A. Paccagnella

Radiation induced leakage current and stress induced leakage current in ultra-thin gate oxides

Low-field leakage current has been measured in thin oxides after exposure to ionising radiation. This Radiation Induced Leakage Current (RILC) can be described as an inelastic tunnelling process mediated by neutral traps in the oxide, with an energy loss of about 1 eV. The neutral trap distribution is influenced by the oxide field applied during irradiation, thus indicating that the precursors of the neutral defects are charged, likely to be defects associated with trapped holes. The maximum leakage current is found under zero-field condition during irradiation, and it rapidly decreases as the field is enhanced, due to a displacement of the defect distribution across the oxide towards the cathodic interface. The RILC kinetics are linear with the cumulative dose, in contrast with the power law found on electrically stressed devices.

A model of radiation induced leakage current (RILC) in ultra-thin gate oxides

An analytical model of Radiation Induced Leakage Current (RILC) has been developed for ultra-thin gate oxides submitted to high dose ionizing radiation. The model is based on the solution of the Schrodinger equation for a simplified oxide band structure, where RILC occurs through electron trap-assisted tunneling. The values of the model parameters have been calibrated by comparing the transmission probabilities obtained in this model with those obtained through the WKB method in the actual oxide band structure. No free fitting parameter has been introduced, and all physical constant values have been selected within the values found in literature. Different trap distributions have been considered as candidates, but the comparison between simulated and experimental curves have indicated that a double gaussian distribution in space and in energy grants the best fit of the experimental results for different ionizing particles, oxide fields during irradiation, radiation doses, and oxide thickness. Excellent matching has been found for both positive and negative RILC by using a single trap distribution. The trap density linearly increases with the radiation dose and decreases with the oxide field during irradiation. The trap distribution is spatially symmetrical in the oxide, centered in the middle of the oxide thickness, and is not modified as the cumulative dose increases.

Radiation Effects in Flash Memories

We review ionizing radiation effects in Flash memories, the current dominant technology in the commercial non-volatile memory market. A comprehensive discussion of total dose and single event effects results is presented, concerning both floating gate cells and peripheral circuitry. The latest developments, including new findings on the mechanism underlying upsets due to heavy ions and destructive events, are illustrated.

Low field leakage current and soft breakdown in ultra-thin gate oxides after heavy ions, electrons or X-ray irradiation

The excess leakage current across ultra-thin dielectrics has been studied for different ionizing radiation sources. Namely, X-rays, 8 MeV electrons, and three ion beams with different LETs values have been used on large area MOS capacitors with 4-nm thick oxides. Small oxide fields were applied during irradiation, reaching 3 MV/cm at most. For ionizing radiation with relatively low LET (<10 MeV cm/sup 2//mg), only Radiation Induced Leakage Current (RILC) was observed, due to the formation of neutral defects mediating electron tunneling via a single oxide trap. For high LET values, instead, the gate leakage current could be described by an empirical relation proper of Soft Breakdown (SB) phenomena detected after electrical stress. Moreover, the typical random telegraph signal noise feature of this Radiation induced Soft Breakdown (RSB) currents was observed during and after irradiation. RSB can be attributed to conduction through a multi-defect path across the oxide, produced by the residual damage of dense ion tracks. The oxide field applied during irradiation enhances the RSB intensity, but RSB can be achieved even for irradiation at zero field, being LET the main factor leading to RSB activation. The dose dependence of both RILC and QB have been investigated, showing a quasi linear kinetics with the cumulative dose. We have also studied the effect of modifying the angle of incidence of the ion beam on the intensity of the gate leakage current.

Anomalous charge loss from floating-gate memory cells due to heavy ions irradiation

We are presenting new data on the charge loss in large floating gate (FG) memory arrays subjected to heavy ion irradiation. Existing models for charge loss from charged FG and generation-recombination after a heavy ion strike are insufficient to justify (or in contrast with) our experimental results. In particular, the charge loss is by far larger than predicted by existing models, it depends on the number of generated holes, not on those surviving recombination, and it is larger for FGs with larger threshold voltage before irradiation. We show that these data can be explained as the effect of two different mechanisms. The first one is a semi-permanent multi trap-assisted tunneling (TAT), which closely resembles anomalous stress induced leakage current (SILC) in electrically stressed devices. The second mechanism is a transient phenomenon responsible for the largest part of the lost FG charge. Detailed physical modeling of this mechanism is still not available, owing to the limited knowledge of the physical background under these phenomena, but three possible models are explored and discussed.

Transient conductive path induced by a Single ion in 10 nm SiO/sub 2/ Layers

Large charge loss can happen in isolated conductive lines when hit by a single high linear energy transfer (LET) ion. We have demonstrated this phenomenon by using floating gate (FG) memory arrays, which allowed us to study it on the basis of a large statistical set of data. Charge loss is by far larger than that expected from a simple generation-recombination model. FGs hit by ions experience a charge loss linearly dependent on ion LET and on the electric field. We are proposing a semi-empirical model based on the idea that a conductive path assimilable to a resistance connects the FG to the substrate during the time (10/sup -14/ s) needed for electrons to escape the tunnel oxide. The model is fully consistent with a broad range of theoretical and experimental results, and has excellent fitting capabilities.

Data retention after heavy ion exposure of floating gate memories: analysis and simulation

Floating gate (FG) memories are the most important of current nonvolatile memory technologies. We are investigating the long-term retention issues in advanced Flash memory technologies submitted to heavy ion irradiation. Long tails appear in threshold voltage distribution of cells hit by ions after they have been reprogrammed. This phenomenon is more pronounced in devices with smaller gate area. Results are explained by a new physics-based model of the leakage current flowing through the damaged oxides of FG memory cells. The model calculates the trap-assisted tunneling current through a statistically distributed set of defects by using electron coupling to oxide phonons. The model is used to fit experimental data and to discuss retention properties after heavy ions exposure of future devices, featuring thinner tunnel oxide.

Angular Dependence of Heavy Ion Effects in Floating Gate Memory Arrays

Single, high energy, high LET, ions impacting on a Floating gate array on grazing or near-grazing angles lead to the creation of long traces of FGs with corrupted information. Up to 30 consecutive devices can be involved in the trace left by a single ion. We demonstrate that charge collection at multiple nodes can be expected as the technology advances. One of the major implications is that the widely adopted cosine law should be used with great care when dealing with modern devices, with sizes smaller than 100 nm.

Effect of different total ionizing dose sources on charge loss from programmed floating gate cells

We irradiated programmed Floating Gate (FG) memory arrays with different radiation sources, including 10 keV X-rays, /sup 60/Co /spl gamma/-rays, and 27 MeV protons. After irradiation, FGs experience a net charge loss which can degrade the stored information in terms of MOSFET threshold voltage. The charge loss is the result of two different phenomena: charge generation/recombination in the oxides and photoemission from the FG. The threshold voltage shift in irradiated devices depends on the radiation source: strong dose enhancement phenomena were found after X-ray irradiation, whereas proton results closely follow /spl gamma/-ray results.

Collapse of MOSFET drain current after soft breakdown and its dependence on the transistor aspect ratio W/L

Gate-oxide soft breakdown (SB) can have a severe impact on MOSFET performance even when not producing any large increase of the gate leakage current. The SB effect on the MOSFET characteristics strongly depends on the channel width W: drain saturation current and MOSFET transconductance dramatically drop in transistors with small W after SB. As W increases, the SB effect on the drain current fades. The drain saturation current and transconductance collapse is due to the formation of an oxide defective region around the SB spot, whose area is much larger than the SB conductive path. Similar degradation can be observed even in heavy ion irradiated MOSFETs where localized damaged oxide regions are generated by the impinging ions without producing any increase of gate leakage current.Gate oxide soft breakdown (SB) can have a severe impact on MOSFET performance even when not producing any large increase of the gate leakage current. The soft breakdown effect on the MOSFET characteristics strongly depends on the aspect ratio W/L: drain saturation current and MOSFET transconductance dramatically drop in transistors with small W/L after soft breakdown. As W/L increases, the SB effect on the drain current fades. The drain saturation current and transconductance collapse are due to the formation of an oxide defective region around the SB spot, the area of which is much larger than the SB conductive path. Similar degradation can be observed even in heavy ion irradiated MOSFETs where localized damaged oxide regions are generated by the impinging ions without producing, any increase of gate leakage current.