Abstract:
A shielding layer outside a sensing region I of a CMOS image sensor includes a stack of a first monochromatic color filter layer and a second monochromatic color filter layer. Such a two-layered monochromatic color filter acts as a shielding layer, and the amount of black photoresist needed is decreased. Therefore, a process of CMOS image sensor fabrication is simplified and the cost of fabrication is decreased. The black pigment is prevented from remaining and causing contamination.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is a divisional of application Ser. No. 10/908,219 filed May 3, 2005. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention is related to a shielding layer outside a pixel region of an optical device and a method for forming the same, and particularly, related to a shielding layer outside a sensing region of a CMOS device and a method for forming the same.  
         [0004]     2. Description of the Prior Art  
         [0005]     A complementary metal-oxide semiconductor (CMOS) image sensor is fabricated with a conventional CMOS fabricating processes. Therefore, the CMOS image sensor can easily be integrated with its control circuitry. Thus the cost of the CMOS image sensor is cheaper than a charge-coupled device. In addition, the CMOS image sensor also has advantages of small size, high quantum efficiency, and low read-out noise.  
         [0006]     The CMOS image sensor separates (or classifies) incident light into a combination of light of different wavelengths. The light of different wavelengths are received by respective sensing elements and are transferred into digital signals of different intensities. For example, the CMOS image sensor can consider incident light as a combination of red, blue, and green light. Those wavelengths are subsequently received by photodiodes, and then transformed into digital signals. However, in order to separate incident light, a monochromatic color filter array (CFA) must be set above every optical sensor element.  
         [0007]     In addition, in order to decrease noise, all light received by the CMOS image sensor should come from the CFA. In other words, light coming from intervals between monochromatic color filters and that coming from regions outside the sensing regions should be blocked. Please refer to  FIG. 1 .  FIG. 1  is a sectional view of a conventional CMOS image sensor. In  FIG. 1 , a region I that includes a CFA, including monochromatic color filters  28 ,  30 ,  32 , is a sensing region, and a left side of the sensing region I is the peripheral circuit region II which is outside the sensing region. As shown in  FIG. 1 , there is a patterned metal layer  14 , so as to shield sensing elements  32 ,  34 ,  36  on a semiconductor substrate  40 , from light scattered from the intervals of the CFAs  28 ,  30 ,  32 . In another words, only on the regions beneath the intervals of the CFAs  28 ,  30 ,  32  is the metal using for shielding. Outside the CMOS sensing region I, there are shielding elements  22  to block light from regions outside the sensing region I. In addition, there are metal pads  24  for connecting outside the sensing region I. Since the metal pad  24  can shield light, there is no shielding element above the metal pad  13 .  
         [0008]     In conventional fabrication processes, after the base elements, such as the metal layer  14  and the planar layer  20 , are formed, a CFA can be formed on the nitride layer  12 . In order to form the CFA, a first monochromatic color filter layer made by photosensitive resin is formed. Following that, an exposing and developing process is applied on the monochromatic color filter layer to obtain a desired pattern, and then dyeing of the patterned monochromatic color filter layer with a first color is performed, so as to form a patterned first monochromatic color filter layer  26 . Alternatively, photoresist dyed with the first color can also be used to form a first monochromatic color filter layer, after which an exposing and developing process is performed on it so as to form the patterned first monochromatic color filter layer  26 . After the first monochromatic color filter layer  26  is formed, a curing process may be performed to strengthen the first monochromatic color filter layer  26 . After the first monochromatic color filter  26  is formed, the process above is repeated to form a patterned second monochromatic color filter layer  28 , and a patterned third monochromatic color filter layer  30 . Those monochromatic color filters  26 ,  28 ,  30  all together form the CFA.  
         [0009]     After the CFA is formed, a shielding layer is formed on the peripheral circuit region  11 , which is outside the sensing region I, with similar process. In other words, a black photosensitive material layer is formed outside the sensing region I, and is then exposed and developed, so as to form a shielding layer  22  impervious to light. At last, an insulation layer  16  is formed on shielding layer  22 , and the CFAs  26 ,  28 ,  30 , to facilitate the fabrication of the lens  18 . Parts of the insulation layer  16  are then removed to expose the metal pads  24  and other regions that need to be exposed. In addition, a metal pad  24  may not exist in the peripheral region II due to a different layout design. In such a case, the shielding layer  22  should be able to cover the whole peripheral circuit region II, which is outside the sensing region I.  
         [0010]     Even though a shielding layer of a conventional CMOS image sensor is able to shield lights efficiently, a black photoresist material is expensive and the black pigment can cause problems. In addition, only the shielding layer is made of the black photoresist, therefore an extra exposing, developing and curing process and an extra mask is needed to form the shielding layer  22 . As a result, a more economic and convenient shielding layer is needed to decrease the fabrication cost of the CMOS image sensor.  
       SUMMARY OF THE INVENTION  
       [0011]     An object of the claimed invention is to provide an improved shielding layer in an optical device and a method for forming the same, so as to decrease the cost of forming a shielding layer outside the sensing region of a CMOS image sensor.  
         [0012]     According to the claims of the present invention, a shielding layer of an optical device is disclosed. The optical device is a CMOS image sensor, and the shielding layer is formed on a semiconductor substrate. In addition, the shielding layer includes a stack of a first monochromatic color filter layer and a second monochromatic color filter layer.  
         [0013]     The device according to the present invention and the CFA can be formed at the same time using the same mask according to the method of the present invention. Therefore no extra mask is needed. In addition, the device according to the present invention does not include black photoresist. Therefore, the present invention can avoid the problem of remaining black pigment and decrease the fabrication cost.  
         [0014]     These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]      FIG. 1  is a sectional view of a conventional CMOS image sensor.  
         [0016]      FIG. 2  is a sectional view of a CMOS image sensor according to the present invention.  
         [0017]      FIG. 3  is a sectional view of the CMOS image sensor of  FIG. 2 .  
         [0018]      FIG. 4  is a sectional view of a CMOS image sensor of  FIG. 2 .  
         [0019]      FIG. 5  is a graph illustrating ranges of light that can pass through red, green, and blue monochromatic color filters.  
         [0020]      FIG. 6  is a schematic view of the structure of an LCoS illustrating an embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0021]     Compared to a conventional shielding layer of an optical device, the shielding layer according to the present invention not only can shield light efficiently but also has the advantage of low fabrication cost. In other words, fabricating the shielding layer according to the present invention needs fewer masks, no black photoresist, and thus is able to avoid the contamination of black pigment.  
         [0022]     Consider a CMOS image sensor for example regarding the preferred embodiment of the present invention. Please refer to  FIG. 2  to  FIG. 4 .  FIG. 2  to  FIG. 4  are sectional views of the CMOS image sensor.  FIG. 2  to  FIG. 4  illustrate a preferred embodiment for fabricating the according to the method of the present invention. As shown in  FIG. 2 , the CMOS image sensor can be divided into a sensing region I that senses incident lights, and a peripheral circuit region II outside the sensing region I. The sensing region I includes color filter array (CFA) and sensing elements  232 ,  234 ,  236  on the substrate  240 . Before fabricating the shielding layer according to the present invention, the sensing elements  232 ,  234 ,  236  and other elements under the CFA are formed. Among those elements, a patterned metal layer  214  is formed under where the CFA will be formed. The patterned metal layer  214  is used to prevent light from being scattered through the intervals of the CFA. Therefore the pattern of the metal layer is dependent on the pattern of the CFA, and metal only exists in areas under the intervals of the CFA. In addition, a metal pad  204  may be formed on the peripheral circuit region II according to requirements. After the metal layer  214 , the metal pad  204 , and other metal interconnects are formed, a planar layer  220  is formed on the metal layer  214  so as to facilitate the performing of the subsequent process. Following that, a nitride layer  212  is alternatively formed on the planar layer  220  as a passivation layer.  
         [0023]     After the above process is completed, the shielding layer according to the present invention is formed. According to the present invention, while forming the first monochromatic color filter layer  206  in the sensing region I, another first monochromatic color filter layer  262  is formed in the peripheral circuit region II simultaneously. Following that, as shown in  FIG. 3 , while forming a second monochromatic color filter layer  208  in the sensing region I, another second monochromatic color filter layer  282  is formed outside the peripheral circuit region II simultaneously, and the second monochromatic color filter layer  282  is stacked onto the first monochromatic color filter layer  262  so as to form the shielding layer according to the present invention. Lastly, as shown in  FIG. 3 , a third monochromatic color filter layer  210  is formed in the sensing region I so as to complete the fabrication of the CFA.  
         [0024]     The stack of the first monochromatic color filter layer  262  and the second monochromatic color filter layer  282  is the shielding layer according to the present invention. Please refer to  FIG. 5 .  FIG. 5  is a graph illustrating the wavelengths of light that can pass through red, green, and blue monochromatic color filters. According to  FIG. 5 , there is only a small range of light, shown as area A, that can pass through the red monochromatic color filter and then pass through the blue monochromatic color filter. In other words, a stack of a red monochromatic color filter and a blue monochromatic color filter can filter out most visible light. Therefore, when the first monochromatic color filter layer  262  and the second monochromatic color filter layer  282  are a red monochromatic color filter and a blue monochromatic color filter respectively, the shielding layer according to the present invention can shield most visible light. As a result, the shielding layer according to the present invention is able to replace the conventional shielding layer made of black photoresist.  
         [0025]     In addition, monochromatic color filters of other colors can also be stacked together to form a shielding layer. For example, when the first monochromatic color filter layer  262  and the second monochromatic color filter layer  282  are a red monochromatic color filter and a green monochromatic color filter respectively, there is only a range of light, shown as area B, that can pass through the shielding layer. Therefore, a shielding layer constructed with a red monochromatic color filter and a green monochromatic color filter is also workable, even though its performance may not be as good as that constructed with a red monochromatic color filter and a blue monochromatic color filter. Similarly, a single monochromatic color filter can also be used as a shielding layer. However, the performance of the single layer is limited, and thus is not as good as the two-layered one. In addition, red, green, and blue monochromatic color filters can all be stacked together to form a three-layered shielding layer. This kind of shielding layer has the best shielding performance. However, the more layers used, the thicker the shielding layer. If the shielding layer is too thick, there can be problems in the subsequent packing and wiring processes. Therefore a two-layered shielding layer is the preferred embodiment of the present invention for its better performance in light shielding and thickness. However, all these are design considerations that can change to meet the requirements of specific products, constructions, and layout designs, so as to achieve the best arrangement.  
         [0026]     It has to be noted that the fabrication process of the shielding layer is not limited to the above process. For example, the shielding layer according to the present invention can be formed after the monochromatic color filters of the CFA are formed. In such a case, the using of black photoresist is also avoided and thus contamination is reduced.  
         [0027]     In addition to the CMOS image sensor, some liquid crystal on silicon (LCoS) displays also use CFAs to separate light. Please refer to  FIG. 6 .  FIG. 6  illustrates a sectional of the lower part of an LCoS display. As shown in  FIG. 6 , an LCoS includes a semiconductor substrate  622  and a pixel electrode  624  that can also serve as a reflector. There is a CFA composed of a plurality of monochromatic color filters  606 ,  608 ,  610 . A pixel region of the LCoS includes the monochromatic color filters  606 ,  608 ,  610 . Outside the pixel region, a shielding layer according to the present invention, that is the stack of a first monochromatic color filter layer  662  and a second monochromatic color filter layer  682 , is formed to prevent light reflected by the pixel electrode  624  from emitting to the region outside the pixel region.  
         [0028]     Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.