GROWTH AND EVALUATION OF DIELECTRIC LAYERS FOR FABRICATING THIN-FILM RC STRUCTURES

Abstract

The growth and study of dielectric layers are receiving greater attention because of their wide and diversified applications in the fabrication of Thin-film distributed RC networks, interfacial devices, integrated circuits and in surface passivation of semiconductors etc. The performance of these devices and in particular, of those made with amorphous semiconductors is highly dependent on the quality of the insulating films used in them. Most of the amorphous semiconductors have very high resistivities. This imposes a condition on the resistivity of the dielectric layer to be still higher. Several oxide and nitride films such as silicon dioxide (SiO ), silicon nitride (Si_N, ) have been studied to meet these requirements but none of them resulted in stable high-quality dielectric layers. The use of alumina (AlpCL) layers as a dielectric has proved to be more promising as it is free from the instabilities like migration of foreign ions at elevated temperatures and high electric fields. Accepting this fact, a process of growing highquality alumina dielectric layers by wet anodization technique was developed. The electrolytic bath used consists of boric acid buffered with ammonium hydroxide and diluted with ethylene glycol. The properties of the alumina layers grown with this bath were found superior as compared to those grown in tartaric acid bath under the similar environmental

conditions in the laboratory. The anodization process was optimised by studying the effect of pH and the concentration of the boric acid in the electrolytic bath. Several Al-AlgCL-Al MINI structures were fabricated to study the quality and reproducibility of the as-.grown alumina dielectric layers. An arrangement for vacuum holding of the substrate on brass stage and a micromanupulator with two platinum ball tipped probes was constructed to make electrical contacts with the base aluminum film and top aluminum counter electrode of MIM structure. The whole arrangement was enclosed in light-tight grounded metallic box. The quality of the dielectric layer was determined by measuring its electrical parameters like resistivity, di electric constant and breakdown field. The dependence of the capacitance density and dissipation factor on frequency and the anodization voltage were also studied and reported in the thesis. The dissipation factor was strongly dependent on the post-baking of the alumina layers. The as-grown high-quality low-loss alumina dielectric layers were used to fabricate the distributed RC (RC) structures. The RC structures provide the characteristics such as sharper cut-off in filters and large phase-shift with less attenuation which can not be achieved with the circuits employing finite numbers of lumped elements. Fabrication of RC structures particularly for applications in audio frequency range where large values of Rpand Cj are required, is difficult

as they need dielectric layers of large area free from defects. Several uniformly distributed RC (URC) structures were fabricated using the as-grown high-quality alumina as a dielectric layer in between a top resistive film and a bottom aluminum conducting film. The top resistive film was deposited on the alumina layer by flash evaporation of nichrome through the mechanical mask. The characteristics of notch network made with this URC structure in coniunction with lumped resistor have been studied. Unfortunately the RC structures fabricated with alumina dielectric layers showed high leakage and, some times, even short circuit from the top resistive film to the bottom conducting film when the measurements were conducted on the structures after a month or so for determining the stability of the films with time. The use of silicon-monooxide as a protettive film over the alumina dielectric layer was thought to be the solution of the problem. Thus the URC structures were fabricated with composite dielectric layer consisting of alumina covered with thin layer of silicon-monooxide as a protective layer. These URC structures were used to form different circuits such as one port two-terminal networks', low-pass, high-pass, band pass and notch filters; attenuators and phase-compensating networks. The effect of variation of lumped load across out put terminals on the characteristics of the above filter networks was also thoroughly investigated. Adetailed study of the effect of variation of lumped resistor on notchfrequency as well as notch-depth is also presented. The

geometrical tapering of RC structure results in very useful improvement in cut-off rate of attenuation versus frequency characteristics of the filters and produce more phase shift with less attenuation than obtained with a URC structure. The exponentially tapered RC (ERG) structures with different degree of tapering were fabricated with the composite di electric layers to study the effect of tapering on the characteristics of the above networks fabricated with URC structures. The experimental results were compared with the theoretically computed ones to check their authentisity and accuracy. The analysis of RC networks made with RC structure in conjunction with lumped and active elements using con ventional methods is very complicated and lengthy. Avery simple and straight forward technique for analysing above mentioned experimentally-studied and other networks was developed and is presented in the thesis. The three..terminal two port RC structure is replaced by pi and tee lumpedequivalents. The three impedances of each lumped-equivalent model are expressed in terms of the characteristic function of RC structure. To make the analysis more easier the new and modified characteristic functions are used. It has been shown that the clearer insight into the mechanism of null and the optimum notch-parameters of the notch network can be obtained by analysing only one of each of the impedances of the pi and tee lumped-equivalents. The scope for further work in this field is also discussed in the thesis.