Tokyo — (Business Wire) — kioxia Corporation recently announced that it will use specially designed semi-circular floating gate (FG) units to develop the world’s first  three-dimensional (3D) semi-circular grid dividing flash memory unit structure “twin BICS flash”. Compared with traditional circular charge trap (CT) cells, twin BICS flash can achieve excellent program slope and larger program / window cleaning in smaller cell size. These features make this new cell design a powerful candidate for more than four bits per cell (QLC) to significantly improve storage density and reduce stack layers. The technology was announced at the IEEE International Conference on electronic devices (IEDM) held in San Francisco, California on December 11.
3D flash memory technology has achieved high density at low cost by increasing the number of cell stack layers and realizing multi-layer stack deposition and high aspect ratio etching. In recent years, as the number of cell layers exceeds 100, it becomes more and more challenging to control the trade-off between etching profile control, size uniformity and productivity. In order to overcome this problem, Kaixia developed a new semi-circular cell design by splitting the gate electrode in the traditional circular cell to reduce the cell size (compared with the traditional circular cell), so that higher density storage can be realized on a small number of cell layers.
Thanks to the curvature effect, the circular control gate provides a program window larger than the plane gate and reduces the saturation problem. In the curvature effect, the carrier injection through the tunnel dielectric is enhanced and the electron leakage of the block (BLK) dielectric is reduced. In this grid cell design, the circular control grid is symmetrically divided into two semicircular gates to take advantage of the significant improvement of program / erase dynamics. As shown in Fig. 1, the conductive storage layer is combined with high-k BLK dielectric to improve charge capture efficiency and achieve high coupling ratio, so as to obtain program window and reduce electron leakage from FG, so as to alleviate the saturation problem. The experimental program / erase characteristics in Fig. 2 show that the semi-circular FG unit with BLK based on high-k shows significant gain in program slope and program / wipe window on larger circular CT units. Semicircular FG cells with excellent program / erase characteristics are expected to obtain relatively tight QLC VT distribution under small cell size. In addition, the integration of low trap Si channels enables each unit to have more than four bits, for example, a five layer unit (PLC) as shown in Fig. 3. These results confirm that the semicircular FG element is a feasible scheme to pursue higher density.
Looking forward to the future, Kaixia is committed to the research and development of flash memory innovation, which will include continuing the development of twin BICS flash and seeking its practical application. During IEDM 2019, kioxia also published six other papers focusing on the company’s intensive R & D activities in the field of flash memory.