Patent Publication Number: US-2021183936-A1

Title: Image sensor

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the priority benefit of Taiwan application serial no. 108145336, filed on Dec. 11, 2019. The entirety of the above-mentioned patent application is hereby incorporated by reference herein and made a part of this specification. 
     BACKGROUND 
     Technical Field 
     The disclosure relates to a sensing device, and in particular, to an image sensor. 
     Description of Related Art 
     Compared with a charge coupled device (CCD), a complementary metal-oxide-semiconductor image sensor (CMOS image sensor, CIS) has the advantages of low operating voltage, low power consumption, high operating efficiency, capability of random access and the like, and may also be integrated into the existing semiconductor technology so as to be manufactured in batches, so the application range is very wide. 
     Pixel photosensitive elements of the CIS are mainly composed of PN diodes, and the intensity of image signals generated after light sensing depends on an area of a photosensitive region and the intensity of the incident light. For the back-side illuminated (BSI) CIS widely used in the current market, transistors, capacitors and metal circuit layers thereof are all built at bottom layers of the pixel photosensitive elements. Therefore, sizes of the pixel photosensitive regions of the BSI-CIS are almost equal to sizes of the pixels so as to greatly improve the photosensitivity. 
     SUMMARY 
     The invention is directed to an image sensor capable of effectively improving photosensitivity. 
     The image sensor of the invention includes a semiconductor substrate, a plurality of micro-lenses, a plurality of color filters, an interconnection structure and a reflecting layer. The semiconductor substrate has a first surface and a second surface opposite to each other. The semiconductor substrate includes a plurality of sensing pixels arranged in an array, and each of the plurality of sensing pixels includes a plurality of photosensitive elements. The plurality of micro-lenses are located over the first surface of the semiconductor substrate. The plurality of color filters are located between the semiconductor substrate and the plurality of micro-lenses. The interconnection structure is located over the second surface of the semiconductor substrate and is electrically coupled to the plurality of photosensitive elements. The reflecting layer is located between the interconnection structure and the plurality of photosensitive elements and is configured to reflect all or a portion of light passing through the plurality of photosensitive elements back to the plurality of photosensitive elements. The interconnection structure includes a plurality of circuit layers stacked alternately, and the reflecting layer is located at a same level as one of the plurality of circuit layers closest to the semiconductor substrate. 
     Based on the above, the image sensor according to the embodiments of the invention may enable all or a portion of light passing through the photosensitive elements to be irradiated to the photosensitive elements again by the reflecting layer. Therefore, the light entering the sensor may be collected more efficiently so as to improve the photosensitivity of the image sensor. 
     To make the features and advantages of the invention clear and easy to understand, the following gives a detailed description of embodiments with reference to accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic cross-sectional diagram of an image sensor according to a first embodiment of the invention. 
         FIG. 2  is a schematic cross-sectional diagram of an image sensor according to a second embodiment of the invention. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
       FIG. 1  is a schematic cross-sectional diagram of an image sensor according to a first embodiment of the invention. An image sensor  100  of the present embodiment may be applied to various high-pixel electronic devices (such as cameras, mobile phones and computers) for image shooting, and may achieve full-color image sensing. For example, the image sensor  100  of the present embodiment may be applied to an electronic device with 12 million pixels or 64 million pixels. When the image sensor  100  is applied to a camera of a mobile phone with 64 million pixels, the size of each pixel may be 1.4*1.4 μm 2  or 0.7*0.7 μm 2 , but the invention is not limited thereto. 
     Referring to  FIG. 1 , the image sensor  100  includes a semiconductor substrate  110 , a plurality of micro-lenses  120 , a plurality of color filters  130 , an interconnection structure  140  and a reflecting layer  150 . The semiconductor substrate  110  has a first surface  110   a  and a second surface  110   b  opposite to each other. The semiconductor substrate  110  includes a plurality of sensing pixels P arranged in an array, and each of the plurality of sensing pixels P includes a plurality of photosensitive elements  112 . The plurality of micro-lenses  120  are located over the first surface  110   a  of the semiconductor substrate  110 . The plurality of color filters  130  are located between the semiconductor substrate  110  and the plurality of micro-lenses  120 . The interconnection structure  140  is located over the second surface  110   b  of the semiconductor substrate  110  and is electrically coupled to the plurality of photosensitive elements  112 . The reflecting layer  150  is located between the interconnection structure  140  and the plurality of photosensitive elements  112  and is configured to reflect all or a portion of light L passing through the plurality of photosensitive elements  112  back to the plurality of photosensitive elements  112 . 
     Specifically, the image sensor  100  of the present embodiment is a back-side illuminated complementary metal-oxide-semiconductor image sensor (BSI-CIS), the first surface  110   a  of the semiconductor substrate  110  may be referred to as a back surface, and the second surface  110   b  of the semiconductor substrate  110  may be referred to as a front surface (or an active surface). The light (or radiation) L enters the back surface (namely the first surface  110   a ) of the semiconductor substrate  110 , and enters the photosensitive elements  112  through the back surface (namely the first surface  110   a ) to perform an image sensing function. However, a portion of the light L may pass through the photosensitive elements  112  and may not be effectively sensed. Therefore, the image sensor  100  of the present embodiment may enable all or a portion of the light L passing through the photosensitive elements  112  to be irradiated to the photosensitive elements  112  again by the reflecting layer  150 . Therefore, the light L entering the image sensor  100  may be collected more efficiently so as to improve the sensing sensitivity of the image sensor  110 . 
     In the present embodiment, the semiconductor substrate  110  may be made of the following materials: a suitable elemental semiconductor, such as crystalline silicon, diamond or germanium; a suitable compound semiconductor, such as gallium arsenide, silicon carbide, indium arsenide or indium phosphide; or a suitable alloy semiconductor, such as silicon germanium carbide, gallium arsenic phosphide or gallium indium phosphide. The semiconductor substrate  110  may be a p-type substrate or an n-type substrate. For example, when the semiconductor substrate  110  is the p-type substrate, the semiconductor substrate  110  may be doped with a p-type doping agent (such as boron), and when the semiconductor substrate  110  is the n-type substrate, the semiconductor substrate  110  may be doped with an n-type doping agent (such as phosphorus or arsenic). 
     The semiconductor substrate  110  may include a plurality of isolation structures  114  to define a plurality of active regions in the semiconductor substrate  110 . The isolation structures  114  extend from the first surface  110   a  of the semiconductor substrate  110  to the second surface  100   b  of the semiconductor substrate  110 . The plurality of photosensitive elements  112  is respectively formed in the plurality of active regions defined in the semiconductor substrate  110 . For example, the isolation structures  114  may include deep trench isolation (DTI) structures to isolate the plurality of photosensitive elements  112  from each other, so that the light signal interference between adjacent photosensitive elements  112  may be significantly reduced. However, in other embodiments, the isolation structures  114  may also include shallow trench isolation (STI) structures, implant isolation structures or other isolation structures. The photosensitive elements  112  may include photo-diodes. The photo-diodes may include at least one p-type doped region, at least one n-type doped region, and a p-n junction formed between the p-type doped region and the n-type doped region. A method for forming the photosensitive elements  112  may be an ion implantation method. Specifically, when the semiconductor substrate  110  is the p-type substrate, the n-type doping agent (such as phosphorus or arsenic) may be doped in the active regions to form an n-type well, and the p-n junction formed in the semiconductor substrate  110  may perform an image sensing function. Similarly, when the semiconductor substrate  110  is the n-type substrate, the p-type doping agent (such as boron) may be doped in the active regions to form a p-type well. When a reversed bias is applied to the p-n junction of the photosensitive elements  112 , the p-n junction is sensitive to incident light. At this time, the photosensitive elements  112  are in a floating high impedance state. After being irradiated by light for a period of time, the photosensitive elements  112  may generate a current, and a resulting pressure difference is an image signal. That is, the light received or detected by the photosensitive elements  112  may be converted into a photo-current, and then, an image signal may be generated and output. 
     Furthermore, the image sensor  100  may also include one or more pixel transistors (not shown in the figure) on the active surface (namely the second surface  110   b ) of the semiconductor substrate  110 . For example, the pixel transistor may include a transfer transistor configured to transfer the charges generated in the photosensitive elements  112  out of the photosensitive elements  112  for reading. Furthermore, the pixel transistor may also include other transistors, such as a source-follower transistor, a row select transistor, a reset transistor, and the like. For the purpose of clarity, the semiconductor elements are not shown in  FIG. 1 . 
     The color filters  130  are disposed on the first surface  110   a  of the semiconductor substrate  110 , and each of the plurality of color filters  130  respectively corresponds to each of the plurality of photosensitive elements  112 . The color filters  130  allow transmission of the light having a specific wavelength range, and simultaneously block the light having a wavelength exceeding the specific range. For example, the plurality of color filters  130  may include a red light filter R, a green light filter G and a blue light filter B. The red light filter R allows red light to pass through, so that the red light is received by the photosensitive elements  112  located below the red light filter R. The green light filter G allows green light to pass through, so that the green light is received by the photosensitive elements  112  located below the green light filter G. The blue light filter B allows blue light to pass through, so that the blue light is received by the photosensitive elements  112  located below the blue light filter B. The image sensor  100  of the present embodiment is suitable for sensing the light having a light wavelength within a visible light range. 
     The plurality of micro-lenses  120  is disposed on the plurality of color filters  130 , and each of the plurality of micro-lenses  120  respectively corresponds to each of the plurality of color filters  130 . The plurality of micro-lenses  120  may form a micro-lens array. The center points of the plurality of micro-lenses  120  are substantially aligned with the center points of the plurality of color filters  130  in a vertical direction respectively. The micro-lenses  120  may be configured to focus the incident light L to the photosensitive elements  112 . After the light L is refracted by the micro-lenses  120 , the light L may substantially enter the reflecting layer  150  vertically, and the reflecting layer reflects the light L back to the photosensitive elements  112  to improve the light collection efficiency. Because the light L is almost vertically incident, the light L is not reflected to other adjacent photosensitive elements  112  so as to reduce the noise interference. 
     As shown in  FIG. 1 , the interconnection structure  140  is disposed on the active surface (namely the second surface  110   b ) of the semiconductor substrate  110  and is electrically coupled to the photosensitive elements  112 , so that signals generated from the photosensitive elements  112  may be transmitted to other elements for processing. In the present embodiment, the interconnection structure  140  includes an interlayer dielectric (ILD) layer  142  and a plurality of circuit layers  144  stacked alternately in the ILD layer  142 . The material of the ILD layer  142  includes silicon oxide, silicon nitride, silicon oxynitride, phosphosilicate glass (PSG), borophosphosilicate glass (BPSG), spin-on glass (SOG), fluorinated silica glass (FSG), carbon-doped silicon oxide (such as SiCOH), polyimide, or a combination thereof. The materials of the circuit layers  144  include conductive materials, such as metals. Four or five circuit layers  144  may be disposed, but the invention is not limited thereto. In other embodiments, the interconnection structure  140  may include more or less circuit layers  144 . Specifically, one of the plurality of circuit layers  144  closest to the semiconductor substrate  110  may be a metal one layer, and the circuit layers  144  stacked on the metal one layer may be sequentially a metal two layer, a metal three layer, and so on. Taking five circuit layers  144  as an example, one of the plurality of circuit layers  144  closest to the semiconductor substrate  110  may be a metal one layer, and one of the plurality of circuit layers  144  furthest from the semiconductor substrate  110  may be a metal five layer. According to the present embodiment, one of the plurality of circuit layers  144  closest to the semiconductor substrate  110  may be disposed on the metal five layer, one of the plurality of circuit layers  144  furthest from the semiconductor substrate  110  may be disposed on the metal one layer, and so on. In the present embodiment, because the interconnection structure  140  is disposed on the opposite side (namely the second surface  110   b ) of a light incidence surface (namely the first surface  110   a ), namely below the photosensitive elements  112 , the interconnection structure  140  does not block the light L from irradiating on the photosensitive elements  112 . 
     In the present embodiment, the reflecting layer  150  may be a sheet metal layer, the reflecting layer  150  extends continuously in a direction parallel to the semiconductor substrate  110 , and orthographic projections of the plurality of photosensitive elements  112  on the reflecting layer  150  are located within a range of the reflecting layer  150 . In other words, viewing from top to bottom, the plurality of photosensitive elements  112  may overlap the reflecting layer  150 , so that the light L passing through the photosensitive elements  112  may be reflected by the reflecting layer  150  and irradiated to the photosensitive elements  112  again. In the present embodiment, the reflecting layer  150  may be formed in a same process as one of the plurality of circuit layers  144  closest to the semiconductor substrate  110  (namely the metal one layer). In other words, the reflecting layer  150  may be located at a same level as one of the plurality of circuit layers  144  closest to the semiconductor substrate  110  (namely the metal one layer), and the reflecting layer  150  and the circuit layers  144  may include a same material (such as metal). It should be noted that in order to clearly show the reflecting layer  150 , a circuit layer (namely a metal one layer) at the same level as the reflecting layer  150  is not shown. 
     Because the reflecting layer  150  may be formed together with the circuit layers  144 , that is, the existing process may be utilized for manufacturing the reflecting layer  150 , additional process steps are not needed, high process compatibility is realized, and no additional cost is increased. Furthermore, because the reflecting layer  150  may be a metal layer, the reflection of the light L on the reflecting layer  150  may be mirror reflection, and the light L is not easy to scattered, the vertically incident light L may be vertically reflected to the photosensitive elements  112  to prevent the light L from being scattered to other adjacent photosensitive elements  112  so as to reduce the noise interference. 
     In the present embodiment, the reflecting layer  150  is electrically disconnected from the photosensitive elements  112 . In an embodiment, the reflecting layer  150  may be coupled to a power voltage (VDD) or a grounding voltage (GND), so that the reflecting layer  150  may serve as signal shielding to reduce signal interference and disturbance between the photosensitive elements  112  and the circuit layers  144 . However, in other embodiments, the reflecting layer  150  may also be in electrical floating. 
       FIG. 2  is a schematic cross-sectional diagram of an image sensor according to a second embodiment of the invention. Referring to  FIG. 2 , an image sensor  200  of the present embodiment is similar to the image sensor  100  in  FIG. 1 , so that details thereof are omitted herein. Compared with  FIG. 1 , a reflecting layer  250  of the image sensor  200  in  FIG. 2  may include a plurality of separated reflecting blocks  252 , and an orthographic projection of each of a plurality of photosensitive elements  112  on the reflecting layer  250  is respectively located within a range of each of the plurality of reflecting blocks  252 . In other words, viewing from top to bottom, one of the plurality of photosensitive elements  112  may overlap one of the plurality of reflecting blocks  252 , so that the light L passing through the photosensitive elements  112  may be reflected by the reflecting layer  250  and irradiated to the photosensitive elements  112  again. Furthermore, the reflecting layer  250  may also include a plurality of connecting lines (not shown) connected among the plurality of reflecting blocks  252 . 
     In conclusion, the image sensor according to the embodiments of the invention may enable all or a portion of the light passing through the photosensitive elements to be irradiated to the photosensitive elements again by the reflecting layer. Therefore, the light entering the image sensor may be utilized more efficiently so as to improve the sensing sensitivity of the image sensor. 
     Although the invention is described with reference to the above embodiments, the embodiments are not intended to limit the invention. A person of ordinary skill in the art may make variations and modifications without departing from the spirit and scope of the invention. Therefore, the protection scope of the invention should be subject to the appended claims.