On the Reliability of FeFET On-Chip Memory

Ferroelectric Field-Effect Transistor (FeFET) is a promising future technology for non-volatile on-chip memories. It is rapidly attracting an ever-increasing attention from industry. The key advantage of FeFETs is full compatibility with the existing CMOS fabrication process beside their very low power consumption. To enable ultra-dense memories, 1-FeFET AND Arrays were proposed in which a memory cell is formed from merely a single FeFET. All access transistors, which are traditionally needed to operate memory cells, are removed. However, this imposes a new challenge of
indirect write disturbances
. Neighboring memory cells are indirectly degraded whenever a
direct write operation
occurs to a particular FeFET cell. Only recently the impact of such indirect disturbances on the FeFET reliability was experimentally investigated at device (i.e., transistor) level. However, to explore and properly judge the feasibility of 1-FeFET AND Arrays for on-chip memories, investigating only the reliability of individual cells is indeed insufficient. Bridging the gap between the device level and system (i.e., chip) level is inevitable. In the presence of indirect disturbances, the position of a write access within the array plays a key role, which is governed by the running workloads. In addition, whether the write operation flips the previously stored value or not also plays an important role with regards to reliability. Hence, running workloads, which determine not only the position of the memory cells to be written but also the values written to them, plays an essential role in determining 1-FeFET AND Array reliability over time. Therefore, studying the reliability of FeFETs only at the device level (as done in state of the art) is insufficient. In this work, we investigate, for the first time, the reliability of FeFET memories from device to system level. To achieve that, we develop a unified model capturing the impact of both
indirect
disturbances and
direct
writes on the reliability of FeFET cells. Our study at system level then employs the unified model in the context of application workloads. We investigate different array sizes, write voltages, write methods and a wide range of workloads using the example of CPU caches as an example of on-chip memory. We demonstrate that indirect write disturbances are the dominate effect degrading the reliability of FeFET memories. For most cells, it contributes over 90 percent to the overall induced degradation. This provides guidelines for researchers at both device and circuit level to optimize the FeFET reliability further while considering the
hidden
impact of indirect write disturbances.