Static information storage and retrieval – Interconnection arrangements – Transistors or diodes
Reexamination Certificate
2000-08-30
2003-06-03
Tran, Andrew Q. (Department: 2824)
Static information storage and retrieval
Interconnection arrangements
Transistors or diodes
C365S063000, C365S051000, C365S230030, C365S230040, C365S202000
Reexamination Certificate
active
06574128
ABSTRACT:
FIELD
The present invention relates to integrated circuit memories and, more particularly, to DRAMs and other such memories that are organized in rows and columns.
BACKGROUND
Of the many different kinds of integrated circuit memories, the dynamic random access memory (DRAM) is the commonest.
FIG. 1
shows a block diagram of an early DRAM chip, showing some of the basic architecture. In this drawing, memory array
161
is divided into two parts, A and B, with associated sense amplifiers
162
adjacent the array. Addresses to be accessed are received in the address input buffer
163
and are sent to the row decoder
164
and column decoder
165
. Data to be stored is received on the data-in buffer
166
, and data to be output is sent to the data-out buffer
167
.
At the level of the DRAM array itself, seen in
FIG. 2
, memory cells are arranged in rows (also called word lines) and columns (bit lines). Each cell can be referenced by its unique word line/bit line address, with each Cell
x,y
including a storage capacitor C
x,y
and a pass transistor T
x,y
to provide access to the capacitor. When the pass transistor is turned on, it connects the capacitor to a bit line, which in turn connects the selected cell in a given column to a sense amplifier. The bit lines are arranged as complementary pairs (commonly referred to as a digit and digit bar pair), and each pair is attached to a p-sense amplifier and an n-sense amplifier (which together are often referred to as simply a sense amplifier). In the array shown, even numbered digit pairs (BL
0A
/BL
0B
and BL
2A
/BL
2B
) would be connected to sense amplifiers at the bottom of the array, while odd numbered digit pairs (BL
1A
/BL
1B
and BL
3A
/BL
3B
) would be connected to sense amplifiers at the top of the array. The p-sense amplifier, seen in
FIG. 3
, is composed of two cross-coupled p-channel transistors
1410
and
1420
. Likewise, an n-sense amplifier, seen in
FIG. 4
, is composed of cross-coupled n-channel transistors
1310
and
1320
. The p-channel and n-channel sense amplifiers together form a bi-stable latch which may be fired or floated by controlling connections to high and low potentials. When the cell is accessed for a read or refresh operation, the sense amplifier amplifies the very weak signal from the cell capacitor to provide a useable output signal, and restore the voltage on the capacitor to its maximum high or low value.
In a precharged state, all word lines are at a low level, keeping all storage capacitors isolated from their respective bit lines by the access transistors. All of the bit lines are held at a level typically midway between the high and low potential and shorted together in pairs by an equilibration circuit (not shown).
In operation, the following events take place sequentially: (a) a word line is taken high, allowing a charge share to take place between the storage capacitors of the selected cells and one of each bit line pair; (b) the sharing of charge from the storage capacitors causes a small change higher or lower in the level of the connected digit, which then is at a different level than the other digit in the digit pair; (c) the sense amplifiers are fired, pulling the lower of the two digits comprising a digit pair to the low potential and the higher digit to the high potential; (d) reads (or writes) may reference (or change) the latched state of the digit pair and sense amplifiers; (e) the word line is returned to the low precharged state; and (f) the sense amplifiers are floated and the digit lines are shorted to their original starting state.
One of the possible sources of errors in a dense memory array is externally received transient signals. Another is cross-coupling: whenever two closely-spaced conductors run in parallel for a significant distance, a certain degree of parasitic coupling will exist between the conductors. To alleviate both of these problems, one common architecture is “twisted” bit lines. This is generally accomplished by taking two adjacent bit line pairs (e.g., BL
0A
/BL
0B
and BL
1A
/BL
1B
) and causing them to cross over each other so that the same bit lines are not adjacent to each other for their entire length. This twisting is evident in FIG.
2
.
One inconvenient result of the progress in shrinking memory cells is that the “pitch” of a memory array (i.e., the center-to-center spacing between adjacent columns or adjacent rows) can become too small to contain the needed peripheral circuits. An example of this is seen in the sense amplifiers used to read the cells. Where previously a sense amplifier could be constructed within the width of a bit line pair, the decreasing pitch of shrinking DRAM arrays now means that a sense amplifier needs the width of two bit line pairs for its construction. This is generally handled by putting sense amplifiers at both ends of the array, with one half of the bit line pair being connected to sense amplifiers at a first end of the array, and a second half of the bit line pairs being connected to sense amplifiers at the other end.
As memory cells have continued to shrink, one constraint which becomes more important is parasitic capacitance. As the size of individual cells is reduced, the strength of the electrical signals associated with them is also reduced. At the same time, the number of cells attached to a bit line increases and the length of the bit line itself gets longer, which increases the parasitic capacitance associated with the bit line. With a lower capacitance on the cell and a higher capacitance on the bit line, it takes longer for a useful signal to develop on the line, which slows the response time of the memory, a critical factor. An additional effect of this shrinking capacitance ratio is that the potential difference between digits which the sense amplifier must detect becomes smaller.
One approach which has been tried to reduce bit line capacitance is segmented bit lines. In this approach, the individual cells are not connected to the entire bit line but merely connected to a segment. Segment selection switches connect the appropriate segment for a selected cell to the bit line and to the sense amplifier. However, this approach requires additional area for the segment selection switches and typically also requires an additional layer of metallization.
Another approach is to use isolation gates. U.S. Pat. No. 5,369,622 to McLaury, which is commonly owned by the assignee of this application, explains how an older, single pitched sense amplifier memory can be made to save power by controlling existing isolation devices in a novel manner by isolating unnecessary portions of the bit line during read/write operations.
FIG. 5
shows an example of a bit line pair in array A
x
, such as is shown in this patent. This array contains a single n-sense amplifier for each digit pair, but two p-sense amplifiers at either end of the array A
x
, each used with either subarray A or subarray B. Although this drawing shows a single bit line, the same layout is repeated across the array. Isolation lines isolate_a and isolate_b selectively isolate the non-selected subarray (and its associated p-sense amplifier) from the n-sense amplifier, which is still connected to the selected subarray and its associated p-sense amplifier. This isolation favorably changes the capacitance ratio between the bit line and the cell being read/written, but since the I/O path is connected to only one end of the bit line, it is necessary to turn on the isolation devices to allow full connection of the bit line to I/O devices. McLaury recognized that if the selected cells were in the subarray which was directly connected to the I/O path, then it was possible to save power by controlling the timing on existing isolation devices so that the subarray which was distant from the I/O is kept isolated when it was not involved in the read/write operation.
However, virtually all DRAM memories are now made with dual pitched sense amplifiers shared between two array blocks. An example of this is seen in FIG.
6
. In this drawing, a row of n-sense amplifiers and
Leffert Jay & Polzglaze P.A.
Micro)n Technology, Inc.
Tran Andrew Q.
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