System architecture and method for three-dimensional memory

In one embodiment, a chip-level architecture is provided comprising a monolithic three-dimensional write-once memory array and at least two of the following system blocks: an Error Checking & Correction Circuit (ECC); a Checkerboard Memory Array containing sub arrays; a Write Controller; a Charge Pump; a Vread Generator; an Oscillator; a Band Gap Reference Generator; and a Page Register/Fault Memory. In another embodiment, a chip-level architecture is provided comprising a monolithic three-dimensional write-once memory array, ECC, and smart write. The monolithic three-dimensional write-once memory array comprises a first conductor, a first memory cell above the first conductor, a second conductor above the first memory cell, and a second memory cell above the second conductor, wherein the second conductor is the only conductor between the first and second memory cells.

BACKGROUND

Field of the Invention

This invention is directed to a chip-level architecture used in combination with a monolithic three-dimensional write-once memory array.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS

The following 23 documents are hereby incorporated by reference:1. “512 Mb PROM with 8 Layers of Antifuse/Diode Cells” (to be presented as ISSCC 2003/Session 16/Non-Volatile Memory/Paper 16.4 at the 2003 IEEE International Solid-State Circuits Conference (3 pages)).2. 23 pages of slides (first slide labeled “Agenda;” last slide labeled “Summary”), which are reproduced herein asFIGS. 1-23.3. U.S. Pat. No. 6,034,882 to Johnson et al. (“Vertically Stacked Field Programmable Nonvolatile Memory and Method of Fabrication”).4. U.S. Pat. No. 6,420,215 to Knall et al. (“Three Dimensional Memory Array and Method of Fabrication”).5. U.S. Pat. No. 6,525,953 to Johnson (“Vertically-Stacked, Field Programmable, Nonvolatile Memory and Method of Fabrication”).6. U.S. Pat. No. 6,952,043 to Vyvoda et al. (“Electrically Isolated Pillars in Active Devices”).7. U.S. patent application Ser. No. 10/326,470 to Herner et al. (“An Improved Method for Making High-Density Nonvolatile Memory”).8. U.S. Patent Application Publication No. US 2003-0120858 A1 to March et al. (“Memory Devices and Methods for Use Therewith”).9. U.S. Pat. No. 6,534,403 (“Contact and Via Structure and Method of Fabrication”).10. U.S. Pat. No. 6,574,145 (“Memory Device and Method for Sensing while Programming a Non-Volatile Memory Cell”).11. U.S. Pat. No. 6,567,287 (“Memory Device with Row and Column Decoder Circuits Arranged in a Checkerboard Pattern under a Plurality of Memory Arrays”).12. U.S. Pat. No. 6,928,590 (“Memory Device and Method for Storing Bits in Non-Adjacent Storage Locations in a Memory Array”).13. U.S. Patent Application Publication No. US 2003-0115518 A1 (“Memory Device and Method for Redundancy/Self-Repair”).14. U.S. Pat. No. 6,591,394 (“Three-Dimensional Memory Array and Method for Storing Data Bits and ECC Bits Therein”).15. U.S. Pat. No. 6,486,728 to Kleveland (“Multi-Stage Charge Pump”).16. U.S. Pat. No. 6,385,074 to Johnson et al. (“Integrated Circuit Structure Including Three-Dimensional Memory Array”).17. U.S. Pat. No. 6,661,730 to Scheuerlein et al. (“Partial Selection of Passive Element Memory Cell Sub-Arrays for Write Operation”).18. (a) U.S. Pat. No. 6,618,295 to Scheuerlein (“Method and Apparatus for Biasing Selected and Unselected Array Lines when Writing the Memory Array”). (b) U.S. Pat. No. 6,504,753, which has the same specification (excluding the claims).19. (a) U.S. patent application Ser. No. 09/896,468 to Scheuerlein (“Current Sensing Method and Apparatus Particularly Useful for a Memory Array of Cells Having Diode-Like Characteristics”). (b) U.S. Pat. No. 6,522,594 to Scheuerlein (“Memory Array Incorporating Noise Detection Line”), which has the same specification (excluding the claims).20. U.S. Pat. No. 6,407,953 to Cleeves et al. (“Memory Array Organization and Related Test Method Particular Well Suited for Integrated Circuits Having Write Once Memory Arrays”).21. U.S. Pat. No. 6,515,904 to Moore et al. (“Method and System for Increasing Programming Bandwidth in a Non-Volatile Memory Device”).22. U.S. Pat. No. 6,856,572 to Scheuerlein et al. (“Multiheaded Decoder Structure Utilizing Memory Array Line Driver with Dual Purpose Driver Device”).23. U.S. Pat. No. 6,781,878 to Kleveland et al. (“A Dynamic Sub Array Group Selection Scheme”).

This invention is directed to a chip-level architecture used in combination with a monolithic three-dimensional write-once memory array. For example, the architecture can be used with any of such memory devices taught in document numbers 3 through 7 in the above list.

The Chip Block Diagram in the above-listed document number 2, which is reproduced herein asFIG. 13, exemplifies a preferred embodiment of the architecture showing the functional organization of:

an Error Checking & Correction Circuit (ECC) described in documents 8, 13, and 14 above;

a Checkerboard Memory Array described in document 11 above;

a Smart Write Controller described in documents 10, 13, and 21 above;

a Charge Pump exemplified in documents 15 and 16 above;

a Vread Generator exemplified in document 2 above with a circuit schematic (seeFIGS. 19-20herein);

an Oscillator;

a Band Gap Reference Generator (precision reference generator); and

a Page Register/Fault Memory described in document 13 above.

Document numbered 17 in the above list shows the user data for one programming request stored temporarily in a register called a page register, programmed into memory cells in multiple sub arrays of the memory device.

Document numbered 18(a) in the above list describes the biasing for selected and unselected array lines of a selected sub array, unselected sub arrays have all their memory lines held to a common voltage preferably the ground voltage of the chip. Document 18(b) describes a method and apparatus for discharging memory lines of a selected sub array to the voltages required for memory lines in an unselected sub array.

Document numbered 19(a) in the above list describes current sensing method used to sense the state of memory cells on the memory lines of the sub arrays. Document numbered 19(b) in the above list describes a sub array incorporating a noise detection line used in combination with the above sensing method for reliable sensing of small signals from cells in the sub arrays.

The memory device has multiple sub arrays in a checkerboard arrangement. Circuits as described in more detail in document 18(a) are provided that bias the memory lines of selected arrays so they can be accessed for read and write operations, and other circuits as described in document 18(b) that bias the memory lines of unselected arrays so the state of cells in unselected arrays are not disturbed during read or write operations. By these means, the power dissipation of the memory device is much lower than if all cells in the memory were biased for selectivity. The number of selected sub arrays can be modified by control circuits on the memory as described in document 23 in the above list.

The sub arrays have read and write sensing circuitry connected to array lines in one direction (i.e., sensing lines). The circuitry is shared between adjacent subarrays by the method described for checkerboard arrays. The circuitry uses current sensing methods and noise cancellation lines described in documents 18 and 19, particularly the sensing circuit shown in the Read Sense Amplifier figure of document 2 (which is reproduced herein asFIG. 17) and described in document 1, to allow large sub arrays with reliable sensing. All the sensing circuits for subarrays in a column are connected together by means of shared bi-directional data busses and control lines on a layer of interconnection metal above the memory cells. The data busses and control lines are preferably substantially parallel to one another and parallel to the sensing lines in the memory array. They connect the sub arrays to control circuitry in the smart write controller. The data bus is bi-directionally controlled in both read and write operations to reduce the number of wires required. The smart write controller transfers data between the selected memory sub arrays and a register called a page register/fault memory during read and write operations. Thereby, the data from the page register is written to or read from a set of cells distributed across all the selected sub arrays.

The selected sub arrays are preferably all in a horizontal group called a stripe and within the sub arrays the selected cells are preferably all in a single row, i.e., the row decoders in each of the selected sub arrays are coordinated. One of the sub arrays contains the calculated bits for ECC protection of the data as described in document 14. Two other sub arrays located at the left and right ends of the horizontal stripe, and preferably smaller than the user data subarrays, contains supplemental data including flags and other redundancy control bits that thereby can be located in the same single row with the user data. Preferably, each row contains the page register data, as well as ECC data and redundancy control bits. The spreading of the data to many sub arrays improves the effectiveness of the ECC and the coordination of the row decoders facilitates a self-repair mechanism further described in document 13.

The smart write control circuitry achieves variable bandwidth transfers to the sub arrays as described in document 17 and 23. The smart write control circuitry also collects detected errors during programming using the method described in document 10 and steers them to the fault memory portion of the register to activate the self repair mechanism which will reprogram the data from the page register in a row of redundant cells preferably in the same set of selected sub arrays.

The exemplary Vread Generator provides a voltage to which a selected word line is driven during a read operation. Pairs of transistors, labeled N3, per collection of sub-arrays are preferably spatially distributed throughout the die to achieve reduced voltage drop along the reference node Vread. This Vread generator, which provides a voltage to which a word line is driven in a 3-D array, having a distributed portion and a localized portion, can be used alone or in combination with any other elements disclosed herein. It is further described in document number 1 in the above-identified list.

Each memory line has two control transistors, as shown in document 2, the foil titled, “Die Organization 2/3, which is reproduced herein as FIG.10.” These are the “two transistors” referred to in the “Tile Organization” foil of document 2, which is reproduced herein asFIG. 8, while “epsilon” is the amortized cost of the row decoders and bias circuits. This die organization can be used alone or in combination with any other elements disclosed herein.

Other combinations of particular interest include, as examples, 3-Dimensional

1. Smart write controller and oscillator.2. Smart write controller, a collection of memory subarrays, and a bi-directional connection between the memory subarrays and the smart write controller.a. Where in a preferred embodiment, during a write operation, information is transferred bi-directionally, specifically, data is transferred to the subarray for programming cells in the subarray and programming success is indicated to the smart write controller on the bi-directional connection.3. Smart write and checkerboard.a. Where in a preferred embodiment, wiring above memory cells connects the subarrays to the write controller.b. Where in a preferred embodiment, a bi-directional connection is used between the memory subarrays and the smart write controller.4. Set of selected subarrays containing the combination of user data, ECC data, and redundancy control bits5. Checkerboard and ECC.6. ECC and smart write.7. ECC and on-the-fly redundancy.8. Vread generator with distributed output.9. Smart write plus dummy bit lines.10. Data from the page register is distributed in a corresponding physical row in each of the subarraysa. Preferably, each row contains the page register data, as well as ECC data and redundancy control bits.11. Die organization having two control (driver) transistors per memory line, plus row decoders and bias circuits that are shared amongst memory lines, details of which are provided in document numbered 22 above, optionally in combination with smart write.
Three-Dimensional Memory Devices
Pillar Three-Dimensional Memory Devices

In one embodiment of the present invention, a conductor layer (say, conductor layer number J) runs north-to-south, and adjacent conductor layers (numbers J−1 and J+1) run east-to-west. Wherever a conductor's vertical projection on layer (J) crosses over a conductor on layer (J−1), a memory cell pillar is created. Similarly, wherever a conductor's projection on layer (J+1) crosses a conductor on layer (J), a memory cell pillar is created. Memory cell pillars are defined and patterned by the intersection (crossover) of the conductors, and so the pillars are selfaligned to the conductors. Selfalignment is an extremely important advantage, because it lets the photolithographic patterns of the memory cell be designed without including any extra allowance for misalignment tolerances. Thus the pattern features of our selfaligned memory cell may be made smaller, resulting in a smaller cell area, which gives higher density and lower cost.

For purposes of illustrating the selfaligned fabrication of these pillars, consider an embodiment which uses four sequential layers of material (a “layer stack”) to fabricate the steering element and the state change element. In this illustrative example the steering element consists of a polycrystalline silicon PN junction diode, and the state change element consists of a poly-oxide-poly dielectric rupture antifuse. Other embodiments are set forth in the body of this application.

In this embodiment, a pillar contains four layers of material in a layer stack, deposited sequentially as shown inFIG. 24(a): (1) a layer of P+ doped polysilicon40; (2) a layer of N-doped polysilicon41; (3) a layer of silicon dioxide42; (4) a layer of N+ doped polysilicon43. Layers (40) and (41) form a PN junction diode (the steering element), and layers (41-43) form a poly-oxide-poly dielectric rupture antifuse. In this embodiment the stack of four materials which together create the memory cells are referred to as the “layer stack”45. There are also a conductor layer below and above the layer stack45which is patterned as will be described. These are shown as conductors46and48inFIG. 24(a).

An alternate stack is shown inFIG. 24(a) as stack450. Again it includes conductors at the ends of the stack, specifically460and480which may be fabricated from any conductive material such as a metal or a polysilicon. The steering element in stack450comprises a first layer400of P+ doped semiconductor such as microcrystalline silicon, and a second layer410of N doped semiconductor such as microcrystalline silicon.

The state change element comprises the layer420. Layer420may be an amorphous silicon layer used to form an antifuse. This layer has a nominal high resistance, however, after a large current is passed through it for programming, its resistance will be substantially lower. The layer430is shown as an N+ layer to provide good electrical contact to the overlying conductor480. Layer430could be amorphous, microcrystalline or polysilicon but the processing methods need to be low temperature to maintain the amorphous structure in layer420.

Another stack405is also shown inFIG. 24(a). It comprises an N-polysilicon layer400, a silicon dioxide layer402and an N+ polysilicon layer403. Again, the layers400or403could be microcrystalline or amorphous semiconductor layers. The stack405is sandwiched between the conductors406and408. Here the steering element is a Schottky diode formed by the metal of conductor406and the layer400. The state change element is an antifuse formed by layer402. By way of example, layers406and408may be titanium silicide or aluminum with a thickness of approximately 1000 A. The layers400,402and403may be 500 A, 80 A, and 500 A in thickness, respectively.

The fabrication sequence for the memory cell is schematically illustrated inFIGS. 24(b)-12(g). After deposition and before patterning, the layer stack45(or the stacks450and405) is a continuous sheet that extends across the entire integrated circuit (indeed across the entire wafer) such as shown inFIG. 24(b). Conceptually the selfalignment method is a two-etch-step procedure: In the first etch step, this layer stack (a continuous sheet) is patterned into long straight strips running (say) east-to-west, by etching them with the same patterning step that etches the east-to-west conductors on the conductor layer below. After deposition and planarization of an interlevel dielectric, a second conductor and layer stack is deposited. This stack is patterned into long straight strips running north south. Etching used to pattern the north-to-south lines continues until the first layer stack has also been etched through the steering element. This results in pillars formed on the east-to-west running lines. The resulting pillars are perfectly aligned to both the conductor below and the conductor above since both the pillars and the conductors are etched simultaneously. In alternate embodiments the semiconductor layers within the layer stack (45or450or405) may be deposited as microcrystalline or polycrystalline, and then laser treated to improve crystallinity and enhance the dopant activation.

The cross-section of the pillar will be rectangular with one dimension being equal to the width of the bottom conductors and the other dimension equal to the width of the top conductors. If these conductors have equal width then the cross-section will be square.

The patterning in both east-to-west and north-to-south uses well-known photolithographic steps widely used in the semiconductor industry and may use either wet or dry etching. Also, the silicon used in the cells and when used for the conductors may be doped insitu or after being deposited, for example, by ion implantation.

Of course other patterning technologies may be used rather than etching, for example “liftoff” technology or “Damascene” technology or an additive rather than subtractive patterning technology may be employed instead of etching. But ideally the layer stack should be patterned in two separate steps, once with the mask that defines the conductors below, and again with the mask that defines the conductors above. This holds true regardless of the specific fabrication techniques used to pattern the various layers.

In practice a large number of vertically stacked memory cells are built, and each conductor layer is selfaligned to both the layer stack below, and the layer stack above. Therefore the etching steps which selfalign the conductors to the pillars, must etch away material from three different layers: the layer stack above, the conductor layer, and the layer stack below.

The processing may begin with a wafer that may have received prior processing steps, for example, CMOS transistors may be fabricated in the monocrystalline substrate for the peripheral circuitry. An insulator then is deposited, and preferably, planarized (using chemical-mechanical polishing (“CMP”), resist etchback planarization, or any of a number of other technologies for planarization). The first conductor layer is deposited such as layer46ofFIG. 24(b), and then the first layer stack45is deposited.FIG. 24(b) shows the wafer at this stage.

Next, the mask which defines the features on the conductors1layer is applied, and these features are etched into both the pillar layer stack45and the conductors1layer46below. An insulator is deposited on the wafer and planarized, using CMP or other planarizing technology.FIG. 24(c) shows the wafer at this stage. Note in particular that the pillar layer stack and bottom layer have, been etched into long continuous strips (46aand45a) and (46band45b), not isolated individual pillars. Also note that the edges of the pillar layer stack45aand45bare aligned to the edges of the conductor46aand46blayer, since both were etched at the same time with the same mask. Note the conductors generally comprise coplanar conductors, such as aluminum or other metals, silicides, or doped silicon conductors, for each level.

While not shown inFIG. 24(c) or the other figures, the dielectric fills the voids between the strips (and pillars) and thus adds support to the array. Also it should be noted that the planarization must reveal the upper surface of the strips so that the conductor layer that follows contacts the strips. The planarized dielectric also forms the layers through which the vias and vertical conductors of FIG. 13 in U.S. Pat. No. 6,034,882 pass.

Next, the second conductor layer50(“conductors2”) is deposited, and the second pillar stack51(“stack2”) is deposited.FIG. 24(d) shows the wafer at this stage. Note that the planarization automatically gives a selfaligned contact between a pillar layer stack (such as45b) and the subsequent conductor layer (such as50) above it.

Now, the conductors2mask is applied, and its features are etched downward into three distinct strata: pillarstack2(51), conductors2layer50, and pillarstack1(45aand45b). (This etch stops below the steering element within45aand45b, providing a unique circuit path through the memory cell). An insulator is deposited on the wafer and planarized (using CMP or other means).FIG. 24(e) shows the wafer at this stage. Note that the conductors2mask+etch has completed the definition of the individual pillars (45a1,45a2,45b1and45b2) in the layerstack1. Also note that these pillars in the layerstack1layer are aligned to both the conductors1layer (46a,46b) and to the conductors2layer (50a,50b), thereby achieving the goal of selfalignment.

Next, the third conductor layer52(“conductors3”) is deposited, and the third pillar layerstack53(“layerstack3”) is deposited.FIG. 24(f) shows the wafer at this stage.

Now, the conductors3mask is applied, and its features are etched downwards into layers stack3, conductors3, and stack2. (This etch stops below the steering element of layer stack2and is intended to leave the conductor2layer intact.) An insulator is deposited on the wafer and planarized (using CMP or other means).FIG. 24(g) shows the wafer at this stage. The conductors3mask+etch has completed the definition of the individual pillars in the layerstack2layer (such as51a1,51a2,51b2).FIG. 24(g) shows that (N+1)=3 conductor layers and hence (N+1)=3 masking steps, are required to pattern (N=2) layers of pillar layerstack (not counting the interlevel via layers which are used in the peripheral circuits but not in the memory array). The wafer is now ready to receive more stack layers and conductor layers, at the discretion of the manufacturer.

In one possible embodiment of an array of the invented memory cells the pillars are vertically stacked directly above one another as illustrated inFIG. 24. Note that pillars are lined up in vertically aligned stacks. However, because of selfalignment, this vertical stacking of pillars directly above one another is not a requirement.

Memory cell pillars are automatically formed wherever a conductor on conductor layer (J+1) crosses over a conductor on conductor layer (J). This is true even if the conductor layers are not lined up directly above one another, giving vertical stacks of pillars. In fact it may be preferred that the pillars not be stacked vertically; that is they are offset from one another, as illustrated in FIG. 7 in U.S. Pat. No. 6,034,882. CompareFIG. 5(vertical stacks of pillars) to FIG. 7 in U.S. Pat. No. 6,034,882 (pillars offset from one another) to see the effect. Offset or staggered pillar placement such as shown in FIG. 7 in U.S. Pat. No. 6,034,882, may be advantageous in practice. It may help give a smoother wafer surface, more suited to planarization and polishing.

In the foregoing sequence of steps, electrode or conductor material is etched along with device material. Since most plasma metal etches also etch polysilicon, a practical combination of materials that enables such dual etching would be aluminum and polysilicon, for example. Control of the etching process may be effected, if desired, through the use of etch chemistries that are selective (e.g., preferentially etching polysilicon, but stopping on aluminum), or through the use of barrier materials that are not etched by the etchants that remove electrode and device material. The state change element may also be used as an etch stop, particularly if it is an oxide rupture type.

Refractory metals such as molybdenum and tungsten are compatible with conventional CVD deposition temperatures for Si and may be used for the conductors. Metal suicides are compatible with even higher temperatures used to activate dopants in Si. Even heavily doped Si itself can be used as a conductor. The choice may be dictated based on resistivity and integration concerns including etch characteristics.

The planarization described after the first half-step of the foregoing is necessary to form self-aligned contacts to the half-etched cells (i.e., the lines running in the east-west direction in the foregoing example). Such planarization may be effected through a variety of means well known in the art, such as chemical-mechanical polishing (CMP), etched-back spin-on dielectric layers, and etched-back spin-on polymers, to cite three well-known examples. To tolerate the possibility of excessive over-polishing or over-etching that may occur during planarization, a second planarization may be performed after deposition of an electrode layer to insure a planar electrode surface for subsequent deposition of device material layers.

The foregoing process sequence exploits self-alignment to reduce the required alignment tolerances between the pillar and the conductors. This embodiment may be substituted with an embodiment involving one or more additional photomasking steps to explicitly define the pillar itself, rather than defining it using the intersection of two conductor photomasking steps, as is done in the self-aligned process. This may be advantageous in various processes that could exploit the explicitly defined sidewalls that would result from such a process. For example, solid-phase crystallization of amorphous silicon could be used to form the steering element layer stack. The free energies of the sidewalls would be expected to favor the formation of a single crystal or grain within the steering element, which may be advantageous in some system embodiments.

Another process that could exploit explicitly defined sidewalls is laser-induced crystallization. Again, the free energies of the sidewalls would be expected to favor the formation of a single crystal or grain within the steering element.

In processes involving the explicit definition of the pillar, a photomasking step would be used to define a bottom conductor. This would be etched. Then, the layer stack required to form the state change and steering elements would be deposited. Another photomasking step would be used to define the pillar, which would be etched. After this etch, an insulating material would be deposited and planarized as in the self-aligned cell, exposing the top of the pillar to form a self-aligned contact. The top conductor would then be deposited and the process would be repeated for subsequent levels of cells as required.

The order of masking steps in the above process could also be reversed. For example, the pillar could be formed prior to patterning the bottom conductor. In this process, the entire layer stack for the bottom conductor, the steering element, and the state change element would be deposited. The pillar would then be lithographically defined and etched down through the steering element. The bottom conductor would then be defined and etched. This structure would be passivated using a planarized insulator contacting scheme, as described above. In all three processes, the self-aligned contact could also be replaced by an explicit contact forming photomasking step.

The various device fabrication steps may result in the presence of residual chemicals or dangling bonds that may degrade device characteristics. In particular, device leakage can result from the presence of such dangling bonds or chemicals (e.g., incompletely removed photoresist). A low-temperature (e.g., <400 C.) plasma oxidation exposure may be used to grow a clean-up oxide on the edges of the device pillar, thereby passivating edge traps. The growth of the oxide is self-limiting because the oxygen species diffuse only slowly through previously grown oxide, resulting in extremely uniform oxide thickness and, therefore, improved manufacturability. (Plasma oxidation may also be used to form an anti-fuse layer.) Oxide deposition may also be used to passivate the surface, for example, either alone or in conjunction with a grown oxide.

Because, in the foregoing for some embodiments, device material (e.g., polysilicon) is deposited after electrode material (e.g., metals), it is desirable to deposit and process the device material at the lowest practical temperatures to widen the selection of suitable metals. As an example, insitu doped polysilicon may be deposited at low temperatures using LPCVD (low pressure chemical vapor deposition), PECVD (plasma-enhanced chemical vapor deposition), PVD (physical vapor deposition), or UHVCVD (ultra high vacuum chemical vapor deposition). An alternative is to deposit undoped polysilicon, followed by doping and activation using a low temperature process. (Traditional activation steps such as long thermal anneals expose the wafer to potentially unacceptably high temperatures.) It may also be desirable in some cases to substitute microcrystalline or amorphous silicon or crystallized amorphous silicon for the polysilicon to enable low temperature fabrication.

Another concern is the possibility of diffusion of electrode material (e.g., metal) into the device layer during processing. Low temperature processing helps to reduce the severity of this problem, but may be insufficient to solve it completely. To prevent this problem, a number of barrier materials may be employed. Examples include titanium nitride (TiN), tantalum (Ta) or tantalum nitride (TaN), among many that are well known to the art.

In one embodiment of the cell, a thin dielectric layer is employed as an antifuse element. In such a cell, good uniformity of dielectric thickness, as well as a low film defect density (e.g., of pinholes in the dielectric) are among highly desirable properties. The quality of the dielectric may be enhanced through a variety of means, such as rotating (continuously or periodically) the substrate and/or source during deposition; forming the dielectric by thermal means using plasmas or low-temperature growth chemistries; or by employing liquid-phase dielectric deposition means.

It is desirable to reduce the number of masking steps that involve critical alignment tolerances. One method for reducing the number of masking steps is to employ vias that interconnect several electrode layers. The vias may be rectangular, rather than square, to allow a relaxation in alignment tolerances. For example, to interconnect metal lines in several layers running in the x-direction, the x-edge via size may be made substantially looser than the pitch of the x-lines in the y-direction, resulting in a rectangular via. Vias are discussed in conjunction with FIGS. 12 and 13 in U.S. Pat. No. 6,034,882.

Overview of the Structure of the Invented Memory Array

The invented memory array is fabricated on several levels and, for instance, may have eight levels of storage. Each level includes a first plurality of parallel spaced-apart rail-stacks running in a first direction and a second plurality of rail-stacks or conductors (depending on the embodiment) running in a second direction. Generally, the first rail-stacks run perpendicular to the second conductors/rail-stacks and hence form a right angle at their intersections. (In the invented array as well as in the prior art, conductors at one level are shared with the next level, hence the term “level” may not be precisely descriptive.)

The use of rail-stacks is a departure from prior art three-dimensional memories where conductors alone were used in lieu of rail-stacks, and where discrete cells (e.g., pillars) were formed at the intersections of the lines. As will be seen, a bit is stored at each of the intersections of rail-stacks. However, there is no apparent individual memory cell at the intersections, rather memory cells are defined by the rail-stacks and intermediate layers. This makes it easier to fabricate the invented array as will be seen. When the array is fabricated all the bits are in the zero (or one) state and after programming, the programmed bits are in the one (or zero) state.

In the embodimentFIG. 25several rail-stacks are illustrated in the partial cross-section of the invented array. For instance, rail-stack1600is shown at one height and a half rail-stack1800is shown at a second height above the first height. Also, half rail-stacks are disposed between rail-stack1600and a substrate1000. These lower rail-stacks run in the same direction as the half rail-stack1800. A bit is stored at the intersection of rail-stacks and, for instance, a “cell” is present between the rail-stacks and layers shown within the bracket1700and another within the bracket1900. Each of these brackets span a memory level.

The array is fabricated on a substrate1000which may be an ordinary monocrystalline silicon substrate. Decoding circuitry, sensing circuits, and programming circuits are fabricated in one embodiment within the substrate1000under the memory array using, for instance, ordinary MOS fabrication techniques. (These circuits may also be fabricated above the substrate.) Vias are used to connect conductors within the rail-stacks to the substrates to allow access to each rail-stack in order to program data into the array and to read data from the array. For instance, the circuitry within the substrate1000may select rail-stack1600and the rail stack1800in order to either program or read a bit associated with the intersection of these rail-stacks. (In the case of the embodiments ofFIG. 29some conductors are not part of rail-stacks; these conductors are also coupled to the substrate circuits.)

As shown inFIG. 25, an insulating layer1200is formed over the substrate in order that the array may be fabricated above the substrate. This layer may be planarized with, for instance, chemical-mechanical polishing (CMP) to provide a flat surface upon which the array may be fabricated.

Following this, a conductive layer1400is formed on the substrate. As will be seen, conductive layers are used within the rail-stacks and these layers and the resultant conductors may be fabricated from elemental metals such as tungsten, tantalum, aluminum, copper or metal alloys may be used such as MoW. Metal suicides may also be used such as TiSi2, CoSi2 or a conductive compound such as TiN, WC may be used. A highly doped semiconductor layer such as silicon is also suitable. Multiple layer structures may be used selecting one or more of the above.

Following the deposition of a conductive layer, a layer of semiconductor material (layer1500) such as silicon is formed over the conductive layer. This is typically a polysilicon layer, however, an amorphous layer may be used. Other semiconductor materials may be used such as Ge, GaAs, etc. In the embodiment ofFIG. 25this semiconductor layer is highly doped and, as will be seen, forms one-half a diode. After masking and etching steps, half rail-stacks are formed. These rail-stacks are “half” or partial rail-stacks since they are approximately half the thickness of the rail-stacks used in the next level.

Following this, in the embodiment ofFIG. 25, a material for the antifuses used to program the array is deposited. In one embodiment, the layer2000is a dielectric such as silicon dioxide which is deposited by chemical vapor deposition (CVD) in a blanket deposition over the half rail-stacks and a dielectric fill, filling the space between the rail-stacks. In another embodiment the layer2000is grown on the upper surface of the silicon layer1500and only exists on the rail-stacks.

Now a full set of memory array rail-stacks is formed on the layer2000. This comprises first the deposition of a lightly doped silicon layer2100doped with a conductivity type dopant opposite to that used for the silicon layer1500, a heavily doped silicon layer2200doped also opposite to the layer1500, a conductive layer2300and a heavily doped silicon layer2400doped with the same conductivity type dopant as layers2100and2200. After masking and etching, the rail-stacks shown inFIG. 25, such as rail-stack1600are formed. These rail-stacks are, as illustrated, in a direction perpendicular to the rail-stacks above and below them.

While not shown inFIG. 25but as will be described later, the spaces between the rail-stacks after they are defined, are filled with a dielectric such as silicon dioxide. Then the rail-stacks and fill are planarized by CMP. In another embodiment spin-on-glass (SOG) is used to fill the voids, in this case chemical planarization can be used, for example, plasma etching. Other fill and planarization methods can be used.

After formation of the rail-stacks another antifuse layer2600is formed, for instance, from a dielectric such as silicon dioxide, silicon nitride, silicon oxynitride, amorphous carbon or other insulating materials or combinations of materials. (Also an updoped layer of silicon may be used for the antifuse layer.)

Now another layer of rail-stacks are defined and only half rail-stacks are shown inFIG. 25at this upper level. This half rail-stack comprises a silicon layer2800doped with a conductivity type dopant opposite to that of layer2400. This is a lightly doped layer. Another silicon layer3000is formed on layer2800and this layer is doped with the same conductivity type dopant as layer2800, however, it is more heavily doped. Then a conductive layer3100is formed above the layer3000.

Half rail-stacks are used at the very upper-most level of the array and at the very lowest level of the array. In between the half rail-stacks the full rail-stacks, such as rail-stack1600, are used throughout the array.

It should be noted that the silicon layers disposed on the conductive layers extend the entire length of the rail-stacks in the embodiment ofFIG. 25and are uninterrupted except possibly where vias are used to provide a conductive path to the substrate1000.

InFIG. 25a path3200is illustrated from a lower conductor in level1700to an upper conductor in this level found in the rail-stack1800. This path is accessed in one embodiment through decoding circuitry in the substrate for both programming and reading of data into and from the array for one bit.

For instance, to program the bit, a relatively high voltage, 5-20V is applied between the conductors generally so as to forward-bias the “diode” between these conductors. This relatively high voltage causes a breach-in the layer2600creating a diode. Without this high voltage, the layer2600remains an insulator. Thus, by selecting pairs of conductors, diodes can be selectively formed so as to program the array. While programming the array with the layers adjacent to the antifuse material being forward-biased is currently preferred, it is also possible to program using a reverse-biasing potential.

To sense the data programmed into the array, a voltage lower than that for programming is used. This voltage is applied so as to forward-bias the diode of the cell being accessed and thus allowing a sense amplifier to determine whether or not the layer2600is intact between the rail-stacks. Note that “sneak” or parasitic paths in the array which would interfere with the sensing will include a reverse-biased diode.

Embodiment of FIG.27

In the cross-section elevation view ofFIG. 27, one embodiment is illustrated which corresponds to the embodiment shown inFIG. 25. InFIG. 27the half rail-stacks ofFIG. 25are not illustrated. Three complete levels3500,3600and3700of the array are illustrated inFIG. 27. Below layer3800ofFIG. 27other rail-stacks or half rail-stack are used. Also above layer6500, a full or half rail-stack is used.

The rail-stack3comprising layers3800through4100includes a lightly doped n− layer3800, a heavily doped n+ layer3900, a conductor layer4000and n+ layer4100. The fabrication of these rail-stacks will be discussed in more detail in conjunction withFIG. 26AthroughFIG. 26G. An antifuse layer4200which for the embodiment ofFIG. 27is a blanket deposition covers all of the rail-stacks formed below layer4200as well as the fill filling the voids between the rails. As mentioned, the layer4200is a deposited silicon dioxide layer in one embodiment.

It should be noted that n+ layers sandwich the conductor layer4000. These highly doped layers provide ohmic transitions to prevent unintended Schotky formation.

The layers above and below conductor4000are not symmetrical for the embodiment illustrated in that an n− layer3800is used below the conductor4000and not above the conductor4000. Only a single lightly doped layer (in conjunction with a heavily doped layer) is needed to define a diode; the thickness of this lightly doped layer is important in controlling the break-down voltage and resistance of the diode so formed. The layer4100, a heavily doped semiconductor layer, and the fill are planarized after the rail-stacks are defined and then a blanket deposition of the antifuse layer4200is formed on the layer4100. (The lines4300inFIG. 27are used to indicate that the antifuse layer4200and like layers are not etched with the rail-stack below it and thus extend over the entire array for the illustrated embodiment.)

One advantage to the layer4200and the other like layers in the structure, such as layers5100,5600and6500, is that since they are an unbroken deposition, sidewall leakage (into the rail-stacks below) will be minimized, limiting electrical problems during reading and writing. When subsequent conductive material is deposited, it is unable to reach the sides of the rail-stacks below it because of this blanket deposition of the antifuse layer. For instance, path4900which would allow silicon from layer5200to cause a parasitic path does not exist because of the unbroken blanket deposition of the antifuse layer5100.

Rail-stacks4comprising layers4400,4500,4600and4700are formed on the antifuse layer4200. Layer4400is lightly doped with a p-type dopant for the embodiment illustrated followed by a p+ layer4500, a conductive layer4600and a p+ layer4700. After these layers are deposited, they are masked and etched to define the rail-stacks. Then the voids between these rail-stacks, such as void5000, are filled with a dielectric. The fill dielectric is planarized along with a portion of p+ layer4700. Planarization is done at this point in the fabrication since there is generally poor control over the thickness and contour of the fill. The fill tends to build up on the rail-stacks when a non-spin-on deposition is used. This is followed by a blanket deposition of layer5400.

The process is now repeated this time beginning with an n− layer5200followed by an n+ layer5300, a conductive layer5400and n+ layer5500. Again after defining the rail-stacks5, the voids are filled and the surface is planarized. Another antifuse layer5600is deposited. The process is repeated for the rail-stacks6this time beginning with a p− layer6100, p+ layer6200, conductive layer6300, p+ layer6400. Again after defining the rail-stacks, filling the void6000and then planarizing, another antifuse layer6500is deposited.

As shown by the path6600, when a large enough voltage is applied between conductors4600and5400the antifuse layer5100, at the intersection of layers4700and5200is breached creating a diode at the intersection. As mentioned, this is selectively done throughout the array to program the array. The conductor5400is therefore a bit line for the “cells” above and below it, for instance path6700indicates another possible current path for another “cell” where the conductor5400is again a bit line during sensing.

It should be noted that with the reversal of the p− and n− layers at each successive rail-stack, planarization always occurs on a heavily doped layer such as layer4700and layer5500. Moreover, the lightly doped layers are always formed on relatively planar surfaces, consequently their thickness can be more easily controlled. This, as mentioned, allows the characteristics of the diode (once the intermediate antifuse layer is breached) to be more reliably controlled.

Processing Flow for the Embodiment ofFIG. 27

The process flow for forming rail-stack5ofFIG. 27is illustrated inFIGS. 26A-26H. It will be apparent that the rail-stacks for the other embodiment (FIGS. 28 and 29) are similarly processed.

First, as shown inFIG. 26Aan antifuse layer5100is deposited. This typically is 50-200 Å of silicon dioxide which can be deposited with any one of very well-known processes. Following this, a silicon layer5200is deposited which is typically 1000-4000 Å thick and formed with a CVD process where a phosphorous dopant is deposited along with the deposition of for instance, the polysilicon semiconductor material or where the dopant is ion implanted following the deposition of the layer. This layer is doped to a level of 5×1016−1018/cm3.

Now, as shown inFIG. 26Ban n+ layer5300is deposited again using CVD. This layer may be approximately 300-3000 Å thick and in one embodiment is doped to a level of >1019/cm3.

Throughout this application two adjacent silicon layers are often shown such as layers5200and5300, with different doping. These layers may be formed with one deposition and then using ion implantation step at two different energy levels to obtain the two doping levels.

A conductive layer which may be 500-1500 Å thick is formed using any one of numerous well-known thin film deposition process such as sputtering. A refractory metal may be used or a silicide of a refractory metal. Also as mentioned aluminum or copper can be used, or more simply the heavily doped silicon can be the conductor.

Next another semiconductor layer of, for instance, polysilicon approximately 1500-2000 Å thick is formed again doped to a level of >1019/cm3. This is shown as layer5500inFIG. 26D; after planarization its thickness is between 300 Å and 2000 Å thick.

A masking and etching step is now used to define rail-stacks, such as rail-stacks6900,7000and7100shown inFIG. 26E. Note that when comparing this view to the view of rail-stack5ofFIG. 27, the view inFIG. 26Eis taken from the side and consequently shows the individual rail-stacks. An ordinary masking and etching step for instance using plasma etching, may be used. Etchants can be used that stop on the antifuse layer thus preventing this layer from being etched away. Thus, layer5100can be considered an etchant stop layer depending on the specific etchants used.

Now as shown inFIG. 26F, the spaces between the rail-stacks are filled with a dielectric such as formed with a HDPCVD process.

Chemical-mechanical polishing is then employed to planarize the upper surface of the rail-stacks shown inFIG. 26Fin one embodiment. Chemical etching can also be used as mentioned with certain dielectrics. This planarization can reduce the thickness of the layer5500to approximately 500 Å, thus this layer ends up being of approximately the same thickness as the layer5300.

Next as shown inFIG. 26Hanother antifuse layer5600is formed on the planarized surface7500. Since the layer5600is deposited over all the rail-stacks and the filler material and remains unetched, it forms a barrier to the migration of the materials subsequently deposited that might make their way along the sides of the rail-stacks such as along path7900. Thus the layer5600helps prevent the parasitic paths and potential shorts that may occur with prior art memories.

It should be noted that inFIG. 27while the antifuse layer is shown as a blanket layer covering the rail-stacks and fill, it is possible also to fabricate each level where the antifuse layer is in fact grown from a semiconductor layer. For instance, an oxidation step may be used to grow a silicon dioxide layer from layers4100,4700,5500and6400. This grown layer would then be in lieu of the antifuse layers shown inFIG. 27.

The Embodiment of FIG.28

For the embodiment ofFIG. 28each rail-stack begins with a conductor such as layer8000ofFIG. 28. An n+ semiconductor layer8100and an n− layer8200are formed on layer8000. Next a layer of antifuse material8300is formed. Then a p+ layer8400of semiconductor material is deposited (e.g., silicon with boron dopant) on the antifuse. When the rail-stacks are formed, for instance for rail-stack200ofFIG. 28, the antifuse layer8300is etched as well as layers8000,8100,8200and8400.

The voids between the rail stacks are now filled and planarization is done, planarizing the fill with the upper surface of the layer8400. Following the completion of the rail-stack2the next rail-stacks are formed shown as rail-stacks3inFIG. 28. This comprises a conductor layer8500, p+ layer8600, p− layer8700, antifuse layer8800and n+ layer8900. Again masking and etching occur. This etching also etches the exposed regions of layer8400which does not appear in the view ofFIG. 28, but this will be apparent shortly when region9500of the next stack is discussed. Now filling and planarization occurs and the next layer of rail-stacks are formed shown as rail-stack400. As illustrated, this comprises a conductive layer9000, n+ layer9100, n− layer9200, antifuse layer9300, and p+ layer9400. Once again masking, etching, filling and planarization occur.

Unlike the embodiment ofFIG. 27, when rail-stacks at any particular height are formed, etching must occur on one layer of the rail-stack immediately below the rail-stack being defined. For instance, when rail-stack4is etched the layer8900of rail-stack3is etched away where it is not covered by rail-stack4as shown by region9500. This etching is used to remove all of the semiconductor material between the adjacent conductors and consequently prevent a path, such as path9600shown inFIG. 28. This etching also occurs to layer8400which, as mentioned, is not seen inFIG. 28. In this connection the antifuse layer8800can be used as an etchant stop, although this is not necessary. No harm is done if etching does occur through the layer8800since the antifuse layer is only needed at the intersections of the rail-stacks. Note the etching of the region9500is done in alignment with overlying rail-stacks and consequently no additional masking is required.

As was the case with the earlier embodiment, the order of the n and p doped layers alternate with each successive rail-stack. Moreover, the rail-stacks at any given height include both p and n layers. In contrast, for the embodiment ofFIG. 27, at any particular height, the rail-stacks are doped with either an n type or p type dopant but not both.

Embodiment of FIG.29

In the embodiment ofFIG. 29, alternate levels of rail-stacks running in a first direction and intermediate layers of conductors are running in a second direction are used. For instance as shown inFIG. 29, the conductors3,5and7run in a first direction whereas the rail-stacks4and6run in a second direction.

In this embodiment each of the rail-stacks is symmetrical about a conductor such as conductor10900of rail-stack4. The conductor is sandwiched between two n+ layers10800and11000. More lightly doped outer layers10700and11100are disposed on these more heavily doped layers.

In fabrication the conductors such as conductors10500, are first formed, for instance, on the substrate. The spaces between these conductors may be filled and planarization may occur. Then an antifuse layer10600, n− layer10700, n+ layer10800, conductive layer10900, n+ layer11000and n− layer11100are deposited. Rail-stacks are then defined by masking and etching. The voids between the rail-stacks are then filled with a dielectric. Then planarization of the filling material and the upper surface of layer11100is performed. Following this, antifuse layer11200is deposited over the entire array. Now additional conductors are formed such as conductors11300. Each level in this array is between a metallic conductor such as conductor10500, and a sandwich conductor such as conductor10900. Thus there are four memory levels shown inFIG. 29, levels10000,10100,10200and10300.

Programming in this array causes the formation of Schottky diodes consequently, the conductors such as conductors10500and11300must be of a suitable material to allow formation of a Schottky diode. For instance, aluminum and some refractory metal or silicides may be used.

Other Embodiments

In the above description a conductor is shared by two levels. An array may be fabricated where there are two conductors for each level that are not shared with other levels. A dielectric may be used to separate each such level. Also while above diodes on alternate levels “point” in the same direction, this is not necessary. For instance, a shared conductor may have diodes point-in from above and point-out from below. This requires different driving circuitry in the substrate.

Any term, concept, feature, drawing, method, apparatus, system, etc. or portion thereof, described in the above-listed documents can be used alone or in combination to support claims of this or any other non-provisional patent application(s) claiming the benefit of U.S. provisional application No. 60/446,910. Additionally, the material described in the above-listed documents provides only some of many possible implementations. For this reason, the above-listed documents are intended by way of illustration and not by way of limitation. It is only the claims, including all equivalents, in this and any other non-provisional patent application(s) that claim the benefit of U.S. provisional application No. 60/446,910 that are intended to define the scope of inventions supported by the above-listed documents. The following claims express the statements made in this paragraph.