Device quality low temperature gate oxide growth using electron cyclotron resonance plasma oxidation of silicon

Jaju, Vishwas
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According to Moore's Law, which has been true so far, transistors density on a chip area doubles every two years. This increase in number of transistors requires reduced device dimensions or to say scaling by a factor of ∼0.7. As the device dimensions are shrinking, high temperature processes used in CMOS device fabrication, e.g. thermal oxidation of silicon to grow gate oxide, are becoming incompatible with CMOS processing. High temperature processes could change the impurity profile in Si, produce stresses on Si wafer, and also result in high thermal budget. In the past, many attempts have been made to reduce the oxidation temperature of Si by using high pressures of oxidizing gases, rapid thermal oxidation process, and plasma-assisted oxidation. In all these processes, plasma-assisted oxidation process, because of possible low processing temperatures, has gained considerable attention. However devices using plasma oxide have suffered with low oxidation rates, high interface defect density, plasma-induced damage and reliability issue.;This thesis presents the successful development of low temperature (∼100°C) silicon dioxide, which can be reliably used as a gate oxide in CMOS processing. Silicon dioxide was grown using an electron cyclotron resonance (ECR) plasma of oxygen/helium gaseous mixture. For the first time, MOSFET devices were fabricated using ECR plasma grown silicon dioxide as the gate oxide. Hot carrier induced (HCI) channel degradation of this gate oxide was studied and compared with thermally grown gate oxide. It was found that the high temperature annealing of ECR oxide improves channel resistance to HCI degradation.;In order to reduce the interface defect density and lower the processing temperature fluorine was added to the ECR plasma oxidation process. Small amount of fluorine gas in O2/He plasma was found to enhance the oxidation rate significantly and lower the defect density by an order of magnitude. Most importantly, fluorine incorporated ECR oxide grown at low temperature showed comparable HCI degradation as that of the thermal oxide with having to anneal it at high temperature.;MOS devices were fabricated to evaluate the oxide breakdown strength, interface defect density and oxide charges. Interface defect density (D it), for ECR oxide grown using 10% O2/He, was ∼1x10 11 cm-2eV-1, while for thermal oxide Dit was ∼3x1010 cm-2 eV-1. Defect density of the ECR oxide was significantly reduced by the inclusion of small amounts of fluorine during ECR plasma processing.;Plasma diagnosis was performed using optical emission spectroscopy (OES) and Langmuir probe. It was found that ECR plasma of pure oxygen gas has only O2+ ions and reactive oxygen (O*) present in the plasma. Langmuir probe was used to study the plasma density and plasma sheath potential. Fourier Transform Infrared (FTIR) spectroscopy was used to identify the bonds in plasma grown silicon dioxide. X-ray Photoelectron Spectroscopy (XPS) was used to examine the chemical composition of the oxide.;The effect of temperature, pressure and DC bias on growth rate was studied. Higher densities of reactive species were observed at lower pressures. Plasma oxide growth showed very little dependence on the temperature. It was demonstrated that a positive bias significantly increases the growth rate and high negative biases halts the oxidation process. Results from these process parameters helped in understanding the oxide growth mechanism and are presented in the results section.

Electrical and computer engineering;Electrical engineering