Multiple bit line column redundancy

Details Multiple bit line column redundancy Multiple bit line column redundancy

2000-08-21

2002-12-17

Zarabian, A. (Department: 2824)

Static information storage and retrieval

Read/write circuit

Bad bit

C365S230030, C365S185110

Reexamination Certificate

active

06496425

ABSTRACT:

TECHNICAL FIELD OF THE INVENTION
The present invention relates generally to semiconductor memory devices, and in particular, the present invention relates to column redundancy in a multiple bit line architecture for non-volatile semiconductor memory devices.
BACKGROUND OF THE INVENTION
Memory devices are typically provided as internal storage areas in the computer. The term memory identifies data storage that comes in the form of integrated circuit chips. In general, memory devices contain an array of memory cells for storing data, and row and column decoder circuits coupled to the array of memory cells for accessing the array of memory cells in response to an external address.
There are several different types of memory. One type is RAM (random-access memory). This is typically used as main memory in a computer environment. RAM refers to read and write memory; that is, you can repeatedly write data into RAM and read data from RAM. This is in contrast to ROM (read-only memory), which generally only permits the user in routine operation to read data already stored on the ROM. Most RAM is volatile, which means that it requires a steady flow of electricity to maintain its contents. As soon as the power is turned off, whatever data was in RAM is lost.
Computers almost always contain a small amount of ROM that holds instructions for starting up the computer. Unlike RAM, ROM generally cannot be written to in routine operation. An EEPROM (electrically erasable programmable read-only memory) is a special type of non-volatile ROM that can be erased by exposing it to an electrical charge. Like other types of ROM, EEPROM is traditionally not as fast as RAM. EEPROM comprise a large number of memory cells having electrically isolated gates (floating gates). Data is stored in the memory cells in the form of charge on the floating gates. Charge is transported to or removed from the floating gates by programming and erase operations, respectively. Yet another type of non-volatile memory is a Flash memory. A Flash memory is a type of EEPROM that can be erased and reprogrammed in blocks instead of one byte at a time. Many modern PCs have their BIOS stored on a flash memory chip so that it can easily be updated if necessary. Such a BIOS is sometimes called a flash BIOS. Flash memory is also popular in modems because it enables the modem manufacturer to support new protocols as they become standardized.
A typical Flash memory comprises a memory array that includes a large number of memory cells arranged in row and column fashion. Each of the memory cells includes a floating gate field-effect transistor capable of holding a charge. The cells are usually grouped into blocks. Each of the cells within a block can be electrically programmed in a random basis by charging the floating gate. The charge can be removed from the floating gate by a block erase operation. The data in a cell is determined by the presence or absence of the charge in the floating gate.
A synchronous DRAM (SDRAM) is a type of DRAM that can run at much higher clock speeds than conventional DRAM memory. SDRAM synchronizes itself with a CPU's bus and is capable of running at 100 MHZ, about three times faster than conventional FPM (Fast Page Mode) RAM, and about twice as fast EDO (Extended Data Output) DRAM and BEDO (Burst Extended Data Output) DRAM. SDRAMs can be accessed quickly, but are volatile. Many computer systems are designed to operate using SDRAM, but would benefit from non-volatile memory.
As memory sizes continue to increase, satisfying the demands for high-speed access of memory arrays becomes increasingly difficult. To meet these demands, designers are turning to more elaborate memory array architectures. These more elaborate memory architectures introduce problems of their own. One such problem is the implementation of redundancy.
Redundancy is a method of incorporating spare or redundant devices on a semiconductor die that can be used to replace defective devices. As an example, a memory device may have redundant columns of memory cells. If a memory cell is determined to be defective, the column containing the defective memory cell is replaced by a redundant column by redirecting the address of the defective column to the redundant column in a manner known in the art. However, more elaborate memory architectures may be incapable of repair through such straightforward replacements.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for alternate memory array architectures and redundancy organizations in semiconductor memory devices.
SUMMARY OF THE INVENTION
The above-mentioned problems with memory devices and other problems are addressed by the present invention and will be understood by reading and studying the following specification.
Memory devices are described having multiple bit line column redundancy suited for high-performance memory devices, with particular reference to synchronous non-volatile memory devices. Such memory devices include blocks of memory cells arranged in columns with each column of memory cells coupled to a local bit line. Such memory devices further include global bit lines having multiple local bit lines selectively coupled to each global bit line, with each global bit line extending to local bit lines in each memory block of a memory sector. Global bit lines are coupled to sensing devices generally in pairs. Repair of one or more defective columns of memory cells within a sector is effected by providing a redundant grouping of memory cells having a redundant sense amplifier, global bit lines and local bit lines. Access requests directed to memory cells within a grouping of memory cells containing a defective column are redirected to a redundant grouping of memory cells.
For one embodiment, the invention provides a memory array. The memory array includes a first memory block having columns of memory cells coupled to a plurality of first local bit lines, and a second memory block having columns of memory cells coupled to a plurality of second local bit lines. The memory array further includes at least one primary grouping of memory cells having a primary sensing device coupled to a first primary global bit line and a second primary global bit line. Each primary global bit line is coupled to at least two primary local bit lines from the plurality of first local bit lines and at least two primary local bit lines from the plurality of second local bit lines through selective coupling devices. The memory array further includes at least one redundant grouping of memory cells having a redundant sensing device coupled to a first redundant global bit line and a second redundant global bit line. Each redundant global bit line is coupled to at least two redundant local bit lines from the plurality of first local bit lines and at least two redundant local bit lines from the plurality of second local bit lines through selective coupling devices. For another embodiment, the memory array further includes a comparator coupled to the primary groupings of memory cells and the redundant groupings of memory cells for selectively accessing one of the redundant groupings of memory cells when an address applied to the memory array corresponds to a memory cell in a known defective primary grouping of memory cells. For still another embodiment, the memory array further includes addressing circuitry coupled to the primary groupings of memory cells and the redundant groupings of memory cells for decoding an address applied to the memory array, for determining whether the address is directed to a memory cell in a known defective primary grouping of memory cells, and for addressing one of the redundant groupings of memory cells when the address is directed to a memory cell in a known defective primary grouping of memory cells. For yet another embodiment, access requests to a memory cell in a known defective primary grouping of memory cells of the memory array ar

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