J. Wyss

SIRAD: an irradiation facility at the LNL Tandem accelerator for radiation damage studies on semiconductor detectors and electronic devices and systems

This article describes the essential features of the SIRADfacility of the INFN Laboratori Nazionali di Legnaro. This facility, located at the 15 MV Tandem accelerator, is dedicated to radiation damage studies (bulk damage, total dose and Single Event Effects) induced by protons and heavy ions on semiconductor detectors, electronic devices and systems. SIRADis at present routinely used by groups involved in detector development for elementary particle physics, in electronic device physics and in space applications. # 2001 Elsevier Science B.V. All rights reserved.

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.

Heavy ion irradiation of thin gate oxides

We have studied the gate leakage current after heavy ion irradiation of MOS capacitors with thin gate oxides. In 3-nm and 4-nm oxides radiation-induced soft breakdown (RSB) occurs even after ion fluences as small as 100 ion hits on the device surface. The RSB conductive paths likely reproduce the ion hit distribution: some of them can drive a substantial fraction of the whole gate leakage current. The bias applied during irradiation enhances the RSB current intensity but no critical field exists to ignite the RSB, which is observed also under flat-band. The irradiated 3-nm oxides show smaller current variations and random telegraph signal (RTS) noise than the 4-nm oxides, owing to the higher current driven in fresh devices by direct tunneling conduction. The RTS noise increases with the radiation dose; it can be described successfully neither by a Levy nor by a Gaussian distribution. In 6.5-nm and 10-mn thick oxides the defect clusters generated by heavy ion irradiation can produce RSB and RILC (radiation induced leakage current), which have not been observed after low LET irradiation or electrical stresses.

Thin oxide degradation after high-energy ion irradiation

We have investigated the degradation induced by I- and Si-ions on 10- and 3-nm-thick oxide MOS capacitors. Ten-nanometer oxides were biased at low electric field (/spl les/3.3 MV/cm) during irradiation up to 100 Mrad(Si). DC radiation induced leakage current (RILC) has been observed after irradiation, and the differences of RILC characteristics between 10-nm and thinner oxides are discussed. In 10-nm oxides, RILC is attributed to multitrap assisted tunneling, which is reduced by subsequent Fowler-Nordheim electron injection. The density of the radiation-induced positive charged defect, the positive charge recombination by Fowler-Nordheim electron injection, and the negative charge trapping in radiation-induced neutral electron traps have been also addressed. On the other side, radiation-induced soft breakdown (RSB) is triggered by I-ions in 3-nm oxides at low doses (<1 krad(Si)) for moderate applied electric fields (4.4 MV/cm). Silicon ion irradiation is unable to produce RSB and RILC in 10-nm oxides, but it can generate a peculiar RILC in 3-nm oxides.

Radiation damage of standard and oxygenated silicon diodes irradiated by 16-MeV and 27-MeV protons

The effects of irradiation by 16- and 27-MeV protons on standard and oxygenated silicon diodes, processed by different technologies, have been investigated. The acceptor creation rate /spl beta/ can be lower for standard diodes than for state-of-the art oxygenated devices, suggesting that the role of oxygen is more complex than expected and must be folded with the technology of the fabrication process. In addition, we show the inaccuracy of the /spl beta/ normalization by the nonionizing energy loss factor not only for oxygenated diodes but also for standard nonoxygenated devices.

Observation of an energy dependence of the radiation damage on standard and oxygenated silicon diodes by 16, 21, and 27 MeV protons

Abstract First measurement of the energy dependence of the radiation damage induced by low-energy protons on standard and oxygen enriched diodes is presented. The current damage constant α is always insensitive to the oxygen content and increases for lower energy protons, whereas the acceptor creation rate β for both types of diodes slowly decreases for lower proton energies, this effect being amplified when the fluences are normalized to their 1xa0MeV neutron equivalent values. The dependence from the proton energy of the normalized β values is in open disagreement with the currently accepted NIEL hypothesis. Irradiations and measurements have been performed at the INFN Laboratorio Nazionale di Legnaro.

Lithium ion irradiation of standard and oxygenated silicon diodes

The next generation silicon detectors for future very high luminosity colliders or a possible LHC upgrade scenario will require radiation-hard detectors for fluences up to 10/sup 16/ 1-MeV equivalent neutrons/cm/sup 2/. These high fluences present strong constraints because long irradiation times are required at the currently available proton irradiation facilities. Energetic (58 MeV) Lithium ions present a nonionizing energy loss higher than protons and neutrons, and could consequently be a new promising radiation source for investigating the radiation hardness of silicon detectors up to very high particle fluences. Starting from this premise, we have investigated the degradation, as measured by the leakage current density increase and depletion voltage variations in the short- and long-term characteristics, induced by 58 MeV Li ions in state-of-the-art silicon diodes processed by two different manufacturers on standard and oxygenated silicon substrates. Finally, the correlation between the radiation damage induced by 58 MeV Li ions and 27 MeV protons is discussed.

Low- and high-energy proton irradiations of standard and oxygenated silicon diodes

Oxygenated and standard (not oxygenated) silicon diodes processed by two different manufacturers (ST Microelectronics and Micron Semiconductor) have been irradiated by low (27 MeV) and high- (24 GeV) energy protons. The leakage current density increase rate (/spl alpha/) and its annealing do not show any significant dependence on oxygenation and are the same for both manufacturers. Oxygenation improves the radiation hardness by decreasing the acceptor introduction rate (/spl beta/) and mitigating the depletion voltage (V/sub dep/) increase. Nevertheless, standard ST diodes present /spl beta/ values lower than Micron standard devices and close to oxygenated devices, whose /spl beta/s are similar for both manufacturers. The amplitude of the V/sub dep/ reverse annealing is reduced by oxygenation, which in addition delays the electrically active defect increase, at least for high-energy protons. Oxygenation is consequently the best approach for silicon substrate radiation hardening.

Ion electron emission microscopy at the SIRAD single event effect facility

The SIRAD facility at the 15 MV Tandem accelerator of the INFN Legnaro Laboratory is dedicated to characterizing the global sensitivity of electronic devices and systems to single event effects (SEE) due to ion impacts over a wide range of linear energy transfer. To map out device sensitivity with micrometric resolution, an ion electron emission microscope will be used to localize the impact point of each ion, with spatial resolution better than 1 μm, by imaging the secondary electrons emitted by the device. A fast position sensitive detector will handle rates >104 Hz. The system will be fully ion-impact reconstruction efficient for ions with Z⩾8.

Neutron irradiation effects on standard and oxygenated silicon diodes

Silicon diodes processed on standard and oxygenated silicon substrates by two different manufacturers have been irradiated by neutrons in a nuclear reactor and by the /sup 9/Be(d,n)/sup 10/B nuclear reaction. The leakage current density (J/sub D/) increase is linear with the neutron fluence. J/sub D/ and its annealing curve at 80/spl deg/C do not present any sizeable dependence on substrate oxygenation and/or manufacturing process. On the contrary, standard devices from one manufacturer present the lowest acceptor introduction rate (/spl beta/) for the effective substrate doping concentration (N/sub eff/), showing that the /spl beta/ dependence on the particular process can be important, overtaking the small substrate oxygenation effect. Finally, the average saturation value of the N/sub eff/ reverse annealing is slightly lower for the oxygenated samples, pointing out a positive effect of the substrate oxygenation even for devices irradiated by neutrons.