each piezoelectric element 24 so that a peristaltic movement from the 1 side toward the outlet 22 side is generated in the chamber 23.

FIG. 5 is a schematic sectional view showing the operation of the piezoelectric element 24 when the chamber 23 is perturbed. As shown by the solid line in the figure, for example, at a certain point in time, a predetermined first piezoelectric element 24a is driven so as to be convex toward the chamber 23 (the lower side in the figure),
The second piezoelectric element 24b adjacent to the outlet portion 22 side of FIG. 4a (right side in the figure) is moved to the opposite side of the chamber (upper side in the figure).
Is driven to be convex. At the next point in time, the first piezoelectric element 24a is returned to a flat state while the second piezoelectric element 24b is driven to be convex toward the chamber 23, as shown by the broken line in FIG. The third piezoelectric element 24c adjacent to the outlet 22 side is driven so as to project toward the opposite side of the chamber. Such driving of the piezoelectric element 24 is performed from the piezoelectric element 24 closest to the inlet 21 to the outlet 2.
The operation is sequentially performed up to the piezoelectric element 24 on the second side, and when the piezoelectric element 24 on the side of the outlet section 22 is reached, the process returns to the piezoelectric element 24 on the side of the inlet section 21 and is repeated. In this case, the chamber 23 perturbs from the inlet 21 to the outlet 22 by the piezoelectric element 24 functioning as an actuator. When the chamber 23 perturbs in this manner, the fluid pressed by the vibrating membrane 25 convex to the chamber 23 side is sequentially sucked toward the outlet 22 by the movement of the vibrating membrane 25 convex to the opposite chamber side. Therefore, the liquid or gas in the chamber 23 is transported from the inlet 21 to the outlet 22. Since the fluid is conveyed by the peristaltic motion of the chamber 23, both the inlet 21 and the outlet 22 are formed in a tapered shape in which the cross-sectional area gradually increases toward the surface 31 of the first layer 1. Even if it is, from the entrance 21 to the exit 2
A micro pump capable of continuously transporting the fluid to the second pump can be configured.

In the micropump manufacturing method, the second layer 2 having an etching characteristic different from that of the first layer 1 is laminated on the first layer 1, and a third layer 2 having an etching characteristic different from that of the second layer 2 is formed.
The layer 3 is laminated on the second layer 2. By adopting such a so-called sandwich structure, the chamber 23 which can be slid from the inlet 21 toward the outlet 22 is
The second layer 2 can be formed by etching. When the chamber 23 is permissible in this manner, the fluid is conveyed by this peristalsis as described above, so that the inlet 21 and the outlet 2 constituting the micropump are provided.
2 can be formed by anisotropic wet etching from the surface 31 of the first layer 1. Therefore, the inlet part 21, the outlet part 22, and the chamber 23 of the micropump can be formed by simply performing etching, film formation, and the like on the substrate 50 without joining any members such as the substrate. As a result, it is possible to supply a micropump having stable quality and performance without causing a problem such as a decrease in reliability and a decrease in pump performance which are caused by joining of members in the conventional manufacturing method. .

Further, a second process for forming the chamber 23 is performed.
The etching of the layer 2 is performed through the pores 39 provided in the third layer 3. The cross-sectional area of the pore 39 can be made sufficiently smaller than the area of the chamber 23. For this reason, by forming the SiN layer 6 by a plasma CVD method or the like, the chamber 23 and the outside can be connected to each other after the etching of the second layer 2 by setting the small cross-sectional area so that the pores 39 can be easily filled by the film formation. Communication can be easily cut off. In the above description, the cross section of the pore 39 is a square having a side of about 1.2 μm. Pore 3
By making the cross section of 9 such a size, the communication can be cut off by film formation by the plasma CVD method.

A plurality of the pores 39 are arranged in a range substantially the same as the range in which the chamber 23 is formed (about 11 mm × 11 mm). The adjacent distance between the pores 39 is about 1.8 μm. Therefore, the chamber 23 is formed by wet etching in a short time so as to be substantially the same as the arrangement range of the fine holes 39. Therefore, it is possible to accurately control the shape of the chamber 23 formed by etching, and to obtain stable fluid transfer performance of the micropump.

Although the specific embodiments of the present invention have been described above, the present invention is not limited to the above embodiments, and can be implemented with various modifications within the scope of the present invention. In the above, in the step shown in FIG. 1E, a SiN film 36 of about 0.3 μm is formed on the first layer 1 side of the substrate 50, and the inlet 21 and the outlet 22 are
To cover. If the formation of the SiN film 36 is omitted, the etchant (BHF) is supplied to the second layer 2 also from the inlet 21 and the outlet 22 in the step shown in FIG. Therefore, for example, the chamber 23 to be formed
In the case where the area of the second layer 2 is relatively large, the formation of the SiN film 36 in the step shown in FIG. 1E is omitted, and the second layer 2 is also etched from the entrance 21 and the exit 22. You may. Further, for example, when the inlet 21 and the outlet 22 are close to each other, the chamber 23 is formed by wet etching through the inlet 21 and the outlet 22 without providing the pores 39 in the third layer 3. May be formed. In addition to providing a large number of pores 39 as shown in FIG. 2, for example, when the chamber 23 is relatively small, a single pore is provided and wet etching is performed on the second layer 2. You may do so. In such a case, it is preferable to provide a fine hole at a central position between the inlet 21 and the outlet 22. In this way, the structure of the micropump can be made symmetrical so that good pump performance can be reliably maintained.

In the above description, the third layer 3 is made of single crystal silicon. However, it is only necessary that the third layer 3 has an etching characteristic different from that of the second layer 2. For example, the third layer 3 is made of SiO2. For example, polycrystalline silicon may be used as an example. The material of the third layer 3 can be appropriately selected in consideration of the etching characteristics of the second layer 2 and the ease of manufacturing the substrate 50.

Further, the first layer 1 was made of single-crystal silicon, and anisotropic wet etching was performed on the first layer 1 to form an inlet 21 and an outlet 22 having a tapered longitudinal section. However, the shape of the inlet 21 and the outlet 22 is not limited to the tapered vertical cross section as shown, as long as the fluid can be transferred by peristaltic movement of the chamber 23. Then, for example, the inlet 21 or the outlet 2 whose cross-sectional area is constant from the chamber 23 toward the surface 31 of the first layer 1.
When forming No. 2, the inlet 21 or the outlet 22 may be formed by dry etching instead of anisotropic wet etching as shown in FIG. Also S
In the case where a predetermined pumping performance can be obtained even if the etching area is larger than the etching parts 32 and 33 provided in the iN layer 4, the inlet part 21 or the outlet part 22 is formed using isotropic wet etching. You may. And entrance 2
When both the first portion 1 and the outlet portion 22 are formed by a method other than anisotropic wet etching, the first layer 1 may be made of a material other than single-crystal silicon.

In the above description, the second layer 2 is made of SiO2. However, the second layer 2 only needs to have different etching characteristics from the first layer 1 and the third layer 3; Can form the second layer 2. For example, KOH of silicon heavily doped with impurities such as boron
Since the rate is also much lower than the KOH rate of single crystal silicon, the second layer 2 may be formed of boron-doped silicon. Such a boron-doped substrate can also be obtained in an SOI substrate manufacturing process.

In place of the substrate 50 having the structure of Si / SiO2 / Si as described above, Si / SiN / SiO
A substrate having a 2 / Si structure may be used. Of these, the SiN layer is removed in the step shown in FIG.

Further, in the above description, the substrate 50 in which the first layer 1 and the third layer 3 constitute both surfaces is used, but another layer is laminated on the surface side of the first layer 1 and the third layer 3. May be used. A portion of such a layer that needs to be removed may be appropriately removed by etching or the like during the process. Further, in the above description, the first layer 1, the second layer 2, the third layer
Although the substrate 50 in which the layers 3 are directly laminated is used, a substrate in which a layer made of a predetermined material is interposed between the layers may be used. Such an intervening layer may be removed by etching or the like as necessary.

In the above description, the pores 39 having a square cross section are formed, and one side of the cross section is set to about 1.2 μm. It is preferable that the cross section of the pore 39 is selected so that one side thereof is in a range of about 0.8 to 1.2 μm. When the thickness is about 1.2 μm or less, the opening can be easily and reliably shielded by film formation using a plasma CVD method or the like.
When the thickness is more than μm, the etching solution such as BHF
This is because it can be easily supplied to the second layer 2 via Further, the cross-sectional shape is not limited to a square shape, and the pores 39 may have a circular cross-sectional shape. In this case as well, if the distance between the opposing side walls of the pores 39 is set to about 1.2 μm or less, the opening is formed by the plasma CV.
The film can be easily and reliably shielded by the film formation using the method D or the like. For example, in the case of a circular cross section, the diameter may be about 1.2 μm or less.

In the above description, the piezoelectric element 24 is used as an actuator for perturbing the chamber 23. However, as the actuator, for example, an electrode to which a voltage is applied by a predetermined drive circuit may be used. When a predetermined voltage is applied to such an electrode, an electrostatic attraction is generated between the electrode and the first layer 1, and the vibrating film 25 in that portion is pulled toward the first layer 1. Therefore, a voltage is sequentially applied to the electrodes from the inlet 21 toward the outlet 22 to thereby perturb the chamber 23. Further, the peristaltic movement of the chamber 23 described with reference to FIG. 5 is an example, and it is a matter of course that the chamber 23 may be perturbed by a different method.

[0032]

According to the micropump manufacturing method of the present invention,
The inlet, the outlet, and the chamber constituting the micropump can be formed in the raw material without joining the members. Therefore, it is possible to reliably avoid the problems of reliability and pump performance degradation that may occur with joining of members.

[Brief description of the drawings]

FIG. 1 is a schematic cross-sectional process diagram for explaining a micropump manufacturing method according to an embodiment of the present invention.

FIG. 2 is a schematic cross-sectional process diagram for explaining the micropump manufacturing method.

FIG. 3 is a partial plan view of a mask used in the micropump manufacturing method.

FIG. 4 is a transmission plan view of a micropump manufactured by the above manufacturing method.

FIG. 5 is a schematic sectional view illustrating peristaltic movement of a chamber in the micropump.

FIG. 6 is a schematic cross-sectional view of a micropump manufactured by a conventional manufacturing method.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 1st layer 2 2nd layer 3 3rd layer 21 Inlet part 22 Outlet part 23 Chamber 39 Pore 50 Substrate

Claims (3)

[Claims] A first layer, a material having an etching characteristic different from that of the first layer, a second layer laminated on the first layer, and a material having an etching characteristic different from the second layer. A first step of preparing a raw material having a third layer laminated on a second layer, and an entrance portion which is etched from the second layer side of the first layer and reaches a surface on the second layer side; A second step of forming an outlet part in the first layer, and etching at least a second layer existing between the inlet part and the outlet part, so that the inlet part and the outlet part communicate with each other. A step of forming a chamber that can be peristalted from the outlet to the outlet. 2. The method according to claim 2, further comprising: forming a pore in the third layer prior to the third step, and communicating the second layer with the outside of the raw material through the pore. In the third step, the etching of the second layer for forming the chamber is performed by wet etching through the pores formed in the fourth step, and further, after the third step, 5. The method according to claim 1, further comprising a fifth step of interrupting communication via the first pump. 3. The micropump manufacturing method according to claim 2, wherein in the third step, the distance between the opposing side walls of the pores is about 1.2 μm or less.