Abstract:
An image sensor includes a substrate having photosensitive areas; an insulator spanning at least a portion of the substrate; and a first and second layer of a multi-layer metallization structure, wherein the first layer forms light shield regions over selected portions of the photosensitive area as well forming circuit interconnections and barrier regions to prevent spiking into the substrate or gates at contacts in the non-imaging area; and the second layer spanning the interconnections and barrier regions of the first layer only over the non-imaging areas and the second layer overlays edges of the first layer.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This is a divisional of application Ser. No. 11/068,358 filed Feb. 28, 2005 which is a divisional of application Ser. No. 10/833,386, filed Apr. 28, 2004 now U.S. Pat. No. 6,878,919. 
    
    
     FIELD OF THE INVENTION 
     The invention relates generally to the field of solid-state image sensors, and more particularly to the process of forming a lightshield and the interconnection layers for a solid-state image sensor. 
     BACKGROUND OF THE INVENTION 
     Image sensors are made of an array of pixels. Within each pixel, some regions are specifically designed to be photosensitive, and other regions are protected from light by a lightshield. Regions are protected from light because light absorbed in these protected regions causes degraded performance through mechanisms such as color crosstalk, smear, or reduced blooming control. 
     In U.S. Pat. No. 6,867,062 by Eric G. Stevens, a thin lightshield process is described for providing a lightshield from one of the layers of a bi-layer metallization process. The aluminum layer in this process is usually patterned with a chlorine-based plasma chemistry which leaves chlorine-containing residue on the wafers after the etch. Further, this residue may react with the aluminum or TiW, especially where the aluminum and TiW meet, causing corrosion of these films, and degradation of their electrical properties or optical light-shielding properties. In addition, U.S. Pat. No. 6,867,062 requires that the etch of the bottom layer of the bi-layer metal be masked in some regions by the top layer of the bi-layer metallization. This requirement may restrict the use of certain metals for the bi-layer metallization. 
     Consequently, a need exists for producing image sensors that overcome the above-described drawbacks. 
     SUMMARY OF THE INVENTION 
     The present invention is directed at overcoming the problems described above. The invention resides in a method for forming an image sensor comprising (a) a substrate having photosensitive areas; (b) an insulator spanning at least a portion of the substrate; and (c) a first and second layer of a multi-layer metallization structure, wherein the first layer forms light shield regions over selected portions of the photosensitive area as well forming circuit interconnections and barrier regions to prevent spiking into the substrate or gates at contacts in the non-imaging area; and the second layer spanning the interconnections and barrier regions of the first layer only over the non-imaging areas and the second layer overlays edges of the first layer. 
     The above and other objects of the present invention will become more apparent when taken in conjunction with the following description and drawings wherein identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. 
     ADVANTAGEOUS EFFECT OF THE INVENTION  
     The present invention has the advantage of a thin lightshield and an interconnect metallization layer using a process that minimizes corrosion of the aluminum and TiW layers. A second advantage is that the patterned second layer of a bi-layer metallization is not used as a mask for the etch of the first layer of the bi-layer metallization. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a side view in cross section of an image sensor of the present invention illustrating initial steps in producing the image sensor; 
         FIG. 2  is a drawing illustrating a step in the manufacturing process after  FIG. 1 ; 
         FIG. 3  is a drawing illustrating a step in the manufacturing process after  FIG. 2 ; 
         FIG. 4  is a drawing illustrating a step in the manufacturing process after  FIG. 3 ; 
         FIGS. 5   a  and  5   b  are an alternative embodiment of the present invention; 
         FIG. 6  is also an alternative embodiment of the present invention; and 
         FIG. 7  is a perspective view of a digital camera for illustrating a typical commercial embodiment to which the ordinary consumer is accustomed. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to  FIG. 1 , a typical image sensor  10  consists of an array of photosensitive elements or pixels  30  in an image area. Within each pixel are regions that are exposed to light so that an electrical signal may be created in response to the incident light. In addition, there are regions within the pixel, which are prevented from receiving light because the light will degrade the imaging performance. A typical image sensor  10  also provides dark reference pixels  32  that are insensitive to light because they are covered with a light shield. The signal from these dark reference pixels  32  are used in the signal processing portion of the camera to indicate the signal of photosensitive pixels when no light is incident upon them. In addition, interconnects are provided within the image sensor to electrically connect various parts of the imager and to provide means to connect the imager to external circuits. 
     Referring to  FIG. 1 , there is shown an initial stage of forming an image sensor  10  of the present invention. This stage includes providing a substrate  20  having a plurality of photosensitive sites  30  that convert incident light into charge packets. An insulator  40  spans and covers the substrate  20  and includes an opening  45  therethrough for forming a contact hole, and the first layer  60  of a bi-layer metallization structure is deposited on the insulator  40 . In the preferred embodiment, the first layer of the bi-layer metallization is a titanium and tungsten alloy, and a bi-layer metallization is described. However other metals or combination of metals and/or their compounds can be used. The important properties of this first layer are that it is opaque in order to be used as a light shield, and that the metal can be used as part of a bi-layer metallization process where this first layer provides a barrier preventing the interaction of the silicon substrate with the upper and more conductive layer. Other first layers may be tungsten, or tungsten silicide, or molybdenum, or molybdenum silicide. Photoresist is selectively disposed on the TiW layer (not shown) to form a mask to prevent the etching of the underlying TiW layer. The exposed regions of the TiW layer are then etched using a fluorine-based plasma etchant. 
     Referring to  FIG. 2 , there is shown the resulting cross-section after the selective etching of the titanium and tungsten alloy layer  60 , and the removal of the photoresist. The titanium and tungsten alloy  60  covers those regions of the pixel that should not be exposed to light for forming a light shield. The titanium and tungsten alloy  60  may also cover dark reference pixels  32 . In addition, the titanium and tungsten alloy  60  remains where metallization interconnects and bus lines, generally region  80 , are to be provided. The present invention includes the capability to separately pattern the titanium and tungsten alloy  60  so that it may be used as a local interconnect to electrically connect different parts of the imager that are not required to conduct high current levels, or to connect parts of the imager that are very close to each other, or other instances where the high conductivity of aluminum is not required. This local interconnect has the advantage of lower capacitive coupling to other parts of the imager and its circuitry because the total interconnect height is less than the bi-layer metallization. An example is shown in  FIG. 6 . An aluminum layer  90  (see  FIG. 4 ) will be used in combination with the titanium and tungsten alloy  60  as the interconnect for other circuits elements, as will be described hereinbelow. A floating diffusion  100  is connected via the titanium and tungsten alloy  60  to a gate of a transistor  75  that forms a portion of an image sensor output structure. 
     Referring to  FIG. 3 , after patterning the titanium and tungsten alloy  60 , a layer of aluminum or alloy of aluminum  90  such as an alloy of aluminum and silicon, or an alloy of aluminum, silicon, and copper is deposited. This aluminum alloy layer  90  covers the patterned titanium and tungsten alloy  60  and the insulator  40  where the titanium and tungsten alloy  60  have been removed. It is noted for clarity that the combination of the titanium and tungsten alloy  60  and the aluminum  90  form the bus line  80 . 
     Referring to  FIG. 4 , next, photoresist is selectively disposed spanning and covering the aluminum alloy layer  90  that is in the non-imaging areas, such as the dark reference pixels  32 , and the interconnect region or bus line  80 . This photoresist then masks a chlorine-based etch of the aluminum alloy layer  90 . The chlorine-based plasma etch selectively etches aluminum layer  90 , but does not etch the titanium and tungsten alloy  60 , nor the insulator  40 . The photoresist is then removed. The aluminum alloy  90  no longer covers the imaging area or photosensitive site  30 . The aluminum  90  does cover the titanium and tungsten alloy  60  over interconnect region  80 , and therefore forms the bi-metal interconnect wiring used for bus lines and other electrical connections. In the bi-layer structure, the aluminum alloy  90  can cover both the top and the sides of the titanium and tungsten alloy  60  so that corrosion of the interconnect wiring is minimized. In particular, it is noted that the aluminum alloy covers the edges  95  of the titanium and tungsten alloy  60 . The aluminum alloy  90  may also be patterned to cover the light shielded dark reference pixels  32  with or without the underlying titanium and tungsten alloy layer  60  if the titanium and tungsten alloy layer  60  does not provide sufficient opacity in this region. The aluminum alloy  90  does not cover the local interconnections made with the titanium and tungsten alloy  60  only. For clarity of understanding, it is noted that the titanium and tungsten alloy  60  form a barrier region to prevent intermixing between the gate region and the aluminum layer  90  and to prevent intermixing of source and drain regions with the aluminum layer  90 . The remaining steps needed for completion of a commercially usable image sensor are well known in the art and need not and will not be discussed in detail herein. 
     A second embodiment provides the same advantages, but instead of a continuous bi-layer metallization, the titanium and tungsten alloy  60  is patterned so that it is placed only in the contact holes and an overlap around the contact holes. The overlap of the contact holes ensures that the contact hole is completely covered by the titanium and tungsten layer within alignment variations of the process. The aluminum alloy alone is used for the interconnect layer in regions away from the contact hole. In this embodiment, the conductivity of the interconnect is about the same as the first embodiment, and junction spiking and electromigration at the contact holes is prevented by the barrier layer (the titanium and tungsten alloy), but reduces the thickness of the metallization interconnect over much of the device.  FIGS. 5   a  and  5   b  show an example where the titanium and tungsten alloy  60  are patterned to cover only the contact hole  105 , while the aluminum alloy  90  alone is used in regions away from the contact hole  105 . 
     Referring to  FIG. 7 , there is shown an electronic device, such as a digital camera  110 , for illustrating a typical commercial embodiment for the image sensor  10  of the present invention. 
     The invention has been described with reference to a preferred embodiment. However, it will be appreciated that variations and modifications can be effected by a person of ordinary skill in the art without departing from the scope of the invention. 
     PARTS LIST 
     
         
           10  image sensor 
           20  substrate 
           30  photosensitive elements (image area) or pixels 
           32  dark reference pixels 
           40  insulator 
           45  opening 
           60  titanium and tungsten alloy layer (1 st  layer) 
           75  transistor 
           80  interconnect region or bus line 
           90  aluminum alloy layer 
           95  edges 
           100  floating diffusion 
           105  contact hole 
           110  digital camera