Memory cell with programmable antifuse technology

Abstract: A memory cell (10) comprising a first antifuse (A1) operable to place the memory cell (10) in a non-volatile state. In one embodiment, the memory cell (10) comprises a pair of cross-coupled inverters (I1, I2). The first antifuse (A1) is connected between an output (B) of one of the cross-coupled inverters and ground and is operable to place the memory cell in a first non-volatile state. A second antifuse (A2) is connected between an output (B) and a supply voltage (Vcc) and is operable to place the memory cell (10) in a second non-volatile state. Only one of the antifuses, (A1 or A2) is programmed in memory cell (10).

Description: FIELD OF THE INVENTION The invention relates generally to the field of non-volatile memories and more specifically to memory cells of the type defined in the precharacterizing portion of claim 1 and to a method as defined in the precharacterizing portion of claim 7. BACKGROUND OF THE INVENTION Semiconductor memories may be divided into two classes, volatile and non-volatile. In volatile memories the stored data is lost when power is removed, and in non-volatile memories the stored data is not lost when power is lost. A volatile CMOS type memory cell comprising a pair of cross coupled inverters and two pass transistors is disclosed in US-A-4 524 377. Because of the need for static non-volatile memory in calculators, watches, game products, and other commercial devices, there has been a increasing demand for these type of memories. Typical non-volatile memories include Read-Only Memories (ROMs), Erasable Programmable Read-Only Memories (EPROMs), Flash EPROMs, and Electrically Erasable Programmable Read-Only Memories (EEPROMs). In a ROM, the contents are defined at the time of fabrication by selective patterning of one or more fabricated layers. However, a ROM cannot be rewritten if the user wants to modify the data stored in the ROM. Rather the user must re-specify the desired memory contents and the revised chips must be fabricated with new information stored. Erasable Programmable Read Only Memories (EPROM) are erased by exposing the memory array to ultraviolet light which allow the charge stored on a Floating Gate Avalanche Injection Metal Oxide Semiconductor (FAMOS) device floating gate to discharge to the surrounding conductive materials, thereby erasing the state of the cell To re-write the memory, an elevated voltage supply is required to tunnel charge to the desired FAMOS floating gates. The drawbacks of EPROMs include long erase times (~ 20 minutes), expensive windowed packaging, process complications due to the high programming voltage required, and the fact that EPROMS must be removed from the system to be reconfigured. Electrically Erasable Programmable Read Only Memories (EEPROM) are electrically erasable by the application of an elevated voltage which overwrites the charge on the floating gate of the FAMOS device. This has obvious advantages over the EEPROM technology in that devices can be quickly erased while still in the system and can be purchased in inexpensive plastic packages. EEPROMs have the disadvantage of requiring the system designer to route the elevated programming voltage as well as the normal supply voltage to their design. The erasing is typically done in large blocks, so while prototyping the user must completely reload the memory contents when any change is made. Flash technology is the latest successful entrant into the non-volatile memory business. Flash memories are similar to EEPROMs except that the erasing voltage is the same as the normal power supply. Again, the erasing is typically done in large blocks, so while prototyping the user must completely reload the memory contents when any change is made. A memory cell of the type defined in the precharacterizing portion of the independent claims is known from EP-A-0 156 135. That document discloses a CMOS memory cell including two CMOS inverter circuits, wherein the output node of each respective CMOS inverter is connected to the gate of the n-channel transistor in the other inverter circuit. Four selectively severable nodes are provided in the metallization interconnecting the four transistors in the circuit so as to allow for wiring level programming into a preconditioned binary one or zero state. The preconditioned CMOS memory cell will assume a preselected binary state when power is turned on. Thereafter, the cell can be accessed for normal binary one and zero selective storage. Severing of the metallization nodes is effected by melting the metallization line using a laser beam. In this context, reference is further made to US-A-4 418 401 which discloses an asymmetric RAM cell which will have a predictable initial storage state when pulsed drain voltage is turned on and yet after the initial turn-on interval, will operate in a symmetric fashion storing either binary ones or zeros. Thus, an initial prestored set of information can be permamently provided in a memory array made up of such cells, by orienting each individual cell at the time of manufacture so as to selectively represent either a binary one or zero. Patent Abstracts of Japan, Volume 14, No. 78, P-1006, JP-1294296, discloses a Static Random Access Memory, wherein a memory cell of the RAM is provided with a read-only memory cell. When an arbitrary storage information initial value is written in the read-only memory cell at the time of forming the RAM, the steady state of the memory cell of the RAM is settled. In accordance with the present invention, a memory cell of the type defined in claim 1 is provided. There is further provided a method of forming a memory device as defined in claim 7. The first programmable antifuse is operable to place the memory cell in a first non-volatile state, such as a logic "1". A second programmable antifuse is connected between an output of a second one of the pair of cross-coupled inverters and the ground node. The second programmable antifuse is operable to place the memory cell in a second non-volatile state, such as a logic "0". An advantage of the invention is providing a memory cell with programmable antifuse technology such that it is initially very easily re-writeable volatile memory which may subsequently be made non-volatile. A volatile memory is useful during the development phase of the system which the memory is in. Once the system has been debugged and the system specification is finalized, the memory may be made non-volatile and read-only by programming the appropriate antifuse. Useful applications include circuits such as microprocessor micro-code and BIOS instruction chips. These and other advantages will be apparent to those of ordinary skill in the art upon reference to the specification in conjunction with the drawings and claims. BRIEF DESCRIPTION OF THE DRAWINGS In the drawings: Figure 1 is a schematic diagram of a six transistor memory cell with programmable antifuse technology according to the first preferred embodiment of the invention. Figure 2 is a block diagram of an memory array device according to the preferred embodiments of the invention. Figure 3 is a schematic diagram of a five transistor memory cell with programmable antifuse technology according to the second preferred embodiment of the invention. Figure 4 is a schematic diagram of a four transistor two resistor memory cell with programmable antifuse technology according to the third preferred embodiment of the invention. Corresponding numerals and symbols in the different figures refer to corresponding parts. DETAILED DESCRIPTION OF THE INVENTION The preferred embodiments of the invention will now be described in conjunction with several CMOS memory cell configurations. It will be apparent to those skilled in the art that the invention may be applied to devices such as SRAMs (Static Random Access Memories), shift registers, sequentially accessible memories and other random access memories, which may each realize the benefits thereof. The preferred embodiments of the invention allow the system designer to use a memory system in their design which can be treated exactly like a standard READ/WRITE Random Access Memory (RAM) during system development. The chips operate as a normal SRAM until the user decides to permanently configure the stored data and change the chip to a non-volatile ROM (Read Only Memory). After this, the Read operation still is performed as a standard SRAM (Static Random Access Memory), but writing new and different data is not possible. This avoids the erasing step required with prior art non-volatile memory technologies. It would then be very convenient to implement the final SRAM contents in a non-volatile Read Only Memory once the final contents of the SRAM is decided upon. Although this invention is one-time-programmable once the decision is made to make the SRAM contents non-volatile, this is consistent with the majority of system requirements which consume most of the non-volatile memory. It is noted that much of the EPROM market is satisfied with low-cost window-less plastic packages which make the EPROMs one time programmable. This is acceptable once the final system specification is completed. A first preferred embodiment of the invention is shown in Figure 1. The memory cell 10 of Figure 1 is similar to prior art SRAM memory cells in that it comprises two cross-coupled inverters, I1 and I2, and two pass transistors, 20 and 30. Each of the inverters I1 and I2 comprise a p-channel transistor (22, 26) connected between Vcc and an output node (B,A) and an n-channel transistor (24, 28) connected between the output node (B,A) and ground. Pass transistor 20 is connected between a bitline BL and the input to inverter I1. Pass transistor 30 is connected between a bitline BL_ and the input to inverter I2. The gates of pass transistors 20 and 30 are connected to a wordline signal WL. The memory cell 10 of Figure 1 is different from prior art SRAMs in that it contains an antifuse A1 connected between the output of inverter I1 and ground and antifuse A2 connected between the output of inverter I1 and Vcc. The antifuses are normally open devices which would not influence normal memory cell operation except for a small amount of additional capacitance and depending on the antifuse technology employed, a small amount of leakage current through the antifuses which the transistors could be sized to supply. Methods for forming antifuses are well known in the art. Referring to Figure 2, many memory cells 10 of figure 1 may be combined to form a memory array such as in SRAM 34. SRAM 34 may then be placed in a system which may be under development. In the development phase of operation, the memory cell 10 of Figure 1 would operate like a typical SRAM cell The value stored in cell 10 could be changed at any time and would be lost during a power down condition. Then, when the design of a system is finalized, the cell 10 could be made non-volatile or fixed by blowing either antifuse A1 or A2. If antifuse A1 were blown, the cell 10 would permanently store a logic value of "1". If antifuse A2 were blown, the cell 10 would permanently store a value of "0". Antifuse A1 may be blown by providing a high voltage on both BL_ and WL. In addition, a low voltage is provided on BL so that current does not shunt from node A to ground through transistor 28. The voltage applied to BL_ (minus some threshold drop across pass transistor 30) is applied to node B and will thus be applied across the terminals of antifuse A1. If the voltage is high enough, the antifuse is blown. Node B is thus connected to GND, transistor 28 is turned off and transistor 26 is turned on. Node A is accordingly high and the cell has a stored value of '1'. Alternatively, antifuse A2 may be programmed by placing a high voltage on WL, a low voltage on BL_ and raising Vcc until the voltage across the terminals of antifuse A2 is large enough to program antifuse A2. Once antifuse A2 is programmed, node B is connected to Vcc turning transistor 28 on and transistor 26 off. Node A is accordingly at a low voltage and a "0" is stored in the Cells 10. It should be noted that only one of the antifuses, A1 or A2, is programmed. The voltage required to program the antifuses will depend on the design and technology of the antifuse and on the transistor characteristics. Typical voltages are in the range of 10 to 20 volts, depending on the antifuse technology used. Emerging antifuse technologies may provide even lower programming voltages. Programming some antifuse technologies, such as that taught by U.S Patent No. 4,899,205 assigned to Actel which use a time-dependent-dielectric-breakdown mechanism may require the voltage to be held high for durations in the milli-second range. Other antifuses, such a amorphous silicon may take micro-second range pulse durations. A second preferred embodiment is shown in Figure 3. Memory cell 50 is similar to a typical five transistor memory storage cell in that it comprises cross-coupled inverters I3 and I4 and pass transistor 40. As in the above case, each inverter I3 and I4 includes a p-channel transistor (42, 46) connected between Vcc and an output node (D,C) of the respective inverter (I3,I4). Pass transistor 40 is connected between a memory cell input IN and node C. Node C is also the output node OUT of the memory cell 50. Cell 50 is different from prior art structures in that it contains antifuse A3 connected between node D and ground and antifuse A4 connected between node C and ground. In operation, a number of cells 50 may be combined to form a memory device such as the SRAM 34 shown in Figure 2. As in the first preferred embodiment, memory cell 50 operates as a typical memory cell during the development stage of a system. Once the design is finalized, one of the antifuses A3 or A4 is programmed to permanently stored the desired value. Antifuse A3 is programmed if a high output is desired and antifuse A4 is programmed if a low output is desired. Still referring to Figure 3, Antifuse A3 may be programmed by placing a low voltage on IN, and high voltage on WL which turns on p-channel transistor 42 and raising Vcc in order to provide a wide voltage differential across the terminals of antifuse A3. Antifuse A3 is thus programmed providing a connection between node D and ground. This, in turn, causes the output OUT to be permanently high as p-channel transistor 46 is held on, pulling node OUT up to Vcc. Antifuse A4 may be programmed by placing a high voltage on IN and WL. The voltage differential between node C and ground and thus across the terminals of antifuse A4 is large enough to program the antifuse A4 to provide a connection between node C (OUT) and ground. A third preferred embodiment is shown in Figure 4. Cell 72 is similar to prior art four transistor - two resistor SRAM cells except that antifuses AS and A6 have been added. Cell 72 contains resistors 62 and 66 connected between Vcc and nodes F and E, respectively. N-channel transistors 64 and 68 are connected between GND and nodes F and E, respectively. Pass transistor 60 is connected between node E and BL and pass transistor 70 is connected between node F and BL_. The gates of pass transistors 60 and 70 are connected to WL. Antifuse A5 is connected between node F and GND and may be used to store a "1" in cell 72. Antifuse A5 may be programmed by placing a high voltage on BL_ and WL and a low voltage on BL. Antifuse A6 is connected between node E and GND and may be used to store a "0" in cell 72. Antifuse A6 may be programmed by placing a high voltage on BL and WL and a low voltage on BL_. While this invention has been described with reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications and combinations of the illustrative embodiments, as well as other embodiments of the invention, will be apparent to persons skilled in the art upon references to the description.

Claims: A memory cell comprising a first programmable circuit (A1; A3; A5) operable to place said memory cell (10; 50; 72) in a non-volatile state and a pair of cross-coupled inverters (I1, I2; I3, I4; I5, I6), wherein said first programmable circuit (A1; A3; A5) is connected between an output of a first one of said pair of cross-coupled inverters (I1, I2; I3, I4; I5, I6) and a ground node, characterized in that said first programmable circuit (A1; A3; A5) is a first antifuse, an antifuse being defined as a device which is initially an open circuit between a pair of leads, which circuit is closed upon application of a sufficient voltage between the leads of the antifuse. The memory cell of claim 1, further comprising: a second programmable antifuse (A4; A6) connected between an output of a second one of said pair of cross-coupled inverters (I3, I4; I5, I6) and said ground node. The memory cell of claim 1, further comprising: a second programmable antifuse (A2) connected between said output of said first one of said pair of cross-coupled inverters (I1, I2) and a supply voltage. The memory cell of claim 1, wherein each inverter of said pair of cross-coupled inverters (I1, I2; I3, I4) comprises a p-channel transistor (22, 26; 42, 46) and an n-channel transistor (24, 28; 44, 48) connected in series between a supply voltage and said ground node. The memory cell of claim 1, wherein each inverter of said pair of cross-coupled inverters (I5, I6) comprises a resistor (62, 66) and an n-channel transistor (64, 68) connected in series between a supply voltage and said ground node. A memory device comprising a plurality of said memory cells (10; 50; 72) of any of claims 1 to 5, the antifuse of each of said memory cells (10; 50; 72) being programmable. A method of forming a memory device (34), comprising the steps of: providing a plurality of memory cells (10; 50; 72) each comprising a pair of cross-coupled inverters (I 1 , I 2 ; I 3 , I 4 ; I 5 , I 6 ) having first and second output nodes; providing, for each of said plurality of memory cells (10; 50; 72), a first programmable circuit (A 1 ; A 3 ; A 5 ) connected between said first output node of each of said plurality of memory cells and a ground node; programming said first programmable circuit (A 1 ; A 3 ; A 5 ) in at least one first memory cell of said plurality of memory cells (10; 50; 72) to convert said at least one first memory cell from a volatile memory cell to a non-volatile memory cell, characterized in that said first programmable circuit (A1; A3; A5) is a first antifuse, an antifuse being defined as a device which is initially an open circuit between a pair of leads, which circuit is closed upon application of a sufficient voltage between the leads of the antifuse. The method of claim 7, wherein said step of programming said first antifuse (A 1 ; A 3 ; A 5 ) comprises the step of providing a programming voltage to said first output node of said at least one first memory cell through a first bitline. The method of claim 8, further comprising the step of grounding said second output node of said at least one first memory cell through a second bitline. The method of any of claims 7 to 9, further comprising the steps of: providing, for each of said plurality of memory cells (50; 72), a second antifuse (A 4 ; A 6 ) connected between said second output node of each of said plurality of memory cells (50; 72) and said ground node; and programming said second antifuse (A 4 ; A 6 ) in at least one second memory cell of said plurality of memory cells (50; 72) by providing a programming voltage to said second output node of said at least one second memory cell to convert said memory device from a volatile memory device to a non-volatile memory device. The method of claim 10, wherein said step of programming said second antifuse (A 6 ) comprises the steps of: providing said programming voltage to said second output node of said at least one second memory cell through a first pass transistor and a first bitline; and grounding said first output node of said at least one second memory cell through a second pass transistor and a second bitline. The method of claim 10, wherein said step of programming said second antifuse comprises the steps of: grounding said first output node of said at least one transistor through a first bitline; and providing said programming voltage to said second output node of said at least one second memory cell through a voltage supply line. The method of any of claims 7 to 9, further comprising the steps of: providing a second antifuse (A 2 ) for each of said plurality of memory cells (10), said second antifuse (A 2 ) connected between said first output node of each of said plurality of memory cells (10) and a supply voltage; and programming said second antifuse (A 2 ) in at least one second memory cell by grounding said first output node and raising the supply voltage.

Inventors: HARWARD MARK G, HARWARD, MARK G.
Applicants: TEXAS INSTRUMENTS INC, TEXAS INSTRUMENTS INCORPORATED
Application year: 1995
Main classification: G11C7
IPC codes: H01L27, G11C17, H01L21, G11C7