We demonstrate that infrared femtosecond laser pulses with intensity above two-photon ionization threshold of crystalline silicon (c-Si) induce charge transport through the tunnel oxide in floating gate Metal-Oxide-Semiconductor (MOS) transistor devices. With repeated irradiations of Flash memory cells, we show how the laser-produced free-electrons naturally redistribute on both sides of the tunnel oxide until the electric field of the transistor is suppressed. This ability enables to determine in a nondestructive, rapid and contactless way the flat band and the neutral threshold voltages of the tested device. The physical mechanisms including nonlinear ionization, quantum tunneling of free-carriers, and flattening of the band diagram are discussed for interpreting the experiments. The possibility to control the carriers in memory transistors with ultrashort pulses holds promises for fast and remote device analyses (reliability, security, defectivity) and for new developments in the growing field of ultrafast microelectronics.

The behaviour of semiconductor materials and devices subjected to femtosecond laser irradiation has been under scrutiny, for many reasons, during the last decade. In particular, recent works have shown that the specific functionality and/or geometry of semiconductor devices, among which non-volatile memory (NVM) devices hold a special place, could be used to improve the knowledge about ultrafast laser-semiconductor interactions. So far, such an approach has been applied to draw conclusions about the spatio-temporal properties of laser propagation in bulk materials. Here, by comparing the evolution of the electrical characteristics of Flash cells under the cumulative effect of repeated femtosecond laser pulses with first-order physical considerations and TCAD (Technology Computer Aided Design) simulations, we clearly establish the role of the carriers created by nonlinear ionization on the functionality of the structures. The complete electrical analysis informs indirectly on the energy of the laser-produced free-carriers which, to date, was almost inaccessible by an experimental method applicable to the bulk of a material. Establishing the link between the carrier energy and laser parameters is of major importance to improve the comprehension of the nonlinear ionization mechanisms associated to intense laser-semiconductor interactions and applied in various fields from microelectronics to laser micromachining.

In this paper we present the behavior of a single nonvolatile Flash floating gate memory cell when it is irradiated, from the backside, by femtosecond laser pulses. For the first time we show that the memory cell state can change using this type of stimulation. The measurements were carried out with an experimental setup with an ad hoc probe station built around the optical bench. We present the experimental results using different memory bias conditions to highlight the charge injection in the floating gate. Then, we study the cell degradation to check the state of the tunnel oxide and the drain-bulk junction. The aim is to understand the failure mechanisms and use this technique for accelerated reliability tests. Finally we report the experimental results achieved for different laser energies.

The present paper proposes to investigate the effect of short pulsed Program/Erase signals on the functioning of Flash memory transistors. Usually, electrical operations related to said devices involve the application of single long pulses to various terminals of the transistor to induce various tunneling effects allowing the variation of the floating gate charge.  According to the literature, the oxide degradation occurring after a number of electrical operations, leading to loss of performance and reliability, can be reduced by replacing DC stress by AC stress or by reducing the time spent under polarization by the MOS-based devices. After a brief presentation of the functioning of the Flash memory transistors tested in this work, the experimental setup used to replace standard electric signals with short pulses will be described. Electrical results showing the benefits of programming and erasing non-volatile memories with short pulses will then be presented.

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