Semiconductors
Design considerations for embedded NVM in high-radiation environments
By Amir Regev, VP technology development, Weebit Nano N
on-Volatile Memory (NVM) plays a critical role in many electronic devices today. Any device that boots up, runs code, senses its surroundings, processes or stores data uses some kind of NVM. In these products, NVM stores and loads the operating system (OS), program files and data, and is also responsible for functions such as configuration and trimming.
Semiconductor technologies used in high-radiation environments have special requirements, and NVM is no exception. The types and levels of radiation which may affect NVM depend on the specific application, environment and duration of exposure. On the high end of the spectrum, devices such as orbiting satellites are constantly exposed to a harsh irradiation environment which is not protected by our atmosphere and so must be designed to withstand high doses of more than 1 Mrad of irradiation. On the other end, radiation tolerant devices such as those used in medical applications and low-orbit space deployments must typically be designed to handle more than 300 krads.
When selecting an embedded NVM for applications in high-radiation environments such as aerospace or medical devices, designers must consider whether radiation will impact its operation. Some types of NVM perform better than others.
The challenges of flash in high- radiation environments
Today, flash is the most common NVM used in electronic products, and it presents a challenge for designers of radiation-tolerant devices since it is particularly sensitive to even relatively low doses of radiation. Embedded floating gate memories such as flash store data as an electrical charge, so problems can be caused by the direct interaction of ionizing radiation with their stored charge.
Damage can be based on cumulative 40 May 2023
Figure 1: potential radiation effects on electronics
radiation exposure, known as total ionizing dose, or TID, which can build up over time and can potentially lead to overall degraded performance such as threshold shifts, increased device leakage, timing changes, etc. It can also alter the charge levels stored in the flash memory cell, causing read/write circuitry to malfunction.
Flash can also be impacted by displacement damage from charged particles, which can permanently alter the atomic structure of the material they strike. For example, if the atoms forming the bits are displaced, memory functionality can be lost. Single event effects (SEEs) are also a potential problem. If a particle strike happens during a read or write operation sequence, it could require the routine to restart or may even require a complete power cycle.
Given its radiation sensitivity, using flash memory for rad-hard applications today introduces additional complexities to the design process. Designers must add error correction code (ECC) and also integrate additional memory cells to provide redundancy for continued operation in case of failures. These measures increase
Components in Electronics
the overall die size, power consumption and cost, and can also increase latency. Research has shown that the sensitivity of floating gate memories increases with the move to smaller process geometries.
Radiation-tolerant NVMs New NVM technologies are now entering the mainstream which overcome many of the challenges associated with flash memory, including its sensitivity to radiation. NVM alternatives to flash include Resistive Random Access Memory (ReRAM or RRAM), Phase Change Memory (PCM), Magneto Resistive RAM (MRAM), and Ferroelectric RAM (FRAM). These NVMs are coming to market at an opportune time, as economic and technical challenges make it prohibitive for most applications to embed flash memories into modern SoCs. ReRAM is becoming a leading alternative to flash for applications including those in high radiation environments. Unlike flash, ReRAM is resilient to ionizing radiation, SEE damage and displacement damage, since there is no direct interaction between the radiation and the storage mechanism of the technology.
ReRAM technology
At Weebit Nano, we recently teamed with the Nino Research Group (NRG) in the University of Florida’s Department of Materials Science and Engineering to study the effects of radiation on Weebit ReRAM technology under various conditions. In the first studies, we tested Weebit ReRAM arrays under Gamma irradiation at the University of Florida Training Reactor (UFTR). Gamma radiation was produced using a Cs-137 source with a radiation rate of 702 rad/min. Samples were held in a rotating chamber and were stressed from 0.5 Mrad radiation dose up to 10 Mrad of radiation dose.
ReRAM samples consisted of 16kb 1Transistor-1Resistor (1T1R) arrays integrated in the back-end-of-line (BEOL) of a 130nm CMOS test vehicle. Each sample was previously cycled up to 1k cycles, and then partly programmed in a High Resistive State (HRS) and partly in a Low Resistive State (LRS). After irradiation, cell resistances were read out and reprogrammed to check data retention and memory functionality. The study showed that both HRS and LRS distribution were preserved, confirming that
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