Structure of a stacked memory cell, in particular a ferroelectric cell

[0002] The present invention refers to a stacked type of memory cell structure. In particular, the memory cell described is of the ferroelectric non-volatile type, but the same structure can be used for DRAM cells.

[0003] As is known and shown in FIG. 1, a ferroelectric cell 1 is composed of a MOS transistor 2 and a capacitor 3 having, as a dielectric, a ferroelectric material, for example PZT (PbZr_1−xTi_xO₃, perovskite) or SBT (SrBi₂Ta₂O₉, layered perovskite). In detail, in the ferroelectric cell 1, the NMOS-type transistor 2 has a source terminal 4 connected to a bit line BL, a gate electrode 5 connected to a word line WL and a drain terminal 6 connected to a first plate 7 of the capacitor 3. A second plate 8 of the capacitor 3 is connected to a plate line PL.

[0004] The cell is capable of storing binary information thanks to the hysteresis characteristics of the ferroelectric material which is sandwiched between the plates 7 and 8 and which, when there are no applied voltages, can assume two bias states depending on the sign of previously-applied voltage across the capacitor 3 terminals.

[0005] All currently-known ferroelectric cells can be classified into two families: strapped cells and stacked cells.

[0006] In strapped cells, an embodiment of which is shown in FIG. 2, the capacitor 3 is constructed above a field oxide region 10 that delimits an active area 11 of the substrate 12 in which the conductive regions (source 13 and drain 14) of the transistor 2 are formed. In detail, the first plate 7 of the capacitor 3 is here placed on top and is made of a square- or rectangular-shaped region of conductive material (for example, platinum), connected to the drain region 14 of the transistor 2 through a metallic connection line 16; the second plate 8 of the capacitor 3 is here placed underneath and is made by a band of conductive material (for example, platinum again) which runs perpendicular to the drawing plane and forms a plate line PL, connected to other capacitors of adjacent cells; a dielectric region 17, of ferroelectric material, is sandwiched between the first plate 7 and the second plate 8. The gate electrode 5 of the transistor 2 is made of a band of polycrystalline silicon which runs perpendicular to the drawing plane and forms a word line WL.

[0007] In stacked cells, an embodiment of which can be seen in FIG. 3, the capacitor 3 is constructed above the active area 11, directly above the drain region 14 of the transistor 2. In this case, the first plate 7 of the capacitor 3 is placed underneath and is made by a square- or rectangular-shaped region of conductive material (for example, platinum) connected to the drain region 14 through a contact 23 formed in an opening of a protective layer 24 (for example BPSG) and the second plate 8, of conductive material, is placed above and is connected to a metalization band 25 defining the plate line PL.

[0008] A titanium/titanium nitride region 26 runs below the first plate 7 to help the adhesion of the first plate 7 of the capacitor 3 on the protective layer 24.

[0009] The architecture of a array 28 of ferroelectric stacked or strapped cells 1 is shown in FIG. 4. It will be noted that the ferroelectric cells 1 are placed on rows and columns and are coupled so that the cell pairs 27 are placed parallel to bit lines BL; the transistors 2 of each cell pair 27 have common source regions, connected to the same bit line BL; and the capacitors 3 belonging to the cell pairs 27 adjacent in a parallel direction to the bit lines BL are connected to adjacent plate line pairs PL.

[0010] Ferroelectric stacked cells 1 are currently preferred, since they are the only ones capable of meeting the scalability requirements of new CMOS technologies. In stacked cells, the layout rules on the capacitor 3 design are crucial for the optimization of the cell.

[0011] There are therefore several known embodiments for stacked cells, apart from that shown in FIG. 3, in which both plates 7,8 and dielectric region 17 are defined using a single mask and forming the plate line PL via a special metallic band. For example, according to another known arrangement, the first (lower) electrode 7 is formed by a separately-shaped conductive region, while the dielectric region 17 and the second (upper) electrode 8 are mutually aligned and shaped using a single mask.

[0012] In all these cases, the connection of at least one of the plates 7,8 with the same mask used for the connection of the ferroelectric material composing the dielectric region 17 is critical; for example, during connection, slightly volatile components are formed, and these can be redeposited along the capacitor edge and damage its active zone, causing a decay in the ferroelectric properties of the capacitor, with an increase in edge losses and lower voltage strength.

[0013] On the other hand, the separate definition of the three parts constituting the capacitor 3 (first and second plates 7,8 and dielectric region 17), which would allow the problem presented by current manufacturing processes to be solved, causes an increase in overall dimensions that is in conflict with present trends towards miniaturization. In fact, in making definition masks, account must be taken of both manufacturing tolerances (at present, with a 0.35 μm process, equal to 0.2 μm) and the minimum distances between the lower adjacent plates and the upper adjacent plates (for example, equal to 0.4 μm). In particular, with the conditions given above, it would be necessary for the lower plate 7 to be wider, with respect to the dielectric region 7, by an amount at least equal to the manufacturing tolerance (at least 0.2 μm) on each side; similarly, it would be necessary for the dielectric region 17 to be wider, with respect to the upper plate 8, by the same amount; therefore, taking into account the minimum distance between the lower plates 7, the resulting overall dimensions for the capacitors 3 and consequently for the ferroelectric cells 1 are excessive.

[0014] An object the present invention is making a stacked memory cell, without the described disadvantages. According to the present invention, a stacked memory cell is realized as claimed in Claim 1.

[0015] In practice, according to the invention, the dielectric regions of at least two adjacent cells in the direction of the bit lines are no longer separate. In particular, the dielectric region 17 can be continuous and shared between the two adjacent capacitors belonging to pairs of adjacent cells. In this way, the layout definition rules for capacitor scalability are given only by the distance between two lower adjacent electrodes and by the lateral space (enclosure) between upper electrode and lower electrode. This allows, with the same cell area, maximizing the working area of the capacitor compared with the layout of the arrangement where three different masks for definition of the capacitor are used and the dielectric region is divided between adjacent cells in the direction of the bit lines. This causes an increase in signal amplitude (proportional to the active area of the capacitor) supplied by each cell to the sense amplifier during reading. Alternatively, it is possible to obtain a reduction in the area occupied by the ferroelectric cells.

[0016] For a better understanding of the invention, an embodiment thereof will now be described, merely as a non-limiting example and with reference to the enclosed drawings, in which:

[0017] FIG. 1 shows the equivalent electrical diagram of a known ferroelectric cell.

[0018] FIG. 2 shows a cross-section through a first known type of ferroelectric cell.

[0019] FIG. 3 shows a cross-section through a second known type of ferroelectric cell.

[0020] FIG. 4 shows the known architecture of a memory cell array of the type under consideration.

[0021] FIG. 5 shows a cross-section through a slice of semiconductor material in which cells have been constructed according to the invention, taken along section line V-V in FIG. 7.

[0022] FIG. 6 shows a transverse section of ferroelectric cells according to the invention, taken along section line VI-VI in FIG. 5.

[0023] FIG. 7 shows the layout of two cells adjacent to each other, according to the invention.

[0024] FIGS. 5-7 show stacked cells 1 constructed according to the invention. The present description refers to ferroelectric cells in particular, without being restricted to these.

[0025] In detail, FIG. 5 shows a pair of cells 27 and a single ferroelectric cell 1 adjacent to the pair of cells 27 in the direction of the bit lines (horizontal direction in FIG. 5). Pairs of adjacent cells 27 are insulated from each other by a thick oxide layer 30 (field oxide) which delimits, inside a P-type substrate 31, active areas 32 in which two drain regions 33 and a common N⁺type source region 34 are formed. In a known way, gate electrodes 36 are formed above the substrate 31 and are insulated therefrom by a thin oxide layer 37. The gate electrodes 36 are preferably made by bands comprising a double layer of polycrystalline silicon and tungsten silicide running perpendicular to FIG. 5 and defining word lines; the gate electrodes 36 are also laterally flanked by oxide spacers 38.

[0026] Less doped drain extension regions 39 are formed in the substrate 31 below spacers 38 and a protective oxide layer 40 covers the surface of the substrate 31.

[0027] A first insulating layer 41 (for example, made of Boron Phosphorous Silicon Glass, or BPSG) runs above the protective oxide layer 40 and has openings into which run first and second contacts 43, 44 made of conductive material, for contacting the drain regions 33 and, respectively, the source regions 34.

[0028] Above the first insulating layer 41, there are formed lower plates 50 (corresponding to the first plates 7 in FIG. 3) electrically connected to the first contacts 43; preferably, the lower plates 50 are composed of a lower portion 50amade of titanium and an upper portion 50bmade of platinum. Bands of ferroelectric material 51 run, without gaps, above and between adjacent pairs of lower plates 50, belonging to different pair of cells 27; in addition, the bands of ferroelectric material 51 continuously run above and between the lower plates 50 of adjacent cells in the cross section of FIG. 6, as also shown in the layout of FIG. 7.

[0029] Above the bands of ferroelectric material 51, there are first bands of conductive material 52 forming upper plates (corresponding to the second plates 8 in FIG. 3) of the capacitors 3; the first bands of conductive material 52 compose plate lines PL and, as is clear from FIG. 7, have a smaller width compared to the lower plates 50.

[0030] Above the first insulating layer 41, there are also formed first contact regions 54, overlying and directly connected to the second contacts 44; the first contact regions 54 are also made preferably using a double layer of titanium and platinum.

[0031] A second insulating layer 55 is formed above the first insulating layer 41, covering the first bands of conductive material 52; openings are formed through the second insulating layer 55, into which third contacts 56 run (FIG. 5), directly connected to the first contact regions 54, and openings into which fourth contacts 57 run (FIG. 6), directly connected to the first bands of conductive material 52. Above the second insulating layer 55 metalization lines 60 are formed that define the bit lines BL and are directly connected to the third contacts 56; in addition, second contact regions 61 are formed (FIG. 6) directly connected to the fourth contacts 57. The metalization lines 60 and the second contact regions 61 are formed in the first metalization level.

[0032] Above the second insulating layer 55, completely covering the metalization lines 60, there runs a third insulating layer 63, showing openings into which fifth contacts 65 run (FIG. 6) directly connected to the second contact regions 61; above the third insulating layer 63, third contact regions 66 are formed, directly connected to the fifth contacts 65 and made in the second level of metalization, as well as second bands of conductive material 67, connected, in a way not shown, to the word lines forming the gate electrodes 36.

[0033] A passivation layer 70 completely covers the device.

[0034] Purely by way of example, possible values relative to the regions which compose the ferroelectric cell 1 are given hereinbelow. The area of the capacitor 2 is in the range between 1 and 5 μm²; the lower titanium portion 5Oaof the lower plate is 20 nm thick; the upper platinum portion 50bhas a thickness between 100 and 200 nm; the layer of ferroelectric material 51 has a thickness between 50 and 250 μm, and is preferably equal to 100 nm; the first bands of conductive material 52 are approximately 200 nm thick.

[0035] The process for manufacturing the ferroelectric cells 1 shown in FIGS. 5-7 is as follows. After having made the transistors 2, having covered with the first insulating layer 41 and having made the first and second contacts 43, 44, in a way known per se, a layer of titanium and then a layer of platinum are made (for example, by sputtering deposition); the lower plates 50 are defined via a first mask (connection of platinum and titanium layers); a layer of ferroelectric material is laid and then a layer of platinum is laid (for example, by sputtering deposition). Then, using a second mask, the first bands of conductive material 52 (connection of platinum layer) are defined and, using a third mask, the bands of ferroelectric material 51 are defined.

[0036] The advantages that can be obtained with the present invention are as follows. First of all, the ratio between capacitor 3 area and total cell 1 area is maximized. In fact, the restrictive rules in the cell design must now take into account only the distance between the two lower plates 50 belonging to adjacent pairs of cells 27 and the enclosure between the edges of the lower plate 50 and of the first band of conductive material 52 of each cell.

[0037] In addition, critical points in the steps of photolithography and ferroelectric material connection are removed.

[0038] Studies by the applicant have shown that the parasitic ferroelectric capacitor formed between two capacitors 2 belonging to two pairs of adjacent cells 27 on the same row does not prejudice the correct operation of the ferroelectric cells 1, given the increased thickness of the dielectric between the two plates 50 and the reduced facing area of the two plates 50 themselves. In addition, the ratio between the active capacitance (capacitance of capacitors 3) and the parasitic capacitance improves with the reduction in thickness of the lower plate 50 and of the layer of ferroelectric material 51 and therefore tends to be reduced with the increase in miniaturization of the devices, expected in the next technological generations.

[0039] Finally, it is evident that modifications and variations can be made to the described cell, still remaining within the scope of the present invention.

[0040] For example, the layer of ferroelectric material 51 can run over the whole surface of the first insulating layer 41 except for the zones where the first contact regions 54 have to be made (openings 75 in FIG. 5); the materials can differ from those described, as long as they are suitable for the functions required; the exact arrangement of the areas and their dimensions can differ from those described above.