Abstract:
A method for fabricating an image sensor is provided. A substrate is provided, and then a plurality of photoresist patterns is formed on the substrate. The photoresist patterns are arranged in a first array and defined by a plurality of photomask patterns arranged as a photomask pattern array, wherein a top view of each photoresist pattern has a substantially square shape and a distance between two neighboring photoresist patterns decreases from a center of the first array toward an edge of the first array. Besides, each photomask pattern includes a transparent portion and an opaque portion, wherein an area proportion of the transparent portion included in a photomask pattern increases from the center toward the edge of the photomask pattern array. Then, a thermal reflow step is performed to convert the photoresist patterns into a plurality of microlenses arranged in a second array.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation application of U.S. patent application Ser. No. 12/548,670 filed on Aug. 27, 2009. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     This invention relates to a method for fabricating an image sensor, wherein the image sensor is applied in an electronic image recording apparatus. 
     2. Description of Related Art 
     Electronic image recording apparatuses, such as charge-coupled device (CCD) image recording apparatuses and complementary metal oxide semiconductor (CMOS) image recording apparatuses, have been widely used for image recording. As shown in  FIG. 1A , the core of such an image recording apparatus typically includes an image sensor chip  10  and lenses  12  disposed thereover. The sensor chip  10  is formed with photosensitizing devices (not shown) like CCDs or photodiodes. The lenses  12  are fit in a lens barrel  14 . The incident light  16  irradiates the chip  10  through the lenses  12 . 
     The chip  10  is schematically illustrated in  FIG. 1B  in a magnified view, having a photosensitizing plane  110  defined by the photosensitizing devices and an interconnect dielectric layer  120 . To improve the sensitivity of the photosensitizing devices, a planarization layer  130  and microlenses  140  with focusing capability are usually formed over the interconnect dielectric layer  120 . 
     However, as shown in  FIGS. 1A-1B , since the incident angle of the central incident light  16   a  is zero, the incident angle of non-central incident light  16   b  increases toward the edge of the chip  10  and the curvatures and the focal lengths of the respective microlenses  140  are the same, the distance between the focus position of the incident light  16   b  and the photosensitizing plane  110  increases toward the edge of the chip  10 . Thus, the sensitivity of the photosensitizing device decreases toward the chip edge, which is a cause of the distortion in image recording. 
     SUMMARY OF THE INVENTION 
     Accordingly, this invention provides a method for fabricating an image sensor to solve the problem of non-uniform sensitivity of the photosensitizing devices. 
     The method for fabricating an image sensor of this invention is described below. A substrate is provided, and then a plurality of photoresist patterns is formed on the substrate. The photoresist patterns are arranged in a first array, wherein a top view of each photoresist pattern has a substantially square shape and a distance between two neighboring photoresist patterns decreases from a center of the first array toward an edge of the first array. Then, a thermal reflow step is performed to convert the photoresist patterns into a plurality of microlenses arranged in a second array. 
     The focal length of microlens increases from the center of the array toward the edge of the array. Thus, all incident lights from the array center to the array edge can be focused at the photosensitizing plane, so that the sensitivity of the photosensitizing devices is uniformized. 
     In order to make the aforementioned and other objects, features and advantages of the present invention comprehensible, a preferred embodiment accompanied with figures is described in detail below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  illustrates the arrangement of the image sensor chip, the lenses and the lens barrel in an electronic image sensor in the prior art, and  FIG. 1B  schematically illustrates a magnified view of the image sensor chip. 
         FIG. 2  schematically illustrates the microlens array formed in a second case of a first embodiment and the paths of the incident lights at the array center and the array edge respectively. 
         FIG. 3  schematically illustrates the microlens array formed in a second embodiment and the paths of the incident lights at the array center and the array edge respectively. 
         FIG. 4A  schematically illustrates the photoresist pattern array as the precursor of the microlens array in a first case of the first embodiment of this invention, and  FIG. 4B  illustrate the IV-IV′ cross sections of the photoresist patterns in  FIG. 4A  and the vertical cross sections of the microlenses formed through the thermal reflow step. 
         FIG. 5A  schematically illustrates the photoresist pattern array as the precursor of the microlens array in the second case of the first embodiment of this invention, and  FIG. 5B  illustrate the V-V′ cross sections of the photoresist patterns in  FIG. 4A  and the vertical cross sections of the microlenses formed through the thermal reflow step. 
         FIGS. 6A and 6B  schematically illustrate the top views of two examples of the photomask pattern array defining the microlens array in the second case of the first embodiment of this invention. 
         FIGS. 7A ,  7 B and  7 C schematically illustrate the top views of the photomask patterns defining a central microlens and an edge microlens respectively according to three examples of the second embodiment of this invention. 
         FIGS. 8A and 8B  schematically illustrate the variations of the photoresist patterns as the precursor of the microlenses from the array center toward the edge according to two examples of a third embodiment of this invention. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     This invention is further explained with the following first to third embodiments, which are however not intended to restrict the scope of this invention. For example, the first sub-patterns and/or the second sub-patterns of a photoresist pattern for forming a chessboard-type microlens may have shapes other than those shown in  FIGS. 8A-8B . 
       FIG. 2  schematically illustrates the microlens array formed in the second case of the first embodiment and the paths of the incident lights at the array center and the array edge respectively. The microlens array formed in the first embodiment can be called a cushion-type microlens array, which is characterized in that each microlens therein has different curvatures between the X or Y direction and the diagonal direction. 
     As shown in  FIG. 2 , in the second case of the first embodiment, the height of the microlens  200  decreases from the array center toward the array edge, and the curvature of the same also decreases from the array center toward the array edge, so that the focal length of the same increases from the array center toward the array edge. Since the incident angle of the incident light increases from the array center toward the array edge ( 16   b &gt; 16   a ), all incident light passing the microlenses  200  from the array center to the array edge can be focused at the photosensitizing plane  110 . 
       FIG. 3  schematically illustrates the microlens array formed in the second embodiment and the paths of the incident lights at the array center and the array edge respectively. The microlens array formed in the second embodiment can be called a continuous-type microlens array, as described in U.S. patent application Ser. No. 11/970,936 filed on Jan. 8, 2008, being characterized in that any two neighboring microlenses therein are connected with each other and each microlens has substantially the same curvature in the vertical cross-sectional views of all directions. Any two neighboring photoresist patterns among the photoresist patterns as the precursor of the continuous-type microlenses are connected with or close to each other, so that any two neighboring microlenses are connected with each other. 
     As shown in  FIG. 3 , any two neighboring microlenses  300  are connected with each other. The heights of the microlenses  300  are substantially the same, but the curvature of the same decreases from the array center toward the array edge, so that the focal length of the same increases from the array center toward the array edge. Since the incident angle of the incident light increases from the array center toward the array edge ( 16   b &gt; 16   a ), all incident light passing the microlenses  300  from the array center to the array edge can be focused at the photosensitizing plane  110 . 
     In the first embodiment of this invention concerning the cushion-type microlens array, microlenses are formed by reflowing a plurality of separate photoresist patterns previously formed on the planarization layer. The top view of each photoresist pattern has a substantially square shape, so that each microlens has different curvatures in the X or Y direction and the diagonal direction. 
     Referring to  FIG. 4A  and the IV-IV′ cross-sectional view in  FIG. 4B , in the first case of the first embodiment, all the photoresist patterns  400  with a substantially square shape in the top view have the same height, but the area thereof increases from the array center to the array edge, so that the distance between two neighboring photoresist patterns  400  decreases from the array center to the array edge. Because the photoresist patterns  400  have the same height and the area thereof increases from the array center to the array edge, the microlenses  410  formed from the photoresist patterns  400  have the same height, and the curvature thereof decreases from the array center toward the array edge so that the focal length thereof increases from the array center toward the edge. Since the incident angle of the incident light also increases from the array center toward the array edge, all incident light passing the microlenses  410  from the array center to the array edge can be focused at the photosensitizing plane. Moreover, for the distance between two neighboring photoresist patterns  400  decreases from the array center to the array edge  110 , any two neighboring microlenses  410  apart from the array center by a distance larger than a certain value are connected with each other, and the thickness of the connection part gradually increases toward the array edge. 
     Referring to  FIG. 5A  and the V-V′ cross-sectional view in  FIG. 5B , in the second case of the first embodiment, all the photoresist patterns  500  have the same area, but the height thereof decreases from the array center to the array edge, so that the height and the curvature of the microlenses  200  formed from the photoresist patterns  500  decrease from the array center toward the array edge and the focal length increases from the array center toward the array edge. 
     In the second case of the first embodiment, the height decrease of the photoresist patterns  500  from the array center toward the array edge may be achieved by increasing the transparency of the photomask patterns defining the photoresist patterns  500  from the array center toward the array edge. The transparency increase may be achieved by including a transparent portion and an opaque portion in each photomask pattern and making the area proportion of the transparent portion in the photomask pattern increases from the array center toward the array edge. The area proportion of the transparent portion in a photomask pattern may be varied with the methods shown in  FIGS. 6A-6B . 
     Referring to  FIG. 6A , each photomask pattern  602  includes an opaque portion  604  constituted of a plurality of block opaque regions, and a transparent portion  606  constituted of a plurality of transparent line regions between the block opaque regions. The number of the transparent line regions of the photomask patterns  602  increases from the array center toward the edge, so that the area proportion of the transparent portion  606  in the photomask pattern  602  increases from the array center toward the array edge and the transparency of the photomask pattern  602  increases from the array center toward the array edge. 
     Referring to  FIG. 6B , each photomask pattern  612  includes an opaque portion  614 , and a transparent portion  616  constituted of a plurality of transparent dot regions in the opaque portion  614 . The number of the transparent dot regions in the photomask patterns  612  increases from the array center toward the edge, so that the area proportion of the transparent portion  616  in the photomask pattern  612  increases from the array center toward the edge and the transparency of the photomask pattern  612  increases from the array center toward the array edge. 
     On the other hand, in the second embodiment of this invention concerning the continuous-type microlens array, the photomask pattern for defining a microlens may have a transparency distribution where the transparency increases from the center of the photomask pattern toward the edge of the same. Such a transparency distribution may be made by disposing certain concentric transparent scattering rings. Three examples of the photomask patterns with transparent scattering rings are shown in  FIGS. 7A-7B . 
     Referring to  FIG. 7A , as compared to the edge photomask pattern  702 , the central photomask pattern  702  additionally has two smaller transparent scattering rings  706 , while the widths of the opaque portion  704  between the common transparent scattering rings  706  of them are substantially the same, so that the transparency increase rate from the center of the edge photomask pattern  702  toward the edge of the same is lower than that from the center of the central photomask pattern  702  toward the edge of the same. Hence, as compared with the case of the central photomask pattern  702 , the center-to-edge height difference of the photoresist pattern  708  defined by the edge photomask pattern  702  is smaller, so that the curvature of the corresponding microlens  300  is smaller. The variation of the curvature of the microlenses  300  from the array center to the array edge can be controlled by adjusting the number and widths of the additional smaller transparent scattering rings  706 . 
     Referring to  FIG. 7B , the central opaque portions  714  of the photomask patterns  712  from the array center to the array edge have the same diameter D 1 , while the width of the transparent scattering rings  716  decreases from the array center to the array edge (W 1 ′&lt;W 1 ) and the width of the annular opaque portions  714  between the scattering rings  716  increases from the array center to the edge. Thereby, the closer a photomask pattern  712  is to the array edge, the lower the transparency increase rate from its center to its edge. Thus, the self center-to-edge height difference of the photomask pattern  718  decreases from the array center to the edge, so that the curvature of the microlenses  300  formed from the photoresist patterns  718  by thermal reflow decreases from the array center to the edge. Meanwhile, the height of the connection portion between two neighboring photomask patterns  718  increases from the array center to the array edge. 
     Referring to  FIG. 7C , the central opaque portions  724  of the photomask patterns  722  from the array center to the array edge have the same diameter D 1  and the widths of the transparent scattering rings  726  are not varied, but the width of the annular opaque portions  724  between the scattering rings  726  increases from the array center to the edge (D 2 ′&gt;D 2 , D 3 ′&gt;D 3 ). Thereby, the closer a photomask pattern  722  is to the array edge, the lower the transparency increase rate from its center to edge. Thus, the self center-to-edge height difference of the photomask pattern  728  decreases from the array center to the array edge, so that the curvature of the microlenses  300  formed from the photoresist patterns  728  by thermal reflow decreases from the array center to the edge. Meanwhile, the height of the connection portion between two neighboring photomask patterns  728  increases from the array center to the array edge. 
       FIGS. 8A and 8B  schematically illustrate the variations of the photoresist patterns as the precursor of the microlenses from the array center toward the edge according to two examples of the third embodiment of this invention. The microlens formed in this embodiment can be called a chessboard-type microlens, which typically includes two first sub-microlenses arranged diagonally and two second sub-microlenses arranged diagonally. The shapes of the first and the second sub-microlenses before the thermal reflow, i.e., the shapes of the first and second sub-photoresist patterns as the precursors of the first and the second sub-microlenses, are different in the top view. 
     To make all incident light passing the microlenses from the array center to the array edge be focused at the photosensitizing plane, the focal length of such chessboard-type have to be increased from the array center to the edge. This may be achieved by increasing the area of the first or second sub-photoresist pattern from the array center to the array edge. 
     Referring to  FIG. 8A , a photoresist pattern  800  as the precursor of a chessboard-type microlens includes two first sub-patterns  802  arranged diagonally and two second sub-patterns  804  arranged diagonally. The first sub-patterns  802  are formed in a first lithography process, the second sub-patterns  804  are formed in a second lithography process, and the first sub-patterns  802  overlap with the second sub-patterns  804 . In the top view, a first sub-pattern  802  has a substantially circular shape, and a second sub-patterns  804  substantially has an octangular shape corresponding to a square shape that is cut at four corners thereof. 
     In the example of  FIG. 8A , the focal length of the chessboard-type microlenses is increased from the array center to the array edge in the following manner. The center-to-center distance between the two first sub-patterns  802  and that between the two second sub-patterns  804  in any photoresist pattern  800  are fixed. Meanwhile, the radius of the substantially circular first sub-pattern  802  is increased from the array center to the array edge (R′&gt;R), so that the area of the first sub-pattern  802  increases from the array center to the array edge. 
     In the example of  FIG. 8B , the focal length of the chessboard-type microlenses is increased from the array center to the array edge in the following manner. The center-to-center distance between the two first sub-patterns  812  and that between the two second sub-patterns  814  in any photoresist pattern  810  are fixed. Meanwhile, the area of the cut corners of the square shape corresponding to the second sub-pattern  814  is decreased from the array center to the array edge ( 2 D′ 2 &lt; 2 D 2 ), so that the area of the second sub-pattern  814  increases from the array center to the array edge. 
     Since the focal length of the microlenses formed in this invention increases from the center to the edge of the image sensor while the incident angle of the incident light increases from the center to the edge of the image sensor, all incident light from the center to the edge of the image sensor can be focused at the photosensitizing plane. Thus, the sensitivity of the photosensitizing devices can be uniformized, so that the distortion in image recording can be reduced as compared to the prior art. 
     This invention has been disclosed above in the preferred embodiments, but is not limited to those. It is known to persons skilled in the art that some modifications and innovations may be made without departing from the spirit and scope of this invention. Hence, the scope of this invention should be defined by the following claims.