Abstract:
In conventional membrane infrared (IR) sensors, little to no attention has been paid toward transmissivity of IR near metal traces. Here, because the substrate of an integrated circuit carrying the sensor is used as a visible light filter, reflection of IR radiation back into the substrate can affect the operation and reliability of the IR sensor. As a result, an arrangement is provided that reduces the area occupied by metal lines by reducing the pitch and compacting the routing so as to reduce the effects from the reflection of IR radiation by metal traces.

Description:
This Application is a Divisional of prior application Ser. No. 12/878,752, filed Sep. 9, 2010, now U.S. Pat. No. 8,436,304, issued May 7, 2013. 
    
    
     TECHNICAL FIELD 
     The invention relates generally to infrared (IR) sensors and, more particularly, to reducing IR reflectivity for an IR membrane sensor. 
     BACKGROUND 
     In conventional membrane IR sensors, little to no attention has been paid toward transmissivity of IR near metal traces. Because of the arrangement of these membrane sensors, reflection of IR was generally not a factor. However, when the sensor absorber is behind the metal traces (for example, a Wafer Level Chip Scale Package or WCSP package) the metal traces can substantially block or reflect the IR radiation away from the absorber and can affect the operation and reliability of the IR sensor. Therefore, there is a need to increase the transmissivity of a membrane IR sensor that uses the substrate as a visible light filter. 
     An example of a conventional system is PCT Publ. No. WO2010029488. 
     SUMMARY 
     A preferred embodiment of the present invention, accordingly, provides an apparatus. The apparatus comprising a substrate; a plurality of polysilicon traces formed over the substrate, wherein each polysilicon trace has a first width, and wherein the plurality of polysilicon traces occupy a first area; a recess formed in the substrate below at least a portion of each polysilicon trace; a first dielectric layer formed over the polysilicon traces; a plurality of metal traces formed over the first dielectric layer, wherein each metal trace has a second width, and wherein the second width is less than the first width, and wherein the plurality of metal traces occupy a second area, and wherein each metal trace is associated with at least one of the polysilicon traces to form a pair of electrodes for a thermopile, and wherein the horizontal spacing between adjacent metal traces is reduced so that the second area is substantially less than first area; and a second dielectric layer formed over the metal traces. 
     In accordance with a preferred embodiment of the present invention, the metal traces are formed in a single metallization layer. 
     In accordance with a preferred embodiment of the present invention, the first dielectric layer is further comprised of a plurality of dielectric layers. 
     In accordance with a preferred embodiment of the present invention, the plurality of dielectric layers are formed of silicon dioxide. 
     In accordance with a preferred embodiment of the present invention, the metal traces are formed of a plurality of metallization layers. 
     In accordance with a preferred embodiment of the present invention, the first dielectric layer is further comprised of a plurality of dielectric layers. 
     In accordance with a preferred embodiment of the present invention, the plurality of dielectric layers are formed of silicon dioxide. 
     In accordance with a preferred embodiment of the present invention, the second area is less than about three-quarters the first area. 
     In accordance with a preferred embodiment of the present invention, an apparatus is provided. The apparatus comprises a substrate; an infrared (IR) sensor formed over a first portion of the substrate, wherein the IR sensor includes a thermopile and a plurality of sections, and wherein each section includes: a plurality of polysilicon traces formed over the substrate, wherein each polysilicon trace has a first width, and wherein the plurality of polysilicon traces occupy a first area; a recess formed in the substrate below at least a portion of each polysilicon trace; a dielectric layer formed over the polysilicon traces; and a plurality of metal traces formed over the dielectric layer, wherein each metal trace has a second width, and wherein the second width is less than the first width, and wherein the plurality of metal traces occupy a second area, and wherein each metal trace is associated with at least one of the polysilicon traces to form a pair of electrodes for a thermopile, and wherein the horizontal spacing between adjacent metal traces is reduced so that the second area is substantially less than first area; a second dielectric layer formed over the metal traces; and an IR absorber formed over the second dielectric layer; and active circuitry that is formed over a second portion of the substrate, wherein the active circuitry is in communication with the IR sensor. 
     In accordance with a preferred embodiment of the present invention, an apparatus is provided. The apparatus comprises an integrated circuit (IC) including: a substrate; an IR sensor formed over a first portion of the substrate, wherein the IR sensor includes a thermopile and a plurality of sections, and wherein each section includes: a plurality of polysilicon traces formed over the substrate, wherein each polysilicon trace has a first width, and wherein the plurality of polysilicon traces occupy a first area; a recess formed in the substrate below at least a portion of each polysilicon trace; a first dielectric layer formed over the polysilicon traces; a plurality of metal traces formed over the dielectric layer, wherein each metal trace has a second width, and wherein the second width is less than the first width, and wherein the plurality of metal traces occupy a second area, and wherein each metal trace is associated with at least one of the polysilicon traces to form a pair of electrodes for a thermopile, and wherein the horizontal spacing between adjacent metal traces is reduced so that the second area is substantially less than first area; a second dielectric layer formed over the metal traces; and an IR absorber formed over the second dielectric layer; and active circuitry that is formed over a second portion of the substrate, wherein the active circuitry is in communication with the IR sensor; a plurality of solder balls that are each secured to the IC; and a circuit board having a plurality of contact pad, wherein each contact pad is secured to at least one of the solder balls. 
     The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and the specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a block diagram of an example of a system in accordance with a preferred embodiment of the present invention; 
         FIG. 2  is block diagram of an example of an integrated circuit (IC) of  FIG. 1 ; 
         FIG. 3  is an block diagram of an example of the IR sensor of  FIG. 2 ; 
         FIG. 4  is a plan view of an example of a section of the IR sensor of  FIG. 3 ; 
         FIG. 5  is a cross-sectional view section of  FIG. 4  along section line A-A; 
         FIG. 6  is a plan view of another example of a section of the IR sensor of  FIG. 3 ; and 
         FIG. 7  is a cross-sectional view section of  FIG. 6  along section line B-B. 
     
    
    
     DETAILED DESCRIPTION 
     Refer now to the drawings wherein depicted elements are, for the sake of clarity, not necessarily shown to scale and wherein like or similar elements are designated by the same reference numeral through the several views. 
     Turning to  FIG. 1 , an example of a system  100  in accordance with a preferred embodiment of the present invention can be seen. As shown, an IC  102  is secured to a circuit board  112  by a WCSP package, where solder balls  114  are secured to both the IC  102  and contact pads (not shown) on the circuit board  112 . Within IC  102  is an IR sensor  102  and active circuitry  104 , which are formed over a substrate  104 . For this sensor, the substrate  104  (which is generally formed of silicon) operates as a visible light filter, allowing IR radiation to pass to sensor  108 . 
     In  FIG. 2 , a plan view of an example of IC  102  can be seen. As shown, the sensor  108  is located substantially at the center of IC  102 . Surrounding the sensor  108  is the active circuitry  106 , which is in communication with the sensor  108 . This active circuitry  106  extends over the solder balls  114 , where communication can be established with an underlying circuit board (i.e., circuit board  112 ). Additionally, there is an IR filter  109  formed over the IR sensor  102 . 
     Looking to  FIG. 3 , a plan view of an example of IR sensor  108  can be seen. In this view, all of the layers of the IR sensor  108  are shown for the sake of illustration. As shown, the IR sensor  108  is substantially surrounded by an isolation ring  120  that separates the sensor  108  from the surrounding active circuitry  106 . Generally, IR sensor  108  operates based on the Peltier effect, where a “hot junction” is formed over the center of sensor  108  and a “cold junction” is formed along the perimeter of the sensor  108 . To do this, a dielectric stack  122  is formed over a portion of the center of the sensor  108 , where openings (i.e., opening  114 ) are used to etch away a portion of the substrate underlying the dielectric stack  122 . Electrodes  116 , then, couple the “hot junction” and “cold junction” together forming a portion of a thermopile as shown for section  118 . 
     Turning to  FIGS. 4 and 5 , an example of section  118  (referred to as  118 -A) can be seen in greater detail. As with  FIG. 3 , the layers of section  118 -A can be seen for the purposes of illustration. As shown, the electrodes are generally comprised of polysilicon traces  202 - 1  to  202 - 11  and metal traces  204 - 1  to  204 - 11 , where each metal trace  204 - 1  to  204 - 11  is associated with polysilicon trace  202 - 1  to  202 - 11 , respectively. The polysilicon traces  202 - 1  to  202 - 11  are generally parallel to one another over substrate  104  with recess or cavity  208  being formed under at least a portion of each polysilicon trace  202 - 1  to  202 - 11 . The polysilicon traces  202 - 1  to  202 - 11  (which are each about 1.2 μm wide) occupy a generally rectangular area (which is typically between 360 μm 2  to about 1755 μm 2 ). The pitch and routing of the metal traces  204 - 1  to  204 - 11  (which each have a width of about 0.4 μm wide) is reduced so that the area occupied is substantially less than the area occupied by its corresponding polysilicon traces  202 - 1  to  202 - 11 . For example, the area occupied by the metal traces  204 - 1  to  204 - 11  is about 283 μm 2 , which is about three-quarters of the area occupied by polysilicon traces  202 - 1  to  202 - 11 . As the polysilicon traces  202 - 1  to  202 - 11  and metal traces  204 - 1  to  204 - 11  become longer, the area occupied by metal traces  204 - 1  to  204 - 11  becomes increasingly smaller as compared to the area occupied by polysilicon traces  202 - 1  to  202 - 11 . Additionally, as shown metal traces  202 - 1  to  202 - 11  are separated from polysilicon traces  204 - 1  to  201 - 11  by a dielectric (such as silicon dioxide), which may be formed of one or more layers. An IR absorber  109  covers the thermopile  118 . 
     Turning to  FIGS. 6 and 7 , another example of section  118  (referred to as  118 -B) can be seen. Here, the metal traces  304 - 1  to  304 - 15  are formed of metallization layers  306 - 1  to  306 - 4  and are routed over top of one another, being separated from one another by a dielectric  120  (i.e., silicon dioxide) that can be formed in one or more layers. Because multiple metallization layers  306 - 1  to  306 - 15  have been used, the area occupied by metal layers  304 - 1  to  304 - 15  is much less than metal layers  204 - 1  to  204 - 11  (which use one metallization layer). In this example, the area occupied by metal layers  304 - 1  to  304 - 15  is less than half of the area occupied by polysilicon traces  302 - 1  to  302 - 15 . 
     By using this reduced metal pitch and routing, the overall reflective area of metal traces (i.e.,  204 - 1  to  204 - 11 ) can be greatly reduced. As an example, the total reflective area for the IR sensor  108  can be reduced by as much as 50%, improving the responsivity of the IR sensor  108 . 
     Having thus described the present invention by reference to certain of its preferred embodiments, it is noted that the embodiments disclosed are illustrative rather than limiting in nature and that a wide range of variations, modifications, changes, and substitutions are contemplated in the foregoing disclosure and, in some instances, some features of the present invention may be employed without a corresponding use of the other features. Accordingly, it is appropriate that the appended claims be construed broadly and in a manner consistent with the scope of the invention.