Abstract:
An improved architecture which reduces the adverse impact of the disturb pulse on non-selected ferroelectric memory cells is disclosed. The architecture provides plateline selection switches for selectively coupling memory groups on the selected side of the memory block to the plateline and decoupling the non-selected side of the memory block from the plateline. By decoupling the non-selected side of the memory block from the plateline, the plate pulse does not adversely affect the memory cells in the non-selected side of the memory block.

Description:
BACKGROUND OF INVENTION  
         [0001]    Ferroelectric metal oxide ceramic materials such as lead zirconate titanate (PZT) have been investigated for use in ferroelectric semiconductor memory devices. Other ferroelectric materials including strontium bismuth tantalate(SBT) can also be used. FIG. 1 shows a ferroelectric memory cell with a transistor  142  and a capacitor  144 . The capacitor comprises a ferroelectric metal ceramic layer  190  sandwiched between first and second electrodes  192  and  193 . The electrodes typically are formed from a noble metal such as platinum. Other conductive materials or conductive oxides, such as strontium ruthenium oxide (SRO) or iridium oxide (IrO), are also useful. Electrode  192  is coupled to a plateline (not shown) and electrode  193  is coupled to the transistor, which selectively couples or decouples the capacitor from a bitline (not shown), depending on the state of a wordline (not shown) coupled to a gate of the transistor.  
           [0002]    The capacitor uses the hysteresis polarization characteristic of the ferroelectric material for storing information. The logic value stored in the memory cell depends on the polarization of the capacitor. To change the polarization a voltage, which is greater than the switching voltage (coercive voltage) needs to be applied across the capacitor&#39;s electrodes. The polarization of the capacitor depends on the polarity of the voltage applied. An advantage of the ferroelectric capacitor is that it retains its polarization state after power is removed, resulting in a non-volatile memory cell.  
           [0003]    Referring to FIG. 2, a pair of bitlines (bitline BL and bitline complement/BL) is shown. Each of the bitlines includes first (e.g., left)and second (e.g., right) groups of memory cells  110   a - b  or  110   c - d . The memory cells of a group, each with a transistor  142  coupled to a capacitor in parallel, are coupled in series. Such memory architectures are described in, for example, Takashima et al., “High Density Chain ferroelectric random access Memory (chain FRAM)”, IEEE Jrnl. of Solid State Circuits, vol.33, pp.787-792, May 1998, which is herein incorporated by reference for all purposes. A sense amplifier is coupled to the bitlines to facilitate access to the memory cell.The gates of the cell transistors can be gate conductors which are coupled to or serve as wordlines. A selection transistor  130  is provided to selectively couple one end of the group to its respective bitline (e.g.,  130   a  couples group  110   a  to BL;  130   b  couples group  110   b  to BL;  130   c  couples group  110   c  to/BL; and  130   d  couples group to/BL). A plateline is coupled to the other end of the group (e.g., PL or/PL). The groups on the left and right side on the same bitline share the same plateline (e.g., groups on BL are coupled to PL and groups on/BL are coupled to/PL). Numerous bitline pairs are addressed via wordlines to form a memory block.  
           [0004]    The selection transistors are controlled by different bitline select (BS) control signals. For example, BS 0  and BS 2  respectively control selection transistors  130   a  and  130   b  to selectively couple or decouple memory groups  110   a  and  110   b  to BL; BS 1  and BS 3  respectively control selection transistors  130   c  and  130   d  to selectively couple or decouple memory groups  110   c  and  110   d  to/BL. During a memory access, one cell is selected from one of the bitline pair by selecting one of the wordlines (e.g., one of the wordlines WL 0 -WL 15  is selected). Depending on which group the cell is located, the corresponding BS signal is activated. For a group on the left side of the platelines, BS 0  or BS 1  is activated depending on whether the group is coupled to BL or/BL. Otherwise, BS 2  or BS 3  is activated to couple a group located on the right side of the platelines.  
           [0005]    An access plate pulse, for example, about 2.5V is provided on either PL or/PL, depending on whether the selected cell is located on BL or/BL, after the appropriate bitline select signal is activated. This pulse creates an electric field across the capacitor of the selected cell, which is sensed by the sense amplifier. However, with respect to the non-selected cells of the non-selected group on the other side of the platelines, this pulse acts as a disturb pulse, which can have a negative effect on the polarization of the capacitors of the non-selected. For example, due to coupling mechanism and leakage of transistors, the capacitors of the non-selected cells on the other side of the platelines see a small plate pulse. This pulse causes an inelastic travel of the polarization along the hysteresis curve, slowly decreasing the remnant polarization of the ferroelectric material. As a result, read signal strength is decreased, adversely affecting reliability and service life of the device.  
           [0006]    From the foregoing discussion, it is desirable to provide an improved ferroelectric memory architecture which decreases the negative effects of the disturb pulse.  
         SUMMARY OF INVENTION  
         [0007]    The invention relates to integrated circuits (ICs) in general, and more particularly to ferroelectric memory ICs having a series architecture. In one embodiment, the IC includes first and second bitlines which form a bitline pair. The first bitline includes first and second memory groups and the second bitline includes third and fourth memory groups. A memory group comprises first and second ends with a plurality of memory cells serially coupled between the ends. A first end of a memory group is provided with a bitline selection switch for selectively coupling the memory group to its respective bitline. The first and third memory groups form one section of the bitline pair and the second and fourth groups form a second section of the bitline pair.  
           [0008]    Plateline selection switches are provided at the second ends of the first and second memory groups to selectively couple the groups to a first plateline. In one embodiment, plateline selection switches are provided at the second ends of the third and fourth memory groups to selectively couple the groups to a second plateline. By providing plateline selection switches for the memory groups, the plate pulse is experienced by the section of the bitline in which the selected memory cell is located. The non-selected section is isolated from the pulse due to the plateline switches. This reduces the adverse affects of the disturb pulse on memory cells in the non-selected section of the bitline pair. 
       
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0009]    [0009]FIG. 1 shows a conventional ferroelectric capacitor;  
         [0010]    [0010]FIG. 2 shows a bitline pair of a ferroelectric memory block with grouped architecture;  
         [0011]    [0011]FIG. 3 shows an embodiment of the invention; and  
         [0012]    [0012]FIG. 4 shows a cross-sectional view of one embodiment of the invention. 
     
    
     DETAILED DESCRIPTION  
       [0013]    Referring to FIG. 3, a bitline pair comprising first and second bitlines BL and/BL, each including first and second groups ( 410   a - b  or  410   c - d ) of memory cells  140 , is shown. In one embodiment, a group comprises  8  memory cells coupled in series. Groups having other number of memory cells are also useful. Preferably, the number of cells within a group is equal to 2 y , where y is equal to a whole number≧1. A memory cell includes a transistor  142  coupled to a capacitor  144  in parallel. The transistor, for example is an n-FET and the capacitor is a stacked capacitor. Other types of transistors (e.g. p-FETs) or capacitors (e.g., trench) are also useful. Coupling a memory group to its respective bitline is a selection transistor  130 . A first plateline PL is commonly coupled to the first and second memory groups of BL; a second plateline/PL is commonly coupled to the first and second memory groups of/BL.  
         [0014]    A plurality of bitline pairs can be interconnected via wordlines to form a memory block. For example, the gates of the cell transistors can be gate conductors, which are coupled to or serve as wordlines. The memory block is separated into first (left) and second (right) sections  102  and  103 , each comprising a group of bitline. As shown, wordlines WL 0  to WL 7  and bitline select signals BS 0  and BS 1  are used to address memory groups in the first section and wordlines WL 8  to WL 15  and BS 2  to BS 3  are used to address memory groups on the second section.  
         [0015]    In accordance with the invention, only the memory groups in the section (either the left or right) in which the selected memory cell is located are coupled to the respective platelines PL and/PL. For example, if the decoded row address of the memory access is equal to one of the wordlines WL 0  to WL 7 , the memory groups in the first section are coupled to the platelines. On the other hand, the memory groups in the second section are coupled to the platelines if the decoded address is equal to one of the wordlines WL 8  to WL 15 .  
         [0016]    In one embodiment, a section selection (SS) transistor  460  is provided between the end of a memory group and a plateline (either PL or/PL), enabling the memory group to be selectively coupled or decoupled to the plateline. The SS transistors, for example, are n-FETs. Other types of transistors, such as p-FETs are also useful. In one embodiment, a first section select signal PLSL controls the SS transistors coupled to memory groups in the first section and a second section select signal PLSR controls the SS transistors coupled to the memory groups in the second section.  
         [0017]    During a memory access to a memory cell in the first section, an active PLSL (e.g., logic 1) and inactive PLSR (e.g., logic 0) are provided to couple the memory groups in the first section to and decoupling the memory groups in the second section from the platelines. For a memory access to a memory cell in the second section, an inactive PLSL and active PLSR is provided.  
         [0018]    The PLSL and PLSR signals can be normally active or normally inactive. If normally active, the selection signal of the non-selected side should be deactivated prior to the generation of the access pulse. If normally inactive, the selection signal for the selected side should be activated prior to the generation of the access pulse. Through the use of SS transistors the access pulse is not seen by memory cells in the non-selected section of the block. This reduces the adverse effects of the access pulse on memory cells in the non-selected section of the memory block. Additionally, reducing the number of cells coupled to the plateline advantageously enables smaller plateline drivers (space reduction) as well as lower power consumption since the capacitive load is reduced by about half.  
         [0019]    In one embodiment of the invention, the selection transistors are controlled by different control signals. The four groups of the bitline pair, as illustrated, are each controlled by a different control signal. For example, BS 0  and BS 2  respectively control selection transistors  130   a  and  130   b  to selectively couple one of the groups  410   a  or  410   b  to BL. Likewise, BS 1  and BS 3  respectively control selection transistors  130   c  and  130   d  to selectively couple one of the groups  110   c  or  110   d  to/BL. Providing common bitline select signals to control the selection transistors of the groups coupled to the same bitline can also be useful.  
         [0020]    When different bitline selectsignals are used for each group, the PLSL and PLSR signals can be derived from the bitline select signals. In one embodiment, an active PLSR signal is derived from either an active BS 2  or BS 3  (e.g., PLSR=(BS 2  U BS 3 )) and an active PLSL signal is derived from either an active BS 0  or BS 1  (e.g., PLSL=(BS 0  U BS 1 )).  
         [0021]    [0021]FIG. 4 shows a cross-sectional view of portions of two ferroelectric memory groups  610   a - b  that are couple to a bitline in accordance with one embodiment of the invention. As shown, the groups are formed on a semiconductor substrate  601 , such as silicon. Other types of semiconductor substrates can also be used. The memory group comprises, for example,  8  memory cells  140 . Memory groups of other sizes are also useful. Preferably, the number of cells within a group is equal to 2 y  where y is a whole number≧1. More preferably, y is from 2 to 5. A memory cell comprises a transistor  142  coupled to a ferroelectric capacitor  144 . The transistors are coupled to wordlines.  
         [0022]    In one embodiment, the transistors of the memory cells within the group share a diffusion region. Sharing of the diffusion region advantageously reduces surface area required. The capacitors of adjacent memory cells are interconnected. As shown, two adjacent capacitors share a common electrode  610  to form a capacitor pair. Two non-common electrodes  620  of adjacent capacitors from adjacent capacitor pairs are coupled to a coupling interconnect  667  via studs  663 . Preferably, the common electrode is the lower electrode while the non-common electrode is the upper electrode. A first common diffusion region  648  of a memory cell transistor is coupled to the common electrode of a capacitor pair via a contact stud  670  and the coupling interconnect is coupled to a second common diffusion region  649  via contact stud  674 .  
         [0023]    The memory groups are coupled to a plateline (either PL or/PL) at first adjacent ends of the memory group via respective SS transistors  680   a - b . Illustratively, the memory groups are coupled to PL. The PLSL signal line is coupled to the gate of, for example, SS transistor  680   a  while the PLSR signal line is coupled to SS transistor  680   b . Based on which section is selected, one of the groups is coupled to the PL via the SS transistor. In one embodiment, the SS transistors of the two memory groups share a common diffusion region  687  which is coupled to PL via contact stud  682  while the other diffusion region of the SS transistors is shared with a cell transistor. In one embodiment, the SS transistor is located in an area between the last cell transistor and plateline. This area, for example, is occupied by dummy capacitors  690 . By placing the SS transistors in the area already occupied by the dummy capacitors advantageously enables the invention to be implemented without the need of additional chip area or reducing the need for additional chip area.  
         [0024]    Illustratively, the taller contacts (e.g., contacts  674 ) are formed in two process steps. Other schemes can also be used to form the different types of studs. The first step forms lower portion (e.g.,  674   a ) along with the studs  470 . The second process step forms the upper portion (e.g.,  674   b ). Such studs can also be formed using a single process step. Additional structures (not shown) such as support logic, passivation layers, and package may be included to complete the IC.  
         [0025]    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.