Abstract:
An MOS device is provided having a channel-stop implant placed between active regions and beneath field oxides. The channel-stop dopant material is a p-type material of atomic weight greater than boron, and preferably utilizes solely indium ions. The indium ions, once implanted, have a greater tendency to remain in their position than boron ions. Subsequent temperature cycles caused by, for example, field oxide growth do not significantly change the initial implant position. Thus, NMOS devices utilizing indium channel-stop dopant can achieve higher pn junction breakdown voltages and lower parasitic source/drain-to-substrate capacitances. Furthermore, the heavier indium ions can be more accurately placed than lighter boron ions to a region just below the silicon layer which is to be consumed by subsequent field oxide growth. By fixing the peak concentration density of indium at a depth just below the field oxide lower surface, channel-stop implant region is very shallow. Small dispersions in range allow for more precise control of the indium atoms just below the field oxide, further from the inner bulk material of the underlying substrate.

Description:
This application is a continuation of application Ser. No. 08/253,411, filed Jun. 3, 1994 now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to semiconductor processing and more particularly to a p-type field region implant which substantially maintains its position during subsequent temperature cycles relative to adjacent n-type source and drain regions. 
     2. Description of the Relevant Art 
     Fabrication of a MOSFET device is well known. Generally speaking, MOSFETs are manufactured by placing an undoped polycrystalline material or “polysilicon” material over a relatively thin gate oxide, and implanting the polysilicon and adjacent source/drain regions with an impurity dopant material. If the impurity dopant material is n-type, then the resulting MOSFET is an NMOSFET (“NMOS”) device. Conversely, if the impurity dopant material is p-type, then the resulting MOSFET device is a PMOSFET (“PMOS”) device. 
     There are numerous MOSFETs devices spaced across a single piece of silicon. Each device must be electrically isolated from other devices. Generally speaking, MOSFETS are self-isolated, as long as the source-substrate and drain-substrate pn junctions are held in reverse bias. The self-isolation property of MOSFETs devices represents a substantial area savings for NMOS and PMOS circuits compared to junction-isolated bipolar circuits. 
     Once the NMOS (and/or PMOS) devices are formed in silicon, they must then be interconnected by metal conductors placed across oxide in regions between the devices. Oxide formed between active devices is generally referred to as “field oxide”. Field oxide is distinguishable from “gate oxide” in that gate oxide is formed in the active regions between source and drain and between polysilicon gate and underlying silicon. 
     The metal or poly conductors extending over field oxide often carry significant voltage levels. It is important that the conductor voltage not activate the parasitic channel regions underlying field oxide. The threshold voltage in the field region underlying the field oxides must therefore be kept higher than any possible operating voltage on the overlying conductors. One way in which to prevent channels in the field region. is to increase the thickness of the field oxide. Unfortunately, thick field oxide can present large disparities in the upper surface elevation leading to poor planarization and possible step coverage problems. Another, more suitable way in which to maximize field region threshold voltage is to implant the field region prior to field oxide growth. The field region can be implanted with a dopant type matching that of the underlying substrate (or tub). Implantation of the field region is often referred to as “channel-stop implant”. The combination of channel-stop implant with adequate field oxide thickness can provide isolation for PMOS or NMOS devices to prevent channel formation in the field region. 
     The field oxide must be selectively grown only in the field regions and not in the active regions in which the active channels of the MOSFETs are formed. A popular method in which to selectively grow field oxide is often referred to as local oxidation of silicon (“LOCOS”). LOCOS methodology begins by covering the active regions with a thin layer of silicon nitride which prevents oxidation from occurring beneath the nitride. After the nitride layer has been etched away in the field regions, and prior to field oxide growth, the silicon in the field regions is selectively implanted with the channel-stop dopant. Thus, the field or channel-stop region becomes self-aligned to the field oxide. 
     Growth of field oxide can often present step-coverage limitation, and can be overcome to some degree by a selective oxidation approach. If the silicon is etched after the nitride layer is patterned, the field oxide can then be grown until it forms a planar surface with the silicon substrate. Etch-back of silicon in the field regions is often referred to as the “fully recessed” isolation oxide process. If the field oxide is grown without prior etch-back, the resulting field oxide will only be “partially (or semi) recessed”. In the semi recessed process, the field oxide step height is larger than in the fully recessed process, but nonetheless, has a gentle upward slope from the silicon juncture area, some of which is consumed by oxide growth. Consumption of silicon during oxide growth provides a gentle upward slope at the outer edge of the areas in which nitride is removed. Thus, the edges of the field oxide slope upward at their juncture with the edge of the nitride layer. 
     While channel-stop implant in the field region is necessary to prevent channel formation therein, conventional channel-stop implant can, in and of itself, present problems. In NMOS circuits, a p-type implant of boron is generally used in the field region. After field oxide growth, boron is supposed to reside primarily below the field oxide. Unfortunately, due to the high diffusivity of boron (i.e., due to its small atomic weight and size), implanted boron atoms readily segregate and move laterally toward adjacent arsenic-implanted source and drain regions. Boron atoms may also diffuse into the growing field oxide or deeper within the substrate. Lateral (diffusion parallel to the substrate upper surface) or non-lateral (diffusion perpendicular to the substrate upper surface) is primarily caused by heat cycles occurring after boron is initially placed. Heat cycles occur during field oxide growth and are a necessary part of that growth. 
     Any segregation or diffusion of boron from its implanted area laterally to adjacent n-type (arsenic) source and drain regions can cause high source/drain-to-substrate (or tub) capacitances and/or reduction in source/drain-to-substrate (or tub) n+p junction breakdown voltages. See, e.g., Wolf, “Silicon Processing for the VLSI Era”, Volume 2: Process Integration (Lattice Press, 1990), pp. 20-22. Generally speaking, breakdown voltage is inversely proportional to the doping concentration of the lighter-doped side of the p+n junction. Thus, increasing the doping of the p-type substrate (or tub) will reduce the breakdown voltage of the n-type source and drain regions adjoining the substrate. Source/drain-to-substrate capacitances are directly proportional to the doping concentration of the lighter-doped side. Increasing the doping of the p-type substrate will increase the parasitic capacitance of the source and drain regions leading to slower operation. 
     As described in Wolf, conventional research into minimizing lateral and non-lateral diffusion has focused primarily upon the field oxide step. Using high pressure oxidation (“HIPOX”) to grow the field oxide allows the oxide growth temperature to be reduced thereby reducing the diffusion length of boron. Research effort has also focused upon co-implanting germanium ions with boron ions to exploit the fact that boron diffuses with a low diffusivity in the presence of implanted germanium. By lowering the growth temperature of field oxide and/or co-implanting germanium with boron, research appears to indicate an increase in field threshold voltage with the same or lower dosage of boron. 
     Instead of merely depositing boron at high concentrations necessary to offset any lateral diffusions or at deep depths in order for the ions not to be absorbed by the growing field oxide, researchers point to changing the field oxide growth step or co-implanting germanium. Using HIPOX to grow the field oxide requires the oxide growth chamber be retrofitted with pressure equipment. Retrofitting the oxide chamber can oftentimes be costly. Moreover, each wafer run requires the oxide chamber to be pressurized and then de-pressurized leading to lower wafer throughput. If the chamber is under pressure during oxide growth, disturbance of and ingress of unwanted particles can occur. Still further, if germanium is co-implanted with boron, boron must be implanted as the source material followed by germanium implantation. The two step implant process can further decrease wafer throughput and add to the complexity of implant source retrofit. 
     It would be advantageous to avoid retrofit of the wafer fabrication line. Specifically, maximum throughput entails a single implant step with non-pressurized field oxide growth. P-type channel-stop implant with minimal lateral and non-lateral diffusivity is a target outcome yet to be achieved by existing wafer fabrication equipment and process flow. 
     SUMMARY OF THE INVENTION 
     The problems outlined above are in large part solved by the field region or channel-stop region implant methodology of the present invention. Instead of using boron (a mainstay in conventional NMOS channel-stop implants), the present invention utilizes indium implant. Indium, being of larger atomic mass and weight than boron, has a greater tendency to remain in its implanted position than that of boron. Specifically, indium can be implanted at a more controlled elevational depth in the field region than conventional boron. Indium is optimally implanted just below the silicon consumed by field oxide growth. Thus, indium is purposefully placed in a shallow region just below the resulting field oxide. Once placed, indium remains at or near its initial implant position and does not segregate and migrate to a substantial extent. Accordingly, indium does not diffuse laterally and non-laterally to the extent boron does and therefore will not cause high source/drain-to-substrate capacitances and/or low source/drain-to-substrate n+p junction breakdown voltages. 
     Indium is a p-type material from Group IIIA of the Periodic Table with an atomic weight of 114.82 as opposed to boron of the same Group IIIA having an atomic weight of 10.81. Indium therefore has a proportional decrease in its migration ability from its implant point. Indium can be implanted at a lower dosage and at a shallow depth necessary to overcome the problems with conventional high dosage boron implanted at deeper depths. Still further, indium can be easily substituted for boron using, for example, an indium bromide or indium chloride source material. Indium is implanted in a single step, and there is no need for modification of the field oxide growth chamber. Importantly, channel-stop implant of indium can be used in existing semiconductor fabrication equipment without additional processing steps and the disadvantages of conventional boron, boron/germanium and/or HIPOX. 
     Broadly speaking, the present invention contemplates a method for fabricating a semiconductor. The method comprises the steps of providing an opening to a field region of a semiconductor substrate upper surface. Indium ions are then implanted through the opening and into the field region. Next, a field oxide is grown upon the field region. The step of providing an opening to the field region comprises three substeps. First, a pad oxide is grown upon the substrate upper surface. Second, silicon nitride is deposited upon the pad oxide. Third, silicon nitride and pad oxide are selectively removed to produce the opening. The implanting step comprising ionizing elemental indium and placing the ions of indium into the field region at an exemplary dose less than or equal to 5×10 13  atoms/cm 2  at an implant energy greater than or equal to 200 keV. The exemplary placement depth at peak concentration is, therefore, approximately 950 Angstroms. The step of growing field oxide comprises subjecting the exposed field region to oxygen in a steam ambient at a temperature between 900° C. to 1100° C. for two to four hours. The resulting field oxide is grown to a thickness of 0.2 μm to 1.0 μm. 
     The present invention further contemplates a method for minimizing segregation and diffusion of dopant placed into field regions of a NMOS semiconductor device, comprising the steps of providing access to a field region at the upper surface of a semiconductor substrate. P-type ions are then inserted into the field region. The p-type ions are of larger atomic weight than boron and are inserted at a peak concentration dopant depth of, for example, 950 Angstroms relative to the upper surface with a ΔRp of approximately 230 Angstroms. A field oxide is then grown upon the field region. Growing of field oxide consumes silicon at the upper surface of the semiconductor substrate to a depth shallower than the dopant depth. A gate oxide can then be grown in an active region adjacent to the field oxide region. Deposited upon the gate oxide is a polysilicon gate conductor which allows for self-aligned implanting of source and drain regions of n-type ions into the active region adjacent the implanted p-type ions. The polysilicon gate conductor and n-type implanted source and drain regions comprise an NMOS semiconductor device. The p-type ions are of a sufficient atomic weight so as to limit their segregation and movement from the field region into the active region during the growing of field oxide. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other objects and advantages of the invention will become apparent upon reading the following detailed description and upon reference to the accompanying drawings in which: 
     FIGS. 1-6 are cross-sectional views of a semiconductor substrate undergoing process steps of LOCOS and indium implant according to the present invention; 
     FIG. 7 is a top plan view of an MOS device embodying an active region and surrounding field region according to the present invention; 
     FIG. 8 is a cross-sectional view along plane  8 — 8  of FIG. 7; 
     FIG. 9 is a cross-sectional view along plane  9 — 9  of FIG. 8; 
     FIG. 10 is a partial atomic detailed view along area  10  of FIG. 8; 
     FIG. 11 is a graph of concentration density versus depth of indium implanted into the field region according to the present invention; and 
     FIG. 12 is a graph of concentration density versus depth of indium in the field region after field oxide growth according to the present invention. 
    
    
     While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that the drawings and detailed description thereto are not intended to limit the invention to the particular form disclosed, but on the contrary, the intention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the present invention as defined by the appended claims. 
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to FIG. 1, a cross-sectional view of a partial semiconductor substrate  10  is shown. Substrate  10  is preferably manufactured as p-type and has an upper surface upon which a pad-oxide layer  12  is placed. Pad oxide  12  is thermally grown on a pre-cleaned bare silicon surface to a thickness less than 100 nm. Preferably, pad oxide  12  is as thin as possible yet thick enough to cushion the transition of stresses between the substrate and the subsequently deposited nitride layer. In most cases, it is preferred that pad oxide  12  be at least 30 percent of the overlying nitride layer thickness. 
     FIG. 2 illustrates, in a subsequent step, silicon nitride layer  14  chemical vapor deposition (CVD) deposited upon pad oxide  12 . Silicon nitride is assumed to oxidize at a fairly slow rate by one or both of the following reactions: 
     
       
         Si 3 N 4 +6H 2 O→3SiO 2 +6H 2 +2N 2   
       
     
     
       
         Si 3 N 4 +6H 2 O→3SiO 2 +4NH 3   
       
     
     Oxygen and water vapor diffuse very slowly through silicon nitride preventing oxidizing species from reaching the silicon surface under the nitride. In addition, the nitride itself oxides very slowly as the field oxide is grown in a subsequent step. Silicon nitride films, however, exhibit high tensile stress thereby requiring an underlying pad oxide  12  to combat those stresses and avoid dislocation generation. 
     FIG. 3 illustrates a subsequent step needed to selectively remove nitride  14  and underlying pad oxide  12  leaving openings or access locations to underlying field regions  16 . All other regions (or active regions) where active devices will be formed are covered or masked by the remaining pad oxide  12  and nitride  14 . Non-patterned, initial nitride  14  and oxide  12  are removed in numerous ways according to photolithography steps. A resist layer is placed on the non-patterned nitride  14  and selectively exposed. Polymerized resist remains and non-polymerized resist is removed allowing openings to underlying, to-be-removed nitride  14  and oxide  12 . Nitride and oxide can be etched used either wet etch or dry etch (plasma) process. A preferred etch having anisotropic capability is dry etch. Shown in FIG. 3 is resist layer  18  which remains on the non-removed nitride layer  14 . 
     The resist layer  18  forms a barrier, along with nitride  14  against implantation of channel-stop ions into the underlying active region, as shown in FIG.  4 . As such, p-type dopant ions of heavier atomic mass and weight than boron, and from Group IIIA are implanted into field regions (or channel-stop regions)  16 . Indium ions, of atomic weight  115  are used as the channel-stop implant species. The ion implanter can utilize indium chloride or indium bromide as a vaporized source material. The ion implanter of indium ions can place the indium at an exemplary dose less than or equal to 5×10 13  atoms/cm 2  at an implant energy greater than or equal to 200 keV. The exemplary placement depth is approximately 950 Angstroms as measured at the concentration peak density of pre-implant. The above dosage, implant energy and concentration peak density range are merely exemplary values and can be varied depending upon desired application. For example, dosages can vary in order to adjust field threshold and/or source/drain-to-substrate capacitances or breakdown voltages. The parameters are varied in order to achieve a concentration peak density depth just below the field oxide which is formed in subsequent steps shown in FIGS. 5 and 6, and detailed below. Channel-stop implant  20  is therefore used to prevent inversion (channel formation) in the field oxide regions  16  so as to electrically isolate each MOS device. 
     Indium ions are p-type ions and are therefore used to provide channel-stop dopant  20  between active areas. Each active area is an NMOS device configured to receive n-type source/drain dopants (such as arsenic). 
     After channel-stop implant  20  is formed, field oxide  22  is thermally grown using a wet oxidation process. Suitable parameters for growing 0.2 μm to 1.0 μm field oxide  22  are provided in a steam ambient at a temperature between 900° C. to 1100° C. for two to four hours. During growth, field oxide  22  will extend perpendicular to substrate  10  upper surface as well as parallel (lateral) to the upper surface. Lateral growth of field oxide  22  causes it to grow under and lift the edges of nitride  14 . Depending upon the thickness of pad oxide  12 , more or less nitride  14  will be lifted, as shown in FIG.  5 . Lateral extension of field oxide  22  is a well-known phenomenon often referred to as “bird&#39;s beak”. Field oxide  22  is grown in a heated chamber after resist  18  is removed. 
     Field oxide  22  not only grows laterally but also grows perpendicular to the upper surface of substrate  10 . Approximately two-thirds of field oxide  22  grows outward from the initial substrate upper surface and one-third of the ensuing oxide  22  extends into or consumes substrate  10 . Thus, field oxide  22  remains slightly recessed but also extends outward from the substrate upper surface. Consumption of silicon within substrate  10  causes a large portion of the upper surface of field regions  16  to be consumed. Therefore, it is important that indium ions be implanted at a concentration peak density just below the lower extent of the ensuing oxide  22 . As shown in FIG. 5, channel-stop dopant  20  peak concentration is preferably within 50 Angstroms below the lower boundary of field oxide  22 . Suitable peak concentration range of 950 Angstroms therefore dictates a field oxide down to approximately 900-950 Angstroms below the silicon upper surface. By carefully controlling the implant species during the step shown in FIG. 4, channel-stop dopant  20  can be relatively thin and of light dosage for the advantages stated above. After field oxide  22  is grown, oxide-protecting nitride layer  14  as well as underlying pad oxide  12  is removed in the active region, designated as reference numeral  24 . Active region  24  is then ready for subsequent photolithography steps necessary to produce an NMOS device. 
     Referring now to FIG. 7, an active device (doped as NMOS) is shown in a top plan view. MOS device  26  is formed according to the self-aligned process in which n-type dopant is introduced in active region  24  on opposite sides of a polysilicon gate  28 . Surrounding active region  24  is field oxide  22  grown to the specified thickness described above. 
     Referring now to FIG. 8, a cross-sectional view of device  26  is shown along plane  8 — 8  of FIG.  7 . Specifically, active region  24  of NMOS device  26  includes n-type source and drain regions  30 . Source and drain regions  30  are implanted on opposite sides of polysilicon gate  28  and underlying gate oxide  32 . Source and drain regions  30  are also implanted between polysilicon gate  28  and field oxides  22 . 
     Source and drain regions  30 , in accordance with the self-aligned process are adjacent to channel-stop dopant  20 . Thus, it is important that the p-type channel-stop dopant  20  not be allowed to laterally diffuse into the n-type source and drain regions  30 . Any cross-diffusions can jeopardize the integrity of the source, drain and channel-stop regions. As shown in FIG. 9, any lateral diffusion into the active region from channel-stop dopant area  20  can cause a reduction in the gate width (W) of the NMOS device. Gate width is generally defined by the circuit designer to provide adequate drive of the active device. If the gate width decreases, drive capability will be reduced. Thus, it is important that channel-stop dopant  20  maintain its implanted position at the edge of field oxide  22 . Any decrease in gate width caused by lateral diffusion of channel-stop dopant  20  can deleteriously effect the turn-on capacity and therefore the drive capability of the ensuing device. 
     Referring now to FIG. 10, a detailed view along area  10  of FIG. 8 is shown. In particular, FIG. 10 illustrates atomic interaction of indium atoms  34  within channel-stop dopant region  20  and arsenic atoms  36  within source/drain region  30 . Due to their large atomic weight, indium atoms  34  do not easily segregate and/or migrate through the silicon lattice to the adjoining source/drain region  30 . Indium atoms are of larger atomic mass than the silicon atoms within the lattice and do not diffuse according to interstitial or substitutional movement mechanisms normally associated with diffusivity. Movement is very constrained along specific planes of the crystalline silicon lattice. Likewise, arsenic atoms are of larger atomic weight than the silicon atoms within the lattice and do not readily migrate from their source/drain region  30  to abutting channel-stop dopant region  20 . Any migration which occurs is of fairly short diffusion length and is much shorter than boron diffusion length. 
     Referring to FIGS. 11 and 12, implant concentrations before and after field oxide growth, respectively, is shown. FIG. 11 illustrates a peak concentration density level  38  for the indium atoms implanted within field regions (channel-stop dopant regions). Concentration peak density  38  corresponds to a depth which is preferably less than 100 Angstroms below the lower boundary of the silicon consumed in the subsequent field oxide growth step. The lower boundary of field oxide is shown by reference numeral  40 . 
     FIG. 12 illustrates that while oxide  22  is growing, silicon is being consumed and the impurities (such as p-type indium atoms) that were in the silicon are likely to be redistributed. The manner in which the distribution occurs depends upon the segregation coefficient of the indium impurity, oxidation enhanced diffusion from interstitial silicon injected from the grown oxide, and the relative diffusion coefficients of the impurity in silicon and in silicon dioxide. Segregation coefficient M is defined as the ratio of equilibrium concentration of impurity on the silicon side to that on the oxide side of the interface. As oxidation proceeds, the impurities can either be rejected by the oxide or depleted from the silicon to the oxide. Due to the high diffusivity coefficient and segregation coefficient of boron, boron has a tendency to become depleted from the silicon and built-up within the oxide area, as shown in phantom line  42 . Thus, boron impurity concentration in the silicon just below field oxide  22  is depleted and must be counteracted by enhancing the implant dosage level. Increasing the dosage level of boron causes the problems described above and is to be avoided, if at all possible. Using indium instead of boron, and due to its lower diffusivity and segregation coefficient, allows for a rejection of indium atoms by the overlying field oxide  22 , as shown by line  44 . A build-up of indium ions just below field oxide lower surface  40  results from the advantages of using indium ions rather than boron. Not only can indium ions be more closely controlled and implanted due to their larger atomic weight to a concentration peak density just below lower edge  40 , but also a build-up of indium ions naturally arises during the field oxide thermal cycle shown in FIG.  12 . As such, indium ions can be implanted at a relatively light dosage, much less than boron, to achieve a tightly controlled, thin layer of indium ions just below the lower surface region  40 . Lateral diffusion and deeper diffusion of indium ions can therefore be avoided. 
     It will be appreciated to those skilled in the art having the benefit of this disclosure that this invention is believed to be capable of NMOS applications which utilize a channel-stop dopant material in the field regions. It is also to be understood that the form of the invention shown and described is to be taken as a present preferred embodiment. Various modifications and changes may be made without departing from the spirit and scope of the invention as set forth in the claims. For example, modifications can be made to each and every processing step as would be obvious to a person skilled in the art having benefit of this disclosure, provided the modifications achieve the result set forth in the claims. It is therefore intended that the following claims be interpreted to embrace all such modifications and changes.