Efficient dual port DRAM cell using SOI technology

[0002] The present invention relates to dynamic random access memory (“DRAM”) cells, particularly to a dual port DRAM cell having reduced architecture utilizing silicon on insulator (“SOI”) structure.

[0003] As the data processing speed of computer systems increase, the need for faster memory speeds also increases. Conventional (or single port) DRAMs, due to their high density and relatively low manufacturing cost, are presently used for the majority of memory applications. Conventional DRAMs, however, are not feasible for many growing memory applications. While cost effective, conventional DRAMs have a lower operating speed relative to other memory types.

[0004] In a typical single port configuration, as shown in FIG. 1, a DRAM cell 100 consists of an access transistor 101, a storage capacitor 120, a digit line 111 and a wordline 106. During a write access, a wordline enable signal is asserted on wordline 106 thereby turning on transistor 101. A data signal is provided on digit line 111. This signal is routed through transistor 101 and stored in capacitor 120. During a read access, a wordline enable signal is asserted on wordline 106 to turn on transistor 101. The data signal stored in capacitor 120 is routed to digit line 111 through transistor 101. This data signal is amplified by a sense amplifier (not shown) and then provided to the device initiating the read access.

[0005] As noted, the single port cell DRAM illustrated in FIG. 1 while having many advantages is relatively slow. As a result, faster memory devices have been developed which include dual-port DRAMs (called video RAMs, or VRAMs, in some instances). Dual-port DRAMs provide speed advantages over conventional DRAMs by providing an additional input/output port to the memory array. In conventional DRAMs, read and write operations may never occur simultaneously, as both operations occur through a single random access port, as discussed above. In contrast, in a dual-port DRAM, the second port (most often a serial port) is provided in addition to the random access port. Data is read and written by transferring an entire array row at one time between a serial shift register and the array.

[0006] FIG. 2 is a schematic diagram of a conventional dual-port DRAM cell 200. Cell 200 consists of write access transistor 201, read access transistor 202 and storage capacitor 220. The operation of cell 200 is similar to that of single port DRAM cell 100, except the read and write accesses are performed at two separate dedicated ports. The write port is defined by the write digit line 211 and write wordline 206. The read port is defined by the read digit line 212 and read wordline 205.

[0007] During a write access, a write wordline enable signal is asserted on write wordline 206, thereby turning on write transistor 201. A data signal is provided to capacitor 220 through write digit line 211 and write transistor 201. During a read access, a read wordline enable signal is asserted on read wordline 205. If a logic high data value is stored in capacitor 220, read transistor 202 turns on and the logic high wordline enable signal on read wordline 205 is transmitted to read digit line 212. If a logic low data value is stored in the capacitor 220, transistor 202 is not turned on, and read digit line 212 is left floating (i.e. at a logic low value).

[0008] However, even though dual port DRAMs have higher operating speeds, their cells are more complex in design and are larger in size relative to a conventional single port DRAM cell. Typically, a dual port DRAM cell occupies 16 square features (F²). Hence, what is needed is a dual port DRAM with a reduced structure size.

[0009] The present invention proposes a dual port DRAM SOI structure with reduced architecture of no more than 8 F²by utilizing two separate metallization layers above and below an upper surface of the silicon on insulator layer.

[0010] In a memory cell of the invention, each storage capacitor is connected by an associated access transistor, to one of two bit lines. Thus, a first access transistor connects the capacitor through an upper bit line contact to a first digit line which runs above the upper surface of the silicon on insulator layer while a second access transistor connects the capacitor through a lower bit line contact to a second digit line which runs below the upper surface of the silicon on insulator layer.

[0011] The above advantages and features of the invention will be more clearly understood from the following detailed description which is provided in connection with the accompanying drawings.

[0012] FIG. 1 illustrates a schematic of a single port memory cell;

[0013] FIG. 2 illustrates a schematic of a dual port memory cell;

[0014] FIG. 3 illustrates a cross-sectional view of an exemplary embodiment of a dual port memory cell in accordance with the present invention;

[0015] FIG. 4 illustrates a top-plan view of the FIG. 3 embodiment; and

[0016] FIG. 5 illustrates a processor based system utilizing the exemplary embodiment of the present invention.

[0017] The present invention will be described as set forth in FIGS. 3-5. Other embodiments may be utilized and structural or logical changes may be made without departing from the spirit or scope of the present invention. Although the invention is illustrated in the drawings in connection with dual port DRAM cells, the invention may also be practiced using any multi-port memory cell as well. Like items are referred to by like reference numerals throughout the drawings.

[0018] The term SOI used in the following description may include any silicon-on-insulator structure that has an exposed silicon surface. Structure must be understood to include, any SOIs fabricated by techniques such as bonded wafer method, SIMOX (separation by implanted oxygen), FIPOS (full isolation by oxidized silicon), laser recrystallization method, and lateral epitaxial method. When reference is made to SOI in the following description, previous process steps may have been utilized to form regions or junctions in or on the base semiconductor layer or foundation.

[0019] Although it will be appreciated that additional techniques can be used to measure memory cell sizes which may produce different results from the technique described and used to determine cell size herein, the intended definition of feature is minimum realizable process dimension, but in actual fact equates to a dimension that is half of the wordline (row) and/or digit line (column) pitch.

[0020] An exemplary embodiment of the invention is now illustrated in connection with FIGS. 3, 4 and 5. FIG. 3 shows a cross section of the dual port DRAM cells 300 of the present invention. An array of dual port DRAM cells 300 are built from orthogonal stripes of active area 24 and wordline polysilicon 16. This memory cell, employing a buried capacitor structure, places the digit lines 14a,14bphysically above and below the storage capacitor 12, respectively and above and below the upper surface of the SOI layer 9. The digit lines 14aand 14bare constructed from either metal or polysilicon and digit line contacts 22, 23, formed using metal or a polysilicon plug through insulating layers 27-29 (for contact 22) and SOI layer 9 and insulating layer 31 (for contact 23) are used to connect the digit lines 14aand 14bto respective access transistors 11 and 13. The memory cell capacitor 12 is formed with polysilicon (poly2) 30 connected to a polysilicon bottom cell plate, an oxide-nitride-oxide (ONO) dielectric 34, and a sheet of polysilicon (poly3) 32 which forms the common node shared by all memory cell capacitors 12. The capacitor shape can be simple, such as a rectangle, or complex, such as concentric cylinders or stacked discs. Many other materials can also be used in place of those used for the poly plug 30, polysilicon cell plates and dielectric, as well known in the art.

[0021] Each storage capacitor 12 has on its opposite sides, separate access transistors. One of the access transistors utilizing gate 11 connects through an upper bit line contact 22 to the upper digit line 14awhich runs above the upper surface of the SOI layer 9. The second access transistor utilizing gate 13 connects through a lower bit line contact 23 to a second digit line 14bwhich runs below the upper surface of the SOI layer 9. In other words, capacitor 12 is connected, during one of a read and write operation, to a first bit line contact 22 and then to a second bit line contact 23, during the other of a read and write operation, with the second bit line contact 23 being spatially located in a plane of the memory cell 300 which is different from a plane of the memory cell 300 in which the first bit line contact 22 is located. Hence, by fabricating the DRAM array utilizing two separate metallization layers (14a,14b) above and below the upper surface of the SOI layer, the overall cell size is reduced by two linear features (one width and one space) such that the final cell size is eight square features (8F²) instead of sixteen square features (16F²).

[0022] FIG. 4 shows a top plan view of the dual port DRAM cell of the present invention. As shown, memory cells 300 are constructed in pairs to allow sharing of the digit line contact 22. Sharing a contact significantly reduces overall cell size. The memory cells consist of an active area rectangle 24, a pair of polysilicon wordlines 16, a single digit line contact 22, a metal or polysilicon digit line 14, and cell capacitors 12 formed with oxide-nitride-oxide dielectric between two layers of polysilicon. For some processes, the wordline polysilicon is silicided to reduce the sheet resistance, permitting longer wordline segments without reducing speed.

[0023] The digit line pitch 26 (width plus space) dictates the active area pitch and capacitor pitch. The active area width and the field oxide width are set to maximize transistor drive and minimize transistor to transistor leakage. The field oxide technology greatly impacts this balance. A thicker field oxide or a shallower junction depth will enable wider transistor active area. Also, the wordline pitch 28 (width plus space) dictates the space available for the digit line contact, transistor length, active area space, poly width, and capacitor length.

[0024] An imaginary box 30 drawn around the memory bit defines the cell's outer boundary. Along the x-axis, this box will include ½ digit line contact feature, 1 wordline feature, 1 capacitor feature, 1 wordline feature, and ½ digit line contact feature, which totals to 4 features. Along the y-axis, this box contains two ½ field oxide features and 1 active area feature, which totals to 2 features. Hence, the area of the memory bit is, as defined herein, (4F*2F)=8 F². Thus, for example, a 0.25 μm process having wordline and digit line pitches of 0.6 μm yields a memory bit size that is 8(0.3 μm)²=0.72 μm².

[0025] Hence, by fabricating the DRAM array utilizing two separate metallization layers (14a,14b), above and below the access transistors (and above and below the SOI layer), the overall cell size is reduced by two linear features (one width and one space) such that the final cell size is eight square features (8F²).

[0026] A processor system which may employ the dual port memory cell of the invention is illustrated in FIG. 5. As shown in FIG. 5, the processor system, such as a computer system, for example, comprises a central processing unit (CPU) 510, for example, a microprocessor, that communicates with one or more input/output (I/O) devices 540, 550 over a bus 570. The computer system 500 also includes random access memory (RAM) 560, a read only memory (ROM) 580 and may include peripheral devices such as a floppy disk drive 520 and a compact disk (CD) ROM drive 530 which also communicates with CPU 510 over the bus 570. The RAM 560 may be constructed as an integrated circuit which includes the dual port DRAM cell 300 as described above. It may also be desirable to integrate the processor 510 and memory 560 on a single IC chip.

[0027] Although the invention has been described above in connection with exemplary embodiments, it is apparent that many modifications and substitutions can be made without departing from the spirit or scope of the invention. Accordingly, the invention is not to be considered as limited by the foregoing description, but is only limited by the scope of the appended claims.