JP3979439B2 - Manufacturing method of semiconductor optical lens - Google Patents

Description

The present invention relates to a method for manufacturing an optical lens from a semiconductor substrate.

  Document 1 (Japanese Patent Publication 2000-263556) discloses a method of manufacturing a mold for a micro optical lens. This mold provides a semiconductor substrate, forms an insulating mask on the upper surface of the semiconductor substrate, forms one or more openings in the insulating mask, immerses the semiconductor substrate in an electrolytic solution, and insulates It is manufactured by anodizing the upper surface of the base material in a portion not covered with a mask, thereby modifying this portion into a porous region. Then, the concave curved surface is left on the upper surface of the base material by removing the porous region. The concave curved surface is filled with an ultraviolet curable resin and cured to form a convex lens. Although this prior art discloses forming a porous region, this method uses an insulating mask with openings so that the porous region develops isotropically from the center of each opening. Is. Accordingly, the resulting concave surface is limited to one having a substantially uniform radius of curvature. Due to this limitation, the prior art methods are not suitable for producing optical lenses with non-uniform curvature radii, i.e. with complex surface shapes.

Further, it is recognized that a minute projection appears on the concave curved surface that appears when the porous region is removed, because a relatively uneven interface remains between the porous region and the semiconductor substrate. Since the unevenness of the lens is impaired by the unevenness, it is desired to smoothen the unevenness in order to manufacture a lens having excellent transparency.
JP 2000-263556 A

  The present invention has been made in view of the above problems, and provides a method for manufacturing a semiconductor optical lens having various surface shapes and excellent transparency.

The method according to the present invention includes a step of forming an anode on the lower surface of the semiconductor substrate using a semiconductor substrate having a flat upper surface and a flat lower surface, and placing the semiconductor substrate in an electrolytic solution. Next, an electric current is passed between the anode and the negative electrode in the electrolytic solution, and the upper surface of the semiconductor substrate is anodized so as to have different depths depending on the location, and a porous layer is formed in the upper surface. Thereafter, the porous layer is removed from the semiconductor substrate, leaving a curved surface on the upper surface. In this manufacturing method, comprising the smoothing process for smoothing the minute projections remaining on the surface. For this reason, the lens is finished into a smooth curved surface and the transparency is improved. In addition, the anode has a predetermined electric field intensity distribution that varies depending on the portion of the semiconductor substrate, and has an anode pattern that is designed to form a desired lens shape. Can be easily formed. Furthermore, the anodizing process proceeds on the upper surface of the semiconductor substrate that is completely exposed to the electrolytic solution, that is, the surface not masked with the material that restricts the anodizing process, and the shape of the curved surface is basically a semiconductor. Since it is controlled by the electric field intensity distribution according to the anode pattern on the lower surface of the base material, it is possible to easily develop a porous layer with a precisely controlled contour and shape, and a lens with a precise surface shape Can be formed.

  Therefore, the manufacturing method of the present invention can be used to manufacture an optical lens, particularly an aspherical lens, having a precisely planned shape and excellent transparency.

  In this manufacturing method, after removing the porous layer, it may be used to immerse the semiconductor substrate in an appropriate etching solution. As a result, fine protrusions are preferentially etched away, and a smooth curved surface can be imparted to the lens.

  Alternatively, it is possible to form an oxide layer on the curved surface layer using thermal oxidation. In this case, the oxidation action proceeds preferentially with the fine protrusions, and the fine protrusions are selectively oxidized, so that the oxide layer does not reach the fine protrusions and the semiconductor substrate side deeply. Therefore, by removing the oxide layer by etching, a lens having a smooth curved surface and a smooth surface can be obtained.

  Furthermore, plasma etching and laser ablation methods can be used in the same manner, and in any case, the fine protrusions can be removed preferentially to smooth the surface of the lens.

  The above smoothing treatment can form a curved surface having a mean square surface roughness (RMS) of 200 nm or less that can reduce the transmittance of the lens due to the surface roughness to 10% or less, and a sufficient lens. Can be satisfied.

  Furthermore, the present invention provides a solution for facilitating the above smoothing process by minimizing the protrusions formed at the interface between the semiconductor substrate and the porous layer. For this reason, while forming the porous layer, the parameter for anodizing the upper surface of the semiconductor substrate is changed with time, and the porosity at the interface with the semiconductor substrate is reduced compared to other parts. To be done. As a result, it is possible to control the porous layer to develop with a relatively large porosity until the porous layer advances to the planned interface with the semiconductor substrate, and to reduce the porosity at the interface. As a result, the porous layer can be efficiently formed and removed, and the irregularity of the protrusions or the surface can be minimized by reducing the porosity.

  The above parameter can be the current density of the current that flows between the anode and the negative electrode, and this current density is the thickness of the porous layer that has a thickness that affects the flattening of the curved surface. Decrease in the final stage.

  Further, the concentration of the electrolytic solution can be controlled alone or in combination with the control of the current density, and this concentration can be increased at the final formation of the porous layer.

  The above and other advantages will become more apparent from the following details described with reference to the accompanying drawings.

  With reference to FIGS. 2-6, the manufacturing method of the plano-convex lens which concerns on preferable embodiment is demonstrated regarding the manufacturing method of the semiconductor optical lens of this invention. As shown in FIGS. 2 to 4, the lens L is formed in a shape in which a flange F is integrated, and this flange is used for mounting the lens on a device such as the optical sensor 200. The optical sensor 200 is a general usage form of the lens. As shown in FIGS. 4 and 5, the optical sensor 200 includes a sensor element such as a pyroelectric element 210 and a circuit module 220 associated therewith, and receives light through the lens. The pyroelectric element 210 is held by a circuit module 220, the circuit module is supported by a base 230 having a plurality of terminal pins 240, and the lens L is supported by a cover 250.

  The lens is formed of a semiconductor material such as silicon (Si), germanium (Ge), silicon carbide (SiC), gallium arsenide (GaAs), gallium phosphide (GaP), or indium phosphide (InP). In this embodiment, a p-type Si semiconductor substrate 10 is used, and a plano-convex lens is manufactured by selectively anodizing the semiconductor substrate 10. Anodization uses an anodizing apparatus 100, which is configured to immerse the semiconductor substrate 10 in an electrolytic solution 140, as shown in FIG. 6, and has a positive electrode 120 and a negative electrode immersed in the solution. A regulator 130 for regulating the current flowing between the electrodes 110 is provided. The positive electrode 120 is disposed so as to be in contact with the lower surface of the semiconductor substrate 10 and allows anodization to proceed at different depths in the upper surface of the semiconductor substrate that opposes the negative electrode 110. Both the positive electrode 120 and the negative electrode 110 are formed of platinum or other metals.

In this embodiment, the semiconductor substrate 10 having a low resistance of several Ωcm to several hundred Ωcm is selected. For example, a lens is formed from the flat semiconductor substrate 10 of 0.5 mm thickness and 80 Ωcm through the processes of FIGS. After cleaning, the semiconductor substrate 10 is processed so as to form the conductive layer 20 on the entire lower surface (FIG. 7B). The conductive layer 20 is, for example, aluminum, and is deposited on the semiconductor substrate 10 by sputtering or other methods to have a uniform thickness of 1 μm. Next, after a resist pattern is formed on the conductive layer 20 by a photolithography process, a part of the resist pattern is removed by wet etching, so that the circular opening 22 having a diameter of 2 mm corresponding to the diameter of the lens is left in the conductive layer 20. A structure is formed, and an anode having an anode pattern in which the conductive layer 20 is integrated with the semiconductor substrate 10 in this integrated structure is defined (FIG. 7C). Etching is not limited to wet etching, and dry etching may be used. Hereinafter, the phrase “anode” will be used in place of the conductive layer where it appears appropriate to describe the present invention. Thereafter, the anode 20 is brought into contact with the positive electrode 120, the semiconductor substrate 10 is immersed in the electrolytic solution 140 of the anodizing apparatus 100, and a current is passed between the anode 20 and the negative electrode 110, The upper surface of the semiconductor substrate 10 is selectively anodized according to the anode pattern, thereby developing the porous layer 30 on the upper surface of the semiconductor substrate 10 (FIG. 7D). The current is adjusted to a predetermined current density, for example, 30 mA / cm ² by the regulator 130, and is continuously supplied for a predetermined time, for example, 120 minutes. Thereafter, the porous layer 30 and the anode 20 are removed by etching to obtain a lens (FIG. 7E).

The electrolytic solution used is an aqueous solution in which hydrogen fluoride (HF) and ethanol are mixed at an appropriate ratio. In the anodizing process, the following chemical reaction proceeds.
Si + 2HF + (2-n ) h + (R) SiF 2 + 2H + + n × e -
SiF ₂ + 2HF (R) SiF ₄ + H ₂
SiF ₄ + 2HF (R) SiH ₂ F ₆
Here, h ⁺ represents a hole, and e ⁻ represents an electron.
As soon as the Si substrate is anodized, the oxidized portion reacts with the electrolytic solution and is removed, thereby forming a porous layer on the upper surface of the semiconductor substrate 10. Therefore, the anodic oxidation process proceeds without being delayed by the oxidized portion, so that the porous layer 30 having a deeper thickness can be developed, and a lens having a relatively large thickness can be manufactured. .

  As schematically shown in FIG. 8A, the in-plane electric field strength, that is, the current density is distributed so as to change according to the anode pattern. In this figure, black arrows indicate paths of positive currents flowing through the semiconductor substrate, and white arrows indicate paths of electrons flowing through the semiconductor substrate. Since the in-plane current density increases from the center of the opening 22 toward the periphery, the thickness of the obtained porous layer 30 gradually increases from the center of the opening toward the periphery. Therefore, by removing this porous layer 30, a plano-convex lens can be obtained. If necessary, the anode 20 is also removed. The in-plane electric field strength distribution is mainly determined by the anode pattern. Secondary, the resistance value and thickness of the semiconductor substrate 10, the resistance value of the electrolytic solution 140, the distance between the semiconductor substrate 10 and the negative electrode 110, And the planar configuration of the negative electrode (arrangement of the negative electrode in a plane parallel to the substrate). Therefore, various lens shapes can be easily provided by appropriately combining these parameters with the anode pattern. In such anodization, the porous layer can be continuously developed without being delayed by the oxidized portion formed in the semiconductor substrate. A lens can be manufactured, and the freedom degree of lens design can be improved.

  The resistance value of the electrolytic solution 140 can be adjusted by the concentration of the aqueous hydrogen fluoride (HF) solution and / or the mixing ratio of HF and ethanol. As shown in FIG. 8A, the negative electrode 110 is designed in a pattern that matches the anode pattern, or, as shown in FIG. 10 may be slightly shifted toward the center side of the opening 22 of the anode pattern in a plane parallel to 10. This shift amount is adjusted in relation to the current density and the distance from the semiconductor substrate 10.

  In the anodizing process, the regulator 130 functions to keep the current density basically constant, but at the final stage of the anodizing process, the current density is decreased to reduce the development rate of the porous layer 30. It is preferable. In this method, the porosity of the porous layer formed by lowering the current density of anodic oxidation is reduced. Thereby, the unevenness | corrugation of the curved surface after a porous layer removal can be reduced, and the surface of a lens becomes smoother. Such adjustment of the current density is performed by detecting a current level or a voltage level.

  In order to remove the porous layer 30 and the anode 20, an alkaline solution such as KOH, NaOH, TMAH (tetramethylammonium hydroxide), or HF solution is used.

Figure 10 (A) and (B), shows a control for reducing the current density at the final stage of anodizing, the current density I1 (e.g., 30 mA / cm ²⁾ from I2 (e.g., 3mA / cm ² ) Continuously or gradually in a stepwise manner from I1 to I2. As a result, the anodic oxidation rate is reduced, and as shown schematically in FIG. 1 (A), the porosity at the interface with the base material is made smaller than that in the other parts. Only the fine protrusions are left on the curved surface of the semiconductor substrate that appears after the layer 30 is removed. Here, FIG. 1A exaggerates the change in porosity in order to explain the above anodic oxidation treatment, and the size and distribution of the pores do not reflect actual ones. It is desirable to reduce the porosity only in the final stage of forming the porous layer 30 in consideration of the decrease in the formation speed of the porous layer if the porosity is reduced by reducing the current density. In order to satisfy the requirement of rapidly removing the porous layer 30 by etching and simultaneously minimizing the protrusions remaining on the curved surface of the semiconductor substrate, an anodic oxidation time T1 at a large current density I1; A time T2 for anodization at a small current density I2 is appropriately selected. For example, T1 and T2 are 30 minutes and 10 minutes, respectively.

  In combination with, or instead of, the control of the current density, the concentration of the electrolytic solution, for example, the HF concentration is controlled, and as shown in FIGS. It is also possible to step up to M2 stepwise or continuously. When the concentration of the electric field solution is increased, smaller pores are formed, the porosity is reduced, and small fine protrusions remain on the surface of the semiconductor substrate 10. Also in this case, the times T1 and T2 are appropriately selected to facilitate the formation and removal of the porous layer 30, and at the same time, leave only fine protrusions at the interface between the porous layer 30 and the semiconductor substrate 10. Like that.

As shown in FIG. 1B, the fine protrusions 12 remaining on the semiconductor substrate 10 have an average height of 1 μm and intervals of several μm. Such surface irregularities are removed by a smoothing process after removing the porous layer 30. In the present embodiment, the semiconductor substrate 10 (shown in FIG. 1B) from which the porous layer 30 has been removed is, for example, KOH, NaOH, TMAH (tetramethylammonium hydroxide), or an HF-based solution. It is immersed in a dry etching solution. In this process, the fine protrusions 12 on the semiconductor substrate are selectively removed by etching as shown by the broken lines in FIG. 1C, and the semiconductor substrate 10 is smoothed as shown in FIG. Leave a good surface.
Example An aluminum conductive layer 20 having a thickness of 1 μm was formed by sputtering on a p-type Si substrate 10 having a diameter of 100 mm, a thickness of 0.5 mm, and a resistance value of 80 Ωcm. The conductive layer 20 was sintered at 420 ° C. for 20 minutes, and then masked with a resist pattern having a plurality of windows having a diameter of 2 mm by photolithography. Next, the unmasked portion of the conductor layer 20 was removed by wet etching to form a plurality of openings 22 having a diameter of 2 mm in the conductive layer, that is, the anode 20, as shown in FIG. After removing the resist pattern, the semiconductor substrate was placed in the anodizing apparatus of FIG. 5 containing an electrolytic solution having a mixing ratio of 50% aqueous solution of hydrogen fluoride (HF) and ethanol of 1: 1. This semiconductor substrate 10 was anodized at a current density of 30 mA / cm ² for 3 hours. As a result, the thickness of the porous layer 30 corresponding to the conductive layer 20 is 0.3 mm, and the thickness gradually decreases toward the center of the opening 20 in a plane parallel to the semiconductor substrate. The porous layer 30 and the conductive layer 20 were then etched away with a 10% KOH solution for 15 minutes to form a plurality of plano-convex lenses. Next, the semiconductor substrate 10 was immersed in an HF solution for 1 hour, and fine protrusions 12 remaining on the surface of the semiconductor substrate 10 were selectively removed by etching. Thereafter, the semiconductor substrate 10 was cut into a plurality of lenses. The etching rate for removing the porous layer 30 was 10 times or more the etching rate for removing fine protrusions on the semiconductor substrate 10. The lens thus obtained had a thickness of 0.195 mm and a mean square surface roughness (RMS) of 50 nm.

  FIG. 12 shows the relationship between the mean square surface roughness (RMS) and the transmittance (%) of infrared rays having a wavelength of 5 μm. The wavelength of 5 μm to 10 μm is an infrared wavelength band radiated from a human, and the transmittance for 5 μm was verified in consideration of using an infrared sensor for human body detection. In the graph of FIG. 12, black dots indicate the measured surface roughness and transmittance, and the curve indicates a theoretical relationship represented by the following formula.

Here, σ indicates the surface roughness. From this graph, the measured transmittance decreases as the surface roughness increases, as in the theoretical relationship. In actual use, it is desirable that the decrease in infrared transmittance due to scattering on the surface unevenness is within 10%. From this viewpoint, the lens needs to have a transmittance of 42% or more, and the mean square surface roughness of the lens. Is 200 nm or less. The average square surface roughness (RMS) of the lens manufactured in the above example is 50 nm, which satisfies this requirement sufficiently, and the above method is performed so that the mean square surface roughness (RMS) is 200 nm or less. The lens manufactured in is sufficient for practical use.

  In order to smooth the lens surface, as shown in FIGS. 13A to 13E, as another method, the surface of the semiconductor substrate is thermally oxidized, and then an oxide layer is formed. Can be removed by etching. First, as shown in FIG. 13A, the porous layer 30 is formed on the surface layer portion of the semiconductor substrate 10, and between the porous layer 30 and the remaining semiconductor substrate 10, as described above. A curved interface corresponding to the opening 22 of the anode 20, that is, the anode pattern is given. Next, the porous layer 30 is removed by etching to obtain the structure of FIG. 13B in which the fine protrusions 12 remain on the upper surface of the semiconductor substrate 10. Next, by heating the semiconductor substrate 10 using an oxidation furnace, an oxide film 14 is developed on the upper surface of the semiconductor substrate as shown in FIG. In this case, an oxide film of about 0.4 μm is formed by performing treatment at 1000 ° C. for 200 minutes while introducing water vapor into the oxidation furnace. In the thermal oxidation, as shown in FIG. 13 (D), oxidation in which Si becomes SiO2 uniformly proceeds along the normal direction on the surface of the substrate, and the protrusions are preferentially oxidized compared to other portions. Thus, smoothing is achieved without surface irregularities. Therefore, it is possible to form the oxide layer 14 including all the protrusions 12 and not including other portions by controlling thermal oxidation. Thereafter, the oxide layer 14 is removed by etching to give a smooth surface to the semiconductor substrate as shown in FIG.

Furthermore, in the present invention, plasma etching can be used to remove fine protrusions from the upper surface of the semiconductor substrate 10. In this case, the semiconductor substrate 10 from which the porous layer 30 has been removed is exposed to a plasma stream, and the fine protrusions 12 remaining on the semiconductor substrate are selectively and preferentially removed. Sputter etching is performed when argon gas is used, and reactive ion etching is performed when CF ₄ gas or a mixed gas of CF ₄ and O ₂ is used.

  Furthermore, the above smoothing treatment can also be performed by laser ablation. In this case, fine projections are selectively and preferentially removed by irradiating the surface of the semiconductor substrate with laser light.

  The above-described anodizing treatment that changes the current density and the concentration of the electrolytic solution is preferable, but the present invention is not limited to this, and anodizing treatment that does not control these parameters can be used as well. Is.

  Furthermore, the present invention should not be limited to the use of a silicon substrate, but other semiconductor materials can be used in combination with a specific electrolytic solution, as shown in the table below.

In the table above, mask materials that can be used in combination with a semiconductor material and an electrolytic solution are shown.

(A)-(D) are sectional drawings which show the process of the manufacturing method of the semiconductor lens which concerns on preferable embodiment of this invention. The perspective view of the plano-convex lens created based on preferable embodiment of this invention. Sectional drawing of the said lens. (A) (B) is sectional drawing which shows the apparatus which uses the said lens, respectively. The exploded perspective view of the above-mentioned device. Sectional drawing of the anodizing apparatus used in order to perform the manufacturing method of this invention. (A)-(E) are sectional drawings which show the manufacture process of the said lens. (A) and (B) are schematic diagrams showing an electric field intensity distribution that appears in the process of forming the lens. The top view which shows the anode pattern formed in the lower surface of a semiconductor base material. (A) (B) is a graph which shows the control for developing a porous layer. (A) (B) is a graph which shows another control for developing a porous layer. The graph which shows the relationship between the transmittance | permeability of the said lens, and surface roughness. (A)-(E) is sectional drawing which shows another smoothing process about the said lens surface.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Semiconductor substrate 12 Protrusion 14 Oxide layer 20 Anode 30 Porous layer 110 Negative electrode 140 Electrolytic solution

Claims (9)

A semiconductor optical lens manufacturing method for manufacturing a semiconductor optical lens by removing a part of a semiconductor substrate,
Prepare a semiconductor substrate having a flat upper surface and a flat lower surface facing each other,
Forming an anode on the lower surface of the semiconductor substrate;
The above semiconductor substrate is placed in an electrolytic solution,
A current is passed between the anode and the negative electrode in the electrolytic solution, and the anodization treatment on the upper surface is advanced to a different depth depending on the location, thereby forming a porous layer in the upper surface,
Removing the porous layer from the semiconductor substrate, leaving a curved surface on the upper surface,
The anode is configured to form a predetermined electric field strength distribution that varies from place to place along the upper surface and the lower surface of the semiconductor substrate, whereby the depth varies depending on the electric field strength distribution. Giving different porous layers described above,
The anode has an anode pattern that is designed to form a desired lens shape,
The method further includes a smoothing step of smoothing fine protrusions remaining on the curved surface. 2. The semiconductor according to claim 1, wherein, in the smoothing step, after removing the porous layer, the semiconductor substrate is immersed in an etching solution to remove fine protrusions remaining on the curved surface. Manufacturing method of optical lens . In the smoothing step, after removing the porous layer, the surface layer portion of the curved surface is thermally oxidized to form an oxidized layer therein, and then the oxidized layer is removed by etching. The method of manufacturing a semiconductor optical lens according to claim 1, wherein fine protrusions remaining on the substrate are removed. 2. The semiconductor optical lens according to claim 1, wherein, in the smoothing step, after removing the porous layer, the curved surface is exposed to plasma to remove fine protrusions remaining on the curved surface. Production method. 2. The semiconductor optical according to claim 1, wherein, in the smoothing step, after removing the porous layer, the curved surface is irradiated with laser light to remove fine protrusions remaining on the curved surface. Lens manufacturing method. 6. The semiconductor optical lens according to claim 2, wherein the surface roughness of the curved surface is 200 nm or less in terms of mean square surface roughness (rms) by the smoothing step. Production method. In the step of forming the porous layer, the porosity at the interface with the semiconductor substrate is made smaller than the other portions by changing the parameters of the anodizing treatment on the upper surface of the semiconductor substrate with time. The method of manufacturing a semiconductor optical lens according to claim 1, wherein the semiconductor optical lens is manufactured. 8. The parameter according to claim 7, wherein the parameter is a current density of a current flowing between the anode and the negative electrode, and the current density is decreased in a final stage of anodizing treatment for forming the porous layer. The manufacturing method of the semiconductor optical lens of description. 8. The method of manufacturing a semiconductor optical lens according to claim 7, wherein the parameter is a concentration of the electrolytic solution, and the concentration is increased in a final stage of anodizing treatment for forming the porous layer .

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