A 3.3V single-supply 32Mb flash memory realizing a 512B per sector progranderase unit features serial sector read, sector program and sector erase modes. By usingAND cells and connecting one sense & latch (SL) circuit to every data line (DL) pair, these modes can handle data strictly sector by sector (512B) [l]. The same sector size for both programming and erasing simplifies the rewrite operation to a small number of sectors and prevents system performance degradation.The operation of the AND memory cell is shown in Figure 1. Injection of electrons to the floating gate by FN tunneling raises the Vth of the memory cell. To lower the Vth, electrons are emitted to the drain by tunnel emission.The bits selected by one word line (WL) correspond to the data of 64 memory cells (b1ock)in one sector are connected in parallel by a common diffusion area. In serial read mode, all data in the selected sector are sensed by the SL simultaneously. These data are then output synchronized with an external serial clock. The access time of a serial read is 36ndB. As this SL can be used as a data register in program mode, up to 512B of data can be buffered with no additional chip area. The direct connection of buffer and memory cells requires no access penalty such as for data transfer from individual buffers in the peripheral circuit to the memory array. The SL is also used for bit-by-bit control of memory Vth during auto program verify and auto erase verify operations.Operation of the serial read mode are illustrated in Figure 2. DLs inthe selectedmemoryarrayareprechargedto lV, whileunselected DLs are precharged to the reference level of O.5V by M1 and M2.Then if the cell has low Vth, the DL will be eventually discharged below the reference level. SL compares the reference and data levels and read the data as the programmed state (low Vth), or "0". The same operation takes place in the program verify and erase verify modes, except that in program mode DLs are selectively precharged according to the data latched in SL (Figure 2 SPC0,l). This selective precharge is used for bit-by-bit verify.As DL pitch decreases, coupling noise from adjacent DLs becomes significant. Especially noise during the discharge phase leads to erroneous reading of "1" data as " 0 data. To reduce this noise, the operation is divided into two phases, one for the DLs with even addresses and one for DLs with odd addresses. The inactive DLs work as shield lines in each phase of operation. For DL space ofless than lpm, the coupling noise is reduced by 20 to 30%.One of the advantages of the AND-ceU is that it is suitable for both high-speed operation and highdensity application. To increase flexibility, the chip also has a random byte read mode. Any one byte can be read randomly with an access time of 751x3. In random-read mode, the data path is switched as shown in Figure 3. The current through the selected memory cell is converted to voltage by a current-voltage converter. The reference current passing through dummy memory cells is also converted and used to com...
We show that the maximum reoxidation time is one of the useful parameters for determining the optimum processing condition of reoxidized nitrided oxide (RONO) film. The maximum reoxidation time is defined as the time in which the thickness of a nitrided oxide film increases 10%. Since many RONO process parameters have a close relationship to this parameter, it is easy to determine an optimum RONO condition. Using such an optimized condition of RONO film formation, that is, performing the nitridation of oxide in 50% NH3 ambient at 1050°C for 30 s and following it with reoxidation in O2 ambient at 1100°C for 120 s, we formed 9-nm-thick RONO film as the EEPROM tunnel dielectric. The film is found effective not only in improving the endurance to more than 106 cycles, but also in causing no degradation at all to the characteristics of the 1M-bit EEPROM devices actually fabricated.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.