Abstract:
A method of creating a lateral overflow drain, anti-blooming structure in a charge coupled device, the method includes steps for self-aligning a peripheral edge of the lateral overflow drain to an edge of the thick field oxide, whereby the overflow drain is substantially covered by the field oxide.

Description:
FIELD OF THE INVENTION 
     The invention relates generally to the field of image sensors and, more particularly, to such image sensors having the edge of a lateral overflow drain self aligned to a gate of the field oxide layer. 
     BACKGROUND OF THE INVENTION 
     Currently known and utilized full frame image sensors, such as in U.S. Pat. No. 5,130,774, include lateral overflow drains for preventing blooming, as is well known in the art. These drains are typically formed underneath the CCD gate electrodes and are limited in performance by surface breakdown. This is because, as the dose of the drain is increased to improve its conductivity, it&#39;s breakdown voltage drops. Therefore, there is a maximum dose and correspondent maximum amount of drain-limited blooming protection that can be provided for a given minimum tolerable breakdown voltage for any given device process. 
     To avoid this surface breakdown limitation, the drain can be placed underneath the thick field oxide that is typically used for channel-to-channel isolation between the vertical CCDs of these devices as described in U.S. patent application Ser. No. 09/945,034, A LATERAL OVERFLOW DRAIN, ANTI-BLOOMING STRUCTURE FOR CCD DEVICES HAVING IMPROVED BREAKDOWN VOLTAGE, by Edmund K Banghart and Eric G. Stevens. By placing the field oxide layer over the lateral overflow drain, the surface electric field is reduced in inverse proportion to the thickness. In implementing such a device, the lateral overflow drain should be aligned to the edge of the field oxide later. If the drain is not fully covered by the field oxide later, its breakdown voltage will be reduced and limited by the portion of the drain that protrudes out beneath the thinner gate dielectric. If, on the other hand, the drain is placed too far underneath the field oxide layer, connection to it via the buried channel may be lost. This is because the buried channel is typically self-aligned to the field oxide edge by implanting it after the field oxide growth. This may render the structure nonfunctional. Although the latter limitation could be eliminated by implanting the buried channel prior to the growth of the field oxide layer, the former problem would still exist. 
     Consequently, a need exists for overcoming the above-described shortcomings by providing a process wherein the lateral overflow drain is underneath and self aligned to one edge of the field oxide layer. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to overcoming one or more of the problems set forth above. Briefly summarized, according to one aspect of the present invention, the invention resides in a method for creating a lateral overflow drain, anti-blooming structure in a charge coupled device, the method comprising the steps of (a) providing a substrate of a first conductivity type for the charge coupled device; (b) providing a layer of oxide abutting the substrate; (c) providing a layer of nitride abutting the oxide; (d) providing a hard mask abutting the nitride with an etched away portion having a dimension which substantially equals a combined dimension of heavily doped first and second conductivity type subsequently implanted regions in the substrate; (e) placing photoresist in a portion of the etched away portion which remaining etched away portion includes a dimension substantially equal to the first conductivity type subsequently implanted region in the substrate; (f) implanting ions of the first conductivity type through the remaining etched away portion and into the substrate for creating a channel stop; (g) removing the photoresist and placing a second photoresist layer in a portion of the etched away portion wherein a remaining etched away portion includes a dimension substantially equal to the second conductivity type subsequently implanted region in the substrate and wherein the remaining etched away portion is adjacent the implanted channel stop; (h) implanting ions of the second conductivity type through the remaining etched away portion and into the substrate for forming the lateral overflow drain; (i) etching a portion of the nitride so that a peripheral portion of the etched away portion is substantially aligned with a peripheral portion of the second conductivity type; (j) growing a thick field oxide in the etched away portion of the nitride layer so that the lateral overflow drain is covered by the thick field oxide 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 following advantage of self-aligning a peripheral edge of the lateral overflow drain to an edge of the field oxide, whereby the overflow drain is substantially fully covered by the field oxide. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a view in cross section of an initial stage in the process of creating an image sensor of the present invention; 
     FIG. 2 is a view in cross section of a subsequent stage of FIG. 1; 
     FIG. 3 is a view in cross section of a subsequent stage of FIG. 2, 
     FIG. 4 is a view in cross section of a subsequent stage of FIG. 3; 
     FIG. 5 is a view in cross section of a subsequent stage of FIG. 4; 
     FIG. 6 is a view in cross section of a subsequent stage of FIG. 5; 
     FIG. 7 is a view in cross section of a subsequent stage of FIG. 6; 
     FIG. 8 is a view in cross section of a subsequent stage of FIG. 7; and 
     FIG. 9 is a view in cross section of the final stage of creating the image sensor of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIG. 1, there is shown a cross sectional view of the initial phase of creating a full-frame image sensor  10  of the present invention. The image sensor  10  includes a substrate  20  having a 300 angstrom thick (for example) layer of oxide  30  thereon, and a 300 angstrom thick (for example) layer of nitride  40  deposited on the oxide layer. A hard mask  50 , preferably low temperature oxide although any suitable substitute may be used, is placed over the nitride  40 , and a first, removable layer of photoresist  60  is deposited on the hard mask  50 . 
     In going from FIG. 1 to FIG. 2, the photoresist  60  is patterned and the hard mask  50  is etched with an opening  70  whose width W lod +W chst  substantially equals the combined width of the to-be-implanted channel stop and lateral overflow drain. The remaining photoresist  60  is then removed. 
     Referring to FIG. 3, a second, removable layer of photoresist  80  is deposited on a portion of the hard mask  50  and in a portion of the etched-out area  70  and atop the nitride  40 . As may be apparent to those skilled in the art, the thickness of the photoresist  80  and hard mask  50  are of sufficient thickness to block the subsequent implants. The dimension W chst  is substantially equal to the width of the heavily doped, p-type channel stop  90  which is then implanted in the substrate  20 . 
     Referring to FIG. 4, the second layer of photoresist  80  is removed and a third, removable layer of photoresist  100  is patterned over the hard mask  50  which is adjacent the channel stop  90  and in the etched-out portion  70  over the channel stop  90 . The lateral overflow drain  110  is then implanted in the substrate  20  via a heavily doped n type conductivity. 
     Referring to FIG. 5, the photoresist  100  is removed, and the nitride layer  40  is etched so that the channel stop  90  and lateral overflow drain  110  are exposed via opening  75 . Those skilled in the art may recognize that the nitride layer  40  may be etched in a previous stage of the process, the etching at this stage is only the preferred embodiment. It is instructive to note that a peripheral portion of the etched away portion is substantially aligned with a peripheral portion of the lateral overflow drain. Referring briefly to FIG. 6, the hard mask  70  is removed. 
     In going from FIG. 6 to FIG. 7, a thick field oxide  120  is grown in the opening  75  of the nitride layer  40  on top of substrate  20 . As used herein thick field oxide means a field oxide layer that is thicker than the ONO layers. At the same time as the field oxide is grown, a top-gate oxide layer  130  is grown on nitride layer  40 , thereby completing the oxide-nitride-oxide (ONO) gate dielectric stack. Referring to FIG. 8, a buried channel (n type conductivity)  140  is implanted and well-known techniques are used for completing the image sensor  10  as shown in FIG.  9 . As can be readily seen, the completed image sensor  10  includes a barrier (p type conductivity)  150  for forming a barrier over which the overflow charges will flow, and a polysilicon top layer gate electrode  160 . 
     The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention. 
     For example, although this invention has been described using a p-type silicon substrate and channel stops with an n-type buried channel and overflow drain, an n-type substrate could be used by using the opposite conductivity types for the various other implants. Also, other gate electrode materials, such as indium-tin oxide, could be used. It is also to be understood that, although the drawings showing only one CCD and its corresponding lateral, there are a plurality of such CCDs and their corresponding lateral overflow drain, anti-blooming structure. 
     PARTS LIST 
       10  image sensor 
       20  substrate 
       30  oxide 
       40  nitride 
       50  hard mask 
       60  photoresist 
       70  opening 
       75  opening 
       80  photoresist 
       90  channel stop 
       100  photoresist 
       110  lateral overflow drain 
       120  filed oxide 
       130  top-gate oxide layer 
       140  buried channel 
       150  barrier 
       160  polysilicon