Abstract:
A method of forming a memory device on a substrate having memory, LV and HV areas, including forming pairs of spaced apart memory stacks in the memory area, forming a first conductive layer over and insulated from the substrate, forming a first insulation layer on the first conductive layer and removing it from the memory and HV areas, performing a conductive material deposition to thicken the first conductive layer in the memory and HV areas, and to form a second conductive layer on the first insulation layer in the LV area, performing an etch to thin the first conductive layer in the memory and HV areas and to remove the second conductive layer in the LV area, removing the first insulation layer from the LV area, and patterning the first conductive layer to form blocks of the first conductive layer in the memory, LV and HV areas.

Description:
RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 62/106,531, filed Jan. 22, 2015. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to non-volatile memory cell arrays. 
     BACKGROUND OF THE INVENTION 
     It is well known in the art to form split-gate memory cells as an array of such cells. For example, U.S. Pat. No. 7,868,375 discloses an array of memory cells, where each memory cell includes a floating gate, a control gate, a select gate, an erase gate, all formed on a substrate with a channel region defined between a source and drain regions. For efficient use of space, the memory cells are formed in pairs, with each pair sharing a common source region and erase gate. 
     It is also known to form both low voltage and high voltage logic devices on the same wafer die as the array of memory cells. Such logic devices can include transistors each having a source and drain, and a poly gate controlling the conductivity of the channel region between the source and drain. Conventional logic device formation includes forming the poly gate first (preferably forming the memory cell erase and select gates, and forming the logic device poly gates, using the same poly deposition processing), followed by an LDD implant to form the source and drain regions, whereby the source/drain regions are self-aligned to the poly gate. The poly gate blocks prevent any of the implant from reaching the channel region under the gate. High voltage logic devices are designed to operate at higher voltages, and are typically made by using a higher LDD implant energy so that the source/drain regions formed thereby have a higher breakdown voltage. 
     One problem is that as device geometries continue to scale down to smaller sizes, the poly gates for the logic devices are becoming too thin to effectively block the HV LDD implant, which can penetrate through the relatively thin poly gate and into the channel region (which adversely affects performance). The conventional solution is to lower the implant energy for the HV LDD implant to prevent such poly layer penetration. However, the lower implant energy results in a lower gated-diode breakdown voltage, thus undesirably limiting the operational voltage of high voltage transistors. 
     BRIEF SUMMARY OF THE INVENTION 
     The aforementioned problems and needs are addressed by a method of forming a memory device. The method includes providing a silicon substrate (wherein the substrate has a memory area, an LV area and an HV area which are insulated from each other by insulation material extending into a surface of the substrate, and wherein the substrate has a first conductivity type), forming pairs of spaced apart memory stacks on the substrate and in the memory area (wherein each memory stack includes a floating gate disposed over and insulated from the substrate, and a control gate disposed over and insulated from the floating gate), forming a first conductive layer over and insulated from the substrate in the memory area, the LV area and the HV area (wherein the first conductive layer extends up and over the pairs of memory stacks), forming a first insulation layer on the first conductive layer in the memory area, the LV area and the HV area, removing the first insulation layer from the memory area and the HV area, while maintaining the first insulation layer in the LV area, performing a conductive material deposition to thicken the first conductive layer in the memory area and the HV area, and to form a second conductive layer on the first insulation layer in the LV area, performing an etch to thin the first conductive layer in the memory area and the HV area and to remove the second conductive layer in the LV area (wherein a top surface of the first conductive layer in the memory area and the HV area is higher than a bottom surface of the first insulation layer in the LV area), removing the first insulation layer from the LV area, and patterning the first conductive layer to form blocks of the first conductive layer in the memory area, the LV area and the HV area (wherein the blocks of the first conductive layer in the LV area have a height less than that of the blocks of the first conductive layer in the HV area). 
     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 
         FIGS. 1A-1P  are side cross sectional views illustrating the steps of forming the memory device of the present invention. 
         FIG. 2A  is a side cross sectional view of a simulation illustrating the desirable implantation that results from a sufficiently thick HV gate. 
         FIG. 2B  is a side cross sectional view of a simulation illustrating the undesirable implantation that results from an insufficiently thick HV gate. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention is a technique for forming low and high voltage logic devices on the same wafer die as the memory cell array, whereby sufficiently high voltage implantation can be employed to fabricate the high voltage logic devices without penetrating the high voltage logic device poly gate. 
     Referring to  FIGS. 1A-1P  there are shown cross-sectional views of the steps in the process to make a memory cell and low/high voltage logic devices on the same wafer. The process begins by forming a layer of silicon dioxide (oxide)  12  on a substrate  10  of P type single crystalline silicon. Thereafter a first layer  14  of polysilicon (or amorphous silicon) is formed on the layer  12  of silicon dioxide, as illustrated in  FIG. 1A . The first layer  14  of polysilicon is subsequently patterned in a direction perpendicular to the view of  FIG. 1A . 
     Another insulating layer  16 , such as silicon dioxide (or even a composite layer, such as ONO (oxide, nitride, oxide)) is formed on the first layer  14  of polysilicon. A second layer  18  of polysilicon is then formed on the oxide layer  16 . Another insulating layer  20  is formed on the second layer  18  of polysilicon and used as a hard mask during subsequent dry etching. In the preferred embodiment, the layer  20  is a composite NON layer, comprising silicon nitride  20   a , silicon dioxide  20   b , and silicon nitride  20   c . The resulting structure is shown in  FIG. 1B . The hard mask may instead be a composite layer of silicon oxide  20   b  and silicon nitride  20   c  (omitting nitrite  20   a ). The hard mask may also instead be formed with a thick silicon nitride layer  20   a  only. 
     Photoresist material (not shown) is coated on the structure, and a masking step is performed exposing selected portions of the photoresist material. The photoresist is developed and using the photoresist as a mask, the structure is etched. Specifically, the composite layer  20 , the second layer  18  of polysilicon, and the insulating layer  16  are anisotropically etched, until the first layer  14  of polysilicon is exposed. The resultant structure is shown in  FIG. 1C . Although only two “stacks” (S 1  and S 2 ) are shown, it should be clear that there are a number of such “stacks” that are separated from one another. 
     Silicon dioxide  22  is formed on the structure. This is followed by the formation of silicon nitride layer  24 . The silicon nitride  24  is anisotropically etched leaving a composite spacer  26  (which is the combination of the silicon dioxide  22  and silicon nitride  24 ) alongside each of the stacks S 1  and S 2 . Formation of spacers is well known in the art, and involves the deposition of a material over the contour of a structure, followed by an anisotropic etch process, whereby the material is removed from horizontal surfaces of the structure, while the material remains largely intact on vertically oriented surfaces of the structure (with a rounded upper surface). The resultant structure is shown in  FIG. 1D . 
     A layer of oxide is formed over the structure, followed by an anisotropical etch leaving spacers  30  of the oxide alongside the stacks S 1  and S 2 . A photoresist  28  is formed over the regions between the stacks S 1  and S 2 , and other alternating pairs of stacks S 1  and S 2 . For the purpose of this discussion, the region between the pair of stacks S 1  and S 2  will be called the “inner region” and the regions outside of the inner region (i.e. between adjacent pairs of stacks S 1  and S 2 ) will be referred to as the “outer regions”. The exposed spacers  30  in the outer regions are removed by isotropic etch. The resulting structure is shown in  FIG. 1E . 
     After the photoresist  28  is removed, the exposed portions first polysilicon  14  in the inner and outer regions are anisotropically etched. Part of oxide layer  12  will also be etched (removed) during the poly over-etching. A thinner layer of remaining oxide will preferably stay on the substrate  10  so as to prevent damage to the substrate  10 . The resultant structure is shown in  FIG. 1F . 
     A layer of oxide is formed over the structure, followed by an anisotropical etch leaving spacers  31  of the oxide alongside the stacks S 1  and S 2  and a layer  33  of oxide on substrate  34 . Another oxide layer is formed over the structure, thickening spacers  31  and layer  33 . Photoresist material  32  is then formed and masked leaving openings in the inner regions between the stacks S 1  and S 2 . Again, similar to the drawing shown in  FIG. 1E , the photoresist is between other alternating pairs of stacks. The resultant structure is subject to an ion implant (i.e. into exposed portions of substrate  10 ) forming source regions  34  therein (i.e. regions of conductivity type different than that of the substrate). The oxide spacers  31  adjacent to the stacks S 1  and S 2  and oxide layer  33  in the inner region are then removed by e.g. a wet etch. The resultant structure is shown in  FIG. 1G . 
     The photoresist material  32  in the outer regions of the stacks S 1  and S 2  is removed. A high-temperature thermal annealing step is applied to activate the ion implant and to complete the formation of the source junction (i.e. first or source region  34 ). Silicon dioxide  36  is then formed everywhere. The structure is once again covered by photoresist material  38  and a masking step is performed exposing the outer regions of the stacks S 1  and S 2  and leaving photoresist material  38  covering the inner region between the stacks S 1  and S 2 . An oxide anisotropical etch followed by isotropic wet etch are performed, to remove oxide  36  and oxide  33  from the outer regions of stacks S 1  and S 2 , and possibly to reduce the thickness of the oxide spacers  31  in the outer regions of the stacks S 1  and S 2 . The resultant structure is shown in  FIG. 1H . Each stack includes a floating gate  14  disposed over and insulated from the substrate, and a control gate  18  formed over and insulated from the floating gate  14 . The inner region (between stacks S 1  and S 2 ) includes a tunnel oxide  36  formed on the substrate and along the sidewalls of the stacks S 1  and S 2 ). 
     An oxide layer is formed on the surface of the substrate (or some of oxide  33  is left remaining after the previous oxide etch). After the photo resist  38  is removed, a polysilicon layer is formed over the structure, followed by the formation of a blocking oxide layer  44  on the poly layer  42 . The resulting structure is shown in  FIG. 1I , which is an expanded view showing the memory cell areas  46  (i.e., those area(s) of the wafer in which memory cells will be formed), LV (low voltage) logic device areas  48  (i.e., those area(s) of the wafer die in which the low voltage logic devices will be formed), and HV (high voltage) logic device areas  50  (i.e., those area(s) of the wafer die in which the high voltage logic devices will be formed). Areas  46 ,  48 ,  50  are separated from each other by STI insulation regions  52  (insulation filled trenches formed into the substrate). 
     Photoresist  54  is formed over the structure, and patterned using photolithography and a photoresist etch so that the photoresist only remains in the LV logic device area  48 . An oxide etch is then performed to remove the blocking oxide from the memory cell area(s)  46  and the high voltage logic device area(s)  50 , as illustrated in  FIG. 1J . After the photo resist  54  is removed, a second layer of polysilicon  56  is deposited over the structure, which thickens the poly  42  in the memory cell area(s)  46  and the HV logic device area(s)  50 , and results in dummy poly  56  over the blocking oxide  44  in the LV logic device area(s)  48 , as illustrated in  FIG. 1K . 
     A poly CMP (chemical-mechanical polish) etch is performed to reduce the thickness of the combined logic poly  42  and dummy poly  56 , as illustrated in  FIG. 1L . A further poly etch reduces the thickness of the poly  42  in the memory cell area(s)  46  and HV logic device area(s)  50 , but this thickness (e.g.  1000 A or thicker) is greater than the thickness of the poly  42  under the blocking oxide  44  in the LV logic device area(s)  48 . All of the dummy poly  56  over blocking oxide is removed by this poly etch. The resulting structure is shown in  FIG. 1M . 
     An oxide etch is used to remove the blocking oxide  44 . An N+ poly pre-doping implant can be performed (e.g. arsenic or phosphorus). Photo resist  58  can be formed and used to block this implant in those regions that it is not desired (e.g. P-tab/P-well pick-up), as illustrated in  FIG. 1N . 
     After the photo resist  58  is removed, new photo resist  60  is formed over the structure, and patterned using photolithography and photo resist etch to remove the photo resist  60  except for the memory cell, LV logic device and HV logic device areas  46 ,  48 ,  50 . A poly etch is then used to pattern the poly  42  by removing the exposed portions of poly  42 , leaving blocks of the poly  42   a  and  42   b  in the memory cell area(s)  46  that are the select gates and the erase gates respectively for the memory cells, leaving blocks of the poly  42   c  in the LV logic device area(s)  48  that are the conductive gates for the LV logic devices, and leaving blocks of the poly  42   d  in the HV logic device area(s)  50  that are the conductive gates for the HV logic devices. The thicknesses (height) of the select/erase gate poly blocks  42   a / 42   b  and HV logic device gate poly blocks  42   d  are greater than that of the LV logic device gate poly blocks  42   c , as shown in  FIG. 1O . 
     After the photo resist  60  is removed, and after an optional poly re-ox process is performed, memory cell LDD and LV logic device LDD implants are performed to form the drain regions  62  in the memory cell area(s)  46  and the source/drain regions  64 / 66  in the LV logic device area(s)  48 . Then, photo resist  68  is formed over the memory cell and LV logic device areas  46 / 48 , leaving the HV logic device area(s)  50  exposed. A HV logic device LDD implant is performed to form the high voltage source/drain regions  70 / 72  in the HV logic device area(s)  50 . The relatively thick gate poly  42   d  in the HV logic device area(s)  50  prevents penetration of the implant into the underlying channel regions under poly  42   d . The resulting structure is shown in  FIG. 1P . 
     Simulations show that the added thickness of the HV logic device poly gate effectively prevents undesirable implant penetration of the poly gate into the channel region. As shown in  FIG. 2B , using a 500 Å thick poly gate  74  and a phosphorous implant, some of the phosphorous penetrated the poly gate  74  and was implanted into the channel region  76  under the poly gate  74  (see channel implant  78 ). However, as shown in  FIG. 2A , using the same implant energy, a  1000 A thick poly gate  75  effectively blocked the implant from reaching the channel region  76 . Moreover, the thicker poly gate in the HV logic device area(s) is formed using the same poly processing steps as that for forming the poly gates in the LV logic device area(s) and the memory cell select gates. 
     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, but rather in any order that allows the proper formation of the pairs of memory cells and associated logic devices. Lastly, single layers of material could be formed as multiple layers of such or similar materials, and vice versa. 
     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.