Memories
Reliability and Radiation Effects on Advanced CMOS Technologies

 

 

We investigate radiation effects in volatile and non-volatile memories:

Floating-gate memories

For these devices the control circuitry, rather than the array itself, has been found to be the most radiation-sensitive part of the device in most studies. However, radiation induced phenomena in the FG array are becoming more and more important with the advances in technology.
Radiation induced phenomena in FG memories are interesting because of their reliability implications, and because of some peculiar characteristics which makes them interesting vehicles to study the radiation-matter interaction:

charge loss happens in 10fs in out-of-equilibrium conditions;

  • even FGs not hit by ions can experience charge loss;
  • an ion crossing the device under grazing incidence can lead to a track of FGs losing charge;
  • the ion leaves a track of defects which endanger successive retention cababilities of the cell (Radiation Induced Leakage Current, RILC);
  • RILC has a noticeable erratic behavior;
  • TID performances depend on the source (protons, x-rays, gamma rays);
  • high energy protons result in both TID and SEE.

SRAMs

We investigate SEU and TID effects in SRAM memories as a function of radiation type, supply voltage, radiation incidence angle, and temperature. We test commercial SRAMs from different vendors under neutron radiation at ISIS facility (both with high-energy and thermal neutrons), alpha particles emitted by a portable 241Am source, heavy ions, and x rays at LNL accelerator. A ‘cold finger’ enables experiments at low temperatures (down to liquid Nitrogen) while a controlled heater reaches temperatures up to hundreds °C. Our first experiments with alpha particles, heavy ions, and neutrons show that there is not a clear trend of SEU dependence on temperature. In fact, the trend depends on the memory vendor and technology. In some samples, we detect frequent SELs that make the tests under radiation exposure difficult to carry out.
To understand the mutual relationship existing between SEE and TID effects and between radiation and electrical stresses, we expose some samples to x rays and to wear-out before heavy-ions radiation. Then, HSPICE simulations allow us to accurately predict and model the observed behavior.

Peripheral circuitry

Real-time experiments with Flash memories (manufactured by both STMicroelectronics and other commercial vendors) allow us to understand what problems may originate when ionizing radiation strikes the different parts of a Flash memory under operating conditions. Not only errors on Floating Gate cell array (see section Floating-gate memories), but also effects on the complex peripheral and control circuitry of these devices are studied. Irradiating the memories shielding the different parts of the die and thanks to the confidential information provided by STMicroelectronics, we are able to shed light on the origin of the different kind of errors we encounter and to calculate a cross section that takes into account the impact of operating conditions. According to the way the memory is used (frequent reads, rather than continuous program/erase operations), the device may experience different kinds of errors, from SEUs in the Page Buffer latches used during read, to degradations of the charge pumps, to SEFIs in the finite state machine, accompanied by sudden bursts in supply current, besides the corruption of the bit stored in the Floating Gate cells.

Phase Change Memories (PCM)

We are studying the effects of ionizing radiation on Phase Change Memories (PCM). We evaluate the effects of different radiation sources: heavy-ions, X-rays, protons and electrons. Our aim is to identify the effects of the interaction between ionizing radiation and the chalcogenide material (GST) used to store the information in PCM. We also investigate the combined effects of radiations and high temperature, performing irradiation experiments at high temperature. The retention characteristic after irradiation are evaluated by high temperature accelerated retention tests.
At a higher level, we study the effects of ionizing radiation on the CMOS components of completed PCM array chips. 

 

RECENT RESULTS

The page buffer plays a fundamental role in determining the soft error rate of a NAND Flash memory.