Abstract:
A CMOS image sensor and a method for fabricating the same in which characteristics of the image sensor are not affected even if a profile of microlenses is varied, so as to obtain a more reliable device. The CMOS image sensor of the present invention includes color filter layers formed over a semiconductor substrate, a planarization layer formed on the color filter layers, and microlenses formed of the same material as that of the planarization layer on the planarization layer, the microlenses positioned to correspond to the color filter layers respectively.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]     This application claims the benefit of the Korean Patent Application No. P2004-114791, filed on Dec. 29, 2004, which is hereby incorporated by reference as if fully set forth herein.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to a complementary metal-oxide semiconductor (CMOS) image sensor and a method for fabricating the same, and more particularly, to a CMOS image sensor and a method for fabricating the same in which better reliability of a device is obtained.  
         [0004]     2. Discussion of the Related Art  
         [0005]     Generally, an image sensor is a semiconductor device that converts optical images to electrical signals. An image sensor may include a charge coupled device (CCD) or a CMOS image sensor.  
         [0006]     The CCD includes a plurality of photodiodes (PD) arranged in a matrix to convert optical signals to electrical signals, a plurality of vertical charge coupled devices (VCCDs) formed between the photodiodes in a vertical direction to transfer charges generated by the respective photodiodes in a vertical direction, a plurality of horizontal charge coupled devices (HCCDs) transferring the charges transferred by the VCCDs in a horizontal direction, and a sensing amplifier sensing the charges transferred in a horizontal direction to output electrical signals.  
         [0007]     The aforementioned CCD has the drawbacks of requiring a complicated driving mode, high power consumption, and multistage photolithographic processes.  
         [0008]     Furthermore, it is difficult to integrate a control circuit, a signal processing circuit, and an analog-to-digital converter in a CCD chip. This prevents the manufacture of slim products.  
         [0009]     To overcome the drawbacks of CCD, the CMOS image sensor has been given much attention as an image sensor for next generation.  
         [0010]     The CMOS image sensor employs a switching mode that sequentially detects outputs of unit pixels using MOS transistors by forming the MOS transistors corresponding to the number of the unit pixels on a semiconductor substrate using a control circuit and a signal processing circuit as peripheral circuits.  
         [0011]     The CMOS image sensor sequentially detects electrical signals of each unit pixel using a switching mode to display images by forming photodiodes and MOS transistors in unit pixels.  
         [0012]     The CMOS image sensor has the advantages of low power consumption and a simple fabrication process that involves only a relatively small number of photolithographic process steps.  
         [0013]     Furthermore, since the CMOS image sensor allows a control circuit, a signal processing circuit and an analog-to-digital converter to be integrated in its chip, it has the advantage of allowing for the manufacture of slim products.  
         [0014]     In fact, CMOS image sensors have found a wide use in various applications such as digital still camera and digital video camera.  
         [0015]     The CMOS image sensor is divided into a 3T type, a 4T type, and 5T type depending on the number of transistors. The 3T type CMOS image sensor is comprised of a photodiode and three transistors while the 4T type CMOS image sensor is comprised of a photodiode and four transistors.  
         [0016]      FIG. 1  is an exemplary circuit diagram illustrating a general 3T type CMOS image sensor, and  FIG. 2  is an exemplary layout illustrating a typical 3T type CMOS image sensor.  
         [0017]     A unit pixel of the 3T type CMOS image sensor, as shown in  FIG. 1 , includes a photodiode PD and three nMOS transistors T 1 , T 2  and T 3 . A cathode of the photodiode is connected to a drain of the first nMOS transistor T 1  and a gate of the second nMOS transistor T 2 .  
         [0018]     Sources of the first and second nMOS transistors T 1  and T 2  are connected to a power line supplied with a reference voltage VR. A gate of the first NMOS transistor T 1  is connected to a reset line supplied with a reset signal RST.  
         [0019]     A source of the third nMOS transistor T 3  is connected to a drain of the second nMOS transistor T 2 , its drain is connected to a reading circuit (not shown) through a signal line, and its gate is connected to a heat selection line supplied with a heat selection signal SLCT.  
         [0020]     The first nMOS transistor T 1  is called a reset transistor Rx, the second nMOS transistor T 2  is called a drive transistor Dx, and the third nMOS transistor T 3  is called a selection transistor Sx.  
         [0021]     In a unit pixel of a 3T type CMOS image sensor, as shown in  FIG. 2 , a photodiode  20  is formed in a wide portion of an active area  10 , and gate electrodes  120 ,  130 , and  140  of three transistors overlapped with one another are formed in the remaining portion of active area  10 .  
         [0022]     The reset transistor Rx is formed by the gate electrode  120 , the drive transistor Dx is formed by the gate electrode  130 , and the selection transistor Sx is formed by the gate electrode  140 .  
         [0023]     Source and drain areas of each transistor are formed by implanting impurity ions into the active area  10  at each transistor except for the portions below the gate electrodes  120 ,  130  and  140 .  
         [0024]     A power voltage Vdd is applied to the source and drain areas between the reset transistor Rx and the drive transistor Dx, and the source and drain areas at one side of the selection transistor Sx are connected to a reading circuit (not shown).  
         [0025]     Although not shown, each gate electrode  120 ,  130  and  140  is connected to a signal line. Each signal line is provided with a pad at one end to be connected to an external driving circuit.  
         [0026]     Process steps performed after the formation of each signal line provided with the pad will be described as follows.  
         [0027]      FIG. 3A  to  FIG. 3D  are sectional views illustrating a method for fabricating CMOS image sensor according to the related art.  
         [0028]     First, as shown in  FIG. 3A , an insulating layer  101  (for example, oxide layer) such as a gate insulating layer or a dielectric interlayer is formed on a semiconductor substrate  100 . A metal pad  102  of each signal line is formed on the insulating layer  101 .  
         [0029]     A passivation layer  103  is formed on an entire surface of the insulating layer  101  and on the metal pad  102 .  
         [0030]     Subsequently, the passivation layer  103  is selectively removed to partially expose the surface of the metal pad  102 . A cap oxide layer  104  is then formed over the entire surface of the semiconductor substrate  100 .  
         [0031]     As shown in  FIG. 3B , color filter layers  105  are formed on the cap oxide layer  104  to correspond to each photodiode area (not shown).  
         [0032]     Subsequently, a planarization layer  106  is formed over the entire surface of the semiconductor substrate  100 . The planarization layer  106  is then selectively removed by a photolithographic process to remain only on the color filter layers  105 .  
         [0033]     As shown in  FIG. 3C , microlenses  107  are formed on the planarization layer  106  to correspond to the respective color filter layers  105 .  
         [0034]     As shown in  FIG. 3D , a blanket etching process is performed on the entire surface of the cap oxide layer  104  to expose the surface of the metal pad  102  and to form a pad opening portion  108 .  
         [0035]     During this blanket etching process of the cap oxide layer  104  however, the microlenses  107  are damaged. For this reason, a profile of the microlenses  107  is varied as shown by “A” in  FIG. 3D .  
         [0036]     Variation of the profile of the microlenses affects the characteristics of the image sensor and deteriorates the reliability of the device.  
       SUMMARY OF THE INVENTION  
       [0037]     Accordingly, the present invention is directed to a CMOS 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.  
         [0038]     One advantage of the present invention is that it can provide a CMOS image sensor and a method for fabricating the same, in which characteristics of the image sensor are not affected even if a profile of microlenses is varied, so as to obtain a more reliable device.  
         [0039]     Additional examples of advantages and features of the invention will be set forth in part in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from practice of the invention.  
         [0040]     To achieve these and other advantages and in accordance with an embodiment of the invention, as embodied and broadly described herein, a CMOS image sensor according to the present invention includes color filter layers formed over a semiconductor substrate, a planarization layer formed on the color filter layers, and microlenses formed on the planarization layer, wherein the microlenses are formed of the same material as that of the planarization layer, and are positioned to correspond to the color filter layers.  
         [0041]     In another aspect of the present invention, a method for fabricating a CMOS image sensor includes forming color filter layers over a semiconductor substrate, forming a planarization layer on the color filter layers, and forming microlenses on the planarization layer, wherein the microlenses are formed of the same material as that of the planarization layer in such a way that the microlenses are positioned to correspond to the color filter layers.  
         [0042]     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  
       [0043]     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 principles of the invention.  
         [0044]     In the drawings:  
         [0045]      FIG. 1  is an equivalent circuit diagram illustrating a typical CMOS image sensor:  
         [0046]      FIG. 2  is a layout illustrating a typical CMOS image sensor;  
         [0047]      FIG. 3A  to  FIG. 3D  are sectional views illustrating a method for fabricating a related art CMOS image sensor;  
         [0048]      FIG. 4  is a sectional view illustrating a CMOS image sensor according to an embodiment of the present invention; and  
         [0049]      FIG. 5A  to  FIG. 5E  are sectional views illustrating a method for fabricating a CMOS image sensor according to an embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0050]     Reference will now be made in detail to some 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.  
         [0051]      FIG. 4  is a sectional view illustrating an exemplary CMOS image sensor according to the present invention.  
         [0052]     As shown in  FIG. 4 , a CMOS image sensor according to the present invention includes an insulating layer  201  formed on a semiconductor substrate divided into an active area and a pad area, a metal pad  202  formed in the pad area on the insulating layer  201 , a passivation layer  203  formed with a pad opening portion  208  to partially expose a surface of the metal pad  202 , a cap oxide layer  204  formed in the active area on the passivation layer  203 , color filter layers  205  formed on the cap oxide layer  204 , a planarization layer  206  formed on the color filter layers  205 , and microlenses  207 , formed of the same material as that of the planarization layer  206 , on the planarization layer  206  to correspond to the color filter layers  205 .  
         [0053]     The planarization layer  206  and the microlenses  207  are formed of either photoresist layers or TEOS layers.  
         [0054]      FIG. 5A  to  FIG. 5E  are sectional views illustrating a method for fabricating the CMOS image sensor according to the preferred embodiment of the present invention.  
         [0055]     As shown in  FIG. 5A , the insulating layer  201  such as a gate insulating layer or a dielectric interlayer is formed on the semiconductor substrate  200 . A metal pad  202  of each signal line is formed on the insulating layer  201 .  
         [0056]     The metal pad  202  may be formed of the same material as that of gate electrodes  120 ,  130  and  140  shown in  FIG. 2  and on the same layer as the gate electrodes  120 ,  130  and  140 . Alternatively, the metal pad  202  may be formed of a material different from that of the gate electrodes  120 ,  130  and  140  and connected through a separate contact hole. In most cases, the metal pad  202  may be formed of aluminum (Al).  
         [0057]     A surface of the metal pad  202  is treated with UV ozone or mixture of solutions to improve corrosion resistance of the metal pad  102 .  
         [0058]     The passivation layer  203  is formed over an entire surface of the semiconductor substrate  200  including the metal pad  202 . The passivation layer  203  is then selectively removed to partially expose the surface of the metal pad  202 .  
         [0059]     Subsequently, the cap oxide layer  204  is formed over the entire surface of the semiconductor substrate  200  including the passivation layer  203 .  
         [0060]     The cap oxide layer  204  has a thickness of approximately 300 Å-800 Å. The cap oxide layer  204  protects the metal pad  202  from corrosion, which may be caused by a developing solution during the exposure and development processes performed during the formation of the color filter layers.  
         [0061]     As shown in  FIG. 5B , the color filter layers  205  are formed on the cap oxide layer  204  to correspond to each photodiode area (not shown).  
         [0062]     The color filter layers  205  are formed as follows. A blue color resist is deposited and then patterned by photolithography to form a blue color filter layer. A green and red color filter layers are then respectively formed in the same manner.  
         [0063]     As shown in  FIG. 5C , a planarization layer  206  is formed over the entire surface of the semiconductor substrate  200  including the color filter layers  205 . The planarization layer  206  may be formed of the same material as that of the microlenses that will be formed later, which may be a photoresist or TEOS material.  
         [0064]     Subsequently, the planarization layer  206  is selectively removed by a photolithographic process to remain only in an area that does not include the metal pad.  
         [0065]     UV baking may additionally be performed over the planarization layer  206 .  
         [0066]     As shown in  FIG. 5D , the same material as that of the planarization layer  206  may be deposited over the entire surface of the semiconductor substrate  200  including the planarization layer  206 , and a microlens pattern may be formed by a photolithography process.  
         [0067]     Subsequently, the microlens pattern undergoes a reflow process at a predetermined temperature to form microlenses  207  of a semispherical shape.  
         [0068]     As shown in  FIG. 5E , a blanket etching process is performed on the entire surface of the semiconductor substrate  200  to selectively remove the cap oxide layer  204  on the pad area, thereby forming the pad opening portion  208 .  
         [0069]     During the blanket etching process of the cap oxide layer  204 , the microlenses  207  are damaged. For this reason, a profile of the microlenses  207  is varied, in which the microlenses  207  become thin. However, since the planarization layer  206  is formed of the same material as that of the microlenses  207 , the planarization layer  206  between the microlenses  207  may be selectively etched to form a V-shape groove of a desired depth. In this manner, the varied profile of the microlenses  207  can be compensated.  
         [0070]     As described above, the CMOS image sensor and the method for fabricating the same according to the present invention has many advantages.  
         [0071]     Since the thin portion of the microlenses, which is caused by the blanket etching process of the cap oxide layer, can be compensated by removing the planarization layer formed between the microlenses, it is possible to prevent deterioration of the reliability of the device.  
         [0072]     In addition, since the planarization layer is formed of the same material as that of the microlenses, it is possible to prevent a space of the microlenses from becoming large.  
         [0073]     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 invention. 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.