PATENT DOCUMENT

Publication Number: US-10868203-B1
Application Number: US-201916288128-A
Country: US
Kind Code: B1

Title: Film-based image sensor with planarized contacts

Abstract:
An imaging apparatus includes a semiconductor substrate, an array of pixel circuits formed on the semiconductor substrate and including respective pixel electrodes. A layer of a photosensitive medium overlies the pixel electrodes and has a lower surface in electrical contact with the pixel electrodes, and is configured to convert incident photons into charge carriers, which are collected by the pixel electrodes. A planar conductive top electrode, which is at least partially transparent, overlies an upper surface of the photosensitive medium. A bias circuit is formed on the semiconductor substrate and configured to provide a bias potential for application to the photosensitive medium. A bias contact extends from the bias circuit to the top electrode so as to apply the bias potential to the top electrode while contacting the top electrode along a plane that is parallel to the upper surface of the photosensitive medium.

Claims:
The invention claimed is: 
     
       1. Imaging apparatus, comprising:
 a semiconductor substrate; 
 an array of pixel circuits formed on the semiconductor substrate and comprising respective pixel electrodes; 
 a layer of a photosensitive medium, which overlies the pixel electrodes and has a lower surface in electrical contact with the pixel electrodes, and which is configured to convert incident photons into charge carriers, which are collected by the pixel electrodes; 
 a planar conductive top electrode, which is at least partially transparent, overlying an upper surface of the photosensitive medium; 
 a bias circuit formed on the semiconductor substrate and configured to provide a bias potential for application to the photosensitive medium; and 
 a bias contact extending from the bias circuit to the top electrode so as to apply the bias potential to the top electrode while contacting the top electrode along a plane that is coplanar with the upper surface of the photosensitive medium. 
 
     
     
       2. The imaging apparatus according to  claim 1 , wherein the layer of the photosensitive medium comprises a quantum film. 
     
     
       3. The imaging apparatus according to  claim 1 , and comprising an insulating layer overlying the bias circuit and at least partially surrounding the photosensitive medium, wherein the bias contact extends through the insulating layer. 
     
     
       4. The imaging apparatus according to  claim 3 , wherein the insulating layer comprises silicon dioxide.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of U.S. Provisional Patent Application 62/662,229, filed Apr. 25, 2018, which is incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to optoelectronic devices, and particularly to image sensors. 
     BACKGROUND 
     Hybrid image sensors have a photosensitive layer overlaid on and connected to pixel circuitry on a silicon chip. For example, the photosensitive layer may comprise a photosensitive film, such as a film containing quantum dots (known as a quantum film). 
     A typical structure of a hybrid image sensor comprises a photosensitive layer, top and bottom conductive layers serving respectively as top and bottom electrodes to the photosensitive layer, and pixel circuitry. The photosensitive layer can be designed, for example, as a blanket photo-resistive layer with linear signal output as a function of an applied voltage, or with non-linear response to the applied voltage, similar to a photodiode response. The top electrode (or electrodes) on the photosensitive layer is typically common for a group of pixels or all pixels of the array and at least partially transparent to the incoming light, and is coupled to an electrode contact that provides the required bias voltage. 
     U.S. Pat. No. 8,558,286, whose disclosure is incorporated herein by reference, describes a photodetector along with corresponding materials, systems, and methods. The photodetector comprises an integrated circuit and at least two optically sensitive layers. A first optically sensitive layer is over at least a portion of the integrated circuit, and a second optically sensitive layer is over the first optically sensitive layer. Each optically sensitive layer is interposed between two electrodes. The two electrodes include a respective first electrode and a respective second electrode. The integrated circuit selectively applies a bias to the electrodes and reads signals from the optically sensitive layers. The signal is related to the number of photons received by the respective optically sensitive layer. 
     SUMMARY 
     Embodiments of the present invention that are described hereinbelow provide improved image sensors and methods for their manufacture. 
     There is therefore provided, in accordance with an embodiment of the present invention, an imaging apparatus, including a semiconductor substrate, an array of pixel circuits formed on the semiconductor substrate and including respective pixel electrodes, and a layer of a photosensitive medium, which overlies the pixel electrodes and has a lower surface in electrical contact with the pixel electrodes, and which is configured to convert incident photons into charge carriers, which are collected by the pixel electrodes. The imaging apparatus further includes a planar conductive top electrode, which is at least partially transparent, overlying an upper surface of the photosensitive medium, a bias circuit formed on the semiconductor substrate and configured to provide a bias potential for application to the photosensitive medium, and a bias contact extending from the bias circuit to the top electrode so as to apply the bias potential to the top electrode while contacting the top electrode along a plane that is parallel to the upper surface of the photosensitive medium. 
     In an embodiment, the layer of the photosensitive medium includes a quantum film. 
     In a further embodiment, the imaging apparatus includes an insulating layer overlying the bias circuit and at least partially surrounding the photosensitive medium, wherein the bias contact extends through the insulating layer. In another embodiment, the insulating layer includes silicon dioxide. 
     There is further provided, in accordance with another embodiment of the invention, an imaging apparatus, including a semiconductor substrate, an array of pixel circuits formed on the semiconductor substrate and including respective pixel electrodes, and a layer of a photosensitive medium, which overlies the pixel electrodes and has a lower surface in electrical contact with the pixel electrodes, and which is configured to convert incident photons into charge carriers, which are collected by the pixel electrodes. The imaging apparatus further includes a bias circuit formed on the semiconductor substrate and configured to provide a bias potential for application to the photosensitive medium, an insulating layer overlying the bias circuit and at least partially surrounding the photosensitive medium, a planar conductive top electrode, which is at least partially transparent, overlying an upper surface of the photosensitive medium and the insulating layer, and a bias contact extending from the bias circuit through the insulating layer so as to contact and apply the bias potential to the top electrode. 
     In an embodiment, the insulating layer includes silicon dioxide. 
     In yet another embodiment, the layer of photosensitive medium includes a quantum film. In a further embodiment, the quantum film has a thickness of at least 1 μm. 
     There is also provided, in accordance with an embodiment of the invention, a method for fabricating an imaging apparatus, including forming an array of pixel circuits and a bias circuit on a semiconductor substrate, depositing a first insulating layer over the semiconductor substrate, and forming in the first insulating layer an array of pixel electrodes in electrical contact with respective pixel circuits, and a first bias contact in electrical contact with the bias circuit. The method further includes depositing a second insulating layer over the first insulating layer, extending the bias contact through the second insulating layer, opening a cavity in the second insulating layer by removing a part of the second insulating layer that covers the array of pixel contacts, depositing a layer of a photosensitive medium in the cavity with a lower surface of the photosensitive medium in electrical contact with the pixel electrodes, and depositing a conductive and at least partially transparent layer over the photosensitive medium and the second insulating layer so that a plane of the conductive layer that is parallel to the upper surface of the photosensitive medium is in electrical contact with the bias contact. 
     The present invention will be more fully understood from the following detailed description of the embodiments thereof, taken together with the drawings in which: 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic sectional view of an image sensor, in accordance with an embodiment of the invention; and 
         FIG. 2  is a flowchart that schematically illustrates the fabrication process flow of the image sensor shown in  FIG. 1 , in accordance with an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Hybrid image sensors typically comprise a layer of a photosensitive medium, such as a quantum film (QF), which is sandwiched between an at least partially transparent, conductive top electrode (comprising, for example, ITO (indium-tin oxide)) and bottom electrodes, also referred to as pixel electrodes. Each of the bottom electrodes defines a pixel of the image sensor and is connected to a corresponding pixel circuit. The top electrode is coupled to a bias contact, also referred to as a bias node, which provides the top electrode with a bias potential for the photosensitive layer. 
     In hybrid image sensors that are known in the art, the bias contact and bottom electrodes are formed by the same process steps. Consequently, the top of the bias contact is level with the top of the bottom electrodes, at the bottom of the photosensitive layer. In this contact configuration, the edge of the photosensitive layer is cut, using etching or similar methods known in the art, in order to provide space for a conducting path from the top electrode to the bias contact. This conducting path typically comprises a thin layer of metal, such as aluminum, extending from the top of the device to the bias contact. This sort of metal layer contacts the top electrode only through a sidewall, which contacts the edge of the top electrode and extends along the edge of the photosensitive layer down to the bias contact. This kind of edge contact is highly resistive, which may limit the charge/discharge response time of the photosensitive layer due to a high RC-factor. 
     Another problem that can arise when the edge of a photosensitive film, such as a quantum film, is handled in this manner is that the film has a tendency to peel off the underlying layers, particularly when the thickness of the film is greater than 0.5 μm. This tendency limits the permissible thickness of the film and can therefore constrain the quantum efficiency, which increases with the film thickness. 
     The embodiments of the present invention that are described herein address the problems described above and may thus enable the fabrication of image sensors with a faster response time and possibly higher quantum efficiency. 
     The disclosed embodiments extend the bias contacts up to the level of the top electrode. This enables a direct connection between the bias contact and the bottom surface of the top electrode along a plane that is parallel to the upper surface of the photosensitive medium, and thus the generation of a large contact area between the bias contact and the top electrode. This sort of connection can reduce the contact resistance (in some instances by multiple orders of magnitude) as compared to that of a conventional edge contact. Consequently, the RC-factor of the conducting path from the top electrode to the bias contact is lower, with a concomitant decrease in the charge/discharge response time of the image sensor. 
     In the disclosed embodiments the extension of the bias contact is achieved by depositing and patterning an additional insulating layer (such as SiO 2 ) and extending the bias contact through this layer. The photosensitive layer is formed in a cavity in the additional insulating layer, so that it is coplanar with the insulating layer and of comparable thickness, and the insulating layer at least partially surrounds the photosensitive medium. For an embodiment wherein the photosensitive layer comprises a photosensitive film, such as a quantum film, the insulating layer protects the edges of the film, thus reducing the risk of peeling. By an appropriate adjustment of the fabrication process, a quantum film with a thickness exceeding 1 μm is achievable, improving its quantum efficiency relative to sensors that use thinner film layers. 
       FIG. 1  is a schematic sectional view of an image sensor  20 , in accordance with an embodiment of the invention. Image sensor  20  is fabricated on a silicon substrate  22 , on which pixel circuits  23 , a bias circuit  25 , and additional electrical conductors (not shown) are fabricated using methods of integrated circuit technology that are known in the art. A first insulating layer  24 , typically comprising silicon dioxide (SiO 2 ), is coated on silicon substrate  22 . A layer of a photosensitive medium of image sensor  20  is formed over first insulating layer  24 . In the pictured embodiment, the photosensitive medium comprises a quantum film (QF)  26 . In alternative embodiments the photosensitive medium may comprise, for example, elemental semiconductors, compound semiconductors, colloidal nanocrystals, epitaxial quantum wells, epitaxial quantum dots, organic photoconductors, bulk heterojunction organic photoconductors, or any other suitable photosensitive material that is known in the art. These materials may be hybrid-bonded to image sensor  20 , and may form, for example, photoconductors, p-n junctions, heterojunctions, Schottky-diodes, quantum well stacks, quantum wires, quantum dots, phototransistors, or combinations of these devices connected in series and parallel. 
     Pixel circuits  23  are coupled to QF  26  by first metal contacts  28 , typically comprising aluminum or copper, which extend through first vias  30  (passing through first insulating layer  24 ) to respective bottom electrodes  32  (also referred to as pixel electrodes). Each bottom electrode  32  electrically contacts the lower surface of QF  26  and defines a pixel  34  in the QF. QF  26  converts incident photons into charge carriers, which are collected by bottom electrodes  32 . Typically (although not necessarily), pixels  34  form a two-dimensional matrix, arranged in rows and columns. Each row and column typically comprises hundreds or thousands of pixels  34 . However, for the sake of clarity, only three pixels  34  are shown in  FIG. 1 , with one of them being a “dark pixel”  34   a  used for calibration purposes. QF  26  is formed within an opening in a second insulating layer  36 , of a thickness h, where the second insulating layer is formed over first insulating layer  24  and typically comprises SiO 2 , as well. Since QF  26  is surrounded by second insulating layer  36 , the QF may also be deposited to a thickness h with a reduced risk of peeling off the underlying layers, and is effectively limited only by the etch aspect ratio of second vias  38 , etched through second insulating layer  36 . By depositing second insulating layer  36  to a thickness exceeding 1 μm, also the thickness of QF  26  may exceed 1 μm, thus enabling a high quantum efficiency for the film. 
     Second metal contacts  40  extending through second vias  38  in second insulating layer  36  and connecting through corresponding vias in first insulating layer  24  connect bias circuit  25  to a bias pad  42  at the level of the top of QF  26 . Bias pad  42  contacts a top electrode  44 , overlying QF  26 , along a plane that is parallel to the upper surface of the QF so as to apply a bias potential provided by bias circuit  25  to the top electrode. The specific first metal contact  28  that is in electrical contact with bias circuit  25 , its extension by second metal contact  40 , and bias pad  42  together form a bias contact  45 , which couples top electrode  44  to bias circuit  25 . 
     Bond pads  46  and bond pad contacts  48  may be formed on top of second insulating layer  36  and connected to the circuits on silicon substrate  22  through similar metal contacts and vias, forming a bond contact  49 . (Several bond pads  46  are formed for the image sensor to provide inputs and outputs for electrical potentials and signals to and from the sensor, but for the sake of clarity only one is shown in  FIG. 1 ). Bond pads  46  are further coupled to any required external circuitry. 
     A top encapsulation layer  50  seals QF  26 . Top electrode  44 , comprising conductive and at least partially transparent material such as ITO, covers QF  26  (which is under top encapsulation layer  50 ) and second insulating layer  36 . Top electrode  44  is in electrical contact with bias pad  42  of bias contact  45  through the bottom surface of the electrode rather than through its edge. This contact configuration ensures a much larger contact area than an edge contact and yields a contact resistance between bias contact  45  and top electrode  44  that is substantially lower than for an edge contact. This reduced resistance ensures a low RC factor and a fast response of sensor  20 . 
     Each pixel  34  (except for dark pixel  34   a ) is covered by a micro-lens  52  for enhanced light collection. Micro-lenses  52  are coated by an anti-reflection coating  54  in order to reduce the light loss due to reflections from the interfaces between the micro-lenses and air. Pixels  32  may be optionally covered with an array of color filters, located between the pixel array and the micro-lens array. The addition of a color filter array depends on the wavelengths of interest and the application of the imaging apparatus. Micro-lenses  52  are isolated from top electrode  44  by a passivation layer  58 . 
     Metallic optical shield  56  covers one of pixels  34  for the purpose of generating “dark pixel”  34   a.    
       FIG. 2  is a flowchart  100  that schematically illustrates the fabrication process flow of image sensor  20  shown in  FIG. 1 , in accordance with an embodiment of the invention. All of the process steps described in the flowchart use techniques of semiconductor fabrication that are known in the art. References to specific items utilize the labels from  FIG. 1 . The first step in the flowchart, a silicon processing and first insulating layer deposition step  102 , comprises the steps of fabricating pixel circuits  23 , bias circuit  25 , and the additional electrical conductors in the silicon substrate, as well as depositing first insulating layer  24 . 
     In a first via patterning step  104 , first vias  30  are etched through first insulating layer  24 . In a first metal contact patterning step  106 , the top ends of the metal filling first vias  30  are patterned. In a first metal contact filling step  108 , first vias  30  are filled with a metal, typically comprising aluminum or copper. In a bottom electrode deposition step  110  and a bottom electrode patterning step  112 , bottom electrodes  32  are respectively deposited and patterned on top of first metal contacts  28 , over which QF  26  will be deposited in a later QF deposition step. In second insulating layer deposition step  114 , second insulating layer  36  is deposited on top of first insulating layer  24 . In a second via patterning step  116 , vias are etched through second insulating layer  36  for bias contact  45  and for bond contact  49 , connecting to corresponding first vias  30 . In a second metal contact patterning step  118  and a second metal contact filling step  120 , second vias  38  are filled with a metal and patterned similarly to the filling of first vias  30  and patterning of first metal contacts  28 . In a contact deposition step  122 , contacts are deposited and patterned on top of second metal contacts  40 , thus defining bias pad  42  and bonding pad contact  48 . 
     In a second insulating layer patterning step  124 , a cavity is etched through second insulating layer  36  down to the level of bottom electrodes  32  (pixel electrodes). QF  26  is deposited in this cavity in a QF deposition step  126 . In an encapsulation deposition step  128 , top encapsulation layer  50  is deposited over QF  26 , and in an encapsulation patterning step  130 , it is etched outside the pixel array to uncover bias pad  42 . In a top electrode deposition step  132 , top electrode  44  is deposited over the area of QF  26  and second insulating layer  36 . Top electrode  44  makes electrical contact along its lower surface with bias contact  45  through its bias pad  42 . In a top electrode patterning step  134 , top electrode layer  44  is removed from bond pad  46  contact area. 
     In a metal deposition step  136  and a metal patterning step  138 , metal pads are respectively deposited and patterned to form metallic optical shield  56  and bond pad  46 . In a passivation deposition step  140 , passivation layer  58  is deposited over the entire surface of the image sensor. A color filter array may optionally be coated and patterned over the pixel array in a color filter coating and patterning step  142 . In a micro-lens deposition and patterning step  144 , micro-lens material is deposited and patterned to form one micro-lens  52  over each individual pixel  34 . In an anti-reflection coating step  146 , one or more thin layers of SiO 2  or SiO x N y  (silicon oxynitride) are deposited over the surface of micro-lenses  52  to serve as anti-reflection coating  54  for the lenses. In a passivation patterning step  148 , passivation layer  58  is opened at bond pad  46  in order to provide access for coupling any external circuitry to the bond pad. 
     It will be appreciated that the embodiments described above are cited by way of example, and that the present invention is not limited to what has been particularly shown and described hereinabove. Rather, the scope of the present invention includes both combinations and subcombinations of the various features described hereinabove, as well as variations and modifications thereof which would occur to persons skilled in the art upon reading the foregoing description and which are not disclosed in the prior art.

Metadata:
Filing Date: 20190228
Publication Date: 20201215
Grant Date: 20201215
Priority Date: 20180425
Inventors: LEE, HONG-WEI
Assignee: APPLE INC
CPC Classifications: [{"code": "H10F39/8063", "inventive": false, "first": false, "tree": "[]"}, {"code": "H10F39/8053", "inventive": false, "first": false, "tree": "[]"}, {"code": "H10F39/805", "inventive": false, "first": false, "tree": "[]"}, {"code": "H10F77/933", "inventive": true, "first": false, "tree": "[]"}, {"code": "H10F77/306", "inventive": true, "first": false, "tree": "[]"}, {"code": "H10F77/247", "inventive": true, "first": false, "tree": "[]"}, {"code": "H10F77/206", "inventive": true, "first": false, "tree": "[]"}, {"code": "H10F71/138", "inventive": true, "first": false, "tree": "[]"}, {"code": "H10F39/811", "inventive": true, "first": false, "tree": "[]"}, {"code": "H10F39/182", "inventive": true, "first": false, "tree": "[]"}, {"code": "H10F39/024", "inventive": true, "first": false, "tree": "[]"}, {"code": "H10F71/00", "inventive": true, "first": false, "tree": "[]"}, {"code": "H10F77/143", "inventive": true, "first": false, "tree": "[]"}, {"code": "H10F77/413", "inventive": true, "first": false, "tree": "[]"}, {"code": "H10F77/331", "inventive": true, "first": false, "tree": "[]"}, {"code": "H10F77/50", "inventive": true, "first": false, "tree": "[]"}, {"code": "H10F77/93", "inventive": true, "first": false, "tree": "[]"}, {"code": "H10F39/191", "inventive": true, "first": false, "tree": "[]"}, {"code": "H10F39/182", "inventive": true, "first": false, "tree": "[]"}, {"code": "H10F39/811", "inventive": true, "first": false, "tree": "[]"}, {"code": "H10F39/8063", "inventive": false, "first": false, "tree": "[]"}, {"code": "H10F39/8053", "inventive": false, "first": false, "tree": "[]"}, {"code": "H10F39/8057", "inventive": false, "first": false, "tree": "[]"}, {"code": "H10F77/1433", "inventive": true, "first": true, "tree": "[]"}, {"code": "H10F39/809", "inventive": true, "first": true, "tree": "[]"}, {"code": "H01L31/022408", "inventive": true, "first": false, "tree": "[]"}, {"code": "H01L31/02005", "inventive": true, "first": false, "tree": "[]"}, {"code": "H01L27/14621", "inventive": false, "first": false, "tree": "[]"}, {"code": "H01L31/022475", "inventive": true, "first": false, "tree": "[]"}, {"code": "H01L31/02161", "inventive": true, "first": false, "tree": "[]"}, {"code": "H01L27/14636", "inventive": true, "first": false, "tree": "[]"}, {"code": "H01L27/1462", "inventive": false, "first": false, "tree": "[]"}, {"code": "H01L31/1884", "inventive": true, "first": false, "tree": "[]"}, {"code": "H01L27/14627", "inventive": false, "first": false, "tree": "[]"}, {"code": "H01L31/035218", "inventive": true, "first": true, "tree": "[]"}, {"code": "H01L27/14645", "inventive": true, "first": false, "tree": "[]"}, {"code": "H01L27/14685", "inventive": true, "first": false, "tree": "[]"}]
Family ID: 73746806