Abstract:
A semiconductor device has recesses formed in the substrate during removal of the anti-reflective coating (ARC) because these recess locations are exposed during the etching of the ARC. Although the etchant is chosen to be selective between the ARC material and the substrate material, this selectivity is limited so that recesses do occur. A problem associated with the formation of these recesses is that the source/drains have further to diffuse to become overlapped with the gate. The result is that the transistors may have reduced current drive. The problem is avoided by waiting to perform the ARC removal until at least after formation of a sidewall spacer around the gate. The consequent recess formation thus occurs further from the gate, which results in reducing or eliminating the impediment this recess can cause to the source/drain diffusion that desirably extends to overlap with the gate.

Description:
FIELD OF THE INVENTION  
         [0001]    This invention relates to integrated circuits and more particularly to integrated circuits with a recess in the substrate.  
         RELATED ART  
         [0002]    In the manufacture of integrated circuits one of the problems that has become more significant as dimensions have become smaller is recesses in the substrate that occur under normal processing. The recesses in the substrate occur primarily as a consequence of the substrate being exposed during the etching away of some portion of a layer of material that was over the substrate. An etchant is applied to the substrate for some amount of time during and/or after the layer that is being etched has been removed. One example is that there is a situation in which there is exposed substrate at the onset of an etch of another material in a different location. Another example is that a thin layer over the substrate is etched through during an etch of a material elsewhere so that the substrate becomes exposed part way through the etch of the material elsewhere. Another example is that a layer over the substrate is being etched and after the substrate becomes exposed, the etch continues as an over-etch to ensure that the layer that is desired to be removed is completely removed. The etchant that is chosen desirably does not significantly etch semiconductor substrates, but as a practical matter such etchants are very difficult to work with. Consequently the layers that are desired to be removed are removed by an etchant that does have some etching effect on the semiconductor substrate, typically silicon. Such a process is shown in FIGS.  1 - 9 .  
           [0003]    Shown in FIG. 1 is a semiconductor device  10  useful in making an integrated circuit comprising a substrate  12 , a polysilicon gate  14 , an anti-reflective coating (ARC)  16  of nitride, and a thin oxide  18  which is between gate  14  and substrate  12  as well as extending in areas adjacent to gate  14 . In order to remove nitride ARC  16 , an etchant, such as a halogen based material such as fluorine and chlorine, is used. These etchants also etch silicon although at not as fast a rate as nitride is etched. The result of removing ARC  16  is a recess surface  22  shown in FIG. 2. Shown in FIG. 3 is device  10  after formation of a sidewall spacer  24 . Sidewall spacer  24  is formed of oxide and occurs as a result, as is commonly known, of applying a relatively conformal layer and subsequently etching it with an anisotropic etch. This causes a further recess in substrate  12  aligned with sidewall spacer  24 . Shown in FIG. 4 is formation of source/drain region  26  and source/drain  28  using sidewall spacer  24  as a mask. This implant is commonly called the extension implant and has a relatively lower doping concentration than a subsequent heavy source/drain implant.  
           [0004]    Shown in FIG. 5 is device  10  after deposition of an oxide liner  30  and a nitride layer  32 . Nitride layer  32  is then etched back as is liner  30  resulting in sidewall spacer  34  and liner portion  38 . During this processing, source/drain regions  26  and  28  diffuse, expanding the area of source/drain regions  26  and  28 . Shown in FIG. 7 is device  10  after a heavy implant to form heavily-doped regions  40  and  42  using sidewall spacer  34  as a mask. Shown in FIG. 8 is continued expansion of source/drain regions  26  and  28  as well as diffusion of regions  40  and  42  due to standard processing.  
           [0005]    Shown in FIG. 9 is device  10  after formation of silicide regions  48  and  50  which extend under regions  40  and  42 . This also shows a completed diffusion of regions  49  and  51 , which are the remaining portions of regions  26  and  28 . These regions may not extend all the way to gate oxide  20 . With the regions  49  and  51  not fully extending to be in contact with gate oxide  20 , there is some additional space between gate  44  and the channel formed between regions  49  and  51  so that current passing between regions  49  and  51  is less than it would be if they had diffused in closer proximity to gate  20 . This is a disadvantage and is a direct result of the additional distance the diffusion must travel due to the recess of substrate  12  adjacent to gate  44 . Salicide region  46  is also formed on top of gate  14  and consumes a significant amount of gate  14  to leave a gate that is a combination of a region  44  of polysilicon and a region  46  of silicide.  
           [0006]    Thus, there is a need to reduce the adverse effects of a recess that occurs in the substrate during normal processing. This problem continues to get worse as dimensions decrease and voltages decrease. The ability to completely invert the channel and provide optimum current between source and drain is compromised if the source and drain do not have the proper overlap with the overlying gate. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]    FIGS.  1 - 9  show sequential cross-sections of a semiconductor device according to the prior art;  
         [0008]    FIGS.  10 - 18  are sequential cross-sections of a semiconductor device made according to one embodiment of the invention; and  
         [0009]    FIGS.  19 - 25  are sequential cross-sections of a semiconductor device made according to another embodiment of the invention.  
     
    
     DESCRIPTION OF THE INVENTION  
       [0010]    A problem with recess in the substrate is overcome by waiting until later in the process to remove the nitride anti-reflective coating (ARC) so that the recess that occurs has much less impact with regard to the source and drain moving in to close proximity to the gate dielectric and overlapping with the gate. One way this is achieved is by waiting until the sidewall spacer stack that is utilized for masking the heavy source/drain implant is in place before removing the nitride ARC. In an alternative, the nitride ARC is removed after formation of the sidewall spacer that is used for the source/drain extension implant and in such case the nitride ARC is removed with a wet etch.  
         [0011]    Shown in FIG. 10 is a device  60  after formation of a sidewall spacer  70  as an alternative to the structure shown in FIG. 2. The structure of FIG. 10 follows the device structure shown in FIG. 1. Device  60  comprises a substrate  62 , a gate  64 , which may be made of polysilicon and is a type of patterned conductive layer, a gate oxide  66 , an ARC  16 , which may be nitride, and a sidewall spacer  70 . Preferable material for substrate  62  is silicon and for sidewall spacer  70  is oxide. ARC  16  could be of some other effective anti-reflective material than nitride as well. Gate  64  could be materials other than polysilicon also. Sidewall spacer  70  results from an oxide layer that is relatively conformal being anisotropically etched. As a consequence of this anisotropic etch will be a recess  71  of substrate  62 . This is a consequence of the necessary over-etch to ensure that all of the layer that is being used to form the sidewall spacer is removed except where the sidewall spacer is to be formed. Since the only exposure of the substrate is during an over-etch time, the recess is relatively small. Shown in FIG. 11 is device  60  after a source/drain extension implant forming source/drain region  72  and source/drain region  74  adjacent to sidewall spacer  70  which surrounds gate  64 .  
         [0012]    Shown in Shown in FIG. 12 is device  60  after formation of liner  76 , a layer  78 , and a layer  80 . Layer  76 ,  78  and  80  are all typically dielectric materials. Layer  76  is preferably oxide, layer  78  is preferably nitride, and layer  80  is preferably oxide, but instead of a typical dielectric may be amorphous silicon. Shown in FIG. 13 is sidewall spacer  82  formed from layer  80  using an anisotropic etch. This exposes layer  78  of nitride in areas adjacent to sidewall spacer  82  including an area over gate  64  and ARC  68  as well as a portion of layer  76  which functions as a liner. Shown in FIG. 14 is device  60  after a nitride etch has been performed so that uncovered portions of layer  78  are removed to leave nitride portions  84  around gate  64 . This also has the effect of removing the portion of layer  76  above ARC  68  to leave a portion  86  of layer  76 . During this processing regions  72  and  74  diffuse toward each other and toward being under gate  64 . With the relatively small amount of recess of substrate  62 , the diffusion process is effective in overcoming that small amount of recess. The removal of nitride continues until ARC  68  has been removed which also causes a reduction in the height of sidewall spacer  84  to leave sidewall spacer  88 . Sidewall spacer  88  is slightly lower than polysilicon  64  due to over-etching which is necessary to be certain that all of ARC  68  has been removed. A relatively large recess in substrate  62  aligned with sidewall spacer  82  occurs primarily during the etch of ARC  68 . This etch is preferably a dry etch because of its superior defectivity characteristics over that of a wet etch. The dry etch will result in a greater recess in substrate  62  than if a wet etch had been used. In this case, however, the relative difference is not material because the recess is significantly removed from the area where it would have a negative impact on the ability of source/drain regions  72  and  74  to become overlapped with gate  64 .  
         [0013]    Shown in FIG. 16 is device  60  after a heavy source/drain implant resulting in heavily doped source/drain regions  90  and  92  aligned to sidewall spacer  82  which acts as an implant mask. If sidewall spacer  82  is chosen to be amorphous silicon, it should be removed after this implant. Shown in FIG. 17 is device structure  60  after a silicide step forms silicide regions  94  and  96  that is also aligned to sidewall spacer  82 . If sidewall spacer  82  was chosen to be amorphous silicon, it should be removed before this step of forming silicide. In the depicted example, sidewall spacer  82  is oxide. Shown in FIG. 18 are portions  100  and  102  of source/drain regions  72  and  74 , respectively, that have diffused sufficiently to overlap gate  64 . The relatively small recess caused during the over-etch in the formation of sidewall spacer  70  is all that needs to be overcome so that source/drain regions  100  and  102  overlap gate  64 . The recess caused by the etching away of ARC  16  is not visible in the final device structure shown in FIG. 18. The formation of silicide in the area of the recession removes the evidence that there was even a recess present. Thus it is seen that by moving the location of the relatively large recessed area caused as a result of the removal of the ARC layer by a dry etch further away from the gate area, this relatively large recessed area does not impact the distance that the source/drain must diffuse to obtain the desired overlap.  
         [0014]    Shown in FIG. 19 is a device structure  110  is shown as a beginning point for another embodiment comprised of a non-volatile memory (NVM) transistor  111  and a regular transistor  113  both of which are formed in a substrate  112 . Transistor  111 , as shown in FIG. 19, comprises a gate oxide  130 , a floating gate  114 , an interlayer dielectric  120 , and a control gate  118 . Regular transistor  113  comprises a gate oxide  132  and a gate  116 . Over control gate  118  is an ARC layer  126  and over gate  116  is an ARC layer  128 . These are two transistors are formed simultaneously and are shown as transistors that would occur as a result of formation of sidewall spacers  122  and  124  and analogous to FIG. 10. Thus there is a recess in the surface of substrate  112  shown as  134  and  136  in FIG. 19. This recess is caused by the over-etch in the formation of sidewall spacer  122 . Shown in FIG. 20 is device structure  110  after ARC layers  126  and  128  have been removed using a wet etch. By using a wet etch the recess shown in  134  and  136  in FIG. 120 is significantly less than it would be if a dry etch were used. A typical wet etch chemistry is phosphoric acid. A typical dry etch for nitride is CF4+HBO. The wet etch is effective in this situation because sidewall spacer  122  protects interlayer dielectric  120 . A wet etch without sidewall spacer  122  protecting interlayer dielectric  120  would degrade dielectric layer  120  and cause a problem between the storage element  114  and the control gate  118 . It is important that there not be leakage between storage element  114 , which in this depicted case is a floating gate, and control gate  118 . With the protection of sidewall spacer  122 , the wet etch will not harm interlayer dielectric  120 . This also shows the resulting transistor  113  with ARC  128  removed.  
         [0015]    Shown in FIG. 21 is device structure  110  after an extension implant using sidewall spacer  122  as a mask and sidewall spacer  124  as a mask. The resulting source/drain extension regions  138 ,  140 ,  142 , and  144  are formed. Shown in FIG. 22 is device structure  110  after deposition of a liner  146  and a nitride layer  148 . Nitride layer  148  is then anisotropically etched to form sidewall spacer  150  and sidewall spacer  152 . Liner  146  is substantially, if not completely, removed in those areas where it is exposed as a consequence of the removal of nitride layer  148  in the forming of sidewall spacers  150  and  152 . Shown in FIG. 24 is device structure  110  after a heavy implant to form heavily doped source/drain regions  154 ,  156 ,  158 , and  160  using sidewall spacers  150  and  152  as a mask.  
         [0016]    Shown in FIG. 25 is device structure  110  after silicide formation to form silicide regions  170 ,  172 ,  174 , and  176 . Thus the source/drain regions  142  and  144  have, to a large extent, been consumed by silicide regions  170 ,  172 ,  174 , and  176 . Similarly, gate regions  114  and  116  have been somewhat consumed by silicide regions  164  and  168  respectively. This leaves a polysilicon portion  167  for transistor  111  and a polysilicon portion  166  for transistor  113 . Source/drain portions  178 ,  180 ,  182 , and  184  expand and diffuse sufficiently to overlap gate regions  167  and  166  although there is a recess to overcome caused by removal of the ARC. Such ARC removal is by wet etch so that the amount of the recess is significantly less then that of a dry etch. Although the dry etch is preferred, in the case of a non-volatile memory the significance of having sufficient overlap is greater than for a regular transistor. Thus it is more important that the overlap between the floating gate, the area that has charge storage in it, to have good overlap in the source/drain area. Also, by having the ARC removed after formation of sidewall spacer  122 , the location of the recess does not have as severe of an impact as for the case depicted in FIGS.  1 - 9  in which the ARC removal occurs prior to formation of such sidewall spacer. In the case of FIGS.  1 - 9 , the sidewall spacer  24  is formed after removal of the ARC layer.