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

Maxime Chambonneau (1) , Sarra Souiki Souiki-Figuigui (2) , Philippe Chiquet (3) , Vincenzo Della Marca , Jérémy Postel-Pellerin (3) , Pierre Canet , Jean-Michel Portal (3) , David Grojo (1)

1 LP3 - Laboratoire Lasers, Plasmas et Procédés photoniques
2 CEA-LETI - Commissariat à l'énergie atomique et aux énergies alternatives - Laboratoire d'Electronique et de Technologie de l'Information
3 IM2NP - Institut des Matériaux, de Microélectronique et des Nanosciences de Provence

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 interaction between focused infrared femtosecond laser pulses and bulk crystalline silicon (c-Si) results in the formation of microplasmas 1 by multiphoton ionization. 2, 3 Thereby, one could expect to reach the critical electron density of c-Si by simply increasing the laser intensity. However, a recent theoretical study has demonstrated that this density cannot be reached with femtosecond laser irradiation due to plasma defocusing phenomenon which prevents the material from any permanent modification. 4 It is thus natural to investigate the possibility to handle the free-carriers produced in this regime of self-limited excitation. The most likely field where such a study can be applied is microelectronics. In this context, several studies have been conducted during the last decade on laser-transistor interactions. 5-7 Although these works have shown an impact of the laser on the device, irradiations at 1064 nm were involved implying only modest free-carrier production (ionization) at the transistor region. To solve this issue, El-Mamouni et al. employed a longer wavelength performing laser experiments relying on localized two-photon ionization on fin field-effect transistors (FinFETs). 8 Due to the nonlinear nature of the ionization of c-Si by photons with sub-bandgap energy, the volume of the subsequent plasma is extremely confined with a high electron density. Nevertheless, FinFETs are multigate structures making difficult the evaluation of the laser effect on each gate individually. Among all the existing microelectronic devices, one of the most relevant that is able to store laser-produced free-carriers is the Flash memory due to the possibility to evaluate the number of carriers trapped inside the floating gate (FG) of the device. 9-11 This latter is the place where electrons can be stocked, defining the logical state of the memory. When no charge is stored in the FG, the particular value of the threshold voltage () of the Flash cell is called the neutral threshold voltage (0). This state used to be reached after ultraviolet (UV) frontside exposure imparting to electrons sufficient energy to migrate inside or outside the floating gate. 12 However, this technique is not applicable anymore for advanced technologies because the stacks of metal used for electrical circuitry prevent UV penetration. Since the knowledge of 0 is crucial for the characterization and the reliability assessment of Flash memories, 13 the elaboration of a nondestructive, rapid and contactless experimental method for determining this parameter is essential.

Domaines: Science des matériaux [cond-mat.mtrl-sci]; Optique [physics.optics]

Maxime Chambonneau, Sarra Souiki Souiki-Figuigui, Philippe Chiquet, Vincenzo Della Marca, Jérémy Postel-Pellerin, et al.. Suppressing the memory state of floating gate transistors with repeated femtosecond laser backside irradiations. Applied Physics Letters, 2017, 110, pp.161112 - 161112. ⟨hal-01655116⟩