Abstract:
A complementary metal-oxide semiconductor (CMOS) image sensor and a method of fabricating the same are disclosed. In a complementary metal-oxide semiconductor (CMOS) image sensor including a photodiode receiving irradiated light and generating electric charges, a plurality of conductive circuits each formed in different layers, a plurality of interlayer dielectrics insulating the conductive circuits, and a micro-lens formed of the interlayer dielectrics and focusing the irradiated light to the photodiode, the CMOS image sensor includes a lens formed in a dome shape on any one of the interlayer dielectrics and re-focusing the light focused by the micro-lens to the photodiode.

Description:
[0001]     This application claims the benefit of the Korean Patent Application No. 10-2004-0046536, filed on Jun. 22, 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 an image sensor, and more particularly, to a complementary metal-oxide-metal (CMOS) image sensor and a method of fabricating the same. Although the present invention is suitable for a wide scope of applications, it is particularly suitable for enhancing light focusing characteristics.  
         [0004]     2. Discussion of the Related Art  
         [0005]     Generally, an image sensor is a semiconductor device, which converts an optic image to an electric signal. More specifically, a charge coupled device (CCD) is a device having a plurality of metal-oxide-metal (MOS) capacitors each formed within a proximate range from one another, and wherein a carrier electric charge is stored in and transmitted to each capacitor. A complementary MOS (CMOS) image sensor is a device forming a number of MOS transistors corresponding to the number of pixels by using the CMOS technology, which uses a control circuit and a signal processing circuit as peripheral circuits, and adopting a switching method, which uses the MOS transistors to sequentially detect each output.  
         [0006]     However, the CCD is disadvantageous in that the driving method is complicated and consumes a large amount of power, and the fabrication process is complicated having too many mask process steps. Furthermore, a one-chip circuit cannot be easily formed because the signal processing circuit cannot be realized in the CCD chip. Recently, in order to overcome such disadvantages, a CMOS image sensor using sub-micron CMOS fabrication technology has been under development. The CMOS image sensor includes a photodiode and a MOS transistor formed within the unit pixel and uses a switching method to sequentially detect each signal, thereby representing an image. Herein, power consumption can be reduced by using the CMOS fabrication technology, and the number of masks is reduced to  20 , as compared to the CCD fabrication process requiring 30 to 40 masks, thereby simplifying the fabrication process. Also, a plurality of signal processing circuits and one-chip circuits can be formed. For such reasons, the CMOS image sensor is being considered as the next generation image sensor.  
         [0007]      FIG. 1  illustrates a circuit diagram of a unit pixel of a general CMOS image sensor, which includes a photodiode (PD) and four MOS transistors. The unit pixel includes a photodiode (PD) receiving light so as to form photocharge, a transfer transistor (Tx) transferring photocharge collected in the photodiode (PD) to a floating diffusion (FD) area, a reset transistor (Rx) setting an electrical potential of the floating diffusion (FD) area to a desired value and outputting electric charge, thereby resetting the floating diffusion (FD) area, a drive transistor (Dx) acting as a source follow buffer amplifier, and a select transistor (Sx) allowing addressing of the switching function. Also, a load transistor for reading an output signal is formed outside of the unit pixel.  
         [0008]      FIG. 2  illustrates a cross-sectional view of a unit pixel of a related art CMOS image sensor and, more particularly, the main parts related to light-focusing. Referring to  FIG. 2 , the related art CMOS image sensor includes a semiconductor substrate  11 , a field oxide layer  12  formed on the semiconductor substrate  11  to define an active area, and a photodiode (PD)  13  and a plurality of transistors  14  formed in the active area of the semiconductor substrate  11 . Then, an interlayer dielectric  15  is formed on the entire structure including the photodiode (PD)  13  and the transistors  14 . And, a plurality of metal circuits  16 ,  17 , and  18  is formed on the interlayer dielectric  15  so as to form the unit pixel. The metal circuits  16 ,  17 , and  18  are aligned so as to prevent light being irradiated to the photodiode (PD) from being blocked.  
         [0009]     Interlayer dielectrics  19 ,  20 , and  21  are formed between each layer of metal circuits  16 ,  17 , and  18  for electric insulation. And, although not shown in the drawing, a passivation layer for protecting the device from humidity and scratches, R, G, and B color filter layers for representing a color image, and an overcoating layer are formed on the last interlayer dielectric  21 . And, a micro-lens  22  having a dome structure and formed of a photoresist material is formed on the overcoating layer. The micro-lens  22  deposits a photoresist layer on the overcoating layer, which is patterned so that the photoresist remain on the photodiode (PD). Then, the photoresist is treated with a baking process so as to induce the photoresist to leak, thereby obtaining a desired curvature. The above-described micro-lens  22  plays an important role of focusing the irradiated light and transmitting the focused light to the photodiode (PD). However, as high density devices are being developed, the metal circuits should be formed in different layers, thereby increasing the thickness of the interlayer dielectrics. Thus, the micro-lens  22  becomes insufficient for appropriately focusing and transmitting light to the photodiode (PD).  
         [0010]      FIGS. 3A  to  3 D illustrate problems shown in the related art CMOS image sensor. Referring to  FIG. 3A , the related art CMOS image sensor the focused light passing through the micro-lens  22  is concentrated only on the center portion of the photodiode (PD), which causes a difference in amount of light between the edge portion and the center portion, thereby generating a dark current in the edge portion of the photodiode. Thus, when the irradiated light is converted into an electric signal, as shown in  FIG. 3B , and outputted through a display device, the picture image is distorted at the edge portion and the clearness of the image is deteriorated.  
         [0011]     Furthermore, referring to  FIG. 3C , when the light is not irradiated in a direction parallel to the axis of the micro-lens and is irradiated to have an inclination, the light cannot be accurately focused on the photodiode and is focused more on another area, thereby reducing the intensity of the signal. Therefore, as shown in  FIG. 3D , the outputted image may be focused to one side, and problems of distorted image and unclearness at the edge portion may occur. As the size of the unit pixel is reduced, the thickness of the upper structures, such as the interlayer dielectrics  15 ,  19 ,  20 , and  21  and the metal circuits  16 ,  17 , and  18  formed on the photodiode, are increased, as compared to the size of the photodiode. Such problems are aggravated as the number of metal circuits increases and the thickness of the structure is increased due to the high integration of the device, thereby causing problems in device integration. This problem may be resolved if the micro-lens  22  is formed on a lower portion, instead of the uppermost portion. However, if a later (or following) process is performed after forming the micro-lens  22  using the photoresist as material, the photoresist may leak due to the heat of the following process, and so such method cannot be actually applied.  
       SUMMARY OF THE INVENTION  
       [0012]     Accordingly, the present invention is directed to a CMOS image sensor and a method of fabricating the same that substantially obviate one or more problems due to limitations and disadvantages of the related art.  
         [0013]     An object of the present invention is to provide a CMOS image sensor and a method of fabricating the same that can easily focus light to enhance signal distortion or deterioration of signal characteristics.  
         [0014]     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 having ordinary skill 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.  
         [0015]     To achieve these objects and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, in a complementary metal-oxide semiconductor (CMOS) image sensor including a photodiode receiving irradiated light and generating electric charges, a plurality of conductive circuits each formed in different layers, a plurality of interlayer dielectrics insulating the conductive circuits, and a micro-lens formed of the interlayer dielectrics and focusing the irradiated light to the photodiode, the CMOS image sensor includes a lens formed in a dome shape on any one of the interlayer dielectrics and re-focusing the light focused by the micro-lens to the photodiode.  
         [0016]     Herein, the lens may be formed of a nitride layer, and lens may include a dome-shaped oxide layer, and a nitride layer formed on a surface of the oxide layer.  
         [0017]     In another aspect of the present invention, in a method of fabricating a complementary metal-oxide semiconductor (CMOS) image sensor, wherein the CMOS image sensor includes a photodiode receiving irradiated light and generating electric charges, a plurality of conductive circuits each formed in different layers, a plurality of interlayer dielectrics insulating the conductive circuits, and a micro-lens formed of the interlayer dielectrics and focusing the irradiated light to the photodiode, the method includes forming a lens for re-focusing the light focused by the micro-lens to the photodiode, wherein the step of forming a lens includes a first step of randomly forming one of the interlayer dielectrics and forming a nitride layer on an entire surface thereof, a second step of depositing a photoresist layer on the nitride layer, and patterning the photoresist layer so that a set portion of photoresist remains in a corresponding area, a third step of selectively removing the nitride layer by using the patterned photoresist layer as a mask, and removing the photoresist layer, a fourth step of rounding corner portions of the selectively removed nitride layer by using a high frequency bias sputter etch process, and a fifth step of forming the nitride layer in a dome shape by using a wet-etch process.  
         [0018]     The method of fabricating a complementary metal-oxide semiconductor (CMOS) image sensor may include before the second step, a step of performing a reflow process on the patterned photoresist. And, the fourth step includes a radio frequency bias sputter etch process using helium (He) and a radio frequency bias sputter etch process using silver (Ar), which are performed simultaneously.  
         [0019]     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  
       [0020]     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 embodiments of the invention and together with the description serve to explain the principle of the invention. In the drawings:  
         [0021]      FIG. 1  illustrates a circuit diagram of a unit pixel of a general CMOS image sensor;  
         [0022]      FIG. 2  illustrates a cross-sectional view of a unit pixel of a related art CMOS image sensor;  
         [0023]      FIGS. 3A  to  3 D illustrate problems shown in the related art CMOS image sensor;  
         [0024]      FIG. 4  illustrates a cross-sectional view showing a process step for fabricating a CMOS image sensor according to an embodiment of the present invention;  
         [0025]      FIGS. 5A  to  5 D illustrate cross-sectional views showing process steps for fabricating a lens according to an embodiment of the present invention;  
         [0026]      FIGS. 6A and 6B  illustrate cross-sectional views showing the process steps for fabricating the CMOS image sensor according to the present invention, wherein a photoresist reflow process is included;  
         [0027]      FIG. 7  illustrates a graph showing characteristics of a radio frequency (RF) sputter etch process;  
         [0028]      FIG. 8  illustrates a combination of a radio frequency (RF) bias sputter etch using helium (He) and a radio frequency (RF) bias sputter etch using silver (Ar); and  
         [0029]      FIGS. 9A and 9B  illustrate cross-sectional views showing the process steps for fabricating the lens according to another embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0030]     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.  
         [0031]      FIG. 4  illustrates a cross-sectional view showing a process step for fabricating a CMOS image sensor according to an embodiment of the present invention. Referring to  FIG. 4 , the CMOS image sensor according to the present invention further includes a dome-shaped lens  43  formed of a nitride group material between a micro-lens  42  on an uppermost layer and a photodiode (PD). More specifically, a field oxide layer  32  for defining an active area and a field area is formed on the semiconductor layer  31 , and a photodiode (PD)  33  and a plurality of transistors  34  are formed on the active area of the semiconductor layer  31 . Also, an interlayer dielectric  35  is formed on the entire surface of the structure including the photodiode (PD)  33  and the transistors  34 , and a plurality of metal circuits  36 ,  37 , and  38  forming the unit pixel is formed on the interlayer dielectric  35  in different layers. The metal circuits  36 ,  37 , and  38  are aligned so as to prevent the light being irradiated to the photodiode (PD) from being blocked.  
         [0032]     Interlayer dielectrics  39 ,  40 , and  41  are formed between each layer of metal circuits  36 ,  37 , and  38  for electric insulation. And, although not shown in the drawing, a passivation layer for protecting the device from humidity and scratches, R, G, and B color filter layers for representing a color image, and an overcoating layer are formed on the last interlayer dielectric  41 . And, a micro-lens  42  having a dome structure and formed of a photoresist material is formed on the overcoating layer. Then, a dome-shaped lens  43  formed of a nitride group material is formed on the interlayer dielectric  39  between the photodiode (PD)  33  and the micro-lens  42 , which can easily transmit the light passing through the micro-lens  42  to the photodiode (PD). Herein, the lens  43  is formed of a nitride group material, which is highly heat resistant, instead of a photoresist material because the photoresist material may leak due to the temperature of a following process. The above description is an example of the lens  43  being formed on the interlayer dielectric  39 . However, the lens  43  may also be formed on different interlayer dielectrics  35  or  37  between the photodiode (PD) and the micro-lens  42 .  
         [0033]     The method of fabricating the lens  43  will now be described in detail. The members parts other than the lens  43  are identical to those described above, and the description of the other members parts will, therefore, be omitted for simplicity.  FIGS. 5A  to  5 D illustrate cross-sectional views showing process steps for fabricating a lens according to an embodiment of the present invention. And,  FIGS. 6A and 6B  illustrate cross-sectional views showing the process steps for fabricating the CMOS image sensor according to the present invention, wherein a photoresist reflow process is included.  
         [0034]     Referring to  FIG. 5A , a sub layer  50  having a field oxide layer, a photodiode, a transistor, a metal circuit, and an interlayer dielectric formed thereon is deposited on a nitride layer  51 . A photoresist layer  52  is deposited on the nitride layer  51 . And, then, the photoresist layer  52  is selectively patterned by using light exposure and development processes, thereby leaving the photoresist only on a specific portion for forming the lens. Subsequently, as shown in  FIG. 5B , by using the patterned photoresist  52  as a mask, the nitride layer  51  is treated with a dry-etch process, so as to remove the photoresist  52 .  
         [0035]     In order to enhance efficiency, after patterning the photoresist  52 , the patterned photoresist  52  is treated with a baking process, so as to perform a reflow process on the photoresist  52 . Then, as shown in  FIG. 6A , a curvature is formed on the photoresist  52 . And, when the nitride layer  51  therebelow is etched by using the photoresist  52  as the mask, a slight curvature is formed in advance on the nitride layer  51 , as shown in  FIG. 6B .  
         [0036]     Thereafter, referring to  FIG. 5C , a radio frequency sputter etch is performed. Due to its characteristics, the RF bias sputter etch has a greater etch rate on the corner portions, and so the corners of the nitride layer  51  are etched to have rounded edges, as shown in  FIG. 5C .  
         [0037]      FIG. 7  illustrates a graph showing characteristics of a radio frequency (RF) sputter etch process. More specifically,  FIG. 7  illustrates a graph showing etch rates in accordance with the degrees of the radio frequency (RF) bias sputter etch process using helium (He) and the radio frequency (RF) bias sputter etch process using silver (Ar). And,  FIG. 8  illustrates a combination of a radio frequency (RF) bias sputter etch using helium (He) and a radio frequency (RF) bias sputter etch using silver (Ar).  
         [0038]     Generally, the RF bias sputter etch process shows a different etch rate depending upon the degree. Therefore, as shown in  FIG. 7 , when performing the RF bias sputter etch rate using helium (He), the etch rate is at a maximum level at 60°. And, when performing the RF bias sputter etch rate using silver (Ar), the etch rate is at a maximum level at 45°.  
         [0039]     Therefore, in order to increase the sensitivity of the curvature of the lens, the RF bias sputter etch process using helium (He) and the RF bias sputter etch process using silver (Ar) are performed in combination. Then, as shown in  FIG. 8 , when the etching process is repeated by performing the RF bias sputter etch process using helium (He) along a direction of 60° and by performing the RF bias sputter etch process using silver (Ar) along a direction of 45°, the curvature of the etched nitride layer becomes more smooth. Subsequently, referring to  FIG. 5D , a wet-etch process is performed so as to wear out the corner portions of the nitride layer  51 , thereby forming a dome-shaped lens  51   a.    
         [0040]      FIGS. 9A and 9B  illustrate cross-sectional views showing the process steps for fabricating the lens according to another embodiment of the present invention. Herein, a dome-shaped oxide layer  53  is formed, and a nitride layer  51  is deposited on the oxide layer  53 , thereby forming a double-layered lens  51   a  of oxide layer and nitride layer.  
         [0041]     As described above, the CMOS image sensor and the method of fabricating the same has the following advantages. A lens is provided between a micro-lens and a photodiode, so as to transmit light to the photodiode, thereby enhancing light-focusing efficiency of the image sensor. Also, a lens is formed between the micro-lens and the photodiode, so as to allow the light to be transmitted to the entire surface of the photodiode, thereby preventing image distortion at corner portions of the image and unclearness of the image, which is caused by dark current, from occurring.  
         [0042]     Furthermore, light passing through the lens is focused once again, thereby allowing a certain amount of variation in the fabrication process of the micro-lens. And, thus, a margin of the micro-lens fabrication process can be enhanced. And, finally, even when the micro-lens and the photodiode become spaced further apart, the lens allows the light to be focused on the photodiode, thereby improving the degree of integration of the CMOS image sensor.  
         [0043]     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.