Abstract:
Provided is a solid-state CMOS image sensor, specifically a CMOS image sensor pixel that has stacked photo-sites, high sensitivity, and low dark current. In an image sensor including an array of pixels, each pixel includes: a standard photo-sensing and charge storage region formed in a first region under a surface portion of a substrate and collecting photo-generated carriers; a second charge storage region formed adjacent to the surface portion of the substrate and separated from the standard photo-sensing and charge storage region; and a potential barrier formed between the first region and a second region underneath the first region and diverting the photo-generated carriers from the second region to the second charge storage region.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a solid-state image sensor; and, more particularly to a complementary metal oxide semiconductor (CMOS) image sensor with stacked photo-sites, which result in a compact pixel layout, high sensitivity, and low dark current. The vertical photo-site arrangement obviates the need for utilization of standard light-absorbing color filters to sense colors and increases the sensor pixel density. 
     DESCRIPTION OF RELATED ARTS 
     Typical image sensors sense light by converting impinging photons into electrons that are integrated (collected) in sensor pixels. After completion of an integration cycle, collected charge is converted into a voltage, which is supplied to output terminals of the sensor. In a CMOS image sensor, charge-to-voltage conversion is accomplished directly in the pixels themselves and the analog pixel voltage is transferred to the output terminals through various pixel addressing and scanning schemes. The analog signal can also be converted on-chip to a digital equivalent before reaching the chip output. 
     The pixels have incorporated therein a buffer amplifier, typically a source follower, which drives sense lines that are connected to the pixels by suitable addressing transistors. After the charge-to-voltage conversion is completed and the resulting signal transferred out from the pixels, the pixels are reset in order to be ready for accumulation of new charge. In pixels that are using floating diffusion (FD) as a charge detection node, the reset is accomplished by turning on a reset transistor that momentarily conductively connects the FD node to a voltage reference. 
     This step removes the collected charge. However, the removal generates kTC-reset noise as is well known in the art. kTC noise has to be removed from the signal by the correlated double sampling (CDS) signal processing technique in order to achieve a desired low noise performance. The typical CMOS sensors that utilize the CDS concept need to have four transistors (4T) in the pixel. 
     An example of the 4T pixel circuit can be found in U.S. Pat. No. 5,991,184 issued to J. W. Russell et al. By introducing switching pulses into a Vdd bias line, it is possible to eliminate a select transistor from the pixel and achieve CDS operation with only 3T in the pixel as described by Masahiro Kasano in an article entitled “A 2.0 μm Pixel Pitch MOS Image Sensor with an Amorphous Si Film Color Filter,” in Digest of Technical Papers ISCC, Vol. 48, February 2005, pp. 348-349. The larger number of transistors in each pixel may become a disadvantage when the pixel size needs to be reduced in order to build low cost and high-resolution image sensors. Standard 3T pixels cannot use the CDS concept for kTC noise suppression and thus, some other methods need to be used to minimize the adverse effects of this noise. 
     The color sensing in most single chip CMOS and CCD image sensors is accomplished by placing various light absorbing and color transmitting filters on top of the pixels in a predetermined pattern. Thus, the different pixels in a given pixel sub-group or a sub-array become sensitive only to a certain wavelength band of the spectrum. Hence, the pixel sub-groups form single color super pixels. The signal from the “color sensitive” sub-group pixels is then used to construct the color super-pixel signal using various interpolating and color signal-processing methods in an attempt to recover the resolution that has been unavoidably lost in this scheme. An example of a typical color pixel pattern can be found in U.S. Pat. No. 3,971,065 issued to B. E. Bayer. Another example of the color filter arrangement can be found in the article by Masahiro Kasano already mentioned above. All these approaches to the color sensing may have a principal disadvantage of sacrificing the resolution as mentioned above and sacrificing sensitivity by absorbing light in color filters. 
       FIG. 1  is a cross-sectional view illustrating a standard photo-site of a typical 4T pixel and an associated pixel circuit. Particularly, a pinned photodiode light-sensing element and a simplified diagram of the associated pixel circuit are illustrated in  FIG. 1 . A p-type silicon substrate  101  has a shallow trench isolation (STI) region  102 , obtained by forming a trench through etching the substrate  101  to a certain depth and filling the trench with a silicon dioxide  103  layer. The silicon dioxide layer  103  also covers the remaining surface of the pixel. A shallow p+-type doped region  104  passivates the walls and the bottom of the STI region  102  as well as the surface of the pixel. A photo-generated charge is collected in the n-type doped region  105  of the pinned photodiode. When a charge integration cycle is completed, the charge from the STI photodiode region (i.e., the n-type doped region  105 ) is transferred to the floating diffusion (FD)  106  by turning a gate  107  momentarily on. The FD  106  is reset by a first transistor  118  to a suitable potential (i.e., Vdd), and the FD potential changes are sensed by a second transistor  114 . A capacitor Cs  119 , connected between a Vdd node  117  and a FD node  113 , is used to adjust a conversion gain of the pixel. The pixel is addressed via a select transistor  115 . Control signals are supplied to the pixel via a transfer gate bus Tx  112 , a reset gate bus Rx  120  and an address gate bus Sx  121 . The output from the pixel is supplied to a pixel column bus  116 . When photons  122  impinge on the pixel, the photons penetrate into the silicon bulk depending on their wavelengths and create electron-hole pairs. Electrons are generated both in a depleted region  109  and in an undepleted region of the substrate  101 . The electrons  110  generated in the undepleted region of the substrate  101  then diffuse to the edge of a depletion region  109  where they are quickly swept into a potential well located in the n-type doped region  105 . The electrons generated in the neutral undepleted region can also diffuse laterally and contribute to cross-talk between the pixels. For this reason, the depletion region is formed with a certain depth Xd  111 , so that the above mentioned unwanted phenomenon could be minimized. 
     While functioning well, this pixel has no ability to separate charge according to the depth of charge generation and thus according to the wavelength of the photons that have created the charge. As a result, it is necessary to place color filters on top of the pixels to absorb certain portions of the spectrum in order to create the color sensing ability. The absorption of light causes loss of sensitivity, which is an unwanted side effect of this method of color sensing. 
     One solution to this limitation has been found and is already pursued by several companies, for example, by Foveon as can be learned in U.S. Pat. No. 6,894,265 issued to R. B. Merrill et al. In this approach, three photo-diodes are placed on top of each other inside the silicon bulk and photo-generated carriers are collected at different depths depending on a wavelength of impinging light. A voltage signal is then obtained by connecting these buried photodiodes to circuits located on top of the silicon surface, and charge is sensed, processed, and reset via a typical scheme. One advantage of this approach is that no resolution is sacrificed by placing the color filter covered pixels side by side and no photons need to be absorbed in the color filters. However, it may not be easy to form the photodiodes that are buried deeply in the silicon bulk. Also, it may be difficult to sense charge collected in the buried photodiodes by circuits located on top of the silicon without adding noise. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a CMOS image sensor with stacked photo-sites, which sense color by vertically separating photo-generated carriers, so that the CMOS image sensor has an advantage of providing two or more color-coded signals without using conventional light absorbing color filters. Placing suitable potential barriers under a typical pinned photodiode structure achieves this goal and other objects of the invention. 
     In accordance with an aspect of the present invention, there is provided an image sensor including an array of pixels, each pixel including: a standard photo-sensing and charge storage region formed in a first region under a surface portion of a substrate and collecting photo-generated carriers; a second charge storage region formed adjacent to the surface portion of the substrate and separated from the standard photo-sensing and charge storage region; and a potential barrier formed between the first region and a second region underneath the first region and diverting the photo-generated carriers from the second region to the second charge storage region. 
     In accordance with another aspect of the present invention, there is provided an image sensor including a pixel array, wherein: the pixel array includes a group of pixels with first color filters and a group of pixels with second color filters, which are arranged in the form of a checkerboard pattern; and the pixels with the first color filters have standard photo-sites and the pixels with the second color filters have stacked photo-sites. 
     The above described exemplary embodiments of the present invention address usual difficulties and provide a simpler and more practical solution for color sensing with less resolution loss than in the typical approach and with minimum loss of light sensitivity. For instance, U.S. Pat. No. 6,894,265 issued to Richard B. Merrill et al. teaches one typical approach of forming the buried photodiode and collecting and storing charge in the deep silicon bulk. On the contrary to the typical approach, a special potential barrier is placed under the standard pinned photodiode, and thus, it is possible do divert the photo-generated carriers from the deep bulk and direct the photo-generated carriers to flow in a narrow region to the surface of the silicon substrate where the photo-generated carriers can be easily collected and stored for readout. 
     The carriers from the bulk can thus be conveniently stored in a suitable structure next to the carriers generated and stored in the standard photodiode near the silicon substrate surface. It is thus not necessary to form buried photodiodes and collect and store charge deep in the bulk of the silicon, which is often difficult to access, read, and reset. It is also possible to place the special potential barrier in different depths in different pixels and thus make the pixels sensitive to different light spectral regions. Each pixel can thus provide two or more differently coded color signals instead of one. The resolution is not sacrificed as much as in the typical approach and the light sensitivity is also not sacrificed, since no color absorbing filters or not as many color absorbing filters are used. Storing all the photo-generated charge close to the silicon surface makes possible to share some of the low noise readout and reset circuitry that is located there and thus achieve high performance with very small pixel sizes. This approach is thus much simpler and easier to implement in the current CMOS technology with high yield. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other objects and features of the present invention will become better understood with respect to the following description of the exemplary embodiments given in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a simplified cross-sectional view illustrating a standard pinned photodiode photo-site typically used in a 4T CMOS image sensor pixel and associated pixel circuits; 
         FIG. 2  is a simplified cross-sectional view illustrating a photo-site of associated circuits having a pinned photodiode with an underlying potential barrier in accordance with a first embodiment of the present invention; 
         FIG. 3  is a simplified cross-sectional view illustrating associated circuits having a pinned photodiode with an underlying potential barrier in accordance with a second embodiment of the present invention; 
         FIG. 4  is a diagram illustrating circuits where the charge packets can be read independently using a CDS readout method in accordance with a third embodiment of the present invention; 
         FIG. 5  is a diagram illustrating pixels with stacked photo-sites, in which cyan and magenta color filters with micro-lenses are placed on top of the pixels in accordance with a fourth embodiment of the present invention; 
         FIG. 6  is a diagram illustrating pixels with stacked photo-sites, in which cyan color filters with micro-lenses are placed on top of a group of the pixels next to a group of the pixels without color filters and the two groups of the pixels have different conversion gains depending on a value of a pixel capacitor in accordance with a fifth embodiment of the present invention. 
         FIG. 7  is a diagram illustrating pixels, in which micro-lenses are placed on top of the pixels without absorbing color filters and the different pixels have different depths of a potential barrier Xb and different values of conversion gain in accordance with a sixth embodiment of the present invention; 
         FIG. 8  is a diagram illustrating a color filter arrangement in a two-dimensional pixel array for magenta and cyan on top of the pixels to achieve an improved and more compact color sensing in accordance with a seventh embodiment of the present invention; 
         FIG. 9  is a diagram illustrating a pixel arrangement in a two-dimensional pixel array without any color filters on top of the pixels to achieve an improved and more compact color sensing in accordance with an eighth embodiment of the present invention; and 
         FIG. 10  is a diagram illustrating an arrangement of two pixels respectively with a standard photo-site and a stacked photo-site in accordance with a ninth embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings. 
       FIG. 2  is a simplified cross-sectional view illustrating a pixel with a stacked photo-site and a potential barrier and readout circuits associated with the pixel in accordance with a first embodiment of the present invention. 
     According to the first embodiment of the present invention, the pixel has an ability to separate charge according to the depth of charge generation and thus sense color. A substrate  201  has a shallow STI region  202 , obtained by forming a trench through etching the substrate  201  to a certain depth and filling the trench with a silicon dioxide layer  203 . The silicon dioxide layer  203  also covers the entire surface of the pixel. Herein, the substrate  201  is a p-type silicon substrate. A shallow p+-type doped region  204  passivates the walls and the bottom of the STI region  202  as well as the surface of the pixel to minimize a dark current generation. However, in this pixel, a p+-type doped barrier  223  is placed into the pixel at a depth Xb  225 . This p+-type doped barrier  223  separates the pixel into two distinct regions. A photo-generated charge  208  is generated within the depth Xb  225 , which is typically depleted and, the photo-generated charge  208  is collected and stored in an n-type doped region  205  of a pinned photodiode. Charge  210  generated below the p+-type doped barrier  223  in an undepleted region of the substrate  201  diffuses around the p+-type doped barrier  223  into the edge of a depletion region  209  and is collected and stored in a FD  206 . 
     Since the depletion region is made shallower than the depletion region of the typical pixel, it is necessary to add a charge cross-talk barrier  224  into the above structure to minimize the lateral charge diffusion and thus the pixel cross-talk. As for another approach of reducing the cross-talk, a method of making the STI isolation trench deeper is well known to those skilled in the art and thus, will not be discussed here any further. The pixel according to the first embodiment has an ability to detect and separately store charge generated at different depths according to the wavelength of light generating the charge and thus inherently sense color without necessity of light absorbing filters on top of the pixel. A circuit for processing signals from this pixel is substantially identical to the circuits typically known in the art. A first transistor  218  resets a node  213  after a second transistor  214  senses an electric potential of the FD node. A third transistor  215  is a select transistor that connects a pixel signal to a column sense line  216 . The charge, which corresponds to light with longer wavelengths, is collected on the FD  206 . A transfer gate Tx  207  is briefly pulsed to transfer charge collected in a pinned photodiode region to the FD  206 . This charge corresponds to light with shorter wavelengths. The remaining control signals are supplied to the pixel via a reset gate bus Rx  220  and an address gate bus Sx  221 . A conversion gain of this pixel is adjusted by selecting the suitable value for a capacitor Cs  219  that is connected between the node  213  and another node Vdd  217 . 
     When photons  222  impinge on the pixel, they penetrate into the silicon bulk depending on the wavelengths of the photons  222  and create the corresponding electron-hole pairs at the corresponding depths. The pixel according to the first embodiment of the present invention has an ability to sense charge according to the depth of charge generation and thus sense color. As being made clear in the above detailed description, this effect is accomplished without the necessity of forming an additional n-type bulk charge storage region under the pinned photodiode. Only a potential barrier formed by the p+-type doped layer, which does not store charge is added to the pixel. The charge generated below this potential barrier is diverted away from the pinned photodiode and flows into another storage region located at the surface of the substrate. Multiple storage regions can also be formed at the silicon surface. Such an exemplary arrangement will be described with reference to  FIG. 3  hereinafter. 
       FIG. 3  illustrates a cross-sectional view of an improved pinned photodiode and a simplified schematic diagram of an associated pixel circuits in accordance with a second embodiment of the present invention. The pixel has an ability to separate charge according to the depth of charge generation and thus sense color. A substrate  301  has a shallow STI region  302 , obtained by forming a trench through etching the substrate  301  to a certain depth and filling the trench with a silicon dioxide layer  303 . The substrate  301  may be a p-type silicon substrate. The silicon dioxide layer  303  also covers the entire surface of the pixel as in the typical pixel structure. A shallow p+-type doped region  304  passivates the walls and the bottom of the STI region  302  as well as the surface of the pixel to minimize the dark current generation. In this pixel, same as in the first embodiment, a p+-type doped barrier  313  is placed at a depth Xb  315  into the pixel together with a cross-talk barrier  314 . The P+-type doped barrier  313  separates the pixel into two distinct regions as described in the first embodiment, and photo-generated charge  326  generated within the depth Xb  315  (typically depleted) is collected and stored in an n-type doped region  305  of a pinned photodiode. Charge  312  generated below the p+-type doped barrier  313  in an undepleted region of the substrate  301  diffuses around the p+-type doped barrier  313  into the edge of a depletion region  310  and is collected and stored in a special potential well  314  under a gate  308  instead in a FD  306 . The potential well  311 , formed by applying a suitable bias to gates  307 ,  308 , and  309 , stores the charge in a CCD fashion so that the charge can be transferred to the FD  306  and read using the CDS concept, same as the charge stored in the pinned photodiode. The CDS readout concept is well known to those skilled in the art and is used to remove kTC noise from the signal generated by the destructive charge readout of the FD  306 . The rest of the circuit is the same as in the first embodiment with a source follower transistor (SF)  317  sensing a FD node potential, a select transistor  318  connecting an output to a column bus  328 , and a reset transistor  321  resetting the FD  306 . The pixel also uses a capacitance Cs  319 , which is connected between a node  316  and another node Vdd  320 , to adjust a conversion gain of the pixel. The control signals are supplied to the pixel by a reset gate bus Rx  326 , a select gate bus Sx  325 , and three transfer gate buses Tx 1   324 , Tx 2   323 , and Tx 3   322 . 
     Another method for suppressing kTC noise can be used with the above-described pixels, such as a parametric reset, an active reset, or a negative feedback reset that can be introduced into the node  316 . All these techniques are well known to those skilled in the art and will not be discussed here any further. 
     It is also clear to those skilled in the art that the depth Xb  315  of the P+-type doped barrier  313  can be changed from pixel to pixel and thus, different pixels can have different color sensitivity. For example, when ion implantation of boron with energy of approximately 150 keV is used to form the p+-type doped barrier  313 , severing as a potential barrier, the p+-type doped barrier  313  is formed at a depth of approximately 0.4 um. This depth is suitable for the separation of charge created by blue light from charge created by yellow light. On the other hand, when the boron with ion implant energy of approximately 1.2 MeV is used, the p+-type doped barrier  313  is formed at a depth of approximately 2.0 um. This depth is suitable for separation of charge created by cyan light from charge created by red light. Hence, it is possible to extract red (R), green (G) and blue (B) color signals or cyan (Cy), magenta (Mg) and yellow (Ye) color signals from these two pixels by suitable signal processing circuits. It is thus not necessary to use the light absorbing filters placed on top of the pixel and sacrifice the sensor light sensitivity. The processing of color coded pixels and extraction of the R, G, B or other combinations of color signals from such pixels is a well developed technique in the art, and therefore will not be discussed here any further. 
     There are many other combinations of the pinned photodiode arrangements and charge storage wells that can be used with the pixel according to the second embodiment of the present invention. For the simplicity of description one such possibility and another embodiment of the present invention is shown only in a simplified circuit diagram form in  FIG. 4 . 
       FIG. 4  is a simplified circuit diagram illustrating a stacked pixel where charge from a shallow depleted region is stored in a pinned photodiode  401  and charge from a deep undepleted region is directed to another pinned photodiode  402 . The pinned photodiodes  401  and  402  interface with a common FD charge detection node  408  via respective charge transfer gates  403  and  404 . The rest of the circuit is the same as in the first and second embodiments where transistors  405 ,  406  and  407  are a SF transistor  405 , an address transistor and a reset transistor, respectively. Control signals are supplied to the pixel via a reset gate bus Rx  410 , an address gate bus Sx  413 , and two transfer gate buses Tx 1   411  and Tx 2   412 . The pixel Vdd bias is supplied to a terminal  414  and the ground reference is a terminal  415 . The pixel has a capacitor Cs  416  to adjust a conversion gain of the pixel. 
       FIG. 5  is a simplified cross sectional view illustrating a stacked pixel array in which all pixels have substantially an identical construction. A substrate  501  contains an array of substantially identical stacked pixels  502  incorporated with potential barriers at a depth Xb  507  and cross-talk barriers. The substrate  501  has an oxide layer  503  formed on top of the surface of the substrate  501 . Cyan and magenta filters  504  and  505  are formed on top of the oxide layer  503 . Micro-lenses  506  are also formed on top of the cyan and magenta color filters  504  and  505  to improve pixel aperture efficiency. Since each pixel can deliver two color-coded signals, it is easily seen that the pixels with the cyan filters  504  supply blue and green color information while the pixels with the magenta filters  505  supply blue and red color information. The pixels in the present embodiment have substantially identical values for capacitances Cs and thus, substantially an identical conversion gain, and saturate at a nearly identical output level when white light impinges on a sensor. Since only the complementary color filters are always used with the stacked pixels, it is clear to those skilled in the art that the sensitivity of such sensors has improved two times in comparison with the standard Bayer sensor configuration. The resolution is also improved two times in comparison with the standard case, since the total pixel density is twice the standard case. 
       FIG. 6  is a simplified cross-sectional view illustrating a stacked pixel array in which neighboring pixels have different conversion gains. A substrate  601  contains an array of stacked pixels  602  and  603  incorporated with potential barriers at a depth Xb  607  with cross-talk barriers, but with different capacitors Cs 1  and Cs 2  and thus different conversion gains. An oxide layer  604  is formed on top of the surface of the substrate  601  and only cyan color filters  605  are formed on top of the oxide layer  604 . Micro-lenses  606  are also formed on top of a group of the pixels  602  with the color filters  605  as well as on another group of the pixels  603  with no color filters. It is clear to those skilled in the art that the group of the pixels  602  with the cyan filters  605  supplies the blue and green color information while the other group of the pixels  603  with no color filter supplies the blue and yellow color information. The sensitivity can be further improved and different conversion factor values can be used to balance the signal levels in each pixel to make sure that the pixel output saturates at nearly the same level for each pixel when white illumination impinges on a sensor. Interference color filters can be used in this embodiment instead of pigment type color filters, which significantly reduce the height of the pixel structure above the photodiode surface and thus improves the pixel performance for wide light incidence angles. 
       FIG. 7  is a simplified cross-sectional view illustrating a stacked pixel array in which neighboring pixels have different conversion gains and different barrier depths in the neighboring pixels. A substrate  701  contains an array of stacked pixels  702  and  703  incorporated with potential barriers at respective depths Xb 2   707  and Xb 1   706  and also cross-talk barriers. Different capacitors Cs 1  and Cs 2  provide different conversion gains for the pixels  702  and  703 . The substrate  701  includes an oxide layer  704  formed on top of the surface of the substrate  701  and has no color filters. Micro-lenses  705  are deposited on top of the oxide layer  704  to improve the pixel aperture efficiency. It is again clear to those skilled in the art that a group of the pixels  703  with the shallow barriers formed at the depth Xb 1   706  supply the blue and yellow color information while a group of the pixels  702  with the deep barriers formed at the depth Xb 2   707  supply cyan and red color information. The pixel sensitivity can be improved and different conversion gain factor values can be used to balance the signal levels in each pixel to make sure that the pixel output saturates at nearly the same level for each pixel when white illumination impinges on a sensor. The complete elimination of the color filters reduces the height of the pixel structure above the photodiode surface to its minimum and thus, achieves a maximum pixel performance for wide light incidence angles with maximum light sensitivity. 
     A top view of one possible filter and pixel arrangement is shown in  FIG. 8  for a sensor with cyan and magenta color filters and in  FIG. 9  for a sensor without any color filters.  FIG. 8  illustrates a corner of a pixel array  801  with a block of four pixels arranged in a certain pattern. For instance, the four pixels can be arranged in a checkerboard pattern. Pixels  802  have cyan color filters Cy on top, and other pixels  803  have magenta color filters Mg on top. All of the pixels  802  and  803  have substantially an identical barrier depth Xb incorporated therein and an identical value for a capacitor Cs.  FIG. 9  illustrates a corner of a pixel array  901  with a block of four pixels arranged in a certain pattern. For instance, the four pixels can be arranged in a checkerboard pattern. Pixels  902  have shallow barriers formed at a depth Xb 1  incorporated therein with a capacitor Cs 1 , and other pixels  903  have deep barriers formed at another depth Xb 2  incorporated therein with another capacitor Cs 2 . It is also possible to combine the standard shared circuit color pixel with the stacked pixel concept in a manner that one photo-site of the shared circuit pixel is a stacked photo-site and the other is a standard photo-site. This arrangement is shown schematically in  FIG. 10 . 
       FIG. 10  is a diagram illustrating a corner of a pixel array  1001  with a block of pixel pairs  1002  with shared readout and reset circuits as shown in  FIG. 4 . The shared pixel pair  1002 , however, includes a stacked photo-site  1004  and a standard photo-site  1003 . The stacked photo-site  1004  has a magenta color filter Mg formed on top while the standard photo-site  1003  has a green color filter Gr formed on top. Other filter arrangements are also possible where the standard photo-site has a red filter formed on top and the stacked photo-site has a cyan filter formed on top. All these combinations improve the sensitivity as well as the resolution in comparison with the typical standard approach. 
     Other pixel arrangements with more than two barrier depths and more than two values of the capacitance Cs are, of course, possible as is clear to those skilled in the art. 
     On the basis of the exemplary embodiments of the present invention, the pixel with stacked photo-sites has an ability to detect two color-coded signals without using any light absorbing color filters on top of the pixel. The image sensors constructed using the stacked pixels have higher pixel densities, higher resolution and higher sensitivity. 
     The present patent application contains subject matter related to the Korean patent application No. KR 2005-0068469, filed in the Korean Patent Office on Jul. 27, 2005, the entire contents of which being incorporated herein by reference. 
     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.