A novel technique is described which was used to study the intrinsic breakdown mechanism in films of thermal SiO2 in the thickness range 30–300 Å. It was determined that high-field and high electron injection current conditions existing in the films just prior to breakdown result in the generation of a very high density of defects which behave electrically as stable electron traps. These traps are most likely generated close to the injecting electrode. The internal field in the oxide due to trapped electrons can approach 3×107 V/cm which appears to be the maximum field strength which Si-O bonding can withstand. At all temperatures between 77 and 393 °K, the breakdown mechanism is intimately related to the rate of generation of the electron traps. No evidence was found to support the impact ionization breakdown model. The technique is also described as a tool for yield measurements, with important implications for long-term reliability of MOS IC’s.
Films of thermal SiO2 in the thickness range 30−300 Å were stressed at fields approaching breakdown. Intrinsic breakdown was observed to be preceded by generation of a very high density of electron traps. These traps must be energetically located at least 4 eV below the SiO2 conduction band and may be assocated with broaken Si-O bonds. The ultimate breakdown strength of ultrathin films was found to be (2.8±0.4) ×107 V/cm. The present work suggests a new mechanism to explain intrinsic breakdown in films of thermal SiO2.
The storage of positive charge in the SiO2 insulator of MIS devices has been studied at both 300 and 80 K. It has been found that additional charge is stored in the oxide as a result of low−temperature x irradiation and behaves differently from that induced by room−temperature irradiation. This additional charge may be removed from the oxide by photodepopulation techniques, field emission, and thermal annealing. The portion of the charge which is present at both 300 and 80 K is shown to be insensitve to these treatments under the same experimental conditions. The experimental data indicate that the observed behavior is not due to positive ion transport within the oxide and strongly suggests that hole transport is occurring. Models for the trapping sites and the role of surface states are discussed.
Internal photoemission studies of the Si-sapphire interface of N-channel transistors fabricated on silicon-on-sapphire (SOS) indicate that the leakage current observed in such devices after exposure to ionizing radiation is due to holes trapped in the sapphire close to the silicon interface. These holes can be removed by recombination with electrons photoinjected into the sapphire from the silicon. The much smaller preirradiation N-channel leakage cannot be removed by electron photoinjection and is therefore thought to have a different origin.
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