Patent Publication Number: US-8993046-B2

Title: Method for fabricating image sensors

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of U.S. application Ser. No. 12/837,637, filed Jul. 16, 2010, now U.S. Pat. No. 8,324,701, the entireties of which are incorporated by reference herein. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to an image sensor, and more particularly to an image sensor with a disconnected color filter structure and fabrication method thereof. 
     2. Description of the Related Art 
     An image sensor, as a kind of semiconductor device, transforms optical images into electrical signals. Image sensors can be generally classified into charge coupled devices (CCDs) and complementary metal oxide semiconductor (CMOS) image sensors. Among these image sensors, a CMOS image sensor comprises a photodiode for detecting incident light and transforming it into electrical signals, and logic circuits for transmitting and processing the electrical signals. 
     A conventional method for manufacturing a CMOS image sensor comprising microlenses is hereinafter described referring to  FIGS. 1A to 1C . 
     First, referring to  FIG. 1C , a conventional CMOS image sensor is provided. The CMOS image sensor comprises a light sensing part  13  comprising a photodiode  11  for accepting incident light, and for generating and accumulating electric charges, a protecting layer  21  formed on a structure of the light sensing part  13 , a color filter array  23 , and a plurality of microlenses  27 . 
     In a conventional method of manufacturing such a structured CMOS image sensor, as shown in  FIG. 1A , the protecting layer  21  with a silicon nitride base is formed on a semiconductor substrate  10  that comprises the light sensing part  13  comprising the photodiodes  11 . Then, as shown in  FIG. 1B , the color filter array  23  (with a connected color filter structure) is formed on the protecting layer  21 . Here, the color filter array  23  is formed in a primary color system, i.e., comprising a red filter (R), a green filter (G), and a blue filter (B), using photoresist materials containing a red, green, or blue pigment, respectively. Formation of each color filter involves a series of coating, exposure and development processes according to photolithography techniques. Alternatively, the color filter array  23  can be formed in a complementary color system comprising cyan, yellow, and magenta filters. 
     Then, as shown in  FIG. 1C , a photoresist layer is applied, exposed, and developed on the color filter array  23 ; thus forming a plurality of photoresist patterns. These photoresist patterns are then thermally reflowed and cured to form lenses; thus resulting in a plurality of microlenses  27 . 
     According to the conventional method, the microlenses  27  are formed with a distant from each other by about 0.2 μm to 0.5 μm (due to the connected color filter structure), for the purpose of preventing formation of bridges between the microlenses  27  during the curing and reflowing processes of corresponding photoresist patterns. However, a gap between the microlenses  27  results, such that at least some light incident between the microlenses  27  is loss, and resolution of color signals may be decreased to a level that is less than optimal due to oblique light incident to adjacent pixels. 
     Additionally, the microlenses  27  are formed by a coating, photolithography and thermal process. Thus, the materials of the microlenses  27  are limited to only photo type materials. 
     BRIEF SUMMARY OF THE INVENTION 
     One embodiment of the invention provides an image sensor comprising a pixel sensor, a protecting layer formed on the pixel sensor, a color filter array comprising a plurality of color filters formed on the protecting layer, wherein two adjacent color filters have a gap therebetween (e.g., a disconnected color filter structure), and a gapless microlens array comprising a plurality of microlenses formed on the color filter array. 
     The protecting layer comprises silicon nitride. The color filter in the plurality of color filters comprises a first gap along a row direction, a second gap along a column direction and a third gap along a diagonal direction. The first gap is similar to the second gap. The third gap is larger than the first gap and the third gap is larger than the second gap. The color filter is polygonal or rectangular. The microlens comprises photoresist or thermoplastic resins. The microlens on the color filter has a height determined by the first gap, the second gap and the third gap. The microlens has a slope determined by the first gap, the second gap and the third gap. 
     In the disclosed image sensor, the disconnected color filter structure improves light sensitivity to and condensing efficiency of incident light. Since the light sensitivity can be improved and an oblique light incident to adjacent pixels can be reduced or prevented, it is possible to realize clearer images using an image sensor, for example a CMOS image sensor, manufactured according to the invention. 
     One embodiment of the invention provides a method for fabricating an image sensor comprising providing a pixel sensor, forming a protecting layer on the pixel sensor, forming a color filter array comprising a plurality of color filters on the protecting layer, wherein two adjacent color filters have a gap therebetween, coating a transparent material on the color filter array, and hardening the transparent material to form a gapless microlens array comprising a plurality of microlenses. 
     The protecting layer comprises silicon nitride. The color filter array is formed by a photolithography process. The color filter in the plurality of color filters comprises a first gap along a row direction, a second gap along a column direction and a third gap along a diagonal direction. The first gap is similar to the second gap. The third gap is larger than the first gap and the third gap is larger than the second gap. The transparent material comprises photoresist or thermoplastic resins. The transparent material is hardened by a thermal reflow and curing process. The microlens on the color filter has a height determined by the first gap, the second gap and the third gap. The microlens on the color filter has a slope determined by the first gap, the second gap and the third gap. 
     In the disclosed fabrication method, a gapless microlens array is formed by mere coating and a thermal process without an additional photolithography process. Thus, the materials of the microlenses are not limited to photo type materials. Alternatively, non-photo type materials may also be used, improving process window. Additionally, in accordance with various process requirements (for example formation of various distances from the microlens to the diode), the microlens profile (e.g., height and curve) may be easily altered by adjusting the gap size. 
     One embodiment of the invention provides an image sensor comprising a pixel sensor, a color filter array comprising a plurality of color filters formed on the pixel sensor, wherein two adjacent color filters have a gap therebetween and the color filter in the plurality of color filters comprises a first gap along a row direction, a second gap along a column direction and a third gap along a diagonal direction, and a gapless microlens array comprising a plurality of microlenses formed on the color filter array, wherein the microlens on the color filter has a height and a slope determined by the first gap, the second gap and the third gap. 
     A detailed description is given in the following embodiments with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention can be more fully understood by reading the subsequent detailed description and examples with references made to the accompanying drawings, wherein: 
         FIGS. 1A to 1C  are cross-sectional views illustrating a conventional method for manufacturing a CMOS image sensor; 
         FIGS. 2A to 2D  are cross-sectional views illustrating one embodiment of a method for manufacturing an image sensor according to the invention. 
       FIG.  2 B′ is a top view of  FIG. 2B . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The following description is of the best-contemplated mode of carrying out the invention. This description is made for the purpose of illustrating the general principles of the invention and should not be taken in a limiting sense. The scope of the invention is best determined by reference to the appended claims. 
     One embodiment of an image sensor according to the invention is described with reference to FIGS.  2 D and  2 B′. 
     Referring to  FIG. 2D , an image sensor comprises a pixel sensor  100 , a protecting layer  201 , a color filter array  203  comprising a plurality of color filters and a gapless microlens array  207  comprising a plurality of microlenses. The protecting layer  201  is formed on the pixel sensor  100 . The color filter array  203  is formed on the protecting layer  201 . The gapless microlens array  207  is formed on the color filter array  203 . Specifically, in the color filter array  203 , two adjacent color filters  203 ′ have a gap (X 1 , Y 1  and D 1 ) therebetween (e.g., a disconnected color filter structure), as shown in FIG.  2 B′. 
     The protecting layer  201  may comprise silicon nitride. The gap X 1  between two adjacent color filters  203 ′ along a row direction is substantially similar to the gap Y 1  between two adjacent color filters  203 ′ along a column direction in the color filter array  203 . The gap D 1  between two adjacent color filters  203 ′ along a diagonal direction is substantially larger than the gap X 1  between two adjacent color filters  203 ′ along a row direction or the gap Y 1  between two adjacent color filters  203 ′ along a column direction in the color filter array  203 . The color filter  203 ′ is alternatively polygonal or rectangular. For instance, determination of the gap D 1 , is dependent upon such polygonal color filters. The color filter array  203  may comprise a primary color system, i.e., comprising a red filter (R), a green filter (G), and a blue filter (B). Alternatively, the color filter array  203  may comprise a complementary color system comprising cyan, yellow, and magenta filters. The microlens may comprise photoresist or thermoplastic resins. The microlens has a height substantially determined by the sizes of the gaps X 1 , Y 1  and D 1 . The microlens has a curve substantially determined by the sizes of the gaps X 1 , Y 1  and D 1 . 
     In the disclosed image sensor, the disconnected color filter structure improves light sensitivity to and condensing efficiency of incident light. Since the light sensitivity can be improved and an oblique light incident to adjacent pixels can be reduced or prevented, it is possible to realize clearer images using an image sensor, for example a CMOS image sensor, manufactured according to the invention. 
     Hereinafter, one embodiment of a manufacturing method for a CMOS image sensor according to the invention is described with reference to  FIGS. 2A to 2D . 
     Referring to  FIG. 2A , a protecting layer  201  with a silicon nitride base (e.g., which may comprise silicon nitride) is formed on a pixel sensor  100 . 
     Next, as shown in  FIG. 2B , a color filter array  203  comprising a plurality of color filters is formed on the protecting layer  201 . Here, the color filter array  203  may comprise a primary color system, i.e., comprising a red filter (R), a green filter (G), and a blue filter (B), using a photoresist material containing a red, green, and blue pigment, respectively. Formation of each color filter may involve performing, at least three times, the photolithography process, which comprises coating, exposure and development of each individual photoresist material. Alternatively, the color filter array  203  is formed by methods such as inject printing. Specifically, in the color filter array  203 , two adjacent color filters form a gap therebetween, such as, formation of the disconnected color filter structure, as shown in FIG.  2 B′. FIG.  2 B′ is a top view of  FIG. 2B . In FIG.  2 B′, the gap between two adjacent color filters  203 ′ along a row direction represents X 1  gap. The gap between two adjacent color filters  203 ′ along a column direction represents Y 1  gap. The gap between two adjacent color filters  203 ′ along a diagonal direction represents D 1  gap. Alternatively, X 1  gap is similar to Y 1  gap. D 1  gap is larger than X 1  gap and Y 1  gap. Additionally, the color filters  203 ′ are alternatively polygonal or rectangular. For instance, determination of the gap D 1 , is dependent upon such polygonal color filters facilitate. This embodiment exemplifies the primary color filter system, however, the color filter array  203  can be alternatively formed in a complementary color system comprising cyan, yellow, and magenta filters. 
     Next, a transparent material  205  is coated on the color filter array  203 , as shown in  FIG. 2C . Then, as shown in  FIG. 2D , the transparent material  205  is hardened, for example by a thermal reflow and curing process, to form lenses having a desired curvature; thus resulting in a microlens array  207  comprising a plurality of microlenses. The formed microlens array  207  is gapless due to the disconnected color filter structure. In the microlens formation process, no photolithography process is required. Thus, in addition to photo type materials such as photoresist, non-photo type materials such as thermoplastic resins may be alternatively used. Furthermore, in order to meet the thermal reflow property, any thermal flowable materials with a melting point of about 120 to 180° C. are appropriate for use as the transparent material  205 . Additionally, the microlens  207  has a height substantially determined by the sizes of X 1 , Y 1  and D 1  gaps. The microlens  207  has a curve substantially determined by the sizes of X 1 , Y 1  and D 1  gaps. 
     In the disclosed fabrication method, the gapless microlens array is formed by a mere coating and a thermal process without an additional photolithography process. Thus, the materials of the microlenses are not limited to photo type materials. Alternatively, non-photo type materials may also be used, improving process window. Additionally, in accordance with various process requirements (for example formation of various distances from the microlens to the diode), the microlens profile (e.g., height and curve) may be easily altered by adjusting the gap size. 
     While the invention has been described by way of example and in terms of preferred embodiment, it is to be understood that the invention is not limited thereto. To the contrary, it is intended to cover various modifications and similar arrangements (as would be apparent to those skilled in the art). Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.