Destructive events in NAND Flash memories irradiated with heavy ions
Introduction
Flash memories have experienced a fast growth in the mainstream market during the last years, due to their massive diffusion in portable devices, such as smart-phones, digital cameras, and mp3 players. In addition, the space community has demonstrated an increased interest in commercial-off-the-shelf Flash components [1], [2]. In fact, these memories offer a smart solution for non-volatile storage in space missions: they are cheap, non-volatile, highly integrated, and with sizes that are not available in rad-hard devices. However, they are quite sensitive to ionizing radiation and before using them in the harsh space environment, their susceptibility to radiation must be carefully investigated.
An extensive literature covers the two broad families of radiation effects that can be observed in Flash memories: Single Event Effects (SEE) [1], [3] and Total Ionizing Dose (TID) [1], [4]. Due to the complexity of Flash memories and the large number of heterogeneous building blocks (floating gate cells, buffers, charge pumps, onboard microcontroller, row decoder), their response to radiation can feature different signatures and it is quite often difficult to interpret [5]. Charge pumps (CP) have always been one of the most radiation-soft blocks, from the standpoint of both SEE and TID [1]. In fact, these circuits use relatively high voltages (∼20 V), necessary to program and erase (and in the most advanced devices, also read) the floating gate (FG) memory cells. Consequently, CP transistors feature thick oxides, subject to high electric fields, which are more prone to radiation effects, such as charge trapping and Single Event Gate Rupture.
Recently, a new destructive phenomenon was observed in state-of-the-art Flash memories during heavy-ion irradiation [6], occurring even in stand-by mode. Failure analysis performed on the irradiated samples showed damage to the CP region. The loss of functionality has been associated to the appearance of spikes in the supply current [7], even though no clear evidence exists as of this correlation. The same devices irradiated by other groups, do not show the occurrence of current spikes [8].
The purpose of this contribution is to shed some light on this phenomenon, presenting new experimental data collected with an ad hoc testing procedure. Based on these results, some insight will be provided on the LET and flux dependence, and a new interpretation and physical mechanisms for the observed behavior will be presented.
Section snippets
Studied devices and experimental procedure
In this work we studied 90-nm multi-level cell (two-bit-per-cell) NAND Flash memories manufactured by Samsung (part number K98GG08UOMPCB0), with 3.3-V supply voltage and 8-Gbit size. During irradiation, we selectively protected some of the device functional blocks (either FG cells or CP) with aluminum shields to identify the sensitive regions. We performed heavy-ion irradiations at the SIRAD line of the TANDEM accelerator at Laboratori Nazionali di Legnaro, Padova, Italy. The Linear Energy
Results and discussion
If we expose a memory (M1) to high-LET ions (e.g., Iodine) at a flux of 8 × 103 ions/cm2/s, with no shields, and measure the standby current during irradiation, we observe the behavior shown in Fig. 2. The standby current during irradiation increases several times, sometimes exceeding the maximum datasheet specification by more than 300 times. The current usually goes spontaneously back either to the nominal value (0.3 mA) or to a higher value (25 mA), a few seconds after the spike. An Iodine
Conclusions
We provided new insight on current spikes and permanent loss of functionality in 90-nm two-bit-per-cell commercial NAND Flash memories irradiated with heavy ions in stand-by mode. We analyzed the behavior of the irradiated devices as a function of the LET and flux of impinging particles and we selectively shielded either the charge pumps or the floating gate matrix. In addition, we applied a diagnostic procedure after the observed events, to analyze the device behavior. Our experimental data
Acknowledgments
The authors would like to thank Mario Tessaro, Serena Mattiazzo, and Devis Pantano for their precious help during irradiations at the Laboratori Nazionali di Legnaro.
This work has been partially supported by MIUR under the FIRB Project No. RBIP06YSJJ.
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