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KIOXIA develops a new 3D semicircle FLASH unit structure "Twin BiCS FLASH"

Published :12/17/2019 1:36:48 AM

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Tokyo -- Kioxia Corporation (Kioxia Corporation) has announced that it will develop the world's first "Twin BiCS FLASH" three-dimensional (3D) semicircular grille FLASH unit structure using a specially designed semicircular floating gate (FG) unit.Twin BiCS FLASH delivers excellent program slopes and larger program/window cleaning at a smaller unit size than traditional round charge trap (CT) units.These features make this new cell design a strong candidate for more than four bits per cell (QLC) to significantly increase storage density and reduce stack layers.The technology was announced December 11 at the IEEE international conference on electronic devices (IEDM) in San Francisco, California

3D flash technology has achieved high density at low cost per unit by increasing the number of cell stack layers and realizing multilayer stack deposition and high depth-width ratio etching.In recent years, as the number of unit layers has exceeded 100, it has become increasingly challenging to control the trade-offs between etching profile control, size uniformity, and productivity.To overcome this problem, kai developed a new semicircular cell design by splitting the gate electrode in the traditional circular cell to reduce the cell size (compared to the traditional circular cell), thus achieving higher density storage on a smaller number of cell layers.


Because the curvature effect, the circular control gate provides a larger program window than the flat gate and reduces the saturation problem, in which carrier injection through the tunnel dielectric is enhanced while reducing electron leakage in the block (BLK) dielectric.In this grill-type unit design, the circular control gate is symmetrically divided into two semicircular gates to take advantage of the dramatic improvement in program/erasure dynamics.As shown in figure 1, the conductive storage layer is combined with a high k BLK dielectric to improve the efficiency of charge capture and achieve a high coupling ratio to obtain program Windows and reduce electron leakage from FG, thus easing the saturation problem.The experimental program/erase characteristics in figure 2 show that the semi-circular FG unit with a high-k BLK based BLK shows significant gains in the program slope and program/erase window on the larger circular CT unit.Semicircular FG units with excellent program/erasability characteristics are expected to achieve relatively tight QLC Vt distribution at smaller unit sizes.In addition, the integration of low-trap Si channels enables each cell to have more than four bits, for example, the five-tier cell (PLC) shown in figure 3.These results confirm that the semicircular FG unit is a feasible scheme to pursue higher bit density.


Looking ahead, kaixia's efforts to develop FLASH innovations will include continuing Twin BiCS FLASH development and looking for practical applications.During IEDM 2019, Kioxia published six other papers highlighting the company's intensive research and development activities in flash memory