Abstract:
A test circuit for testing differential read signals during a memory access is disclosed. The test circuit is coupled to a pair of bitlines. During a read access, a selected memory cell produces a differential read signal on the bitlines. When the test circuit is activated, the magnitude of the differential read signal is varied. This enables easy testing of read signal margins in, for example, memory ICs.

Description:
BACKGROUND OF INVENTION  
         [0001]    [0001]FIG. 1 shows a conventional 2T2C ferroelectric random access memory cell  105 . The memory cell comprises first and second ferroelectric capacitors  140   a  and  140   b , each having a ferroelectric material, such as lead zirconate titanate (PZT), located between first and second plates. First plates of the capacitors are commonly coupled to a plateline  170  while second plates or the capacitors are coupled to respective bitlines  150   a  and  150   b  of a bitline pair via cell transistors  130   a  and  130   b . The gates of the transistors of a memory cell are commonly coupled to a wordline  160 . The bitline pair includes additional memory cells, forming a column of memory cells.  
           [0002]    A sense amplifier having inverting and non-inverting terminals is coupled to one end of the bitline pair. One bitline (BL) is coupled to the non-inverting terminal of the sense amplifier while the other bitline (/BL) is coupled to the inverting terminal. The other end of the bitlines of the bitline pair is coupled to ground. Each bitline has a biltine capacitance. The bitline capacitance usually originates from parasitic capacitances caused by, for example, wire to wire coupling or junction capacitance. In some applications, a capacitor may be coupled to the bitline to provide the bitline with the desired bitline capacitance value. The capacitor, for example, can be formed by a gate oxide capacitance. The bitline capacitance is needed for the cell capacitor to produce a read signal on the bitline. For a ferroelectric capacitor, the magnitude of the read signal depends on the polarization direction of the ferroelectric material. For example, a first polarization direction produces a read signal equal to a first voltage level (e.g., V LO ) while the other direction produces a read signal equal to a second voltage level (e.g., V HI )  
           [0003]    The two capacitors of a 2T2C memory cell are always in the opposite state. One bitline will have a read signal equal to V LO  and the other V HI  when a memory cell is read. The two signals produce-a differential read signal (e.g., difference between V LO  and V HI ). Depending on whether the differential signal is positive or negative, a logic 1 or logic 0 is stored in the cell. By storing the bit of information in opposite states in two capacitors, the two read signals from a cell are compared with each other. This eliminates the need of a reference voltage to perform a read. The absence of a reference voltage, however, makes it difficult to vary the sensing window (e.g., difference between V LO  and V HI ) for performing signal margin tests during reliability testing of the IC. From the foregoing discussion, it is desirable to provide signal margin test circuit for 2T2C memory ICs.  
         SUMMARY OF INVENTION  
         [0004]    The invention relates generally to ICs and more particularly, to a test circuit which varies the read signals on bitlines in ICs with memory cells to perform read signal margin test. In one embodiment, the IC comprises first and second bitlines coupled to a sense amplifier. A plurality of memory cells are coupled to bitlines. During a read access, a selected memory cell produces a differential read signal on the bitlines for sensing by the sense amplifier. In accordance with the invention, a test circuit is coupled to the bitlines. The test circuit, when activated during test mode, varies the magnitude of the differential read signal. 
       
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0005]    [0005]FIG. 1 shows a conventional 2T2C ferroelectric memory cell;  
         [0006]    [0006]FIG. 2 shows an embodiment of the invention;  
         [0007]    [0007]FIG. 3 shows a timing diagram of a test read access in test in accordance with one embodiment of the invention;  
         [0008]    [0008]FIG. 4 shows an alternative embodiment of the invention; and  
         [0009]    [0009]FIG. 5 shows a timing diagram of a test read access in test in accordance with another embodiment of the invention. 
     
    
     DETAILED DESCRIPTION  
       [0010]    [0010]FIG. 2 shows a column of 2T2C memory cells in accordance with one embodiment of the invention. As shown, the column includes a pair of complementary bitlines having first and second bitlines  150   a  and  150   b . A sense amplifier  180  is coupled to first ends of the bitlines. In one embodiment, the bitline coupled to the non-inverting terminal of the sense amplifier is referred to as the bitline true (BL). The other bitline, which is coupled to the inverting terminal of the sense amplifier, is referred to as bitline complement (/BL).  
         [0011]    The bitline pair includes a plurality of 2T2C memory cells  105 , such as those described in FIG. 1. The 2T2C memory cells, in one embodiment, are ferroelectric memory cells. Other types of 2T2C memory cells can also be used. A 2T2C ferroelectric cell includes first and second cell transistors  130   a  and  130   b . In one embodiment, the first transistor is coupled to BL and a first ferroelectric capacitor  140   a . The second transistor is coupled to /BL and a second ferroelectric capacitor  140   b . A wordline  160  is coupled to the gates of the cell transistors and a plateline  170  is coupled to the capacitors.  
         [0012]    Second ends of the bitlines are coupled to ground. As previously discussed, each bitline includes a bitline capacitance C BL . The capacitance of each bitline should be closely matched with each other. Furthermore, depending on the application, this biltine capacitance can be augmented by bitline capacitors  195 . As shown, the capacitors are located near the second ends of the bitlines. It is understood that the capacitors can be located anywhere along the bitlines. For purposes of discussion, C BL  is the total capacitance on the bitline (e.g., parasitic bitline capacitance and capacitance from capacitor  195 , if present). The value of the read signal on a bitline is equal to C cap /C BL , where C cap  is equal to the effective capacitance of the capacitor and C BL  is the bitline capacitance. For a ferroelectric capacitor, C cap  depends on the polarization of the ferroelectric material of the capacitor.  
         [0013]    In accordance with the invention, a reduced signal test (RST) circuit  220  is coupled to the bitline pair. The RST circuit facilitates signal margin testing of the IC by varying the sensing window. This is achieved by manipulating the differential read signal. In one embodiment, the differential read signal is manipulated by varying the total capacitance on the bitlines. Preferably, the differential read signal is manipulated by varying the total capacitance on one of the bitlines.  
         [0014]    In one embodiment, the RST circuit comprises first and second test capacitors  222   a - b  coupled to respective first and second bitlines. A test capacitor is coupled in parallel to a bitline capacitor. First input terminal  221   a  is coupled to the first test capacitor and second input terminal  221   b  is coupled to the second test capacitor. The input terminals receive first and second test signals M and /M. In one embodiment, the test signals are complementary. That is, one is a logic 1 and the other is a logic 0.  
         [0015]    During normal operation, both test signals are inactive (e.g., logic 0) to disable the RST circuit. When the RST circuit is disabled, the test capacitors only passively contribute to the bitline capacitance. As such, the test capacitors of the RST circuit do not interfere with the normal operations of the memory cells. In test mode, an active input test signal is provided (e.g., logic 1). In one embodiment, an active input test signal is provided at one of the inputs while the other input terminal receives an inactive input signal (e.g., inactive or logic 0). For example, an active signal can be provided on the input terminal M and the inactive signal on input terminal /M during test mode. Alternatively, an active signal can be provided on the input terminal /M and the inactive signal on input terminal M.  
         [0016]    As the input signal transitions from a logic 0 to a logic 1 voltage level across the test capacitor, an additional amount of charge is added to the bitline, resulting in an increase in the read signal. The amount of electric charge depends on the magnitude of the active signal and capacitor. By providing an active test signal associated with the capacitor coupled to the bitline having a V LO  read signal, V LO  can be increased. Increasing V LO  decreases the difference between V LO  and V HI , which in turn reduces the sensing window.  
         [0017]    In one embodiment, the capacitance of the test capacitors is selected to increase the read signal by an amount less than the magnitude of the differential read signal (e.g., less than V HI V LO  ) for a given logic  1  signal. The capacitance is selected to reduce the sensing window by the desired amount to perform signal margin test. For example, the test capacitor reduces the sensing window by 1/2. Other values between V HI  and V LO  are also useful, e.g., 1/3. The amount of the increase of the read signal can be adjusted by either the size of the test capacitor or the magnitude of the voltage applied to the input terminal of the test capacitor.  
         [0018]    [0018]FIG. 3 shows the timing diagram of the read signals on the bitlines during test mode in accordance with one embodiment of the invention. To read from a memory cell, a pulse is provided on the plateline and a wordline is activated. The pulse causes an electric field across the capacitors, producing read signals on the bitlines at t 0 . At t 1 , the full read signals are developed on the bitlines. One capacitor produces a read signal equal to V LO  and the other capacitor produces a read signal equal to V HI . The two signals together form the differential read signal.  
         [0019]    At t M , an active test signal is provided at the input terminal associated with the capacitor coupled to the bitline having the V LO  read signal. The active input test signal causes V LO  to increase to V LOTest . Without the active test signal, the read signal would have remained at V  LO (indicated by the dotted line). The sense amplifier is activated at t 2 , amplifying the signals to the full bitline voltage levels. After the signals have been amplified, the sense amplifier is switched off at t 3  and the read cycle terminates at t 4 .  
         [0020]    Through the use of the RST circuit in accordance with the invention, the normal sensing window  384  can be manipulated to form a smaller test sensing window  385 . This facilitates performing signal margin tests on the IC. The signal margin can be selected to anticipate the expected aging of the memory cell, and thus reducing the read signal of the device over the whole lifetime. For example, if an IC fails the signal margin test (e.g., fails to amplify the read signal using the test sensing window,) it would be rejected.  
         [0021]    In an alternative embodiment shown in FIG. 4, the test circuit  420  comprises first and second set of capacitors coupled to respective first and second bitlines. In one embodiment, an input terminal is associated with each capacitor for receiving a test control signal (e.g., input terminal  427  for first set of capacitors and input terminals  428  for the second set of capacitors). In one embodiment, a set of capacitors comprises n capacitors when n is a whole number≧2. The capacitors are coupled in parallel to the bitline. Preferably, the capacitors within a set have different capacitances. Having different capacitances allows the set of test capacitors to vary the read signal by 2 n −1 times. This, in turn, can vary the sensing window by 2 n −1 times. More preferably, the two sets of test capacitors each have the same number of capacitor with corresponding capacitance values.  
         [0022]    In one embodiment, the capacitance of the test capacitors is selected to increase the read signal by an amount less than the magnitude of the differential read signal (e.g., less than V HI V LO ). Preferably, the total capacitance of the capacitors increases the read signal by an amount less than the magnitude of the differential read signal. The capacitance of a capacitor within a set is selected to reduce the sensing window by the desired amount to perform signal margin test.  
         [0023]    Illustratively, a set of capacitors comprises first and second test capacitors ( 422   a  and  423   a  or  422   b  and  423   b ). Preferably, the capacitors within a set have different capacitances. In one embodiment, the first capacitors of the sets have a first capacitance value and the second capacitors have a second capacitance value. The first capacitance value, for example, is less than the second capacitance value. For example, the first capacitors are ½ the size of the second capacitors. Other size ratios are also useful.  
         [0024]    For a given magnitude of the active input signal, the first capacitance value increases the magnitude of the read signal by a first level and the second capacitance increases the read signal by a second level. In one embodiment, the second level is greater than the first level. The combination of the first and second capacitances increases the read signal by a third level. The first, second, and third levels are less than the magnitude of the differential read signal.  
         [0025]    During test mode, one or more active test signals are provided to a set of capacitors of one of the bitlines. For example, one or more active test signals are provided to the test capacitors on BL (e.g., M0, M1, or both M0 and M1). The active test signal or signals increase the magnitude of the read signal accordingly.  
         [0026]    [0026]FIG. 5 shows a timing diagram of the read signals on the bitlines during test mode in accordance with another embodiment of the invention. Illustratively, the read signals are produced with an RST circuit having two sets of capacitors, each with first and second capacitors and corresponding input terminals (e.g., M0 and M1; /M0 and /M1). A read access is initiated at t 0 . At t 1 , the full read signals V LO  and V HI  are developed on the bitlines. At t M  one or more active test input signals are provided at input terminals of the RST associated with the capacitor set coupled to the bitline having the V LO  read signal. The test input signals increases the magnitude of the V LO  read signal. For example, an active first input signal increases the V LO  signal to V LO1 , an active second input signal increases V LO  to V LO2 , and active first and second signals increase the V LO  signal to V LO3 . The sense amplifier is activated at t 2 , amplifying the signals to the full bitline voltage levels. After the signals have been amplified, the sense amplifier is switched off at t 3  and the read cycle terminates at t 4 .  
         [0027]    While the invention has been particularly shown and described with reference to various embodiments, it will be recognized by those skilled in the art that modifications and changes may be made to the present invention without departing from the spirit and scope thereof. The scope of the invention should therefore be determined not with reference to the above description but with reference to the appended claims along with their full scope of equivalents.