Millimeter-wave and terahertz technology for integrated circuits

Abstract

In recent years, there has been rapid growth in the use of millimeter-wave or Terahertz-wave frequencies for various applications like communication, imaging, medical sciences and space instrumentation. As the semiconductor processing technologies have enhanced from past years and with state of the art TMIC (Terahertz Monolithic Integrated Circuits) offering increased cut-off frequencies (>1 THz) of HEMT / HBT transistors, these applications have become even more feasible and can now be integrated onto a single chip for low cost and compact production. The work carried out in this thesis mainly deals with the development of passive structures such as transmission lines, antenna, couplers and power dividers, which are compatible to available TMIC processes using GaN on low resistivity silicon as a substrate. Techniques to reduce ohmic contact resistance for GaN HEMT technology was also investigated. To reduce losses caused by the substrate and to enhance performance of the integrated antenna at THz frequencies, passive structures were shielded using silicon nitride and metal, in addition to a layer of low dielectric material. Transmission lines were designed with operational frequencies up to 1 THz, in order to demonstrate losses presented by several dielectric mediums- air, BCB (Benzocyclobutene) and SiO2 (Silicon-dioxide). BCB and SiO2 were also investigated to evaluate antenna performance. For this, various types of antenna were designed - rectangular, circular, three variants of stack antenna (double rectangular, double circular, rectangular-circular), array antenna and stack array antenna. These designs were studied at two different frequencies, 300 and 650 GHz. Both simulated and measured results are presented, which show the importance of using low dielectric materials at THz frequencies. Other passive structures, such as couplers and power dividers were designed using the shielding technique mentioned above. Here, four different hybrid junction couplers (branch line, cascaded branch line, rat race, curved rat race) and one power divider (Wilkinson) were designed at three frequencies: 90, 300, and 650 GHz. The results presented by both antenna and couplers showed the viability of the shielding technique used. The optimisation of ohmic contact formation on AlGaN/GaN layers on LR Si has been studied by way of etching into the substrate under metal contacts. This investigation compared various etch patterns to standard, unetched, contacts. The depth of these patterns was varied to be above and below the substrates conductive channel and Ti/Al/Ni/Au metal schemes was used. A contact resistance below 0.3 Ω.mm was achieved using a grid etch pattern