Abstract:
Retrograde wells are formed by implanting through nitride films ( 40 ). Nitride films ( 40 ) are formed after STI ( 20 ) formation. By selectively masking a portion of the wafer with photoresist ( 47 ) after portions of a retrograde well are formed ( 45, 50, 55,  and  60 ) the channeling of the subsequent zero degree implants is reduced.

Description:
This application claims priority under 35 USC §119(e)(1) of provisional application Ser. No. 60/239,209, filed Oct. 10, 2000. 
    
    
     FIELD OF THE INVENTION 
     The invention is generally related to the field of semiconductor device fabrication and more specifically to a method for reducing channeling and latchup in shallow trench isolation structures by implanting through nitride. 
     BACKGROUND OF THE INVENTION 
     In current VLSI technology for fabricating submicron CMOS integrated circuits, high energy ion implantation is used for making retrograde wells. In retrograde wells the carrier concentration deep in the silicon substrate is greater than the carrier concentration at the surface. In general, retrograde wells are useful for the following reasons: their ability to put high concentration of dopants at specific desired locations, their low thermal budget (no high thermal budget well formation required), their reduced cost and processing complexity by having all the channel implants done at the same mask level, and their improvement in soft-error immunity. The high energy (MeV) implants require some changes in the processing steps compared to the non-MeV implant processing. The MeV implant requires photoresist that is a couple of microns thick to prevent the ions from penetrating it in the masked-off regions. For such thick photoresist, the channel implants need to be done at zero degrees. At any other angle, the implant will shadowed in the device region which could lead to a reduction in device performance and/or device failure. Shadowing also causes the well implant to be shifted laterally from its zero degree implant position. Implants done at zero degrees in (100) silicon result in channeling. At other angles, the implants will be dechanneled depending on the tilt angle of the implant. The dechanneled dopant profiles differ from the channeled implants in the tail region of the profile. In case of a boron channeled implant, there is a prominent second peak; for a completely dechanneled boron implant, the peak is small or absent. In the case of phosphorus channeled implant, there is a long tail; for a completely dechanneled phosphorus implant, the tail is very small or absent. Since the total implanted dose is the same in all implants, the nonchanneled implants will have higher peak concentrations compared to the channeled implants. Thus, the net effect of increasing the implant tilt angle is to cause the well doping profiles to be shifted laterally, decrease the dopant channeling tail, and increase dopant peak concentration. 
     Given the current feature sizes of shallow trench isolation (STI) structures, it is necessary to use zero degree high energy implants to form the retrograde well structures. For zero degree high energy implants, the center of the silicon wafer or substrate has channeling implants, and the left and right edges have nonchanneling implants. This causes a variation in the punch through voltage, leakage current, and latchup characteristics of transistors fabricated in these substrates. In general, this variation in transistor characteristics occurs in both 6-in and 8-in wafers with the effect being more pronounced in the larger wafers. 
     As the feature size of the transistors decrease, the spacing between transistors will also decrease. This reduction in spacing is accomplished by reducing the feature size of the STI structures. As the STI feature size is reduced, the effect on the transistor characteristics of channeling during retrograde well formation will become more pronounced. This channeling could eventually lead to complete failure of the integrated circuit. There is therefore a great need for a method of formation of retrograde wells with reduced dopant channeling. 
     SUMMARY OF THE INVENTION 
     The instant invention is a method of forming retrograde wells in silicon integrated circuit fabrication with reduced channeling effects. The method comprises forming silicon nitride films on the surface of the wafer after the formation of the isolation structures. By performing the retrograde well implants through these silicon nitride layers, channeling is reduced. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings: 
     FIGS. 1A-1E are cross-sectional diagrams illustrating an embodiment of the instant invention 
     FIGS. 2A-2C are cross-sectional diagrams illustrating a further embodiment of the instant invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A silicon substrate  10  may be single-crystal silicon or an epitaxial silicon layer formed on a single crystal substrate as shown in FIG.  1 A. Shallow trench isolation structures  20 , pad oxide structures  30 , and silicon nitride structures  40 ,  41  are formed in the substrate  10  using standard processing techniques. In an embodiment of the instant invention, the pad oxide  30  is formed by thermally growing or depositing a film of silicon oxide. A silicon nitride film is formed on the pad oxide film and the structure is patterned and etched to form the isolation trench structures. The trenches are filled with a dielectric material, usually silicon dioxide, to form the STI structures  20 . The structure shown in FIG. 1A is formed after chemical mechanical polishing (CMP) is used to remove excess silicon dioxide material which was formed during the trench filling process. 
     As shown in FIG. 1B, a photoresist film  43  is formed and patterned on the structure of FIG. 1A to expose the area where the first well structure will be formed. The photoresist film must be thick enough not to be penetrated by the implanted ion species. In the case of an n-well, a n-type dopant is implanted through the exposed nitride structure  41  to form the n-well region  60 . In one embodiment of the instant invention the implant used to form the n-well region comprises a phosphorous implant with a dose of 2×10 12  cm 2 -6×10 12  cm 2  with energies of 300 KeV-500 KeV. The p-channel channel stop implants comprise a n-type dopant and are also performed through the exposed nitride structure  41  to form region  55  in FIG.  1 B. In an embodiment of the instant invention, the p-channel stop implant comprises a phosphorous implant with a dose of 2×10 12  cm 2 -6×10 12  cm 2  with energies of 100 KeV-500 KeV. Forming the N-well region  60  and the P-channel stop region  55  by implanting through the nitride region  41  reduces the channeling of the zero degree implanted species that would otherwise have taken place. Following the formation of the N-well region  60  and the P-channel stop region  55 , the nitride region  41  is removed. In an embodiment of the instant invention this is accomplished using a reactive ion etching (RIE) process. The punch through implant and the threshold voltage implant are then performed through the exposed oxide region  30  to form regions  50  and  45  respectively. In an embodiment of the instant invention the punch through implant comprises a phosphorous implant with a dose of 2×10 12  cm 2 -6×10 12  cm 2  with energies of 100 KeV-300 KeV and the threshold voltage implant comprises a phosphorous implant with a dose of 2×10 12  cm 2 -6×10 12  cm 2  with energies of 20 KeV-80 KeV. The regions  45 ,  50 ,  55 , and  60  which are formed using different implant conditions represent regions of various dopant concentrations. 
     Following the above described implants, the resist film  43  is removed and a new resist film is formed and patterned forming the resist structure  47  illustrated in FIG.  1 D. The P-well implant and the N-channel stop implant are performed through the exposed nitride structure  40  to form regions  80  and  75  respectively. In an embodiment of the instant invention, the P-well implant comprises a boron implant with a dose of 1×10 13  cm 2 -5×10 13  cm 2  with energies of 100 KeV-700 KeV and the n-channel stop implant comprises a boron implant with a dose of 3×10 12  cm 2 -1×10 13  cm 2  with energies of 50 KeV-500 KeV. The nitride structure is then removed and the punch through implant and the threshold voltage implant are performed through the exposed oxide film  30  to form the regions  70  and  65  respectively. In an embodiment of the instant invention the punch through implant comprises a boron implant with a dose of 1×10 12  cm 2 -8×10 12  cm 2  with energies of 20 KeV-120 KeV and the threshold voltage implant comprises a boron implant with a dose of 10×10 11  cm 2 -3×10 12  cm 2  with energies of 2 KeV-50 KeV. The regions  65 ,  70 ,  75 , and  80  which are formed using different implant conditions represent regions of various dopant concentrations. Implant the p-well and the n-channel punch through regions through the nitride structure  40  reduces the channeling of the zero degree implants. 
     Illustrated in FIGS. 2A-2C is a further embodiment of the instant invention. Shown in FIG. 2A is a silicon substrate  10  with STI structures  20 , pad oxide  30 , a nitride structure  40 , and a photoresist film  43 . The n-well region  60 , n-channel stop region  55 , punch through region  50 , and threshold voltage region  45  are formed as described above. Following the formation of regions  45 ,  50 ,  55 , and  60 , the p-well region  80  is formed by implanting a p-type dopant species through the photoresist film  43 , nitride structure  40 , and pad oxide  30 . This p-well region  80  is shown in FIG.  2 B. The p-well region  80  formed in this way will have reduced channeling and will be self aligned to the n-well region  60  and the punch through region  55 . This implant will also form the p-type region  90 . Because of the high implant energy required to penetrate the resist film  43  and the nitride structure  40 , region  90  will be positioned below the n-well region  60  and will have no effect on the operation of devices fabricated in the n-well region  60 . Following formation of the p-well region  80 , the photoresist structure  43  is removed and the channel stop, punch through, and threshold voltage implants are performed through the now exposed nitride structure  40 . These p-type implants will form regions  75 ,  70 , and  65  respectively. These regions will also be self aligned to regions  60  and  55 . In addition by implanting through the nitride structure  40  the channeling of the ions is reduced. These implants will also form regions  95 ,  100 , and  105 . These p-type regions will be offset from the n-type regions  60 ,  55 , and  50 . The amount of offset will be determined by the implant conditions and the thickness of the nitride structure  40 . Since the implanted p-type ions will compensate the n-type regions  60 ,  55 ,  50 , and  45 , it is important that the implant conditions and nitride thickness be such that devices fabricated in the n-well region  60  following all the implants function correctly.