Abstract:
To fabricate a semiconductor memory, one or more pairs of first structures are formed over a semiconductor substrate. Each first structure comprises (a) a plurality of floating gates of memory cells and (b) a first conductive line providing control gates for the memory cells. The control gates overlie the floating gates. Each pair of the first structures corresponds to a plurality of doped regions each of which provides a source/drain region to a memory cell having the floating and control gates in one or the structure and a source/drain region to a memory cell having floating and control gates in the other one of the structures. For each pair, a second conductive line is formed whose bottom surface extends between the two structures and physically contacts the corresponding first doped regions. In some embodiments, the first doped regions are separated by insulation trenches. The second conductive line may form a conductive plug at least partially filling the region between the two first structures.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to semiconductor technology, and more particularly to nonvolatile memories. 
     FIGS. 1-8 illustrate fabrication of a conventional nonvolatile stacked-gate flash memory described in U.S. Pat. No. 6,013,551 issued Jan. 11, 2000 to J. Chen et al. Silicon oxide layer  108  (“tunnel oxide”) is grown on P-type silicon substrate  150 . Doped polysilicon  124  is deposited over oxide  108 . Polysilicon  124  will provide floating gates for memory cell transistors. 
     Mask  106  is formed over the structure. Polysilicon  124 , oxide  108 , and substrate  150  are etched through the mask openings. Trenches  910  are formed in the substrate as a result (FIG.  2 ). 
     As shown in FIG. 3, the structure is covered with dielectric which fills the trenches. More particularly, silicon oxide  90  is grown by thermal oxidation. Then silicon oxide  94  is deposited by PECVD (plasma enhanced chemical vapor deposition). Then thick silicon oxide layer  96  is deposited by SACVD (subatomspheric chemical vapor deposition). 
     The structure is subjected to chemical mechanical polishing (CMP). Polysilicon  124  becomes exposed during this step, as shown in FIG.  4 . 
     As shown in FIG. 5, ONO (silicon oxide, silicon nitride, silicon oxide) layer  98  is formed on the structure. Silicon  99  is deposited on top. Then tungsten silicide  100  is deposited. 
     Then a mask is formed (not shown), and the layers  100 ,  99 ,  98 ,  124  are patterned (FIG.  6 ). Layer  124  provides floating gates, and layers  99 ,  100  provide control gates and wordlines. 
     Then mask  101  is formed over the structure, as shown in FIG.  8 . Silicon oxide etch removes those portions of oxide layers  90 ,  94 ,  96  which are exposed by mask  101 . After the etch, the mask remains in place, as dopant is implanted to form source lines  103 . 
     Other implantation steps are performed to properly dope the source and drain regions. 
     Alternative memory structures and fabrication methods are desirable. 
     SUMMARY 
     To fabricate a semiconductor memory, one or more pairs of first structures are formed over a semiconductor substrate. Each first structure comprises (a) a plurality of floating gates of memory cells and (b) a first conductive line providing control gates for the memory cells. The control gates overlie the floating gates. Each pair of the first structures corresponds to a plurality of doped regions each of which provides a source/drain region to a memory cell having the floating and control gates in one of the structures and a source/drain region to a memory cell having floating and control gates in the other one of the structures. For each pair, a second conductive line is formed whose bottom surface extends between the two structures and physically contacts the corresponding first doped regions. In some embodiments, the first doped regions are separated by insulation trenches. The second conductive line may form a conductive plug at least partially filling the region between the two first structures. 
     Other features and advantages of the invention are described below. The invention is defined by the appended claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1-7 are cross section illustrations of a prior art flash memory at different stages of fabrication. 
     FIG. 8 is a top view of the memory of FIGS. 1-7. 
     FIG. 9A is a top view of a memory according to some embodiments of the present invention. 
     FIGS. 9B,  9 C are cross section illustrations of the memory of FIG.  9 A. 
     FIG. 10A is a circuit diagram of the memory of FIG.  9 A. 
     FIG. 10B is a top view of the memory of FIG.  9 A. 
     FIGS. 11,  12 A are cross section illustrations of the memory of FIG. 9A at different stages of fabrication. 
     FIG. 12B is a top view of the structure of FIG.  12 A. 
     FIGS. 13-15,  16 A,  16 B,  17 A,  17 B,  18 A,  18 B,  19 A,  19 B,  20 A,  20 B,  21 ,  22 A,  22   23  are cross section illustrations of memory embodiments of the present invention. 
     FIG. 24 is a top view of a memory embodiment of the present invention. 
     In the drawings, the reference numbers are used as indicated in the following table. The list of the reference numbers in this table is not exhaustive. The description of the features is not complete, and is not limiting. For example, silicon dioxide can be replaced with other insulators. Not all of the functions described for a reference number have to be present in the invention, and also functions not described can be present. 
     
       
         
               
               
             
               
               
             
           
               
                   
               
               
                 Reference 
                   
               
               
                 Number 
                 Feature 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 98 
                 Insulator isolating the floating gates 124 from the control 
               
               
                   
                 gates 128 
               
               
                 108 
                 Tunnel oxide 
               
               
                 124 
                 Floating gates 
               
               
                 128 
                 Control gates 
               
               
                 130 
                 Bitlines 
               
               
                 134 
                 Bitline regions of memory cells 
               
               
                 138 
                 Bitline contacts to memory cells 
               
               
                 144 
                 Source line regions (doped regions in the substrate) 
               
               
                 150 
                 Isolated substrate region 
               
               
                 520S 
                 Polysilicon source lines 
               
               
                 520W 
                 Wordlines 
               
               
                 710 
                 Stacks including the floating and control gates 
               
               
                 720 
                 Silicon nitride at the top of stacks 710 
               
               
                 903 
                 Silicon nitride on sidewalls of stacks 710 
               
               
                 904 
                 Photoresist mask used to pattern the floating gate polysilicon 
               
               
                   
                 124 and the isolation trenches 
               
               
                 905 
                 Substrate 
               
               
                 910 
                 Isolation trench 
               
               
                 1010 
                 Insulation in isolation trenches 
               
               
                 1103 
                 N-region isolating the substrate region 150 from below 
               
               
                 1105 
                 N-region isolating the substrate region 150 laterally on all 
               
               
                   
                 sides 
               
               
                 1203 
                 Silicon nitride that serves as a stop layer during the etch of 
               
               
                   
                 trench insulation 1010 
               
               
                 1510 
                 Silicon dioxide insulating the floating gate sidewalls 
               
               
                 1810 
                 Gate oxide for select transistors 
               
               
                 2013 
                 Photoresist mask for patterning source line regions 144 
               
               
                 2110 
                 Deep source line implant 
               
               
                 2401 
                 Source line and bitline region implant 
               
               
                   
               
             
          
         
       
     
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     The description of the preferred embodiments is illustrative and not limiting. The invention is not limited by any particular dimensions, materials, processing steps, doping levels, crystal orientation, layer thicknesses, layouts, or any other features, unless expressly stated otherwise. 
     FIG. 9A is a top view of a flash memory array of self-aligned triple-gate memory cells  120 . FIG. 9B illustrates a cross section of the array along the line  9 B— 9 B in FIG.  9 A. FIG. 9C illustrates a cross section along the line  9 C— 9 C in FIG.  9 A. FIG. 10A is a circuit diagram of the array. FIG. 10B is a top view illustrating some additional features. 
     In FIGS. 9A,  10 A,  10 B, bitlines  130  extend horizontally. The bitlines are formed from a conductive layer overlying the memory cells (for example, aluminum or tungsten, not shown in FIGS. 9B,  9 C). The bitlines contact the memory cells&#39; bitline regions  134  in contact regions  138 . Source lines  520 S extend vertically between the adjacent row structures  710 . The source lines  520 S physically contact the memory cells&#39; source line regions  144 . Each row structure  710  includes a conductive control gate line  128  (e.g. doped polysilicon) extending vertically and providing control gates for a row of memory cells. Floating gates  124  (made of doped polysilicon, for example) underlie the control gates  128 . Each floating gate extends between adjacent isolation trenches  910 . Trenches  910  extend horizontally between the bitlines  130 . 
     Each structure  710  is a self-aligned stack. 
     Conductive wordlines  520 W (e.g. doped polysilicon) are perpendicular (or at some other angle) to the bitlines. Each wordline  520 W provides select gates for a row of memory cells. Each wordline  520 W is a self-aligned sidewall spacer formed over a sidewall of a corresponding stack  710 . Wordlines  520 W are insulated from the adjacent control gates  128  and floating gates  124  by silicon nitride spacers  903  and silicon dioxide  1510 . Layers  903 ,  1510  can be formed without a mask. 
     As shown in FIG. 10A, each row of memory cells has two cells  120  between each two adjacent bitlines  130 . Each row has a control gate line  128  and a wordline  520 W. Two adjacent memory rows share a source line  144 . In each memory cell  120 , an NMOS select transistor  120 S and a floating gate transistor  120 F are connected in series. The gate of the select transistor  120 S is provided by wordline  520 W. The control gate of the transistor  120 F is provided by line  128 . 
     Each cell  120  can be erased by Fowler-Nordheim tunneling of electrons from its floating gate  124  (FIG. 9B) through silicon dioxide  108  to source line region  144  or substrate region  150 . (Region  150  contains the channel regions of the memory cells.) The cell can be programmed by source-side hot electron injection. The term “source-side hot electron injection” assumes that a cell&#39;s bitline region  134  is called a “source”. At other times, this region is called a drain, and the source line region  144  is called a source. Each of regions  134 ,  144  may also be called a source/drain region. The invention is not limited by any particular terminology. 
     The beginning fabrication stages for one embodiment of the memory of FIGS. 9A-10B are identical to the respective fabrication stages of a memory described in U.S. patent application Ser. No. 09/640,139 filed on Aug. 15, 2000 by H. T. Tuan et al., entitled “Nonvolatile Memory Structures and Fabrication Methods” (now U.S. Pat. No. 6,355,524), incorporated herein by reference. More particularly, the memory can be formed in and over an isolated P-type region  150  of monocrystalline silicon substrate  905  (FIG.  11 ). In one embodiment, region  150  is formed as follows. N type dopant is implanted into substrate  905  by ion implantation through a mask opening to form an N-region  1103  which insulates the region  150  from below. In a separate ion implantation step or series of steps, using another mask (not shown), N type dopant is implanted to form an N-region  1105  completely surrounding the region  150  on all sides. In some embodiments, this step creates also N wells (not shown) in which peripheral PMOS transistors will be formed for peripheral circuitry. Such circuitry may include sense amplifiers, input/output drivers, decoders, voltage level generators. 
     Regions  1103 ,  1105  are at a voltage equal to or above the voltage of substrate region  150  during memory operation. The areas  1107  of substrate  905  that surround the regions  1103 ,  1105  are at some voltage equal to or below the voltage of the regions  1103 ,  1105 . In some embodiments, the regions  150 ,  1103 ,  1105  are shorted together, and the region  1107  is at ground. 
     The invention is not limited to a particular region  150  isolation technique, or to memories having an isolated substrate region. 
     As shown in FIG. 12A, silicon dioxide  108  (tunneling oxide) is grown on substrate  905  by thermal oxidation. In some embodiments, the oxide is grown to a thickness of 9 nm. 
     Conductive polysilicon layer  124  is formed on oxide  108 . In some embodiments, polysilicon  124  is deposited to a thickness of  120  nm by LPCVD (low pressure chemical vapor deposition), and is lightly doped (N type) during or after deposition. Layer  124  will provide the floating gates and, possibly, other circuit elements as needed for the peripheral circuitry. Such elements may include interconnects, transistor gates, resistors, capacitor plates. 
     Silicon nitride  1203  is deposited over polysilicon  124 . In some embodiment, nitride  1203  is deposited to a thickness of 120 nm by LPCVD. 
     Photoresist mask  904  is formed photolithographically over nitride  1203 . Nitride  1203  and polysilicon  124  are etched through the mask openings to form strips extending in the bitline direction through the memory array. In the top view of FIG. 12B, the “BL” axis indicates the bitline direction. The “WL” axis indicates the wordline direction. 
     A misalignment of mask  904  does not affect the cell geometry and hence may have to be accommodated, if at all, only at the array boundaries and in the peripheral areas (the areas in which the peripheral circuitry is located). 
     After the polysilicon etch, oxide  108  and substrate region  150  are etched through the openings in mask  904  to form isolation trenches  910  (FIG.  13 ). Isolation trenches for the peripheral circuitry (not shown) are also formed in this step. In some embodiments, the trench depth is 0.25 μm. 
     Then mask  904  is removed. 
     Whenever a masked etch of two or more layers is described herein, it is assumed, unless stated otherwise, that only the top layer may be etched using the mask. After the top layer is etched, the mask may be removed, and the remaining layers may be etched with the top layer as a mask, or even without a mask. For example, after the etch of nitride  1203 , the mask  904  may be removed, and then polysilicon  124 , oxide  108  and substrate  150  can be etched with nitride  1203  as a mask. Nitride  1203  may also be etched but is not completely removed. 
     Trench insulation  1010  (FIG. 13) fills the trenches  910  and covers the wafer. In some embodiments, insulation  1010  is formed as follows. A 13.5 nm layer of silicon dioxide is grown on the exposed surfaces of trenches  910  by a well-known RTO (rapid thermal oxide) process. Then a 480 nm layer of silicon dioxide is deposited by chemical vapor deposition (CVD) using high density plasma (HDP). 
     Trench insulation  1010  is subjected to chemical mechanical polishing (CMP) and/or some blanket etch process, until silicon nitride  1203  is exposed (FIG.  14 ). Nitride  1203  acts as a stop layer during this step. Then nitride  1203  is removed (by a wet etch, for example). Optionally, insulation  1010  is etched down also. The resulting structure may have a planar top surface as shown in FIG.  15 . Alternatively, the etch of insulation  1010  may expose the sidewalls of polysilicon  124 . This may improve the efficiency of the memory cells, as explained in the aforementioned U.S. patent application Ser. No. 09/640,139. 
     Then insulation  98  is formed. See FIGS. 9B,  9 C,  16 A,  16 B. FIGS. 16A,  16 B show memory array cross sections by planes parallel to the bitlines. In FIG. 16A, the cross section is taken between trenches  910 . In FIG. 16B, the cross sectional plane passes through a trench  910 . 
     Similarly, FIGS. 17A,  18 A,  19 A,  20 A,  21 ,  22 A,  23  illustrate cross sections taken between the trenches. FIGS. 17B,  18 B,  19 B,  20 B,  22 B illustrate cross sections taken along a trench  910 . 
     In some embodiments, the insulation  98  is ONO (oxide-nitride-oxide). 
     Layer  128  is formed on insulation  98 . In some embodiments, layer  128  is polysilicon deposited by LPCVD and doped N+ or P+ during or after deposition. In other embodiments, layer  128  is polysilicon covered by tungsten suicide. Other conductive materials can also be used. 
     A photoresist layer (not shown) is deposited and patterned photolithographically into a mask that contains strips extending in the wordline direction over the memory array. This mask defines stacks  710  (FIGS. 9A,  9 B,  9 C,  16 A,  16 B). This mask can also be used to pattern the polysilicon  128  and silicon nitride  720  in the peripheral areas (not shown) as described in the aforementioned U.S. patent application Ser. No. 09/640,139. Layer  128  may provide transistor gates, interconnects, and other features in the peripheral areas. A misalignment of this resist mask does not change the geometry of the memory cells and hence may have to be accommodated only at the boundaries of the memory array and in the peripheral areas. 
     Layers  720 ,  128 ,  98 ,  124 ,  108  are etched to define the stacks  710 . The resulting memory array cross sections are shown in FIGS. 16A,  16 B. 
     The structure is oxidized (e.g. by RTO, i.e. rapid thermal oxidation). As a result, silicon dioxide  1510  (FIGS. 17A,  17 B) is grown on the exposed surface of substrate region  150  to a thickness of 5 nm. This operation also results in oxidation of the exposed sidewalls of polysilicon layers  124 ,  128 . The horizontal thickness of oxide  1510  on the polysilicon sidewalls is 8 nm. 
     A thin conformal layer  903  of silicon nitride (FIGS. 18A,  18 B) is deposited to a 20 nm thickness by LPCVD. Layer  903  is etched anisotropically without a mask to form spacers over the sidewalls of stacks  710 . 
     This etch also removes exposed portions of oxide  1510 . Silicon dioxide is regrown on substrate region  150 . This oxide, shown at  1810  in FIG. 18A, will provide gate dielectric for the select transistors. An exemplary thickness of oxide  1810  is 5 nm. 
     In some embodiments, either nitride  903  or oxide  1510  is omitted. 
     A conductive layer  520 . 1  (FIGS. 19A,  19 B) is formed over the wafer. In some embodiments, layer  520 . 1  is polysilicon deposited by LPCVD and heavily doped during or after deposition. An exemplary thickness of layer  520 . 1  is 50 to 100 nm. Other thicknesses can also be used. 
     Photoresist mask  2013  is formed over the wafer and patterned photolithographically to expose the areas in which the source line regions  144  will be formed. See also FIGS. 20A,  20 B. In the embodiment of FIGS. 19A,  19 B, the mask exposes regions extending throughout the memory array between two adjacent stacks  710 . The longitudinal edges of mask  2013  can be positioned anywhere over the respective stacks  710 , so their positioning is not critical if the mask alignment tolerance is not more than one half of the width of a stack  710 . In some embodiments, the minimal feature size is 0.14 μm. The mask alignment tolerance is 0.07 μm. The width of each stack  710  is 0.14 μm, that is, twice the alignment tolerance. 
     Polysilicon  520 . 1  and oxide  1810  are removed from the areas exposed by the mask. Trench insulation  1010  in the exposed areas may be slightly reduced in thickness during the etch of oxide  1810 . 
     After the oxide etch, mask  2013  remains in place as N type dopant (e.g. phosphorus) is implanted into the wafer to heavily dope (N+) the source line regions  144 , as shown by arrows  2110  in FIG.  20 A. This is a “deep” implant done to enable the source lines to carry high voltages for erase and/or programming operations. The deep implant will also provide a suitable overlap between the doped source line regions and the floating gates  124  when the dopant diffuses laterally (as shown in FIG.  20 A). 
     In some embodiments, the dopant does not penetrate the insulation  1010 , so the bottoms of trenches  910  are not doped (see FIG.  20 B). Whether or not the dopant penetrates the insulation  1010 , insulation  1010  prevents the dopant from coming close or reaching the N-region  1103  (FIG.  11 ). Therefore, a high leakage current or a short between the source lines  144  and the region  1103  is avoided. In some embodiments, the top surface of region  1103  at the end of fabrication (after thermal steps) is about 1 μm below the top surface of substrate  905  (of region  150 ). The trench depth is 0.25 μm. 
     Then the resist  2013  is removed. Polysilicon  520 . 1  protects the oxide  1810  over the bitline regions  134  during the removal of resist  2013  and a subsequent wafer cleaning operation. 
     In some embodiments, the resist  2013  is removed before the implant  2110 . Polysilicon  520 . 1  acts as a mask during the implant. 
     In some embodiments, the implant  2110  is performed before the etch of polysilicon  520 . 1  or oxide  1810 . The implant is performed through the polysilicon or the oxide or both. In some embodiments, layer  520 . 1  is omitted. 
     Conductive polysilicon layer  520 . 2  (FIG. 21) is formed. In some embodiments, polysilicon  520 . 2  is deposited by LPCVD to a thickness of 300 nm, and is heavily doped during or after deposition. The dopant type (N+ or P+) is the same as for layer  520 . 1 . Layers  520 . 1 ,  520 . 2  are subjected to a blanket anisotropic etch (e.g. RIE) to form spacers  520 W over the sidewalls of stacks  710  on the side of the bitline regions  134  (FIGS. 22A,  22 B). Layers  520 . 1 ,  520 . 2  are etched off the top of stacks  710 . The vertical thickness of nitride  720  and polysilicon layers  520 . 1 ,  520 . 2 , can be adjusted to control the width of the polysilicon spacers. 
     Polysilicon plugs  520 S formed by polysilicon  520 . 2  fill the gaps between adjacent stacks  710  on the side of source line regions  144 . Each polysilicon plug  520 S forms a source line extending through the memory array and physically contacting the underlying source line regions  144 . The bottom surface of each plug  520 S physically contacts the trench insulation  1010 . We will sometimes refer to polysilicon layers  520 . 1 ,  520 . 2  collectively as layer  520 . 
     In addition to the wordlines and source lines, layer  520  can provide interconnects, transistor gates, and other circuit elements for the peripheral circuitry. For that purpose, layer  520  can be masked in the peripheral areas before it is etched. No such masking is needed over the memory array. 
     In some embodiments, polysilicon  520 . 2  does not entirely fill the regions between adjacent stacks  710  over the source line regions  144 . Polysilicon  520 . 2  may be recessed relative to the top of the stacks  710 . In some embodiments, polysilicon  520 . 2  forms spacers over the sidewalls of stacks  710  over the regions  144 . In this case, a source line  520 S consists of two such spacers shorted together by regions  144 . 
     A blanket N+ implant  2401  (FIG. 23) is performed to dope the bitline regions  134 . Stacks  710 , polysilicon  520 , and trench insulation  1010  mask the substrate during this implant. Polysilicon  520  is also implanted during this step. 
     This implant does not penetrate insulation  1010 , so the bitline regions  134  are not shorted together. 
     Memory fabrication can be completed using known techniques. Insulating layers (not shown) can be deposited. Contact openings such as  138  (FIG. 9A) can be formed. Conductive materials can be deposited and patterned to provide bitlines and other features as needed. 
     The gates of peripheral transistors can be formed from polysilicon layer  128  or  520 . See the aforementioned U.S. patent application Ser. No. 09/640,139. In some embodiments, some of the peripheral transistor gates or other features are formed using layer  128 , while other peripheral gates or features are formed using layer  520 . 
     In some embodiments, source lines  520 S are silicided to reduce their resistance. The silicidation can be performed using the source line silicidation techniques described in U.S. patent application Ser. No. 09/640,139. 
     FIG. 24 illustrates another flash memory array according to the present invention. Each isolation trench  910  extends between adjacent source line regions  144  but does not cross the source line regions. The boundaries of the isolation trenches are shown at  910 B. 
     This memory can be fabricated as follows. The substrate doping and the trench isolation can be performed as described in U.S. patent application Ser. No. 09/640,139. For example, trenches  910  can be defined by resist  904  (FIG. 12A) or by a combination of resist  904  with another resist layer. 
     The remaining fabrication steps can be identical to those described above in connection with FIGS. 16A-23. 
     In some embodiments of FIGS. 9A through 24, a memory cell is programmed (rendered non-conductive) via source-side hot electron injection. See W. D. Brown et al., “Nonvolatile Semiconductor Memory Technology” (1998), pages 21-23. 
     A memory cell can be erased using Fowler-Nordheim tunneling from floating gate  124  to source line region  144  or to substrate region  150 . 
     A memory may have multiple memory arrays, each with its own bitlines and wordlines. Different arrays may be fabricated in the same substrate region  150  or in different isolated regions  150  in the same integrated circuit. 
     The invention is not limited to the embodiments described above. The invention is not limited to any particular erase or programming mechanisms (e.g. Fowler-Nordheim or hot electron injection). The invention covers non-flash EEPROM memories and other memories, known or to be invented. The invention is not limited to the materials described. In particular, control gates, select gates, and other conductive elements can be formed from metals, metal suicides, polycides, and other conductive materials and their combinations. Silicon dioxide and silicon nitride can be replaced with other insulating materials. P and N conductivity types can be interchanged. The invention is not limited to any particular process steps or order of steps. For example, in some embodiments, thermal oxidation of silicon can be replaced with depositing silicon dioxide or some other insulator by chemical vapor deposition or some other technique, known or to be invented. The invention is not limited to silicon integrated circuits. Other embodiments and variations are within the scope of the invention, as defined by the appended claims.