Abstract:
A method of forming a memory device on a semiconductor substrate having a memory region (with floating and control gates), a first logic region (with first logic gates) and a second logic region (with second logic gates). A first implantation forms the source regions adjacent the floating gates in the memory region, and the source and drain regions adjacent the first logic gates in the first logic region. A second implantation forms the source and drain regions adjacent the second logic gates in the second logic region. A third implantation forms the drain regions adjacent the control gates in the memory region, and enhances the source region in the memory region and the source/drain regions in the first logic region. A fourth implantation enhances the source/drain regions in the second logic region.

Description:
RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Application No. 62/172,319, filed Jun. 8, 2015, and which is incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to non-volatile memory cells, and more particularly to a method of forming such cells on the same wafer as logic devices. 
       BACKGROUND OF THE INVENTION 
       [0003]    Split-gate type memory cell arrays are known. For example, U.S. Pat. No. 5,029,130, which is incorporated herein by reference for all purposes, discloses a split gate memory cell and its formation, which includes forming source and drain regions in the substrate with a channel region there between. A floating gate is disposed over and controls the conductivity of one portion of the channel region, and the control gate is disposed over and controls the conductivity of the other portion of the channel region. The control gate extends up and over the floating gate. 
         [0004]    It is also known to form high voltage logic devices on the same wafer (substrate) as the split-gate memory cell array.  FIGS. 1A-10A, 1B-10B and 1C-10C  show the steps in forming high voltage logic devices (e.g. 12 volt logic devices) on the same wafer as the split gate memory cells. A semiconductor substrate  10  is masked (i.e. photo resist is deposited, selectively exposed using a mask, and selectively removed, using a photolithographic process, leaving portions of the underlying material covered by remaining photo resist while leaving other portions of the underlying material (here the substrate) exposed). The exposed substrate portions are etched away leaving trenches that are then filled with dielectric material  12  (e.g. oxide) to form isolation regions in the memory cell region  14  of the wafer (see  FIG. 1A ), in the NMOS logic region  16  of the wafer (see  FIG. 1B ) and in the PMOS logic region  18  of the wafer (see  FIG. 1C ), all shown after the photo resist is removed. The wafer is then masked again, but this time to cover the NMOS logic and memory cell regions  16  and  14  with photo resist  20 , while leaving the PMOS logic region  18  exposed. A high voltage NWEL implant is then performed on the exposed PMOS logic region  18 , as shown in  FIGS. 2A, 2B and 2C . The photo resist  20  blocks the implantation from the memory cell and NMOS logic regions  14  and  16  of the wafer. The photo resist  20  is removed. The wafer is then masked to cover the PMOS logic region  18  with photo resist  22 , but leaving the NMOS logic and memory cell regions  16  and  14  exposed. A high voltage PWEL implant is performed on the exposed NMOS logic and memory cell regions  16  and  14  as shown in  FIG. 3A, 3B and 3C . 
         [0005]    After the photo resist  22  is removed, a layer of oxide  24  (FG oxide) is formed on the substrate  10 , a layer of polysilicon  26  (FG poly) is formed on oxide  24 , and a layer of nitride  28  (FG nitride) is formed on poly layer  24 , as shown in  FIGS. 4A, 4B and 4C . The wafer is masked, leaving photo resist  30  on the wafer except on selected locations of the nitride  28  which are left exposed in the memory cell region  14 . The exposed nitride  28  is etched using an appropriate nitride etch to expose portions of poly layer  26 , as shown in  FIGS. 5A, 5B and 5C . The exposed portions of the FG poly layer  26  are oxidized using an oxidation process, forming oxide areas  32  on the FG poly  26 .  FIGS. 6A, 6B and 6C  show the resulting structure after the photo resist  30  is removed. A nitride etch is used to remove the remaining nitride layer  28 . An anisotropic poly etch is used to remove exposed portions of the poly layer  26 , leaving blocks of polysilicon  26  underneath the oxide areas  32  in the memory cell region  14  (which will constitute the floating gates of the memory cells), as shown in  FIGS. 7A, 7B and 7C . 
         [0006]    An oxide layer  34  is formed over the structure. After additional masking and implant steps (logic NWEL, IO NWEL, logic PWEL, IO PWEL, LLVOX and LVOX), a layer of polysilicon is deposited over the wafer. The structure is masked leaving portions of the poly layer exposed, which are then removed by a poly etch. The remaining portions of the poly layer constitute the control gates  36   a  in the memory cell region  14 , logic gate  36   b  in the NMOS logic region  16 , and logic gate  36   c  in the PMOS logic region  18 . The resulting structure is shown in  FIGS. 8A, 8B and 8C  (after the photo resist has been removed). The structure is masked again leaving only portions of the memory cell region between pairs of adjacent floating gate poly blocks  26  exposed by photo resist  38 . An implantation is performed to form source regions  40  in the portions of the substrate between the floating gate poly blocks  36   a,  as shown in  FIGS. 9A, 9B and 9C . 
         [0007]    After the photo resist  38  is removed and after additional masking and implant steps (logic NLDD, IO NLDD, logic PLDD and IO PLDD), the wafer is masked again, leaving the PMOS logic and memory cell regions  18  and  14  covered by photo resist, but leaving the NMOS logic area  16  exposed. An LDD implantation is then performed on the NMOS logic region  16 . The photo resist is removed. The wafer is masked again, leaving the NMOS logic and memory cell regions  16  and  14  covered by photo resist, but leaving the PMOS logic region  18  exposed. An LDD implantation is then performed on the PMOS logic region  18 . After photo resist removal, the wafer is masked covering portions of the structure with photo resist but leaving the NMOS logic region  16  exposed and those portions of the memory cell region  16  adjacent the control gate poly blocks  36   a  exposed. An N+ implantation is used to form the source/drain regions  44  and  45  in the NMOS logic region  16  and drain regions  46  in the memory cell region  14 . The photo resist is removed. The wafer is masked leaving just the PMOS logic region  18  exposed by photo resist, and a P+ implantation is used to form the source/drain regions  48  and  49  in the PMOS logic region  18 . 
         [0008]    The photo resist is removed. The process continues by forming insulation spacers  50 , silicide layers  52  on the poly blocks  36   a,    36   b  and  36   c  and on all the source/drain regions, and insulation layers  54 - 57 , as shown  FIGS. 10A, 10B and 10C . This back end processing includes at least two more masking steps (silicide blocking to limit silicide formation, and back end processing to create the contacts  58  through the insulation over the drain regions in the memory cell region and over the source/drain regions in the logic device regions). 
         [0009]    The above technique produces non-volatile memory cells (each with a source  40 , drain  46 , floating gate  26 , control gate  36   a ) on the same substrate as high voltage NMOS logic devices (each with a logic gate  36   b,  source  44  and drain  45 ) and high voltage PMOS logic devices (each with a logic gate  36   c,  source  48  and drain  49 ). It would be desirable to reduce the complexity and cost of manufacturing the memory cells and logic devices, including the number of masking steps used. 
       BRIEF SUMMARY OF THE INVENTION 
       [0010]    The aforementioned problems and needs are addressed by a method of forming a memory device that includes: 
         [0011]    providing a semiconductor substrate having a memory region, a first logic region and a second logic region; 
         [0012]    forming a pair of spaced apart floating gates in the memory region; 
         [0013]    forming a pair of control gates in the memory region, wherein each control gate has a first portion adjacent to one of the floating gates and a second portion that extends up and over one of the floating gates; 
         [0014]    forming a first logic gate in the first logic region; 
         [0015]    forming a second logic gate in the second logic region; 
         [0016]    forming a first photo resist that covers the second logic region and portions of the substrate adjacent to the control gates in the memory region, but not the first logic region and not a portion of the substrate between the pair of floating gates; 
         [0017]    performing a first implantation that forms a source region in the substrate between the pair of floating gates, a source region in the substrate adjacent a first side of the first logic gate, and a drain region in the substrate adjacent a second side of the first logic gate opposite the first side of the first logic gate; 
         [0018]    removing the first photo resist; 
         [0019]    forming a second photo resist that covers the first logic region and the memory region, but not the second logic region; 
         [0020]    performing a second implantation that forms a source region in the substrate adjacent a first side of the second logic gate and a drain region in the substrate adjacent a second side of the second logic gate opposite the first side of the second logic gate; 
         [0021]    removing the second photo resist; 
         [0022]    forming a third photo resist that covers the second logic region, but not the memory region and not the first logic region; 
         [0023]    performing a third implantation that forms drain regions in the substrate adjacent the control gates; 
         [0024]    removing the third photo resist. 
         [0025]    Other objects and features of the present invention will become apparent by a review of the specification, claims and appended figures. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0026]      FIGS. 1A-10A  are side cross sectional views illustrating conventional steps for forming memory cells in a memory cell region of the wafer. 
           [0027]      FIGS. 1B-10B  are side cross sectional views illustrating conventional steps for forming a logic device in an NMOS logic region of the wafer. 
           [0028]      FIGS. 1C-10C  are side cross sectional views illustrating conventional steps for forming a logic device in a PMOS logic region of the wafer. 
           [0029]      FIGS. 11A-23A  are side cross sectional views illustrating steps for forming memory cells in a memory cell region of the wafer. 
           [0030]      FIGS. 11B-23B  are side cross sectional views illustrating steps for forming a logic device in an NMOS logic region of the wafer. 
           [0031]      FIGS. 11C-23C  are side cross sectional views illustrating steps for forming a logic device in a PMOS logic region of the wafer. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0032]    It has been discovered that by reducing the operating voltages on the logic devices (i.e. from 12 volts to 5 volts), significant reduction on the complexity and cost of manufacturing the memory cells and logic devices can be achieved. In fact, the number of masking steps can be reduced significantly. 
         [0033]      FIGS. 11A-23A, 11B-23B and 11C-23C  show the steps in forming high voltage logic devices (e.g. 5 volt logic devices) on the same wafer (substrate) as the split gate memory cells according to the present invention. A semiconductor substrate  60  is masked (i.e. photo resist is deposited, selectively exposed using a mask, and selectively removed, using a photolithographic process, leaving portions of the underlying material covered by remaining photo resist while leaving other portions of the underlying material (here the substrate) exposed). The exposed substrate portions are etched away leaving tranches that are then filled with dielectric material  62  (e.g. oxide) to form isolation regions in the memory cell region  64  of the wafer (see  FIG. 11A ), in the NMOS logic region  66  of the wafer (see  FIG. 11B ) and in the PMOS logic region  68  of the wafer (see  FIG. 11C ). After the photo resist is removed, the wafer is then masked again, but this time to cover the PMOS logic region  68  with photo resist  70 , but leaving the memory cell and NMOS logic regions  64  and  66  exposed. A 5V PWEL implant is then performed on the exposed memory cell and NMOS logic regions  64  and  66  (e.g., to form P-wells in the N type substrate in the memory cell region  64  and NMOS logic region  66 ), as shown in  FIGS. 12A, 12B and 12C . The photo resist blocks the implantation from the PMOS logic region  68  of the wafer. 
         [0034]    After the photo resist  70  is removed, a layer of oxide  72  (FG oxide) is formed on the wafer, a layer of polysilicon  74  (FG poly) is formed on oxide  72 , and a layer of nitride  76  (FG nitride) is formed on poly layer  74 , as shown in  FIGS. 13A, 13B and 13C . The wafer is masked, leaving photo resist  78  on the wafer except on selected portions of the nitride  76  which are left exposed in the memory cell region  64 . The exposed nitride  76  is etched using an appropriate nitride etch to expose portions of poly layer  74 , as shown in  FIGS. 14A, 14B and 14C . The exposed portions of poly layer  74  are oxidized using an oxidation process, forming oxide areas  80  on the FG poly.  FIGS. 15A, 15B and 15C  show the resulting structure after the photo resist  78  is removed. A nitride etch is used to remove the remaining nitride layer  76 . An anisotropic poly etch is used to remove the poly layer  74  except those portions underneath the oxide areas  80  in the memory cell region  74 , leaving blocks of polysilicon  74  that will constitute the floating gates of the memory cells, as shown in  FIGS. 16A, 16B and 16C . 
         [0035]    The wafer is then masked to cover the NMOS logic region  66 , and the memory cell region (except for those areas between adjacent FG poly blocks), with photo resist  82 . 
         [0036]    An implant (5V PMOS/PH) is performed on those areas left exposed by the photo resist  82 , as shown in  FIGS. 17A, 17B and 17C . After the photo resist  82  is removed, an oxide layer  84  is formed on the structure and the wafer. After additional masking and implant steps (Core PWEL for logic NMOS and LVOX for open core oxide region), a layer of polysilicon is deposited over the wafer. The structure is masked leaving portions of the poly layer exposed, which are then removed by a poly etch. The remaining portions of the poly layer constitute the control gates  86   a  in the memory cell region  64 , and the logic gates  86   b  and  86   c  in the NMOS and PMOS logic regions  66  and  68  respectively. The resulting structure is shown in  FIGS. 18A, 18B and 18C  (after the photo resist has been removed). 
         [0037]    After an additional masking and implant step (Core NLDD for logic NMOS and LDD), the structure is masked again leaving only the NMOS region  66  and those areas between adjacent floating gate poly blocks  74  in the memory cell region  64  exposed by photo resist  87 , followed by a 5V NLDD implantation to form the source regions  88  in the portions of the substrate between the floating gate poly blocks  74  in the memory cell region  64  and to form the source and drain regions  90  and  91  in the NMOS logic region  66 , as shown in  FIGS. 19A, 19B and 19C . After the photo resist  87  is removed, and after an additional masking and implant step (Core PLDD), the structure is masked to leave only the PMOS logic region  68  exposed from photo resist  92 . This is followed by a 5V PLLD PH implantation to form source and drain regions  94  and  95  in the PMOS logic region  68 , as shown in  FIGS. 20A, 20B and 20C . The purpose of the NLDD and PLLD implants is to mitigate the effect of hot carrier injection (HCI) damage and make the effective channel length shorter. 
         [0038]    After photo resist  92  is removed, the structure is masked to cover PMOS logic region  66  with photo resist  96 , which is followed by an implantation (NNII−N+) to enhance the source region  88  and form drain regions  101  in the memory cell region  64 , and enhance the source and drain regions  90  and  91  in the NMOS logic region  66 , as shown in  FIGS. 21A, 21B and 21C . After the photo resist  96  is removed, the wafer is masked with photo resist  98  except for the PMOS logic region  68 , and a P+ implantation is used to enhance the source/drain regions  94 / 95  in the PMOS logic region  68 , as illustrated in  FIGS. 22A, 22B and 22C . 
         [0039]    The process continues by forming insulation spacers  100  (e.g. by oxide deposition and etch), silicide layers  102  on the poly blocks  86   a,    86   b  and  86   c  and on all source/drain regions, and insulation layers  104 - 107 , as shown in  FIGS. 23A, 23B and 23C . This back end processing includes at least two more masking steps (silicide blocking to limit silicide formation, and back end processing for etching through insulation layers  104 - 107  to create contact holes  108  through the insulation over the drain regions in the memory cell region and over the source/drain regions in the logic device regions). 
         [0040]    By forming high voltage logic devices that operate at a lower voltage (e.g. 5 volts) than done in the prior art (e.g. 12 volts), it allows for certain logic region implantations to be shared with the memory cell region that could not be shared before. These different sharing arrangements allow for a reduction of masking steps from  22  down to  15  in forming the memory cells and logic devices on the same wafer. 
         [0041]    It is to be understood that the present invention is not limited to the embodiment(s) described above and illustrated herein, but encompasses any and all variations falling within the scope of the appended claims. For example, references to the present invention herein are not intended to limit the scope of any claim or claim term, but instead merely make reference to one or more features that may be covered by one or more of the claims. Materials, processes and numerical examples described above are exemplary only, and should not be deemed to limit the claims. Further, as is apparent from the claims and specification, not all method steps need be performed in the exact order illustrated or claimed. Additionally, the above method is illustrated with an N type substrate and P wells formed in the memory cell region and the NMOS logic region. However, a P type substrate can be used, in which case an N well can be formed in the PMOS logic region. Lastly, single layers of material could be formed as multiple layers of such or similar materials, and vice versa. 
         [0042]    It should be noted that, as used herein, the terms “over” and “on” both inclusively include “directly on” (no intermediate materials, elements or space disposed there between) and “indirectly on” (intermediate materials, elements or space disposed there between). Likewise, the term “adjacent” includes “directly adjacent” (no intermediate materials, elements or space disposed there between) and “indirectly adjacent” (intermediate materials, elements or space disposed there between), “mounted to” includes “directly mounted to” (no intermediate materials, elements or space disposed there between) and “indirectly mounted to” (intermediate materials, elements or spaced disposed there between), and “electrically coupled” includes “directly electrically coupled to” (no intermediate materials or elements there between that electrically connect the elements together) and “indirectly electrically coupled to” (intermediate materials or elements there between that electrically connect the elements together). For example, forming an element “over a substrate” can include forming the element directly on the substrate with no intermediate materials/elements there between, as well as forming the element indirectly on the substrate with one or more intermediate materials/elements there between.