Michael Lisiansky

Radiation Tolerance of NROM Embedded Products

Radiation tolerance of NROM memories is demonstrated at the level of industrial 4 Mbit memory embedded modules, specifically not designed for operation in radiation harsh environments. The memory fabricated in 0.18 um technology remains fully functional after total ionization doses exceeding 100 krad. The tests were performed by irradiating with γ-rays ( 60Co source) and 10 MeV 11B ions in active (during programming/erase and read-out) and passive (no bias) modes. Comprehensive statistics were obtained by using large memory arrays and comparison of the data with the parameters of irradiated single cells allowed deep understanding of the physical phenomena in the irradiated NROM devices for both moderate (<;1 Mrad) and large (> 1 Mrad) TID. The obtained data is currently employed in the design of the new generation of NROM memories, having improved radiation tolerance.

Ionizing Radiation Effects on Non Volatile Read Only Memory Cells

Threshold voltage (<i>V</i>_th) and drain-source current (<i>I</i>_DS) behaviour of nitride read only memories (NROM) were studied both in situ during irradiation or after irradiation with photons and ions. <i>V</i>_th loss fluctuations are well explained by the same Weibull statistics regardless of the irradiation species and total dose. Results of drain current measurements in-situ during irradiation with photons and ions reveal a step-like increase of <i>I</i>_DS with the total irradiation dose. A brief physical explanation is also provided.

Compact instrumentation for radiation tolerance test of flash memories in space environment

Aim of this work is the description of a test equipment, designed to be integrated on board of a microsatellite, able to investigate the radiation tolerance of non-volatile memory arrays in a real flight experiment. An FPGA-based design was adopted to preserve a high flexibility degree. Besides standard Program/Read/Erase functions, additional features such as failure data screening and latch-up protection have been implemented. The instrument development phase generated, as a by-product, a non-rad-hard version of the instrument that allowed performing in-situ experiments using 60Co and 10 MeV Boron irradiation facilities on Ground. Preliminary measurement results are reported to show the instrumentation potential.

Threshold Voltage Variability of NROM Memories After Exposure to Ionizing Radiation

Threshold voltage (Vth) behavior of nitride read-only memories (NROMs) was studied after irradiation with photons (γ- and X-rays), light and heavy ions. Both programmed and nonprogrammed single cells were investigated. The data suggest that two main physical phenomena are contributing to Vth variation and that the Vth loss and the variability can be modeled by a Weibull statistics with a shape parameter k ~ 2.2 regardless of the irradiation species and total dose. The same peculiarities were found in large memory arrays, confirming the results from single-cell studies but with significantly larger statistics. Hence, once the irradiation dose is known, the Vth loss distribution can be obtained, thus providing a predictive model of the radiation tolerance of NROM memory arrays.

Extending endurance of NROM memories to over 10 million program/erase cycles

We report on NROM (nitride read only) memory with enhanced endurance/retention. A novel “refresh” is introduced into the cycling algorithm to exclude parasitic electron trapping in the memory transistor. Negative gate pulses are applied when the drain voltage in the erase procedure reaches the threshold value. The memory stack is optimized to allow injection of holes from the substrate through the bottom oxide (BOX). More than 10 million program/erase (P/E) cycles with excellent retention are easily achieved.