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Title: Radiation effects and reliability of dielectrics in CMOS transistors and resistive memories
Author: Morgan, Katrina
ISNI:       0000 0004 5361 1876
Awarding Body: University of Southampton
Current Institution: University of Southampton
Date of Award: 2015
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Many industries heavily rely upon advances in electronic devices. As development of electronics continues, new structures and new materials are being utilised. The reliability of these new technologies therefore need to meet the same high levels as the traditional technologies that they are replacing. Industries such as space and nuclear in particular, face an additional challenge affecting the reliability of their electrical devices; radiation. Ionizing radiation in particular can damage dielectric layers in devices such as metal-oxide-semiconductor (MOS) transistors and resistive memories. In either case, controlling the radiation effects of dielectrics is essential for the reliability of these devices. High-k MOS capacitors have been fabricated, analysed and irradiated and compared to a reference silicon dioxide MOS capacitor. Hafnium oxide and aluminium oxide were used for the dielectric layer, with Al and TiN used for the top electrode. C-V measurements indicated the high quality of the TiN=HfO2=Si structure in particular, with an interfacial equivalent oxide thickness (EOT) of less than half the total EOT of 1.9 nm. The TiN=HfO2=Si capacitor showed superior capacitance and leakage current properties when compared to silicon dioxide capacitors verifying the successful fabrication of high quality, high-k MOS capacitors. The ionizng radiation results showed a voltage shift of less than 100 mV up to 3 Mrad(Si) for the TiN=HfO2=Si capacitors. These radiation hardness levels are in the same order of magnitude as silicon dioxide structures. High-k dielectrics can therefore be used as a replacement for silicon dioxide gate oxides without decreasing the radiation hardness of the device, whilst simultaneously achieving reduced leakage current. Two types of TiN/HfOx/TiN resistive memory cells have been fabricated where the top 200 nm TiN electrode has been deposited by two different sputtering methods; reactive, using a titanium target in a nitrogen environment, and non-reactive, using a titanium nitride target. Characterization of the materials shows that the reactive TiN is singlephase stoichiometric TiN with a sheet resistance of 7/square. The non-reactive TiN has a sheet resistance of 300/square and was found to contain significant amounts of oxygen. The resistive switching behaviour differs for both memory cells. The reactive stoichiometric TiN device results in bipolar switching with a ROFF/RON ratio of 50. The non-reactive TiN results in unipolar switching with a ROFF/RON ratio of more than 103. These results show that an oxygen rich layer between the top electrode and insulator affects the ROFF value. It supports the theory of oxygen vacancies leading to the formation of conductive filaments. Resistive random access memory based on TiN/HfOx/TiN has been fabricated, with the stoichiometry of the HfOx layer altered through control of atomic layer deposition (ALD) temperature. Sweep and pulsed electrical characteristics were extracted before and after 60Co gamma irradiation. Monoclinic HfOx deposited at 400 °C did not result in resistive switching. Deposition at 300 °C and 350 °C resulted in cubic HfOx which switched successfully.
Supervisor: De Groot, Cornelis Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: TK Electrical engineering. Electronics Nuclear engineering