Abstract:
A data retention monitor for a memory cell including a voltage source and a voltage comparator. The voltage source is adapted to provide a selectable voltage to the memory cell. The selectable voltage includes a read voltage and a test voltage, with the test voltage being greater than the read voltage. The voltage comparator is adapted to compare a voltage of the memory cell with a reference voltage after the provision of the selectable voltage to the memory cell. The memory cell retains data when the memory cell voltage generated at least in part by the test voltage is substantially equal to the reference voltage.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates generally to semiconductor memories, and more particularly to a system and method for monitoring memory content of a memory cell. 
     One type of memory utilizes a phase change material that may be, in one application, electrically switched between amorphous and crystalline states, or between different detectable states across the entire spectrum between completely amorphous and completely crystalline states. 
     The operation of a phase change memory (PCM) is based on a resistance change caused by the different states of the phase change material. Typical materials suitable for the phase change material include, but are not limited to, various chalcogenide glasses, such as GeSbTe. The advantages of phase change memories include high write throughput and single-bit random write access. 
     When the memory is set in either a crystalline, semi-crystalline, amorphous, or semi-amorphous state having a respective resistance value, that state is retained until reprogrammed, even if power is removed. The state is retained because the programmed resistance represents a phase or physical state of the material (e.g., crystalline or amorphous). The state of the phase change materials is thus generally non-volatile. 
     A first low resistive state, i.e. crystalline state or semi-crystalline state, is achieved by a small power being applied to the memory compared to the second state, i.e. amorphous state or semi-amorphous state, where a high power pulse is used to melt and quench the material into an amorphous state. It is possible to program the cell into mid-level states by application of medium power levels. 
     SUMMARY OF THE INVENTION 
     A data retention monitor for a memory cell including a voltage source and a voltage comparator. The voltage source is adapted to provide a selectable voltage to the memory cell. The selectable voltage includes a read voltage and a test voltage, with the test voltage being greater than the read voltage. The voltage comparator is adapted to compare a voltage of the memory cell with a reference voltage after the provision of the selectable voltage to the memory cell. The memory cell retains data when the memory cell voltage generated at least in part by the test voltage is substantially equal to the reference voltage. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of a data retention monitor in accordance with an embodiment of the invention; 
         FIG. 2  is a flow chart showing a write verification test in accordance with an embodiment of the invention; 
         FIG. 3  is a flow chart showing a background memory test in accordance with an embodiment of the invention; and 
         FIG. 4  is a flow chart showing a foreground memory test in accordance with an embodiment of the invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A. Overview 
     Phase change memory cells are typically read without triggering a state change of the memory cell. A concern for phase change memory is read disturb and data loss for memory cells that are in the amorphous state due to crystallization of the phase change material at elevated temperatures. Phase change occurs when the threshold current at the threshold voltage of the memory cell is exceeded. As a result, the read current may be limited to avoid triggering. However, limiting the read current to less than the threshold current reduces performance. Use of a higher current while limiting the applied voltage to be less than the threshold voltage results in the memory cell being sensitive to variations in the threshold voltage. Such an approach may reduce margin and inadvertently trigger the memory cell, possibly causing the cell to change (read disturb) or to be misread (non-repeating “soft error”). In order to reduce read disturb in PCMs, a PCM cell should be rewritten if it is in the process of shifting from the amorphous state to the crystalline state, or if it is shifting between semicrystalline states. 
     In a typical PCM, the resistance of each PCM cell varies based on its state, generally ranging from 100 KΩ in the crystalline state to 1 MΩ in the amorphous state. The number of discrete resistance values assigned to each PCM cell determines the number of states a given PCM cell has. As the resistance of the PCM cell varies, a voltage on a data line caused by the read voltage for the PCM cell varies. During increased ambient temperature the resistance of the PCM cell is reduced to an amorphous state, resulting in an increased current through the PCM cell; correspondingly, temperature of the phase change material increases, thereby enhancing crystallization. Therefore, using a greater read voltage accelerates read disturb. A PCM cell that is close to losing its stored information may be detected by reading data from the PCM cell with a standard voltage, measuring the data line voltage, and subsequently comparing the data line voltage at a greater read voltage. Alternatively, such a PCM cell may be detected by reading the data with a greater read voltage and comparing the voltage on the data line with a reference voltage. 
     B. Data Retention Monitor 
       FIG. 1  shows a data retention monitor  100  according to an embodiment of the invention. As shown, data retention monitor  100  includes a memory cell  140 . While only a single memory cell  140  is shown, data retention monitor  100  can be applied to a memory array of any size. To read data in memory cell  140 , a read voltage  145  of voltage source  180  is applied to data line  135  via switch  130 , clamp  120 , and switch  160 . A voltage measured at node  170  represents the data stored in memory cell  140 . Clamp  120  has two inputs, a first input being the read voltage, a second input being a voltage from the data line  135  via feedback  125 . In an embodiment, the feedback voltage is typically 450 mV to 500 mV. As the temperature of the device increases, either due to operation or the environment, the voltage needed to read the data in memory cell  140  decreases due to the decreasing resistance of the memory cell  140 . 
     Memory cell  140  is programmable into one of at least two memory states by applying an electrical signal to data line  135 . The electrical signal alters the phase of the memory material between a substantially crystalline state and a substantially amorphous state. As noted above, the electrical resistance of the memory cell  140  in the substantially amorphous state is greater than the resistance of the memory cell in the substantially crystalline state. 
     B1. Write Data 
     To write data to memory cell  140 , the phase of the memory cell  140  may be altered by the current source  110  providing a high current that results in a heating of the memory cell  140 . In some applications, the high current is approximately 2 mA. The current melts at least a part of the phase change material and, when the charge is removed, the temperature drops quickly and the melted portion of the material solidifies in the amorphous state having a higher resistance. 
     Alternatively, data may be written to the memory cell  140  by the current source  110  applying a low current that energizes the phase change material so that the phase change material is allowed to arrange itself into a more crystalline state having a lower resistance. In some applications, the low current is approximately 1.2 mA. 
     It should be noted that heat alone tends to alter the state of the memory cell. 
     B2. Weak Cell Test 
     To verify the integrity of memory cell  140 , data retention monitor  100  performs a weak cell test. The weak cell test determines if a lower than expected resistance is present in the memory cell  140  under test. If a lower resistance is present, a higher current will flow through the cell and the cell will fail the weak cell test. Before a weak cell test is performed, the content (state) of memory cell  140  is determined. In an embodiment, the current state of memory cell  140  is used to set the value of Vref  165 . 
     To perform the weak cell test, switch  130  selects weak cell filter voltage  155  from voltage source  180  and applies it to data line  135  via clamp  120  and switch  160 . The weak cell filter voltage  155  is higher than the read voltage  145  in order to stress the memory cell  140 . A comparator  150  compares the voltage on the data line  135  at node  170  with a reference voltage Vref  165 . If the voltage at node  170  is substantially equal to reference voltage Vref  165 , the memory cell  140  passes the weak cell test and does not need to be rewritten. Conversely, when the voltage at node  170  is not substantially equal to reference voltage Vref  165 , the memory cell  140  fails the weak cell test and needs to be rewritten in the manner described in section B1 above. It should be noted that different values of Vref are used for different memory states. 
     Alternatively, the weak cell test may be performed by using the voltage at node  170  during the read operation as the reference voltage Vref  165 . This voltage initially read from the cell  140  is compared with the voltage at node  170  due to weak cell filter voltage  155 . A rewrite is required if the two voltages differ by at least a predetermined amount. 
     The read, write, and verification parameters vary with environmental conditions. In an embodiment, at 85° C., a breakdown voltage of memory cell  140  is about 1V, the read voltage  145  is typically within an approximate range of 50 mV to 800 mV, and the weak cell filter voltage  155  is typically within an approximate range of 50 mV to 1V greater than read voltage  145 . As the temperature of memory cell  140  increases, lower read and weak cell voltages are needed. 
     The weak cell test is an aggressive readout that destroys the data in memory cell  140  if the memory cell  140  is close to failing. Therefore, prior to performing a weak cell test, the data is either read from memory cell  140  or recently written into memory cell  140  so the data is known. During the weak cell test, a normal cell will retain the semiamorphous or amorphous state. If a memory cell fails the weak cell test, the state of the memory cell becomes more crystalline. In an embodiment, the weak cells are tracked so that if a memory cell has a higher than normal failure rate, that cell can be skipped in future write steps. 
     At higher temperatures, a memory cell  140  is more prone to failure as the higher temperatures tend to cause crystallization of the PCM material. The higher current provided to the PCM during the weak cell test mimics a high temperature operation of the PCM. This test ensures that the PCM will operate at all desired temperatures. Additionally, by tracking cells that are more prone to failure, those cells can be avoided. 
     C. Operational Tests 
     There are three operational tests for the data retention monitor  100  of  FIG. 1 , a write verification test, a background memory test, and a foreground memory test, as will be explained below with respect to  FIGS. 2-4 . In a typical application, a plurality of memory cells are arranged in a array. 
     C1. Write Verification Test 
     A write verification test  200  in accordance with an embodiment of the invention is shown in  FIG. 2 . A write verification test involves performing a weak cell test to verify the content of the memory cell after data is written to a memory cell. The data written to the memory cell is used to determine Vref. 
     The write verification test begins by selecting a memory cell  140 , which includes a phase change material (S 210 ). After the memory cell  140  is selected, data is written to the selected memory cell  140  (S 220 ) by supplying a current from current source  110 , as described above in section B1. After the data is written to the memory cell  140  (S 220 ), a weak cell test is performed (S 240 ) in the manner described above in section B2. If the memory cell  140  passes the weak cell test (S 250 ), the process repeats with selection of the next memory cell  140  in the array (S 210 ). If the memory cell  140  fails the weak cell test (S 250 ), on the other hand, the data is first rewritten to the memory cell  140  (S 260 ), as described above in section B1, and then the process repeats with selection of the next memory cell  140  in the array (S 210 ). 
     Optionally, to verify that the data has been written to memory cell  140 , the data can be read from memory cell  140  (S 230 ) and saved in another location in case the data in the memory cell is destroyed during the weak cell test. To read the data, if required, a read voltage  145  is selected from voltage source  180  by switch  130  and applied to data line  135  via clamp  120  and switch  160 . A voltage at node  170  is representative of the data in memory cell  140 . This data can then be saved to another location. It should be noted that the step of reading data from the memory cell (S 230 ) is not required in all embodiments because the data written into the memory (S 220 ) is typically still available to be rewritten. 
     C2. Background Memory Test 
       FIG. 3  is a flow chart showing a background memory cell test  300  in accordance with an embodiment of the invention. A background memory test is a test in which a maintenance check is performed in a memory array independent of read or write operations. 
     The background memory test  300  begins by selecting a memory cell  140 , which includes a phase change material (S 310 ). After the memory cell  140  is selected, it is determined whether the memory cell  140  is in use, i.e., being read from or written to (S 320 ). In an embodiment, the system makes this determination by analyzing the state of switch  160 ; if the switch  160  is turned on, the memory cell  140  is in use, and if the switch  160  is not turned on, the memory cell  140  is not in use. 
     If the memory cell  140  is in use, the next memory cell  140  in the memory cell array is selected (S 310 ) so as to not interfere with the normal operation of the memory cell  140 . On the other hand, if the memory cell  140  is not in use, data is read from the memory cell  140  (S 330 ). To read the data, a read voltage  145  is selected from voltage source  180  by switch  130  and applied to data line  135  via clamp  120  and switch  160 . A voltage at node  170  is representative of the data in memory cell  140 . The data read from memory cell  140  is used to determine the value used for Vref. 
     Next, a weak cell test is performed (S 340 ) in the manner described above in section B2. If the memory cell  140  passes the weak cell test (S 350 ), the process is repeated by selecting a next memory cell  140  in the array to test (S 310 ). Alternatively, if the memory cell  140  fails the weak cell test, the data is rewritten to the memory cell  140  (S 360 ) in the manner described above in section B1, and after rewriting the data, the process is repeated by selecting the next memory cell  140  in the array (S 310 ). 
     C3. Foreground Memory Test 
       FIG. 4  depicts a foreground memory test  400  in accordance with an embodiment of the invention. A foreground memory test is a test in which a weak cell test to verify memory cell content is performed with a memory cell is read. 
     The foreground memory test  400  begins by selecting a memory cell  140 , which includes a phase change material (S 410 ). Next, data is read from the memory cell  140  (S 420 ). In an embodiment, the data is temporarily stored to another location. To read the data a read voltage  145  is selected from voltage source  180  by switch  130  and applied to data line  135  via clamp  120  and switch  160 . A voltage at node  170  is representative of the data in memory cell  140 . The data read from memory cell  140  is used to determine the value used for Vref. The system then performs a weak cell test (S 430 ) as described above in section B2. If the cell passes the weak cell test (S 440 ), the next memory cell  140  is selected for a read operation (S 410 ). If the memory cell  140  fails the weak cell test, the data that was read is rewritten to the memory cell  140  (S 450 ), as described above in section B1, and then the next memory cell  140  is selected (S 410 ). 
     It should be noted that in operation, the system can switch between the operating modes. The system can switch between reading and writing to memory cells and verifying those cells. Additionally, it should be noted that in an embodiment, different cells in an array can be simultaneously read, written to, or verified. In an embodiment of the invention, after a cell fails the weak cell test, the repair step of rewriting the data may be scheduled to be performed at a later time or may be repeated several times if necessary to restore the contents of a weak cell. 
     PCMs function in all situations where a non-volatile memory is used including cell phones, SIM cards, data storage medium, ID cards, access control cards, charge cards, hotel access cards, passports, metro cards, and the like. In these environments, the PCM must operate in an approximately 85° C. environment. In other situations such as automobile alarms, a PCM is expected to function in 150° C. or higher environments. Automotive applications include alarms, fuel injection controls, and the like. In modern automobiles, portions of the electronics for the alarm are mounted in the oil pan. In this way, a thief is unable to access the control portion or memory portion of the alarm to disable it. However, the extreme conditions in the engine may deteriorate the PCM. Data loss at elevated temperatures is mostly attributed to gradual crystallization. Hence, the above-described approach may be used to filter those cells. Therefore, the weak cell test ensures that data is not lost in these extreme conditions. 
     Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that a variety of alternate and/or equivalent implementations may be substituted for the specific embodiments shown and described without departing from the scope of the present invention. This application is intended to cover any adaptations or variations of the specific embodiments discussed herein. Therefore, it is intended that this invention be limited only by the claims and the equivalents thereof.