Abstract:
An image sensor with optical guard rings is provided. The optical guard rings are embedded in a stacked inter-metal dielectric layer between the sensor areas, that is, around each pixel. The refraction index (RI) of the optical guard rings is smaller than that of the stacked inter-metal dielectric layer. Therefore, the incident light with a large angle of incidence is blocked by the optical guard rings and cannot arrive at the adjacent pixels.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     This invention relates generally to an image sensor. In particular, it relates to an image sensor with optical guard rings to prevent cross-talk issue between adjacent pixels.  
         [0003]     2. Description of the Related Art  
         [0004]     Solid state image sensors are necessary components in many optoelectronic devices, including digital cameras, cellular phones, and toys. Conventional solid-state image sensors for color analog or digital video cameras are typically charge-coupled device (CCD) or complementary metal oxide semiconductor (CMOS) photodiode array structures which comprise a spectrally photosensitive layer below one or more layers patterned in an array of color filters and above which resides a surface-layer array of microlens elements. The elementary unit of the image sensor is defined as a pixel. The basic technology used to form the CMOS image sensor is common to both types sensors.  
         [0005]     The CMOS image sensor comprises a photo detector for detecting light and a logic circuit for converting the detected light into an electric signal representing data regarding the detected light. The fill factor, sometimes referred to as the aperture efficiency, is the ratio of the size of the light-sensitive area to the size of the total pixel size. Although efforts have been made to increase the fill factor of the image sensor and thereby increase the sensor sensitivity, further increases in the fill factor are limited because the associated logic circuitry cannot be completely removed. In order to increase the sensitivity of the light, a microlens formation technology has been used to converge and focus the incident light onto the photo detector by changing the path of the light that reaches the lens of the photo detector. In order for the image sensor to detect and provide a color image, it typically must include both a photo detector for receiving the light and generating and accumulating charge carriers and a color filter array (CFA), i.e., a plurality of color filter units sequentially arranged above the photo detector. The CFA typically uses one of two alternative three-color primary configurations, either red R, green G and blue B (RGB) or yellow Y, magenta M and cyan C (CMY). A plurality of microlenses are positioned above the color filter array to increase the photo-sensitivity of the image sensor.  
         [0006]      FIG. 1  shows a traditional image sensor disposed in the substrate. The incident light  30  may not effectively focus on the photodiode  12  and may transmit to the adjacent photodiode  12 ′. As the pixel size is shrunk and multi-layer metal is used to reduce sensor cost, the cross-talk issue is more serious due to light scattering coming from metal layers  16  and  20 . Thus, for black and white sensors in particular, image resolution is degraded. Furthermore, color correction is more difficult for color sensors.  
       SUMMARY OF THE INVENTION  
       [0007]     Accordingly, it is the object of this invention to provide an image sensor to prevent cross-talk problem occurring between adjacent pixels.  
         [0008]     To achieve the above objects, an image sensor with optical guard rings around sensor areas is provided. The optical guard rings with low RI material are embedded in the stacked inter-metal dielectric layer between the sensor areas. The refraction index (RI) of the optical guard rings is smaller than that of the stacked inter-metal dielectric layer. Therefore, the incident light with a large angle of incidence is blocked by the optical guard rings and cannot arrive at the adjacent pixels. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]      FIG. 1  shows a traditional image sensor without optical guard ring disposed in the substrate.  
         [0010]      FIGS. 2A  to  2 D illustrate a method for fabricating an image sensor in accordance with one embodiment of the present invention.  
         [0011]      FIG. 3  shows a layout of a portion of a pixel of an image sensor with optical guard rings formed in the stacked inter-metal dielectric layer.  
         [0012]      FIG. 4  shows an image sensor with optical guard rings formed by two etching and coating processes.  
         [0013]      FIG. 5  shows an image sensor with optical guard rings formed in lower inter-metal dielectric layer. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0014]      FIGS. 2A  to  2 D illustrate a method for fabricating an image sensor in accordance with the present invention by significantly modifying the conventional image sensor fabrication process to include the formation of guard rings with low refraction index (RI).  
         [0015]     As shown in  FIG. 2A , a sensor area, such as a photodiode  102 , is formed in a semiconductor substrate  100 . Other elements, such as transistors (not shown), are also formed in or on the semiconductor substrate  100 . An interlayer dielectric layer (ILD layer)  104 , which is a pre-metal light transmitting dielectric layer, is formed on the semiconductor substrate  100  to cover those elements.  
         [0016]     An interconnection process is performed to form multi-layer metal lines (not shown) except above the photodiode  102  and multi-layer insulators isolating the metal lines. The multi-layer insulators, also referred as a stacked inter-metal dielectric layer, are shown in figures as a layer  106 . For example, a first metal layer is deposited on the ILD layer  104  and selectively patterned to form a first metal line on the ILD layer  104  except above the photodiode  102 . After a first inter-metal dielectric layer for insulating adjacent metal lines from each other is deposited and planarized on the first metal line, a second metal layer is deposited on the first inter-metal dielectric layer and selectively patterned and etched to generate a second metal line positioned generally above the first metal line. A second inter-metal dielectric layer for insulating the adjacent metal lines from each other is deposited and planarized. The following interconnection process is then performed.  
         [0017]     As shown in  FIG. 2B , an energy sensitive layer, such as photoresist layer  108 , is deposited on the stacked inter-metal dielectric layer  106  and patterned utilizing conventional photolithography process to form an optical guard ring pattern. The patterned photoresist layer  108  with the optical guard ring pattern is used as a mask for etching the stacked inter-metal dielectric layer  106 , thereby forming openings between pixels, that is, around each pixel boundary.  
         [0018]     As shown in  FIG. 2C , a low RI and light transmitting insulating layer  112  is formed on the stacked inter-metal dielectric layer  106  filling the openings  110 . The low RI and light transmitting insulating layer  112  is preferably an organic spin-on material, for instance SiLK (manufactured by Dow), which has a refraction index (RI=1.35) which is smaller than that of the silicon oxide (RI=1.46) usually used to form the stacked inter-metal dielectric layer  106 . The low RI and light transmitting insulating layer  112  over the stacked inter-metal dielectric layer  106  is then removed by etching back or CMP (chemical mechanical polishing), as shown in  FIG. 2D . The low RI and light transmitting insulating layer  112  is embedded in the stacked inter-metal dielectric layer  106  to form an optical guard ring. An example of the layout of the optical guard ring  112  and the photodiode  102  is shown in  FIG. 3 , wherein the optical guard ring  112  is disposed around the photodiode  102  to prevent cross-talk to the adjacent pixels. The shape of the photodiode  102  is not limited to that in  FIG. 3 .  
         [0019]      FIG. 2D  shows that the optical guard rings  112  may be embedded in the stacked inter-metal dielectric layer  106  in such a way as to penetrate the entire layer. In another embodiment, the optical guard rings  112  may be embedded in the stacked inter-metal dielectric layer  106  in such a way that only parts of the stacked inter-metal dielectric layer  106  are penetrated. In the former, the optical guard rings  112  can be formed by one etching process (as shown in  FIG. 2D ), or formed by two or more etching processes (as shown in  FIG. 4 ). As shown in  FIG. 4 , the optical guard rings  112   a  is formed in the lower stacked inter-metal dielectric layer  106   a  by the first etching process, and the optical guard rings  112   b  is formed in the upper stacked inter-metal dielectric layer  106   b  by the second etching process. In the latter, the optical guard rings  112   a  can be formed in the lower stacked inter-metal dielectric layer  106   a , while the upper stacked inter-metal dielectric layer  106   b  is formed on the lower stacked inter-metal dielectric layer  106   a  without optical guard rings therein, as shown in  FIG. 5 .  
         [0020]     The optical guard rings  112 ,  112   a  and  112   b  used to prevent cross-talk between adjacent pixels in the present invention have a refraction index smaller than the stacked inter-metal dielectric layer  106 ,  106   a  and  106   b  respectively, as shown in  FIGS. 2D, 4  and  5 . Typically, the incident angel of the incident light is preferably larger than 19°. For example, if the stacked inter-metal dielectric layer has RI=1.46 and the optical guard rings have RI=1.35, the incident light is completely reflected when the incident angel is larger than sin −1  (1.36/1.45), i.e. 67°.  
         [0021]     The image sensor having optical guard rings to prevent incident light from transmitting to different pixels, e.g., cross-talk, improves image resolution for black and white image sensors or color correction for color image sensors.  
         [0022]     The foregoing description of the preferred embodiments of this invention has been presented for purposes of illustration and description. Obvious modifications or variations are possible in light of the above teaching. The embodiments were chosen and described to provide the best illustration of the principles of this invention and its practical application to thereby enable those skilled in the art to utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. All such modifications and variations are within the scope of the present invention as determined by the appended claims when interpreted in accordance with the breadth to which they are fairly, legally, and equitably entitled.