Memory devices are typically provided as internal storage areas in a computer. One type of memory used to store data in a computer is random access memory (RAM). RAM is typically used as main memory in a computer environment. Most RAM is volatile. That is, RAM generally requires a steady flow of electricity to maintain its contents. As soon as the power is turned off, all data stored in the RAM is lost.
Another type of memory is a non-volatile memory. A non-volatile memory is a type of memory that retains stored data when power is turned off. A flash memory is a type of non-volatile memory. An important feature of a flash memory is that it can be erased and reprogrammed in blocks instead of one byte at a time. Each block of memory in a memory array of the flash memory comprises rows and columns of memory cells. Many modern computers have their basic I/O system (BIOS) stored on flash memory chips.
As with other memory devices, defects can occur during the manufacture of a flash memory array having rows and columns of memory cells. Typical defects can include bad memory cells, open circuits, shorts between a pair of rows and shorts between a row and column. Shorts typically occur because of the large number of rows and columns of memory cells that have to be placed in close proximity to each other on an integrated circuit wafer. Defects can reduce the yield of the flash memory device. A way to resolve this problem, without discarding the memory device, is to incorporate redundant elements in the memory that selectively replace the defective elements. For example, redundant columns are a common form of redundant elements used in flash memory to replace defective memory cells coupled to a primary column. Redundant rows are another common form of redundant elements used in flash memory to replace defective memory cells coupled to a primary row.
After a memory die has been manufactured, it is tested for defects. Generally with volatile memory, redundant circuitry or redundant fuse circuitry is used to selectively route address requests directed to the defected elements to the redundant elements. Redundant circuitry can comprise registers of banks of multiple electrical fuses that are selectively “blown” (i.e. open circuited) to store the address of the defective elements in the primary array. Thereafter, the redundant circuitry compares each address request with addresses stored in the banks of electrical fuses. When the redundant circuitry discovers an address request that matches an address in a bank of electric fuses, the address request is redirected to an associated redundant element instead of the defective element in the primary array. The electrical fuses are generally blown by one of two methods to store the address of the defective elements. The first is known as the Ohm heating method. This method involves driving a substantial current through a fuse to melt the fuse's conductive material. The other method is known as the laser method. The laser method uses a laser to cut a fuse's conductive material. Anti-fuse circuitry having anti-fuse registers can also be used to store the address of defective elements in the primary array. Anti-fuses are normally open and short circuited (closed) when programmed.
Some memory devices, including some flash memory devices, utilize registers of non-volatile memory to store addresses of defective elements in the primary array. As with registers of banks of multiple fuses, the non-volatile registers are generally contained in redundant circuitry or a redundant circuit. The redundant circuit compares address requests to addresses stored in the registers. If an address request matches an address stored in a register, the redundant circuit directs or maps the address request to an associated redundant element instead of the defective element.
Generally, the use of redundant elements works well, however, problems can occur if the defect involves shorts between two rows or shorts between a row and a column in a primary memory block array. This is because, even though a redundant row or column is read to or written to instead of the defective row or column, the short is still embedded in the primary memory array and the defect can effect other elements in the primary array. For example, a row that is shorted to another row may result in problems during the pre-program and soft program cycles of an erase operation. A method is needed to determine the type of defect the redundant elements are designated to replace and to selectively deal with the specific type of defect during an erase operation.
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 a flash memory that has the ability to implement and manage a redundant circuit in a flash memory to deal with specific defects during an erase operation.