KIOXIA develops new Twin BiCS Flash cell structure

KIOXIA Europe GmbH announced the development of the world’s first split-gate 3D semi-circular flash cell technology based on the Twin BiCS Flash, a floating-gate FG semi-circular cell structure. New structure features improved write performance and a wider write/erase window at a much smaller size than traditional round charge trap CT cells.

Fig. 1. Formed semicircular FG cells: a – cross-section b – top view

With these characteristics, the new cell design is seen as a future-proof solution that outperforms the four-bit-per-cell QLC storage technology, allowing for significantly higher storage densities and fewer layers. The new technology was presented at the IEEE International Electron Devices Meeting IEDM , held on December 11 in San Francisco, California, USA.

3D flash memory technology has achieved high write density at a low cost per bit thanks to an increased number of layers of cells and the implementation of a multilayer structure and long-elongation etch. In recent years, as the number of cell layers has exceeded a hundred, finding a compromise between etch profile management, size uniformity and manufacturing efficiency has become increasingly challenging. To address this, KIOXIA developed a new semicircular cell design by separating the gate output in a traditional circular cell to reduce the size and create a higher density memory with fewer layers. Circular control gate with curvature effect that improves carrier injection through tunnel dielectric and reduces electron leakage into blocking dielectric BLK provides wider write window and reduces saturation effect compared to flat gate.

Fig. 2. Experimental write/erase performance of semi-circular FG cells vs. circular CT cells

In this design, the circular control gate is symmetrically divided into two semicircular gates, greatly improving the write/erase dynamics. As shown in Fig. 1, a conductive storage layer combined with a high permeability blocking dielectric is used to improve charge capture efficiency. The result is a high coupling coefficient to increase the write window, as well as reduced electron leakage from the floating gate, thus eliminating problems caused by saturation. Experimental write/erase performance on Fig. 2 shows that semicircular cells with a floating gate FG and a high permeability blocking dielectric provide a significant improvement in write performance and a wider write/erase window compared to larger round charge trap CT cells.

Fig. 3. Simulated Vt distributions after writing with calibrated parameters

FG semicircular cells, with better write/erase performance, are expected to have a relatively dense QLC Vt distribution at small cell sizes. In addition, the use of a silicon channel with a low trap concentration makes it possible to store more than four bits of data in a single cell and implement, for example, five-level cells PLC , as shown in Fig. 3. These results confirm that semicircular FG cells can serve as a practical solution to increase data storage density.

From now on, KIOXIA’s research and development into innovative flash memory will include further development of Twin BiCS Flash technology and finding practical applications. At IEDM 2019, KIOXIA also presented six other publications demonstrating the company’s high level of research and development activity in flash memory.