GROWTH AND STUDIES OF INSULATING FILMS FOR SILICON DEVICES

Abstract

In recent years the growth and or deposition of insulating films on the surface of a semiconductor material has received considerable attention as it is used for passivation, masking, isolation, dielectric purposes (gate dielectric) etc. Quality of the insulating films govern the performance of the device and to a some extent the packing density in integrated circuits. Silicon is by far the most widely used semiconductor material for fabrication of discrete, integrated circuits, LSI, VLSI, etc. due to its well established technology and its native oxide possess the outstanding qualities. Various techniques have been developed for the growth of oxide films on silicon such as thermal oxidation, plasma anodization and wet anodization. Thermal oxidation of silicon is the most commonly used technique for the growth of oxide film, due to superior Si-SiC>2 interface properties. However temperature involved in oxidation generates stacking faults and dislocations; produces stress and wafer warpage; and changes the impurity profiles previously formed in the substrate. With the advent of VLSI's it was realised that for further increasing the packing density on the single chip some low temperature technique should be evolved for the growth of oxide films so that the high temperature effects may be minimized.

In order to avoid the undesired effects of thermal oxidation and the encouraging results of anodic oxide films on GaAs, there is a renewed interest in growing thin insulating films on silicon by anodic oxidation. Present thesis deals with the growth and or deposition of insulating films on silicon at low temperatures. Due to the ease in fabrication, simplicity of structure and the sensitivity of the C-V characteristics to physical properties, MOS structures were used to study the influence of process parameters on the properties of insulating films. The material used in the investigation was n-type epitaxially grown silicon wafer of <111> orientation having resistivity of 6 to 8 ohm-cm. The resistivity of the substrate as 0.005 ohm-cm. A stringent cleaning procedure based n hydrogen peroxide solutions was used for surface preparation of samples. The electrolytic bath used for anodization was freshly prepared 0.04N KNO 3 solution in Ethylene-Glycol. Anodization parameters were optimised experimentally with a view to grow thin compact Si02 layers for applications in integrated circuit technology. The anodization was carried out at constant current density followed by a constant voltage mode. Anodically oxidised samples were annealed in H2 atmosphere, ohmic contacts were made at the back of anodized samples. The MOS structure was completed by evaporating ultrapure Al through a metal mask over the grown oxide film in a vacuum coating unit. The suitability w o (iii of the grown SiO film for devices was analysed by studying C-V, G-V and I-V curves. Interfacial and electrical properties such as surface state density, dielectric breakdown strength and dielectric constant were determined to assess the quality of grown oxide film. Experimental results reveal that the quality of anodically grown film of the order of 300 A° is comparable with that of thermally grown oxide films. It was also noted that the dielectric properties of the grown films deteriorates as the film thickness increases. During investigations it was observed that the anodiza tion of silicon samples at higher current density in an electrolytic bath of KNO 3 in Ethylene-Glycol mixed with small quality of water at elevated temperature results in porous films. The characteristics of such anodically grown films vary with the humidity of the surrounding environment. The capacitance of the such MOS structure was measured as a function of relative humidity. Results show that the porous SiO films can be used to realise humidity sensors. The proposed sensors may be suitable to realise I.C. sensors as it can be fabricated on silicon I.C. chips as an integral part of the circuit. Insulating A1?03 films were also deposited on silicon samples and on anodically grown layers by reactive evaporation of Aluminium to fabricate and study the MAS and MAOS struc tures. Aluminium oxide has higher dielectric constant,

higher density and large impermeability for impurity diffusion, further A1203 films gives a positive values of threshold voltage and stops fast diffusing alkali ions. Evaporation parameters and the annealing temperature were optimized for the fabrication of MAS and MAOS samples. From the measurements and results it was concluded that A1203 .film depo sited by reactive evaporation method is useful for use as gate insulator in MOS devices alone or as an auxiliary dielectric with SiO,,. Essentially the anodization and reactive evaporation technique here used is a low temperature fabrication technique and may be suitable in VLSI and MOS technology. With the scaling down of device dimensions the use of thin Si02 films with high dielectric breakdown strength are finding importance in MOS technology. Therefore anodically grown Si02 films of thickness of the order of 300 A0 is a promising substitute of the thermally grown films for VLSI and MOS technology.