Abstract:
A dynamic random access memory includes a plate line; a digit line; a memory cell selectively coupled between the digit line and the plate line; sense circuitry selectively coupled to the memory cell to read the memory cell and capable of applying a first voltage from the plate line to the digit line; equilibration circuitry selectively coupling the plate line to an equilibration voltage less than the first voltage and selectively coupling the digit line to the equilibration voltage; and control circuitry configured to cause the equilibration circuitry to couple the plate line to the equilibration voltage while the memory cell is being accessed. A method of manufacturing a dynamic random access memory includes providing control circuitry configured to operate in a specified manner. A method of operating a dynamic random access memory includes turning on one equilibration transistor, while another equilibration transistor is off, so that a plate line equilibrates to a voltage defined by the equilibration voltage source during accessing of a memory cell.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This is a Continuation of U.S. patent application Ser. No. 09/353,307, filed Jul. 14, 1999, and titled “Memory Array Architecture, Method of Operating a Dynamic Random Access Memory, and Method of Manufacturing a Dynamic Random Access Memory”. 
    
    
     TECHNICAL FIELD 
     The invention relates to memory devices. More particularly, the invention relates to memory array architectures and to dynamic cell plate sensing. 
     BACKGROUND OF THE INVENTION 
     A DRAM memory cell includes a MOS access transistor and a storage capacitor. The access transistor is located between the memory cell capacitor and a digit line. The digit line is coupled to a plurality of memory cell transistors. Typically, either metal or polysilicon is used to form the digit line. The memory cell holds one bit of binary information, as stored electric charge in the cell capacitor. Given a bias voltage of Vcc/2 on the capacitor&#39;s common node, a logical one is represented by +Vcc/2 volts across the capacitor and a logical zero is represented by −Vcc/2 volts across the capacitor. 
     The access transistor has a gate coupled to a word line. The word line is coupled to a plurality of memory cells, and is an extended segment of the same polysilicon used to form the access transistor&#39;s gate. The word line is physically orthogonal to the digit line. 
     Digit lines are typically fabricated as pairs. The digit lines are initially equilibrated at Vcc/2 volts, and all word lines are initially at zero volts, which turns off the memory cell access transistors. To read a memory cell, its word line transitions to a voltage that is at least one transistor Vth above Vcc. This elevated word line voltage level is referred to as Vccp or Vpp. When the word line voltage exceeds one Vth above the digit line equilibrate voltage (Vcc/2) and the memory cell access transistor turns on, the memory cell capacitor begins to discharge onto a digit line. Reading or accessing a memory cell results in charge being shared between the memory cell capacitor and the digit line capacitance. This sharing of charge causes the digit line voltage to either increase for a stored logic one or decrease for a stored logic zero. Ideally, the access will only modify the active digit line, leaving its complement digit line unaltered. Thus, differential voltage develops between the two digit lines. 
     After the cell access is complete, a sensing operation is performed by a differential sense amplifier. The sense amplifier typically includes a cross-coupled PMOS transistor pair and a cross-coupled NMOS transistor pair. A signal voltage develops between the digit line pair when the memory cell access occurs. While one digit line contains charge from the cell access, the other digit line serves as a reference for the sensing operation. The sense amplifier firing generally occurs sequentially rather than concurrently. The N-sense-amp fires first and the P-sense-amp second. 
     In another memory architecture, a common plate of the array of memory cell capacitors is used as a reference for the sense amplifier circuitry. In this type of architecture, the common plate is held at a predetermined voltage during operation. 
     The invention relates most particularly to memory architectures of a type having individual cell plate lines instead of the type where the cell plate line is the upper contact of the memory cell capacitor and all upper contacts are tied together on cell poly. 
     Attention is directed to the following patents, which describe dynamic cell plate sensing and which are incorporated herein by reference: U.S. Pat. No. 5,862,072 to Raad et al.; U.S. Pat. No. 5,862,089 to Raad et al.; and U.S. Pat. No. 5,821,895 to Manning. Attention is also directed to U.S. Pat. No. 5,841,691 to Fink, which relates to cell plate generators, and which is incorporated herein by reference. 
     Two problems exist in the individual plate line architecture. First, if both the bit line and the plate line were latched by a common sense amp, adjacent un-accessed storage cells can be corrupted. Specifically, a zero in an un-accessed storage cell could be coupled down to −DVC 2 . This would then turn on the access device of the un-accessed storage cell thereby leaking the zero up. Another problem which can occur in this type of architecture, is that the digit line and the plate line, upon firing of a word line, will move in opposite directions and thereby place, if allowed to, a full Vcc across the memory cell. This undesirably places a full Vcc across the nitride which serves as the cell dielectric. This can lead to breakdown and other problems which render the device inoperative. 
     SUMMARY OF THE INVENTION 
     The invention provides a method of operating a dynamic random access memory. One method includes turning on one equilibration transistor, while another equilibration transistor is off, so that a plate line equilibrates to a voltage defined by an equilibration voltage source during accessing of a memory cell. 
     One aspect of the invention provides a dynamic random access memory. The dynamic random access memory includes a plate line, a digit line, and a memory cell selectively coupled between the digit line and the plate line. Sense circuitry is selectively coupled to the memory cell to read the memory cell. The sense circuitry is capable of applying a first voltage from the plate line to the digit line. Equilibration circuitry selectively couples the plate line to an equilibration voltage less than the first voltage and selectively couples the digit line to the equilibration voltage. Control circuitry is configured to cause the equilibration circuitry to couple the plate line to the equilibration voltage while the memory cell is being accessed. 
     Another aspect of the invention provides a method of manufacturing a dynamic random access memory. The method includes providing control circuitry configured to turn on the first and second equilibration transistors to couple the digit line and plate line to the equilibration voltage source; turn off the first and second equilibration transistors, causing the digit line and plate line to float; turn on the access transistor; read the memory cell with the sense circuitry by determining the amount of charge on the capacitor; and turn on the second equilibration transistor, while the first equilibration transistor is off, so that the plate line equilibrates to the voltage defined by the equilibration voltage source. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Preferred embodiments of the invention are described below with reference to the following accompanying drawings. 
     FIG. 1 is a block diagram of a system including a memory array. 
     FIG. 2 is a general schematic diagram of a dynamic digit line memory array layout for a memory array of the type shown in FIG.  1 . 
     FIG. 3 is a timing diagram for the memory array layout of FIG.  2 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     This disclosure of the invention is submitted in furtherance of the constitutional purposes of the U.S. Patent Laws “to promote the progress of science and useful arts” (Article 1, Section 8). 
     This invention relates to binary logic. The terms “low” and “high,” as used herein, refer generally to the false and true binary logic levels, respectively. Signals are generally considered to be active when they are high, however, an asterisk (*) following the signal name indicates that the signal is negative or inverse logic (active low). 
     FIG. 1 shows a system  10 , such as a computer, including a DRAM  12  and access circuitry  14  for accessing the DRAM  12 . The access circuitry  14  for accessing the DRAM  12  can be a microprocessor, memory controller, a chip set, or other external system. The DRAM  12  includes input/output connections including address lines A 0 -Ax via which the external access circuitry  14  accesses the DRAM. 
     The DRAM  12  has a memory array  16  and associated circuitry for reading from and writing to the memory array. The memory array  16  has rows and columns of individual memory cells. The DRAM  12  includes a row decoder  18  which decodes a row address from an address signal provided on address lines A 0 -Ax, and addresses the corresponding row of the memory array  16 . The DRAM  12  also includes column decoder  20  which decodes a column address from an address signal provided on address lines A 0 -Ax, and addresses the corresponding column of the memory array  16 . 
     The DRAM  12  further includes I/O circuitry  22  including a data input buffer and a data output buffer. Data stored in the memory array  16  is selectively transferred to outputs DQ 1 -DQx through the data output buffer. Similarly, the data input buffer is used to receive data from DQ 1 -DQx and transfer the data to the DRAM array. 
     The DRAM  12  further includes sense amplifier circuitry  26  to sense and amplify data stored on the individual memory cells of the memory array  16 . The DRAM  12  also includes control circuitry  28  to monitor the memory circuit inputs and control reading and writing operations. 
     The DRAM  12  further includes input and output connections described as follows. Output enable (OE*) enables the output buffer of the I/O circuitry  22  of the DRAM. Write enable (WE*) is used to select either a read or write operation when accessing the DRAM. Row address strobe (RAS*) input is used to clock in the row address bits. Column address strobe (CAS*) input is used to clock in the column address bits. Address input lines A 0 -Ax are used to identify a row and column address. DRAM data input/output lines DQ 1 -DQx provide data input and output for the DRAM. A clock signal can be provided by the microprocessor for operating the memory circuit in a synchronous mode. 
     It will be understood that the above description of a DRAM is intended to provide a general understanding of the memory and is not a complete description of all the elements and features of a DRAM. Further, the present invention is equally applicable to other types of memory circuits, such as memory circuits of various sizes, and is not intended to be limited to the DRAM described above. 
     FIG. 2 is a general schematic diagram of a dynamic digit line memory array  30 . The memory array  30  has two memory cells  32  and  34  which share a common p-sense amplifier circuit  36 , and an n-sense amplifier  38 . Memory cell  32  includes a capacitor  40  for selectively storing a charge. The capacitor  40  has a capacitor plate coupled to a plate line  42  and another capacitor plate selectively coupled to a digit line  48  via an access transistor  50 . The plate line  42  could also be referred to as a digit line (e.g. a second digit line, or a complementary digit line). Memory cell  34  has a capacitor  44  for selectively storing a charge. The capacitor  44  has a capacitor plate coupled to plate line  46  and another capacitor plate selectively coupled to a digit line  52  via an access transistor  54 . 
     The digit lines  48  and  52 , which define an array column, and the cell plate lines  42  and  46  are coupled to sense amplifier circuitry  72  through isolation transistors  56 ,  58 ,  60 , and  62 . Isolation transistor  56  has a gate “ISOB,” isolation transistor  58  has a gate ISOB_PL, isolation transistor  60  has a gate ISOA, and isolation transistor  62  has a gate ISOA_PL. 
     An equilibration transistor  64  is coupled between the plate line  42  and the digit line  48 , and an equilibration transistor  66  is coupled between the plate line  46  and the digit line  52 . The equilibration transistor  64  has a gate EQA, and the equilibration transistor  66  has a gate EQB. An equilibration transistor  68  is coupled between the plate line  42  and an equilibration voltage DVC 2 , and an equilibration transistor  70  is coupled between the plate line  46  and DVC 2 . DVC 2  is typically Vcc÷2. The equilibration transistor  68  has a gate EQ_PLA and the equilibration transistor  70  has a gate EQ_PLB. The equilibration transistors  64 ,  66 ,  68 , and  70  are controlled by their respective gates. 
     For purposes of discussion, only memory cell  34  and the right side of FIG. 2 will be discussed. Operation of the memory cell  32  and left side of FIG. 2 is similar. When transistor  70  is turned on, by firing its gate EQ_PLB, this takes plate line  46  to equilibration voltage DVC 2 . When the equilibration transistor  66  is turned on, by firing its gate EQB, the transistor  66 , in effect, shorts digit line  52  and plate line  46  together. When transistor  70  is turned on and transistor  66  is also turned on the voltage DVC 2  is passed onto both digit line  52  and plate line  46  thereby equilibrating them to a common potential. 
     Isolations transistors  60  and  62  control which of the digit and plate lines are routed to sense circuitry  72 . The sense circuitry  72  includes first and second nodes  74  and  76 . The transistor  62 , which is gated by ISOA_PL, selectively couples plate line  46  to node  76  of the sense circuitry  72 . The transistor  60 , which is gated by ISOA, selectively couples digit line  52  to the node  74  of the sense circuitry  76 . 
     Refer now to both FIG.  2  and FIG.  3 . To read the memory cell  34 , the digit line  52  and plate line  46  are first equilibrated by the equilibration transistors. EQB and EQ_PLB are both high until a point designated event  100  (FIG.  3 ). This means that prior to event  100 , digit line  52  and plate line  46  (FIG. 2) are equilibrated. This is also indicated in the timing diagram (FIG. 3) where DIGIT_LINE_RIGHT and PLATE_LINE_RIGHT (shown in FIG. 3 as DIGIT/PLATE_LINE_RIGHT) are shown as being at voltage level DVC 2  while EQB and EQ_PLB are high. Both digit line  52  and plate line  46  are maintained at DVC 2  which is the potential provided by transistor  70  (FIG.  2 ). 
     Prior to firing word line WORD_LINE_RIGHT at a event  110 , the equilibration transistors  66  and  70  are turned off, thereby allowing digit line  52  and plate line  46  to float. This is indicated by the trailing edge of the EQB and EQ_PLB signals at  100 . Now with the digit line  52  and plate line  46  floating, transistor  54  is turned on by firing WORD_LINE_RIGHT at event  110 . 
     The transistor  54  is the access transistor for the memory cell  34 . When the word line WORD_LINE_RIGHT fires, the transistor  54  is turned on and the memory cell  34  can be read from and written to. Typically, during operations, a charge, representing either a logic “1” or a logic “0” is stored on capacitor  54 . Reading the memory cell  34  involves sensing the charge on the capacitor  44  after firing WORD_LINE_RIGHT. Firing WORD_LINE_RIGHT dumps the charge stored on the capacitor  44 . More particularly, reading or accessing the memory cell  34  results in charge being shared between the memory cell capacitor  44  and the digit line capacitance. This sharing of charge causes the digit line voltage to increase and plate line voltage to decrease for a stored logic one or causes the digit line voltage to decrease and plate line voltage to increase for a stored logic zero. Thus, differential voltage develops between the digit line  52  and plate line  46 . 
     Assume we are accessing and restoring for a logic one. After the transistor  54  is turned on (event  110  in FIG.  3 ), the voltage on digit line  52  increases, the voltage on plate line  46  decreases, then transistor  62  is turned off (event  111  in FIG. 3) after a differential voltage appears between the digit line  52  and the plate line  46 . Then, the sense circuitry  72  is fired (event  116  in FIG.  3 ). Then, transistor  70  is turned on to equilibrate the plate line  46  (event  120  in FIG.  3 ). 
     In an alternative embodiment (not shown), the plate line  46  is equilibrated before the sense circuitry  72  is fired (e.g., event  120  occurs before event  116 ). 
     Firing of the sense circuitry  72 , while transistor  54  is turned on, starts the digit line  52  moving toward Vcc (event  112  in FIG.  3 ). Sensing which is accomplished by sense circuitry  72  is generally an amplification of the differential voltage between the digit lines  52  and plate line  46 . Sensing is necessary to properly read the cell data and to refresh the memory cell. 
     The sense circuitry  74  includes a cross-coupled NMOS pair of transistors defining the n-sense amplifier  38 , and a cross-coupled PMOS pair of transistors defining the p-sense amplifier  36 . The NMOS pair  38  has a common node labeled RNL* (n-sense latch node). Similarly, the PMOS pair  36 , has a common node labeled ACT (active pull-up node). 
     In operation, RNL* is biased to Vcc/2, and ACT is biased to Vss or signal ground. Because the digit line  52  and plate line  46  are both initially equilibrated at, for example, Vcc/2, both transistors of NMOS pair  38  are off. Similarly, both transistors of PMOS pair  36  are off. When the memory cell  34  is accessed, a signal develops across the digit line  52  and plate line  46 . 
     In the illustrated embodiment, the n-sense and p-sense amplifiers are fired at the same time. In an alternative embodiment, the sense amplifiers can be fired sequentially, with the n-sense amp  38  being fired first, followed by the p-sense amp  36 . 
     The n-sense amp  38  is fired by bringing RNL* toward ground. As the voltage difference between RNL* and the digit and plate lines approaches the threshold voltage, the NMOS transistor whose gate is connected to the higher voltage line begins to conduct. This conduction occurs first in the subthreshold region and then in the saturation region as the gate-to-source voltage exceeds the threshold voltage. This conduction causes the low-voltage line to be discharged toward the RNL* voltage. Ultimately RNL* will reach ground, and the line will be brought to ground potential. Note that the other NMOS transistor will not conduct; its gate voltage is derived from the low-voltage line which is being discharged towards ground. 
     With one typical approach, sometime after n-sense amp  38  fires, ACT will be brought toward Vcc and activate the p-sense amp, which operates in a complementary fashion to the n-sense amp. With the low-voltage line approaching ground, there is a strong signal to drive the appropriate PMOS transistor into conduction. This conduction, again moving from subthreshold to saturation, charges the high-voltage line toward ACT, ultimately reaching Vcc, in one embodiment. Because the access transistor  54  of the memory cell remains on, the memory cell capacitor  44  is refreshed during the sensing operation. The voltage, and hence the charge, which the capacitor  44  held prior to accessing, is restored to a full level; e.g., Vcc for a logic 1, and ground for a logic 0. In an alternative embodiment, conduction to full rail voltages is not required; for example, instead of charging between ground and Vcc, a range between any two particular voltages can be employed. For example, a range between ground and Vcc/2 can be employed. 
     A write operation is similar to the sensing and restore operations, except that a write driver circuit included in the control circuit  28  determines what data is placed into the cell  34 . 
     After the sense circuitry  72  is fired, a column select line (not shown) in column decoder  20  then fires which passes the potentials of the plate line  46  and digit line  52  to I/O circuitry  22 . 
     When the sense circuitry  72  fires, digit line  52  moves more dramatically toward Vcc as indicated at  118  in FIG.  3 . At this point (after firing of the sense circuitry  72 ) if nothing were done, the plate line  46  would move towards ground potential as indicated in FIG. 3 by dashed line L 1 . 
     If the plate line were allowed to reach ground, that would mean that an entire full Vcc would be provided between the plate line  46  and digit line  52 . This is undesirable. 
     Therefore, after the sense circuitry  72  is fired, transistor  70  is turned on at  120  (FIG. 3) by firing EQ_PLB. As can be seen from FIG. 3, when the transistor  70  is turned on, this enables DVC 2  to be provided onto the plate line  46 . Referencing now the timing diagram of FIG. 3, when this happens, rather than the plate line moving in the direction indicated by dashed line L 1 , it equilibrates back to DVC 2  as shown at  122 . By doing this, the plate line is maintained at a desirable potential relative to the digit line so that an undesired potential is not created. 
     This solves the problems associated with corruption of a zero in an un-accessed storage cell (see Background of the Invention). 
     Operation of the memory cell  32  and circuitry shown in the left side of FIG. 2 is similar to that of operation of the memory cell  54  and circuitry shown on the right side of FIG. 2, described above. 
     In compliance with the statute, the invention has been described in language more or less specific as to structural and methodical features. It is to be understood, however, that the invention is not limited to the specific features shown and described, since the means herein disclosed comprise preferred forms of putting the invention into effect. The invention is, therefore, claimed in any of its forms or modifications within the proper scope of the appended claims appropriately interpreted in accordance with the doctrine of equivalents.