Patent Publication Number: US-8119435-B2

Title: Wafer level processing for backside illuminated image sensors

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional application of U.S. patent application Ser. No. 12/169,791, filed Jul. 9, 2008 now U.S. Pat. No. 7,859,033, which is related to the inventions described in commonly-assigned U.S. patent applications , entitled “Color Filter Array Alignment Mark Formation in Backside Illuminated Image Sensors,” , entitled “Backside Illuminated Image Sensor with Shallow Backside Trench for Photodiode Isolation,” entitled “Backside Illuminated Image Sensor with Reduced Dark Current,” which are concurrently filed herewith. The disclosures of these related applications are incorporated by reference herein in their entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to electronic image sensors for use in digital cameras and other types of imaging devices, and more particularly to processing techniques for use in forming backside illuminated image sensors. 
     BACKGROUND OF THE INVENTION 
     A typical electronic image sensor comprises a number of light sensitive picture elements (“pixels”) arranged in a two-dimensional array. Such an image sensor may be configured to produce a color image by forming an appropriate color filter array (CFA) over the pixels. Examples of image sensors of this type are disclosed in U.S. patent application Publication No. 2007/0024931, entitled “Image Sensor with Improved Light Sensitivity,” which is incorporated by reference herein. 
     As is well known, an image sensor may be implemented using complementary metal-oxide-semiconductor (CMOS) circuitry. In such an arrangement, each pixel typically comprises a photodiode and other circuitry elements that are formed in a silicon sensor layer on a silicon substrate. One or more dielectric layers are usually formed above the silicon sensor layer and may incorporate additional circuitry elements as well as multiple levels of metallization used to form interconnects. The side of the image sensor on which the dielectric layers and associated levels of metallization are formed is commonly referred to as the frontside, while the side having the silicon substrate is referred to as the backside. 
     In a frontside illuminated image sensor, light from a subject scene is incident on the frontside of the image sensor, and the silicon substrate is relatively thick. However, the presence of metallization level interconnects and various other features associated with the dielectric layers on the frontside of the image sensor can adversely impact the fill factor and quantum efficiency of the image sensor. 
     A backside illuminated image sensor addresses the fill factor and quantum efficiency issues associated with the frontside dielectric layers by thinning or removing the thick silicon substrate and arranging the image sensor such that light from a subject scene is incident on the backside of the image sensor. Thus, the incident light is no longer impacted by metallization level interconnects and other features of the dielectric layers, and fill factor and quantum efficiency are improved. 
     Backside illuminated image sensors can be difficult to process due to the thinning or removal of the silicon substrate. Important processing issues that are not adequately addressed by conventional techniques include use and attachment of temporary carrier wafers, proper configuration and placement of image sensor covers for optimum image quality, and formation of interconnects between image sensor bond pads and solder balls or other package contacts. Accordingly, certain conventional processing techniques, such as those disclosed in, for example, U.S. patent application Publication No. 2007/0194397, entitled “Photo-Sensor and Pixel Array with Backside Illumination and Method of Forming the Photo-Sensor,” can suffer from increased die size, higher cost, and decreased image quality. 
     A need therefore exists for improved processing techniques for forming backside illuminated image sensors. 
     SUMMARY OF THE INVENTION 
     Illustrative embodiments of the invention provide backside illuminated image sensors that are easier to process, and exhibit improved performance relative to conventional sensors. 
     In accordance with one aspect of the invention, a process of forming a backside illuminated image sensor is provided. The process is a wafer level process for forming a plurality of image sensors each having a pixel array configured for backside illumination, with the image sensors being formed utilizing an image sensor wafer. The image sensor wafer comprises a substrate, an oxide layer formed over the substrate, a sensor layer formed over the oxide layer, and one or more dielectric layers formed over the sensor layer, with the sensor layer comprising photodiodes or other photosensitive elements of the pixel arrays. The process includes the steps of attaching a temporary carrier wafer to a frontside surface of the dielectric layer; removing the substrate to expose a backside surface of the oxide layer; forming color filter arrays on the backside surface of the oxide layer; attaching a transparent cover sheet comprising transparent covers overlying respective ones of the color filter arrays; removing the temporary carrier wafer; forming openings in the dielectric layer to expose respective bond pad conductors; forming redistribution metal conductors in electrical contact with the respective bond pad conductors through the respective openings; forming a redistribution passivation layer over the redistribution metal conductors; forming openings in the redistribution passivation layer to expose portions of respective ones of the redistribution metal conductors; and forming contact metallizations in electrical contact with the respective redistribution metal conductors in respective ones of the openings in the redistribution passivation layer. A resulting processed wafer is subsequently separated into the plurality of image sensors by dicing the wafer. 
     Each of the transparent covers provided by the transparent cover sheet may comprise a central cavity arranged over its corresponding color filter array and may further comprise peripheral supports secured to the backside surface of the oxide layer. 
     The above-described wafer level process may include additional steps in a given embodiment of the invention. For example, the process may further comprise the step of forming microlenses over respective color filter elements of the color filter arrays, the step of adding a protective film layer to the transparent cover sheet, and the step of attaching solder balls to respective ones of the contact metallizations formed in the respective openings in the redistribution passivation layer. 
     In accordance with another aspect of the invention, a backside illuminated image sensor comprises a sensor layer having a plurality of photosensitive elements of a pixel array, an oxide layer adjacent a backside surface of the sensor layer, and at least one dielectric layer adjacent a frontside surface of the sensor layer. A color filter array is formed on a backside surface of the oxide layer, and a transparent cover is attached to the backside surface of the oxide layer overlying the color filter array. Redistribution metal conductors are in electrical contact with respective bond pad conductors through respective openings in the dielectric layer. A redistribution passivation layer is formed over the redistribution metal conductors, and contact metallizations are in electrical contact with respective ones of the respective redistribution metal conductors through respective openings in the redistribution passivation layer. 
     A backside illuminated image sensor in accordance with the invention may be advantageously implemented in a digital camera or other type of imaging device, and provides improved performance in such a device without significantly increasing image sensor die size or cost. For example, use of a transparent image sensor cover with a central cavity and peripheral supports provides better image quality than conventional arrangements. Also, the illustrative embodiments overcome the drawbacks of conventional techniques with regard to use and attachment of temporary carrier wafers, and formation of interconnects between image sensor bond pads and solder balls or other package contacts. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other objects, features, and advantages of the present invention will become more apparent when taken in conjunction with the following description and drawings wherein identical reference numerals have been used, where possible, to designate identical features that are common to the figures, and wherein: 
         FIG. 1  is a block diagram of a digital camera having a backside illuminated image sensor configured in accordance with an illustrative embodiment of the invention; 
         FIGS. 2 through 9  are cross-sectional views showing portions of a backside illuminated image sensor at various steps in an exemplary process for forming such an image sensor, in accordance with an illustrative embodiment of the invention; and 
         FIG. 10  is a plan view of an image sensor wafer comprising multiple image sensors formed using the exemplary process of  FIGS. 2 through 9 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention will be illustrated herein in conjunction with particular embodiments of digital cameras, backside illuminated image sensors, and processing techniques for forming such image sensors. It should be understood, however, that these illustrative arrangements are presented by way of example only, and should not be viewed as limiting the scope of the invention in any way. Those skilled in the art will recognize that the disclosed arrangements can be adapted in a straightforward manner for use with a wide variety of other types of imaging devices and image sensors. 
       FIG. 1  shows a digital camera  10  in an illustrative embodiment of the invention. In the digital camera, light from a subject scene is input to an imaging stage  12 . The imaging stage may comprise conventional elements such as a lens, a neutral density filter, an iris and a shutter. The light is focused by the imaging stage  12  to form an image on an image sensor  14 , which converts the incident light to electrical signals. The digital camera  10  further includes a processor  16 , a memory  18 , a display  20 , and one or more additional input/output (I/O) elements  22 . 
     Although shown as separate elements in the embodiment of  FIG. 1 , the imaging stage  12  may be integrated with the image sensor  14 , and possibly one or more additional elements of the digital camera  10 , to form a compact camera module. 
     The image sensor  14  is assumed in the present embodiment to be a CMOS image sensor, although other types of image sensors may be used in implementing the invention. More particularly, the image sensor  14  comprises a backside illuminated image sensor that is formed in a manner to be described below in conjunction with  FIGS. 2 through 9 . The image sensor generally comprises a pixel array having a plurality of pixels arranged in rows and columns and may include additional circuitry associated with sampling and readout of the pixel array, such as signal generation circuitry, signal processing circuitry, row and column selection circuitry, etc. This sampling and readout circuitry may comprise, for example, an analog signal processor for processing analog signals read out from the pixel array and an analog-to-digital converter for converting such signals to a digital form. These and other types of circuitry suitable for use in the digital camera  10  are well known to those skilled in the art and will therefore not be described in detail herein. Portions of the sampling and readout circuitry may be arranged external to the image sensor, or formed integrally with the pixel array, for example, on a common integrated circuit with photodiodes and other elements of the pixel array. 
     The image sensor  14  will typically be implemented as a color image sensor having an associated CFA pattern. Examples of CFA patterns that may be used with the image sensor  14  include those described in the above-cited U.S. patent application Publication No. 2007/0024931, although other CFA patterns may be used in other embodiments of the invention. As another example, a conventional Bayer pattern may be used, as disclosed in U.S. Pat. No. 3,971,065, entitled “Color Imaging Array,” which is incorporated by reference herein. 
     The processor  16  may comprise, for example, a microprocessor, a central processing unit (CPU), an application-specific integrated circuit (ASIC), a digital signal processor (DSP), or other processing device, or combinations of multiple such devices. Various elements of the imaging stage  12  and the image sensor  14  may be controlled by timing signals or other signals supplied from the processor  16 . 
     The memory  18  may comprise any type of memory, such as, for example, random access memory (RAM), read-only memory (ROM), Flash memory, disk-based memory, removable memory, or other types of storage elements, in any combination. 
     Functionality associated with sampling and readout of the pixel array and the processing of corresponding image data may be implemented at least in part in the form of software that is stored in memory  18  and executed by processor  16 . 
     A given image captured by the image sensor  14  may be stored by the processor  16  in memory  18  and presented on display  20 . The display  20  is typically an active matrix color liquid crystal display (LCD), although other types of displays may be used. The additional I/O elements  22  may comprise, for example, various on-screen controls, buttons or other user interfaces, network interfaces, memory card interfaces, etc. 
     Additional details regarding the operation of a digital camera of the type shown in  FIG. 1  can be found, for example, in the above-cited U.S. patent application Publication No. 2007/0024931. 
     It is to be appreciated that the digital camera as shown in  FIG. 1  may comprise additional or alternative elements of a type known to those skilled in the art. Elements not specifically shown or described herein may be selected from those known in the art. As noted previously, the present invention may be implemented in a wide variety of other types of digital cameras or imaging devices. Also, as mentioned above, certain aspects of the embodiments described herein may be implemented at least in part in the form of software executed by one or more processing elements of an imaging device. Such software can be implemented in a straightforward manner given the teachings provided herein, as will be appreciated by those skilled in the art. 
     The image sensor  14  may be fabricated on a silicon substrate or other type of substrate. In a typical CMOS image sensor, each pixel of the pixel array includes a photodiode and associated circuitry for measuring the light level at that pixel. Such circuitry may comprise, for example, transfer gates, reset transistors, select transistors, output transistors, and other elements, configured in a well-known conventional manner. 
     As indicated above,  FIGS. 2 through 9  illustrate the process of forming the backside illuminated image sensor  14  in one embodiment of the present invention. It should be noted that these figures are simplified in order to clearly illustrate various aspects of the present invention, and are not necessarily drawn to scale. A given embodiment may include a variety of other features or elements that are not explicitly illustrated but would be familiar to one skilled in the art as being commonly associated with image sensors of the general type described. 
       FIG. 2  shows a portion of an image sensor wafer  200  at the completion of a number of initial steps of an exemplary CMOS process flow. The image sensor wafer  200  at this stage comprises a silicon substrate  202 , a buried oxide (BOX) layer  204  formed over the substrate, a silicon sensor layer  206  formed over the oxide layer, and dielectric layers  208 ,  210  formed over the sensor layer. The dielectric layer  208  is an interlayer dielectric (ILD), and the dielectric layer  210  is an intermetal dielectric (IMD) that separates multiple levels of metallization. The ILD and IMD layers may alternatively be viewed as collectively forming a single dielectric layer. Various image sensor features such as interconnects, gates or other circuitry elements may be formed within a given dielectric layer using conventional techniques. 
     As will be described, the image sensor wafer  200  is further processed to form a plurality of image sensors each having a pixel array configured for backside illumination. The portion of the image sensor wafer  200  as shown in  FIG. 2  generally corresponds to a particular one of the image sensors, and includes a pixel array area  212  surrounded by periphery areas  214  and bond pad areas  216 . The areas  212  and  214  include conductors  218  associated with various levels of metallization in the IMD layer  210 . Similarly, the bond pad areas  216  include conductors  220  associated with multiple IMD metallization levels. The sensor layer  206  includes photodiodes and associated circuitry of the pixel array. 
     The image sensor wafer  200  has a frontside and a backside as indicated in  FIG. 2 . As described previously herein, the frontside refers generally to the side of an image sensor on which dielectric layers and associated levels of metallization are formed, while the side having the silicon substrate is referred to as the backside. The terms “frontside” and “backside” will be used herein to denote particular sides of an image sensor wafer or an image sensor formed from such a wafer, as well as sides of particular layers of the image sensor wafer or corresponding image sensor. For example, the sensor layer  206  has a frontside surface  206 F and a backside surface  206 B. As mentioned above, the illustrative embodiments relate to backside illuminated image sensors, that is, image sensors in which light from a subject scene is incident on the photodiodes or other photosensitive elements of the pixel array from a backside of the sensor. 
     It should be noted that terms such as “on” or “over” when used in conjunction with layers of an image sensor wafer or corresponding image sensor are intended to be construed broadly, and therefore should not be interpreted to preclude the presence of one or more intervening layers or other intervening image sensor features or elements. Thus, a given layer that is described herein as being formed on or formed over another layer may be separated from the latter layer by one or more additional layers. 
     The image sensor wafer  200  illustrated in  FIG. 2  is an example of a silicon-on-insulator (SOI) wafer. In such a wafer, the thickness of the silicon sensor layer  206  may be approximately 1 to 6 micrometers (μm), and the thickness of the buried oxide layer  204  may be approximately 0.1 to 0.5 μm, although other thicknesses may be used. The silicon substrate  202  is typically substantially thicker than the sensor layer or buried oxide layer. Alternative embodiments of the invention may utilize other types of wafers to form backside illuminated image sensors, such as, for example, epitaxial wafers or bulk semiconductor wafers that do not include a buried oxide layer, although an SOI wafer generally provides a smoother surface for backside processing. 
     The image sensor wafer  200  is further processed in the manner shown in  FIGS. 3 through 9  in order to form the backside illuminated image sensor  14  of  FIG. 1 . 
     In the step illustrated in  FIG. 3 , a temporary carrier wafer  300  is attached to a frontside surface of the IMD layer  210  using epoxy  302 . The temporary carrier wafer  300  may comprise, for example, a type of wafer commonly referred to as a handle wafer. 
     The silicon substrate  202  is then removed, resulting in the image sensor wafer structure as illustrated in  FIG. 4 . The substrate may be removed using, for example, grinding, polishing or etching techniques, in any combination. Typically, the substrate is removed in its entirety, exposing the buried oxide layer at the backside of the wafer. In an alternative embodiment, such as one involving an epitaxial or bulk semiconductor wafer, the substrate may be thinned rather than completely removed. 
     After removal of the substrate, the structure is flipped over and CFAs and associated microlenses are formed on the backside surface of the buried oxide layer  204  as shown in  FIG. 5 . The temporary carrier wafer  300  serves as a substrate at this stage, providing support for the structure after the removal of the original substrate  202 . Generally, each of the pixel arrays of the image sensor wafer has a corresponding CFA  500  which includes color filter elements  502  that are arranged over respective photosensitive elements  510  of the sensor layer  206 . As indicated previously herein, the photosensitive elements may comprise photodiodes. Such elements may be part of the image sensor wafer  200  as shown in  FIG. 2 , and formed in a conventional manner, but are omitted from that figure and other figures herein for clarity of illustration. Also associated with each of the color filter elements  502  of the CFA  500  is a corresponding microlens  504 . The color filter elements and associated microlenses may be aligned with the photosensitive elements of the sensor layer using alignment marks, although such marks are not shown. Advantageous techniques for forming alignment marks in a backside illuminated image sensor are disclosed in the above-cited U.S. patent application. 
     A glass cover sheet  600  is then attached to the backside surface of the buried oxide layer  204  as illustrated in  FIG. 6 . The cover sheet  600  generally comprises a plurality of glass covers overlying respective ones of the CFAs  500  of the image sensor wafer  200 . In the portion of the image sensor wafer shown in  FIG. 6 , a given such glass cover overlies the CFA  500  formed on the backside surface of the buried oxide layer. Each of the glass covers may be viewed as generally comprising a central cavity  602  arranged over its corresponding CFA and further comprising peripheral supports  604  secured to the backside surface of the oxide layer  204  via epoxy  606 . Although the cover sheet is formed of glass in this example, other types of transparent materials may be used in other embodiments. The glass cover sheet is generally attached to the wafer as a single sheet which is divided into separate covers when the image sensors are diced from the wafer as will be described below. The central cavity  602  in the present embodiment is at least about 25 μm deep, and more particularly on the order of 50 μm deep, as measured from the top of the CFA  500  to an inside lower surface of the overlying glass cover. Each of the peripheral supports may be at least about 300 μm wide, as measured after dicing of the wafer but in the same cross-sectional direction shown in  FIG. 6 . Again, other dimensions may be used in other embodiments. 
     As illustrated in  FIG. 7 , a protective film layer  700  is added to the glass cover sheet  600 . The structure is then flipped, and the temporary carrier wafer  300  is removed. Thus, the glass cover sheet previously attached to the image sensor wafer  200  provides sufficient support for the remaining process steps to allow the temporary carrier wafer to be removed. 
       FIG. 8  illustrates the image sensor wafer  200  after a number of additional process steps. Initially, openings are formed in the IMD layer  210  to expose respective ones of the bond pad conductors  220 . Then, redistribution layer (RDL) metal conductors  800  are formed in electrical contact with the respective bond pad conductors  220  through the respective openings in the IMD layer. An RDL passivation layer  802  is formed over the RDL metal conductors  800 , and openings  804  are formed in the RDL passivation layer to expose portions of the underlying RDL metal conductors. Contact metallizations  806  are then formed in electrical contact with the RDL metal conductors  800  in respective ones of the openings  804  in the RDL passivation layer. The contact metallizations in this example are referred to as under-ball metallization (UBM), and are also denoted as UBM metallizations in the figure. The routing of the RDL metal allows the use of solder balls in arrangements in which the bond pad conductors would otherwise be too close to one another to accommodate the solder ball pitch. 
       FIG. 9  shows the addition of solder balls  900  in the openings  804  in contact with the contact metallizations  806 . The image sensor wafer  200  has also been flipped, and the protective film layer  700  has been removed. The term “solder ball” as used herein is intended to be construed generally so as to encompass solder bumps or other solder arrangements. 
     The processed image sensor wafer  200  is now ready for dicing into a plurality of image sensors  14  configured for backside illumination. Alternatively, dicing can be done earlier in the process, for example, prior to the addition of the solder balls  900  and the removal of the protective film layer  700  from the glass cover sheet  600 . 
       FIG. 10  shows a plan view of a given processed image sensor  1000  prior to dicing. The image sensor wafer  1000  comprises a plurality of image sensors  1002 . The image sensors  1002  are formed through wafer level processing operations of the type described in conjunction with  FIGS. 2 through 9 . The image sensors are then separated from one another by dicing the wafer along dicing lines  1004 . A given one of the image sensors  1002  corresponds to image sensor  14  in digital camera  10  of  FIG. 1 . 
     The above-described illustrative embodiments advantageously provide an improved processing arrangement for forming a backside illuminated image sensor. For example, handling issues relating to the thinning or removal of the silicon substrate are addressed in an efficient manner, without substantially increasing the die size or cost of the resulting image sensor. Also, the use of a glass cover with a cavity overlying the CFA elements and microlenses of a given pixel array provides enhanced image quality. Moreover, the use of RDL metal conductors to facilitate placement of solder balls provides for a more efficient packaging arrangement. 
     The invention has been described in detail with particular reference to certain illustrative embodiments thereof, but it will be understood that variations and modifications can be effected within the scope of the invention as set forth in the appended claims. For example, the invention can be implemented in other types of image sensors and digital imaging devices, using alternative materials, wafers, layers, process steps, etc. These and other alternative embodiments will be readily apparent to those skilled in the art. 
     PARTS LIST 
       10  digital camera 
       12  imaging stage 
       14  backside illuminated image sensor 
       16  processor 
       18  memory 
       20  display 
       22  input/output (I/O) elements 
       200  image sensor wafer 
       202  substrate 
       204  buried oxide (BOX) layer 
       206  sensor layer 
       206 B sensor layer backside surface 
       206 F sensor layer frontside surface 
       208  interlayer dielectric (ILD) 
       210  intermetal dielectric (IMD) 
       212  pixel array area 
       214  periphery area 
       216  bond pad area 
       218  conductors 
       220  conductors 
       300  temporary carrier wafer 
       302  epoxy 
       500  color filter array (CFA) 
       502  color filter element 
       504  microlens 
       510  photosensitive elements 
       600  glass cover sheet 
       602  central cavity 
       604  peripheral support 
       606  epoxy 
       700  protective film 
       800  redistribution layer (RDL) conductors 
       802  RDL passivation layer 
       804  RDL openings 
       806  under-ball metallization (UBM) 
       900  solder balls 
       1000  image sensor wafer 
       1002  image sensors 
       1004  dicing lines