Abstract:
In order to efficiently form microlenses wide in light receiving surfaces, microlenses are manufactured according to the following process. A first light transmitting film on which columnar projections are formed with a predetermined interval is formed on a semiconductor substrate. A second light transmitting film made of a material same as that of the first light transmitting film is laminated on a surface of the first light transmitting film, and a planar shape of the projection is enlarged to make a separation between the projections narrower. Argon ions are irradiated onto the second light transmitting film to round off a corner of the second light transmitting film, and thereby a lens is formed.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The invention relates to a method for manufacturing a microlens and a method for manufacturing a solid-state image pickup device equipped with microlenses.  
         [0003]     2. Description of the Related Art  
         [0004]     Recently, demand for higher pixel density is high in CCD image pickup devices and CMOS image pickup devices. A simple increase in the number of pixels results in an increase in a size of the image pickup device. However, in a small-size image pickup device that is incorporated in a mobile device such as a portable telephone, an increase in the size of the image pickup device cannot be permitted. Accordingly, in a small-size image pickup device, the higher pixel density is realized by making the area of each light receiving pixel smaller.  
         [0005]     When an area of each of light receiving pixels is made smaller, since an area receiving light corresponding to a subject becomes smaller, the sensitivity of the image pickup device deteriorates. As a countermeasure to this, a configuration where a microlens is formed for each light receiving pixel of the image pickup device is known. Since the microlens can condense light from an area larger than an area of the light receiving pixel on the corresponding light receiving pixel to generate information electric charges corresponding to an amount of condensed light, the sensitivity of the image pickup device can be improved.  
         [0006]     In an image pickup device provided with the microlenses, in order to improve the sensitivity, it is necessary to form microlenses having wide light receiving areas to efficiently make use of light incident on the image pickup device. However, it has been difficult to efficiently form microlenses with wide light receiving surfaces on a substrate on which light receiving pixels are formed.  
       SUMMARY OF THE INVENTION  
       [0007]     In this regard, the invention provides a method for manufacturing a microlens having a wide light receiving surface and a method for manufacturing an image pickup device provided with microlenses having wide light receiving surfaces.  
         [0008]     The manufacturing method according to the invention includes; forming a first light transmitting film with projections formed at a predetermined separation on a substrate; forming a second light transmitting film on the first light transmitting film; and irradiating gas ions toward the second light transmitting film. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]      FIG. 1A  is a sectional view showing a state where a first light transmitting film is formed in the forming microlenses in a first embodiment.  
         [0010]      FIG. 1B  is a sectional view showing a state where a mask is formed with a photoresist in the forming microlenses in the first embodiment.  
         [0011]      FIG. 1C  is a sectional view showing a state where the first light transmitting film is etched in the forming microlenses in the first embodiment.  
         [0012]      FIG. 2A  is a sectional view showing a state after the photoresist is removed in the forming microlenses in the first embodiment.  
         [0013]      FIG. 2B  is a sectional view showing a state where a second light transmitting film is formed in the forming microlenses in the first embodiment.  
         [0014]      FIG. 2C  is a sectional view showing a lens shape obtained by irradiating gas ions in the forming microlenses in the first embodiment.  
         [0015]      FIG. 3A  is a plan view in a state where projections of the first light transmitting film are formed in the forming microlenses in the first embodiment.  
         [0016]      FIG. 3B  is a plan view showing a state where a second light transmitting film is formed in the forming microlenses in the first embodiment.  
         [0017]      FIG. 3C  is a plan view showing a state after the gas ions are irradiated in the forming microlenses in the first embodiment.  
         [0018]      FIG. 4A  is a sectional view showing a state where the photoresist is removed in the forming microlenses in a second embodiment.  
         [0019]      FIG. 4B  is a sectional view showing a state where a second light transmitting film is formed in the forming microlenses in the second embodiment.  
         [0020]      FIG. 4C  is a sectional view showing a lens shape obtained by irradiating gas ions in the forming microlenses in the second embodiment. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0021]      FIGS. 1A, 1B ,  1 C,  2 A,  2 B and  2 C are diagrams for explaining steps in the forming an image pickup device provided with microlenses in a first embodiment according to the invention. Firstly, as shown in  FIG. 1A , a first light transmitting film  12  is formed on a surface of a semiconductor substrate  10 . Here, a plurality of light receiving pixels is formed on a surface of the semiconductor substrate  10 . The light receiving pixels can be formed according to a well-known manufacturing method. The first light transmitting film  12  is made of a light transmitting material; for instance, a silicon nitride film or a silicon oxide film can be used. The first light transmitting film  12  can be formed by use of various kinds of deposition technologies such as a CVD (Chemical Vapor Deposition) method, a PVD (Physical Vapor Deposition) method and so on.  
         [0022]     In the next place, as shown in  FIG. 1B , at a position where a lens is being formed on a surface of the first light transmitting film  12 , a photoresist film  16  that becomes a mask to the etching mentioned below is formed. When a plurality of light receiving pixels is formed on the semiconductor substrate  10  as in the configuration described here, a photoresist film  16  is formed on a top of each of positions where the plurality of light receiving pixels is formed. After a photoresist is coated on the first light transmitting film, the photoresist is patterned with an exposing device to form photoresist films  16  at positions corresponding to the lenses.  
         [0023]     Then, as shown in  FIG. 1C , the first light transmitting film  12  on which the photoresist films  16  are formed is etched. The etching process may be any of a dry process and a wet process. An amount of etching can be determined in accordance with a necessary height of a lens. In a first embodiment, with the dry etching process, etching is preferably applied only in a direction vertical to a surface of the semiconductor substrate  10 .  
         [0024]     Subsequently, as shown in  FIG. 2A , the photoresist films  16  are removed. Thus, in the first light transmitting film  12 , pillar projections  14  are formed. A shape of the projection  14  can be determined in conformity with a shape of a plurality of light receiving pixels formed on the semiconductor substrate  10 . For instance, when the light receiving pixel is formed in a rectangular shape, the projection  14  is preferably formed into a rectangular parallelepiped.  
         [0025]     In the next place, as shown in  FIG. 2B , a second light transmitting film  18  is formed on the first light transmitting film  12  provided with the projection  14 . The second light transmitting film  18  is formed, by use of a CVD method, with a substantially uniform thickness, on an exposed surface of the first light transmitting film  12  provided with the projections  14 . In the formation of the second light transmitting film  18 , other than the CVD method, any deposition method that can form a film with a substantially uniform thickness on an exposed surface can be used. The second light transmitting film  18  is preferably formed of a light transmitting material same as that of the first light transmitting film  12 . When the first light transmitting film  12  is formed of a silicon nitride film, the second light transmitting film  18  is also formed of a silicon nitride film.  
         [0026]     Thereafter, gas ions are irradiated onto the second light transmitting film  18  having projections corresponding to the projections  14  formed on the semiconductor substrate  10 . The gas ions are irradiated with an intention of rasping off corners of the projections. Here, the gas ions are preferably ions of an inert gas. As the inert gas ions, argon ions can be used; however, other inert gas ions may be used. When argon ions are irradiated on the first and second light transmitting films  12  and  18 , an argon ion plasma is generated, and an electric field is applied to the generated plasma to allow the argon ions to irradiate (bombard) the second light transmitting film  18 . At this time, the kinetic energy of the argon ions is controlled in its magnitude so that bonds of surface atoms or molecules of the second light transmitting film  18  may be broken to allow recombining with other atoms or molecules in an irradiation direction (so that the surface atoms or molecules may move only toward the proximity of the projection  14 ).  
         [0027]     In the argon ion-irradiated first and second light transmitting films, as shown in  FIG. 2C , the corners of the second light transmitting film  18  formed on the projection  14  are rasped off and the rasped portion is displaced in the proximity of the projection. Thus, a curved portion is formed on the second light transmitting film  18  on the projection  14 , and thereby the first and second light transmitting films combine to form a lens.  
         [0028]     By undergoing a step of irradiating gas ions, in a portion of the second light transmitting film  18  where the projection  14  is not located as well, a curved portion is extended, and thereby a lens with a wide light receiving surface can be efficiently formed.  
         [0029]     After the projections  14  are formed on the first light transmitting film, the argon ions are irradiated to rasp off corners of the projections  14 , whereby microlenses can be formed as well. In this case, in order to form a lens with a wide light receiving surface, a distance W between the projections  14  is necessary to be designed optimally so as to bring adjacent lenses into contact with each other. However, the distance W between the projections  14  is restricted by exposure technology.  
         [0030]     On the other hand, in the first embodiment, the second light transmitting film is formed on the first light transmitting film  12  provided with the projections  14 . Accordingly, a distance W′ between adjacent projections can be made smaller than a distance W between the projections  14 . At this time, by controlling a film thickness of the second light transmitting film, the distance W′ between adjacent projections can be controlled. Accordingly, in a lens formed by combining the first and second light transmitting films, a light receiving surface can be made larger by irradiating the argon plasma, resulting in improving the sensitivity of an image pickup device.  
         [0031]     In the first embodiment, a rectangular parallelepiped projection  14  is formed on a rectangular light receiving pixel. However, the method is not restricted thereto. For instance, in the case of the light receiving pixel having a hexagonal shape, when a projection  14  having a hexagonal columnar shape is formed, a lens with a wide light receiving surface in accordance with a shape of the light receiving pixel can be efficiently formed.  
         [0032]      FIGS. 3A, 3B  and  3 C, respectively, are plan views showing a manufacturing step of microlenses according to the first embodiment. In  FIG. 3A , projections  14  formed on a first light transmitting film are shown. The projections  14  are formed on tops of a plurality of light receiving pixels formed on a surface of a semiconductor substrate  10 . A sectional diagram along a line X-X′ in the drawing corresponds to  FIG. 2A .  FIG. 3B  shows a state where a second light transmitting film  18  is formed on the first light transmitting film  12  on which the projections  14  are formed. The second light transmitting film  18  is formed with a substantially uniform film thickness on an exposed surface of the first light transmitting film  12  on which the projections  14  are formed. A sectional diagram along a Y-Y′ line in the drawing corresponds to  FIG. 2B .  FIG. 3C  shows a state after the gas ions are irradiated onto the second light transmitting film  18 . Thus, a curved portion is formed in the second light transmitting film  18  on the projection  14 , and thereby the first and second light transmitting films  12  and  18  formed in a lens shape can be obtained. A sectional diagram along a Z-Z′ line in the drawing corresponds to  FIG. 2C .  
         [0033]      FIGS. 4A, 4B  and  4 C, respectively, are diagrams for explaining steps for forming an image pickup device provided with microlenses in a second embodiment according to the invention. As shown in  FIG. 4A , a first light transmitting film  12  on which projections  14  are formed is formed on a semiconductor substrate  10 . A second embodiment is different from the first embodiment in a point in that projections  14  are formed in taper. A taper-like projection  14  in the second embodiment can be formed by applying the wet etching or the dry etching to the first light transmitting film on which a photoresist film shown in  FIG. 1B  is formed.  
         [0034]     Later steps are similar to that of the first embodiment. As shown in  FIG. 4B , a second light transmitting film  18  is formed on the first light transmitting film  12  on which the taper-like projections  14  are formed. The second light transmitting film  18  is formed with a substantially uniform film thickness on an exposed surface of the first light transmitting film  12  on which the taper-like projections  14  are formed. In the next place, the gas ions are irradiated to the second light transmitting film  18 . Thus, as shown in  FIG. 4C , a curved portion is formed on the second light transmitting film  18  on the taper-like projection  14 , and thereby the first and second light transmitting films are formed into a lens shape.  
         [0035]     In the second embodiment, when the projection  14  is formed to be taper-like, a curvature of the curved portion of the first and second light transmitting films  12  and  18  that are formed in a lens shape can be controlled. Accordingly, a lens having a desired curvature can be efficiently formed.  
         [0036]     As described above according to the embodiments, according to the invention, a microlens having a wide light receiving surface can be efficiently formed and thereby light incident on a image pickup device can be efficiently utilized; accordingly, the sensitivity of the image pickup device can be improved.