Abstract:
The present invention provides a method for manufacturing a microlens in a semiconductor substrate having a first surface and a second surface, comprising the steps of preparing the semiconductor substrate, forming a first resist layer approximately cylindrical in form on the first surface of the semiconductor substrate, reflowing the first resist layer by heat treatment while holding the semiconductor substrate in such a manner that the first surface is normal to a vertical line and placed below the second surface, thereby to deform the first resist layer into a second resist layer approximately hemispherical in form, and simultaneously etching the second resist layer and the semiconductor substrate by means of anisotropic etching to form the corresponding lens in the semiconductor substrate.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a method for manufacturing a microlens formed in a semiconductor substrate. 
     A microlens has been greatly used in optical devices such as an optical communication connector, a CCD (Charge Coupled Device) image pickup element, etc. Since the microlens is small in shape, a manufacturing method such as a reflow method, a gray-scale mask method or the like, which is different from grinding or molding used in the normal lens manufacture, has generally been used. The reflow method will now be explained in brief. First, a resist pattern approximately cylindrical in form is formed on a bedding or base substrate by photolithography. Thereafter, the substrate is heated to fluidize a resist, whereby a resist pattern approximately hemispherical in form is formed by surface tension. Subsequently, such a resist pattern is etched by anisotropic dry etching simultaneously with the base substrate, and the pattern approximately hemispherical in form is transferred onto the base substrate, whereby a microlens is fabricated. 
     The inventions related to microlens manufacturing methods have been described in, for example, patent documents 1 (Japanese Unexamined Patent Publication No. Hei 5(1993)-136460 (refer to the 4th page and FIG. 2) and 2 (Japanese Unexamined Patent Publication No. Hei 7(1995)-106237)(refer to the fourth page and FIGS. 4 through 7). 
     According to the microlens manufacturing method described in Japanese Unexamined Patent Publication No. Hei 5(1993)-136460, a resist pattern approximately hemispherical in form, which is formed on a base substrate (GaAs) by potting, is used as an etching mask, and the resist pattern and the base substrate are simultaneously etched to fabricate a microlens (refer to a second embodiment). 
     According to the microlens manufacturing method described in Japanese Unexamined Patent Publication No. Hei 7(1995)-106237, a plurality of first resist patterns formed on a base substrate in zigzags are deformed into hemispherical form by heat treatment. Thereafter, a plurality of hemispherical second resist patterns are further formed in spaces defined among the thermosetted or heat-cured first resist patterns. Thus, a CCD converging microlens whose lens-to-lens distance has been shortened is fabricated. 
     In the reflow method, the thickness of the post-reflow resist pattern, i.e., the thickness of its top portion approximately hemispherical in form, and the applied thickness of resist prior to the formation of the pattern by photolithography are placed in a predetermined proportionality relation. The thickness of the post-reflow resist pattern has heretofore been controlled by adjusting the applied thickness of resist. 
     With miniaturization of a lens and an improvement in its precision, however, a mere adjustment to the applied thickness of resist is becoming difficult to form a resist pattern necessary for desired lens formation. This is because demands contradictory to each other with respect to the resist thickness exist in two processes necessary for manufacture of a microlens, i.e., a photolithography process and a dry etching process respectively. In terms of processing of the resist by photolithography, there is a need to make thin the applied thickness of resist for the purpose of lens miniaturization. This is because optical transmittance is reduced upon exposure when the applied thickness of resist is thick, and a development remainder, so-called scum is apt to occur upon development. On the other hand, there is a need to ensure a predetermined thickness as the thickness of the resist pattern approximately hemispherical in form, which serves as the etching mask, in terms of the formation of the lens pattern by dry etching. It cannot be said that the thinning of the resist is a desirable direction. Thus, there is a need to increase the thickness of the resist pattern approximately hemispherical in form, which serves as the etching mask in the dry etching process, while meeting the demand for thinning of the applied thickness of resist in the photolithography process. 
     The microlens manufacturing method described in Japanese Unexamined Patent Publication No. Hei 5(1993)-136460 needs not to perform the photolithography-based resist processing necessary for the normal reflow method because the resist pattern approximately hemispherical in form is formed by potting. Therefore, a description about the adjustment to the thickness of the post-reflow resist is not especially shown in Japanese Unexamined Patent Publication No. Hei 5(1993)-136460. 
     The microlens manufacturing method described in Japanese Unexamined Patent Publication No. Hei 7(1995)-106237 has the fear that since the resist pattern approximately hemispherical in form is formed by the reflow method, a problem about such a resist thickness as already mentioned above arises with the miniaturization of the lens. However, the adjustment to the thickness of the post-reflow resist is not described in Japanese Unexamined Patent Publication No. Hei 7(1995)-106237 in particular. 
     SUMMARY OF THE INVENTION 
     With the foregoing in view, it is therefore an object of the present invention to provide a method for manufacturing a microlens, which is capable of increasing the thickness of a post-reflow resist while the applied thickness of resist is being thinned. 
     According to one aspect of the present invention, for attaining the above object, there is provided a method for manufacturing a microlens formed in a semiconductor substrate having a first surface and a second surface, comprising the steps of preparing the semiconductor substrate, forming a first resist layer approximately cylindrical in form on the first surface of the semiconductor substrate, reflowing the first resist layer by heat treatment while holding the semiconductor substrate in such a manner that the first surface is normal to a vertical line and placed below the second surface, thereby to deform the first resist layer into a second resist layer approximately hemispherical in form, and simultaneously etching the second resist layer and the semiconductor substrate by means of anisotropic etching to form the corresponding lens in the semiconductor substrate. 
     According to the microlens manufacturing method according to the present invention, when the first resist layer approximately cylindrical in form is reflowed, it is heat-treated with the first and second surfaces of the semiconductor substrate being turned upside down, i.e., the first surface of the semiconductor substrate formed with the first resist layer being directed downward vertically. Consequently, the fluidized second resist layer approximately hemispherical in form is pulled down by gravitation, so that its thickness increases. Thus, a predetermined thickness of the second resist layer that serves as an etching mask can be ensured while the applied thickness of resist is thinned. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       While the specification concludes with claims particularly pointing out and distinctly claiming the subject matter which is regarded as the invention, it is believed that the invention, the objects and features of the invention and further objects, features and advantages thereof will be better understood from the following description taken in connection with the accompanying drawings in which: 
         FIGS. 1A through 1E  are production process views of a microlens according to a first embodiment of the present invention; and 
         FIGS. 2A through 2E  are production process views of a microlens according to a second embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Preferred embodiments of the present invention will hereinafter be described in detail with reference to the accompanying drawings. 
     First Preferred Embodiment 
       FIGS. 1A through 1E  are process sectional views showing a method for manufacturing a microlens, according to a first embodiment of the present invention. 
     As shown in  FIG. 1A , a semiconductor substrate  101  for fabricating the microlens is first prepared. The semiconductor substrate  101  is a silicon substrate, for example. 
     Next, as shown in  FIG. 1B , a resist is applied onto the semiconductor substrate  101  to form a resist pattern  102  shaped in the form of a substantially circular cylinder by using the known photolithography. Now, the applied thickness of the resist is suitably set according to the size of a formed lens. When, however, a lens having a diameter having approximately 100 μm, for example is formed, the applied thickness of the resist may be set to approximately 10 to 50 μm. 
     Next, as shown in  FIG. 1C , the semiconductor substrate  101  formed with the resist pattern  102  is placed on a hot plate  103  and turned upside down, i.e., so that a surface thereof with the resist pattern  102  formed thereon being directed downward vertically. Then, thermal treatment is effected thereon at a glass transition temperature or higher of the resist. Thus, the resist pattern  102  approximately cylindrical or columnar in form is reflowed to deform into a resist pattern  104  shaped in the form of a substantially hemisphere. Thermal treatment conditions are set to 300 seconds at 160° C. -180° C., for example, and may suitably be set according to resist materials to be used. Since the fluidized resist pattern  104  is pulled down by gravitation in the present thermal treatment process, its thickness can be increased. 
     Next, as shown in  FIG. 1D , the resist pattern  104  and the semiconductor substrate  101  are simultaneously etched by anisotropic dry etching such as Reactive Ion Etching (RIE). The etching executed here makes use of, for example, a mixed gas of CF 4  (tetrafluoromethane) and O 2  as an etching gas. If a mixture ratio of CF 4  and O 2  is adjusted in the present etching process in such a manner that a selective ratio of the resist pattern  104  and the semiconductor substrate  101  reaches approximately 1, then the resist pattern  104  and the semiconductor substrate  101  are etched at approximately equal rates so that the resist pattern  104  approximately hemispherical in form is transferred onto the semiconductor substrate  101 . 
     The etching further proceeds and a microlens  100  having substantially the same curvature as the shape of the resist pattern  104  is completed as shown in  FIG. 1E  in a state in which the resist pattern  104  has perfectly been removed. 
     Incidentally, the present embodiment is not limited to the manufacture of the microlens. When it is necessary to form a substantially hemispherical pattern similar to the microlens, the present embodiment can also be applied to other semiconductor devices or MEMS (Micro Electro Mechanical Systems) or the like. 
     According to the microlens manufacturing method according to the first embodiment, when the resist pattern  102  approximately cylindrical in form is reflowed, it is heat-treated by means of the hot plate with the surface formed with the resist pattern  102  being directed downward vertically, so that the fluidized resist pattern  104  approximately hemispherical in form is pulled down by gravitation, thereby leading to an increase in its thickness. Thus, since the predetermined thickness of the resist pattern  104  that serves as an etching mask can be ensured while the applied thickness of resist is being thinned, a fine and high-precision microlens can be fabricated. Since the fine and high-precision microlenses can be manufactured, high integration of a CCD lens array using those is also enabled. 
     Second Preffered Embodiment 
       FIGS. 2A through 2E  are process sectional views showing a method for manufacturing a microlens, according to a second embodiment of the present invention. 
     As shown in  FIG. 2A , a semiconductor substrate  201  for fabricating the microlens is first prepared. The semiconductor substrate  201  is a silicon substrate, for example. 
     Next, as shown in  FIG. 2B , a resist is applied onto the semiconductor substrate  201  to form a resist pattern  202  approximately cylindrical in form by using the known photolithography. Now, the applied thickness of the resist is suitably set according to the size of a formed lens. When, however, a lens having a diameter of approximately 100 μm, for example is formed, the applied thickness of the resist may be set to approximately 10 to 50 μm. 
     Next, as shown in  FIG. 2C , the semiconductor substrate  201  formed with the resist pattern  202  is fixed to a predetermined support device  203  and turned upside down, i.e., so that a surface thereof with the resist pattern  202  formed thereon being directed downward vertically. Then, thermal treatment is effected thereon at a glass transition temperature or higher of the resist by the radiation of energy lines such as infrared rays. Thus, the resist pattern  202  approximately cylindrical in form is reflowed to deform into a resist pattern  204  approximately hemispherical in form. Incidentally, an electron beam is used in place of the use of the infrared rays as the energy lines for heat treatment. Since the fluidized resist pattern  204  is pulled down by gravitation in the present thermal treatment process, its thickness can be increased. 
     Next, as shown in  FIG. 2D , the resist pattern  204  and the semiconductor substrate  201  are simultaneously etched by anisotropic dry etching such as Reactive Ion Etching (RIE). The etching executed here makes use of, for example, a mixed gas of CF 4  and O 2  as an etching gas. If a mixture ratio of CF 4  and O 2  is adjusted in the present etching process in such a manner that a selective ratio of the resist pattern  204  and the semiconductor substrate  201  reaches approximately 1, then the resist pattern  204  and the semiconductor substrate  201  are etched at approximately equal rates so that the resist pattern  204  approximately hemispherical in form is transferred onto the semiconductor substrate  201 . 
     The etching further proceeds and a microlens  200  having substantially the same curvature as the shape of the resist pattern  204  is completed as shown in  FIG. 2E  in a state in which the resist pattern  204  has perfectly been removed. 
     Incidentally, the present embodiment is not limited to the manufacture of the microlens. When it is necessary to form an approximately hemispherical pattern similar to the microlens, the present embodiment can also be applied to other semiconductor devices or MEMS or the like. 
     The microlens manufacturing method according to the second embodiment can bring about an advantageous effect similar to that of the microlens manufacturing method according to the first embodiment. That is, when the resist pattern  202  approximately cylindrical in form is reflowed, heat treatment is effected thereon by radiation of the energy lines such as the infrared rays with the surface formed with the resist pattern  202  being directed downward vertically, so that the fluidized resist pattern  204  approximately hemispherical in form is pulled down by gravitation, thereby leading to an increase in its thickness. Thus, since the predetermined thickness of the resist pattern  204  that serves as an etching mask can be ensured while the applied thickness of resist is being thinned, a fine and high-precision microlens can be fabricated. Since the fine and high-precision microlenses can be manufactured, high integration of a CCD lens array using those is also enabled. 
     While the preferred forms of the present invention have been described, it is to be understood that modifications will be apparent to those skilled in the art without departing from the spirit of the invention. The scope of the invention is to be determined solely by the following claims.