US20010039089A1 - Memory array having a digit line buried in an isolation region and method for forming same - Google Patents
Memory array having a digit line buried in an isolation region and method for forming same Download PDFInfo
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- US20010039089A1 US20010039089A1 US09/900,341 US90034101A US2001039089A1 US 20010039089 A1 US20010039089 A1 US 20010039089A1 US 90034101 A US90034101 A US 90034101A US 2001039089 A1 US2001039089 A1 US 2001039089A1
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- H—ELECTRICITY
- H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
- H10B—ELECTRONIC MEMORY DEVICES
- H10B12/00—Dynamic random access memory [DRAM] devices
- H10B12/30—DRAM devices comprising one-transistor - one-capacitor [1T-1C] memory cells
- H10B12/31—DRAM devices comprising one-transistor - one-capacitor [1T-1C] memory cells having a storage electrode stacked over the transistor
- H10B12/318—DRAM devices comprising one-transistor - one-capacitor [1T-1C] memory cells having a storage electrode stacked over the transistor the storage electrode having multiple segments
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- H—ELECTRICITY
- H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
- H10B—ELECTRONIC MEMORY DEVICES
- H10B12/00—Dynamic random access memory [DRAM] devices
- H10B12/01—Manufacture or treatment
- H10B12/02—Manufacture or treatment for one transistor one-capacitor [1T-1C] memory cells
- H10B12/03—Making the capacitor or connections thereto
- H10B12/033—Making the capacitor or connections thereto the capacitor extending over the transistor
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
- Y10S—TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y10S257/00—Active solid-state devices, e.g. transistors, solid-state diodes
- Y10S257/905—Plural dram cells share common contact or common trench
Definitions
- the invention relates generally to memory arrays, and more particularly to a memory array having one or more bit, i.e., digit, lines buried in the isolation regions such as silicon-trench-isolation (STI) regions.
- STI silicon-trench-isolation
- DRAM Dynamic Random Access Memory
- one known technique for reducing the layout area of a Dynamic Random Access Memory (DRAM) array is to stack storage capacitors above memory cells.
- the memory cells are formed in adjacent pairs, where each pair shares a common source/drain region that is connected to a respective digit line. Because the digit lines are disposed above the stack capacitors, and thus above the common source/drain regions, conductive vias are needed to connect the digit lines to the respective common source/drain regions. Therefore, these vias must extend through or adjacent to the plates of the stacked storage capacitors.
- a problem with such a stacked-capacitor memory array is that the area of each memory cell, and thus the area of the memory array itself, often cannot be reduced without reducing the capacitances of the stacked capacitors beyond acceptable limits. Because capacitance is proportional to the overlap area of the capacitor plates, the plates of the stacked capacitors must have an overlap area that is large enough to give these capacitors the desired storage capabilities. But the vias that connect the common source/drain regions to the digit lines also have minimum dimensions that are proportional to the minimum feature size of the utilized semiconductor process. Therefore, because a via extends through a hole in a respective pair of stacked-capacitor plates, the minimum total area of a plate is the sum of the minimum required overlap area and the minimum required cross-section area of the intersecting via.
- the article “Buried Bit-Line Cell for 64 MB DRAMS,” proposes burying the bit-lines in the substrate. But, because these bit-lines are formed after the field oxide regions and because the contacts between the bit-lines and the respective memory cells are also buried in the substrate, the resulting reduction in memory-cell area falls short of the maximum obtainable reduction for a given minimum feature size.
- Another problem is that, even if it were possible to reduce the area of such a memory array by the maximum obtainable reduction, it would be difficult, if not impossible, to implement a folded-digit-line architecture in such a reduced-area array.
- the word lines have minimum dimensions that are dictated by the minimum feature size. Therefore, if the area of a memory cell is reduced too much, adjacent word lines may become short-circuited to each other, thus rendering the memory array defective.
- a memory array includes a semiconductor substrate, an isolation trench located in the substrate, and a conductor that is located in the trench.
- the array also includes a memory cell that is coupled to the conductor in the trench.
- the conductor is a digit line that is coupled to a source/drain region of the memory cell.
- the conductor in the isolation trench is a digit line
- no digit-line vias are required.
- the freed-up space can be used to increase the size, and thus the capacitances, of the stacked capacitors. Or, it can be used to reduce the size of the memory cells without reducing these capacitances.
- FIGS. 1 - 9 show a first embodiment of a method for forming a memory array according to the invention.
- FIGS. 1, 2B, 3 , and 4 B are cross-sectional views
- FIGS. 2A, 4A, 7 , and 8 are top plan views
- FIGS. 5, 6 and 9 are isometric views.
- FIGS. 10 - 17 show a second embodiment of a method for forming a memory array according to the invention.
- FIGS. 10 and 11B are cross-sectional views
- FIG. 11A is a top plan view
- FIGS. 12 - 17 are isometric views.
- FIGS. 18 - 19 are cross-sectional views showing an embodiment of a method for forming sub-lithographic word lines in a memory array according to the invention.
- FIGS. 20 - 31 show an embodiment of a method for forming stacked capacitors in a memory array according to the invention.
- FIGS. 20, 22, 23 A, and 24 A- 31 are cross-sectional views and
- FIGS. 21A, 21B, and 23 B are top plan views.
- FIG. 32 is a schematic diagram of a memory circuit that includes an embodiment of a memory array according to the present invention.
- FIG. 33 is a block diagram of an electronic system that incorporates a memory circuit according to the invention.
- FIGS. 1 - 9 show a first embodiment of a method for forming a memory array having buried digit lines.
- a pad structure 10 is conventionally formed on a semiconductor substrate 12 , which is formed from a material such as silicon.
- the pad structure 10 includes a gate insulator 14 that is disposed on the substrate 12 , a gate conductor 16 that is disposed on the insulator 14 , and a protective pad 18 that is disposed on the gate conductor 16 .
- the gate insulator 14 is silicon dioxide
- the gate conductor 16 is polysilicon
- the protective pad 18 is silicon nitride.
- FIG. 2A which is a top plan view
- FIG. 2B which is a cross section taken alone lines 2 B of FIG. 2A
- a plurality of buried digit lines 20 are formed in the substrate 12 .
- a photoresist mask (not shown) that exposes the patterns for a pair of isolation trenches 22 a and 22 b is conventionally formed on the pad 18 .
- the exposed regions of the pad 18 are conventionally etched, and then the mask is conventionally stripped.
- the remaining regions of the pad 18 mask the unexposed regions of the layers 14 and 16 and the substrate 12 while the exposed regions are conventionally etched to form the trenches 22 a and 22 b.
- the trenches 22 a and 22 b extend approximately 0.3 microns ( ⁇ m) into the substrate 12 .
- a sacrificial oxide layer (not shown) is conventionally grown in the trenches 22 a and 22 b and then conventionally stripped to smoothen the trench walls.
- a thin passivation oxide (not shown) is then conventionally grown in the trenches 22 a and 22 b.
- An oxide layer 24 is then conventionally formed in the trenches 22 a and 22 b.
- the layer 24 may be deposited using chemical vapor deposition (CVD) such as in the well-known TEOS process, or may be thermally grown. Because the alignment tolerance is approximately 1 ⁇ 3 the minimum line width allowed by the process, in one embodiment, the maximum thickness of the layer 24 is 1 ⁇ 3 of the trench width. This ensures proper alignment between the buried digit lines 20 a and 20 b and the respective source/drain contact straps (not shown in FIGS. 2A and 2B) as discussed below. Thus, in an embodiment where the trench width (before the formation of the layer 24 ) is approximately 40 nanometers (nm), the thickness of the layer 24 is approximately 10-15 nm, leaving trench openings that are approximately 10-20 nm wide.
- CVD chemical vapor deposition
- the trenches 22 a and 22 b are conventionally filled with a conductive material to form the digit lines 20 a and 20 b.
- a conductive material In one embodiment, heavily doped polysilicon, tungsten, or another suitable material is CVD deposited in the trenches 22 a and 22 b and on the pad 18 .
- CMP chemical-mechanical polishing
- the digit lines 20 a and 20 b are then conventionally recessed below the surface of the pad 18 and conventionally capped with an insulator.
- the digit lines 20 a and 20 b are etched to approximately 70 nm below the surface.
- silicon dioxide is CVD deposited and then polished back to the surface of the pad 18 to form the oxide caps 26 a and 26 b.
- the caps 26 a and 26 b, together with the layer 24 encapsulate the lines 20 a and 20 b, respectively.
- FIG. 4A which is a top plan view
- FIG. 4B which is a cross section of FIG. 4A taken along lines 4 B
- a plurality of isolation segments 28 are conventionally formed to define active areas 29 of the substrate 12 .
- a mask (not shown) is formed to expose the regions of the pad 18 where the segments 28 are to be formed.
- the pad 18 is then etched and the mask is removed.
- the exposed portions of the conductor 16 and the insulator 14 are etched to expose respective portions of the substrate 12 .
- the etching of the insulator 14 has a minimal effect on the thickness of the caps 26 a and 26 b.
- the exposed regions of the substrate 12 are etched to the desired depth, which is approximately 3 ⁇ m in one embodiment.
- a sacrificial oxide (not shown) is grown in the etched regions of the substrate 12 .
- the sacrificial oxide is stripped, and then a thin passivation oxide (not shown) is grown in the etched regions of the substrate 12 .
- An oxide such as TEOS is CVD deposited to fill the recesses in the substrate 12 , layers 14 and 16 , and pad 18 . The oxide is then polished back to the surface of the pad 18 to form the isolation segments 28 .
- a mask 30 is conventionally formed to provide a pattern for etching the gates of the transistors in the memory cells.
- the exposed regions of the pad 18 and the conductor 16 are conventionally etched to form the transistor gates 32 .
- the exposed regions of the substrate 12 are then conventionally implanted with a dopant to form the common and uncommon source/drain regions 34 and 36 , respectively.
- the regions of the insulator 14 that are on the source/drain regions 34 and 36 are conventionally removed.
- a mask 37 is conventionally formed, and openings 38 are conventionally formed therein.
- the openings 38 expose the common source/drain regions 34 and the respective regions of the isolation trenches 22 a and 22 b that are adjacent to the common regions 34 .
- the exposed regions of the insulator caps 26 a and 26 b are then etched to expose the respective underlying regions of the digit lines 20 a and 20 b.
- FIG. 8 which is a top plan view
- FIG. 9 which is an isometric view
- the masks 30 and 37 are conventionally removed, and an insulator layer is conventionally formed on all of the exposed surfaces.
- a layer of silicon dioxide is CVD deposited on all of the exposed surfaces, and the thickness of this layer is approximately 20-50 nm.
- the layer is then anisotropically etched, using a conventional technique such as reactive ion etching (RIE), to form insulative sidewalls 40 on the exposed vertical sidewalls of the gates 32 .
- a conductive material 41 is then conventionally deposited to fill in the spaces above the exposed regions of the digit lines 20 a and 20 b and the regions 34 and 36 .
- the conductor 41 is then conventionally polished back to the surface of the pad 18 .
- respective regions of the conductor 41 form straps 42 , which electrically couple the common source/drain regions 34 to the respective adjacent digit lines 20 a and 20 b. Because the digit lines 20 a and 20 b are buried in the isolation trenches 22 a and 20 b, no digit lines need be formed above the memory cells, and thus no digit-line vias need be formed. The absence of these vias allows more space for the plates of the stacked capacitors (not shown in FIGS. 8 and 9), and thus allows one to either increase the capacitance of the capacitors, or to reduce the dimensions of the memory cells and thus the overall size of the memory array.
- each active area 29 is formed a pair of memory cells that each include a respective one of the regions 36 and that share the common source/drain region 34 with the other cell.
- the substrate 12 is doped with a P-type dopant such as Boron, and the regions 34 and 36 are doped with an N-type dopant such as phosphorous or arsenic.
- the isolation segments 28 are staggered such that the segments on one side of a trench 22 are approximately halfway between the respective adjacent segments 28 on the other side of the same trench. This allows the gates 32 to also be staggered, and the word lines (not shown in FIG. 9) to be laid out in a folded-digit-line architecture as discussed below.
- FIGS. 10 - 17 show a second embodiment for forming a memory array according to the, invention.
- like numbers refer to like structures in FIGS. 1 - 9 .
- FIG. 10 is a cross-sectional view
- the gate conductor and the gate oxide are not formed on the substrate 12 at the beginning of the process. Instead, a thin thermal oxide layer 50 is conventionally formed on the silicon substrate 12 , and a pad nitride 52 is conventionally formed on the oxide 50 .
- FIG. 11A which is a top plan view
- FIG. 11B which is a cross section of FIG. 11A taken along lines 11 B
- FIG. 12 which is an isometric view
- the digit lines 20 a and 20 b, trenches 22 a and 22 b, insulator layer 24 , insulator caps 26 a and 26 b , and isolation segments 28 are formed in a manner that is similar to that described above in conjunction with FIGS. 1 - 4 B.
- the pad nitride 52 and the layer 50 are conventionally removed, and a gate insulator 54 is conventionally formed on the exposed areas of the substrate 12 .
- the insulator 54 is silicon dioxide and is either grown or CVD deposited.
- a gate conductor 56 is conventionally formed on the insulator 54 .
- the conductor 56 is polysilicon that is CVD deposited. The conductor 56 is then polished back so that it is substantially even with the surfaces of the trenches 22 a and 22 b and the isolation segments 28 .
- the conductor 56 is then conventionally etched such that it becomes recessed with respect to the surfaces of the isolation trenches 22 a and 22 b and the isolation segments 28 .
- the conductor 56 is recessed approximately 100 nm.
- a nitride layer 58 is conventionally formed on the conductor 56 and then polished back to be substantially even with the surfaces of the trenches 22 and the segments 28 .
- the mask 30 is then conventionally formed as discussed above in conjunction with FIG. 5.
- FIG. 16 which is an isometric view
- the gate segments 32 , the common source/drain regions 34 , and the uncommon source/drain regions 36 are formed in a manner similar to that described above in conjunction with FIG. 6.
- FIG. 17 which is an isometric view
- the sidewalls 40 , conductive material 41 , and straps 42 are formed in a manner similar to that described above in conjunction with FIGS. 7 - 9 .
- FIG. 32 is a block diagram of one embodiment of a memory circuit 60 , which includes memory banks 62 a and 62 b. These memory banks each incorporate a memory array according to the invention, like the ones shown in FIGS. 9, 17, 19 , or 31 .
- the memory circuit 60 is a synchronous DRAM (SDRAM), although it may be another type of memory in other embodiments.
- SDRAM synchronous DRAM
- FIGS. 18 and 19 show one embodiment for forming sub-lithographic word lines for reduced-area memory arrays, such as those shown in FIGS. 9 and 17.
- Such sub-lithographic word lines have widths that are less than the minimum feature size of the process, and thus allow such memory arrays to be constructed with a folded-digit-line architecture without the word lines being electrically shorted together.
- such reduced-area memory arrays can be constructed with a shared-digit-line architecture using conventional process technology to form conventional word lines.
- the techniques shown in FIG. 18 and FIG. 19 are not required to form a shared-digit-line architecture.
- FIG. 18 is a cross section of a portion of the memory array shown in FIG. 17, although it is understood that the formation of the sub-lithographic word lines for the memory array of FIG. 9 occurs in a similar manner.
- the conductive material 41 is conventionally etched back such that it becomes recessed with respect to the surface of the pad 58 . In one embodiment, the material 41 is recessed approximately 100 nm.
- an insulator layer 88 such as an oxide, is conventionally grown or deposited and then polished back to the surface of the pad 58 to give the structure shown in FIG. 18.
- a 90 is conventionally formed on the layer 88 and the pad 58 .
- the mandrel 90 is formed from intrinsic, i.e., undoped. polysilicon.
- the mandrel is then conventionally polished to smoothen its upper surface.
- a groove 92 which has sidewalls 94 a and 94 b, is conventionally etched into the mandrel 90 .
- the sidewall 94 a is over a midsection of the isolation segment 28
- the sidewall 94 b is over a midsection of the gate segment 32 .
- a conventional anisotropic etch removes the exposed region of the pad 58 , and thus exposes a region of the gate 32 .
- a conductive material such as polysilicon is conventionally formed in the groove 92 .
- the conductive material is then anisotropically etched to leave conductive sidewalls that become the sub-minimum dimension word lines 96 and 98 .
- the mandrel 90 is then removed.
- the mandrel 90 and the word lines 96 and 98 are conventionally polished or etched to make the shape of the word lines 96 and 98 rectangular, and to center the word line 98 over the respective gate 32 .
- FIGS. 20 - 31 show one embodiment of a method for forming stacked capacitors in a reduced-area memory array that is similar to those shown in FIGS. 9 and 17, where the memory array has the sub-lithographic word lines formed as discussed above in conjunction with FIGS. 18 and 19.
- a silicon substrate 100 provides a strong base for the semiconductor layers of a memory array 102 .
- the isolation segments 104 which are similar to the segments 28 of FIG. 17, provide support and isolation between the devices in the array 102 .
- N+ diffusion regions 106 , 108 , and 110 which are similar to the regions 36 , 34 , and 36 of FIG. 17, respectively, are formed by introducing any suitable N-type dopant into the substrate 100 .
- the N-type dopant, such as phosphorous, is typically introduced by diffusion or ion implantation.
- the transistor gates 112 and 114 which are similar to the gates 32 of FIG.
- gate oxide 116 and 118 typically comprise polysilicon, and are respectively separated from the substrate 100 by thin layers of gate oxide 116 and 118 , which are similar to the layer 54 of FIG. 17, in order to limit the gate current to a negligible amount.
- the N+ diffusion region 106 , gate 112 , channel region 120 , and N+ diffusion region 108 define a first transistor.
- the N+ diffusion region 110 , gate 114 , channel region 122 , and N+ diffusion region 108 define a second transistor.
- the center N+ diffusion region 108 acts as a common source or drain, and the N+ diffusion regions 106 and 110 act as independent sources or drains depending upon the voltage applied to these regions.
- the transistors of the array 102 are enhanced NMOS transistors.
- any transistor configuration suitable for memory-cell access may readily be used.
- these transistors are shown as exemplary only.
- any suitable semiconductor device may be formed in the substrate 100 without departing from the scope of the invention.
- the array 102 includes contact regions that can be formed from any appropriate conductive material such as polysilicon. These contact regions are coupled to the N+ diffusion regions. For example, a contact region 124 is coupled to the N+ diffusion region 108 , while contact regions 126 and 128 are coupled to the N+ diffusion regions 106 and 110 , respectively.
- Contact insulating layers 130 include a conventional thin-film insulator such as silicon nitride and insulate the contact regions 124 , 126 , and 128 .
- the array 102 also includes word lines 132 and 134 , which extend normal to the substrate 100 and are formed outwardly from the gates 112 and 114 , respectively. These word lines are sub-lithographic word lines, and are thus similar to the word line 98 of FIG. 19.
- the word lines 132 and 134 are formed from polysilicon, but in other embodiments, they are formed from other suitable conductive materials such as conventional metals.
- the sub-lithographic, edge-defined word lines 132 and 134 are formed outwardly from the device gates 112 and 114 in a manner similar to that described above in conjunction with FIGS. 18 and 19.
- “Passing” word lines 136 which are similar to the word line 96 of FIG. 19, form a second pair of word lines that provide a conductive path to adjacent memory cells in the array 102 .
- FIG. 21A which is a top view of the integrated circuit 102 , shows the interconnection of the memory cells of the array 102 . Specifically, FIG. 21A shows how the word lines 132 and 134 are coupled to the gates 112 and 114 , respectively, within a memory cell 140 . FIG. 21A also shows how the passing conductors 136 pass through the memory cell 140 and are coupled to the device gates 142 and 144 of adjacent memory cells 146 and 148 , respectively. Note that the memory cells 146 and 148 are only partially shown.
- the word lines 132 and 134 are capped with an insulator 150 and are lined with a sidewall insulator 152 .
- An insulator 154 insulates the gates 112 and 114 .
- Any suitable semiconductor insulator material such as silicon dioxide, may be used for the insulators 150 , 152 , and 154 .
- an oxide layer is CVD deposited and directionally etched to form the insulator linings 152 .
- intrinsic polysilicon 156 is deposited and conventionally polished back along with the top portions of the linings 152 , such that the tops of the word lines 132 , 134 , and 136 are exposed. Then, a thermal oxide is grown on these exposed portions to form the caps 150 , and the structure is again polished back to give the structure shown in FIG. 20.
- a material with a high degree of etch selectivity is used.
- this suitable material such as the intrinsic polysilicon 156
- the high degree of etch selectivity of a material such as intrinsic polysilicon 156 is advantageous because it allows intricate etching without disturbing the surrounding semiconductor regions.
- FIG. 21B which is a top plan view
- a photoresist and a mask are used to reveal the plurality of semiconductor memory cells formed on the substrate 100 .
- a photoresist is applied to the entire array 102 .
- Masked areas 158 illustrate the areas of the photoresist 160 that are covered by a mask and therefore are not hardened when exposed to ultraviolet light.
- the intrinsic polysilicon 156 between the word lines 132 and 134 and the passing word lines 136 is removed by selectively etching the intrinsic polysilicon 156 .
- three stud holes 162 are created in the array 102 .
- the stud holes 162 extend into the array 102 and toward the substrate 100 , and ultimately expose the contact insulating layers 130 .
- the regions of the intrinsic polysilicon 156 that are covered by the mask are not etched.
- FIG. 23A which is a cross-sectional view
- a second mask is formed that allows the layers 130 that overly the layers 126 and 128 to be etched, thus exposing the regions 126 and 128 . Small regions of the layers 130 remain between the insulator 152 adjacent to the passing word lines 136 and the contact regions 126 and 128 , respectively. The second mask is then removed.
- an insulator such as silicon dioxide
- CVD deposited on the walls of the openings between the word lines 132 and 134 and the passing conductors 136 .
- This step creates sleeves 180 , which line the insulator 152 and the intrinsic polysilicon 156 , but which cover the exposed surfaces of the contact regions 126 and 128 .
- These sleeves 180 are advantageous because they reduce the sizes of the stud holes 162 , and thus reduce the parasitic capacitances of the conductive connections between the active regions of the substrate 100 and the stacked capacitors that will be formed.
- the formation of the sleeves 180 is followed by an anisotropic etch, such as a dry reactive ion etch (RIE), that removes the recently deposited oxide from all horizontal surfaces but leaves it on the vertical surfaces. This removes the insulator from the recently exposed contact regions 126 and 128 . It is necessary to correctly time the etch so that it does not inadvertently etch the horizontal oxide layers 154 , which insulate the bases of the word lines 132 and 134 and the gates 112 and 114 . Thus, as a result of the directional etch, the stud holes 162 are aligned with the insulating sleeve 180 .
- RIE dry reactive ion etch
- the next step is to fill the two side stud holes 162 with a conductive material, such as doped polysilicon 182 , by conventional CVD.
- a conductive material such as doped polysilicon 182
- an insulating material such as silicon dioxide 184
- the doped polysilicon 182 and the oxide 184 are conventionally polished so that they are substantially flush with the oxide caps 150 .
- the doped polysilicon 182 provides conductive paths to the contact regions 126 and 128 , respectively. In this manner, the conductive paths formed by the doped polysilicon 182 are bounded by the word lines 132 and 134 and the passing word lines 136 .
- the center contact region 124 because it is connected to the buried digit line, can be covered with the oxide 184 instead of being filled with a conductive material that will eventually form a conductive digit-line via.
- FIG. 24B which is a cross-sectional view
- the remaining portions of the intrinsic polysilicon 156 that were hidden by the mask 160 are selectively etched.
- An insulator 186 which may be any conventional insulator, such as silicon dioxide, is deposited on the entire wafer to fill the void regions where the intrinsic polysilicon 156 was removed.
- the insulator 186 is then conventionally polished so that it is substantially planar with the oxide caps 150 , the doped polysilicon regions 182 , and the oxide region 184 .
- the resulting formation as shown in FIG. 24B is virtually identical to that shown in FIG. 24A, with the exception that the intrinsic polysilicon 156 has been replaced with the oxide filler 186 .
- a thick layer of intrinsic polysilicon 188 is CVD deposited on the entire wafer. This layer should be at least 0.5 ⁇ m thick.
- a thin mask 190 is created by depositing a conventional thin-film insulator, such as silicon nitride, on the thick layer of intrinsic polysilicon 188 .
- the thin mask 190 should be approximately 500 angstroms thick.
- FIG. 26 which is a cross-sectional view
- a resist is applied to the wafer and is used to define openings 192 over the doped polysilicon 182 .
- These outer openings 192 will be used to form the stacked capacitors. Therefore, in one embodiment, the sizes and shapes of the outer openings 192 are designed to maximize the capacitor size and minimize the contact size.
- the intrinsic polysilicon 188 is etched to create two hollow regions 194 .
- the thin-film insulator 190 acts as a mask, so a new mask and resist need not be applied.
- this etch has an isotropic component, such that it is slightly nondirectional. The isotropic component effectively enlarges the size of the hollow regions 194 relative to the outer holes 192 in the insulator 190 .
- the thin mask layer 190 is removed.
- a conductive material such as N+ polysilicon
- the conductive material forms conductive liners in the regions 194 . These liners are the respective bottom plates 200 and 202 for the stacked capacitors.
- the N+ polysilicon is conventionally polished to guarantee that the plates 200 and 202 are not shorted together over the intrinsic polysilicon 188 .
- the remaining intrinsic polysilicon 188 is selectively etched in a conventional manner after the conductive material that forms the plates 200 and 202 is polished. This step exposes the oxide filler 186 , as well as the oxide 184 .
- a dielectric material 204 which is any suitable dielectric material, such as tantalum pentoxide, is deposited. In other embodiments, any suitable dielectric material may be used.
- an upper plate conductor 206 is deposited on the dielectric material 204 . In one embodiment, platinum is used as the plate conductor 206 . In other embodiments, any suitable conductor may be used.
- an insulator 210 which is any suitable insulator, such as silicon dioxide, is deposited after the capacitor materials are formed. The insulator 210 is then conventionally polished to smoothen its surface.
- the bottom plates 200 and 202 of the capacitors are shown to not extend over the oxide 184 , in another embodiment, these plates extend over the oxide 184 to increase the plate area, and thus the capacitance of, the stacked capacitors. In yet another embodiment, the space between the word lines 132 and 134 may be reduced, thus reducing the width of the memory cells and the overall area of the memory array 102 .
- the memory circuit 60 includes an address register 64 , which receives an address from an ADDRESS bus.
- a control logic circuit 66 receives a clock (CLK) signal, receives clock enable (CKE), chip select ( ⁇ overscore (CS) ⁇ ), row address strobe ( ⁇ overscore (RAS) ⁇ ), column address strobe ( ⁇ overscore (CAS) ⁇ ), and write enable ( ⁇ overscore (WE) ⁇ ) signals from the COMMAND bus, and communicates with the other circuits of the memory device 60 .
- CLK clock
- CKE clock enable
- CS chip select
- RAS row address strobe
- CAS column address strobe
- WE write enable
- a row-address multiplexer 68 receives the address signal from the address register 64 and provides the row address to the row-address latch-and-decode circuits 70 a and 70 b for the memory bank 62 a or the memory bank 62 b, respectively.
- the row-address latch-and-decode circuits 70 a and 70 b activate the word lines of the addressed rows of memory cells in the memory banks 62 a and 62 b, respectively.
- Read/write circuits 72 a and 72 b read data from the addressed memory cells in the memory banks 62 a and 62 b, respectively, during a read cycle, and write data to the addressed memory cells during a write cycle.
- a column-address latch-and-decode circuit 74 receives the address from the address register 64 and provides the column address of the selected memory cells to the read/write circuits 72 a and 72 b.
- the address register 64 , the row-address multiplexer 68 , the row-address latch-and-decode circuits 70 a and 70 b, and the column-address latch-and-decode circuit 74 can be collectively referred to as an address decoder.
- a data input/output (I/O) circuit 76 includes a plurality of input buffers 78 .
- the buffers 78 receive and store data from the DATA bus, and the read/write circuits 72 a and 72 b provide the stored data to the memory banks 62 a and 62 b, respectively.
- the data I/O circuit 76 also includes a plurality of output drivers 80 .
- the read/write circuits 72 a and 72 b provide data from the memory banks 62 a and 62 b, respectively, to the drivers 80 , which in turn provide this data to the DATA bus.
- a refresh counter 82 stores the address of the row of memory cells to be refreshed either during a conventional auto-refresh mode or self-refresh mode. After the row is refreshed, a refresh controller 84 updates the address in the refresh counter 82 , typically by either incrementing or decrementing the contents of the refresh counter 82 by one. Although shown separately, the refresh controller 84 may be part of the control logic 66 in other embodiments of the memory device 60 .
- the memory device 60 may also include an optional charge pump 86 , which steps up the power-supply voltage V DD to a voltage V DDP .
- the pump 86 generates V DDP approximately 1-1.5 V higher than V DD .
- the memory circuit 60 may also use V DDP to conventionally overdrive selected internal transistors.
- FIG. 33 is a block diagram of an electronic system 212 , such as a computer system, that incorporates the memory circuit 60 of FIG. 32.
- the system 212 also includes computer circuitry 214 for performing computer functions, such as executing software to perform desired calculations and tasks.
- the circuitry 214 typically includes a processor 216 and the memory circuit 60 , which is coupled to the processor 216 .
- One or more input devices 218 such as a keyboard or a mouse, are coupled to the computer circuitry 214 and allow an operator (not shown) to manually input data thereto.
- One or more output devices 220 are coupled to the computer circuitry 214 to provide to the operator data generated by the computer circuitry 214 . Examples of such output devices 220 include a printer and a video display unit.
- One or more data-storage devices 222 are coupled to the computer circuitry 214 to store data on or retrieve data from external storage media (not shown). Examples of the storage devices 222 and the corresponding storage media include drives that accept hard and floppy disks, tape cassettes, and compact disk read-only memories (CD-ROMs).
- the computer circuitry 214 includes address data and command buses and a clock line that are respectively coupled to the ADDRESS, DATA, and COMMAND buses, and the CLK line of the memory device 60 .
Abstract
Description
- The invention relates generally to memory arrays, and more particularly to a memory array having one or more bit, i.e., digit, lines buried in the isolation regions such as silicon-trench-isolation (STI) regions.
- To accommodate continuing consumer demand for integrated circuits that perform the same or additional functions and yet have a reduced size as compared with available circuits, circuit designers continually search for ways to reduce the size of the memory arrays within these circuits without sacrificing array performance. For example, one known technique for reducing the layout area of a Dynamic Random Access Memory (DRAM) array is to stack storage capacitors above memory cells. Typically, the memory cells are formed in adjacent pairs, where each pair shares a common source/drain region that is connected to a respective digit line. Because the digit lines are disposed above the stack capacitors, and thus above the common source/drain regions, conductive vias are needed to connect the digit lines to the respective common source/drain regions. Therefore, these vias must extend through or adjacent to the plates of the stacked storage capacitors.
- A problem with such a stacked-capacitor memory array is that the area of each memory cell, and thus the area of the memory array itself, often cannot be reduced without reducing the capacitances of the stacked capacitors beyond acceptable limits. Because capacitance is proportional to the overlap area of the capacitor plates, the plates of the stacked capacitors must have an overlap area that is large enough to give these capacitors the desired storage capabilities. But the vias that connect the common source/drain regions to the digit lines also have minimum dimensions that are proportional to the minimum feature size of the utilized semiconductor process. Therefore, because a via extends through a hole in a respective pair of stacked-capacitor plates, the minimum total area of a plate is the sum of the minimum required overlap area and the minimum required cross-section area of the intersecting via.
- To solve this problem, the article “Buried Bit-Line Cell for 64 MB DRAMS,” proposes burying the bit-lines in the substrate. But, because these bit-lines are formed after the field oxide regions and because the contacts between the bit-lines and the respective memory cells are also buried in the substrate, the resulting reduction in memory-cell area falls short of the maximum obtainable reduction for a given minimum feature size.
- Another problem is that, even if it were possible to reduce the area of such a memory array by the maximum obtainable reduction, it would be difficult, if not impossible, to implement a folded-digit-line architecture in such a reduced-area array. In such an architecture, there are typically four word lines that extend over a memory cell pair, as compared with two word lines in a shared-digit-line architecture. Like the vias, the word lines have minimum dimensions that are dictated by the minimum feature size. Therefore, if the area of a memory cell is reduced too much, adjacent word lines may become short-circuited to each other, thus rendering the memory array defective.
- In accordance with one aspect of the invention, a memory array includes a semiconductor substrate, an isolation trench located in the substrate, and a conductor that is located in the trench. The array also includes a memory cell that is coupled to the conductor in the trench. In another embodiment of the invention, the conductor is a digit line that is coupled to a source/drain region of the memory cell.
- Thus, where the conductor in the isolation trench is a digit line, no digit-line vias are required. The freed-up space can be used to increase the size, and thus the capacitances, of the stacked capacitors. Or, it can be used to reduce the size of the memory cells without reducing these capacitances.
- FIGS.1-9 show a first embodiment of a method for forming a memory array according to the invention. FIGS. 1, 2B, 3, and 4B are cross-sectional views, FIGS. 2A, 4A, 7, and 8 are top plan views, and FIGS. 5, 6 and 9 are isometric views.
- FIGS.10-17 show a second embodiment of a method for forming a memory array according to the invention. FIGS. 10 and 11B are cross-sectional views, FIG. 11A is a top plan view, and FIGS. 12-17 are isometric views.
- FIGS.18-19 are cross-sectional views showing an embodiment of a method for forming sub-lithographic word lines in a memory array according to the invention.
- FIGS.20-31 show an embodiment of a method for forming stacked capacitors in a memory array according to the invention. FIGS. 20, 22, 23A, and 24A-31 are cross-sectional views and FIGS. 21A, 21B, and 23B are top plan views.
- FIG. 32 is a schematic diagram of a memory circuit that includes an embodiment of a memory array according to the present invention.
- FIG. 33 is a block diagram of an electronic system that incorporates a memory circuit according to the invention.
- FIGS.1-9 show a first embodiment of a method for forming a memory array having buried digit lines. Referring to FIG. 1, a
pad structure 10 is conventionally formed on asemiconductor substrate 12, which is formed from a material such as silicon. Thepad structure 10 includes agate insulator 14 that is disposed on thesubstrate 12, agate conductor 16 that is disposed on theinsulator 14, and aprotective pad 18 that is disposed on thegate conductor 16. In one embodiment, thegate insulator 14 is silicon dioxide, thegate conductor 16 is polysilicon, and theprotective pad 18 is silicon nitride. - Referring to FIG. 2A, which is a top plan view, and FIG. 2B, which is a cross section taken
alone lines 2B of FIG. 2A, a plurality of buried digit lines 20 are formed in thesubstrate 12. For clarity, only thedigit lines isolation trenches pad 18. Next, the exposed regions of thepad 18 are conventionally etched, and then the mask is conventionally stripped. The remaining regions of thepad 18 mask the unexposed regions of thelayers substrate 12 while the exposed regions are conventionally etched to form thetrenches trenches substrate 12. - Next, a sacrificial oxide layer (not shown) is conventionally grown in the
trenches trenches - An
oxide layer 24 is then conventionally formed in thetrenches layer 24 may be deposited using chemical vapor deposition (CVD) such as in the well-known TEOS process, or may be thermally grown. Because the alignment tolerance is approximately ⅓ the minimum line width allowed by the process, in one embodiment, the maximum thickness of thelayer 24 is ⅓ of the trench width. This ensures proper alignment between the burieddigit lines layer 24 is approximately 10-15 nm, leaving trench openings that are approximately 10-20 nm wide. - Next, the
trenches digit lines trenches pad 18. Using conventional chemical-mechanical polishing (CMP) techniques, the structure is then polished so that the surfaces of thedigit lines pad 18, which is used as a polish stop. - Referring to FIG. 3, which is a side view, the
digit lines pad 18 and conventionally capped with an insulator. In one embodiment, thedigit lines pad 18 to form the oxide caps 26 a and 26 b. Thus, thecaps layer 24, encapsulate thelines - Referring to FIG. 4A, which is a top plan view, and FIG. 4B, which is a cross section of FIG. 4A taken along
lines 4B, a plurality ofisolation segments 28 are conventionally formed to defineactive areas 29 of thesubstrate 12. In one embodiment, a mask (not shown) is formed to expose the regions of thepad 18 where thesegments 28 are to be formed. Thepad 18 is then etched and the mask is removed. Next with the remaining regions of thepad 18 acting as a mask, the exposed portions of theconductor 16 and theinsulator 14 are etched to expose respective portions of thesubstrate 12. Because theinsulator 14 is significantly thinner than thecaps insulator 14 has a minimal effect on the thickness of thecaps substrate 12 are etched to the desired depth, which is approximately 3 μm in one embodiment. Next, a sacrificial oxide (not shown) is grown in the etched regions of thesubstrate 12. The sacrificial oxide is stripped, and then a thin passivation oxide (not shown) is grown in the etched regions of thesubstrate 12. An oxide such as TEOS is CVD deposited to fill the recesses in thesubstrate 12, layers 14 and 16, andpad 18. The oxide is then polished back to the surface of thepad 18 to form theisolation segments 28. - Referring to FIG. 5, which is an isometric view, a
mask 30 is conventionally formed to provide a pattern for etching the gates of the transistors in the memory cells. - Referring to FIG. 6, which is an isometric view, the exposed regions of the
pad 18 and theconductor 16 are conventionally etched to form thetransistor gates 32. Using theinsulator 14 as a screening layer, the exposed regions of thesubstrate 12 are then conventionally implanted with a dopant to form the common and uncommon source/drain regions insulator 14 that are on the source/drain regions - Referring to FIG. 7, which is a top plan view, a
mask 37 is conventionally formed, andopenings 38 are conventionally formed therein. Theopenings 38 expose the common source/drain regions 34 and the respective regions of theisolation trenches common regions 34. The exposed regions of the insulator caps 26 a and 26 b are then etched to expose the respective underlying regions of thedigit lines - Referring to FIG. 8, which is a top plan view, and FIG. 9, which is an isometric view, the
masks gates 32. Aconductive material 41 is then conventionally deposited to fill in the spaces above the exposed regions of thedigit lines regions conductor 41 is then conventionally polished back to the surface of thepad 18. - Thus, respective regions of the
conductor 41 form straps 42, which electrically couple the common source/drain regions 34 to the respectiveadjacent digit lines digit lines isolation trenches - Furthermore, within each
active area 29 is formed a pair of memory cells that each include a respective one of theregions 36 and that share the common source/drain region 34 with the other cell. In one embodiment, thesubstrate 12 is doped with a P-type dopant such as Boron, and theregions - Still referring to FIG. 9, in the illustrated embodiment, the
isolation segments 28 are staggered such that the segments on one side of a trench 22 are approximately halfway between the respectiveadjacent segments 28 on the other side of the same trench. This allows thegates 32 to also be staggered, and the word lines (not shown in FIG. 9) to be laid out in a folded-digit-line architecture as discussed below. - FIGS.10-17 show a second embodiment for forming a memory array according to the, invention. In these Figures, like numbers refer to like structures in FIGS. 1-9.
- Referring to FIG. 10, which is a cross-sectional view, one main difference between this embodiment and that described with reference to FIGS.1-9 is that the gate conductor and the gate oxide are not formed on the
substrate 12 at the beginning of the process. Instead, a thinthermal oxide layer 50 is conventionally formed on thesilicon substrate 12, and apad nitride 52 is conventionally formed on theoxide 50. - Referring to FIG. 11A, which is a top plan view, FIG. 11B, which is a cross section of FIG. 11A taken along
lines 11B, and FIG. 12, which is an isometric view, thedigit lines trenches insulator layer 24, insulator caps 26 a and 26 b, andisolation segments 28 are formed in a manner that is similar to that described above in conjunction with FIGS. 1-4B. - Referring to FIG. 13, which is an isometric view, the
pad nitride 52 and thelayer 50 are conventionally removed, and agate insulator 54 is conventionally formed on the exposed areas of thesubstrate 12. In one embodiment, theinsulator 54 is silicon dioxide and is either grown or CVD deposited. - Referring to FIG. 14, which is an isometric view, a
gate conductor 56 is conventionally formed on theinsulator 54. In one embodiment, theconductor 56 is polysilicon that is CVD deposited. Theconductor 56 is then polished back so that it is substantially even with the surfaces of thetrenches isolation segments 28. - Referring to FIG. 15, which is an isometric view, the
conductor 56 is then conventionally etched such that it becomes recessed with respect to the surfaces of theisolation trenches isolation segments 28. In one embodiment, theconductor 56 is recessed approximately 100 nm. Next, anitride layer 58 is conventionally formed on theconductor 56 and then polished back to be substantially even with the surfaces of the trenches 22 and thesegments 28. Themask 30 is then conventionally formed as discussed above in conjunction with FIG. 5. - Referring to FIG. 16, which is an isometric view, the
gate segments 32, the common source/drain regions 34, and the uncommon source/drain regions 36 are formed in a manner similar to that described above in conjunction with FIG. 6. - Referring to FIG. 17, which is an isometric view, the
sidewalls 40,conductive material 41, and straps 42 are formed in a manner similar to that described above in conjunction with FIGS. 7-9. - FIG. 32 is a block diagram of one embodiment of a
memory circuit 60, which includesmemory banks memory circuit 60 is a synchronous DRAM (SDRAM), although it may be another type of memory in other embodiments. - FIGS. 18 and 19 show one embodiment for forming sub-lithographic word lines for reduced-area memory arrays, such as those shown in FIGS. 9 and 17. Such sub-lithographic word lines have widths that are less than the minimum feature size of the process, and thus allow such memory arrays to be constructed with a folded-digit-line architecture without the word lines being electrically shorted together. Of course, such reduced-area memory arrays can be constructed with a shared-digit-line architecture using conventional process technology to form conventional word lines. Thus, the techniques shown in FIG. 18 and FIG. 19 are not required to form a shared-digit-line architecture.
- FIG. 18 is a cross section of a portion of the memory array shown in FIG. 17, although it is understood that the formation of the sub-lithographic word lines for the memory array of FIG. 9 occurs in a similar manner. First, the
conductive material 41 is conventionally etched back such that it becomes recessed with respect to the surface of thepad 58. In one embodiment, thematerial 41 is recessed approximately 100 nm. Next, aninsulator layer 88, such as an oxide, is conventionally grown or deposited and then polished back to the surface of thepad 58 to give the structure shown in FIG. 18. - Referring to FIG. 19, which is a cross-sectional view, a90 is conventionally formed on the
layer 88 and thepad 58. In one embodiment, themandrel 90 is formed from intrinsic, i.e., undoped. polysilicon. The mandrel is then conventionally polished to smoothen its upper surface. Next, agroove 92, which has sidewalls 94 a and 94 b, is conventionally etched into themandrel 90. Thesidewall 94 a is over a midsection of theisolation segment 28, and thesidewall 94 b is over a midsection of thegate segment 32. Then, a conventional anisotropic etch removes the exposed region of thepad 58, and thus exposes a region of thegate 32. Next, a conductive material such as polysilicon is conventionally formed in thegroove 92. The conductive material is then anisotropically etched to leave conductive sidewalls that become the sub-minimum dimension word lines 96 and 98. In one embodiment, themandrel 90 is then removed. In another embodiment, themandrel 90 and the word lines 96 and 98 are conventionally polished or etched to make the shape of the word lines 96 and 98 rectangular, and to center theword line 98 over therespective gate 32. - FIGS.20-31 show one embodiment of a method for forming stacked capacitors in a reduced-area memory array that is similar to those shown in FIGS. 9 and 17, where the memory array has the sub-lithographic word lines formed as discussed above in conjunction with FIGS. 18 and 19.
- Referring to FIG. 20, a
silicon substrate 100 provides a strong base for the semiconductor layers of amemory array 102. Theisolation segments 104, which are similar to thesegments 28 of FIG. 17, provide support and isolation between the devices in thearray 102.N+ diffusion regions regions substrate 100. The N-type dopant, such as phosphorous, is typically introduced by diffusion or ion implantation. Thetransistor gates gates 32 of FIG. 17, typically comprise polysilicon, and are respectively separated from thesubstrate 100 by thin layers ofgate oxide layer 54 of FIG. 17, in order to limit the gate current to a negligible amount. In this configuration, theN+ diffusion region 106,gate 112,channel region 120, andN+ diffusion region 108 define a first transistor. Similarly, theN+ diffusion region 110,gate 114,channel region 122, andN+ diffusion region 108 define a second transistor. - The center
N+ diffusion region 108 acts as a common source or drain, and theN+ diffusion regions array 102 are enhanced NMOS transistors. Alternatively, any transistor configuration suitable for memory-cell access may readily be used. Furthermore, these transistors are shown as exemplary only. In an alternate embodiment, any suitable semiconductor device may be formed in thesubstrate 100 without departing from the scope of the invention. - The
array 102 includes contact regions that can be formed from any appropriate conductive material such as polysilicon. These contact regions are coupled to the N+ diffusion regions. For example, acontact region 124 is coupled to theN+ diffusion region 108, whilecontact regions N+ diffusion regions layers 130 include a conventional thin-film insulator such as silicon nitride and insulate thecontact regions - The
array 102 also includesword lines substrate 100 and are formed outwardly from thegates word line 98 of FIG. 19. In one embodiment, the word lines 132 and 134 are formed from polysilicon, but in other embodiments, they are formed from other suitable conductive materials such as conventional metals. - The sub-lithographic, edge-defined
word lines device gates word line 96 of FIG. 19, form a second pair of word lines that provide a conductive path to adjacent memory cells in thearray 102. - FIG. 21A, which is a top view of the
integrated circuit 102, shows the interconnection of the memory cells of thearray 102. Specifically, FIG. 21A shows how the word lines 132 and 134 are coupled to thegates memory cell 140. FIG. 21A also shows how the passingconductors 136 pass through thememory cell 140 and are coupled to thedevice gates adjacent memory cells memory cells - Referring again to FIG. 20, the word lines132 and 134 are capped with an
insulator 150 and are lined with asidewall insulator 152. Aninsulator 154 insulates thegates insulators mandrel 90 is removed, an oxide layer is CVD deposited and directionally etched to form theinsulator linings 152. Then,intrinsic polysilicon 156 is deposited and conventionally polished back along with the top portions of thelinings 152, such that the tops of the word lines 132, 134, and 136 are exposed. Then, a thermal oxide is grown on these exposed portions to form thecaps 150, and the structure is again polished back to give the structure shown in FIG. 20. - In order to form stacked capacitors outwardly from the
substrate 100, a material with a high degree of etch selectivity is used. As discussed above, this suitable material, such as theintrinsic polysilicon 156, is deposited between the word lines 132 and 134 and thepassings word lines 136 by a conventional process such as CVD. The high degree of etch selectivity of a material such asintrinsic polysilicon 156 is advantageous because it allows intricate etching without disturbing the surrounding semiconductor regions. - Referring to FIG. 21B, which is a top plan view, a photoresist and a mask are used to reveal the plurality of semiconductor memory cells formed on the
substrate 100. First, a photoresist is applied to theentire array 102.Masked areas 158 illustrate the areas of thephotoresist 160 that are covered by a mask and therefore are not hardened when exposed to ultraviolet light. After exposing the resist and the mask, theintrinsic polysilicon 156 between the word lines 132 and 134 and the passing word lines 136 is removed by selectively etching theintrinsic polysilicon 156. - Referring to FIG. 22, three
stud holes 162 are created in thearray 102. The stud holes 162 extend into thearray 102 and toward thesubstrate 100, and ultimately expose thecontact insulating layers 130. The regions of theintrinsic polysilicon 156 that are covered by the mask are not etched. - Referring to FIG. 23A, which is a cross-sectional view, a second mask is formed that allows the
layers 130 that overly thelayers regions layers 130 remain between theinsulator 152 adjacent to the passingword lines 136 and thecontact regions - Referring to FIG. 23B, which is a top view of the
array 102 after thecontact regions conductors 136. This step createssleeves 180, which line theinsulator 152 and theintrinsic polysilicon 156, but which cover the exposed surfaces of thecontact regions sleeves 180 are advantageous because they reduce the sizes of the stud holes 162, and thus reduce the parasitic capacitances of the conductive connections between the active regions of thesubstrate 100 and the stacked capacitors that will be formed. - The formation of the
sleeves 180 is followed by an anisotropic etch, such as a dry reactive ion etch (RIE), that removes the recently deposited oxide from all horizontal surfaces but leaves it on the vertical surfaces. This removes the insulator from the recently exposedcontact regions gates sleeve 180. - Referring to FIG. 24A, which is a cross-sectional view, the next step is to fill the two side stud holes162 with a conductive material, such as doped
polysilicon 182, by conventional CVD. Then, an insulating material, such assilicon dioxide 184, is conventionally deposited in thecenter stud hole 162. The dopedpolysilicon 182 and theoxide 184 are conventionally polished so that they are substantially flush with the oxide caps 150. The dopedpolysilicon 182 provides conductive paths to thecontact regions polysilicon 182 are bounded by the word lines 132 and 134 and the passing word lines 136. One difference between this structure and that which could be used where the digit lines are not buried but are formed in an upper conductive layer is that here, thecenter contact region 124, because it is connected to the buried digit line, can be covered with theoxide 184 instead of being filled with a conductive material that will eventually form a conductive digit-line via. - Referring to FIG. 24B, which is a cross-sectional view, the remaining portions of the
intrinsic polysilicon 156 that were hidden by themask 160 are selectively etched. Aninsulator 186, which may be any conventional insulator, such as silicon dioxide, is deposited on the entire wafer to fill the void regions where theintrinsic polysilicon 156 was removed. Theinsulator 186 is then conventionally polished so that it is substantially planar with the oxide caps 150, the dopedpolysilicon regions 182, and theoxide region 184. The resulting formation as shown in FIG. 24B is virtually identical to that shown in FIG. 24A, with the exception that theintrinsic polysilicon 156 has been replaced with theoxide filler 186. - At this point in the fabrication of the stacked capacitors, the process has effectively provided conductive paths to the active regions of the substrate, where these conductive paths are disposed between the sub-lithographic word lines. The remaining steps in the process as discussed below form the stacked capacitors.
- Referring to FIG. 25, which is a cross-sectional view, a thick layer of
intrinsic polysilicon 188 is CVD deposited on the entire wafer. This layer should be at least 0.5 μm thick. Next, athin mask 190 is created by depositing a conventional thin-film insulator, such as silicon nitride, on the thick layer ofintrinsic polysilicon 188. Thethin mask 190 should be approximately 500 angstroms thick. - Referring to FIG. 26, which is a cross-sectional view, a resist is applied to the wafer and is used to define
openings 192 over the dopedpolysilicon 182. Theseouter openings 192 will be used to form the stacked capacitors. Therefore, in one embodiment, the sizes and shapes of theouter openings 192 are designed to maximize the capacitor size and minimize the contact size. - Referring to FIG. 27, which is a cross-sectional view, the
intrinsic polysilicon 188 is etched to create twohollow regions 194. During this step, the thin-film insulator 190 acts as a mask, so a new mask and resist need not be applied. In one embodiment, this etch has an isotropic component, such that it is slightly nondirectional. The isotropic component effectively enlarges the size of thehollow regions 194 relative to theouter holes 192 in theinsulator 190. After etching, thethin mask layer 190 is removed. - Referring to FIG. 28, which is a cross-sectional view, a conductive material, such as N+ polysilicon, is deposited on the
array 102. The conductive material forms conductive liners in theregions 194. These liners are therespective bottom plates plates plates intrinsic polysilicon 188. - Referring to FIG. 29, which is a cross-sectional view, the remaining
intrinsic polysilicon 188 is selectively etched in a conventional manner after the conductive material that forms theplates oxide filler 186, as well as theoxide 184. - Referring to FIG. 30, which is a cross-sectional view, a
dielectric material 204, which is any suitable dielectric material, such as tantalum pentoxide, is deposited. In other embodiments, any suitable dielectric material may be used. Next, anupper plate conductor 206 is deposited on thedielectric material 204. In one embodiment, platinum is used as theplate conductor 206. In other embodiments, any suitable conductor may be used. - Referring to FIG. 31, which is a cross-sectional view, an
insulator 210, which is any suitable insulator, such as silicon dioxide, is deposited after the capacitor materials are formed. Theinsulator 210 is then conventionally polished to smoothen its surface. - Although in the illustrated embodiment the
bottom plates oxide 184, in another embodiment, these plates extend over theoxide 184 to increase the plate area, and thus the capacitance of, the stacked capacitors. In yet another embodiment, the space between the word lines 132 and 134 may be reduced, thus reducing the width of the memory cells and the overall area of thememory array 102. - The
memory circuit 60 includes anaddress register 64, which receives an address from an ADDRESS bus. Acontrol logic circuit 66 receives a clock (CLK) signal, receives clock enable (CKE), chip select ({overscore (CS)}), row address strobe ({overscore (RAS)}), column address strobe ({overscore (CAS)}), and write enable ({overscore (WE)}) signals from the COMMAND bus, and communicates with the other circuits of thememory device 60. A row-address multiplexer 68 receives the address signal from theaddress register 64 and provides the row address to the row-address latch-and-decode circuits memory bank 62 a or thememory bank 62 b, respectively. During read and write cycles, the row-address latch-and-decode circuits memory banks write circuits memory banks decode circuit 74 receives the address from theaddress register 64 and provides the column address of the selected memory cells to the read/write circuits address register 64, the row-address multiplexer 68, the row-address latch-and-decode circuits decode circuit 74 can be collectively referred to as an address decoder. - A data input/output (I/O)
circuit 76 includes a plurality of input buffers 78. During a write cycle, thebuffers 78 receive and store data from the DATA bus, and the read/write circuits memory banks O circuit 76 also includes a plurality ofoutput drivers 80. During a read cycle, the read/write circuits memory banks drivers 80, which in turn provide this data to the DATA bus. - A refresh counter82 stores the address of the row of memory cells to be refreshed either during a conventional auto-refresh mode or self-refresh mode. After the row is refreshed, a
refresh controller 84 updates the address in therefresh counter 82, typically by either incrementing or decrementing the contents of therefresh counter 82 by one. Although shown separately, therefresh controller 84 may be part of thecontrol logic 66 in other embodiments of thememory device 60. - The
memory device 60 may also include anoptional charge pump 86, which steps up the power-supply voltage VDD to a voltage VDDP. In one embodiment, thepump 86 generates VDDP approximately 1-1.5 V higher than VDD. Thememory circuit 60 may also use VDDP to conventionally overdrive selected internal transistors. - FIG. 33 is a block diagram of an
electronic system 212, such as a computer system, that incorporates thememory circuit 60 of FIG. 32. Thesystem 212 also includescomputer circuitry 214 for performing computer functions, such as executing software to perform desired calculations and tasks. Thecircuitry 214 typically includes aprocessor 216 and thememory circuit 60, which is coupled to theprocessor 216. One ormore input devices 218, such as a keyboard or a mouse, are coupled to thecomputer circuitry 214 and allow an operator (not shown) to manually input data thereto. One ormore output devices 220 are coupled to thecomputer circuitry 214 to provide to the operator data generated by thecomputer circuitry 214. Examples ofsuch output devices 220 include a printer and a video display unit. One or more data-storage devices 222 are coupled to thecomputer circuitry 214 to store data on or retrieve data from external storage media (not shown). Examples of thestorage devices 222 and the corresponding storage media include drives that accept hard and floppy disks, tape cassettes, and compact disk read-only memories (CD-ROMs). Typically, thecomputer circuitry 214 includes address data and command buses and a clock line that are respectively coupled to the ADDRESS, DATA, and COMMAND buses, and the CLK line of thememory device 60. - From the foregoing it will be appreciated that, although specific embodiments of the invention have been described herein for purposes of illustration, various modifications may be made without deviating from the spirit and scope of the invention. For example, although the invention is described with respect to digit lines in a memory circuit, other types of conductors, such as world lines or other circuit interconnections, can be formed in the isolation trenches of other types of circuits. Thus, in these other circuits, the invention can be used to add another layer of wiring with little or no increase in the layout area. Accordingly, the invention is not limited except as by the appended claims.
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US09/900,341 US6417040B2 (en) | 1997-04-25 | 2001-07-05 | Method for forming memory array having a digit line buried in an isolation region |
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US09/900,341 Expired - Lifetime US6417040B2 (en) | 1997-04-25 | 2001-07-05 | Method for forming memory array having a digit line buried in an isolation region |
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Also Published As
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US5892707A (en) | 1999-04-06 |
US6417040B2 (en) | 2002-07-09 |
US6306703B1 (en) | 2001-10-23 |
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