OPTIMIZATION OF MANUFACTURING TECHNOLOGY FOR EPITAXIAL-PLANAR VARICAP

Abstract

A varicap is a semiconductor diode that uses the dependence of the barrier capacitance p-n transition on the reverse voltage during its operation. This dependence is called the volt-farad characteristic. Varicaps are used in circuits for electronic adjustment of the frequency of an oscillating circuit, in amplifying parametric circuits, in frequency dividers and multipliers, in controlled phase shifters, etc. However, despite the widespread use, the cost of varicaps remains relatively high due to the low output of suitable diodes, which is explained by the high level of reverse current of the devices. The article discusses the causes and mechanisms of degradation of the reverse characteristics of the varicap. It is shown that the reason for the low yield of varicaps is the significant influence on their reverse characteristics of structural defects and foreign impurities and the quality of the surface of varicap structures. It has been established that the main reason for the low percentage yield of suitable investigated varicaps is epitaxial and oxidation defects of the packaging, as well as surface effects due to impurities. Epitaxial packaging defects are formed in silicon structures during the growth of epitaxial layers, oxidation packaging defects – in the process of high-temperature operations. In order to prevent the formation of structural defects, it was necessary to choose an effective method of gettering. Since structural defects are formed starting from the epitaxy process, it is obvious that the getter region must be created in the substrates on which the epitaxial layers will be deposited. The conducted studies showed that the most effective method of preventing the formation of structural defects in epitaxial layers is the creation of a getter region on the reverse side of the substrates by means of its laser treatment. As a result of the conducted research, it was established that annealing silicon wafers at a temperature of 1100°C in an argon environment after thermal oxidation makes it possible to significantly reduce the amount of the total charge in the SiO2 film. The proposed manufacturing technology of varicap structures using hetering with the help of processing the reverse side of the plates with a laser and annealing the plates in an argon environment is considered in detail. The experimental results of the study of the effect on the inverse characteristic of the varicap of the process of gettering and annealing of structures in an argon medium are presented, and the possible mechanisms of this effect are also analyzed. The effectiveness of the proposed technology using gettering and annealing of silicon wafers in reducing the level of reverse currents and increasing the output of suitable devices is shown.