Abstract:
A method for manufacturing a microlens formed on a semiconductor substrate includes the steps of preparing the semiconductor substrate, forming an insulating film, which has high etching selectivity with the semiconductor substrate, on the semiconductor substrate, forming a first resist layer, which has an opening that exposes a part of the insulating film, on the insulating film, forming a lens forming portion by eliminating a part of the insulting film, using the first resist layer as a mask, forming a second resist layer, which has roughly cylindrical shape, on the lens forming portion surrounded by the insulating film, transforming the second resist layer into a third resist layer that has roughly hemispheric shape by reflowing the second resist later with a heat treatment, and forming a lens on the semiconductor substrate by etching the third resist layer, the semiconductor substrate, and the insulating film simultaneously with anisotropic etching.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method for manufacturing a microlens. 
     A microlens is extensively used in optic devices, such as an optical communication connector and a charge coupled device (CCD) image sensor. The polishing method or the mold method is used to manufacture a normal lens, but other methods, such as the reflow method or the grayscale mask method is generally used to manufacture a microlens. This is because a microlens is minute, compared to a normal lens. The following is a brief explanation of the reflow method. First, a nearly cylindrical resist pattern is formed on a base substrate with photolithography. Then, the resist pattern is fluidized by heating the base substrate, and a nearly hemispheric resist pattern is formed by the surface tension of the fluidized resist pattern. Next, the nearly hemispheric resist pattern and the base substrate are simultaneously etched with anisotropic dry etching. Then a nearly hemispheric pattern is printed on the base substrate, and thus a microlens is formed. 
     2. Background Information 
     Japanese Patent Publication JP-A-05-136460 (especially page 4 and FIG. 2), and Japanese Patent Publication JP-A-06-104480 (especially pages 3-5 and FIG. 4), each show a method for manufacturing a microlens. 
     As the second embodiment of a method for manufacturing a microlens, the Japanese Patent Publication JP-A-05-136460 describes a method for manufacturing a microlens by using a nearly hemispheric resist pattern, which is formed on a base substrate (GaAs) with the potting method, as an etching mask, and by etching the resist pattern and the base substrate simultaneously. Also, the Japanese Patent Publication JP-A-06-104480 describes a method for manufacturing a microlens basically with the hitherto known reflow method. However, a structural feature of the microlens is that the surrounding portion of the microlens is formed so that its height is higher than the top of the microlens. Because of this structure, the microlens is protected from damage caused by external factors. The following is a brief explanation of a method for manufacturing a microlens described in this Japanese Patent Publication. First, a base of a negative resist is formed on the surrounding part of the side of a base substrate (InP) where a microlens is formed. Then, a positive resist is formed on both the base of the negative resist and the central part of the base substrate where a microlens is formed, and the bowl-shaped positive resist is formed by fluidizing the positive resist with a thermal treatment. Next, a microlens is formed by etching this resist pattern and the base substrate simultaneously with anisotropic dry etching. At this time, if the etch selectivity between the positive resist and the base substrate nearly equals one and the etch selectivity between the negative resist and the base substrate nearly equals to three, the height of the surrounding portion of the microlens is formed higher than the top of the microlens. 
     In the reflow method, the nearly cylindrical resist pattern formed by photolithography is fluidized with a high temperature heat treatment (i.e., a reflow treatment), and it is transformed into a nearly hemispheric resist pattern. In this reflow treatment, some combination of the heat treatment&#39;s temperature, the thickness of the resist, and the material of the resist can change the size of the resist pattern at the end of the treatment. Especially, as an example of the size change, it often happens that the size of the resist pattern becomes larger than its desired size because of thermal sag. In this case, there is a possibility that the lens properties will vary and furthermore the miniaturization of the lens will be prevented. 
     In the method for manufacturing a microlens described in Japanese Patent Publication JP-A-05-136460, the nearly hemispheric resist pattern is formed by the potting method. Therefore, it is thought that the size of the resist pattern in this method tends to vary more widely than that in the reflow method. Also, in the method for manufacturing a microlens described in the Japanese Patent Publication JP-A-06-104480, a heretofore known reflow method is used to form a microlens. Therefore, it is thought that the size of the resist pattern tends to vary. 
     In view of the above, it will be apparent to those skilled in the art from this disclosure that there exists a need for an improved method for manufacturing a microlens. This invention addresses this need in the art as well as other needs, which will become apparent to those skilled in the art from this disclosure. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to resolve the above-described problem, and to provide a method for manufacturing a microlens having stable properties and shape. 
     In accordance with the present invention, a method for manufacturing a microlens formed on a semiconductor substrate comprises the steps of preparing the semiconductor substrate, forming an insulating film, which has high etching selectivity with the semiconductor substrate, on the semiconductor substrate, forming a first resist layer, which has an opening that exposes a part of the insulating film, on the insulating film, forming a lens forming portion by eliminating a part of the insulting film, using the first resist layer as a mask, forming a second resist layer, which has roughly cylindrical shape, on the lens forming portion surrounded by the insulating film, transforming the second resist layer into a third resist layer that has roughly hemispheric shape by reflowing the second resist later with a heat treatment, and forming a lens on the semiconductor substrate by etching the third resist layer, the semiconductor substrate, and the insulating film simultaneously with anisotropic etching. 
     According to the method for manufacturing a microlens of the present invention, a nearly hemispheric third resist layer is formed by reflowing a second resist layer that is surrounded by an insulating film. Therefore, it is possible to prevent the size of the third resist layer from varying, even if the size of the third resist layer increases due to thermal sag, because the size of the third resist layer is defined by the diameter of a lens forming portion that is surrounded by an insulating film. This makes it possible to manufacture a microlens whose properties and shape are stable. 
     These and other objects, features, aspects, and advantages of the present invention will become apparent to those skilled in the art from the following detailed description, which, taken in conjunction with the annexed drawings, discloses a preferred embodiment of the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
       Referring now to the attached drawings which form a part of this original disclosure: 
         FIG. 1  shows diagrams of the steps in a microlens manufacturing method in accordance with the first embodiment of the present invention; 
         FIG. 2  shows diagrams of the steps in a microlens manufacturing method in accordance with the first embodiment of the present invention; 
         FIG. 3  shows diagrams of the steps in a microlens manufacturing method in accordance with the second embodiment of the present invention; 
         FIG. 4  shows diagrams of the steps in a microlens manufacturing method in accordance with the second embodiment of the present invention; 
         FIG. 5  is a view of diagrams showing two types of photo masks. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Selected embodiments of the present invention will now be explained with reference to the drawings. It will be apparent to those skilled in the art from this disclosure that the following descriptions of the embodiments of the present invention are provided for illustration only and not for the purpose of limiting the invention as defined by the appended claims and their equivalents. 
     Referring now to the drawings, preferred embodiments of the present invention will be described in detail. 
     First Embodiment 
       FIGS. 1 and 2  are cross-section diagrams to explain a method for manufacturing a microlens in accordance with the first embodiment of the present invention. 
     First, as shown in  FIG. 1(A) , a semiconductor substrate  101  is prepared in order to manufacture a microlens. For example, a silicon substrate is used as the semiconductor substrate  101 . 
     Next, as shown in  FIG. 1(B) , an insulating film  102  is formed on the semiconductor substrate  101 . For example, the thickness of the insulating film  102  is 1 μm. Preferably, the insulating film  102  is made of substance that has an etch selectivity with silicon, which is the substance of the semiconductor substrate  101 . For example, a silicon dioxide (SiO 2 ) film or a silicon nitride (SiN) film, which are generally used for manufacturing a semiconductor device, may be used as the substance of the insulating film  102 . 
     Next, as shown in  FIG. 1(C) , a lens forming portion  102   a  is formed by etching the insulating film  102  with a heretofore known photolithography etchant, so that it exposes a part of surface of the semiconductor substrate  101 . A diameter D 1  of the lens forming portion  102   a  is formed to be larger than a diameter D 2  of a resist pattern  103  by Δd, in consideration of the registration between the lens forming portion  102   a  and the resist pattern  103  that is formed within the lens forming portion  102   a  in the next process ( FIG. 1  (D)) and the variation of the size of the resist pattern  103 . That is, the diameter D 1  of the lens forming portion  102   a  is expressed in the form of a equation: D 1 =D 2 +Δd. Also, as shown in  FIG. 1(D) , the distance from the left and right sides of the resist pattern  103  to both the left and right sides of the insulating film  102  is Δd/2, respectively. The Δd can be adjusted depending on the size of the diameter D 2  of the resist pattern  103 . For example, if the resist pattern  103  is formed so that it has a diameter D 2  of 100 μm, the value of Δd may be set to be nearly 1 μm. 
     Next, as shown in  FIG. 1(D) , a nearly cylindrical resist pattern  103  is formed on a part of the semiconductor substrate  101 , which is exposed as the lens forming portion  102   a , with a heretofore known photolithography process. The thickness of the resist pattern  103  can be set depending on the size of the lens to be formed. For example, if a lens is formed so that it has a diameter of nearly 100 μm, the thickness of the resist pattern  103  may be set to be nearly 10 μm. Also, the diameter D 2  of the resist pattern  103  may be set according to the size of the lens to be formed. 
     Next, as shown in  FIG. 2(A) , a heat treatment is conducted at a temperature higher than the glass transition temperature of the resist, and the nearly cylindrical resist pattern  103  is transformed into a nearly hemispheric resist pattern  104  by reflowing it. For example, as the heat treatment conditions, the temperature and duration of the heat treatment may be set as 170–190° C. and 30–300 seconds, respectively. The conditions may be changed according to the type of resist that is used. In this heat treatment process, the size of the resist pattern  104  does not exceed the diameter D 1  of the lens forming portion  102   a , even if the size of the resist pattern  104  becomes larger than that of the resist pattern  103  because of thermal sag. This is because the resist pattern  103  is surrounded by the insulating film  102 . In other words, the size of the resist pattern  104  is defined by the wall of the insulating film  102 . 
     Next, as shown in  FIG. 2(B) , the resist pattern  104 , the semiconductor substrate  101 , and the insulating film  102  are etched simultaneously by anisotropic dry etching, such as reactive ion etching (RIE). For example, a mixed gas of tetrafluomethane (CF 4 ) and oxygen (O 2 ) are used in the etching process. In this etching process, if the mixture ratio of CF 4  and O 2  is adjusted so that the etch selectivity between the resist pattern  104  and the semiconductor substrate  101  is set to be a value of nearly 1, the resist pattern  104  and the semiconductor substrate  101  are etched at the nearly same ratio. Through the etching process, the nearly hemispheric resist pattern  104  is printed on the semiconductor substrate  101 . On the other hand, in the same etching conditions, the etch selectivity of the semiconductor substrate  101  made of silicon and the insulating film  102  made of silicon dioxide film is defined as a value of 5–10. Therefore, the etching process on the area in which the insulating film  102  is formed proceeds slowly. In other words, the insulating film  102  functions as an etching mask toward the semiconductor substrate  101 . In general, the smaller the area to be etched, the higher the etching ratio. As shown in  FIG. 2(B) , a part of the semiconductor substrate  101  is covered with the insulating film  102 , which functions as an etching mask toward the semiconductor substrate  101 . Because of this, the exposed area of the semiconductor substrate  101  can be decreased, and the lowering of the etching ratio with respect to the semiconductor substrate  101  can be inhibited. 
     As shown in  FIG. 2(C) , in the state in which the etching process further proceeds and the resist pattern  104  is completely eliminated, a lens portion  105  is formed which has nearly the same curvature with the resist pattern  104 . Also, in the area that was covered with the insulating film  102 , a peripheral lens portion  106  is formed so that its height is higher than that of a basolateral lens part  107 . Also, if the initial film thickness of the insulating film  102  is set so that it is completely etched in this etching process, a process to eliminate the remaining portion of the insulating film  102  is not needed. Thus, a microlens  100  is formed. 
     According to the method for manufacturing a microlens of the first embodiment, the nearly hemispheric resist pattern  104  is formed by conducting the reflow in a state in which the surrounding portion of the nearly cylindrical resist pattern  103  is covered with the insulating film  102 . Therefore, even if the size of the resist pattern  104  becomes larger than that of the resist pattern  103  because of thermal sag, it is defined according to the diameter D 1  of the lens forming portion  102   a  that is surrounded by the insulating film  102 . Because of this, variation in the size of the resist pattern  104  can be inhibited from becoming larger, and a microlens having stable properties and shape can be manufactured. Also, in the process of forming a lens-shaped resist pattern by etching the semiconductor substrate  101  (FIG.  2 (B)), the lowering of the etching ratio with respect to the semiconductor substrate  101  can be prevented and the duration of the etching process can be shortened, by forming the insulating film  102  on the surface of the semiconductor substrate  101 . 
     Second Embodiment 
       FIGS. 3 and 4  are cross-section diagrams to explain a method for manufacturing a microlens in accordance with the second embodiment of the present invention. 
     First, as shown in  FIG. 3(A) , a semiconductor substrate  201  is prepared in order to manufacture a microlens. For example, a silicon substrate is used as the semiconductor substrate  201 . 
     Next, as shown in  FIG. 3(B) , an insulating film  202  is formed on the semiconductor substrate  201 . For example, the thickness of the insulating film  202  is 1 μm. Preferably, the insulating film  202  is made of substance that has an etch selectivity with silicon, which is the substance of the semiconductor substrate  201 . For example, a silicon dioxide film (SiO 2 ) film and a silicon nitride (SiN) film, which are generally used for manufacturing a semiconductor device, may be used as the substance of the insulating film  202 . 
     Next, as shown in  FIG. 3(C) , a negative resist is applied on the insulating film  202 , and a exposure process and a development process is conducted with a MASK-A, and a resist pattern  203  with an opening  203   a  is formed. As shown in  FIG. 5(A) , the MASK-A is a photo mask that has a light-resistant body in the area corresponding to an area where a lens is formed in the later process (i.e., a lens forming portion  202   a  shown in  FIG. 3(D) ). Also, when a negative resist is exposed to light, a chemical reaction is caused in its exposed portion. This chemical reaction makes its exposed portion insoluble toward a developer. Because of this, the exposed portion of the negative resist is left as a resist pattern. Therefore, when an exposure process and a development process are conducted toward the negative resist with the MASK-A, the resist pattern  203 , as shown in  FIG. 3(C) , is formed. It is also possible to form the resist pattern  203  by applying a positive resist on the insulating film  202  and using a MASK-B shown in  FIG. 5(B) . The MASK-B is a photo mask that has a light-resistant body in the area except for the area where a lens is formed in the later process (i.e., a lens forming portion  202   a  shown in  FIG. 3(D) ). Also, when a positive resist is exposed to light, a chemical reaction is caused in its exposed portion. This chemical reaction makes its exposed portion soluble toward a developer. Because of this, the exposed portion of the negative resist is dissolved in a developer and the unexposed portion of it is left as a resist pattern. Therefore, when an exposure process and a development process are conducted toward the positive resist with the MASK-B, the resist pattern  203 , as shown in  FIG. 3(C) , is formed. 
     Next, as shown in  FIG. 3(D) , the insulating film  202  is partially eliminated by using the resist pattern  203  as an etching mask, and a lens forming portion  202   a  is formed which exposes a part of the surface of the semiconductor substrate  201 . A diameter D 1  of the lens forming portion  202   a  is formed to be larger than a diameter D 2  of a resist pattern  204  by Δd, in consideration of the registration between the lens forming portion  202   a  and the resist pattern  204  that is formed within the lens forming portion  202   a  in the next process ( FIG. 4  (A)) and the variation of the size of the resist pattern  204 . That is, the diameter D 1  of the lens forming portion  202   a  is expressed in the form of an equation: D 1 =D 2 +Δd. Also, as shown in  FIG. 4(A) , the distance from the left and right sides of the resist pattern  204  to both the left and right sides of the insulating film  202  is Δd/2, respectively. The Δd can be adjusted depending on the size of the diameter D 2  of the resist pattern  204 . For example, if the resist pattern  204  is formed so that it has a diameter D 2  of nearly 100 μm, the value of Δd may be set to be nearly 1 μm. 
     Next, as shown in  FIG. 4(A) , a positive resist is applied on the semiconductor substrate  201  and the insulating film  202 , and an exposure process and a development process are conducted with the MASK-A, which is used for manufacturing the resist pattern  203  ( FIG. 3(C) ). Through these processes, a nearly cylindrical resist pattern  204  is formed on a part of the semiconductor substrate  201  that is exposed through the lens forming portion  202   a . As described above, the MASK-A shown in  FIG. 5(A)  is a photo mask that has a light-resistant body in the area corresponding to the lens forming portion  202   a . Also, the positive resist is a type of resist in which the portion exposed to light is dissolved in a developer because of a chemical reaction in the exposed part, and the portion not exposed to light is left as a resist pattern. Therefore, when an exposure process and a development process are conducted toward the positive resist with the MASK-A, the resist pattern  204  shown in  FIG. 4(A)  is formed. It is also possible to form the resist pattern  204  by applying a negative resist on the semiconductor substrate  201  and the insulating film  202 , and using the MASK-B shown in  FIG. 5(B) . As described above, the MASK-B is a photo mask that has a light-resistant body in the area except for the area corresponding to the lens forming portion  202   a . Also, the negative resist is a type of resist in which the portion exposed to light becomes insoluble toward a developer because of a chemical reaction in the exposed part, and the portion exposed to light is left as a resist pattern. Therefore, when an exposure process and a development process are conducted toward the negative resist with the MASK-B, the resist pattern  204  shown in  FIG. 4(A)  is formed. The thickness of the resist pattern  204  can be set according to the size of the lens to be manufactured. For example, when a lens with a diameter of nearly 100 μm is manufactured, the thickness of the resist pattern  204  may be set to be nearly 10 μm. Also, the diameter D 2  of the resist pattern  204  may be set according to the size of the lens to be manufactured. 
     Next, as shown in  FIG. 4(B) , a heat treatment is conducted at a temperature higher than the glass transition temperature of the resist, the nearly cylindrical resist pattern  204  is transformed into the nearly hemispheric resist pattern  205  by reflowing it. For example, as the heat treatment conditions, the temperature and duration of the heat treatment may be set as 170–190° C. and 30–300 seconds, respectively. The conditions may be changed according to the type of resist that is used. In this heat treatment process, the size of the resist pattern  205  does not exceed the diameter D 1  of the lens forming portion  202   a , even if the size of the resist pattern  205  becomes larger than that of the resist pattern  204  because of thermal sag. This is because the resist pattern  205  is surrounded by the insulating film  202 . In other words, the size of the resist pattern  205  is defined by the wall of the insulating film  202 . 
     Next, as shown in  FIG. 4(C) , the resist pattern  205 , the semiconductor substrate  201 , and the insulating film  202  are etched simultaneously by anisotropic dry etching, such as reactive ion etching (RIE). For example, a mixed gas of tetrafluoromethane (CF 4 ) and oxygen (O 2 ) are used in the etching process. In this etching process, if the mixture ratio of CF 4  and O 2  is adjusted so that the etch selectivity between the resist pattern  205  and the semiconductor substrate  201  is set to be a value of nearly 1, the resist pattern  205  and the semiconductor substrate  201  are etched at the nearly same ratio. Through the etching process, the nearly hemispheric resist pattern  205  is printed on the semiconductor substrate  201 . On the other hand, in the same etching conditions, the etch selectivity of the semiconductor substrate  201  made of silicon and the insulating film  202  made of silicon dioxide film is defined as a value of 5–10. Therefore, the etching process on the area in which the insulating film  202  is formed proceeds slowly. In other words, the insulating film  202  functions as an etching mask toward the semiconductor substrate  201 . In general, the smaller the area to be etched, the higher the etching ratio. As shown in  FIG. 4(B) , a part of the semiconductor substrate  201  is covered with the insulating film  202 , which functions as an etching mask toward the semiconductor substrate  201 . Because of this, the exposed area of the semiconductor substrate  201  can be decreased, and the lowering of the etching ratio with respect to the semiconductor substrate  201  can be inhibited. 
     As shown in  FIG. 4(D) , in the state in which the etching process further proceeds and the resist pattern  205  is completely eliminated, a lens portion  206  is formed which has nearly the same curvature with the resist pattern  205 . Also, in the area that was covered with the insulating film  202 , a peripheral lens portion  207  is formed so that its height is higher than that of a basolateral lens part  208 . Also, if the initial film thickness of the insulating film  202  is set so that it is completely etched in this etching process, a process to eliminate the remaining portion of the insulating film  202  is not needed. Thus, a microlens  200  is formed. 
     According to the method for manufacturing a microlens of the second embodiment, the nearly hemispheric resist pattern  205  is formed by conducting the reflow in a state in which the surrounding portion of the nearly cylindrical resist pattern  204  is covered with the insulating film  202 . Therefore, even if the size of the resist pattern  205  becomes larger than that of the resist pattern  204  because of thermal sag, it is defined according to the diameter D 1  of the lens forming portion  202   a  that is surrounded by the insulating film  202 . Because of this, variation in the size of the resist pattern  205  can be inhibited from becoming larger, and a microlens having stable properties and shape can be manufactured. Also, in the process of forming a lens-shaped resist pattern by etching the semiconductor substrate  201  (FIG.  4 (C)), the lowering of the etching ratio with respect to the semiconductor substrate  201  can be prevented and the duration of the etching process can be shortened, by forming the insulating film  202  on the surface of the semiconductor substrate  201 . In addition, when different types of resists are used in the process of patterning the insulating film  202  ( FIG. 3(C) ) and the process of forming the cylindrical resist pattern  204  ( FIG. 4(A) ) (e.g., when a negative resist is used for patterning the insulating film  202  and a positive resist is used for forming the resist pattern  204 ), the same type of the photo mask can be used for these processes. Therefore, the number of the photo masks can be decreased, and manufacturing costs can be reduced. 
     This application claims priority to Japanese Patent Application No. 2004-298609. The entire disclosure of Japanese Patent Application No. 2004-298609 is hereby incorporated herein by reference. 
     The terms of degree, such as “nearly”, used herein mean a reasonable amount of deviation of the modified term such that the end result is not significantly changed. For example, the terms can be construed as including a deviation of at least ±5% of the modified term if this deviation would not negate the meaning of the word it modifies. 
     While only selected embodiments have been chosen to illustrate the present invention, it will be apparent to those skilled in the art from this disclosure that various changes and modifications can be made herein without departing from the scope of the invention as defined in the appended claims. Furthermore, the foregoing descriptions of the embodiments according to the present invention are provided for illustration only, and not for the purpose of limiting the invention as defined by the appended claims and their equivalents. Thus, the scope of the invention is not limited to the disclosed embodiments.