Abstract:
A photo sensing structure and methods for forming the same. The structure includes (a) a semiconductor substrate and (b) a photo collection region on the semiconductor substrate. The structure also includes a funneled light pipe on top of the photo collection region. The funneled light pipe includes (i) a bottom cylindrical portion on top of the photo collection region of the photo collection region, and (ii) a funneled portion which has a tapered shape and is on top and in direct physical contact with the bottom cylindrical portion. The structure further includes a color filter region on top of the funneled light pipe.

Description:
BACKGROUNG OF THE INVENTION 
   1. Technical Field 
   The present invention relates to pixel sensors, and more specifically, to pixel sensors that have funneled light pipes. 
   2. Related Art 
   Some advanced pixel sensors implement vertical light pipes with micro-lenses, wherein the micro-lenses are used to focus light into the light pipes. Therefore, there is a need for a pixel sensor structure that does not have the micro-lenses of the prior art. 
   SUMMARY OF THE INVENTION 
   The present invention provides a pixel sensor structure, comprising (a) a semiconductor substrate; (b) a photo collection region on the semiconductor substrate; and (c) a funneled light pipe on top of the photo collection region, wherein the funneled light pipe comprises (i) a bottom cylindrical portion on top of the photo collection region of the photo collection region, and (ii) a funneled portion which has a tapered shape and is on top and in direct physical contact with the bottom cylindrical portion. 
   The present invention also provides a semiconductor structure fabrication method, comprising providing a structure that includes (i) a semiconductor substrate, (ii) a photo collection region on the semiconductor substrate, and (iii) a BEOL (Back End Of Line) layer on the photo collection region and the semiconductor substrate; etching the BEOL layer so as to form a funneled cavity in the BEOL layer, wherein a cross-section of the funneled cavity has a tapered shape; after said etching the BEOL layer so as to form the funneled cavity in the BEOL layer, further etching the BEOL layer through the funneled cavity so as to form a cylindrical cavity in the BEOL layer, wherein the cylindrical cavity are directly above the photo collection region and directly beneath the funneled cavity; and forming a funneled light pipe in the cylindrical cavity and the funneled cavity. 
   The present invention also provides a photo sensing structure, comprising (a) a semiconductor substrate; (b) a photo collection region on the semiconductor substrate; (c) a BEOL (Back End Of Line) layer on the semiconductor substrate and the photo collection region; and (d) a funneled light pipe on top of the photo collection region and in the BEOL layer, wherein the funneled light pipe comprises (i) a bottom cylindrical portion on top of the photo collection region of the photo collection region, (ii) a funneled portion which has a tapered shape and is on top and in direct physical contact with the bottom cylindrical portion, and (iii) a light reflective layer on side walls of the bottom cylindrical portion and the funneled portion. 
   The present invention provides a pixel sensor structure that does not have the micro-lens of the prior art. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIGS. 1A-1I  show cross-section views of a pixel sensor going through different fabrication steps of a fabrication process, in accordance with embodiments of the present invention. 
       FIGS. 2 ,  3 , and  4  show other embodiments of the pixel sensor of  FIG. 1I , in accordance with embodiments of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIGS. 1A-1I  show cross-section views of a pixel sensor  100  going through different fabrication steps of a fabrication process, in accordance with embodiments of the present invention. 
   With reference to  FIG. 1A , in one embodiment, the fabrication process starts out with a semiconductor substrate  110 . Illustratively, the semiconductor substrate  110  comprises a semiconductor material such as silicon Si, germanium Ge, etc. 
   Next, in one embodiment, four photo collection region  112   a ,  112   b ,  112   c , and  112   d  are formed on top of the semiconductor substrate  110  as shown in  FIG. 1A . Illustratively, the four photo collection region  112   a ,  112   b ,  112   c , and  112   d  are formed by using any conventional method. In one embodiment, the four photo collection region  112   a ,  112   b ,  112   c , and  112   d  are photo diodes or photo gates  112   a ,  112   b ,  112   c , and  112   d , respectively. 
   Next, with reference to  FIG. 1B , in one embodiment, a nitride layer  116  is formed on top of the semiconductor substrate  110  and the photo diodes  112   a ,  112   b ,  112   c , and  112   d . More specifically, the nitride layer  116  can be formed by CVD (Chemical Vapor Deposition) of silicon nitride on top of the structure  100  of  FIG. 1A . 
   Next, in one embodiment, a dielectric layer  122  is formed on top of the nitride layer  116 . Illustratively, the dielectric layer  122  comprises an electrically insulating material such as USG (Undoped Silicate Glass). 
   Next, in one embodiment, metal lines  124  are formed in the dielectric layer  122 . Illustratively, the metal lines  124  comprise copper, aluminum, or any other electrically conductive metal. In one embodiment, the metal lines  124  are formed by using a conventional method. 
   Next, in one embodiment, a nitride layer  126  is formed on top of the dielectric layer  122 . Illustratively, the nitride layer  126  is formed by CVD of silicon nitride on top of the dielectric layer  122 . The dielectric layer  122 , the metal lines  124 , and the nitride layer  126  are collectively referred to as an interconnect layer  120 . 
   Next, with reference to  FIG. 1C , in one embodiment, interconnect layers  130 ,  140 , and  150  similar to the interconnect layer  120  are formed in that order on top of each other to provide interconnect multi-layers  155  as shown in  FIG. 1C . The interconnect multi-layers  155  can also be referred to as a BEOL (Back End Of Line) layer  155 . In one embodiment, the formation of each of the interconnect layers  130 ,  140 , and  150  is similar to the formation of the interconnect layer  120 . In one embodiment, the nitride layers  126 ,  136 , and  146  separate the adjacent interconnect layers  120 ,  130 ,  140 , and  150 . 
   Next, with reference to  FIG. 1D , in one embodiment, a patterned photo-resist layer  160  is formed on top of the nitride layer  156 . In one embodiment, the patterned photo-resist layer  160  is formed by using a conventional lithographic process. 
   Next, with reference to  FIG. 1E , in one embodiment, the patterned photo-resist layer  160  is used as a blocking mask to etch the interconnect multi-layers  155  stopping at the nitride layer  146  to form funnels  164   a ,  164   b ,  164   c , and  164   d  in the interconnect multi-layers  155 . This etching step is represented by arrows  162  and hereafter is referred to as the etching step  162 . In one embodiment, the etching step  162  is performed isotropically such that the cross-section of each of side walls  165   a ,  165   b ,  165   c , and  165   d  of the funnels  164   a ,  164   b ,  164   c , and  164   d , respectively, has a shape of a concave hyperbola as shown in  FIG. 1E . 
   Next, with reference to  FIG. 1F , in one embodiment, the patterned photo-resist layer  160  is used as a blocking mask to further etch through the interconnect multi-layers  155  stopping at the nitride layer  116  to form cavities  168   a ,  168   b ,  168   c , and  168   d . This etching step is represented by arrows  166  and hereafter is referred to as the etching step  166 . In one embodiment, the etching step  166  is an anisotropic etching process. Because the etching step  166  is anisotropic, so side walls  169   a ,  169   b ,  169   c , and  169   d  of the cavities  168   a ,  168   b ,  168   c , and  168   d , respectively, are vertical. The funnel  164   a  and the cavity  168   a  can be collectively referred to as a funneled pipe  164   a ,  168   a . Similarly, the funnel  164   b  and the cavity  168   b  can be collectively referred to as a funneled pipe  164   b ,  168   b . The funnel  164   c  and the cavity  168   c  can be collectively referred to as a funneled pipe  164   c ,  168   c . The funnel  164   d and the cavity  168   d  can be collectively referred to as a funneled pipe  164   d ,  168   d.    
   Next, in one embodiment, the patterned photo-resist layer  160  is removed by using a wet etching step, resulting in the structure  100  of  FIG. 1G . Alternatively, the patterned photo-resist layer  160  is removed by using an oxygen based plasma etch. 
   Next, with reference to  FIG. 1H , in one embodiment, the funneled pipes  164   a ,  168   a ;  164   b ,  168   b ;  164   c ,  168   c ; and  164   d ,  168   d  (in  FIG. 1G ) are filled with a transparent material so as to form funneled light pipes  170   a ,  170   b ,  170   c , and  170   d , respectively. Illustratively, the funneled light pipes  170   a ,  170   b ,  170   c , and  170   d  are formed by depositing the transparent material on top of the entire structure  100  of  FIG. 1G  (including in the funneled pipes  164   a ,  168   a ;  164   b ,  168   b ;  164   c ,  168   c  and  164   d ,  168   d ) and then polishing by a CMP (Chemical Mechanical Polishing) step to remove excessive transparent material outside the funneled pipes  164   a ,  168   a ;  164   b ,  168   b ;  164   c ,  168   c  and  164   d ,  168   d . In an alternative embodiment, the funneled pipes  164   a ,  168   a ;  164   b ,  168   b ;  164   c ,  168   c ; and  164   d ,  168   d  (in  FIG. 1G ) are filled with a spin-on photo-resist, and then the excessive photo-resist outside the funneled pipes  164   a ,  168   a ;  164   b ,  168   b ;  164   c ,  168   c  and  164   d ,  168   d  can be removed by using a standard lithographic process. In one embodiment, the spin-on photo-resist is a clear material. 
   In one embodiment, the transparent material of the funneled light pipes  170   a ,  170   b ,  170   c , and  170   d  has a refractive index: (a) which is higher than the refractive index of the material of the dielectric layers  122 ,  132 ,  142 , and  152  surrounding the funneled light pipes  170   a ,  170   b ,  170   c , and  170   d , and (b) but which is lower than the refractive index of the material of the nitride layer  116  above the photo diodes  112   a ,  112   b ,  112   c , and  112   d . In one embodiment, the transparent material of the funneled light pipes  170   a ,  170   b ,  170   c , and  170   d  can be BPSG (boro-phospho-silicate glass), or silicon nitride. 
   In an alternative embodiment, the side walls  165   a ,  165   b ,  165   c ,  165   d ,  169   a ,  169   b ,  169   c , and  169   d  of the funneled pipes  164   a ,  168   a ;  164   b ,  168   b ;  164   c ,  168   c  and  164   d ,  168   d  are coated with a light reflective material (such as aluminum) so as to form a light reflective layer (not shown) before the funneled light pipes  170   a ,  170   b ,  170   c , and  170   d  are formed as described above. More specifically, the aluminum layer is formed by depositing aluminum on top of the entire structure  100  of  FIG. 1G  (including on the side walls  165   a ,  165   b ,  165   c ,  165   d ,  169   a ,  169   b ,  169   c , and  169   d  of the funneled pipes  164   a ,  168   a ;  164   b ,  168   b ;  164   c ,  168   c  and  164   d ,  168   d ) by CVD and then etching back to remove excessive aluminum outside the funneled pipes  164   a ,  168   a ;  164   b ,  168   b ;  164   c ,  168   c  and  164   d ,  168   d . As a result, the aluminum layer remains on the side walls  165   a ,  165   b ,  165   c ,  165   d ,  169   a ,  169   b ,  169   c , and  169   d  after the etching step. In this alternative embodiment, because of the aluminum layer on the side walls  165   a ,  165   b ,  165   c ,  165   d ,  169   a ,  169   b ,  169   c , the refractive index of the transparent material does not need to be higher than the refractive index of the material of the dielectric layers  122 ,  132 ,  142 , and  152 . 
   In yet another alternative embodiment, the side walls  165   a ,  165   b ,  165   c ,  165   d ,  169   a ,  169   b ,  169   c , and  169   d  of the funneled pipes  164   a ,  168   a ;  164   b ,  168   b ;  164   c ,  168   c  and  164   d , 168   d  can be first coated with a nitride film (not shown) so as to form a “cladding” and then an oxide material or a clear polymer can be used to fill the funneled pipes  164   a ,  168   a ;  164   b ,  168   b ;  164   c ,  168   c  and  164   d ,  168   d  as described above. 
   Next, with reference to  FIG. 1I , in one embodiment, CFA (Color Filter Array) regions  180   a ,  180   b ,  180   c , and  180   d  are formed on top of the funneled light pipes  170   a ,  170   b ,  170   c , and  170   d , respectively. More specifically, the CFA regions  180   a  and  180   c  comprise a green color filter material that allows only green photons to pass through it. The CFA region  180   b  comprises a blue color filter material that allows only blue photons to pass through it. The CFA region  180   d  comprises a red color filter material that allows only red photons to pass through it. In one embodiment, the CFA regions  180   a ,  180   b ,  180   c , and  180   d  are formed as follows. First, the green CFA regions  180   a  and  180   c  are formed by using any conventional method. Then, in a similar manner, the blue CFA region  180   b  and the red CFA region  180   d  are formed in turn. The resulting structure  100  is shown in  FIG. 1I . It should be noted that the green, red, blue colors are used for illustration only and other colors can be used. In one embodiment, the arrangement of the CFA regions  180   a ,  180   b ,  180   c , and  180   d  can be different. 
   In one embodiment, the operation of the pixel sensor  100  of  FIG. 1I  is as follows. Assume that a light beam (not shown) which comprises blue, red, and green photons is incident on the surface  186  of the structure  100  of  FIG. 1I . The CFA regions  180   a ,  180   b ,  180   c , and  180   d  ensure that only green photons pass through the green CFA regions  180   a  and  180   c , only blue photons pass through the blue CFA region  180   b , and only red photons pass through the red CFA region  180   d . FIG.  1 I′ shows paths of photons in the funneled light pipe  170   a  of  FIG. 1I , for illustration. With reference to FIGS.  1 I and  1 I′, some of the green photons that pass through the CFA region  180   a  (like photon  182 ) will travel down the funneled light pipe  170   a  to arrive at the photo diode  112   a  without hitting the side walls  165   a  and  169   a  of the funneled light pipe  170   a . Some others of the green photons that pass through the CFA region  180   a  (like a photon  184 ) will hit the side walls  165   a  and  169   a  at possibly different incident angles. In the representative case of the photon  184 , the photon  184  travels along a path i and hits the side wall  165   a  at an incident angle θ (θ is the angle between the path i and an imaginary line n, called a normal line, that is perpendicular to the side wall  165   a  at the incident point of the photon  184 ). If the incident angle θ of the photon  184  is less than a critical angle θ 0  (not shown), the photon  184  will refract into the BEOL layer  155 . The critical angle θ 0  is determined by the mathematical formula: 
             θ   0     =       sin     -   1       ⁡     (       n     dielectric   ⁢           ⁢   material         n     transparent   ⁢           ⁢   material         )             
wherein n dielectric material  is refractive index of the material of the dielectric layers  122 ,  132 ,  142 , and  152  and n transparent material  is refractive index of the transparent material of the funneled light pipe  170   a . If the incident angle θ of the photon  184  is greater than the critical angle θ 0 , the photon  184  will bounce back (i.e., reflect) into the funneled light pipe  170   a . Then, the photon  184  can travel down the funneled light pipe  170   a  and arrive at the photo diode  112   a , or hit the side walls  165   a  and  169   a  one or more times at possibly different incident angles (not shown). If these incident angles are also greater than the critical angle θ 0 , the photon  184  will travel down the funneled light pipe  170   a  and arrive at the photo diode  112   a . The greater n transparent material  is, the smaller the critical angle θ 0  is, and therefore, the more green photons (like the photon  184 ) that arrive at the photo diode  112   a . Blue photons of the light beam that pass through the blue CFA region  180   b  will travel down along the funneled light pipe  170   b  and reach the photo diodes  112   b  in a similar manner. Red photons of the light beam that pass through the red CFA region  180   d  will travel down along the funneled light pipe  170   d  and reach the photo diodes  112   d  in a similar manner. As a result, the greater n transparent material  is, the more photons of the light beam that arrive at the photo diodes  112   a ,  112   b ,  112   c  and  112   d . It should be noted that the description above is for the case where there is no light reflective coating layer on side walls  165   a ,  165   b ,  165   c ,  165   d ,  169   a ,  169   b ,  169   c , and  169   d . If the side walls  165   a ,  165   b ,  169   a ,  169   b ,  169   c , and  169   d  of the funneled pipes  164   a ,  168   a ;  164   b ,  168   b ;  164   c ,  168   c  and  164   d ,  168   d  are coated with the light reflective material (such as aluminum) and then filled with the transparent material as describe above with reference to  FIG. 1H , the photon  184  will reflect back regardless of the incident angle θ.
 
     FIG. 2  shows a cross-section view of a pixel sensor  200 , in accordance with embodiments of the present invention. In one embodiment, the pixel sensor  200  is similar to the pixel sensor  100  of  FIG. 1I , except that the cross-section of each of side walls  265   a ,  265   b ,  265   c , and  265   d  of the funnels  264   a ,  264   b ,  264   c , and  264   d  has a shape of a convex hyperbola as shown in  FIG. 2  (as opposed to the concave hyperbolic shape of the side walls  165   a ,  165   b ,  165   c , and  165   d  of the funnels  164   a ,  164   b ,  164   c , and  164   d , respectively, as shown in  FIG. 1I ). For simplicity, similar regions and layers will have the same reference numeral. In one embodiment, the convex hyperbolic side walls  265   a ,  265   b ,  265   c , and  265   d  of the funnels  264   a ,  264   b ,  264   c , and  264   d , respectively, are formed by etching with a changing component of chemical substance or another chemical substance. In one embodiment, the convex hyperbolic side walls  265   a ,  265   b ,  265   c , and  265   d  of the funnels  264   a ,  264   b ,  264   c , and  264   d  are formed by polymerizing RIE process (fluorocarbon chemistry with CHF3 or C4F8 for example), and then the lower portions of light pipes are formed by non-polymerizing RIE process (CF4 or CHF3/02 or C4F8/O2). 
   In one embodiment, the operation of the pixel sensor  200  is similar to the operation of the pixel sensor  100  of  FIG. 1I  as described above. More specifically, when a light beam (not shown) is incident on the surface  286  of the structure  200 , most of the photons of the light beam that pass through the CFA regions  180   a ,  180   b ,  180   c , and  180   d  will arrive at the photo diodes  112   a ,  112   b ,  112   c  and  112   d , respectively. 
     FIG. 3  shows a cross-section view of a pixel sensor  300 , in accordance with embodiments of the present invention. In one embodiment, the pixel sensor  300  is similar to the pixel sensor  100  of  FIG. 1I , except that the cross-section of each of side walls  365   a ,  365   b ,  365   c , and  365   d  of the funnels  364   a ,  364   b ,  364   c , and  364   d  is a slant straight line as shown in  FIG. 3 . In one embodiment, the straight side walls  365   a ,  365   b ,  365   c , and  365   d  of the funnels  364   a ,  364   b ,  364   c , and  364   d , respectively, are formed by etching with a changing component of chemical substance or another chemical substance. In one embodiment, the straight funnels  364   a ,  364   b ,  364   c , and  364   d  are formed by polymerizing RIE process (fluorocarbon chemistry with CHF3 or C4F8 for example), and then the lower portions of light pipes are formed by non-polymerizing RIE process (CF4 or CHF3/O2 or C4F8/O2). In one embodiment, the straight funnels  364   a ,  364   b ,  364   c , and  364   d  can also be formed by anisotropic RIE to form non-tapered light pipe (including lower portions) and followed by sputter etch (in Ar for example) to form tapered upper portions of light pipes. 
   In one embodiment, the operation of the pixel sensor  300  is similar to the operation of the pixel sensor  100  of  FIG. 1I  as described above. More specifically, when a light beam (not shown) is incident on the surface  386  of the structure  300 , most of the photons of the light beam that pass through the CFA regions  180   a ,  180   b ,  180   c , and  180   d  will arrive at the photo diodes  112   a ,  112   b ,  112   c , and  112   d , respectively. 
     FIG. 4  shows a cross-section view of a pixel sensor  400 , in accordance with embodiments of the present invention. In one embodiment, the formation of the pixel sensor  400  is similar to the formation of the structure  100  of  FIG. 1H , except for the formation of funneled light pipes  168   a ,  480   a ;  168   b ,  480   b ;  168   c ,  480   c ; and  168   d ,  480   d . More specifically, the cavities  168   a ,  168   b ,  168   c , and  168   d  of the funneled light pipes  168   a ,  480   a ;  168   b ,  480   b ;  168   c , 480   c ; and  168   d , 480   d  are filled with the transparent material which is then etched back down to the filled cavities  168   a ,  168   b ,  168   c , and  168   d . Next, in one embodiment, CFA funneled regions  480   a ,  480   b ,  480   c , and  480   d  are formed in the funnels  164   a ,  164   b ,  164   c , and  164   d , respectively, by using any conventional method, resulting in the structure  400  of  FIG. 4 . More specifically, the funnels  164   a  and  164   c  are filled with a green color filter material to form the green CFA funneled regions  480   a  and  480   c  that allow only green photons to pass through them. Then, the funnel  164   b  is filled with a blue color filter material to form the blue CFA funneled region  480   b  that allows only blue photons to pass through it. Then, the funnel  164   d  is filled with a red color filter material to form the red CFA funneled region  480   d  that allows only red photons to pass through it. 
   In one embodiment, the operation of the pixel sensor  400  of  FIG. 4  is similar to the operation of the pixel sensor  100  of  FIG. 1I . It should be noted that the CFA funneled regions  480   a ,  480   b ,  480   c , and  480   d  play two roles: (a) the role of color filter regions (similar to the role of the CFA regions  180   a ,  180   b ,  180   c , and  180   d  of  FIG. 1I ) and (b) the role of funneled regions (similar to the role of the filled funnels  164   a ,  164   b ,  164   c , and  164   d  of  FIG. 1I ). 
   FIG.  4 ′ shows a cross-section view of a pixel sensor  400 ′, in accordance with embodiments of the present invention. In one embodiment, the formation of the pixel sensor  400 ′ is similar to the formation of the pixel sensor  400  of  FIG. 4 , except that micro-lenses  490   a ,  490   b ,  490   c , and  490   d  are formed on top of the CFA funneled regions  480   a ,  480   b ,  480   c , and  480   d , respectively. The micro-lenses  490   a ,  490   b ,  490   c , and  490   d  are used to focus light into the CFA funneled regions  480   a ,  480   b ,  480   c , and  480   d , respectively. It should be noted that the micro-lenses  490   a ,  490   b ,  490   c , and  490   d  can be applied to all the embodiments, including with and without color filter arrays (like the CFA regions  180   a ,  180   b ,  180   c , and  180   d  of  FIG. 1I ). 
   In the embodiments described above, with reference to  FIGS. 1A-1I , there are four photo diodes  112   a ,  112   b ,  112   c , and  112   d . In general, the pixel sensor  100  can have N photo diodes, and wherein N is a positive integer. 
   In the embodiments described above, with reference to  FIG. 1E , the etching step  162  stops at the nitride layer  146  of the interconnect layer  140 . In an alternative embodiment, the etching step  162  stops before the nitride layer  146  is exposed to surrounding ambient. In yet another alternative embodiment, the etching step  162  etches through the nitride layer  146  and stops at the nitride layer  136 . In general, the etching step  162  can stop at anywhere in the interconnect multi-layers  155 . 
   In the embodiments described above, the side walls of the funnels  164   a ,  164   b ,  164   c , and  164   d  ( FIG. 1G ), the funnels  264   a ,  264   b ,  264   c , and  264   d  ( FIG. 2 ), and the funnels  364   a ,  364   b ,  364   c , and  364   d  ( FIG. 3 ) have a hyperbolic shape. Alternatively, they have a parabolic shape. 
   While particular embodiments of the present invention have been described herein for purposes of illustration, many modifications and changes will become apparent to those skilled in the art. Accordingly, the appended claims are intended to encompass all such modifications and changes as fall within the true spirit and scope of this invention.