Abstract:
A sensor die that lowers the lens requirements by the use of a variable thickness distribution over the sensor die. The sensing portion of the sensor die has a different number of layers than the non-sensing portion of the sensor die. By reducing the thickness of each layer and/or eliminating one or more unnecessary layers in the sensing portion, the thickness of the sensing portion is reduced to lower the amount of stray light and allow an increase in chief ray angle as well as a decrease in the F-number of the imaging system coupled to the sensor die without compromising the image quality of the sensor die. Also, the thickness reduction provides the design engineers with extra allowance in handling the chief ray angle and F-number for a given lens requirements.

Description:
BACKGROUND 
     1. Field of the Invention 
     The present invention relates to CMOS image sensor design and more particularly to lowering imaging lens requirements of CMOS image sensors. 
     2. Background of the Related Art 
     There has been an increase of digital image devices using CMOS image sensors. A conventional CMOS image sensor requires a matching imaging lens unit which includes one or more lens elements to direct incoming light to its sensor surface and generate an image on the sensor. In general, the chief ray angle for a pixel at the center of the sensor surface is zero, while the chief ray angles for pixels at corners and/or edges of the sensor surface reach to a considerable amount. To mitigate the negative effects of large chief ray angles and consequently generate a uniform image over the entire sensor surface, the imaging system of the CMOS image sensor may be customized under given imaging lens requirements. 
     In general, the imaging lens requirements may refer to the limitations on the number of lens elements and size of the imaging lens unit (assembly). A complex imaging lens unit having multi lenses may require a complicated optical design and a high manufacturing cost. Thus, it is desirable to minimize the number of imaging lens elements without degrading the performance of the imaging system. 
     The overall physical dimension of the imaging system may determine the size of imaging lens unit. Thus, the lens size requirement may become more significant as the size of the imaging device decreases as in typical mobile applications. For this reason, the mobile sensor device engineers often make significant efforts to minimize the lens size without compromising the performance of the imaging system. 
     However, the conventional approaches to change the number of lens elements and size of the imaging lens unit in an effort to lower the lens requirements may encounter additional difficulties as these requirements may be accompanied by a considerable amount of alteration and customization of the entire CMOS image sensor layout. Thus, there is a need for an improved methodology that lowers the lens requirements without modifying the overall imaging system significantly and compromising the image quality and performance of the sensor. 
     SUMMARY 
     The present invention provides a sensor die that lowers the lens requirements by the use of a variable thickness distribution over the sensor die. The sensing portion of the sensor die has a different number of layers than the non-sensing portion of the sensor die. Also, by reducing the thickness of each layer and/or eliminating one or more unnecessary layers in the sensing portion, the thickness of the sensing portion is reduced to provide the design engineers with extra allowance in handling chief ray angles and F-number of the imaging system without compromising the image quality of the sensor die. 
     In one aspect of the present invention, a sensor die includes: a sensing potion having a plurality of pixels, each of the plurality of pixels having a first set of layers and a microlens; and a non-sensing portion having a second set of layers, the second set of layers having a different number of layers than the first set of layers. 
     In another aspect of the present invention, a sensor die formed on a substrate having a plurality of passive components includes: a sensing potion including a plurality of pixels, each of the plurality of pixels having a microlens and a first set of layers that comprises: a photodiode partially embedded on the substrate; a first insulting layer on top of the photodiode and the substrate; a plurality of metal layers on top of the first insulating layer, the photodiode and the plurality of passive components connected to at least one of the plurality of metal layers; a plurality of middle insulating layers, each of the plurality of metal layers sandwiched by corresponding two of the plurality of middle insulating layers; a passivation layer on top of the plurality of middle insulating layers; a color filter; and a planar layer on top of the color filter; and a non-sensing portion having a second set of layers, the second set of layers having a different number of layers than the first set of layers. 
     In yet another aspect of the present invention, an imaging device includes a sensor die that comprises: a sensing potion including a plurality of pixels, each of the plurality of pixels having a first set of layers and a microlens; and a non-sensing portion having a second set of layers, the second set of layers having at least one more layer than the first set of layers. The imaging device further includes an imaging lens unit that includes one or more lens elements to direct incoming light to the sensor die. 
     In still another aspect of the present invention, a sensor die includes: a sensing potion comprising a plurality of pixels, each of the plurality of pixels having a first set of layers and a microlens; and a non-sensing portion having a second set of layers, the thickness of the second set of layers being larger than that of the first set of layers. 
     These and other features, aspects and advantages of the present invention will become better understood with reference to the following drawings, description and claims. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic diagram of an exemplary image module assembly in accordance with one embodiment of the present invention. 
         FIG. 2   a  is a top view of the CMOS sensor die shown in  FIG. 1 . 
         FIG. 2   b  is a side cross sectional view of a conventional CMOS sensor die. 
         FIGS. 3   a - b  are schematic diagrams of a sensor die having shifted microlenses in accordance with one embodiment of the present invention. 
         FIG. 4   a  is a schematic diagram of an exemplary embodiment of a conventional sensor pixel. 
         FIG. 4   b  is a schematic diagram of an exemplary embodiment of a sensor pixel in accordance with one embodiment of the present invention. 
         FIG. 5  is a side cross sectional view of an exemplary embodiment of a sensor die in accordance with one embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Foregoing described embodiments of the invention are provided as illustrations and descriptions. They are not intended to limit the invention to precise form described. In particular, it is contemplated that functional implementation of invention described herein may be implemented equivalently in hardware, software, firmware, and/or other available functional components or building blocks. Other variations and embodiments are possible in light of above teachings, and it is thus intended that the scope of invention should not be limited by this Detailed Description, but rather by Claims following. 
     Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Although any methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present invention, the preferred methods and materials are now described. 
     It must be noted that, as used herein and in the appended claims, the singular forms “a”, “an”, and “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to “a pixel” includes a plurality of such pixels, i.e., pixel array, and equivalents thereof known to those skilled in the art, and so forth. 
       FIG. 1  is a schematic diagram of an exemplary image module assembly  100  in accordance with one embodiment of the present invention. The image module assembly  100  may be included in a digital image device, such as digital image camera and cellular phone with imaging capabilities. The image module assembly  100  includes: an outer case having barrel members  102   a - b  and a bottom  102   c ; a lens unit (or, equivalently lens assembly)  103  having one or more lens elements  104   a - c ; and a CMOS sensor die  106 . The sensor die  106  may contain more than a hundred thousand, or even more than a million pixels, and the detailed structure of the pixels will be described in connection with  FIGS. 2   a - b . In one embodiment, the width and length of the sensor die  108  may be about, but not limited to, 5 mm. 
     The lens elements  104   a - c  may direct incoming optical rays  108  to form an image on the sensor die  106 . A chief ray  110  may be at the center of a light ray pencil  111  collected by a pixel. Likewise, a chief ray angle  114  may be defined as an angle between a normal to the surface of the sensor die  106  and a chief ray  116 . The chief ray angle of the center pixel of the sensor die  106  may be zero, while that of a corner pixel (e.g., the angle  114 ) may be up to 25-30 degrees. As will be explained later, the chief ray angle  114  may be determined by several factors including the lens requirements. A marginal ray angle  112  may be the largest angle between two rays within a light ray pencil collected by a pixel. For simplicity, only three lens elements  104   a - c  are shown in  FIG. 1 . However, it should be apparent to those of ordinary skill that the present invention can be practiced with any number of lens elements that satisfy the limiting lens requirements specific to the module assembly  100 . 
     As mentioned above, the lens requirement, i.e., the number of lens elements  104   a - c  and the size of the lens unit  103 , may be closely related to the design factors of the image module assembly  100 , such as, the physical dimension of the module assembly  100  (e.g., the height  122  and width  118 ), the chief ray angle  114 , the distance  120  between the lens element  104   c  and surface of the sensor die  106 , and the physical dimension of the sensor die  106 . For example, the size of the imaging lens elements  104   a - c  may become the major factor limiting the module dimension. The module height  122  and footprint size may be limited by the lens size especially when the sensor resolution gets higher with a high number of imaging lens elements. 
     The lens requirements may also affect the chief ray angle of corner pixels and the F-number of the compound imaging lens unit  103  having the lens elements  104   a - c . For example, for a fixed sensor die dimension and the distance  120 , a decrease in the diameter of imaging lens elements  104   a - c  may yield an increase in the chief ray angle while a decrease in the number of lens elements may yield an increase in the F-number to maintain the image quality. The chief ray angle of the corner pixel (e.g.,  114 ) may determine the image uniformity, while the F-number may determine the marginal ray angle  112 , and as a consequence, the overall module sensitivity. As the major goals in module design may be maximizing the chief ray angle of corner pixels and minimizing the F-number of the compound imaging system without degrading the image quality, the imaging system design may be directed toward lowering the lens requirements. In one embodiment of the present invention, in contrast to the conventional approaches to modify the imaging lens elements  104   a - c , the thickness of the sensor die  106  may be changed to generate an effect equivalent to lowering the lens requirement. 
       FIG. 2   a  is a top view of the CMOS sensor die  106  shown in  FIG. 1 . As illustrated in  FIG. 2   a , the sensor die  106  includes: a sensing portion  204  having a plurality of pixels and a non-sensing portion  204  having circuits to process the signal generated by the sensing portion  204 , wherein each pixel may include a microlens  206 . 
       FIG. 2   b  is a side cross sectional view of a conventional CMOS sensor die  230 . As illustrated in  FIG. 2   b , the conventional sensor die  230  includes a sensing portion  232  and a non-sensing portion  234 . The sensing portion  232  may includes a plurality of pixels  222  located on a substrate  218 , preferably a silicon substrate, and each pixel includes: a photodiode  220  partially embedded in the substrate layer  218 ; four transparent insulating layers  208   a - d ; four metal layers  210   a - d , the four metal layers being insulated by the four transparent insulating layers  208   a - d  and connected to the photodiode  220 ; a passivation layer  212 , the passivation layer being a transparent insulating layer and having a flat top surface; a color filter  216  for transmitting a specific wavelength or wavelength band of light to the photodiode  220 ; a planar layer  214  for providing a flat surface; and a microlens  206  for focusing light rays to the photodiode  220 . The sensor die  230  may further include a plurality of passive components (such as transistors, resistors and capacitors) partially embedded in the silicon substrate layer  218 , which are not shown in  FIG. 2   b  for simplicity. The metal layers  210   a - d  may function as connecting means for the photodiodes  220  and passive components to the non-sensing area of the sensor die  230 , where the signals from the photodiodes and passive components may be transmitted using a column transfer method. Further details of the sensor die  230  are disclosed in U.S. patent application Ser. No. 11/004,465 entitled “Microlens alignment procedures in CMOS image sensor design” filed Dec. 2, 2004, which is hereby incorporated herein by reference in its entirety. 
     Each color filter  216  filters light rays directed to its corresponding photodiode  220  and transmits light rays of only one wavelength or wavelength band. In one embodiment, a RGB color system may be used, and consequently, a color filter  216  may be one of three types. In the RGB system, signals from three pixels are needed to form one complete color. However, it is noted that the number of types of filters can vary depending on the color system applied to the sensor die  230 . 
       FIGS. 3   a - b  are schematic diagrams of the sensor die  106  having shifted microlenses in accordance with one embodiment of the present invention. In  FIGS. 3   a - b , only microlenses  304  and  324  and photodiodes  302  and  322  are shown for simplicity. However, it should be apparent to those of ordinary skill that other components, such as a color filter, may be inserted between the microlens  304  (or  324 ) and the photodiode  302  (or  322 ) and shifted with respect to the photodiode  302 . The microlenses  304  and  324  may correspond to pixels at the center and edge of the sensor die  106 , respectively. As the chief ray angle of the center pixel may equal zero, the microlens  304  may not be shifted with respect to the photodiode  302  in  FIG. 3   a . However, the chief ray angle  332  of the edge pixel may be up to 25-30 degrees and, as a consequence, the microlens  324  of the pixel near the edge of the sensor die  106  may be shifted by a distance S  330  to collect the light rays  328  as shown in  FIG. 3   b . More detailed explanation of the photodiode shifting techniques are disclosed in the previously referenced U.S. patent application Ser. No. 11/004,465 entitled “Microlens alignment procedures in CMOS image sensor design” filed Dec. 2, 2004. 
     The thickness T  326  may be determined by a set of parameters including the number and thicknesses of the metal layers, transparent insulating layers, passivation layer, color filter and planar layer. The distances S  330  may be calculated by the equation:
 
 T *tan θ/ n   —   eq=S   (1)
 
where θ and n_eq represent the chief ray angle  332  and the equivalent refractive index of the layers from microlens vertex to the photodiode surface, respectively. The shift distance S  330  may decrease as the thickness T  326  decreases for a fixed chief ray angle. Likewise, for a fixed shift distance S  330 , a decrease in the thickness T  326  may allow an increase in the chief ray angle  332 . Thus, the stack height, or equivalently the thickness T  326 , may be an ultimate limit to the chief ray angle  332 .
 
     As describe above in connection with  FIG. 1 , the F-number of the compound imaging system may be related to the marginal ray angle  309 . For a given dimension of the photodiode  302 , a decrease in the thickness T  326  may yield an increase in the marginal ray angle  309 , which in turn may allow a decrease in the F-number of the compound imaging system. Thus, the thickness T  326  may be an ultimate limit to the F-number of the imaging system. As the major goals in module design may be maximizing the chief ray angle of the corner pixel and minimizing the F-number of the compound imaging system without degrading the image quality, it is desirable to decrease the thickness T  326 . Also, as the maximum chief ray angle and the minimum F-number of the compound imaging system are limited by the lens requirements, the decrease in the thickness T  326  may have an effect equivalent to lowering the lens requirements. 
     Another advantage of thin sensor die may be realized by the use of thick microlenses. As the thickness T  326  decreases, the focal length of a microlens (e.g.,  304 ) may decrease, and as a consequence, the thickness of the microlens may increase. In general, thicker microlenses may be fabricated with relative easy than thinner microlenses. Furthermore, the surface roughness of the thicker microlenses may have less negative effect on the image intensity than the thinner microlenses. 
       FIG. 4   a  is a schematic diagram of an exemplary embodiment  400  of the conventional sensor pixel  222  (shown in  FIG. 2   b ). The sensor pixel  400  may be on a substrate  420  and include: a photodiode  416 ; four transparent insulating layers  404   a - d ; four metal layers  406   a - d , the four metal layers being insulated by the four transparent insulating layers  404   a - d  and connected to the photodiode  416 ; a passivation layer  408 , the passivation layer being a transparent insulating layer and having a flat top surface; a color filter  410  for transmitting a specific wavelength or wavelength band of light to the photodiode  416 ; a planar layer  412  for providing a flat surface; and a microlens  414  for focusing light rays to the photodiode  416 . Typically, the thicknesses  418  and  419  may be about 7 and 10 microns, respectively. 
       FIG. 4   b  is a schematic diagram of an exemplary embodiment  420  of a sensor pixel in accordance with one embodiment of the present invention. The sensor pixel  420  includes: a photodiode  436 ; two transparent insulating layers  424   a - b ; two metal layers  426   a - b ; a passivation layer  428 , the passivation layer being a transparent insulating layer and having a flat top surface; a color filter  430  for transmitting a specific wavelength or wavelength band of light to the photodiode  436 ; a planar layer  432  for providing a flat surface; and a microlens  434  for focusing light rays to the photodiode  436 . The sensor die  420  may be similar to the sensor pixel  400  except a difference that the sensor pixel  420  has a smaller number of metal and insulating layers to reduce its thickness. The thicknesses  438  and  439  may be about, but not limited to, 3 and 5 microns, respectively. In one embodiment, to reduce the thicknesses  438  and  439  further, the thickness of each layer of the pixel  420  may be decreased. In  FIG. 4   b , only two metal layers and two insulating layers are shown for simplicity. However, it is noted that the present invention may be practiced with any number of metal and insulting layers. 
       FIG. 5  is a side cross sectional view of an exemplary embodiment  500  of a sensor die in accordance with one embodiment of the present invention. As illustrated in  FIG. 5 , the sensing portion  520  of the sensor die  500  may have a smaller number of metal and transparent insulating layers than the non-sensing portion  522 . The sensing portion  520  includes: a substrate  502 ; photodiodes  504 ; two metal layers  508   a - b ; three transparent insulating layers  506   a - c ; a passivation layer  512 ; a planar layer  510 ; color filters  514 ; and microlenses  516 . The non-sensing portion  522  may include: the substrate  502 ; three metal layers  508   a - c ; four transparent insulating layers  506   a - d ; the passivation layer  512 ; and the planar layer  510 . In one embodiment, the two insulating layers  506   a  and  506   d  may be made of one dielectric material. 
     Typically, as illustrated in  FIG. 2   b , each layer of the conventional sensor die  230  may have a uniform thickness distribution over the portions  232  and  234 . Thus, in a conventional sensor die  230 , some of the layers may be required by only one of the two portions  232  and  234 , and become unnecessary to the other. For example, the non-sensing portion  522  may require a metal layer  508   c  coupled to a plurality of passive components (such as transistors, resistors and capacitors) embedded in the substrate  502 , which are not shown in  FIG. 5  for simplicity. As the sensing portion  520  may not need the metal layer  508   c , a corresponding part of the metal layer  508   c  may be eliminated as illustrated in  FIG. 5 . 
     In general, the top metal layer of a conventional sensor die  404   d  may be configured to reduce the amount of stray light that otherwise would be collected by the photodiode  416  and its neighboring photodiodes. However, by eliminating unnecessary layers, the thickness  418  may be reduced, and as a consequence, the amount of stray light may be reduced to make the top metal layer  418  dispensable. In  FIG. 5 , the sensor die  500  does not have a metal layer configured to block the stray light, which may further reduce the thicknesses  438  and  439 . In one embodiment, the number of imaging lens elements  104   a - c  for a VGA (640×480 resolution) system may be reduced from 3 to 2, where the number of imaging lens elements for multi mega pixel system may be reduced from 4 to 3. In another embodiment, the lens diameter and thickness may be decreased by at least 15%. 
     As explained in connection with  FIGS. 3   a - b , a decrease in stack height may allow an increase in the maximum chief ray angle of corner pixels and a decrease in the minimum F-number of compound imaging system. Typically, the maximum chief ray angle and the minimum F-number of a conventional sensor die may be about 15-20 degrees and 2.8-3.5, respectively. In one embodiment, a reduced stack height may allow the maximum chief ray angle of 25-30 degrees and the minimum F-number of 1.7-2.5, respectively. 
     For clarity explanation, only three metal layers are shown in  FIG. 5 . However, it should be apparent to those of ordinary skill that the present invention may be practiced with any number of metal layers. Also, it is should be apparent to those of ordinary skill that the difference in the number of layers between the sensing and non-sensing portions can vary without deviating from the present teachings. 
     Those skilled in the art will appreciate that the methods and designs described above have additional applications and that the relevant applications are not limited to those specifically recited above. It should be understood that the foregoing relates to exemplary embodiments of the invention and that modifications may be made without departing from the spirit and scope of the invention as set forth in the following claims.