Read-only memory having anti-fuse elements

Abstract: A one-time, voltage-programmable, read-only memory array in which individual memory cells comprise an insulated-gate, field-effect transistor, the channel of which provides, through a voltage-programmable anti-fuse element, a current path between a reference voltage line (34) and a bitline (48+49). The array comprises a semiconductor substrate having a series of parallel, alternating, minimum-pitch field isolation regions (31) and active area strips (32), a series of parallel, minimum-pitch wordlines (33A-33F) overlying and perpendicular to the field isolation region (31) and active area strips, the wordlines being insulated from the active areas by a gate dielectric layer (42) and being dielectrically insulated on their edges and upper surfaces, source/drain junction regions (50) between each wordline pair and field isolation strip pair, a reference voltage line (34) between and coextensive with every other wordline pair that makes anti-fusible contact to each subjacent pair of cell junctions along its length, anti-fusible contact for each cell being made within a trench that extends below junction depth, and is lined with conformal silicon nitride dielectric layer (46) that breaks down when subjected to a programming voltage.

Description: This invention relates to a one-time, voltage-programmable read only memory (PROM) array according to the pre-characterizing part of claim 1. BACKGROUND OF THE INVENTION Read Only Memories (ROM) are commonly used to store programs in computer systems. Although mask-programmed ROMs are the least expensive when manufactured in high volume, they have the disadvantages of high initial design cost and long lead time to manufacturing. Programmable ROMs (PROMs), of which there are many types, have the advantage of being field programmable. Bipolar PROMs, which incorporate bipolar fusible links or vertical shorting junction elements, have typically been used for high-speed applications, despite their relatively high manufacturing cost, high power consumption, and low circuit density. Fusible links are particularly problematic. Not only are high current values required to blow the fusible links during programming, but the photolithographic and etch steps required for their formation must be precisely controlled so as to minimize process variability. Unless link size and shape is controlled within a narrow range, it will not function effectively both as a conductor if not blown and as a completely open circuit if blown. Where speed is not of paramount importance, electrically-programmable ROMs (EPROMs) employing floating gate technology have been typically employed. Such EPROMs feature greater circuit density than bipolar EPROMs, due to a more compact cell design. However, the manufacture of floating gate EPROMs requires complicated processing. Additionally, data is retained in the floating gate, which is vulnerable to leakage and ultraviolet radiation, either of which may result in the permanent loss of data. Various other PROM cells have been proposed that utilize antifuse elements for the programmable link. Generally, the antifuse elements are essentially capacitors, consisting of two conductive or semiconductive materials separated by a dielectric layer. During programming, the dielectric layer is broken down by the application of a programming voltage that is significantly greater than the normal read voltage, thereby electrically connecting the conducting materials. For PROM cells described in U.S. Pat. Nos. 3,634,929 an isolated diode is used as the selection element and a capacitor is used as the programmable antifuse element. Such designs suffer from several disadvantages. Reverse bias leakage will result in high background current values. In addition, the programming operation tends to stress the dielectric on cells which are to remain unshorted. Finally, it is difficult to balance the need to utilize a read voltage which will be sufficient to trigger the column sense amps without exerting undue stress on the dielectric of the unshorted cells during the useful life of the device. The PROM cells described in U.S. Pat. Nos. 4,322,822 and 4,507,757 utilize a programmable antifuse capacitor in combination with a MOS transistor, which functions as the selection element. The first referenced patent discloses an electrically-programmable memory array having capacitor devices formed in anisotropically etched V-grooves, which provide enhanced dielectric breakdown at the apex of the groove. After breakdown (i.e., programming), a cell exhibits a low resistance to a grounded substrate. This cell structure is somewhat difficult to manufacture, in that it requires a highly-conductive, grounded substrate. Additionally, the design does not lend itself to high density circuit layout. The second referenced patent ('757) discloses an electrically-programmable memory array having a thin-oxide avalanche fuse element which is programmed at voltage below the oxide breakdown level. Such a design is extremely susceptible to manufacturing process variability, since the avalanche phenomenon is difficult to control. US-A-4 499 557 discloses a PROM in accordance with the precharacterizing part of claim 1. The anti-fuse element is in the form of an amorphous alloy deposited between a reference voltage line and the adjacent junction of the field-effect transistor (Figs. 7-9 and 13, 14 of US '557). EP-A-0 250 078 discloses a programmable interconnect device for use in integrated circuits, wherein anti-fuses are formed by means of a silicon nitride layer. EP-A-0 355 768 discloses a memory structure in which a wiring forming a read data line is made of polycrystalline silicon and a superjacent refractory metal silicide layer. The invention, as defined by claim 1, is an improved one-time, voltage-programmable, read-only memory array in which individual memory cells comprise an insulated-gate, field-effect transistor, the channel of which provides, through a voltage-programmable anti-fuse element, a current path between a reference voltage line and a bitline. The function of the cell may be compared with that of a conventional dynamic random access memory (DRAM) cell. In a DRAM cell, charge stored within a cell capacitor is accessed through an insulated-gate field effect transistor (the access transistor), which is turned on by activating a single word line. The cell capacitor has a node called the cell plate, which is electrically common to all other cells within the array. The voltage of the cell plate is held to approximately V CC /2 (V CC being the voltage supplied to the chip). A "1" is stored in the capacitor of a memory cell by charging the cell capacitor's uncommon node, or storage-node plate, to V CC through the access transistor. Likewise, a "0" is stored by discharging the uncommon node to V SS (generally ground potential) through the access transistor. The charge on the capacitor is read by first equilibrating the cell plate and all digit lines to V CC /2. After the equilibration voltage is disconnected from the digit lines and cell plate, the access transistor is turned on, dumping the charge stored in the cell's capacitor to an interconnected digit line. If the uncommon node of the cell capacitor had been charged to V CC , the voltage on that digit line will be increased slightly, whereas if the uncommon node had been grounded to V SS , the voltage on that digit line will be decreased slightly. An adjacent digit line, to which no charge has been dumped during this read cycle, is used as a reference. The voltage differential between the two digit lines will be within a range of approximately 200 to 400mV. This differential voltage is then amplified by cross-coupled N-channel and P-channel transistors (differential sense amplifiers), which pull the digit line having the slightly lower voltage to V SS and the digit line having the slightly higher voltage to V CC . Once this has occurred, the voltages on the digit lines is passed out of the array to a column decoder and read by an output buffer. Within the subject PROM array, cells have access transistors, but no cell capacitor. A reference voltage line common to all cells within the array, provides a current source which is accessible through a particular cell access transistor only if the programmable antifuse link within that cell is shorted. Since a current source is accessed through the cell transistor, differential sense amps are not required to sense a charge differential between two bitlines. Instead, a far simpler current sense amp may be used to determine cell logic states. For the preferred embodiment of the invention, the array comprises a semiconductor substrate having a series of parallel, alternating, minimum-pitch field isolation region and active area strips, a series of parallel, minimum-pitch wordlines overlying and perpendicular to the field isolation region and active area strips, the wordlines being insulated from the active areas by a gate dielectric layer and being dielectrically insulated on their edges and upper surfaces, source/drain junction regions between each wordline pair and field isolation strip pair, a reference voltage line between and coextensive with every other wordline pair that makes anti-fusible contact to each subjacent pair of cell junctions along its length, anti-fusible contact for each cell being made within a trench that extends below junction depth, and is lined with conformal silicon nitride dielectric layer that breaks down when subjected to a programming voltage. A series of minimum pitch bitlines, which run parallel to the wordlines, completes the memory array. Each bitline makes direct contact with each pair of cell junctions along its length. The array is characterized by a non-folded bitline architecture. Brief Description of the Drawings Figure 1A is a proposed schematic symbol for a non-shorted antifuse element; Figure 1B is a proposed schematic symbol for a shorted antifuse element; Figure 2 is an abbreviated circuit schematic for the improved PROM array; Figure 3 is a top plan view of a portion of the improved PROM array; and Figure 4 is a cross-sectional view through broken line 4-4 of Figure 3, which fully depicts a single memory cell within the array. Preferred Embodiment of the Invention Since the charge storage characteristics of a capacitor are typically immaterial to antifuse element performance, the schematic representation of such elements using capacitor symbols is somewhat misleading. Given this realization, antifuse elements have been variously represented with gate-like and fuse-like symbols. Because of the need within this disclosure to distinguish between shorted and non-shorted antifuse elements, a new symbol is proposed. Figure 1A depicts a non-shorted antifuse element, which consists of two curved lines separated by a gap. Figure 1B, on the other hand, depicts a shorted antifuse element, the difference being a dot which bridges the gap between the two curved lines. Referring now to the abbreviated circuit schematic of Figure 2, the improved PROM array is shown as comprising a series of non-intersecting wordlines (WL1, WL2, WL3, and WL4), a series of reference voltage lines (RL1 and RL2), each of which is located between every wordline pair, and a series of non-intersecting bitlines (BL1, BL2, BL3, and BL4), which intersects the series of wordlines. One memory cell is associated with each wordline-bitline intersection. For example, cell C1 is associated with the intersection of wordline WL1 and bitline BL1, cell C7 with the intersection of wordline WL3 and BL2, and cell 14 with the intersection of wordline WL2 and BL4. Within each cell, a reference voltage line is coupled to a bitline via an antifuse link and a cell access transistor which is gated by the wordline associated with that cell. For example, in cell C1, reference voltage line RL1 is coupled to bitline BL1 through antifuse link A1 and access transistor Q1, which is gated by wordline WL1. If the antifuse link within a cell is shorted (i.e., conductive), then current from the reference voltage line will flow to the bitline. However, if the antifuse link is unshorted (i.e., an open circuit), no current will flow to the bitline. A "1" is permanently written to a cell within the array by applying a voltage that is greater than the breakdown voltage of the antifuse link to the bitline associated with that cell, and activating the wordline associated with the cell. In this schematic, cells C1, C4, C8, C9, C11, C12, C14, C15, and C16 are depicted as having shorted antifuse links, while the remainder of the cells are depicted as having unshorted links. The preferred embodiment of the improved PROM array will now be described with reference to both Figure 3, which is a top plan view thereof and Figure 4, which is a cross-sectional view through broken line 4-4 of Figure 3. Although the process utilized to fabricate the preferred embodiment of the array is not within the scope of this disclosure, an abbreviated description of the process will be provided that should be sufficient to enable a semiconductor process engineer having ordinary skill in his art to fabricate the array. Features within the Figures will be given an item number either between 31 and 39 if they are more readily discernable in Figure 3 or between 41 and 50 if they are more readily discernable in Figure 4. An array fabrication process is employed, which, at least through transistor formation, is very similar--if not identical--to processes employed to fabricate stacked-capacitor DRAM arrays. The array depicted will be fabricated utilizing N-channel devices. However, a functional array may also be fabricated on an N-type substrate by reversing implant conductivity types. Following the creation of a series of parallel, linear field oxide regions 31 in a lightly-doped P-type silicon substrate 41, a gate oxide layer 42 is thermally grown on the surface of the active areas 32 (the substrate regions between field oxide regions 31). A series of wordlines 33A, 33B, 33C, 33D, 33E, 33F....33n (where n is the total number of wordlines in the array) is patterned from a silicon dioxide covered, refractory-metal-silicided first polycrystalline silicon (poly) layer. Thus each conductive portion of a wordline comprises a poly layer 43 and a silicide layer 44. Silicide layer 44 is covered by a silicon dioxide layer 45. Source and drain regions 50A, 50B are then created by implanting an N-type impurity such as phosphorus, arsenic, or both. Wordline sidewall spacers are utilized to appropriately offset the implants from the channel edges. From this point, the array fabrication process departs from DRAM process flows. Active area regions between every other wordline are then subjected to an anisotropic etch which trenches the substrate below transistor junction depth. These trenches are then lined with a conformal silicon nitride dielectric layer 46 that is approximately 50 to 100Å in thickness. A series of parallel reference voltage lines 34 is then created in the interwordline gaps where the silicon nitride lined trenches have been created. The array is then covered with a dielectric isolation layer 47. A self-aligned bitline contact etch is then performed at the array locations 35, which are identified in Figure 3 with a circle having an "X" therein. A series of parallel bitlines which intersect wordlines 33 and reference voltage lines 34 are then patterned from a refractory-metal-silicided second poly layer. Bitlines are thus formed which have a second poly layer component 48 and a refractory metal silicide layer component 49. Referring now to figure 4, exclusively, it will be noted that the trench in which reference voltage line 34 rests serves a dual function. First, it electrically isolates the cells associated with wordline 33C from those associated with wordline 33D. Additionally, it provides a relatively large surface area for breakdown of the silicon nitride dielectric layer 46 to occur during high voltage programming. The programmable antifuse link consists of the silicon nitride dielectric layer 46 between the reference voltage line 34 and the adjacent transistor junction 50B. In order to program a particular cell, a selected digitline and a selected wordline are both held at a programming potential of approximately 10-12 volts, while unselected bitlines, unselected wordlines, and reference voltage lines are held at ground potential. When this programming voltage is applied to a bitline and wordline pair, the access transistor of the cell so selected is completely turned on, and the dielectric in the antifuse link ruptures (i.e., breaks down), forming a permanent conductive path from the reference voltage line to the adjacent transistor junction. There are two basic methods for reading the logic state stored within a particular cell. For the first method, a selected bitline is held at ground potential. The voltage applied to the remaining bitlines, although theoretically irrelevant from a logic standpoint, are also held at ground potential to eliminate potential inter-circuit current leakage. A selected wordline is held within a range of approximately 4-6 volts, while unselected wordlines are held at ground potential. Voltage reference lines are held within a range of approximately 2-3 volts. If the antifuse link in a selected cell is shorted, detectable current will flow from the reference voltage line to the selected bitline. If, on the other hand, the antifuse link in the selected cell is not shorted, no current will flow to the selected bitline. For the second method of reading the cell logic state, a selected bitline is held at a voltage level of 2-3 volts, while unselected bitlines are once again held at ground potential to eliminate the possibility of inter-circuit current leakage. A selected wordline is held within a range of approximately 4-6 volts, while unselected wordlines are held at ground potential. Reference voltage lines are grounded. If the antifuse link in a selected cell is shorted, detectable current will flow from the selected bitline to the reference voltage line. If, on the other hand, the antifuse link in the selected cell is not shorted, no current will flow to the reference voltage line. This second read method has an advantage over the first method, in that reference voltage line may remain permanently grounded. Although only a single embodiment of the improved PROM array has been disclosed, it will be obvious to those having ordinary skill in the art that changes and modifications may be made thereto without departing from the scope of the invention as claimed. For example, instead of trenching the substrate beneath the reference voltage lines and placing the silicon nitride dielectric layer beneath the reference voltage line, the substrate beneath the bitline contact may be trenched and the silicon nitride placed beneath the bitline contact. However, such a modified cell design, although workable in theory, would make programming a far more complicated task.

Claims: A one-time, voltage-programmable, read-only memory (PROM) array having a semiconductor substrate (41) and individual memory cells each of which comprises an insulated-gate field-effect transistor (33 + 42 + 50A + 50B), the channel of which provides, through a voltage-programmable anti-fuse element (46), a current path between a reference voltage line (34) and a bitline (48 + 49), characterized in that the antifuse element (46) is formed in a trench which extends vertically into the substrate below the depth of an adjacent junction (50B) of the field-effect transistor, wherein shorting of the antifuse element during high-voltage programming occurs on a substantially vertical surface of said trench. The PROM array of claim 1, wherein the antifuse element is a silicon nitride layer (46) interposed between the reference voltage line (34) and said adjacent junction of the field-effect transistor (50B), said junction having been formed in said semiconductor substrate (41). The PROM array of claim 1 or 2, which further comprises a series of substantially parallel wordlines (33A-33F) patterned from a dielectrically-coated conductive layer. The PROM array of claim 3, wherein said conductive layer comprises a conductively doped polycrystalline silicon layer (43) and a superjacent refractory metal silicide layer (44). The PROM array of claim 4, wherein programming means are provided for programming a particular cell within the array such that a logic value of "1" will be permanently stored therein, by applying a low voltage V L to the reference voltage line and a high voltage V H to the bitline (48 + 49) associated with that cell to be programmed, such that V H -V L is greater than the breakdown voltage of the silicon nitride dielectric layer (46); said programming means further activating the wordline associated with the cell to be programmed so that substantially the entire potential V H -V L is applied accross the antifuse link which pertains to that cell, thus causing it to break down and form a permanent conductive path between the reference voltage line (34) and the adjacent transistor junction (50B). The PROM array of claim 4, wherein programming means are provided for programming a particular cell within the array, such that a logic value of "1" will be pennanently stored therein, by grounding the reference voltage lines (34) within the array; applying a voltage higher than the breakdown voltage of the silicon nitride dielectric layer (46), to the bitline (48 + 49) associated with the cell to be programmed, i.e. the selected bitline; and applying a voltage to the wordline (33) associated with the cell to be programmed, i.e. the selected wordline, that is at least sufficient to activate the cell access transistor of the cell to be programmed. The PROM array of claim 6, wherein said programming means applies a voltage to the selected bitline (48 + 49) which is substantially equal to the voltage applied to the selected wordline. The PROM array of claim 7, wherein the voltage applied to the selected bitline and the selected wordline is within a range of 10-12 volts. The PROM array of claim 7, wherein reading means are provided for reading the logic value stored within a particular cell within the array by applying a voltage to the wordline associated with the cell to be read that is sufficient to activate the transistors associated therewith; maintaining unselected wordlines at a potential insufficient to activate the transistors associated therewith; applying a reference voltage to the reference voltage lines (34); placing the bitlines (48 - 49) at ground potential; and determining whether or not current is flowing from the cells's reference voltage line to the bitline associated with the cell to be read. The PROM array of claim 9, wherein activation voltage for a selected wordline during a cell read operation is within a range of 4-6 volts. The PROM array of claim 10, wherein reference voltage lines (34) are held at a potential with a range of 2-3 volts. The PROM array of claim 6, wherein reading means are provided for reading the logic value stored within a particular cell within the array by applying a voltage to the wordline associated with the cell to be read, i.e. the selected wordline, that is sufficient to activate the transistors associated therewith; maintaining unselected wordlines at a potential insufficient to activate the transistors associated therewith; grounding the reference voltage lines; applying a voltage of 2-3 volts to the bitline associated with the cell to be read, i.e. the selected bitline; determining whether or not current is flowing from the selected bitline to the cell reference voltage line. The PROM array of claim 12, wherein activation voltage for a selected word line during a cell read operation is within a range of 4-6 volts. The PROM array of any of claims 1 to 13, which is further characterized by a non-folded bitline architecture.

Inventors: LEE RUOJIA, LOWREY TYLER A, LEE, RUOJIA, LOWREY, TYLER A.
Applicants: MICRON TECHNOLOGY INC, MICRON TECHNOLOGY, INC.
Application year: 1992
Main classification: G11C17
IPC codes: H01L27, G11C17