Growth Behavior And Stability Of High K ZrO₂-based Films

As astounding progress of ULSI being made, the feature size of transistors shrink rapidly down to the sub-0.1μm level and the SiO₂ gate oxide thickness scale below 2 nm. At such a thickness, conventional thermal SiO₂ is no longer applicable because of the excess direct tunneling leakage current, which directly results in an increase of power consuming and a decrease in the control ability of gate voltage. Thus, high-dielectric constant materials have recently gained considerable attention as a possible alternative. Replacement of a thin SiO₂ layer by the thicker material with higher dielectric constant (k>3.9) will reduce the gate leakage current and improve the reliability while keeping the capacitance equivalent oxide thickness constant. Among high k materials, ZrO₂ has attracted much attention as one of the most promising candidates due to its high bulk dielectric constant, wide energy bandgapand offsets.In this thesis, the reactive radio frequency magnetron sputtering has been applied to the fabrication of high k oxide films based on ZrO₂. With the analysis of atomic force microscopy, high-resolution transmission electron microscopy (HRTEM), varied angle spectroscopic ellipsometry, and measurement of capacitance-voltage (C-V), current-voltage (I-V) curves, the growth behavior and thermal stabilities of the films have been studied systematically. The results are summarized as follow:1. Oxygen partial pressure is one of important factors of affecting the growth behavior of ZrO₂ films. Upon increasing the oxygen ratio from 7% to 100% at the total pressure of 3 Pa, the phase transition of the films is a-ZrO₂ (amorphous)→a-ZrO₂ with few m-ZrO₂ (monoclinic)→m-ZrO₂+t-ZrO₂ (tetragonal)→m-ZrO₂. The reason of phase transition during film growth can be attributed to the decrease of deposition rate with the increase of oxygen ratio. A critical value of oxygen ratio can be expected to influence the thickness of the interfacial layer between ZrO₂ film and Si substrate. For the oxygen ratio below the critical value, the thichness of the interfacial layer is a constant. For the oxygen ratio higher than the critical value, however, the interfacial layer has an increase in thickness with the oxygen ratio. The influence of oxygen ratio on the interfacial layer in thickness is relative to the crystalinity of the deposited films. With the measurement of physical properties, the crystalinity is suggested to determine the dielectric constant of the deposited films. An amorphous film with optimized properties (higher dielectric constant²5, larger optical band gap, lower leakge current and smoother surface morphology) has been fabricated at the oxygen ratio of 15%.2. Deposition temperature is another important factor of affecting the growth behavior of ZrC>2 films. In the range of room temperature to 550°C, the phase transition of the films is a-ZrO2 (below 250°C) -? m-ZrO2 with few a-ZrO2 (450°C) -> m-ZrO2 with few t-ZxCh. (550°C). The different mechanisms dominate the growth behavior of the films deposited at different temperatures, non-diffusive mechanism at room temperature, limited diffusion mechanism in the range of 250 450 °C, and the mechanism of preferred orientation growth at 550 °C, respectively.3. In the annealed films in air, a phenomenon of oxygen diffusion along Z1O2 grainboundaries is first observed in the cross-section HRTEM image of the interfacial layer between ZrO2 film and Si substrate. The oxygen diffusion along Z1O2 grainboundaries is suggested to be the mechanism inducing the interfacial reaction during annealing process. An AI2O3 transition layer has been first employed to improve the thermal stability of the interface between Z1O2 film and Si substrate. The AI2O3 transition layer is revealed to be smooth in atomic scale and to prevent oxygen diffusion effectively.4. ZrC>2 films doped with different contents of AI2O3 have been fabricated with co-sputtering methods. Doping with low Al content results in the crystallization of Zr-Al-0 film at low temperature. Doping with high Al content, however, enables the thermal stability of the amorphous Zr-Al-0 films increase. In the case of Zr/Al atomic ratio of 5:4, the as-deposited film is amorphous with a little phase separation and has a crystallizing temperature as high as 800°C. After annealed at 800°C, the film has a very low leakage current, ixlO^AJcm2 and lxlO"5A/cm2 at 1.5V and -1.5V, respectively, being lower about six orders in magnitude than a pure SiO2 layer in the same electrical thickness of ² run.5. With the consideration in the formation of amorphous state and in the electrical requirment of high-A: gate film, a film with Zr-Al-Ti-0 compositions is suggested to be possible in the application to replace SiC>2 layer. The fabrication technology of Zr-Al-Ti-0 films has been studied and optimized. The results indicate that amorphous Zr-Al-Ti-0 films can be easy obtained over a wide range of compositions. The optimized film is a uniform amorphous and has a crystallizing temperature as high as 850°C. The film is expected to have a reasonable dielectric constant.