Abstract:
A wire-bondable image sensor having an integral contaminant shadowing reduction structure is described. In one aspect, an image sensor includes a substrate that has a side supporting at least one imaging area and at least one wirebonding area. Light detectors are constructed and arranged to receive light through the imaging area. Bond pads are exposed in the wirebonding area for connecting to respective bond wires. A contaminant shadowing reduction structure on the imaging area has an exposed contaminant displacement surface over the imaging area and separated from the imaging area by a distance of at least 300 μm. The contaminant shadowing reduction structure is substantially transparent to radiation within an operative wavelength range specified for the image sensor. Methods of making the above-mentioned image sensor also are described.

Description:
BACKGROUND  
       [0001]     Image sensors typically include a one-dimensional linear array or a two-dimensional array of light sensitive regions (often referred to as “pixels”) that generate electrical signals that are proportional to the intensity of the light respectively received in the light sensitive regions. Solid-state image sensors are used in a wide variety of different applications, including digital still cameras, digital video cameras, machine vision systems, robotics, guidance and navigation applications, and automotive applications.  
         [0002]     One class of image sensors is based on charge-coupled device (CCD) technology. In a common implementation, a CCD image sensor includes an array of closely spaced metal-oxide-semiconductor (MOS) diodes. In operation, a sequence of clock pulses is applied to the MOS diodes to transfer charge across the imaging area. Another class of image sensors is based on active pixels sensor (APS) technology. Each pixel of an APS image sensor includes a light sensitive region and sensing circuitry. The sensing circuitry includes an active transistor that amplifies and buffers the electrical signals generated by the associated light sensitive region. In a common implementation, APS image sensors are made using standard complementary metal-oxide-semiconductor (CMOS) processes, allowing such image sensors to be readily integrated with standard analog and digital integrated circuits.  
         [0003]     An individual image sensor chip oftentimes is mounted inside a camera module package, which protects the image sensor against damage from environmental hazards that may arise after the image sensor chip has been packaged. The image sensor chip commonly is electrically connected to the leads of the camera module package through bond wires that are attached to bond pads on the topside of the image sensor chip or through solder bumps on the backside of the image sensor chip. Although solder-bump bonded electrical connections provide improved data rate performance due to their shorter length, wirebonded electrical connections are favored in terms of cost and throughput.  
         [0004]     In addition to post-packaging-generated contamination, the performance and the yield of image sensors also are compromised by the presence of contaminants, such a dust and other particles, that are generated during fabrication, dicing, and packaging of the image sensors. The presence of these contaminants tend to damage various components of the image sensors, including the pixels in the image (or pixel) areas, the bond pads, and the electrical traces on the surfaces of the image sensors that carry signals to and from bonding pads.  
         [0005]     Various approaches have been proposed for protecting wire-bondable image sensors against damage and defects that are caused by contaminants that are generated prior to the dicing and packaging of individual image sensor chips. In one approach, a photo-etchable cover wafer is attached to a semiconductor wafer containing a plurality of image sensor dice prior to dicing and packaging of individual image sensor chips. The cover wafer protects the image sensor substrate from environmental hazards, such as particulate contaminants, moisture, processing agents such as solvents, and inadvertent scratching of the image sensor substrate. In another approach, a thin, transparent, water-repellant and oil-repellant resin is applied over the image sensors prior to dicing and packaging. The low surface energy of the resin layer is purported to substantially prevent dust from attaching to the resin layer and to readily allow any dust that manages to attach to the resin layer to be removed easily by means of a cotton swab or the like.  
         [0006]     The proposed image sensor protection approaches described above appear to reduce the damage caused by contaminants during fabrication and packaging of the image sensors. These approaches, however, do not address the additional need to reduce the adverse impact caused by the presence of contaminants that interfere with the reception of light by the pixels in the imaging area and thereby cause defects in the images captured by the image sensors.  
       SUMMARY  
       [0007]     In one aspect, the invention features an image sensor that includes a substrate having a side supporting at least one imaging area and at least one wirebonding area. Light detectors are constructed and arranged to receive light through the imaging area. Bond pads are exposed in the wirebonding area for connecting to respective bond wires. A contaminant shadowing reduction structure on the imaging area has an exposed contaminant displacement surface over the imaging area and separated from the imaging area by a distance of at least 300 μm. The contaminant shadowing reduction structure is substantially transparent to radiation within an operative wavelength range specified for the image sensor.  
         [0008]     In another aspect, the invention features a method fabricating an image sensor. In accordance with this inventive method, image sensor dice are formed on a wafer. Each of the dice comprises light detectors constructed and arranged to receive light through a respective imaging area and bond pads in a wirebonding area. A contaminant shadowing reduction structure is formed on the dice. The contaminant shadowing reduction structure has an exposed contaminant displacement surface over the imaging areas and separated from the imaging areas by a distance of at least 300 μm. The contaminant shadowing reduction structure is substantially transparent to radiation within an operative wavelength range specified for the image sensor. Regions of the contaminant shadowing reduction structure over the wirebonding areas of the dice are removed to expose the bond pads. The dice are separated into respective image sensor chips.  
         [0009]     Other features and advantages of the invention will become apparent from the following description, including the drawings and the claims. 
     
    
     DESCRIPTION OF DRAWINGS  
       [0010]      FIG. 1A  is a diagrammatic top view of an embodiment of an image sensor chip.  
         [0011]      FIG. 1B  is a diagrammatic sectional view of the image sensor chip embodiment shown in  FIG. 1A  taken along the line  1 B- 1 B.  
         [0012]      FIG. 2  is a diagrammatic sectional view of a particulate contaminant on a portion of the imaging area of the image sensor chip embodiment shown in  FIG. 1B .  
         [0013]      FIG. 3  is a diagrammatic sectional view of a particulate contaminant displaced from a portion of the imaging area of the image sensor chip embodiment shown in  FIG. 1B  by a distance L.  
         [0014]      FIG. 4  is a diagrammatic sectional view of a contaminant shadowing reduction structure over the imaging area of embodiment of an image sensor chip.  
         [0015]      FIG. 5  is a flow diagram of an embodiment of a method of making an image sensor.  
         [0016]      FIGS. 6A and 6B  are diagrammatic sectional views of a portion of an embodiment of a wafer during different stages of the method of  FIG. 5 .  
         [0017]      FIG. 6C  is a diagrammatic perspective view of a portion of an embodiment, of a wafer containing a plurality of dice each of which includes an imaging area with an overlying contaminant shadowing reduction structure.  
         [0018]      FIG. 7  is a flow diagram of an embodiment of a method of making an image sensor.  
         [0019]      FIGS. 8A-8D  are diagrammatic sectional views of a portion of an embodiment of a wafer during different stages of the method of  FIG. 7 .  
         [0020]      FIG. 9  is a flow diagram of an embodiment of a method of making an image sensor.  
         [0021]      FIGS. 10A-10D  are diagrammatic sectional views of a portion of an embodiment of a wafer during different stages of the method of  FIG. 9 . 
     
    
     DETAILED DESCRIPTION  
       [0022]     In the following description, like reference numbers are used to identify like elements. Furthermore, the drawings are intended to illustrate major features of exemplary embodiments in a diagrammatic manner. The drawings are not intended to depict every feature of actual embodiments nor relative dimensions of the depicted elements, and are not drawn to scale.  
         [0023]     The embodiments that are described in detail below include a contaminant shadowing reduction structure that reduces the adverse effects of contaminants that interfere with the reception of light by the pixels in the imaging area and thereby cause defects in the images captured by the image sensors. The contaminant shadowing reduction structure includes a contaminant displacement surface that prevents contaminants, whether generated during the process of manufacturing the image sensors or generated during operation of devices incorporating the images sensors, from producing a significant shadowing on the underlying imaging area. In this way, the contaminant shadowing reduction structure improves image capturing performance and the manufacturing yields associated with these embodiments.  
         [0024]      FIGS. 1A and 1B  diagrammatically show an embodiment of a prior art image sensor chip  10  that includes an imaging area  12  and a surrounding wirebonding area  14  that are supported on one side of a substrate  16 . The imaging area  12  includes a plurality of light detectors and associated readout circuitry for detecting incoming light received through the imaging area  12 . Each light detector typically corresponds to a pixel of the imaging area  12 . The imaging area  12  also may include a color filter array above the light detectors, as well as other known optical components and circuit elements. The wirebonding area  14  includes a plurality of bond pads  18  that are exposed for connecting to respective terminals of an optoelectronic device module or package through respective bond wires. In a typical implementation, the substrate  16  is a semiconductor substrate (e.g., silicon). The structures that are formed in substrate  16  may be fabricated in accordance with any semiconductor device fabrication process, including CMOS, bipolar CMOS (BiCMOS), and bipolar junction transistor fabrication processes.  
         [0025]      FIG. 2  shows a particulate contaminant  20  on a portion of the imaging area  12  of the image sensor chip  10  shown in  FIG. 1B . The particulate contaminant  20  blocks incoming light  22  that is directed toward the photosites (or pixels)  24  in the portion of the imaging area  12  that is located directly underneath the particulate contaminant  20 . That is, the particulate contaminant  20  casts a shadow  26  on the underlying photosites  24  and the pixel data generated by these photosites will correspond to dark pixels. Common image processing techniques are available for interpolating values for isolated dark pixels from neighboring pixels. Such pixel correction techniques, however, typically break down when more than a few pixels in a neighborhood are defective, in which case the images captured by the image sensor  10  will include noticeable defects in pixel regions corresponding to the pixels that are shadowed by the particulate contaminant  20 .  
         [0026]     In general, the particulate contaminant  20  may be generated during the process of manufacturing the image sensor  10  or during operation of a device incorporating the images sensor  10  (e.g., during movement of a lens in a camera incorporating the image sensor  10 ). Image sensor  10  typically is manufactured in a clean room environment that is characterized by particulate contaminants with diameters of about 30 μm (micrometers) or less. The particulate contaminants that are generated during use of the image sensor  10 , on the other hand, typically are about 100 μm in diameter and, in some cases, may by about 300 μm or greater in diameter. Since pixel sizes commonly are on the order of about 3 μm by 3 μm or less, the types of particulate contaminants that typically might become deposited on the imaging area  12  may cast shadows on regions of imaging area  12  that range from about 10 pixels by 10 pixels up to about 100 pixels by 100 pixels. To the extent that such contaminants cannot be reduced, the image capturing performance and manufacturing yields associated with the image sensors will be reduced. Consequently, it is highly desirable to reduce the shadowing impact of such contaminants.  
         [0027]      FIG. 3  shows that the shadowing impact of the particulate contaminant  20  on the underlying photosites  26  of imaging area  12  is reduced by increasing the separation distance L between the particulate contaminant  20  and the photosites  24 . In the geometric optics ray tracing example shown in  FIG. 3 , the separation distance L at which the particulate contaminant  20  casts only a point shadow on the photosites  24  is given by:  
             L   =     D     2   ⁢           ⁢   tan   ⁢           ⁢     (       1   2     ⁢     θ   CONE       )                 (   1   )               
 where D is the diameter of the particulate contaminant  20  and θ CONE  is the cone angle, which is given by: 
 θ CONE =2×sin −1 ( NA )  (2)  
 where NA is the numerical aperture of a lens that focuses the light  28  onto the photosites  24 . Since NA≅(2×f/#) −1  for a typical NA&lt;0.25, where f/# is the f-number of the lens, the separation distance L may be expressed in terms of the f-number as follows:  
             L   =     D     2   ⁢           ⁢   tan   ⁢           ⁢     (       sin     -   1       ⁡     (       (     2   ×     f   /   #       )       -   1       )       )                 (   3   )               
 Assuming a typical particulate diameter D=100 μm and a typical f-number of 2.8 for common digital camera applications, the separation distance L is 275 μm, which on the order of about 300 μm to one significant digit. A larger separation would further reduce the shadowing impact of the particulate contaminant  20 . 
 
         [0028]     To summarize, the simple geometric optic ray tracing model presented in  FIG. 3  shows that a separation distance of at least 300 μm between the particulate contaminant  20  and the photosites  24  in the imaging area  12  significantly reduces the shadowing effect of the particulate contaminant  20  on the underlying photosites  24 .  
         [0029]      FIG. 4  shows an image sensor  30  that embodies the insights revealed by the model shown in  FIG. 3  to improve the image capturing performance and the manufacturing yields that are associated with the image sensor  30 . In particular, image sensor  30  includes over the imaging area  12  a contaminant shadowing reduction structure  32  that is substantially transparent to radiation within an operative wavelength range (e.g., 390 nm to 770 nm) specified for the image sensor  30 . The contaminant shadowing reduction structure  32  has a contaminant displacement surface  34  that is separated from the imaging area  12  by a distance S of at least 300 μm. Therefore, contaminants that are generated during the dicing and packaging fabrication stages or during operation of the image sensor  30 , and that adhere in the incoming light path of the image sensor  30 , necessarily will be separated from the imaging area  12  by a distance of at least 300 μm. As a result, the adverse shadowing effect of particulate contaminants with expected sizes up to 100 μm in diameter will be reduced substantially.  
         [0030]     It is noted that the shadowing impact of particulate contaminants with sizes larger than 100 μm also will be reduced substantially relative to approaches in which potential particulate contaminants are separated from the imaging areas by a smaller distance.  
         [0031]     Exemplary implementations of the image sensor  30  are described below.  
       EXAMPLE 1  
       [0032]     Referring to  FIGS. 5 and 6 A- 6 C, a first implementation of the image sensor  30  is fabricated as follows.  
         [0033]     A photoresist layer  40  is applied over a plurality of image sensor dice  41  on a wafer  42  (block  44 ;  FIG. 6A ). The photoresist layer  40  is applied in one or more coats using a spin coating process. In this implementation, the applied photoresist layer  40  has a thickness of at least 300 μm. The photoresist layer  40  is substantially transparent to radiation within an operative wavelength range (e.g., the visible wavelength range) that is specified for the image sensors. An exemplary photoresist material is the NANO™ SU-8 2000 negative tone photoresist, which is available from MicroChem Corporation of Newton, Mass. U.S.A. In some implementations, at least one infrared light absorbing dye is incorporated in the photoresist. The infrared light absorbing dye preferably exhibits strong absorption in the wavelength range of 630 nm to 930 nm. Exemplary types of suitable infrared light absorbing dyes are anthraquinone dyes. After the photoresist layer  40  has been applied, the photoresist layer  40  may be soft-baked to evaporate the solvent and densify the film.  
         [0034]     The photoresist layer  40  is patterned so that the remaining regions  48  of the photoresist layer  40  are disposed over the imaging areas  12  and the areas over the bond pads  18  are free of photoresist (block  46 ;  FIG. 6B ). The photoresist layer  40  is patterned using a photolithographic process, which may include exposing the photoresist layer  40 , performing a post-exposure bake to selectively cross-link the exposed portions of the photoresist layer  40 , and developing the unexposed portions of the photoresist layer  40 .  
         [0035]     In some implementations, the remaining, cross-linked portions  48  of the photoresist layer  40  are optionally hard-baked to additionally cross-link (or cure) the remaining portions  48  of the photoresist layer  40  (block  50 ).  FIG. 6C  shows a perspective view of a portion of the wafer  42  containing a plurality of dice  41  each of which includes an imaging area with an overlying contaminant shadowing reduction structure consisting of a respective one of the cured portions  48  of the photoresist layer  40 .  
         [0036]     After the remaining cross-linked portions  48  of the photoresist layer  40  have been cured (block  50 ), the dice  41  are separated (block  54 ). The dice  41  may be separated using any known sawing or etching process that is suitable for cutting through the wafer  42 .  
       EXAMPLE 2  
       [0037]     Referring to  FIGS. 7 and 8 A- 8 D, a second implementation of the image sensor  30  is fabricated as follows.  
         [0038]     A photoresist layer  60  is applied over a plurality of image sensor dice  61  on a wafer  42  (block  62 ;  FIG. 8A ). The photoresist layer  60  is applied in one or more coats using a spin coating process. In this implementation, the applied photoresist layer  60  has a thickness of at least 300 μm. An exemplary photoresist material is the NANO™ SU-8 2000 negative tone photoresist, which is available from MicroChem Corporation of Newton, Mass. U.S.A. After the photoresist layer  60  has been applied, the photoresist layer  60  may be soft-baked to evaporate the solvent and densify the film.  
         [0039]     The photoresist layer  60  is patterned so that the remaining regions  63  of the photoresist layer  60  are disposed over the wirebonding areas  14  and the imaging areas  12  are free of photoresist (block  64 ;  FIG. 8B ). The photoresist layer  60  is patterned using a photolithographic process, which may include exposing the photoresist layer  60 , performing a post-exposure bake to selectively cross-link the exposed portions of the photoresist layer  60 , and developing the unexposed portions of the photoresist layer  60 .  
         [0040]     An epoxy layer  66  is applied over the patterned photoresist layer  60  (block  68 ;  FIG. 8C ). The epoxy layer  66  is applied in one or more coats using a spin-on coating process. In this implementation, the applied epoxy layer  66  has a thickness of at least 300 μm over the imaging areas  12 . The epoxy layer  66  is substantially transparent to radiation within an operative wavelength range (e.g., the visible wavelength range) that is specified for the image sensors. In some implementations, at least one infrared light absorbing dye is incorporated in the epoxy layer  66 . The infrared light absorbing dye preferably exhibits strong absorption in the wavelength range of 630 nm to 930 nm. Exemplary types of suitable infrared light absorbing dyes are anthraquinone dyes.  
         [0041]     The patterned photoresist layer  60  is removed using a liftoff process (block  70 ;  FIG. 8D ). The liftoff process may be any type of liftoff process that is suitable for the photoresist layer  60 , including an immersion, spray or spray-puddle liftoff process. A proprietary developer solution or other solvent-based developers may be used in the liftoff process. After the patterned photoresist layer  60  has been removed, the remaining regions  72  of the epoxy layer  66  are disposed over the imaging areas  12  and the areas over the bond pads  18  are free of photoresist and epoxy material, as shown in  FIG. 8D .  
         [0042]     In some implementations, the remaining portions  72  of the epoxy layer  66  are optionally hard baked to cure the remaining portions  72  of the epoxy layer  66  (block  74 ). The resulting cured portions  72  of the epoxy layer  66  appear much like the cured portions  48  of the photoresist layer  40  shown in  FIG. 6C .  
         [0043]     After the remaining portions  72  of the epoxy layer  66  have been cured (block  74 ), the image sensor dice  61  are separated (block  76 ). The dice  61  may be separated using any known sawing or etching process that is suitable for cutting through the wafer  42 .  
       EXAMPLE 3  
       [0044]     Referring to  FIGS. 9 and 10 A- 10 E, a third implementation of the image sensor  30  is fabricated as follows.  
         [0045]     A patterned photoresist layer  80  is applied over a plurality of image sensor dice  81  on a wafer  42  (block  82 ;  FIG. 10A ). The photoresist layer  80  is applied in one or more coats using a spin coating process. In this implementation, the applied photoresist layer  80  has a thickness ranging from 10 μm to at least 300 μm. An exemplary photoresist material is the NANO™ SU-8 2000 negative tone photoresist, which is available from MicroChem Corporation of Newton, Mass. U.S.A. After the photoresist layer  80  has been applied, the photoresist layer  80  may be soft-baked to evaporate the solvent and densify the film.  
         [0046]     The photoresist layer  80  is patterned so that the remaining regions  83  of the photoresist layer  80  are disposed over the wirebonding areas  14  and the imaging areas  12  are free of photoresist. The photoresist layer  80  is patterned using a photolithographic process, which may include exposing the photoresist layer  80 , performing a post-exposure bake to selectively cross-link the exposed portions of the photoresist layer  80 , and developing the unexposed portions of the photoresist layer  80 .  
         [0047]     An epoxy layer  84  is deposited over the patterned portions  83  of the photoresist layer  80  (block  86 ;  FIG. 10B ). The epoxy layer  84  is applied in one or more coats using a spin-on coating process. In this implementation, the applied epoxy layer  84  has a thickness ranging from 10 μm to at least 300 μm. The epoxy layer  84  is substantially transparent to radiation within an operative wavelength range (e.g., the visible wavelength range) that is specified for the image sensors.  
         [0048]     In some implementations, at least one infrared light absorbing dye is incorporated in the epoxy layer  84 . The infrared light absorbing dye preferably exhibits strong absorption in the wavelength range of 630 nm to 930 nm. Exemplary types of suitable infrared light absorbing dyes are anthraquinone dyes. In these implementations, the epoxy layer  84  has a thickness of at least 200 μm so that a sufficient quantity of the infrared light absorbing dye is dispersed throughout the epoxy layer  84  to exhibit a specified level (e.g., greater than 80%) of infrared light passing through the epoxy layer  84  in the target absorption range is absorbed. The density and light absorption properties of the infrared light absorbing dye should not detrimentally affect the transmission of light in the operative wavelength range that is specified for the image sensors.  
         [0049]     A glass wafer  88  is applied over the epoxy layer  84  (block  90 ;  FIG. 10C ). In one implementation, the glass wafer is at least 500 μm thick. A soft bake process may be performed to at least partially cure the epoxy material and thereby adhere the glass wafer  88  to the epoxy layer  84 .  
         [0050]     A dicing process is used to cut through the glass wafer along die lines between the individual image sensor dice  81  (block  92 ;  FIG. 10D ). Any known sawing or etching process that is suitable for cutting through the glass wafer  88  may be used in the dicing process. In this process, the patterned photoresist layer  80  protects the bond pads  18  and other metal traces in the wirebonding areas  14  against damage that otherwise might occur during the dicing process. During the dicing process, portions of the epoxy layer  84  and a top portion of the patterned photoresist layer  80  under the die lines may be removed as shown in  FIG. 10D  without detrimentally affecting the underlying bond pads  18  and other metal traces.  
         [0051]     The remaining portions  83  of the patterned photoresist layer  80  are removed using a liftoff process (block  94 ;  FIG. 10E ). The liftoff process may be any type of liftoff process that is suitable for the photoresist layer  80 , including an immersion, spray or spray-puddle liftoff process. A proprietary developer solution or other solvent-based developers may be used in the liftoff process. After the patterned portions  83  of the photoresist layer  80  have been removed, the remaining regions of the epoxy layer  84  and the glass wafer  88  are disposed over the imaging areas  12  and the areas over the bond pads  18  in the wirebonding areas  14  are free of photoresist and epoxy material, as shown in  FIG. 10E .  
         [0052]     In some implementations, the remaining portions  95  of the epoxy layer  84  are optionally hard baked to cure the remaining portions of the epoxy layer  84  (block  96 ). The resulting cured portions  95  of the epoxy layer  84  and the overlying portions of the glass wafer  88  appear similar to the cured portions  48  of the photoresist layer  40  shown in  FIG. 6C .  
         [0053]     After the remaining portions  95  of the epoxy layer  84  have been cured (block  96 ), the image sensor dice  81  are separated (block  98 ). The dice  81  may be separated using any known sawing or etching process that is suitable for cutting through the wafer  42 .  
         [0054]     Other embodiments are within the scope of the claims.