Suppressing the memory state of floating gate transistors with repeated femtosecond laser backside irradiations

Abstract

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.

Phenomenological modelling of non-volatile memory threshold voltage shift induced by nonlinear ionization with a femtosecond laser

“…It can thus be concluded that the only consequence of individual laser shots, after thermodynamic equilibrium is reached again, lies in a change of the floating gate charge and the potential distribution in the vertical direction of the injection zone. In our previous works 18,19 no damage of the tunnel oxide (surface or volume) has been observed, even at laser energies much higher than the ones employed in this study. This can indicate the effect of Anode Hole Injection (AHI) can here be neglected, especially since the biases applied on the Control Gate (and thus on the Floating Gate) are low.…”

“…This last equation, when combined with Eqs (5) and (6), allows a deeper understanding of the experimental results displayed in Figs 4 and 5. At a given laser intensity, and considering the fact that | V FG | = f (| V CG |) is an increasing function (see supplementary material of our previous work 19 ), the oxide electric field and thus the transmission coefficient of tunneling electrons will be enhanced as | V CG | gets higher. These considerations should explain why, as seen in Fig.…”

“…This basic model, in the light of which the subsequent experimental results will be interpreted, also relies on a few assumptions backed up by physical considerations or results discussed in our previous studies 18,19 . More specifically, it will be considered that femtosecond laser irradiation will not affect the electrical properties of the tunnel oxide because of the high value of the SiO 2 bandgap (around 9 eV) that would require ten-photon absorption to come into play for laser energy deposition at 1300-nm wavelength.…”

“…Since the feasibility of through-wafer two-photon absorption resulting from femtosecond laser irradiation has been demonstrated 9,10 , transient and charge collection measurements performed on semiconductor devices such as bipolar transistors, Fin Field-effect transistors (FinFETs), and diodes have been compared to numerical simulations in order to be able to emulate and further understand SEEs thanks to this method 11–17 . Our recent works have shown that the peculiarities of other semiconductor devices, such as Flash memory cells, can be used as a way to characterize the carriers generated by femtosecond laser irradiation in semiconductors 18,19 . Flash memory cells can be described, as depicted in Fig.…”

“…We have recently shown that femtosecond laser irradiation of initially programmed and erased Flash cells could provoke their progressive shifting towards the opposite electrical state 18,19 . As the biasing conditions of the cells were neither compatible with hot carrier injection nor with Fowler-Nordheim conduction during the laser shots, charge carrier transit through the injection zone, for laser energies below the threshold for irreversible device degradation, was attributed to the tunneling of energetic carriers created in the substrate and the floating gate.…”