Abstract:
A method and apparatus is disclosed for sensing the resistance state of a Programmable Conductor Random Access Memory (PCRAM) element using complementary PCRAM elements, one holding the resistance state being sensed and the other holding a complementary resistance state. A sense amplifier detects voltages discharging through the high and low resistance elements to determine the resistance state of an element being read.

Description:
FIELD OF THE INVENTION 
     The invention relates to a method and apparatus for sensing the resistance of a Programmable Conductor Random Access Memory (PCRAM) element. 
     BACKGROUND OF THE INVENTION 
     PCRAM devices store binary data as two different resistance values, one higher than the other. The resistance value represents a particular binary value of logic “0” or logic “1”. When sensing the resistance value of a PCRAM device, it is common to compare the resistance of a memory cell undergoing a read operation with resistance of a reference cell to determine the resistance value of the cell being read and thus its logic state. Such an approach is disclosed in U.S. Pat. No. 5,883,827. However, this approach has some limitations. 
     If the reference cell is defective and a column of memory cells within an array uses a same defective reference cell, the entire column of memory cells will have erroneous resistance readings. In addition, specialized circuitry is required to write a resistance value into the reference cell, and a sense amplifier circuit for such an arrangement tends to be complex and large. 
     Typically, sensing schemes for PCRAM devices also tend to have a unique architecture which is different from that normally employed in typical DRAM circuits. Although PCRAM&#39;s differ from DRAM&#39;s in that they store binary values in resistive memory elements rather than as charges on capacitors, and although PCRAM&#39;s are non-volatile, where the capacitor structures employed in DRAM&#39;s are volatile, nevertheless it would be desirable if the read and write circuits for both devices were as similar as possible so that existing DRAM memory device architectures could be easily adapted to read and write PCRAM devices. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention provides a PCRAM memory device and its method of operation which utilizes a read architecture similar to that employed in some DRAM memory devices. A pair of complementary PCRAM memory cells comprising first and second programmable conductor memory elements are employed, each connected to respective access transistors. During a write operation, the first and second memory elements are written with complementary binary values, that is: if the first memory element is written to a high resistance state, then the second memory element is written to a low resistance state; whereas if the first memory element is written to a low resistance state, the second memory element is written to a higher resistance state. 
     During a read operation of, for example, the first memory element, a sense amplifier is connected so that its respective inputs are coupled to receive respective precharge voltages which discharge through the first and second memory elements. A sense amplifier reads the discharging voltages through the two memory elements to determine which is the larger voltage, thus determining the resistance (high or low) and logic state (high or low) of the memory cell being read. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other features and advantages of the invention will become more apparent from the following detailed description of exemplary embodiments of the invention which are provided in connection with the accompanying drawings in which: 
     FIG. 1 shows an exemplary PCRAM device; 
     FIG. 2 is a schematic diagram depicting one aspect of the invention; 
     FIG. 3 is a schematic diagram depicting an additional aspect of the invention; 
     FIG. 4 is a schematic diagram depicting an additional aspect of the invention; 
     FIG. 5 shows the discharge rate characteristics of capacitors employed in the invention; 
     FIG. 6 shows the invention utilized in a computer system. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention employs a sense amplifier architecture which is somewhat similar to that employed in some conventional DRAM devices to sense the resistance states of PCRAM memory cells. In the invention, a binary value is stored as a resistance value in a first PCRAM cell while its complement resistance value is stored in a second PCRAM cell. During readout of the first PCRAM cell, both PCRAM cells are used to discharge a precharge voltage into respective inputs of a sense amplifier which reads the discharge voltages to determine the resistance and thus the binary value stored in the first PCRAM cell undergoing a read operation. 
     FIG. 1 illustrates an exemplary cell arrangement provided within a portion of a PCRAM memory device constructed in accordance with the invention. A PCRAM memory element  102  is illustrated which has a chalcogenite glass body and lower  103  and upper  104  conductors. As is well known, a programmable conductor memory element has two stable resistance states: one high resistance and one low resistance. Normally, when at rest the memory has a high resistance state, but it can be programmed to a low resistance state by suitably applying bias voltages to the conductors  103  and  104 . Typically, the low resistant state of a PCRAM memory element is characterized by a dendrite growth through the chalcogenite glass body or along the surface of the chalcogenite glass body between the conductors  103  and  104 . A high resistant state is present when there is no such dendrite growth. The grown dendrite is relatively non-volatile in that it will remain in place for a relatively long time, e.g. days or weeks, after the bias voltage is removed. 
     As further shown in FIG. 1, the PCRAM memory element  102  is coupled by a conductive plug  101  to an access transistor  207  which is driven by a word line  105  which forms the gate structure of transistor  207 . The access transistor is coupled through conductive plug  101  to one of the conductors  103  of the PCRAM memory element. The other conductor  104  of the PCRAM element is connected by a common cell plate  109  to a bias voltage, which is common to other PCRAM memory elements provided in the memory device. 
     FIG. 1 illustrates a common PCRAM architecture in which two adjacent memory cells  207 ,  211  are coupled to a common digit line  118 . Thus, FIG. 1 also shows another access transistor  211  driven by a word line  107  which is connected through conductive plug  99  to another PCRAM memory element  104 , which in turn is also connected also to the common cell plate  109 . Access transistor  211  also has one terminal connected to digit line  118 . 
     FIG. 2 shows an electrical schematic arrangement of a memory array employing the cell architecture illustrated in FIG.  1 . Thus, the top portion of FIG. 2 illustrates the transistors  207  and  211  coupled to the respective PCRAM memory elements  102  and  106  with the access transistors  207  and  211  coupling the memory elements  102  and  106  to the digit line  118 . 
     As also illustrated in FIG. 2, a complementary digit line D 1 *  120  is also provided in the memory array, to which another set of access transistors is connected which are in turn connected to other PCRAM memory elements. To simplify discussion, a single complementary pair of PCRAM cells is illustrated as  300 . It includes transistor  207  and associated PCRAM memory element  102 , which is coupled to the digit line  118  (D 1 ), and an access transistor  209  and associated PCRAM memory element  124 , which are coupled to digit line  120  (D 1 *). 
     During a write operation, a row line  104 , which is coupled to transistor  207  and a row line  113  which is coupled to transistor  209  are activated such that if PCRAM memory element  102  is written to a high resistance state, PCRAM element  124  is written to a low resistance state, and vice versa. In this way, PCRAM memory elements  102  and  124  are accessed together and always store complementary resistance digit values. Thus assuming that PCRAM memory element  102  is the primary element which is being written to and read from, a sense amplifier  210  which is coupled to the digit lines  118  and  120  will read the value of PCRAM memory element  102  by comparing a discharging precharge voltage on digit line  118  to the discharging precharge voltage on digit line  120  during a memory read operation. 
     Thus, prior to a memory read, a precharge voltage is applied to complementary digit lines  118  and  120  by a precharge circuit  301 . The precharge circuit is activated by a logic circuit on a precharge line which activates transistors  305  to supply a voltage, for example, Vcc/2, to both digit lines  118  and  120 . 
     An equilibrate circuit  303  may also be provided which is activated by an equilibrate signal after the precharge circuit is activated to ensure that the voltages on lines  118  and  120  are the same. The voltages on lines  118  and  120  are held by a parasitic capacitance of the lines. After precharge and equilibrate (if present) circuits are activated, a read operation may be conducted on the complimentary cell pair  300 . This read operation is illustrated in greater detail in FIG. 3, which is a simplification of the sense amplifier  210  input path. 
     Parasitic capacitance for the complementary digit lines  118  and  120  are illustrated as C 1  and C 1 *. The respective access transistors  207  and  209  are illustrated as connected to their respective word lines  105  and  113 . The PCRAM memory elements  102  and  124  are also illustrated. As noted, a binary value is stored, for example, in memory PCRAM memory element  102  as a resistance value. It will be either a high resistance value or a low resistance value, and the complementary resistance value will be stored in PCRAM memory element  124 . 
     During a read operation, the precharge voltage applied to the complementary digit lines  118  and  120  is allowed to discharge through the access transistors  207  and  209  and through the respective resistance values of the PCRAM memory elements  102  and  124 . Because the resistance values will be different, one high and one low, the voltages on the digit lines D 1  and D 1 * ( 118 ,  120 ) will begin to diverge during a read operation. Although the voltage initially applied to the complementary digit lines  118  and  120  is a voltage of Vcc/2, during a read operation this voltage actually is slightly higher by approximately 0.3 mV due to the presence of the parasitic capacitance C 1  and C 1 * on the digit lines  118  and  120 , as well as gate-drain capacitance inherent within transistors  207  and  209 . 
     FIG. 5 illustrates the voltages on the complementary digit lines  118  and  120  during a read operation. The activation of the word lines  105  and  113  is illustrated as a pulse signal, and initially the voltage of Vcc/2+ approximately 0.3 mV which exists on both digit lines D 1  and D 1 * begins to decay. Because one PCRAM memory element, e.g.  102 , has a higher resistance than the other, the voltage on the digit line associated with the lower resistance value, e.g.  124 , will decay faster than the voltage on the digit line coupled to the higher resistance value, e.g. D 1 . This is illustrated in FIG.  5 . 
     The divergence of the two voltages on the lines D 1  and D 1 * progressively increases. At a predetermined time after the word lines  105  and  113  are activated, the sense amplifier  210  is activated. The sense amplifier can have an architecture typically employed in a DRAM arrangement which is illustrated in FIG.  4 . Such a sense amplifier includes an Nsense amplifier latch  302  and a Psense amplifier latch  304 . This structure is illustrated in FIG.  4 . 
     Reverting back to FIG. 5, the N sense amplifier is fired first at a time t 1 . When the Nsense amplifier fires, the digit line which has the lower voltage, e.g. D 1 * in the example, is immediately pulled to ground. Thereafter, the Psense amplifier is fired at a time t 2  which drives the higher voltage line, e.g. D 1 , to Vcc. Accordingly at a time t 2 , the sense amplifier  210  outputs a value of Vcc indicating the high resistant state for the PCRAM memory element  102 . 
     Although FIG. 5 illustrates the signal timing which occurs when PCRAM memory element  102  has a higher resistance than memory element  104 , obviously the signal levels are reversed if PCRAM memory element  102  has a low resistance state and PCRAM memory element  124  has a high resistance state. That is, the signal diagrams illustrated in the FIG. 5 would have the digit line D 1 * going towards Vcc and the digit line D 1  going towards ground. 
     FIG. 5 also illustrates another aspect of the invention. As shown, the voltage for row lines  105 ,  113  increases from near ground level to a positive voltage near Vcc for a read operation. This voltage then returns to near ground level before the sense amplifier is enabled (before t 1 ). As a result, there is no rewriting of a read PCRAM memory element. If such rewriting of a PCRAM cell is desired, then the voltage on row line  105 ,  113  having a memory element which is written to a low resistance state, may be at a voltage level near Vcc during operation of the sense amplifier  210 , which will automatically rewrite (refresh) the read cell to the low resistance state. 
     Because programmable contact memory elements are resistive rather than capacitive memory elements, it is possible they will take longer to pull the digit lines up to Vcc and to ground than a typical capacitive memory element found within a DRAM. Supposing that to be true, older DRAM sense amplifier designs that run somewhat slower than the latest generation of DRAM sense amplifiers could also be used with PCRAM memory cells. The advantage of doing so would be that these older DRAM sense amplifiers have already been shown to perform effectively, and their test infrastructure is already confirmed. Consequently, a hybrid memory consisting of PCRAM memory elements using DRAM sense amplifiers can be produced having the advantages of PCRAM technology, yet being producible quickly and inexpensively. 
     Although FIG. 2 shows the complementary programmable contact memory element  102  and  106  and associated access transistors and digit lines D and D* as being provided in the same memory array, the complementary memory elements, access transistors and digit lines may also be provided in respective different memory arrays. 
     FIG. 6 is a block diagram of a processor-based system  400  utilizing a PCRAM memory device  200  constructed in accordance with one of the embodiments of the present invention. The processor-based system  400  may be a computer system, a process control system or any other system employing a processor and associated memory. The system  400  includes a central processing unit (CPU)  402 , e.g., a microprocessor, that communicates with the PCRAM memory device  408  and an I/O device  404  over a bus  420 . It must be noted that the bus  420  may be a series of buses and bridges commonly used in a processor-based system, but for convenience purposes only, the bus  420  has been illustrated as a single bus. A second I/O device  406  is illustrated, but is not necessary to practice the invention. The processor-based system  400  also includes read-only memory (ROM)  410  and may include peripheral devices such as a floppy disk drive  412  and a compact disk (CD) ROM drive  414  that also communicates with the CPU  402  over the bus  420  as is well known in the art. 
     One or more memory devices  200  may be provided on a plug-in memory module  256 , e.g. SIMM, DIMM or other plug-in memory module, for easy connection with or disconnection from the bus  420 . While the invention has been described and illustrated with reference to specific exemplary embodiments, it should be understood that many modifications and substitutions can be made without departing from the spirit and scope of the invention. Accordingly, the invention is not to be considered as limited by the foregoing description but is only limited by the scope of the appended claims.