Radiation effects and reliability of dielectrics in CMOS transistors and resistive memories

Abstract

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/HfO_x/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 7Ω/square. The non-reactive TiN has a sheet resistance of 300Ω/square and was found to contain signifificant 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 R_OFF value. It supports the theory of oxygen vacancies leading to the formation of conductive filaments.

Resistive random access memory based on TiN/HfO_x/TiN has been fabricated, with the stoichiometry of the HfO_x layer altered through control of atomic layer deposition (ALD) temperature. Sweep and pulsed electrical characteristics were extracted before and after ⁶⁰Co gamma irradiation. Monoclinic HfO_x deposited at 400ºC did not result in resistive switching. Deposition at 300ºC and 350ºC resulted in cubic HfO_x which switched successfully. Both stoichiometric HfO₂ and sub-oxides HfO_2-x result in similar memory characteristics. All memory cells are shown to be radiation hard up to 10 Mrad(Si), independent of stoichiometry.

Amorphous silicon carbide Cu/a-SiC/Au resistive memory cells are measured using the pulsed voltage technique and exhibit the highest R_off/R_on ratio recorded for any resistive memory. The switching kinetics are investigated and fitted to a numerical model, using thermal conductivity and resistivity properties of the dielectric. The SET mechanism of the Cu/a-SiC/Au memory cells is found to be due to ionic motion without thermal contributions, whereas the RESET mechanism is found to be due to thermally assisted ionic motion. The conductive filament diameter is extracted to be Φ~4 nm. The high thermal conductivity and resistivity for the Cu/a-SiC/Au memory cells result in slow switching but with high thermal reliability and stability, showing potential for use in harsh environments. Radiation properties of SiC memory cells were investigated. No change was seen in DC sweep or pulsed switching nor in conductive mechanisms, up to 2 Mrad(Si) using ⁶⁰Co gamma irradiation.

High-k metal gate MOS capacitors, valence change memory (VCM) and electrochemical metallization memory (ECM) cells have all shown high tolerance to ionizing radiation with negligible change seen in device parameters. This indicates that the radiation sensitive region within a memory circuit is the select device used to address the memory cell, such as transistors, and not the memory cell itself. In particular, within transistors, the gate oxide is essentially radiation hard, even when using high-k dielectrics, due to the thin layer. Therefore the areas within a circuit that are susceptible to ionizing radiation damage remain to be the buried oxides and isolation oxides.

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Identifiers

ORCID for Katrina Morgan: orcid.org/0000-0002-8600-4322

ORCID for C.H. De Groot: orcid.org/0000-0002-3850-7101

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