Non-volatile one time programmable memory

[0001] The present invention relates generally to memory devices and in particular the present invention relates to a flash memory devices.

[0002] Memory devices are typically provided as internal, semiconductor, integrated circuits in computers or other electronic devices. There are many different types of memory including random-access memory (RAM), read only memory (ROM), dynamic random access memory (DRAM), synchronous dynamic random access memory (SDRAM), and flash memory.

[0003] Flash memory devices have developed into a popular source of non-volatile memory for a wide range of electronic applications. Flash memory devices typically use a one-transistor memory cell that allows for high memory densities, high reliability, and low power consumption. Common uses for flash memory include personal computers, personal digital assistants (PDAs), digital cameras, and cellular telephones. Program code and system data such as a basic input/output system (BIOS) are typically stored in flash memory devices for use in personal computer systems.

[0004] Flash memory devices can have a areas of memory that are one time programmable (OTP) memory blocks. For example, in a NOR-type flash memory device, the OTP area could be the mini-array that is used by the manufacturer to store data such as the locations of redundant columns, lock bits for specific memory blocks, or other data used by the device's controller.

[0005] These mini-arrays, however, take up valuable real estate on the memory die that can be used for the main memory array. This results in an additional cost to the chip manufacturer when the manufacturer is already selling the devices at a small profit margin in order to remain competitive.

[0006] 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 one time programmable memory that does not require additional lock circuitry.

[0007] The above-mentioned problems with flash memories and other problems are addressed by the present invention and will be understood by reading and studying the following specification.

[0008] The present invention encompasses a lockable one time programmable memory area in a flash memory device. The method determines the erase status of the memory area. If the erase status indicates that the memory area is programmed, further programming to the memory area is prevented.

[0009] Further embodiments of the invention include methods and apparatus of varying scope.

[0015] In the following detailed description of the invention, reference is made to the accompanying drawings that form a part hereof and in which is shown, by way of illustration, specific embodiments in which the invention may be practiced. In the drawings, like numerals describe substantially similar components throughout the several views. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention. Other embodiments may be utilized and structural, logical, and electrical changes may be made without departing from the scope of the present invention. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present invention is defined only by the appended claims and equivalents thereof.

[0016] While the subsequently described embodiments refer to a NAND flash memory device, the present invention is not limited to such an architecture. The present invention can be implemented in NOR or other types of non-volatile memory configurations.

[0017] FIG. 1 illustrates a simplified diagram of one embodiment for a NAND flash memory array of the present invention. The memory array of FIG. 1, for purposes of clarity, does not show all of the elements typically required in a memory array. For example, only two bitlines are shown (BL1 and BL2) when the number of bitlines required actually depends upon the memory density. The bitlines are subsequently referred to as (BL1–BLN).

[0018] The array is comprised of an array of floating gate cells 101 arranged in series strings 104, 105. Each of the floating gate cells 101 are coupled drain to source in each series chain 104, 105. A word line (WL0–WL31) that spans across multiple series strings 104, 105 is coupled to the control gates of every floating gate cell in a row in order to control their operation. The bitlines (BL1–BLN) are eventually coupled to sense amplifiers (not shown) that detect the state of each cell.

[0019] In operation, the wordlines (WL0–WL31) select the individual floating gate memory cells in the series chain 104, 105 to be written to or read from and operate the remaining floating gate memory cells in each series string 104, 105 in a pass through mode. Each series string 104, 105 of floating gate memory cells is coupled to a source line 106 by a source select gate 116, 117 and to an individual bitline (BL1–BLN) by a drain select gate 112, 113. The source select gates 116, 117 are controlled by a source select gate control line SG(S) 118 coupled to their control gates. The drain select gates 112, 113 are controlled by a drain select gate control line SG(D) 114.

[0020] Each cell can be programmed as a single bit per cell (SBC) or multiple bits per cell (i.e., multilevel cell—MLC). Each cell's threshold voltage (V_t) determines the data that is stored in the cell. For example, in a single bit per cell, a V_t of 0.5V might indicate a programmed cell while a V_t of −0.5V might indicate an erased cell. The multilevel cell may have multiple V_t windows that each indicate a different state. Multilevel cells take advantage of the analog nature of a traditional flash cell by assigning a bit pattern to a specific voltage range stored on the cell. This technology permits the storage of two or more bits per cell, depending on the quantity of voltage ranges assigned to the cell.

[0021] During a typical prior art programming operation, the selected wordline for the flash memory cell to be programmed is biased with a programming pulse at a voltage that is greater than 16V. A verification operation with a wordline voltage of 0V is then performed to determine if the floating gate is at the proper voltage (e.g., 0.5V). The unselected wordlines for the remaining cells are typically biased at approximately 10V during the program operation. Each of the memory cells is programmed in a substantially similar fashion.

[0022] In a NOR configuration, the cells are arranged in a matrix. The gates of each floating gate memory cell of the array matrix are connected by rows to wordlines and their drains are connected to column bitlines. The source of each floating gate memory cell is typically connected to a common source line. Still other embodiments can use other architectures.

[0023] The above-described NAND and NOR flash memory configurations are arranged into memory arrays that are typically broken down into memory subsets referred to as memory blocks. Some of the memory blocks may be designated as one time programmable (OTP) blocks so that the memory device user can program data into these blocks for permanent storage. For example, a manufacturer of a cellular telephone may program telephone and system identification information into these blocks so that the consumer cannot change the information. A memory device manufacture might have one particular block of memory that is designated as an OTP block.

[0024] FIG. 2 illustrates a flowchart of one embodiment of a method of the present invention for programming OTP memory blocks without using a lock bit. This method uses a verify operation to determine if any of the OTP block has been programmed. In one embodiment, the verify operation is substantially similar to an erase verify operation. Since an erased memory block is all logical ones, the verify operation looks for a logical zero to determine that data has been written to the OTP area. In an alternate embodiment, an erased condition might be all logical zeros. In this case, the verify operation looks for a logical one to determine that data has been written to the OTP area.

[0025] When it is desired to write data to the OTP area, a verify operation is performed on the entire OTP block 201. If any data has been written to the area 203, the block is locked 205 to any further programming. If the OTP block is erased 203, data can be written to the area 207. Once the data has been written, the OTP block is now considered locked.

[0026] FIG. 3 illustrates a flowchart of an alternate embodiment of the present invention for programming OTP memory blocks. This embodiment uses a lock bit or lock page to indicate when the OTP block is locked to further programming. In the alternate embodiment, the OTP block can be written to as many times as desired until the memory is locked. The lock bit or lock page can be located in any memory block of the memory device. The embodiments of the present invention do not require the lock bit or lock page to be in the same block as the OTP block.

[0027] Initially, a verify operation may be performed on the lock page or lock bits to determine if they have been programmed 301. In one embodiment, an entire page of memory (e.g., 16 k bits) is used as a lock page. When any one bit of the page (e.g., page 0) has been programmed with any data, the entire OTP block is locked. The programmed address and the data written is not important as long as the verify operation can determine that data has been programmed. Alternate embodiments can use other pages of the OTP block to perform as lock pages. For example, a user space block or a redundant memory block may be used. In still other embodiments, particular bits of a lock page or a particular lock pattern may be used to lock the OTP block. For example, a logical “1010” sequence might be programmed into page 0 in order to lock the OTP block. When this sequence is detected during the verify operation, the OTP block is considered locked.

[0028] If there is data programmed into the lock page or lock bits 303, the OTP memory block is locked from further programming 305. If there are no programmed bits in the lock page 303, the OTP block has not been locked and data can be written to this area 307. When the user is done writing data 309 and desires to lock the OTP area, data is programmed into the lock page 311 or the lock bits of the page are programmed. If it is not desired to lock the OTP block 309, data can be subsequently written to and/or erased from the OTP block until the lock page/bits have been programmed.

[0029] The embodiments of FIGS. 2 and 3 refer to the OTP area as being a memory block. However, alternate embodiments can use the methods of the present invention to lock multiple memory blocks or even an entire memory device. The size of memory to be locked is not limited by the present invention.

[0030] FIG. 4 illustrates a flowchart of one embodiment for locking an OTP area from being erased. Since it is typically desirable that an OTP memory block not be changed in any way once it is programmed as desired, this method provides a user with the ability to keep others from erasing the OTP memory. In one embodiment, this method is part of the embodiments of the above-described method for locking the OTP area. In an alternate embodiment, the erase inhibit method is executed as a stand-alone method.

[0031] The verify operation is performed 401 on the lock page or the lock bits, depending on the embodiment used. If the page/bits is erased 403, the OTP area is considered unlocked and can be erased 407. If the page/bits have been programmed in some way or if the lock pattern is present, again depending on the embodiment, the OTP is locked and an erase operation is prevented.

[0032] FIG. 5 illustrates a functional block diagram of a memory device 500 that can incorporate the flash memory array with the lockable OTP block of the present invention. The memory device 500 is coupled to a processor 510. The processor 510 may be a microprocessor or some other type of controlling circuitry. The memory device 500 and the processor 510 form part of an electronic system 520. The memory device 500 has been simplified to focus on features of the memory that are helpful in understanding the present invention.

[0033] The memory device includes an array of flash memory cells 530 as described above with reference to FIG. 1. The memory array 530 is arranged in banks of rows and columns. The control gates of each row of memory cells is coupled with a wordline while the drain and source connections of the memory cells are coupled to bitlines. As is well known in the art, the connections of the cells to the bitlines determines whether the array is a NAND architecture or a NOR architecture.

[0034] An address buffer circuit 540 is provided to latch address signals provided on address input connections A0–Ax 542. Address signals are received and decoded by a row decoder 544 and a column decoder 546 to access the memory array 530. It will be appreciated by those skilled in the art, with the benefit of the present description, that the number of address input connections depends on the density and architecture of the memory array 530. That is, the number of addresses increases with both increased memory cell counts and increased bank and block counts.

[0035] The memory device 500 reads data in the memory array 530 by sensing voltage or current changes in the memory array columns using sense/buffer circuitry 550. The sense/buffer circuitry, in one embodiment, is coupled to read and latch a row of data from the memory array 530. Data input and output buffer circuitry 560 is included for bi-directional data communication over a plurality of data connections 562 with the controller 510. Write circuitry 555 is provided to write data to the memory array.

[0036] Control circuitry 570 decodes signals provided on control connections 572 from the processor 510. These signals are used to control the operations on the memory array 530, including data read, data write (program), and erase operations. The control circuitry 570 may be a state machine, a sequencer, or some other type of controller. In one embodiment, the control circuitry 570 is responsible for executing the embodiments of the OTP lock method of the present invention.

[0037] The flash memory device illustrated in FIG. 5 has been simplified to facilitate a basic understanding of the features of the memory. A more detailed understanding of internal circuitry and functions of flash memories are known to those skilled in the art.

[0038] In summary, the embodiments of the present invention provide a way to lock one time programmable areas of flash memory devices without additional circuitry to take up valuable real estate on the memory die. The OTP memory block, or other lockable areas, is user lockable with the present invention so that the user can store important data that is desired to be made unchangeable to consumers or other users. Once locked, the OTP memory block and the lock page/bits cannot be programmed or erased.

[0039] Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that any arrangement that is calculated to achieve the same purpose may be substituted for the specific embodiments shown. Many adaptations of the invention will be apparent to those of ordinary skill in the art. Accordingly, this application is intended to cover any adaptations or variations of the invention. It is manifestly intended that this invention be limited only by the following claims and equivalents thereof.