DETAILED DESCRIPTION OF THE INVENTION

[0022] One or more specific embodiments of the present invention will be described below. In an effort to provide a concise description of these embodiments, not all features of an actual implementation are described in the specification. It should be appreciated that in the development of any such actual implementation, as in any engineering or design project, numerous implementation-specific decisions must be made to achieve the developers' specific goals, such as compliance with system-related and business-related constraints, which may vary from one implementation to another. Moreover, it should be appreciated that such a development effort might be complex and time consuming, but would nevertheless be a routine undertaking of design, fabrication, and manufacture for those of ordinary skill having the benefit of this disclosure.

[0023] The terms “wafer” and “substrate” are to be understood as interchangeable and as including silicon, silicon-on-insulator (SOI) or silicon-on-sapphire (SOS), doped and undoped semiconductors, epitaxial layers of silicon supported by a base semiconductor foundation, and other semiconductor structures. Furthermore, when reference is made to a “wafer” or “substrate” in the following description, previous process steps may have been utilized to form regions, junctions or material layers in or on the base semiconductor structure or foundation. In addition, the semiconductor need not be silicon-based, but could be based on silicon-germanium, germanium, gallium arsenide, or other known semiconductor materials.

[0024] FIG. 1 is a partial schematic illustrating an integrated circuit, such as a memory device 10 , incorporating an array of memory cells which may be fabricated in accordance with an exemplary embodiment of the present invention. The memory device 10 may be an MRAM device or a PCRAM device, for example. In the exemplary embodiment, the memory device 10 includes a number of memory bits 12 arranged in a grid pattern comprising a number of rows and columns. As can be appreciated, the number of memory cells (and corresponding rows and columns) may vary depending on system requirements and fabrication technology. Each memory bit 12 indudes an access device and a storage device. In the present exemplary embodiment, the access device comprises a field-effect transistor (FET) 14 and the storage device comprises memory cell 16 . The access device 14 is implemented to provide controlled access to the memory cell 16 . In the exemplary memory bit 12 , the FET 14 includes a drain terminal 18 and a source terminal 20 , along with a gate terminal 22 for controlling conduction between the drain and source terminals 18 , 20 . The memory cell 16 is coupled between one of the source/drain terminals 18 , 20 and a reference voltage (illustrated as a ground potential).

[0025] It should be noted that although the above description depicts the terminal of the access device that is coupled to the memory cell 16 as the source 20 and the other non-gate terminal of the access device as the drain 18 , during read and write operations, the FET 14 may be operated such that each of the terminals 18 , 20 operates at one time or another as a source or a drain. Accordingly, for purposes of further discussion, it should be recognized that whenever a terminal is identified as a source or a drain, it is only for convenience purposes. Thus, during operation of the FET 14 either terminal could be a source or a drain depending on the manner in which the FET 14 is being controlled by the voltages applied to the terminals 18 , 20 , 22 of the FET 14 .

[0026] As previously described, the memory array of device 10 is arranged in a series of rows and columns. To implement data storage capabilities in each memory bit 12 , an electrical charge is placed on the drain 18 of the FET 14 via a corresponding bitline (BL). By controlling the voltage at the gate 22 via the wordline (WL), a voltage potential may be created across the FET 14 such that the electrical charge at the drain 18 can flow to the memory cell 16 .

[0027] The bitlines BL are used to read from, and write data to, the memory bits 12 . The wordlines WL are used to activate the FET 14 to access a particular row of memory bits 12 . The memory device 10 includes an address buffer 24 , a row decoder 26 , and column decoder 28 to control the wordlines WL and bitlines BL. The address buffer 24 controls the row decoder 26 and the column decoder 28 such that the row decoder 26 and column decoder 28 selectively access memory bits 12 in response to address signals provided on the address bus 30 during read and write operations. The address signals are typically provided by an external controller such as a microprocessor or other memory controller. The column decoder 28 may also include sense amplifiers and input/output circuitry to further enable data to be read from and written to the memory bits 12 via the bitlines BL.

[0028] In one exemplary mode of operation, the memory device 10 receives an address of a particular memory bit 12 at the address buffer 24 . The address buffer 24 passes a row address to the row decoder 26 and a column address to the column decoder 28 . The row decoder 26 selectively activates a particular wordline WL to activate the FETs 14 of each memory bit 12 connected to the selected wordline WL. The column decoder 28 selects the bitline (or bitlines) BL of the memory bit 12 corresponding to the requested column address. For a write operation, data received by input/output circuitry is coupled to the selected bitline (or bitlines) BL and is stored in memory cell 16 through the FET 14 . The information corresponds to binary data (i.e., a logical “1” or “0”). For a read operation, data stored in the selected memory bit 12 , represented by the potential state stored in the memory cell 16 , is coupled to the selected bitline (or bitlines) BL and amplified by the sense amplifier; a corresponding output is provided to the input/output circuit in the column decoder 28 .

[0029] The memory array of the memory device 10 may be fabricated using a variety of technologies. One particularly advantageous technique for fabricating the memory bits 12 is now described with reference to FIGS. 2 - 10 . To provide a high density memory device 10 , the channel of the FET 14 is fabricated perpendicular to the surface of a wafer (i.e., vertically-oriented) rather than parallel to the surface. Advantageously, the vertically-oriented access FET 14 occupies less space than would be occupied by FETs produced by other techniques.

[0030] In addition, by incorporating vertically-oriented access FETs 14 , the memory bits 12 are less susceptible to alpha-radiation. The memory bits 12 have increased radiation hardness, and reduced potential for soft errors imparted by alpha particles produced, for example, from packaging materials, over bits with a non-vertically oriented access FET 14 .

[0031] FIG. 2 illustrates a semiconductor substrate 32 , made of material such as silicon (Si). More specifically, the substrate 32 is a P-doped silicon. To fabricate the vertically oriented access FETs 14 , the substrate 32 is subtractively etched to produce silicon pillars 33 . As described further below, the pillars 33 will eventually form the channels of the FETs 14 . The pillars 33 may be formed through any one of a number or commonly known etching techniques, such as plasma etching, ion beam etching, or reactive ion etching (RIE). Each pillar 33 has a height of approximately 2.0 μm and a diameter of approximately 0.2 μm, for example. In the present embodiment, each pillar 33 may be generally cylindrical such that the top view of each pillar 33 is generally circular. This shape facilitates the eventual fabrication of annular rings around the pillars 33 , as described further below. Alternatively, pillars having other geometric cross-sectional shapes, such as rectangles, squares, or ellipses, may be implemented to construct the channel of the FETs 14 .

[0032] As can be appreciated, the specific heights and thicknesses of the features and materials described herein are exemplary in nature and for purposes of illustration. Accordingly, the exemplary dimensions provided herein are in no way meant to limit the scope of the present invention. Further, while the present exemplary embodiment illustrates pillars 33 that are perpendicular to the surface of the substrate 32 , the pillars 33 may extend away from the surface of the substrate 32 at other angles. For instance, in an alternate embodiment, the pillars 33 may be fabricated at an angle in the range of approximately 45° to approximately 90° with respect to the surface of the substrate 32 .

[0033] After formation of the pillars 33 , an isolation layer, such as an oxide layer 34 , is applied on top of the substrate 32 , as illustrated in FIG. 3 . The isolation layer can include any number of non-conductive materials such as oxide, silicon dioxide, silicon nitride, etc. The oxide layer 34 is applied by chemical vapor deposition (CVD), for example. The oxide layer 34 is disposed at a thickness of approximately 0.2 μm. As can be appreciated by those skilled in the art, the oxide layer 34 is disposed over the entire surface of the substrate 32 . A photoresist is used to facilitate the patterning and etching of the oxide layer 34 such that the oxide layer 34 is not present on top of the pillars 33 . It should be understood that those skilled in the art readily understand the deposition, masking, and etching techniques used to construct the patterns illustrated with reference to FIGS. 2 - 10 .

[0034] FIG. 4 illustrates the deposition of the material used to create the bitlines BL discussed previously with respect to FIG. 1 . A polysilicon layer 36 is disposed on the oxide layer 34 . The polysilicon layer 36 can be over-doped such that when the substrate 32 is baked or otherwise heated, the concentration gradient between the polysilicon layer 36 and the substrate 32 will cause dopants to diffuse from the polysilicon layer 36 into the substrate 32 , forming n+ contact regions 38 at the silicon/polysilicon interface as illustrated in FIG. 4 . The silicon/polysilicon interface generally is near the base of the pillar 33 , where the n+ contact regions 38 are formed by so-called “out-diffusion.” The contact regions 38 may also be formed using gas diffusion or ion implant techniques. The polysilicon layer 36 may be disposed at a thickness of approximately 0.4 μm, for example.

[0035] FIG. 5 illustrates a cross-sectional top view of the structure illustrated with reference to FIG. 4 taken along cut lines 5 - 5 . The polysilicon layer 36 is patterned to provide annular rings around the silicon pillars 33 . Thus, the polysilicon layer 36 , which forms the bitlines BL of the memory array (described with reference to FIG. 1 ), extends to connect all of the silicon pillars 33 in a given column of the array. As previously described, the silicon pillar 33 also includes n+ contact regions 38 formed from the polysilicon layer 36 . Advantages of the annular ring pattern around the silicon pillars 33 used to form the bitline polysilicon layer 36 , are discussed below.

[0036] Referring to FIG. 6 , after deposition of the polysilicon layer 36 and the formation of the n+ contact regions 38 , another isolation layer, such as an oxide layer 40 , is disposed on the polysilicon layer 36 . The oxide layer 40 electrically isolates the polysilicon layer 36 from layers subsequently disposed over the oxide layer 40 . As with the oxide layer 34 , the oxide layer 40 is deposited, patterned and etched to provide a structure as illustrated in FIG. 6 . The thickness of the oxide layer 36 may be approximately 0.2 μm, for example.

[0037] FIG. 6 further illustrates a thin gate oxide layer 42 which is disposed or grown around the pillar 33 to facilitate the functionality of the gate 22 ( FIG. 1 ) of the FET 14 . The gate oxide layer 42 may be grown to a thickness of approximately 60 angstroms by any one of a number of conventional techniques. It should be noted that for purposes of etch selectivity, the oxide layer 40 and the gate oxide 42 may comprise different insulated materials with respect to one another, such as oxide, silicon dioxide, silicon nitride, TEOS, etc.

[0038] Referring to FIGS. 7 and 8 , after the deposition of the oxide layer 40 and the growth of the gate oxide layer 42 , another layer of polysilicon is deposited, patterned, and etched to form the wordline polysilicon layer 44 . The thickness of the wordline polysilicon layer 44 extending upwardly from the surface of the substrate 32 in the direction of the pillar 33 may be about 0.8 μm, for example. The thickness of the wordline polysilicon layer 44 extending outwardly from the surface of the pillar 33 may be about 0.1 to about 0.2 μm, for example. The wordline polysilicon layer 44 is patterned such that the polysilicon material completely surrounds the pillar 33 . As illustrated in FIG. 8 , the wordline polysilicon layer 44 is patterned such that it runs perpendicular to the bitline polysilicon layer 36 . The advantages of the annular ring pattern around the silicon pillars 33 used to form the wordline polysilicon layer 44 will be discussed further below.

[0039] Referring to FIG. 9 , after the deposition, patterning, and etching of the wordline polysilicon layer 44 , a dielectric layer 46 , such as silicon dioxide or silicon nitride, is deposited over the entire structure such that each pillar 33 is covered completely with dielectric material. The dielectric layer 46 may be disposed by chemical vapor deposition (CVD), for example. Thus, the dielectric layer 46 may be deposited to a thickness of more than 1.0 μm, in the present exemplary embodiment, such that the dielectric layer 46 is deposited to a height approximately coplanar with the height of the pillars 33 .

[0040] After deposition of the dielectric layer 46 , the surface of the structure may be planarized such as by chemical-mechanical polishing/planarization (CMP). The surface of the structure is planarized to a point where the thin gate oxide layer 42 is removed from the top of the pillar 33 thereby exposing the silicon pillar 33 . Finally, an n+ contact region 48 is formed at the top of the pillar 33 . The n+ contact region 48 may be formed through gas diffusion or ion implant techniques, for example. The n+ contact region 48 forms the source 20 ( FIG. 1 ) that will be coupled to the memory cell 16 formed in subsequent processing steps discussed below and illustrated with respect to FIG. 10 .

[0041] The completed access device (FET 14 ) is illustrated in FIG. 9 . The silicon pillar 33 forms a channel of the FET 14 . By completely surrounding the channel (i.e., pillar 33 ) with the wordline polysilicon layer 44 , the gate 22 ( FIG. 1 ) functions as a channel with increased drive capabilities over access devices having conventional gate structures. The annular structure of the bitline polysilicon layer 36 offers advantages of compact layout and efficiency.

[0042] To complete the memory bit 12 ( FIG. 1 ), a storage device, such as the memory cell 16 , is fabricated. FIG. 10 illustrates a completed memory bit structure 12 incorporating an exemplary memory cell 16 .

[0043] One technique for fabricating the memory cell 16 begins by disposing a sacrificial oxide layer (not shown) on top of the structure described above and illustrated with reference to FIG. 9 . The oxide layer is disposed at a thickness at least as high as what later will be the height or vertical thickness of the memory cell 16 . For instance, the thickness of the sacrificial oxide may be approximately 2.0 μm. Once the sacrificial oxide is deposited, holes are drilled or etched in the oxide to create wells which are vertical with respect to the surface of the substrate. After the wells are created, the layers making up the memory cell 16 may be disposed in the wells. A lowest layer 50 is deposited to make contact with the n+ contact region 48 of the pillar 33 . This interface provides the connection of the FET 14 to the memory cell 16 .

[0044] Memory cell 16 can be fabricated by various methods, depending on the type of memory cell used in the memory device 10 . An MRAM cell generally will include a free magnetic layer, a pinned magnetic layer, and a magnetic tunnel junction barrier disposed between the two magnetic layers. A description of MRAM cells in general, and an exemplary method of fabricating MRAM cells, are disclosed in U.S. Pat. No. 6,358,756, issued Mar. 19, 2002 and of common assignment with the present invention, the entire disclosure of which is incorporated herein by reference.

[0045] Memory cell 16 also can be a PCRAM cell. A PCRAM cell generally will include a lower contact layer on which is disposed a PCRAM bit. PCRAM bits are resistance variable memory elements in which a metal containing layer is formed between a first chalcogenide glass layer and a second glass layer. One or both of the glass layers may be doped with a metal and one or more metal containing layers may be provided between the glass layers. An exemplary method of manufacturing a PCRAM cell is disclosed in pending U.S. patent application Ser. No. 10/120,521, filed Apr. 12, 2002 and of common assignment with the present invention, the entire disclosure of which is incorporated herein by reference.

[0046] FIG. 11 illustrates a bitline BL according to another embodiment of the invention. More specifically, FIG. 11 illustrates a cross-sectional top view of an alternate embodiment of the structure illustrated with reference to FIG. 4 taken along cut lines 5 - 5 . As can be seen, a polysilicon layer 36 A is patterned to provide semi-annular rings around the silicon pillars 33 . The polysilicon layer 36 A, which forms the bitlines of the memory array described with reference to FIG. 1 , extends to connect each of the silicon pillars 33 in a single column. As previously described, the silicon pillar 33 also includes n+ contact regions 38 A which may be formed by out diffusion from the polysilicon layer 36 A. Thus, the present exemplary embodiment provides a polysilicon layer 36 A which is patterned to surround only a portion of the pillar 33 . Advantageously, the alternate exemplary embodiment illustrated with reference to FIG. 11 may provide for further pitch reduction and thus, reduction in the area of each memory bit and overall die size.

[0047] Similarly, the wordline WL, may be patterned to provide a semi-annular ring. FIGS. 12 and 13 illustrate an alternate embodiment of the wordline WL, incorporating semi-annular rings. FIG. 12 illustrates an alternative view of the structure illustrated in FIG. 7 . Thus, after the deposition of the oxide layer 40 and the growth of the gate oxide layer 42 , a polysilicon layer is disposed, patterned and etched to form the polysilicon layer 44 A having semi-annular rings. The wordline polysilicon layer 44 A extends in a direction perpendicular to the page and thus, the view of the structure illustrated in FIG. 12 includes the polysilicon layer 44 A on only one side of the pillar 33 . However, the wordline polysilicon layer 44 A is patterned about a portion of the pillar 33 , as further illustrated with respect to FIG. 13 .

[0048] FIG. 13 illustrates a cross-sectional top view of the alternate embodiment illustrated in FIG. 12 taken along cut line 13 - 13 . As can be seen, the wordline polysilicon layer 44 A is patterned to provide semi-annular rings around the silicon pillars 33 . The wordline polysilicon layer 44 A is patterned such that it runs perpendicular to the bitline polysilicon layer 36 . The semi-annular wordline polysilicon layer 44 A may be implemented along with the semi-annular bitline polysilicon layer 36 A described with reference to FIG. 11 . Further, while FIGS. 11 - 13 illustrate semi-annular rings, it should be evident that an annular ring may extend around any desirable portion (e.g., more than or less than half) of the pillar 33 . For instance, it may be advantageous to provide annular rings that extend around only a quarter to a third of the circumference of the pillar 33 . Alternatively, it may be advantageous to provide annular rings that extend around two-thirds to three-quarters of the circumference of the pillar 33 , for example.

[0049] FIG. 14 illustrates an alternate embodiment of the structure illustrated in FIG. 9 , implementing an alternate technique for fabricating the wordline WL. FIG. 15 illustrates a cross-sectional top view of the alternate embodiment illustrated in FIG. 14 taken along the cut line 15 - 15 . In the present exemplary embodiment, the wordline polysilicon layer 44 is replaced with a thin gate conductor layer 44 B and a thick signal conductor layer 44 C. As can be seen, the thin gate conductor layer 44 B completely surrounds the pillar 33 . The thin gate conductor layer 44 B may have a thickness extending from the surface of the pillar 33 of less than 0.1 μm, for example. After the deposition, patterning and etching of the thin conductor layer 44 B, a dielectric layer 46 A may be disposed. Unlike the embodiment illustrated in FIG. 9 , however, the dielectric layer 46 A is not disposed to cover the entire pillar 33 . The dielectric layer 46 A is disposed such that a portion of the pillar remains uncovered, as illustrated in FIG. 14 .

[0050] Next, the thick signal conductor layer 44 C is disposed, patterned and etched to form the wordline WL. The gate conductor layer 44 B is electrically coupled to the signal conductor layer 44 C. In one exemplary embodiment, the gate conductor layer 44 B and the signal conductor layer 44 C are each polysilicon layers. However, as can be appreciated, the gate conductor layer 44 B and the signal conductor layer 44 C may be different materials. For instance, the gate conductor layer 44 B may be a polysilicon layer, while the signal conductor layer 44 C may be a tungsten layer. To complete the structure, a dielectric layer 46 B may be disposed to a thickness sufficient to cover the pillars 33 , and the surface of the structure may be planarized, as previously described. Advantageously, by providing a thin gate conductor layer 44 B coupled to a thick signal conductor layer 44 C, a smaller pitch between structures may be implemented, thereby reducing cell size and overall die size.

[0051] As can be appreciated, while the present wordline and bitline structures are described as being fabricated through deposition techniques, other processes, such as a damascene process may implemented to form the wordlines and bitlines in accordance with the present techniques. Further, while the present exemplary embodiments have illustrated the annular gate structures with respect to DRAM memory devices, the present techniques may be implemented in a number of other applications, such as flash memory cells, SRAM memory cells, anti-fuse devices, image sensors and simple logic gates, for example.

[0052] FIG. 16 illustrates an exemplary processing system 900 , which may incorporate memory devices 10 of the present invention into one of more of the memory modules 908 described below. The processing system 900 includes one or more processors 901 coupled to a local bus 904 . A memory controller 902 and a primary bus bridge 903 also are coupled to local bus 904 . The processing system 900 may include multiple memory controllers 902 and/or multiple primary bus bridges 903 . The memory controller 902 and the primary bus bridge 903 may be integrated as a single device 906 .

[0053] The memory controller 902 is also coupled to one or more memory buses 907 . Each memory bus accepts memory components 908 . The memory components 908 may be a memory card or a memory module. Examples of memory modules include single inline memory modules (SIMMs) and dual inline memory modules (DIMMs). The memory components 908 may include one or more additional devices 909 . For example, in a SIMM or DIMM, the additional device 909 might be a configuration memory, such as a serial presence detect (SPD) memory. The memory controller 902 may also be coupled to a cache memory 905 . The cache memory 905 may be the only cache memory in the processing system. Alternatively, other devices, for example, processors 901 may also include cache memories, which may form a cache hierarchy with cache memory 905 . If the processing system 900 include peripherals or controllers which are bus masters or which support direct memory access (DMA), the memory controller 902 may implement a cache coherency protocol. If the memory controller 902 is coupled to a plurality of memory buses 907 , each memory bus 907 may be operated in parallel, or different address ranges may be mapped to different memory buses 907 .

[0054] The primary bus bridge 903 is coupled to at least one peripheral bus 910 . Various devices, such as peripherals or additional bus bridges may be coupled to the peripheral bus 910 . These devices may include a storage controller 911 , a miscellaneous I/O device 914 , a secondary bus bridge 915 communicating with a secondary bus 916 , a multimedia processor 918 , and a legacy device interface 920 . The primary bus bridge 903 may also coupled to one or more special purpose high speed ports 922 . In a personal computer, for example, the special purpose port might be the Accelerated Graphics Port (AGP), used to couple a high performance video card to the processing system 900 .

[0055] The storage controller 911 couples one or more storage devices 913 , via a storage bus 912 , to the peripheral bus 910 . For example, the storage controller 911 may be a SCSI controller and storage devices 913 may be SCSI discs. The I/O device 914 may be any sort of peripheral. For example, the I/O device 914 may be a local area network interface, such as an Ethernet card. The secondary bus bridge 915 may be used to interface additional devices via another bus to the processing system. For example, the secondary bus bridge may be an universal serial port (USB) controller used to couple USB devices 917 via to the processing system 900 . The multimedia processor 918 may be a sound card, a video capture card, or any other type of media interface, which may also be coupled to one additional devices such as speakers 919 . The legacy device interface 920 is used to couple at least one legacy device 921 , for example, older styled keyboards and mice, to the processing system 900 .

[0056] The processing system 900 illustrated in FIG. 16 is only an exemplary processing system with which the invention may be used. While FIG. 16 illustrates a processing architecture especially suitable for a general purpose computer, such as a personal computer or a workstation, it should be recognized that well known modifications can be made to configure the processing system 900 to become more suitable for use in a variety of applications. For example, many electronic devices which require processing may be implemented using a simpler architecture which relies on a CPU 901 coupled to memory components 908 . These electronic devices may include, but are not limited to audio/video processors and recorders, gaming consoles, digital television sets, wired or wireless telephones, navigation devices (including system based on the global positioning system (GPS) and/or inertial navigation), and digital cameras and/or recorders. The modifications may include, for example, elimination of unnecessary components, addition of specialized devices or circuits, and/or integration of a plurality of devices.

[0057] While the invention may be susceptible to various modifications and alternative forms, specific embodiments have been shown by way of example in the drawings and have been described in detail herein. However, it should be understood that the invention is not intended to be limited to the particular forms disclosed. Rather, the invention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the following appended claims.