Abstract:
A comparator-type sense amplifier compares a constant voltage that was read out of a FeRAM cell to a sequence of reference voltage levels. A multiple-comparison operation includes (a) reading out data to a bit line, (b) applying a first/next reference voltage, (c) comparing the bit line voltage to the applied reference voltage, and (d) repeating steps (b) and (c) one or more times. The multiple comparison operation can be used to characterize operation of an FeRAM cell, predict or detect an FeRAM cell that may introduce a bit error, or to read a multi-bit value from an FeRAM cell.

Description:
BACKGROUND 
     A conventional FeRAM has memory cells containing ferroelectric capacitors. Each ferroelectric capacitor contains a ferroelectric material sandwiched between conductive plates. To store data in a memory cell, a write operation applies write voltages to the plates of the ferroelectric capacitor to polarize the ferroelectric material in a direction associated with the data bit being written. A persistent polarization remains in the ferroelectric material after the write voltages are removed, which in turn maintains charge on the conductive plates. 
     A conventional read operation for a FeRAM cell connects one plate of a ferroelectric capacitor to a bit line and raises the other plate to a read voltage. If the persistent polarization in the ferroelectric capacitor is in a direction corresponding to the read voltage, the read voltage causes a relatively small current through the ferroelectric capacitor, resulting in a small voltage change on the bit line. If the persistent polarization initially opposes the read voltage, the read voltage flips the direction of the persistent polarization, discharging the plates and resulting in a relatively large current and voltage increase on the bit line. A sense amplifier can sense the stored value from the resulting bit line current or voltage. 
     FIG. 1A illustrates a portion of a conventional FeRAM  100  that includes memory cells  110  arranged in rows and columns to form a memory array. Only one column and two rows of memory cells  110  are shown in FIG. 1A for simplicity of illustration, but a typical FeRAM array may include hundreds or thousands of columns of memory cells with a similar number of rows. Each memory cell  110  of FeRAM  100  includes a ferroelectric capacitor  112  and a select transistor  114 . Each select transistor  114  has a gate connected to a word line  116  corresponding to the row containing the memory cell and a source/drain connected to a bit line  120  corresponding to the column containing the memory cell. 
     A conventional read operation accessing a selected memory cell  110  in FeRAM  100  biases a plate of the selected memory cell to a plate voltage Vp (e.g., 3 V), and activates a selected word line  116  to turn on a select transistor  114  thereby electrically connecting the selected ferroelectric capacitor to bit line  120 . The difference between the plate voltage and the initial bit line voltage forces the persistent polarization in the selected ferroelectric capacitor into a first state. Bit line  120  acquires a voltage that depends on the initial polarization state of the selected memory cell  110 . In particular, if the selected memory cell was in a second state having a persistent polarization in a direction opposite to the persistent polarization of the first state, forcing the memory cell from the second state into the first state causes a relatively large current to bit line  120  and a corresponding rise in the bit line voltage. If the selected memory cell was already in the first state, a relatively small current flows to bit line  120 . 
     A sense amplifier  130  connected to the bit line  120  compares the bit line voltage to a reference voltage Vref. A reference voltage generator (not shown) can generate reference voltage Vref at a level that is above the bit line voltage read out when the selected memory cell  110  has the first polarization state and below the bit line voltage read out when the selected memory cell  110  has the second polarization state. In sense amplifier  130 , cross-coupled transistors drive bit line  120  to a logic level (high or low) depending on whether the bit line voltage was greater or less than reference voltage Vref. A bit thus read has a value indicated by the voltage on the bit line after operation of the sense amplifier. 
     FIG. 1B illustrates an alternative memory  100 ′ in which each memory cell  110 ′ includes two ferroelectric capacitors  112  and  112 ′ connected through respective select transistors  114  and  114 ′ to respective bit lines  120  and  120 ′. A write operation forces ferroelectric capacitor  112 ′ to a polarization state that is complimentary to the polarization state of ferroelectric capacitor  112 . A read operation applies plate voltage Vp to both ferroelectric capacitors  112  and  112 ′ and activates a selected word line  116  to turn on select transistors  114  and  114 ′ and electrically connect selected ferroelectric capacitors  112  and  112 ′ to bit lines  120  and  120 ′, respectively. The read operation thus forces both ferroelectric capacitors  112  and  112 ′ to the first polarization state. The bit line  120  or  120 ′ connected to the ferroelectric capacitor  112  or  112 ′ initially in the second polarization state rises to a higher voltage. Sense amplifier  130  drives the bit lines  120  and  120 ′ connected to the memory cell initial in the second polarization state to complementary voltage, where the voltage on bit line  120  indicates the data bit read from the memory cell. 
     A read operation for FeRAM cell  110  or  110 ′ of FIG. 1A or  1 B generally requires a write-back operation to restore a persistent polarization of a ferroelectric capacitor to the second state if the read operation forced the ferroelectric capacitor from the second state to the first state. In FeRAMs like  100  and  100 ′, sense amplifier  130  drives the bit lines  120  and  120 ′ to voltage suitable for the write-back operations. However, the driven voltage can interfere with uses of FeRAM that may require comparing a read-out bit line voltage to multiple different reference voltages. For example, an on-chip bit failure prediction, detection, and correction method might need to compare the read-out voltage from a memory cell to a series of reference voltages to determine whether the polarization state of the memory cell provides a bit line voltage large enough for an accurate read operation. 
     Using conventional read operations for comparisons to multiple reference voltages requires repeating the steps of reading a voltage out of a selected memory cell  110  to a bit line and sense amplifier, applying the first or next reference voltage to the sense amplifier, and comparing the read-out voltage to the applied reference voltage. Repetition of these operations is generally too slow for on-chip bit failure correction techniques. Additionally, write-back operations and time dependent failure mechanisms in ferroelectric materials make the charge delivered to a bit line or the voltage read out from a FeRAM cell vary from access to access, particularly because the polarization state of the FeRAM cell is refreshed between comparisons. The comparisons to different reference voltages thus may be inconsistent. 
     In view of the limitations to current read processes for FeRAM, improved processes and circuits for performing multiple comparisons are desired. 
     SUMMARY 
     In accordance with an aspect of the invention, a read out voltage from a ferroelectric capacitor is compared to multiple reference voltages using a sense amplifier that does not disturb the read out voltage on a bit line. Accordingly, a fast series of comparisons can be performed to characterize the performance of an FeRAM cell, to anticipate or detect a bit error, or to read a multi-bit or multi-level value from a single ferroelectric capacitor. 
     The multiple-comparison operation includes reading out a voltage to a bit line that is otherwise floating and is coupled to a gate of a transistor in a sense amplifier. The read out voltage can be maintained while a series of reference voltages are applied to the sense amplifier for a series of sensing or comparison operations. When the series of operations is complete, a write-back operation restores the polarization state in the selected FeRAM cell. 
     One specific embodiment of the invention is a device including a bit line connected to FeRAM cells, a reference voltage generator capable of generating a series of voltage levels, and a sense amplifier connected to the bit line and the reference voltage generator. The sense amplifier, which can be a comparator-type sense amplifier, is capable of comparing the voltage on the bit line to each of the series of voltage levels without changing the voltage on the bit line. The device may further include an error detection circuit that predicts, detects, or corrects bit errors based on signals indicating results of comparing the voltage on the bit line to the series of voltage level. Alternatively, the FeRAM cell stores a multi-bit value, and the results of multiple comparisons of the voltage on the bit line to the series of voltage levels indicate the multi-bit value. 
     Another embodiment of the invention is a process including setting a voltage on a bit line according to a polarization state of an FeRAM cell and comparing the voltage on the bit line to each of multiple reference voltages, while keeping the voltage on the bit line constant throughout multiple comparisons. Applying a first voltage to a first plate of a ferroelectric capacitor in the FeRAM cell and connecting a second plate of the ferroelectric capacitor to the bit line while the bit line is floating can set the voltage on the bit line. The voltage on the bit line generally depends on an amount of current that flows through the ferroelectric capacitor. After completion of comparing the voltage on the bit line to each of the multiple reference voltages, writing back a data value read from the FeRAM cell can restore the polarization state of the FeRAM cell. Results from comparing the voltage on the bit line to each of the multiple reference voltages can characterize the operation of the FeRAM cell, indicate whether the FeRAM cell is operating properly, or indicate a multi-bit value stored in the FeRAM cell. 
     Yet another embodiment of the invention is a multiple-comparison operation including: (a) reading data out of a FeRAM cell to establish a bit line voltage on a first input node of a sense amplifier; (b) applying a first/next reference voltage to a second input node of the sense amplifier; (c) comparing the bit line voltage to the applied reference voltage, and (d) repeating steps (b) and (c) one or more times while keeping voltage on the first input of the sense amplifier constant. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1A and 1B are circuit diagrams of portions of FeRAM devices used in convectional read operations. 
     FIG. 2 is a circuit diagram of a portion of an FeRAM suitable for a multiple-comparison operation in accordance with an embodiment of the invention. 
     FIG. 3 contains timing diagrams indicating signal levels in the FeRAM of FIG. 2 multiple-comparison operation in accordance with an embodiment of the present invention. 
     Use of the same reference symbols in different figures indicates similar or items. 
    
    
     DETAILED DESCRIPTION 
     In accordance with an aspect of the invention, an access to an FeRAM cell preserves the voltage read out to a bit line while multiple reference voltages are compared to age read from the FeRAM cell. Accordingly, the multiple comparisons are quick to make an on-chip bit error detection and correction process practical. Further, the comparison operation can characterize the performance of an FeRAM cell when he design of an FeRAM, and FeRAM devices that store multiple bits or levels of ion in a single FeRAM cell can use a multiple-comparison operation to quickly read it value from an FeRAM cell. 
     FIG. 2 shows a portion of an FeRAM  200  capable of implementing a multiple-comparison operation in accordance with an embodiment of the present invention. FeRAM  200  contains a conventional array of FeRAM cells  110 , which are organized into rows and columns. Each FeRAM cell  110  includes a ferroelectric capacitor  112  and a select transistor  114 , which can be fabricated using conventional techniques. Bit lines  120  (only one of which is shown in FIG. 2) connect to select transistors of FeRAM cells  110  in respective columns of the memory array. Word lines  116  connect to the gates of select transistors  114  in respective rows of the memory array. 
     In the illustrated embodiment, sense amplifier  230  is a comparator-type sense amplifier having a separate write-back circuit  240 . As illustrated in FIG. 2, sense amplifier  230  includes p-channel transistors MP 1 , MP 2 , MP 3 , MP 4 , and MP 5  and n-channel transistors MN 1 , MN 2 , MN 3 , and MN 4 . Transistor MP 1 , which serves to activate and deactivate sense amplifier  230  in response to an enable signal/EN, is between a supply voltage VDD and transistors MP 2  and MP 3 . Transistors MP 2 , MP 4 , and MN 1  are connected in series between transistor MP 1  and ground, and transistors MP 3 , MP 5 , and MN 2  are similarly connected in series between transistor MP 1  and ground. Transistors MN 3  and MN 4  are connected in parallel with transistors MN 1  and MN 2 , respectively, and respond to enable signal/EN by grounding respective nodes N 1  and N 2  in preparation for comparison operations. 
     The gates of transistors MP 1  and MP 2  are connected to receive input signals respectively from bit line  120  and a reference voltage generator  250  during a multiple-comparison operation. Reference voltage generator  250  can be any circuit capable of generating a reference voltage Vref having a series of different voltage levels, which are to be compared to the bit line voltage. A voltage difference between the bit line voltage and the reference voltage Vref determines whether transistor MP 2  or MP 3  is more conductive, which in turn influences whether the voltage on node N 1  between transistors MP 5  and MN 2  or the voltage on node N 2  between transistors MP 4  and MN 1  rises more quickly when sense amplifier  240  is activated. 
     The gates of transistors MP 4 , MP 5 , MN 1 , and MN 2  are cross-coupled, so that transistors MP 4 , MP 5 , MN 1 , and MN 2  amplify a voltage difference between node N 1  and node N 2 . As a result, an output signal OUT from node N 1  is complementary to an output signal/OUT from node N 2 . 
     A data output circuit  260  receives complementary output signals OUT and /OUT from sense amplifier  230  and holds comparison results from the comparisons of the bit line voltage read to the series of reference voltage levels. As described further below, the comparison values indicate the data value read from the selected FeRAM cell  110 . Typically, data output circuit  260  is a storage circuit such as a latch, flip-flop, or buffer for storage of binary data indicating comparison results. 
     Write-back circuit  240  is connected to a data output circuit  260 . In FIG. 2, write-back circuit  240  is a tri-state inverter that drives bit line  120  in response to a write-back signal WB and a signal from data output circuit  260  indicating the value to be written back. Write-back circuit  240  thus drives bit line  120  to an appropriate voltage for writing a value from data output circuit  260  to the selected FeRAM cell. Plate voltage Vp is typically at an intermediate level (e.g., about 3 volts) for the write-back operation. 
     FIG. 3 shows timing diagrams for selected signals in memory  200  of FIG. 2 during a multiple-comparison operation in accordance with an embodiment of the invention. To prepare sense amplifier  230  for the operation, active-low enable signal/EN is initially high causing transistor MP 1  to shut off power in sense amplifier  230  and transistors MN 3  and MN 4  to ground nodes N 1  and N 2 . 
     To begin the multi-comparison operation, the plate voltage Vp is raised to a read level (e.g., 3 V) and a row decoder (not shown) activates a select signal (e.g., signal SEL 0 ). The activated select signal turns on a select transistor  114  and electrically connects the selected ferroelectric capacitor  112  to bit line  120 . The difference between the plate voltage Vp and the initial bit line voltage (e.g., ground) forces the selected ferroelectric capacitor  112  into a polarization state with polarization corresponding to the voltage difference between the plates of the selected ferroelectric capacitor  112 . If the selected ferroelectric capacitor was already in the polarization state corresponding to the read voltage difference, bit line  120 , which is floating, rises to a voltage V 0 . If the selected ferroelectric capacitor was initially in the polarization state opposing the read voltage difference, bit line  120  rises to a higher voltage V 1 . The process of reading out a voltage V 0  or V 1  from the selected ferroelectric capacitor  112  typically takes about 10 to 30 ns. 
     Reference voltage Vref is applied to the gate of transistor MP 3  at a first level VR 1  when the bit line voltage is applied to the gate of transistor MP 2 . When enable signal/EN is activated (drops low), sense amplifier  230  performs a first comparison of the bit line voltage read out of the selected FeRAM cell to reference voltage Vref. The comparison operation can typically be performed in about 5 to 10 ns. The first comparison result indicates whether the bit line voltage is greater or less than the first level VR 1 , and complementary output signals OUT and /OUT indicate a first binary value representing the comparison result, which can be temporarily stored in data output circuit  260 . 
     Without changing the bit line voltage, the multi-comparison operation deactivates enable signal/EN and changes the reference voltage Vref to the next level VR 2  before reactivating enable signal/EN for a second comparison. The second comparison result indicates whether the bit line voltage is greater or less than the second level VR 2 , and data output circuit  260  temporarily stores a second binary value indicating the result of the second comparison. 
     Further comparisons can be conducted in the same fashion. In particular, the multi-comparison operation deactivates enable signal/EN (high) and changes the reference voltage to the next level without changing the bit line voltage and then reactivates enable signal/EN for the next comparison. Each comparison indicates whether the bit line voltage is greater or less than a corresponding level of reference voltage Vref, and data output circuit  260  temporarily stores binary values indicating the result of the comparisons. FIG. 3 illustrates the example of a process performing three comparisons, but more generally, a multiple-comparison operation can perform two or more comparisons. 
     After the comparisons to all of the desired reference voltage levels VR 1 , VR 2 , . . . , the enable signal/EN is deactivated (high), and write-back signal WB is activated (high). A signal from data output circuit  260  controls the voltage that write-back circuit  240  drives on bit line  120  so that the original data value is rewritten to the selected FeRAM cell  110 . The multi-comparison operation thus performs a single write-back operation for multiple comparison operations, which is much faster than performing write-back operations after each comparison. 
     The comparison values from data output circuit  260  can also be used for a variety of purposes in different applications. An on-chip bit error detection circuit  270  can use the comparison values to determine whether accessing the selected FeRAM cell provides a bit line voltage that is high enough for a reliable read operation. Reference voltage level VR 1  can be, for example, the desired bit line voltage V 1  for a FeRAM having its polarization state changed during a read operation, while the other reference voltage levels VR 2  and VR 3  are lower than voltage V 1  but higher than the voltage V 0  that results when the access does not change the polarization state of the selected FeRAM cell. A multiple-comparison operation that finds a bit line voltage lower than level VR 1  but higher than level VR 2  or VR 3  indicates that the selected FeRAM cell was storing a data value corresponding to the polarization state flipped but did not provide the expected bit line voltage. An error signal could then be generated, or the FeRAM cell could be replaced with a redundant FeRAM cell to prevent data errors. FIG. 2 shows an error detection block  270  representing a peripheral circuit that detects errors from the comparison results from data output circuit  260 . 
     Another use of the multiple-comparison operation is for storage of more than a binary value in an FeRAM cell. If memory  200  had polarization states of different polarization magnitudes and corresponding to different data values, the levels VR 1 , VR 2 , . . . of reference voltage Vref can be the boundaries of ranges for bit line voltages corresponding to the different polarization states. The comparison results from data output circuit  260  indicate a voltage range for the bit line voltage read out of the selected FeRAM cell and therefore indicate the data value stored in the selected FeRAM cell. 
     The multiple-comparison operations as described above allow on-chip bit failure prediction, detection, and correction and provide a very fast (up to 40 times faster than convention techniques) and efficient way to capture charge distributions. Additionally, the multiple-comparison operation reduces or eliminates fatigue and imprint problems that may arise from repeated read and write-back operations. 
     Although the invention has been described with reference to particular embodiments, the description is only an example of the invention&#39;s application and should not be taken as a limitation. Various adaptations and combinations of features of the embodiments disclosed are within the scope of the invention as defined by the following claims.