Selection of substrate material for hybrid microwave integrated circuits (HMICs)

Abstract

This paper presents systematic approach of material selection for substrate material in hybrid microwave integrated circuits (HMICs). The  performance of HMICs starts deteriorating at microwave frequencies because the  circuit dimensions become comparable to the wavelength. So, it is important to design HMICs in such a way that its performance remains unaffected even at high frequencies. One way to fulfil this requirement is to fabricate a  proper device. For this, substrate material in HMICs plays an important role. But there are different types of materials available which can be used as substrate material of HMICs. The commonly reported materials are: Silicon (Si), Gallium Arsenide (GaAs), Indium Phosphide (InP), Alumina (Al₂O₃), Beryllia, Fused Silica, Sapphire, Woven PTFF/ Glass, PTFE/Glass, Polyolefin, and Ferrite/Granite. As each material is having its own advantages and limitations; therefore, it is important to choose the best possible material out of all possible materials. For this purpose two decision making techniques are used. Firstly, Ashby’s approach as the Multi-Objective Decision Making (MODM) technique is used, and then the Technique for Order Preference by Similarity to Ideal Solution (TOPSIS) as the Multiple Attribute Decision Making (MADM) technique is used. Ashby’s approach provides effective material selection technique to select the  best material based on their material and performance indices for better device performance. While TOPSIS approach is a decision making approach which is basically used to determine the  best alternative which should have the shortest Euclidean distance (S*) from the ideal solution (A*) and the largest Euclidean distance (S-) from the negative ideal solution (A-). For MICs, the material indices considered and evaluated are surface roughness, relative dielectric constant, loss tangent, thermal conductivity, and dielectric strength. It is observed that Sapphire followed by high-purity Alumina (99.5 percent pure or better) are good candidates for substrate material in hybrid MICs. The  surface roughness of Sapphire is better than that of Alumina, though the  dielectric strength of both these materials is the  same. A comparison between two different methods for material selection shows consistency between the  two methods of material selection. Both methodologies show that Sapphire is the  most suitable substrate material for hybrid MICs.