Patent Publication Number: US-8980674-B2

Title: Image sensor with improved dark current performance

Description:
PRIORITY DATA 
     The present application is a divisional patent application of U.S. patent application Ser. No. 13/295,145, filed on Nov. 14, 2011 now U.S. Pat. No. 8,697,472 issued Apr. 15, 2014, entitled “Image Sensor with Improved Dark Current Performance”, the disclosure of which is hereby incorporated by reference in its entirety. 
    
    
     BACKGROUND 
     Semiconductor image sensors are used to sense radiation such as light. Complementary metal-oxide-semiconductor (CMOS) image sensors (CIS) and charge-coupled device (CCD) sensors are widely used in various applications such as digital still camera or mobile phone camera applications. These devices utilize an array of pixels in a substrate, including photodiodes and transistors, that can absorb radiation projected toward the substrate and convert the sensed radiation into electrical signals. 
     A back side illuminated (BSI) image sensor device is one type of image sensor device. These BSI image sensor devices are operable to detect light projected from the backside. The BSI image sensors may have metal shields formed over reference pixels on the back side, so as to prevent light from reaching the reference pixels. Traditional semiconductor image sensor devices may experience large amounts of stress fluctuations caused at least in part by the existence of these metal shields. The stress within the image sensor devices can result in poor dark current performance of the image sensor. 
     Therefore, while existing semiconductor image sensors have been generally adequate for their intended purposes, they are not entirely satisfactory in every aspect. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Aspects of the present disclosure are understood from the following detailed description when read with the accompanying figures. It is emphasized that, in accordance with the standard practice in the industry, various features are not drawn to scale. In fact, the dimensions of the various features may be arbitrarily increased or reduced for clarity of discussion. 
         FIG. 1  is a flowchart illustrating a method for fabricating an image sensor device according to various aspects of the present disclosure. 
         FIGS. 2-5  are diagrammatic fragmentary cross-sectional side views of an image sensor device at various stages of fabrication in accordance with various aspects of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     It is to be understood that the following disclosure provides many different embodiments, or examples, for implementing different features of the invention. Specific examples of components and arrangements are described below to simplify the present disclosure. These are, of course, merely examples and are not intended to be limiting. Moreover, the formation of a first feature over or on a second feature in the description that follows may include embodiments in which the first and second features are formed in direct contact, and may also include embodiments in which additional features may be formed interposing the first and second features, such that the first and second features may not be in direct contact. Various features may be arbitrarily drawn in different scales for the sake of simplicity and clarity. 
     Illustrated in  FIG. 1  is a flowchart of a method  10  for fabricating a semiconductor image sensor device according to various aspects of the present disclosure. Referring to  FIG. 1 , the method  10  begins with block  12  in which a plurality of radiation-sensing components is formed in a semiconductor substrate. The substrate includes a black level correction region. The method  10  continues with block  14 , in which a first compressive layer is formed over the substrate. The method  10  continues with block  16  in which a metal device is formed on the first compressive layer. The metal device is formed over the black level correction region of the substrate. The method  10  continues with block  18  in which a second compressive layer is formed on the metal device and on the first compressive layer. It is understood that additional processing steps may be performed before, during, or after the method  10  of  FIG. 1 . But for the sake of simplicity, these additional processing steps are not discussed in detail herein. 
       FIGS. 2 to 5  are diagrammatic fragmentary sectional side views of various embodiments of an apparatus that is a back side illuminated (BSI) image sensor device  30  at various stages of fabrication according to aspects of the method  10  of  FIG. 1 . The image sensor device  30  includes an array or grid of pixels for sensing and recording an intensity of radiation (such as light) directed toward a back-side of the image sensor device  30 . The image sensor device  30  may include a charge-coupled device (CCD), complimentary metal oxide semiconductor (CMOS) image sensor (CIS), an active-pixel sensor (APS), or a passive-pixel sensor. The image sensor device  30  further includes additional circuitry and input/outputs that are provided adjacent to the grid of pixels for providing an operation environment for the pixels and for supporting external communication with the pixels. It is understood that  FIGS. 2 to 5  have been simplified for a better understanding of the inventive concepts of the present disclosure and may not be drawn to scale. 
     With reference to  FIG. 2 , the image sensor device  30  includes a substrate  40 , hereinafter referred to as a device substrate. The device substrate  40  is a silicon substrate doped with a p-type dopant such as Boron (for example a p-type substrate). Alternatively, the device substrate  40  could be another suitable semiconductor material. For example, the device substrate  40  may be a silicon substrate that is doped with an n-type dopant such as Phosphorous or Arsenic (an n-type substrate). The device substrate  40  could include other elementary semiconductors such as germanium and diamond. The device substrate  40  could optionally include a compound semiconductor and/or an alloy semiconductor. Further, the device substrate  40  could include an epitaxial layer (epi layer), may be strained for performance enhancement, and may include a silicon-on-insulator (SOI) structure. 
     Referring back to  FIG. 2 , the device substrate  40  has a front side (also referred to as a front surface)  50  and a back side (also referred to as a back surface)  60 . For a BSI image sensor device such as the image sensor device  30 , radiation is projected from the back side  60  and enters the substrate  40  through the back surface. The device substrate  40  also has an initial thickness  70 . In some embodiments, the initial thickness  70  is in a range from about 100 microns (um) to about 3000 um, for example between about 500 um and about 1000 um. 
     A plurality of dielectric trench isolation (STI) structures is formed in the substrate  40 . In some embodiments, the STI structures are formed by the following process steps: etching openings into the substrate  40  from the front side  50 ; filling the openings with a dielectric material such as silicon oxide, silicon nitride, silicon oxynitride, a low-k material, or another suitable dielectric material; and thereafter performing a polishing process—for example a chemical mechanical polishing (CMP) process—to planarize the surface of the dielectric material filling the openings. In some embodiments, deep trench isolation (DTI) structures may be formed. The formation processes for the DTI structures may be similar to the STI structures, though the DTI structures are formed to have greater depths than the STI structures. In certain embodiments, doped isolation structures may also be formed. These doped isolation structures may be formed by one or more ion implantation processes. The doped isolation structures may be formed to replace or to supplement the STI or DTI structures. 
     A plurality of pixels is formed in the substrate  40 . The pixels contain radiation-sensing doped regions. These radiation-sensing doped regions are formed by one or more ion implantation processes or diffusion processes and are doped with a doping polarity opposite from that of the substrate  40  (or the doped region  140 ). Thus, in the embodiment illustrated, the pixels contain n-type doped regions. For a BSI image sensor device such as the image sensor device  30 , the pixels are operable to detect radiation, such as an incident light  75 , that is projected toward device substrate  40  from the back side  60 . 
     In some embodiments, the pixels each include a photodiode. A deep implant region may be formed below each photodiode in some embodiments. In other embodiments, the pixels may include pinned layer photodiodes, photogates, reset transistors, source follower transistors, and transfer transistors. The pixels may also be referred to as radiation-detection devices or light-sensors. The pixels may be varied from one another to have different junction depths, thicknesses, widths, and so forth. It is understood that each pair of adjacent or neighboring pixels may be separated from each other by a respective one of the isolation structures discussed above. For the sake of simplicity, neither the pixels nor the isolation structures are specifically illustrated herein. 
     Referring now to  FIG. 3 , an interconnect structure  80  is formed over the front side  50  of the device substrate  40 . The interconnect structure  80  includes a plurality of patterned dielectric layers and conductive layers that provide interconnections (e.g., wiring) between the various doped features, circuitry, and input/output of the image sensor device  30 . The interconnect structure  80  includes an interlayer dielectric (ILD) and a multilayer interconnect (MLI) structure. The MLI structure includes contacts, vias and metal lines. For purposes of illustration, a plurality of conductive lines  90  and vias/contacts  95  are shown in  FIG. 3 , it being understood that the conductive lines  90  and vias/contacts  95  illustrated are merely exemplary, and the actual positioning and configuration of the conductive lines  90  and vias/contacts  95  may vary depending on design needs and manufacturing concerns. 
     The MLI structure may include conductive materials such as aluminum, aluminum/silicon/copper alloy, titanium, titanium nitride, tungsten, polysilicon, metal silicide, or combinations thereof, being referred to as aluminum interconnects. Aluminum interconnects may be formed by a process including physical vapor deposition (PVD) (or sputtering), chemical vapor deposition (CVD), atomic layer deposition (ALD), or combinations thereof. Other manufacturing techniques to form the aluminum interconnect may include photolithography processing and etching to pattern the conductive materials for vertical connection (for example, vias/contacts  95 ) and horizontal connection (for example, conductive lines  90 ). Alternatively, a copper multilayer interconnect may be used to form the metal patterns. The copper interconnect structure may include copper, copper alloy, titanium, titanium nitride, tantalum, tantalum nitride, tungsten, polysilicon, metal silicide, or combinations thereof. The copper interconnect structure may be formed by a technique including CVD, sputtering, plating, or other suitable processes. 
     Still referring to  FIG. 3 , a buffer layer  100  is formed on the interconnect structure  80 . In the present embodiment, the buffer layer  100  includes a dielectric material such as silicon oxide. Alternatively, the buffer layer  100  may optionally include silicon nitride. The buffer layer  100  is formed by CVD, PVD, or other suitable techniques. The buffer layer  100  is planarized to form a smooth surface by a CMP process. 
     Thereafter, a carrier substrate  110  is bonded with the device substrate  40  through the buffer layer  100 , so that processing of the back side  60  of the device substrate  40  can be performed. The carrier substrate  110  in the present embodiment is similar to the substrate  40  and includes a silicon material. Alternatively, the carrier substrate  110  may include a glass substrate or another suitable material. The carrier substrate  110  may be bonded to the device substrate  40  by molecular forces—a technique known as direct bonding or optical fusion bonding—or by other bonding techniques known in the art, such as metal diffusion or anodic bonding. 
     Referring back to  FIG. 3 , the buffer layer  100  provides electrical isolation between the device substrate  40  and the carrier substrate  110 . The carrier substrate  110  provides protection for the various features formed on the front side  50  of the device substrate  40 , such as the pixels formed therein. The carrier substrate  110  also provides mechanical strength and support for processing of the back side  60  of the device substrate  40  as discussed below. After bonding, the device substrate  40  and the carrier substrate  110  may optionally be annealed to enhance bonding strength. 
     Still referring to  FIG. 3 , after the carrier substrate  110  is bonded, a thinning process  120  is then performed to thin the device substrate  40  from the backside  60 . The thinning process  120  may include a mechanical grinding process and a chemical thinning process. A substantial amount of substrate material may be first removed from the device substrate  40  during the mechanical grinding process. Afterwards, the chemical thinning process may apply an etching chemical to the back side  60  of the device substrate  40  to further thin the device substrate  40  to a thickness  130 , which is on the order of a few microns. In some embodiments, the thickness  130  is greater than about 1 um but less than about 3 um. It is also understood that the particular thicknesses disclosed in the present disclosure are mere examples and that other thicknesses may be implemented depending on the type of application and design requirements of the image sensor device  30 . 
     Referring now to  FIG. 4 , a compressive layer  150  is formed over the device substrate  40  from the back side  60 . The compressive layer  150  has a thickness  160 . In some embodiments, the thickness  160  is less than about 500 Angstroms. In some embodiments, the compressive layer  150  contains silicon oxide. The compressive layer  150  is operable to deliver a compressive stress to layers above and/or below. In general, compressive stress is a type of stress on a material that leads to contraction of the material, meaning the length of the material tends to decrease in the compressive direction. Compressive stress is opposite from tensile stress, which is a type of stress on a material that leads to expansion of the material, which means the length of the material tends to increase in the tensile direction. The compressive layer  150  herein also serves as a buffer layer between the device substrate  40  and the layers to be formed over the substrate  40  from the back side  60 . 
     A compressive layer  170  is formed over the compressive layer  150 . The compressive layer  170  also delivers a compressive stress to layers above and/or below. The compressive layer  170  has a thickness  180 . In some embodiments, the thickness  180  is greater than about 300 Angstroms but less than about 3000 Angstroms. In some embodiments, the compressive layer  170  contains silicon nitride. The silicon nitride material may be tuned to be compressively stressed by adjusting parameters such as a Radio-Frequency (RF) power range and a SiH 4  gas content during the formation of the silicon nitride. In certain embodiments, the compressive stress of the compressive film is greater than about −10×10 8  dyne/centimeters 2  in magnitude. Stated differently, the absolute value of the compressive stress of the compressive film is greater than about 10×10 8  dyne/centimeters 2 . 
     A metal device  200  is formed on a portion of the compressive layer  170 . The metal device  200  may be formed using a suitable deposition process and patterning process known in the art. The metal device  200  is formed in a region of the image sensor device  30  known as a black level correction region  210 . The black level correction region  210  contains one or more reference pixels formed in the device substrate  40  that need to be kept optically dark. Therefore, the metal device  200  is operable to block light penetration from the back side  60  so that the reference pixel below (formed in the substrate  40 , not illustrated herein) can be kept dark. Thus, the metal device  200  may also be referred to as a metal shield. In some embodiments, the metal device  200  contains AlCu. The metal device  200  has a thickness  220 . In some embodiments, the thickness  220  is greater than about 600 Angstroms but less than about 3500 Angstroms. In the embodiments illustrated herein, the metal device  200  is a tensile device. 
     In addition to the black level correction region  210 , the image sensor device  30  also has an array region  230 , which contains “regular” pixels that are operable to detect light and should not be kept dark. Thus, no light-blocking devices are formed over the compressive layer  170  in the array region  230 . 
     Referring now to  FIG. 5 , a compressive layer  240  is formed over the metal device  200  and over the exposed surface of the compressive layer  170 . In other words, the compressive layer  240  is formed in both the black level correction region  210  and the array region  230 . The sidewall of the metal device  200  is covered by the compressive layer  240 . The compressive layer  250  delivers a compressive stress to the metal device  200 . The compressive layer  240  has a thickness  250 . In some embodiments, the thickness  250  is greater than about 1000 Angstroms but less than about 4000 Angstroms. In some embodiments, the compressive layer  250  contains silicon oxide. 
     A compressive layer  260  is formed over the compressive layer  240 . The compressive layer  260  is formed in both the black level correction region  210  and the array region  230 . The compressive layer  260  delivers a compressive stress to the layers below. The compressive layer  260  has a thickness  270 . In some embodiments, the thickness  270  is greater than about 1200 Angstroms but less than about 3500 Angstroms. In some embodiments, the compressive layer  260  contains silicon nitride. Similar to the compressive layer  170 , the silicon nitride material of the compressive layer  260  may be tuned to be compressively stressed by adjusting parameters such as a Radio-Frequency (RF) power range and a SiH 4  gas content during the formation of the silicon nitride. In certain embodiments, the compressive stress of the compressive film is greater than about −20×10 8  dyne/centimeters 2  in magnitude. 
     The embodiments discussed above offer advantages over conventional image sensor devices, for example advantages in dark current performances. However, it is understood that not all advantages are necessarily discussed herein, and other embodiments may offer different advantages, and that no particular advantage is required for all embodiments. 
     Dark current is a common type of image sensor defect and may be defined as the existence of pixel current when no actual illumination is present. In other words, the pixel “detects” light when it is not supposed to. Dark current defects may be caused by stress. In more detail, conventional image sensors may experience excessive amounts of internal stress. The excessive stress may induce a bandgap of a charge carrier to be narrowed, which may result in leakage current. This issue is particular severe in the black level correction region of an image sensor, where a metal device (blocking the reference pixels) may cause a dominant level of tensile stress. Such tensile stress caused by the metal device is not adequately alleviated by conventional image sensors, and therefore dark current defects often times plague conventional image sensors. 
     In comparison, the image sensor device  30  discussed above utilized a unique and optimized film stacking scheme to reduce the stress of the metal device  200  in the black level region  210 . For example, a nitride-containing compressive layer  170  is formed below the metal device  200  according to various embodiments. As discussed above, compressive stress and tensile stress are opposite one another. The compressive layer  170  delivers a compressive stress to the metal device  200  above, thereby reducing the tensile stress of the metal device  200 . In addition, the amount of stress delivered by a layer to another layer depends on the relative thicknesses of these layers. Here, the metal device  200  is somewhat thick (e.g., being thousands of Angstroms thick). Thus, to ensure a sufficient amount of compressive stress can be delivered to the metal device  200 , the nitride-containing compressive layer  170  is configured to have a relatively large thickness too (e.g., being thousands of Angstroms thick). 
     Conventional image sensors also fail to protect the sidewalls of the metal devices in the black level region. The lack of sidewall protection of the metal device also leads to leakage current that can degrade the performance of the image sensor. In comparison, the image sensor device  30  discussed according to various embodiments above utilizes an oxide-containing compressive layer  240  to protect the sidewall of the metal device  200 . Such sidewall protection reduces the current leakage defects and improves the dark current performance of the image sensor device  200 . Moreover, the oxide-containing compressive layer  240  is also configured to deliver a compressive stress to the metal device  200 , thereby further reducing the tensile stress of the metal device  200 . Once again, to ensure that a sufficient amount of compressive stress can be delivered to the metal device  200 , the compressive layer  240  is also designed to have be relatively thick, for example being thousands of Angstroms thick. 
     It is understood that the compressive layer  150  and the compressive layer  260  are also operable to deliver compressive stress to the metal device  200 , even though they are not in direct physical contact with the metal device  200 . Through the compressive layers  150 ,  170 ,  240 , and  260 , the total amount of compressive stress delivered to the metal device  200  can substantially balance out the tensile stress of the metal device  200 . Stated differently, the metal device  200  may approach a stress-free state. As such, the energy bandgap will not be substantially altered, thereby reducing leakage current and improving dark current performance. 
     Additional fabrication processes may be performed to complete the fabrication of the image sensor device  40 . For example, a color filter layer may be formed on the back side  60  of the substrate  40 . The color filter layer may contain a plurality of color filters that may be positioned such that the incoming radiation is directed thereon and therethrough. The color filters may include a dye-based (or pigment based) polymer or resin for filtering a specific wavelength band of the incoming radiation, which corresponds to a color spectrum (e.g., red, green, and blue). Thereafter, a micro-lens layer containing a plurality of micro-lenses is formed over the color filter layer. The micro-lenses direct and focus the incoming radiation toward specific radiation-sensing regions in the device substrate  40 . The micro-lenses may be positioned in various arrangements and have various shapes depending on a refractive index of a material used for the micro-lens and distance from a sensor surface. The device substrate  40  may also undergo an optional laser annealing process before the forming of the color filter layer or the micro-lens layer. For reasons of simplicity, the color filters and the micro-lenses are not specifically illustrated herein. 
     It is understood that the sequence of the fabrication processes described above is not intended to be limiting. Some of the layers or devices may be formed according to different processing sequences in other embodiments than what is shown herein. Furthermore, some other layers may be formed but are not illustrated herein for the sake of simplicity. For example, an anti-reflection coating (ARC) layer may be formed over the back side  60  of the substrate  40  before the formation of the color filter layer and/or the micro-lens layer. 
     It is also understood that the discussions above pertain mostly to a pixel region of the image sensor device  30 . In addition to the pixel region, the image sensor  30  also includes a periphery region, a bonding pad region, and a scribe line region. The periphery region may include digital devices, such as application-specific integrated circuit (ASIC) devices or system-on-chip (SOC) devices, or other reference pixels used to establish a baseline of an intensity of light for the image sensor device  30 . The bonding pad region is reserved for the formation of bonding pads, so that electrical connections between the image sensor device  30  and external devices may be established. The scribe line region includes a region that separates one semiconductor die from an adjacent semiconductor die. The scribe line region is cut therethrough in a later fabrication process to separate adjacent dies before the dies are packaged and sold as integrated circuit chips. For the sake of simplicity, the details of these other regions of the image sensor device  30  are not illustrated or described herein. 
     One of the broader forms of the present disclosure involves an image sensor device that includes: a semiconductor substrate that contains a plurality of radiation-sensing regions, the substrate having a first side and a second side opposite the first side; a first compressive layer disposed on the second side of the substrate; a metal device disposed on a portion of the first compressive layer; and a second compressive layer disposed on the metal device. 
     In some embodiments, the metal device is a tensile device. 
     In some embodiments, the metal device is disposed over a black level correction region of the substrate, the black level correction region containing a reference pixel. 
     In some embodiments, the radiation-sensing regions are operable to sense radiation projected toward the substrate from the second side. 
     In some embodiments, the first compressive layer includes a compressively-stressed silicon nitride material. 
     In some embodiments, the second compressive layer includes a compressively-stressed silicon oxide material. 
     In some embodiments, the second compressive layer is disposed on a sidewall of the metal device and on the first compressive layer. 
     In some embodiments, the image sensor device further includes: a compressive silicon oxide layer disposed between the substrate and the first compressive layer; and a compressive silicon nitride layer disposed on the second compressive layer. 
     In some embodiments, the second compressive layer has a stress greater than about −10×10 8  dyne/centimer 2  in magnitude; and the compressive silicon nitride layer has a stress greater than about −20×10 8  dyne/centimer 2  in magnitude. 
     Another one of the broader forms of the present disclosure involves an image sensor device that includes: a semiconductor substrate that includes an array region and a black level correction region, wherein the array region contains a plurality of radiation-sensitive pixels, and the black level correction region contains one or more reference pixels, and wherein the substrate has a front side and a back side; a first compressively-stressed layer formed on the back side of the substrate, the first compressively-stressed layer containing silicon nitride; a metal shield formed on the compressively-stressed layer, wherein the metal shield is formed over at least a portion of the black level correction region; and a second compressively-stressed layer formed on the metal shield and the first compressively-stressed layer, wherein the second compressively-stressed layer contains silicon oxide, and wherein a sidewall of the metal shield is protected by the second compressively-stressed layer. 
     In some embodiments, the radiation-sensitive pixels are configured to detect light that enters the substrate from the back side. 
     In some embodiments, the metal shield is a tensile device. 
     In some embodiments, the image sensor device further includes: a third compressively-stressed layer formed between the substrate and the first compressively-stressed layer; and a fourth compressively-stressed layer formed on the second compressively-stressed layer. 
     In some embodiments, the third compressively-stressed layer contains an oxide material; and the fourth compressively-stressed layer contains a nitride material. 
     Still another of the broader forms of the present disclosure involves a method of fabricating an image sensor device, the method includes: forming a plurality of radiation-sensing components in a semiconductor substrate, the substrate including a black level correction region; forming a first compressive layer over the substrate; forming a metal device on the first compressive layer, the metal device being formed over the black level correction region of the substrate; and forming a second compressive layer on the metal device and on the first compressive layer. 
     In some embodiments, the substrate has a front surface and a back surface; the radiation-sensing components are operable to sense light that enters the substrate from the back surface; and the first compressive layer is formed over the back surface of the substrate. 
     In some embodiments, the black level correction region contains one or more reference pixels. 
     In some embodiments, the metal device is a tensile device. 
     In some embodiments, the first compressive layer contains silicon nitride; and the second compressive layer contains silicon oxide. 
     In some embodiments, a portion of the second compressive layer is formed on a sidewall of the metal device. 
     The foregoing has outlined features of several embodiments so that those skilled in the art may better understand the detailed description that follows. Those skilled in the art should appreciate that they may readily use the present disclosure as a basis for designing or modifying other processes and structures for carrying out the same purposes and/or achieving the same advantages of the embodiments introduced herein. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the present disclosure, and that they may make various changes, substitutions and alterations herein without departing from the spirit and scope of the present disclosure.