Patent Publication Number: US-2004042249-A1

Title: Random access memory device and method for driving a plate line segment therein

Description:
BACKGROUND OF THE INVENTION  
       [0001] 1. Field of the Invention  
       [0002] The present invention relates generally to a random access memory device and, more particularly, to a nonvolatile ferroelectric random access memory device with a segmented plate line and a method for driving a plate line segment.  
       [0003] 2. Description of Related Art  
       [0004] Random access memory devices are well known in the art. One type of random access memory is a ferroelectric random access memory (FRAM), which employs a ferroelectric capacitor as the storage element for each memory cell. A FRAM stores a logic state based on the electrical polarization of the corresponding ferroelectric capacitor. When a sufficient voltage potential difference (i.e., above the switching threshold or coercive voltage level) is applied to the plates of the ferroelectric capacitor, the ferroelectric material of the capacitor is polarized in the direction of the electric field.  
       [0005] Typically, one plate of a ferroelectric capacitor is coupled to a bit line via an access transistor and the other plate is coupled to a plate line. For example, FIG. 1 illustrates a FRAM circuit having a conventional segmented plate line scheme. As shown, a global plate line (GPL) drives a number of segmented plate lines (PL00 through PLNN) via access transistors, a word line decoder (WL DEC) drives a number of word lines (WL0 through WLN), and a sense amplifier/column decoder (labeled Sense Amplifier &amp; Column Decoder Circuit) is coupled to the ferroelectric capacitors via corresponding bit lines and access transistors.  
       [0006] In standby, the word lines and the segmented plate lines are at a low voltage level. During operation, one word line activates one row of the segmented plate lines which is driven by the global plate line. A drawback of this scheme is that a segmented plate line pull-down circuit (one of which is circled in FIG. 1 and labeled PLPD) is required, which increases the loading of the word line and the overall size of the circuitry (i.e., chip area). Thus, there is a need for an improved FRAM device having fewer components and having word lines with less loading.  
       [0007] Another FRAM circuit is shown in FIG. 2 having a segmented plate line scheme as disclosed in U.S. Pat. No. 6,201,727, which is incorporated herein by reference in its entirety. As shown in FIG. 2 along with the respective timing diagram shown in FIG. 3, in standby, all segmented plate lines (PLS0 through PLSm) are coupled to ground by driving a signal on PRCHG control lines high. During access, a signal is driven high on an SEL control line, and a word line (one of WL0 through WLm) is activated, resulting in the selected word line charging one of the local segmented plate lines (one of PLS0 through PLSm). One drawback of this scheme is that the extent of the loading of the word line and the plate lines can limit the voltage level ramp-up, which in turn can limit device operation speed. Thus, the operational speed can be limited. As a result, there is a need for an improved FRAM device having reduced loading of the word lines and plate lines and, further, having memory cells with improved access time and cycle time.  
       SUMMARY OF THE INVENTION  
       [0008] The present invention seeks to meet these needs of the prior art by providing, in accordance with one aspect, random access memory (RAM) devices with reduced loading on the word lines and plate lines. The reduced loading of the word lines and plate lines can enable increased voltage level ramp-up times on the word lines and plate lines without area penalty.  
       [0009] The present invention further seeks to address the prior-art needs by providing a method of operating a random access memory device wherein plate lines are charged by a local plate line decoder or other circuit, instead of being charged directly by the row decoder. Moreover, pull-down circuits are not connected, so the area penalty is small. Consequently, by operation of the row decoder selecting local plate lines instead of actually charging them, access times and cycle times of the random access memory device can be increased.  
       [0010] To achieve these and other advantages and in accordance with a purpose of the present invention, as embodied and broadly described herein, the invention provides random access memory devices and methods for operating the devices. For example, in accordance with one aspect of the present invention, a random access memory device includes a number of memory cells, with word lines, plate lines, and bit lines coupled to the memory cells. A switch, controlled by a word line, couples one end of the plate line to a first global plate line, while another switch, controlled by a second global plate line, couples the one end of the plate line to a reference voltage. The plate lines are charged by the first global plate line, which improves operational speed of the device, reduces loading of the word lines, and improves the access time and cycle time of the memory cells.  
       [0011] In accordance with another aspect of the present invention, a random access memory device includes a plurality of memory cells, a word line, a plate line, a plurality of bit lines, a first global plate line, a second global plate line, a first switch circuit, and a second switch circuit. The word line, plate line, and bit lines are coupled to the memory cells, and each of the memory cells is arranged at an intersection of the word line and a corresponding bit line. The first switch circuit, responsive to a voltage potential asserted on the word line, couples one end of the plate line to the first global plate line. The second switch circuit, responsive to a voltage potential asserted on the second global plate line, couples the one end of the plate line (and the first switch circuit) to a reference voltage.  
       [0012] In accordance with yet another aspect of the present invention, a random access memory device includes a memory cell array, a plurality of word lines and a row decoder. The row decoder selects one of the word lines and activates the selected word line. The memory cell array is divided into a plurality of memory blocks, with each of the memory blocks including a plurality of memory cells, a plurality of plate line segments, a plurality of bit lines, a first global plate line, a second global plate line, a local plate line decoder, a plurality of first switch circuits, and a plurality of second switch circuits. In the memory block, the memory cells are arranged at intersections of the word lines and the bit lines. The plate line segments couple to the memory cells, and one end of each of the plate line segments is coupled to a corresponding one of the first switch circuits and the second switch circuits. The first switch circuits further are coupled to the first global plate line, and the second switch circuits are further coupled to reference voltages. In this case, each of the first switch circuits is controlled by a corresponding word line, and each of the second switch circuits is controlled by the second global plate line.  
       [0013] Furthermore, in accordance with another aspect of the present invention, a method for driving a plate line in a random access memory device is disclosed. The method includes asserting a first voltage potential on a word line to switch a first switch circuit off to decouple a plate line from a first global plate line; asserting a second voltage potential on a second global plate line to switch a second switch circuit on to charge the plate line with a reference voltage; asserting a third voltage potential on the word line to switch the first switch circuit on so as to couple the plate line to the first global plate line; asserting a fourth voltage potential on the second global plate line to switch the second switch circuit off so as to decouple the plate line from the reference voltage; and asserting a signal on the first global plate line such that the plate line has a plate line voltage.  
       [0014] In each of the foregoing aspects, the present invention provides a switch that is controlled by a word line and that couples one end of a plate line to a global plate line, and further provides another switch that is controlled by a second global plate line and that couples the one end of the plate line to a reference voltage. The plate lines are charged by the first global plate line, which can improve operational speed of the device, reduce loading of the word lines, and improve the access time and cycle time of the memory cells.  
       [0015] Any feature or combination of features described herein are included within the scope of the present invention provided that the features included in any such combination are not mutually inconsistent as will be apparent from the context, this specification, and the knowledge of one of ordinary skill in the art.  
       [0016] Additional advantages and aspects of the present invention are apparent in the following detailed description and claims. It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0017]FIG. 1 shows a circuit having a conventional segmented plate line scheme;  
     [0018]FIG. 2 shows another circuit having a conventional segmented plate line scheme;  
     [0019]FIG. 3 shows a timing diagram for the circuit of FIG. 2;  
     [0020]FIG. 4 shows a circuit having a segmented plate line scheme in accordance with an embodiment of the present invention; and  
     [0021]FIG. 5 shows an exemplary timing diagram for the circuit of FIG. 4 in accordance with an embodiment of the present invention. 
    
    
     [0022] The preferred embodiments of the present invention and their advantages are best understood by referring to the detailed description that follows.  
     DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS  
     [0023] Reference will now be made in detail to the presently preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers are used in the drawings and the description to refer to the same or like parts. It should be noted that the drawings are in simplified form and are not to scale. In reference to the disclosure herein, for purposes of convenience and clarity only, directional terms, such as, top, bottom, left, right, up, down, above, below, beneath, rear, and front, are used with respect to the accompanying drawings. Such directional terms should not be construed to limit the scope of the invention in any manner.  
     [0024] Although the disclosure herein refers to certain illustrated embodiments, it is understood that these embodiments are presented by way of example and not by way of limitation. The intent of the following detailed description is to cover all modifications, alternatives, and equivalents as may fall within the spirit and scope of the invention as defined by the appended claims. It is to be understood and appreciated that the process steps and structures described herein do not cover a complete process flow for the structure and operation of, for example, the random access memory device and associated structures. The present invention can be practiced in conjunction with various memory devices and associated components that are used in the art, and only so much of the commonly practiced components and method steps are included herein as are necessary to provide an understanding of the present invention.  
     [0025]FIG. 4 shows a circuit  400  for a random access memory having a segmented plate line scheme in accordance with an embodiment of the present invention. The circuit  400  includes a memory cell array that stores data information and is divided into a plurality of memory blocks  402 , which are separately referenced as memory blocks  402 (1) through  402 ( k ) with k representing the number of the last memory block  402  in the memory cell array. A plurality of word lines  404  (also labeled WL1 through WLn), which are separately referenced as word lines  404 (1) through  404 ( n ) with n representing the number of the last word line  404 , are arranged in parallel in a row direction and extend through the memory blocks  402 .  
     [0026] Each memory block  402  has a plurality of plate line segments  406  (also labeled PLS1 through PLSn and also referred to herein as local plate lines), which are referenced separately as plate line segments  406 (1) through  406 ( n ) with n representing the number of the last plate line segment  406 . The plate line segments  406  are arranged in parallel in the row direction within each memory block  402 . A plurality of bit lines  408  (also labeled BL1 through BLm), which are referenced separately as bit lines  408 (1) through  408 ( m ) with m representing the number of the last bit line  408 , are arranged in parallel in a column direction. Thus, there are m bit lines in each memory block  402 . The plate line segments  406 (1) through  406 ( n ) in the respective memory blocks  402  correspond to the word lines  404 (1) through  404 ( n ), respectively.  
     [0027] A plurality of memory cells are disposed within each memory block  402 , generally arranged in rows, with each memory cell having an access transistor  10  and a ferroelectric capacitor  12 . Each access transistor  10  has a gate coupled to one of the word lines  404  corresponding to the row of the associated memory cell. Each ferroelectric capacitor  12  has one plate coupled to a corresponding bit line  408  via a corresponding access transistor  10  and another plate coupled to a corresponding plate line segment  406 .  
     [0028] One end of each plate line segment  406  is connected to a first global plate line  418  (also labeled FGPL and separately referenced  418 (1) through  418 ( k )) through a corresponding transistor  422  (e.g., an NMOS transistor) and to a reference voltage through a corresponding transistor  424  (e.g., an NMOS transistor). The transistors  422  and  424  are separately referenced as transistors  422 (1) through  422 ( n ) and transistors  424 (1) through  424 ( n ), respectively, and are associated with the corresponding plate line segments  406 (1) through  406 ( n ) within each memory block  402 .  
     [0029] Each transistor  422  (referred to herein also as a first switch circuit) performs a switching operation responsive to a voltage potential asserted on a corresponding word line  404 . Each transistor  424  (referred to herein also as a second switch circuit) performs a switching operation responsive to a voltage potential asserted on a second global plate line  420  (also labeled SGPL and separately referenced  420 (1) through  420 ( k )). A row decoder  414  determines which word line  404  to select and drive (i.e., assert the appropriate voltage potential on each word line  404 ) in response to a received row address.  
     [0030] A local plate line (PL) decoder  412  (separately referenced as  412 (1) through  412 ( k )) receives a global plate line  410  (also labeled GPL) and determines the appropriate voltage potential to assert on the associated first global plate line  418  and the associated second global plate line  420 . As shown in FIG. 4, the first global plate lines  418 (1) through  418 ( k ), the second global plate lines  420 (1) through  420 ( k ), and the local plate line decoders  412 (1) through  412 ( k ) are associated respectively with the memory blocks  402 (1) through  402 ( k ).  
     [0031] As described herein, each of the memory blocks  402 (1) through  402 ( k ) has the same general structure. It should also be understood that the first global plate lines  418  charge the associated plate line segments  406  through the corresponding transistors  422 . Consequently, the word lines  404  only control the access transistors  10  and the transistors  422  rather than drive (or charge) the plate line segments  406 . Thus, the loading of the word lines  404  is reduced as compared to conventional segmented plate line schemes. Furthermore, the techniques described herein are applicable to various types of random access memories employing line-driven schemes, such as for example plate-line driven or bit-line driven operation schemes.  
     [0032]FIG. 5 shows an exemplary timing diagram  500  for the circuit of FIG. 4 in accordance with an embodiment of the present invention. The timing diagram  500  illustrates waveforms for signal timing and control signal behavior for a plate-line driven example for the circuit  400 . The timing diagram  500  includes a waveform  502  for the word line  404 (1) (labeled WL1), a waveform  504  for the first global plate line  418  (labeled FGPL), a waveform  506  for the second global plate line  420  (labeled SGPL), a waveform  508  for the plate line segment  406 (1) (labeled PLS1), a waveform  510  for a sense amplifier enable signal (labeled SAE), and a waveform  512  for one of the bit lines  408 (1) through  408 ( m ) along with its corresponding reference bit line (labeled BLj, BLj NOT).  
     [0033] In terms of general operation for this example, in standby, the second global plate line  420  is pulled high (i.e., a high logical voltage level is asserted on the second global plate line  420  by the local plate line decoder  412 ) and the first global plate line  418  is pulled low (i.e., a low logical voltage level is asserted on the first global plate line  418  by the local plate line decoder  412 ). This configuration is illustrated in FIG. 5 by the initial values of the waveforms  504  and  506 . It should be understood that the first global plate line  418  and the second global plate line  420  have a 1800 phase difference. Thus, each of the plate line segments  406  is charged with a reference voltage. In this exemplary embodiment, the reference voltage is at a ground voltage level (labeled GND in FIG. 4), but the reference voltage level may be set to a level other than ground depending upon the application. Thus, for this example, each of the plate line segments  406  is grounded.  
     [0034] During operation of the circuit  400 , the row decoder  414  decodes received signals (row address lines to the row decoder are not shown) and asserts, for example, a high logical voltage level on the word line  404 (1). In other words, the row decoder  414  activates the word line  414 (1), as illustrated by the waveform  502 , which switches on the transistor  422 (1) and couples the first global plate line  418 (1) to the plate line segment  406 (1). The second global plate line  420  is pulled to a low logical voltage level and the first global plate line  418  is pulled to a high logical voltage level by the local plate line decoder  412  due to a signal received through the global plate line  410 . Accordingly, the plate line segment  406 (1) is driven to a high logical voltage level by the first global plate line  418 . Specifically, the first global plate line  418  charges the plate line segment  406 (1). At the same time, the other plate line segments  406 (2) through  406 ( n ) remain at a low logical voltage level because the corresponding word lines  404 (2) through  404 ( n ) apply a low logical voltage level to the gates of the associated transistors  422 (2) through  422 ( n ) and prevent the first global plate line  418  from charging the plate line segments  406 (2) through  406 ( n ).  
     [0035] The bit lines  408  and reference bit lines (not shown) are driven to different levels (as shown in FIG. 5 by the waveform  512 ), and a sense amplifier/column decoder  416  (also referred to herein as a sense amplifier) is enabled to sense the data values from the bit lines  408  and reference bit line. The sense amplifier  416  is enabled, for example, after receipt of a sense amplifier enable (SAE) signal as illustrated by the waveform  510 . Subsequently, after the data is read out of the selected memory cells, it is necessary to write back the data into the ferroelectric capacitors  12  through the associated access transistors  10  if the data is obtained through a destructive read.  
     [0036] For example, the sense amplifier/column decoder  416  selects the appropriate bit lines  408  in response to a column address, detects and amplifies a signal on the selected bit lines  408 , and provides a corresponding output data signal during a read cycle. During a write cycle, the sense amplifier/column decoder  416  is used as a driver for writing either a logic one or a logic zero into the memory cells via the bit lines  408 .  
     [0037] In accordance with embodiments of the present invention, the plate line segments  406  are not driven directly by the word lines  404 . Thus, the loading of the word lines  404  is decreased and the word line voltage level ramp-up speeds can be increased. Furthermore, the local plate line decoder  412  provides the decoding for and drives the respective first global plate line  418  and the second global plate line  420 . Thus, the loading of the local plate line decoder  412  can be less than prior art global plate lines and its voltage level ramp-up speed to drive the plate line segments  406  can be increased. Because the voltage level ramp-up speeds of the word line  404  and the plate line segments  406  can be increased, the operational speed of the RAM device, such as exemplified by the circuit  400 , is also increased relative to conventional schemes.  
     [0038] Embodiments described above illustrate but do not limit the invention. It should also be understood that numerous modifications and variations are possible in accordance with the principles of the present invention. For example, the number of memory blocks can be varied from one to many, depending on the amount and characteristics of the memory required. Furthermore, the number of memory cells within each row and the number of memory cells within a memory block may vary, which in turn determines the number of word lines, plate line segments, and bit lines. The row decoder may be a single decoder or divided into a number of decoders for the memory blocks. Similarly, a single local plate line decoder may be associated with each memory block or one or more of the local plate line decoders may be combined to provide switching control for more than one memory block. Accordingly, the scope of the invention is defined only by the following claims.  
     [0039] The above-described embodiments have been provided by way of example, and the present invention is not limited to these examples. Multiple variations and modification to the disclosed embodiments will occur, to the extent not mutually exclusive, to those skilled in the art upon consideration of the foregoing description. Such variations and modifications, however, fall well within the scope of the present invention as set forth in the following claims.