Flash Memories:Flash Memory Array Structures.

Flash Memory Array Structures

NOR Type Array

In general, most of the Flash memory array, as shown in Figure 54.25(a), is the NOR-type array [49–61]. In this array structure, two neighboring memory cells share a bit-line contact and a common source line. Therefore, a half the drain contact size and half the source line width is occupied in the unit memory cell. Since the memory cell is connected to the bit-line directly, the NOR-type array features random access and lower series resistance characteristics. The NOR-type array can be operated in a larger read current and thus a faster read operation speed. However, the drawback of the NOR-type array is the large cell area per unit cell. In order to maintain the advantages in NOR-type array and also reduce the cell size, there were several efforts to improve the array architectures. The major improvement in the NOR-type array is the elimination of bit-line contacts—the employment of buried bit-line configuration [52]. This concept evolves from the contactless EPROM proposed by Texas Instruments Inc. in 1986 [89]. By using this contactless bit-line concept, the memory cell has a 34% size reduction.

AND Type Families

Another modification of the NOR-type array accompanied by a different operation mode is the AND-type array. In the NOR-type array, the CHEI is used as the electron injection scheme. However, owing to the considerations of power consumption and series resistance contributed by the buried bit-line/ source, both the programming and erase operations utilize FN tunneling to eliminate the above concerns. Some improvements and modifications based on the NOR-type array have been proposed, including DIvided-bitline NOR (DINOR) proposed by Mitsubishi Corp. [65,68], Contactless NOR (AND)

proposed by Hitachi Corp. [64,66], Asymmetrical Contactless Transistor (ACT) cell by Sharp Corp.[69], and Dual String NOR (DuSNOR) by Samsung Corp. [70] and Macronix, Inc. [67]. The DINOR architecture employs the main bit-line and sub-bit-line configuration to reduce the disturbance issue during FN programming. The AND and DuSNOR structures consist of strings of memory cells with n+ buried source and bit-lines. String-select and ground-select transistors are attached to the bit and source line, respectively. In DuSNOR structure, a smaller cell size can be realized because every two adjacent cell strings share a source line. Although a smaller cell size can be obtained utilizing the buried bit-line and source line, the resistance of the buried diffusion line would degrade the read performance. The read operation consideration will be the dominant factor in determining the size of a memory string in the AND and DuSNOR structures.

NAND Type Array

In order to realize a smaller Flash memory cell, the NAND structure was proposed in 1987 [90]. As shown in Figure 54.25(b), the memory cells are arranged in series. It was reported that the cell size of the NAND structure is only 44% of that in the NOR-type array under the same design rules. The operation mechanisms of a single memory cell in the NAND architecture is the same as NOR and AND architectures.

However, the programming and read operations are more complex. Besides, the read operation speed is lower than that in the NOR-type structure because a number of memory cells are connected in series.

Originally, the NAND structure was operated with CHEI programming an FN tunneling through the channel region [90]. Later on, edge FN ejection at drain side was employed [62,63]. However, owing to reliability concerns, operations utilizing the bi-polarity write/erase scheme were then proposed to reduce the oxide damage [71–78]. Owing to the memory cells in the NAND structure being operated by FN write and erase, in order to improve the FN operation efficiency and reduce the operation voltage, the booster plate technology on the NAND structure was proposed by Samsung Corp. [77]