Abstract:
A semiconductor device includes a plurality of bitlines arranged in an array, the plurality of bitlines being grouped in pairs and at least some of the bitlines include a twist. A twist region is disposed along the plurality of bitlines wherein the twist region occupies layout area designated for the twists. An equalizer element is disposed in the twist region for equalizing a pair of bitlines.

Description:
BACKGROUND 
     1. Technical Field 
     This disclosure relates to semiconductor devices, and more particularly, to a semiconductor memory device with bitline equalization in twisted bitline regions. 
     2. Description of the Related Art 
     Many current dynamic random access memory (DRAM) devices employ bitlines (BL) for reading and writing data to storage capacitors of memory cells. Many DRAM designs employ a bitline for each 512 memory cells (e.g., 512 bits/BL). To reduce bitline to bitline coupling effects, a bitline twist is introduced. These twists consume layout space. 
     Referring to FIG. 1, a schematic diagram of semiconductor memory bitline architecture is illustratively shown. The bitline architecture shows a memory array  10  having a plurality of bitlines  12 . Bitlines  12  are paired. Each pair of bitlines includes a bitline true (BLt) and a bitline complement (BLc). Each pair is indexed in FIG. 1 to designate the pair, e.g., BLc i  and BLt i , where i = 1 ,  2 ,  3 , . . . for pair number  1 , pair number  2 , etc. Each pair is coupled to a sense amplifier (SA)  14 . Every other BL pair includes a twist  16 . Twists  16  are employed to reduce bitline coupling between adjacent bitlines, i.e., a portion of BLt&#39;s and BLc&#39;s are separated from adjacent or nearby BLt&#39;s and BLc&#39;s to prevent cross-talk when the adjacent or nearby BLt&#39;s and BLc&#39;s are simultaneously activated. In one example, the layout area consumed by twists  16  includes a width of 6 times the pitch of a wordline  18 . Wordlines  18  in a region  20  are dummy wordlines, which are inactive as a consequence of twists  16  in this region. The twist region  20  has no active functionality, it merely serves to cross true and complement BL&#39;s every other BL-pair. 
     Because sense amplifiers  14  sense charge differences between BL&#39;s, the BL&#39;s need to be equalized after each active cycle. Equalizing transistors  22  are connected across each pair of BL&#39;s (e.g., BLc 1  and BLt 1 ). Transistors  22  are activated by an equalize signal (EQU) to permit conduction between each pair of BL&#39;s before the next active cycle of the BL&#39;s. Transistors  22  are located near sense amplifiers  14  on one end of each pair of BL&#39;s  12 . The time needed to equalize the BL&#39;s, especially for long bitlines (e.g., 512 bits/BL architectures), before the next activate (ACT) command can be issued limits chip performance. 
     Referring to FIG. 2, an illustrative timing diagram is shown for employing BL&#39;s. The following signals are represented: a global clock signal (CLK), a row address strobe (RAS), a column address strobe (CAS) a write enable (WE), an address line (ADR) and an output line signal (DQ). The following command are also shown: activate (ACT), precharge (PRE), write (WRT) and read (READ). tRP (or row address strobe (RAS) to Precharge) represents the time it takes to equalize BL&#39;s before the ACT command. 
     Therefore, a need exists for reducing the time needed to equalize bitlines without chip layout penalty. A further need exists for a layout, which employs the inactive region caused by bitline twists. 
     SUMMARY OF THE INVENTION 
     A semiconductor device of the present invention includes a plurality of bitlines arranged in an array, the plurality of bitlines being grouped in pairs and at least some of the bitlines include a twist. A twist region is disposed along the plurality of bitlines wherein the twist region occupies layout area designated for the twists. An equalizer element is disposed in the twist region for equalizing a pair of bitlines. 
     In other embodiments, the equalizer element preferably includes a transistor coupled between a pair of bitlines. The device may include additional equalizer elements disposed near sense amplifiers of the plurality of bitlines, the additional equalizer elements being located outside the twist region. The additional equalizer elements preferably include transistors. The twist region is preferably centrally disposed along a length of the plurality of bitlines and the equalizer element and the additional equalizer element each equalize the pair of bitlines. The device may include a plurality of equalizer elements wherein the equalizer elements and the additional equalizer elements are enabled by a same equalize signal. The equalizer element may include a dummy component of the semiconductor device adapted to perform an equalizing function for the pairs of bitlines. The twist region may includes a longitudinal axis disposed substantially perpendicular to a length of the bitlines. The twist region may include a width perpendicular to the longitudinal axis and occupies an area of at least one wordline pitch. Every other pair of bitlines may include a twist, or all of the pairs of bitlines may include a twist. 
     Another semiconductor device of the present invention includes a plurality of bitlines arranged in an array, the plurality of bitlines being grouped in pairs and at least some of the pairs of bitlines include a twist. A twist region is disposed along the plurality of bitlines wherein the twist region occupies layout area designated for the twists. A first equalizer element is disposed in the twist region for equalizing a pair of bitlines. A second equalizer element is disposed in the twist region for equalizing bitlines between pairs of bitlines. 
     In other embodiments, the first equalizer element may include a transistor coupled between each pair of bitlines. Each pair of bitlines may include a true bitline and a complement bitline, and the second equalizer element may include a transistor coupled between two adjacent true bitlines or two adjacent complement bitlines. A third equalizer element may be disposed near a sense amplifier of the plurality of bitlines, the third equalizer element being located outside the twist region. The third equalizer element preferably includes a transistor. The first, second and third equalizer elements are preferably enabled by a same equalize signal. The twist region is preferably centrally disposed along a length of the plurality of bitlines and the first and second equalizer elements each equalize the pair of bitlines. The first equalizer element may include a dummy component of the semiconductor device adapted to perform an equalizing function for the pairs of bitlines. The second equalizer element may also include a dummy component of the semiconductor device adapted to perform an equalizing function for the pairs of bitlines. The twist region preferably includes a longitudinal axis disposed substantially perpendicular to a length of the bitlines. The twist region may include a width perpendicular to the longitudinal axis and occupy an area of at least about one wordline pitch. 
     These and other objects, features and advantages of the present invention will become apparent from the following detailed description of illustrative embodiments thereof, which is to be read in connection with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     This disclosure will present in detail the following description of preferred embodiments with reference to the following figures wherein: 
     FIG. 1 is a schematic diagram showing a conventional bitline architecture with dummy wordlines in a twist region; 
     FIG. 2 is a timing diagram showing delay time for an equalize function in accordance with the prior art; 
     FIG. 3 is a schematic diagram showing a bitline architecture with equalizing elements in a twist region in accordance with the present invention; 
     FIG. 4 is a timing diagram showing tRP reduced by an entire cycle for an equalize function in accordance with the present invention; 
     FIG. 5 a schematic diagram of a wordline stitch gap is shown providing a region to connect an equalize line to equalize transistors function in accordance with the present invention; 
     FIG. 6 is a schematic diagram showing a bitline architecture with equalizing elements on both sides of a twist region in accordance with the present invention; and 
     FIG. 7 is a schematic diagram showing a bitline architecture with equalizing elements between bitline pairs and between adjacent bitline pairs in a twist region in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The present invention provides an architecture or layout, which employs the inactive region caused by bitline twists by placing equalizing transistors in this region. The centrally located position of the equalizing transistors permits a significant reduction in equalizing time and reclaims layout area normally not used due to the bitline twists. Advantageously, by employing the present invention, chip performance is increased, and chip layout penalty is reduced. The present invention will be described in terms of an illustrative dynamic random access memory (DRAM) device. However, the present invention should not be construed as limited to the described architecture or device type, as other architectures (e.g., multiple twist regions between two sense amplifier banks, multiple equalize transistors per bitline pair, bitline twists for all bitline pairs, etc.) and device types (e.g., synchronous DRAM, embedded DRAM, static RAM (SRAM), etc.) are also contemplated. 
     Referring now in specific detail to the drawings in which like reference numerals identify similar or identical elements throughout the several views, and initially to FIG. 3, a schematic diagram of a memory array  100  is shown in accordance with one embodiment of the present invention. Memory array  100  includes a plurality of memory cells (not shown) which are selectively connected to bitlines  102  to read or write data thereto. Wordlines  104  are employed to activate an access transistor (not shown) for each memory cell to permit a write or read operation to be performed to either charge or discharge a storage capacitor (not shown) of the memory cell, as is known in the art. 
     Bitlines  102  are paired. Each bitline pair includes a bitline true (BLt) and a bitline complement (BLc) as illustratively shown in FIG. 3 (FIG. 6, and FIG.  7 ). Each pair is indexed in the FIGS. to designate the pair, e.g., BLc i  and BLt i , where i = 1 ,  2 ,  3 , . . . for pair number  1 , pair number  2 , etc. Each bitline pair is coupled to a sense amplifier (SA)  104  at one end of bitline  102 . In this example, every other BL pair includes a twist  106 . However, in other embodiments, all bitlines may be twisted or multiple twist regions may be included between two sense amplifier banks. As described above, twists  106  are employed to reduce bitline coupling between adjacent bitlines, e.g., a portion of BLt&#39;s and BLc&#39;s are separated from neighboring pairs to prevent cross-talk when adjacent or nearby BLs are simultaneously activated. In one example, the layout area consumed by twists  106  includes an integer multiple of wordline pitch, e.g., 6 times the pitch of a wordline  104 . The layout area consumed by twists  106  may also include non-integer multiples of wordline pitch. 
     Sense amplifiers  114  sense charge differences between bitline pairs (BLt and BLc). The bitlines need to be equalized after each active cycle. In accordance with the present invention, equalizing elements  112  (e.g., transistors, switches, or other selective connection)) are connected across each pair of BL&#39;s (e.g., BLc and BLt) in twist region  110 . In this embodiment, transistors  116  are disposed near each sense amplifier  114 . Transistors  112  and  116  are activated by an equalize signal (EQU) to permit conduction between each pair of bitlines  102  before the next active cycle of the bitlines  102 . Transistors  112  and  116 , when conducting, short BLt&#39;s and BLc&#39;s of each bitline pair. In one embodiment, the number of transistors  112  may be increased in region  110 , and transistor  116  may be omitted. Alternately, transistor  116  can be omitted without increasing the number of transistors  112 . In other embodiments, multiple equalize transistors per bitline pair may be employed. Advantageously, in accordance with the present invention, transistors  112  may employ dummy devices normally found in a region  110 . In other words, access transistors which are inactive because of their location in twist region  110  and which would have otherwise been employed with memory cells may be adapted as equalization transistors  112 . Alternately, transistors  112  may be formed during the fabrication of transistors  116  of memory array  100 , by processes known in the art. 
     Transistors  112  and  116  conduct as controlled by EQU and, as such, provide multiple electrical paths between bitline pairs (BLt and BLc). Although one transistor  112  is shown for each bitline pair, multiple transistors may also be employed to couple the bitline pairs in accordance with EQU. 
     In this embodiment, transistors  112  are located in twist region  110 . Advantageously, twist region  110  is located at approximately midway along bitlines  102  between sense amplifiers and ends  115  of the bitlines. One benefit of having additional equalize transistors  112  includes reducing the RC constant (e.g., time delay) needed for equalization. As described above with reference to FIG. 2, the time needed to equalize the BL&#39;s, especially for long bitlines (e.g., 512 bits/BL architectures), before the next activate (ACT) command can be issued, limits chip performance. In the illustrative embodiment of FIG. 3, the time needed to equalize the BL&#39;s is roughly cut in half, since each transistor (transistor  112  and  116 ) has to equalize half of the bitline pair. Reducing tRP (see FIG. 4) increases chip performance by decreasing the amount of time needed before the next activate command (ACT). This amount of time need not be an entire cycle and may include other length of time. 
     Referring to FIG. 4, an illustrative timing diagram is shown for employing BL&#39;s in accordance with the present invention. The following signals are represented: a global clock signal (CLK), a row address strobe (RAS), a column address strobe (CAS) a write enable (WE), an address line (ADR) and an output line signal (DQ). The following commands are also shown: activate (ACT), precharge (PRE), write (WRT) and read (READ). tRP represents the time it takes to equalize BL&#39;s before the ACT command. Comparing FIG. 4 with FIG. 2, tRP is reduced by one-half in the embodiment shown. The decrease in tRP is preferably at least a few nanoseconds depending on the chip design, and clock specifications. A performance boost of even a few nanoseconds is significant considering that millions of access cycles are performed in a few seconds. The performance boost could be distributed to increase the overall access speed or speed yield of the semiconductor chip. 
     A plurality of different options may be realized for the layout of the present invention. These options depend on for example, a particular twist design or design rule constraints (See e.g., Dong-Sun Min, “Multiple Twisted Dataline Techniques for Multigigabit DRAM&#39;s,” IEEE Journal of Solid State Circuits, Vol. 34, No. 6, June 1999, and U.S. Pat. No. 5,534,732 to DeBrosse et al.). One skilled in the art would be able to adapt the present invention to many different designs. For example, in the illustrative embodiment described above, sufficient space is provided if at least a 6 wordline pitch space is consumed by the bitline twist. Equalize transistors  112  may be placed in larger or smaller regions. In addition, access to gates of equalize transistors  112  may be provided in a wordline stitch gap. 
     Referring to FIG. 5, a wordline stitch gap  170  is shown in between groups of bitlines  172 . Wordline stitch gap  170  is a layout area in an array  174  where wordlines  176  are connected to metal lines  178 . No bitlines  180  are present in these gaps  170  which permits connections between the EQU line and equalize transistors  112  (FIG. 3) to global metal lines  178 . 
     Transistors  112  are illustratively shown in FIG. 3 on the left side but may be located on the right side or alternate between the left side and the right side as shown in FIG.  6 . In other embodiments, all equalizing transistors ( 112  and  116 ) are located in the twist region  110 . In addition, multiple equalizing elements (transistors) may be positioned in twist region  110  and employed to couple a same bitline pair. 
     Referring to FIG. 7, a schematic diagram of another embodiment of the present invention is shown. In this embodiment, additional equalize transistors  212  are provided in region  110 . Transistors  212  are connected across adjacent BLt&#39;s and adjacent BLc&#39;s, respectively, between bitline pairs. These additional transistors  212  provide greater equalization between bitline pairs to ensure that charge imbalance is removed from all bitlines. Transistors  212  are enabled by EQU. When activated transistors  212 ,  112  and  116  conduct causing all connected bitlines short against each other. 
     Having described preferred embodiments for bitline twist with equalizer function (which are intended to be illustrative and not limiting), it is noted that modifications and variations can be made by persons skilled in the art in light of the above teachings. It is therefore to be understood that changes may be made in the particular embodiments of the invention disclosed which are within the scope and spirit of the invention as outlined by the appended claims. Having thus described the invention with the details and particularity required by the patent laws, what is claimed and desired protected by Letters Patent is set forth in the appended claims.