A CMOS-compatible Non-volatile Memory Element using Fishbone-in-cage Capacitor

Abstract

This paper proposes a new non-volatile memory element that can be fabricated with a standard CMOS process and programmed and erased without a large current consumption. This paper also proposes a characteristics measurement circuit for the proposed memory element. Recently, self-powered sensor chips using on-chip solar cells as micro energy harvesters have been studied. For such sensor chips, however, non-volatile memory is indispensable to retain the data during nighttime. We propose a new memory element that consists of a Fishbone-in-Cage Capacitor (FiCC) and an NMOS to realize the double-gate structure of flash memory without using dedicated fabrication processes. We also develop a circuit for measuring the threshold voltage (V T ) of the memory element to clarify the feasibility of using FN tunneling for programming and erasing operations to reduce the supply current. From measurement results, we show that V T shifts to 4.5 V by applying a 5 V programming voltage for 5 sec, and the V T shift remains observable for approximately 13 days. It is also seen that only a slight degradation appears after 25,000 program-erase cycles. We also investigated a non-volatile memory array architecture with bit cells, each of which consists of a pair of proposed memory elements. By writing the memory element pair complementarily, and reading with a differential amplifier, a small difference in V T s can be sensed.

Nonvolatile Storage Cells Using FiCC for IoT Processors with Intermittent Operations

“…First, the store (write) time to the nonvolatile memory is shorter. In [27], the write time is 5 seconds, but only 0.1 seconds in this measurement. When the write operation is over 1 secthe data is written to the nonvolatile memory even if the holding value of the latch is low.…”

“…The erase operation is performed by pulling electrons out of the FG using FNtunneling by applying an opposite voltage between CG and Drain. By applying the write voltage of 5.0 V for 5 seconds to the CG of a nonvolatile memory using FiCC, the thresh-old voltage can be increased to 4 V, and the threshold voltage shift can be observed for approximately 13 days [27]. It is also shown that the memory characteristics do not change significantly after 25,000 write and erase cycles.…”

“…FiCC [27] was proposed to address those issues. FiCC is an MFC that consists of outer electrodes and inner electrodes.…”

“…Therefore, the data retention time can be extended by lowering the voltage applied to the CG during the restore operation. This suggests that changing the voltage applied to the CG during the restore operation can shorten the time required to store (write) data to the nonvolatile memory using FiCC and extend its lifetime The retention time of the nonvolatile memory is much shorter than [27] because of these two reasons. First, the store (write) time to the nonvolatile memory is shorter.…”

“…Inspired by the nonvolatile memory element using Fishbone-in-Cage Capacitor (FiCC) [27], a sort of metal fringe capacitor, this paper proposes to adopt it for an NV-SCM and an NV-FF, presents layout designs and measurement results in them, and shows the effectiveness of nonvolatile system using them. They can be fabricated in a conventional CMOS process without any additional mask.…”