Abstract:
The manufacture of multi-level optical imagers and the resulting imagers are described. Multiple levels of metallization are prepared, each level having a via. The vias are aligned and a material having a higher refractive index than its surrounds is positioned within the vias to form an optical channel. The higher refractive index material may be an optical plug. A lens is mounted at one end of the optical channel and a photoconversion device is mounted at the other end.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to the field of metal optical imagers, and more particularly to optical channels utilized in multi-level metal optical imagers. 
     BACKGROUND 
     The semiconductor industry currently uses different types of semiconductor-based imagers, such as, for example, complementary metal oxide semiconductor (CMOS) imagers, charge coupled devices (CCDs), photodiode arrays, charge injection devices and hybrid focal plane arrays, among others. The above noted examples of semiconductor imagers incorporate solid state pixels that receive an image, from lenses or other light-receiving structures, with sensors and convert that image to electrical signals. 
     Semiconductor imagers all require at least one level and up to three levels of metallization to connect transistors for the pixels to the circuitry that drives the pixels. The levels of metallization must be properly insulated and are generally within an intralayer dielectric material. An undesirable aspect of incorporating plural levels of metallization is that the light attenuates as the distance between the lens and the sensor increases. 
     SUMMARY 
     The various embodiments of the invention are directed to improving the transmission of light to a light device of a pixel cell of a digital imager or from a pixel cell of a display device. Embodiments of the present invention provide an optical imager that includes a lens, a light device, a first dielectric material, and a second dielectric material extending through the first dielectric material. The second dielectric material exhibits a refractive index greater than the refractive index exhibited by the first dielectric material and serves as an optical channel optically connecting the lens with the light device. 
     Other embodiments of the present invention provide an optical coupling between a lens and a light device. The optical coupling includes a via extending between the lens and the light device, a first optical channel section extending through a portion of the via, and at least a second optical channel section extending through the remainder of the via. The optical channel sections exhibit a refractive index greater than the material surrounding the optical channel sections. 
     Other embodiments of the present invention provide a multi-level optical imager that includes a plurality of intermediate structures built over a pixel cell, a lens, and a light device within the pixel cell. Each intermediate structure includes a dielectric material portion, a stop layer capping the dielectric material portion, and an optical channel section extending through the dielectric material portion and the stop layer. The optical channel sections are aligned with each other to form an optical channel optically connecting the lens with the light device. 
     Other embodiments of the present invention provide an optical imager including a lens, a light device, and at least one metallized layer structure. The at least one metallized layer structure includes a first dielectric material, a metallized portion within the first dielectric material, and a second dielectric material extending through the first dielectric material. The second dielectric material exhibits a refractive index greater than the refractive index exhibited by the first dielectric material and serving as an optical channel optically connecting the lens with the light device. 
     Other embodiments of the present invention provide a method of manufacturing an optical imager. The method includes forming a base intermediate structure over a pixel cell, the base intermediate structure including a first optical channel section within a dielectric material, forming one or more secondary intermediate structures on the base intermediate structure, each of the secondary intermediate structures including a second optical channel section aligned with the first optical channel section to form an optical channel, and optically connecting a lens at one end of the optical channel with a light device of the pixel cell at the other end of the optical channel. 
     Other embodiments of the present invention provide a method of forming a multi-level optical imager. The method includes the acts of forming a base intermediate structure over a pixel cell, forming one or more secondary intermediate structures over the base intermediate structure, and optically connecting a lens at one end of an optical channel with a light device at the other end of the optical channel. The act of forming the base intermediate structure includes forming a first dielectric material portion including a first dielectric material over the light device in the pixel cell, forming a stop layer on the first dielectric material, forming a via through the stop layer and the first dielectric material to the light device, filling the via with a second dielectric material to form a first optical channel section that exhibits a refractive index greater than the refractive index exhibited by the first dielectric material, and planarizing the second dielectric material with a surface of the stop layer. The act of forming one or more secondary intermediate structures over the base intermediate structure includes patterning a metallized portion over the base intermediate structure, forming a second dielectric material portion including the first dielectric material over the metallized portion, forming a stop layer over the second dielectric material portion, forming a via through the stop layer and the second dielectric material portion, filling the via with the second dielectric material to form a second optical channel section in alignment with the first optical channel section, and planarizing the second dielectric material portion with a surface of the stop layer. The second optical channel section exhibits a refractive index greater than the refractive index exhibited by the second dielectric material. 
     Other embodiments of the present invention provide an optical imager including a lens, a light device, a first dielectric material having a via, and an optical plug introduced in the via of the first dielectric material. The optical plug serves as an optical channel optically connecting the lens with the light device. 
     Other embodiments of the present invention provide a multi-level optical imager that includes a first intermediate structure, a second intermediate structure, a lens and a light device. The first intermediate structure includes a first dielectric material portion, a first stop layer over the first dielectric material portion, and a first optical channel section extending through the first dielectric material portion and the first stop layer. The first optical channel section includes an optical plug. The second intermediate structure includes a second dielectric material portion, a second stop layer over the second dielectric material portion, and a second optical channel section extending through the second dielectric material portion and the second stop layer to the optical plug. The first intermediate structure is mounted over the light device The first and second optical channel sections are aligned with each other to form an optical channel optically connecting the lens with the light device. 
     Other embodiments of the present invention also provide a method of forming a multi-level optical imager that includes the acts of forming a first intermediate structure over a light device of a pixel cell, forming a second intermediate structure over the first intermediate structure, and optically connecting a lens with the light device. The act of forming a first intermediate structure includes forming a first dielectric material portion over the pixel cell, forming a first stop layer over the first dielectric material portion, forming a first via through the first stop layer and the first dielectric material portion to the light device, and plugging the first via with an optical plug to form a first optical channel section that exhibits a refractive index greater than the refractive index exhibited by the first dielectric material portion. The act of forming a second intermediate structure includes patterning a metallized portion on the first intermediate structure, forming a second dielectric material portion over the metallized portion, forming a second stop layer over the second dielectric material portion, forming a second via through the second stop layer and the second dielectric material portion, wherein the second via is aligned with the first optical channel section, filling the second via with a second dielectric material to form a second optical channel section that exhibits a refractive index greater than the refractive index exhibited by the second dielectric material portion, and planarizing the second dielectric material with a surface of the second stop layer. 
     The inventions can be used to not only channel light to a pixel cell of a digital imager, but also to channel emitted light from pixel cells of a display device. These and other features of the invention will be more readily understood from the following detailed description of the invention which is provided in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a view of a single metallized level patterned within a low refractive index material in accordance with a first exemplary embodiment of the invention. 
         FIG. 2  is a view of the low refractive index material of  FIG. 1  capped with a stop layer. 
         FIG. 3  is a view of the low refractive index material of  FIG. 1  with a via extending therethrough. 
         FIG. 4  is a view of a higher refractive index material capping the low refractive index material of  FIG. 1 . 
         FIG. 5  is a view of the low refractive index material of  FIG. 1  with the higher refractive index material polished down to the stop layer. 
         FIG. 6  is a view of a multi-level metal optical imager including the single metallized level of  FIG. 1 . 
         FIG. 7  illustrates a process of forming The multi-level metal optical imager of  FIG. 6 . 
         FIG. 8  is a view of an intermediate structure having a low refractive index material and a higher refractive index optical plug in accordance with a second exemplary embodiment of the invention. 
         FIG. 9  is a view of a multi-level optical imager including the intermediate structure of  FIG. 8 . 
         FIG. 10  illustrates a process of forming the multi-level metal optical imager of  FIG. 9 . 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The various embodiments described herein illustrate the inventions in the context of channeling exterior light onto a pixel cell photosensor of a digital imager. However, the invention also can be used with a digital display device having light emitting pixel cells to channel light from the pixels cells to the exterior of the display device. 
       FIGS. 1–6  illustrate the formation of a multi-level metal optical imager, and  FIG. 7  describes the process of forming such an imager. The process described with reference to  FIGS. 1–7  can be used in the manufacture of, for example, CMOS imagers charge. coupled devices (CCDs), photodiode arrays, charge injection devices, hybrid focal plane arrays, and other types of digital imagers, as well as display devices. At an initial Step  300  ( FIG. 7 ), a low refractive index dielectric material  12  is deposited over a light device  22  formed in a pixel cell  23  ( FIG. 1 ) of a substrate. For clarity of illustration, only the light device  22  of the pixel cell  23  is illustrated. It should be appreciated that the light device  22  may be a photoconversion device (such as a photodiode or a photogate) for a digital imager, or the light device  22  may be a light emitting device for a digital display device. 
     The dielectric material  12  may be planarized through any suitable process, such as a chemical mechanical polishing process. The dielectric material  12  is preferably a material exhibiting a low dielectric constant (K), meaning one with a dielectric constant no greater than about 4.0. Suitable examples of a material for forming a low dielectric K, low refractive index material  12  are optically transparent materials having a low refractive index that are compatible with semiconductor processing, such as, for example, HOSP™ (K of 2.5), GX-3™ (K of 2.65), and NANOGLASS® (K of 2.2), all marketed by Honeywell. However, it should be appreciated that higher dielectric constant materials (those with K greater than about 4.0) may also be suitable as the dielectric material  12 . 
     Next, at Step  305 , a polish stop layer  14  is formed on the dielectric material  12  ( FIG. 2 ). The polish stop layer  14  is formed of a material which is adapted to inhibit chemical-mechanical polishing, such as, for example, silicon nitride. The polish stop layer  14  includes a surface  15 . At Step  310 , a via  16  is formed through the polish stop layer  14  and the dielectric material  12  ( FIG. 3 ) down to the light device  22 . Preferably, the via  16  is formed by the use of a mask and an etching material. Specifically, the via  16  is first patterned with a mask and then etched into the polish stop layer  14  and the dielectric material  12 . The polish stop layer  14  serves to prevent the etching material from etching too much laterally into the dielectric material  12 . 
     At Step  315 , a second dielectric material  18  is formed over the polish stop layer  14  ( FIG. 4 ). Preferably, the second dielectric material  18  is deposited on the surface  15  of the polish stop layer  14 , allowing the second dielectric material  18  to fill the via  16 . The second dielectric material  18  has a higher refractive index than the dielectric material  12 . Examples of suitable high refractive index materials include silicon dioxide, silicon nitride, silicates, phosphosilicates, methylsiloxanes, and organic polymers, and other optically transparent materials compatible with semiconductor processing, where the refractive indices range from 1.39 to 1.83. It is important that the ratio of the refractive indices of the first and second dielectric materials  12  and  18  be sufficient to cause total internal reflection. The equation for total internal reflection is:
 
sin (θ)= N   1   /N   2 
 
where θ is the critical angle, N 1  is the material having the higher refractive index (namely the second dielectric material  18 ), and N 2  is the material with the lower refractive index (the first dielectric material  12 ).
 
     Table 1 provides a non-exclusive selection of suitable N 1  and N 2  materials 
     
       
         
               
               
               
               
               
             
           
               
                 1 
               
               
                   
               
               
                 Refractive 
                 Refractive 
                   
                   
                   
               
               
                 Index for N 1   
                 Index for N 2   
                 Critical 
                 N 1  Dielectric 
                 N 2  Dielectric 
               
               
                 Material 
                 Material 
                 Angle 
                 Constant 
                 Constant 
               
               
                   
               
             
             
               
                 1.63 
                 1.43 (Silicon 
                 60 
                 about 10 
                 4.5 
               
               
                 Alumina 
                 dioxide 
               
               
                 1.43 (Silicon 
                 1.39 
                 76 
                 4.5 
                 Variable 
               
               
                 dioxide 
                 (Oxysilane 
               
               
                 1.83 (Silicon 
                 1.43 (Silicon 
                 51 
                 7.5 
                 4.5 
               
               
                 nitride 
                 dioxide 
               
               
                   
               
             
          
         
       
     
     At Step  320 , the second dielectric material  18  is planarized down to the surface  15  of the polish stop layer  14 , removing the second dielectric material  18  from everywhere except within the via  16 . The structure thus formed is a base intermediate structure  24  ( FIG. 5 ). Next, at Step  325 , one or more secondary intermediate structures are built or stacked on top of the base intermediate structure  24  and each other to form an optical channel  28  ( FIG. 6 ). As shown in  FIG. 6 , two secondary intermediate structures  124  and  224  are shown to have been built on the intermediate structure  24  to form the optical channel  28  including optical channel sections  26 ,  126 ,  226 . The secondary intermediate structures  124  and  224 , which respectively include polish stop layers  114  and  214 , are built in a similar manner as the base intermediate structure  24 . However, the dielectric materials  112  and  212  each also isolate patterned metallization portions  10  which may be provided over the polish stop layers  14  and  114 . 
     It should be appreciated that the dielectric materials  112  and  212  may be formed of the same or a different material than the dielectric material  12 . The limit on the number of intermediate structures, and hence, the number of levels of metallization is controlled by the sensitivity of the transistor/amplifier combination of the pixel cell  23  and the absorption of the dielectric material used as the optical channel  28 . 
     To ascertain whether there is total internal reflection, the ratio of refractive indices of each material in each intermediate structure is checked. Thus, the aforementioned equation becomes:
 
sin (θ A )= N   26   /N   12 
 
sin (θ B )= N   126   /N   112 
 
sin (θ C )= N   226   /N   212 .
 
It should be appreciated that, while it is preferable in certain instances for the critical angle θ A  to equal the critical angles θ B  and θ C , there may be reasons why the critical angles differ. Obviously, if the dielectric materials  112  and  212  are made of a different material than the dielectric material  12 , then the critical angles θ B  and θ C  likely will be different than the critical angle θ A .
 
     In an alternative aspect of the process illustrated in  FIG. 7 , Step  325  (stacking or building the intermediate structures  24 ,  124 ,  224  to form the optical channel  28 ) can immediately follow Step  305  (positioning the stop layers  14 ,  114 ,  214  over, respectively, the dielectric materials  12 ,  112 ,  212 ) and then a via can be formed (Step  310 ), such as by etching or drilling, through all the intermediate structures  24 ,  124 ,  224  down to the light device  22 . Afterward, the via can be filled with the higher refractive index material (Step  315 ) to form the optical channel  28 . 
     At Step  330 , a lens  20  is formed at one end of the optical channel  28  and optically connected through the optical channel  28  with the light device  22 , which may be a photodiode or photogate. The optical channel  28  acts as an optical pathway between the lens  20  and the light device  22 . The structure thus formed has a plurality of metallized layers, two being shown in  FIG. 6 , with an optical channel connection between the lens  20  and the light device  22 . The structure illustrated in  FIG. 6  includes three intermediate structures ( 24 ,  124 ,  224 ) with first, second and third dielectric material portions  12 ,  112 ,  212  surrounding the optical channel  28  which has a greater refractive index than the dielectric material surrounding it. It should be appreciated that the exemplary structure illustrated in  FIG. 6  may have fewer or more than the three intermediate structures  24 ,  124 ,  224  shown. 
     Next will be described with reference to  FIGS. 8–10  a second exemplary embodiment of the invention. The second exemplary embodiment follows the process illustrated and described with reference to  FIGS. 1–7  through Step  310 . Specifically, a single low refractive index dielectric material  12  is capped off with a polish stop layer  14  and a via  16  is etched therethrough. At this point, the second exemplary embodiment diverges from the previously described process by depositing a high refractive index plug  426  within the via  16  at Step  515  ( FIG. 10 ) to form a base intermediate structure  424  ( FIG. 8 ). The high refractive index plug  426  has a higher refractive index than the material used to form the optical channel sections  126 ,  226 . Thereafter, the fabrication process proceeds similarly as the process described above with respect to the  FIGS. 1–7 . Specifically, one or more intermediate layers  124 ,  224  are built over the base intermediate structure  424 . The exemplary structure illustrated in  FIG. 9  stacks two such layers  124 ,  224 , but fewer or more may be employed. 
     The plug  426  should have a refractive index greater than the refractive indices of both the dielectric material  12  and the optical channel sections  126  and  226 . Further, the ratio of the respective refractive indices of the plug  426  to the optical channel section  226  and the dielectric material  12  should be sufficient to cause total internal reflection. Preferably, the plug  426  is formed of a material which exhibits a refractive index similar to the photoconversion device  22 , which serves to effectively move the interface with photoconversion device  22  upward. Preferable materials from which the plug  426  is formed include aluminum oxide (Al 2 O 3 ), silicon nitride (Si 3 N 4 ), and silicon nitride hydrogen (Si 3 N 4 :H). 
     As noted, the intermediate layers  124  and  224  are built on the intermediate structure  424  in the manner described previously, and the lens  20  and photoconversion device  22  are optically connected through the thus created optical channel  428  (which includes the optical channel sections  126  and  226  and the plug  426 ) to form a multi-level metal optical imager  230 . Since the plug  426  effectively moves the interface of the photoconversion device  22  upwards the length of one of the intermediate structures, a multi-level metal optical imager  230  can be constructed which includes the plug  426  and a plurality of metallized layers which may exceed two such layers. 
     While the invention has been described in detail in connection with exemplary embodiments known at the time, it should be readily understood that the invention is not limited to such disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. For example, it should be appreciated that the dielectric material layers  12 ,  112 ,  212  all may be formed of the same material or formed of different materials. Also, it should be appreciated that while the polish stop layers  14 ,  114 ,  214  have all been described as being formed of silicon nitride, some or all of the polish stop layers  14 ,  114 ,  214  may instead be formed of another material capable of inhibiting chemical-mechanical polishing. Alternatively, the polish stop layers  14 ,  114 ,  214  may be omitted entirely. In addition, although the various embodiments of the invention are described with respect to channeling exterior light onto pixel cells of a digital imaging device, the various embodiments would also be used with light emitting devices of display devices to channel light from the light emitting devices to the exterior of the display device. Accordingly, the invention is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims.