Patent Publication Number: US-2015084152-A1

Title: Photodiode

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an image sensor; in particular, to a photodiode image sensor. 
     2. Description of Related Art 
     Complementary metal-oxide-semiconductor (CMOS) image sensor includes an active pixel matrix or an image sensor cell array. The conventional image sensor cell includes a photodiode for sensing the intensity of the illumination and transforming the received optical signal into digital signal. The digital signal may be then received by the transistor arranged adjacently to the image sensor cell. 
     The abovementioned transistor as well as the other additional devices arranged in the peripheral region, such as the control and signal processing circuits and the logic circuit and so on, are integrated to be a photodiode-type CMOS image sensor. In order to reduce the cost and simplify the manufacturing processes, each of steps for manufacturing the circuits arranged in the peripheral region of the photodiode-type CMOS image sensor is also used to form the transistor arranged in the main region so that the circuit and the photodiode-type CMOS image sensor are fabricated in the same processing step. 
     However, the abovementioned fabricating method usually results in unfavorable effect on the electrical properties of the image sensor cell configured in the main region for sensing light. Specifically, the recombination centers may be generated due to the Si dangling bond defects at the interface between the semiconductor and the oxide, and thus reducing the lifetime of the minority carrier and generating the leakage current. In addition, the silicide (self-aligned silicidation) is formed in the peripheral region to be serve as the electrodes, for example, the gate, source and drain electrodes of the CMOS logic circuit, in the meanwhile, the silicide is also formed on the surface of the photodiode, and the impact resulted from the abovementioned defects may be reinforced. As a result, the dark current in the image sensor cell may increase, and the signal-to-noise ratio (SNR) would be decreased, which impact the sensitivity of the image sensor. 
     As the semiconductor processing techniques are developed, the demands for minimizing the device and precisely controlling the fabrication of the CMOS device may be satisfied. However, the smaller the distance between the devices, the more obvious the interference between the devices. As the result, the fabrication of the shallow trench isolation (STI) between the devices becomes more and more important. In the structure of the conventional photodiode, the defects existing between the isolating layer enclosing the photodiode and the active region may lead to the occurrence of the dark current, so does the Si dangling bond defects existing in the portion of the side region of the photodiode or near to the surface of the Si substrate. That is to say, according to the surface physics, there are dangling bonds formed at the interface between the isolating layer and the active region. As a result, when the carriers drift to the interface, some of the carriers would be trapped and released at certain energy level randomly. It may lead to the generation of the dark current which may result in the worse image quality captured by the image sensor and smaller dynamic range. 
     SUMMARY OF THE INVENTION 
     The object of the present invention is to decrease the leakage current, which results from lattice mismatch at the interface between the isolating layer and the N-well in conventional photodiode. The lattice mismatch is caused by a strain which may result from a difference in coefficient of thermal expansion between the isolating layer and the n-well or generated while fabricating the isolating layer. 
     In order to achieve the aforementioned objects, according to an embodiment of the present invention, a photodiode, the isolating layer and the n-well of which are separated by a distance, is provided. 
     The photodiode includes a first-type substrate. A second-type doped well is formed within the first-type substrate. A second-type doped region is formed within the second-type doped well. In the first-type substrate, an isolation region is formed to act as an isolation device in the photodiode, and separated from the second-type doped well. A protective layer is formed on the upper surface of the first-type substrate to cover the second-type doped well. A contact conductor penetrates through the protective layer and includes a contact layer and a conductive strip. The contact layer is formed on a bottom end of the conductive strip, and in contact with the second-type doped region to make electrical connection. 
     Compare to the conventional photodiode, the isolation region is separated from the second-type doped well in the photodiode of the instant disclosure. As a result, the defects at the interface between the isolation region and the first-type substrate are also separated from the active region by a distance, and the interference of the dark current for the active region may be avoid. 
     In order to further the understanding regarding the present invention, the following embodiments are provided along with illustrations to facilitate the disclosure of the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a top view of a photodiode according to an embodiment of the instant disclosure; 
         FIG. 2  shows a cross-sectional view of the photodiode taken along a line A-A shown in  FIG. 1 ; 
         FIG. 3  shows a cross-sectional view of a photodiode according to an embodiment of the instant disclosure; and 
         FIG. 4  shows a cross-sectional view of a photodiode according to an embodiment of the instant disclosure 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The aforementioned illustrations and following detailed descriptions are exemplary for the purpose of further explaining the scope of the present invention. Other objectives and advantages related to the present invention will be illustrated in the subsequent descriptions and appended drawings. 
     Please refer to  FIG. 1  and  FIG. 2 .  FIG. 1  shows a top view of photodiode according to an embodiment of the instant disclosure, and  FIG. 2  shows a cross-sectional view of the photodiode taken along a line A-A shown in  FIG. 1 . The photodiode  100  of the instant disclosure includes a first-type substrate  102 , a second-type doped well  118 , a second-type doped region  119 , a depletion region  109 , a P-N junction  107 , an isolation region  106 , a contact layer  120 , a contact conductor  103  and a protective layer  104 . The first-type substrate  102  has an upper surface  105  for receiving the light. The second-type doped well  118  is defined within the first-type substrate  102  for doping the impurities, and the P-N junction  107  is formed between the first-type substrate  102  and second-type doped well  118 . The second-type doped region  119  is defined within the second-type doped well  118  and is doped by the same type impurities, which extends from a surface layer to the bulk of the second-type doped well  118 . 
     In one embodiment, the abovementioned first-type substrate  102  is P-substrate, and the second-type doped well  118  is N-well. The second-type doped region  119  is N-type doped region defined within the second-type doped well  118 , and has higher concentration of the donor impurities than that of the second-type doped well  118 . 
     The abovementioned depletion region  109 , the region enclosed by the break line shown in  FIG. 2 , is defined by a peripheral region of the P-N junction  107  between the first-type substrate  102  and the second-type doped well  118 . 
     The isolation region  106 , which act as an isolating structure of the photodiode  100 , is formed in the abovementioned first-type substrate  102  and separated from the second-type doped well  118 . Specifically, there is a space  212  between the isolation region  106  and second-type doped region  119  in the instant disclosure. The space  212  is a partial region of the depletion region  109 , of course, the structure of the space  212  is the same as that of the depletion region  109 . In accordance with the embodiment of the instant disclosure, the isolation region  106  of which is separated from the second-type doped region  119 . In other words, the isolation region  106  may not be in contact with the P-N junction  107  between the first-type substrate  102  and second-type doped well  118 . The isolation region  106  also limits the range of the depletion region  109  at the lateral side. The isolation region  106  may be made of silicon nitride (Si 3 N 4 ) or silicon dioxide (SiO 2 ), and fabricated by the process of the local oxidation of silicon (LOCOS), shallow trench isolation (STI) and field oxide, and so forth. 
     The protective layer  104  is formed on an upper surface  105  of the abovementioned first-type substrate  102 , and covers the second-type doped well  118 . In addition, the contact conductor  103  is formed on the upper surface  105  of the first-type substrate  102 , and covers the second-type doped region  119 . For example, the contact conductor  103  is a contact plug. In one embodiment, the contact conductor  103  includes a contact layer  120  and a conductive strip  121 . The conductive strip  121  is formed on the contact layer  120 . Specifically, the protective layer  104  has an opening (not labeled) to expose the second-type doped region  119 , and the contact layer  120  and the conductive strip  121  are formed on the second-type doped region  119  through the opening. In other words, the contact layer  120  is electrically connected to and in contact with the second-type doped region  119  through the opening. 
     The protective layer  104  is made of an insulating transparent material, such as epoxy. When a light irradiate on the upper surface  105  of the second-type doped well  118 , and the energy of the photons would be absorbed so that a plurality of the electron-hole pairs is generated in the depletion region  109 . Each of the electron-hole pairs is separated into electrons and holes by photovoltaic effect, and then a current is generated. The generated current can be collected and flow to CMOS circuit through the contact conductor  103  disposed on the second-type doped region  119 . 
     Please refer to  FIG. 2 . The abovementioned isolation region  106  and the second-type doped region  119  are separated from each other by the space  212 . In one embodiment, the space  212  has a width of larger than 50 μm. While the isolation region  106  is fabricated by the processes, such as etching, chemical mechanical polishing (CMP), low pressure chemical vapor deposition system, and so on, the lattice structure of the first-type substrate  102  at the interface between the first-type substrate  102  and the isolation region  106  would be damaged by a mechanical stress, and leading to the formation of the defects, such as dislocation. Since the isolation region  106  may not overlap the second-type doped well  118 , these defects at the interface is far from the second-type doped well  118 , and the leakage current near to the second-type doped well  118  may be decreased. 
     Please refer to  FIG. 3 , which illustrates a cross-sectional view of a photodiode and for explanation of the formation of the space shown in  FIG. 2 . In one embodiment of the instant disclosure, after forming the isolation region  106  within the first-type substrate  102  and before ion implantation, a passivation layer  213  is formed on the upper surface  105  of the first-type substrate  102 . The passivation layer  213  superimposing on the space  212  and the isolation region  106  can be serve as a mask for the following ion implantation to prevent the impurities from doping into the space  212  between the isolation region  106  and the second-type doped region  119 . As a result, the impurities may be blocked by the passivation layer  213  during ion implantation, and may not diffuse into the isolation region  106 . The leakage current at the interface between the isolation region  106  and the second-type doped region  119  may be decreased. 
     Please refer to  FIG. 2 . In this embodiment of the instant disclosure, a photodiode  100  is provided. The protective layer  104  is formed on the upper surface  105  of the first-type substrate  102 . The protective layer  104  covers the second-type doped well  118 . The contact conductor  103  includes the contact layer  120  and the conductive strip  121 , and the contact layer  120  is formed on a bottom end of the conductive strip  121 . When the contact conductor  103  penetrates through the protective layer  104 , the contact conductor  103  is connected to and in contact with the second-type doped region  119  by the contact layer  120 . 
     The abovementioned contact layer  120  is a silicide layer which is fabricated by self-aligned silicidation. Various kinds of metals or alloys can be chose to form the silicide. For example, the metal is Ti, Co, Ni, Pd, Pt, and the alloy is Ti/W, Ti/Mo, Co/W or Co/Mo. 
     The silicide at the surface of the photodiode  100  may become an origin which may induce the leakage current or induce the formation of the recombination center at the surface, thus leading to less photocurrent collection. In one embodiment of the instant disclosure, in the photodiode  100 , a covering range of the contact layer  120  is limited within a bottom area of the conductive strip  121 , where the bottom area faces to the upper surface  105 . A portion of the contact layer  120  formed on the surface of the photodiode  100  and not covered by the conductive strip  121  may be removed. In other words, in the top view, the silicide (the portion of the contact layer  120 ) extending out of the range covered by the conductive strip  121  would be removed to attenuate the effect of the portion of the contact layer  120  on leakage current. 
     In addition, the absorption depth of the incident light in the photodiode depends on the wavelength of the incident light. When the incident light penetrates the photodiode, if the incident light with shorter wavelength, it would be absorbed near the surface layer of the photodiode, whereas the incident light with longer wavelength would more deeply penetrate into the photodiode, i.e., the incident light with longer wavelength has deeper absorption depth. The photodiode-type CMOS image sensor has a good sensitivity for the infrared light, which has the wavelength ranging between 700 nm to 800 nm in the light spectrum. In addition, the photodiode-type CMOS image sensor has grater quantum efficiency when irradiated by the incident light having wavelength of 850 nm. Moreover, the spectral response of the photodiode-type CMOS image sensor to incident light rises as the wavelength increasing in a spectral response curve. Because the incident light with longer wavelength has deeper penetration length, the photons in the incident light may penetrate into the P-N junction  107  and transfer their energy to electrons. After absorbing the energy of the photons, the electron-hole pairs at the P-N junction  107  may be generated and decoupled by the internal electric-field of the P-N junctions, thus, the transformation efficiency at the P-N junction  107  is higher. When the incident light has shorter wavelength, the energy of the incident light is absorbed at the surface layer, there are more recombination centers at which than in the body, the generated electron-hole pairs are easily recombined again, and thus decreasing the responsivity. However, for the photodiode for detecting the infrared light or the light with longer wavelength, if the photodiode absorbs the light with shorter wavelength, for example, blue light, the electron-hole pairs generated by the blue light at the surface layer may result in the noise, and affect the sensitivity of the photodiode. 
     In one embodiment of the instant disclosure, for the photodiode  100  for majorly sensing the infrared light, the structure of the protective layer  104  is adjusted. Please refer to  FIG. 4 . The protective layer  104  is formed from a stack including a transparent conductive oxide layer  214  and a polysilicon layer  215 . In one example, the transparent conductive oxide layer  214  is disposed on the polysilicon layer  215 . In one embodiment, the polysilicon has a thickness of 0.1 μm. The polysilicon layer  215  and the transparent conductive oxide layer  214  are electrically connected to the first-type substrate  102 . The polysilicon layer  215  is connected to ground by an electrode  216 . When the incident light with short wavelength irradiates the photodiode, before penetrating into the first-type substrate  102 , the incident light is absorbed within the transparent conductive oxide layer  214  or in the polysilicon layer  215 . A photoelectric current generated in the transparent conductive oxide layer  214  or in the polysilicon layer  215  is leaded to ground through the electrode  216 . The arrangement of the transparent conductive oxide layer  214  and the polysilicon layer  215  can assist to filter the stray light with shorter wavelength. 
     The abovementioned transparent conductive oxide layer  214  is made of conductive metal oxide. In one preferred example, the conductive metal oxide is indium tin oxide (ITO). 
     The polysilicon layer  215  is formed on the upper surface  105  of the first-type substrate  102 , and encloses the contact conductor  103 . The transparent conductive oxide layer  214  superimposes on the polysilicon layer  215 . The arrangement of the polysilicon layer  215  can make the peak concentration of the second-type doped well  118  move deeper into the first-type substrate  102  and create an absorption tail structure to enhance the absorption efficiency. 
     The descriptions illustrated supra set forth simply the preferred embodiments of the present invention; however, the characteristics of the present invention are by no means restricted thereto. All changes, alternations, or modifications conveniently considered by those skilled in the art are deemed to be encompassed within the scope of the present invention delineated by the following claims.