Riichiro Shirota

Fast and accurate programming method for multi-level NAND EEPROMs

For the replacement of conventional hard disks by NAND EEPROMs, a very high density and a high programming speed are required. An increased density can be achieved by using multi-level memory cells. With the new method, using staircase programming pulses combined with a bit-by-bit verify, a very narrow threshold voltage distribution of 0.7 V, necessary for 4-level or 2-bit operation, and a high programming speed of 300 /spl mu/s/page or 590 ns/byte can be obtained.

A NAND structured cell with a new programming technology for highly reliable 5 V-only flash EEPROM

A programming technology is proposed to improve the endurance and read retention characteristics of NAND-structured EEPROM cells programmed by Fowler-Nordheim tunneling of electrons. Erasing and writing are accomplished uniformly over the whole channel area instead of nonuniform erasing at the drain. To achieve programming over the whole channel area, a new device structure is also proposed. The high-voltage pulses can be easily generated on a chip from a single 5-V power supply because the direct current due to the avalanche breakdown does not flow. The gate length of the memory transistor is 1.0 μm. Using 1.0 μm rules, the cell size per bit is 11.7 μm2

A 0.67 /spl mu/m/sup 2/ self-aligned shallow trench isolation cell (SA-STI cell) for 3 V-only 256 Mbit NAND EEPROMs

An ultra high-density NAND-structured memory cell, using a new Self-Aligned Shallow Trench Isolation (SA-STI) technology, has been developed for a high performance and low bit cost 256 Mbit flash EEPROM. The SA-STI technology results in an extremely small cell size of 0.67 /spl mu/m/sup 2/ per bit, 67% of the smallest flash memory cell reported so far, by using a 0.35 /spl mu/m technology. The key technologies to realize a small cell size are (1) 0.4 um width Shallow Trench Isolation (STI) to isolate neighboring bits and (2) a floating gate that is self-aligned with the STI, eliminating the floating-gate wings. Even though the floating-gate wings are eliminated, a high coupling ratio of 0.65 can be obtained by using the side-walls of the floating gate to increase the coupling ratio. Using this self-aligned structure. A reliable tunnel oxide can be obtained because the floating gate does not overlap the trench corners, so enhanced tunneling at the trench corner is avoided. Therefore, the SA-STI cell combines a low bit cost with a high performance and a high reliability, such as the fast programming (0.2 /spl mu/sec/byte), fast erasing (2 msec), good write/erase endurance (>10/sup 6/ cycles), and excellent read disturb characteristics(>10 years). This paper describes the process technologies and the device performance of the SA-STI cell, which can be used to realize NAND EEPROMs of 256 Mbit and beyond.<<ETX>>

A 4 Mb NAND EEPROM with tight programmed V/sub t/ distribution

Described is a 5-V-only 4-Mb (512K*8 b) NAND EEPROM (electrically erasable programmable ROM) with tight programmed threshold voltage (V/sub t/) distribution, controlled by a novel program-verify technique. A tight programmed V/sub t/ distribution width of 0.8 V for the 4 Mb cell array is achieved. By introducing a compact row-decoder circuit, a die size of 7.28 mm*15.31 mm is achieved using 1.0 mu m design rules. A unique twin p-well structure has made it possible to realize low-power 5 V-only erase/program operation easily and to achieve 100 K-cycle endurance. >

A 125-mm/sup 2/ 1-Gb NAND flash memory with 10-MByte/s program speed

A single 3-V only, 1-Gb NAND flash memory has been successfully developed. The chip has been fabricated using 0.13-/spl mu/m CMOS STI technology. The effective cell size including the select transistors is 0.077 /spl mu/m/sup 2/. To decrease the chip size, a new architecture is introduced. The in-series connected memory cells are increased from 16 to 32. Furthermore, as many as 16 k memory cells are connected to the same wordline. As a result, the chip size is decreased by 15%. A very small die size of 125 mm/sup 2/ and an excellent cell area efficiency of 70% are achieved. As for the performance, a very fast programming and serial read are realized. The highest program throughput ever of 10.6-MByte/s is realized: 1) by quadrupling the page size and 2) by newly introducing a write cache. In addition, the garbage collection is accelerated to 9.4-MByte/s. In addition, the write cache accelerates the serial read operation and a very fast 20-MByte/s read throughput is realized.

Trapped hole enhanced stress induced leakage currents in NAND EEPROM tunnel oxides

The stress induced tunnel oxide leakage current occurring in NAND EEPROM memory cells after a large number of WRITE/ERASE (W/E) cycles has been investigated for different W/E pulses. A model for the stress induced leakage current is proposed in which the presence of both holes and neutral oxide traps are a necessary condition for the stress induced leakage current to occur.

A quick intelligent program architecture for 3 V-only NAND-EEPROMs

A quick program/program verify architecture with an intelligent verify circuit for 3-V-only NAND-EEPROMs is described. The verify circuit, which is composed of two transistors, provides a simple, intelligent program algorithm for 3-V-only operation. The total programming time is reduced to 50%. By using intelligent verify circuits, the memory cells which require more time to reach the program state are automatically detected. Verify-read, the modification of program data, and data reload are performed simultaneously. The chip size penalty is estimated to be only 1% for a 16-Mb NAND-EEPROM.<<ETX>>

A novel side-wall transfer-transistor cell (SWATT cell) for multi-level NAND EEPROMs

A multi-level NAND Flash memory cell, using a new Side-WAll Transfer-Transistor (SWATT) structure, has been developed for a high performance and low bit cost Flash EEPROM. With the SWATT cell, a relatively wide threshold voltage (Vth) distribution of about 1.1 V is sufficient for a 4-level memory cell in contrast to a narrow 0.6 V distribution that is required for a conventional 4-level NAND cell. The key technology that allows this wide Vth is the Transfer Transistor which is located at the side wall of the Shallow Trench Isolation (STI) region and is connected in parallel with the floating gate transistor. During read, the Transfer Transistors of the unselected cells (connected in series with the selected cell) function as pass transistors. So, even if the Vth of the unselected floating gate transistor is higher than the control gate voltage, the unselected cell will be in the ON state. As a result, the Vth distribution of the floating gate transistor can be wider and the programming can be faster because the number of program/verify cycles can be reduced. Furthermore, the SWATT cell realizes a very small cell size of 0.67 /spl mu/m/sup 2/ for a 0.35 /spl mu/m rule. Thus, the SWATT cell combines a small cell size with a multi-level scheme to realize a very low bit cost. This paper describes the process technology and the device performance of the SWATT cell, which can be used to realize NAND EEPROMs of 512 Mbit and beyond.

A novel gate-offset NAND cell (GOC-NAND) technology suitable for high-density and low-voltage-operation flash memories

This paper describes a novel scaled and low-voltage-operation NAND EEPROM technology with a G_ate-O_ffset NAND C_ell (GOC-NAND), which is free from program disturbance in a self-boosted program. In GOC-NAND, novel source/drain engineering is introduced for the first time. The program disturbance is decreased by two decades of magnitude in 0.1 /spl mu/m generation, without area penalty and additional process steps. Furthermore, the program disturbance is not increased by scaling and low voltage operation. Therefore, GOC-NAND is indispensable technology for gigabit-scaled NAND EEPROMs.

A 4-Mbit NAND-EEPROM with tight programmed V t distribution

The authors describe a 4-Mb NAND-EEPROM with tight Vt (threshold voltage) distribution which is controlled by a novel program verify technique. A tight Vt distribution width of 0.6 V for the entire 4-Mb cell array is achieved, and read margin is improved. A unique twin p-well structure has made it possible to realize low-power 5-V-only erase/program operation easily compared with the previous design