JPWO2003081303A1 - Optical element manufacturing method - Google Patents

Abstract

Provided is a method of manufacturing an optical element that forms a plurality of etching holes whose hole widths are modulated in a depth direction from a main surface of a semiconductor substrate. Such a manufacturing method includes a preparation step of preparing a semiconductor substrate having a main surface and having a specific resistance modulated in the depth direction from the main surface, and a defining step of defining a hole forming region on the main surface of the semiconductor substrate And an etching step of forming an etching hole by immersing the main surface in the electrolytic solution, electrochemically etching the semiconductor substrate in the depth direction from the hole forming region with the semiconductor substrate at a high potential with respect to the electrolytic solution. .

Description

TECHNICAL FIELD The present invention relates to a method for manufacturing an optical element, and more particularly to a method for manufacturing an optical element comprising a semiconductor substrate having a plurality of holes or grooves.
BACKGROUND ART In recent years, optical elements such as a wavelength filter element, a polarization separation element, and an optical waveguide using a photonic band gap have been proposed. The photonic band gap is a phenomenon in which a light wave having a predetermined wavelength band depending on the period cannot exist in a periodic structure in which the refractive index varies periodically. For example, in a multilayer optical filter, when the multilayer period represented by the optical length is one-half of the wavelength of the incident light, incident light near that wavelength cannot be transmitted and is reflected, and the multilayer optical filter Acts as a filter.
FIG. 12 is a top view of an optical element denoted as a whole by 500, which is described in JP-T-11-509644. FIG. 13 is a cross-sectional view of the optical element 500 of FIG. 12 when viewed in the XII-XII direction. Optical element 500 is made of n-type silicon substrate 1 and has a plurality of holes 3 formed in a direction substantially perpendicular to main surface 2. The diameter of the hole 3 is modulated in the depth direction. From the side close to the main surface 2, the region 31 where the diameter of the hole 3 is large, the region 32 where the diameter of the hole 3 is small, and the diameter of the hole 3 again. Is a large region 33.
The optical element 500 has a lattice region 4 in which such hole portions 3 are regularly arranged, and a defect region 5 provided so as to divide the lattice region 4. Here, the defect region 5 is a region where the holes 3 constituting the lattice region 4 are not formed in a line.
In the optical element 500, the grating region 4 has a periodic structure in which the refractive index periodically changes two-dimensionally, and has a photonic band gap with respect to light propagating in a direction parallel to the main surface 2. That is, light in the wavelength band corresponding to the photonic band gap hardly transmits the grating region 4 in such a direction. For this reason, the light wave excited inside the defect region 5 cannot enter the lattice structure 4.
On the other hand, in the region 32, the volume occupied by the silicon substrate is large and the volume occupied by the air filling the hole 3 is small compared to the regions 31 and 33 formed above and below the region 32. For this reason, the area 32 has a higher average refractive index than the areas 31 and 33. The light is confined in the region 32 sandwiched between the region 31 and the region 33 by total reflection due to the difference in average refractive index.
Based on such a principle, the defect region 5 provided in a linear shape acts as an optical waveguide having the region 32 as a core, and guides light in a direction perpendicular to the paper surface in FIG.
In the method for manufacturing the optical element 500, first, an n-type silicon substrate 1 having a specific resistance of 1 Ω · cm is prepared. Subsequently, pits having a periodic pattern corresponding to the positions where the holes 3 are formed are formed on the main surface 2 of the silicon substrate 1. At this time, no pit is provided in the defect area 5.
Next, with the main surface 2 of the silicon substrate 1 in contact with the electrolyte solution of 3 wt% hydrofluoric acid, light is incident from the opposite side to the main surface 2 to generate holes in the silicon substrate 1. . Further, a voltage of 3 V is applied between the silicon substrate 1 and the electrolytic solution. As a result, holes generated in the silicon substrate 1 are attracted to the electric field concentration portion near the pit provided on the main surface 2. Electrochemical etching proceeds using these holes, and holes 3 are formed starting from the pits. By changing the light irradiation amount, the current density is sequentially changed from large to small to large. The cross-sectional area of the hole 3 formed by etching varies depending on the current density: large, small, and large. As a result, the hole 3 whose diameter is modulated in the depth direction as shown in FIG. 13 is formed.
However, in such a manufacturing method, the depth and diameter of each of the regions 31, 32, and 33 are not as designed because etching conditions such as the temperature and concentration of the electrolytic solution and the amount of light irradiation change with time. As a result, when the optical element 500 is used as an optical waveguide, there arises a problem that the photonic bandgap wavelength and the light transmission characteristics deviate from desired values. Such a problem also becomes a problem when the optical element 500 is used as a polarizing element or an optical filter.
DISCLOSURE OF THE INVENTION Accordingly, the present invention relates to an optical element manufacturing method in which the hole depth and diameter can be formed with the designed dimensions in a method for manufacturing an optical element in which the hole diameter and groove width are different in the depth direction. The purpose is to provide a manufacturing method.
That is, the present invention is a method of manufacturing an optical element that forms a plurality of etching holes whose hole widths are modulated in the depth direction from the main surface of a semiconductor substrate. A preparation step of preparing a semiconductor substrate whose specific resistance is modulated in the direction; a defining step of defining a hole forming region on the main surface of the semiconductor substrate; And an etching step of forming an etching hole by electrochemically etching the semiconductor substrate in the depth direction from the hole forming region with the substrate at a high potential.
By using this manufacturing method, an optical element having a structure as designed can be obtained regardless of variations in manufacturing conditions such as the temperature of the electrolytic solution. In addition, the manufacturing yield is improved, and the optical element can be manufactured at low cost.
Japanese Patent Application Laid-Open No. 2001-272566 describes a method of manufacturing a photonic crystal in which the diameter of a hole is periodically modulated by laminating and etching media having different etching characteristics. However, such a manufacturing method includes a step of forming a hole by RIE or the like, and a hole expanding step of expanding the diameter of the hole, and the hole is formed by only one electrochemical etching as in the present invention. This is different from the method of forming an etching hole having a modulated width.
The preparatory step may be a step of preparing a semiconductor substrate including a low specific resistance layer, a high specific resistance layer, and a low specific resistance layer in this order from the main surface of the semiconductor substrate in the depth direction.
Further, the preparation step may be a step of preparing a semiconductor substrate whose specific resistance is modulated in a sine wave shape in the depth direction from the main surface of the semiconductor substrate.
The preparatory step is a step of preparing a semiconductor substrate in which the ratio of the maximum value to the minimum value of the modulated specific resistance is approximately 1.01 or more and approximately 1000 or less.
The maximum value of the specific resistance is preferably about 1 Ω · cm or more and about 5000 Ω · cm or less, and the specific value of the specific resistance is preferably about 0.01 Ω · cm or more and 100 Ω · cm or less.
The defining step may be a step of forming a pit by etching a predetermined position on the main surface of the semiconductor substrate to form a hole forming region.
Further, the defining step may be a step of etching the main surface of the semiconductor substrate into a substantially parallel stripe to form a hole forming region.
The etching step may further include a step of irradiating the semiconductor substrate with light.
The cross section parallel to the main surface of the etching hole may be substantially circular.
The cross section parallel to the main surface of the etching hole may be substantially rectangular.
Best Mode for Carrying Out the Invention (Embodiment 1)
FIG. 1 is a top view of an optical element denoted as a whole by 100 according to the present embodiment. 2 is a cross-sectional view of the optical element 100 of FIG. 1 when viewed in the II direction. In FIG. 1, 2, the same code | symbol as FIG. 12, 13 shows the same or an equivalent location.
As shown in FIG. 2, the optical element 100 includes an n-type silicon substrate 1. The silicon substrate 1 includes three types of silicon layers 11, 12, and 13 having different specific resistances. The silicon layers 11 and 13 are low resistance layers, and have a specific resistance of 5 Ω · cm and a film thickness of 5 μm, for example. On the other hand, the silicon layer 12 is a high resistance layer, and has a specific resistance of 10 Ω · cm and a film thickness of 2 μm, for example. The silicon layers 11, 12, and 13 are formed by, for example, chemical vapor deposition. The silicon substrate 1 below the silicon layer 13 only needs to be conductive. For example, the specific resistance may be about 100 Ω · cm.
The optical element 100 has a plurality of holes 3 formed in a direction substantially perpendicular to the main surface 2. A region corresponding to each of the silicon layers 11, 12, and 13 is a region 31 in which the diameter of the hole 3 is large, a region 32 in which the diameter of the hole 3 is small, and a region 33 in which the diameter of the hole 3 is large. The diameter of the hole 3 is approximately 0.05 μm to approximately 1.95 μm, and is approximately 0.25 μm in the regions 31 and 33 of the optical element 100 and approximately 0.35 μm in the region 32.
As shown in FIG. 1, the optical element 100 includes a lattice region 4 in which such hole portions 3 are regularly arranged and a defect region 5 provided so as to divide the lattice region 4. Here, in the lattice region 4, the pitch of the holes 3 is approximately 0.2 μm to approximately 2.0 μm, and in the optical element 100, the pitch is approximately 0.5 μm. In addition, the defect region 5 is a region where the holes 3 constituting the lattice region 4 are not formed in a row.
In the optical element 100, the grating region 4 has a periodic structure in which the refractive index periodically changes two-dimensionally, and has a photonic band gap with respect to light propagating in a direction parallel to the main surface 2. That is, light in the optical wavelength band corresponding to the photonic band gap can hardly pass through the grating region 4 in this direction. For this reason, the light wave excited inside the defect region 5 cannot enter the lattice structure 4.
On the other hand, in the region 32, the volume occupied by the silicon substrate is large and the volume occupied by the air filling the hole 3 is small compared to the regions 31 and 33 formed above and below the region 32. For this reason, the area 32 has a higher average refractive index than the areas 31 and 33. The light is confined in the region 32 sandwiched between the region 31 and the region 33 by total reflection due to the difference in average refractive index.
As a result, the defect region 5 provided in a line shape acts as an optical waveguide having the region 32 as a core, and guides light in a direction substantially perpendicular to the paper surface in FIG.
Next, a method for manufacturing the optical element 100 according to the present embodiment will be described with reference to FIGS. The manufacturing method includes the following steps 1 to 3.
Step 1: As shown in FIG. 3, an n-type silicon substrate 1 having a specific resistance of approximately 100 Ω · cm is prepared. Subsequently, three types of silicon layers 11, 12, and 13 having different specific resistances are sequentially formed on the silicon substrate 1 by, for example, chemical vapor deposition. By controlling the doping concentration, each silicon layer has a predetermined specific resistance.
The silicon layers 11 and 13 are low resistance layers and have a specific resistance of about 0.01 Ω · cm to about 100 Ω · cm, and preferably about 0.5 Ω · cm to about 50 Ω · cm. Here, it is set to 5 Ω · cm. When the specific resistance is smaller than this value, the etching of the hole 3 becomes isotropic, the adjacent holes 3 are connected, and the strength of the optical element 100 is reduced. On the other hand, if the specific resistance is larger than this value, the shape of the hole 3 becomes irregular and light scattering in the optical element 100 increases. The film thickness of the silicon layers 11 and 13 is approximately 5 μm.
On the other hand, the silicon layer 12 is a high resistance layer and has a specific resistance of about 1.01 Ω · cm to about 1000 Ω · cm, and preferably about 2 Ω · cm to about 100 Ω · cm. Here, it is set to 10 Ω · cm. The film thickness is approximately 2 μm.
However, it is necessary to select the specific resistance of each layer so that the specific resistance of the silicon layer 12 is approximately 1.01 to 1000 times the specific resistance of the silicon layers 11 and 13. When the ratio of the specific resistance is reduced, the contrast of the refractive index between the respective layers is lowered, and a sufficient photonic band gap effect cannot be obtained as an optical waveguide or an optical filter described later. On the other hand, when the specific resistance ratio increases, the etching of the hole 3 becomes irregular, and light is scattered in the optical element 100.
Note that the preferred specific resistance and film thickness are substantially the same regardless of the conductivity type of the silicon layer.
Step 2: Pits 35 are formed on the main surface 2 as shown in FIG. The pits 35 are formed as a periodic pattern so as to correspond to the arrangement of the holes 3 shown in FIG. For example, the pit 35 is formed by patterning a silicon oxide film or a silicon nitride film formed on the main surface 2 using an electron beam lithography technique to form an opening, and then immersing the pit 35 in an alkaline solution such as potassium hydroxide. The main surface 2 is formed by anisotropic etching.
Step 3: As shown in FIG. 5, the main surface 2 of the silicon substrate 1 is brought into contact with an electrolytic solution 36 of 4 wt% hydrofluoric acid. Further, a voltage is applied to the electrode 37 immersed in the electrolytic solution so that the silicon substrate 1 has a potential approximately 1V higher. Further, light 38 is irradiated from the back surface of the silicon substrate 1 (the side opposite to the main surface 2). In the case where the silicon substrate 1 and the silicon layers 11, 12, and 13 are p-type, holes are present as majority carriers in the silicon substrate 1 and the like, and thus light 38 does not have to be irradiated.
In the state shown in FIG. 5, holes in the silicon substrate 1 are attracted to the vicinity of the pit 35, and electrochemical etching proceeds using the holes, and formation of the hole 3 starts from the pit 35.
Here, the etching is mainly performed near the tip of the hole 3. Further, the etching amount is affected by the specific resistance of silicon near the tip of the hole 3. That is, the diameter of the hole 3 is increased in the low resistance silicon layer, while the diameter of the hole 3 is decreased in the high resistance silicon layer.
When the etching progresses and the depth of the hole 3 becomes deeper, the tip of the hole 3 moves from the low resistance silicon layer 11 to the high resistance silicon layer 12 as shown in FIG. The silicon layer 13 is moved to. In such an etching process, the specific resistance of the silicon layer near the tip of the hole 3 is sequentially changed to 5 Ω · cm (silicon layer 11), 10 Ω · cm (silicon layer 12), and 5 Ω · cm (silicon layer 13). For this reason, the diameter of the hole 3 formed by etching also changes from large to small to large. That is, the diameter of the hole 3 is modulated in the depth direction, and from the side close to the main surface 2, the region 31 in which the diameter of the hole 3 is large, the region 32 in which the diameter of the hole 3 is small, and the diameter of the hole 3 is large. The hole 3 constituted by the region 33 can be formed.
The hole 3 can be formed using a mask formed by patterning a silicon oxide film or the like without forming pits.
The optical element 100 as shown in FIG. 2 is completed by performing the above processes 1-3. In such a manufacturing method, the lengths of the regions having different diameters of the hole 3 (the depths of the regions 31, 32, and 33) are determined according to the film thicknesses of the silicon layers 11, 12, and 13. Unaffected by changes in temperature and concentration over time. The diameter of the hole 3 is also determined by the specific resistance of the silicon layers 11, 12 and 13. Therefore, an optical element having a photonic band gap with dimensions as designed and having stable light propagation characteristics for light having a wavelength of about 0.6 μm to about 2.0 μm, for example, used for optical communication 100 can be obtained.
Further, the yield in the manufacturing process is improved, and the optical element 100 having desired characteristics can be manufactured at low cost.
(Embodiment 2)
FIG. 6 is a top view of an optical element denoted as a whole by 200 according to the present embodiment. FIG. 7 is a cross-sectional view of the optical element 200 of FIG. 6 when viewed in the VII-VII direction. 6 and 7, the same reference numerals as those in FIGS. 1 and 2 indicate the same or corresponding portions.
As shown in FIG. 6, in the optical element 200, the holes 23 formed in a direction substantially perpendicular to the main surface 2 are provided so as to be evenly distributed over the entire surface. Moreover, as shown in FIG. 7, each hole 23 has a shape in which the diameter is periodically modulated in the depth direction.
The optical element 200 having such a structure can obtain the same effect as an optical element in which the average refractive index is periodically changed in the depth direction (direction perpendicular to the main surface 2). For this reason, a photonic band gap corresponding to the period is formed, and for example, it acts as an optical filter that does not transmit light having a wavelength band used for optical communication of approximately 0.6 μm to approximately 2.0 μm.
Next, a method for manufacturing the optical element 200 according to the present embodiment will be briefly described. The manufacturing method includes the following steps 1 to 3.
Step 1: An n-type silicon substrate 1 is prepared. FIG. 8 shows the relationship between the depth from the main surface of the silicon substrate 1 and the specific resistance. As can be seen from FIG. 8, the silicon substrate 1 has a structure in which the specific resistance changes in a sine wave shape having a maximum value of about 6 Ω · cm, a minimum value of about 4 Ω · cm, and a period of about 0.5 μm. Yes. The period in the depth direction may be set to another value in the range of approximately 0.2 μm to approximately 2.0 μm. Such a structure can be formed, for example, by depositing a silicon layer by a CVD method while changing the amount of supplied dopant. The specific resistance of the silicon substrate 1 having a depth of 1.8 μm or more was made constant at about 10 Ω · cm. The maximum value and the minimum value are the maximum value and the minimum value of the specific resistance in the region where the specific resistance is modulated in a sine wave shape. In the first embodiment, the specific resistance of the high specific resistance layer and the low specific resistance layer is referred to.
It should be noted that the ratio of the maximum value to the minimum value of the specific resistance of the silicon substrate 1 needs to be selected so as to be approximately 1.01 to approximately 1000 times, as in the first embodiment.
Step 2: Similar to the first embodiment, pits are formed on the main surface 2 of the silicon substrate 1. In the present embodiment, the pits are formed so as to be evenly distributed on the main surface. The distance between adjacent pits is set in a range of approximately 0.2 to approximately 2.0 μm, and is approximately 0.5 μm here.
Step 3: The main surface 2 of the silicon substrate 1 is brought into contact with an electrolyte solution of 2 wt% hydrofluoric acid using the same configuration as in Step 3 (FIG. 5) of the first embodiment. Further, a voltage is applied so that the potential of the silicon substrate 1 is about 0.8 V higher than the potential of the electrolytic solution. Further, in order to supply holes necessary for electrochemical etching, light having an intensity of 100 mW / cm ² is irradiated. If the silicon substrate 1 is p-type, light irradiation is not necessary.
In such a state, holes are attracted in the vicinity of the pits provided on the main surface 2 and the electrochemical etching proceeds as in the first embodiment. In the etching process, the diameter of the hole 3 is increased as the specific resistance is decreased, and the diameter of the hole 3 is decreased as the specific resistance is increased. As a result, an optical element 200 in which the diameter of the hole 3 is modulated corresponding to the distribution of the specific resistance in the depth direction as shown in FIG. 7 is obtained.
By using this manufacturing method, the optical element 200 having a structure as designed can be obtained regardless of variations in manufacturing conditions such as the temperature of the electrolytic solution. In addition, the manufacturing yield is improved, and the optical element 200 can be manufactured at low cost.
(Embodiment 3)
FIG. 9 is a top view of an optical element denoted as a whole by 300 according to the present embodiment. FIG. 10 is a cross-sectional view of the optical element 300 of FIG. 9 when viewed in the XX direction. 9 and 10, the same reference numerals as those in FIGS. 1 and 2 indicate the same or corresponding portions.
As shown in FIG. 9, the optical element 300 has a plurality of grooves 6 formed in a direction substantially perpendicular to the main surface 2. The pitch of the groove 6 is selected from the range of about 0.2 μm to about 2.0 μm. Moreover, as shown in FIG. 10, each groove part 6 has a shape in which the groove width is periodically modulated in the depth direction.
The optical element 300 having such a structure acts as a polarization selection element that selectively transmits polarized light according to the vibration direction of the polarized light. That is, in the structure in which the groove width is periodically modulated in the depth direction, the average refractive index periodically varies in the depth direction, and polarized light having a vibration direction parallel to the groove portion 6 and the groove portion 6 The average refractive index is different from that of polarized light having a vertical vibration direction. For this reason, a photonic band gap occurs in different wavelength bands between polarized light having a vibration direction parallel to the groove 6 and polarized light having a vibration direction perpendicular to the groove 6. As a result, the wavelength band through which light does not pass varies depending on the vibration direction of polarized light, and in a certain wavelength band, the polarization selection element transmits only polarized light in one vibration direction.
Next, a method for manufacturing the optical element 300 according to the present embodiment will be briefly described. The manufacturing method includes the following steps 1 to 3.
Step 1: A p-type silicon substrate 1 is prepared. FIG. 11 shows the relationship between the depth from the main surface of the silicon substrate 1 and the specific resistance. As can be seen from FIG. 11, the silicon substrate 1 has a structure in which the specific resistance changes in a sine wave shape having a maximum value of about 6 Ω · cm, a minimum value of about 4 Ω · cm, and a period of about 0.5 μm. Yes. The period in the depth direction may be set to another value in the range of approximately 0.2 μm to approximately 2.0 μm. Such a structure can be formed, for example, by depositing a silicon layer by a CVD method while changing the amount of supplied dopant. The specific resistance of the silicon substrate 1 having a depth of 1.8 μm or more gradually increases and finally becomes constant at about 10 Ω · cm.
Note that the ratio of the maximum value to the minimum value of the specific resistance of the silicon substrate 1 needs to be selected so as to be approximately 1.01 times to approximately 1000 times, as in the first embodiment.
Step 2: Shallow initial grooves arranged at substantially equal intervals as shown in FIG. 9 are formed on the main surface 2 of the silicon substrate 1. The pitch of the initial grooves is selected from a range of approximately 0.2 μm to approximately 2.0 μm, and is approximately 0.5 μm here. The initial groove is formed by forming a mask such as silicon oxide on the main surface 2 of the silicon substrate 1 and then anisotropically etching the main surface 2 using a potassium hydroxide solution.
Step 3: The main surface 2 of the silicon substrate 1 is brought into contact with an electrolytic solution of 5 wt% hydrofluoric acid using the same structure as in Step 3 (FIG. 5) of the first embodiment. Further, a voltage is applied so that the potential of the silicon substrate 1 is about 0.8 V higher than the potential of the electrolytic solution. If the silicon substrate 1 is n-type, light irradiation is required.
Similar to the formation of the hole 3 in the first and second embodiments described above, also in the formation of the groove 6, the groove width is increased in a region having a small specific resistance, and the groove width is decreased in a region having a large specific resistance. As a result, a structure in which the groove width is modulated corresponding to the specific resistance distribution as shown in FIG. 10 is obtained.
By using such a manufacturing method, the optical element 300 having a structure as designed can be obtained regardless of variations in manufacturing conditions such as the temperature of the electrolytic solution. Further, the manufacturing yield is improved, and the optical element 300 can be manufactured at low cost.
In the first to third embodiments, the case where either the n-type or the p-type is used as the silicon substrate has been described, but any conductivity type substrate may be used.
INDUSTRIAL APPLICABILITY The present invention provides a method for manufacturing an optical element having a structure as designed, and can be applied to the manufacture of optical elements such as optical waveguides and optical filters.
[Brief description of the drawings]
FIG. 1 is a top view of the optical element according to the first embodiment of the present invention.
FIG. 2 is a sectional view of the optical element according to the first embodiment of the present invention.
FIG. 3 is a sectional view of the optical element manufacturing process according to the first embodiment of the present invention.
FIG. 4 is a cross-sectional view of the manufacturing steps of the optical element according to the first embodiment of the present invention.
FIG. 5 is a cross-sectional view of the manufacturing process of the optical element according to the first embodiment of the present invention.
FIG. 6 is a top view of the optical element according to the second embodiment of the present invention.
FIG. 7 is a sectional view of an optical element according to the second embodiment of the present invention.
FIG. 8 shows the relationship between the depth from the main surface of the silicon substrate and the specific resistance.
FIG. 9 is a top view of the optical element according to the third embodiment of the present invention.
FIG. 10 is a sectional view of an optical element according to the third embodiment of the present invention.
FIG. 11 shows the relationship between the depth from the main surface of the silicon substrate and the specific resistance.
FIG. 12 is a top view of a conventional optical element.
FIG. 13 is a cross-sectional view of a conventional optical element.

Claims (7)

A method of manufacturing an optical element that forms a plurality of etching holes whose hole widths are modulated in a depth direction from a main surface of a semiconductor substrate,
A preparation step of preparing a semiconductor substrate having a main surface and having a specific resistance modulated in a depth direction from the main surface;
A defining step for defining a hole forming region on the main surface of the semiconductor substrate;
An etching process in which the main surface is immersed in an electrolytic solution, the semiconductor substrate is set to a high potential with respect to the electrolytic solution, and the semiconductor substrate is electrochemically etched in the depth direction from the hole forming region to form an etching hole. The manufacturing method of the optical element characterized by the above-mentioned. The step of preparing is a step of preparing a semiconductor substrate including a low specific resistance layer, a high specific resistance layer, and a low specific resistance layer in order from the main surface of the semiconductor substrate in a depth direction. 2. The production method according to 1. The manufacturing method according to claim 1, wherein the preparation step is a step of preparing a semiconductor substrate in which the specific resistance is modulated in a sine wave shape in a depth direction from the main surface of the semiconductor substrate. 2. The step of preparing the semiconductor substrate according to claim 1, wherein the preparing step is a step of preparing a semiconductor substrate having a ratio of a maximum value to a minimum value of the modulated specific resistance being approximately 1.01 or more and approximately 1000 or less. Manufacturing method. The maximum value of the specific resistance is about 1 Ω · cm or more and about 5000 Ω · cm or less, and the minimum value of the specific resistance is about 0.01 Ω · cm or more and 100 Ω · cm or less. Item 5. The production method according to Item 4. 2. The manufacturing method according to claim 1, wherein the defining step is a step of forming a pit by etching a predetermined position on the main surface of the semiconductor substrate to form the hole forming region. The manufacturing method according to claim 1, wherein the defining step is a step of etching the main surface of the semiconductor substrate into a substantially parallel stripe shape to form the hole forming region.

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