Abstract:
A method of ion implantation is provided. The method comprising: providing a substrate; forming a masking image having a sidewall on the substrate; forming a blocking layer on the substrate and on the masking image; and performing a retrograde ion implant through the blocking layer into the substrate, wherein the blocking layer substantially blocks ions scattered at the sidewall of the masking layer.

Description:
This application is a divisional of Ser. No. 10/083,062; filed on Feb. 26, 2002 now U.S. Pat. No. 6,610,585. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to the field of semiconductor processing; more specifically, it relates to a method for forming a retrograde ion implant. 
     BACKGROUND OF THE INVENTION 
     Modern semiconductor devices such as N channel field effect transistors (NFETs) and P-channel field effect transistors (PFETs) require careful tailoring of the dopant concentration profile in the channel region of the device in order to control voltage (V T ), off currents (I OFF ) and short channel effects (SCE). For an NFET, the channel is formed by control of the P-well dopant profile concentration. For a PFET, the channel is formed by control of the N-well dopant profile concentration. Control of the respective N or P-well profile is accomplished by performing at least one low-voltage and low-dose shallow ion implant and at least one high-voltage and high-dose ion retrograde implant, both of the same dopant type. A shallow implant is one in which the implanted species remain relatively close to the silicon surface. A retrograde implant is one in which the highest dopant concentration of the implanted species occurs a distance below the silicon surface. The channel/well profile tailoring ion implant processes may be best understood by reference to FIGS. 1A and 1B. 
     FIGS. 1A and 1B are partial cross-sectional views illustrating a related art method of forming a P-well or an N-well. In FIG. 1A, formed in a substrate  100  is shallow trench isolation (STI)  105 . Formed on a top surface  110  of silicon substrate  100  is a thin oxide layer  115 . Formed on a top surface  120  of STI  105  is a photoresist image  125 . A low-voltage and low-dose ion implantation of ion species “X,” where “X” represents boron for a P-well or phosphorus for an N-well, is performed. Ions  130 A pass through thin oxide layer  115  and penetrate into substrate  100  forming a shallow portion  135  of well  140 . Ions  130 B striking photoresist image  125  are absorbed by photoresist image  125 . Ions  130 C, striking near sidewall  145  of photoresist image  125  are deflected by atoms in the photoresist but image lack sufficient energy to pass through the sidewall of the photoresist image. 
     In FIG. 1B, a high-voltage and high-dose ion implantation of ion species “X,” where “X” represents boron or for a P-well or phosphorus for an N-well, is performed. Ions  150 A pass through thin oxide layer  115  and penetrate into substrate  100  forming a deep portion  155  of well  135 . Ions  150 B striking photoresist image  125  are absorbed by the photoresist image. Ions  150 C, striking near sidewall  145  of photoresist image  125  penetrate into the photoresist image, are deflected by atoms in photoresist image  125 , and have sufficient energy to escape through sidewall  145 , pass through thin oxide layer  115  and penetrate into an edge region  160  of well  140 . Edge region  160  extends a distance “W” into well  140  measured from resist sidewall  145 . Edge region  160  extends a depth “D” measured from a top surface  165  of thin oxide layer  115 . Obviously P-wells or N-wells away from photoresist image  125  are not effected and do not have edge regions, “D” can range from about near zero to 0.5 microns and “W” can range from about near zero to 1.2 microns. The V T  of NFETs and PFETs devices fabricated in wells adjacent to photoresist image  125  can differ from the V T  of NFETs and PFETs fabricated in wells away from (non-adjacent) by as much as about 20 to 120 millivolts. The concentration of dopant in the shallow portion  135  of well  140  in edge region  160  can be ten times the concentration of dopant in the rest of shallow portion  135  of well  140 . 
     Since devices fabricated away from edge region  160  or in wells away from a resist sidewall, which will not have an edge region, their V T  will not be increased. Integrated circuits fabricated from a mix of edge and non-edge NFETs and PFETs will have some slow devices and some fast devices. Integrated circuits fabricated from a mix of edge and non-edge NFETs and PFETs and will often exhibit asymmetric behavior. 
     Therefore, what is needed is a method of forming retrograde ion implants that dose not cause increased dopant concentrations in edge regions of P-wells and N-wells. 
     SUMMARY OF THE INVENTION 
     A first aspect of the present invention is a method of ion implantation comprising: providing a substrate; forming a masking image having a sidewall on the substrate; forming a blocking layer on the substrate and on the masking image; and performing a retrograde ion implant through the blocking layer into the substrate, wherein the blocking layer substantially blocks ions scattered at the sidewall of the masking layer. 
     A second aspect of the present invention is a method of ion implantation comprising: providing a substrate; forming blocking layer on the substrate; forming a masking image having a sidewall on the blocking layer; and performing a retrograde ion implant through the blocking layer into the substrate, wherein the blocking layer substantially blocks ions scattered at the sidewall of the masking layer. 
     A third aspect of the present invention is a method of ion implantation comprising: providing a substrate; forming a first blocking layer on the substrate and a second blocking layer on the first blocking layer; forming a masking image having a sidewall on the second blocking layer; and performing a retrograde ion implant through the first and second blocking layer into the substrate, wherein the second or first and second blocking layers substantially blocks ions scattered at the sidewall of the masking layer. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     The features of the invention are set forth in the appended claims. The invention itself, however, will be best understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings, wherein: 
     FIGS. 1A and 1B are partial cross-sectional views illustrating a related art method for forming a P-well or an N-well; 
     FIG. 2 is a flowchart of processing steps for forming a P-well or an N-well according to a first embodiment of the present invention; 
     FIGS. 3A and 3B are partial cross-sectional views illustrating the ion implant steps of FIG. 2; 
     FIG. 4 is a flowchart of processing steps for forming a P-well or an N-well according to a second embodiment of the present invention; 
     FIGS. 5A and 5B are partial cross-sectional views illustrating the ion implant steps of FIG. 4; 
     FIG. 6 is a flowchart of processing steps for forming a P-well or an N-well according to a third embodiment of the present invention; 
     FIGS. 7A and 7B are partial cross-sectional views illustrating the ion implant steps of FIG. 6; 
     FIG. 8 is a flowchart of processing steps for forming a P-well or an N-well according to a fourth embodiment of the present invention; and 
     FIGS. 9A and 9B are partial cross-sectional views illustrating the ion implant steps of FIG.  7 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A low-voltage ion implant generally results in a shallow ion implant. Shallow implants are often performed at low dose as well as low voltage. In a shallow ion implant, the implanted species remain relatively close to the substrate surface and the highest dopant concentration of the implanted species occurs at or very near the substrate surface. In this disclosure ion implants performed at a voltage of less than about 100 Kev and at a dose of less than about 5E13 atoms/cm 2  are considered shallow ion implants. 
     A high-voltage ion implant generally results in a retrograde ion implant provided any blocking layer is sufficiently thin. Retrograde ion implants are often performed at high-dose as well as high voltage. In a retrograde ion implant the highest dopant concentration of the implanted species occurs a distance below the substrate surface. In this disclosure ion implants performed at a voltage of equal to or greater than about 100 Kev and at a dose of about equal to or greater than 5E13 atoms/cm 2 . The present invention is also applicable to shallow ion implants of low-energy and high-dose as well as to retrograde implants of high-energy and low-dose though the dopant concentration of the shallow portion of a P-well or an N-well formed by a shallow high-dose ion implant well would not be effected as much by scattering from a retrograde low-dose ion implant. 
     It has been determined that the amount of ion scattering of high-voltage and high-dose ion implants of boron and phosphorus is about the same for ion incident angles in the range of about 0° to 10° and increase significantly above about 10° with boron scattering more than phosphorus. 
     The ion implantation steps, both low-energy and low dose and high-energy and high-dose for all embodiments of the present invention, are performed at an incident angle between about 0° to 10° with 7° being most commonly used, though the invention is applicable to any angle between 0° and 90°. The incident angle is measured from a line normal to the surface being implanted. 
     While the present invention will be described in terms of a retrograde boron or phosphorus implant to form either a P-well or an N-well respectively, the invention is equally applicable to a retrograde implant of other ion species containing atoms of arsenic, germanium or indium used alone or in combination with each other and in combination with boron and/or phosphorus. Also one skilled in the art would realize that ion species containing boron or phosphorus could be implanted, for example, BF 2   + , and that the terms boron and phosphorus are intended to include all ion species containing boron or phosphorus. 
     The present invention is also applicable to other substrates such as sapphire, ruby, SiGe and silicon-on-insulator (SOI). 
     FIRST EMBODIMENT 
     Referring to FIGS. 2,  3 A and  3 B, FIG. 2 is a flowchart of processing steps for forming a P-well or an N-well according to a first embodiment of the present invention and FIGS. 3A and 3B are partial cross-sectional views illustrating the ion implant steps of FIG.  2 . Referring to FIG,  3 A, in step  170  of FIG. 2, STI  105  is formed in substrate  100  and thin oxide layer  115  formed on top surface  110  of the silicon substrate. Depending upon the technology, thin oxide layer  115  may be explicitly formed or may be formed as a result of the shallow trench isolation (STI) processes previously performed. In one example, thin oxide layer  115  is about 40 to 60 Å thick. Both STI  105  and thin oxide layer  115  are optional. 
     Referring to FIG. 3A, in step  175  of FIG. 2, photoresist image  125  is formed on top surface  120  of STI  105  by any one of a number of photolithographic methods known to one skilled in the art. While the example of a photoresist image is used, other masking images formed from masking layers comprised of materials other than photoresist may be employed in this and subsequent embodiments of the present invention. In one example, photoresist image  125  is either positive or negative photoresist and is about 0.8 to 2.2 microns thick. 
     Referring to FIG. 3A, in step  180  of FIG. 2, a low-voltage and low-dose ion implantation of ion species “X,” where “X” represents boron or for a P-well or phosphorus for an N-well, is performed. Ions  130 A, striking thin oxide layer  115  pass through the thin oxide layer and penetrate into substrate  100  forming shallow portion  135  of well  140 . Ions  130 B striking photoresist image  125  are absorbed by the photoresist image. Ions  130 C, striking photoresist image  125  near sidewall  145  of the photoresist image pass into the photoresist image and are deflected by atoms in the photoresist image. Ions  130 C lack sufficient energy to escape through sidewall  145  of photoresist image  125  or if they do escape, to pass through thin oxide layer  115 . 
     Referring to FIG. 3B, in step  185  of FIG. 2, a blocking layer  190  is formed over thin oxide layer  115  and photoresist image  125 . It is not necessary that blocking layer cover sidewall  145  of photoresist image  125 . Of course, when blocking layer  190  covers sidewall  145 , the possibility exists for scattering of ions off the blocking layer itself, so the thickness of the blocking layer needs to take this into account as well. 
     Referring to FIG. 3B, in step  195  of FIG. 2, a high-voltage and high-dose ion implantation of ion species “X,” where “X” represents boron or for a P-well or phosphorus for an N-well, is performed. Ions  150 A striking blocking layer  190  pass through the blocking layer and through thin oxide layer  115  and penetrate into substrate  100  forming deep portion  155  of well  140 . Ions  150 B striking blocking layer  190 , pass through the blocking layer, penetrate into photoresist image  125  and are absorbed by the photoresist image. Ions  150 C striking blocking layer  190  near sidewall  145  of photoresist image  125  pass through the blocking layer, penetrate into the photoresist image and are deflected by atoms in the photoresist image. Ions  150 C have sufficient energy to pass through sidewall  145  of photoresist image  125  but not through blocking layer  190  and are absorbed by the blocking layer. 
     A blocking layer substantially blocks ions scattered at the sidewall of a masking image from penetrating into the substrate by absorbing a significant portion of the scattered ions alone or in combination with overlaying or underlaying layers. Substantial blocking may be determined to have occurred when little or no difference in the V T  of edge devices and the V T  of non-edge devices can be measured or when the difference in edge device V T  and non-edge device V T  is within a preset limit. Alternatively, substantial blocking may be determined to have occurred when under similar processing conditions except for the presence or absence of a blocking layer, the V T  of edge devices fabricated without the use of a blocking layer is measurably different (or different within a preset limit) from the V T  of edge devices fabricated with the use of a blocking layer. Secondary ion mass spectroscopy (SIMS) analysis may also be used by comparing structures implanted away from resist edges with structures implanted near or next to resist edges. 
     That a given layer will exhibit substantial blocking can also be predicted by combining a theoretical determination of the amount of energy remaining to deflected ions with data from range tables or calculations using range equations of the material and thickness of the blocking layer such that a predetermine percentage of the total number of deflected ions do not penetrate into the substrate. 
     Blocking layer  190  must be thin enough to allow ions  150 A to pass through but thick enough to block ions  150 C from passing through, ions  150 C having lost energy by collisions with atoms within photoresist image  125 . In one example, blocking layer  190  is formed from any one of several organic anti-reflective coating (ARC) materials or other conformal materials well known in the art and is about 900 to 3600 Å thick. 
     Referring to FIG. 3B, in step  200  of FIG,  2 , resist image  125  and blocking layer  190  are removed. 
     SECOND EMBODIMENT 
     Referring to FIGS. 4,  5 A and  5 B, FIG. 4 is a flowchart of processing steps for forming a P-well or an N-well according to a second embodiment of the present invention and FIGS. 5A and 5B are partial cross-sectional views illustrating the ion implant steps of FIG.  4 . Referring to FIG,  5 A, in step  205  of FIG. 4, STI  105  is formed in substrate  100  and thin oxide layer  115  formed on top surface  110  of the silicon substrate. In one example, thin oxide layer  115  is about 40 to 60 Å thick. Both STI  105  and thin oxide layer  115  are optional. 
     Referring to FIG. 5A, in step  210  of FIG. 4, a blocking layer  215  is formed over thin oxide layer  115  and STI  105 . In one example, blocking layer  215  is an organic material such as polyimide or photoresist and is about 1000 to 3000 Å thick. 
     Referring to FIG. 5A, in step  220  of FIG. 4, photoresist image  125  is formed on a top surface  225  of blocking layer  215 . Photoresist image  125  is aligned over STI  105 . Photoresist image  125  may be formed by any one of a number of photolithographic methods known to one skilled in the art. In one example, photoresist image  125  is either positive or negative photoresist and is about 0.8 to 2.0 microns thick. 
     If blocking layer  215  is formed from a photoresist material then photoresist image  125  is formed from a photoresist of opposite polarity from that of the blocking layer. For example, if blocking layer  215  is formed from positive resist, then photoresist image  125  is formed from negative resist. If blocking layer  215  is formed from negative resist, then photoresist image  125  is formed from positive resist. 
     Referring to FIG. 5A, in step  230  of FIG. 4, a high-voltage and high-dose ion implantation of ion species “X,” where “X” represents boron or for a P-well or phosphorus for an N-well, is performed. Ions  150 A striking blocking layer  215  pass through the blocking layer, through thin oxide layer  115  and penetrate into substrate  100  forming deep portion  155  of well  135 . Ions  150 B striking blocking layer  215 , pass through the blocking layer, penetrate into photoresist image  125  and are absorbed by the photoresist image. Ions  150 C, striking blocking layer  215  near sidewall  145  of photoresist image  125  pass through blocking the layer, are deflected by atoms in the photoresist image and have sufficient energy to pass through sidewall  145  of the photoresist image but not through the blocking layer and are absorbed by the blocking layer. 
     Blocking layer  215  must be thin enough to allow ions  150 A to pass through but thick enough to block ions  150 C from passing through, ions  150 C having lost energy by collisions with atoms within photoresist image  125 . 
     Referring to FIG. 5B, in step  235  of FIG. 4, blocking layer  215  (see FIG. 5A) is thinned to form a thinned portion  215 A of blocking layer  215  where the blocking layer is not protected by photoresist image  125 . In one example, thinned portion  215 A of blocking layer  215  is about 0 to 1000 Å thick and the thinning was accomplished by any one of well known reactive ion etch (RIE) processes. Photoresist image  125  (see FIG. 5A) is also thinned by the RIE process to form thinned photoresist image  125 A, so it is the combination of the thickness of thinned portion  215 A of blocking layer  215  and the thickness of thinned photoresist image  215 A that must be sufficient to block low voltage ion  130 A. 
     Referring to FIG. 5B, in step  240  of FIG. 4, a low-voltage and low-dose ion implantation of ion species “X,” where “X” represents boron or for a P-well or phosphorus for an N-well, is performed. Ions  130 A, striking thinned blocking layer  215 A pass through the thinned blocking layer, pass through thin oxide layer  115  and penetrate into substrate  100  forming shallow portion  135  of well  140 . Ions  130 B striking photoresist image  125  are absorbed by the photoresist image. Ions  130 C, striking photoresist image  125  near sidewall  145  of the photoresist image are deflected by atoms in the photoresist image but lack sufficient energy to escape the photoresist image or if they do escape, to penetrate thinned portion  215 A of blocking layer  215 . 
     Referring to FIG. 5B, in step  245  of FIG. 4, resist image  125  thinned portion  215 A and blocking layer  215  are removed. 
     THIRD EMBODIMENT 
     Referring to FIGS. 6,  7 A and  7 B, FIG. 6 is a flowchart of processing steps for forming a P-well or an N-well according to a third embodiment of the present invention and FIGS. 7A and 7B are partial cross-sectional views illustrating the ion implant steps of FIG.  6 . Referring to FIG,  7 A, in step  250  of FIG. 6, STI  105  is formed in substrate  100  and thin oxide layer  115  formed on top surface  110  of the silicon substrate. In one example, thin oxide layer  115  is about 40 to 60 Å thick. Both STI  105  and thin oxide layer  115  are optional. 
     Referring to FIG. 7A, in step  255  of FIG. 6, a blocking layer  260  is formed over thin oxide layer  115  and STI  105 . In one example, blocking layer  260  is formed from silicon oxide, silicon nitride, polysilicon, borosilicate glass (BSG), boro-phosphorus-silicate glass (BPSG), quartz, tetraethoxysilane (TEOS) oxide or high density plasma (HDP) oxide and is about 200 to 3600 Å thick. 
     Referring to FIG. 7A, in step  265  of FIG. 6, photoresist image  125  is formed on a top surface  270  of blocking layer  260 . Photoresist image is  125  is aligned over STI  105 . Photoresist image  125  may be formed by any one of a number of photolithographic methods known to one skilled in the art. In one example, photoresist image  125  is either positive or negative photoresist and is about 1.2 to 2.2 microns thick. 
     Referring to FIG. 7A, in step  275  of FIG. 6, a high-voltage and high-dose ion implantation of ion species “X,” where “X” represents boron or for a P-well or phosphorus for an N-well, is performed. Ions  150 A striking blocking layer  260  pass through the blocking layer and through thin oxide layer  115  and penetrate into substrate  100  forming deep portion  155  of well  140 . Ions  150 B striking resist image  125 , penetrate into the photoresist image and are absorbed by the photoresist image. Ions  150 C, striking photoresist image  125  near sidewall  145  of the photoresist image penetrate into the photoresist image, are deflected by atoms in the photoresist image and have sufficient energy to pass through sidewall  145  of the photoresist image. Ions  150 C do not have sufficient energy to pass through blocking layer  260  and are absorbed by the blocking layer. 
     Blocking layer  260  must be thin enough to allow ions  150 A to pass through but thick enough to block ions  150 C from passing through, ions  150 C having lost energy by collisions with atoms within photoresist image  125 . 
     Referring to FIG. 7B, in step  280  of FIG. 6, portions of blocking layer  260  not protected by resist image  125  are removed. 
     Referring to FIG. 7B, in step  2985  of FIG. 6, a low-voltage and low-dose ion implantation of ion species “X,” where “X” represents boron or for a P-well or phosphorus for an N-well, is performed. Ions  130 A, striking thin oxide layer  115  pass through the thin oxide layer and penetrate into substrate  100  forming shallow portion  135  of well  140 . Ions  130 B striking photoresist image  125  are absorbed by the photoresist image. Ions  130 C, striking photoresist image  125  near sidewall  145  of the photoresist image are deflected by atoms in the photoresist image but lack sufficient energy to escape the photoresist image or if they do escape, to penetrate thin oxide layer  115 . 
     Referring to FIG. 7B, in step  290  of FIG,  6 , resist image  125  and blocking layer  260  are removed. 
     FOURTH EMBODIMENT 
     Referring to FIGS. 8,  9 A and  9 B, FIG. 8 is a flowchart of processing steps for forming a P-well or an N-well according to a fourth embodiment of the present invention and FIGS. 9A and 9B are partial cross-sectional views illustrating the ion implant steps of FIG.  7 . Referring to FIG. 9A, in step  295  of FIG. 8, STI  105  is formed in substrate  100  and thin oxide layer  115  formed on top surface  110  of the silicon substrate. In one example, thin oxide layer  115  is about 40 to 60 Å thick. Both STI  105  and thin oxide layer  115  are optional. 
     Referring to FIG. 9A, in step  300  of FIG. 8, a first blocking layer  305  is formed over thin oxide layer  115  and STI  105  and a second blocking layer  310  is formed on top surface  315  of first blocking layer  305 . In one example, first blocking layer  305  is formed from silicon nitride or polysilicon and is 100 to 500 Å thick and second blocking layer  310  is formed from borosilicate glass (BSG), boro-phosphorus-silicate glass (BPSG), quartz, tetraethoxysilane (TEOS) oxide, high density plasma (HDP) oxide or polysilicon and is about 500 to 2500 Å thick. 
     Referring to FIG. 9A, in step  320  of FIG. 8, photoresist image  125  is formed on a top surface  325  of second blocking layer  310 . Photoresist image is  125  is aligned over STI  105 . Photoresist image  125  may be formed by any one of a number of photolithographic methods known to one skilled in the art. In one example, photoresist image  125  is either positive or negative photoresist and is about 1.2 to 2.2 microns thick. 
     Referring to FIG. 9A, in step  330  of FIG. 8, a high-voltage and high-dose ion implantation of ion species “X,” where “X” represents boron or for a P-well or phosphorus for an N-well, is performed. Ions  150 A striking second blocking layer  310  pass through second blocking layer, pass through first blocking layer  305 , pass through thin oxide layer  115  and penetrate into substrate  100  forming deep portion  155  of well  140 . Ions  150 B striking resist image  125 , penetrate into the photoresist image and are absorbed by the photoresist image. Ions  150 C, striking photoresist image  125  near sidewall  145  of the photoresist image penetrate into the photoresist image, are deflected by atoms in the photoresist image, have sufficient energy to pass through sidewall  145  of the photoresist image but not through second blocking layer  310  or first and second blocking layer  305  and  310  and are absorbed by the blocking layer(s). 
     First and second blocking layers  305  and  310  must be thin enough to allow ions  150 A to pass through but thick enough to block ions  150 C from passing through, ions  150 C having lost energy by collisions with atoms within photoresist image  125 . 
     Referring to FIG. 9B, in step  335  of FIG. 8, portions of second blocking layer  340  not protected by resist image  125  are removed. First blocking layer  305  acts as an etch stop during the etching of second blocking layer  310 . 
     Referring to FIG. 9B, in step  340  of FIG. 8, a low-voltage and low-dose ion implantation of ion species “X,” where “X” represents boron or for a P-well or phosphorus for an N-well, is performed. Ions  130 A, striking first blocking layer  305 , pass through first blocking layer  305 , pass through thin oxide layer  115  and penetrate into substrate  100  forming shallow portion  135  of well  140 . Ions  130 B striking photoresist image  125  are absorbed by the photoresist image. Ions  130 C, striking photoresist image  125  near sidewall  145  of the photoresist image are deflected by atoms in the photoresist image but lack sufficient energy to escape the photoresist image or if they do escape, to penetrate first blocking layer  305 . 
     Referring to FIG. 9B, in step  345  of FIG,  8 , resist image  125 , second blocking layer  310  and first blocking layer  305  are removed. 
     The description of the embodiments of the present invention is given above for the understanding of the present invention. It will be understood that the invention is not limited to the particular embodiments described herein, but is capable of various modifications, rearrangements and substitutions as will now become apparent to those skilled in the art without departing from the scope of the invention. Therefore, it is intended that the following claims cover all such modifications and changes as fall within the true spirit and scope of the invention.