Optimal Cell Structure/Operation Design of 3D Semicircular Split-gate Cells for Ultra-high-density Flash Memory

Three-dimensional (3D) semicircular split-gate floating-gate (FG) cells have been successfully developed for expanding the capability of multiple bits per cell. Large program/erase (P/E) window, tight V_th distributions and superior data retention characteristics were realized by optimizing read operation and FG structures. As a result, V_th distributions of four bits/cell (quadruple-level cell: QLC) and five bits/cell (penta-level cell : PLC) were experimentally demonstrated.

3D flash memory technology has responded to the market demand for higher bit density with low cost and larger capability by increasing the number of cell stack layers ^[1]. Aggressive cell stacking structure is requiring multi-layer stack deposition and high aspect ratio etch. So the fundamental trade-offs among etch profile control, size uniformity and productivity have been severe. To overcome this issues, we proposed split-gate flash memory with semicircular FG cells (Twin BiCS FLASH) ^[2] and continued to refine the cell characteristics. Control gate (CG) split structure has several challenge such as leakage current of back-side cell, P/E window reduction due to parasitic leakage at gate-edge and random telegraph noise (RTN) increase due to smaller cell area. We overcome these challenges by optimizing read operation and cell structures and realize large P/E window, tight V_th distributions and superior data retention characteristics ^[3].

This split-gate cell has a structure in which FG and CG are divided while the channel is connected. Therefore, when reading a selected cell, leakage current through the channel of the back-side cell should be suppressed. By optimizing back-side gate bias, this leakage is cut-off and the current of selected cell have been obtained correctly, as shown in (Figure 2). By this optimization, V_th of selected cells can be read regardless of the state of the back-side cells (Figure 3).

In the split-gate cell, reduction of parasitic leakage at gate-edge which caused by fringing field effect is essential to extract the maximum P/E window. This fringing field effect is suppressed by FG structure engineering, and large P/E window of more than 10 V is obtained as shown in (Figure 4). RTN increase due to cell size reduction is also challenge of split-gate cells, however adoption of FG makes RTN of small-size cell comparable to conventional circular charge trap (CT) cell (Figure 5)

The FG in split-gate cells is completely isolated for each cells, so charge migration across adjacent cells is suppressed compare to CT cells. As a result, constant data retention characteristics is obtained regardless of the V_th difference with adjacent cells (Figure 6).

By optimizing the cell operation and structure, the distributions of QLC and PLC have been experimentally demonstrated by the split-gate cell arrays for the first time (Figure 7).

Twin BiCS FLASH, in which the control gate of the conventional circular cell is divided into the semicircular shape, can achieve both cell size reduction and multiple bits per cell. This cell structure is a promising candidate to boost bit density with minimum increase in stacking layer number.