FERROELECTRIC MEMORY STRUCTURE

Abstract

A ferroelectric memory structure including a first conductive line, a second conductive line, and a memory cell is provided. The second conductive line is disposed on the first conductive line. The memory cell is disposed between the first and second conductive lines. The memory cell includes a switch device and a ferroelectric capacitor structure. The switch device is disposed between the first and second conductive lines. The ferroelectric capacitor structure is disposed between the first conductive line and the switch device. The ferroelectric capacitor structure includes ferroelectric capacitors electrically connected. Each of the ferroelectric capacitors includes a first conductive layer, a second conductive layer, and a ferroelectric material layer. The second conductive layer is disposed on the first conductive layer. The ferroelectric material layer is disposed between the first conductive layer and the second conductive layer. The ferroelectric material layers in the ferroelectric capacitors have different top-view areas.

Claims

1. A ferroelectric memory structure, comprising: a first conductive line; a second conductive line disposed on the first conductive line; and a memory cell disposed between the first conductive line and the second conductive line and comprising: a switch device disposed between the first conductive line and the second conductive line; and a ferroelectric capacitor structure disposed between the first conductive line and the switch device and comprising ferroelectric capacitors electrically connected, wherein each of the ferroelectric capacitors comprises: a first conductive layer; a second conductive layer disposed on the first conductive layer; and a ferroelectric material layer disposed between the first conductive layer and the second conductive layer, wherein the ferroelectric material layers in the ferroelectric capacitors have different top-view areas.

2. The ferroelectric memory structure according to claim 1, wherein the ferroelectric capacitors are connected in series.

3. The ferroelectric memory structure according to claim 1, wherein the ferroelectric capacitors are connected in parallel.

4. The ferroelectric memory structure according to claim 1, wherein the ferroelectric capacitors are located between the same first conductive line and the same switch device.

5. The ferroelectric memory structure according to claim 1, wherein a material of the ferroelectric material layer comprises hafnium zirconium oxide, lead zirconate titanate, strontium titanium oxide, barium titanate, or bismuth ferrite.

6. The ferroelectric memory structure according to claim 1, wherein the switch device comprises a bipolar junction transistor, a diode, or a metal oxide semiconductor field effect transistor.

7. The ferroelectric memory structure according to claim 6, wherein the switch device is the bipolar junction transistor, and the switch device comprises: a first semiconductor layer; a second semiconductor layer disposed on the first semiconductor layer; and a third semiconductor layer disposed between the first semiconductor layer and the second semiconductor layer, wherein the first semiconductor layer and the second semiconductor layer have a first conductive type, and the third semiconductor layer has a second conductive type.

8. The ferroelectric memory structure according to claim 7, a material of the first semiconductor layer and a material of the second semiconductor layer comprise one of a P-type oxide semiconductor and an N-type oxide semiconductor, and a material of the third semiconductor layer comprises the other of the P-type oxide semiconductor and the N-type oxide semiconductor.

9. The ferroelectric memory structure according to claim 8, wherein the P-type oxide semiconductor comprises cobalt oxide, nickel oxide, strontium copper oxide, copper aluminum oxide, copper indium oxide, or copper gallium oxide.

10. The ferroelectric memory structure according to claim 8, wherein the N-type oxide semiconductor comprises indium gallium zinc oxide, zinc oxide, or indium zinc oxide.

11. The ferroelectric memory structure according to claim 1, comprising a plurality of the first conductive lines, a plurality of the second conductive lines, and a plurality of the memory cells to form a memory array.

12. The ferroelectric memory structure according to claim 11, comprising a plurality of the memory arrays arranged in a stack.

13. A ferroelectric memory structure, comprising: a first conductive line; a second conductive line disposed on the first conductive line; and a memory cell disposed between the first conductive line and the second conductive line and comprising: a switch device disposed between the first conductive line and the second conductive line; and a ferroelectric capacitor structure disposed between the first conductive line and the switch device and comprising ferroelectric capacitors electrically connected, wherein each of the ferroelectric capacitors comprises: a first conductive layer; a second conductive layer disposed on the first conductive layer; and a ferroelectric material layer disposed between the first conductive layer and the second conductive layer, wherein the ferroelectric material layers in the ferroelectric capacitors have different thicknesses.

14. The ferroelectric memory structure according to claim 13, wherein the ferroelectric capacitors are connected in series.

15. The ferroelectric memory structure according to claim 13, wherein the ferroelectric capacitors are connected in parallel.

16. The ferroelectric memory structure according to claim 13, wherein the ferroelectric capacitors are located between the same first conductive line and the same switch device.

17. The ferroelectric memory structure according to claim 13, wherein a material of the ferroelectric material layer comprises hafnium zirconium oxide, lead zirconate titanate, strontium titanium oxide, barium titanate, or bismuth ferrite.

18. The ferroelectric memory structure according to claim 13, wherein the switch device comprises a bipolar junction transistor, a diode, or a metal oxide semiconductor field effect transistor.

19. The ferroelectric memory structure according to claim 18, wherein the switch device is the bipolar junction transistor, and the switch device comprises: a first semiconductor layer; a second semiconductor layer disposed on the first semiconductor layer; and a third semiconductor layer disposed between the first semiconductor layer and the second semiconductor layer, wherein the first semiconductor layer and the second semiconductor layer have a first conductive type, and the third semiconductor layer has a second conductive type.

20. The ferroelectric memory structure according to claim 19, a material of the first semiconductor layer and a material of the second semiconductor layer comprise one of a P-type oxide semiconductor and an N-type oxide semiconductor, and a material of the third semiconductor layer comprises the other of the P-type oxide semiconductor and the N-type oxide semiconductor.

21. The ferroelectric memory structure according to claim 20, wherein the P-type oxide semiconductor comprises cobalt oxide, nickel oxide, strontium copper oxide, copper aluminum oxide, copper indium oxide, or copper gallium oxide.

22. The ferroelectric memory structure according to claim 20, wherein the N-type oxide semiconductor comprises indium gallium zinc oxide, zinc oxide, or indium zinc oxide.

23. The ferroelectric memory structure according to claim 13, comprising a plurality of the first conductive lines, a plurality of the second conductive lines, and a plurality of the memory cells to form a memory array.

24. The ferroelectric memory structure according to claim 23, comprising a plurality of the memory arrays arranged in a stack.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0031] The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention.

[0032] FIG. 1 is a schematic perspective view illustrating a ferroelectric memory structure according to some embodiments of the invention.

[0033] FIG. 2A to FIG. 2D are schematic perspective views illustrating a ferroelectric capacitor structure and a conductive line in FIG. 1 according to some embodiments of the invention.

[0034] FIG. 3A is a cross-sectional view taken along section line I-I′ in FIG. 2A.

[0035] FIG. 3B is a cross-sectional view taken along section line II-II′, section line III-III′, and section line IV-IV′ in FIG. 2B.

[0036] FIG. 3C is a cross-sectional view taken along section line V-V′ in FIG. 2C.

[0037] FIG. 3D is a cross-sectional view taken along section line VI-VI′, section line VII-VII′, and section line VIII-VIII′ in FIG. 2D.

[0038] FIG. 4 is a schematic perspective view illustrating a ferroelectric memory structure according to another embodiments of the invention.

[0039] FIG. 5A to FIG. 5D are schematic perspective views illustrating a ferroelectric capacitor structure and a conductive line in FIG. 4 according to another embodiments of the invention.

[0040] FIG. 6A is a cross-sectional view taken along section line IX-IX′ in FIG. 5A.

[0041] FIG. 6B is a cross-sectional view taken along section line X-X′ and section line XI-XI′ in FIG. 5B.

[0042] FIG. 6C is a cross-sectional view taken along section line XII-XII′ in FIG. 5C.

[0043] FIG. 6D is a cross-sectional view taken along section line XIII-XIII′ and section line XIV-XIV′ in FIG. 5D.

DESCRIPTION OF THE EMBODIMENTS

[0044] FIG. 1 is a schematic perspective view illustrating a ferroelectric memory structure according to some embodiments of the invention. FIG. 2A to FIG. 2D are schematic perspective views illustrating a ferroelectric capacitor structure and a conductive line in FIG. 1 according to some embodiments of the invention. FIG. 3A is a cross-sectional view taken along section line I-I′ in FIG. 2A. FIG. 3B is a cross-sectional view taken along section line II-II′, section line III-III′, and section line IV-IV′ in FIG. 2B. FIG. 3C is a cross-sectional view taken along section line V-V′ in FIG. 2C. FIG. 3D is a cross-sectional view taken along section line VI-VI′, section line VII-VII′, and section line VIII-VIII′ in FIG. 2D. In addition, in FIG. 2A to FIG. 2D, some components in FIG. 3A to FIG. 3D are omitted to clearly illustrate the configuration relationship between the components in FIG. 2A to FIG. 2D.

[0045] Referring to FIG. 1, FIG. 2A to FIG. 2D, and FIG. 3A to FIG. 3D, a ferroelectric memory structure 10 includes a conductive line 100, a conductive line 102, and a memory cell 104. In some embodiments, the ferroelectric memory structure 10 may be a ferroelectric random access memory (FRAM) structure. The ferroelectric memory structure 10 may be located on a substrate. In the following embodiments, in order to simplify the drawings, the substrate is not shown. In addition, the conductive line 102 is disposed on the conductive line 100. The material of the conductive line 100 and the material of the conductive line 102 may each include a conductive material such as metal (e.g., tungsten, aluminum, or copper).

[0046] In some embodiments, the ferroelectric memory structure 10 may include a plurality of the conductive lines 100, a plurality of the conductive lines 102, and a plurality of the memory cells 104 to form a memory array MA. In the memory array MA, the conductive lines 100 may extend in direction D1 and may be arranged in direction D2, and the conductive lines 102 may extend in direction D2 and may be arranged in direction D1. Furthermore, the direction D1 intersects the direction D2. In some embodiments, the ferroelectric memory structure 10 may include a plurality of the memory arrays MA arranged in a stack, so that the ferroelectric memory structure 10 may be a three-dimensional (3D) memory structure.

[0047] The memory cell 104 is disposed between the conductive line 100 and the conductive line 102. The memory cell 104 includes a switch device 106 and a ferroelectric capacitor structure 108. The switch device 106 is disposed between the conductive line 100 and the conductive line 102. In some embodiments, the switch device 106 may be a BJT, a diode, a MOSFET, or other suitable switch devices.

[0048] In the present embodiment, the switch device 106 may be the BJT, and the switch device 106 may include a semiconductor layer 110, a semiconductor layer 112, and a semiconductor layer 114, but the invention is not limited thereto. The semiconductor layer 112 is disposed on the semiconductor layer 110. The semiconductor layer 114 is disposed between the semiconductor layer 110 and the semiconductor layer 112.

[0049] The semiconductor layer 110 and the semiconductor layer 112 may have a first conductive type. The semiconductor layer 114 may have a second conductive type. The first conductive type and the second conductive type are different conductive types. Hereinafter, the first conductive type and the second conductive type may be one and the other of the P-type conductive type and the N-type conductive type, respectively. For example, when the first conductive type is the P type and the second conductive type is the N type, the switch device 106 may be a PNP BJT. Moreover, when the first conductive type is N-type and the second conductive type is P-type, the switch device 106 may be an NPN BJT.

[0050] In some embodiments, the material of the semiconductor layer 110 and the material of the semiconductor layer 112 may be one of a P-type oxide semiconductor and an N-type oxide semiconductor, and the material of the semiconductor layer 114 may be the other of the P-type oxide semiconductor and the N-type oxide semiconductor. For example, when the switch device 106 is the PNP BJT, the material of the semiconductor layer 110 and the material of the semiconductor layer 112 may be the P-type oxide semiconductor, and the material of the semiconductor layer 114 may be the N-type oxide semiconductor. In addition, when the switch device 106 is the NPN BJT, the material of the semiconductor layer 110 and the material of the semiconductor layer 112 may be the N-type oxide semiconductor, and the material of the semiconductor layer 114 may be the P-type oxide semiconductor.

[0051] In some embodiments, the P-type oxide semiconductor may be a transparent conducting metal oxide. For example, the P-type oxide semiconductor may include cobalt oxide (CoO.sub.x), nickel oxide (NiO.sub.x), strontium copper oxide (SrCu.sub.2O.sub.x), copper aluminum oxide (CuAlO.sub.2), copper indium oxide (CuInO.sub.2), or copper gallium oxide (CuGaO.sub.2). In some embodiments, the N-type oxide semiconductor may include a transparent amorphous oxide semiconductor or an ionic amorphous oxide semiconductor. For example, the N-type oxide semiconductor may include indium gallium zinc oxide (IGZO), zinc oxide (ZnO), or indium zinc oxide (IZO).

[0052] The ferroelectric capacitor structure 108 is disposed between the conductive line 100 and the switch device 106. In some embodiments, the ferroelectric capacitors 116 may be located between the same conductive line 100 and the same switch device 106. In the present embodiment, although the ferroelectric capacitor structure 108 is disposed between the conductive line 100 and the switch device 106, the invention is not limited thereto. In other embodiments, the ferroelectric capacitor structure 108 may be disposed between the conductive line 102 and the switch device 106.

[0053] The ferroelectric capacitor structure 108 includes ferroelectric capacitors 116 electrically connected. In the present embodiment, the number of the ferroelectric capacitors 116 electrically connected in the ferroelectric capacitor structure 108 is, for example, three, but the invention is not limited thereto. As long as the ferroelectric capacitor structure 108 includes at least two ferroelectric capacitors 116 electrically connected, it falls within the scope of the invention. Each of the ferroelectric capacitors 116 includes a conductive layer 118, a conductive layer 120, and a ferroelectric material layer 122. The conductive layer 120 is disposed on the conductive layer 118. The material of the conductive layer 118 and the material of the conductive layer 120 may each include a conductive material such as metal. The ferroelectric material layer 122 is disposed between the conductive layer 118 and the conductive layer 120. Furthermore, the ferroelectric material layers 122 in the ferroelectric capacitors 116 are separated from each other. The ferroelectric material layer 122 may be used as a ferroelectric insulating layer. The material of the ferroelectric material layer 122 may include hafnium zirconium oxide (HZO), lead zirconate titanate (PZT), strontium titanium oxide (STO), barium titanate (BTO), or bismuth ferrite (BFO).

[0054] In some embodiments, as shown in FIG. 2A, FIG. 2C, FIG. 3A, and FIG. 3C, the ferroelectric capacitors 116 may be arranged in a stack on the conductive line 100. In the ferroelectric capacitor structure 108, the stacking sequence of the ferroelectric capacitors 116 may be adjusted according to the product requirement and is not limited to the stacking sequence in the figure. For example, in FIG. 2A and FIG. 3A, the ferroelectric capacitor 116a, the ferroelectric capacitor 116b, and the ferroelectric capacitor 116c are sequentially stacked on the conductive line 100, but the invention is not limited thereto. In other embodiments, the position of the ferroelectric capacitor 116a and the position of the ferroelectric capacitor 116c may be interchanged, that is, the ferroelectric capacitor 116c, the ferroelectric capacitor 116b, and the ferroelectric capacitor 116a may be sequentially stacked on the conductive line 100. In some embodiments, as shown in FIG. 2B, FIG. 2D, FIG. 3B, and FIG. 3D, the ferroelectric capacitors 116 may be horizontally arranged on the conductive line 100.

[0055] In some embodiments, as shown in FIG. 2A, FIG. 2B, FIG. 3A, and FIG. 3B, the ferroelectric material layers 122 in the ferroelectric capacitors 116 have different top-view areas, so that the ferroelectric capacitors 116 can have different impedances (e.g., capacitances). For example, as shown in FIG. 2A, FIG. 2B, FIG. 3A, and FIG. 3B, the top-view area of the ferroelectric material layer 122a may be larger than the top-view area of the ferroelectric material layer 122b, and the top-view area of the ferroelectric material layer 122b may be larger than the top-view area of the ferroelectric material layer 122c. In some embodiments, as shown in FIG. 2A, FIG. 2B, FIG. 3A, and FIG. 3B, the ferroelectric material layers 122 in the ferroelectric capacitors 116 may have the same thickness, but the invention is not limited thereto.

[0056] In some embodiments, as shown in FIG. 2C, FIG. 2D, FIG. 3C, and FIG. 3D, the ferroelectric material layers 122 in the ferroelectric capacitors 116 have different thicknesses, so that the ferroelectric capacitors 116 can have different impedances (e.g., capacitances). For example, as shown in FIG. 2C, FIG. 2D, FIG. 3C, and FIG. 3D, the thickness of the ferroelectric material layer 122a may be greater than the thickness of the ferroelectric material layer 122b, and the thickness of the ferroelectric material layer 122b may be greater than the thickness of the ferroelectric material layer 122c. In some embodiments, as shown in FIG. 2C, FIG. 2D, FIG. 3C, and FIG. 3D, the ferroelectric material layers 122 in the ferroelectric capacitors 116 may have the same top-view area, but the invention is not limited thereto.

[0057] That is, the top-view areas and/or the thicknesses of the ferroelectric material layers 122 in the ferroelectric capacitors 116 may be adjusted to make the ferroelectric capacitors 116 have different impedances (e.g., capacitances).

[0058] In some embodiments, as shown in FIG. 3A and FIG. 3C, the ferroelectric capacitors 116 may be connected in series. For example, in FIG. 3A and FIG. 3C, the conductive layer 120a of the ferroelectric capacitor 116a may be electrically connected to the conductive layer 118b of the ferroelectric capacitor 116b, and the conductive layer 120b of the ferroelectric capacitor 116b may be electrically connected to the conductive layer 118c of the capacitor 116c, so that the ferroelectric capacitors 116 (e.g., ferroelectric capacitor 116a, ferroelectric capacitor 116b, and ferroelectric capacitor 116c) may be connected in series, but the invention is not limited thereto. In other embodiments, two adjacent ferroelectric capacitors 116 may share a conductive layer, so that the ferroelectric capacitors 116 (e.g., ferroelectric capacitor 116a, ferroelectric capacitor 116b, and ferroelectric capacitor 116c) may be connected in series. For example, when the conductive layer 118b and the conductive layer 118c are omitted, the ferroelectric capacitor 116a and the ferroelectric capacitor 116b may share the conductive layer 120a, and the ferroelectric capacitor 116b and the ferroelectric capacitor 116c may share the conductive layer 120b, so that the ferroelectric capacitors 116 (e.g., ferroelectric capacitor 116a, ferroelectric capacitor 116b, and ferroelectric capacitor 116c) may be connected in series.

[0059] In addition, in FIG. 3A and FIG. 3C, the conductive layer 118a of the ferroelectric capacitor 116a may be electrically connected to the conductive line 100, and the conductive layer 120c of the ferroelectric capacitor 116c may be electrically connected to the semiconductor layer 110 of the switch device 106. In some embodiments, as shown in FIG. 3A and FIG. 3C, the conductive layer 118a of the ferroelectric capacitor 116a may be directly connected to the conductive line 100, and the conductive layer 120c of the ferroelectric capacitor 116c may be directly connected to the semiconductor layer 110 of the switch device 106, but the invention is not limited thereto. In other embodiments, the conductive layer 118a of the ferroelectric capacitor 116a may be electrically connected to the conductive line 100 by a conductive member (e.g., contact) (not shown). In other embodiments, the conductive layer 120c of the ferroelectric capacitor 116c may be electrically connected to the semiconductor layer 110 of the switch device 106 by a conductive member (e.g., contact) (not shown).

[0060] In some embodiments, as shown in FIG. 3B, the ferroelectric capacitors 116 may be connected in parallel. For example, as shown in FIG. 3B, when the ferroelectric capacitors 116 are separated from each other, the conductive layers 118 of the ferroelectric capacitors 116 may be electrically connected to the same conductive line 100 respectively, and the conductive layers 120 of the ferroelectric capacitors 116 may be electrically connected to the semiconductor layer 110 of the same switch device 106 respectively, so that the ferroelectric capacitors 116 (e.g., ferroelectric capacitor 116a, ferroelectric capacitor 116b, and ferroelectric capacitor 116c) may be connected in parallel, but the invention is not limited thereto. In other embodiments, the conductive layer 118a of the ferroelectric capacitor 116a, the conductive layer 118b of the ferroelectric capacitor 116b, and the conductive layer 118c of the ferroelectric capacitor 116c may be connected to each other to form a single conductive layer, and the conductive layer 120a of the ferroelectric capacitor 116a, the conductive layer 120b of the ferroelectric capacitor 116b, and the conductive layer 120c of the ferroelectric capacitor 116c may be connected to each other to form a single conductive layer, so that the ferroelectric capacitors 116 (e.g., ferroelectric capacitor 116a, ferroelectric capacitor 116b, and ferroelectric capacitor 116c) may be connected in parallel.

[0061] In FIG. 3B, the conductive layers 118 of the ferroelectric capacitors 116 may be directly connected to the same conductive line 100, but the invention is not limited thereto. Furthermore, in FIG. 3B, the conductive layers 120 of the ferroelectric capacitors 116 may be directly connected to the semiconductor layer 110 of the same switch device 106, but the invention is not limited thereto. In other embodiments, the conductive layers 118 of the ferroelectric capacitors 116 may be electrically connected to the same conductive line 100 by a conductive member (e.g., contact) (not shown), respectively. In other embodiments, the conductive layers 120 of the ferroelectric capacitors 116 may be electrically connected to the semiconductor layer 110 of the same switch device 106 by a conductive member (e.g., contact) (not shown), respectively.

[0062] In some embodiments, as shown in FIG. 3D, the ferroelectric capacitors 116 may be connected in parallel. For example, as shown in FIG. 3D, when the ferroelectric capacitors 116 are separated from each other, the conductive layers 118 of the ferroelectric capacitors 116 may be electrically connected to the same conductive line 100 respectively, and the conductive layers 120 of the ferroelectric capacitors 116 may be electrically connected to the semiconductor layer 110 of the same switch device 106 respectively, so that the ferroelectric capacitors 116 (e.g., ferroelectric capacitor 116a, ferroelectric capacitor 116b, and ferroelectric capacitor 116c) may be connected in parallel, but the invention is not limited thereto. In other embodiments, the conductive layer 118a of the ferroelectric capacitor 116a, the conductive layer 118b of the ferroelectric capacitor 116b, and the conductive layer 118c of the ferroelectric capacitor 116c may be connected to each other to form a single conductive layer.

[0063] In FIG. 3D, the conductive layers 118 of the ferroelectric capacitors 116 may be directly connected to the same conductive line 100, but the invention is not limited thereto. In other embodiments, the conductive layers 118 may be electrically connected to the same conductive line 100 by a conductive member (e.g., contact) (not shown), respectively. Moreover, in FIG. 3D, the conductive layers 120 of the ferroelectric capacitors 116 may be directly connected to the semiconductor layer 110 of the switch device 106 or electrically connected to the semiconductor layer 110 of the switch device 106 by a conductive member (e.g., contact 124). As shown in FIG. 3D, the conductive layer 120a of the ferroelectric capacitor 116a may be directly connected to the semiconductor layer 110 of the switch device 106, but the invention is not limited thereto. In other embodiments, the conductive layer 120a of the ferroelectric capacitor 116a may be electrically connected to the semiconductor layer 110 of the switch device 106 by a conductive member (e.g., contact) (not shown). In addition, as shown in FIG. 3D, the conductive layer 120b of the ferroelectric capacitor 116b and the conductive layer 120c of the ferroelectric capacitor 116c may be electrically connected to the semiconductor layer 110 of the switch device 106 by the contact 124, respectively.

[0064] Furthermore, the ferroelectric memory structure 10 may further include other required dielectric layers (for isolation) and/or other required interconnect structures (for electrical connection), and the description thereof is omitted here.

[0065] Hereinafter, Table 1 is used to illustrate various storage states of the memory cell 104 of the ferroelectric memory structure 10. By controlling the voltages applied to the conductive line 100 and the conductive line 102, the ferroelectric capacitor 116 may have a polarization state of “positive (+) direction” or a polarization state of “negative (−) direction”. When the ferroelectric capacitor 116 has the polarization state of “positive (+) direction”, the ferroelectric capacitor 116 may have a low impedance Z.sub.L and is regarded as storing a first data (e.g., data “0”). When the ferroelectric capacitor 116 has the polarization state of “negative (−) direction”, the ferroelectric capacitor 116 may have a high impedance Z.sub.H and is regarded as storing a second data (e.g., data “1”). In the present embodiment, the impedance is, for example, a capacitance, but the invention is not limited thereto. Thus, when the memory cell 104 is operated, by controlling the voltages applied to the conductive line 100 and the conductive line 102, the ferroelectric capacitor 116a may have a low impedance Z.sub.L1 or a high impedance Z.sub.H1, the ferroelectric capacitor 116b may have a low impedance Z.sub.L2 or a high impedance Z.sub.H2, and the ferroelectric capacitor 116c may have a low impedance Z.sub.L3 or a high impedance Z.sub.H3. Therefore, a single memory cell 104 may have 8 different storage states (i.e., “000”, “001”, “010”, “011”, “100”, “101”, “110”, and “111”). That is, a single memory cell 104 may store 3 bits of data.

TABLE-US-00001 TABLE 1 storage state 000 001 010 011 100 101 110 111 series series series series series series series series series impedance impedance impedance impedance impedance impedance impedance impedance impedance of Z.sub.L1, of Z.sub.L1, of Z.sub.L1, of Z.sub.L1, of Z.sub.L1, of Z.sub.L1, of Z.sub.L1, of Z.sub.L1, Z.sub.L2, and Z.sub.L2, and Z.sub.L2, and Z.sub.L2, and Z.sub.L2, and Z.sub.L2, and Z.sub.L2, and Z.sub.L2, and Z.sub.L3 Z.sub.L3 Z.sub.L3 Z.sub.L3 Z.sub.L3 Z.sub.L3 Z.sub.L3 Z.sub.L3 parallel parallel parallel parallel parallel parallel parallel parallel parallel impedance impedance impedance impedance impedance impedance impedance impedance impedance of Z.sub.L1, of Z.sub.L1, of Z.sub.L1, of Z.sub.L1, of Z.sub.L1, of Z.sub.L1, of Z.sub.L1, of Z.sub.L1, Z.sub.L2, and Z.sub.L2, and Z.sub.L2, and Z.sub.L2, and Z.sub.L2, and Z.sub.L2, and Z.sub.L2, and Z.sub.L2, and Z.sub.L3 Z.sub.L3 Z.sub.L3 Z.sub.L3 Z.sub.L3 Z.sub.L3 Z.sub.L3 Z.sub.L3

[0066] Based on the above embodiments, in the ferroelectric memory structure 10, a single memory cell 104 includes the ferroelectric capacitors 116 electrically connected, and the ferroelectric material layers 122 in the ferroelectric capacitors 116 have different top-view areas or different thicknesses. Therefore, a single memory cell 104 can store multiple bits of data, thereby increasing the bit density of the memory device.

[0067] FIG. 4 is a schematic perspective view illustrating a ferroelectric memory structure according to another embodiments of the invention. FIG. 5A to FIG. 5D are schematic perspective views illustrating a ferroelectric capacitor structure and a conductive line in FIG. 4 according to another embodiments of the invention. FIG. 6A is a cross-sectional view taken along section line IX-IX′ in FIG. 5A. FIG. 6B is a cross-sectional view taken along section line X-X′ and section line XI-XI′ in FIG. 5B. FIG. 6C is a cross-sectional view taken along section line XII-XII′ in FIG. 5C. FIG. 6D is a cross-sectional view taken along section line XIII-XIII′ and section line XIV-XIV′ in FIG. 5D. In addition, in FIG. 5A to FIG. 5D, some components in FIG. 6A to FIG. 6D are omitted to clearly illustrate the configuration relationship between the components in FIG. 5A to FIG. 5D.

[0068] Referring to FIG. 1 to FIG. 6D, the difference between the ferroelectric memory structure 20 in FIG. 4 to FIG. 6D and the ferroelectric memory structure 10 in FIG. 1 to FIG. 3D is described as follows. In the ferroelectric memory structure 20, the number of the ferroelectric capacitors 116 of the ferroelectric capacitor structure 208 is two. For example, the ferroelectric capacitor structure 208 may include the ferroelectric capacitor 116a and the ferroelectric capacitor 116b. Furthermore, in FIG. 6A and FIG. 6C, since the ferroelectric capacitor structure 208 does not include the ferroelectric capacitor 116c in FIG. 3A and FIG. 3C, the conductive layer 120b of the ferroelectric capacitor 116b may be electrically connected to the semiconductor layer 110 of the switch device 106.

[0069] Moreover, the same or similar components in the ferroelectric memory structure 20 and the ferroelectric memory structure 10 are denoted by the same or similar symbols, and the same or similar content in the ferroelectric memory structure 20 and the ferroelectric memory structure 10 may be referred to the description of the ferroelectric memory structure 10 in the foregoing embodiment, and the description thereof is omitted here. In addition, the ferroelectric memory structure 20 may further include other required dielectric layers (for isolation) and/or other required interconnect structures (for electrical connection), and the description thereof is omitted here.

[0070] Hereinafter, Table 2 is used to illustrate various storage states of the memory cell 204 of the ferroelectric memory structure 20. By controlling the voltages applied to the conductive line 100 and the conductive line 102, the ferroelectric capacitor 116 may have a polarization state of “positive (+) direction” or a polarization state of “negative (−) direction”. When the ferroelectric capacitor 116 has the polarization state of “positive (+) direction”, the ferroelectric capacitor 116 may have a low impedance Z.sub.L and is regarded as storing a first data (e.g., data “0”). When the ferroelectric capacitor 116 has the polarization state of “negative (−) direction”, the ferroelectric capacitor 116 may have a high impedance Z.sub.H and is regarded as storing a second data (e.g., data “1”). In the present embodiment, the impedance is, for example, a capacitance, but the invention is not limited thereto. Thus, when the memory cell 204 is operated, by controlling the voltages applied to the conductive line 100 and the conductive line 102, the ferroelectric capacitor 116a may have a low impedance Z.sub.L1 or a high impedance Z.sub.H1, and the ferroelectric capacitor 116b may have a low impedance Z.sub.L2 or a high impedance Z.sub.H2. Therefore, a single memory cell 204 may have 4 different storage states (i.e., “00”, “01”, “10”, and “011”). That is, a single memory cell 204 may store 2 bits of data.

TABLE-US-00002 TABLE 2 storage state 00 01 10 11 series series series series series impedance impedance impedance impedance impedance of of of of Z.sub.L1 and Z.sub.L2 Z.sub.L1 and Z.sub.H2 Z.sub.H1 and Z.sub.L2 Z.sub.H1 and Z.sub.H2 parallel parallel parallel parallel parallel impedance impedance impedance impedance impedance of of of of Z.sub.L1 and Z.sub.L2 Z.sub.L1 and Z.sub.H2 Z.sub.H1 and Z.sub.L2 Z.sub.H1 and Z.sub.H2

[0071] Based on the above embodiments, in the ferroelectric memory structure 20, a single memory cell 204 includes the ferroelectric capacitors 116 electrically connected, and the ferroelectric material layers 122 in the ferroelectric capacitors 116 have different top-view areas or different thicknesses. Therefore, a single memory cell 204 can store multiple bits of data, thereby increasing the bit density of the memory device.

[0072] In summary, in the ferroelectric memory structure of the aforementioned embodiments, a single memory cell includes ferroelectric capacitors that are electrically connected and have different impedances (e.g., capacitances). Therefore, a single memory cell can store multiple bits of data, thereby increasing the bit density of the memory device.

[0073] Although the invention has been described with reference to the above embodiments, it will be apparent to one of ordinary skill in the art that modifications to the described embodiments may be made without departing from the spirit of the invention. Accordingly, the scope of the invention is defined by the attached claims not by the above detailed descriptions.