Abstract:
Disclosed are a complementary metal oxide semiconductor (CMOS) device and a method for fabricating the same. The CMOS image sensor includes: a photodetector; a microlens formed on the photodetector; an insulating passivation layer formed on the microlens to protect the microlens; and an oxide layer with a refraction index lower than that of the microlens formed between the microlens and the insulating passivation layer. The method for fabricating a CMOS image sensor includes the steps of: forming a photodetector on a substrate; forming a microlens on the photodetector; forming an oxide layer having a refraction index lower than the microlens on the microlens; and forming an insulating passivation layer for protecting the microlens on the oxide layer.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to a complementary metal oxide semiconductor (CMOS) image sensor; and more particularly, to a CMOS image sensor including a microlens capable of efficiently collecting a light and an upper structure of the microlens.  
       DESCRIPTION OF RELATED ARTS  
       [0002]     In general, a complementary metal oxide semiconductor (CMOS) image sensor is a semiconductor device that converts an optical image to an electrical signal. A charge coupled device (CCD) and the CMOS image sensor are typical examples of the image sensors.  
         [0003]     In the image sensor, the charge coupled device (CCD) is a semiconductor device that each of metal-oxide-silicon (MOS) capacitors are placed in close proximity and charge carriers are stored in and transferred to the capacitors. The CMOS image sensor is a semiconductor device adopting a switching method for sequentially detecting an output by making and using MOS transistors as many as the number of pixels based on CMOS technology using peripheral circuits such as control circuits and signal processing circuits.  
         [0004]      FIG. 1  is a circuit diagram illustrating a unit pixel of a conventional CMOS image sensor.  
         [0005]      FIG. 1  is a circuit diagram illustrating the unit pixel provided with one photodiode (PD) and four MOS transistors for the conventional CMOS image sensor. The unit pixel is formed with a photodiode (PD)  100  for generating photo-generated charges by receiving a light, a transfer transistor for transferring the photo-generated charges collected at the photodiode  100  to a floating diffusion region  102 , a reset transistor  103  for setting electric potentials of the floating diffusion region and discharging charges, thereby resetting the floating diffusion region  102 , a drive transistor  104  for serving a role of a source follower buffer amplifier by that a voltage of the floating diffusion region is transferred to a gate and a select transistor  105  for serving a role in addressing and switching. Outside of the unit pixel, a load transistor  106  is formed to read an output signal.  
         [0006]      FIG. 2  is a cross-sectional view illustrating a unit pixel of a conventional CMOS image sensor.  
         [0007]     If examining the unit pixel of the conventional image sensor with reference to  FIG. 2 , a plurality of inter-layer insulation layers  13 ,  14  and  15  are sequentially formed on a photodetector.  11  formed on a substrate  10 . More specifically, the plurality of inter-layer insulation layers are classified as a first inter-layer insulation layer  13 , a second inter-layer insulation layer  14  and a third inter-layer insulation layer  15 . A first interconnection line  16  is placed between the first inter-layer insulation layer  13  and the second inter-layer insulation layer  14 . A second interconnection line  17  is placed between the second inter-layer insulation layer  14  and the third inter-layer insulation layer  15 . Herein, a reference numeral  12  denotes a device isolation layer.  
         [0008]     Furthermore, there are a plurality of planarization layers  18  and  20  on top of the third inter-layer insulation layer  15 . Herein, a first planarization layer is denoted with a reference numeral  18  and a second planarization layer is denoted with a reference numeral  20 . A color filter  19  is formed between the first planarization layer  18  and the second planarization layer  20 . Herein, a reference numeral  19 A denotes an adjacent color filter.  
         [0009]     A microlens  21  is formed on the second planarization layer  20  and a low temperature insulating passivation layer  22  is formed thereon.  
         [0010]     The conventional image sensor forms the first interconnection line  16  and the second interconnection line  17  on an upper structure of the photodetector  11  and a passivation layer is formed thereon. Afterwards, a planarization process is performed before forming the color filter  19  and then, the color filter  19  is formed. Then, the planarization process is performed thereon  19  one more time.  
         [0011]     Thereafter, the microlens  21  is formed and then, the low temperature insulating passivation layer  22  is deposited for protecting a photoresist layer that is a main component of the microlens from external contamination and preventing a metal etch damage particularly generated during a bump process.  
         [0012]     However, when an incident light passes the microlens through the low temperature insulating passivation layer, a difference in a refraction index between two materials is not large. Thus, the light incident on edges of the microlens cannot be collected to the photodetector  11 , thereby frequently generating a case that the light gets incident on the metal interconnection lines surrounding the pixel.  
         [0013]     That is, the light passing the edges of the microlens is not collected to the photodetector  11 . Instead, the light is transferred to the surrounding metal interconnection line or even to the pixels adjacent to the metal interconnection line. Accordingly, a cross talk between the pixels is generated, thereby decreasing photosensitivity while the light reaches the photodetector  11  that is a photo-detecting unit.  
       SUMMARY OF THE INVENTION  
       [0014]     It is, therefore, an object of the present invention to provide a complementary metal oxide semiconductor (CMOS) device capable of preventing a decrease in photosensitivity generated by a low temperature insulating passivation layer formed for protecting a microlens.  
         [0015]     In accordance with one aspect of the present invention, there is provided a complementary metal oxide semiconductor (CMOS) image sensor, including: a photodetector; a microlens formed on the photodetector; an insulating passivation layer formed on the microlens to protect the microlens; and an oxide layer with a refraction index lower than that of the microlens formed between the microlens and the insulating passivation layer.  
         [0016]     In accordance with another aspect of the present invention, there is provided a method for fabricating a CMOS image sensor, including the steps of: forming a photodetector on a substrate; forming a microlens on the photodetector; forming an oxide layer having a refraction index lower than the microlens on the microlens; and forming an insulating passivation layer for protecting the microlens on the oxide layer. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]     The above and other objects and features of the present invention will become better understood with respect to the following description of the preferred embodiment given in conjunction with the accompanying drawings, in which:  
         [0018]      FIG. 1  is a circuit diagram illustrating a unit pixel of a conventional complementary metal oxide semiconductor (CMOS) device;  
         [0019]      FIG. 2  is a cross-sectional view illustrating a conventional CMOS image sensor;  
         [0020]      FIGS. 3A  to  3 E are cross-sectional views illustrating a unit pixel of a CMOS image sensor in accordance with the preferred embodiment of the present invention; and  
         [0021]      FIGS. 4A  to  4 C are graphs comparing an experiment data used for a unit pixel of a CMOS image sensor fabricated as shown in  FIGS. 3A  to  3 E with that used for a unit pixel of a conventional CMOS image sensor. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0022]     Hereinafter, detailed descriptions on preferred embodiments of the present invention will be provided with reference to the accompanying drawings.  
         [0023]      FIGS. 3A  to  3 E are cross-sectional views illustrating a unit pixel of a complementary metal oxide semiconductor (CMOS) image sensor in accordance with the preferred embodiment of the present invention.  
         [0024]     As shown in  FIG. 3A , the CMOS image sensor according to the present invention forms a photodetector  31  on a substrate  30 .  
         [0025]     Subsequently, a first inter-layer insulation layer  33 , a second inter-layer insulation layer  34  and a third inter-layer insulation layer  35  are sequentially formed thereon. A first interconnection line  36  is formed between the first inter-layer insulation layer  33  and the second inter-layer insulation layer  34  and a second interconnection line  37  is formed between the second inter-layer insulation layer  34  and the third inter-layer insulation layer  35 .  
         [0026]     Subsequently, a first planarization layer  38  is formed and then, a color filter  39  is formed on an upper structure of the photodetector  31 . Herein, a reference numeral  39 A denotes an adjacent color filter.  
         [0027]     Subsequently, a second planarization layer  40  is formed on the color filter  39 .  
         [0028]     Next, a microlens  41  of which a refraction index (n) is approximately 1,592 is formed on the color filter  39 . Subsequently, a spin-on-glass (SOG) based oxide layer  42  of which a refraction index (n) is approximately 1.41, at a wavelength of approximately 450 nm is formed in order to cover the microlens  41 . Herein, the oxide layer  42  is coated in a thickness ranging from approximately 4,000 Å to approximately 5,000 Å. Also, an insulation layer having a refraction index (n) less than approximately 1.5 can be used instead of the SOG based oxide layer.  
         [0029]     Subsequently, a photoresist layer  43  is formed on the oxide layer  42 .  
         [0030]     Next, as shown in  FIG. 3B , the photoresist layer  43  is selectively removed, thereby forming a photoresist pattern  43 A.  
         [0031]     Subsequently, as shown in  FIG. 3C , the oxide layer  42  is selectively removed by using the photoresist pattern  43 A as an etch mask. Herein, the photoresist pattern  43 A uses a negative photoresist layer and the oxide layer  42  except a pixel is selectively removed by using a mask for opening the pixel portion.  
         [0032]     Subsequently, as shown in  FIG. 3D , the photoresist pattern  43 A is removed.  
         [0033]     Next, as shown in  FIG. 3E , the low temperature insulating passivation layer  45  of which a refraction index (n) is approximately 1.55 at a wavelength of approximately 450 nm is formed on the oxide layer  42 A. The low temperature insulating passivation layer  45  is formed in a thickness raging from approximately 2,000 Å to approximately 4,000 Å.  
         [0034]     In case of the image sensor using 0.18 μm technology, as a height difference of an insulation layer formed on the photodetector  31  is reduced than before, an amount of the incident light increases, thereby improving the photosensitivity. However, there is a problem of generating a difference between the photosensitivity in edges and the photosensitivity in the center.  
         [0035]     The above difference is caused by a phenomenon that the edges of the unit pixel are defocused. It is possible to generate this phenomenon if an incidence angle of the light is controlled. The CMOS image sensor according to the present invention controls a refraction angle of the light in order to collect the light incident on the photodetector much better.  
         [0036]     The unit pixel of the CMOS image sensor in accordance with the present invention makes the light passing trough the low temperature insulating passivation layer incident on the microlens  41  through the SOG based oxide layer  42 A.  
         [0037]     The light passing through the microlens  41  is collected to the photodetector  31 . At this time, a path to collect the light to the photodetector  41  is decided based on refraction indexes of the oxide layer  42 A and the microlens  41 .  
         [0038]     An incidence angle determined by the light passing through two materials having different refraction indexes is decided on the Snell&#39;s Law, i.e., n i sin Θi=n r Sin Θr. Herein, n i  denotes the refraction index of the oxide layer  42 A and n r  denotes the refraction index of the microlens  41 . Accordingly, the larger the difference between the refraction index of the microlens  41  and the refraction index of the oxide layer  42 A is, the more the light is refracted. Thus, the light is collected to the photodetector  31  much better.  
         [0039]     For the unit pixel included in the conventional CMOS image sensor, the insulating passivation layer  45  is formed directly on the microlens  41 . However, since the difference between the refraction index of the low temperature insulating passivation layer  45  and the refraction index of the microlens  41  is not large, the refraction angle of the light becomes small. Thus, the light passing through the microlens  41  is not well collected to the photodetector. Particularly, in case of the light passing through the edges of the microlens  41 , a degree that the light is collected to the photodetector is much worse.  
         [0040]     However, the CMOS image sensor in accordance with the present invention forms the SOG based oxide layer  42  between the microlens  41  and the insulating passivation layer  45 . Accordingly, when the light passing through the insulating passivation layer  45  passes the microlens  41 , the light is transferred to the photodetector  31  by being more refracted to the photodetector  31  for the refraction index of the light. Particularly, the light passing through the edges of the microlens  41  in accordance with the present invention is refracted much more toward the photodetector compared with conventional image sensor, thereby improving a light collecting ability.  
         [0041]     In the present invention, the SOG based oxide layer  42 A is used as a layer capable of efficiently collecting the light since the refraction index of the SOG based oxide layer provides a big difference from the refraction index of the photoresist layer used as the microlens  41 . Furthermore, any layers having a lower refraction index, i.e., n&lt;1.5, than the refraction index of the microlens, i.e., n=1.592, can be used in the present invention.  
         [0042]     Meanwhile, to try to make the refraction index of the oxide layer  42 A less than the refraction index of the microlens  41 , the refraction index of the oxide layer  42 A gets larger than the refraction index of the insulating passivation layer  45 .  
         [0043]     Before being transferred to the microlens  41 , the incident light passes through the insulating passivation layer  45  and the oxide layer  42 A. At this time, when the light gets incident on the oxide layer  42  having a small refraction index from the insulating passivation layer  45  having a large refraction index, there is a possibility that the light is refracted to the opposite side of the photodetector  31 . However, in this case, since the light gets incident vertically, i.e., in an angle of 90°, a phenomenon that the light is refracted in the opposite side of the photodetector is not happened.  
         [0044]     Accordingly, in order not to produce the above problem, the present invention planarizes the oxide layer  42 A surrounding the microlens and forms the insulating passivation layer  45  thereon.  
         [0045]     Table 1 shown below and  FIGS. 4A  to  4 C illustrates an experiment data about photosensitivity of both a unit pixel of a CMOS image sensor fabricated as described in  FIGS. 3A  to  3 E and a unit pixel of a conventional image sensor. Herein, photosensitivity is referred as a white sensitivity.  
         [0046]     Table 1 illustrates that the data about photosensitivity used for the conventional image sensor and the image sensor in accordance with the present invention. Herein, the above data about photosensitivity illustrates each case of a red pixel, a blue pixel and a green pixel, respectively. More particularly, the above data indicates photosensitivity in edges and the center of a microlens. Furthermore, for each unit pixel, ratios of the red pixel and the blue pixel with respect to the green pixel are illustrated.  
         [0047]     Referring to Table 1, a control group indicates a case that only low temperature insulating passivation layer is formed on a microlens in accordance with the conventional image sensor and a SOG deposition experimental group indicates a case that a SOG based oxide layer is formed between a microlens and an insulating passivation layer in accordance with the present invention. Hereinafter, the SOG deposition experimental group is expressed as an experimental group.  
                                                                                   TABLE 1                                           SOG Deposition                   Experimental Group               SOG 5,000 Å + Low               temperature   Control Group           Scheme   insulating layer   Low insulating layer           Condition   2,000 Å   2,000 Å            Test Item   Wafer ID   18   19   20   21                    W-GREEN Sensitivity CENTER   mV/lux sec   733   705   733   731       W-RED Sensitivity CENTER   mV/lux sec   477   457   468   464       W-BLUE Sensitivity CENTER   mV/lux sec   485   466   553   557       W-GREEM TO GREEN _Ratio   —   1.01   1.01   1.01   1.01       CENTER       W-RED TO GREEN _Ratio   —   0.652   0.665   0.607   0.604       CENTER       W-BLUE TO GREEN _Ratio   —   0.663   0.666   0.758   0.764       CENTER       W-GREEN_Sensitivity_EDGE   mV/lux sec   412   398   314   308       W-RED_Sensitivity_EDGE   mV/lux sec   339   327   234   228       W-BLUE_Sensitivity_EDGE   mV/lux sec   336   324   273   270       W-GREEN TO   —   1   1   1   1       GREEN_RATIO_EDGE       W-RED TO GREEN_RATIO_EDGE   —   0.823   0.822   0.745   0.740       W-BLUE TO GREEN_RATIO_EDGE   —   0.816   0.814   0.869   0.877                  
 
         [0048]     In the control group, a thickness of the low temperature insulating passivation layer is approximately 8,000 Å and in the experimental group, a thickness of the SOG based oxide layer is approximately 5,000 Å and a thickness of the low temperature insulating passivation layer is approximately 2,000 Å.  
         [0049]      FIG. 4A  illustrates a graph illustrating a data about photosensitivity in the center of a microlens for each red, blue and green unit pixel of a conventional CMOS image sensor and of a CMOS image sensor in accordance with the present invention.  FIG. 4B  is a graph illustrating a data about photosensitivity in edges of a microlens for each red, blue and green unit pixel of a conventional CMOS image sensor and of a CMOS image sensor in accordance with the present invention.  
         [0050]      FIG. 4C  is a graph illustrating the data shown in  FIG. 4A and 4B  at the same time.  
         [0051]     First, if examining photosensitivity in the center of the microlens, there is almost no change in the photosensitivity difference between the control group and the experimental group in case of the green and red pixels. For instance, for the control group, the photosensitivity of the green pixel ranges from approximately 731 mV/lux sec to approximately 733 mV/lux sec and the photosensitivity of the red pixel ranges from approximately 464 mV/lux sec to approximately 468 mV/lux sec. For the experimental group, the photosensitivity of the green pixel ranges from approximately 705 mV/lux sec to approximately 733 mV/lux sec and the photosensitivity of the red pixel ranges from approximately 457 mV/lux sec to approximately 477 mV/lux sec.  
         [0052]     Furthermore, in case of the blue pixel, the photosensitivity of the experimental group is decreased by approximately 60 mV/lux sec to approximately 80 mV/lux sec compared with the control group. For instance, for the control group, the photosensitivity of the blue pixel ranges from approximately 553 mV/lux sec to approximately 557 mV/lux sec. For the experimental group, the photosensitivity of the blue pixel ranges from approximately 466 mV/lux sec to approximately 485 mV/lux sec.  
         [0053]     In case of the experimental group, the photosensitivity ratio of the blue pixel to the green pixel and the photosensitivity ratio of the,red pixel to the green pixel are shifted in almost the same values.  
         [0054]     In general, in the CMOS image sensor, it is preferred that the photosensitivity ratio of the red pixel to the green pixel and the photosensitivity ratio of the green pixel to the blue pixel are almost the same. Thus, it is possible to obtain a good image quality produced by processed information when the red pixel and the blue pixel have almost the same photosensitivity.  
         [0055]     In order to collect a light incident on the edges of the microlens to a photodetector, the CMOS image sensor in accordance with the present invention includes an oxide layer having a refraction index lower than the refraction index of the microlens. As a result, the photosensitivity ratio of the red pixel to the green pixel and the photosensitivity ratio of the blue pixel to the green pixel become almost the same, thereby improving the CMOS image sensor.  
         [0056]     Meanwhile, if examining the photosensitivity in the edges of the microlens, in case of the green pixel and the red pixel, the photosensitivity of the control group increases by approximately 100 mV/lux sec compared with the photosensitivity of the experimental group. For instance, for the control group, the photosensitivity of the green pixel ranges from approximately 308 mV/lux sec to approximately 314 mV/lux sec and the photosensitivity of the red pixel ranges from approximately 228 mV/lux sec to approximately 234 mV/lux sec. For the experimental group, the photosensitivity of the green pixel ranges from approximately 398 mV/lux sec to approximately 412 mV/lux sec and the photosensitivity of the red pixel ranges from approximately 327 mV/lux sec to approximately 339 mV/lux sec.  
         [0057]     Furthermore, in case of the blue pixel, the photosensitivity of the experimental group increases approximately 60 mV/lux sec compared with the photosensitivity of the control group. Accordingly, since the photosensitivity of the blue pixel increases less than the photosensitivity of the red pixel as much as approximately 40 mV/lux sec with respect to the red pixel of which the photosensitivity increases by approximately 100 mV/lux sec, the photosensitivity of the blue pixel is shifted in a relatively similar level with the photosensitivity of the red pixel.  
         [0058]     Thus, in case of the experimental group, the photosensitivity ratio of the blue pixel to the green pixel and the photosensitivity ratio of the red pixel to the green pixel are shifted in almost the same value.  
         [0059]     In summary, by forming the SOG based oxide layer  42 A between the microlens  41  and the insulating passivation layer  45  for the CMOS image sensor in accordance with the present invention, the photosensitivity in the edges of the microlens is greatly increased without changing the photosensitivity of the center of the microlens. The CMOS image sensor in accordance with the present invention provides effects of increasing the photosensitivity of the red pixel and the photosensitivity of the blue pixel as much as approximately 100 mV/lux sec and increasing the photosensitivity of the blue pixel as much as approximately 60 mV/lux sec.  
         [0060]     Furthermore, the CMOS image sensor fabricated by forming the SOG based oxide layer between the microlens  41  and the insulating passivation layer  45  serves a role in shifting the photosensitivity of the blue pixel to make the photosensitivity of the blue pixel approach to the photosensitivity ratio of the blue pixel to the green pixel and of the red pixel to the green pixel.  
         [0061]     Furthermore, since a characteristic of a dead zone is lowered from approximately −4 mV to approximately −2.8 mV, a defect caused by black fine dots appearing on an image can be reduced below the half level of the defect.  
         [0062]     The present invention makes a light to be collected to a photodetector at the maximum extent, thereby improving photosensitivity of a unit pixel.  
         [0063]     Furthermore, a CMOS image sensor in accordance with the present invention reduces a difference between photosensitivity in the center of a unit pixel and photosensitivity in edges of a unit pixel and thus, a more reliable image processing with respect to a light is possible through the CMOS image sensor in accordance with the present invention.  
         [0064]     The present application contains subject matter related to the Korean patent application No. KR 2004-0072280, filed in the Korean Patent Office on Sep. 9, 2004 the entire contents of which being incorporated herein by reference.  
         [0065]     While the present invention has been described with respect to certain preferred embodiments, it will be apparent to those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the invention as defined in the following claims.