Abstract:
A self aligned, lateral-overflow drain antiblooming structure that is insensitive to drain bias voltages and therefore has improved insensitivity to process variations. The length of the antiblooming barrier regions are easily adjusted and determined by photolithography. The self aligned, lateral-overflow drain (LOD) antiblooming structure results in a design that saves space, and hence, improves overall sensor performance. In this structure, an antiblooming potential barrier is provided that is smaller (in volts) than the barriers that separate the pixels from one another so that excess charge will flow preferentially into the LOD as opposed to the adjacent pixels.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     The present invention relates to antiblooming structures used within solid state image sensors, and more particularly, to self aligned antiblooming structures. 
     2. Background of the Invention 
     Blooming is a well known phenomenon that occurs in solid-state image sensors when the number of photocarriers generated by the incident radiation exceeds that of the storage capacity of the element, or pixel. These excess carriers then spill over, or &#34;bloom&#34;, into adjacent photosites thereby degrading the integrity of the image. Many types of structures have been proposed in the past (such as U.S. Pat. No. 5,130,774) which discloses a device that provide sinks for these excess carriers, either laterally or vertically, adjacent the photodetector elements. 
     It is important to maintain high quantum efficiency and charge capacity. Therefore, it is important that antiblooming structures do not take up so much space that there is a resulting degradation in the quantum efficiency and charge capacity of the device. Many conventional antiblooming structures are inherently subject to level-to-level misalignment. The extra space taken up within these antiblooming structures to compensate for the level-to-level misalignment can result in a reduction in performance of the sensor. 
     Some of the more recent disclosures are contained in U.S. Pat. Nos. 5,349,215 issued to Anagnostopoulos et al. (hereinafter referred to as Anagnostopoulos); 5,130,774 issued to Stevens et al. (hereinafter referred to as Stevens); 5,118,631 issued to Dyck et al. (hereinafter referred to as Dyck); and 4,593,303 also issued to Dyck et al. (hereinafter referred to s Dyck &#34;303&#34;), describe relatively modern approaches at antiblooming structure design. Another important factor in the performance of these antiblooming structures is the length (in microns) of the blooming channel&#39;s barrier region. The length of the blooming channel&#39;s barrier regions in Anagnostopoulos and Stevens are unaffected by alignment, but they are not self aligned to the drain. The extra amount of area that must be added to compensate for misalignment becomes an important factor for small pixel size devices. Dyck discloses a self aligned structure, but offers little flexibility in adjusting the length of these barrier regions since this length depends on lateral diffusion of the barrier-region implant. 
     The length of the barrier regions in this structure is typically only about 0.5 μm. Although this is very short, thus conserving the pixel&#39;s surface area, it makes the structure susceptible to the so-called DIBL (drain-induced, barrier lowering) effect. This effect can reduce the antiblooming barrier height dramatically thereby resulting in reduced charge capacity and hence, lower dynamic range. This also makes the barrier height sensitive to the LOD voltage. Hence, this voltage may need to be adjusted on a part-to-part basis due to process variations. Also, changing the length of this region requires changing the process (by varying Dt). 
     As can be seen by the foregoing discussion, there remains a need within the art for an antiblooming structure design that can offer the advantages of self alignment and that solves the problems associated with short antiblooming barrier lengths. 
     SUMMARY OF THE INVENTION 
     The present invention disclosure describes a self aligned, lateral-overflow drain (LOD) antiblooming structure. The self aligned aspect of the design saves space, and hence, improves overall sensor performance. In this structure, a potential barrier is provided that is smaller (in volts) than the barriers that separate the pixels from one another so that excess charge will flow preferentially into the LOD as opposed to the adjacent pixels. 
     The present invention details a method of manufacturing an antiblooming structure for image sensors having a semiconductor substrate of a first conductivity type with a series of masking layers contained on a major surface of the substrate. It is necessary to define an antiblooming barrier region and a drain area adjacent the antiblooming barrier region on the major surface of the substrate and to remove the masking layers from the antiblooming barrier region and the drain area, using conventional etching techniques, to create at least a pair of spaced walls within the masking layers while leaving certain masking layers not being etched from the major surface within the spaced walls. A barrier region of the first conductivity type, like the substrate, is implanted through unremoved portions of the masking layers and within the spaced walls such that the barrier region is self aligned with the spaced walls. Locally Oxidized Silicon (LOCOS) regions are formed on the unremoved portions of the masking layers and within the spaced walls such that the LOCOS regions are self aligned with the barrier regions on the major surface. A second masking is performed to create a reverse image of the previous masking by removing any remaining masking layers that were used to define the LOCOS regions and covering the major surface of the substrate with a photoresist material prior to patterning the photoresist leaving a portion of the LOCOS regions and the previously defined drain region uncovered. Then anisotropically etching the remaining masking material within the drain region and a second conductivity type, that is opposite in conductivity from the first conductivity type, is implanted into the area defined by the photoresist and the drain region. 
     In the first embodiment of this invention, p-type antiblooming barrier regions are implanted through openings in a masking layer that are defined by photolithography. Then, an image reversal technique is used to form an opening that is self aligned to the inner edges of the first openings in a second masking layer through which the n-type LOD region is implanted. An optional p-type &#34;container&#34; implant may be provided for at this point. 
     In the second embodiment of the invention, the n-type LOD regions are implanted first through same opening in the second masking layer as formed above. Then, the first masking layer is etched off and the etch continues down through the second masking layer after which the p-type antiblooming barrier regions are implanted. In this embodiment, this barrier implant can be formed underneath the n-type LOD region thereby forming a &#34;containing&#34; implant to reduce the depletion depth underneath the LOD. 
     It is an object of this invention to provide an LOD antiblooming structure that is self aligned. 
     It is also an object of the present invention to provide an antiblooming structure having a barrier region with a channel length that is defined photolithographically and, therefore, easily adjusted without modification of the process. 
     It is still further an object of the present invention to provide an antiblooming structure wherein the antiblooming barrier height is fixed by processing and virtually insensitive to the bias voltage on the LOD. 
     It is still further an object of the invention to describe a method and apparatus for the manufacturing an antiblooming structure for image sensors characterized by: providing a semiconductor substrate of a first conductivity type, having a series of layers used for masking purposes contained on a major surface of the substrate; defining a first pattern upon the major surface; etching away a first preselected set of the series of layers defined by the first pattern to form at least a pair of spaced walls; implanting antiblooming barrier regions of the first conductivity type in at least one area defined by the spaced walls; forming a mask within and defined by the spaced walls; defining a second pattern that employs the mask to form at least a portion of the second pattern, to create the second pattern that is essentially self aligned with at least one wall of the pair of spaced walls; etching away a second predetermined set of the series of layers according to the pattern defined by the second pattern to create an area that is self aligned with the spaced walls; and implanting within the substrate between the spaced walls a second conductivity type that is opposite the first conductivity type such that it is adjacent and self aligned to the barrier region(s) to form a drain region of the antiblooming structure. 
     It is further the object of the invention to provide a method of constructing an antiblooming structure whereby the steps relating to the implanting of the drain and the antiblooming barrier regions can be essentially reversed. 
     The above and other objects of the present invention will become more apparent when taken in conjunction with the following description and drawings wherein like characters indicate like parts and which drawings form a part of the present invention. 
     The present invention has advantages over the prior art in that it provides a self aligned antiblooming channel regions that can be created using conventional etching techniques. The self aligned channels, also, provide for superior antiblooming protection because of their potential for greater antiblooming channel width. Additionally, the present invention offers higher quantum and charge capacity. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross section through a CCD channel showing the initial layer stack used in the manufacturing process of antiblooming structure for the present invention. 
     FIG. 2 illustrates the process steps for defining the antiblooming barrier region opening within the masking layers of FIG. 1 and implanting barrier regions. 
     FIG. 3 illustrates the removal of the photoresist prior the local oxidation of the deposited silicon (LOCOS) creating edges of these regions that are self aligned to the barrier region&#39;s edges. 
     FIG. 4 illustrates masking and silicon nitride removal such that only a portion of the deposited silicon layer situated between the two barrier regions is exposed and anisotropically etched down to the dielectric stack, thereby forming a self aligned opening between the barrier regions through which a self aligned n-type LOD is then implanted. 
     FIG. 5 illustrates the buried channel, gate electrode, and antiblooming structure of the completed CCD image sensor once the remaining layers are stripped from the device of FIG. 4 and the device is completed in a conventional manner. 
     FIG. 6 shows a cross section of the second embodiment of the present invention at a stage of processing similar to that as shown in FIG. 4 but without the p-type barrier implants of the first embodiment. 
     FIG. 7 is an illustration of the device of the second embodiment at a point in the process where the photoresist and LOCOS regions have been removed, the deposited silicon is anisotropically etched down to the dielectric stack forming an opening in the deposited silicon and the antiblooming barrier regions are implanted self aligned through this opening in the polysilicon masking layer. 
     FIG. 8 illustrates the completed device of the second embodiment after the remaining layers have been stripped off and the device has been completed in the usual manner. This figure shows the buried channel, gate electrode, and antiblooming structure of the CCD image sensor. 
     FIG. 9 is an illustration of the single crystalline silicon substrate and various layers used for another embodiment of the present invention without the deposited silicon layers of the previous embodiments. 
     FIG. 10 is an illustration of placing p-type barrier implants into the configuration of FIG. 9. 
     FIG. 11 is a diagram showing the growing of LOCOS regions self aligned to the barrier regions of FIG. 10 on the single crystalline substrate. 
     FIG. 12 shows the alternate scheme at the point in the process where the n-type LOD is implanted self aligned to the inner edges of the LOCOS regions of FIG. 11. 
     FIG. 13a is a top view of the implanted-barrier, true-two-phase CCD device with the antiblooming structure of the present invention containing blooming channels in both phases. FIG. 13b is a device similar to FIG. 13a a without the self-alignment features within the antiblooming channels. 
     FIG. 14 is an alternate configuration of the present invention wherein blooming channels are only provided in one phase of the two-phase CCD. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     It has been discovered that a self aligned, lateral-overflow drain antiblooming structure is insensitive to drain bias voltages and, therefore, has improved insensitivity to process variations. The length of the antiblooming barrier regions within such a structure can be easily adjusted and determined by photolithography. 
     FIGS. 1 through 5 show a process by which the first embodiment of the invention is manufactured. In the first embodiment of this invention, p-type antiblooming barrier regions are implanted through openings in a masking layer that are defined by photolithography. Then, an image reversal technique is used to form an opening that is self aligned to the inner edges of the first openings in a second masking layer through which the n-type LOD region is implanted. Also, an optional p-type &#34;container&#34; implant may be provided for at this point. 
     Although these figures show a n-type buried channel CCD, a p-type buried channel CCD could just as easily be formed by reversing the conductivity type of the appropriate layers as would be apparent to one skilled in the art. Additionally, different options for the substrate used include substrate layers with epitaxial layers, substrate layers without epitaxial layers, substrate with wells and substrates without wells. 
     Referring now to FIG. 1, which is a cross section as seen through the layers used to construct the CCD channel and antiblooming structure in the manufacturing process, p-type silicon substrate 12 has placed on a major surface 2 a dielectric layer 14 that is preferably made from an oxide-nitride-oxide stack (SiO 2  -Si 3  N 4  -SiO 2 , commonly known as ONO). As would be apparent to those skilled in the art, depending on the etch chemistry employed, other materials may be employed for layer 14. A deposited silicon layer 16 (preferably polysilicon or amorphous) is placed on top of the dielectric layer and a masking layer 18 of Si 3  N 4  is then placed on the deposited silicon layer 16. The series of layers is completed by a photoresist layer 20. 
     FIG. 2 illustrates the steps within the manufacturing process used to define the antiblooming barrier region openings 22 in the first masking layer 18 of Si 3  N 4 . Using conventional photolithography and etching techniques, the barrier region openings 22 are formed within the photoresist layer 20 and masking layer 18 etching away a first preselected set of the series of layers defined by the first masking layer 18 to form at least a pair of spaced walls. Boron ions (or other p-type species) of sufficient energy are then implanted through deposited silicon layer 16, as indicated by the arrows, to form antiblooming barrier regions 23 within the substrate 12 as defined by openings 22. Thereby, implanting antiblooming barrier regions of the same conductivity type as the substrate in at least the area defined by the spaced walls defining barrier region openings 22. 
     It is envisioned by the present invention that by etching away a second predetermined set of the series of layers according to the pattern defined by the second pattern to create an area that is self aligned with the spaced walls. Referrings to FIG. 3, the photoresist layer 20 is removed and the depositied silicon layer 16 is locally oxidized to form LOGOS 25 within the barrier region openings 22 using conventional techniques that are well known within the art, thereby forming a mask within and defined by the space walls. The edges of these LOCOS 25 regions are therefore self aligned to the edges of the barrier region openings 22. It will be apparent to those skilled in the art that only one LOCOS region 25 can be created within only one barrier region opening 22, the preferred embodiment, however, envisions that two LOCOS region 25 be created within two barrier region openings 22. 
     Referring now to FIG. 4, by implanting within the substrate, and between the spaced walls, a conductivity type that is opposite that used in forming the substrate, such that it is adjacent and self aligned to the barrier regions(s) to form a drain region for the antiblooming structure, the Si 3  N 4  layer 18 has been removed and another photoresist layer 30 is patterned such that it leaves a portion of the LOCOS regions 25 uncovered and the region situation between the two barrier regions is exposed. Thereby, defining a second pattern that employs LOCOS regions 25 as a mask to form at least a portion of the second pattern, and moreover, to create the second pattern such that is self aligned with at least one wall of the pair of spaced walls. This exposed region of deposited silicon is anisotropically etched down to the dielectric layer 14, thereby, forming an implant opening 31 between the barrier regions that are self aligned to their inner edges. An n-type lateral overflow drain (LOD) 32 is then implanted at sufficiently low energy so that it is self aligned to implant opening 31. Also, an optional p-type &#34;container&#34; implant (not shown) can be provided at this time to reduce the depletion depth under the LOD 32 if desired. 
     FIG. 5 shows the device after remaining layers have been stripped off and the device is completed in the usual manner. Here, the photoresist layer 30, the masking layer 18, the deposited silicon layer 16 and the LOCOS regions 25 have all been removed. Preferably the dielectric layer 14 is removed and replaced with a fresh layer of gate dielectric material. Gate electrode 37 is placed on insulating layer 38. As shown in FIG. 5, an n-type buried channel 35 is used in the completed device. 
     A second embodiment of the invention is envisioned, whereby, the n-type LOD regions are implanted first through the same opening in the second masking layer as formed above in the first embodiment. Then, the first masking layer is etched off and an anisotropic etch continues down through the second masking layer after which the p-type antiblooming barrier regions are implanted. In this embodiment, this barrier implant can be formed underneath the n-type LOD region thereby forming a &#34;containing&#34; implant to reduce the depletion depth underneath the LOD. 
     FIGS. 6 through 8 illustrate the steps required to construct the second embodiment of this invention. Again, an n-channel CCD is shown. 
     FIG. 6 shows a cross section of the device at the stage of processing as shown in FIG. 4 of the first embodiment. Note that the p-type barrier implants 23 as shown in FIG. 5 have not been completed at this stage of the process as shown in FIG. 6. Also note that the nitride layer 18 has not been removed as was done in FIG. 4. The LOCOS regions 25 and n-type LOD 32 are formed in a manner similar to that shown for the first embodiment above. 
     As seen in FIG. 7, the photoresist layer 20 and LOCOS regions 25 are removed from the structure as shown in FIG. 6 and the polysilicon layer 16 is anisotropically etched down to layer 14. Next, the p-type antiblooming barrier regions 42 are implanted through this opening 43 in the deposited silicon masking layer 16. 
     FIG. 8 shows the completed device of the second embodiment. The remaining layers are stripped off and the device is completed in the usual manner with an n-type buried channel 45. 
     FIGS. 9 through 12 show an alternate scheme wherein the deposited silicon layer (polysilicon or other) is not used and the LOCOS regions 52 are grown on the single crystalline substrate 12. 
     FIG. 9 is an illustration of the single crystalline silicon substrate and various layers used for still another embodiment of the present invention without the deposited silicon layers of the previous embodiments. This is apparent from the various layers to construct the device. 
     FIG. 10 is an illustration of implanting p-type antiblooming barrier regions into the substrate of the configuration shown in FIG. 9. Conventional etching techniques are employed to form a pair of spaced walls within which these p-type barrier implants are formed. 
     FIG. 11 is a diagram showing the growth of LOCOS regions on a single crystalline substrate within the spaced walls that are self aligned to the antiblooming barrier regions of FIG. 10. 
     FIG. 12 shows the alternate scheme at the point in the process where the n-type LOD is implanted at sufficiently low energies such that it is self aligned to the inner edges of the LOCOS regions of FIG. 11. The n-type LOD implant 55 is therefore, masked by these thick, LOCOS regions 52 that are self aligned to the edges of the antiblooming barrier regions 53. Note that the procedure of implanting the antiblooming barrier regions after the LOD regions 55 as shown in FIGS. 6 through could also be employed here. 
     FIG. 13a is a top view of the implanted-barrier, true-two-phase CCD device with the antiblooming structure of the present invention. Antiblooming channels appear in each phase as with U.S. Pat. No. 5,130,774. The antiblooming drains have the antiblooming regions on either side consistent with the previous discussing of the present invention. 
     Still referring to FIG. 13a, there are additional advantages of the present invention which will be described further below. The amount of antiblooming protection (XAB) can be shown to be given by the relationships below. 
     
         X.sub.AB =1+2α[1+(W.sub.AB L.sub.CCD /L.sub.AB W.sub.CCD)e.sup.δV/nV t]                            Equation 1 
    
     Where: 
     α is charge in adjacent, unilluminated pixel as a fraction of charge in the illuminated pixel at the onset of blooming. (Typically defined to be 0.5); 
     W AB , L AB  is the width and length of the antiblooming barrier region, respectively; 
     W CC , L CCD  are the width and length of the CCD&#39;s barrier regions, respectively; 
     δV is the potential barrier height difference between the antiblooming barrier region and the CCD barrier region; 
     n is the nonideality factor (typically about 1.0 for LOD); 
     V t  is the thermal voltage, kT/q, equal to approximately 26 mV at room temp. 
     k is Boltzman&#39;s constant. 
     T is absolute temperature in degrees kelvin. 
     q is the charge of an electron. 
     Therefore for δV greater than 50 to 75 mV, which represents two to three times kT/q at room temperature, and with α=0.5, 
     
         X.sub.AB≈ (W.sub.AB L.sub.CD /L.sub.AB W.sub.CCD)e.sup.δV/nV tEquation 2 
    
     From the above relationships, it is clearly evident that the amount of antiblooming is proportional to the width of the antiblooming channel and inversely proportional to the length. Prior art devices have alignment tolerances that require spacing. This tolerance space occurs at the expense of space used, otherwise, for antiblooming channel width, for example. These tolerances can be seen in FIG. 13b, which is an illustration of a similar device to that of FIG. 13a, however the device of 13b is without the self alignment feature of the antiblooming channels taught by the present invention. The width of the antiblooming channel within the relationship indicated by Equation 1 and Equation 2 is W2 (62) on FIG. 13b. W2 (62) is narrowed by an amount equal to twice the tolerance width, indicated as T w  (63). This is corrected in the present invention by creating a self aligned antiblooming channel that does not require alignment tolerances. This results in an increase in the width of the antiblooming channel and increased antiblooming protection. 
     Referring once again to the device of FIG. 13a there are isolation regions 58 between phases within the CCD. The isolation regions 58 are constructed to receive both the antiblooming region implants 68 (not shown) and the CCD barrier region implants 69 (not shown). The CCD barrier implants are conventionally used to create an implanted barrier two phase device. These isolation regions prevent inadvertent transfer of charge into the LOD during normal charge transfer between phases. These isolation regions are present in the same relative positions under all phases 1 and 2 of the CCD. The self aligned antiblooming regions are naturally employed to construct these isolation regions resulting in self aligned isolation regions. 
     Referring now to FIG. 14, which is an alternate configuration to which the present invention can be employed, wherein only one phase (phase 2 in this case) has an antiblooming channel. This configuration is discussed in U.S. Pat. No. 5,349,215. The blooming channel under phase 1 is eliminated by implanting these regions with both the antiblooming and CCD barrier region implants, thereby effectively turning these regions off (as done to form the CCD-to-LOD isolation regions as discussed above). This has effect of forcing excess charge into the second phase of the CCD in order for any antiblooming of the excess charge to take place. The construction of such a device is enhanced via self alignment as taught by the present invention. 
     Additional variations of the present invention could include heavy p-type channel stops adjacent to each CCD channel to allow the device to run in the accumulation mode as described in U.S. Pat. No. 5,115,458. Another variation would be to employ the present invention upon CCD devices having more than two phases. 
     While there has been shown what is considered to be the preferred embodiment of the invention, it will be manifest that many changes and modifications may be made therein without departing from the essential spirit of the invention. It is intended, therefore, in the annexed claims, to cover all such changes and modifications as may fall within the true scope of the invention. 
     Parts List: 
     2 major structure 
     10 CCD 
     12 p-silicon substrate 
     14 Dielectric layer 
     16 Deposited silicon layer 
     18 masking (Si 3  N 4 ) layer 
     20 photoresist layer 
     22 Antiblooming barrier region openings 
     23 p-type antiblooming barrier implants 
     25 LOCOS region of FIG. 4 and 6 
     30 photoresist layer 
     31 LOD implant opening 
     32 LOD 
     35 n-type channel 
     37 gate electrode 
     38 Dielectric layer 
     42 p-type antiblooming barrier implant 
     43 opening 
     45 N-channel 
     52 LOCOS region of FIG. 11 
     53 Antiblooming barrier region 
     55 LOD 
     58 isolation regions 
     59 L length of antiblooming channel 
     60 W1 width of antiblooming channel 
     62 W2 width of prior art antiblooming channel 
     63 T w , the antiblooming width tolerance 
     65 T L , the antiblooming length tolerance 
     68 Antiblooming region implants 
     69 CCD region implants