Abstract:
A method for aligning a microlens array in a sensor die to resolve non-symmetric brightness distribution and color balance of the image captured by the sensor die. The method includes performing a pre-simulation to simulate a microlens array alignment in a silicon die and to determine a shrink-factor and de-centering values, calculating the error in a real product&#39;s alignment in process and image offset, performing a post simulation based on offset calculation on the real product and re-design of the microlens alignment, and repeating the steps of calculating the error and performing the post-simulation until a satisfactory brightness distribution is obtained. The sensor die has sensor pixels, each pixel comprising a photodiode and a microlens for directing incoming light rays to the photodiode, wherein optical axis of the microlens is shifted with respect to optical axis of the photodiode by a preset amount determined by at least one iteration of alignment process.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS/INCORPORATION BY REFERENCE 
     This application is a continuation of U.S. application Ser. No. 11/313,976, filed Dec. 20, 2005, which is a divisional of U.S. application Ser. No. 11/004,465, filed Dec. 2, 2004. The disclosure of U.S. application Ser. No. 11/313,976 is hereby incorporated herein by reference in its entirety. 
    
    
     BACKGROUND 
     1. Field of the Invention 
     The present invention relates to CMOS image sensor design and more particularly to microlens alignment procedures 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 to have certain ray angle incident on its sensor surface to generate acceptable image data output. In an effort to mitigate the ray angle requirement, the CMOS sensor may be customized to accept incident rays at large angles, especially for the pixels at corners and edges thereof. 
     One common practice of CMOS sensor customization is shifting a microlens array of the sensor to match the incident rays at large angles. However, in the application of the shifting technique, the non-symmetric nature of the CMOS sensor pixel layout may create non-symmetric brightness distribution over the image output, where the non-symmetric nature may be more pronounced at the corners and edges of the image output. In addition, such non-symmetric brightness may be accompanied by improper color balance, i.e., the color of the image of a white light source is not white over the entire image output. 
     To resolve the appearance of non-symmetric brightness distribution over the image output, the existing approaches have attempted several symmetric layouts for each pixel of the CMOS sensor. Such approaches may impose many limitations and restrictions to a layout designer and some tradeoff may be necessary to accommodate the symmetric layout with additional silicon real estate. In addition, the entire CMOS pixel layout may be modified upon unsatisfactory image output, which requires lengthy and expensive turn-around processes. Furthermore, the existing approaches cannot correct the non-symmetric nature that may be rooted in other sources, such as chemical contamination occurred during the CMOS process for producing a silicon die, electrical field generated by metal layers of the CMOS sensor, imperfect masks used in the CMOS process and other unknown sources. Thus, there is a need for an improved method for resolving non-symmetric brightness distribution. 
     SUMMARY 
     The present invention provides a method for aligning a microlens array in a sensor die to resolve non-symmetric brightness distribution and improper color balance of images captured by the sensor die. 
     In one aspect of the present invention, a method for aligning a microlens array in a sensor die includes the steps of (a) performing a pre-simulation to simulate a microlens array alignment in a silicon die and to determine a shrink-factor; (b) designing a new photo-mask for the microlens array based on the shrink-factor; (c) producing a sample silicon die using the new photo-mask; (d) capturing an image of a collimated white light source using the sample silicon die; (e) evaluating uniformity of brightness distribution of the image; and in case of unsatisfactory brightness distribution; (f) calculating error in alignment of the sample silicon die and a de-centering value; (g) performing a post-simulation based on the error to tune the shrink-factor and the de-centering value; (h) designing a new photo-mask for the microlens array based on the shrink-factor and the de-centering value; and (i) repeating the steps (c)-(h) until a satisfactory brightness distribution is obtained. 
     In another aspect of the present invention, a sensor die for digital imaging includes: a processing area; and a sensing area, comprising: a plurality of sensor pixels, comprising: a silicon substrate having a photodiode and a plurality of passive components; a first insulting layer on top of the silicon 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 middle insulating layers sandwiched between two neighboring ones of the plurality of metal layers; a first insulating planar layer on top of the plurality of metal layers; a color filter; a second insulating planar layer on top of the color filter; and a microlens to direct incoming light to the photodiode through the color filter; wherein an optical axis of the microlens is shifted with respect to an optical axis of the photodiode by a preset amount determined by at least one iteration of alignment process, each of the at least one iteration including a pre-simulation to determine a shrink-factor and a post-simulation to tune the shrink-factor. 
     In yet another aspect of the present invention, an imaging device includes: a sensor die for digital imaging, comprising: a processing area; and a sensing area, comprising: a plurality of sensor pixels, each of the plurality of sensor pixels comprising: a silicon substrate having a photodiode and a plurality of passive components; a first insulting layer on top of the silicon 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 middle insulating layers sandwiched between two neighboring ones of the plurality of insulating layers; a first insulating planar layer on top of the plurality of metal layers; a color filter; a second insulating planar layer on top of the color filter; and a microlens to direct incoming light to the photodiode through the color filter; wherein an optical axis of the microlens is shifted with respect to an optical axis of the photodiode by a preset amount determined by at least one iterative alignment process, each of the at least one iterative alignment process including a pre-simulation to determine a shrink-factor and a post-simulation to tune the shrink-factor. 
     In still another aspect of the present invention, a computer readable medium carries one or more sequences of instructions for aligning a microlens array in a sensor die, wherein execution of the one or more sequences of instructions by one or more processors causes the one or more processors to perform the steps of: (a) performing a pre-simulation to simulate a microlens array alignment in a silicon die and to determine a shrink-factor; (b) designing a new photo-mask for the microlens array based on the shrink-factor; (c) producing a sample silicon die using the new photo-mask; (d) capturing an image of a collimated white light source using the sample silicon die; (e) evaluating uniformity of brightness distribution of the image; and in case of unsatisfactory brightness distribution; (f) calculating error in alignment of the sample silicon die and an image de-centering value; (g) performing a post-simulation based on the error to tune the shrink-factor; (h) designing a new photo-mask for the microlens array based on the shrink-factor and the de-centering value; and (i) repeating the steps (c)-(h) until a satisfactory brightness distribution is obtained. 
     In another aspect of the present invention, a system for aligning a microlens array in a sensor die includes: means for performing a pre-simulation to simulate a microlens array alignment in a silicon die and to determine a shrink-factor; means for designing a new photo-mask for the microlens array based on the shrink-factor; means for producing a sample silicon die using the new photo-mask; means for capturing an image of a collimated white light source using the sample silicon die; means for evaluating uniformity of brightness distribution of the image; and in case of unsatisfactory brightness distribution; means for calculating error in alignment of the sample silicon die and an image brightness center offset; means for performing a post-simulation based on the error to tune the shrink-factor; means for designing a new photo-mask for the microlens array based on the shrink-factor and the image brightness center offset; and means for repeating the steps of producing a sample silicon die to the step of designing a new photo-mask until a satisfactory brightness distribution is obtained. 
     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 image module assembly in accordance with one embodiment of the present teachings. 
         FIG. 2   a  is a top view of a portion of a silicon die in accordance with one embodiment of the present teachings. 
         FIGS. 2   b ,  2   c  and  2   d  are a front, perspective and side view of the portion in  FIG. 2   a , respectively 
         FIG. 3  is a detailed layout of two metal layers and photodiodes in accordance with one embodiment of the present teachings. 
         FIGS. 4   a  and  4   b  illustrate ray acceptance angles for a sensor with a non-shifted microlens array. 
         FIG. 5  is an image of a white light source captured by an image sensor having a sensor die, where the brightness of the image is non-uniform across the sensor die. 
         FIGS. 6   a  and  6   b  illustrate ray acceptance angles for a sensor with a shifted microlens array in accordance with one embodiment of the present teachings. 
         FIG. 7  is a top view of a sensor die with a microlens array, elements of which are shifted to achieve ray angle match over the entire sensor die in accordance with one embodiment of the present teachings. 
         FIG. 8  is a flow chart of an iterative process for aligning a microlens array in a sensor die in accordance with one embodiment of the present teachings. 
         FIG. 9  shows an image captured by a sensor die having a microlens array aligned with de-centering offset following the steps in  FIG. 8 . 
     
    
    
     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 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 microlens” includes a plurality of such microlens, i.e., microlens array, and equivalents thereof known to those skilled in the art, and so forth. 
     One common practice of CMOS sensor customization is shifting a microlens array of the sensor to match incident rays at large angles. However, in the application of the shifting technique, the non-symmetric nature of the CMOS sensor layout may create non-symmetric brightness distribution over the image output, where the non-symmetric nature may be more pronounced at the corners and edges of the image output. In addition, such non-symmetric brightness may be accompanied by improper color balance, i.e., the color of the image of a white light source is not white over the entire image output. The present inventor provides a simple, yet effective way to resolve the appearance of non-symmetric brightness in the image by introducing off-center alignment (aligned with de-centering values) between a microlens array and a sensor pixel array of the CMOS sensor. 
       FIG. 1  is a schematic diagram of an image module assembly  100  (or, equivalently a lens/sensor assembly) in accordance with one embodiment of the present teachings. The lens/sensor assembly  100  may be included in digital image devices, such as digital image camera and cellular phone with imaging capabilities. As illustrated, the lens/sensor assembly  100  includes: a sensor die  108  as an image sensor; and a lens assembly  102  having several pieces of lenses and iris (no shown in  FIG. 1  for simplicity) assembled in a lens barrel, the lens assembly forming an image on the surface of the sensor die  108 . In one embodiment, the width and length of the sensor die  108  is about, but not limited to, 5 mm. 
     Optical rays  104   a - c , exemplary optical rays from the lens assembly  102 , are directed to sensor pixels (the sensor pixels will be explained later) at the center, near the left edge and near the right edge of the sensor die  108 , respectively, and angled with respect to the surface normal of the sensor die  108  by chief ray angles  106   a - c , respectively. The chief ray angles  106   b  and  106   c  may be as large as 30 degrees, while the chief ray angle  106   a  is about zero degree. 
     The sensor die  108 , a type of CMOS image sensor, is a piece of silicon that includes an integrated circuit (IC) to function as an image sensor. The IC comprises a processing area and a sensing area that may have from several hundred thousands to millions of identical sensor pixels. Hereinafter, for simplicity, the sensor die  108  refers to its sensing area only.  FIG. 2   a  is a top view of a portion  109  of the silicon die  108  in accordance with one embodiment of the present teachings, where only 9 sensor pixels  110  are shown for simplicity.  FIGS. 2   b  and  2   d  are the front and side views of the portion  109  in  FIG. 2   a , respectively, showing multiple layers  112 - 138  of the silicon die  108 .  FIG. 2   c  is a perspective view of the portion  109  in  FIG. 2   a , focusing on several key features of the layers. 
     As shown in  FIGS. 2   b - 2   d , each pixel  110  includes: a silicon substrate layer  112 ; a photodiode  114  forming a portion of and being underneath the surface of the silicon substrate layer  112 ; a plurality of passive components  115  (such as transistors, resistors and capacitors) underneath the surface of the silicon substrate layer  112 ; four transparent insulating layers  116 ,  120 ,  124  and  128 ; four metal layers  118 ,  122 ,  126  and  130 , the four metal layers being insulated by the four transparent insulating layers  116 ,  120 ,  124  and  128 , and connected to the photodiode  114  and/or the plurality of passive elements  115 ; a first planar layer  132 , the first planar layer being a transparent insulating layer and having a flat top surface; a color filter  134  for passing a specific wavelength or wavelength band of light to the photodiode  114 ; and a microlens  138  for focusing light rays to the photodiode  114 . A microlens array  139  in  FIG. 2   c  comprises the identical microlens  138 . 
     In one embodiment of the present teachings, the photodiode  114  and the plurality of passive elements  115  may be formed by a semiconductor etching process, i.e., etching the surface of the silicon substrate layer  112  and chemically depositing intended types of material on the etched area to form the photodiode  114  and the plurality of passive elements  115 . 
     As mentioned, the color filter  134  filters light rays (such as  104  in  FIG. 1 ) directed to its corresponding photodiode  114  and transmits light rays of only one wavelength or wavelength band. In one embodiment of the present teachings, a RGB color system may be used, and consequently, a color filter array (CFA)  135  (shown in  FIG. 2C ) comprises three types of filters  134 . 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 in the CFA  135  can vary depending on the color system applied to the silicon die  108 . 
     The metal layers  118 ,  122 ,  126  and  130  function as connecting means for the photodiodes  114  and passive components  115  to the processing area of the silicon die  108 , where the signals from the photodiodes and passive components are transmitted using a column transfer method. In  FIGS. 2   c  and  2   d , for the purpose of illustration, exemplary connections  119  and  121  are shown, where the connections  119  and  121  link the metal layer  118  to the photodiode  114  and one of the passive components  115 , respectively. However, it should be apparent to the one of ordinary skill that connections between the four metal layers ( 118 ,  122 ,  126  and  130 ) and the photodiode  114  and the passive components  115  can vary depending on the overall layout of the silicon die  108 . Also, the number of metal layers depends on the complexity of the layout of metal layers and, as a consequence, a different layout of the silicon die may have different number of metal layers. 
       FIG. 3  is a detailed layout  300  of the metal layers and photodiodes of the silicon die  108  in accordance with one embodiment of the present teachings, where a top view of only two metal layers  118 ,  122  and photodiodes  114  are shown for simplicity. The metal layers  118  and  122  may be formed of an opaque material, such as aluminum, and define the shape of openings  302  through which the light rays directed to each photodiode  114  are collected. As shown in  FIG. 3 , the shape of the opening  302  may not have any axis of symmetry. Tn addition, the layout of two other metal layers  126  and  130 , when superimposed on top of the layout  300 , would make the opening  302  be further non-symmetric. The effective light collecting area of the non-symmetric opening  302  varies as the angle of light rays with respect to the surface normal of the opening  302  changes. Consequently, the intensity of electric signal from the photodiode  114  may be a function of the chief ray angle  106  (shown in  FIG. 1 ). 
     As illustrated in  FIG. 1 , each of the optical rays  104   a - c  is angled with respect to the surface normal of the sensor die  108 .  FIGS. 4   a  and  4   b  illustrate ray acceptance angles  406  for a sensor die  408  with a non-shifted microlens array  404 , where the optical axis of a microlens in each pixel  410  coincides with the optical axis of a photodiode  402  in the pixel. (Hereinafter, the optical axis of a photodiode refers to an axis normal to the surface of the photodiode and passes through the geometric center of the photodiode.) In  FIGS. 4   a - b , for simplicity, only photodiodes  402  and a microlens array  404  are shown. As illustrated in  FIG. 4   a , most of the light rays  104   a  are collected by a photodiode  402   a  that is located at the center of the sensor die  408 . Thus, the light ray acceptance angle  406   a  is same as that of incoming light rays  104   a . In contrast, as shown in  FIG. 4   b , some portion of the optical rays  104   b  are not collected by a photodiode  402   b  that is located near the right edge of the silicon die  408 , i.e., the photodiode  402   b  has a limited ray acceptance angle  406   b . Such limited ray acceptance angle, when combined with the non-symmetric nature of the opening  302 , may result non-uniform brightness distribution of an image on the sensor die  408 , as shown in  FIG. 5 .  FIG. 5  shows an image  500  of a white light source captured by an image sensor having the sensor die  408 , where the brightness of image  500  is non-uniform across the sensor die  408 . In addition, the image  500  may not be a color balanced, i.e., the color of the image is not white over the entire sensor die. 
       FIGS. 6   a  and  6   b  illustrate ray acceptance angles  606  for a sensor die  608 , where the optical axis of a microlens  604  in each pixel  610  has been shifted with respect to the optical axis of a photodiode  602  of the pixel in accordance with one embodiment of the present teachings. In  FIG. 6   a , the optical axis of a microlens  604   a  in a pixel coincides with the optical axis of a photodiode  602   a  of the same pixel, where the pixel is located at the center of the sensor die  608 . However, as shown in  FIG. 6   b , the optical axis of a microlens  604   b  in a pixel located near the right edge of the sensor die  608  has been shifted by a distance  612  with respect to the optical axis of a photodiode  602   b  in an effort to improve the ray acceptance angle  606   b . The light ray acceptance angles  606   a  and  606   b  are equal to those of the incoming light rays  104   a  and  104   b , respectively. 
     In this embodiment, a pre-simulation has been performed to calculate the distance  612  for each pixel  610  and simulate the ray acceptance angles  606  by a basic optical method, such as a conventional optical ray trace technique. Based on the calculated distance  612 , a “shrink-factor” is calculated, where the shrink-factor is the ratio of the dimension of the microlens array  604  to that of the silicon die  608 . Hereinafter, the term “shrinking” means reducing the size of microlens array  604  based on the calculated shrink-factor. Actual shrinking is realized by reducing the area of each microlens and gaps between neighboring microlenses, while the thickness of the microlens may be kept unchanged. As the microlens array  604  is formed using a photo-mask in a photo-processing of the silicon die  608 , shrinking is implemented by scaling the photo-mask. 
       FIG. 7  is a top view of a sensor die  700  with a microlens array  702 , the elements of which are shifted to achieve ray angle match over the entire sensor die  700  in accordance with one embodiment of the present teachings. (In  FIG. 7 , for simplicity, only the array of microlens  702  and photodiodes  704  are shown.) As shown in  FIG. 7 , each microlens  702  has been shifted toward the center  712  of the sensor die  700  so that each of light spots  706  is located within the corresponding photodiode  704 , which improves the ray acceptance angle, and subsequently, the brightness distribution of image on the silicon die  700 . The shifting of each microlens is more pronounced near sensor edges  710  and corners  708  than the center  712 . As mentioned above, the shifting of each microlens is implemented by scaling a photo mask of the microlens array  702 . 
     The pre-simulation based on the optical ray trace technique may not provide a perfect prediction of the optical characteristics of the pixels due to the complexity of the layout of the silicon die. Also, the accuracy of the pre-simulation is limited as the pixel size keeps shrinking. Furthermore, it is quite difficult, if not possible, to simulate the effects of additional factors that may contribute to simulation error. The factors include; chemical contamination occurred during the CMOS process for producing a silicon die; electrical field generated by metal layers; and imperfect masks used in the CMOS process as well as other unknown sources. Thus, upon completion of shrinking based on the pre-simulation and production of a sample product of the silicon die, an image of a collimated white light source captured by the sample product should be analyzed to access any error in the sample silicon die&#39;s alignment and image center offset (or, equivalently, de-centering value). If the brightness distribution of the captured image of the collimated white light source is not satisfactory, further adjustment via an iteration process may be performed. 
       FIG. 8  is a flow chart  800  of an iterative process for aligning a microlens array in a sensor die in accordance with one embodiment of the present teachings. At step  802 , a pre-simulation is performed to simulate a microlens array alignment and to determine a shrink-factor as described above. Based on the shrink-factor, a new photo-mask for the microlens array is designed at step  804 . Next, a sample silicon die is produced using the new photo-mask at step  806 . Subsequently, an image of a collimated white light source is captured by the produced sample silicon die at step  808 . At step  810 , the uniformity of brightness distribution on the captured image is evaluated. In this step, the distribution of both intensity and color balance of the image are evaluated. In case of unsatisfactory distribution, the error in alignment of the sample silicon die and image center offset can be calculated at step  814 . Next, at step  816 , based on the calculated error, post-simulation may be performed in the same manner as the pre-simulation of step  802  to tune the shrink-factor and de-centering values. Subsequently, a new photo-mask for the microlens array is designed based on the tuned shrink-factor at step  818  and the steps  806 - 818  are repeated until a satisfactory image is obtained to end the iterative process at step  812 . 
       FIG. 9  shows an image  900  captured by a sensor die having a microlens array aligned with de-centering following the steps in  FIG. 8 . As can be noticed, the image  900  shows enhanced uniformity of brightness and symmetric color balance over the entire sensor die compared to the image  500  in  FIG. 5 . Thus, the iteration process, the steps  806 - 818  of  FIG. 8 , allows a sensor designer to tune the shrink-factor in a high precision and effectively improve the image quality. 
     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