Patent Publication Number: US-2005139945-A1

Title: Image sensor and method for fabricating the same

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
      This application claims the benefit of Korean Application No. P2003-101699 filed on Dec. 31, 2003, which is hereby incorporated by reference as if fully set forth herein.  
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to image sensors, and more particularly, to an image sensor, in which, instead of a traditional microlens array over the color filter array, a microlens pattern is arranged newly under a color filter array, which can substitute a function of the microlens array, to enable to shorten a total distance of travel of a converged light finally reaching to the photodiode, that improves intensity and focus of the light finally reaching to the photodiode array, to improve a low luminance performance of the image sensor completed finally, significantly; and a method for fabricating the same.  
      2. Discussion of the Related Art  
      Recently, as electric and electronic technologies are developed rapidly, a variety of electronic products having image sensor technologies applied thereto, such as video cameras, digital cameras, PCs with built-in miniature cameras, cellular phones with built-in miniature cameras, and so on, are developed, and spread widely.  
      Traditionally, though Charge Coupled Devices (CCD) have been used as the image sensors, because the CCD has many disadvantages in that a high driving voltage and an additional separate supporting circuit may be required, a process cost is high, and so on, presently it is a trend that use of the CCD is reduced, drastically.  
      Recently, as image sensors that can replace the CCD, Complementary Metal Oxide Semiconductor (CMOS) image sensors attract much interest. Different from the present CCD, because the CMOS image sensors are fabricated based on CMOS circuit technologies, the CMOS image sensors have advantages in that low voltage driving is possible, no additional supporting circuit is required, the process cost is low, and so on.  
      Referring to  FIG. 1 , such a related art image sensor, for an example, the CMOS image sensor, is provided with a microlens array  7  for converging a light from an exterior lens  100 , a color filter array  6  for converting the light converged by the microlens array  7  into a color light, a planarizing layer  5  on the color filter array  6  for planarizing a base of the microlens array  7  to induce uniform light transmission, a light transmission layer  4  for transmission of the light converted into the color light at the color filter layer  7  toward a photodiode array  3 , and the photodiode array  3  on an active region of a semiconductor substrate  1  defined by an active cell isolation layer  2 , for receiving the light passed through the light transmission layer  4 , to produce, and store photo charges.  
      In this instance, by using its own curvature, the microlens array  7  passes a light incident on a point p 1  in a straight line, and refracts lights incident on points p 2 , and p 3  at an angle, so that all the lights passed through the exterior lens  100  are focused on the photodiode array  3 .  
      As described before, in the related art image sensor, the light converged by the microlens array  7  is provided to the photodiode array  3  through the color filter array  6 , the light transmission layer  4 , and so on. That is, there is a substantial distance for the light converged at the microlens array  7  and reaching to the photodiode array  3 .  
      Thus, if there is a substantial distance between the microlens array  7  and the photodiode array  3 , an intensity and a focus of the light incident on the photodiode array  3  can not but be distorted in proportion to the distance, and as a result of this, a low luminance performance of the image sensor can not but be dropped, significantly.  
      Of course, if such a drop of low luminance performance of the image sensor is left as it is without taking any proper countermeasure, the image finally formed by the image sensor can not but have a substantially low quality.  
     SUMMARY OF THE INVENTION  
      Accordingly, the present invention is directed to an image sensor and a method for fabricating the same that substantially obviates one or more problems due to limitations and disadvantages of the related art.  
      An object of the present invention is to provide an image sensor having a microlens pattern under a color filter array, for minimizing a total distance of travel by focused light to a photodiode array.  
      Another object of the present invention is to provide an image sensor that minimizes the distance-proportional distortion in light intensity and focus to the photodiode array, and that may have improved low luminance performance.  
      Another object of the present invention is to provide an image sensor having a microlens pattern under a color filter array, that may provide some protection for the microlens array.  
      A further object of the present invention is to provide an image sensor having a significantly improved reproduced image quality.  
      Additional advantages, objects, and features of the invention will be set forth in part in the description which follows and in part will become apparent to those skilled in the art upon examination of the following or may be learned from practice of the invention. The objectives and other advantages of the invention may be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.  
      To achieve these objects and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, a method of making an image sensor includes the steps of forming a photodiode array in an active region of a semiconductor substrate defined by an active cell isolation film; forming a light transmission layer on the photodiode array; forming a microlens pattern on the light transmission layer, the microlens pattern being adapted to converge an external light; and forming a color filter array over the microlens pattern.  
      In another aspect of the present invention, an image sensor includes a color filter array for converting an external light into a color light; a microlens pattern under the color filter array for converging the color light passing through the color filter array; a photodiode array in an active region of a semiconductor substrate for receiving the light converged at the microlens pattern, adapted to produce and store photo charges; and a light transmission layer over the photodiode array for supporting the microlens pattern and the color filter array, and transmitting the light converged at the microlens patterns toward the photodiode array.  
      It is to be understood that both the foregoing general description and the following detailed description of the present invention are exemplary and explanatory and are intended to provide further explanation of the invention as claimed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this application, illustrate embodiment(s) of the invention and together with the description serve to explain the principle of the invention. In the drawings;  
       FIG. 1  illustrates a section of an example of a conventional image senor;  
       FIG. 2  illustrates a section of an example of an image senor in accordance with a preferred embodiment of the present invention; and  
      FIGS.  3 A˜ 3 E illustrate sections showing the steps of a method for fabricating an image sensor in accordance with a preferred embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF TH INVENTION  
      Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts.  
      Referring to  FIG. 2 , the image sensor, for an example, a CMOS image sensor, includes a color filter array  16  for converting a light from an exterior lens  100  into a color light, a light transmission layer  14  for transmission of the light converted into the color light at the color filter layer  16  toward a photodiode array  13 , and the photodiode array  13  on a semiconductor substrate  11  at an active region defined by an active cell isolation layer  12 , for receiving the light passed through the light transmission layer  14 , to produce, and store photo charges.  
      The light transmission layer  14  having a PMD insulating film, metal wiring, interlayer insulating film, and so on, is over the semiconductor substrate  11  to cover the photodiode array  13  and support the color filter array  16 . In this case, there is a planarizing layer  15  on the light transmission layer  14  for planarizing a base of the color filter array  16  to induce uniform transmission of the light.  
      As shown, in the image sensor of the present invention, instead of eliminating the traditional microlens array from a location on or over the color filter array  16 , a microlens pattern  20  is arranged under the color filter array  16  for effective convergence of the light passed through the color filter array  16  in substitution of a function of the related art microlens array.  
      In this case, preferably, the microlens pattern  20  includes a first lens pattern  21  comprising an oxide (e.g., a conventional silicon dioxide) for passing a light from the color filter array  16  toward the photodiode array  13  in a substantially straight line or direction (e.g., substantially normal to the planar upper surface of the photodiode  13 ), and a second lens pattern  22  of nitride (for example, SiN) on sidewalls of the first lens pattern  21  so as to cover the sides in a rounded shape, for refracting the light passed through the color filter array  16  toward the photodiode array  13 .  
      Preferably, the first lens pattern  21  has a thickness of 11,000 Å˜14,000 Å, and the second lens pattern  22  has a thickness of 6,000 Å˜8,000 Å.  
      Of course, as is known widely, the nitride film has a refractive index greater than the oxide film. Therefore, as shown in the drawing, if the microlens pattern  20  has a structure in which the second lens pattern  22  of nitride covers sidewalls of the first lens pattern  21  of oxide in a rounded shape, the light incident on a point p 4  passes the first lens pattern  21  in a straight line, and the light incident on points p 5  and p 6  refract at an angle, and pass through the second lens pattern  22  such that all the light passing through the exterior lens  100  can be converged toward the photodiode array  13  without problem, at the end.  
      In summary, by newly arranging the microlens pattern  20  under the color filter array  16 , for performing essentially the same function as a traditional microlens, a total distance of travel of a focused light to the photodiode array  13  can be minimized.  
      The microlens array, which converges a light, over the color filter array in the related art can not but transmit a converged light to the photodiode array through the color filter array, the light transmission layer, and so on, and as a result of which intensity and a focus of the light finally reaching to the photodiode array can not but be distorted in proportion to the distance, to drop a low luminance performance of the image sensor completed finally, substantially at the end.  
      However, the microlens pattern  20 , which converges a light, under the color filter array  16  in the present invention enables the converged light to transmit toward the photodiode array  13  naturally after passing through a short distance of the light transmission layer  14 , such that the intensity and focus of the light reaching to the photodiode array  13  finally can be maintained to be at an optimum state, to improve the low luminance performance of the image sensor completed finally, at the end.  
      When the distance of travel of the converged light is minimized, to optimize the intensity and focus of the light reaching to the photodiode array  13 , a quality of the image reproduced finally by the image sensor can be improved, significantly.  
      A method for fabricating the foregoing image sensor will be described in detail.  
      Referring to  FIG. 3A , an STI process (Shallow Trench Isolation process), or an LOCOS process (LOCal Oxidation of Silicon process), or the like is performed, to form an active cell isolation film  12  in the semiconductor substrate to define an active region of a semiconductor substrate  11 . In this case, a P-type epitaxial layer (not shown) may be formed on the semiconductor substrate  11 , such as a heavily doped P ++  type single crystal silicon substrate depending on a situation for increasing a size (depth) of a depletion region.  
      Then, ions are injected to define a P-type impurity layer, an N-type impurity layer, and so on in the semiconductor substrate  11  at the active region, to form a photodiode array  13  for producing, accumulating photo charges.  
      Next, referring to  FIG. 3B , deposition, etching, and the like are repeated, to form a light transmission layer  14  having, for an example, a PMD insulating film, metal wirings, an interlayer insulating film, and so on on the semiconductor substrate  11  inclusive of the photodiode array  13 .  
      Of course, a structure, and a fabricating sequence of the light transmission layer  14  can be varied with situations.  
      Chemical vapor deposition is performed, to form an oxide film layer  21   a  on the light transmission layer  14  to a thickness of, for an example, 11000 Å˜14000 Å, and a photoresist pattern  201  is formed on the oxide film layer  21   a  for defining a first lens pattern to be formed, later.  
      Referring to  FIG. 3C , an exposure process and a development process, and the like are performed based on the photoresist pattern  201 , to form first lens patterns  21  spaced from one another on the light transmission layer  14 .  
      Then, referring to  FIG. 3D , after a chemical vapor deposition is performed, to form a nitride film layer  22   a  on the light transmission layer  14  inclusive of the first lens pattern  21 , dry etching having an anisotropic characteristic, for an example, reactive ion etching, is performed targeting at the nitride film layer  22   a , to form second lens patterns  22  at opposite sides of the first lens pattern  21  to a thickness of, for an example, 6000 Å˜8000 Å.  
      Upon finishing above process, the microlens pattern  20  including the first lens pattern  21  of oxide, and the second lens pattern  22  of nitride covering the opposite sides of the first lens pattern  21  in rounded shapes is formed on the light transmission layer  14 .  
      Upon finishing fabrication of the microlens pattern  20 , depending on a situation, an ozone-TEOS (Tetra Ortho Silicate Glass) process, an atmospheric pressure chemical vapor deposition process, a plasma chemical vapor deposition process, a high density plasma chemical vapor deposition process (HDP CVD process), or the like is performed selectively, to form a planarizing layer  15  on the light transmission layer  14  to cover the microlens pattern  20 , and a chemical mechanical polishing process is performed, to polish the planarizing layer, smoothly.  
      Then, deposition, patterning, and so on are performed, to form a color filter array  16  on the planarizing layer  15 , to finish fabrication of the image sensor intended to obtain from the present invention.  
      As has been described, instead of the traditional microlens array over the color filter array, the microlens pattern arranged newly under the color filter array, which can substitute a function of the microlens array, permits to shorten a total distance of travel of a converged light finally reaching to the photodiode, that improves intensity and focus of the light finally reaching to the photodiode array, to improve a low luminance performance of the image sensor completed finally, significantly.  
      The minimized travel distance of the converged light, which optimizes intensity and focus of the light finally reaching to the photodiode array, permits to improve a quality of the image reproduced by the image sensor, significantly.  
      It will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the spirit or scope of the inventions. Thus, it is intended that the present invention covers the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.