Patent Publication Number: US-2019189694-A1

Title: Image Sensor and Forming Method Thereof

Description:
CROSS REFERENCES TO RELATED APPLICATIONS 
     This application claims the benefit of priority to Chinese Patent Application No. CN201711351450.5, entitled “Image Sensor and Forming Method Thereof”, filed with SIPO on Dec. 15, 2017, the contents of which are incorporated herein by reference in its entirety. 
     TECHNICAL FIELD 
     The present disclosure relates to the technical field of semiconductor devices, and in particular, to an image sensor and a forming method thereof. 
     BACKGROUND 
     An image sensor is a semiconductor device for converting optical image signals into electrical signals. Among a variety of image sensors, complementary metal oxide semiconductor (CMOS for short) image sensors have been widely applied because of their advantages of small size, low power consumption and low cost. 
     The existing CMOS image sensors for mobile phones mainly include two type devices: front-side illumination (FSI for short) CMOS image sensors and back-side illumination or back illumination (BSI for short) CMOS image sensors, which have different requirements. The back-side illumination CMOS image sensors on mobile phones are characterized with more demanding photoelectric conversion effect (i.e., high quantum conversion efficiency). 
     However, in practical applications, after light arrives at a photosensitive diode (also referred to as a photodiode) of a CMOS image sensor, certain wavelength light, e.g., red light with longer wavelength, cannot be fully absorbed by the existing CMOS sensor because of the narrow silicon band gap window from the photosensitive diode. Light with longer wavelength like red will penetrate the sensor and miss the photo-electric conversion process in the photosensitive diode, resulting in loss of quantum efficiency of the device. There is a need to improve the quantum efficiency of an existing image sensor. 
     SUMMARY 
     The present disclosure provides an image sensor, comprising: a semiconductor substrate, the semiconductor substrate has photodiodes; and a dielectric layer, the dielectric layer is located on a surface of the semiconductor substrate; and photoelectric conversion films formed in the dielectric layer, wherein the positions of the photoelectric conversion films are in one-to-one correspondence with the positions of the photodiodes, so that light passing through the photodiodes is transmitted to the corresponding photoelectric conversion films. 
     Optionally, the photoelectric conversion films are organic photoelectric conversion films. 
     Optionally, MOS transistors are further formed in the semiconductor substrate, and the dielectric layer covers gates of the MOS transistors. 
     Optionally, a photosensitive area of each photoelectric conversion film is not smaller than a photosensitive area of the corresponding photodiode. 
     Optionally, an output end of each photodiode is electrically connected with the corresponding photoelectric conversion film. 
     Optionally, an edge of each photoelectric conversion film is bent toward the corresponding photodiode. 
     The present disclosure further provides a forming method of an image sensor, comprising: providing a semiconductor substrate, the semiconductor substrate having photodiodes therein; and forming a dielectric layer on a surface of the semiconductor substrate, the dielectric layer having photoelectric conversion films therein, wherein the positions of the photoelectric conversion films are in one-to-one correspondence with the positions of the photodiodes, so that light passing through the photodiodes is transmitted to the corresponding photoelectric conversion films. 
     Optionally, the photoelectric conversion films are organic photoelectric conversion films. 
     Optionally, forming a dielectric layer on a surface of the semiconductor substrate comprises: forming a first dielectric layer on the surface of the semiconductor substrate; etching the first dielectric layer to form grooves, the positions of the grooves being in one-to-one correspondence with the positions of the photodiodes; filling the grooves with the photoelectric conversion films; and forming a second dielectric layer, the second dielectric layer covering the photoelectric conversion films and the first dielectric layer, wherein the dielectric layer comprises the first dielectric layer and the second dielectric layer. 
     Optionally, an edge of each photoelectric conversion film is bent toward the corresponding photodiode. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a cross sectional view of an image sensor structure according to an embodiment of the present disclosure. 
         FIG. 2  shows a flow chart of a forming method of an image sensor according to an embodiment of the present disclosure. 
         FIG. 3  to  FIG. 8  is a step by step illustration in the forming process of the image sensor according to an embodiment of the present disclosure. 
         FIG. 9  illustrates the working principle of a single pixel unit in the image sensor according to an embodiment of the present disclosure. 
         FIG. 10  shows a schematic circuitry diagram of the pixel unit shown in  FIG. 9 . 
         FIG. 11  shows the time sequence diagram of the circuitry for the pixel unit shown in  FIG. 9 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The foregoing objectives, features, and advantages of the present disclosure will become more apparent from the following detailed description of specific embodiments of the disclosure in conjunction with the accompanying drawings. In the detailed description of the embodiments of the present disclosure, for convenience of description, the schematic diagram will be partially enlarged not according to an ordinary ratio, and the schematic diagram is only an example, which should not limit the protection scope of the present disclosure. In addition, three-dimensional space dimensions of length, width, and depth should be comprised in actual production. 
     As described in the background, the existing image sensor cannot completely absorb incident light and has low quantum efficiency. 
     In order to solve the above technical problem, an embodiment of the present disclosure provides an image sensor, comprising: a semiconductor substrate, the semiconductor substrate has photodiodes therein; and a dielectric layer, the dielectric layer is located on a surface of the semiconductor substrate, and photoelectric conversion films are formed in the dielectric layer, wherein the positions of the photoelectric conversion films are in one-to-one correspondence with the positions of the photodiodes, so that light passing through the photodiodes is transmitted to the corresponding photoelectric conversion films. 
       FIG. 1  shows a cross sectional view of an image sensor according to an embodiment of the present disclosure. The image sensor is a BSI CMOS image sensor, or a FSI CMOS image sensor. 
     Next, the back-side illumination CMOS image sensor will be described in detail as an example. 
     Specifically, referring to  FIG. 1 , the image sensor  100  includes a semiconductor substrate  110 , wherein the semiconductor substrate  110  may have photodiodes  111 ; and a dielectric layer  120 , wherein the dielectric layer  120  is located on the bottom surface of the semiconductor substrate  110 , and photoelectric conversion films  121  is formed in the dielectric layer  120 ; the photoelectric conversion film  121  is under the photodiode  111 , such that light l 1  passing through the photodiode  111  can be transmitted to the corresponding photoelectric conversion film  121 . 
     More specifically, since the image sensor  100  is a back-side illumination CMOS image sensor, the photodiodes  111  may be formed in the back surface of the semiconductor substrate  110 , and the dielectric layer  120  may be located on the front surface of the semiconductor substrate  110 . At a position shown in  FIG. 1 , the photoelectric conversion films  121  are located under the corresponding photodiodes  111 . This is determined based on a transmission path of incident light l 2 , which first arrives at the photodiodes  111  when entering the image sensor  100 . Then, part of the light with longer wavelength in red or near infrared, not absorbed by the photodiode because of the narrow silicon band gap window arrives at the photoelectric conversion films  121 . 
     Further, the image sensor  100  may further comprise grids  112 . The grids  112  define openings corresponding to the photodiodes  111  within the semiconductor substrate  110 . 
     Further, the image sensor  100  may further comprise color filter  160  and lenses  170 . The color filter  160  and the lenses  170  are in correspondence to the openings defined by the grids  112 . 
     Preferably, the lenses  170  may be micro lenses. 
     Further, the color filter  160  may be red color filter, green color filter or blue color filter. 
     In a preferred example, the grids  112  are at least flush with the color filter  160  to better avoid light crosstalk. 
     As a nonrestrictive embodiment, for color filter of different colors, the photosensitive thickness and/or the photosensitive area of the photoelectric conversion films  121  corresponding to the photodiodes  111  at the openings thereof may be different. 
     For example, for red light with a longer wavelength, the thickness of the photosensitive layer in photoelectric conversion film  121  corresponding to the photodiode  111  arranged in the opening where the red color filter lens is located may be thicker to sufficiently block red light projecting through the photodiode  111 . 
     Preferably, the photoelectric conversion films  121  may be organic photoconductive thin-films (OPF for short). Preferably, an active layer in the organic photoelectric conversion film contains polymer compound, which may contain one polymer compound, or two or more polymer compounds. The polymer compound may be an electron donor compound and/or an electron acceptor compound. In order to improve the charge transport property of the active layer, the electron donor compound and the electron acceptor compound may be used together in the active layer. Preferably, the active layer contains a conjugated polymer compound and a fullerene derivative. For example, an organic thin film containing a conjugated polymer compound and a fullerene derivative maybe used as the active layer. 
     Further, MOS transistors may be further formed within the semiconductor substrate  110 , and the dielectric layer  120  may cover gates  130  of the MOS transistors. 
     As a nonrestrictive example, the dielectric layer  120  may be an inter layer dielectric (ILD) layer as an isolator between the semiconductor substrate  110  and a first layer of metal. 
     Further, the photosensitive area of the photoelectric conversion film  121  may be larger than the photosensitive area of the corresponding photodiode  111  to ensure that all the light l 2  penetrating the photodiode  111  can be captured. For example, in a plane where a surface of the semiconductor substrate  110  is located, a planar area of the photoelectric conversion film  121  is not smaller than the photosensitive area of the corresponding photodiode  111  (e.g., located above the photoelectric conversion film  121 ). 
       FIG. 2  shows a flow chart of a formation method of an image sensor according to an embodiment of the present disclosure. A formation process is used for forming at least part of a structure in the image sensor  100  shown in  FIG. 1  above. 
     Specifically, in this embodiment, the forming method of the image sensor comprises the following steps: 
     Step S 101 , providing a semiconductor substrate, the semiconductor substrate has photodiodes disposed in one surface. 
     Step S 102 , forming a dielectric layer on this surface of the semiconductor substrate, photoelectric conversion films are provided in the dielectric layer. 
     The photoelectric conversion films are in one-to-one correspondence to the photodiodes, so that light passing through the photodiodes is transmitted to the corresponding photoelectric conversion films. Meanwhile, in this embodiment, each photoelectric conversion film is located under its corresponding photodiode. 
     As a nonrestrictive embodiment, the semiconductor substrate may be a silicon substrate suitable for a back-side illumination CMOS image sensor. The forming method of the image sensor  100  shown in  FIG. 1  will be described in detail below with reference to  FIGS. 3 to 8 . 
     Referring to  FIG. 3 , a semiconductor substrate  110  is provided first, in which photodiodes  111  and MOS transistors  130  are provided. The adjacent photodiodes  111  are separated by a grid  112  made of a shielding material, so as to prevent light crosstalk and electron crosstalk. 
     Next, a dielectric material  1201 ′ is deposited on a surface (e.g., front surface) of the semiconductor substrate  110  to cover gates  130  of the MOS transistors, which exposes the surface of the semiconductor substrate  110 . 
     Preferably, the dielectric material  1201 ′ is a silicon oxide or silica material. 
     Preferably, the shielding material  112  is an insulating material such as an oxide. or 
     The shielding material can also be some metals that prevent light crosstalk. 
     Further, referring to  FIG. 4 , a surface of the dielectric material  1201 ′ is planarized to form a first dielectric layer  1201  on a surface of the semiconductor substrate  110 . 
     Preferably, the planarization is achieved based on a chemical mechanical polishing process. 
     Further, referring to  FIG. 5 , a surface of the first dielectric layer  1201  is spin-coated with a photoresist (e.g., a lithography photoresist) and then exposed to pattern of photoelectric conversion films  121  on the surface of the first dielectric layer  1201 . The first in the dielectric layer  1201  to form grooves  123 , and all the grooves  123  formed on the surface of the first dielectric layer  1201  prepare for deposition of the photoelectric conversion films  121 . In addition, the grooves  123  align to the photodiodes  111 . 
     Preferably, the grooves  123  are formed by dry etching process. 
     Further, referring to  FIG. 6 , a photoelectric conversion film material  121 ′ is formed on a surface of the first dielectric layer  1201  filling the grooves  123 . Specifically, the photoelectric conversion film material  121 ′ may be formed by a coating method with a solution containing the active layer forming material mentioned above and a solvent, and for example, may be formed by a coating method using a solution containing a conjugated polymer compound, a fullerene derivative and a solvent. The solvent may be a hydrocarbon series solvent such as toluene or xylene, a halogenated saturated hydrocarbon series solvent such as carbon tetrachloride, chloroform or methylene chloride, a halogenated unsaturated hydrocarbon series solvent such as chlorobenzene, dichlorobenzene or trichlorobenzene, or an ether solvent such as tetrahydrofuran or tetrahydropyrane. The coating method for the solution of the active layer forming material may be, for example, a spin-coating method. 
     Further, referring to  FIG. 7 , a surface of the photoelectric conversion film material  121 ′ is planarized until the first dielectric layer  1201  is exposed. The photoelectric conversion films  121  are patterned to align with grooves  123 . 
     A planarization method of the photoelectric conversion film material  121 ′ includes various suitable methods such as CMP. 
     Further, referring to  FIG. 8 , a dielectric material is deposited on the exposed surfaces of the first dielectric layer  1201  and the photoelectric conversion films  121 ; the dielectric material  1201 ′ then is planarized to form a second dielectric layer on the exposed surfaces of the first dielectric layer  1201  and the photoelectric conversion films  121 . The second dielectric layer covers the photoelectric conversion films  121  and the first dielectric layer  1201 , wherein the first dielectric layer  1201  and the second dielectric layer jointly form the dielectric layer  120 . 
     Preferably, the first dielectric layer  1201  and the second dielectric layer deposited in this step may be made of the same dielectric material  1201 ′. 
     Further, after a device structure shown in  FIG. 8  is obtained, flip the device over, now referring back to  FIG. 1 , a metal interconnection structure  140  may be formed on the surface of the dielectric layer  120  at the same side of the substrate. The color filter  160  and the lenses  170  are placed on the other surface (i.e., a surface opposite to the dielectric layer  120 ) of the semiconductor substrate  110 . 
     As a nonrestrictive embodiment, in  FIG. 1 , a third dielectric layer  150  may be formed between the color filter  160  and the semiconductor substrate  110  to seal and protect the semiconductor substrate  110 . Preferably, the third dielectric layer  150  may be made of a high dielectric constant (High-K) material. 
     Further, referring to  FIG. 1 , the metal interconnection structure  140  may comprise three metal layers  141 , and via holes  142 . 
     Further, the metal interconnection structure  140  may be electrically connected with the gates  130  of the MOS transistors through connecting wires  122 . Preferably, the connecting wires  122  are copper wires. 
     Alternatively, the metal interconnection structure  140  may be prepared on another substrate in advance, and is integrally bonded to a surface of the dielectric layer  120  after the device structure shown in  FIG. 8  is formed. 
     As a nonrestrictive embodiment, edges of each photoelectric conversion film  121  may be bent to wrap the corresponding photodiode  111 , so that light l 1  passing through each photodiode  111  is not reflected or refracted to next photodiodes  111  and absorbed by the their photoelectric conversion films  121 , so as to better avoid crosstalk between adjacent pixels (i.e., the photodiodes  111 ). 
     For example, when the grooves  123  shown in  FIG. 5  are formed, stepwise etching may be adopted, and etching depth at the edges of each groove  123  may be deeper than etching depth at other regions of the grooves, so that the edge of the groove  123  is bent toward the corresponding photodiode  111 . 
     Or, the etching depth may keep uniform, but each groove  123  is patterned to bend at edges through lithography. 
     Further, an output end of each photodiode  111  may be electrically connected with the corresponding photoelectric conversion film  121 , so that photo-generated charges collected by the photodiode  111  and the corresponding photoelectric conversion film  121  are gathered together to be transmitted within an exposure period to avoid an image trailing phenomenon. 
     Alternatively, a surface (e.g., a surface irradiated by the light l 1 ) of the photoelectric conversion film  121  may also be made wavy, which can also avoid light crosstalk between adjacent pixels. 
       FIG. 9  shows a prospective view of a single pixel unit of the image sensor in a working mode according to an embodiment of the present disclosure. The pixel unit may comprise the photodiode  111  and the corresponding photoelectric conversion film  121  in the image sensor  100  shown in  FIG. 1  above. 
     Specifically, the photodiode  111  may be located in a depletion region of the image sensor, and the corresponding photoelectric conversion film  121  is located on a surface of the photodiode  111 . On a transmission path of incident light l 2 , light is absorbed mostly by the photodiode  111  and only the longer wavelength light like red or infrared passes through to reach the photoelectric conversion film  121 . 
     More specifically, the semiconductor substrate  110  may be a lightly doped P-type substrate, a P-type well may be formed on the semiconductor substrate  110 , and a grid of shallow trench isolation (STI) regions is formed in the P-type wells. The distance between the photodiode  111  and the neighboring STI region is measured through a recessed distance. 
     Further, an output end of one photodiode  111  and the corresponding photoelectric conversion film  121  may be connected to agate  130  of an MOS transistor in the image sensor. Preferably, the gate  130  is located on a floating (FD) node of the pixel unit. 
     Referring to  FIG. 9  and  FIG. 10 , taking a 4T (transistor) type image sensor as an example, each pixel unit, consisting of a photodiode  111  and a corresponding photoelectric conversion film  121 , in the image sensor  100  shown in  FIG. 1  may be electrically connected with agate  130  of a corresponding MOS transistor (being a transmission wire TG in this embodiment, also referred to as a transmission wire TX) formed in the semiconductor substrate  110 . Thus, the potential on the floating node can directly determine the potential on the gate of a source follower wire SF, and further determine the final output current. 
     Further, in this embodiment, through a time sequence shown in  FIG. 11  generated by the circuitry of  FIG. 10 , within one image period, by the photons are collected by photodiode  111  and photo-generated electrons are converted by the corresponding photoelectric conversion film  121 , finally collected by the floating node via the transmission wire TG. 
     Further, in the embodiment, in addition to the transmission wire TG and the source follower wire SF, the 4T type image sensor further comprises a reset RS (also referred to as RSVT) and a gating wire SE, wherein a source of the gating wire SE is connected to an output. 
     Further, in this embodiment, when time sequence shown in  FIG. 11  is between T 1  and T 4 , the pixel unit is in an exposure (light collection) phase, and the transmission wire TG is in an off state. 
     Further, during the period of T 2  to T 7 , the gating wire SE is turned on, and the pixel unit performs a readout operation. 
     Further, during the period of T 2  to T 3 , the gating wire SE and the reset wire RS are both in on state to reset the floating node. 
     Further, during the period of T 4  to T 5 , when gating wire SE is on, the transmission wire TG is also turned on to read out light integral signals accumulated by the pixel unit during the time period of T 1  to T 4  (i.e., collect the converted photo-generated charges). 
     Further, during the period of T 6  to T 7 , after the light integral signals are transferred, the gating wire SE, the reset wire RS and the transmission wire TG are turned on at the same time to reset the photodiode  111  and the photoelectric conversion film  121  and transfer all the photo-generated charges remaining in the pixel unit to the source of the gating wire SE, so as to prevent this frame of signals from producing an image trailing influence on output of a next frame. 
     From the above, the image sensor obtained by adopting the solution of this embodiment can achieve photoelectric conversion through the photodiodes, and also can capture the incident light (i.e., the light leaked by the photodiodes) penetrating the photodiodes through the photoelectric conversion films and convert the incident light into photo-generated charges, so as to ensure that the incident light can be completely absorbed and effectively improve the quantum efficiency of the image sensor. 
     Further, the photoelectric conversion films and the photodiodes are in one-to-one correspondence, which can effectively avoid light crosstalk between adjacent pixels. 
     Further, an output end of each photodiode is electrically connected with the corresponding photoelectric conversion film, so that the photo-generated charges collected by each photodiode and the corresponding photoelectric conversion film are gathered together to be transmitted within an exposure period to avoid an image trailing phenomenon. 
     Further, an edge of each photoelectric conversion film is bent toward the corresponding photodiode so as to better avoid light crosstalk between adjacent pixels. 
     The forming method provided by the present disclosure forms the photoelectric conversion films in the dielectric layer formed on the surface of the semiconductor substrate in the process of forming the image sensor, so as to capture the light leaked by the corresponding photodiodes, thereby improving the quantum efficiency of the finally formed image sensor. 
     Although the present disclosure is disclosed as above, the present disclosure is not limited thereto. Any person skilled in the art can make various changes and modifications without departing from the spirit and scope of the present disclosure. Therefore, the scope defined by the claims shall prevail the protection scope of the present disclosure.