Reliability and 1/f noise properties of MOSFETs with nitrided oxide gate dielectrics

This thesis is broadly concerned with the electrical properties of MOS devices with thin (12 nm) nitrided oxide and reoxidized nitrided oxide gate dielectrics and specifically with the reliability and 1/f-noise properties of MOSFETs with such dielectrics. Nitrided oxides are formed by low pressure annealing of thermal silicon dioxide films in ammonia and reoxidized nitrided oxides are formed by low pressure annealing of nitrided oxides in oxygen. The annealing is accomplished at 950 'C in a custom-built low pressure furnace system operating at a pressure of 0.1 or 0.01 atmospheres for anneal times ranging from 15 minutes to 10 hours. Previous researchers have shown that sufficiently heavy nitridations of silicon dioxide provide resistance to interface state generation under electrical stress, insensitivity to ionizing radiation, and a barrier to various dopants and contaminants. However, the nitridation process, which is typically performed at atmospheric pressure and at high temperatures (2 1000 *C), is known to introduce a large number of electron traps and a high fixed charge density. The process of reoxidation has also been shown to be somewhat effective in reducing nitridation-induced electron traps. Thus, reoxidized nitrided oxides show promise as reliable dielectrics but suffer from high fixed charge which reduces the inversion layer mobility of devices. In this thesis, we demonstrate that light, low pressure nitridations coupled with reoxidations can be used to reduce fixed charge and trap densities to levels approaching those of silicon dioxide, while maintaining better reliability under electrical stress as well as under ionizing radiation. A capacitor study indicates that a reoxidized nitrided oxide process reduces interface state generation under high-field stress by more than a factor of 25 compared to silicon dioxide. While the process of nitridation and reoxidation degrades the pre-stress interfacial characteristics of oxide, we are able to achieve fixed charge densities as low as 2-3 x 1011 cm 2 and midgap interface state densities as low as , 3 x 1010 cm 2 eV1, while maintaining the improvement in reliability. A transistor study indicates that a low pressure reoxidized nitrided oxide process results in devices with a projected operating life that is one order of magnitude larger than for silicon dioxide devices. We achieve this with only a ~20% degradation in the electron and hole inversion layer mobilities. Since nitridation introduces oxide traps near the band edges of Si, this thesis also investigates the use of 1/f noise measurements as means of characterizing these traps. Two extensions of the basic number fluctuation model, which attributes noise to tunneling of channel electrons to and from interfacial oxide traps, are considered. In the first case, the effect of a nonuniform oxide trap distribution in space and energy is analyzed. It is theoretically shown that a nonuniform distribution of oxide traps can give rise to a gate voltage dependence in the magnitude and exponent, y, of the 1/fP spectrum. In the second case, an extension of the 1/f noise theory based on the McWhorter tunneling number fluctuation model is considered which includes both number fluctuations and correlated mobility fluctuations. Both the number and mobility fluctuations arise from the same physical mechanism involving electronic tunneling transitions between interfacial oxide traps and the MOSFET channel. The trapped electrons result in coulombic scattering of channel carriers causing mobility fluctuations. The model includes the dependence of coulombic scattering on the distance of the trapped charge from the interface and considers the consequence of such a scattering dependence on the shape of the 1/f noise spectrum. It is shown that the correlated model also predicts a gate voltage dependence in the magnitude and exponent of 1/fr noise even for the case of uniform trap distributions. Both the above noise models are then used to analyze 1/f noise data on oxide devices to extract the oxide trap density and distribution in space and energy. The number fluctuation model is also applied to characterizing the effect of ammonia and oxygen annealing of the gate oxide on the 1/f noise properties of nand p-channel MOSFETs. It is shown that nitridation increases the interfacial electron trap density in the oxide near the conduction band of silicon by a factor of 2-10 over control oxide devices. Reoxidation is shown to decrease the nitridationinduced interfacial electron trap density under certain conditions. Analysis of p-channel results indicate that the nitridation process increases the hole trap density near the valence band of Si by a factor of 2-6 over control oxide devices. Hole traps due to nitridation have not been previously observed. Reoxidation is shown to reduce the nitridation-induced interfacial hole trap density to values approaching those found in oxide. Thesis Supervisor: Professor Charles. G. Sodini Title: Associate Professor of Electrical Engineering and Computer Science