Floating Gate (FG) memories, and in particular Flash, are the most important player in nowadays nonvolatile memory arena [1]. The basic structure of all FG memories relies on an insulated polysilicon layer (that is, the FG) interposed between the substrate and the Control Gate, CG (see Fig.1). By accumulating a net charge in the FG we can modify the threshold voltage VTH of the FG transistor, thus storing a bit of information or more. Electrons or holes in the insulated FG cannot escape from it (at least in principle), hence granting a permanent storage of information. Electrons and holes can be injected (emitted) in (from) the FG by using Channel Hot Electron (CHE) injection or Fowler-Nordheim (F-N) tunneling [1][2]. Flash memories feature extremely high density, while maintaining good speed and reliability, but also a complex control circuitry. Single Event Effect (SEE) performances of Flash memories has been studied by several authors [3][4][5]. The most radiation sensitive part of commercial Flash memories is the complex circuitry external to the FG memory cell array [3]. For converse, the loss of the charge stored in the floating gate of a programmed cell and the consequent threshold voltage shift ÄVTH have been less frequently investigated in literature [6][7][8]. By using specially designed instrumentation and devices, we have recently shown that FG charge loss upon heavy ion irradiation is not negligible [7][8], even when it does not lead to a read error at the circuit output. The charge loss subsequent to a single heavy ion strike appears to be due to two parallel mechanisms. The first is a prompt one, taking place in times shorter than those elapsed between irradiation and measurement, and it appears as the responsible for the main part of the charge loss [8]. The second mechanism, which is the main subject of this paper, is active over long times (days and weeks) and is responsible for the slow discharge of some of the hit FGs [8]. In this context, the aim of this paper is to investigate the long-term retention issues in two advanced Flash memory technologies submitted to heavy ion irradiation.
Data retention of irradiated FG memories / Cellere, G; Larcher, Luca; Paccagnella, A; Modelli, A; Candelori, A.. - STAMPA. - (2004), pp. 1-4. (Intervento presentato al convegno 2nd SIRAD Workshop tenutosi a INFN National Laboratory, Legnaro (PD, Italy) nel April 2004).
Data retention of irradiated FG memories
LARCHER, Luca;
2004
Abstract
Floating Gate (FG) memories, and in particular Flash, are the most important player in nowadays nonvolatile memory arena [1]. The basic structure of all FG memories relies on an insulated polysilicon layer (that is, the FG) interposed between the substrate and the Control Gate, CG (see Fig.1). By accumulating a net charge in the FG we can modify the threshold voltage VTH of the FG transistor, thus storing a bit of information or more. Electrons or holes in the insulated FG cannot escape from it (at least in principle), hence granting a permanent storage of information. Electrons and holes can be injected (emitted) in (from) the FG by using Channel Hot Electron (CHE) injection or Fowler-Nordheim (F-N) tunneling [1][2]. Flash memories feature extremely high density, while maintaining good speed and reliability, but also a complex control circuitry. Single Event Effect (SEE) performances of Flash memories has been studied by several authors [3][4][5]. The most radiation sensitive part of commercial Flash memories is the complex circuitry external to the FG memory cell array [3]. For converse, the loss of the charge stored in the floating gate of a programmed cell and the consequent threshold voltage shift ÄVTH have been less frequently investigated in literature [6][7][8]. By using specially designed instrumentation and devices, we have recently shown that FG charge loss upon heavy ion irradiation is not negligible [7][8], even when it does not lead to a read error at the circuit output. The charge loss subsequent to a single heavy ion strike appears to be due to two parallel mechanisms. The first is a prompt one, taking place in times shorter than those elapsed between irradiation and measurement, and it appears as the responsible for the main part of the charge loss [8]. The second mechanism, which is the main subject of this paper, is active over long times (days and weeks) and is responsible for the slow discharge of some of the hit FGs [8]. In this context, the aim of this paper is to investigate the long-term retention issues in two advanced Flash memory technologies submitted to heavy ion irradiation.
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