A. Candelori

Drain current decrease in MOSFETs after heavy ion irradiation

In this work, we have focused our attention on MOSFETs, which are the real basic elements of all CMOS applications. We have studied the immediate and latent effects produced by heavy ion irradiation on MOSFETs with ultrathin gate oxide, even after electrical stresses subsequent to irradiation. We found that a single ion can generate a physically damaged region (PDR) localized in the Si-SiO/sub 2/ interface, which may hamper the surface channel formation. In order to generate a PDR the ion hit must be close enough to MOSFET borders, i.e., in correspondence with the STI or the LDD spacer. Consequently, if both MOSFET W and L are large enough only few ion hits may give place to a PDR, mitigating the radiation damage. Finally we have developed an original model to describe the impact of the PDR on channel conductance in the ohmic linear region. On the basis of this model, we predict a PDR size around 0.2-1 /spl mu/m.

Radiation hardness of silicon detectors for high-energy physics applications

Oxygenated and standard (not oxygenated) silicon diodes processed by CNM and IRST have been irradiated by 27 MeV protons and compared with standard devices from ST Microelectronics. As expected, the leakage current density increase rate (/spl alpha/) and its annealing do not show any significant dependence on starting material, oxygenation and/or device processing. On the contrary, oxygenation improves the radiation hardness by decreasing the acceptor introduction rate (/spl beta/) and mitigating the depletion voltage (V/sub dep/) increase, with the /spl beta/ parameter depending also on starting material and/or effects related to device processing for standard diodes. Finally, these results are included in a general review on the state of the art for silicon detector radiation hardening, confirming the good performance of the considered technologies.

Statistical model for radiation-induced wear-out of ultra-thin gate oxides after exposure to heavy ion irradiation

In this work, we present an original model to explain the accelerated wear-out behavior of irradiated ultra-thin oxides. The model uses a statistical approach to the breakdown occurrences based on a nonhomogeneous Poisson process. By means of our model, we can estimate the number and the time evolution of those damaged regions produced by ion hits that generate breakdown spots during high field stresses after irradiation, including the dependence on the oxide field. Also, by using the proposed model, we have studied the wear-out dependence on the stress voltage, gate area, and ion fluence. In particular, by studying the stress voltage dependence of wear-out acceleration, it is feasible to extrapolate the device lifetime even at low operating voltage.

SEU sensitivity of virtex configuration logic

This paper presents a strategy of investigation of SEU upset mechanisms in the configuration logic of Virtex I devices. Measurement procedures specifically addressing configuration logic and a hardware set up for radiation testing are described. The results of a heavy ion radiation test are then presented. Previously unreported failure mechanisms have been observed and classified and their corresponding cross sections measured.

Lithium ion irradiation effects on epitaxial silicon detectors

Diodes manufactured on a thin and highly doped epitaxial silicon layer grown on a Czochralski silicon substrate have been irradiated by high energy lithium ions in order to investigate the effects of high bulk damage levels. This information is useful for possible developments of pixel detectors in future very high luminosity colliders because these new devices present superior radiation hardness than nowadays silicon detectors. The reverse current increase, the variation of the depletion voltage, and their annealing characteristics, as well as the charge collection properties, are presented and discussed.

Heavy ion effects on configuration logic of Virtex FPGAs

A heavy ion radiation test has been performed to evaluate the SEU sensitivity of Virtex devices. Differently from previous radiation tests, the one here described specifically addresses configuration logic. Previously unreported failure mechanisms have been observed and classified and their corresponding cross sections measured.

Semiconductor materials and detectors for future very high luminosity colliders

Recent results from the CERN RD50 Collaboration for the development of radiation-hard detectors for the LHC upgrade (Super-LHC) and in general for very high luminosity colliders are reviewed, summarized and discussed. Particularly, the attention is focused on emerging technologies (Czochralski and Magnetic Czochralski silicon, thinned detectors, highly doped thin epitaxial layer on Czochralski substrate, pre-irradiated silicon, p-type substrate devices) and new detector structures (3D, 3D-STC, Semi-3D, and Stripixel).

Improvement in breakdown characteristics with multiguard structures in microstrip silicon detectors for CMS

Abstract To obtain full charge collection the CMS silicon detectors should be able to operate at high bias voltage. We observed that multiguard structures enhance the breakdown performance of the devices on several tens of baby detectors designed for CMS. The beneficial effects of the multiguard structures still remains after the strong neutron irradiation performed to simulate the operation at the LHC.

Electron irradiation effects on thin MOS capacitors

Abstract The charge trapping properties of 8 nm thick oxide layers of metal-oxide-semiconductor capacitors have been studied before and after 8 MeV electron irradiation as a function of the radiation dose. The positive charge induced by ionizing radiation increases with the radiation dose without saturation up to 20 Mrad(Si). It does not disappear at room temperature even 10 days after the irradiation, but it annihilates by recombination if electrons are injected into the oxide by Fowler–Nordheim tunneling. After the positive charge recombination, we have observed, by capacitance–voltage and current–voltage measurements, increased electron trapping in the irradiated oxides, due to the neutral traps generated by radiation. The negative trapped charge increases linearly with the radiation dose, and its centroid is close to the center of the oxide but shifted to the gate/SiO 2 interface. Finally, the oxide degradation of irradiated and unirradiated devices by constant current stress has been measured: differences between irradiated and unirradiated devices disappear for fluences larger than 10 19 electrons/cm 2 .

Modifications of Fowler-Nordheim injection characteristics in /spl gamma/ irradiated MOS devices

In this work we have investigated how gamma irradiation affects the tunneling conduction mechanism of a 20 nm thick oxide in MOS capacitors. The radiation induced positive charge is rapidly compensated by the injected electrons, and does not impact the gate current under positive injection after the first current-voltage measurement. Only a transient stress induced leakage current at low gate bias is observed. Instead, a radiation induced negative charge has been observed near the polysilicon gate, which enhances the gate voltage needed for Fowler-Nordheim conduction at negative gate bias. No time decay of this charge has been observed. Such charges slightly modify the trapping kinetics of negative charge during subsequent electrical stresses performed at constant current condition.