Patent Publication Number: US-9902120-B2

Title: Wide-angle camera using achromatic doublet prism array and method of manufacturing the same

Description:
RELATED APPLICATIONS 
     This application is related to co-filed patent application Ser. No. 14/616,936 titled “Wide-Angle Camera Using Achromatic Doublet Prism Array and Method of Manufacturing the Same”. 
     BACKGROUND 
     There are several ways to capture a wide-angle image; one of them is based on N×N lens array system that provides compact and small size camera module as compared to a more conventional camera module that uses a single lens. The lens array technique uses a prism and other optical components to form an optical system with increased viewing angle. However, the use of the prism causes severe chromatic aberration that lowers the modulation transfer function (MTF) of the optical system significantly and thereby reduces resultant image quality. 
     SUMMARY OF THE INVENTION 
     Optical systems and manufacturing methods thereof disclose a prism-based optical system with reduced chromatic aberration. Based upon wafer-level fabrication, a novel achromatic doublet prism array has two asymmetric prisms that improve optical resolution while not unduly complicating the wafer-level fabrication process. As used herein, the term “two asymmetric prisms” means that the shape of the first prism to the second prism is asymmetric. That is, those two prisms are inversely bonded to each other. The concept of asymmetry is discussed in greater detail below. 
     In one embodiment, a wide-angle camera has a sensor with a plurality of pixel sub-arrays and an array of optical elements on a first side of a substrate where each of the optical elements is capable of forming an image from a field of view onto a different one of the pixel sub-arrays. The wide-angle camera also includes an array of achromatic doublet prisms on a second side of the substrate, where each of the achromatic doublet prisms is aligned to provide a viewing angle with a different one of the optical elements, such that the sensor captures a wide-angle field of view while having a compact format. 
     In another embodiment, in a compact format wide-angle camera of the type having an array of optical elements and a corresponding array of single prisms that cooperate to capture a wide field-of-view, where the array of optical elements is formed on a first side of a substrate and the array of single prisms is formed on a second side of the substrate, and each of the single prisms is aligned with a different one of the optical elements and causes chromatic aberration, the improvement includes implementing the array of single prisms as an array of achromatic doublet prisms formed using wafer-level fabrication onto the second side of the substrate such that each achromatic doublet prism is aligned with a different one of the optical elements, the array of achromatic doublet prisms and the array of optical elements cooperating to capture the wide field-of-view with reduced chromatic aberration. 
     In another embodiment, a method of manufacturing an achromatic doublet prism array having a N×N number of sections, includes: forming an array of first prisms each located in one of the N×N sections, composed of a first material, onto a substrate; and, forming an array of second prisms each located in one of the N×N sections, composed of a second material different than the first material, atop the array of first prisms. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  shows one exemplary wide-angle camera using an achromatic doublet prism array, in an embodiment. 
         FIG. 2  shows a front view of the camera of  FIG. 1  illustrating the achromatic doublet prism array with nine elements in a three-by-three array, in an embodiment. 
         FIG. 3  is a side cross section A-A through the camera of  FIGS. 1 and 2  illustrating three exemplary sub-cameras, in an embodiment. 
         FIG. 4  shows the sub-camera of  FIG. 3  in further exemplary detail, in an embodiment. 
         FIG. 5  shows an MTF through field graph illustrating exemplary optical performance of the sub-camera of  FIGS. 3 and 4 , in an embodiment. 
         FIG. 6  shows a spot diagram generated by simulation of the sub-camera of  FIGS. 3 and 4  when configured as described for  FIG. 5 , in an embodiment. 
         FIG. 7  shows one prior art wafer-level lens that has three substrates and five surfaces forming an image on a sensor array. 
         FIG. 8  is an MTF through field graph illustrating optical performance of the prior art wafer-level lens of  FIG. 7 . 
         FIG. 9  is a spot diagram illustrating optical performance of the wafer-level lens of  FIG. 7 . 
         FIG. 10  shows another prior art wafer-level lens that is similar to the wafer-level lens of  FIG. 7  but includes a single prism. 
         FIG. 11  is an MTF through field graph illustrating optical performance of the prior art wafer-level lens of  FIG. 10 . 
         FIG. 12  is a spot diagram illustrating optical performance of the wafer-level lens of  FIG. 10 . 
         FIG. 13  is a flowchart illustrating one exemplary method for fabricating a wide-angle camera with an achromatic doublet prism array. 
         FIG. 14  depicts a perspective view of an exemplary camera assembly, including an array of achromatic doublet prisms stacked on lens array assembly, imaging sensor array, and imaging substrate, in one embodiment. 
         FIGS. 15A-C  are cross sectional schematic diagrams illustrating the steps of the method of  FIG. 13 . 
         FIG. 16  is a cross sectional schematic diagram illustrating one exemplary camera with a 2×2 achromatic doublet prism array formed by the method of  FIG. 13 . 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       FIG. 1  shows a side cross-section of one exemplary wide-angle camera  100  using an achromatic doublet prism array  102 .  FIG. 2  shows a front view of camera  100  illustrating achromatic doublet prism array  102  with nine achromatic doublet prisms  202 ( 1 )-( 9 ) in a three-by-three array. Camera  100  is shown within a device  108  selected from the group including a smart phone, a personal camera, a wearable camera, and so on. Camera  100  is suitable for any application that requires a compact image capture device with a wide-angle field of view. Camera  100  also includes a lens array  104  and a sensor array  106 . Lens array  104  and achromatic doublet prism array  102  facilitate capture of a wide-angle  110  field of view  111  by camera  100 . 
       FIG. 3  is a side cross section A-A through camera  100  illustrating three exemplary achromatic doublet prisms  202 ( 2 ),  202 ( 5 ), and  202 ( 8 ), corresponding optical elements  302 ( 2 ),  302 ( 5 ), and  302 ( 8 ), and corresponding pixel sub-arrays  304 ( 2 ),  304 ( 5 ), and  304 ( 8 ), respectively. Each achromatic doublet prism  202 , corresponding optical element  302 , and corresponding pixel sub-array  304  forms a sub-camera  306 , where camera  100  has nine such sub-cameras. In the example of  FIG. 3 , sub-camera  306  includes achromatic doublet prism  202 ( 2 ), corresponding optical element  302 ( 2 ), and corresponding pixel sub-array  304 ( 2 ). 
       FIG. 4  shows sub-camera  306  of  FIG. 3  in further exemplary detail. Optical element  302 ( 2 ) is a five surface wafer-level lens structure having a first substrate  406  with a first lens  408 , a second substrate  410  with a second lens  412  and a third lens  414 , and a third substrate  416  with a fourth lens  418  and a fifth lens  420 . Substrates  406 ,  410 , and  416  are for example glass. Although lenses  408 ,  412 ,  414 ,  418 , and  420  are shown in this example, other optical elements with more or fewer lenses, or lenses of different types, may be used without departing from the scope hereof. 
     Each achromatic doublet prism  202  is formed of two asymmetric prisms. Achromatic doublet prism  202 ( 2 ) has a first prism  402  having a low Abbe number (V1) and high refractive index (n1), and a second prism  404  having a high Abbe number (V2) and a low refractive index (n2). For example, in  FIG. 4 , first prism  402  has an angle, refractive index (n1) and an Abbe number (V1) of 13.6 degrees, 1.6, and 30, respectively; whereas second prism  404  has an angle, refractive index (n2), and Abbe number (V2) of −17.2 degrees, 1.5, and 57, respectively. It should be appreciated that these values may vary without departing from the scope hereof. Achromatic doublet prism  202 ( 2 ) functions to modify the viewing angle of optical element  302 ( 2 ) and sub-camera  306 . Configuration of each achromatic doublet prism  202  is selected to alter the viewing angle of the corresponding sub-camera  306  such that camera  100  captures wide-angle  110  field of view  111 . Achromatic doublet prism  202 ( 2 ) is formed directly onto a surface, opposite lens  408 , of first substrate  406 , as is discussed in further detail below, thereby reducing manufacturing time and cost. Further, the use of achromatic doublet prism  202  significantly improves the optical resolution of camera  100  such that it is comparable in quality to cameras formed without a prism. 
     To achieve a compact camera with wide-angle capability, an achromatic doublet prism having two asymmetric prisms made from two different optical materials with different Abbe numbers is used. The Abbe number of the first prism is lower that the Abbe number of the second prism. These prisms are formed combined on a first substrate using wafer-level fabrication, for example using method  1300  as discussed in further detail below. Geometry of each of each achromatic doublet prism is based upon its position within the array. 
     Assume the Abbe number of first prism is V1, and the Abbe number of second prism is V2, the refractive index of the first prism is n1, and the refractive index second prism is n2. If the following two constraints are satisfied, high optical performance is achieved for each sub-camera  306  (i.e., achromatic doublet prism  202 ( 2 ) and optical element  302 ( 2 )). 
     Constraint 1: V2&gt;V1, V2&gt;50 and V1&lt;35 (d line, wavelength is 587 nm). 
     Constraint 2: n2&lt;n1, n2&lt;1.52 and n1&gt;1.58 (d line, wavelength is 587 nm). 
     The angle of combination surface  403  between first prism  402  and second prism  404  depends upon the matching of the refractive index of different materials of first prism  402  and second prism  404 . For example, the respective angles of first prism  402  and second prism  404  may be different than 13.6 and −17.2 degrees as illustrated in  FIG. 4 , but the angle of first prism  402  is preferably negative compared to the angle of second prism  404 . 
       FIG. 5  shows an MTF through field graph  500  illustrating exemplary optical performance of sub-camera  306  (i.e., achromatic doublet prism  202 ( 2 ) and optical element  302 ( 2 )) of  FIGS. 3 and 4 . First prism  402  has an Abbe number (V1) of 30 and material of first prism  402  has a refractive index (n1) of 1.6 (d line, at 587 nm). Second prism  404  has an Abbe number (V2) of 57 and is made of a material with a refractive index (n2) of 1.51 (d line, at 587 nm).  FIG. 6  shows a spot diagram  600  generated by simulation of sub-camera  306  (i.e., achromatic doublet prism  202 ( 2 ) and optical element  302 ( 2 )) of  FIGS. 3 and 4  when configured as described for  FIG. 5 . 
     For comparison, exemplary prior art optical configurations are tested and compared with MTF through field graph  500  and spot diagram  600  of achromatic doublet prism  202 ( 2 ) and optical element  302 ( 2 ) of  FIGS. 3 and 4 . 
       FIG. 7  shows one prior art wafer-level lens  700  that has three substrates  702 ( 1 )-( 3 ) and five surfaces  704 ( 1 )-( 5 ) forming an image on a sensor array  706 . Wafer-level lens  700  is similar to optical element  302 ( 2 ) of  FIG. 3 . Of note, wafer-level lens  700  does not include any prism and therefore does not have wide field-of-view capability. 
       FIG. 8  is an MTF through field graph  800  illustrating optical performance of prior art wafer-level lens  700  of  FIG. 7 .  FIG. 9  is a spot diagram  900  illustrating optical performance of wafer-level lens  700  of  FIG. 7 . MTF through field graph  800  and spot diagram  900  illustrate typical performance of wafer-level lens  700 . 
       FIG. 10  shows another prior art wafer-level lens  1000  that is similar to wafer-level lens  700  of  FIG. 7  but has an added single prism  1002  configured with a surface of substrate  702 ( 1 ) opposite surface  704 ( 1 ). Single prism  1002  has an Abbe number (V D ) of 62.6, and is made from a material with a refractive index (n) of 1.5168 (d line, at 587 nm). Of note, single prism  1002  provides wide angle capability to wafer-level lens  1000 . 
       FIG. 11  is an MTF through field graph  1100  illustrating optical performance of prior art wafer-level lens  1000  of  FIG. 10 .  FIG. 12  is a spot diagram  1200  illustrating optical performance of wafer-level lens  1000  of  FIG. 10 . MTF through field graph  800  and spot diagram  900  illustrate typical performance of wafer-level lens  700 . As shown in graph  1100  and diagram  1200 , the addition of single prism  1002  results in severe chromatic aberration that significantly lowers the optical resolving capability of wafer-level lens  1000  as shown in  FIGS. 11 and 12  as compared to  FIGS. 8 and 9 . Thus, using a single prism, as shown in wafer-level lens  1000  results in poor quality images. 
     However, comparing MTF through field graph  500 ,  FIG. 5 , and spot diagram  600 ,  FIG. 6 , with prior art MTF through field graph  800  ( FIG. 8 ) and spot diagram  900  ( FIG. 9 ) clearly shows that use of achromatic doublet prism  202  in sub-camera  306  of  FIG. 3  results in a significant improvement in optical performance over prior art wafer-level lens  1000  of  FIG. 10 . 
       FIG. 13  is a flowchart illustrating one exemplary method  1300  for fabricating a wide-angle camera with an achromatic doublet prism array.  FIG. 14  depicts a perspective view of camera  100  of  FIG. 1 , including achromatic doublet prism array  102  stacked on lens array  104  and imaging sensor array  106 , which is illustratively shown formed on an imaging substrate  1408 , in one embodiment.  FIGS. 15A-C  are a cross sectional schematic diagrams illustrating the steps of method  1300 , of  FIG. 13  to form a plurality of cameras  100  on a wafer. In particular,  FIG. 15A  shows exemplary use of molds  1500 ,  1508  to form first and second prisms onto a substrate  1506 , and  FIG. 15B  shows substrate  1506  combined with a lens array assembly  1514  and an image sensor array  1516  that is then diced to form each individual camera assembly  1400 .  FIGS. 13 through 15B  are best viewed together with the following description. 
     For the purposes of discussion of  FIGS. 13-15B , reference is made to manufacturing three-by-three array camera assemblies  1400 . However, it should be appreciated that method  1300  may apply to any N×M camera assembly array, where N and M are positive integer values. 
     In step  1302 , method  1300  generates a first mold corresponding to an array of first prisms. In one example of step  1302 , first mold  1500  is generated for forming an array of first prisms  402 . First mold  1500  is configured with a plurality of areas  1502  corresponding to the desired formation of first prisms  402 . In  FIG. 15A , mold  1500  is shown forming two achromatic doublet prism arrays  1402  that each correspond to cross section line B-B of  FIG. 14 . Moreover, as each area  1502  correlates to a given section of the achromatic double prism, each area  1502  may have a different formation based on a desired formation of the first prism in that section. In the example illustrated in  FIG. 15 , the cross section of first mold  1500  correlates to first prisms  402  of achromatic doublet prisms  202 ( 2 ),  202 ( 5 ) and  202 ( 8 ) of achromatic doublet prism array  102  of  FIGS. 1 and 14 , where area  1502  is shaped and sized to form each first prism  402  therein. 
     In step  1304 , method  1300  forms, using the first mold, an array of first prisms, composed of a first material, onto a first substrate. In one example of step  1304 , first material is disposed into areas  1502 ( 1 )-( 4 ) to form first prisms  404 ( 1 )-( 4 ), respectively, on substrate  406 . First material may be ultra-violet (UV) curable material. Substrate  406  may be glass, plastic, silicon, or other optically transparent material. 
     In optional step  1306 , first material is cured to finalize formation of first prisms  1504 . 
     In step  1308 , method  1300  removes the first mold. In one example of step  1308 , first mold  1500  is removed leaving first prisms  402  on substrate  406 . 
     In optional step  1310 , method  1300  generates a second mold corresponding to an array of second prisms. In one example of step  1310 , second mold  1508  is generated for forming an array of second prisms  404 . Mold  1508  includes at least one area  1510  corresponding to the desired formation of second prisms  404 . Mold  1508  corresponds to mold  1500  and forms a plurality of cameras  100  on a wafer for example. Each section of area  1510  correlates to a given section of the achromatic double prism array  102 , wherein each section of area  1510  may have a different shape and size based upon the shape and size or a corresponding second prism  404  of achromatic prism array  102 . In the example illustrated in  FIG. 15A , the cross section of mold  1510  correlates to sub-cameras  306 ( 2 ),  306 ( 5 ) and  306 ( 8 ) of camera  100  of  FIGS. 1, 2 and 3 , including an area  1510  for forming second prism  404  therein. Were cross section line B-B to cross through sections  202 ( 1 )- 202 ( 3 ), for example, the surface of mold  1510  would differ to match the desired formation of second prisms  404  of those sub-cameras  306 . 
     In step  1312 , method  1300  forms, using the second mold, an array of second prisms, composed of a second material different from the first material, onto the first prisms. In one example of step  1312 , second material is disposed into area  1510  to form second prisms  404 ( 1 )-( 6 ), respectively, on first prisms  402 . In the example shown in  FIG. 15A , which applies to a 3×3 array, the center of the array corresponding to sub-camera  306 ( 5 ) only includes second material and no first prism. Therefore, at this section, second material is formed onto substrate  406 . Second material may be a ultra-violet (UV) curable material. 
     In optional step  1314 , second material is cured to finalize formation of second prisms  404 . 
     In step  1316 , method  1300  removes the second mold. In one example of step  1316 , second mold  1508  is removed leaving second prisms  404  above first prisms  402  and substrate  406 . 
     In optional step  1318 , method  1300  stacks first and second prism arrays formed in steps  1302 - 1316  on a lens array assembly. In one example of step  1318 , substrate  406 , having first prisms  402  and second prisms  404  located thereon is stacked onto lens array assembly  104  and image sensor array  106 . In the example of  FIG. 15B , an additional lens (e.g., lens  408 .  FIG. 4 ) has been formed onto a second side of substrate  406  prior to stacking. 
     In optional step  1320 , method  1300  dices the stacked array to form individual cameras. In one example of step  1320 , achromatic prism array  102 , substrate  406 , lens array  104 , and image sensor array  106  are diced (e.g., along dicing line  1518 ) to form individual cameras  100 , as shown in  FIG. 15C . 
     Steps  1301  and  1317  are optional. If step  1301  is included, then step  1317  is not included. If step  1317  is included, then step  1301  is not included. In each of optional steps  1301  and  1317 , an optional lens array is fabricated on a second side of the substrate. In one example of steps  1301  and  1317 , lenses  408  are fabricated onto a second side of substrate  406 . That is, if included, lenses  408  may be fabricated onto a second side of substrate  406  either before or after fabrication of achromatic doublet prism array  102 . 
     In the examples of  FIGS. 1 through 15 , because sub-camera  306 ( 5 ) requires no modification of its corresponding field of view, no first prism  402  is included. In other words, assuming camera  100  if formed of a symmetrical N×N array of sub-cameras  306 , when N is odd, the center achromatic doublet prism of the achromatic doublet prism array  102  may not include first prism  402 , but may include material corresponding to other second prisms  404 . When N is even, the center sub-cameras optionally include a first prism. For example, in a 4×4 array, the center four sub-cameras of camera  100  may only include the second material. Alternatively, in a 4×4 array, the center four sections may include both a first and second prism.  FIG. 16  is a cross sectional schematic diagram illustrating one exemplary camera  1600  with a 2×2 achromatic doublet prism array  1602  formed by method  1300  of  FIG. 13 . Camera  1600  has an achromatic doublet prism array  1602 , a lens array  1614 , and a sensor array  1616 . In the example of  FIG. 16 , camera  1600  is formed as a 2×2 array of sub-cameras, and therefore has no central sub-camera, wherein each sub-camera includes both a first and second prism  1604 ,  1606 . 
     As illustrated in  FIGS. 15A-C , second material forming second prisms  404  may be made from a single contiguous layer of material that encapsulates each of first prisms  402 . Advantageously, this saves time and precision of aligning second prisms  404  with first prisms  402 . Alternatively, second mold  1508  may be configured such that only a top surface of first prisms  402  is covered with second material of the respective second prisms  404 , in a similar manner to  FIGS. 3 and 4 . Advantageously, this saves money on the amount of material used in forming second the prism array. 
     Changes may be made in the above methods and systems without departing from the scope hereof. It should thus be noted that the matter contained in the above description or shown in the accompanying drawings should be interpreted as illustrative and not in a limiting sense. The following claims are intended to cover all generic and specific features described herein, as well as all statements of the scope of the present method and system, which, as a matter of language, might be said to fall therebetween.